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The occurrence and behaviour of rare earth and associated
elements in lateritic regolith profiles in Western Australia
XIN DU
BSc (EnvSc)
This thesis is presented for the degree of Doctor of Philosophy
in Soil Science of the University of Western Australia
School of Earth and Environment
Faculty of Natural and Agricultural Sciences
2012
i
Statement of candidate contribution
This thesis contains published work and/or work prepared for publication, some of
which has been coauthored. The bibliographical details of the work and where it
appears in the thesis are outlined below (with percentage contributions from coauthors
in parentheses).
i. Du, X. (70%), Rate, A.W. (15%), & Gee, M. (15%), 2012. Redistribution and
mobilization of titanium, zirconium and thorium in an intensely weathered lateritic
profile in Western Australia. Chemical Geology 330-331, 101-115.
ii. Du, X. (70%), Rate, A.W. (15%), & Gee, M. (15%), 2012 (In press). Particle size
fractionation and chemical speciation of REE in a lateritic weathering profile in
Western Australia. Explore.
iii. Du, X. (70%), Rate, A.W. (15%), & Gee, M. (15%), 2011. Translocation and
fractionation of rare earth elements within intensely weathered lateritic profiles in
Western Australia. Mineralogical Magazine 75, 784. Oral presentation in the 2011
Goldschmidt Conference, Prague, Czech Republic. available at:
http://goldschmidt.info/2011/abstracts/finalPDFs/784.pdf
iv. Du, X. (70%), Rate, A.W. (15%), & Gee, M. (15%), 2010. Geochemical
mass-balance in intensely weathered soils, Darling Range, Western Australia. In:
Gilkes, R.J. & Prakongkep, N. (Editors). Proceedings of the 19th World Congress
of Soil Science: Soil Solutions for a Changing World. IUSS: Brisbane, Australia.
(Published on DVD; ISBN 9780646537832; available at:
https://events.ccm.com.au/ei/viewpdf.esp?id=126&file=E%3A%5Ceventwin%5Cdocs
%5Cpdf%5Csoil2010Abstract01769%2Epdf)
We hereby declare that the individual authors have granted permission to the candidate
(Xin Du) to use the results presented in these publications.
Student Signature
Coordinating Supervisor Signature
iii
ABSTRACT
The nature of the redistribution and fractionation of rare earth elements (REE) during
supergene weathering is not fully understood, especially in lateritic weathering which
is characterised by intense weathering and formation of ferruginous materials.
Therefore, the geochemical characteristics and mode of occurrence of REE were
investigated in four lateritic regolith profiles (GE, MQ I, MQ II and JG) developed on
granitoids with dolerite dykes in Western Australia. The outcomes of this study are
important factors to consider when using REE as tracers for the regolith weathering,
pedogenesis and sedimentation.
A high deficiency of REE relative to parent granitoids is typical of the regolith studied,
especially in the GE and JG profiles, suggesting high mobility of REE. Breakdown of
weathering-susceptible light REE (LREE)-rich minerals, such as allanite and/or
REE-rich fluorocarbonate facilitates depletion of LREE at early stages of weathering.
The released REE either are partially leached away by solutions, or precipitate as
secondary phosphates (e.g. rhabdophane and florencite). These secondary phosphates
play an important role in sequestering REE and hence limiting their further mobility,
especially for LREE. Trace to minor amounts of REE are associated with clay minerals,
Fe oxides/oxyhydroxides and organic ligands, and thus are retained in regolith, as
revealed by the sequential extraction experiment.
Heavy REE (HREE)-rich minerals, such as zircon, are relatively weathering-resistant
and thus HREE hosted by these minerals are not susceptible to inter-horizon transport.
Residual accumulation of weathering-resistant minerals is the main control on the
retention of HREE in intensely weathered regolith. The occurrence of REE is
dominated by mineral phases (the residual species) in intensely weathered lateritic
regolith as revealed by the sequential extraction. Therefore, the abundance, stability
and composition of LREE-rich secondary phosphates and HREE-rich
weathering-resistant minerals control the fractionation of REE in lateritic regolith, both
of which are largely affected by the weathering conditions.
iv
Cerium fractionated from the other REE and showed positive anomalies in the
duricrust of GE (Ce*=6.1) and JG (Ce
*=25.3) profiles. In the JG profile, neoformed
poorly crystalline (hydr)oxide phases enriched in Ce, Zr and Th were observed as a rim
attaching onto: (i) the wall of Al/Fe-rich pores in the duricrust, and (ii) the boundary
layer between Al-rich and Fe-rich rims in iron nodules of the upper ferruginous zone.
This suggests that: (i) Ce fractionates from other REE as hydrous cerianite
(CeO2∙nH2O) and precipitates with Fe oxyhydroxides during oxidative processes;
(ii) Zr and Th mobilize at the sampling scale during lateritization which is attributed to
the breakdown of thorite and REE-rich fluorocarbonates during initial weathering.
In the JG profile, trace concentrations of Yb (0.02-0.12 wt%) were determined in iron
cores and clay layers of iron nodules and minor concentrations of REE were associated
with extracted Fe oxide/oxyhydroxide species. Fine-grained (<10 µm) REE-bearing
phosphates were incorporated in crystalline Fe oxides in the duricrust, or occurred in
the clay layer of iron nodules. These imply that: (i) translocation of REE occurred both
at mineral and profile scales; and (ii) Fe oxides/oxyhydroxides are important in
redistribution and fractionation of REE at advanced stages of lateritization.
High concentrations of REE reside in the silt and clay size fractions, inferring that
formation of secondary minerals and adsorption by clay minerals augmented REE
concentrations in fine particle fractions. The sand size fraction had the lowest
concentrations but the highest mass of REE, indicating the dilution effect of quartz and
the importance of weathering-resistant minerals in retention of REE.
In addition, redistribution of Th into secondary phosphates as a trace component and
strong partitioning into gravel rather than matrix showed translocation of Th both at
mineral and profile scales. Absence of primary sphene crystals and observation of
dissolved ilmenite and rutile reflect the mobility of Ti at the mineral assemblage scale.
Fluctuation of Ti/Zr in the ferruginous zone in contrast to the consistency of Zr/Hf
throughout the JG profile (within the range of parent granitoids) implies that Ti and Zr
fractionate from each other during extreme weathering and advanced lateritization.
v
ACKNOWLEDGEMENTS
This thesis was completed with financial support from the International Research Fees
Scholarship (China Scholarships) and a Top-Up Scholarship provided by the
University of Western Australia (UWA), China Scholarship Council (CSC), and an
analytical funding (In-Kind Student Support) provided by the Association of Applied
Geochemists (AAG).
I would like to gratefully thank my PhD supervisors, A/Prof Andrew W. Rate and
A/Prof Mary Gee in the University of Western Australia for their expert assistance,
guidance, mentoring, and encouragement throughout my candidature. You helped me
with sample collection even in hot summer with flies all-around and helped me without
any complaints in correcting my grammatical mistakes and provided expert
commentary on my thesis. I feel very honoured to have had the opportunity to learn
from and be inspired by such exceptional scientists.
Thank you to our talented analytical chemist Michael Smirk, who provided me with
analytical assistance and practical advice; and I am thankful to Rick Pearce and Tracey
Quinn for their analytical expertise to help me analysing the trace elements in
sequential extractions and particle size fractions; I also want to thank Mr Frank
Nemeth, who helped me with the preparation of polished thin sections and polished
mounts with patience.
I am very grateful to David Adams, Janet Muhling and Peter Duncan from the Centre
for Microscopy, Characterisation and Analysis (CMCA) for their assistance, teaching
and sharing their expertise with me during my microscopy and microprobe analysis.
I would like to acknowledge the Australian Synchrotron for beamtime allocation which
allowed the completion of the Synchrotron X-ray Powder Diffraction (SXRD) and
Synchrotron X-ray Fluorescence Microscopy (SXFM) work in this thesis. I would also
like to thank Dr Justin Kimpton, the beamline scientist of SXRD, Dr David Paterson
and Dr Daryl Howard, the beamline scientists of SXFM and Dr Chris Ryan, the
scientist who developed the Maia detector and Geopixe software. Without your kind
vi
support, this work would not have been completed.
I would also like to thank Prof. Bob Gilkes and Prof. Martin Fey for their great advice
and comments on my research during my candidate.
In addition, thank you to all the lovely UWA Soil Science administration staff, Gail,
Karen and Margaret, for being so helpful and supportive during my study and thank
you to the amazing soil groups of staff and students, Ursula Salmon, Prof. Peng Bo,
Bree Morgan, Talitha Santini, Andrew Lucas, Yongjun Lu and Dr Nattaporn
Prakongkep and Georgina Holbeche, for sharing your extensive knowledge of the
study and for encouraging me when I was down.
And finally, thank you to my sweet and supportive family, Mum, Dad, Mum-in-law,
Dad-in-law and particularly my husband, Yan, who tolerated my temper when I met
research problems and who cooked delicious food to encourage me. Thank you for
your understanding, support and for making me laugh every day. Thank you.
vii
TABLE OF CONTENTS
1 Introduction ........................................................................................... 1
1.1 General introduction ........................................................................................ 1
1.2 Thesis scope ..................................................................................................... 3
1.3 Thesis structure ................................................................................................ 4
2 Literature review ................................................................................... 9
2.1 An introduction to rare earth elements ............................................................ 9
2.2 Chemistry of the REE .................................................................................... 11
2.3 Data presentation of REE .............................................................................. 11
2.4 REE-hosting minerals in granitoids and weathered regolith ......................... 12
2.5 Weathering intensity and geochemistry of REE ............................................ 13
2.5.1 Proxies for weathering intensity and flux change .................................. 13
2.5.2 Redistribution of REE in weathered regolith ......................................... 15
2.5.3 Fractionation of REE during weathering ................................................ 18
2.5.4 Mineral transformation of REE during weathering ................................ 19
2.6 Geochemical pathways of REE during lateritization .................................... 20
2.6.1 Definition of lateritic profiles ................................................................. 20
2.6.2 A typical lateritic profile ......................................................................... 22
2.6.3 Lateritization ........................................................................................... 22
2.6.4 Geochemical behaviour of REE during lateritization ............................. 23
2.6.5 Anomalies of Ce in lateritic regolith ...................................................... 25
2.6.6 Anomalies of Eu in lateritic regolith ...................................................... 26
2.7 Summary ........................................................................................................ 26
3 Description of the study areas ............................................................ 33
3.1 General geology and climate ......................................................................... 33
3.2 Sampling and profile description ................................................................... 33
4 Redistribution of major elements in lateritic profiles during
intensive weathering in Western Australia ............................................. 39
4.1 Abstract .......................................................................................................... 39
4.2 Key words ...................................................................................................... 39
4.3 Introduction ................................................................................................... 40
4.4 Materials and methods ................................................................................... 41
viii
4.4.1 Analytical methods ................................................................................. 41
4.4.2 Weathering intensity-Chemical Index of Alteration (CIA) .................... 44
4.4.3 Mass balance calculation ........................................................................ 44
4.4.4 Statistical analyses .................................................................................. 45
4.5 Results ........................................................................................................... 45
4.5.1 Weathering intensity of parent rocks and the regolith ............................ 45
4.5.2 Mineralogical properties ......................................................................... 52
4.5.3 Mass balance analysis of elemental loss and gain .................................. 53
4.5.4 Depth functions of pedogenic discontinuities ........................................ 59
4.5.5 Grain size distribution of major elements in MQ II profile .................... 60
4.6 Discussion ...................................................................................................... 60
4.6.1 Significant processes during lateritization .............................................. 60
4.6.2 Genesis and sources of Fe redistribution ................................................ 61
4.6.3 Degrees of lateritization ......................................................................... 64
4.6.4 Principal components analysis ............................................................... 64
4.6.5 Mineralogy and element grain size distribution ..................................... 67
4.6.6 Mobility of Ti and Zr .............................................................................. 68
4.7 Summary of the chapter ................................................................................. 69
5 Redistribution and mobilization of Ti, Zr and Th in an intensely
weathered lateritic profile in Western Australia .................................... 73
5.1 Abstract .......................................................................................................... 73
5.2 Key words ...................................................................................................... 74
5.3 Introduction ................................................................................................... 74
5.4 Materials and methods ................................................................................... 76
5.4.1 Analytical methods ................................................................................. 76
5.4.2 Mass balance calculation ........................................................................ 79
5.5 Results ........................................................................................................... 82
5.5.1 Bulk Ti, Zr and Th concentrations in regolith ........................................ 82
5.5.2 Mass balance of Ti and Th ...................................................................... 85
5.5.3 Mineralogical characteristics of Ti, Zr and Th in the JG profile ............ 86
5.5.4 Grain size distribution of Ti, Zr and Th in the lateritic regolith ............. 94
5.5.5 Partition of Ti, Zr and Th into different extraction species .................... 97
5.6 Discussion ...................................................................................................... 97
5.6.1 Mode of occurrence of Zr and Th in the lateritic regolith ...................... 97
5.6.2 Sources of Zr in poorly crystalline phases in duricrust .......................... 98
ix
5.6.3 Partitioning of Th between gravel and matrix ........................................ 99
5.6.4 Mobility of Ti in the JG profile ............................................................ 100
5.6.5 Geological parent mineralogy vs. weathering conditions .................... 101
5.7 Summary of the chapter ............................................................................... 102
6 Distribution and fractionation of REE in intensely weathered
lateritic profiles in Western Australia ................................................... 103
6.1 Abstract ........................................................................................................ 103
6.2 Key word ..................................................................................................... 104
6.3 Introduction ................................................................................................. 104
6.4 Methods and materials ................................................................................. 105
6.4.1 Analytical methods ............................................................................... 105
6.4.2 Calculation methods ............................................................................. 107
6.5 Results ......................................................................................................... 109
6.5.1 Geochemical data of REE ..................................................................... 109
6.5.2 Mineralogy of REE in the parent rock .................................................. 123
6.5.3 Mineralogy of REE in the regolith ....................................................... 125
6.5.4 REE in grain size fractions and chemical extractions of regolith ......... 132
6.6 Discussion .................................................................................................... 135
6.6.1 Evolution of REE-bearing minerals during intense weathering ........... 135
6.6.2 Reason for stronger depletion of LREE over HREE ............................ 137
6.6.3 Fractionation of REE in weathered regolith ......................................... 139
6.6.4 Ce and Eu anomaly ............................................................................... 139
6.6.5 Grain size fractionation and chemical speciation of REE .................... 140
6.7 Summary of the chapter ............................................................................... 142
7 Mode of occurrence of REE in an intensely weathered lateritic
profile in Western Australia ................................................................... 143
7.1 Abstract ........................................................................................................ 143
7.2 Key words .................................................................................................... 144
7.3 Introduction ................................................................................................. 144
7.4 Materials and methods ................................................................................. 145
7.4.1 Analytical methods ............................................................................... 145
7.4.2 Calculation methods ............................................................................. 147
7.5 Results ......................................................................................................... 149
7.5.1 Bulk geochemical data of REE ............................................................. 149
x
7.5.2 Mineralogy of REE in the parent rock ................................................. 161
7.5.3 Mode of occurrence of REE in lateritic regolith .................................. 162
7.6 Discussion .................................................................................................... 176
7.6.1 Geochemical pathways and fractionation of REE ................................ 176
7.6.2 Enrichment mechanism of Ce in ferruginous zone .............................. 177
7.6.3 Effects of Fe oxides/oxyhydroxides on mode of occurrence of REE... 178
7.7 Summary of the chapter ............................................................................... 179
8 Particle size fractionation and chemical speciation of REE in a
lateritic profile in Western Australia ..................................................... 181
8.1 Abstract ........................................................................................................ 181
8.2 Key words .................................................................................................... 181
8.3 Introduction ................................................................................................. 182
8.4 Materials and methods ................................................................................. 183
8.4.1 Analytical methods ............................................................................... 183
8.4.2 Calculation methods ............................................................................. 184
8.5 Results ......................................................................................................... 185
8.5.1 Concentrations of REE in different particle size fractions ................... 185
8.5.2 Mass loading of REE in different particle size fractions ...................... 185
8.5.3 Speciation of REE from sequential extraction ..................................... 186
8.6 Discussion .................................................................................................... 193
8.7 Summary of the chapter ............................................................................... 197
9 Conclusion and future work ............................................................. 199
9.1 Conclusion ................................................................................................... 199
9.2 Future work ................................................................................................. 201
9.3 Summary ...................................................................................................... 204
10 References ....................................................................................... 207
11 Appendices ...................................................................................... 229
xi
TABLE LIST
Table 2.1 Summary of REE in common minerals in granitoid rocks ............................. 28
Table 2.2 Concentrations of REE in different types of parent rock ................................ 30
Table 2.3 Concentrations of REE in different horizons of lateritic regolith profiles ...... 31
Table 4.1 Selected physical and chemical properties of matrix fractions (<2mm) of the
profiles studied ......................................................................................................... 43
Table 4.2 Concentrations of major elements in gravel and matrix of four lateritic
profiles...................................................................................................................... 47
Table 5.1 Sequential extraction procedures of trace elements in the lateritic regolith.... 77
Table 5.2 Concentrations of Ti, Zr and Th in grain size fractions of the JG profile ....... 81
Table 5.3 Element concentrations of minerals in Figure 5.4 and Figure 5.5 based on
EPMA in parent meta-granitoids and lateritic regolith in the JG profile ................. 88
Table 5.4 Element concentrations from EPMA in Figure 5.8 (a) and (b) ....................... 92
Table 5.5 Concentrations of Zr, Ti and Th in different sequential extraction species ..... 97
Table 6.1 concentrations of REE in parent rock and lateritic regolith of the GE and MQ
profiles.................................................................................................................... 118
Table 6.2 Concentrations of REE and associated elements from EPMA analyses of
representative minerals in parent granitoids from the GE profile .......................... 129
Table 6.3 Concentrations of REE and associated elements from EPMA analyses of
representative minerals in parent granitoids from the MQ profile ......................... 130
Table 6.4 Concentrations of REE and associated elements from EPMA analyses of
representative minerals in lateritic regolith from the MQ profile .......................... 131
Table 6.5 Concentrations of REE in sequential extractions of representative regolith in
the GE and MQ I profiles ....................................................................................... 134
Table 7.1 Concentrations of REE and derived fractionation parameters in parent
meta-granitoids and lateritic regolith from the JG profile ..................................... 154
Table 7.2 Element concentrations from EPMA analyses of representative minerals in
parent meta-granitoids (Figure 7.6) of the JG profile ............................................ 157
Table 7.3 Element concentrations from EPMA analyses of REE-bearing phosphates in
xii
lateritic regolith (Figure 7.8) of the JG profile....................................................... 158
Table 7.4 Element concentrations from EPMA analyses of weathering-resistant
minerals in lateritic regolith of the JG profile ........................................................ 159
Table 7.5 Element concentrations in Figure 7.11, Figure 7.12 & Figure 7.13 of the
duricrust and iron nodules in the JG profile ........................................................... 174
Table 7.6 REE concentrations of random spots in iron core and clay layer in iron
nodules from the A horizon and upper ferruginous zone of the JG profile ............ 175
Table 8.1 Concentrations of REE in grain size fractions of the JG profile ................... 190
Table 8.2 Concentrations of REE in different chemical extractions of representative
regolith in the JG profile ........................................................................................ 192
xiii
FIGURE LIST
Figure 3.1 Sampling sites (a, labelled as box) and sketches of the profiles sampled (b).37
Figure 3.2 Photographs of regolith from selected horizons of the GE profile. ............... 38
Figure 3.3 Photographs of regolith from selected horizons of the MQ I profile ............ 38
Figure 4.1 Ternary A-CN-K and A-FM-CNK plots of regolith samples from four
lateritic profiles (GE, MQ I, MQ II, JG) based on chemical compositions of matrix
and gravel samples. .................................................................................................. 51
Figure 4.2 Semi-quantitative mineralogical composition of regolith samples and parent
granitoids determined by random powder XRD analysis based on weighted
average of matrix and gravel .................................................................................... 55
Figure 4.3 Mass balance of major elements in regolith samples from four lateritic
profiles, based on weighted average concentrations of major elements in matrix
and gravel at each depth, using Zr as the reference element .................................... 56
Figure 4.4 Depth functions of the molar ratio Na/K and concentration ratio
Al2O3/Fe2O3 for MQ two profiles and concentration ratio (Ti/Zr)/10 for four
profiles...................................................................................................................... 57
Figure 4.5 Major element concentrations in grain size fractions of the regolith samples
from the MQ II profile. ............................................................................................ 58
Figure 4.6 The distribution of Al2O3 vs. SiO2 and Al2O3 vs. Fe2O3 in matrix and gravel
from four lateritic profiles ........................................................................................ 63
Figure 4.7 Schellmann SiO2-Al2O3-Fe2O3 diagrams showing different degrees of
lateritization of weathered regolith from four lateritic profiles ............................... 64
Figure 4.8 Principal component analyses of major elements in regolith samples and
parent granitoids from four lateritic profiles ............................................................ 66
Figure 4.9 Principal component factors of regolith samples and parent granitoids from
four lateritic profiles calculated using major element composition. ........................ 66
Figure 4.10 Calculated τ values of Al, Fe and Si referenced to Zr in matrix and gravel
from four lateritic profiles ........................................................................................ 71
Figure 5.1 Variation of Ti, Zr and Th with depth in the JG profile ................................. 84
Figure 5.2 Variation of Ti/Zr, Zr/Hf and Ti/Th with depth in the JG profile .................. 84
xiv
Figure 5.3 Mass balance calculations of Ti and Th against depth in the JG profile,
based on weighted average concentrations in matrix and gravel, using Zr as the
reference element ..................................................................................................... 85
Figure 5.4 Backscatter electron images of Ti-, Zr- and Th- hosting phases in parent
meta-granitoids of the JG profile ............................................................................. 87
Figure 5.5 Backscatter electron images showing Ti retained as ilmenite and Ti oxides
in the ferruginous mottled zone of the JG profile .................................................... 91
Figure 5.6 Diffraction patterns from SXRD showing evidence for transformation of Ti
from ilmenite and rutile in the ferruginous mottled zone (JG4) to anatase in the
duricrust (JG5) of the JG profile .............................................................................. 92
Figure 5.7 (a) The only partially dissolved zircon grain identified in the duricrust
(circled) and (b) a typical fractured, partially metamict zircon grain in the A
horizon (<1 m depth)................................................................................................ 92
Figure 5.8 Neoformed poorly crystalline Zr-hosting phases associated with Ce on pore
walls around Al/Fe matrix in the duricrust of the JG profile ................................... 93
Figure 5.9 Forms of Th persisting in regolith samples of the JG profile ........................ 95
Figure 5.10 Grain size distribution of Zr, Ti and Th in the JG profile. ........................... 96
Figure 6.1 REE distribution patterns of (a) rocks and regolith samples normalized by
the average chondrite; and (b) regolith samples normalized by the parent granitoid
in lateritic GE profile ............................................................................................. 112
Figure 6.2 REE distribution patterns of (a) rocks and regolith samples normalized by
the average chondrite; and (b) regolith samples normalized by the parent granitoid
in lateritic MQ I profile .......................................................................................... 113
Figure 6.3 REE distribution patterns (a) rocks and regolith samples normalized by the
average chondrite; and (b) regolith samples normalized by the parent granitoid in
lateritic MQ II profile. ............................................................................................ 114
Figure 6.4 normalized ratios (La/Sm)PR (LREE/MREE) and (La/Yb)PR (MREE/HREE)
and CIA of regolith samples against depth in three lateritic profiles ..................... 115
Figure 6.5 SiO2-Al2O3-Fe2O3 ternary plots and associated variation of REE
concentrations and ratios against the S/SAF weathering index for the GE profile.121
Figure 6.6 Mass balance calculations of REE against depth for three lateritic profiles,
based on weighted average concentrations of REE in matrix and gravel, using Zr
xv
as the reference element ......................................................................................... 122
Figure 6.7 Backscatter electron images of REE-bearing accessory minerals in parent
granitoids of the GE profile.................................................................................... 124
Figure 6.8 Backscatter electron images of REE-bearing accessory minerals in parent
granitoids of the MQ profiles ................................................................................. 127
Figure 6.9 Backscatter electron images of REE-bearing minerals in regolith of the MQ
profiles.................................................................................................................... 128
Figure 6.10 Concentrations of selected REE (La, Ce, Sm, Dy, and Yb) in grain size
fractions of the MQ II profile. ................................................................................ 133
Figure 6.11 Mass loading of selected REE (La, Ce, Sm, Dy, and Yb) in grain size
fractions of the MQ II profile ................................................................................. 133
Figure 7.1 REE distribution patterns of (a) meta-granitoids and regolith samples
normalized by the average chondrite composition; and (b) regolith samples
normalized by the parent meta-granitoid in the JG profile .................................... 151
Figure 7.2 Normalized ratios (La/Sm)PR (LREE/MREE) and (La/Yb)PR (MREE/HREE)
of regolith samples against depth in the JG profile ................................................ 152
Figure 7.3 Plots of (La/Sm)PR and (La/Yb)PR vs. La for the JG profile, illustrating the
degrees of depletion and fractionation of REE. ..................................................... 152
Figure 7.4 SiO2-Al2O3-Fe2O3 ternary plots and associated variation of REE
concentrations and ratios against the S/SAF weathering index for the JG profile. 156
Figure 7.5 Mass balance calculations of REE against depth in the JG profile, based on
weighted average concentrations of REE in matrix and gravel, using Zr as the
reference element ................................................................................................... 160
Figure 7.6 Backscatter electron images of REE-bearing accessory minerals in parent
meta-granitoids of the JG profile ........................................................................... 164
Figure 7.7 REE distribution patterns of fluorocarbonate and thorite normalized by the
parent meta-granitoids in the JG profile................................................................. 165
Figure 7.8 Backscatter electron images of REE-bearing secondary phosphate minerals
in regolith samples of the JG profile ...................................................................... 166
Figure 7.9 Images of REE-bearing secondary phosphates located in the clay layer of
iron nodules at 1.5m depth in the JG profile .......................................................... 168
xvi
Figure 7.10 Mapping of secondary rhabdophane in iron nodule at 1.5 m depth of the
JG profile................................................................................................................ 169
Figure 7.11 Cerium fractionated from other REE and occurring as a rim along the
Al/Fe-rich pores in the duricrust ............................................................................ 171
Figure 7.12 Cerium fractionated from other REE and occurring as a rim along the
boundary between clay and iron layers in iron nodules ......................................... 172
Figure 7.13 Cerium fractionated from other REE and occurring as joint matrix between
two iron cores within one large nodule .................................................................. 173
Figure 7.14 Backscatter electron images of crystalline Fe oxides intergrown with
micron-size Ce-rich secondary phosphates in the duricrust ................................... 174
Figure 8.1 Concentrations of REE in grain size fractions in the JG profile ................. 188
Figure 8.2 Mass loading of REE in grain size fractions in the JG profile .................... 188
Figure 8.3 Distribution of REE percentages in sequential extractions of the
representative regolith of the JG profile ................................................................ 189
Figure 8.4 Normalized ratios of (La/Sm)PR and (La/Yb)PR in particle size fractions and
sequential extractions in the JG profile .................................................................. 196
xvii
APPENDIX LIST
Appendix 11.1 Abbreviation ......................................................................................... 229
Appendix 11.2 ICP-OES analyses of the reference standards determined repeatedly
with samples for each analysis ............................................................................... 232
Appendix 11.3 R script for principal component analysis of major elements .............. 234
Appendix 11.4 Photographs of polished thin sections of iron nodules mounted on
quartz slides from the JG profile ............................................................................ 235
Appendix 11.5 Detailed operation procedure of the sequential extraction method ...... 236
Appendix 11.6 ICP-MS analyses of the reference standards determined repeatedly with
samples for each analysis ....................................................................................... 240
Appendix 11.7 EPMA detection limits of element concentrations in Ti-, Zr- and Th-
bearing minerals in the JG profile .......................................................................... 242
Appendix 11.8 EPMA detection limits of element concentrations in minerals of parent
granitoids and regolith samples from the GE and MQ profiles ............................. 243
Appendix 11.9 Concentrations of REE in grain size fractions of the MQ II profile .... 245
Appendix 11.10 EPMA detection limits of element concentrations in REE-bearing
minerals from the JG profile .................................................................................. 247
1
1 Introduction
1.1 General introduction
Supergene weathering is an important geochemical process for element cycling in
Earth surface environments, involving water/rock interaction and resulting in many
fundamental chemical changes. In turn, the composition of rock and regolith may
provide useful insights into the chemistry and nature of these interactions, including
the mechanisms of element mobility in crustal environments (McLennan, 1989). As an
index for petrological evolution, regolith weathering, pedogenesis and sediment tracing,
the geochemical cycling of rare earth elements (REE) is worthy of further and
sustained research.
It is widely accepted that REE can mobilize, redistribute and fractionate during
supergene weathering (Aide and Pavick, 2002; Aubert et al., 2001; Banfield and
Eggleton, 1989; Braun et al., 1993; Koppi et al., 1996; Laveuf and Cornu, 2009;
Nesbitt, 1979; Tyler, 2004). Although REE have been widely studied, the geochemical
behaviour of REE during weathering cannot be easily generalized because of: (i) wide
variance of REE-bearing minerals and their relative concentrations of REE; (ii)
different accessibility of these minerals to solutions and variance of solution chemistry;
and (iii) location-specific physicochemical and biological factors during weathering
(Bao and Zhao, 2008; Price et al., 1991). Even though, the mobilization and
fractionation of REE during weathering is proposed to be constrained mainly by the
primary REE-bearing minerals and weathering conditions (Aubert et al., 2001; Braun
et al., 1998; Ji et al., 2004; Nesbitt, 1979).
However, the precise sequence of events, behaviour and fractionation of REE during
rock weathering and pedogenesis are not completely understood, especially during
formation of iron nodules/ferruginous duricrust in intensely weathered lateritic profiles
defined by high concentrations of Fe oxides and oxyhydroxides. Although iron oxides
and oxyhydroxides are known to have high surface areas thus rendering them very
efficient sinks for many cations (e.g. Cu, Ti, V and Zn, etc.) and anions (e.g. phosphate
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
2
and silicate) (Singh and Gilkes, 1992), their effects on the translocation and
fractionation of REE in lateritic soils under superficial weathering and lateritization are
still not well understood.
Diverging views exist with regard to the stage at which REE fractionation starts during
the course of weathering and how weathering intensity affects REE fractionation.
Different mobility of heavy REE (HREE) and light REE (LREE) has been widely
reported in lateritic profiles: e.g. higher mobility of HREE relative to LREE (e.g.
Braun et al., 1993; Huang and Gong, 2001; Ma et al., 2007) in contrast to higher
mobility of LREE over HREE (Beyala et al., 2009; Braun et al., 1990; Ndjigui et al.,
2009; Nesbitt and Markovics, 1997).
The geochemical behaviour of REE during extreme weathering has been far less
studied than incipient or moderate weathering in temperate zones. Elements that are
conserved in temperate zones, such as REE, Ti and Zr, may become mobile during
extreme chemical weathering in tropical regions (Braun et al., 1993). Consequently, a
systematic understanding of the mobilization and translocation of REE and the
commonly considered conservative elements Ti, Zr and Th and their redistribution into
different solid phases is fundamental in order to use REE as a tracer for weathering and
sedimentation and Ti, Zr or Th as immobile elements to assess mass flux or volume
change during supergene weathering.
As the product of intense weathering, lateritic regolith represents one of the most
common superficial formations in the tropics, and is commonly diachronous, extending
over tens of millions of years (Dequincey et al., 2006). The genesis of the ferruginous
horizon of laterite, where polyphases are usually involved during weathering and
lateritization, is poorly understood; and the major and trace element behaviour during
its formation is also difficult to define. In Western Australia, extreme weathered
regolith (e.g. bauxitic) has been widely investigated, including the geological,
geographical and morphological information and geochemical characteristics.
However, the investigation of the geochemical behaviour of REE in intensely
weathered regolith profiles in Western Australia has not been as widely investigated,
Chapter One: Introduction
3
though the extreme weathering intensity may further enhance the mobility of the REE
during the lateritization processes.
Accordingly, this study investigates the abundance and mode of occurrence of REE in
the parent rock and lateritic regolith building on previous work by Sadleir and Gilkes
(1976 and the references therein), Brimhall et al. (1988; 1992) and Anand and Paine
(2002), Anand and Butt (2003; 2010) and Anand et al. (1991) etc.. The aim is to
improve our understanding of the geochemical behaviour and fractionation
mechanisms of REE during intense weathering and lateritization. Associated mobility
of Ti, Zr and Th are also examined, particularly in regards to the ferruginous duricrust
and iron nodules formed under extreme weathering and strong lateritization.
1.2 Thesis scope
This thesis investigates the occurrence, behaviour and fractionation of REE and
associated elements in four intensely weathered lateritic profiles developed on variably
metamorphosed granitoids with dolerite dykes in Western Australia.
Specifically, this thesis addresses the following objectives:
i. To understand more fully the environmental behaviour of major elements in
different solid phases and mineralogical and geochemical processes of lateritic
weathering.
ii. To investigate the mass flux change and mode of occurrence and thus to assess
the mobility of Ti, Zr and Th at different scales under intense lateritic weathering.
iii. To quantify the abundance, redistribution and residence of REE in different
regolith horizons in lateritic profiles and to investigate factors affecting the
mobilization and translocation of REE during intense weathering.
iv. To improve the understanding of the mode of occurrence and fractionation
mechanism of REE during lateritization, especially in iron nodules and ferruginous
duricrust, whose effects on the translocation and fractionation of REE under intense
supergene weathering are still not well understood.
v. To study further the REE redistribution in different grain size fractions and
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
4
chemical speciation in lateritic regolith and to improve understanding of the
translocation and fractionation of REE affected by weathering and particle size sorting
processes.
1.3 Thesis structure
This thesis is composed of nine chapters:
Chapter One (Introduction) provides general background information and introduces
the objectives, scope and structure of this thesis.
Chapter Two (Literature Review) surveys several critical areas of current knowledge
about geochemical behaviour and fractionation of REE during supergene weathering
and lateritization, defines the terminology used in this thesis, and gives a
comprehensive overview of the previous research on the topic.
Chapter Three (Description of the studied areas) describes the geological, geographical
and morphological information of the areas studied. It also describes the regolith
profiles studied (GE, MQ I, MQ II and JG profiles) in terms of horizons, texture,
colour, depth etc. and provides essential details of sampling procedures.
Chapters Four to Eight are five self-contained research papers. Each of these chapters
addresses one primary objective for this study:
Chapter Four (Geochemistry of major elements, objective i) investigates the
mineralogy and geochemistry of protolith and regolith for four lateritic profiles and
quantifies the mass flux change of major elements in different grain size fractions as
well as bulk regolith. This provides important insights into elemental redistribution
into different solid phases and enhances a holistic understanding of the environmental
behaviour of major elements during lateritization.
Following on from this, Chapter Five (mobility of Ti, Zr and Th, objective ii), taking
the JG profile as an example, investigates the mode of occurrence of Ti, Zr and Th in
Chapter One: Introduction
5
ferruginous materials and assesses the mobility of Ti, Zr and Th at the mineral
assemblage and profile scales by the combined use of electron probe micro-analysis
(EPMA) and synchrotron X-ray powder diffraction (SXRD), together with bulk
geochemical data. The source for Zr and Th in the neoformed phases is proposed to be
from the breakdown of thorite and REE-bearing fluorocarbonates rather than zircon
during the early stage of weathering. Strong partitioning of Th into gravel rather than
matrix reflects redistribution of Th at the profile scale. Fluctuation of Ti/Zr ratios in the
ferruginous zone in contrast to the consistency of Zr/Hf ratios throughout the profile
suggests that Ti and Zr fractionated from each other and partitioned between gravel
and matrix during extreme weathering and advanced lateritization. The results in this
chapter prove that Ti, Zr and Th are mobile at a variety of scales, despite their accepted
use as reference elements for studying element mass flux change. This novel
investigation substantially contributes to the current understanding of geochemical
behaviour and partitioning of Ti, Zr and Th under intense supergene weathering.
Chapter Six (Residence and fractionation of REE, objective iii) investigates the
residence, translocation and fractionation of REE in three (GE, MQ I and MQ II)
moderate to intensely weathered lateritic profiles. It addresses the factors controlling
the mobility and affecting the fractionation of REE during intense weathering. Strong
depletion of REE in the regolith suggests high mobility of REE beyond the profile
scale during extreme weathering. Higher depletion of LREE than HREE in regolith
compared with parent granitoids reflects the fundamental controls on the mobility of
REE by the mineralogy of the parent rock and the subsequent characteristics of the
weathering conditions. This is important information to enable confident use of REE as
tracers of geochemical processes in intensely weathered settings. The specifics of REE
depletion and fractionation help us to understand the geochemical pathways of REE
during supergene weathering, and have the potential to be a strategic clue for
mineralogical exploration.
Continuing along a similar theme, Chapter Seven (Mode of occurrence of REE,
objective iv) investigates the mode of occurrence of REE retention in ferruginous
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
6
materials in the JG profile developed over granitoids. Mode of occurrence of REE is
determined by the combined use of EPMA and synchrotron X-ray Fluorescence
Microscopy (SXFM). The occurrence of Ce, associated with Zr and Th, as neoformed
poorly crystalline (hydr)oxide phase forming a rim or coating around an Al-Fe matrix
in the pore system of ferruginous duricrust indicates mobilization and fractionation of
Ce during strong lateritization. To my knowledge, there are no other similar published
studies that identify Ce fractionated from other REE and associated with Zr and Th,
concentrated in ferruginous materials in lateritic regolith. Potential geochemical
signatures and fractionation mechanisms of Ce during lateritization are also explored.
The co-occurrence of Ce, Zr and Th in neoformed phases have been discussed in the
previous chapter (Chapter Five), which investigates the mobility of commonly
considered immobile elements Ti, Zr and Th.
In addition to an extensive study of the mineralogical residence of REE in lateritic
regolith of the JG profile, distribution of REE in grain size fractions and chemical
speciation is also examined in Chapter Eight (objective v). The dominance of REE by
mineral phases resistant to weathering identified in Chapter Seven is consistent with
residual forms in sequential extraction, grain size and electron microprobe data, in
intensely weathered regolith. However, REE can also be retained in the regolith as the
adsorbed ions or complexes/ligands of organic matter, clay minerals, and Fe
oxides/oxyhydroxides. Significant concentrations of REE were determined in the water
soluble, exchangeable and adsorbed species, and these are likely to represent
potentially bioavailable forms of REE. The potential for REE bioavailability is critical
for understanding the behaviour of REE in natural environments or under
anthropogenic influences.
Chapter Nine is a final integrated conclusion, which brings together the relevant
findings from each research paper in addressing the thesis objectives, as well as
identifying possible areas for future research.
Chapter One: Introduction
7
The submission details for each of the chapters are as follows:
(i) Du, X., Rate, A.W., and Gee, M. 2012. Redistribution and mobilization of
titanium, zirconium and thorium in an intensely weathered lateritic profile in
Western Australia. Chemical Geology, 330-331, 101-115.
(ii) Du, X., Rate, A.W., and Gee, M. 2012 (In Press). Particle size fractionation and
chemical speciation of REE in a lateritic weathering profile in Western
Australia. Explore.
(iii) Du, X., Rate, A.W., and Gee, M. 2011. Translocation and fractionation of rare
earth elements within intensely weathered lateritic profiles in Western Australia.
Mineralogical Magazine 75, 784. Oral presentation in the 2011 Goldschmidt
Conference, Prague, Czech Republic; available at:
http://goldschmidt.info/2011/abstracts/finalPDFs/784.pdf
(iv) Du, X., Rate, A.W., and Gee, M. 2010. Geochemical mass-balance in intensely
weathered soils, Darling Range, Western Australia. In: Gilkes, R.J. &
Prakongkep, N. (Editors). Proceedings of the 19th World Congress of Soil
Science: Soil Solutions for a Changing World. IUSS: Brisbane, Australia.
(Published on DVD; ISBN 9780646537832; available at:
https://events.ccm.com.au/ei/viewpdf.esp?id=126&file=E%3A%5Ceventwin%
5Cdocs%5Cpdf%5Csoil2010Abstract01769%2Epdf)
9
2 Literature review
2.1 An introduction to rare earth elements
The Lanthanides, also called rare earth elements (REE), are members of Group IIIA in
the periodic system and share very similar physical and chemical properties because of
their common outer electron shell configuration. The REE include: lanthanum (La),
cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm),
europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),
erbium(Er), thulium(Tm), ytterbium (Yb), and lutetium (Lu). A further element yttrium
(Y) is also a member of Group IIIA and has a similar chemistry to that of the REE, so
is sometimes grouped with the REE.
The REE are lithophile elements (Goldschmidt, 1937) that, excepting Pm, invariably
occur together naturally in all types of crustal rocks (Kabata-Pendias and Pendias,
2001). The terrestrial abundance of the REE decrease with increasing atomic weight,
and, in accordance to the Oddo-Harkins rule (Harkins, 1917), REE with even atomic
numbers are more abundant than those with odd atomic numbers (Kabata-Pendias and
Pendias, 2001). Usually the REE are subdivided into the light REE (LREE; from La to
Sm, those with low atomic numbers and masses) and the heavy REE (HREE; from Gd
to Lu, those with higher atomic numbers and masses). Very occasionally the term
middle REE (MREE), is loosely applied to the elements from around Pm to Ho
(Henderson, 1984).
Although similar slight variations in the physical and chemical properties of REE can
result in differences in solubility of compounds, stability of complexes and different
valences, Ce and Eu being the main examples of the latter. These slight differences
result in fractionation of the REE by many different geochemical processes, as a
consequence REE have traditionally been utilized in studies of the evolution of
petrological systems and magmatic processes (Bea and Montero, 1999; Forster, 1998;
Poitrasson et al., 1996; Taylor and McLennan, 1985), and, because of the potential for
differential leaching (Compton et al., 2003; Huang and Gong, 2001) during weathering,
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
10
REE are also widely used in the study of pedogenesis and weathering processes (Aide
and Smith-Aide, 2003; Laveuf et al., 2008; Lee et al., 2004; Patino et al., 2003; Viers
and Wasserburg, 2004), aqueous systems (Braun et al., 1998; Dupre et al., 1999; Viers
et al., 2000; Viers et al., 1997; Viers and Wasserburg, 2004), and for mineral
exploration (Bierlein et al., 1999; Brugger et al., 2008; Rao et al., 2004; Smith et al.,
2000; Yang et al., 2009).
In order to understand geochemical processes using REE as indices, it is fundamental
to determine REE concentration and characterize REE-bearing minerals. In recent
years, due to advances in instruments such as inductively coupled plasma mass
spectrometry (ICP-MS), laser ablation secondary ionising microprobe and modern
electron microprobes, and greater accessibility to synchrotron techniques and
secondary-ion mass spectrometry ion probe (SIMS), fast and accurate analyses of REE
in bulk samples and minerals with extremely low detection limits, tremendously high
sensitivity, and at micron-scale of spatial resolution can now be achieved (Bidoglio et
al., 1992; Cao et al., 2000; Chen et al., 1993; Schmidt et al., 2007; Williams, 1996),
enhancing studies on REE partitioning during hydrothermal alteration and supergene
weathering (Barrea and Bonzi, 2001; Brugger et al., 2008; Nakai et al., 2001; Schmidt
et al., 2007; Takahashi et al., 2000). For example, micro X-ray fluorescence (µ-XRF)
and micro X-ray absorption near edge structure (µ-XANES) spectroscopy enable
measurement of the distribution and oxidation state of Eu in-situ in scheelite at near
µm-resolution (Brugger et al., 2008); 100 ppm REE diopside glass standards
determined by NanoSIMS have yielded good reproducibility and accuracy and the
spatial resolution can achieve 5-10 µm by this technique (Ito and Messenger, 2009).
Rock weathering, as one of the critical processes in the surficial geochemical cycling
of elements, plays an important role in mobilization and translocation of REE. During
early pedogenesis, the chemical composition of an immature soil will be strongly
controlled by the composition of geological parent materials, whereas the chemical
composition of mature soils reflects the dominant effects of the weathering
environment and processes (Thanachit et al., 2006). These processes mobilize and
Chapter Two: Literature review
11
fractionate REE, and hence REE fractionation patterns can be used as tracers of
pedogenetic processes including dissolution of primary minerals, formation of
secondary minerals, redox processes, transport of material and ion exchange
(Middelburg et al., 1988).
2.2 Chemistry of the REE
Originating in identical external electronic shells (5s, 5p, 6s), the similar chemical and
physical properties of the REE result in their being classed as a geochemically
coherent group. Rare earth elements occur mainly as tri-valent (3+
) ions in nearly all
natural mineral, and only Eu2+
, Sm2+
and Yb2+
are stable in aqueous solutions, while
Ce is also stable as tetra-valent Ce4+
(Cerny et al., 1989). The mobility of REE is
limited because of the very low solubility of their phosphate minerals. Rare earth
elements can form stable complexes with carbonate, fluoride, hydroxide or sulphate
anions in alkaline solutions. The similar radii and oxidation states of the REE allow for
liberal substitution of the REE for each other into various crystal lattices. This
substitution accounts for the characteristic multiple occurrences of REE within a single
mineral (Castor and Hedrick, 2006). However, differences in filling of 4f-orbitals,
from zero electrons (La) to 14 electrons (Lu) results in a regular decrease of the ionic
radius, known as the ‘lanthanide contraction’ (Cerny et al., 1989). The differences in
mass and effective ionic radius lead to differences in typical isomorphic substitutions:
e.g. REE3+
for Cr3+
,V3+
, Fe3+
, Nb3+
, Sc3+
; Ce4+
for Th4+
, U4+
, Zr4+
; and Eu2+
for Ca2+
,
Sr2+
, Pb2+
(Cerny et al., 1989), forming different types of REE-bearing minerals.
2.3 Data presentation of REE
Rare earth element concentrations are usually presented graphically; where the REE
concentrations are normalized to a chosen reference material by dividing the
concentrations of each REE in the sample with the same REE in the reference
materials. The ensuing value is then plotted as the logarithm of the normalized
abundance versus atomic number (referred to as a REE distribution pattern in this
thesis). The reference materials can vary, depending on sample type and the aim of the
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
12
study, however, the mean carbonaceous chondrite values are the most commonly used
(McDonough and Sun, 1995). Other international reference materials include the North
American shale composite (NASC), an average upper crust composition (UCC), and
Post-Archean Australian Shale (PAAS) (Henderson, 1984; McLennan, 1989). In
addition to these commonly used reference materials, a part of the system under
investigation, e.g. parent geological material or one soil horizon from the investigated
profile (e.g. Laveuf et al., 2008), can also be used to normalize the REE concentrations
in order to show the fractionation and mass flux of REE as regolith evolves. In this
thesis, average chondrite (Anders and Grevesse, 1989) and the parent rock were chosen
as reference materials to normalize the REE concentrations to show the
depletion/accumulation and fractionation of REE after persistent intense weathering.
In addition, fractionation of REE in this thesis refers to the variation of concentration
of one particular REE (e.g. Ce) or a group of REE (e.g. HREE), relative to the other
REE or another group of REE (e.g. LREE). To study REE fractionation in as much
detail as possible, the REE are divided into three groups in this thesis (Henderson,
1984): LREE (from La to Nd), MREE (from Sm to Ho) and HREE (from Gd to Lu).
The normalized values (La/Sm)PR and (La/Yb)PR are used for identifying fractionations
between LREE-MREE and LREE-HREE relative to the parent rock (PR). Both Ce and
Eu anomalies were calculated by the following formula (1) and (2) respectively (N
refers to the relevant REE concentration in the reference material):
Ce*=(Ce/CeN)/[(La/LaN)
0.5×(Pr/PrN)
0.5] (1)
Eu*=(Eu/EuN)/[(Sm/SmN)
0.5×(Gd/GdN)
0.5] (2)
2.4 REE-hosting minerals in granitoids and weathered regolith
Rare earth elements occur in more than 200 minerals distributed across a wide variety
of mineral classes (Cerny et al., 1989; Henderson, 1984; Kanazawa and Kamitani,
2006). It is not possible to cover all of these minerals here; rather, discussion is limited
to REE-bearing minerals that are important and widespread during weathering of
granitoids and pedogenesis in regolith developed from such rocks. For a detailed list of
REE-bearing minerals refer to Henderson (1984) or Cerny (1989).
Chapter Two: Literature review
13
The major minerals in granitoids commonly contain negligible contents of REE, e.g.
mica and orthopyroxene. Quartz does not contain REE (Compton et al., 2003; Laveuf
and Cornu, 2009), whereas feldspar may contain negligible to moderate amounts and
commonly has a positive Eu anomaly (Bea, 1996; Condie et al., 1995; Laveuf and
Cornu, 2009). Amphibole and clinopyroxene have appreciable REE concentrations but
low Th and U (Bea, 1996). Garnet is not only a very efficient concentrator of the REE
but is preferentially enriched in HREE (Henderson, 1984), in a similar manner, epidote
is preferentially enriched in HREE and has a moderate to strong positive Eu anomaly
(Table 2.1).
Accessory minerals generally play important roles in the distribution of REE because
of their typically high partition coefficients for REE, indicating that these accessory
minerals act as sites of concentration for REE (Henderson, 1984). This is particularly
important in granitoids, which are abundant in REE and often contain a wide range of
accessory minerals. If these accessory minerals are particularly susceptible to
weathering, then the whole-rock REE pattern may be dramatically changed. Therefore,
the abundance and stability of these REE-bearing minerals are fundamental controls on
the subsequent mobilization and translocation of REE during weathering.
2.5 Weathering intensity and geochemistry of REE
2.5.1 Proxies for weathering intensity and flux change
Weathering modifies rocks and sediments at or near the Earth’s surface by a
combination of physical and chemical processes (Taylor and Shirtliff, 2003). To
quantitatively evaluate the chemical weathering intensity and pedogenesis, different
indices based on whole rock geochemical analyses have been proposed for evaluating
pedogenic processes and the degrees of alteration. In this thesis, three types of proxies
have been used to characterize element mass flux and weathering intensity of regolith:
(i) Chemical Index of Alteration (CIA) (Nesbitt and Young, 1982); (ii) Trace element
concentration ratio; (iii) Mass balance calculation (Brimhall et al., 1991).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
14
Based on the molar proportions of four major elements, the CIA is calculated
according to the formula (3) below (Nesbitt and Young, 1982):
CIA=100×Al2O3/(Al2O3+CaO*+Na2O+K2O) (molar basis) (3)
Where CaO* is CaO associated with the silicate fraction of samples (excludes
carbonates). It assumes that alkali metal elements are leached out from
aluminosilicates, such as feldspar and mica in ionic form, while Al2O3 is residual
during weathering and forms phyllosilicate clay and oxyhydroxide minerals. This
index has been used widely to identify multiple parent materials in soil profiles and
provide background weathering information to improve our understanding of
elemental mobility during weathering.
Trace element concentration ratios are favoured both as means of assessing element
partitioning and flux change during pedogenesis and weathering processes (Sheldon
and Tabor, 2009). The trace element ratios e.g. Ti/Zr, Ti/Th, Zr/Th, Zr/Nb, Ho/Dy,
Lu/Hf, Sm/Nd are considered to be relatively stable during initial and moderate
weathering, but sensitive during advanced stages of weathering (Condie et al., 1995;
Fernández-Caliani and Cantano, 2010; Sheldon and Tabor, 2009) and thus can be used
to evaluate element partitioning during intense weathering.
The mass balance calculation is a commonly used method to assess gains and losses of
various elements in soils, and is extensively used to understand pedogenesis and
investigate weathering intensity (Brimhall et al., 1991). It assumes that an immobile
element (e.g., Zr) behaves conservatively and can be used to calculate the mass flux
change of the other mobile elements (τ) and correct their concentrations for volumetric
strain (ε) during weathering and pedogenesis. The formula is given in Equation (4):
1))((,
,
,
,
, pj
wj
wi
pi
C
C
C
C
ji
(4)
In Equations (4), C represents concentration; subscript i identifies the immobile
element, j identifies the element of interest, w identifies weathered material and p
identifies parent rock. If τi,j = 0, the element has behaved conservatively; if τi,j = −1, the
element j is completely depleted during weathering; positive τi,j values signify absolute
enrichment.
Chapter Two: Literature review
15
This method of mass balance calculation is based on the assumption of a genetic
relationship between regolith and underlying material, and a reference element that is
relatively conservative during weathering in the regolith profile. The most frequently
used immobile elements are Zr, Hf, Sc, Ti, Nb, Ta and Th; however, in intensely
weathered lateritic profiles, it is difficult to define the ‘immobile element’. Although Ti,
Zr and Th have been considered to be the least mobile elements and thus used as
immobile reference elements in previous studies of laterite (e.g. Braun et al., 1993;
Brown et al., 2003; Ji et al., 2004), the mobility of these elements during intense
weathering is still under debate (e.g. Kahmann et al., 2008; Kurtz et al., 2000;
McLennan, 1995; Sheldon and Tabor, 2009).
2.5.2 Redistribution of REE in weathered regolith
The nature of REE redistribution during supergene weathering associated with
mineralogical reactions is not fully understood (McLennan, 1989). This is an important
avenue of research not only because more complete understanding of REE
geochemistry during weathering would provide important additional constraints on the
understanding of the weathering process; but also because documentation of REE
release and subsequent migration could provide insight on the geochemical behaviour
of REE during sedimentation processes and in aqueous geochemistry (McLennan,
1989).
Many field-based studies have demonstrated that REE can mobilize, redistribute and
fractionate during supergene weathering (e.g. Aide and Pavick, 2002; Aubert et al.,
2001; Banfield and Eggleton, 1989; Braun et al., 1993; Koppi et al., 1996; Laveuf and
Cornu, 2009; Nesbitt, 1979; Tyler, 2004), however, there is little agreement regarding
the overall magnitude or potential for fractionation among the REE. In some cases,
there is a failure to clearly address the distinction between small scale REE mobility
associated with mineral reactions and the larger scale transport of REE into or out of
the system under consideration (McLennan, 1989). Although REE have been studied
widely, the geochemical behaviour of REE during weathering cannot be easily
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
16
generalized because of: (i) the wide variance of both REE-bearing minerals and their
concentrations of REE; (ii) the different accessibility of these minerals to solutions and
the variance of solution chemistry; and (iii) the location-specific physicochemical and
biological factors during weathering (Bao and Zhao, 2008; Price et al., 1991). Despite
these uncertainties, the redistribution, mobilization and fractionation of REE during
weathering are proposed to be constrained mainly by the primary REE-bearing
minerals and the weathering conditions (Aubert et al., 2001; Braun et al., 1998; Ji et al.,
2004; Nesbitt, 1979).
The abundance of REE in regolith is usually dependent on the abundance of REE in
parent materials (e.g. Braun et al., 1990; Macfarlane et al., 1994), sites of concentration
in mineral phases (e.g. Braun and Pagel, 1994; Braun et al., 1993), the relative stability
of the mineral phases with respect to fluids (groundwater, penetrating soil water, etc.,
e.g. Braun et al., 1993; Lottermoser, 1990), weathering conditions (e.g. Ji et al., 2004;
Walter et al., 1995) and the types of soil (e.g. Hu et al., 2006; Mourier et al., 2008).
Concentrations of REE in the most common parent rock types are listed in the Table
2.2. Commonly, granitoids have higher REE contents than intermediate and mafic
rocks, while calcareous and ultramafic rocks have the lowest REE contents (Ding et al.,
2002). However, although developed from similar parent materials, regolith which
have undergone different weathering and pedogenic conditions, may also vary
significantly in their REE abundance (Ndjigui et al., 2009).
Generally, REE contents increase with depth (Nesbitt, 1979; Tyler, 2004) or with
decreasing weathering intensity (Ohlander et al., 1996; Taunton et al., 2000a). The
upper part of weathered regolith profiles is usually depleted in REE compared with
accumulation in the bottom part (Braun et al., 1993). Depletion of REE in the upper
part of profiles may reflect acidic conditions which cause REE mobilization, whilst
REE-retaining alkaline conditions dominate in the lower part (Nesbitt, 1979).
The main reason for these different REE redistribution patterns in weathered profiles is
thought to relate to weathering conditions. The mobility of REE is a complex function
of ionic radius and charge in solution, pH, Eh, water flux, history and weathering state
Chapter Two: Literature review
17
of the soil, redox conditions, water table, amount and type of inorganic and organic
ligands, microorganisms, and the nature of secondary and intermediate minerals
formed under different weathering conditions (Aubert et al., 2001; Braun et al., 1998;
Ji et al., 2004; Ma et al., 2004; Price et al., 1991; Taunton et al., 2000a; Taunton et al.,
2000b). The high degree of variability of this wide range of weathering conditions may
lead to variable abundance and translocation of REE in regolith. In addition,
volumetric change during weathering of the parent materials should also be considered
when interpreting REE mobility, thus, relative mass flux change based on mass
balance calculations has an advantage given the volumetric change. Lateral
redistribution of REE in landscapes as a component of catchment scale pedogenetic
processes should also be considered (Sommer et al., 2001; Sommer et al., 2000)
because REE are likely to be subject to horizontal redistribution by groundwater.
In addition, data on the nature and stability of REE complexes at low temperature are
of critical importance to the geochemical exploration for REE in supergene
environments (Wood, 1990). The REE can be mobilized in solution by forming stable
complexes such as carbonate, fluoride, phosphate and oxalate. In neutral and alkaline
solutions (7≤pH≤9), carbonate complexes dominate REE speciation thus enhancing
REE mobilization (Johannesson et al., 1996); conversely, phosphate tends to removal
REE from solution (Johannesson et al., 1995). The complex Ln(CO3)2- (Ln denotes
REE) is strongly enriched in HREE over LREE whereas LnCO3+ (stable in seawater) is
the opposite (Cantrell and Byrne, 1987). At pH 6.5-9.5, LnCO3
+ predominates whereas
at pH ≥9.5 the Ln(CO3)2- complex is favoured (Wood, 1990). When the pH is 2-6.5,
REE occur mainly as simple ions and sulphate complexes (Wood, 1990). However,
lack of systematic experimental data for: (i) all REE across a wide range of pH, (ii) the
presence of multi-complex phases, (iii) the limited investigations of the complexation
of Ce4+
and Eu2+
and stability constants for REE phosphate complexes, and (iv)
conflicting data about the thermodynamics of REE hydrolysis (Wood, 1990), all
restrain further understanding of the mobilization of REE during water/rock interaction
under natural supergene weathering conditions.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
18
2.5.3 Fractionation of REE during weathering
Although fractionation of REE may occur during weathering processes (Koppi et al.,
1996), exactly how weathering intensity exerts effects on REE fractionation, and the
precise sequence of events and fate of REE during weathering is not fully understood.
This information is necessary in order to use REE as tracers for supergene weathering,
pedogenesis and sedimentation processes.
Differences in weathering rates and the formation of more element-specific secondary
minerals (Tyler, 2004) during weathering may also result in REE fractionation in
weathering profiles. However, diverging views exist regarding at which stage the
movement and differentiation of REE start during rock weathering and soil formation
(Zhang et al., 2007). Banfield and Eggleton (1989), Price et al. (1991) and Sharma and
Rajamani (2000) illustrated that REE contents change dramatically during the initial
stages of weathering, while Middelburg et al. (1988) and Duzgoren-Aydin and Aydin
(2009) proposed that the migration and differentiation of REE occurs at advanced
stages of weathering. In early and intermediate weathering, mineral abundances may
control REE abundances (Banfield and Eggleton, 1989; Nesbitt, 1979) but for
advanced weathering in laterite, the relatively greater mobility of the HREE appears to
be more significant (Braun et al., 1993; Braun et al., 1990; Brown et al., 2003).
The lower mobility of LREE compared to HREE commonly results in a significant
relative enrichment of LREE and depletion of HREE in weathering products after
extensive weathering (Braun et al., 1993; Braun et al., 1990; Compton et al., 2003;
Koppi et al., 1996; Nesbitt, 1979), however higher mobility of LREE over HREE have
also been reported in intensely weathered environments (e.g. Beyala et al., 2009; Braun
et al., 1990; Ndjigui et al., 2009; Nesbitt and Markovics, 1997). Light REE are known
to be hydrolysed more easily than HREE, whereas HREE preferentially form more
stable inorganic complexes than LREE, particularly with carbonate, fluoride,
hydroxide or sulphate anions in alkaline solutions (Åström and Corin, 2003) and are
more likely to desorb from clay minerals than LREE. This may explain why HREE are
more prone to mobilization than LREE and translocation to the lower parts of regolith
Chapter Two: Literature review
19
profiles (Aubert et al., 2004; Aubert et al., 2001; Cantrell and Byrne, 1987; Ma et al.,
2007). However, this process is believed to be controlled by pH and the type of REE
complexation (Cantrell and Byrne, 1987; Johannesson et al., 1995; Johannesson et al.,
1996; Wood, 1990).
The relationship between weathering intensity and the mobilization and fractionation
of REE is also unclear. Some previous studies have shown that there is no significant
correlation between the degree of REE fractionation and any of the following;
chemical weathering intensity (by CIA) (Caspari et al., 2006), physical weathering
intensity (by particle size fraction index) (Caspari et al., 2006) and rock weathering
and soil formation (Minarik et al., 1998; Zhang et al., 2007). However, the degree of
inter-horizon transport of REE has been proposed to have great potential to become an
index of weathering intensity (Aide and Christine-Aide, 2012). Then this poses the
question that what is the association between the mobility and fractionation of REE
and the weathering intensity? What factors control REE fractionation during
weathering? And what mechanisms are involved during initial and advanced
weathering? An important research issue, the mobilization and fractionation
mechanisms of REE during weathering, is therefore discussed in detail in this thesis.
2.5.4 Mineral transformation of REE during weathering
The transformation of REE minerals during weathering processes depends on their
susceptibility and the weathering conditions. Rare earth elements-bearing minerals can
be subdivided into three groups according to weathering susceptibility: (i) strongly
resistant to weathering e.g. xenotime and zircon; (ii) moderately resistant to weathering,
e.g. monazite; and (iii) weakly resistant to weathering, e.g. allanite and the
fluorocarbonates (bastnäsite and parisite) (Bao and Zhao, 2008). During initial and
moderate stages of weathering, the weathering-susceptible minerals: feldspar, biotite,
allanite, epidote, apatite etc., preferentially dissolve in low pH solutions. The REE
released during this process may: (i) be lost from the weathering profile via transport in
solution (Condie et al., 1995), (ii) form secondary minerals (Braun and Pagel, 1994),
(iii) be incorporated into or adsorbed onto clay minerals (Vos et al., 2006), and (iv) be
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
20
adsorbed onto Fe/Mn oxyhydroxides (Aide and Smith-Aide, 2003; Aide et al., 1999).
Simultaneously, weathering-resistant heavy minerals such as zircon and rutile are
retained as residuum in weathered regolith, and the REE (especially HREE) included
in these heavy minerals are not expected to be highly mobilized during pedogenesis,
except under intense or extreme weathering. Under extreme weathering conditions,
both secondary and weathering-resistant heavy minerals may partially or completely
alter (Braun et al., 1993; Taunton et al., 2000a; Taunton et al., 2000b). In this process
quartz, being relatively resistant to weathering, acts as a diluent in the regolith (Hardy
and Cornu, 2006). Incorporation or reformation of secondary florencite, rhabdophane
and/or churchite (Braun and Pagel, 1994) is believed to be the main pathway for LREE
retention in weathered regolith (Nedachi et al., 2005).
2.6 Geochemical pathways of REE during lateritization
2.6.1 Definition of lateritic profiles
The study of regolith spans many disciplines of the Earth Sciences (Anand and Paine,
2002), and thus many definitions are confusing and may lead to misunderstanding.
Therefore, it is necessary to define terms from the perspective of regolith geochemistry.
In this thesis the term ‘laterite’ refers to Fe-rich weathering profiles which have
undergone intense supergene weathering (Anand and Butt, 2000; Anand and Butt,
2003; Anand and Butt, 2010; Anand and Paine, 2002); and the other key terminology
for deeply weathered profiles used in this thesis is summarised below, following the
definitions published by Anand and Paine (2002).
A typical laterite profile commonly includes saprolite, mottled clay zone, ferruginous
zone and surface soil. Saprolite refers to nearly isovolumetrically weathered bedrock
retaining the fabric and structure of the parent rock, pseudomorphically replacing the
primary minerals. The mottled zone has macroscopic segregations of subdominant
colour that differ from the surrounding matrix and mottled clay zone is dominantly
composed by secondary clay minerals. The ferruginous zone is composed
predominantly of secondary oxides and oxyhydroxides of Fe (goethite, hematite,
Chapter Two: Literature review
21
maghemite), hydroxides of aluminum (e.g. gibbsite, boehmite) and kaolinite, with or
without quartz. The upper lateritic profile consists of ferruginous mottled zone,
ferruginous duricrust and loose gravel. Ferruginous mottled zone has a goethite rich
halo with sharp or diffuse boundaries, whereas ferruginous duricrust is a cemented
hard layer composed of various Al-Fe secondary segregations that originated from
underlying parent rock. Distinct from the ferruginous duricrust, ferricrete is a product
of cementation and conglomeration of surficial sands and gravel by Fe oxides, where
no genetic relationship between the Fe and the underlying mottled and saprolite zones
is inferred (Anand and Butt, 2010). In this thesis, ‘duricrust’ is used in short for
‘ferruginous duricrust’ since the ferruginous duricrust is the only type of duricrust
present in the lateritic profiles studied. Regolith is used as a collective term for the
weathered and transported materials covering fresh rock, which have been formed by
various geochemical processes e.g. weathering, erosion, transport and/or deposition of
older material.
Laterite represents one of the most common superficial formations in the tropics,
covering approximately 30% of the continents (Dequincey et al., 2002), the formation
of which can extend of 10’s of Ma (Dequincey et al., 2006). Lateritic profiles are
commonly thought to have formed in tropical climates with relatively high
temperatures and seasonal rainfall. However, laterite can also form in wet, cool to cold
climates given sufficient time (Gozzard, 2007). In contrast to common pedogenesis,
during lateritic weathering, lateritic regolith is intensely depleted in base cations and
enriched in iron either in some layers, or throughout the profile, commonly forming
ferruginous zones. In Western Australia, seasonally high rainfall and alternating
arid/humid weathering conditions may have further enhanced the lateritization process
and resulted in a widespread distribution of lateritic profiles. Therefore, studying of
abundance and redistribution of REE in lateritic regolith is significant for
understanding the behaviour and mode of occurrence of REE under intense weathering
conditions and advanced lateritization.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
22
2.6.2 A typical lateritic profile
Zoning in lateritic profiles occurs at variable depths (Bourman, 1993), but a typical
lateritic profile would have: fresh bedrock, saprock, saprolite, a mottled clay zone and
a ferruginous zone (Anand and Paine, 2002). In real situations, however, one or more
zones may be missing from the profile. The saprock and saprolite form the lower part
of the regolith and retain the primary mineral constituents of the bedrock. At this depth,
weathering has been less intense and is nearly isovolumetric, whereas the mottled clay
and ferruginous zones comprising the upper part of the profile have been subjected to
stronger, non-isovolumetric weathering, leaching, cementation and soil-forming
processes, and probably precipitation and erosion (Anand and Butt, 2010; Anand and
Paine, 2002). Thin, depleted (commonly quartz-rich) topsoil may often be present
above the ferruginous zone. In a ferruginous zone, nodules and pisoliths may be
present and they are distinguished by their morphology: nodules are irregular, with
re-entrant surfaces, whereas pisoliths are ellipsoidal or spherical. As the sphericity of
nodules increases they merge with pisoliths (Anand and Butt, 2010) and nodules can
also be formed from cementation of one or more pisoliths. In this study, both nodules
and pisoliths are presented in the Jarrahdale regolith profile (JG) studied, and ‘iron
nodules’ is used to denote both types to simplify the terminology.
2.6.3 Lateritization
The geochemical processes for formation of lateritic profiles/landscape are collectively
called lateritization. Intense weathering is an important process during lateritization,
leading to disaggregation, breakdown of original silicate minerals, dissolution of
primary minerals, and leaching of base cations. At the onset of weathering, any
carbonates are dissolved, sulfides are oxidised, easily weathered Fe-Mg silicates are
hydrolysed, and then most of the readily weathered minerals such as feldspar alter to
kaolinite. Resistant minerals, such as quartz and zircon, remain relatively unaltered.
The intense leaching of base cations and formation of secondary clay minerals leads to
the formation of mottled zone (sometimes known as the ‘pallid zone’ if depleted in Fe).
Secondary clay mineral formation by chemical weathering of the primary minerals is
Chapter Two: Literature review
23
termed ‘kaolinization’ and is one of the fundamental processes during lateritization.
With more intense weathering, silicates are increasingly leached out of the profile, and
more secondary oxides, Fe oxyhydroxides, Al hydroxides, and kaolinite are formed.
Continuous dissolution, precipitation, cementation, and erosion results in formation of
the ferruginous zone (Anand and Paine, 2002); note that enrichment of Fe oxides and
oxyhydroxides during lateritic weathering is referred as ferruginization. This process
leads to accumulation of crystalline Fe oxyhydroxides (e.g. goethite), Fe oxides (e.g.
hematite and maghemite), Al hydroxides (e.g. gibbsite), and Al oxyhydroxides
(e.g. boehmite), in the intensely weathered regolith. Though the geochemistry and
genesis of lateritization have been widely investigated (Bourman, 1993; Brimhall et al.,
1991; Schellmann, 1994), the translocation and fractionation of REE during
lateritization in various secondary mineral phases is still not fully understood (Feng,
2011; Ma et al., 2007).
2.6.4 Geochemical behaviour of REE during lateritization
Quantitative understanding of the nature of the migration-fixation and fractionation
mechanisms of REE caused by sorption of clay or Fe oxyhydroxide is currently
inadequate. Such information is important for interpreting the behaviour of REE as
potential clues tracing the processes of lateritization and weathering.
Mottled clay formed during kaolinization is believed to act as a potential reservoir of
REE in weathered lateritic profiles because of adsorption of REE onto the clay surface
(Laveuf and Cornu, 2009). This is an important secondary REE enrichment process
(Bao and Zhao, 2008; Ohlander et al., 1996) during weathering, however, the main
enrichment of REE takes place during precipitation of secondary REE bearing
minerals (Braun and Pagel, 1994; Braun et al., 1993). Adsorption of REE by clay is
controlled by the nature of the clay minerals, pH, ionic strength, the presence of
additional ligands such as carbonate or organic complexes, surface coverage, and
effects specific to the individual REE (Coppin et al., 2002; Fendorf and Fendorf, 1996;
Koeppenkastrop and Decarlo, 1992; Koeppenkastrop and Decarlo, 1993; Laveuf and
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
24
Cornu, 2009; Piasecki and Sverjensky, 2008; Takahashi et al., 1999). As well as these
controls, differences in clay mineralogy can affect fractionation of REE (Coppin et al.,
2002; Laveuf and Cornu, 2009), potentially explaining the apparently contradictory
signatures of REE adsorbed by clay minerals. Usually, REE adsorption increases with
increasing pH (Coppin et al., 2002), and in high ionic strength solutions HREE are
more sorbed than LREE (Coppin et al., 2002). Clay minerals have a strong affinity for
all REE except Ce (Duzgoren-Aydin and Aydin, 2009). As the most important clay
mineral in lateritic regolith, kaolinite has considerably variable REE concentrations
(Laveuf and Cornu, 2009) and the fractionation of REE by kaolinite sorption is still not
fully understood. The concentrations of REE in different horizons of lateritic regolith
profiles are listed in the Table 2.3.
The upper ferruginous zones in weathered lateritic profiles are commonly depleted in
all REE except Ce, although it has been reported that Fe oxides have high surface areas
rendering them very efficient sinks for heavy metals (Nedel et al., 2010; Singh and
Gilkes, 1992). Iron oxides are known to contain REE, but the concentration does not
correlate with Fe content (Laveuf and Cornu, 2009). It is rare for REE to substitute for
Fe in the lattice of Fe oxides at ambient temperatures and pressures; however,
transformation from ferrihydrite to goethite with Lu3+
and Eu3+
substitution and
incorporation was reported at 70 ºC and pH 13 (Dardenne et al., 2003).
Scavenging of REE by Fe oxides and oxyhydroxides is believed to be mainly affected
by surface complexing, which is strongly pH-dependent (Bau, 1999; Marmier et al.,
1999; Marmier and Fromage, 1999; Piasecki and Sverjensky, 2008), hence sorption of
REE usually increases with increasing pH within a range of 5-7 (Marmier et al., 1999;
Marmier et al., 1997; Marmier and Fromage, 1999; Piasecki and Sverjensky, 2008).
Surface complexing and pH do not affect Y as much as REE (Bau, 1999).
Fractionation of REE by Fe oxyhydroxides shows MREE enrichment at pH>5 in low
salinity solutions when other strong complex ligands are absent (Bau, 1999), indicating
La, Gd, Y and possibly Lu preferentially remain in the solution rather than being
surface-complexed onto Fe oxyhydroxides (Bau, 1999).
Chapter Two: Literature review
25
However, the fractionation between LREE, MREE and HREE in Fe oxides is subject
to debate (Laveuf and Cornu, 2009) and varied fractionation with enrichment of LREE
(Koeppenkastrop and Decarlo, 1993), MREE (Bau, 1999; Land et al., 1999) or HREE
(Elderfield and Greaves, 1981; Marker and Deoliveira, 1994) have been observed. The
differences in REE fractionation induced by Fe oxides probably arise from the
presence of various proportions of different types of Fe oxides and the presence of
other complex ligands (Laveuf and Cornu, 2009). For example, in solutions at
pH 4.0-7.1 with carbonate present, REE sorption by amorphous Fe oxyhydroxides
initially increases with increasing carbonate concentration and then decreases; this
effect was more pronounced for HREE than LREE (Quinn et al., 2006). In addition,
REE are fractionated during adsorption by Fe oxyhydroxides when humate complexes
are present, resulting in MREE enrichment (pH 5.2) rather than the non-preferential
adsorption by Fe oxyhydroxides or humate complexes (Davranche et al., 2004).
Generally, REE contents in amorphous Fe oxyhydroxides are higher than in crystalline
Fe oxides (Land et al., 1999; Laveuf and Cornu, 2009), and operationally defined
amorphous and crystalline Fe phases displaying enrichment of MREE were reported
by Land et al., (1999).
2.6.5 Anomalies of Ce in lateritic regolith
Cerium is one of the ‘unusual’ REE because it can occur in nature as Ce3+
like the
majority of lanthanides or as Ce4+
in oxidizing conditions. Cerium also has very low
elemental mobility, due mainly to the stability and low solubility of CePO4 and CeO2.
In lateritic profiles, topsoil and clay zones usually have either slight positive or no
apparent Ce anomalies, ferruginous zones commonly have a positive Ce anomaly, and
saprolite may show a negative or positive Ce anomaly (Angelica and Dacosta, 1993;
Braun et al., 1993; Braun et al., 1990; Braun et al., 1998; Ndjigui et al., 2009). In the
ferruginous zone, Fe acts as a redox sensitive element, and strong redox mediated
associations between the oxyhydroxide phases are expected within weathered profiles.
During the primary redox change in lateritic weathering, soluble Ce3+
is released by
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
26
chemical weathering from REE-bearing minerals and is oxidized to Ce4+
where pH
ranges from 2.5 to 7.5, and Eh from -0.4 to 1.1V; precipitation as CeO2 in the
ferruginous zones (Angelica and Dacosta, 1993; Braun et al., 1990; Braun et al., 1998;
Takahashi et al., 2000) is most likely in lateritic profiles, however, adsorption by
Fe/Mn oxyhydroxides also has the potential to produce a Ce enrichment (Ndjigui et al.,
2008; Nedel et al., 2010; Ohta et al., 2009; Quinn et al., 2006). The variability of
observed Ce anomalies in saprolite may relate to a change in redox condition induced
by fluctuation of groundwater or movement of the weathering front (Braun et al., 1990;
Ndjigui et al., 2009).
2.6.6 Anomalies of Eu in lateritic regolith
Although Eu anomalies are variable in lateritic weathered profiles (Table 2.3), they are
less studied than Ce anomalies. Negative Eu anomalies in the saprolite zone, or even
throughout the profile (e.g. Braun et al., 1993; Braun et al., 1998), may be caused by
breakdown of plagioclase (Panahi et al., 2000), sphene and allanite (Condie et al.,
1995), or the tetrad effect1 (Feng, 2011). Although the tetrad effect in REE patterns
has been reported widely in different geological samples (e.g. Liu and Zhang, 2005;
Monecke et al., 2002; Takahashi et al., 2002), it is still under debate (McLennan, 1994).
Positive Eu anomalies also exist in lateritic regolith (e.g. Braun et al., 1998; Ndjigui et
al., 2009), and they may relate to the type of parent rock and the redox conditions
during weathering (Ndjigui et al., 2009).
2.7 Summary
Although the geochemical behaviour of REE in supergene settings has been widely
investigated since the 1980s, compared with the studies of REE under high
temperature and high pressure settings it is far less studied, especially during intense
lateritic weathering. Many issues remain unresolved or not fully understood, such as
1tetrad effect: a split of chondrite-normalized REE patterns into four rounded segments which either are convex or
concave and formed M-shaped and W-shaped lanthanide pattern Masuda, A., Kawakami, O., Dohmoto, Y., Takenaka, T., 1987. Lanthanide tetrad effects in nature: two mutually opposite types, W and M. Geochemical Journal 21(3), 119-124.
Chapter Two: Literature review
27
migration-fixation mechanisms and mode of occurrence of REE in lateritic regolith,
the impact of Fe oxyhydroxides on translocation and fractionation of REE during
lateritization, the distribution of REE into different particle size solid phases and the
influence of weathering intensity on mobilization and fractionation of REE etc.. In
addition, many results are controversial and have not been interpreted unambiguously
yet; for example, the preferential mobilization of LREE or HREE during weathering,
and at which stage of weathering mobilization and fractionation of REE starts.
Therefore, the study of the geochemical behaviour and fractionation of REE under
intensely lateritic weathering is important and worthy of further research. This thesis
will improve the understanding of the mode of occurrence, fractionation mechanism
and geochemical behaviour of REE during weathering and lateritization in supergene
settings.
28
Table 2.1 Summary of REE in common minerals in granitoid rocks
Mineral* Formula ΣREE LaN/YbN Ce
* Eu
*
primary minerals
Quartz SiO2 None
Plagioclase (Na, Ca)(Si, Al)4O8 3.0-143 ppm 17.0-1184 0.71-1.16 0.43-32.9
K-feldspar e.g. KAlSi3O8 1.3-43.2 ppm 6.6-242 0.74-1.01 0.41-33.8
Biotite K(Mg,Fe2+
)3[AlSi3O10(OH,F)2] 0.03-1.75 ppm 15.2-17.3 0.11-1.12 3.51-6.82
Muscovite KAl2[AlSi3O10(OH,F)2] 0.08-3.78 ppm 0.96-1.38 0.40-1.04 0-10.7
Amphibole Ca2(Mg,Fe2+
)5Si8O22(OH)2 22.6-203 ppm 1.3-4.1 1.08-1.31 0.42-1.16
Clinopyroxene e.g. (Mn2+
, Mg)2Si2O6 3.6-24.8 ppm 0.6-2.8 0.97-1.15 0.91-1.24
Orthopyroxene e.g. (Mg, Fe2+
)3Al4BeSi3O16 0.05-0.14 ppm 1.36
Garnet e.g. Ca3Al2Si3O12 46.8-268 ppm 0-0.8 0-1.07 0-0.8
Cordierite Mg2Al4Si5O18 0.2-5.2 ppm 1.3-4.7 0.12-1.36 0.78-0.98
Tourmaline e.g. NaAl3Al6(BO3)3(Si6O18)(OH)4 0.7-25.1 ppm 0.8-89.3 0.26-1.71 0-0.42
Minerals with REE non as essential structural ion
Zircon ZrSiO4 76.3ppm-1.28% 0-0.12 0.14-1.20 0-0.88
Apatite Ca5(PO4)3(OH,F,Cl) 0.13%-1.29% 0.10-84.0 0.14-71.1 0.03-1.16
Epidote Ca2(Fe3+
, Al)3(SiO4)3(OH) 25.5-271 ppm 5.8-116 0.27-1.03 1.37-8.31
Sphene(titanite) CaTi(SiO4)(O,OH,F) 0.29%-2.73% 2.9-8.3 0.23-1.04 0.55-1.08
Th-orthosilicate ThSiO4 0.5%-23.0% 0-6.7 5.75 0-0.29
Uraninite UO2 0.75%-0.99% 0.05-0.12 0.41-0.68 0-0.29
Fluorite CaF2 up to 14.1% Ce
Rutile TiO2 tens of ppm
29
Mineral* Formula ΣREE LaN/YbN Ce
* Eu
*
Minerals with REE as essential structural ions
Monazite (La,Ce,Nd)PO4 1.62%-52.7% 5.9-526 0.35-0.87 0-0.73
Allanite (REE,Ca)2(Al,Fe3+
)3(SiO4)3(OH) 13.8%-22.8% 0-327 0.11-1.61 0-0.17
Xenotime (HREE)PO4 0.8%-17.4% 0-0.17 0.13-0.99
Fluocerite (Ce,La)F3 up to 65% predominant Ce
Bastnasite (Ce,La)(CO3)F over 52% predominant LREE
Synchysite (Ce,La) Ca(CO3)2F predominant LREE
Parisite (Ce,La)2Ca(CO3)3F2 predominant LREE
Cerianite CeO2 predominant Ce
Zirkelite (Zr,Ca,Ti,Fe,Mg,REE,U,Th)3O5 ~1.9% HREE predominant HREE
Florencite CeAl3(PO4)2(OH)6 up to 26% predominant LREE
Rhabdophane (Ce,La)PO4∙H2O up to 52% predominant LREE
*There are more than 200 REE-bearing minerals; this table only includes the most common and most important minerals in granitoid rocks.
The data were mainly re-calculated and summarized based on the study by Bea (1996), and some missing data were referred to (Henderson, 1984);
Ce*=(Ce/CeCN)/[(La/LaCN)
0.5×(Pr/PrCN)
0.5];Eu
*=(Eu/EuCN)/[(Sm/SmCN)
0.5×(Gd/GdCN)
0.5]; CN refers to the average chondrite (Anders and Grevesse, 1989).
30
Table 2.2 Concentrations of REE in different types of parent rock
Parent
Rock Location
Element concentrations (ppm) Reference
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu ΣREE
Granite France 16.3 37.5 4.66 17.3 4.14 0.35 3.09 0.49 2.45 0.37 0.82 0.12 0.69 0.09 88.4 (Aubert et al., 2001)
Granite Canada 47.7 94 9.73 33.3 4.88 0.93 3.12 0.42 1.97 0.35 1.08 0.17 1.12 0.17 199 (Panahi et al., 2000)
Syenite Cameroon 69.5 140 65.8 10.7 2.93 6.54 2.66 1.13 0.9 0.13 300 (Braun et al., 1993)
Gneiss Cameroon 11.5 23.0 2.61 9.96 1.90 1.19 1.72 0.33 2.46 0.65 1.98 0.35 2.59 0.39 60.7 (Braun et al., 1998)
Serpentinite Cameroon 0.14 0.34 0.05 0.27 0.07 0.01 0.09 0.02 0.11 0.02 0.08 0.02 0.10 0.02 1.33 (Ndjigui et al., 2009)
Dolomite China 2.78 1.91 0.73 4.09 1.61 0.62 2.75 0.43 1.58 0.21 0.43 0.07 0.39 0.06 17.7 (Ji et al., 2004)
Carbonatite Australia 292 621 287 56.5 16.1 34.5 5.40 4.15 5.55 0.62 1323 (Lottermoser, 1990)
Basalt China 16.6 30.7 3.73 17.2 4.44 1.48 4.91 0.75 4.12 0.79 1.99 0.25 1.53 0.22 88.7 (Ma et al., 2007)
Granodiorite Australia 25.0 57.8 25.4 6.02 1.42 5.73 0.85 1.01 2.89 0.48 127 (Nesbitt, 1979)
Aries
kimberlite Australia 268 433 9.94 2.09 1.04 0.15 714 (Singh and Cornelius, 2006)
chlorite
schists Cameroon 29.9 62.3 7.23 27.6 5.56 1.07 5.03 0.83 5.28 1.10 3.38 0.49 3.24 0.47 154 (Beyala et al., 2009)
PAAS1 38 80 8.9 32 5.60 1.10 4.7 0.77 4.4 1.0 2.9 0.50 2.8 0.50 184 (Nance and Taylor, 1976)
NASC2 32 73 7.9 33 5.7 1.24 5.2 0.85 5.8 1.04 3.4 0.50 3.1 0.48 174 (Haskin and Paster, 1979)
UCC3 30 64 7.1 26 4.5 0.88 3.8 0.64 3.5 0.80 2.3 0.33 2.2 0.32 146 (McLennan et al., 1980)
1PAAS: Post-Arcbean average Australian shale;
2NASC: North American shale composite (post-Archean);
3UCC: Post-Archean upper continental crust.
31
Table 2.3 Concentrations of REE in different horizons of lateritic regolith profiles
PR1 Type PR saprolite saprolite Mottled clay Ferruginous zone
2 Duricrust
3 Reference
lower upper matrix nodule matrix gravel
Serpentinite ΣREE4 1.33 270 271 162 140 (Ndjigui et al., 2009)
(La/Sm)PR 2.12 1.67 2.92 2.49
(La/Yb)PR 0.79 0.49 0.80 0.60
5Ce
* 0.02 1.73 1.57 3.29
Eu* 2.28 2.37 2.26 2.32
Serpentinite ΣREE 1.33 105 110 437 240-742 170 82.1 (Ndjigui et al., 2009)
(La/Sm)PR 2.50 1.42 1.96 1.11-1.36 2.22 1.54
(La/Yb)PR 0.71 0.31 0.44 0.26-0.29 0.47 0.32
Ce* 0.46 0.68 11.3 9.76-24.0 2.46 1.26
Eu* 2.09 2.21 2.3 2.29-2.41 2.14 2.19
Granodioritic
gneiss ΣREE 39.4 21.3-47.4 104-483 (Tripathi and Rajamani, 2007)
(La/Sm)PR 0.58-0.86 0.58-1.04
(La/Yb)PR 0.09-0.16 0.13-0.32
6Ce
* 0.85-1.63 1.21-4.16
Eu* 0.56-0.77 0.30-0.79
Chlorite ΣREE 153.4 194.0 131.1 32.72-100.1(w)
7 (Beyala et al., 2009)
(La/Sm)PR
1.98 1.16 1.32-1.51
(La/Yb)PR
1.38 0.78 0.65-1.04
32
PR1 Type PR saprolite saprolite Mottled clay Ferruginous zone
2 Duricrust
3 Reference
lower upper matrix nodule matrix gravel
Syenite ΣREE 300.3 475.8 445.2 247.7-3035 253.1 193.3(w) 301.5(w) (Braun et al., 1993)
(La/Sm)PR
1.27 1.27 0.94-1.61 1.51 1.46 1.51
(La/Yb)PR
2.37 2.55 1.10-2.26 5.17 3.98 5.98
6Ce
* 1.07 1.28 0.98-19.7 1.09 1.14 1.07
Eu* 0.95 0.90 0.57-0.93 0.84 0.78 0.84
Granodiorite ΣREE 118.4 122.0 100.2-255.0 90.8-165.3(w) 446.5(w) (Dequincey et al., 2002)
(La/Sm)PR
0.91 0.57-1.12 1.28-1.33 1.27(w)
(La/Yb)PR
1.00 0.41-1.24 0.79-0.86 0.91
Ce* 0.94 0.21-1.95 0.73-1.05 6.17
Eu* 1.03 0.86-1.02 0.82-0.91 0.78
1PR: parent rock;
2Ferruginous zone is defined by the dominant composition of secondary Fe oxides and oxyhydroxides, Al hydroxides and kaolinite; and the
3duricrust refers to a hard cemented
layer consisting secondary segregations. In this thesis all ferruginous zones consist of duricrust; however, in the references listed in the table, duricrust has been separated out to
emphasize the concentration variations of REE in different zones.
4ΣREE is the total concentrations of REE;
5Ce
*=(Ce/CePR)/[(La/LaPR)
0.5×(Pr/PrPR)
0.5];Eu
*=(Eu/EuPR)/[(Sm/SmPR)
0.5×(Gd/GdPR)
0.5];
6Data of Pr is missing, so Nd is used when calculating Ce
*;
7w refers to whole rock analysis.
33
3 Description of the study areas
3.1 General geology and climate
The study areas were located in the south-western part of Western Australia and lie
within the Darling Range, slightly east of the Darling Fault and Perth Basin (Figure
3.1). The geological history of the Darling Range can be traced back at least 2600
million years and possibly even further (Gozzard, 2007). Since Paleogene, deep and
intense weathering of exposed rocks of the Darling Plateaus, resulted in a widespread
cover of lateritic materials, and this weathering has continued until geologically recent
times (Gozzard, 2007). This area is part of the vast Yilgarn Craton - an ancient region
of varied rock types that occupies much of the south-western part of Western Australia
(Anand et al., 2006; Anand and Paine, 2002; Gozzard, 2007). Large volumes of
granitoids intruded the metamorphic rocks and other rocks of the Yilgarn Craton
between 2700 and 2600 million years ago and dolerite dykes intrude the granitoids
during development of the Darling Fault in the Mesoproterozoic and Neoproterozoic
(Gozzard, 2007).
The area currently has a Mediterranean climate, with a cool wet season from May to
September and a warm, dry season from November to March, with transition periods
in April and October. Average annual rainfall was ca. 1239.5 mm in the Dwellingup
(Darling Range) from 1934 to 2011 and rainfall mostly occurs in winter (Bureau of
Meterology, 2012).
The vegetation of the areas studied shows marked regional changes based largely on
climate, with local variations of geology, soils, topography and drainage. The Darling
Range in the high-rainfall area currently has open eucalypt forests of jarrah
(Eucalyptus marginata) and marri (Eucalyptus calophylla) (Anand and Paine, 2002).
3.2 Sampling and profile description
The profiles studied are Fe-rich lateritic weathered profiles near outcrops of granitoids
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
34
and dolerites. The dolerites are sub-vertical intrusions that cross cut the metamorphic
fabric of the granitoids and thus called dolerite dykes here. Samples of outcropping
fresh granitoid and dolerite were sampled ca. 5 kg separately at each study site.
Different zones of regolith in each profile were identified based on the physical
properties (e.g. texture, colour, coarse fragment content) and chemical properties (e.g.
mineralogy, Al/Fe concentrations, and organic matter contents). One to two ten-cm
blocks of bulk undisturbed regolith samples per horizon were collected at different
depth and sealed in plastic bags or boxes for transport to the laboratory.
Great Eastern Profile: The first intensely weathered lateritic profile investigated was
adjacent to Great Eastern Highway, Western Australia (31°22'30.95"S, 118°41'27"E),
with very good bedrock (granitoid/dolerite dyke) exposure (Figure 3.1&Figure 3.2).
Sample identities from this location are prefixed with GE. Regolith samples from
different horizons, the parent granitoid and the intrusive dolerite dyke were collected
on 3rd
April, 2009. One outcropped coarse grained granitoid sample (GEPR1B) was
crossed by a late-stage sub-horizontal pegmatite vein (GEPR1A), thus these two
subunits were analysed separately. The profile was ca.12 m deep, including saprolite,
mottled clay, ferruginous duricrust, and A horizon regolith. The saprolite formed from
weathered bedrock with horizons above showing progressive loss of rock fabric
upwards as porosity and the proportion of clay increases. The mottled clay zone was a
pale white kaolinite-rich zone, ca. 3 m thick, with distinct upper boundary with the
ferruginous zone. The ferruginous zone was composed of loose lateritic ferruginous
materials with ferruginous gravel at ca. 7 m depth (GE5), and the cemented
ferruginous duricrust which was a dark red, dense, and hard layer without gravel at ca.
3.5 m depth (GE6).
Mountain Quarry Profiles: The second and third profiles studied were located in
Mountain Quarry (31°54'54" S, 116°3'44" E), on the southern slope of Greenmount
Hill, Western Australia (Figure 3.1). Sample identities are prefixed with MQ. The
second profile (MQ I profile, Figure 3.3) was 3.6 m deep; samples of regolith from
different horizons based on colour and texture were collected on 29th
, May, 2009. The
Chapter Three: Description of the study areas
35
third profile (MQ II profile) was ca. 2 m deep, 10 m away from the second profile,
developed overlying granitoid with stonelines preserved from quartz veins which
imply in-situ weathering below the sub-horizontal component of the stone line at 0.6 m
depth (samples MQ10 to MQ13). Samples of outcropping granitoid and dolerite and
the regolith were also collected at the MQ sampling site.
Jarrahdale profile: The fourth profile was located at the Jarrahdale Railway cutting
(32°17'46"S, 116°5'40"E) at an average elevation of 270 m above sea level in the
Darling Range, 80 km south-east of Perth, Western Australia (Figure 3.1). Sample
identities were prefixed with JG. The lateritic JG profile was ca. 12 m deep overlies
metamorphic basement consisting of granitoid intruded by a dark-coloured dolerite
dyke. The location of intrusive dykes in the granitoids can be mapped from the
overlying duricrust and the profiles developed on granitoid and dolerite are distinctly
different (Gozzard, 2007). The JG regolith is developed overlying granitoids and
divided into seven zones: parent granitoid, saprolite, mottled clay, ferruginous mottled
zone, ferruginous duricrust, upper ferruginous zone and the A horizon. The mottled
clay is pale white kaolinite-rich, consisting of a lower zone (JG2) at 8.6 m depth and an
upper zone (JG3) at 6.5 m depth. The ferruginous duricrust (3 m depth) is gibbsite and
goethite rich. The upper ferruginous zone (JG6, 1.5 m depth) is rich of red iron nodules.
In contrast, the horizon A regolith (JG7-10, <1 m depth) is gravely sandy soil rich of
dark brown to black loose nodules. The sampling of the profile was conducted on the
6th
August 2009. Each zone was identified based on different properties (e.g. texture,
colour, coarse fragment content), sampled at the depth given above in a
ca. 10×10×10 cm cube, put into a sealed plastic box, transported to the laboratory and
air dried. Photographs of each horizon were not taken and thus are not presented in this
thesis.
A number of studies have investigated the geological, morphological and geochemical
characteristics of lateritic bauxite regolith in the Darling Range, and at Jarrahdale the
regolith is widely accepted to have undergone in-situ intense weathering and
lateritization (Anand and Butt, 2010; Anand et al., 1991; Anand and Paine, 2002;
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
36
Brimhall et al., 1992; Brimhall et al., 1994; Gozzard, 2007; Kew and Gilkes, 2007;
Sadleir and Gilkes, 1976). Note that both fresh and weathered euhedral zircons from
lateritic bauxite profiles at Jarrahdale have been dated at ca. 2650 Ma (Brimhall et al.,
1994) indicating upward lithological continuity of the parent meta-granitoid through
the bauxite profile. Rounded zircon, ilmenite, and rutile have been found in the
surficial meter-depth regolith and are proposed to have been transported by wind and
predominantly from a different, much younger source (ca. 700-1150 Ma) (Brimhall et
al., 1992; Brimhall et al., 1994; Brimhall et al., 1988).
37
Figure 3.1 Sampling sites (a, labelled as box) and sketches of the profiles sampled (b). On the map (a) the dashed line labelled Darling Fault represents
the western margin of the Darling Range. In the sketch of regolith profile (b) ‘m’ denotes matrix and ‘g’ denotes gravel.
38
GE1, 12.5 m depth, saprolite GE3, 10 m depth, mottled clay GE5, 7 m depth, lower ferruginous
zone
Figure 3.2 Photographs of regolith from selected horizons of the GE profile.
MQ1, 3.6 m depth, C horizon MQ2, 3.3 m depth, lower B horizon MQ4, 2.2 m depth, upper B horizon
Figure 3.3 Photographs of regolith from selected horizons of the MQ I profile
39
4 Redistribution of major elements in lateritic profiles during
intensive weathering in Western Australia
4.1 Abstract
In order to understand the geochemical behaviour of major elements in different solid
phases in laterite and to investigate geochemical pathways of lateritic weathering, the
redistribution of major elements in matrix (<2 mm) and gravel (>2 mm) in four
intensely weathered lateritic profiles (GE, MQ I, MQ II and JG) in Western Australia
was investigated.
The GE and JG regolith samples were highly weathered with chemical indices of
alteration ca. 99%, nearly complete loss of Na, Ca and Si, and enrichment of Fe in the
ferruginous zone. In the GE and JG profiles, Fe mainly occurred as goethite, hematite
and maghemite, while Al mainly occurred as secondary clay minerals (kaolinite) and
gibbsite in ferruginous zone; gravel was more enriched in Al and Fe but more depleted
in Si than the matrix, which was consistent with gravel having higher weathering
intensity and degree of lateritization than the matrix.
The regolith samples from both MQ profiles were less weathered than the GE and JG
profiles, lack of gibbsite and hematite, and showed weak lateritization. The chemical
indices of alteration ranged from 55% to 92% in both MQ profiles and gravel had
higher concentrations of Si than matrix. The presence of a pedogenic discontinuity in
both MQ profiles was identified from the molar ratio Na/K, concentration ratios
Al2O3/Fe2O3 and Ti/Zr, implying that mass movement had occurred in the upper part of
both profiles during weathering.
Significant depletion of base cations and Si, coupled with enrichment of Fe and Al,
reveal that intense leaching of cations, kaolinization, desilication and ferruginization
took place in lateritic regolith during weathering and lateritization.
4.2 Key words
Major elements; laterite; weathering; mass balance; Western Australia;
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
40
4.3 Introduction
Chemical weathering is one of the critical processes in the geochemical cycling of
elements and translocation of elements from crust to sediments. During early
pedogenesis, the chemical composition of a soil will be strongly controlled by the
composition of geological parent materials, though this influence diminishes in
importance with time (Schaetzl and Anderson, 2005; Thanachit et al., 2006). The
development of a soil reflects the weathering processes associated with the dynamic
environment in which it has formed. The mobilization and redistribution of elements
during weathering follows various pathways as elements behave differently during
various pedogenic processes, including: dissolution of primary minerals, formation of
secondary minerals, redox processes, transport of material and ion exchange
(Middelburg et al., 1988).
Lateritic regolith represents one of the most common superficial formations in the
tropics, and is commonly diachronous, extending over tens of millions of years
(Dequincey et al., 2006). In contrast to common pedogenesis, during lateritic
weathering, regolith is intensely weathered and enriched in Fe, either in some layers or
throughout the profile, commonly forming a hard cap of ferruginous duricrust or
ferruginous gravel. Though many studies have been conducted on lateritic regolith
profiles (e.g. Beauvais, 1999; Brimhall et al., 1991; Brown et al., 2003; Costa, 1997;
Dequincey et al., 2002; Dequincey et al., 2006; Fernández-Caliani and Cantano, 2010),
the mobilization and redistribution of major elements into different grain size fractions
of lateritic regolith during intense weathering are not yet fully understood. A holistic
understanding of elemental behaviour during weathering and lateritization processes
cannot be achieved solely by determination of total elemental concentrations in bulk
regolith. It is essential to determine the relative elemental concentrations in different
solid phases as well, since partitioning of elements into matrix (<2 mm) or gravel
(>2 mm) may reflect the weathering history and weathering processes involved. The
analysis of the geochemical and mineralogical features of lateritic regolith, including
matrix and gravel, has the potential to improve our understanding of weathering and
lateritization (Beauvais, 1999).
A number of studies have been investigated the geological, geographical, morphological
and geochemical characteristics of lateritic bauxite regolith in the Darling Range
(Anand and Butt, 2010; Anand et al., 1991; Anand and Paine, 2002; Brimhall et al.,
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
41
1992; Brimhall et al., 1988; Kew and Gilkes, 2007; Sadleir and Gilkes, 1976).
Accordingly, this study investigates the mobilization and redistribution of major
elements into different solid phases from lateritic regolith building on previous work.
The objective of this study is to investigate the key geochemical and mineralogical
pathways of lateritic weathering and the relative partitioning of major elements into
matrix and gravel in four intensely weathered lateritic profiles in Western Australia.
This information will be helpful to understand the genesis of ferruginous materials
during lateritization.
4.4 Materials and methods
4.4.1 Analytical methods
In this study, chemical compositions of matrix (<2 mm, represented by suffix ‘m’) and
gravel (>2 mm, represented by suffix ‘g’) were analysed separately. Exceptions were
the duricrust in the GE profile (GE6), a very hard cemented material without
corresponding loose matrix or iron nodules, and the saprolite (JG1) and mottled clay
(JG2&3) in the JG profile, both soft pale materials without gravel. These four samples
were crushed and/or ground to ≤ 200 µm and oven dried at 105 °C overnight prior to
chemical analysis.
Pre-treatment of regolith samples included hand-picking roots/rhizomes and sieving
through 2 mm plastic mesh for separation of gravel (>2 mm) from the matrix (<2 mm)
and weighing each subsample separately. The matrix fraction (<2 mm) was used to
determine the pH, total carbon and particle size distribution (Table 4.1). Soil pH was
determined potentiometrically at 23 °C in the supernatant in a 1:5 suspension of soil:
deionised water and 1:5 suspension of soil: 0.01 M CaCl2 (Rayment and Higginson,
1992). Total carbon was determined by Elementar (Vario Macro, Hanau, Germany).
Subsamples of matrix and gravel were ground to ≤ 200 µm and oven dried at 105 °C
overnight. The bulk raw regolith matrix (< 2 mm) from MQ II profile was separated
further into three size fractions without crushing or grinding: clay (<2µm), silt (2-20 µm)
and sand (> 20 µm) by the sedimentation and wet sieving method (Day, 1965) in order
to understand the behaviour of major elements in different particle size fractions. The
particle size fraction limit recommended by the International Society of Soil Science
(ISSS) has been adopted in Australia (Marshall, 1947; Marshall, 2003; Prescott et al.,
1934). Different particle size fractions were rinsed with MilliQ water three times, oven
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
42
dried at 105 °C overnight and ground to ≤ 200 µm.
Fusion beads for elemental analyses were made by mixing 0.1 g (to an accuracy of
0.1 mg) of finely ground sample or reference material with 0.7 g 12:22 Norrish flux
(lithium metaborate:lithium tetraborate=12:22) and heating in a muffle furnace at
1050 °C for 40 minutes. Duplicate fusion beads were made on 10% of samples to check
preparation errors. After cooling, the fusion beads were dissolved in 100 mL of 10%
analytical grade HCl. The major elements were determined by inductively coupled
plasma-optical emission spectroscopy (ICP-OES, Perkin-Elmer Optima 7300DV) at the
University of Western Australia. Certified international standard materials, including
stream sediment reference standards STSD-2, STSD-4 (Canada Centre for Mineral and
Energy Technology, CANMET), an in-house standard reference and 12 blanks were
prepared in the same way as the samples and analysed together with samples to check
the accuracy and precision. The variation between the tested and expected values of the
standards was within 5% (Appendix 11.2). The concentrations of major elements in
matrix and gravel are listed in Table 4.2.
Primary minerals in the weathering products were identified by means of random
powder X-ray diffraction (XRD) from 4 to 70 2 using CuKα radiation and a Philips
PW 1830 diffractometer with a diffracted beam graphite crystal monochromator, after
grinding to <63 µm and homogenisation. The proportion of mineral phases (not include
amorphous or poorly-crystalline phases) were identified semi-quantitatively using the
software Traces (GBC Scientific Equipment). All primary mineral phases were
identified manually and cross-checked with the dataset of the International Centre for
Diffraction Data (ICDD). The main-peak area of each primary mineral was measured,
and the mineral proportion was calculated using the main-peak areas of each primary
mineral divided by the sum of main-peak areas of all primary minerals identified by the
Traces software. The clay fraction (<2 µm) was separated by dispersion and
sedimentation, basally oriented on ceramic slides, air-dried, and then scanned at
1° 2/min from 1 to 30° 2/min. For clay mineral identification, the oriented aggregates
were treated with ethylene glycol. Mineral names were abbreviated according to
Whitney and Evans (2010).
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
43
Table 4.1 Selected physical and chemical properties of matrix fractions (<2mm) of the
profiles studied
Sample1 Depth
2 pH
3 pH
4 clay% silt% sand% TC
5 Description
6
(m) (H2O) (CaCl2) <2µm 2-20µm >20µm
GE
GEA1 0.09 6.05 5.09 1.9 3.2 94.9 4.66 A Horizon (m&g)
GEA2 0.12 6.48 5.03 1.5 2.8 95.7 1.46 A Horizon (m&g)
GEA3 0.23 6.70 5.33 1.9 2.9 95.2 0.80 A Horizon (m&g)
GE6 3.5 6.31 6.12 2.3 7.0 90.7 0.38 Duricrust (g)
GE5 7.0 6.62 5.96 19.3 2.3 78.4 0.21 Lower ferruginous zone
(m&g) GE4 8.4 6.45 5.75 18.9 9.7 71.5 0.04 Mottled clay (m&g)
GE3 10.0 6.79 5.52 22.6 8.1 69.3 0.03 Mottled clay (m&g)
GE2 11.4 6.56 5.26 22.6 7.7 69.7 0.03 Mottled clay (m&g)
GE1 12.5 6.42 4.77 24.7 12.6 62.7 0.07 Saprolite (m&g)
MQ I
MQ9 0.2 6.24 5.19 19.8 6.9 73.2 2.06 A Horizon (m&g)
MQ8 0.5 6.37 5.13 16.1 0.8 83.1 1.17 A Horizon (m&g)
MQ7 0.7 6.27 5.02 35.7 1.9 62.4 0.39 A Horizon (m&g)
MQ6 0.9 5.94 4.91 68.8 0.7 30.5 0.46 B Horizon (m&g)
MQ5 1.1 5.81 5.05 71.9 0.7 27.4 0.39 B Horizon (m&g)
MQ4 2.2 5.47 5.02 25.2 1.2 73.6 0.14 B Horizon (m&g)
MQ3 2.8 5.77 4.60 22.8 2.7 74.5 0.08 B Horizon (m&g)
MQ2 3.3 5.51 4.49 16.5 1.7 81.8 0.06 B Horizon (m&g)
MQ1 3.6 5.88 5.56 0.82 2.1 97.0 0.03 C Horizon (m&g)
MQ II
MQ15 0.08 5.88 4.88 18.4 10.3 71.3 2.04 A Horizon (m&g)
MQ14 0.25 6.15 5.16 30.5 1.7 67.8 0.28 A Horizon (m&g)
MQ13 0.6 5.48 4.83 50.3 0.8 48.9 0.32 A/B Horizon (m&g)
MQ12 1.1 5.45 4.75 25.6 2.0 72.5 0.11 B Horizon (m&g)
MQ11 1.6 5.62 4.62 23.3 2.4 74.3 0.09 B Horizon (m&g)
MQ10 2.0 5.63 4.65 18.0 0.8 81.2 0.07 C Horizon (m&g)
JG
JG7 0.02 5.50 4.83 8.4 6.8 84.8 6.06 A Horizon (m&g)
JG8 0.15 5.59 5.06 7.0 6.5 86.4 2.19 A Horizon (m&g)
JG9 0.3 5.54 5.13 5.8 6.3 87.9 1.06 A Horizon (m&g)
JG10 0.4 5.47 5.06 6.8 6.5 86.7 0.73 A Horizon (m&g)
JG6 1.5 5.60 5.26 4.3 2.5 93.2 0.47 Upper ferruginous zone
(m&g) JG5 3.0 5.08 4.71 6.5 3.5 90.0 0.30 Duricrust (m&g)
JG4 5.0 4.90 4.39 9.0 7.5 83.5 0.20 Ferruginous mottled zone
(m&g) JG3 6.5 4.55 3.96 28.3 7.3 64.4 0.08 Mottled clay (m)
JG2 8.6 3.76 3.45 29.6 8.5 61.8 0.17 Mottled clay (m)
JG1 10.0 3.34 3.15 27.0 15.4 57.7 0.27 Saprolite (m)
1The first two letters of the sample codes identify each profile: Mountain Quarry (MQ) and Great Eastern
Highway (GE) and Jarrahdale granitoid profile (JG). 2depth refers below surface (m);
3pH was determined at 23 °C in a 1:5 suspension of soil: deionised water;
4pH was determined at 23 °C in a 1:5 suspension of soil: CaCl2 solution;
5TC refers to total carbon, determined by vario Macro Elementar Analyser;
6‘m’ denotes matrix and ‘g’ denotes gravel.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
44
4.4.2 Weathering intensity-Chemical Index of Alteration (CIA)
To evaluate the intensity of chemical weathering quantitatively, the elemental
concentrations were converted into oxide concentrations and Chemical Index of
Alteration (CIA) (Nesbitt and Young, 1982) were calculated. The CIA calculates loss of
mobile elements relative to less mobile elements in bulk samples, providing a single
parameter estimate of the intensity of chemical weathering. The formula (Nesbitt and
Young, 1982) is:
CIA=100×Al2O3/(Al2O3+CaO*+Na2O+K2O) (molar basis) (1)
Where CaO* is CaO associated with the silicate fraction of samples (excludes
carbonates). In this study, all regolith have low pH conditions, and no carbonates were
observed by scanning electron microscopy (SEM), and thus all Ca was assumed to be
associated with the silicates.
4.4.3 Mass balance calculation
To quantify net element fluxes from pedogenic weathering, a geochemical mass balance
calculation was used (Brimhall et al., 1991). The formula for normalized concentration
(τi,j) in Equation (2) assumes that an immobile element (e.g. Zr) behaves conservatively
and can be used to correct mobile element concentrations for volumetric strain (ε)
during weathering and pedogenesis.
1))((,
,
,
,
, pj
wj
wi
pi
C
C
C
C
ji
(2)
In Equation (2), C represents concentration, i represents the immobile element, j
represents the element of interest, w represents weathered material and p identifies
parent rock. If τi,j = 0, the element j has behaved conservatively at the sampling scale; if
τi,j = −1, the element j has been depleted completely during weathering; positive τi,j
values signify absolute enrichment.
Equation (2) provides a tool for estimating elemental loss or gain for a profile; however,
mass balance equations have two critical assumptions: a genetic relationship between
regolith and the underlying rock and a fully conserved reference element. In this chapter,
the element Zr was used as a reference element for two main reasons: (i) its existence in
the weathering-resistant, very low-solubility host minerals zircon; (ii) its relatively high
concentrations compared with other high field strength elements, resulting in robust
estimates of mass balance. The mobility of Ti, Zr and Th is discussed in detail in the
next chapter, taking the JG profile as an example.
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
45
4.4.4 Statistical analyses
The converted major oxide composition data were transformed with a centered log-ratio
(CLR) method using CoDaPack software (Reimann et al., 2008) and then subjected to
principal component analysis using R software (R Development Core Team, 2011) to
assess trends in chemical compositions of regolith samples in a more comprehensive
manner and investigate the principal geochemical processes during lateritization. The R
script is listed in Appendix 11.3.
4.5 Results
4.5.1 Weathering intensity of parent rocks and the regolith
The CIA values for parent rock and regolith matrix and gravel are presented in Table
4.2. In the GE profile, the CIA of granitoids was ca. 52%, higher than the dolerite
(36%). Most GE regolith had CIA values above 90%, reflecting extreme weathering
conditions, except the A horizon matrix (GEA1m, 0.1 m depth, CIA=82%). The
duricrust (GE6, 6 m depth) and the upper mottled clay matrix (GE3m, 10 m depth) had
the highest CIA (> 99%). In addition, the gravel CIA was higher than the matrix in the
A horizon regolith, but lower than the matrix from the lower ferruginous zone (3.5 m
depth) to saprolite (12.5 m depth).
In the MQ I profile, the granitoids had similar CIA (ca. 52%) to the granitoids from the
GE profile. The regolith had lower CIA values and thus was less weathered than the GE
regolith. The CIA in MQ I regolith increased from 63% in gravel and 69% in matrix of
the A horizon (0.2 m depth) to ca. 92% in both gravel and matrix at 1.1 m depth (MQ5),
then decreased to 53% in gravel and 56% in matrix of the C horizon (3.6 m depth).
In the MQ II profile, the CIA increased from 58% in gravel and 71% in matrix of the A
horizon to ca. 83% in both gravel and matrix of the B horizon (0.6 m depth), and then
decreased to 55% in gravel and 63% in matrix of the C horizon (2.0 m depth). The CIA
of gravel was less than or equal to CIA in the corresponding matrix in both MQ profiles,
similar to the subsurface regolith of the GE profile.
In the JG profile, however, the CIA of gravel and matrix in the ferruginous zone
(1.5-6.5 m depth) were similar, ranging from 95% to 99%. In the A horizon, the CIA of
gravel (ca. 99%) was higher than matrix (92%-99%), similar to the A horizon regolith
from the GE profile.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
46
Two kinds of outcropping rock were sampled at these sites, loosely classified as either
granitic or doleritic. The most obvious distinction between dolerite and granitoid is that
dolerite has less Si and Zr but more Ti, Fe, Mg, Ca, Mn and P. These variations in
chemical composition were expected to be expressed in regolith (Kew and Gilkes,
2007); the chemical compositions of regolith from the four profiles showed similarity to
the granitoid rather than the dolerite, with Ti and Zr residually enriched (more evidence
from rare earth elements in Chapter Six and Seven). Using A-CN-K and A-CNK-FM
ternary plots, weathering trends in major element composition were identified (Figure
4.1). In the GE and JG profiles, most regolith samples were highly weathered, and the
plots cluster closely at A which reflects the presence of phyllosilicate clay minerals as
the main mineralogical components. In contrast, MQ regolith samples plot separately
from each other, and the weathering trend shown by the regolith chemical compositions
indicates that the regolith was weathered and developed from the granitoid rather than
the dolerite. In ternary plots, the regolith from 2.0 m to 0.6 m depth of the MQ II profile
followed the weathering trend, reflecting in-situ weathering, which is consistent with
the overlying stoneline (Chapter Three). A similar weathering trend was also shown in
the MQ I profile, indicating decreasing Ca and Na and increasing Al and Fe because of
breakdown of primary silicates (mostly feldspar) and formation of secondary clay
minerals and iron oxides. The CIA values of the A horizon regolith in both MQ profiles
were lower than the B and C horizon regolith and did not follow the weathering trends;
this may reflect biogeochemical cycling of Na, K and Ca, or erosion and transportation
of the A horizon regolith as suggested by the stoneline. In the A-CNK-FM plot (Figure
4.1), the MQ regolith showed the depletion of alkaline elements and accumulation of Al
and Fe with increasing weathering intensity.
47
Table 4.2 Concentrations of major elements in gravel and matrix of four lateritic profiles
Sample Depth Proportion1 CIA Al Ca Fe K Mg Na Si S Ti P Mn Zr
Unit m % % % % % % % % % % ppm ppm ppm
d.l.2 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.1 0.1 0.1
Profile GE
Rock
GEBPRA 54.3 7.56 1.01 1.01 2.94 0.14 2.52 34.9 0.00 0.08 89.0 111 106
GEBPRB 53.0 7.51 1.18 1.11 2.86 0.14 2.64 35.4 0.01 0.08 107 129 102
GEPR1A 50.0 5.72 0.42 0.13 2.68 0.01 2.82 38.6 0.02 0.02 11.2 29.5 41.9
GEPR1B 51.3 9.58 2.60 1.55 1.31 0.34 4.02 31.2 0.01 0.16 318 230 183
GEPR2 52.5 7.46 1.14 1.08 2.65 0.15 2.89 34.6 0.01 0.08 131 230 102 3GEPR3 35.9 7.06 8.14 8.35 0.17 4.47 1.28 22.5 0.07 0.61 350 1577 49.0
Regolith
GEA1g 0.20 98.2 27.0 0.07 26.4 0.08 0.03 0.31 3.30 0.04 0.93 250 363 330
GEA1m 0.09 0.80 82.2 2.18 0.11 1.51 0.34 0.03 0.08 35.9 1.68 0.27 61.7 87.8 181
GEA2g 0.54 97.5 26.2 0.18 22.7 0.17 0.04 0.27 4.09 0.04 0.76 157 290 314
GEA2m 0.12 0.46 94.5 3.12 0.03 2.21 0.15 0.02 0.03 36.9 0.80 0.33 28.2 65.9 214
GEA3g 0.56 98.4 24.0 0.02 25.5 0.09 0.03 0.25 3.59 0.04 0.99 151 371 316
GEA3m 0.23 0.44 96.0 2.50 0.02 1.36 0.10 0.02 0.01 38.3 0.42 0.30 87.3 45.0 187
GE6 3.5 W2 99.6 18.0 0.00 20.9 0.09 0.12 0.07 6.20 0.00 0.29 105 92.0 300
GE5g 0.35 92.3 14.0 0.09 27.8 1.14 0.18 0.22 7.46 0.12 0.23 162 186 313
GE5m 7.0 0.65 97.6 17.4 0.01 8.38 0.61 0.13 0.04 14.7 0.00 0.36 63.5 55.6 201
GE4g 0.06 90.7 7.30 0.02 1.72 0.70 0.11 0.21 36.1 0.01 0.16 48.8 55.9 158
GE4m 8.4 0.94 93.8 6.93 0.01 0.75 0.65 0.10 0.02 35.2 0.00 0.17 10.9 37.3 161
GE3g 0.25 98.1 17.5 0.03 1.62 0.11 0.03 0.18 22.5 0.02 0.13 10.7 26.6 127
GE3m 10.0 0.75 99.1 11.6 0.01 1.02 0.14 0.03 0.04 29.0 0.02 0.12 25.6 17.3 116
GE2g 0.08 95.3 6.70 0.05 1.02 0.11 0.02 0.16 37.1 0.01 0.14 20.3 28.9 140
GE2m 11.4 0.92 97.4 6.28 0.01 0.52 0.15 0.05 0.04 35.8 0.09 0.17 10.0 17.9 154
48
Sample Depth Proportion1 CIA Al Ca Fe K Mg Na Si S Ti P Mn Zr
Unit m % % % % % % % % % % ppm ppm ppm
d.l.2 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.1 0.1 0.1
GE1g 0.06 94.1 10.2 0.01 2.82 0.75 0.08 0.09 30.5 0.02 0.14 82.4 48.9 191
GE1m 12.5 0.94 93.5 8.15 0.01 0.76 0.78 0.08 0.02 33.6 0.01 0.18
48.8 31.4 207
Profile MQ
Rock
MQPR1 53.2 7.35 0.72 1.30 3.37 0.41 2.70 33.5 0.03 0.14 230 181 160
MQPR2 51.0 7.55 1.32 1.14 2.92 0.27 2.96 31.0 0.02 0.15 233 154 160 4MQPR3 36.8
7.02 7.55 9.59 0.32 3.77 1.41 21.9 0.09 0.89
465 1705 73.8
Regolith
MQ I
MQ9g 0.21 62.9 7.15 0.49 2.14 2.67 0.16 1.47 33.4 0.00 0.21 121 225 168
MQ9m 0.20 0.79 69.4 7.84 0.45 2.42 2.31 0.19 1.07 28.7 0.06 0.28 183 295 198
MQ8g 0.42 61.5 8.21 0.54 2.64 3.37 0.20 1.77 31.5 0.00 0.21 131 187 158
MQ8m 0.50 0.58 71.1 8.34 0.39 2.64 2.29 0.19 1.10 28.1 0.07 0.30 164 232 186
MQ7g 0.29 77.1 10.3 0.20 4.22 2.23 0.17 1.06 29.7 0.00 0.34 84.0 82.8 172
MQ7m 0.70 0.71 77.1 9.58 0.21 3.58 2.09 0.16 0.95 27.4 0.06 0.34 43.5 70.1 172
MQ6g 0.27 84.8 13.3 0.45 6.19 1.43 0.21 0.68 24.1 0.00 0.45 118 79.8 139
MQ6m 0.90 0.73 89.9 13.0 0.10 5.96 1.11 0.21 0.47 20.9 0.08 0.45 81.2 49.1 130
MQ5g 0.30 92.4 13.4 0.06 6.73 0.83 0.19 0.38 22.8 0.00 0.45 81.4 71.6 124
MQ5m 1.1 0.70 91.9 12.9 0.07 5.94 0.94 0.19 0.34 19.3 0.06 0.42 68.3 40.0 112
MQ4g 0.15 64.9 4.15 0.03 1.14 2.19 0.04 0.59 38.3 0.00 0.09 45.7 32.2 115
MQ4m 2.2 0.85 73.6 8.75 0.07 2.24 2.66 0.09 1.02 29.5 0.07 0.24 42.9 24.8 147
MQ3g 0.15 63.1 6.71 0.07 2.02 3.29 0.10 1.33 34.3 0.00 0.11 8.8 34.7 133
MQ3m 2.8 0.85 68.5 8.83 0.12 2.58 2.82 0.14 1.66 29.7 0.06 0.20 24.2 29.9 154
MQ2g 0.39 55.4 7.01 0.20 1.13 2.78 0.09 2.95 34.7 0.00 0.08 10.0 34.7 135
49
Sample Depth Proportion
1 CIA Al Ca Fe K Mg Na Si S Ti P Mn Zr
Unit m % % % % % % % % % % ppm ppm ppm
d.l.2 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.1 0.1 0.1
MQ2m 3.3 0.61 62.4 7.08 0.20 1.51 2.06 0.11 2.18 30.3 0.05 0.16 21.3 31.4 149
MQ1g 0.41 52.9 7.93 0.78 1.20 3.41 0.16 3.11 33.0 0.01 0.10 55.4 88.3 131
MQ1m 3.6 0.59 56.0 6.77 0.72 1.36 2.47 0.19 2.26 32.5 0.06 0.16 53.0 97.8 163
MQ II
MQ15g 0.42 58.3 7.85 0.66 2.12 3.08 0.20 2.23 32.2 0.00 0.20 104 191 168
MQ15m 0.08 0.58 71.0 8.49 0.44 2.66 2.36 0.30 1.06 30.5 0.04 0.29 248 227 150
MQ14g 0.20 60.1 8.83 0.92 2.35 2.93 0.15 2.23 31.7 0.01 0.21 39.8 107 185
MQ14m 0.25 0.80 73.7 8.41 0.20 2.90 2.30 0.13 0.98 30.8 0.03 0.29 72.4 49.5 193
MQ13g 0.26 83.2 11.4 0.15 4.60 1.51 0.16 0.90 27.0 0.00 0.35 43.4 56.9 150
MQ13m 0.60 0.74 82.7 10.6 0.08 4.09 1.79 0.15 0.75 26.2 0.06 0.33 56.7 33.1 160
MQ12g 0.16 70.9 7.39 0.07 2.87 1.84 0.12 1.43 34.4 0.00 0.18 16.5 41.8 148
MQ12m 1.1 0.84 73.9 9.10 0.09 3.27 2.00 0.14 1.46 27.6 0.06 0.23 17.7 22.2 145
MQ11g 0.26 57.7 6.85 0.18 1.40 3.21 0.08 2.19 35.3 0.00 0.10 13.2 33.2 105
MQ11m 1.6 0.74 67.4 8.20 0.13 2.30 1.91 0.13 2.11 28.9 0.06 0.18 6.6 20.8 133
MQ10g 0.26 54.9 5.81 0.21 1.06 2.81 0.05 2.16 37.3 0.00 0.10 10.0 29.4 108
MQ10m 2.0 0.74 62.5 7.89 0.14 1.68 2.13 0.09 2.61 30.2 0.06 0.17 43.7 15.0 146
Profile JG
Rock
JGPR1 46.8 7.79 1.57 1.41 3.45 0.35 3.72 33.5 0.02 0.13 307 225 160
JGPR2 47.2 7.61 1.51 1.53 3.17 0.33 3.66 32.2 0.02 0.13 252 218 159
Regolith
JG7g 0.24 99.2 23.6 0.03 20.4 0.18 0.03 0.03 6.70 0.03 0.74 184 205 348
JG7m 0.02 0.76 99.0 15.6 0.01 5.25 0.18 0.03 0.01 19.3 0.01 0.74 121 79.1 472
JG8g 0.60 98.7 24.2 0.06 24.2 0.19 0.02 0.08 4.44 0.03 0.89 243 203 354
JG8m 0.15 0.40 92.9 8.52 0.16 2.82 0.54 0.04 0.06 28.8 0.01 0.65 135 305 425
50
Sample Depth Proportion1 CIA Al Ca Fe K Mg Na Si S Ti P Mn Zr
Unit m % % % % % % % % % % ppm ppm ppm
d.l.2 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.1 0.1 0.1
JG9g 0.51 99.3 27.3 0.05 22.2 0.12 0.02 0.03 4.05 0.04 0.77 210 183 354
JG9m 0.3 0.49 92.7 9.04 0.09 3.03 0.65 0.03 0.12 31.0 0.01 0.69 102 209 507
JG10g 0.69 99.3 26.2 0.04 21.5 0.12 0.03 0.05 4.74 0.05 0.67 239 163 346
JG10m 0.4 0.31 92.2 9.35 0.11 3.32 0.78 0.04 0.10 30.5 0.01 0.67 87.2 108 490
JG6g 0.82 99.4 22.9 0.03 23.4 0.08 0.02 0.04 2.04 0.02 0.99 132 168 349
JG6m 1.5 0.18 98.6 21.4 0.00 5.52 0.41 0.04 0.01 13.3 0.02 0.38 80.2 57.5 445
JG5g 0.47 98.6 21.8 0.01 1.79 0.41 0.04 0.01 13.7 0.02 0.33 20.6 34.1 349
JG5m 3.0 0.53 98.1 13.2 0.01 2.49 0.31 0.03 0.02 25.6 0.01 0.44 60.7 50.5 291
JG4g 0.40 95.4 7.09 0.01 0.86 0.46 0.06 0.01 35.4 0.01 0.37 1.6 39.4 292
JG4m 5.0 0.60 95.3 8.05 0.01 1.18 0.54 0.05 0.01 32.2 0.00 0.52 29.0 64.6 482
JG3 6.5 W 94.1 7.50 0.01 0.78 0.63 0.06 0.02 34.2 0.02 0.38 9.0 37.1 341
JG2 8.6 W 86.2 9.73 0.00 0.85 2.18 0.05 0.04 31.4 0.04 0.17 0.8 22.9 164
JG1 10.0 W 64.7 8.35 0.27 0.84 3.93 0.06 1.26 33.6 0.03 0.10 2.1 59.6 105
4Batch 1
RSD5 0.2-0.6 0.0 0.0-0.3 0.0-0.1 0.0 0.0 0.7-1.1 0.0-0.2 0.0 10.1-20.4 3.3-16.3 8.6-8.9
Batch 2
RSD 0.1-0.2 0.0-0.4 0.0-0.1 0.0-0.5 0.0 0.0-0.7 0.0-0.9 0.0 0.0-0.1 0.9-30.2 2.5-17.5 2.6-9.3
Batch 3
RSD 0.0-0.2 0.0-0.3 0.0-0.2 0.0 0.0 0.0-0.3 0.0-0.2 0.0 0.0 7.4-31.4 0.4-8.9 0.9-10.4
1Proportion
refers to the weight proportion of gravel (g) or matrix (m) in percentage; ‘W
2’ refers to this sample without corresponding gravel/matrix and thus determined by
whole-rock analysis; 2d.l. refers to detection limit.
3GEPR3 and
4 MQPR3 are dolerite, whereas other rock samples are granitoid.
4Batch refers to each time determination of element concentrations based on the profile;
5RSD is the range of relative standard deviations (precision) of the duplicates/triplicates analysed by ICP-OES.
51
Figure 4.1 Ternary A-CN-K and A-FM-CNK plots of regolith samples from four lateritic profiles (GE, MQ I, MQ II, JG) based on chemical
compositions of matrix and gravel samples. Dashed line with arrow indicates weathering trend for MQ profiles. (a) and (e) GE profile; (b) and (f) JG
profile; (c) and (g) MQ I profile; (d) and (h) MQ II profile. Triangles represent granitoids and circles represent dolerite, whilst squares indicate regolith
gravel and diamonds indicate matrix samples.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
52
4.5.2 Mineralogical properties
In the GE profile, the parent granitoid (GEPR2) was characteristically identified by
quartz (ca. 67%), muscovite (ca. 4%), K-feldspars (ca. 8%) and albitic plagioclase
(ca. 21%) (Figure 4.2a). The saprock (GEBPRa&b) was composed of similar minerals
to the parent granitoid but in different proportions: quartz (71%-74%), muscovite
(ca. 4%), K-feldspars (ca. 7%) and albitic plagioclase (14%-17%). From saprolite
upwards, anorthitic plagioclase and K-feldspar had been weathered and altered to
kaolinite and gibbsite, whereas quartz and muscovite remained as residual components.
The proportion of kaolinite increased gradually upward from the saprolite (12.5 m depth)
to upper mottled clay (8.4 m depth), and then decreased in the lower ferruginous zone
(7.0 m depth). The concentration of gibbsite increased from the saprolite until duricrust
(3.5 m depth), except in the lower ferruginous zone. Muscovite was absent in the
mottled clay (8.4-11.4 m depth) but occurred in the ferruginous zone, reflecting its
moderate resistance to weathering. Quartz was residually enriched in the saprolite due
to breakdown of feldspar, but was less abundant in the mottled clay and ferruginous
zones due to further desilication and formation of Al hydroxides and Fe oxyhydroxides
under intense weathering. Goethite and hematite were present in the ferruginous zone;
however, significant amounts of hematite were only identified in the duricrust.
In the MQ profiles, the mineralogy followed similar patterns (Figure 4.2b & c):
kaolinite dominated the clay mineral assemblage and the proportion increased gradually
with decreasing depth. In contrast, plagioclase and muscovite decreased upward until
muscovite was completely absent at 1.1 m depth in the MQ I profile; muscovite then
increased again above 1.1 m depth. K-feldspar was also present its lowest proportion
(ca. 4%) at 1.1 m depth in the MQ I profile.
In the JG profile, the mineral characteristics were more complex than the other three
profiles (Figure 4.2d). In the saprolite (10 m depth) and mottled clay (6.5-8.6 m depth),
all plagioclase had been replaced by kaolinite and gibbsite. In the ferruginous zone and
the A horizon, further weathering and degradation resulted in the replacement of
kaolinite by gibbsite and formation of Fe oxyhydroxides. The regolith from the
ferruginous zone and the A horizon was made up of loose matrix with abundant iron
nodules; the iron nodules contained more Al (gibbsite) and Fe oxyhydroxides (goethite)
but less Si (quartz) than the matrix. The loose matrix was composed of heterometric,
sub-rounded to angular quartz grains of low sphericity with lower amounts of
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
53
K-feldspar, kaolinite, gibbsite and boehmite. Iron nodules ranged from 1-3 cm in
diameter and were composites of hematite, maghemite, quartz, gibbsite and boehmite
mineral assemblage. The iron nodules from the ferruginous zone were red and
concentrically zoned, with a core of hematite surrounded by a goethite rim (Fe-rich
layer) and an Al-rich layer, similar to the studies of Anand (2002; 2010) and
Fernandez-Caliani and Cantano (2010). The iron nodules from the A horizon, however,
were non-concentrically zoned, clay matrix cemented with Fe oxides (mainly hematite
and maghemite) without layers, containing less gibbsite but more quartz and the fabric
and texture changed to dark brown/black due to greater organic matter contents
(Appendix 11.4). Quartz grains in the iron nodules appeared strongly corroded and
spongiform, showing dissolution features, similar to those described by Abreu (1990)
and Fernandez-Caliani and Cantano (2010).
4.5.3 Mass balance analysis of elemental loss and gain
Mass balances of major elements were calculated at each sampling depth, based on
weighted average concentrations of major elements in matrix and gravel samples, using
Zr as the reference element. Depth profiles of absolute elemental loss or gain are plotted
in Figure 4.3.
In the GE profile (Figure 4.3a), Al was the least depleted element and enriched in
mottled clay (τ(Zr, Al) = 0.5) at ca. 10 m depth. In the ferruginous zone and the A horizon,
τ(Zr, Fe) ranged from 1.1-5.6, representing Fe enrichment, especially in the duricrust
(τ(Zr, Fe) = 5.6). Silicon was depleted most in the duricrust (τ(Zr, Si) = −0.9) and least in the
mottled clay (τ(Zr, Si) ca. −0.3). The elements Na and Ca were significantly depleted
throughout the profile, with τ(Zr, Na)< −0.98 and τ(Zr, Ca)< −0.95; K and Mg were less
depleted than Na and Ca, with τ(Zr, K) from −0.99 to −0.85 and τ(Zr, Mg) from −0.94 to
−0.60.
In the MQ I profile (Figure 4.3b), τ(Zr, Na) and τ(Zr, Ca) first decreased upwards until 1.1 m
depth, and then increased until the surface soils, consistent with the changes in
plagioclase content. Silicon was slightly enriched in the B and C horizons (τ(Zr, Si)
ranging from 0.01 to 0.1) but depleted in the A and B horizons (τ(Zr, Si) ranging from
−0.1 to −0.2). Aluminium and Fe were enriched throughout the profile with the highest
τ(Zr, Al) = 1.4 and τ(Zr, Fe) = 6.0 at 1.1 m depth.
In the MQ II profile (Figure 4.3c), similar as the MQ I profile, depletion of Na and K
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
54
first increased upwards to 0.6 m depth and then decreased. Aluminium was enriched in
most regolith (except τ(Zr, Al) = −0.05 at 0.25 m depth) and Fe was enriched in all
regolith with τ(Zr, Fe) ranging from 0.5 to 2.5. The changes in mass balance of major
elements at 1.1 m depth in the MQ I profile, and at 0.6 m depth in the MQ II profile,
were consistent with the previous assumption that, above 1.1 m depth in the MQ I
profile and 0.6 m depth in the MQ II profile, transported materials were present.
In the JG profile (Figure 4.3d), Na and Ca showed near-complete loss (τ(Zr, Na) and τ(Zr, Ca)
ca. −0.99) throughout the profile except the saprolite (τ(Zr, Na) = −0.5 and τ(Zr, Ca) = −0.7).
Silicon was enriched in the saprolite (τ(Zr, Si) = 0.6) but depleted upwards until the upper
ferruginous zone (τ(Zr, Si) = −0.95). Iron was depleted in the saprolite (τ(Zr, Fe) = −0.1),
mottled clay (τ(Zr, Fe) ca. −0.7), even the duricrust (τ(Zr, Fe) = −0.3) but strongly enriched
in the upper ferruginous zone (τ(Zr, Fe) = 5.2) and the A horizon (τ(Zr, Fe) average 2.7).
Based on the study of Brimhall et al. (1991), Zr concentrations in the A horizon (<1 m
depth) have been enhanced by aeolian input, and thus τ(Zr) in the A horizon would be
higher without external input, suggesting that calculated Fe enrichments in the A
horizon were conservative estimates.
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
55
Figure 4.2 Semi-quantitative mineralogical composition of regolith samples and parent
granitoids determined by random powder XRD analysis based on weighted average of
matrix and gravel, for: (a) the GE profile; (b) the MQ I profile; (c) the MQ II profile,
and; (d) the JG profile; (Qz: quartz; Ms: Muscovite; Kfs: Potassium feldspar; Fsp:
plagioclase feldspar; kln: kaolinite and halloysite group; Gbs: gibbsite; Gth: goethite;
Hem: hematite; Mgh: maghemite; Boe: boehmite).
56
Figure 4.3 Mass balance of major elements in regolith samples from four lateritic profiles, based on weighted average concentrations of major elements
in matrix and gravel at each depth, using Zr as the reference element (Brimhall et al., 1991): (a) the GE profile; (b) the MQ I profile; (c) the MQ II
profile; (d) the JG profile (the vertical dashed line refers to τ(Zr) = 1.0 without depletion or accumulation relative to the parent granitoid; trend of Ca
was similar to Na, and Mg to K, so not plotted above).
57
Figure 4.4 Depth functions of the molar ratio Na/K and concentration ratio Al2O3/Fe2O3 for MQ two profiles and concentration ratio (Ti/Zr)/10 for four
profiles illustrating the pedogenic discontinuity (shown by the boxed area) at 1.1 m depth in MQ I profile and 0.6 m depth in MQ II profile.
58
Figure 4.5 Major element concentrations in grain size fractions of the regolith samples from the MQ II profile.
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
59
4.5.4 Depth functions of pedogenic discontinuities
Pedogenic discontinuities represent zones of change in physical and chemical
properties primarily originating from processes during soil development, rather than
the parent lithology, which is more commonly referred as lithological discontinuities
(Schaetzl and Anderson, 2005; Tsai and Sang, 2000). In both MQ profiles, the
distribution of major elements in A horizon regolith is not correlated to the weathering
trend (Figure 4.1). In addition, a stone line observed within the MQ II profile may
indicate either the presence of an erosional episode at/near a discontinuity (Parsons and
Herriman, 1966; Ruhe, 1958) or a mass movement with biogenic involvement
(Johnson, 1990; Johnson and Balek, 1991; Lichte, 2000). Either of these reasons
indicates the presence of pedogenic discontinuity in both MQ profiles. Therefore,
depth functions for various pedogenic parameters, including molar ratio Na/K,
concentration ratios Al2O3/Fe2O3 and (Ti/Zr)/10 were prepared for the MQ I and MQ II
profiles (Figure 4.4).
In the MQ profiles, abrupt changes of Na/K, Al2O3/Fe2O3 and (Ti/Zr)/10 occurred at
1.1 m depth of the MQ I profile and at 0.6 m depth of the MQ II profile, including
gravel and matrix. The (Ti/Zr)/10 first increased upwards and then decreased until
surface soil, showing great variations (variation up to > 70% in MQ I profile and > 50%
in MQ II profile relative to the parent granitoid). However, below 1.1 m depth of the
MQ I profile and at 0.6 m depth of the MQ II profile, (Ti/Zr)/10 was relatively
consistent. The depth of abrupt change coincided with the presence of stone line in the
MQ II profile, implying that a mass movement (i.e. erosion and re-deposition with or
without biological recycling) had occurred during the weathering history of these two
profiles.
In the GE and JG profiles, (Ti/Zr)/10 was relatively consistent in the saprolite and
mottled clay zone, close to the parent granitoid (variation < 30%), including both
gravel and matrix; then Ti and Zr in gravel and matrix started to fractionate in the
ferruginous zone, and significantly differentiated from each other in the A horizon. The
variation of (Ti/Zr)/10 in the ferruginous zone showed that Ti and Zr partitioned
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
60
between gravel and matrix at the advanced stage of weathering and lateritization and
further details will be discussed in the next chapter using the JG profile as an example.
4.5.5 Grain size distribution of major elements in MQ II profile
The distribution of major elements into different grain size fractions from the MQ II
profile (Figure 4.5) showed that Al, Fe, Mg, Ti and Na were enriched in the clay and
silt fractions. Potassium and Si, however, were enriched in the sand and gravel
fractions. The silt fraction had the highest Zr concentrations. In the A horizon, gravel
had the highest concentrations of Ca; however, in the B and C horizons, sand was the
main host for Ca.
4.6 Discussion
4.6.1 Significant processes during lateritization
Based on the geochemical and mineralogical data, the regolith from these four lateritic
profiles has experienced moderate to extreme weathering. The mass balance
calculations (Figure 4.3) reflect the substantial loss of alkaline and earth-alkaline
elements in the saprolite of the GE and JG profiles, in agreement with the depletion of
plagioclase (Figure 4.2). Compared with Na and Ca, K and Mg were less depleted and
corresponded to the residual occurrence of muscovite and potassium feldspar. Both
alkalis and Si continued to be lost as weathering intensified. Kaolinite was further
altered into gibbsite and Fe was enriched in the ferruginous zones as goethite and
hematite (Figure 4.2). Similar trends have been reported by Anand and Paine (2002).
At pH above 5, Si (even from quartz) has a higher solubility than Al- and Fe- oxides
(Breemen and Buurman, 1998), and thus would be preferentially removed from the
system. The near-complete loss of Na and Ca and significant enrichment of Al and Fe
in the ferruginous zone indicate that the regolith has undergone an advanced stage of
lateritization. Only residual or partially weathered quartz and some
weathering-resistant minerals remained in the ferruginous regolith.
Both MQ profiles were less weathered than GE and JG profiles (Figure 4.1), and the
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
61
chemical composition of regolith showed intermediate depletion of Si, Na and K and
enrichment of Al and Fe (Figure 4.3). The plots of SiO2 vs. Al2O3 and Fe2O3 vs. Al2O3
(Figure 4.6) clearly show that (i) the regolith was weathered in-situ from granitoids; (ii)
silicon consistently decreased whereas Al and Fe increased with increasing weathering
intensity. Aluminium was residually enriched by dissolution of feldspar and mica and
the formation of kaolinite at the expense of primary minerals (kaolinization) in the B
horizon of both MQ profiles and in the mottled clay zone of the GE and JG profiles
(Figure 4.2). Idealized weathering reactions for dissolution of silicates, e.g. feldspar
and muscovite, and for formation of kaolinite at the early stages of weathering are
presented below:
CaAlSi3O8+H2O+2H+⇌Al2Si2O5(OH)4+Ca
2+ (loss of Ca)
2KAl3Si3O10(OH)2+3H2O+2H+⇌3Al2Si2O5(OH)4+2K
+ (loss of K)
4.6.2 Genesis and sources of Fe redistribution
The nature of formation of iron oxides is generally more dependent on the
environmental conditions at the time of formation than on the particular structures of
the primary mineral from which the Fe was released (Anand and Paine, 2002). Under
anaerobic conditions, Fe can mobilize and redistribute over a range of spatial scales.
This mobilization can be vertical or lateral, at a horizon, or even a landscape, scale.
In the GE and JG profiles, residual accumulation of Fe is likely to be the result of
continuous cyclic dissolution-precipitation processes with redox changes occurring as
a result of changes in regolith water regime (Anand and Butt, 2010; Anand and Paine,
2002; Tripathi and Rajamani, 2007). Redistribution of Fe in the duricrust may be
related to a capillary effect triggered by seasonal fluctuation of the water table (Braun
et al., 1993; Ndjigui et al., 2009). In addition, in the JG profile, the location of intrusive
dykes in the granitoids can be mapped from the overlying duricrust and the profiles
developed on dolerite and granitoid are distinctly different; both of which suggest that
duricrust has formed in-situ (Anand and Paine, 2002; Sadleir and Gilkes, 1976). The
regolith from the GE profile was also formed residually from weathering and
pedogenic processes; the complete profile components (including saprolite, mottled
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
62
clay and ferruginous zone) occur without any discontinuity, erosion or missing zones
and no apparent enrichment of any trace elements derived from external sources in
subsurface regolith (regolith <1 m depth given aeolian input) was observed. Therefore,
Fe enrichment in the GE profile is more likely a result of vertical residual
accumulation rather than lateral movement.
In the MQ profiles, transition zones (at 1.1 m depth in MQ I profile and 0.6 m depth in
MQ II profile) showed the lowest Al2O3/Fe2O3 ratio. It is possible that the Fe released
from primary minerals was partially oxidized by oxygen in penetrating
rainwater/atmosphere and, as a result, was precipitated in the upper part of the regolith
during weathering. With continued weathering and the influence of soil creep or
colluviation, and alternative wetting and drying of the soil due to seasonality or
longer-term changes between humic and arid climates, translocation and redistribution
of Fe was facilitated.
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
63
Figure 4.6 The distribution of Al2O3 vs. SiO2 and Al2O3 vs. Fe2O3 in matrix and gravel
from four lateritic profiles (A_g: gravel of A horizon; A_m: matrix of A horizon;
regolith_g: gravel of subsurface regolith; regolith_m: matrix of subsurface regolith; in
(g) and (h), dolerite had Fe2O3 concentration 13.7 wt%, so not showed).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
64
4.6.3 Degrees of lateritization
To classify the degree of lateritization and demonstrate the effects of weathering
intensities on lateritic regolith, ternary SiO2-Al2O3-Fe2O3 diagrams (Schellmann, 1981)
were plotted (Figure 4.7). The progression from fresh parent granitoids to loss of Si
and relative enrichment of Fe and Al involves various degrees of weathering intensity.
Al2O3 and Fe2O3 only separate under the extreme weathering (strong lateritization).
The GE and JG profile have apparently undergone strong lateritization, whereas both
MQ profiles are still within the ‘weak lateritization’ status, but have both undergone
intense kaolinization.
Figure 4.7 Schellmann SiO2-Al2O3-Fe2O3 diagrams showing different degrees of
lateritization of weathered regolith from four lateritic profiles: (a) the GE profile; (b)
the MQ profiles; (c) the JG profile; regolith include matrix and gravel.
4.6.4 Principal components analysis
In order to systematically analyse the data, chemical compositions of all regolith from
four profiles are standardised and subjected to principal component analyses. Two
principal components extracting 76.0% of variance in major element data are
recognized and plotted in Figure 4.8 and the factor score of each regolith sample is
plotted in Figure 4.9.
Principal Component 1 was controlled by base cations (positive loadings) and
conservative elements such as Ti, Zr, Al and Fe (negative loadings). Parent granitoids
were enriched in base cations (ellipse a in Figure 4.9), with moderate weathered MQ
regolith having some base cation depletion (ellipse b). In contrast, GE and JG regolith
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
65
was characterized by more residually accumulated Zr, Ti, Al and Fe (ellipse c, d, e and
f). This reflects the relative mass flux depletion of base cations in weathered regolith
and relative conservation of Al and Fe in ferruginized regolith during lateritic
weathering. Principal Component 2 was mainly affected by Si, P and Mn. The regolith
of mottled clay and ferruginous mottled zones from the GE and JG profiles contained
high concentrations of Si and separated from the other regolith, indicating initial
residual enrichment of Si due to depletion of base cations without desilication. In
ferruginous zone of the GE and JG profiles, this Si enrichment was weakened by
desilication and formation of gibbsite. The ferruginous surface gravel from the GE and
JG profiles had lower concentrations of Si but higher concentrations of P and Mn than
corresponding matrix, and thus, separated from the matrix and the subsurface regolith.
The C horizon regolith from the MQ I profile and the saprolite from the JG profile (see
arrows) were close to parent granitoids, reflecting: (i) incipient weathering conditions;
(ii) weak depletion of base cations; and (iii) the genetic relationship with the granitoid.
In addition, different types of granitoids were slightly differentiated from each other;
e.g., the coarse grained granite was separated from the late-stage sub-horizontal
pegmatite vein.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
66
Figure 4.8 Principal component analyses of major elements in regolith samples and
parent granitoids from four lateritic profiles. Compositional data were transformed
using centered log-ratios.
Figure 4.9 Principal component factors of regolith samples and parent granitoids from
four lateritic profiles calculated using major element composition.
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
67
4.6.5 Mineralogy and element grain size distribution
The distribution pattern of major elements into different subgroups can be interpreted
by regolith mineral characteristics. In the GE and JG profiles, gravel was more
enriched in Al and Fe but more depleted in Si, whereas matrix was more enriched in Si
but more depleted in Al and Fe (Figure 4.10). This corresponds to the weathering
intensity and mineral composition of gravel and matrix. Both profiles are more
intensely weathered; cyclic reducing and oxidizing conditions likely lead to mottle
formation, with Fe oxides and oxyhydroxides migrating into clay matrix or voids.
Repeated dissolution and cementation results in the formation of gravel, which are
dominated by gibbsite, goethite and hematite. The occurrence of maghemite in
near-surface regolith in the JG profiles may be induced by a combination of heat (bush
fire) to dehydroxylate goethite and organic matter (Anand and Gilkes, 1987; Anand
and Paine, 2002; Perrier et al., 2006). However, in both MQ profiles, Si in gravel was
higher than in matrix, but Al and Fe varied with the depth. The MQ gravel containing
higher Si likely results from less weathering intensity and relatively fast drainage. The
absence of gibbsite in both MQ profiles reveals that kaolinite did not further alter into
gibbsite and no apparent partitioning of Al occurred between matrix and gravel in MQ
regolith profiles. Variation of Fe with depth correlates to the weathering intensity of
matrix and gravel samples. Therefore, the element distribution between matrix and
gravel is a reflection of the mineralogical composition, which is more dependent on the
weathering environment and weathering intensity than the parent lithology.
In addition, the enrichment of Al, Fe, Mg, Ti and Na in silt and clay fractions indicates
that Al released from feldspar, muscovite, etc., in the parent granitoids formed clay
minerals and sesquioxides in the regolith. Iron released from altered magnetite and
other iron-bearing phases in the parent granitoids produced goethite in the regolith.
Due to different charge/ionic radius and hosting minerals, the distribution patterns of
Na and K, Mg and Ca were not the same. These elements inherited from the parent
granitoids, separate from each other during weathering and pedogenic processes
depending on the mineral weathering rate and weathering conditions. The differences
of charge/ionic radius also affect the ability of ion exchange of these elements onto
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
68
clay minerals, Al and Fe oxides/oxyhydroxides and organic matter.
In addition, Si was residually enriched in the sand fraction as quartz, and Zr was
residually enriched in the silt fraction as zircon. Enrichment of Ti in the fine fractions
indicated the physical, and possibly chemical, weathering of ilmenite and rutile or
newly formed anatase (Anand et al., 1991; Anand and Paine, 2002). The geochemical
distribution trends of major elements in different grain size fractions are therefore
mainly controlled by the physical characteristics and chemical stabilities of hosting
minerals inherited from the parent rock and secondary formed minerals in the regolith,
which in themselves are constrained by the weathering conditions.
4.6.6 Mobility of Ti and Zr
Although Ti and Zr are commonly considered to be and most frequently used as the
least immobile elements in weathered profiles (Beyala et al., 2009; Braun et al., 1993;
Brimhall et al., 1991; Nesbitt and Markovics, 1997; Taboada et al., 2006b),
mobilization of Zr induced by eroded zircon and redistribution of Ti during extreme
weathering have been discussed (Anand and Paine, 2002; Anand et al., 2010; Braun et
al., 1993). Titanium occurs in rocks mainly as rutile, ilmenite and sphene, or in the
structure of silicates such as micas, amphiboles and pyroxenes. The susceptibility of
silicate minerals to weathering results in release of some Ti in the early stage of
weathering of igneous rocks and continuing release as weathering proceeds (Anand
and Paine, 2002). Zirconium occurs in rocks largely as zircon, which is very resistant
to weathering and hence Zr is not considered to be transported under low temperature
and low pressure conditions (Henderson, 1984; Linnen et al., 2005; Vos et al., 2006).
In intensely weathered profiles (e.g., laterite), it is difficult to identify the most
appropriate immobile element(s) not subject to dissolution and physical translocation
given different scales.
In the GE and JG profiles, a variation of (Ti/Zr)/10 from the ferruginous zone upwards
was observed, illustrated that either or both Zr or Ti were relatively mobile under
extreme weathering conditions. In the Jarrahdale bauxitic lateritic profile, both rutile
Chapter Four: Redistribution of major elements in lateritic profiles during intensive weathering in Western Australia
69
and zircon have been introduced by aeolian input, inducing higher concentrations of Ti
and Zr in the surface soils (<1 m depth) (Brimhall et al., 1988; Brimhall et al., 1991;
Foo, 1999; McLennan, 1995). The euhedral grains of zircon, observed by scanning
electron microscopy in the regolith of four profiles showed zircon’s stability. However,
eroded ilmenite and rutile, and poorly crystalline Ce, Zr and Th (hydr)oxides, were
observed in the ferruginous zone of the JG profile, suggesting mobility of both Ti and
Zr at the sampling scale during the advanced stages of weathering and lateritization.
The mobilization and redistribution of Ti, Zr and Th will be investigated further in the
next chapter taking the JG profile an example.
4.7 Summary of the chapter
In this chapter, the bulk geochemistry of major elements in matrix and gravel from
four intensely weathered lateritic profiles (GE, MQ I, MQ II and JG) was investigated.
The regolith from the GE and JG profiles had undergone intense weathering and strong
lateritization with the CIA above 99% in the ferruginous zone. Both MQ profiles were
less weathered than the GE and JG profiles with only weak lateritization.
In the GE and JG profiles, gravel was more enriched in Al and Fe but more depleted in
Si than matrix. In both MQ profiles, however, gravel had higher Si than the associated
matrix with varied concentrations of Al and Fe. In the ferruginous gravel of the GE and
JG profiles, Fe was mainly enriched as goethite, hematite and maghemite, while Al
was mainly enriched as secondary clay minerals and gibbsite. In both MQ profiles,
however, lower weathering intensities and relatively fast drainage resulted in the
absence of gibbsite and hematitie and low concentrations of goethite.
Using Zr as the reference element, Ca and Na were near-completely depleted and large
loss of K, Mg and Si also occurred in the GE and JG profiles, indicating breakdown of
plagioclase and incongruent dissolution of potassium feldspar, muscovite and quartz at
advanced stages of weathering. The increasing proportion of gibbsite in the duricrust
and ferruginous gravel of the GE and JG profiles revealed that kaolinite was further
altered into gibbsite and cemented with iron oxides with weathering intensifying.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
70
Taking the MQ II profile as an example, Al, Fe, Mg, Ti and Na were enriched in clay
and silt fractions, K and Si was enriched in sand and gravel fractions and Zr was
enriched in the silt fractions as zircon. The redistribution of elements into different
grain size fractions is mainly controlled by the physical characteristics and chemical
stabilities of hosting minerals inherited from the parent rock and newly formed
secondary minerals in the regolith. Intense leaching of cations, kaolinization,
desilication and ferruginization were identified as significant processes during
lateritization using principal component analysis, and these mechanisms were further
substantiated by the geochemical mass balance calculations and mineralogical
analyses.
71
Figure 4.10 Calculated τ values of Al, Fe and Si referenced to Zr in matrix and gravel from four lateritic profiles (the dashed line indicates τ(Zr) = 0,
without enrichment or depletion).
73
5 Redistribution and mobilization of Ti, Zr and Th in an intensely
weathered lateritic profile in Western Australia
5.1 Abstract
The mobility of titanium, zirconium and thorium, elements commonly considered
insoluble during supergene weathering, is still not well understood, especially in
intensely weathered regolith. Thus, an intensely weathered lateritic profile (JG)
developed on meta-granitoids in Jarrahdale, Western Australia, was investigated. The
mobility of Ti, Zr and Th has been assessed at both mineral assemblage and profile
scale and the mode of occurrence has been investigated through the combined use of
geochemical data from bulk regolith, particle size fractions and sequential extractions,
with in-situ data determined by electron probe micro-analyzer and synchrotron X-ray
powder diffraction.
Neoformed poorly crystalline phases containing trace to minor amounts of Zr, Ce and
Th unassociated with silicates or phosphates were identified on the walls of Al/Fe-rich
pores in the ferruginous duricrust. This implies that some mobilization and
redistribution of Zr and Th occurs within a sample scale. Breakdown of primary thorite
and rare earth element rich fluorocarbonates is thought to be the source for Zr and Th in
the neoformed phases rather than zircon. Thus, the mineral hosts of Zr, Ti and Th in the
parent rock and their relative susceptibility to weathering are the fundamental controls
on subsequent mobility during initial weathering. Trace amounts of Th in secondary
phases, such as rhabdophane and florencite, shows translocation of Th at the mineral
scale; whilst strong partitioning of Th into gravel rather than matrix reflects
redistribution of Th at the profile scale. The absence of primary sphene from the
regolith and the presence partially dissolved ilmenite and rutile grains in the ferruginous
mottled zone suggest mobilization and translocation of Ti at a mineral assemblage scale.
Furthermore, the fluctuation of Ti/Zr in the ferruginous zone is in contrast to the
consistency of Zr/Hf throughout the profile in general (within the range of parent
meta-granitoid). This suggests that Ti and Zr fractionate from each other and partition
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
74
between gravel and matrix during extreme weathering and advanced lateritization. This
study demonstrates that Ti, Zr and Th are mobile at a variety of scales, an important
consideration that is often overlooked when calculating element mass flux in intensely
weathered regolith.
5.2 Key words
Zirconium; titanium; thorium; laterite; weathering; regolith;
5.3 Introduction
The mobility of titanium, zirconium and thorium during fluid/rock interaction has been
discussed extensively (Bednar et al., 2004; Cornu et al., 1999; Duvallet et al., 1999;
Kurtz et al., 2000; Langmuir and Herman, 1980; Melfi et al., 1996; Taboada et al.,
2006a; Taboada et al., 2006b). It is accepted that these so-called ‘immobile’ elements
can be highly mobile during hydrothermal alteration (Jiang, 2000; Rubin et al., 1993;
Vanbaalen, 1993); however, their mobility during supergene weathering is still not well
understood. Despite this uncertainty, Ti, Zr and Th are generally considered to be
immobile and are often used as reference elements to evaluate the mass flux of other
elements (Braun et al., 1993; Brimhall et al., 1991; Gouveia et al., 1993). The
assumption of immobility is because: (i) the main host minerals, ilmenite, rutile and
anatase for Ti, zircon for Zr, and most minerals hosting Th are resistant to weathering
(Henderson, 1984; Linnen et al., 2005; Taboada et al., 2006a; Vos et al., 2006); and (ii)
the solubility of Ti, Zr and Th is very low in the absence of strong complexing ligands
(Brookins, 1988; Kabata-Pendias and Pendias, 2001; Linnen et al., 2005; Taboada et al.,
2006a; Tilley and Eggleton, 2005).
The susceptibility of primary minerals during weathering is the main control on the
subsequent mobility and redistribution of Ti, Zr and Th. Titanium has been known to
become mobile during weathering due to: (i) the alteration from sphene/ilmenite to
secondary rutile and anatase at the mineral assemblage scale (Anand and Paine, 2002;
Schroeder et al., 2002), or (ii) as a dissolved element or organo-metallic compounds at
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
75
centimetre or even profile scale under tropical weathering conditions (Cornu et al.,
1999). It has also been noted that Zr can be released by the breakdown of amphibole
during weathering, mobilized and sorbed as colloidal (hydr)oxides (ZrO2∙nH2O) to the
surface of goethite in the pores and granules of the gibbsite-rich matrix in bauxitic
profiles (Duvallet et al., 1999; Melfi et al., 1996). Amphibole, however, hosts only trace
concentrations of Zr, the main Zr-hosts in igneous rocks are accessory minerals such as
zircon. Despite the acknowledged longevity of zircon in supergene environments, it is
not totally stable, for examples: (i) corroded zircon and partially dissolved
radiation-damaged (metamict) zircon have been found in extremely weathered lateritic
profiles (Braun et al., 1993; Delattre et al., 2007), and (ii) dissolution of zircon can take
place in systems with a pH below 3 and either a naturally high chloride concentration
(Colin et al., 1993), or where organic matter is present (Hodson, 2002). Thorium usually
occurs in rocks and regolith as: (i) a trace constituent in phosphate, oxide and silicate
minerals, e.g. monazite, cerianite, allanite; (ii) in the rare minerals thorite (ThSiO4) or
thorianite (ThO2), or (iii) it is sorbed onto clays and other soil colloids (Langmuir and
Herman, 1980). Thorium, as well as Ti and Zr, is insensitive to redox change and
mainly occurs in its tetravalent form in natural environmental systems (Buettner and
Valentine, 2012; Langmuir and Herman, 1980). Most Th-hosting minerals are highly
resistant to weathering; hence Th has long been considered a very insoluble and
immobile element in natural systems (Langmuir and Herman, 1980). However,
mobilization and redistribution of Th in lateritic soils where dissolution of thorite, and
transformation and precipitation of thorianite, is further enhanced by the presence of
organic matter has been reported, but Th was still considered the least mobile element at
the profile scale (Braun and Pagel, 1994; Braun et al., 1993; Braun et al., 1998). In
addition, breakdown of primary Th-hosting minerals and adsorption of Th onto clays,
oxyhydroxides and organic matter increases the likelihood of mobilization of Th in
supergene environments (Cromieres et al., 1998; Langmuir and Herman, 1980; Reiller
et al., 2002; Seco et al., 2009; Zhang et al., 2006).
The objective of this study is to investigate the mode of occurrence and mobility of Ti,
Zr and Th in intensely weathered lateritic regolith. Therefore, an intensely weathered
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
76
lateritic profile (JG) developed on meta-granitoids in Jarrahdale, Western Australia, was
investigated by integrating the geochemistry of bulk regolith samples, particle size
fractions and sequential extractions with in-situ data from electron probe micro-analyzer
(EPMA) and synchrotron X-ray powder diffraction (SXRD) techniques.
5.4 Materials and methods
5.4.1 Analytical methods
This study was performed on a lateritic profile (JG) developed over meta-granitoid
rocks in Jarrahdale, Western Australia. Regolith samples were separated into two
subsample groups by sieving: gravel (>2 mm, represented by suffix ‘g’) and matrix
(<2 mm, represented by suffix ‘m’), with the exception of saprolite (JG1) and mottled
clay (JG2&3), which had only matrix without gravel. Following this division, the
regolith matrix was further separated, using sedimentation and wet sieving methods
(Day, 1965), into the following three size fractions as recommended by the International
Society of Soil Science (ISSS; Marshall, 1947; Marshall, 2003; Prescott et al., 1934):
clay (<2 µm), silt (2-20 µm) and sand (>20 µm). These particle size fractions were
rinsed with deionised water three times, oven dried at 105 °C overnight (together with
matrix and gravel subsamples), and then ground to ≤ 200 µm prior to fusion. In all
chemical procedures high-purity water (≥18 MΩ.cm, Millipore Milli-Q system),
analytical-grade reagents and acid-washed containers were used.
The chemical species and association behaviour of Ti, Zr and Th in matrix from
saprolite (JG1m), the upper part of the mottled clay (JG3m), and ferruginous duricrust
(JG5m) were assessed by a sequential extraction method to determine the element
partitioning during intense weathering and formation of duricrust. An in-house
laboratory reference soil material was prepared together with selected samples. Regolith
trace elements were operationally defined into five species (modified from Hall et al.,
1996): (i) water soluble, adsorbed, exchangeable and carbonate bound (WAE); (ii)
organic matter and sulphide bound (Org); (iii) amorphous Fe-Mn oxyhydroxides bound
(FeAm); (iv) crystalline Fe-Mn oxide bound (FeCry); and (v) residual species (Res).
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
77
Carbonates are unlikely to be present in the regolith being studied here (due to the low
pH). Sulphides are also scarce in the lateritic regolith, therefore it is assumed that
species Org is mainly hosted by organic matter complexes. A brief summary of the
method used is shown in Table 5.1 and the detailed extraction method and chemical
preparation are listed in the Appendix 11.5. The residual samples and reference
materials were rinsed with deionised water three times, oven dried at 105 °C overnight
and ground to ≤200 µm prior to fusion in order to determine element concentrations.
Table 5.1 Sequential extraction procedures of trace elements in the lateritic regolith
Step Speciation Reagent
i water soluble, adsorbed, and
exchangeable (WAE)
To 1 g of sample, add 20 mL of 1.0 M CH3COONa
(adjust to pH 5 with CH3COOH) at room temperature.
(25°C), shake 6 h, centrifuge for 15min at 3000 rpm;
rinse with 5 mL MilliQ H2O twice, mark 30 mL; repeat.
ii organic matter bound (Org) Add 40 mL 0.1M Na4P2O7 at room temperature (25°C),
shake 1h, centrifuge; repeat; rinse with 5 mL MilliQ H2O
twice, mark 50 mL; repeat.
iii amorphous Fe oxyhydroxide
bound (FeAm)
Add 20 mL 0.25M NH2OH∙HCl in 0.25M HCl, vortex,
water bath at 60°C for 2h, centrifuge; rinse with 5 mL
MilliQ H2O twice, mark 30 mL; repeat.
iv crystalline Fe oxide bound
(FeCry)
Add 30 mL 1.0M NH2OH∙HCl in 25% CH3COOH,
vortex, water bath at 90 °C for 3 h, centrifuge; rinse with
10 mL 25% CH3COOH twice, mark 50 mL; repeat.
v residue (Res) MilliQ water wash residue three times, oven dries at
60 °C. Fuse with 12:22 Norrish flux (Lithium
metaborate/ Lithium tetraborate), dilute with 100 mL
10% HCl.
The fusion beads were made by mixing 0.1 g finely ground sample or reference material
(weighing accuracy 0.1 mg) with 0.7 g 12:22 Norrish flux (lithium metaborate:lithium
tetraborate) and heated in a muffle furnace at 1050 °C for 40 minutes. Duplicate fusion
beads were also made for 10% of the samples to check reproducibility. After cooling,
the fusion beads were dissolved in 100 mL of 10% HCl.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
78
Total concentrations of Ti in the fusion beads and extracted solutions were determined
by inductively coupled plasma-optical emission spectroscopy (ICP-OES) whereas
concentrations of Zr and Th were determined by inductively coupled plasma-mass
spectroscopy (ICP-MS); both analyses took place at Genalysis Laboratory Services of
Intertek Commodities in Maddington, Western Australia. Certified international
standard materials, including stream sediment reference material STSD-2, STSD-4
(Canada Centre for Mineral and Energy Technology, CANMET) and an in-house
standard soil material were prepared in the same way as the samples and analysed
together with the samples to check accuracy and precision. The variation between tested
values and certified values was within 10% of the certified values (Appendix 11.2 &
Appendix 11.6). The total concentrations of Ti, Zr and Th in different grain size
fractions are listed in Table 5.2.
Texture, morphology, and phase composition of individual grains were determined
using polished thin sections of air dried and resin impregnated regolith and outcrop
samples. These polished thin sections were examined using a JEOL JSM-6400 scanning
electron microscope (SEM) with a Link analytic energy dispersive spectrometer (EDS),
utilizing both secondary electron (SE) and back-scattering electron (BSE) imaging at
15kV accelerating voltage with a 3 nA beam current. The chemical composition of
selected representative mineral grains was analysed using a JEOL 8530 field emission
electron probe micro-analyzer (EPMA) at 20 kV accelerating voltage and 5 nA beam
current. Software Probe for EPMA from Probe Software Inc. was used for setting up
and analysing the data. Standard references for microprobe calibration were synthetic
glass 612 from National Institute of Standards and Technology (NIST), in-house
standard synthetic rare earth elements (REE) phosphates, rutile, zircon and thorite;
standard Brazil monazite was also analysed with samples for cross checking. All
microscopy analyses were conducted at the Centre for Microscopy, Characterisation and
Analysis (CMCA), University of Western Australia. The detection limits of EPMA of
element concentrations in minerals are listed in the Appendix 11.7.
In addition, selected regolith samples from the ferruginous mottled zone and duricrust
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
79
were analysed by synchrotron X-ray powder diffraction (SXRD). For these samples, the
matrix fraction was first treated using dispersion and sedimentation to remove the clay
fraction (≤2 µm), and wet sieving to remove coarse sand (>500 µm). The resulting
fraction (2-500 µm) was cleaned in deionised water, oven dried at 50 °C, passed under a
magnet to remove magnetite and added to lithium heteropolytungstate (LST) heavy
liquid to separate the heavy mineral fraction. The heavy mineral fraction was then finely
ground and mounted in capillaries prior to analysis by SXRD at a beam energy of
15.02064 KeV (yielding a wavelength of 0.82616 Å) over an angular range of 4-60° 2θ,
to provide for adequate dispersion/resolution and high peak/background in order to
identify minor constituents. SXRD samples were scanned twice with a 0.5° 2θ-step
difference. SXRD patterns were acquired on beamline 10BM1 (Powder Diffraction) at
the Australian Synchrotron in Melbourne, Australia.
5.4.2 Mass balance calculation
To quantify the net element fluxes from pedogenic weathering, a geochemical mass
balance calculation was used (Brimhall et al., 1991). The formula for normalized
concentration (τi,j) in Equation (1) assumes that an immobile element (e.g., Zr, Th)
behaves conservatively and can be used to correct mobile element concentrations for
volumetric strain during weathering and pedogenesis.
1))((,
,
,
,
, pj
wj
wi
pi
C
C
C
C
ji
(1)
For an immobile element i (τi,j = 0), the volume strain ε can be calculated from the ratios
of density data and the concentration of element i in the regolith and primary rocks:
1)/)(/( ,,, wipiwpwi CC
(2)
In Equations 1 and 2, C represents concentration, ρ represents density, i represents the
immobile element, j represents the element of interest, w represents weathered material
and p identifies parent rock. If τi,j = 0, the element j has behaved conservatively; if τi,j
= −1, the element j is completely depleted during weathering, and positive τi,j values
signify absolute enrichment. If wi, = 0, there is no volume change; wi, > 0 indicates
dilation and wi, < 0 means contraction (collapse).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
80
Although Equation 1 provides a tool for estimating element loss or gain for a profile,
mass balance equations have two critical assumptions: a genetic relationship between
regolith and underlying rock and a fully conserved reference element. Therefore, a
conservative immobile reference element is a prerequisite for mass flux calculation.
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
81
Table 5.2 Concentrations of Ti, Zr and Th in grain size fractions of the JG profile
Element concentrations
depth Ti Zr Th Hf
Unit m wt% ppm ppm ppm
Detection limit 0.01 0.2 0.1 0.1
Methods Fusion/ICP-OES Fusion/ICP-MS Fusion/ICP-MS Fusion/ICP-MS
Upper ferruginous zone 1.5
JG6sand 0.71 246 40.8 7.5
JG6silt 1.12 788 42.6 25.5
JG6clay 0.42 9.6 41.3 0.6
JG6matrix 0.38 445 37.6 12.8
JG6gravel 0.99 349 94.0 9.8
Duricrust 3.0
JG5sand 0.37 182 41.7 5.9
JG5silt 0.97 652 61.3 19.9
JG5clay 0.62 16.9 50.2 0.8
JG5matrix 0.44 291 45.0 8.1
JG5gravel 0.33 349 196 11.0
Ferruginous mottled zone 5.0
JG4sand 0.24 179 23.7 5.8
JG4silt 1.39 1376 66.8 42.4
JG4clay 0.87 35.7 49.1 2.1
JG4matrix 0.52 482 35.6 12.6
JG4gravel 0.37 292 119 7.6
Upper Mottled clay 6.5
JG3sand 0.24 383 36.1 13.3
JG3silt 0.93 914 127 30.4
JG3clay 0.56 171 79.0 5.8
JG3matrix 0.38 341 48.0 10.6
Lower Mottled clay 8.6
JG2sand 0.39 199 153 7.6
JG2silt 0.54 718 287 25.7
JG2clay 0.31 234 220 11.6
JG2matrix 0.17 164 167 4.9
Saprolite 10.0
JG1sand 0.12 128 15.9 4.3
JG1silt 0.23 208 45.1 7.1
JG1clay 0.09 75.6 58.5 2.1
JG1matrix 0.10 105 31.0 2.8
parent meta-granitoids >11.0
JGPR1 0.13 160 16.4 4.12
JGPR2 0.13 159 18.5 5.45
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
82
5.5 Results
5.5.1 Bulk Ti, Zr and Th concentrations in regolith
5.5.1.1 Abundance of Ti, Zr and Th in the parent rock and regolith
All regolith except saprolite had a higher Ti concentration than the parent
meta-granitoid (0.13 wt%; Figure 5.1a). Furthermore, in the A horizon (0-0.4 m depth)
Ti was more abundant in gravel (0.67-0.89 wt%) than in matrix (0.65-0.74 wt%),
whereas in the ferruginous zone (1.5-5 m depth) Ti was more concentrated in matrix
than gravel, excepting the upper ferruginous zone (JG6, 1.5 m depth). The concentration
of Zr varied significantly in the profile (Figure 5.1b), 105 ppm in the saprolite (10 m
depth), 164-341 ppm in the mottled clay (6.5-8.6 m depth), 291-482 ppm in the
ferruginous zone and 346-506 ppm in the A horizon. With the exception of the saprolite
(105 ppm), the regolith samples were enriched in Zr compared with the parent
meta-granitoid (159 ppm). The concentrations of Zr in the A horizon were higher than
in the regolith below, and the concentrations in matrix were higher than in gravel. Both
Ti and Zr concentrations in matrix generally increased upwards, but Th was extremely
enriched in the lower mottled clay matrix (167 ppm) at 8.6 m depth (Figure 5.1c). This
extreme enrichment of Th is not thought to be an analytical error, as analyses of the
particle size fractions from the lower mottled clay showed similar enrichments in all
fractions (sand, silt and clay). Relative to the average concentration in the parent
meta-granitoid (17 ppm), Th was significantly enriched in ferruginous gravel (up to
196 ppm in duricrust) in contrast to weak enrichment in ferruginous matrix (average
26 ppm), implying strong partitioning of Th into gravel during weathering and
lateritization.
5.5.1.2 Variation of ratios of Ti, Zr and Th with depth
During intense weathering and lateritization processes it is difficult to define an
‘immobile’ element, and thus it is instructive to examine element ratios for potential
immobile elements. The (Ti/Zr)/10 value (Figure 5.2a) varies little from saprolite (1.0)
to ferruginous mottled zone (1.1 in matrix and 1.3 in gravel) and remains close to that of
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
83
the meta-parent granitoid (ca. 0.8). In the duricrust and upper ferruginous zone,
however, values in both gravel (1.0-2.8) and matrix (0.9-1.5) are more variable,
suggesting Ti and Zr fractionate from each other and are partitioned between gravel and
matrix at advanced stages of lateritization. This fractionation is most apparent in the
upper ferruginous zone. Compared with (Ti/Zr)/10, (Zr/Hf)/10 remains largely constant
and is within the range of parent meta-granitoids (2.9-3.9) throughout the profile
(Figure 5.2b). In contrast, (Ti/Th)/100 is more variable from the saprolite to horizon A
regolith, especially in gravel from the upper part of the profile (Figure 5.2c). The
constant ratios of Ti/Zr in the lower part of the profile and Zr/Hf throughout the profile
suggest that either these elements have undergone a relative mass flux change at a
similar rate, or they may remain residual during weathering at the investigated scale. It
is difficult to envisage similar rates of element mass flux under persistent intense
supergene weathering, and thus it is more likely that Ti, Zr and Hf are effectively
conservative during initial and moderate weathering. Therefore, as Ti/Zr and Zr/Hf
appear less affected by external processes than Ti/Th, Ti/Zr and Zr/Hf may be more
suitable discrimination ratios for moderate weathering. The fractionation between Ti
and Zr in the ferruginous zone suggests that Ti and Zr partition between gravel and
matrix that subject to extreme weathering and strong lateritization. Given the constant
ratio of Zr/Hf consistent with the parent meta-granitoid and the relatively higher
concentrations than Hf, resulting in robust estimates of mass balance, Zr is used as the
reference element. Similar mass balance calculations have been reported in previous
studies in the lateritic bauxitic profiles in Jarrahdale (Brimhall et al., 1992; Brimhall et
al., 1994).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
84
Figure 5.1 Variation of Ti, Zr and Th with depth in the JG profile (gravel, ‘g’ is
represented by a triangle and matrix, ‘m’ by a circle in this figure and Figure 5.2). Note
that Th is strongly partitioned into gravel and the increase in Th concentration in the
lower mottled clay matrix does not correlate with any similar spike (positive or negative)
in the Zr or Ti.
Figure 5.2 Variation of Ti/Zr, Zr/Hf and Ti/Th with depth in the JG profile. In order to
aid comparison the ratios have been divided by either 10 or 100 as indicated in the
graphs. Note that Zr/Hf remains within the range defined by the parental meta-granitoid
(bounded by dashed lines; the Ti/Zr range is so small it is represented by one line at this
scale).
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
85
5.5.2 Mass balance of Ti and Th
Using Zr as the reference element, Ti and Th were enriched throughout the regolith
profile (Figure 5.3). Values of τ(Zr,Ti) slightly increased from 0.24 in the saprolite to 0.56
in the ferruginous duricrust, and then sharply increased to 2.1 in the upper ferruginous
zone, and averaged 1.3 in the A horizon. Compared with Ti, τ(Zr,Th) showed that Th was
significantly enriched in the duricrust and extremely enriched in the lower mottled clay.
In the ferruginous duricrust (3 m depth), ε(Ti) = −0.5, ε(Zr) = −0.2 and ε(Th) = −0.7, all
suggesting regolith collapse. Collapse can be inferred from the increase in
concentrations of the immobile elements (Ti, Zr and Th) because the loss of mobile
elements is not exactly compensated by an inversely proportional decrease in bulk
density during intense weathering and lateritization (Brimhall et al., 1992).
Figure 5.3 Mass balance calculations of Ti and Th against depth in the JG profile, based
on weighted average concentrations in matrix and gravel, using Zr as the reference
element. As τ(Zr,Ti) and τ(Zr,Th) are both above 0 throughout the profile, this implies mass
flux increase of Ti and Th relative to Zr.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
86
5.5.3 Mineralogical characteristics of Ti, Zr and Th in the JG profile
5.5.3.1 Occurrence of Ti, Zr and Th in parent meta-granitoids
Representative accessory mineral data from the parent meta-granitoid are presented in
Table 5.3. Titanium was predominantly partitioned into ilmenite (FeTiO3, 28-32 wt%,
Figure 5.4a & b) and sphene (CaTiSiO5, also known as titanite, 20-22 wt%, Figure 5.4b).
Zircon grains (ZrSiO4, Figure 5.4c) had high concentrations of Zr (ca. 45-47 wt%), a
minor amount of Th (ca. 0.1 wt%) and varied concentrations of REE (up to 0.3 wt%
total REE). Thorite (ThSiO4, Figure 5.4d) is the main host for Th (18.9-32.9 wt%), and
also contained 7.7-13.7 wt% Zr, 0.6-0.9 wt% Ti and 3.5-5.2 wt% total REE.
In addition to their main host minerals, significant concentrations of Ti, Zr and Th also
occurred in many widely disseminated accessory minerals, for example, REE-rich
fluorocarbonates (Figure 5.4e & f) contained varied concentrations of Ti
(0.02-0.09 wt%), Zr (up to 0.27 wt%) and Th (0.6-6.4 wt%); magnetite 0.03-0.05 wt%
Ti and up to 0.02 wt% Th; and allanite ca. 0.02 wt% Ti and ca. 0.08 wt% Th.
Another Zr-hosting mineral, probably zirkelite, was observed as a string some hundreds
of microns long and one micron wide associated with quartz in the parent
meta-granitoid (Figure 5.4g & h). As this mineral size range is below the spatial
resolution of the electron microprobe, the chemical data could be separated from the Si
interference originating from the quartz.
87
(a) (b) (c) (d)
(e) (f) (g) (h)
Figure 5.4 Backscatter electron images of Ti-, Zr- and Th- hosting phases in parent meta-granitoids of the JG profile: (a) ilmenite surrounded by apatite;
(b) ilmenite intergrown with sphene; (c) zoned euhedral zircon crystal; (d) thorite crystal rich in Zr (a fracture resulted from electron beam impact); (e)
and (f) REE-bearing fluorocarbonates containing Zr and Th; (g) and (h) probable zirkelite ‘string’ associated with quartz. Compositional analyses of
minerals in (b), (c), (d), (e) and (f) are listed in Table 5.3 (Ap: apatite; Fsp: feldspar; Ilm: ilmenite; Py: pyrite; Qz: quartz; Spn: sphene; Zrk: zirkelite).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
88
Table 5.3 Element concentrations of minerals in Figure 5.4 and Figure 5.5 based on
EPMA in parent meta-granitoids and lateritic regolith in the JG profile
Mineral
Sam. No.1 1 2 3 4 5 6 7 8 9
Zrn Thr Fc Fc Ilm Spn Ilm Ilm Rt
El*.(wt%)
2 Fig 5.4(c) Fig 5.4(d) Fig 5.4(e) Fig 5.4(f) Fig 5.4(b) Fig 5.4(b) Fig 5.5(b) Fig 5.5(c) Fig 5.5(d)
Si 15.2 11.3 0.77 5.61 b.d. 13.9 b.d. b.d. 0.25
Zr 45.4 9.79 b.d. 0.27 b.d. b.d. b.d. b.d. 0.38
Ti 0.04 0.76 0.02 0.09 32.2 22.2 33.1 32.1 51.0
Pb 0.07 0.46 b.d. 0.08 b.d. b.d. b.d. b.d. 0.02
Th 0.10 32.9 2.06 2.31 b.d. b.d. 0.01 b.d. 0.14
U 0.22 7.00 0.09 0.23 0.02 b.d. b.d. 0.17 0.14
Al 0.03 0.35 0.48 1.26 b.d. 1.13 b.d. 0.01 2.68
Y 0.13 1.17 0.04 0.08 b.d. b.d. b.d. b.d. b.d.
La b.d. b.d. 20.0 16.2 b.d. b.d. b.d. b.d. b.d.
Ce b.d. 1.83 30.6 23.4 b.d. b.d. b.d. b.d. 0.05
Pr b.d. 0.26 2.51 1.90 b.d. b.d. b.d. b.d. b.d.
Nd 0.11 1.13 5.42 4.33 b.d. b.d. b.d. b.d. b.d.
Sm b.d. 0.45 0.54 0.46 b.d. b.d. b.d. b.d. b.d.
Eu b.d. 0.05 0.19 0.14 b.d. b.d. b.d. b.d. b.d.
Gd 0.02 0.39 0.22 0.15 b.d. b.d. b.d. b.d. b.d.
Dy 0.03 0.17 b.d. b.d. b.d. b.d. 1.04 b.d. b.d.
Yb 0.13 0.25 0.06 0.04 0.06 b.d. b.d. 0.07 0.02
Lu 0.03 0.04 b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Fe 0.64 0.93 1.40 2.90 34.1 0.42 27.9 33.4 3.91
Mg b.d. 0.16 0.36 0.32 0.06 b.d. 0.03 0.07 0.01
Ca 0.08 0.32 4.90 5.26 0.20 19.1 b.d. b.d. 0.04
Sr 0.39 0.01 b.d. b.d. b.d. 0.08 b.d. b.d. b.d.
K 0.01 b.d. b.d. 0.04 b.d. b.d. b.d. b.d. 0.01
P b.d. 0.50 0.02 0.02 b.d. 0.01 b.d. b.d. 0.03
S b.d. 0.03 0.14 0.11 b.d. b.d. b.d. b.d. 0.08
As 0.01 b.d. b.d. b.d. 0.01 b.d. 0.01 0.01 b.d.
F b.d. 1.19 7.09 10.6 b.d. 0.50 b.d. b.d. b.d.
O 33.7 24.4 11.6 14.9 31.5 39.2 30.3 31.3 38.2
total 96.3 95.9 88.5 90.7 98.2 96.6 92.4 97.1 96.9
1No.: Each mineral analysed is allocated with a number; the number allocated is consistent throughout
the thesis. 2Tb, Ho, Er, Tm and Na are below the detection limit (b.d.).
Zrn-zircon; Thr-thorite; Fc-fluorocarbonate; Ilm-ilmenite; Spn-sphene; Rt-rutile.
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
89
5.5.3.2 Residence of Ti, Zr and Th in lateritic regolith
In the regolith, Ti was mainly hosted by ilmenite, rutile and anatase (Figure 5.5 &
Figure 5.6) and primary sphene was not observed. Ilmenite in the ferruginous mottled
samples (Figure 5.5a & b) contained 30-37 wt% Ti and rutile/anatase (Figure 5.5c, d, e
& f) with ca. 51 wt% Ti. Trace concentrations of Ti were incorporated into zircon
(average 0.12 wt%) and secondary phosphate minerals e.g. rhabdophane (ca. 0.01 wt%)
and florencite (ca. 0.05 wt%), and sorbed onto, or co-precipitated with, Al/Fe oxides
(0.04-0.25 wt%).
Generally, the minerals ilmenite, rutile and anatase are resistant to weathering; however,
partially dissolved ilmenite and rutile (Figure 5.6) was identified in the ferruginous
mottled zone. Powder diffraction patterns (SXRD; Figure 5.6) showed that ilmenite and
rutile, although present in the ferruginous mottled zone (JG4, 5 m depth; Chemical
Index of Alteration (CIA)=95%) are not seen in the ferruginous duricrust (JG5, 3 m
depth; CIA=98%); rather, anatase is present. Evidently there is a change in the mineral
location of Ti between the ferruginous mottled zone and the ferruginous duricrust.
In the regolith the only mineral with a high Zr concentration identified by SEM and
EPMA was zircon. No corroded zircon was observed in the regolith profile, except for
two fractured grains, both of which were half partially dissolved and half crystalline
(Figure 5.7): one grain was in the ferruginous duricrust (Figure 5.7a) and the other in
the A horizon (Figure 5.7b). The apparent dissolution may result from physical
weathering (due to metamict areas) rather than chemical dissolution, because half of the
mineral retained its crystalline structure. In addition to zircon, Zr occurs in submicron
poorly crystalline phases associated with Ce, forming a rim and coating around Al/Fe
matrix in the pore system of the ferruginous duricrust (Figure 5.8). Quantitative analysis
by microprobe revealed that the Zr does not always exist with Si as zircon (ZrSiO4), and
the low sum of oxides <100% in many cases most likely reflects a hydrous state (Table
5.4).
In the ferruginous zone, REE-rich fluorocarbonates were absent and most primary
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
90
thorite was also weathered. The REE and Th released during weathering partially
precipitated as secondary REE-bearing phosphates such as rhabdophane and florencite
(Figure 5.9a & b). The concentration of Th in these secondary phosphates varied from
0.08 wt% in florencite up to 9.8 wt% in rhabdophane. Trace concentrations of Th were
also hosted by zircon (up to 0.56 wt%, Figure 5.9c), ilmenite (0.01-0.05 wt%) and
anatase (0.05-0.14 wt%). Thorite (ThSiO4) was rare in the regolith; only one
micron-sized grain was observed in the ferruginous duricrust (Figure 5.9d), with a
significant concentration of Th (ca. 45 wt%) and a minor amount of REE
(ca. 0.41 wt%). In addition, up to 5 wt% Th was determined in neoformed poorly
crystalline phases in ferruginous duricrust (Table 5.4).
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
91
(a) (b)
(c) (d)
(e) (f)
Figure 5.5 Backscatter electron images showing Ti retained as ilmenite and Ti oxides in
the ferruginous mottled zone of the JG profile: (a) and (b) slightly fractured and
decomposed ilmenite; (c) decomposed ilmenite surrounded by Ti oxides; (d), (e) and (f)
are decomposed Ti oxides. Note that (b) is backscatter image from the JEOL
microprobe 8530 and the remainders are backscatter images from the SEM JEOL 6400.
The compositional analyses of minerals (b), (c) and (d) are listed in Table 5.3.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
92
Figure 5.6 Diffraction patterns from SXRD showing evidence for transformation of Ti
from ilmenite and rutile in the ferruginous mottled zone (JG4) to anatase in the duricrust
(JG5) of the JG profile (peaks are only labelled for Ant: anatase; Ilm: ilmenite; Rt: rutile.
Off scale peak at 3.342 d-spacing is quartz which was not totally removed by the
separation procedure).
(a) (b)
Figure 5.7 (a) The only partially dissolved zircon grain identified in the duricrust
(circled) and (b) a typical fractured, partially metamict zircon grain in the A horizon
(<1 m depth).
Table 5.4 Element concentrations from EPMA in Figure 5.8 (a) and (b)
Sam Element concentrations* (wt%)
Si Zr Ti Pb Th U Al Ce Gd Fe S F O Total
(a) 0.15 6.04 0.20 0.07 5.15 0.03 2.24 9.14 0.19 20.7 0.05 0.05 12.7 56.8
(b) 0.29 1.07 0.07 0.03 0.60 0.02 12.7 1.44 0.02 32.7 0.20 0.07 22.1 71.4
*Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu, Mg, Sr, Na, K, P and As were below the detection
limit of the microprobe; the analysis spot was located at the brightest areas of the Ce-mapping
corresponding with the highest concentrations of Ce.
15
20
25
30
35
3 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8
d spacing
Co
un
ts (
tho
us
an
d)
10
15
20
25
Ant
Ilm
RtJG5
JG4
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
93
(a)
(b)
Figure 5.8 Neoformed poorly crystalline Zr-hosting phases associated with Ce on pore
walls around Al/Fe matrix in the duricrust of the JG profile. This co-occurrence of Zr
and Ce is unassociated with silicates or phosphates and thus is most likely (hydr)oxides
(Table 5.4): (a) edge of a ca. 2 mm nodule cemented with clay matrix in the duricrust;
(b) quartz surrounded by and cemented with Al/Fe matrix in the pore system of
duricrust; (CP is the backscatter image).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
94
5.5.4 Grain size distribution of Ti, Zr and Th in the lateritic regolith
In order to exclude the externally sourced Ti and Zr in the A horizon, regolith samples
from 1.5 m to 10 m depth were used to investigate the distribution of Ti, Zr and Th in
different grain size fractions. The results are shown graphically in Figure 5.10 and the
data are presented in Table 5.2.
The silt fraction had the highest concentration of Ti in all regolith samples (Figure
5.10a). In the ferruginous zone, Ti concentration in the clay increased with depth from
1.5 m to 5 m, and then decreased from the mottled clay (6.5 m depth) to the saprolite
(10 m depth).
In all regolith samples except the lower mottled clay zone (8.6 m depth), the silt fraction
had the highest concentration of Zr, whereas the clay fraction contained the lowest
concentration of Zr (Figure 5.10b). In the lower mottled clay zone, silt still had the
highest concentration of Zr and sand had the lowest.
In the ferruginous zone (1.5-5 m depth) gravel had the highest concentrations of Th
whereas the silt fraction contained the highest concentration of Th in the mottled clay
(6.5-8.6 m depth). In the saprolite (10 m depth), the concentration of Th in clay was
slightly higher than both sand and silt (Figure 5.10c).
95
(a) (b)
(c) (d)
Figure 5.9 Forms of Th persisting in regolith samples of the JG profile with/as: (a) secondary REE-bearing mineral rhabdophane; (b) secondary
REE-bearing mineral florencite; (c) substituted with minor concentration in zircon; (d) micron-size grain of thorium orthosilicate mineral (ThSiO4); the
EDS spectra plots in the column on the right correspond to the minerals circled in the SEM backscatter images on the left.
96
Figure 5.10 Grain size distribution of Zr, Ti and Th in the JG profile.
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
97
5.5.5 Partition of Ti, Zr and Th into different extraction species
Sequential extraction showed that Ti predominantly occurred in the Res, with only trace
amounts occurring in WAE and FeAm species in the upper part of the mottled clay
(Table 5.5). In contrast, Zr was more enriched in Org and FeCry than WAE and FeAm in
all regolith samples, though the Res still contained the highest concentration of Zr.
Unlike both Ti and Zr, the WAE and FeCry species contained more Th than Org and
FeAm and the highest concentration of Th was in the Res. In saprolite, a high amount of
Th was also determined in the WAE.
Table 5.5 Concentrations of Zr, Ti and Th in different sequential extraction species
Element concentrations (ppm)
Ti Zr Th
Detection limit 2.00 0.002 0.001
Method ICP-OES ICP-MS ICP-MS
Saprolite
JG1m_WAE b.d. 0.40 8.78
JG1m_Org b.d. 2.03 3.93
JG1m_FeAm b.d. 0.01 0.38
JG1m_FeCry b.d. 3.26 1.42
JG1m_Res 1592 140 9.30
Upper mottled clay
JG3m_WAE 2.00 0.26 2.67
JG3m_Org b.d. 1.32 0.56
JG3m_FeAm 3.00 0.02 0.19
JG3m_FeCry b.d. 8.90 6.69
JG3m_Res 3317 242 29.70
Duricrust
JG5m_WAE b.d. 0.29 1.14
JG5m_Org b.d. 1.72 0.65
JG5m_FeAm b.d. 0.04 0.06
JG5m_FeCry b.d. 7.30 1.62
JG5m_Res 4175 225 36.30
5.6 Discussion
5.6.1 Mode of occurrence of Zr and Th in the lateritic regolith
In the ferruginous duricrust, some Zr occurs in poorly crystalline phases associated with
Ce and Th, forming a rim or coating around Al/Fe matrix in the pore systems (Figure
5.8). This occurrence of Zr and Ce is not associated with Si as zircon (ZrSiO4) or P as
rhabdophane (LnPO4, where Ln denotes REE; Table 5.4), demonstrating that Zr was
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
98
precipitated as oxides or hydroxides (ZrO2∙nH2O or Zr(OH)4) in addition to being
present in zircon in intensely weathered regolith. The geochemical behaviour of Zr
including complexation, mobilization and precipitation, depends on pH and the
presence or absence of organic matter. At a pH of ca. 5, the Zr hydroxy-bicarbonate
(Zr(OH)4-HCO3-H2O) complex, which may be the most significant Zr complex in
natural water, is unstable and possibly decomposes to form Zr(OH)4 (Salminen, 2005;
Vos et al., 2006). When a high organic component is present Zr can also be adsorbed as
colloidal oxides or hydroxides and translocated in the profile (Duvallet et al., 1999).
The sequential extraction (Table 5.5) showed that as well as being hosted by zircon in
residue, Zr was also present in the species of FeCry (7.3 ppm) and Org (1.7 ppm) in the
matrix of ferruginous duricrust (JG5m). Thus, in this case, it is likely that released Zr
was included in neoformed crystalline (hydr)oxides and attached onto the walls of
Al/Fe-rich pores, a process that was enhanced by low pH and the presence of organic
matter.
It is accepted that the geochemical behaviour of Th is dominated by the Th4+
ion
(Langmuir and Herman, 1980), and thus it shows affinity with other tetravalent
elements such as Ce and Zr. This behaviour is seen in this study by the distribution of
Th as a trace component in secondary REE-bearing phosphates (e.g. rhabdophane and
florencite in Figure 5.9) in weathered lateritic regolith. The high concentrations of Th
(5 wt% in Table 5.4) determined in neoformed poorly crystalline phases in ferruginous
duricrust, suggest the formation of insoluble Th (hydr)oxides associating with Ce and
Zr. Trace amounts of Th, associated with the WAE (1.14 ppm) and FeCry species
(1.62 ppm), in the matrix of ferruginous duricrust (Table 5.5) suggest that Th was
affected by sorption or co-precipitation with Al- and Fe- oxides.
5.6.2 Sources of Zr in poorly crystalline phases in duricrust
In this study, the formation of poorly crystalline (hydr)oxide phases containing Zr
indicates some mobility of Zr in the supergene weathering environment. However, the
distance that Zr is mobilized from its original location is not clear, but it is reasonable to
propose that this was less than the sampling scale (centimetre scale) due to: (i) the very
low solubility of Zr(OH)4; (ii) an almost constant Zr/Hf ratio consistent with the parent
meta-granitoid; and (iii) the absence of eroded zircon grains. This then poses the
question: where did the mobilized Zr come from? Limited remobilization of Zr in
supergene environments may occur in strongly acidic and organic-rich media in podzols,
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
99
or F/Cl-rich coastal profiles (Colin et al., 1993). The susceptibility of zircon to
weathering can also be enhanced by mechanical fracturing during deformation and
damage to the crystal lattice (metamictization) from radioactive decay of incorporated U
and Th (Rubin et al., 1993). Therefore, old zircons (with high U and Th concentrations)
in paleosols are more likely to be susceptible to dissolution than undamaged zircon.
The appearance of metamict areas in zircon resembles the effects of chemical
weathering (Rubin et al., 1993) and is visible using a petrological microscope. The
majority of zircon grains in this regolith profile, however, still retained typical crystal
morphology and only two containing metamict areas were observed (Figure 5.7). Thus,
it is unlikely that the Zr contained in the neoformed (hydr)oxides was released from
zircon breakdown. In contrast, other igneous phases incorporating trace amounts of Zr,
such as thorite (up to 13 wt% Zr) and REE-bearing fluorocarbonates (ca. 0.07 wt% Zr
and 7.3 wt% F), would be more likely to break down, thus releasing Zr from the parent
meta-granitoids at the initial stages of weathering. The mobility of Zr would be further
enhanced by the F-rich solution released by the REE-rich fluorocarbonates during
breakdown. Therefore, the residence of Zr not only in zircon but also in other primary
igneous minerals, and the amount and distribution of these mineral phases in the parent
rock, are likely to be significant controls on the mobility of Zr in the lateritic regolith.
5.6.3 Partitioning of Th between gravel and matrix
Unlike Zr, Th has a more complicated mode of occurrence in the lateritic regolith.
Though most Th was contained in the resistant mineral phases (revealed by significant
concentrations of Th hosted by the Res in Table 5.5), trace to minor amounts of Th were
also detected in the WAE, Org, FeAm and FeCry species. In addition, strong partitioning
of Th into gravel in the ferruginous zone reflects the local translocation and
redistribution of Th in the profile. Similar enrichment of Th in iron nodules (gravel) was
also observed in the Nsimi lateritic profile, Cameroon (Braun et al., 2005). However,
the enrichment of Th in gravel is not consistent with the concentrations of Ti and Zr
determined in this study, as concentrations of Ti and Zr were not consistently higher in
gravel than matrix. This indicates that zircon, ilmenite, rutile and anatase were not the
only hosts for Th in gravel. In the duricrust Th also precipitated as Th (hydr)oxides
associating with Zr and Ce (Table 5.4); in the upper ferruginous zone and the A horizon
Th-hosting REE-rich phosphates distributing into iron nodules had been found (Figure
5.9). In addition, from the ferruginous mottled zone to the A horizon, pH varies from
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
100
4.4 to 5.3, concentrations of Al and Fe oxides in gravel are higher than in matrix; both
features may also contribute to the strong partitioning of Th into surface gravel. This is
in agreement with the strong sorption of Th by hematite and gibbsite within a similar
pH range (Cromieres et al., 1998; Zhang et al., 2006). In the mottled clay and saprolite
the silt size fraction had the highest concentrations of Th, which is likely to be the result
of the Th-hosting minerals being dominantly within the range of the silt size fraction
originally (e.g. secondary REE-phosphates). However, the reason for the abnormal
enrichment of Th in the lower mottled clay zone is not clear. To understand more fully
the abnormal accumulation of Th in the lower mottled clay and to evaluate whether this
is observed elsewhere, further research is needed.
5.6.4 Mobility of Ti in the JG profile
The weathering-resistant minerals ilmenite and anatase are the main hosts of Ti in the
profile (Figure 5.5); the absence of the igneous mineral, sphene (present in the parent
meta-granitoid), and the presence of fractured and eroded grains of ilmenite and rutile
are all evidence that Ti is mobile at the mineral assemblage scale. Trace amounts of Ti
were found in the WAE (2 ppm) and FeAm (3 ppm) species in the upper mottled clay
(Table 5.5) as well as with neoformed Zr-(hydr)oxide phases in ferruginous duricrust
(0.07-0.20 wt% in Table 5.4). These amounts, however, are negligible in comparison
with the concentration of Ti in the Res in the upper mottled clay (3317 ppm) and in the
duricrust (4175 ppm). A similar result, where Ti-phases were trapped within neoformed
clay minerals, was noted by Malengreau et al. (1995). In addition, enrichment of Ti
increased in the fine soil fractions (silt and clay) with increasing weathering intensity
(decreasing depth). Enrichment of Ti in the fine fraction was also found in ten
weathering and pedogenetic soil profiles developed on granitic rocks by Taboada et al.
(2006a). The relatively mostly constant Ti/Zr from saprolite to ferruginous mottled zone
suggests that Ti remains largely conservative at the sampling scale in the lower part of
the profile, although mobility of Ti at the mineral scale has been revealed. This implies
that the sphene and ilmenite in parent meta-granitoid break down and are replaced by
secondary ilmenite and rutile in-situ (or nearly so) in the lower part of lateritic regolith
during initial to moderate weathering. The secondary ilmenite and rutile are then altered
into anatase and thus constrain any further mobility of Ti. This is supported by the
intergrowth of ilmenite and Ti oxides in the ferruginous mottled zone (Figure 5.5c).
Similar mineral transformations in intensely weathered lateritic regolith, from
Chapter Five: Redistribution and mobilization of Ti, Zr and Th in an intensely weathered lateritic profile in WA
101
ilmenite/rutile to anatase, have been reported before by Anand et al. (1991; 2002).
Fluctuation of Ti/Zr (in comparison with almost constant Zr/Hf) in the ferruginous
duricrust and upper ferruginous zone indicates that Ti fractionates from Zr during
extreme weathering and advanced lateritization. High concentrations of Ti partitioned
into iron nodules in the upper ferruginous zone (Table 5.2) are consistent with
cementation of Fe oxides and formation of iron nodules in the upper ferruginous zone.
This appears to have resulted in further alteration of ilmenite and rutile into anatase,
which was cemented with Al/Fe oxides and incorporated into iron nodules. Although
zircon and rutile from aeolian input are relatively stable in the upper part of regolith
profile (Brimhall et al., 1992; Brimhall et al., 1988), redox change during lateritization,
and associated changes in pH and Eh have a profound effect on the mobilization and
translocation of Ti that is further enhanced by the involvement of organic matter. This is
supported by the experiment of Thompson et al., (2006) who reported that 10% of total
Ti in a basaltic soil was mobilized as colloids at peak dispersion (related to a change in
pH accompanying redox oscillation); furthermore, Ti and Zr were also observed to be
mobile in the uppermost meter of lateritic regolith in Cameroon, attributed to the
presence of organic colloids (Braun et al., 2005).
5.6.5 Geological parent mineralogy vs. weathering conditions
It is widely known that the factors influencing the mobility of trace elements include: (i)
initial concentration and mineralogical host in the parent rocks, (ii) the susceptibility of
these hosts to subsequent alteration, and (iii) the ability of the solution to transport the
elements released (Rubin et al., 1993). In this study, the susceptibility of primary
igneous hosts of Zr, Ti and Th to weathering (e.g. thorite for Th and Zr, sphene for Ti
and Th, REE-rich fluorocarbonates for REE, Zr and Th) fundamentally controls the
subsequent mobility of these elements, and changes their abundance at the early stages
of weathering. Breakdown of thorite, sphene and REE-rich fluorocarbonates releases Ti,
Zr and Th into solution. Once in solution, Ti, Zr and Th re-enter the solid phase by
formation of new secondary minerals (e.g. ilmenite, rutile, rhabdophane, (hydr)oxide).
Formation of these secondary minerals with very low solubility further limits
mobilization of Ti, Zr and Th. As weathering proceeds, the initial control by igneous
host minerals in the parent rocks diminishes in importance, rather the weathering
intensity and characteristics of the solutions present play an increasingly important role
in translocation of Ti, Zr and Th. Low pH (3.2-5.3 throughout the profile), extreme
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
102
weathering intensity (CIA 65%-99%) and the presence of organic matter enhance the
mobility of Ti, Zr and Th. However, the greater stability of zircon relative to ilmenite
and rutile results in alteration of ilmenite and rutile into anatase, leaving zircon as a
residual mineral. The presence of organic matter, clay minerals and Fe
oxides/oxyhydroxides enhances the mobility of Th by formation and sorption of
complexes at the profile scale.
5.7 Summary of the chapter
A geochemical and mineralogical study of the mobility and mode of occurrence of Ti,
Zr and Th in the intensely weathered lateritic profile at Jarrahdale, Western Australia,
was conducted. The mobilization and redistribution of Zr and Th at the sampling scale
was revealed by neoformed poorly crystalline Zr, Ce and Th (hydr)oxide phases
attaching onto the walls of Al/Fe-rich pores in the ferruginous duricrust. The source for
Zr and Th in these neoformed phases is proposed to be the breakdown of thorite and
REE-rich fluorocarbonates during the early stages of weathering. Distribution into
secondary REE-bearing phosphates (e.g. rhabdophane and florencite) as a trace
component in the regolith showed translocation of Th at the mineral assemblage scale,
whereas strong partitioning of Th into gravel rather than matrix reflects redistribution of
Th at the profile scale. Absence of primary sphene in the regolith and dissolution of
ilmenite and rutile in the ferruginous mottled zone suggest mineral transformation from
sphene, ilmenite and rutile to anatase at the mineral assemblage scale during intense
weathering. The limited range of Ti/Zr from saprolite to ferruginous mottled zone
indicates that Ti is mostly conservative during moderate weathering despite varying in
concentration. The fluctuation of Ti/Zr in the duricrust and upper ferruginous zone
suggests that Ti and Zr fractionate from each other and partition between gravel and
matrix during extreme weathering and advanced lateritization. Therefore, these
commonly considered immobile elements are mobile at a variety of scales, and special
attention should be paid when using these elements to calculate the flux mass,
especially under intensely weathered conditions or where there are particle size sorting
transport processes.
103
6 Distribution and fractionation of REE in intensely weathered
lateritic profiles in Western Australia
6.1 Abstract
Three intensely weathered lateritic profiles (GE, MQ I and MQ II) developed on
granitoids with dolerite dykes in Western Australia were studied to investigate
geochemical behaviour and fractionation mechanisms of rare earth elements (REE)
during intense weathering and lateritization. In three profiles, regolith developed from
the granitoid rather than the dolerite was confirmed by chondrite normalized REE
distribution patterns. Substantial depletion of REE in the regolith was observed,
especially in the GE profile. Chondrite normalized REE distribution patterns of regolith
from three profiles showed light REE (LREE)-enrichment, coupled with higher
depletions of LREE than HREE relative to the parent granitoids.
Monazite, allanite, apatite, zircon, ilmenite and sphene were important REE-hosting
mineral phases in the parent granitoids, with REE-rich fluorocarbonates restricted to the
MQ parent granitoids. Residual monazite and secondary rhabdophane were important
phosphates for retention of LREE, whereas zircon and ilmenite were significant HREE
selective hosts, in weathered MQ regolith. REE released by breakdown of
easy-weathering LREE-rich allanite and fluorocarbonate in parent granitoids at the early
stages of weathering may be partially leached away, or alternatively, be retained in the
regolith by formation of secondary phosphates, e.g. rhabdophane, and hence limited
further mobility.
In addition to being hosted by mineral phases, REE were also retained in weathered
regolith by association with clay minerals, Fe oxides/oxyhydroxides and organic matter.
Among five species of sequential extraction, the water soluble (including adsorbed and
exchangeable) species hosted up to 7.9% of total REE in the C horizon regolith of MQ I
profile; the amorphous Fe oxyhydroxide species contained 3.7% of total REE in the
duricrust of GE profile. A positive Ce anomaly (Ce*=6.1) in the duricrust of GE profile
was likely related to the redox change during formation of the duricrust. All of these
observations suggest that REE can mobilize during weathering and lateritization, to the
extent of becoming highly depleted in intensely weathered lateritic regolith. The
abundance, stability and composition of secondary LREE-rich phosphate minerals and
residual HREE-selective weathering-resistant minerals may control the fractionation of
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
104
REE in intensely weathered regolith.
6.2 Key word
Rare earth elements; laterite; weathering; regolith; fractionation;
6.3 Introduction
Chemical weathering of rocks and the formation of soils are important geochemical
processes, contributing to the redistribution of rare earth elements (REE) in the Earth’s
surface environment. A series of well documented studies have shown that REE can
mobilize and fractionate during weathering (Aubert et al., 2001; Braun et al., 1993;
Duddy, 1980; Feng, 2011; Harlavan and Erel, 2002; Harlavan et al., 2009; Nesbitt,
1979). Although REE have been widely studied, the geochemical behaviour of REE
during weathering cannot be generally applied because of: (i) the wide range of
REE-bearing minerals and their variable concentrations of REE; (ii) different
susceptibility of these minerals to solutions and variable solution chemistry; and (iii)
location-specific physicochemical and biological factors during weathering (Bao and
Zhao, 2008; Price et al., 1991).
Two factors are believed to be the main controls on the mobilization and fractionation
of REE: the type of primary REE-bearing minerals in the protolith and the weathering
conditions (Braun et al., 1990; Braun et al., 1998; Condie et al., 1995; Nesbitt, 1979).
However, the behaviour and fractionation mechanisms of REE during weathering are
still not fully understood. For example, preferential enrichment of LREE over HREE in
lateritic regolith has been widely reported (e.g. Braun et al., 1993; Braun et al., 1998;
Ndjigui et al., 2009), whereas stronger enrichment of HREE than LREE has also been
found in lateritic profiles (e.g. Beyala et al., 2009; Braun et al., 1990; Viers and
Wasserburg, 2004). Lateritic regolith represents one of the most common superficial
formations in the tropics, and is commonly diachronous, extending over tens of millions
of years (Dequincey et al., 2006). Lateritization is therefore of particular significance in
the study of translocation and fractionation of REE (Ji et al., 2004). Therefore, three
intensely weathered lateritic profiles developed over granitoids with a cross-cutting
dolerite dyke in Western Australia were investigated in this study. The aims are to: (i)
determine the abundance and residence of REE in the parent granitoids and lateritic
regolith; (ii) improve the understanding of the geochemical behaviour and fractionation
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
105
mechanisms of REE during intense weathering and lateritization.
6.4 Methods and materials
6.4.1 Analytical methods
This study was performed on regolith samples from three lateritic profiles (GE, MQ I
and MQ II) developed over granitoids with dolerite dyke in Western Australia (Chapter
Three). Regolith samples were separated into two subgroups based on grain size: gravel
(>2 mm, represented by suffix ‘g’) and matrix (<2 mm, represented by suffix ‘m’). The
exception to this subdivision was the duricrust (GE6) in the GE profile, a very hard
cemented material without corresponding loose matrix. Subsamples of gravel and
matrix and crushed duricrust were oven dried at 105 °C overnight, then ground to ≤200
µm prior to fusion in order to determine element concentrations. Following this division,
regolith raw bulk matrix from the MQ II profile was further separated into the following
three size fractions: clay (<2 µm), silt (2-20 µm) and sand (>20 µm) using the
sedimentation pipette and wet sieving methods (Day, 1965). Particle size fraction limit
recommended by the International Society of Soil Science (ISSS) has been adopted in
Australia (Marshall, 1947; Marshall, 2003; Prescott et al., 1934). Different particle size
fractions were rinsed with MilliQ water three times, oven dried at 105 °C overnight,
then ground to ≤200 µm prior to fusion.
To investigate the chemical species and association behaviour of trace elements, a
sequential extraction procedure was performed. Saprolite matrix (GE1m) and ground
duricrust (GE6) from the GE profile and regolith matrix (MQ1m, 3.6 m depth; MQ5m,
1.1 m depth; MQ8m, 0.5 m depth) from the MQ I profile were selected. An in-house
laboratory reference material was analysed together with the selected samples. Regolith
trace elements were operationally divided into five species (modified from Hall et al.,
1996): (i) water soluble, adsorbed, exchangeable and carbonate bound (WAE); (ii)
organic matter bound (Org); (iii) amorphous Fe-Mn oxyhydroxide bound (FeAm); (iv)
crystalline Fe-Mn oxide bound (FeCry); and (v) residual species (Res). Since carbonates
are unlikely to be present in the regolith being studied here due to low pH, they are not
considered relevant in this work, and the acronym AEC
(adsorbed-exchangeable-carbonate) used by Hall et al. (1996) is not used. Sulfides are
also scarce in the lateritic regolith, therefore it is assumed that species Org is mainly
hosted by organic matter complexes. A brief summary of the sequential extraction
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
106
procedures is shown in Table 5.1 and the detailed extraction method and chemical
preparation are listed in the Appendix 11.5. The residual samples and reference
materials were rinsed with MilliQ water three times, oven dried at 105 °C overnight and
ground to ≤200 µm prior to fusion.
Fusion beads for elemental analyses were made by mixing 0.1 g (to an accuracy of
0.1 mg) of finely ground sample or reference material with 0.7 g 12:22 Norrish flux
(lithium metaborate:lithium tetraborate) and heating in a muffle furnace at 1050 °C for
40 minutes. Duplicate fusion beads were made on 10% of samples to check preparation
errors. After cooling, the fusion beads were dissolved in 100 mL of 10% analytical
grade HCl. The trace elements, including REE, in fusion beads of the gravel and matrix,
were analysed after an additional 10-fold dilution with 10 ppb Rh/Ir solution in 10 ml
polypropylene tubes using a Perkin-Elmer Elan 6000 inductively coupled plasma-mass
spectrometry (ICP-MS) at the University of Western Australia. Trace elements,
including REE, in sequential extractions were analysed at the Genalysis Laboratory
Services of Intertek Commodities in Maddington, Western Australia. Certified
international standard materials, including stream sediment reference material STSD-2,
STSD-4 (Canada Centre for Mineral and Energy Technology, CANMET) and an
in-house standard reference were prepared in the same way as the samples and analysed
together with samples to check the accuracy and precision. The variations of REE
between tested values and certified values were within 10% from La to Er and Yb,
except Tm and Lu (variation was within 20%, Appendix 11.6). Therefore, when
calculating fractionation of REE, La/Sm and La/Yb were used rather than La/Lu. The
concentrations of REE in gravel and matrix of regolith samples from three profiles are
listed in Table 6.1.
Texture, morphology, and phase composition of individual grains were determined
using polished thin sections of air dried and resin impregnated regolith and outcrop
samples. These polished thin sections were examined using a JEOL JSM-6400 scanning
electron microscope (SEM) with a Link analytic energy dispersive spectrometer (EDS),
utilizing both secondary electron (SE) and back-scattering electron (BSE) imaging at
15kV accelerating voltage with a 3 nA beam current. Semi-quantitative modal
abundances of REE-bearing accessory minerals in parent rocks were calculated based
on SEM-BSE images of polished thin sections and chemical maps produced by EDS.
The relative volume percentages of REE-bearing minerals were calculated and selected
mineral density data (Deer et al., 1992) were used to convert volume percentage to
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
107
weight percentage. Given that the region of interest had more REE-bearing minerals,
calculated weight percentages will be overestimated. Although the accessory mineral
abundances obtained this way are only semi-quantitative, they can be used as clues for
assessing the mineral control and fractionation of REE in the parent rock. The chemical
composition of representative REE-bearing minerals was analysed by electron probe
micro-analyzer (EPMA, JEOL 8530) at 20 kV accelerating voltage and 5 nA beam
current. Software Probe for EPMA from Probe Software Inc. was used for setting up
and analysing the data. Standard references for microprobe calibration were synthetic
glass 612 from the National Institute of Standards and Technology (NIST), in-house
standard synthetic REE phosphates, rutile, zircon and thorite; standard Brazil monazite
was analysed with samples for cross checking. All microscopy analyses were conducted
at the Centre for Microscopy, Characterisation and Analysis (CMCA), University of
Western Australia. The detection limit of the EPMA for mineral analysis is listed in the
Appendix 11.8.
6.4.2 Calculation methods
6.4.2.1 Fractionation of REE and anomalies of Ce and Eu
In order to study the fractionation of REE, three groups are identified (Henderson,
1984): the light REE (LREE; from La to Nd), the middle REE (MREE: from Sm to Ho)
and the heavy REE (HREE: from Gd to Lu). Regolith REE distribution patterns are
normalized to average chondrite values (Anders and Grevesse, 1989) in order to show
the fractionation of REE during lithogenical weathering and the difference of REE
distribution between granitoids and dolerites; in addition, REE in regolith are also
compared with the parent rock (PR) in order to reveal relative enrichment or depletion.
The normalized ratios (La/Sm)PR and (La/Yb)PR are used for identifying fractionations
between LREE-MREE and LREE-HREE using the composition of the parent rock as a
reference. Cerium and Eu anomalies are calculated using the following equations
(subscript PR refers to parent rock):
Ce*=(Ce/CePR)/[(La/LaPR)
0.5×(Pr/PrPR)
0.5] (1)
Eu*=(Eu/EuPR)/[(Sm/SmPR)
0.5×(Gd/GdPR)
0.5] (2)
6.4.2.2 Weathering intensity-Chemical Index of Alteration (CIA)
To evaluate the intensity of chemical weathering quantitatively, the Chemical Index of
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
108
Alteration (CIA) (Nesbitt and Young, 1982) was used. The CIA calculates loss of
mobile elements relative to Al in bulk samples, providing a single parameter estimate of
the intensity of chemical weathering. The formula (Nesbitt and Young, 1982) is:
CIA=100×Al2O3/(Al2O3+CaO*+Na2O+K2O) (molar basis) (3)
Where CaO* is CaO associated with the silicate fraction of samples (excludes
carbonates).
6.4.2.3 Mass balance calculation
To quantify net elements fluxes from pedogenic weathering, a geochemical mass
balance calculation was used (Brimhall et al., 1991). The formula for normalized
concentration (τi,j) in Equation (4) assumes that an immobile element (e.g., Zr, Th)
behaves conservatively during weathering and pedogenesis.
1))((,
,
,
,
, pj
wj
wi
pi
C
C
C
C
ji
(4)
In Equation (4), C represents concentration, i represents the immobile element, j
represents the element of interest, w represents weathered material and p identifies
parent rock. If τi,j = 0, the element j has behaved conservatively at the sampling scale; if
τi,j = −1, the element j has been depleted completely during weathering; positive τi,j
values signify absolute enrichment.
Equation (4) provides a tool for estimating elemental loss or gain for a profile; however,
mass balance equations have two critical assumptions: a genetic relationship between
regolith and the underlying rock and a fully conserved reference element. Although the
mobility of Ti, Zr and Th is subject to debate (e.g. Braun et al., 1993; Cornu et al.,
1999), Zr is still considered to be conservative in the profiles studied based on the
consistent ratio Zr/Hf in the GE profile and the observation of uncorroded zircon
minerals by SEM in the MQ profiles.
6.4.2.4 Mass loading of REE in grain size fraction
To determine an element’s partitioning into different grain size fractions, a mean
element mass loading was calculated based on its concentration in a selected grain size
of known mass percentage (Sutherland, 2003).
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
109
GSFloading 100 (X i GSi
X i GSii1
n
)
(5)
Where:
Xi is the concentration of REE (ppm) in an individual grain size fraction (e.g. <2 um);
GSi is the mass percentage of an individual fraction, which has limits of 0–100%.
GSFloading is the element mass loading in a selected grain size and the summation of
GSFloading for each sample equals 100%.
Four classes of grain sizes (clay, silt, sand and gravel) were defined in all regolith in the
MQ II profile. Thus, if the REE concentration for a given fraction is very high but it
forms only a small portion of the overall sample mass, the contribution of this fraction
to the total sample REE mass will be minimal.
6.5 Results
6.5.1 Geochemical data of REE
6.5.1.1 REE concentrations and normalized patterns
In all three profiles, uniform REE distribution patterns of regolith samples normalized
to chondrite confirmed that the regolith was developed from weathering of the granitoid
rather than the dolerite (Figure 6.1, Figure 6.2 & Figure 6.3), in agreement with the
major elemental results discussed in Chapter Four; thus, granitoid GEPR2 was selected
as the parent rock in the GE profile, while in both MQ profiles, the average
concentrations of granitoids MQPR1 and MQPR2 were used as the concentrations of
the parent rock. The parent granitoids in three profiles had higher sum of concentrations
of REE (ΣREE, 137 ppm of GE parent granitoid and average 201 ppm of MQ parent
granitoids) than the dolerite (GE dolerite GEPR3 ΣREE=38 ppm; MQ dolerite MQPR3
ΣREE=61 ppm).
In the GE profile, a high deficiency of REE was observed throughout the regolith
profile compared with the parent granitoid (Table 6.1): Up to 96% ΣREE was lost in the
mottled clay (GE3, 10 m depth), followed by 83% loss in the duricrust (GE6, 3.5m
depth); and the saprolite (GE1, 12m depth) showed the least depletion of ΣREE (64%).
Although the REE distribution patterns of regolith normalized by chondrite showed a
LREE-enrichment, higher depletions of LREE than HREE relative to the parent
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
110
granitoids were observed, apart from a positive Ce anomaly (Ce*=6.1) in the duricrust
(GE6, 3.5m depth). The matrix showed more intense depletion of REE than the gravel
in the A horizon, whereas the matrix was less REE depleted than the gravel in the
subsurface regolith (<0.5 m depth) of the GE profile.
In the MQ I profile, the matrix from the A (MQ8-9, <0.5 m depth) and C horizons
(MQ1, 3.6 m depth) was enriched in ΣREE compared to the parent granitoid (Table 6.1).
The matrix generally had higher ΣREE than the gravel, apart from two B horizon
reoglith samples (MQ5-6). Gravel and matrix from the B horizon (0.9-3.3 m depth) lost
more ΣREE than the A (0.2-0.7 m depth) and C horizons (3.6 m depth). The REE
distribution patterns of regolith normalized by chondrite showed LREE-enrichment with
the highest depletion of REE at 1.1 m depth (MQ5). The matrix from MQ5 regolith had
depleted 76% ΣLREE, 67% ΣMREE and 58% ΣHREE, higher than the gravel with
depletion of 69% ΣLREE, 64% ΣMREE and 51% ΣHREE relative to the parent
granitoid.
In the MQ II profile, matrix from the A horizon (MQ15, 0.08 m depth) was enriched in
ΣREE (22%), whereas the regolith below was depleted in ΣREE compared with the
parent granitoid (Table 6.1). The REE distribution patterns normalized by chondrite
showed LREE-enrichment and a general reduction of REE with depth, with a significant
loss of REE below 0.6 m depth. The abundance and distribution of REE in regolith
changed at 1.1 m depth of MQ I profile and at 0.6 m depth of MQ II profile, implying a
mass movement involved in the upper parts of both profiles, and this is in agreement
with the discussion in Chapter Four based on major elemental analyses.
6.5.1.2 Fractionation of REE during intense weathering
In the GE profile, the LREE/MREE ratio (La/Sm)PR in regolith fluctuated with depth
but was mostly below 1.0 throughout the profile, except GE2 (1.2 in gravel and 1.4 in
matrix) at 11 m depth (Figure 6.4a and Table 6.1). The fractionation between La and
Sm in gravel was more severe than or similar to the corresponding matrix in most
regolith, apart from the upper mottled clay (GE4, 8.4 m depth). All regolith had
(La/Yb)PR ≤ 0.7, coupled with high deficiency of REE, suggesting higher loss of LREE
than HREE discussed above. The fluctuation of (La/Yb)PR was correlated negatively to
the index of weathering intensity (e.g. CIA) of regolith samples.
In the MQ I profile, fractionation between La and Sm first increased with depth, and
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
111
then decreased below 1.1 m depth (Figure 6.4b and Table 6.1). Fractionation between
La and Yb was more complexed than La and Sm, especially in the matrix. The highest
depletion of La was at 1.1 m depth with (La/Yb)PR = 0.5, whereas the highest depletion
of Yb at 3.3 m depth with (La/Yb)PR = 1.7.
In the MQ II profile, (La/Sm)PR was below 1.0, except in the surface matrix (MQ15,
0.08 m depth, (La/Sm)PR = 1.1) (Figure 5.4c and Table 6.1). Similar to (La/Sm)PR,
(La/Yb)PR in the B and C horizons was below 1.0 and up to 2.1 in matrix and 1.1 in
gravel of the A horizon.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
112
Figure 6.1 REE distribution patterns of (a) rocks and regolith samples normalized by the
average chondrite; and (b) regolith samples normalized by the parent granitoid in
lateritic GE profile2 (GEPR2-granitoid; GEPR3-dolerite; GEA3-A horizon, 0.23 m
depth; GE6-duricrust, 3.5 m depth; GE5-ferruginous zone below duricrust, 7.0 m depth;
GE3-mottled clay, 10 m depth; GE1-saprolite, 12.5 m depth; ‘g’ denotes gravel and ‘m’
denotes matrix; from GE6 to GE1, weathering intensity decreased; GE2 and GE4
showed similar patterns with GE3, and GEA1 and GEA2 showed similar patterns with
GEA3, so were not plotted).
2 Several REE e.g. Tm appear to have somewhat abnormal values in the REE patterns of GE and MQ profiles
(normalized to the parent granitoid), which probably result from the compounded errors from the measurements and
the plots without log transformation (due to some extremely low concentrations of REE).
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
113
Figure 6.2 REE distribution patterns of (a) rocks and regolith samples normalized by the
average chondrite; and (b) regolith samples normalized by the parent granitoid in
lateritic MQ I profile (MQPR2-granitoid; MQPR3-dolerite; MQ7-A horizon, 0.7 m
depth; MQ5 and MQ3-B horizon, 1.1 m and 2.8 m depth respectively; MQ1-C horizon,
3.6 m depth; ‘g’ denotes gravel and ‘m’ denotes matrix; from MQ6 to MQ1, weathering
intensity decreased; MQ2 and MQ4 showed similar patterns with MQ3, and MQ8 and
MQ9 showed similar patterns with MQ7, so were not plotted).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
114
Figure 6.3 REE distribution patterns (a) rocks and regolith samples normalized by the
average chondrite; and (b) regolith samples normalized by the parent granitoid in
lateritic MQ II profile (MQPR2-granitoid; MQPR3-dolerite; MQ15-A horizon, 0.08 m
depth; MQ13 and MQ12-B horizon, 0.6 m and 1.1 m depth respectively; MQ10-C
horizon, 2.0 m depth; ‘g’ denotes gravel and ‘m’ denotes matrix; from MQ13 to MQ10,
weathering intensity decreased; MQ11 showed similar patterns with MQ12, and MQ14
showed similar patterns with MQ15, so were not plotted).
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
115
Figure 6.4 normalized ratios (La/Sm)PR (LREE/MREE) and (La/Yb)PR (MREE/HREE)
and CIA of regolith samples against depth in three lateritic profiles: (a) GE profile; (b)
MQ I profile; (c) MQ II profile (dashed vertical line refers to no fractionation relate to
the parent granitoid).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
116
6.5.1.3 Variation of REE with lateritization degree
The variation of REE in the GE profile is compared with a second index of weathering
intensity by using the concentration ratio of SiO2/(SiO2+Al2O3+Fe2O3) (S/SAF, Hill et
al., 2000) and to the degree of lateritization by using SiO2-Al2O3-Fe2O3 ternary plots
(Schellmann, 1981), and plotted in Figure 6.5.
The very low concentrations of REE in comparison to the parent rock suggested a major
loss during weathering and lateritization. Cerium did not show an anomaly except at the
strong lateritization stage where Ce*=6.1 in the duricrust. This Ce enrichment co-occurs
with total iron enrichment at a redox gradient in the duricrust (Chapter Four). Ytterbium
(Yb, a HREE) was less depleted than La, Ce, Sm and Y compared with parent
granitoids. Different concentration ranges of each REE among gravel, matrix and parent
granitoids suggested that depletion and partitioning of REE into different size grains
occurred during weathering and lateritization. Compared with parent grantioids,
(La/Yb)PR was lower than (La/Sm)PR, suggesting that fractionation between LREE and
HREE was stronger than fractionation between LREE and MREE under intense
leaching conditions. The (Y/Ho)PR was relatively consistent with changes in S/SAF,
reflecting that these two elements which generally show similar geochemical behaviour,
didn’t fractionate significantly; however, (Y/Ho)PR of regolith average ca. 0.8 relative to
the parent granitoid indicated depletion of both elements during intense weathering.
Since the MQ I and MQ II profiles were still in a weak lateritization stage and greatly
influenced by transported materials, variations of REE against S/SAF do not provide
any more information than the variation of REE against depth already presented above,
so are not plotted.
6.5.1.4 Mass balance calculation of REE
Mass balance of REE is calculated based on the weighted average concentrations of
REE in the matrix and gravel samples with Zr as the reference element, and plotted in
Figure 6.6.
In the GE profile (Figure 6.6a), the A horizon regolith (above 0.5 m depth) had τ(Zr, REE)
values ranging from −0.96 to −0.80; and the ferruginous zone (3.5-5 m depth) had
τ(Zr, REE) ranging from −0.99 to −0.87, apart from τ(Zr, Ce) = 0.9 in the duricrust (3.5 m
depth). The τ(Zr, REE) slightly increased from La to Yb in the same regolith above lower
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
117
mottled clay (11.4 m depth); in the lower mottled clay and saprolite, τ(Zr, Sm) and τ(Zr, Gd)
had the lowest values.
In the MQ I profile (Figure 6.6b), REE were less depleted than the GE profile, and even
slightly enriched in the regolith at 0.5 m depth with τ(Zr, Ce) = 0.09, τ(Zr, Pr) = 0.05 and
τ(Zr, Sm) = 0.05. In the B horizon (0.9-3.3 m depth), REE (not including Lu) were more
depleted (τ(Zr, REE) between −0.72 and −0.19) than the A (τ(Zr, REE) between −0.47 and
0.06) and C horizons (τ(Zr, REE) between −0.21 and 0.05). The C horizon regolith (3.6 m
depth) had τ(Zr, Er) = 0.05 and τ(Zr, Tm) = 0.01.
In the MQ II profile (Figure 6.6c), REE mass flux consistently decreased with depth
except for the C horizon (2.0 m depth). The surface regolith was enriched in REE with
τ(Zr, Ce) = 0.15 and τ(Zr, Pr) = 0.10; however, the regolith below was REE depleted.
11
8
Table 6.1 concentrations of REE in parent rock and lateritic regolith of the GE and MQ profiles
Sample D1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th ΣREE (La/Sm)PR (La/Yb)PR 2Ce* 2Eu*
Unit m ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
GE profile
Rock
GEBPRA 27.5 45.7 4.13 14.1 2.62 0.96 2.70 0.37 2.06 0.40 0.95 0.17 1.22 b.d. 11.1 32.1 103
GEBPRB 31.4 50.1 4.72 16.5 3.06 0.90 3.23 0.41 2.10 0.45 1.21 0.18 1.38 b.d. 12.2 25.6 116
GEPR1A 9.69 14.2 1.19 4.72 1.04 0.09 1.60 0.29 1.94 0.43 1.49 0.23 1.40 b.d. 12.4 10.4 38.4
GEPR1B 29.9 50.4 4.96 17.3 3.66 1.24 4.17 0.60 3.62 0.71 2.01 0.25 1.67 b.d. 19.7 14.2 120
GEPR23 35.5 59.2 5.67 19.0 3.77 0.90 4.68 0.69 3.26 0.63 1.72 0.28 2.12 b.d. 19.1 21.8 137
GEPR3 4.00 10.8 1.37 7.55 2.45 0.82 2.56 0.48 3.03 0.56 1.98 0.30 1.72 b.d.4 17.2 1.38 103
Regolith
GEA1g 5.08 13.8 1.17 4.56 0.97 0.18 0.88 0.15 0.89 0.19 0.53 0.08 0.64 b.d. 4.70 69.7 29.1 0.6 0.5 1.4 0.9
GEA1m 0.09 3.86 8.43 0.78 2.68 0.69 0.15 0.64 0.12 0.82 0.18 0.50 0.11 0.64 b.d. 4.86 7.56 19.6 0.6 0.4 1.2 1.0
GEA2g 5.47 15.0 1.26 4.83 1.06 0.22 1.02 0.17 0.96 0.21 0.61 0.08 0.65 b.d. 4.93 60.0 31.5 0.5 0.5 1.4 1.0
GEA2m 0.12 2.69 5.88 0.55 1.88 0.54 0.13 0.57 0.13 0.59 0.15 0.53 0.09 0.51 b.d. 4.03 7.04 14.2 0.5 0.3 1.2 1.1
GEA3g 5.12 15.1 1.15 4.44 0.96 0.19 0.92 0.15 0.87 0.20 0.59 0.09 0.67 b.d. 4.70 69.4 30.4 0.6 0.5 1.5 0.9
GEA3m 0.23 3.53 7.16 0.63 2.07 0.48 0.13 0.51 0.11 0.89 0.20 0.57 0.09 0.79 b.d. 5.09 6.26 17.2 0.8 0.3 1.1 1.2
GE6 3.5 1.43 17.5 0.32 1.26 0.22 0.11 0.65 0.09 0.48 0.12 0.26 0.07 0.54 b.d. 2.96 94.2 23.1 0.7 0.2 6.1 1.4
GE5g 1.58 4.16 0.41 1.70 0.55 0.21 0.40 0.07 0.45 0.10 0.33 0.06 0.52 b.d. 2.61 110 10.5 0.3 0.2 1.2 2.1
GE5m 7.0 2.69 6.77 0.51 2.41 0.46 0.23 0.69 0.11 0.53 0.12 0.32 0.08 0.53 b.d. 3.18 32.5 15.5 0.6 0.3 1.4 1.9
GE4g 6.88 11.1 0.89 3.06 0.71 0.16 0.67 0.12 0.76 0.21 0.61 0.11 0.74 b.d. 4.64 32.4 26.0 1.0 0.6 1.1 1.1
GE4m 8.4 8.68 14.4 1.04 3.44 1.06 0.21 0.92 0.13 0.94 0.20 0.63 0.12 0.91 b.d. 5.76 34.7 32.7 0.9 0.6 1.1 1.0
GE3g 0.99 1.74 0.16 0.61 0.23 b.d. 0.25 0.06 0.26 0.09 0.24 0.06 0.38 b.d. 1.77 18.1 5.08 0.4 0.2 1.1 0.0
GE3m 10.0 1.30 1.93 0.21 0.64 0.18 0.11 0.19 0.04 0.31 0.09 0.28 0.08 0.48 b.d. 2.31 15.0 5.83 0.8 0.2 0.9 2.7
GE2g 4.57 6.50 0.59 1.99 0.42 0.07 0.41 0.08 0.52 0.12 0.39 0.07 0.50 b.d. 3.01 30.4 16.2 1.2 0.5 1.0 0.8
GE2m 11.4 6.47 9.32 0.96 2.92 0.51 0.15 0.64 0.12 0.64 0.19 0.49 0.10 0.66 b.d. 4.22 31.4 23.2 1.4 0.6 0.9 1.2
GE1g 10.0 16.7 1.56 5.52 1.16 0.17 1.14 0.18 1.10 0.25 0.77 0.13 0.89 b.d. 6.42 59.4 39.5 0.9 0.7 1.0 0.7
GE1m 12.5 12.6 21.1 1.98 6.82 1.41 0.27 1.45 0.21 1.27 0.28 0.94 0.16 1.24 b.d. 8.76 62.0 49.7 0.9 0.6 1.0 0.9
11
9
Sample D1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th ΣREE (La/Sm)PR (La/Yb)PR 2Ce* 2Eu*
Unit m ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
MQ profile
Rock
MQPR1 57.0 89.0 8.27 29.1 4.90 1.08 5.66 0.69 3.95 0.90 2.44 0.39 2.60 0.11 25.6 37.5 206
MQPR2 54.7 84.3 7.65 26.9 4.61 1.15 4.55 0.67 4.07 0.70 2.48 0.35 2.84 0.08 24.6 39.4 195
MQPR3 8.29 17.9 2.44 11.2 4.90 1.23 4.05 0.72 4.25 0.81 2.47 0.30 2.14 0.08 21.6 5.75 60.8
Regolith
MQ I
MQ9g 43.0 74.5 6.57 22.3 3.75 0.69 3.51 0.47 2.73 0.57 1.60 0.24 1.40 0.22 14.6 35.5 162 1.0 1.5 1.1 0.8
MQ9m 0.2 62.9 108 9.66 31.6 4.88 0.80 5.30 0.71 3.52 0.68 2.04 0.23 1.84 0.08 20.1 49.6 232 1.1 1.7 1.1 0.7
MQ8g 42.6 73.0 6.69 22.4 3.83 0.83 3.21 0.49 2.86 0.63 1.67 0.27 1.52 0.25 15.6 38.1 160 0.9 1.4 1.1 1.0
MQ8m 0.5 74.5 126 11.1 37.8 6.74 0.96 6.66 0.92 4.35 0.82 2.53 0.32 2.00 0.15 26.7 54.2 275 0.9 1.8 1.1 0.6
MQ7g 29.0 51.9 4.79 16.5 2.95 0.62 2.54 0.42 2.46 0.57 1.57 0.26 1.63 0.27 13.0 53.9 115 0.8 0.9 1.1 1.0
MQ7m 0.7 44.8 78.2 7.27 24.2 4.25 0.67 4.02 0.54 2.80 0.56 1.81 0.22 1.64 0.09 16.6 57.6 171 0.9 1.3 1.1 0.7
MQ6g 19.7 36.8 3.51 12.6 2.56 0.59 2.26 0.38 2.39 0.55 1.63 0.27 1.74 0.27 12.6 65.3 85.2 0.7 0.6 1.1 1.1
MQ6m 0.9 18.9 37.0 3.33 12.2 2.62 0.57 2.39 0.36 2.80 0.49 1.63 0.24 1.60 0.13 15.2 64.5 84.4 0.6 0.6 1.1 1.0
MQ5g 15.3 28.8 2.61 9.25 1.73 0.36 1.53 0.27 1.72 0.39 1.13 0.20 1.23 0.21 8.4 66.7 64.7 0.8 0.6 1.1 1.0
MQ5m 1.1 10.5 22.9 1.89 6.76 1.70 0.32 1.46 0.25 1.41 0.33 1.06 0.19 1.10 b.d. 8.24 63.5 49.9 0.5 0.5 1.3 0.9
MQ4g 12.0 20.1 1.78 5.99 1.12 0.22 0.94 0.18 1.14 0.30 0.95 0.15 1.01 0.16 8.06 12.9 46.1 0.9 0.6 1.1 1.0
MQ4m 2.2 27.1 48.9 4.05 14.2 2.81 0.31 2.41 0.26 1.36 0.27 1.01 0.16 1.14 0.04 9.47 32.7 104 0.8 1.2 1.1 0.5
MQ3g 15.7 25.1 2.32 7.68 1.55 0.33 1.11 0.18 1.12 0.28 0.82 0.14 0.98 0.19 7.34 19.0 57.5 0.9 0.8 1.0 1.1
MQ3m 2.8 34.3 56.9 5.13 16.2 3.03 0.39 2.72 0.35 1.74 0.36 0.84 0.14 1.09 0.08 10.1 31.7 123 1.0 1.5 1.0 0.6
MQ2g 13.5 22.0 2.02 6.71 1.29 0.25 1.06 0.18 1.07 0.25 0.78 0.14 0.82 0.16 6.16 12.9 50.3 0.9 0.8 1.0 0.9
MQ2m 3.3 32.8 52.2 4.77 16.0 2.93 0.33 2.24 0.33 1.35 0.28 0.86 0.12 0.92 0.05 8.98 24.2 115 1.0 1.7 1.0 0.6
MQ1g 34.9 58.0 5.28 17.7 3.11 0.87 2.83 0.46 2.76 0.63 1.93 0.30 2.00 0.31 16.5 28.4 131 1.0 0.8 1.0 1.3
MQ1m 3.6 54.7 90.3 8.40 27.5 4.45 0.90 4.50 0.71 4.36 0.82 2.78 0.38 2.48 0.28 26.0 40.5 203 1.0 1.1 1.0 0.9
MQ II
MQ15g 41.1 70.9 6.26 21.4 3.78 0.77 3.21 0.50 2.86 0.65 1.84 0.30 1.77 0.30 16.8 35.2 156 0.9 1.1 1.1 1.0
MQ15m 0.08 66.6 115 10.2 32.6 5.37 0.67 5.38 0.67 3.45 0.62 1.90 0.23 1.51 b.d. 19.0 43.9 244 1.1 2.1 1.1 0.6
12
0
Sample D1 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th ΣREE (La/Sm)PR (La/Yb)PR 2Ce* 2Eu*
Unit m ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
MQ14g 34.2 57.2 5.42 18.7 3.33 0.80 2.85 0.45 2.64 0.66 1.79 0.28 1.87 0.31 15.6 37.6 131 0.9 0.9 1.0 1.1
MQ14m 0.25 49.1 81.3 7.79 26.1 4.66 0.81 4.31 0.67 3.34 0.71 2.26 0.32 2.25 0.06 18.7 50.9 184 0.9 1.1 1.0 0.8
MQ13g 23.3 49.4 4.05 14.3 2.62 0.52 2.31 0.38 2.31 0.53 1.64 0.28 1.83 0.30 11.9 48.8 104 0.8 0.6 1.2 0.9
MQ13m 0.6 27.8 54.2 4.20 14.0 3.15 0.52 3.10 0.41 2.20 0.49 1.66 0.23 2.00 0.07 13.3 47.7 114 0.8 0.7 1.2 0.7
MQ12g 8.55 15.5 1.31 4.65 1.01 0.24 0.84 0.16 1.09 0.26 0.82 0.16 1.00 0.18 6.78 20.9 35.8 0.7 0.4 1.1 1.2
MQ12m 1.1 10.7 19.1 1.57 5.33 0.99 0.35 1.28 0.16 0.98 0.23 0.78 0.16 0.85 b.d. 7.36 26.6 42.4 0.9 0.6 1.1 1.4
MQ11g 6.88 10.7 0.96 3.39 0.86 0.30 0.65 0.11 0.68 0.18 0.58 0.11 0.71 0.13 4.74 10.0 26.2 0.7 0.5 1.0 1.8
MQ11m 1.6 7.62 12.9 1.13 4.08 1.16 0.27 0.68 0.14 0.89 0.18 0.63 0.14 0.99 b.d. 6.28 18.8 30.8 0.6 0.4 1.1 1.4
MQ10g 6.05 10.1 0.90 2.97 0.72 0.25 0.47 0.08 0.49 0.12 0.40 0.08 0.52 0.10 3.25 7.12 23.3 0.7 0.6 1.1 1.9
MQ10m 2.0 10.3 16.3 1.43 4.63 1.46 0.20 0.77 0.11 0.83 0.14 0.54 0.11 0.60 b.d. 5.64 13.6 37.4 0.6 0.8 1.0 0.8
RSD5 Min 0.5 0.4 0.1 0.1 0.0 0.0 0.1 0.0 0.2 0.0 0.0 0.0 0.1 0.1 1.1 0.8
RSD Max 2.3 1.2 0.3 1.3 0.3 0.1 0.1 0.0 0.5 0.0 0.1 0.0 0.3 0.2 1.2 1.0
1D denotes depth (m);
2Ce
*=(Ce/CePR)/[(La/LaPR)
0.5×(Pr/PrPR)
0.5]; Eu
*=(Eu/EuPR)/[(Sm/SmPR)
0.5×(Gd/GdPR)
0.5];
3Subscript PR refers to the parent granitoids: GEPR2, average MQPR1 and MQPR2 were used as the parent granitoids in corresponding profile;
4b.d. refers to below detection limit; ‘g’ represents gravel and ‘m’ represents matrix.
5RSD refers to the range of relative standard deviations of the duplicates/triplicates analysed by ICP-MS.
12
1
Figure 6.5 SiO2-Al2O3-Fe2O3 ternary plots and associated variation of REE concentrations and ratios against the S/SAF weathering index for the GE
profile.
12
2
Figure 6.6 Mass balance calculations of REE against depth for three lateritic profiles, based on weighted average concentrations of REE in matrix and
gravel, using Zr as the reference element: (a) GE Profile; (b) MQ I Profile; (c) MQ II Profile (vertical dashed line refers to mass balance τ(Zr,REE) = 0;
Only selected REE are plotted here, as the remaining REE have similar patterns).
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
123
6.5.2 Mineralogy of REE in the parent rock
The parent granitoids of the GE profile contained accessory minerals such as monazite
and allanite, which controlled the abundance and distribution of REE (Figure 6.7 and
Table 6.2). Monazite (<0.05 wt%) was usually incorporated into quartz or feldspar or
intergrown with apatite in GE parent granitoids. However, two different types of
monazite were identified by EPMA (Figure 6.7 and Table 6.2). Type 1 monazite (Figure
6.7a & b) contained an average 52 wt% ΣREE, 5.6 wt% Th and 0.3 wt% U with high
(La/Yb)PR (average 5.9, up to 19.5). Type 2 monazite (Figure 6.7c & d) was
characterized by a much higher concentration of Th (average 23 wt%) than the Type 1,
but lower concentrations of ΣREE (average 23 wt%) and (La/Yb)PR (average 1.0). Both
types of monazite had no apparent Ce anomalies (Ce* ranged from 0.9-1.1) and
moderate Eu anomalies (Eu* ranged from 0.3-0.7).
In addition to monazite, allanite (<0.03 wt%) was another important REE-rich
accessory mineral in GE parent granitoids (Figure 6.7e). The average concentration of
ΣREE in allanite was ca. 18 wt%, lower than Type 1 monazite, and with 0-0.02 wt% Th
and/or U. The ΣREE concentration was dominated by LREE with an average (La/Yb)PR
of 4.9 and without Ce anomaly, but had a moderate negative Eu anomaly (average 0.6).
Other accessory minerals such as zircon and ilmenite hosted trace to minor
concentrations of ΣREE. Zircon (<0.02 wt%) contained 0.15-3 wt% ΣREE, with HREE
predominating (average (La/Yb)PR = 0.01). Ilmenite (<0.03 wt%) contained trace to
minor amounts of REE with a preference for Dy and Yb. Thorite, intergrown with
zircon or as individual grain surrounded by feldspars (Figure 6.7f), also contained a
minor amount of ΣREE (0.3-5.2 wt%). Feldspar may contain negligible concentrations
of ΣREE (ca. 1.2 wt%) with LREE enrichment; in this study the Eu anomaly was not
determined as Sm was below the detection limit of EPMA.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
124
(a) (b)
(c) (d)
(e) (f)
Figure 6.7 Backscatter electron images of REE-bearing accessory minerals in parent
granitoids of the GE profile (scale bar all 10 µm). (a) and (b) Type 1 monazite
surrounded by feldspar; (c) Type 2 monazite surrounded by feldspar; (d) minute grains
of Type 2 monazite included in feldspars; (e) allanite surrounded by feldspars; (f)
hollow thorite surrounded by feldspars. (Aln: allanite; Ap: apatite; Fsp: feldspar; Kfs:
feldspar-K; Mnz: monazite; Qz; quartz; Thr: thorite).
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
125
In the parent granitoids of both MQ profiles, accessory minerals such as monazite,
fluorocarbonate, allanite, apatite, zircon, ilmenite and sphene contained abundant REE
(Figure 6.8 and Table 6.3). Monazite (<0.03 wt%) (Figure 6.8a & b) contained an
average of 51 wt% ΣREE, with a preference for LREE ((La/Yb)PR ranged from 2.7 to
7.5) and negative Eu anomalies (Eu* ranged from 0.2 to 0.8). The concentrations of
ΣREE in MQ monazite were similar to the Type 1 monazite in the GE parent granitoids.
In the MQ parent granitoids, REE-rich fluorocarbonates (<0.02 wt%) (Figure 6.8b)
were observed, occurring as an intergrown mineral with monazite and/or zircon,
incorporated into feldspars and having 50-58 wt% ΣREE with a preference for LREE
(average (La/Yb)PR = 5.4), slightly positive Eu anomalies (Eu* ranged from 1.0-1.4),
and trace to minor amount of Th (0.3-5.3 wt%) and U (ca. 0.06 wt%). These
fluorocarbonates were not observed in GE parent granitoids but observed in the JG
parent granitoids (Chapter Seven). In addition, allanite (<0.01 wt%) (Figure 6.8c) also
hosted average 10 wt% ΣREE with the range of (La/Yb)PR 1.1-2.8.
Apatite (<0.31 wt%) contained 0.1-0.7 wt% ΣREE without apparent LREE or HREE
selectivity. Ilmenite (<0.33 wt%) contained 0-0.6 wt% ΣREE and showed a preference
for Dy (up to 0.47 wt%). Sphene (also called titanite) was observed in MQ parent
granitoids (<0.26 wt%) as an individual crystal or intergrown with ilmenite or allanite
(Figure 6.8d). It contained an average 0.06 wt% Yb. Zircon (<0.03 wt%) contained
ca. 0.1 wt% ΣREE, with preference for Yb.
6.5.3 Mineralogy of REE in the regolith
In the weathered MQ regolith, phosphate phases were identified as the main hosts for
REE, as well as zircon and ilmenite (Figure 6.9 and Table 6.4). Allanite and apatite
were absent in the B horizon regolith. REE-rich phosphates were preferentially enriched
in LREE ((La/Yb)PR average 5.3) with an average 1,000-fold enrichment of REE
(ca. 53 wt% ΣREE) relative to the parent granitoids (average 200 ppm), reflecting
strong mineralogical control of the distribution and fractionation of REE.
Residual monazite and secondary rhabdophane were inferred as the REE-rich
phosphates in the regolith based on chemical composition. Residual monazite (Figure
6.9a, b & c) had higher concentrations of Th (3.8-8.6 wt%) and Pb (0.4-1.1 wt%) with
grain sizes between 10-50 µm, similar to the concentrations of Th (average 6.4 wt%)
and Pb (average 0.6 wt%) in monazite in MQ parent granitoids. Secondary rhabdophane
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
126
(Figure 6.9d, e & f) had lower concentrations of Th (0.4-2.8 wt%) and Pb (0.1-0.3 wt%)
with varied grain size (<60 µm) and the compositional sum of oxides below 100%
(probably due to hydration). Florencite and xenotime were not observed in the MQ
regolith; this does not mean they are not present, as they could have a sub-micron grain
size and be present in very low abundance.
In addition, zircon grains containing average 0.3 wt% ΣREE, with a preference for
HREE as well as Ce, and minor contents of Th (0.1-0.2 wt%) and U (0.1-0.4 wt%) were
observed in the MQ regolith. Thorite was rare and only one grain was found in the C
horizon of MQ II profile (2.0 m depth) (Figure 6.9g), where it contained ca. 9 wt%
ΣREE with a preference for HREE. Ilmenite with 0.02-0.08 wt% Yb was determined by
EPMA, but other REE were below detection limits. Ilmenite was usually intergrown
with rutile/anatase (Figure 6.9h); however, concentrations of REE in Ti-oxides were
below detection limits. In addition, trace concentrations of ΣREE (less than 100 ppm)
were determined in Fe oxides at 1.1 m depth of MQ I profile.
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
127
(a) (b)
(c) (d)
Figure 6.8 Backscatter electron images of REE-bearing accessory minerals in parent
granitoids of the MQ profiles (scale bars vary and are present on each image): (a)
monazite surrounded by quartz; (b) monazite, REE-rich fluorocarbonate together with
zircon, incorporated into feldspars; (c) allanite surrounded by feldspar and sphene; (d)
sphene intergrown with ilmenite, surrounded by feldspars; (Aln: allanite; Fc: REE-rich
fluorocarbonate; Fsp: feldspar; Hbl: hornblende; Ilm: ilmenite; Kfs: feldspar-K; Mnz:
monazite; Qz; quartz; Spn: sphene; Zrn: zircon).
12
8
(a) (b) (c) (d)
(e) (f) (g) (h)
Figure 6.9 Backscatter electron images of REE-bearing minerals in regolith of the MQ profiles (scale bars vary and are present on each image): (a) and
(b) corroded residual Th-rich monazite; (c) Th-rich residual monazite surrounded by hornblende and clay matrix; (d) secondary rhabdophane
surrounded by Fe-rich REE-bearing phosphates incorporated in quartz; (e) and (f) varied grain sizes of Th-poor secondary rhabdophane; (a) REE-rich
thorite; (b) eroded ilmenite and intergrown Ti oxides.
12
9
Table 6.2 Concentrations of REE and associated elements from EPMA analyses of representative minerals in parent granitoids from the GE profile
No. 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
Min. Aln Aln Aln MnzI MnzI MnzI MnzII MnzII MnzII Zrn Zrn Zrn Thr Ilm Fsp
Si 15.9 15.6 16.0 0.38 0.78 0.47 3.19 3.11 4.20 15.8 13.5 13.4 9.83 0.02 18.0 Zr b.d. b.d. b.d. b.d. b.d. b.d. 0.40 0.52 0.18 48.8 43.6 38.0 6.63 b.d. b.d.
Ti 0.21 0.12 0.29 b.d. b.d. b.d. 0.14 0.09 b.d. b.d. 0.26 2.33 b.d. 32.3 0.82
Pb b.d. b.d. b.d. 0.40 0.66 0.82 0.31 0.14 0.07 b.d. 0.03 0.05 3.12 b.d. b.d.
Th 0.02 0.02 b.d. 4.90 5.80 6.32 27.3 25.3 16.2 0.02 0.42 0.73 35.8 b.d. b.d.
U 0.02 0.01 0.02 0.20 0.19 0.35 0.55 0.36 0.36 0.04 0.21 0.48 19.6 b.d. b.d.
Al 10.0 9.73 9.07 b.d. b.d. b.d. b.d. 0.91 1.74 b.d. 0.67 1.27 b.d. b.d. 11.4
Y 0.36 0.29 0.91 0.62 1.50 1.18 2.41 1.80 1.97 b.d. 1.15 1.00 0.93 b.d. b.d.
La 3.99 4.55 4.57 14.3 12.5 13.3 6.15 5.36 6.79 b.d. 0.05 b.d. b.d. b.d. 0.33
Ce 8.02 8.79 7.43 24.5 24.4 23.6 8.22 8.86 12.5 b.d. 0.31 0.30 b.d. b.d. 0.62
Pr 0.82 0.87 0.77 2.42 2.56 2.61 0.83 0.90 1.21 b.d. 0.05 0.07 b.d. b.d. b.d.
Nd 2.68 2.77 2.30 8.18 8.82 7.61 2.67 2.63 3.75 b.d. 0.25 0.30 b.d. b.d. 0.21
Sm 0.40 0.37 0.38 1.25 1.49 1.48 0.62 0.49 0.74 b.d. 0.09 0.09 b.d. b.d. b.d.
Eu 0.07 0.06 0.06 0.19 0.21 0.16 0.10 0.08 0.12 b.d. 0.02 0.01 b.d. b.d. 0.01
Gd 0.33 0.29 0.29 0.81 1.02 1.02 0.57 0.44 0.62 b.d. 0.12 0.13 0.04 b.d. 0.03
Dy b.d. b.d. b.d. 0.20 0.42 0.35 0.43 0.27 0.43 b.d. 0.10 0.06 0.10 0.37 b.d.
Er b.d. b.d. b.d. b.d. 0.02 b.d. 0.08 0.02 0.03 0.03 b.d. b.d. 0.06 b.d. b.d.
Tm b.d. b.d. b.d. b.d. 0.02 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Yb 0.03 0.06 0.14 0.04 0.23 0.21 0.38 0.27 0.36 0.12 0.31 0.23 0.13 0.07 b.d.
Lu b.d. b.d. b.d. 0.02 0.03 0.02 b.d. b.d. 0.02 b.d. 0.04 0.04 b.d. b.d. b.d.
Fe 8.65 8.93 10.2 0.15 0.03 0.03 0.60 0.68 0.59 0.01 1.47 2.76 0.10 31.9 10.1
Ca 8.32 7.88 8.49 0.34 b.d. 0.33 4.10 4.17 2.92 b.d. 0.02 0.25 0.27 b.d. 14.2
Sr 0.05 0.04 0.02 b.d. b.d. b.d. 0.05 0.07 b.d. 0.48 0.38 0.33 b.d. b.d. 0.20
K b.d. b.d. b.d. b.d. b.d. b.d. 0.47 0.61 b.d. b.d. 0.01 0.08 b.d. b.d. b.d.
P 0.01 0.02 0.01 13.6 13.0 13.1 11.3 11.5 12.2 b.d. b.d. b.d. b.d. b.d. 0.01
F 0.18 0.16 0.21 1.40 1.08 1.28 1.09 1.13 0.88 0.14 0.35 0.27 0.38 b.d. b.d.
O 36.3 35.6 35.8 27.4 27.3 26.9 26.0 28.4 30.6 35.2 32.5 32.8 21.6 30.8 40.1
total 96.4 96.3 97.2 101.4 102.1 101.1 98.1 98.1 99.1 100.6 95.9 95.1 98.6 95.5 96.1
Tb and Ho were below detection limit (b.d.); Na was also below the detection limit except No. 18 (0.57 wt%); Fc: REE-rich fluorocarbonate; Mnz: monazite; Aln: allanite; Ilm:
ilmenite; Spn: sphene; Ap: apatite; Zrn: zircon.
13
0
Table 6.3 Concentrations of REE and associated elements from EPMA analyses of representative minerals in parent granitoids from the MQ profile
No. 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
Min. Fc Fc Fc Mnz Mnz Mnz Aln Aln Aln Ilm Ilm Ilm Spn Spn Spn Ap Ap Ap Zrn
Si 1.62 1.88 1.66 1.04 1.24 0.67 16.7 15.6 16.8 b.d. 0.04 0.23 14.3 14.5 14.8 0.15 0.35 0.07 15.7 Zr 1.02 0.71 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 47.8
Ti 0.04 0.04 0.26 b.d. b.d. b.d. 0.05 0.06 0.14 33.6 35.1 38.4 21.8 22.2 20.8 0.04 b.d. b.d. 0.04
Pb 0.18 0.29 0.03 0.63 0.50 0.45 0.02 b.d. b.d. b.d. 0.06 0.15 b.d. b.d. b.d. b.d. b.d. b.d. 0.08
Th 5.26 4.99 0.27 7.91 8.76 5.55 0.02 0.02 0.01 b.d. b.d. 0.07 b.d. b.d. b.d. b.d. 1.96 b.d. 0.24
U 0.07 0.06 0.05 0.23 0.22 0.11 0.02 b.d. 0.02 0.02 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.35
Al 0.48 0.61 0.69 b.d. b.d. b.d. 11.7 10.3 10.7 b.d. b.d. b.d. 1.05 1.02 1.61 b.d. b.d. b.d. b.d.
Y 0.88 0.82 0.23 1.57 2.02 1.07 0.38 0.80 0.09 0.01 b.d. b.d. b.d. b.d. 0.03 0.39 0.51 0.20 0.16
La 22.1 20.5 12.3 14.5 13.6 15.1 1.89 3.87 2.36 b.d. b.d. 0.04 b.d. b.d. b.d. 0.05 0.05 b.d. b.d.
Ce 17.0 19.3 36.6 24.1 22.5 25.7 3.61 6.68 4.03 b.d. b.d. 0.05 b.d. b.d. b.d. 0.15 0.13 b.d. b.d.
Pr 2.31 2.41 2.05 2.15 2.19 2.39 0.38 0.82 0.41 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Nd 7.10 7.52 6.22 6.51 6.84 7.13 1.19 2.00 1.18 b.d. b.d. b.d. b.d. b.d. b.d. 0.12 0.15 0.06 b.d.
Sm 0.74 0.86 0.81 0.97 1.12 1.05 0.10 0.17 0.11 b.d. b.d. b.d. b.d. b.d. b.d. 0.05 0.05 0.03 b.d.
Eu 0.26 0.27 0.21 0.16 0.15 0.17 0.02 0.02 0.02 b.d. b.d. b.d. b.d. b.d. b.d. 0.01 b.d. b.d. b.d.
Gd 0.45 0.49 0.45 0.67 0.71 0.66 0.16 0.31 0.08 b.d. b.d. b.d. b.d. b.d. b.d. 0.07 0.08 0.02 b.d.
Tb b.d. b.d. b.d. b.d. 0.07 0.03 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Dy b.d. b.d. b.d. 0.38 0.44 0.27 b.d. b.d. b.d. b.d. 0.14 0.47 b.d. b.d. b.d. 0.12 0.09 0.04 b.d.
Er b.d. b.d. b.d. 0.07 0.10 0.03 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.02 b.d. b.d. b.d.
Yb 0.19 0.23 0.10 0.18 0.25 0.18 0.09 0.18 0.04 b.d. 0.05 b.d. 0.06 0.06 0.07 0.05 0.02 0.02 0.10
Fe 5.46 4.71 4.70 b.d. b.d. b.d. 8.78 9.51 10.0 27.9 23.5 18.0 0.47 0.38 0.62 0.12 0.48 0.16 0.55
Mg 0.11 0.13 0.20 b.d. b.d. b.d. 0.05 0.11 0.03 0.10 0.03 0.07 b.d. b.d. b.d. b.d. b.d. b.d. 0.01
Ca 2.80 2.71 1.76 b.d. 0.28 0.11 11.1 8.12 11.1 0.05 b.d. b.d. 17.6 17.9 18.4 35.4 35.5 36.2 0.07
K 0.02 0.03 0.08 b.d. b.d. b.d. 0.05 0.06 b.d. b.d. b.d. b.d. 0.04 b.d. b.d. 0.02 0.01 b.d. b.d.
P 0.10 0.13 0.03 12.4 12.1 13.2 b.d. 0.02 0.01 b.d. b.d. b.d. b.d. 0.01 b.d. 18.2 17.6 17.5 b.d.
S 0.09 0.11 0.05 b.d. 0.05 0.01 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.01 b.d. b.d.
F 4.69 5.31 7.19 1.11 b.d. 1.24 b.d. b.d. b.d. b.d. b.d. b.d. 0.38 0.22 1.30 3.29 2.94 4.61 b.d.
O 13.3 13.3 12.1 26.8 27.2 27.4 37.8 35.7 37.5 30.5 30.3 31.2 38.8 39.5 39.2 36.7 36.6 35.4 35.1
total 86.3 87.4 88.0 101.5 100.4 102.5 94.0 94.4 94.8 92.1 89.2 88.7 94.6 95.9 96.9 95.0 96.5 94.3 100.2
Ho, Tm and Na were below the detection limit; Lu was also below the detection limit, except No. 28 monazite (0.03 wt%) and No.32 allanite (0.03 wt%).
13
1
Table 6.4 Concentrations of REE and associated elements from EPMA analyses of representative minerals in lateritic regolith from the MQ profile
No. 44 45 46 47 48 49 50 51 52 53 54 55 56
Min. Mnz Mnz Mnz Rbp Rbp Rbp Ilm Ilm Ilm Zrn Zrn Zrn Thr
Si 0.86 1.09 0.75 0.45 0.26 0.15 b.d. b.d. 0.04 14.8 15.2 15.2 9.76 Zr b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 44.9 46.0 47.1 6.48
Ti b.d. b.d. b.d. b.d. b.d. b.d. 33.5 31.8 30.8 0.06 b.d. b.d. 0.05
Pb 1.05 0.95 0.91 0.05 0.06 0.10 b.d. 0.02 b.d. b.d. b.d. b.d. 0.05
Th 7.24 8.61 7.02 0.70 1.23 1.18 b.d. b.d. b.d. 0.18 0.08 b.d. 38.4
U 0.24 0.26 0.17 0.02 b.d. b.d. 0.02 0.02 b.d. 0.14 0.19 0.13 0.40
Al b.d. b.d. b.d. 0.09 0.04 b.d. 0.09 b.d. b.d. 0.21 0.05 b.d. 0.38
Y 2.14 1.56 1.59 0.13 0.31 0.27 b.d. b.d. b.d. 0.67 0.37 b.d. 3.11
La 13.9 14.4 13.7 15.7 16.0 16.6 b.d. b.d. b.d. b.d. b.d. 0.04 b.d.
Ce 23.5 23.6 24.4 26.8 27.5 28.0 b.d. b.d. b.d. 0.37 0.09 b.d. 1.09
Pr 2.70 2.67 2.77 2.54 2.44 2.75 b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Nd 6.87 6.39 7.09 7.82 7.88 8.41 b.d. b.d. b.d. 0.04 b.d. b.d. 0.46
Sm 1.05 0.90 1.09 1.23 1.13 1.24 b.d. b.d. b.d. 0.02 b.d. b.d. 0.20
Eu 0.06 0.05 0.10 0.20 0.20 0.19 b.d. b.d. b.d. b.d. b.d. b.d. 0.07
Gd 0.68 0.56 0.67 0.89 0.64 0.68 b.d. b.d. b.d. 0.08 0.03 b.d. 0.30
Dy 0.51 0.37 0.37 0.09 0.06 0.14 b.d. b.d. b.d. 0.08 0.03 b.d. 0.23
Er 0.09 0.04 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.03 b.d.
Yb 0.26 0.22 0.20 0.07 0.05 0.14 0.02 0.04 0.05 0.07 0.10 0.09 0.52
Lu 0.03 b.d. 0.02 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.06
Fe 0.08 0.04 0.25 0.03 1.56 b.d. 31.8 35.1 34.8 0.56 0.52 0.07 0.06
Mg b.d. b.d. b.d. b.d. b.d. b.d. 0.10 0.09 b.d. 0.01 b.d. b.d. 0.02
Ca b.d. b.d. b.d. b.d. 0.01 b.d. b.d. b.d. 0.03 b.d. 0.01 b.d. 0.09
Sr b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.41 0.43 0.47 b.d.
P 12.5 12.1 12.7 13.1 12.5 12.8 b.d. b.d. 0.01 b.d. b.d. b.d. 1.29
S b.d. b.d. b.d. b.d. 0.02 b.d. b.d. b.d. b.d. 0.02 b.d. b.d. 0.03
F 1.29 1.12 1.08 1.53 1.02 1.38 b.d. b.d. b.d. 0.15 b.d. b.d. 0.04
O 26.7 26.5 26.9 26.6 26.3 26.3 31.6 31.4 30.6 33.3 33.9 34.0 23.8
total 101.7 101.3 101.7 98.1 99.2 100.1 97.1 98.4 96.3 96.1 97.0 97.2 100.5
Tb, Ho, Tm, Na were below detection limit; K was below the detection limited except No.47 (0.09 wt%). Mnz: monazite; Rbp: rhabdophane; Ilm: ilmenite; Zrn: zircon; Thr: thorite.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
132
6.5.4 REE in grain size fractions and chemical extractions of regolith
In the MQ II profile, the silt fraction (2-20 µm) had the highest concentrations ΣREE,
followed by clay (<2 µm), except the surface regolith which had a higher concentration
of REE in clay than in silt (Figure 6.10, data presented in Appendix 11.9).
Concentrations of ΣREE in each grain size fraction generally decreased with depth until
1.1 m depth, and then increased to saprolite except in gravel (> 2 mm). Concentrations
of ΣREE in gravel decreased with depth until saprolite.
Given the mass percentage of each grain size fraction, sand (0.02-2 mm) contained the
greatest mass of REE, especially HREE at 0.1-0.3 m depth (Figure 6.11). Although silt
generally had the highest concentrations of ΣREE, the relatively low mass percentage
compared with the other fractions minimises its contribution. The clay fraction had the
highest mass of REE, followed by gravel at 0.6-1.1 m depth; these two fractions account
for 71-80% LREE, 70-99% MREE and 65-99% HREE. At 1.6-2.0 m depth, the sand
fraction had the highest mass of REE, composing 44-60% LREE, 37-53% MREE and
38-59% MREE, except Eu. The gravel fraction contained significant mass of Eu
(58-70%) at 1.6-2.0 m depth.
The sequential extraction experiment revealed the REE speciation in representative
lateritic regolith of the GE and MQ I profile (Table 6.5). The Res contained the highest
percentages of ΣREE. In addition to the Res, the WAE hosted minor percentages of
ΣREE (4.6% of ΣREE in the WAE of the GE saprolite and 7.9% in the MQ I C horizon).
The ΣREE percentage associated with the WAE in the saprolite (4.6%) of the GE
profile was higher than the ΣREE percentage associated with the WAE in the duricrust
(2.0%), where the ΣREE percentages in the FeAm (3.7%) and FeCry (2.7%) were higher.
Similarly, the ΣREE percentage associated with the WAE in the C horizon regolith
(7.9%) of the MQ I profile was higher than the ΣREE percentage associated with the
WAE in the A horizon regolith (5.6%), where the ΣREE percentage in the Org was
higher (10.7%), although the Res hosted the highest ΣREE percentage (80%). In the
duricrust of the GE profile, an average of 3.7% ΣLREE, 4.4% ΣMREE and 1.8%
ΣHREE were associated with the FeAm, higher than 3.0% ΣLREE, 1.4% ΣMREE and
0.5% ΣHREE associated with the FeCry.
13
3
Figure 6.10 Concentrations of selected REE (La, Ce, Sm, Dy, and Yb) in grain size fractions of the MQ II profile.
Figure 6.11 Mass loading of selected REE (La, Ce, Sm, Dy, and Yb) in grain size fractions of the MQ II profile (MQ15, 0.1 m depth; MQ14, 0.3 m
depth; MQ13, 0.6 m depth; MQ12, 1.1 m depth; MQ11, 1.6 m depth; MQ10, 2.0 m depth. Only selected REE are plotted here, as the remaining REE
have similar patterns).
0
20
40
60
80
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
La
(p
pm
)
0
40
80
120
160
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Ce
(p
pm
)
0
2
4
6
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Sm
(p
pm
)
sand silt clay matrix gravel
0
2
4
6
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Dy
(p
pm
)
0
2
4
6
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Yb
(p
pm
)
0%
20%
40%
60%
80%
100%
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
La
(w
t%)
0%
20%
40%
60%
80%
100%
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Ce
(w
t%)
0%
20%
40%
60%
80%
100%
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Sm
(w
t%)
gravel sand silt clay
0%
20%
40%
60%
80%
100%
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Dy
(w
t%)
0%
20%
40%
60%
80%
100%
MQ15 MQ14 MQ13 MQ12 MQ11 MQ10
Yb
(w
t%)
13
4
Table 6.5 Concentrations of REE in sequential extractions of representative regolith in the GE and MQ I profiles
Sample Element concentrations (ppm)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Mn Fe ΣREE%1
d.l. 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 1.00 5.00
GE1m WAE 0.518 0.876 0.109 0.377 0.074 0.017 0.063 0.010 0.056 0.013 0.035 0.005 0.034 0.006 0.136 b.d. 30.0 4.56
GE1m Org 0.016 0.024 0.003 0.011 0.004 b.d. 0.002 b.d. 0.003 b.d. 0.002 b.d. 0.004 0.001 0.013 b.d. 125 0.15
GE1m FeAm 0.175 0.514 0.038 0.139 0.028 0.006 0.025 0.004 0.027 0.006 0.018 0.003 0.018 0.003 0.155 2.00 615 2.09
GE1m FeCry 0.055 0.192 0.007 0.025 0.005 0.001 0.005 b.d. 0.006 0.001 0.004 b.d. 0.004 0.001 0.038 3.00 696 0.64
GE1m Res 11.5 18.7 1.90 6.10 1.30 b.d. 1.00 0.20 1.20 0.30 0.90 0.20 1.00 0.20 7.80 26.0 5304 92.6
GE6 WAE 0.086 0.263 0.021 0.079 0.022 0.005 0.017 0.003 0.015 0.003 0.008 0.001 0.006 0.001 0.024 5.00 8.00 1.96
GE6 Org 0.007 0.040 0.003 0.011 0.004 b.d. 0.002 b.d. 0.002 b.d. 0.002 b.d. 0.004 0.001 0.008 b.d. 56.0 0.28
GE6 FeAm 0.059 0.699 0.023 0.094 0.029 0.007 0.023 0.004 0.023 0.004 0.012 0.002 0.011 0.002 0.067 15.0 1272 3.67
GE6 FeCry 0.037 0.626 0.007 0.032 0.010 0.002 0.008 0.001 0.007 0.001 0.004 b.d. 0.004 b.d. 0.022 2.00 5329 2.73
GE6 Res 2.10 17.3 0.45 1.55 0.45 b.d. 0.45 0.10 0.65 0.20 0.55 0.10 0.70 0.10 4.55 22.5 22.5% 91.4
MQ1m WAE 3.473 5.617 0.640 2.081 0.342 0.081 0.281 0.048 0.274 0.058 0.163 0.024 0.131 0.019 1.056 2.00 59.0 7.89
MQ1m Org 0.221 0.361 0.041 0.126 0.022 0.005 0.018 0.003 0.023 0.005 0.016 0.003 0.017 0.003 0.140 b.d. 69.0 0.52
MQ1m FeAm 1.618 2.829 0.301 0.963 0.168 0.037 0.147 0.027 0.167 0.036 0.105 0.017 0.097 0.014 0.977 14.0 1884 3.89
MQ1m FeCry 0.493 0.888 0.093 0.294 0.056 0.015 0.051 0.011 0.073 0.017 0.048 0.008 0.049 0.007 0.435 8.00 1738 1.25
MQ1m Res 38.5 66.0 6.50 19.6 3.10 0.50 2.40 0.50 2.80 0.60 1.80 0.30 2.00 0.30 18.0 67.0 8759 86.4
MQ5m WAE 2.556 5.709 0.583 2.164 0.451 0.126 0.369 0.065 0.387 0.087 0.251 0.039 0.234 0.035 1.316 2.00 b.d. 14.6
MQ5m Org 0.293 0.776 0.066 0.195 0.036 0.009 0.025 0.005 0.035 0.009 0.036 0.010 0.082 0.013 0.244 b.d. 156 1.77
MQ5m FeAm 0.451 2.294 0.115 0.399 0.085 0.024 0.067 0.014 0.094 0.021 0.062 0.010 0.062 0.009 0.365 2.00 1456 4.13
MQ5m FeCry 0.087 0.232 0.021 0.075 0.018 0.005 0.014 0.003 0.020 0.004 0.014 0.003 0.019 0.003 0.072 4.00 5267 0.58
MQ5m Res 19.0 32.0 3.20 9.60 1.60 0.10 1.20 0.20 1.30 0.30 0.90 0.20 1.00 0.20 8.50 29.0 3.52% 79.0
MQ8m WAE 4.316 5.358 0.915 3.378 0.686 0.209 0.661 0.112 0.618 0.134 0.346 0.045 0.230 0.032 2.295 45.0 13.0 5.61
MQ8m Org 7.306 16.82 1.312 3.733 0.670 0.169 0.489 0.099 0.644 0.143 0.429 0.080 0.478 0.066 4.298 21.0 824 10.7
MQ8m FeAm 2.711 5.825 0.473 1.443 0.257 0.063 0.203 0.039 0.234 0.048 0.121 0.016 0.077 0.010 1.055 49.0 2172 3.79
MQ8m FeCry 0.170 0.360 0.030 0.096 0.018 0.005 0.014 0.003 0.018 0.004 0.011 0.002 0.012 0.002 0.095 9.00 4249 0.25
MQ8m Res 67.1 113 11.2 34.3 5.20 0.40 3.50 0.50 3.00 0.60 1.60 0.20 1.40 0.20 17.1 69.0 1.24% 79.7
1ΣREE% refers to percentage of ΣREE in each extraction species; d.l.: detection limit.
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
135
6.6 Discussion
6.6.1 Evolution of REE-bearing minerals during intense weathering
The high deficiencies of REE relative to the parent rock in weathered subsurface
regolith from the three profiles studied, especially in the GE profile, are likely to be the
result of the characteristics of REE inherited from the parent granitoids and the
weathering environment.
Easily-weathered LREE-rich minerals in the parent granitoids, such as allanite and
fluorocarbonate (only observed in MQ granitoids), can quickly break down by
hydrolysis and dissolution during the early stages of weathering. The absence of
fluoroapatite in the MQ regolith suggests its dissolution during weathering. The REE
liberated from dissolving minerals can either be partially leached away in solutions
(Braun et al., 1990), or precipitated in the form of secondary mineral phases (Braun et
al., 1998; Lottermoser, 1990; Nedachi et al., 2005). In the present study, strong
depletion of REE in the GE regolith (Figure 6.1b) suggests leaching of REE during
intense weathering, whereas secondary rhabdophane observed in the B horizon of MQ
regolith (Figure 6.9) reflects the retention of REE by formation of secondary phosphate
minerals. The PO43-
released by dissolution of fluoroapatite plays an important role in
the formation of secondary LREE-rich phosphates according to the following reaction:
Ca5(PO4)3F + 3LREE3+
+ 3H2O → 3(LREE)PO4∙H2O + 5Ca2+
+ F-
(fluoroapatite → rhabdophane)
The breakdown of primary igneous minerals and precipitation of secondary minerals not
only changes the abundance, distribution and fractionation of REE; the growth of
secondary REE-bearing phosphates also constrains further mobility of REE (Braun et
al., 1993). At the same time, weathering-resistant minerals, e.g. monazite, zircon and
ilmenite, become residually enriched in the regolith (Table 6.4). As weathering
intensifies and lateritization proceeds, some REE-bearing weathering-resistant minerals,
e.g. monazite and ilmenite, may be further altered or eroded (Taunton et al., 2000a).
High concentrations of Th and U determined in monazite crystals (Table 6.2 & Table
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
136
6.3) can lead to a disordering of the structure due to self-irradiation and persistent
intensive weathering may enhance the dissolution rate. Weathering and dissolution of
phosphate minerals such as apatite and monazite have important effects on REE budget
in weathered regolith (Aubert et al., 2001). Residual monazite, secondary rhabdophane,
zircon and ilmenite have been shown to be the most important REE-bearing mineral
phases controlling REE concentrations in weathered MQ regolith (Table 6.4).
In addition to weathering-resistant minerals and secondary phosphate minerals, REE
can also be retained in the regolith by sorption to Al- and/or Fe- oxides or organic
matter (Coppin et al., 2002; Cullers et al., 1987; Land et al., 1999; Laveuf and Cornu,
2009; Piasecki and Sverjensky, 2008; Quinn et al., 2006; Sonke and Salters, 2006).
Sorption of REE onto clay minerals was identified by sequential extraction experiments
(Table 6.5): minor concentrations of REE (2.2 ppm ΣREE in the GE saprolite and
13.2 ppm ΣREE in the MQ I C horizon) were found in the WAE species, and the ΣREE
percentage in WAE increased from the C to B horizon in the MQ I profile but decreased
from saprolite to duricrust in the GE profile, both in agreement with the change of the
clay weight proportion in each profile. In addition, 1.0 ppm ΣREE were determined in
the FeAm extraction in the duricrust of GE profile and 3.7 ppm ΣREE determined in B
horizon regolith (MQ5, 1.1 m depth) of MQ I profile (Table 6.5). This suggests that
trace to minor amounts of REE may be conserved in the ferruginous regolith by
sorption onto or coprecipitation with Fe oxyhydroxides.
In addition, the surface regolith in both MQ profiles (0.2-0.5 m depth in MQ I profile
and 0.08 m depth in MQ II profile) accumulated REE at concentrations higher than in
the parent granitoid (Table 6.1); however, the reason for this is unclear. This surface
regolith is thought to include transported materials, so enrichment of REE by lateral
transportation under the influence of soil creep or colluviation has been considered.
However, REE concentrations in most of the weathered subsurface regolith are lower
than parent granitoids, thus transported materials from upper slope is expected to show
similar REE depletion; and given the likely dilation of surface regolith, concentrations
higher than in the parent granitoids cannot be explained solely by lateral transportation.
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
137
Therefore, another mechanism is therefore required, possibly biogeochemical recycling.
Enrichment of La, Ce, Sm, and Th in outer bark and, to a lesser extent, in needles and
twigs of pine trees was reported in southeast of Australia (Arne et al., 1999). In addition,
some ferns, mosses and lichens are also known to accumulate REE (Chiarenzelli et al.,
2001; Tyler, 2004). Further research is required to determine the process, or processes,
causing the accumulation of REE in surface regolith of MQ profiles.
6.6.2 Reason for stronger depletion of LREE over HREE
Subsurface matrix and gravel from the GE and MQ II profile had (La/Yb)PR < 1.0,
coupled with a high deficiency of REE relative to the parent granitoids, showing
stronger depletion of LREE than HREE; subsurface matrix and gravel from the MQ I
profile, however, had varied (La/Yb)PR, reflecting that the fractionation of REE
controlled by grain size was more important in MQ regolith during weathering.
In the GE profiles, stronger depletion of LREE than HREE is believed to result from
extreme weathering conditions. Breakdown of LREE-rich allanite induced LREE
releasing into solution; although REE released may partially precipitate as secondary
mineral rhabdophane and/or florencite, persistent extreme weathering conditions may
further enhance the REE depletion, especially LREE, due to the susceptibility of
LREE-rich phosphates. This is supported by the previous studies: A dissolution
experiment of REE in granitoids by Harlavan and Erel (2002) showed that the
dissolution of allanite dominated the release of REE and weakened the LREE
enrichment in the first stage of weathering. Secondary REE-bearing phosphates
(including rhabdophane and florencite) could be dissolved by organic complexation
and/or uptake by biological cells (Taunton et al., 2000b), especially in the upper profile.
Although the MQ II regolith was less weathered than the GE profile, the relatively
shallow regolith depth (2.0 m depth) may facilitate weathering of the phosphates and
this is supported by the dissolution of phosphates observed by the SEM-BSE images.
The stability of weathering-resistant minerals, e.g. zircon, may enhance the retention of
HREE, especially in highly weathered environments, such as the GE profile.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
138
The C horizon regolith (MQ10) from the MQ II profile is taken as an example to
estimate the residual accumulation of HREE. The concentration of Zr in the bulk sample
was 136.3 ppm (calculated by the weight percentage of gravel and matrix) and thus the
weight percentage of zircon is 0.028% (assuming all Zr is contained by zircon). Given
the average composition of La (0.05%) and Yb (0.22%) in zircon grains in MQ regolith
determined by EPMA, the zircon hosts ca. 0.14 ppm La and ca. 0.63 ppm Yb.
Compared with 9.2 ppm La and 0.58 ppm Yb in this bulk regolith (calculated by the
weight percentage of gravel and matrix), it suggests that zircon dominates the
abundance of Yb in this regolith sample.
In addition to the different stabilities of LREE- and HREE-bearing minerals, pH may
influence REE mobility during weathering as well (Henderson, 1984). REE are well
known to be more easily leached from regolith in acidic conditions than under neutral or
alkaline conditions (Brown et al., 1955; Henderson, 1984; Nesbitt, 1979) and
preferential enrichment of HREE in the lateritic regolith is widely reported to be due to
higher stability of HREE carbonate complexes compared with LREE (Braun et al., 1993;
Braun et al., 1990; Koppi et al., 1996; Laveuf et al., 2008; Nesbitt, 1979).
However, when the pH ranges from 4.6-6.1, (as this study 4.8-6.1 in GE regolith and
4.5-5.6 in MQ regolith), REE can occur as Ln3+
, LnH2PO42+
, LnSO4+, LnF
2+ and/or
LnCO3+, depending on the presence and concentrations of the complex ligands (Wood,
1990). The complex LnCO3+ preferentially enriched in LREE over HREE, whereas
Ln(CO3)2-
is HREE selective (Cantrell and Byrne, 1987), and highly pH-dependent
(Wood, 1990); at pH<6 the predominant REE species will be the Ln3+
ion (Wood, 1990).
Fluoride complexes tend to favour HREE rather than LREE (Wood, 1990); however,
due to their low concentrations and the relatively acidic weathering conditions, fluoride
complexes would not be expected to have been important carriers in the fractionation of
REE during weathering. Although the pH of regolith cannot represent the pH conditions
when weathering took place, it may act as a clue for the imprint of pH on the signature
and fractionation of REE.
Therefore, stronger depletion of LREE over HREE is believed to be a combined
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
139
function of weathering of LREE-rich minerals and residual accumulation of HREE-rich
minerals in the regolith under low pH leaching conditions and persistent moderate to
extreme weathering conditions.
6.6.3 Fractionation of REE in weathered regolith
The differences in weathering rates of REE-bearing minerals, and in the stability of
complexes between LREE, MREE and HREE during weathering, result in REE
fractionation in weathered regolith. Preferential breakdown of LREE-rich accessory
minerals reveals that fractionation of REE starts at the early stage of weathering.
Residual accumulation of weathering-resistant HREE-rich minerals e.g. zircon and
ilmenite may lead to (La/Yb)PR < 1.0 in regolith, especially in intensely weathered
profiles, e.g. the GE regolith and the upper B horizon regolith in both MQ profiles with
high CIA. In the MQ I profile, gravel had (La/Yb)PR < 1.0 whereas matrix had
(La/Yb)PR > 1.0 in the lower part; this may result from the dilution effect of quartz in
gravel (gravel had higher concentration of Si than matrix) and the preferential
enrichment of secondary phosphates in the matrix during the early stages of weathering.
Mineral phases controlling the abundance and fractionation of REE in intensely
weathered regolith is revealed by the sequential extraction experiment (Table 6.5): Res
species hosted >90% REE in saprolite and duricrust of the GE profile, and ca. 86% and
80% REE in the C and A horizon of the MQ I profile. The abundance and stability of
LREE-rich minerals and HREE-rich minerals, however, are greatly affected by the
weathering conditions (e.g. weathering time and intensity). This might be the reason
why the depth profile of (La/Yb)PR has a negative correlation with weathering intensity
in the GE profile which had undergone persistent extreme weathering (Figure 6.4).
6.6.4 Ce and Eu anomaly
In the GE profile, Ce had a positive anomaly in the duricrust (Ce*=6.1) compared with
the parent granitoid, reflecting that Ce fractionated from the other REE and was
enriched in the duricrust. Sequential extraction showed that 0.7 ppm Ce was associated
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
140
with FeAm and 0.63 ppm Ce with FeCry species (Table 6.5), suggesting that Ce likely
precipitated with Fe oxides and/or adsorbed onto Fe oxyhydroxides during formation of
either cerianite (CeO2) under oxidised conditions (Angelica and Dacosta, 1993; Braun et
al., 1990; Braun et al., 1998), or insoluble Ce-rich phosphate when pH changed (Bau,
1999). The formation of cerianite and valence change of Ce can be the result of redox
change which may be related to seasonal fluctuation of the water table under alternating
arid and wet periods (Braun et al., 1993; Ji et al., 2004). This process is consistent with
total iron enrichment and high concentrations of Fe in FeAm (1272 ppm) and FeCry
(5329 ppm) extractions in the duricrust (Table 6.5). In contrast to previous studies, this
occurrence of the positive Ce anomaly is not relevant to reduction and oxidation of Mn
(Duzgoren-Aydin and Aydin, 2009) because the concentration of Mn was very low
(Table 6.5) in both the FeAm (15 ppm) and FeCry (2 ppm), especially when compared to
Fe in the duricrust.
In both MQ profiles, no apparent Ce anomalies were observed in secondary phosphates
(Ce* ranged from 1.0-1.1) or regolith samples (1.0-1.3) compared with the parent
granitoid. Thus Ce is believed to occur mainly in a trivalent form and hence is not
fractionated from the other REE during weathering of the MQ regolith. The MQ
regolith is still in the weak lateritization stage and a lack of persistent intensive leaching
and/or changes in redox environment result in no apparent Ce anomaly.
In addition, weathering of feldspar and sphene may result in negative Eu anomalies in
the saprolite (Bea, 1996; Condie et al., 1995; Panahi et al., 2000); breakdown of
REE-rich fluorocarbonates, which had weak positive Eu anomalies (Eu* ranged 1.0-1.4),
possibly induces negative Eu anomalies as well in the MQ regolith. Due to higher
solubility, Eu (II) will be more easily leached than the other trivalent REE from the
minerals rich in Eu and depleted in the regolith (Van der Weijden and Van der Weijden,
1995).
6.6.5 Grain size fractionation and chemical speciation of REE
In the three profiles studied, REE were more enriched in matrix than gravel, except in
Chapter Six: Distribution and fractionation of REE in intensely weathered lateritic profiles in WA
141
the A horizon of the GE profile (Table 6.1). Matrix enrichment of REE rather than
gravel suggests that cementation of clay and Fe oxides did not play a significant role in
scavenging REE from matrix during gravel formation. The higher concentrations of
REE in silt and clay fractions than the sand fraction of the MQ II profile (Figure 6.10)
may suggest: (i) the importance of secondary REE-bearing minerals in the silt fraction;
(ii) adsorption of REE by clay minerals; and (iii) a dilute effect of quartz in the sand
fraction. In addition, according to mass loading calculations (Figure 6.11), a significant
mass of REE was found in sand fraction at 1.6-2 m and 0.1-0.3 m depth, in contrast to
the large mass of REE contained in clay fraction at 0.6-1.1 m depth in MQ II profile.
This relative enrichment may be for the reason that regolith is less weathered at the
bottom than the upper part of profile, and thus more REE is contained by large
particle-size minerals such as monazite and ilmenite. As weathering proceeds and
advances, the formation of secondary minerals and sorption by clay or Fe
oxyhydroxides becomes important for retention of REE in regolith. Apparently, REE
mobilizes and partitions into different size minerals during weathering.
Significant percentages of REE associated with the WAE extraction may suggest that
some amount of REE is bio-available and occurring as free Ln3+
ion or being adsorbed
in regolith. High concentrations of REE in the WAE extraction may be also related to
relatively low pH in regolith, which favours the conversion of metals from precipitated
forms into dissolved forms (Harter, 1983). In addition, an average of 3.7% ΣLREE, 4.4%
ΣMREE and 1.8% ΣHREE were associated with FeAm in the duricrust of the GE profile,
higher than 3.0% ΣLREE, 1.4% ΣMREE and 0.5% ΣHREE in FeCry (Table 6.5),
suggesting that amorphous Fe oxyhydroxide plays a more important role for retention of
REE than crystalline Fe oxide during ferruginization. Up to 11.9% Ce, 17.1% Er and
23.5% Tm in the Org extraction, higher than the WAE, FeAm and FeCry species, at 0.5 m
depth in MQ I profile (Table 6.5) indicates that organic ligands are particularly
important for hosting REE in the upper part of profile; similar results have been
reported previously (Aubert et al., 2004; Land et al., 1999).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
142
6.7 Summary of the chapter
In this chapter, the distribution and fractionation of REE in three intensely weathered
lateritic profiles (GE, MQ I and MQ II) developed over granitoids with dolerite dykes in
Western Australia were investigated and the conclusions are as follows:
(i) The regolith of all three profiles developed from granitoid rather than dolerite were
confirmed by chondrite normalized REE distribution patterns.
(ii) High deficiencies of REE, especially LREE, were observed in the regolith of three
profiles, especially in the GE and MQ II profiles.
(iii) Preferential weathering of LREE-rich minerals including monazite, and residual
accumulation of weathering-resistant minerals such as zircon, result in a stronger
depletion of LREE than HREE under persistent intense weathering.
(iv) Mineral phases control the fractionation of REE, which greatly depends on the
abundance and stability of LREE-rich minerals (monazite and rhabdophane) and
HREE-rich minerals (zircon and ilmenite) in intensely weathered lateritic regolith.
(v) In addition to mineral phases, trace to minor amounts of REE can be retained in the
regolith by association with clay minerals, Fe oxides/oxyhydroxides and organic matter
as revealed by sequential extractions.
(vi) A positive Ce anomaly (Ce*=6.1) in the duricrust in the GE profile may result from
the redox change during formation of the duricrust.
All this information suggests that REE can be mobilized during weathering and
lateritization, even becoming highly depleted from intensely weathered lateritic regolith.
This is important when using REE as tracers for geochemical processes, especially in
extremely weathered settings.
143
7 Mode of occurrence of REE in an intensely weathered lateritic
profile in Western Australia
7.1 Abstract
The mineralogy and geochemistry of rare earth elements (REE) were studied in an
intensely weathered lateritic profile (JG) developed on meta-granitoids in Jarrahdale,
Western Australia, in order to understand the geochemical pathways of REE during
intense weathering and the effects of Fe oxides and oxyhydroxides on the mode of
occurrence of REE during advanced lateritization. Investigations were made by
combined use of bulk chemical analyses, the electron microprobe and the synchrotron
x-ray fluorescence microprobe.
Great depletion of ΣREE (ca. 94%) in the saprolite was observed, due to near-complete
dissolution of fluorocarbonates, thorite, apatite, etc., suggesting high mobility of REE
during weathering. Evidence for the translocation of REE includes the formation of
secondary phosphate minerals rhabdophane and florencite in regolith. These secondary
phosphate minerals are absent from the parent meta-granitoids and play a significant
role in trapping REE, especially LREE, released from the parent meta-granitoids during
weathering processes. Residual accumulation of the weathering-resistant minerals
zircon, ilmenite, rutile and anatase is also important for retention of REE in regolith,
especially HREE. The abundance and stability of LREE-rich secondary phosphates and
HREE-rich weathering-resistant minerals control the fractionation of REE in intense
weathered lateritic regolith.
In the ferruginous zone, Ce has fractionated from the other REE, showing a high
enrichment (Ce* up to 25 in the duricrust gravel). The occurrence of poorly crystalline
Ce (hydr)oxide phases as a rim along Fe-rich pores in the duricrust or in the boundary
of Al/Fe clay layer in iron nodules in the ferruginous zone is further evidence for this
fractionation, suggesting formation of hydrous cerianite (CeO2∙nH2O) under oxidised
conditions.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
144
In addition, trace concentrations of Yb (0.02-0.12 wt%) were determined in the iron
core and clay layer of iron nodules. Fine-grained (<10 µm) REE-bearing phosphate
crystals were precipitated with crystalline Fe phases in the duricrust, or occurred in
kaolinitic layers of iron nodules. These observations suggest that Fe phases are
important regolith components influencing the redistribution, fractionation and mode of
occurrence of REE in extremely weathered lateritic regolith.
7.2 Key words
Rare earth elements; mode of occurrence; laterite; weathering; iron nodules;
7.3 Introduction
It is well documented that REE can mobilize, redistribute and fractionate during rock
weathering (e.g. Aide and Pavick, 2002; Aubert et al., 2001; Banfield and Eggleton,
1989; Braun et al., 1993; Koppi et al., 1996; Laveuf and Cornu, 2009; Nesbitt, 1979;
Tyler, 2004). However, the precise sequence of events relating to the behaviour of REE
in rock weathering processes and pedogenesis are still not unambiguously understood,
especially in iron nodules/duricrust formed in intensely weathered lateritic profiles,
defined by high concentrations of Fe oxides and oxyhydroxides.
Differences in the weathering rates of REE-bearing minerals and the complex stability
of REE during weathering processes result in REE fractionation. It is proposed that the
fractionation of REE during weathering is predominantly constrained by weathering
conditions and primary REE-bearing minerals in the system (Aubert et al., 2001; Braun
et al., 1998; Ji et al., 2004; Nesbitt, 1979). Diverging views exist on the impact of
weathering intensity on the fractionation of REE, and at which stage of weathering REE
starts to fractionate from each other. For example, Banfield and Eggleton (1989), Price
et al. (1991) and Sharma and Rajamani (2000) demonstrated that the relative
concentrations of different REE change dramatically during the initial stage of
weathering, while Middelburg et al. (1988) and Compton et al. (2003) recognized that
the migration and differentiation of REE occurs at advanced stages of weathering. In
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
145
addition, although iron oxides are known to have high surface areas thus rendering them
very efficient sinks for many cationic trace elements (Singh and Gilkes, 1992), their effects
on the translocation and fractionation of REE in lateritic regolith during weathering and
lateritization are still not well understood.
Therefore, this paper investigates the mode of occurrence and fractionation of REE in
an intensely weathered lateritic profile (JG) developed on meta-granitoid in Jarrahdale
in Western Australia. Geochemical analyses of bulk regolith, electron probe
micro-analyzer (EPMA) and synchrotron X-ray Fluorescence Microscopy (SXFM)
techniques were used for spatial characterization and quantitative analysis of the mode
of occurrence of REE in parent meta-granitoids and regolith samples.
7.4 Materials and methods
7.4.1 Analytical methods
This study was performed on a regolith profile (JG) developed over meta-granitoids in
Jarrahdale, Western Australia (for profile descriptions and sampling details, see Chapter
Three). Regolith samples were separated into two subsample groups based on grain size:
gravel (>2 mm, represented by suffix ‘g’) and matrix (<2 mm, represented by suffix
‘m’). The exception to this subdivision was the saprolite (JG1) and mottled clay (JG2-3),
which had only matrix without gravel. All gravel and matrix subsamples were oven
dried at 105 °C overnight and ground to ≤200 µm prior to fusion in order to determine
element concentrations. In addition, particle size analysis and sequential extraction
experiments were carried out, as described in detail in Chapter Eight.
Fusion beads for elemental analyses were made by mixing 0.1 g (to an accuracy of
0.1 mg) of finely ground sample or reference material with 0.7 g 12:22 Norrish flux
(lithium metaborate:lithium tetraborate) and heating in a muffle furnace at 1050 °C for
40 minutes. Duplicate fusion beads were made on 10% of samples to check
reproducibility and preparation errors. After cooling, the fusion beads were dissolved in
100 mL of 10% analytical grade HCl. The trace elements, including REE, in fusion
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
146
beads of the gravel and matrix were determined after an additional 10-fold dilution with
10 ppb Rh/Ir solution in 10 mL polypropylene tubes by inductively coupled
plasma-mass spectrometry (ICP-MS, Perkin-Elmer Elan 6000, Toronto, Canada) at the
University of Western Australia. The trace elements, including REE, in particle size
fractions and sequential extractions were analysed at Genalysis Laboratory Services,
Maddington, Western Australia. Certified international standard materials, including
stream sediment reference material STSD-2, STSD-4 (Canada Centre for Mineral and
Energy Technology, CANMET) and an in-house standard reference were prepared in
the same way as the samples and analysed together with samples to check accuracy and
precision. The variation of REE between tested values and certified values was within
10%, except Lu (variation was 19.8%, Appendix 11.6). The concentrations of REE in
gravel and matrix of regolith samples from three profiles are listed in Table 7.1.
Texture, morphology, and phase composition of individual grains were determined
using polished thin sections of air dried and resin impregnated regolith and outcrop
samples. These polished thin sections were examined using a JEOL JSM-6400 scanning
electron microscope (SEM) with a Link analytic energy dispersive spectrometer (EDS),
utilizing both secondary electron (SE) and back-scattering electron (BSE) imaging at
15kV accelerating voltage with a 3 nA beam current. Semi-quantitative modal
abundances of REE-bearing accessory minerals in parent rocks were calculated based
on SEM-BSE images of polished thin sections and chemical maps produced by EDS.
The relative volume percentages of REE-bearing minerals were calculated and selected
mineral density data (Deer et al., 1992) were used to convert volume percentage to
weight percentage. Given that the regions examined by SEM had more REE-bearing
minerals, calculated weight percentages will be overestimated. Although the accessory
mineral abundances obtained in this way are semi-quantitative, they can be used as a
basis for assessing the mineral control and fractionation of REE in the parent rock. The
chemical composition of representative REE-bearing minerals was analysed by JEOL
8530 electron probe micro-analyzer (EPMA) at 20 kV accelerating voltage and 5 nA
beam current. Software Probe for EPMA from Probe Software Inc. was used for setting
up and analysing the data. Standard reference materials for microprobe calibration were
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
147
synthetic glass 612 from the National Institute of Standards and Technology (NIST),
in-house standard synthetic REE phosphates, rutile, zircon and thorite; in addition,
standard Brazil monazite was analysed with samples for cross checking. All microscopy
analyses were conducted at the Centre for Microscopy, Characterisation and Analysis
(CMCA), University of Western Australia. The detection limit of the EPMA for
individual elements in mineral grains is presented in the Appendix 11.10. All
microscopy analyses were conducted at the Centre for Microscopy, Characterisation and
Analysis (CMCA), University of Western Australia.
Resin-impregnated polished thin-sections of iron nodules on pure quartz slides
(50.8×25.4 mm, ProSciTech) were also examined using SXFM. The elemental mapping
by SXFM equipped with a Maia detector was conducted on the XFM beamline at the
Australian Synchrotron, Melbourne, Australia. The chemical associations of Fe, Mn Ti,
Ce etc. in iron nodules were imaged by scanning the sample stage using 14.5 keV beam
energy (X-ray wavelength 0.85508 Å) with in-house Ni foils as standards. The pixel
step size was set to 2×2 μm, with a dwell time of 2 ms and beam size of 2 μm.
Post-imaging analyses and full spectra of selected regions were generated to separate
the overlapping contributions from interfering elements using Geopixe software (Ryan
et al., 2010).
7.4.2 Calculation methods
7.4.2.1 Fractionation of REE and anomalies of Ce and Eu
In order to study the fractionation of REE, three groups are identified: the light REE
(LREE; from La to Nd), the middle REE (MREE: from Sm to Ho) and the heavy REE
(HREE: from Gd to Lu) (Henderson, 1984). Regolith REE distribution patterns were
normalized to average chondrite values (Anders and Grevesse, 1989) in order to show
the fractionation of REE during lithogenical weathering; in addition, regolith REE
distribution patterns were compared with the parent rock (PR) in order to reveal relative
enrichment and depletion. The normalized ratios (La/Sm)PR and (La/Yb)PR were used
for identifying fractionations between LREE-MREE and LREE-HREE using the
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
148
composition of the parent rocks as a reference. Cerium and Eu anomalies were
calculated using the following equations (subscript PR refers to parent rock):
Ce*=(Ce/CePR)/[(La/LaPR)
0.5×(Pr/PrPR)
0.5] (1)
Eu*=(Eu/EuPR)/[(Sm/SmPR)
0.5×(Gd/GdPR)
0.5] (2)
7.4.2.2 Weathering intensity-Chemical Index of Alteration (CIA)
The Chemical Index of Alteration (CIA) (Nesbitt and Young, 1982) was used as a
quantitative estimate of the intensity of chemical weathering. The CIA calculates loss of
mobile elements relative to Al in bulk samples, providing a single parameter estimate of
the intensity of chemical weathering. The formula (Nesbitt and Young, 1982) is:
CIA=100×Al2O3/(Al2O3+CaO*+Na2O+K2O) (molar basis) (3)
Where CaO* is CaO associated with the silicate fraction of samples (excludes
carbonates). The CIA values of regolith samples were listed in Table 4.1 in Chapter
Four.
7.4.2.3 Mass balance calculation
A geochemical mass balance calculation (Brimhall et al., 1991) was calculated to assess
the absolute loss or gain of REE during weathering. The formula for normalized
concentration (τi,j) in Equation (4) assumes that an immobile element (e.g. Zr) behaves
conservatively during weathering and pedogenesis.
1))((,
,
,
,
, pj
wj
wi
pi
C
C
C
C
ji
(4)
In Equation (4), C represents concentration, i represents the immobile element, j
represents the element of interest, w represents weathered material and p identifies
parent rock. If τi,j = 0, the element j has behaved conservatively at the sampling scale; if
τi,j = −1, the element j has been depleted completely during weathering; positive τi,j
values signify absolute enrichment.
Equation (4) provides a tool for estimating elemental loss or gain within a profile;
however, mass balance equations have two critical assumptions: a genetic relationship
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
149
between regolith and the underlying rock and a fully conserved reference element.
Although the mobility of Ti, Zr and Th is subject to debate (Braun et al., 1993; Cornu et
al., 1999), Zr is considered conservative at the sampling scale in this study (see Chapter
Five) and is used as the reference element to calculate the mass flux.
7.5 Results
7.5.1 Bulk geochemical data of REE
7.5.1.1 REE concentration and normalized pattern
The concentrations of REE and derived relevant fractionation parameters are presented
in Table 7.1. The parent meta-granitoids contained an average of 102 ppm ΣREE (sum
of concentrations of REE), lower than the average upper continental crust (UCC, 146
ppm, McLennan, 1995).
The A horizon regolith had an average of 65.2 ppm ΣREE in gravel and 58.6 ppm in
matrix. The duricrust contained the highest ΣREE, 236 ppm in gravel and 50.2 ppm in
matrix, and the saprolite contained the lowest (6.45 ppm ΣREE). Yttrium is not a REE
but showed similar behaviour to La, and was more enriched in matrix than gravel,
whereas other REE from Ce to Er were more enriched in gravel than matrix. The ΣREE
decreased with depth in regolith matrix from duricrust (JG5m, 3 m depth,
ΣREE 50.2 ppm) to saprolite (JG1, 10 m depth, ΣREE 6.45 ppm), but were significantly
enriched in gravel from the duricrust (JG5g, 3 m depth, ΣREE 236 ppm, Ce*=25.3) and
ferruginous mottled zone (JG4g, 5 m depth, ΣREE 117 ppm, Ce*=13.9). The saprolite
(JG1) had the most depleted REE, especially LREE and MREE. The chondrite
normalized REE distribution patterns of regolith samples (Figure 7.1) showed a
similarity to the underlying meta-granitoids, suggesting inheritance from the protolith,
and thus the average concentrations of meta-granitoids JGPR1 and JGPR2 were used as
the parent rock concentrations. Although the chondrite-normalized REE distribution
patterns of regolith samples still showed LREE-enrichment, the regolith REE
distribution patterns normalized by the parent meta-granitoid (Figure 7.1) showed
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
150
HREE enrichment, especially the saprolite, suggesting stronger depletion of LREE and
MREE than HREE in this zone.
7.5.1.2 Fractionation of REE during intense weathering
In the JG profile, the LREE/MREE ratio (La/Sm)PR and the LREE/HREE ratio
(La/Yb)PR are plotted versus depth and the La concentration to illustrate the
fractionation of REE with spatial distribution and degrees of depletion (Figure 7.2 &
Figure 7.3).
The saprolite, matrix of ferruginous zone and A horizon regolith showed higher
depletion of LREE than MREE with (La/Sm)PR below 0.8, whereas in the mottled clay
and gravel of ferruginous mottled zone and duricrust, (La/Sm)PR ranged from 1.2-1.4,
suggesting stronger depletion of MREE than LREE and a local redistribution of La and
Sm between gravel and matrix in ferruginous mottled zone and duricrust.
Apart from the upper mottled clay, matrix and gravel from other regolith had (La/Yb)PR
below 0.8, showing stronger depletion of LREE than HREE. In the upper mottled clay
(JG3, 6.5 m depth), the (La/Yb)PR was 1.2, indicating a more severe depletion of HREE
than LREE in this horizon.
The saprolite (10 m depth) showed the strongest depletion of La than other regolith and
thus had significant fractionation of LREE from MREE ((La/Sm)PR = 0.4) and HREE
((La/Yb)PR = 0.2), although it had undergone moderate weathering (CIA = 0.65).
In addition, most regolith samples had negative Eu anomalies (Eu*<1) except the
saprolite (JG1, Eu*=1.0, Table 7.1). The anomaly increased from the saprolite to the
lowest value (0.6) in ferruginous regolith.
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
151
Figure 7.1 REE distribution patterns of (a) meta-granitoids and regolith samples
normalized by the average chondrite composition; and (b) regolith samples normalized
by the parent meta-granitoid in the JG profile; (JGPR2-meta-granitoid; JG10-A horizon,
0.4 m depth; JG6-upper ferruginous zone, 1.5 m depth; JG5-duricrust, 3.0 m depth;
JG3-upper mottled clay, 6.5 m depth; JG1-saprolite, 10 m depth; ‘g’ denotes gravel and
‘m’ denotes matrix).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
152
Figure 7.2 Normalized ratios (La/Sm)PR (LREE/MREE) and (La/Yb)PR (MREE/HREE)
of regolith samples against depth in the JG profile (dashed vertical line at 1.0 shows no
fractionation relative to parent rock).
Figure 7.3 Plots of (La/Sm)PR and (La/Yb)PR vs. La for the JG profile, illustrating the
degrees of depletion and fractionation of REE.
0
2
4
6
8
10
12
0.0 0.5 1.0 1.5(La/Sm)PR
de
pth
(m
)0
2
4
6
8
10
12
0.0 0.5 1.0 1.5(La/Yb)PR
de
pth
(m
)
matrix gravel
(a) (b)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0 5 10 15 20 25 30 35
La (ppm)
(La
/Sm
) PR
JG8g
JG2
JG9m
JG10gJG1
JG3
JG5gJG4g
JG5m
JG4m
JG6m JG8m
JG7gLa more depleted than Sm
JG7m
JG6g
JG10mJG9g
REE fractionation
increases
La depletion increases
JGPR1
JGPR2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 5 10 15 20 25 30 35
La (ppm)
(La
/Yb
) PR
JG9g
JG7gJG6m
JG9mJG6g
JG1
JG3
JG5g
JG2
JG5mJG4m JG7m
JG8m
JG4g
JG8g
JG10g
JG10m
La less depleted than Yb
REE fractionation
increases
La more depleted than Yb
La depletion increases
JGPR2
JGPR1
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
153
7.5.1.3 Variation of REE with lateritization degree
The variation of REE and derived fractionation parameters in the JG profile are
compared with a second index of weathering intensity by using the concentration ratio
SiO2/(SiO2+Al2O3+Fe2O3) (S/SAF) (Hill et al., 2000) and with the degree of
lateritization by using SiO2-Al2O3-Fe2O3 ternary plots (Schellmann, 1981) (Figure 7.4).
Gravel was more enriched in Sm (MREE) and Yb (HREE) than matrix; however, the
concentration of La (LREE) in gravel was similar to matrix; initial scatter of (La/Sm)PR
and (La/Yb)PR suggested that fractionation of REE occurred at the early (weak) stage of
lateritization. (Y/Ho)PR was relatively consistent; however, (Y/Ho)PR in gravel with
strong lateritization was lower than other regolith with lower degrees of lateritization,
which may suggest fractionation of Y and Ho at advanced stages of weathering and
lateritization.
15
4
Table 7.1 Concentrations of REE and derived fractionation parameters in parent meta-granitoids and lateritic regolith from the JG profile
Sample1 unit JG7g JG7m JG8g JG8m JG9g JG9m JG10g JG10m JG6g JG6m JG5g JG5m JG4g JG4m JG3 JG2 JG1 JGPR1 JGPR2 RSD
2
CIA % 99.2 99.1 98.7 92.9 99.3 92.7 99.3 92.2 99.4 98.7 98.6 98.1 95.4 95.3 94.1 86.2 64.7 47.0 47.0
Depth m 0.02 0.15 0.3 0.4 1.5 3.0 5.0 6.5 8.6 10.0
Proportion3 % 0.24 0.76 0.60 0.40 0.51 0.49 0.69 0.31 0.82 0.18 0.47 0.53 0.40 0.60 W
4 W W W W
La ppm 10.1 14.3 12.0 14.9 11.8 12.1 8.52 9.52 7.53 7.78 6.02 6.08 4.98 7.76 10.5 5.22 1.31 25.2 30.6 0.0-0.4
Ce ppm 32.0 28.7 32.3 29.6 34.4 26.4 30.9 21.8 27.0 28.8 224 37.2 107 19.0 14.8 7.48 2.38 44.4 50.4 0.0-0.4
Pr ppm 2.43 2.54 3.07 2.65 2.95 2.24 2.16 1.87 1.72 1.56 0.70 0.89 0.63 1.13 1.26 0.58 0.24 4.14 4.48 0.0-0.1
Nd ppm 9.52 9.16 11.9 10.5 11.5 8.14 8.57 6.72 6.52 5.66 2.35 3.02 1.94 4.27 3.61 1.67 1.09 13.8 14.2 0.0-0.3
Sm ppm 1.96 1.76 2.46 2.00 2.34 1.59 1.80 1.61 1.42 1.06 0.37 0.57 0.31 0.90 0.56 0.31 0.26 2.12 2.05 0.0-0.1
Eu ppm 0.37 0.30 0.46 0.34 0.42 0.31 0.34 0.23 0.28 0.20 0.09 0.10 0.08 0.15 0.12 0.06 0.07 0.42 0.59 0.0
Gd ppm 1.79 1.88 2.06 1.68 2.08 1.50 1.78 1.47 1.41 1.27 1.59 0.67 0.69 0.73 0.54 0.22 0.29 1.75 1.93 0.0-0.1
Tb ppm 0.29 0.27 0.36 0.26 0.34 0.24 0.25 0.25 0.22 0.21 0.08 0.07 0.04 0.12 0.07 0.05 0.03 0.24 0.21 0.0
Dy ppm 1.54 1.58 1.89 1.75 1.75 1.48 1.42 1.26 1.40 1.20 0.38 0.52 0.39 0.70 0.41 0.27 0.19 0.97 0.95 0.0-0.1
Ho ppm 0.33 0.42 0.38 0.35 0.39 0.34 0.31 0.30 0.29 0.27 0.07 0.12 0.08 0.15 0.11 0.07 0.05 0.27 0.23 0.0
Er ppm 0.99 1.03 1.04 0.98 1.06 1.00 0.84 0.86 0.89 0.90 0.23 0.32 0.24 0.56 0.30 0.32 0.19 0.93 0.65 0.0-0.1
Tm ppm 0.15 0.18 0.16 0.18 0.16 0.18 0.14 0.17 0.15 0.18 0.05 0.07 0.04 0.10 0.06 0.06 0.04 0.17 0.14 0.0
Yb ppm 1.06 1.21 1.16 1.23 1.11 1.21 0.97 1.20 1.02 1.10 0.33 0.54 0.35 0.74 0.36 0.51 0.26 1.28 0.93 0.0-0.1
Lu ppm 0.18 0.21 0.20 0.20 0.19 0.19 0.17 0.22 0.18 0.20 0.06 0.08 0.06 0.16 0.09 0.09 0.07 0.26 0.21 0.0
ΣREE ppm 62.6 63.5 69.4 66.6 70.5 56.8 58.2 47.4 50.0 50.4 236 50.2 117 36.5 32.8 16.9 6.45 95.9 108
(La/Sm)PR5 0.38 0.61 0.37 0.56 0.38 0.57 0.35 0.44 0.40 0.55 1.21 0.80 1.21 0.64 1.40 1.25 0.37
(La/Yb)PR 0.38 0.47 0.41 0.48 0.42 0.39 0.35 0.32 0.29 0.28 0.74 0.45 0.56 0.42 1.16 0.41 0.20
15
5
Sample1 unit JG7g JG7m JG8g JG8m JG9g JG9m JG10g JG10m JG6g JG6m JG5g JG5m JG4g JG4m JG3 JG2 JG1 JGPR1 JGPR2 RSD
2
6Ce
* 1.50 1.10 1.23 1.09 1.35 1.17 1.67 1.19 1.73 1.91 25.3 3.69 13.9 1.49 0.94 0.99 0.98
Eu* 0.76 0.64 0.80 0.73 0.74 0.78 0.73 0.58 0.76 0.68 0.43 0.63 0.63 0.71 0.81 0.87 0.95
Y ppm 6.98 11.56 7.76 10.3 7.55 9.77 5.81 9.27 6.42 7.57 2.03 3.20 2.07 4.50 2.70 2.13 1.34 9.72 7.27 0.0-0.2
Th ppm 122 22.0 121 21.9 138 22.3 138 23.9 94.0 37.6 196 45.0 119 35.6 48.0 167 31.0 16.4 18.5 0.2-3.3
Zr ppm 348.3 471.7 353.9 425.0 353.6 506.8 346.1 490.1 349.3 444.7 348.9 291.4 292.0 481.7 341.4 164.0 105.4 159.7 158.8
Hf ppm 8.67 13.2 11.2 11.4 9.95 13.6 10.6 12.3 9.84 12.8 11.0 8.13 7.58 12.6 10.6 4.87 2.81 4.12 5.45
Ti7 ppm 7440 7441 8857 6471 7668 6875 6658 6653 9927 3772 3336 4434 3695 5248 3765 1678 1050 1300 1267
1suffix ‘g’ represents gravel and suffix ‘m’ represents matrix;
2RSD is the range of relative standard deviations of the duplicates/triplicates analysed by ICP-MS;
3proportion refers to weight proportion of matrix and gravel;
4W refers to whole rock analysis;
5Suffix PR refers to the parent meta-granitoids: arithmetic means of concentrations from JGPR1 and JGPR2 were used;
6Ce
*=(Ce/CePR)/[(La/LaPR)
0.5× (Pr/PrPR)
0.5]; Eu
*=(Eu/EuPR)/[(Sm/SmPR)
0.5× (Gd/GdPR)
0.5];
7Concentrations of Ti, Zr and Hf were determined by ICP-OES (details see Chapter Five).
15
6
Figure 7.4 SiO2-Al2O3-Fe2O3 ternary plots and associated variation of REE concentrations and ratios against the S/SAF weathering index for the JG
profile.
15
7
Table 7.2 Element concentrations from EPMA analyses of representative minerals in parent meta-granitoids (Figure 7.6) of the JG profile
No. 3 4 57 58 2 59 5 6 60 61 62 1 63
Min. Fc Fc Fc Fc Thr Ap Ilm Spn Fsp Mag Mag Zrn Zrn
Si 0.77 5.61 2.20 3.09 11.3 0.01 b.d. 13.9 18.4 0.02 0.25 15.2 15.2
Zr b.d. 0.27 b.d. 0.16 9.79 b.d. b.d. b.d. b.d. b.d. b.d. 45.4 47.1
Ti 0.02 0.09 0.03 0.08 0.76 b.d. 32.2 22.2 0.01 0.05 b.d. 0.04 b.d.
Pb b.d. 0.08 0.23 0.19 0.46 b.d. b.d. b.d. b.d. 0.04 b.d. 0.07 0.11
Th 2.06 2.31 0.64 6.44 32.9 b.d. b.d. b.d. b.d. b.d. 0.02 0.10 0.08
U 0.09 0.23 0.18 0.20 7.00 b.d. 0.02 b.d. b.d. 0.03 0.04 0.22 0.33
Al 0.48 1.26 0.44 0.94 0.35 b.d. b.d. 1.13 12.1 0.02 0.02 0.03 b.d.
Y 0.04 0.08 0.06 0.10 1.17 0.02 b.d. b.d. b.d. b.d. b.d. 0.13 0.13
La 20.0 16.2 20.8 16.3 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Ce 30.6 23.4 27.7 24.2 1.83 0.06 b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Pr 2.51 1.90 2.85 2.52 0.26 0.04 b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Nd 5.42 4.33 5.97 5.30 1.13 b.d. b.d. b.d. b.d. b.d. b.d. 0.11 b.d.
Sm 0.54 0.46 0.52 0.55 0.45 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Eu 0.19 0.14 0.06 0.08 0.05 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Gd 0.22 0.15 0.27 0.28 0.39 b.d. b.d. b.d. b.d. b.d. b.d. 0.02 b.d.
Dy b.d. b.d. b.d. b.d. 0.17 b.d. b.d. b.d. b.d. b.d. b.d. 0.03 b.d.
Yb 0.06 0.04 b.d. 0.07 0.25 0.05 0.06 b.d. 0.05 0.04 0.18 0.13 b.d.
Lu b.d. b.d. b.d. b.d. 0.04 b.d. b.d. b.d. b.d. b.d. 0.04 0.03 b.d.
Fe 1.40 2.90 2.96 3.57 0.93 0.02 34.1 0.42 8.59 71.9 72.1 0.64 0.31
Ca 4.90 5.26 2.89 2.68 0.32 38.4 0.20 19.1 15.5 0.02 0.06 0.08 0.01
Sr b.d. b.d. b.d. b.d. 0.01 0.02 b.d. 0.08 0.18 b.d. 2.24 0.39 0.44
K b.d. 0.04 0.04 0.01 b.d. b.d. b.d. b.d. b.d. b.d. 0.03 0.01 b.d.
P 0.02 0.02 0.02 0.05 0.50 17.6 b.d. 0.01 b.d. b.d. 0.02 b.d. b.d.
F 7.09 10.6 6.31 6.77 1.19 4.54 b.d. 0.50 b.d. b.d. 0.45 b.d. b.d.
O 11.6 14.9 12.9 13.5 24.4 36.3 31.5 39.2 40.4 20.7 21.3 33.7 34.1
total 88.5 90.7 87.5 87.4 95.9 97.1 98.2 96.6 95.3 92.9 96.8 96.3 97.8
Tb, Ho and Na were below detection limits (b.d.); Fc: REE-rich fluorocarbonate; Thr: thorite; Ap: apatite; Ilm: ilmenite; Spn: sphene; Fsp: feldspar; Mag: magnetite; Zrn: zircon.
15
8
Table 7.3 Element concentrations from EPMA analyses of REE-bearing phosphates in lateritic regolith (Figure 7.8) of the JG profile
No. 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79
Min Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Rbp Flo d.l.1
Si 0.96 0.72 0.08 1.04 0.74 0.66 1.20 3.34 0.05 0.48 0.32 0.74 0.15 0.03 b.d. 0.46 0.01
Pb 0.46 0.81 0.54 1.21 0.22 0.20 0.26 0.01 0.09 0.91 0.27 1.40 0.15 0.75 0.17 0.13 0.02
Th 9.26 6.04 3.93 7.94 8.94 7.82 10.0 3.24 2.22 5.29 3.50 9.84 4.19 1.91 2.18 3.05 0.02
U 0.16 0.07 0.90 0.19 0.14 0.14 0.15 0.06 0.15 0.31 0.47 0.13 0.58 0.98 0.30 0.02 0.02
Al 0.03 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.10 b.d. 0.24 0.82 b.d. b.d. 18.5 0.01
Y 0.17 0.27 1.61 2.39 0.13 0.81 0.64 0.10 0.79 1.44 0.06 0.43 1.45 1.44 3.02 0.23 0.02
La 12.1 16.4 11.8 10.5 10.1 12.5 12.6 13.0 13.8 14.1 14.1 12.7 11.7 13.6 11.8 8.19 0.04
Ce 23.8 25.5 23.2 21.6 23.0 24.0 23.7 25.0 26.9 24.2 26.6 22.4 22.9 24.4 24.3 16.8 0.04
Pr 2.47 2.25 2.55 2.61 2.75 2.49 2.38 2.72 2.91 2.34 2.73 2.13 2.45 2.42 2.56 1.90 0.04
Nd 8.50 6.57 9.07 8.96 10.7 8.49 8.12 9.08 9.78 7.40 9.23 6.51 8.46 8.48 8.81 1.74 0.04
Sm 1.17 0.76 1.58 1.70 1.80 1.36 1.21 2.07 1.42 1.09 1.29 0.90 1.96 1.47 1.55 0.59 0.02
Eu 0.17 0.15 0.21 0.15 0.18 0.15 0.15 0.17 0.33 0.16 0.15 0.12 0.32 0.35 0.18 0.16 0.01
Gd 0.57 0.34 1.05 0.65 0.57 0.60 0.58 1.07 0.75 0.93 0.89 0.71 1.53 1.36 1.54 0.48 0.02
Dy b.d. b.d. 0.45 0.54 b.d. b.d. b.d. b.d. 0.24 0.27 b.d. b.d. 0.52 0.39 0.74 0.04 0.02
Ho b.d. b.d. b.d. 0.39 b.d. b.d. b.d. 0.42 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.02
Yb 0.14 0.13 0.35 0.35 0.13 0.14 0.14 0.13 0.17 0.25 0.07 0.09 0.33 0.35 0.34 0.14 0.02
Lu b.d. b.d. 0.03 0.04 b.d. b.d. b.d. b.d. b.d. 0.04 b.d. b.d. b.d. 0.04 0.10 b.d. 0.02
Fe 0.59 0.46 0.45 0.60 0.45 b.d. b.d. b.d. 0.09 0.77 0.92 1.08 0.45 0.33 0.49 0.47 0.01
Ca 0.65 0.30 0.24 0.50 0.25 0.63 0.50 0.04 0.12 0.09 0.34 0.92 0.13 0.01 b.d. 0.05 0.01
P 12.2 12.1 12.9 12.3 12.8 12.7 12.3 8.02 13.6 12.5 12.9 12.3 12.0 13.0 13.3 7.71 0.01
S 0.01 b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.08 b.d. b.d. b.d. 0.15 b.d. 0.01 0.06 0.01
F b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 1.15 1.20 1.12 1.37 1.34 1.19 b.d. 0.09
O 26.9 26.5 26.7 27.4 27.5 27.8 27.9 24.3 27.9 26.4 26.9 26.9 25.7 26.1 26.8 31.3
total 100.3 99.4 97.7 100.9 100.3 100.5 101.8 92.8 101.4 100.2 101.9 100.7 97.2 98.8 99.4 92.1
1d.l. refers to detection limit; Tb and Tm, K, Mg and Na were below detection limit (b.d.); Rbp: rhabdophane; Flo: florencite.
15
9
Table 7.4 Element concentrations from EPMA analyses of weathering-resistant minerals in lateritic regolith of the JG profile
No. 8 80 81 7 82 9 83 84 85 86 87 88
Min Ilm Ilm Ilm Ilm Ilm TiO TiO Zrn Zrn Zrn Zrn Thr
Si b.d. 0.01 b.d. b.d. b.d. 0.25 0.16 14.8 14.6 14.3 14.4 9.47
Zr b.d. b.d. b.d. b.d. b.d. 0.38 0.17 44.4 48.0 48.9 47.3 b.d.
Ti 32.1 37.2 30.6 33.1 31.9 51.0 51.9 0.17 b.d. 0.29 0.02 0.03
Pb b.d. 0.02 b.d. b.d. b.d. 0.02 0.05 b.d. b.d. b.d. b.d. 4.07
Th b.d. 0.05 b.d. 0.01 b.d. 0.14 0.05 0.56 b.d. 0.17 b.d. 45.7
U 0.17 b.d. 0.03 b.d. 0.02 0.14 0.31 0.18 0.04 0.10 0.15 19.0
Al 0.01 b.d. b.d. b.d. b.d. 2.68 2.05 0.59 0.01 0.03 0.14 b.d.
Y b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.40 b.d. 0.47 b.d. 0.50
Ce b.d. b.d. b.d. b.d. b.d. 0.05 b.d. 0.27 0.05 0.31 b.d. b.d.
Pr b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.04 b.d. b.d. b.d. b.d.
Nd b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.21 b.d. 0.12 b.d. b.d.
Sm b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.09 b.d. 0.05 b.d. b.d.
Eu b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.03 b.d. 0.02
Gd b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.09 b.d. 0.06 b.d. 0.08
Dy b.d. 0.79 b.d. 1.04 2.18 b.d. b.d. 0.05 b.d. b.d. b.d. 0.08
Yb 0.07 b.d. 0.08 b.d. b.d. 0.02 0.03 0.14 b.d. 0.20 0.14 0.23
Lu b.d. b.d. b.d. b.d. b.d. b.d. b.d. 2.47 b.d. b.d. 0.02 b.d.
Fe 33.4 25.2 34.5 27.9 27.7 3.91 3.59 0.75 1.52 10.6 1.07 0.06
Mg 0.07 0.03 0.03 0.03 0.06 0.01 b.d. b.d. b.d. 0.02 b.d. b.d.
Ca b.d. b.d. b.d. b.d. b.d. 0.04 0.04 b.d. b.d. 0.06 b.d. b.d.
Sr b.d. b.d. b.d. b.d. b.d. b.d. b.d. 0.44 0.50 0.26 0.49 b.d.
P b.d. b.d. b.d. b.d. b.d. 0.03 0.07 b.d. b.d. b.d. b.d. 0.19
S b.d. b.d. b.d. b.d. b.d. 0.08 0.04 b.d. b.d. 0.15 b.d. b.d.
O 31.3 32.3 30.3 30.3 29.7 38.2 38.0 34.0 34.0 34.8 33.6 20.4
total 97.1 95.6 95.5 92.4 91.5 96.9 96.4 99.8 98.7 99.6 97.4 99.8
La, Tb, Ho, Er and Tm, Na and K were below detection limit (b.d.); Ilm: ilmenite; TiO: titanium oxides (rutile/anatase); Zrn: zircon; Thr: thorite.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
160
7.5.1.4 Mass balance of REE
Mass balances of REE were calculated at each sampling depth, based on weighted
average concentrations of matrix and gravel samples, using Zr as the reference element.
Depth profiles of mass fluxes of REE are plotted in Figure 7.5.
The A horizon regolith (above 0.5 m depth) had τ(Zr, La) between −0.9 and −0.8, τ(Zr, Sm)
between −0.7 and −0.5 and τ(Zr, Yb) ca. −0.6. In the ferruginous zone (1.5-5.0 m depth),
τ(Zr, La) was ca. −0.9, τ(Zr, Sm) was between −0.9 and −0.7 and τ(Zr, Yb) was between −0.8
and −0.6; τ(Zr, Ce) had a wide range: from −0.8 in the upper ferruginous zone (1.5 m
depth) increasing to 0.2 in the duricrust (3 m depth), and then decreasing to −0.4 in the
ferruginous mottled zone (5 m depth). In the lower mottled clay zone (8.6 m depth),
τ(Zr, La) was −0.8, close to τ(Zr, Sm), but lower than τ(Zr, Yb) (−0.6); In the saprolite (10 m
depth), τ(Zr, La) was −0.9, suggesting more depletion of La than Sm (τ(Zr, Sm) = −0.8) and
Yb (τ(Zr, Yb) = −0.7).
Figure 7.5 Mass balance calculations of REE against depth in the JG profile, based on
weighted average concentrations of REE in matrix and gravel, using Zr as the reference
element (vertical dashed line refers to mass balance τ(Zr,REE) = 0; Only selected REE are
plotted here, as the remaining REE have similar patterns).
0
2
4
6
8
10
-1.0 -0.5 0.0 0.5
de
pth
(m
)
τ(Zr,La)
τ(Zr,Ce)
τ(Zr,Sm)
τ(Zr,Dy)
τ(Zr,Yb)
τ(Zr,Y)
(enriched)
(depleted)
A horizon
duricrust
saprolite
mottled clay
ferruginous
mottled zone
upper
ferruginous
zone
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
161
7.5.2 Mineralogy of REE in the parent rock
In the parent meta-granitoids of the JG profile, accessory minerals contained abundant
REE (Figure 7.6 and Table 7.2). The REE concentrations of selected REE-bearing
minerals are presented in Table 7.2.
In the JG parent meta-granitoids, fluorocarbonates were the most important accessory
minerals (<0.06 wt%, Figure 7.6a, b & c), containing average 55 wt% ΣREE (103-fold
enrichment of REE above the average value of parent meta-granitoids), with an average
of 2.8 wt% Th and 0.2 wt% U (Table 7.2). Fluorocarbonates were strongly LREE
selective (Figure 7.7), with (La/Yb)PR ranging from 9.4-15.8, and occurred either as ca.
hundred-micron grain size individual crystals, or as (sub)micron grains intergrown with
other minerals (Figure 7.6a & g). Both moderate negative Eu anomalies (Eu*
average
0.7) and positive Eu anomalies (Eu* ca. 2.1) were observed in fluorocarbonates without
a significant Ce anomaly (Ce* ranged from 0.8 to 1.1). These fluorocarbonates may be
produced by reaction with late- or post-magmatic fluids, or occur as secondary
carbonates, the product of the decomposition of allanite and epidote (Bea, 1996).
Thorite (<0.04 wt%, Figure 7.6d) with partial Zr substitution was another important
accessory mineral, containing average 5.0 wt% ΣREE with a preference for HREE
((La/Yb)PR ca. 0.05), slightly positive Ce anomalies (Ce* ca. 1.4), and moderate
negative Eu anomalies (Eu* ca. 0.5). Sphene (also called titanite, <0.65 wt%, Figure
7.6e) was abundant in the JG meta-granitoids, occurring chiefly as individual crystals or
as intergrowths with ilmenite. It contained variable concentrations of ΣREE up to
0.7 wt%, and no systematic preference for LREE or HREE was observed. Zircon
(<0.03 wt%) also contained varied concentrations of ΣREE, from negligible to 0.3 wt%
with HREE predominating (La, however, was below the EPMA detection limit).
Magnetite (<1.0 wt%, Figure 7.6f) contained up to 0.2 wt% ΣREE with a preference for
HREE (La below EPMA detection limit). Apatite (<0.15 wt%, Figure 7.6g & h) had
ca. 0.15 wt% ΣREE without LREE or HREE selectivity and a moderate negative Ce
anomaly was observed (Ce*=0.4). Ilmenite (<0.23 wt%, Figure 7.6e & h) commonly
had a preference for Yb (up to 0.06 wt%). Feldspars contained up to 0.05 wt% ΣREE.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
162
7.5.3 Mode of occurrence of REE in lateritic regolith
7.5.3.1 Mineral phases containing REE in the lateritic regolith
Selected REE-bearing minerals in the duricrust and iron nodules were analysed and
mapped by electron microprobe, and the resulting images are presented in Figure 7.8.
The REE mainly existed in two types of mineral phases in the intensely weathered
regolith: secondary REE-bearing phosphates and primary weathering-resistant minerals;
both are discussed in more detail below.
Secondary REE-bearing phosphates (Figure 7.8 and Table 7.3) are important hosts for
REE in intensely weathered lateritic regolith, especially LREE, as these elements are
essential structural components in these minerals. These REE-bearing phosphates in
regolith are believed to be the secondary weathered products of fluorocarbonates,
allanite, and especially apatite, because no REE-bearing phosphates e.g. monazite, were
observed in the JG parent meta-granitoids. These secondary phosphates were identified
as rhabdophane and florencite based on EPMA analyses, and were the main form of
REE-bearing mineral phases observed in the regolith (Table 7.3), playing an important
role in trapping REE during weathering. Secondary rhabdophane and florencite are
predominantly LREE hosts, as ((La/Yb)PR ranged from 1.2-7.5. Conversely, xenotime is
significant for retaining HREE (Bea, 1996) and was observed as micron-size crystals in
the duricrust; however, this size range is below the spatial resolution of the electron
microprobe, and thus the compositional results cannot be separated from the
interference of Al, Si and Fe in the nearby clay, quartz and Fe oxides. Most of the
REE-bearing secondary phosphates are in the size range 1-10 µm and are distributed in:
(i) in the clay layers rather than the iron cores of iron nodules in the ferruginous zones
(Figure 7.8, Figure 7.9 & Figure 7.10); and (ii) in the clay matrix of iron nodules from
the A horizon (Figure 7.8). Note that iron nodules from the A horizon are different from
the iron nodules in the ferruginous zone, as they are non-concentric, having a kaolinitic
matrix cemented with Fe oxides without layering (Appendix 11.4).
These secondary rhabdophane minerals usually contained a 103-fold enrichment of
ΣREE over the average JG parent meta-granitoids. Rhabdophane lacked an apparent Ce
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
163
anomaly, but had variable Eu anomalies (Eu* ranged from 0.4-1.3), reflecting the strong
mineralogical control on the redistribution of REE in the intensely weathered lateritic
regolith, similar to Braun’s study (1993). High Th concentrations in the secondary
rhabdophane and florencite in JG regolith were also determined, consistent with high Th
concentrations in LREE-rich fluorocarbonates in meta-granitoids.
Weathering-resistant minerals (Table 7.4) such as zircon, ilmenite, rutile and anatase,
present in trace to minor concentrations in lateritic regolith, are also important hosts for
REE. The REE, especially HREE, included in these weathering-resistant minerals are
not commonly expected to be extensively mobilized during pedogenesis, although
erosion and dissolution may occur under very intense weathering (Taunton et al.,
2000a). In weathered regolith, and especially in extremely weathered lateritic regolith,
zircon is the most important of these minerals to host significant concentrations of REE.
Zircon in regolith contained up to 3.4 wt% ΣREE, with a preference for HREE or Ce
(0.05 wt%). Zircon also contained varied concentrations of Th (0-0.56 wt%) and U
(0.04-0.18 wt%). In addition, ilmenite was an important HREE-selective mineral in the
ferruginous regolith, especially for Dy and Yb, containing up to 2 wt% Dy and/or
0.08 wt% Yb whereas other REE were below detection limits. Ilmenite and rutile
concentrated Yb, containing up to 0.03 wt%. Thorite (ThSiO4), though very rare in the
lateritic regolith (only one grain was observed), contained 0.64 wt% ΣREE with a
preference for HREE.
16
4
(a) (b) (c) (d)
(e) (f) (g) (h)
Figure 7.6 Backscatter electron images of REE-bearing accessory minerals in parent meta-granitoids of the JG profile: (a) micron-size fluorocarbonate
intergrown with 100 µm-size fluorocarbonate; (b) and (c) REE-rich fluorocarbonates; (d) thorite rich in REE and Zr; (e) sphene intergrown with
ilmenite surrounded by feldspars; (f) magnetite surrounded by quartz; (g) apatite intergrown with a tiny crystal REE-bearing fluorocarbonate; (h)
apatite intergrown with ilmenite; (Qz-quartz; Ap-apatite; Ilm-ilmenite; Spn-sphene; Fsp-feldspar; Mag-magnetite).
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
165
Figure 7.7 REE distribution patterns of fluorocarbonate and thorite normalized by the
parent meta-granitoids in the JG profile (Fc: fluorocarbonate; Thr: thorite).
0.0
0.2
0.4
0.6
0.8
La Ce Pr Nd Sm Eu Gd Yb
RE
E/p
are
nt
me
ta-g
ran
ito
id
Fc
Fc
Fc
Fc
Thr
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
166
(a) (b)
(c) (d)
(e) (f)
Figure 7.8 Backscatter electron images of REE-bearing secondary phosphate minerals
in regolith samples of the JG profile: (a) and (b) secondary rhabdophane in the clay
layer of iron nodules in the ferruginous zone at 1.5m depth; (c) and (d) secondary
rhabdophane in clay matrix of iron nodules in A horizon at 0.4m depth; (e) and (f) are
secondary florencite locating in the clay layer of iron nodules in ferruginous zone at
1.5m depth; (Zrn-zircon; dark circles are secondary REE-bearing phosphates).
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
167
(a)
(i
(ii
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
168
(b)
(i
(ii
Figure 7.9 Images of REE-bearing secondary phosphates located in the clay layer of
iron nodules at 1.5m depth in the JG profile; element compositions of the phosphate are
listed in the Table 7.3: (a) labelled as the No. 63 rhabdophane; (b) labelled as the No. 64
rhabdophane; (i is backscatter image: the rectangular box indicates the area mapped by
EPMA, and the bright spot in the rectangular box is the fine-grained (<10 µm)
secondary rhabdophane; and (ii is the corresponding microprobe mapping: CP refers to
backscatter scan image, and SL refers to secondary scan image.
16
9
(a) (b) (c)
Figure 7.10 Mapping of secondary rhabdophane in iron nodule at 1.5 m depth of the JG profile. (a) backscatter image; (b) elemental mapping of the
rectangular black box in (a) by EPMA; (c) RGB post-imaging of the rectangular white box in (a), collected by SXFM with Maia 384/96 detector, using
Geopixe software with Ni foils as the in-house reference standards; mapping area 3.6×0.8 mm, scan duration 30 min, Maia run number 18745; (the
blue circles in (a) are the bright spots of Ti minerals in (b); the green circles in (a) are the green spots of Ti minerals in (c); red circle is the micron-size
secondary rhabdophane; element compositions are listed in the Table 7.3 and labelled as the No. 65 rhabdophane).
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
170
7.5.3.2 Mode of occurrence of Ce in the duricrust and iron nodules
Cerium was fractionated from the other REE and significantly enriched in iron nodules
(up to 200 ppm). Element mapping of the duricrust and iron nodules shows that, in
addition to being hosted by secondary phosphate and weathering-resistant minerals, Ce
also precipitated as poorly crystalline phases: (i) as a rim along the Fe-rich pores in the
duricrust (Figure 7.11); (ii) as a rim along the boundary between clay layers (Al-rich)
and iron layers (Fe-rich) in iron nodules (Figure 7.12); and (iii) as joint matrix between
two iron cores within one large nodule (Figure 7.13). Quantitative microprobe analysis
revealed that the concentration of Ce in the rims was up to 1.5 wt% and varied with
location, whereas most of other REE except Gd, Sm and Nd were below detection
limit (Table 7.5). However, the size range of the rims (width<1 µm) is below the spatial
resolution of the microprobe, and therefore the compositional results cannot be
separated from the interference of Al and Fe in the matrix. Despite this problem,
quantitative analysis by microprobe of selected several areas revealed that Ce did not
always exist with P or Si, and the sum of oxides was <100, likely reflecting a hydrous
Ce (hydr)oxide. In addition, a correlation between Th, Zr and Ce was also identified in
both element mapping and chemical analyses (Chapter Five).
In addition to amorphous Fe, crystalline Fe was also observed to be important for
retention of REE, including Ce. A crystalline Fe oxide in the duricrust contained
(sub)micron-size Ce-rich phosphates (Figure 7.14). This may suggest that crystalline Fe
can partially control Ce redistribution by secondary mineral intergrowth, whereas
amorphous Fe controls Ce occurrence by sorption and coprecipitation of Ce
(hydr)oxide.
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
171
(a)
(b)
Figure 7.11 Cerium fractionated from other REE and occurring as a rim along the
Al/Fe-rich pores in the duricrust; (a) is backscatter image with rectangular box
indicating the area mapped by EPMA; and (b) presents the corresponding EPMA
mapping; element concentrations of spot analysis of Ce-rich rim are listed in Table 7.5.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
172
(a)
(b)
Figure 7.12 Cerium fractionated from other REE and occurring as a rim along the
boundary between clay and iron layers in iron nodules; (a) is backscatter image with
rectangular box indicating the area mapped by EPMA; and (b) presents the
corresponding EPMA mapping; element concentrations of spot analysis of Ce-rich rim
are listed in Table 7.5; the slightly paler rectangular zone in (a) resulted from previous
beam scans by the electron microprobe.
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
173
(a)
(b)
Figure 7.13 Cerium fractionated from other REE and occurring as joint matrix between
two iron cores within one large nodule; (a) is backscatter image with rectangular box
indicating the area mapped by EPMA; and (b) presents the corresponding EPMA
mapping; element concentrations of spot analysis of Ce-rich rim are listed in Table 7.5;
the slightly paler rectangular zone in (a) resulted from previous beam scans by the
electron microprobe.
17
4
(a) (b) (c)
Figure 7.14 Backscatter electron images of crystalline Fe oxides intergrown with micron-size Ce-rich secondary phosphates in the duricrust: (a)
crystalline Fe oxides; (b) and (c) Ce-rich secondary phosphates.
Table 7.5 Element concentrations in Figure 7.11, Figure 7.12 & Figure 7.13 of the duricrust and iron nodules in the JG profile
El.*
Element concentrations (wt%)
Ce P Si Al Fe Gd Yb Ca U Th Pb S O total
Figure 7.11 1.16 b.d. 0.64 10.7 49.5 b.d. 0.09 0.01 0.03 0.55 0.05 0.14 25.3 89.3
Figure 7.12 0.49 0.02 0.58 26.8 27.4 0.03 b.d. 0.03 0.03 0.23 0.03 0.06 32.6 88.3
Figure 7.13 1.49 0.06 0.49 28.5 19.4 0.12 b.d. 0.10 0.03 0.68 0.02 0.09 32.1 83.1
*Apart from Ce, Gd and Yb, other REE and Y were below the microprobe detection limit, so are not listed above; the spots for analysis were located at the brightest areas of the
Ce-mapping images.
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
175
7.5.3.3 Determination of Yb onto iron oxide core
In order to understand more fully the effect of Fe oxides on the distribution and
fractionation of REE, 16 random spots in the iron core and clay layer of iron nodules
from the A horizon (0.4 m depth) and the upper ferruginous zone (1.5 m depth) were
analysed by EPMA. Voids in the iron nodules were avoided. Trace concentrations of Yb
(0.02-0.12 wt%) were determined in seven spots, including six spots in the iron core
and one spot in the clay layers (Table 7.6). Only one spot, in the clay layer, contained
trace concentrations of Pr.
Table 7.6 REE concentrations of random spots in iron core and clay layer in iron
nodules from the A horizon and upper ferruginous zone of the JG profile
El.
Concentration (wt%)
iron
core
iron
core
iron
core
iron
core
iron
core
iron
core
clay
layer
Si 0.27 0.26 0.60 0.24 0.26 0.54 0.83
Zr 0.03 b.d. 0.03 0.03 b.d. b.d. b.d.
Ti 0.27 0.08 0.27 0.43 0.83 0.04 0.44
Pb 0.03 0.06 0.04 b.d. 0.03 0.04 b.d.
Th 0.01 b.d. 0.02 0.02 0.08 0.03 b.d.
U 0.03 0.02 0.03 0.03 0.03 0.04 b.d.
Al 3.22 2.99 6.87 15.3 8.48 4.32 35.7
Pr b.d. b.d. b.d. b.d. b.d. b.d. 0.02
Nd b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Yb 0.11 0.04 0.07 0.04 0.02 0.12 0.05
Lu b.d. b.d. b.d. b.d. b.d. b.d. b.d.
Fe 63.0 63.9 58.1 49.8 54.4 57.8 2.42
Ca 0.01 b.d. 0.02 0.01 b.d. 0.02 0.03
Sr b.d. b.d. b.d. b.d. b.d. b.d. 0.01
Na b.d. b.d. b.d. b.d. 0.02 b.d. b.d.
K b.d. b.d. 0.02 0.01 b.d. 0.01 b.d.
P 0.01 0.02 0.03 0.01 0.02 0.01 b.d.
S 0.05 0.02 0.07 0.05 0.14 0.10 0.03
As 0.07 0.06 0.06 0.02 0.04 0.06 b.d.
F b.d. b.d. 0.13 b.d. b.d. 0.15 0.11
O 21.6 21.4 23.8 28.6 24.3 21.2 33.7
total 88.7 88.8 90.2 94.6 88.6 84.57 73.36
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
176
7.6 Discussion
7.6.1 Geochemical pathways and fractionation of REE
Based on bulk geochemistry and the mode of occurrence of REE, abundant easily
weathered accessory minerals in the parent meta-granitoids e.g. fluorocarbonates,
thorite, apatite, are thought to have broken down during the early stages of weathering,
greatly changing the abundance and distribution pattern of REE. In the saprolite (10 m
depth) ca. 94% ΣREE released by dissolution of the accessory minerals has been
leached away or transported via solutions, and only ca. 6% ΣREE is retained; of that 6%,
ca. 5.1% REE is retained in the saprolite by mineral phases (e.g. secondary phosphates
or weathering-resistant minerals), and the remaining ca. 0.9% ΣREE has been retained
by association with other phases e.g. clay minerals, organic matter and Fe
oxides/oxyhydroxides (revealed by sequential extraction in Chapter Eight). Strong
depletion of ΣREE was also shown in the mottled clay zones (6.5-8.6 m depth,
68%-83%) and the A horizon (32%-53%), except for the duricrust (3 m) and the
ferruginous mottled zone (6.5 m depth) because of their anomalous enrichment of Ce
(Table 7.1). Therefore, significant amounts of REE, excluding Ce, have been leached
out of the profile rather than being translocated at the profile scale, suggesting high
mobility of REE under advanced weathering and strong lateritization. This is in contrast
to commonly reported accumulation of REE at the base of lateritic profiles (Beyala et
al., 2009; Braun et al., 1993; Dequincey et al., 2006; Nesbitt, 1979). The acidic
condition found in the weathered matrix of this study (a range of pH 3.2-5.3 from the
saprolite to the ferruginous zone) is lower than or close to the pH of natural rainfall (ca.
4.5-5.6, Charlson and Rodhe, 1982), which may have enhanced strong leaching of REE
during weathering.
Formation of secondary phosphate minerals e.g. rhabdophane and florencite, constrains
further mobility of REE (Braun et al., 1993), especially LREE (because of their LREE
selectivity, with average (La/Yb)PR = 3.2), and play an important role in redistribution
and fractionation of REE. Though Tripathi and Rajamani (2007) proposed that
secondary minerals are not particularly known to produce strong REE fractionation, a
strong preference for LREE in secondary rhabdophane and florencite was observed in
the lateritic JG regolith (Table 7.3). Unlike the LREE-hosting minerals, most
HREE-selective minerals are weathering-resistant and have undergone residual
accumulation in the weathered regolith. Thus, the low values of (La/Yb)PR in the saprolite
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
177
(0.2) and the lower mottled clay (0.4) suggests stronger depletion of LREE than HREE
relative to the parent meta-granitoids. As weathering intensifies, however, some
HREE-rich minerals, e.g. ilmenite, may be partially dissolved. The dissolution and even
removal of ilmenite (Chapter Five) under intense-extreme weathering and strong
lateritization may change the fractionation of REE and increase (La/Yb)PR. This might
be the reason for (La/Yb)PR in the upper mottled clay being higher than the lower
mottled clay (1.2 and 0.4 respectively), as the weathering of upper mottled clay
(CIA=94%) is more intense than lower mottled clay (CIA=86%). Further support for
the partial breakdown of ilmenite changing the fractionation of REE was provided by
the sequential extraction experiments (Chapter Eight): in the saprolite, where ca. 92%
ΣHREE was hosted by mineral phases (Res species), and this value has decreased to
ca. 88% in the upper mottled clay and ca. 82% in the duricrust. This reflects the mineral
control on the translocation of REE and the important effects of weathering on
redistribution and fractionation of REE. Therefore, the abundance and fractionation of
REE in regolith are essentially weighted mean of the abundances and compositions of
LREE-rich secondary phosphates and HREE-rich weathering-resistant minerals, which
are predominantly controlled by the weathering conditions (including weathering
intensity, weathering time, accessibility to solution and pH).
7.6.2 Enrichment mechanism of Ce in ferruginous zone
In the JG profile, significant Ce anomalies were observed in the ferruginous zone (Ce*
ranged from 1.5-25.3), especially in the duricrust (Ce*=25.3 in gravel) relative to the
parent meta-granitoids (Table 7.1); except Ce, the other REE are commonly depleted
(Figure 7.5). This enrichment of Ce is consistent with the total Fe enrichment and the
occurrence of neoformed Ce-(hydr)oxide phases rimming along Fe-rich pores in the
duricrust. The co-accumulation of both Fe (III) and Ce (IV) probably reflects a redox
boundary and existence of oxidising conditions. Similar situations have been reported
previously (Angelica and Dacosta, 1993; Braun et al., 1990; Braun et al., 1998).
During lateritic weathering, goethite, hematite, and maghemite form in the ferruginous
zone. Repetitive dissolution-precipitation of Fe oxyhydroxides produces the duricrust
and concentric iron nodules by seasonal growth during alternative wet-dry periods
(Chapter Four). Under oxidizing conditions, Ce is likely to be fractionated from the
other REE and precipitated as Ce (IV) and attaching onto Fe oxyhydroxides. The net
result of these processes will be positive Ce anomalies in the ferruginous zone
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
178
(including the duricrust).
No apparent Ce anomalies were observed in secondary phosphates in this study (Ce*
ranged from 0.95-1.02), suggesting that the replacement of accessory REE-bearing
minerals by secondary phosphates is not related to a redox gradient. Dissolution of
accessory minerals released REE at the early stages of weathering; some REE
(including Ce) precipitate as secondary phosphates, or complexed by different phases
e.g. clay minerals and retained in regolith, whereas some REE may dissolve in solutions.
Once in solution, pH, Eh and ligand concentrations are important controls on the
solubility of Ce. Fluctuation of water tables can induce redox change; consequently, Ce
may be oxidised to form (hydr)oxides or precipitated with ferric minerals in the
duricrust. Similar situations in lateritic regolith have been reported by Braun et al.,
(1990). Alternatively, repetitive dissolution and precipitation of Fe oxyhydroxides may
shift pH and Eh conditions; in consequence, Ce may fractionate from other REE by
surface precipitation with Fe oxyhydroxides during seasonal growth of iron nodules.
This is supported by the occurrence of Ce as a rim coating along the boundary between
Al-rich and Fe-rich layers in iron nodules.
7.6.3 Effects of Fe oxides/oxyhydroxides on mode of occurrence of REE
Although Fe oxides are known to be efficient sinks for heavy metals due to their large
surface areas (Nedel et al., 2010; Singh and Gilkes, 1992), 58%-82% ΣREE excluding
Ce are depleted in the ferruginous zone of the JG profile, similar to the mottled clay
(67%-83% ΣREE depletion excluding Ce) and the saprolite (93% ΣREE depletion
excluding Ce). This may be the result of persistent intense acidic leaching of
REE-bearing minerals under alternative wet-dry periods.
The mass proportion of amorphous Fe oxyhydroxides was 2.0%, lower than the mass
proportion of crystalline Fe oxides (4.7%), however, amorphous Fe oxyhydroxides
contained a higher mass proportion of ΣREE (4.5%, excluding Ce) than the crystalline
Fe oxides (mass proportion of ΣREE 1.0%, excluding Ce) in the duricrust matrix
(Chapter Eight). This suggests that amorphous Fe oxyhydroxide is more efficient at
scavenging REE than crystalline Fe oxide, a finding that is supported by previous
studies (Compton et al., 2003; Land et al., 1999; Laveuf and Cornu, 2009). In the JG
duricrust (Figure 8.4), amorphous Fe oxyhydroxide has higher (La/Sm)PR (0.16) and
(La/Yb)PR (0.25) than the crystalline Fe oxide (La/Sm)PR (0.37) and (La/Yb)PR (0.49),
Chapter Seven: Mode of occurrence of REE in an intensely weathered lateritic profile in WA
179
reflecting the tendency for amorphous Fe oxyhydroxides to be more selective for Sm
(MREE) and Yb (HREE). The (La/Yb)PR < 1.0 in the amorphous and crystalline Fe
extractions is in consistent with trace concentrations of Yb (0.02-0.12 wt%) determined
in the iron cores and the clay layers (Table 7.6). This finding is supported by Marmier et
al.’s experiments (1997; 1999) showing that surface complexation of Yb occurred on
hematite and magnetite between pH5 and 7.
Although REE showed different degrees of association with Fe oxides and
oxyhydroxides in this study, the mechanism of these associations is proposed to be
different:
(i) Poorly crystalline Ce (hydr)oxide phases as a rim along Fe-rich pores in the duricrust
reflect the oxidation and surface precipitation of Ce with Fe oxyhydroxides when redox
changes.
(ii) The association of REE with extracted Fe species and the determination of trace
concentrations of Yb (0.02-0.12 wt%) in the iron core and the clay layers are likely to
be the result of surface complexation, substitution and/or co-precipitation.
(iii) Minor amounts of REE in the extracted crystalline Fe oxides are likely the result of
surface precipitation of secondary REE-bearing phosphates with Fe oxides during
duricrust formation. This is supported by the observation of micron-size REE-bearing
phosphate crystals occurring within the crystalline Fe oxides in the duricrust (Figure
7.14).
(iv) Alternatively, the occurrence of REE-bearing phosphates in the clay layer of the
iron nodules might result from iron nodules sequestering secondary REE-bearing
phosphates during their seasonal growth and cementation at advanced stages of
lateritization.
Therefore, Fe oxides and oxyhydroxides can play important roles in redistribution and
fractionation of REE during intense weathering and lateritization.
7.7 Summary of the chapter
A lateritic profile (JG) locating in Jarrahdale, Western Australia was investigated for the
mode of occurrence and geochemical behaviour of REE under intense weathering and
advanced lateritization.
In the saprolite, ca. 94% ΣREE was released by dissolution of accessory minerals in the
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
180
parent meta-granitoids e.g. fluorocarbonates, thorite and apatite; ca. 5.1% ΣREE was
retained by formation of secondary phosphates and residual accumulation of
weathering-resistant minerals, and ca. 0.9% ΣREE was retained by association with clay
minerals, organic matter and Fe oxides/oxyhydroxides. The formation of secondary
phosphate minerals e.g. rhabdophane and florencite, which are absent in the parent
meta-granitoids, constrains further mobility of REE, especially LREE. The residual
accumulation of weathering-resistant minerals e.g. zircon and rutile/anatase are
important hosts for retention of HREE, especially in extremely weathered ferruginous
zones. Thus, the abundance and stability of LREE-rich secondary phosphates and
HREE-rich weathering-resistant minerals control the fractionation of REE in intensely
weathered lateritic regolith.
In the ferruginous zone, Ce fractionated from the other REE and was abnormally
enriched (Ce*=25.3 in the duricrust gravel). This Ce enrichment is in agreement with
the occurrence of poorly crystalline Ce (hydr)oxide phases as rims along Al/Fe-rich
pores in the duricrust, or along the boundary of Al/Fe-rich layers in iron nodules of the
ferruginous zone. The fractionation of Ce is the result of surface precipitation of Ce (IV)
phases with Fe oxyhydroxides under oxidization and/or changes of pH and Eh
conditions during advanced stages of lateritization.
Trace concentrations of Yb (0.02-0.12 wt%) determined in the iron cores and clay
layers are consistent with the association between REE and amorphous and crystalline
Fe extracted species. They suggest that Fe phases are effective for retention of REE in
lateritic regolith. Fine-grained (<10 µm) REE-bearing phosphates occurred in the clay
layer of iron nodules, most likely to be the result of sequestering by the iron nodules
during their formation.
Therefore, the significant mineralogical control and high mobility of REE during
intense lateritic weathering are important considerations when using REE as tracers of
geochemical processes in intensely weathered environments. The sensitivity of REE to
weathering conditions, especially Ce to redox change, suggests a potential for REE to
be used as complementary geochemical clues along with Fe to investigate the
lateritization processes and to understand the role of Fe oxides and oxyhydroxides on
scavenging trace metals.
181
8 Particle size fractionation and chemical speciation of REE in a
lateritic profile in Western Australia
8.1 Abstract
The rare earth elements (REE) are commonly used as indicators of geochemical and
pedological processes. To better understand the distribution and partitioning of REE in
different particle size fractions and chemical species, an intensely weathered lateritic
profile developed on meta-granitoids in Jarrahdale, Western Australia was investigated.
High concentrations of REE were found in silt and clay fractions. Given the variation in
mass percentages of different particle size fractions, however, gravel and sand contained
56%-98% of the mass of REE in the ferruginous zone. In the saprolite and mottled clay,
clay had the highest mass loading of light REE (LREE) in contrast to the highest mass
loadings of heavy REE (HREE) found in sand. In the ferruginous zone, gravel was the
predominant host for Ce, whereas most of other REE were contained in the gravel and
sand fractions, suggesting that Ce fractionated from other REE and precipitated with, or
was adsorbed by, Fe oxides/oxyhydroxides during formation of duricrust and iron
nodules. The residual species contained the highest percentages of total REE revealed
by sequential extraction, indicating that the abundance and distribution of REE are
controlled by weathering-resistant minerals in intensely weathered regolith. Water
soluble (including adsorbed and exchangeable) species was the fraction hosting the
second highest percentages of total REE, suggesting the important effect of adsorption
by clay and potential bio-availability. The low pH of the profile is believed to account
for the high proportion of REE in this species. The amorphous Fe oxyhydroxide and
crystalline oxide extractions preferentially hosted LREE and MREE over HREE,
whereas the organic matter species was important in complexing HREE. The
distribution and fractionation of REE in different particle size fractions and chemically
extractable species can be used to better understand geochemical behaviour of REE in
intensely weathered lateritic profiles.
8.2 Key words
Particle size; speciation; rare earth element; laterite; weathering; regolith;
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
182
8.3 Introduction
Physical and chemical weathering of rocks and minerals leads to soil formation. During
this processes, mineral transformations result in a mass flux change of elements within
the mineral assemblage and among particle size fractions, which yields information on
element partitioning and transportation within a profile. The concentration of metals in
soils increases with decreasing particle size (Acosta et al., 2009; Al-Rajhi et al., 1996;
Ljung et al., 2006) because fine particles usually have a larger specific surface area
capable of retaining higher amounts of metals (Wang et al., 2006), or alternatively,
metals are co-precipitated with fine-grained secondary minerals. In the course of
weathering, weathered regolith shifting into smaller particle sizes can result in the
relative accumulation of the REE as refractory elements (Caspari et al., 2006).
However, the substantial influence that the particle size exerts on the abundance and
redistribution of REE in lateritic regolith is not well known. Most of the studies on the
geochemical behaviour of REE during supergene weathering concentrate on bulk
regolith. Therefore, a systematic understanding of the occurrence of REE in different
grain size fractions of lateritic regolith is needed. Understanding grain size effects
would assist pedological interpretation of the fate of REE, and assessment of plant
availability of REE under natural environmental conditions.
Currently, two different approaches are widely used for determining trace element
location and speciation in uncontaminated soils: physical fractionation (e.g. Acosta et
al., 2011; Fichter et al., 1998; Taboada et al., 2006a) and chemical methods (especially
sequential selective extraction, e.g. Aubert et al., 2004; Land et al., 1999). Although the
sequential extraction method suffers from relying on operationally defined fractions and
lack of a standard method for specific trace elements, it is still considered useful for
investigation of element associated phases in soils (Aubert et al., 2004; Cao et al., 2000).
The reactivity or mobility of REE largely depends on their chemical speciation in
weathered profiles, however, few studies have dealt with the speciation of REE in
non-contaminated soils (Aubert et al., 2004), especially in natural weathered profiles.
The objective of this study is to determine the distribution and fractionation of REE in
various particle size fractions and chemical species. The concentrations of REE in
different particle size fractions and chemical species are quantified and fractionation of
REE with respect to particle size distribution and chemical speciation are discussed.
Chapter Eight: Particle size fractionation and chemical speciation of REE in the JG profile in WA
183
8.4 Materials and methods
8.4.1 Analytical methods
This study was performed on a lateritic profile (JG) developed over meta-granitoid
rocks near Jarrahdale, Western Australia. Pre-treatment and analytical methods of the
parent rocks and regolith samples were explained in detail in Chapter Seven. Regolith
samples were separated into two subsample groups: gravel (>2 mm, represented by
suffix ‘g’) and matrix (<2 mm, represented by suffix ‘m’), with the exception of mottled
clay and saprolite, which have only matrix fractions. The fractions of matrix and gravel
were oven dried at 105 °C overnight and ground to ≤ 200 µm prior to fusion in order to
determine trace element concentrations. The regolith matrix was further separated into
the following three size fractions recommended by the International Society of Soil
Science (ISSS) (Marshall, 1947; Marshall, 2003; Prescott et al., 1934): clay (<2 µm),
silt (2-20 µm) and sand (>20 µm) using the sedimentation and wet sieving methods
(Day, 1965). Different particle size fractions were rinsed with MilliQ water three times,
oven dried at 105 °C overnight and ground to ≤ 200 µm prior to fusion.
To investigate chemical species and association behaviour of trace elements, a
sequential extraction procedure was performed. The matrix fraction (< 2 mm) from the
saprolite (JG1m), upper mottled clay (JG3m) and duricrust (JG5m) were selected. An
in-house laboratory reference material was prepared together with selected samples.
Regolith trace elements were extracted as five species (modified from Hall et al., 1996):
(i) water soluble, adsorbed, exchangeable and carbonates bound (WAE); (ii) organic
matter and sulphide bound (Org); (iii) amorphous Fe-Mn oxyhydroxide bound (FeAm);
(iv) crystalline Fe-Mn oxide bound (FeCry); and (v) residual species (Res). Since
carbonates were unlikely to be present in the regolith being studied here due to low pH,
species WAE is considered to include mainly water soluble, adsorbed or exchangeable
elements. Sulfides are also scarce in the lateritic regolith, therefore it is assumed that
species Org is mainly hosted by organic matter complexes. A brief summary of the
method is shown in Table 5.1 in Chapter Five. The residual samples and reference
materials were rinsed with MilliQ water three times, oven dried at 105 °C overnight and
ground to ≤200 µm prior to fusion in order to determine trace element concentrations.
The fusion beads were made by mixing 0.1 g (to an accuracy of 0.1 mg) of finely
ground sample or reference material with 0.7 g 12:22 Norrish flux (lithium
metaborate:lithium tetraborate) and heating in a muffle furnace at 1050 °C for 40
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
184
minutes. Duplicate fusion beads were also made on 10% of samples to check
preparation errors. After cooling, the fusion beads were dissolved in 100 mL of 10%
analytical grade HCl. The trace elements including REE were determined by
inductively coupled plasma-mass spectrometry (ICP-MS) in Genalysis Laboratory
Services of Intertek Commodities in Maddington, Western Australia. Certified
international standard materials, including stream sediment reference material STSD-2,
STSD-4 (Canada Centre for Mineral and Energy Technology, CANMET) and an
in-house standard material were prepared in the same way as the samples and analysed
together with samples to check the accuracy and precision. The variation between tested
values and expected values was within 10% of the certified values. The concentrations
of REE in different particle size fractions and chemical species are given in Table 8.1
and Table 8.2 respectively.
8.4.2 Calculation methods
8.4.2.1 Fractionation of REE
In order to study the fractionation of REE, three groups are identified (Henderson,
1984): the light REE (LREE; from La to Nd), the middle REE (MREE: from Sm to Ho)
and the heavy REE (HREE: from Gd to Lu). The normalized ratios (La/Sm)PR and
(La/Yb)PR were used for identifying fractionations between LREE-MREE and
LREE-HREE using the average composition of parent meta-granitoids as a reference.
8.4.2.2 Calculation of REE mass loading in particle size fraction
To index an element’s partitioning into different particle size fractions, a mean element
mass loading was calculated based on the element’s concentration in a selected grain
size of known mass percentage (Sutherland, 2003).
GSFloading 100 (X i GSi
X i GSii1
n
)
Where:
Xi is the concentration of REE (ppm) in an individual grain size fraction (e.g. <2 µm);
GSi is the mass percentage of an individual fraction, which has limits of 0-100%.
GSFloading is the element mass loading in a selected grain size and the summation of
GSFloading indices for each soil sample equals 100%.
In the ferruginous zone, four classes of particle sizes (clay, silt, sand and gravel) were
Chapter Eight: Particle size fractionation and chemical speciation of REE in the JG profile in WA
185
defined and three in the mottled clay zone and the saprolite (clay, silt and sand). Thus, if
the REE concentration for a given fraction is very high but it forms only a small portion
of the overall sample mass, the contribution of this fraction to the total sample REE
loading will be minimal.
8.5 Results
8.5.1 Concentrations of REE in different particle size fractions
In the lateritic JG profile, silt and clay fractions generally contained the highest
concentrations of REE, except in the saprolite (Figure 8.1). In the ferruginous zone
(from JG6 to JG4, 1.5-5 m depth), clay contained the highest concentrations of LREE
(from La to Nd), followed by the silt fraction. In the duricrust (3 m depth) and
ferruginous mottled zone (5 m depth), however, gravel was abnormally enriched in Ce.
Concentrations of LREE in matrix were slightly higher than in sand in the ferruginous
zone. In the mottled clay (6.5-8.6 m depth) and the saprolite (10 m depth), the relative
concentrations of LREE from high to low were: silt > clay > sand.
MREE (from Sm to Ho) had different distribution patterns between the particle size
fractions. From Sm to Gd, closer to LREE, the highest concentrations were in the clay
fraction in the ferruginous zone but in the silt fraction in the mottled clay zone. From Tb
to Ho, closer to HREE, silt fraction had the highest concentrations except duricrust and
upper ferruginous zone.
HREE (from Er to Lu) and Y showed mostly consistent distribution patterns. The silt
fraction contained the highest concentration of HREE throughout the profile followed
by the clay fraction in the ferruginous zone. In the saprolite and mottled clay, both clay
and sand fractions had similar HREE concentrations.
8.5.2 Mass loading of REE in different particle size fractions
Given the mass percentage of each particle size, the mass loading of selected REE in
each particle size fraction was plotted in Figure 8.2. Although silt and clay fractions had
the highest concentrations of REE, their relatively low mass percentage compared with
other fractions minimized the enrichment.
In the ferruginous zone, gravel dominated the distribution and abundance of Ce, with up
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
186
to 84% Ce in the duricrust. In the upper ferruginous zone (1.5 m depth), gravel and sand
accounted for more than 95% mass of REE, decreasing to ca. 80% in duricrust (3 m
depth) and ca. 60% in ferruginous mottled zone (5 m depth). In the duricrust, the mass
loading of each REE was higher in the clay fraction than the silt fraction. In the
ferruginous mottled zone, however, REE were fractionated: the mass loadings of LREE
and MREE were higher in the clay fraction whereas the mass loadings of HREE were
higher in the silt fraction.
From the upper mottled clay to the saprolite (JG3-JG1, 6.5-10 m depth), the regolith
does not contain gravel. The clay fraction was the most important host for LREE in
these zones (6.5-10 m depth), especially in the upper mottled clay zone (6.5 m depth)
with ca. 48%-50% LREE was in the clay fraction. Higher mass loadings of HREE
(46%-61%), however, were found to be in the sand fraction in the saprolite and mottled
clay. The mass loading of REE in the silt fraction increased with depth from upper
mottled clay to saprolite.
8.5.3 Speciation of REE from sequential extraction
The sequential extraction experiment revealed the percentages of ΣREE (the total REE
concentration) in each chemical species of representative lateritic regolith in the JG
profile (Figure 8.3). Generally, the ΣREE distribution percentage followed the order:
Res > WAE > FeAm > FeCry and Org. The Res and WAE species dominated the
distribution and abundance of REE, accounting for 89%-98% ΣLREE, 87%-97%
ΣMREE and 91%-98% ΣHREE. The saprolite Res had higher percentages of ΣMREE
(85%) and ΣHREE (92%) than upper mottled clay (75% ΣMREE and 88% ΣHREE) and
duricrust (66% ΣMREE and 82% ΣHREE) and the percentages decreased from saprolite
to duricrust. In addition, the saprolite WAE had higher percentage of ΣLREE (13%)
than upper mottled clay (9%) and duricrust (9%). The percentages of ΣMREE (12%)
and ΣHREE (7%) in the saprolite WAE were lower than in the WAE of upper mottled
clay (20% ΣMREE and 9% ΣHREE) and duricrust (21% ΣMREE and 9% ΣHREE).
The duricrust Org had higher percentage of ΣREE (1.5%) than the Org in the saprolite
(0.6%) and upper mottled clay (0.2%), especially HREE. The percentages of total REE
hosted in the FeAm phase of the duricrust (6.6%) were also higher than in the FeAm
phases of the saprolite (1.4%) and upper mottled clay (1.7%). Similarly, the FeCry phase
in the duricrust also had higher percentages of total REE (3.3%) than the total REE
Chapter Eight: Particle size fractionation and chemical speciation of REE in the JG profile in WA
187
percentage in the FeCry of saprolite (0.8%) and upper mottled clay (0.6%). In addition,
in the duricrust the percentage of REE in the FeAm phase (6.6%) was higher than the
percentages of REE in the FeCry (3.3%) and Org (1.5%) phases.
18
8
Figure 8.1 Concentrations of REE in grain size fractions in the JG profile
Figure 8.2 Mass loading of REE in grain size fractions in the JG profile (JG6-upper ferruginous zone, 1.5 m depth; JG5- duricrust, 3 m depth;
JG4-ferruginous mottled zone, 5 m depth; JG3-upper mottled clay, 6.5 m depth; JG2-lower mottled clay zone, 8.6 m depth; JG1-saprolite, 10 m depth.
Only selected REE are plotted here; other REE showed similar patterns).
18
9
Figure 8.3 Distribution of REE percentages in sequential extractions of the representative regolith of the JG profile. (Res: residual; FeCry: crystalline Fe
oxides; FeAm: amorphous Fe oxyhydroxides; Org: organic matter; WAE: water soluble, adsorbed and exchangeable. JG5- duricrust, 3 m depth;
JG3-upper mottled clay, 6.5 m depth; JG1-saprolite, 10 m depth. (Some REE concentrations were below the detection limit of ICP-MS and are not
presented here).
19
0
Table 8.1 Concentrations of REE in grain size fractions of the JG profile
sample Element concentrations (ppm)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y
d.l. 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Upper ferruginous zone JG6sand 7.2 21.9 1.5 5.0 1.1 0.2 1.0 0.2 1.1 0.3 0.8 0.1 0.9 0.2 6.8
1.5 m depth JG6silt 14.6 60.1 3.1 10.3 2.0 0.3 1.8 0.3 2.0 0.5 1.5 0.3 1.8 0.4 13.1
JG6clay 17.2 121 4.3 14.8 2.9 0.6 2.5 0.4 2.2 0.5 1.3 0.2 1.2 0.2 13.1
JG6matrix 7.8 28.8 1.6 5.7 1.1 0.2 1.3 0.2 1.2 0.3 0.9 0.2 1.1 0.2 7.6
JG6gravel 7.5 27.0 1.7 6.5 1.4 0.3 1.4 0.2 1.4 0.3 0.9 0.2 1.0 0.2 5.3
Duricrust JG5sand 5.7 35.8 0.8 2.4 0.5 b.d. 0.4 b.d. 0.5 b.d. 0.3 b.d. 0.4 b.d. 2.5
3 m depth JG5silt 13.8 61.1 2.4 7.7 1.6 0.3 1.1 0.2 1.2 0.3 0.9 0.2 1.2 0.2 7.3
JG5clay 14.9 63.2 2.9 10.1 2.1 0.4 1.5 0.2 1.3 0.3 0.8 0.1 1.0 0.2 5.8
JG5matrix 6.1 37.2 0.9 3.0 0.6 0.1 0.7 0.1 0.5 0.1 0.3 0.1 0.5 0.1 3.2
JG5gravel 6.0 224 0.7 2.4 0.4 0.1 1.6 0.1 0.4 0.1 0.2 0.0 0.3 0.1 2.0
Ferruginous mottled zone JG4sand 4.5 16.0 0.7 2.0 0.4 b.d. 0.3 b.d. 0.3 b.d. 0.2 b.d. 0.3 b.d. 1.8
5 m depth JG4silt 20.2 40.7 3.4 10.8 2.2 0.4 1.7 0.3 1.9 0.5 1.6 0.3 2.2 0.5 12.6
JG4clay 25.1 40.5 4.6 14.8 2.8 0.5 2.0 0.3 1.7 0.4 1.0 0.2 1.1 0.2 7.8
JG4matrix 7.8 19.0 1.1 4.3 0.9 0.1 0.7 0.1 0.7 0.2 0.6 0.1 0.7 0.2 4.5
JG4gravel 5.0 107 0.6 1.9 0.3 0.1 0.7 0.0 0.4 0.1 0.2 0.0 0.4 0.1 2.1
Upper mottled clay JG3sand 7.0 9.9 1.0 2.5 0.4 b.d. 0.3 b.d. 0.4 0.1 0.4 b.d. 0.7 0.2 2.9
6.5 m depth JG3silt 27.0 38.2 3.5 9.7 1.7 0.3 1.2 0.2 1.4 0.4 1.1 0.2 1.7 0.3 10.2
JG3clay 22.6 30.2 2.8 7.7 1.1 0.2 0.8 0.1 0.8 0.2 0.4 b.d. 0.5 0.1 4.0
JG3matrix 10.5 14.8 1.3 3.6 0.6 0.1 0.5 0.1 0.4 0.1 0.3 0.1 0.4 0.1 2.7
Lower mottled clay JG2sand 3.4 4.6 0.4 1.0 0.3 b.d. 0.2 b.d. 0.2 b.d. 0.2 b.d. 0.3 b.d. 1.3
8.6 m depth JG2silt 12.3 17.3 1.5 4.1 0.9 0.1 0.8 0.1 0.8 0.2 0.7 0.1 1.0 0.2 5.6
JG2clay 9.1 12.1 1.0 2.4 0.4 b.d. 0.3 b.d. 0.3 b.d. 0.2 b.d. 0.3 b.d. 1.6
JG2matrix 5.2 7.5 0.6 1.7 0.3 0.1 0.2 0.1 0.3 0.1 0.3 0.1 0.5 0.1 2.1
19
1
sample Element concentrations (ppm)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y
d.l. 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Saprolite JG1sand 0.3 0.6 b.d. 0.3 0.2 b.d. 0.1 b.d. 0.1 b.d. 0.1 b.d. 0.2 b.d. 1.0
10 m depth JG1silt 1.3 2.3 0.2 0.9 0.3 b.d. 0.3 b.d. 0.3 b.d. 0.3 b.d. 0.5 0.1 2.2
JG1clay 1.2 2.0 0.2 0.8 0.2 b.d. 0.1 b.d. 0.1 b.d. 0.1 b.d. 0.2 b.d. 0.8
JG1matrix 1.3 2.4 0.2 1.1 0.3 0.1 0.3 0.0 0.2 0.0 0.2 0.0 0.3 0.1 1.3
Average meta-granitoids 27.9 47.4 4.3 14.0 2.1 0.5 1.8 0.2 1.0 0.2 0.8 0.2 1.1 0.2 8.5
d.l. refers to detection limit; b.d. refers to below detection limit.
19
2
Table 8.2 Concentrations of REE in different chemical extractions of representative regolith in the JG profile
Sample Element concentrations (ppm)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Mn Fe ΣREE%1
d.l. 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.001 1.00 5.00
JG1m WAE 0.140 0.290 0.040 0.149 0.032 0.007 0.021 0.003 0.017 0.004 0.013 0.002 0.018 0.003 2.00 55.00 12.5
JG1m Org 0.008 0.009 0.002 0.006 0.003 b.d. 0.001 b.d. 0.002 b.d. 0.001 b.d. 0.002 b.d. b.d. 130.0 0.6
JG1m FeAm 0.016 0.032 0.004 0.013 0.004 b.d. 0.003 b.d. 0.003 b.d. 0.002 b.d. 0.003 b.d. 3.00 862.0 1.4
JG1m FeCry 0.023 0.014 0.001 0.003 0.001 b.d. 0.001 b.d. 0.001 b.d. b.d. b.d. 0.001 b.d. 4.00 173.0 0.8
JG1m Res2 1.1 1.9 0.2 0.7 0.2 b.d. 0.2 b.d. 0.2 b.d. 0.2 b.d. 0.3 b.d. 14 6773 84.8
JG3m WAE 0.938 0.905 0.168 0.531 0.083 0.018 0.086 0.014 0.070 0.016 0.041 0.006 0.031 0.005 2.00 86.00 9.2
JG3m Org 0.022 0.019 0.004 0.013 0.002 b.d. 0.002 b.d. 0.002 b.d. 0.001 b.d. 0.003 b.d. b.d. 66.00 0.2
JG3m FeAm 0.130 0.200 0.028 0.098 0.019 0.004 0.018 0.003 0.016 0.003 0.010 0.002 0.009 0.002 2.00 519.0 1.7
JG3m FeCry 0.053 0.079 0.008 0.028 0.005 b.d. 0.004 b.d. 0.004 b.d. 0.002 b.d. 0.002 b.d. 3.00 757.0 0.6
JG3m Res 9.2 13 1.1 2.8 0.5 b.d. 0.3 b.d. 0.3 b.d. 0.2 b.d. 0.4 0.2 29 5554 88.3
JG5m WAE 0.422 2.744 0.149 0.586 0.146 0.031 0.102 0.015 0.073 0.014 0.037 0.005 0.030 0.005 3.00 158.0 9.1
JG5m Org 0.031 0.563 0.015 0.058 0.014 0.003 0.009 0.001 0.006 0.001 0.005 0.001 0.012 0.003 b.d. 83.00 1.5
JG5m FeAm 0.127 2.553 0.054 0.221 0.060 0.012 0.042 0.007 0.038 0.008 0.020 0.003 0.020 0.003 b.d. 438.0 6.6
JG5m FeCry 0.049 1.450 0.011 0.039 0.010 0.002 0.006 0.001 0.007 0.001 0.004 b.d. 0.004 b.d. 6.00 1014 3.3
JG5m Res 6..0 27 0.8 2.5 0.5 b.d. 0.3 b.d. 0.4 b.d. 0.3 b.d. 0.4 b.d. 31 19810 79.6
1ΣREE% refers to percentage of ΣREE in each extraction species;
2The detection limit (d.l.) of the Res species, determined by the fusion method, is 0.1 ppm.
Chapter Eight: Particle size fractionation and chemical speciation of REE in the JG profile in WA
193
8.6 Discussion
Although sequential extraction schemes do not extract chemically discrete forms of
elements, the data have revealed variation of the REE distribution in different particle
size fractions and chemical species. Most of REE were hosted by the Res, indicating
that both the abundance and distribution of REE are controlled by weathering-resistant
minerals in intensely weathered regolith. SEM imaging and EPMA analyses show that
LREE are mostly hosted by secondary phosphates ca. 2-20 µm-size, e.g. rhabdophane
and florencite, and HREE are mainly contained in weathering-resistant minerals of
varied grain size (1-100 µm), e.g. zircon and anatase in the lateritic regolith (Chapter
Seven). The high concentration of REE in the silt fraction (2-20 µm) is in good
agreement with the REE-bearing mineral size in the regolith, especially LREE-rich
secondary minerals, indicating morphological and mineralogical change from
REE-bearing accessory minerals e.g. apatite, fluorocarbonates and thorite in the parent
meta-granitoids to secondary rhabdophane and florencite during intense weathering
and lateritization (Chapter Seven). In addition, in the duricrust an abnormal enrichment
of Ce was observed, especially in gravel. It suggests that Ce fractionated from other
REE and less likely to be mobile than the other REE during formation of duricrust and
iron nodules. This agrees with (i) high concentrations of Ce in the FeAm and FeCry in
the duricrust (Table 8.2); and (ii) precipitation and neoformation of Ce-(hydr)oxides as
a rim between Al/Fe layer boundary or along the Al/Fe-rich pore walls (Figure 7.13 in
Chapter Seven).
A significant proportion of REE bound to the WAE species in natural uncontaminated
soils is not common in previous studies; although some of the WAE-extractable REE
may also be colloidal, and therefore the WAE fraction might be overestimated, it
suggests that some amount of REE is bio-available in the regolith studied here. High
deficiency of REE in the profile, especially in the saprolite, may partially attribute to
low soil pH which favours the conversion of metals from precipitated forms into
dissolved forms (Cao et al., 2001; Harter, 1983). In an acidic environment, such as this
(pH ranges from 3.2 to 4.0 from saprolite to mottled clay in Table 4.1), the
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
194
predominant REE species in solution is the free Ln3+
ion (Ln denotes REE). In the
duricrust, REE may also partially occur as LnHCO32+
complexes due to the slightly
higher pH (4.7) and organic complexes due to relatively higher dissolved organic
matter (total carbon 0.30%) than in the mottled clay (total carbon 0.08%). Extraction of
adsorbed or exchangeable REE in a spodosol profile has been reported to be closely
related to pH, in the range 4.2 to 6.5 (Land et al., 1999).
In addition, the high proportion of REE bound to the WAE is probably relevant to high
concentrations of REE in the clay fraction. Kaolinite and halloysite were identified in
the saprolite and mottled clay. The transformation from kaolinite to halloysite during
weathering is accompanied by an increase in hydration, a decrease in Si/Al ratio and an
increasing cation exchange capacity (CEC) (Tari et al., 1999). The clay fraction of the
mottled clay zone had (La/Sm)PR ranging from 1.5-1.7 and (La/Yb)PR from 1.2-1.8
relative to the parent meta-granitoids (Figure 8.4), suggesting that clay acts an
important role in trapping REE, especially LREE. This is also supported by Cullers et
al. (1987) who showed that heavy minerals (biotite, hornblende and sphene) in a soil
developed from a granitic parent material appeared to be altering and making LREE
available to the clay minerals forming in the soil. However, opposite fractionation
(HREE more sorbed than LREE) onto kaolinite has also been reported (Coppin et al.,
2002). Adsorption of REE by clay is controlled by the nature of the clay minerals, pH,
ionic strength, the presence of additional ligands such as carbonate or organic
complexes, surface coverage, and effects specific to the characteristics of the different
REE (Coppin et al., 2002; Fendorf and Fendorf, 1996; Koeppenkastrop and Decarlo,
1992; Koeppenkastrop and Decarlo, 1993; Laveuf and Cornu, 2009; Piasecki and
Sverjensky, 2008; Takahashi et al., 1999). As well as these controls, differences in clay
mineralogy can affect fractionation of REE (Laveuf and Cornu, 2009), potentially
explaining the contradictory signatures of REE adsorbed by clay minerals. Usually,
REE adsorption increases with increasing pH (Coppin et al., 2002), which may explain
the increasing concentrations of REE in the WAE and clay fraction from saprolite to
duricrust. The (La/Sm)PR (ranging from 0.1-0.4) and (La/Yb)PR (ranging from 0.1-0.2)
in all particle size fractions of saprolite suggest that La was substantially fractionated
Chapter Eight: Particle size fractionation and chemical speciation of REE in the JG profile in WA
195
from Sm and Yb and greatly depleted from the saprolite. It is consistent with the
breakdown of LREE-rich accessory minerals (e.g. fluorocarbonates and thorite) at the
early stages of weathering. In addition, a high mass of HREE is observed in sand from
saprolite to upper mottled clay, in contrast to most of LREE being present in clay in
these zones. This may suggest that relatively large-grained (ca. 100 µm) and
weathering-resistant minerals, e.g. zircon, anatase or ilmenite contained significant
amounts of HREE, or alternatively, HREE may be adsorbed onto larger-grained
metal-oxide surfaces, e.g. rutile, hematite (Piasecki and Sverjensky, 2008).
The FeAm and FeCry had higher percentages of LREE and MREE than HREE
throughout the regolith studied. In the duricrust the FeAm phase had a preference for
MREE whereas the FeCry showed a preference for LREE. Since there are negligible
variations in complexation constants for the acetate ligand with various REE (Wood,
1993), the fractionation in extraction of the FeCry is not caused by the extractant
solution (Land et al., 1999) in which the only other solute is NH2OH∙HCl. The reasons
why FeAm and FeCry species show different preference for LREE and MREE is not
clear, but are believed to be related to pH and the presence of other ligands such as
organic complexes (Piasecki and Sverjensky, 2008; Quinn et al., 2006). The
fractionation between LREE, MREE and HREE in Fe oxides is subject to debate
(Laveuf and Cornu, 2009) and varied fractionation with enrichments of LREE
(Koeppenkastrop and Decarlo, 1993), MREE (Bau, 1999; Land et al., 1999) or HREE
(Elderfield and Greaves, 1981; Marker and Deoliveira, 1994) have been observed. For
example, Land et al. (1999), studying a spodosol profile, observed an enrichment of
MREE in the FeAm and a clear HREE enrichment relative to the LREE in FeCry and Org
species. The differences in REE fractionation between species probably also arise from
the various proportions of the different types of Fe- and Mn- oxides present (Laveuf
and Cornu, 2009). The affinity of Ce with Fe oxides indicates surface sorption and
oxidation/coprecipitation of Ce onto Fe oxides, which have been examined by many
authors (Bau and Koschinsky, 2009; Davranche et al., 2004; Nedel et al., 2010).
The Org species plays an important role in complexing HREE in this study, especially
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
196
in the duricrust, in contrast to the FeAm which have a preference for MREE. Affinity of
HREE for organic materials has been observed before (Aubert et al., 2004; Land et al.,
1999). Organic ligands form complexes with HREE which are more stable than those
with LREE (Henderson, 1984; Sonke and Salters, 2006). The proportion of total REE
hosted by the FeAm is higher than both Org and FeCry species, indicating amorphous Fe
oxyhydroxide plays a more important role than other solid components in controlling
the mobility and bioavailability of REE in lateritic regolith.
Figure 8.4 Normalized ratios of (La/Sm)PR and (La/Yb)PR in particle size fractions and
sequential extractions in the JG profile (solid vertical lines indicate ratio=1.0, no
fractionation of REE relative to the parent meta-granitoids).
0
2
4
6
8
10
0.0 0.5 1.0 1.5 2.0
(La/Sm)PR
de
pth
(m)
0
2
4
6
8
10
0.0 0.5 1.0 1.5 2.0
(La/Yb)PRd
ep
th(m
)
sand
silt
clay
matrix
gravel
0
2
4
6
8
10
0.0 0.5 1.0 1.5 2.0
(La/Sm)PR
de
pth
(m)
0
2
4
6
8
10
0.0 0.5 1.0 1.5
(La/Yb)PR
de
pth
(m)
AEC
Org
Am
Cry
Res
Chapter Eight: Particle size fractionation and chemical speciation of REE in the JG profile in WA
197
8.7 Summary of the chapter
A systematic study of particle size fractionation and chemical fractionation of REE in a
lateritic weathered profile developed on meta-granitoids in Jarrahdale, Western
Australia showed that most of the REE (by mass) had partitioned into coarse-grained
material (gravel and sand), despite the high concentrations in fine-grained (silt and clay)
fractions. This partitioning by grain size was not consistent, however, across the REE
series, with significant fractionation occurring; for example, in the lower profile most
LREE mass was in the clay (<2 µm) fraction, but most HREE were associated with
sand (>20 µm). The most significant fractionation of REE was shown by a strong Ce
anomaly in ferruginous duricrust, consistent with formation of both ferruginous
materials and the Ce enrichment by oxidative processes such as precipitation of ferric
minerals. Particle size, sequential extraction, and electron microprobe data were
consistent with REE occurrence being dominated, in intensely weathered regolith, by
mineral phases resistant to weathering. The dominance of residual forms in sequential
extracts supported this conclusion, but the existence of significant REE in water
soluble, exchangeable or adsorbed forms was surprising and was likely to be related to
the low pH of regolith materials. This study demonstrates that the distribution and
fractionation of REE within different particle size fractions and chemically extractable
species can be used as clues for better understanding geochemical behaviour of REE in
intensely weathered lateritic profiles. Both have potential implication for pedological
interpretation of the fractionation of REE during weathering and lateritization,
especially when a particle size sorting process is involved.
199
9 Conclusion and future work
9.1 Conclusion
With this thesis, I set out to improve on the current understanding of geochemical
behaviour and fractionation mechanisms of rare earth elements (REE) during
weathering and lateritization. Four intensely weathered lateritic profiles (GE, MQ I,
MQ II and JG) developed on granitoids with dolerite dykes were investigated.
Substanital depletion of base cations, great loss of Si and enrichment of Al and Fe in
the GE and JG profiles were revealed by mass balance calculations using Zr as the
conservative reference element. Significant geochemical processes e.g. intense
leaching of cations, kaolinization, desilication and ferruginization had occured during
the weathering and lateritization history as suggested by the bulk chemical and
mineralogical data and principal component analysis.
In intensely weathered lateritic regolith, REE (except Ce) were significantly depleted,
compared to the parent granitoids, especially in the GE and JG profiles where the
depletion was up to 94%, reflecting high mobility of REE under extreme weathering.
This is an important consideration when using REE as tracers of geochemical
processes, especially in intensely weathered environments. Stronger depletion, relative
to parent rock, of light REE (LREE) over both middle REE (MREE) and heavy REE
(HREE) was also observed in the regolith, although chondrite-normalized REE
patterns still showed LREE-enrichment.
Breakdown of abundant weathering-susceptible LREE-rich minerals in parent
granitoids e.g. allanite and REE-rich fluorocarbonate contributes to initial depletion of
REE, especially LREE, at early stages of weathering. The REE released may be
partially leached away by solutions, or alternatively, precipitated as secondary
LREE-rich phosphate minerals e.g. rhabdophane and florencite. Formation of
secondary phosphates indicates translocation of REE at the mineral scale, which is
important for retention of REE, especially LREE, as this limits their further mobility
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
200
during weathering. Residual accumulation of weathering-resistant minerals, e.g. zircon,
becomes more important as HREE hosts during advanced weathering and
lateritization.
The importance of stable mineral phases in controlling the occurrence of REE in
intensely weathered regolith is revealed by the dominance of residual species in the
sequential extraction. Therefore, the abundance and fractionation of REE in regolith
essentially correspond to the weighted mean of the abundance and composition of
LREE-rich secondary phosphates and HREE-rich weathering-resistant minerals, which
is closely associated with the weathering conditions (including weathering intensity,
weathering time, accessibility to solution and pH).
In addition to mineral phases, REE were retained in regolith by association with clay
minerals, Fe oxides/oxyhydroxides and organic ligands. Trace to minor amounts of
REE were hosted in water soluble (including adsorbed and exchangeable) species,
amorphous Fe oxyhydroxide and crystalline Fe oxide species, and organic matter
species, as revealed by the sequential extraction. Trace concentrations of Yb
(0.02-0.12 wt%) were substituted with and/or co-precipitated onto the iron nodules.
Fine-grained secondary REE-bearing phosphates were precipitated with crystalline Fe
oxides in the duricrust or incorporated into clay layers of iron nodules. The association
between Fe oxides/oxyhydroxides and REE suggests that Fe oxides and oxyhydroxides
are important for redistribution of REE at advanced stages of weathering.
Positive Ce anomalies in the duricrust of the GE (Ce*=6.1) and JG (Ce
*=25.3) profiles
were observed. This enrichment and fractionation of Ce was evidenced by neoformed
poorly crystalline (hydr)oxides associated with Zr and Th forming a rim on the walls of
Al/Fe-rich pores in the duricrust or along the boundary between Al-rich and Fe-rich
rims in iron nodules. Fractionation of Ce is evidently governed by oxidative processes,
which is consistent with precipitation of the ferric minerals observed in the duricrust.
The absence of any apparent Ce anomalies in secondary phosphates (Ce* ranged from
0.95-1.02) suggests that replacement of accessory REE-bearing minerals by secondary
phosphates will not result in a Ce anomaly.
Chapter Nine: Conclusion and future work
201
In particle size fractionations, silt and clay size fractions generally had higher
concentrations of REE than the sand size fraction, which usually contained lower
concentrations of REE but a higher mass of REE; this indicates the importance of
secondary phosphate formation, adsorption of REE by clay minerals, the dilution effect
of quartz and the presence of weathering-resistant minerals. In sequential extractions of
duricrust, crystalline Fe oxide species showed a preference for LREE, whereas
amorphous Fe oxyhydroxide species favoured MREE and organic matter species
favoured HREE. This suggests the sorption/complexation by different ligands affect
the fractionation of REE during weathering.
In addition, the affinity of Ce with Zr and Th in neoformed phases in the duricrust of
the JG profile suggests that Zr and Th are mobile at the sampling scale. Breakdown of
thorite and REE-bearing fluorocarbonates is believed to be the source for mobile Zr
and Th during early stages of weathering. Redistribution of Th into secondary
phosphates as a trace component and strong partitioning into gravel rather than matrix
showed translocation of Th at mineral assemblage and profile scales. The absence of
primary sphene crystals and the presence of partially weathered ilmenite and rutile in
the ferruginous mottled zone of the JG profile suggests Ti-hosting mineral sphene,
ilmenite and rutile transforms to anatase during intense weathering. The fluctuation of
Ti/Zr in the ferruginous zone, in contrast to the consistency of Zr/Hf throughout the
profile (within the range of the parent granitoids), suggests that Ti and Zr fractionate
from each other and partition between gravel and matrix during extreme weathering
and advanced lateritization. This proves that Ti, Zr and Th are mobile at a variety of
scales, despite their accepted use as reference elements for studying element mass flux
change.
9.2 Future work
First, this research lends support to the hypothesis that Ce fractionates from other REE
and is enriched in the duricrust and iron nodules through neoformation of poorly
crystalline phases associated with Zr and Th. Quantitative analysis revealed that Ce is
not associated with silicate or phosphate in at least two locations, and thus, the most
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
202
likely phase is proposed to be a (hydr)oxide. However, direct determination of the Ce
valence has not been achieved, and the cerianite phase has not been absolutely
identified by SXRD, most likely due to the poorly crystalline morphology and very
low concentration. Furthermore, the size of the rim (sub-micron) is also below the
resolution of the microprobe, and thus it is difficult to avoid interference from Si, Al
and Fe when using this technique. Given these constraints, it was not possible in this
study to conclusively determine the mode of occurrence of Ce, Zr and Th as
(hydr)oxides only. Therefore, it is suggested that better resolution imaging should be
obtained of micro-morphological characteristics of the neoformed phases using nano
secondary ion mass spectrometry (NanoSIMS) and/or transmission electron
microscopy (TEM). Furthermore, electron diffraction patterns may be obtained by
selected area electron diffraction with TEM (SAED-TEM). The oxidation states and
quantification of elements at sub-micron resolution can be achieved by electron
energy-loss spectroscopy (EELS) and synchrotron near edge x-ray absorption fine
structure spectroscopy (NEXAFS). All these information would greatly enhance our
understanding of the nature of Ce-Zr-Th affinity and the influence of Fe oxides and
oxyhydroxides on sequestering of REE. More detailed mineralogical information will
provide insight into geochemical signatures of Ce, Zr and Th during lateritic
weathering and yield useful information for the geochemical history of ferruginization
and for the scavenging mechanism of trace elements by iron nodules.
Second, the interpretation of preferential depletion of LREE and MREE over HREE is
hindered by the inadequate knowledge of aqueous geochemistry of REE. Lack of
systematic experimental complexation data of the whole series of REE at a wide range
of pH, and in the presence of multi-complex phases, constrains further understanding
of the mobilization and fractionation of REE in lateritic regolith. Significantly, REE in
aqueous phases have not been investigated in this study due to time and financial
limitations. The investigation of the abundance and fractionation of REE in soil
solutions, groundwater and surface water will greatly improve our understanding of the
release and migration of REE during water/rock interactions. In addition, by coupling
isotopic data (e.g. Sm and Nd) in soil and water samples, a better understanding of the
Chapter Nine: Conclusion and future work
203
genesis and the evolution history of the lateritic regolith and the geochemical pathways
as a function of water/rock interactions can be achieved.
Third, accumulation of REE in surface soils in both MQ profiles is also worthy of
further research. These topsoils are believed to include transported materials. Though
enrichment of REE by lateral transportation under the influence of soil creep or
colluviation has been considered, biogeochemical recycling is also likely. Thus, a
further study of REE concentrations in above- and below-ground tissues of the surface
plants would test an alternative hypothesis for the enrichment of REE in surface soils.
Fourth, the reason for the abnormal enrichment of Th in the lower mottled clay is not
clear with the current geochemical data. Particle size analysis suggests that this
accumulation is specifically in the clay and silt size fractions, but not quite correlated
to the concentrations of Zr, Ti and REE. Therefore, re-sampling of the regolith is
required to investigate whether this abnormal enrichment is at a profile scale or just a
sampling scale; furthermore, quantitative investigation of (i) Th-hosting mineralogy
based on polished thin section, and (ii) chemical speciation based on the sequential
extraction, are proposed to study the mode of occurrence of Th and the reason for the
repeatable extreme accumulation. The isotopic ratios of Th would also provide an
insight to the source of this enrichment.
Fifth, this study concentrated on lateritic profiles developed from similar granitoids. It
is accepted that the parent rock is a fundamental control on the subsequent
geochemical behaviour of REE during weathering; therefore, comparable research of
the geochemical behaviour of REE developed over contrasting rock types (e.g. dolerite)
would be interesting as a complementary study, and would improve our understanding
of the role of parent rock and weathering conditions in mobilizing REE.
Finally, despite lateral transportation at the sampling scale being considered minor in
this study, lateral redistribution at the landscape scale as a component of pedogenetic
processes have not been investigated. It would be worthwhile to investigate the REE
subject to both vertical and horizontal redistribution at a landscape or catchment scale.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
204
This is particularly relevant and important given the significant depletion of REE in the
intensely in-situ weathered regolith studied. This will further explain whether this
depletion is complete dispersion or translocation of REE and investigate the location of
accumulation of REE at both landscape and catchment scales if possible.
9.3 Summary
In summary, the substantial loss of REE from the four lateritic profiles studied is an
important consideration in the use of REE as tracers in studying pedogenesis and
weathering, especially in intensely weathered regolith. Primary accessory minerals in
the parent rock play a fundamental role in the abundance and mobilization of REE;
however, the subsequent mobility and fate of REE in these minerals is strongly
influenced by the pedological processes and weathering conditions. Primary
weathering-resistant minerals (e.g. zircon and anatase for HREE) and secondary
phosphates (e.g. rhabdophane and florencite for LREE) are predominant hosts for REE
in lateritic regolith and the weighted average of mineral abundance and REE
concentrations essentially dominates the redistribution and fractionation of REE in
regolith horizons and/or profiles. The difficulties in quantifying the abundance and
differentiating the REE signatures of these minerals constituting the soil horizons
restrict the understanding of the mobilization and fractionation of REE at multi scales
in the course of weathering. This thesis therefore applied a variety of suitable bulk and
in situ analytical techniques (e.g., electron microprobe, synchrotron X-ray fluorescence
microprobe, synchrotron X-ray powder diffraction) to arrive at conclusions on the
mobilization, fractionation and mode of occurrence of REE and associated elements at
both mineral and profile scales.
The results suggest that the redistribution of REE in the profiles studied is derived
from successive mobilization steps. First, REE are initially released from differential
weathering of primary accessory minerals, transported in solution and then partially
deposited as secondary phosphates in the reoglith. Second, some weathering-resistant
primary accessory minerals and secondary phosphates are dissolved and altered,
further releasing REE into solution under extreme weathering. Third, REE released in
Chapter Nine: Conclusion and future work
205
solution are absorbed and/or complexed by Fe oxides/oxyhydroxides, organic matter
and clay minerals in different horizons and retained in the regolith.
Variations of REE affinity for Fe oxides/oxyhydroxides demonstrate that the signature
of REE can be affected by Fe oxides and oxyhydroxides:
(i) Cerium was abnormally enriched as poorly crystalline (hydr)oxides
attaching onto the Al/Fe-rich pore walls in the ferruginous duricrust during
ferruginization.
(ii) Minor concentrations of REE (up to 10%) were associated with Fe
oxides/oxyhydroxides species in duricrust matrix extraction and trace
concentrations of Yb (up to 0.12 wt%) were determined by EPMA in cores
of iron nodules.
(iii) Micron-size REE-bearing phosphates co-precipitated with crystalline Fe
oxides in duricrust and distributed in the clay layer of iron nodules.
The association between REE, especially Ce, and Fe oxides/oxyhydroxides contributes
to the understanding of the occurrence and behaviour of REE in lateritic regolith and
sheds lights on the redox change and the weathering and lateritization history.
The importance of secondary phosphate and clay minerals on retention of REE is also
revealed by higher concentrations of REE in silt and clay size fractions than sand size
fraction in particle size analyses, which is consistent with higher concentrations of
REE determined in the Res and WAE than other species. The particle size fractionation
reflects the partitioning of REE being controlled by the size of REE-hosting phases
during weathering. This is an important consideration when a particle size sorting
process (e.g. transportation/mass movement/sedimentation) is involved.
In addition, the mobilization of Ti and Zr at the mineral scale, and Th at the profile
scale, was illustrated. Strong partitioning of Th into gravel, and apparent fractionation
of Ti and Zr in the ferruginous zone, suggest that these relatively conservative
elements can be mobile at different scales under extreme weathering and advanced
lateritization. This is often overlooked when using these elements as internal reference
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
206
elements to calculate element mass flux changes in supergene environment. This study
adds to and improves our understanding of the geochemical behaviour and mode of
occurrence of Ti, Zr and Th during supergene weathering.
207
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229
11 Appendices
Appendix 11.1 Abbreviation
AAG: the Association of Applied Geochemists;
A-CN-K: a ternary plot based on the concentrations of oxides of Al, Ca, Na and K, and
usually used together with the Chemical Index of Alteration to illustrate the
weathering intensity and the weathering trend;
A-CNK-FM: a ternary plot based on the concentrations of oxides of Al, Ca, Na, K, Fe
and Mg;
BSE: backscattering electron;
CANMET: the Canada Centre for Mineral and Energy Technology;
CEC: cation exchange capacity;
CIA: Chemical Index of Alteration, a weathering intensity index based on major
element molar proportion ratio;
CMCA: the centre for microscopy, characterisation and analysis in the University of
Western Australia;
CSC: the China Scholarship Council;
EDS: energy dispersive spectrometer;
EELS: electron energy-loss spectroscopy;
EPMA: electron probe micro-analyser;
FeAm: amorphous Fe oxyhydroxide species determined in sequential extraction
experiment;
FeCry: crystalline Fe oxide species determined in sequential extraction experiment;
GE: the first profile sampled beside the Great Eastern Highway in Western Australia;
GSFloading: the element mass loading in a selected grain size and the summation of the
indices for each soil sample equals 100%.
HREE: heavy rare earth elements according to atomic mass, refers from Eu to Lu in
this thesis;
ICDD: the International Centre for Diffraction Data;
ICP-MS: inductively coupled plasma mass spectroscopy;
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
230
ICP-OES: inductively coupled plasma-optical emission spectroscopy;
ISSS: the International Society of Soil Science;
JG: the fourth profile sampled in the Jarrahdale in Western Australia;
LREE: light rare earth elements according to atomic mass, refers from La to Sm in this
thesis;
MQ: the second and third profile sampled at the Mountain Quarry in Western
Australia;
MREE: middle rare earth elements according to atomic mass, refers from Pm to Ho
when REE was divided into three subgroups;
NanoSIMS: nano secondary ion mass spectrometry;
NASC: North American shales composite;
NIST: the National Institute of Standards and Technology;
Org: organic matter species determined in sequential extraction experiment;
PAAS: Post-Archean Australian Shale;
REE: rare earth elements, also known as lanthanides, sometime referring as ‘Ln’ in
chemical formula;
Res: residual species determined in sequential extraction experiment;
SAED-TEM: selected area electron diffraction with transmission electron microscopy;
S/SAF: the concentration ratio of SiO2/(SiO2+Al2O3+Fe2O3);
SE: secondary electron;
SEM: scanning electron microscope;
SIMS: secondary ion mass spectrometry ion probe;
SiO2-Al2O3-Fe2O3: a ternary plot to quantitatively illustrate the lateritization degree;
STSD: certified international standard stream sediment reference materials from
Canada Centre for Mineral and Energy Technology;
SXFM: synchrotron x-ray fluorescence microscopy;
SXRD: synchrotron x-ray powder diffraction;
TC: the total carbon;
TEM: transmission electron microscopy;
UCC: the upper crust composition;
UWA: the University of Western Australia;
Chapter Eleven: Appendices
231
WAE: water soluble, adsorbed, exchangeable and carbonate species determined in
sequential extraction experiment;
XRD: X-ray diffraction;
μ-XRF: micro X-ray fluorescence spectroscopy;
μ-XANES: micro X-ray absorption near edge fine structure spectroscopy;
23
2
Appendix 11.2 ICP-OES analyses of the reference standards determined repeatedly with samples for each analysis
Reference Major element concentrations
Al
308.215
Ca Fe
238.204
K
766.490
Mg
285.213
Na
589.592
Si
251.611
S Ti P Mn Zr
Unit % % % % % % % % % ppm ppm ppm
d.l. 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.1 0.1 0.1
Batch 11
STSD-2 8.33 2.92 5.07 1.74 1.88 1.29 25.3 0.04 0.47 0.14 0.11 178
STSD-2 8.38 2.94 5.14 1.76 1.88 1.33 25.3 0.05 0.47 0.14 0.11 181
STSD-2 8.05 2.66 4.96 1.61 1.71 1.26 22.1 0.09 0.46 0.12 0.10 171
av2 8.25 2.84 5.06 1.70 1.82 1.29 24.2 0.06 0.47 0.13 0.11 177
error(%)3 -3.2 -0.8 -3.7 -2.2 -2.6 2.5 -3.4 0.0 -3.1 1.0 -0.7 -4.6
RSD4 0.2 0.2 0.1 0.1 0.1 0.0 1.9 0.0 0.0 0.0 0.0 4.8
Batch 2
STSD-2 8.36 2.92 5.11 1.79 1.95 1.31 25.3 0.06 0.45 0.13 0.10 196
STSD-2 8.23 2.99 5.26 1.78 1.93 1.32 25.0 0.07 0.46 0.14 0.10 190
av 8.29 2.96 5.19 1.79 1.94 1.31 25.1 0.06 0.46 0.14 0.10 193
error(%) -2.7 3.4 -1.2 2.7 3.8 4.0 0.1 2.5 -4.5 4.4 -2.7 4.4
RSD 0.1 0.1 0.1 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 3.8
Batch 3
STSD-2 8.93 3.00 5.33 1.78 1.98 1.35 25.3 0.06 0.48 0.13 0.11 184
STSD-2 8.73 3.01 5.36 1.79 1.90 1.27 25.3 0.07 0.48 0.14 0.11 187
STSD-2 8.54 2.93 5.32 1.73 1.94 1.25 24.2 0.06 0.47 0.14 0.11 176
STSD-2 8.68 3.04 5.46 1.76 1.98 1.39 25.2 0.06 0.48 0.14 0.11 174
av 8.72 2.99 5.37 1.77 1.95 1.32 25.0 0.06 0.48 0.14 0.11 180
error(%) 2.4 4.7 2.3 1.4 4.3 4.4 -0.3 2.0 -1.0 4.3 2.1 -2.5
RSD 0.2 0.1 0.1 0.0 0.0 0.1 0.5 0.0 0.0 0.0 0.0 6.5
exp
5 STSD-2 8.52 2.86 5.25 1.74 1.87 1.26 25.1 0.06 0.48 0.13 0.11 185
23
3
Reference Major element concentrations
Al
308.215
Ca Fe
238.204
K
766.490
Mg
285.213
Na
589.592
Si
251.611
S Ti P Mn Zr
Unit % % % % % % % % % ppm ppm ppm
d.l. 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.1 0.1 0.1
OREAS 43p 5.02 0.29 16.78 1.88 0.59 0.19 26.7 0.02 0.29 0.05 0.07 245
OREAS 43p 4.94 0.31 17.25 1.91 0.59 0.10 26.5 0.02 0.29 0.05 0.07 240
av 4.98 0.30 17.01 1.89 0.59 0.14 26.6 0.02 0.29 0.05 0.07 242
exp 5.08 0.31 17.49 1.78 0.57 0.13 26.8 0.02 0.30 0.04 0.06 232
error(%) -1.2 -0.4 -9.1 6.4 1.2 1.1 -1.0 4.6 -1.7 7.3 3.1 5.7
RSD 0.1 0.0 0.3 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.1
1Batch refers to each time determination of major element concentrations by ICP-OES based on the profile;
2av: average value;
3error(%)=[(sample determined value)-(expected value)]/(expected value)×100%;
4RSD: relative standard deviation;
5exp: expected/recommended value.
These footnotes are also used in Appendix 11.6.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
234
Appendix 11.3 R script for principal component analysis of major elements
(‘#’ is the explanation of the command with italic font)
#view data table
major<-read.table("D:/XIN/experimental results(desktop)/analysis/sta/major
oxides.csv",sep=",", header = TRUE)
library(Rcmdr)
str(major)
print(major)
#PCA of centered log ratio transformed major element data
(R cannot recognize subscript, so the the subscript was not used in the oxides)
pc1<-prcomp(~Al2O3_clr +CaO_clr +Fe2O3_clr +K2O_clr +MgO_clr +Na2O_clr
+SiO2_clr +TiO2_clr +ZrO2_clr +P2O5_clr +MnO_clr, scale=TRUE, data=major)
summary(pc1)
print(pc1)
pc1$sd^2 # eigenvalues (variances)
plot(pc1) # scree plot
TypeSymb<-cbind(major$Type)
# biplot PC1 vs. PC2
biplot(pc1, col = c("#000000", "#999999"), xlabs=TypeSymb)
predict(pc1)[,1-8]
write.table(predict(pc1)[,1-8],"D:/XIN/experimental results(desktop)/analysis/sta/x.csv
", sep=",")
Chapter Eleven: Appendices
235
Appendix 11.4 Photographs of polished thin sections of iron nodules mounted on
quartz slides from the JG profile
From left to right are: JG9, the A horizon, 0.3 m depth; JG6, the upper ferruginous
zone, 1.5 m depth; JG10, the A horizon, 0.4 m depth. Nodules from the upper
ferruginous zone were red and concentrically zoned, with a core of hematite
surrounded by goethite and Al-rich rims, whereas nodules from the A horizon were
dark brown-black, non-concentrically zoned, with cemented clay matrix and Fe oxides
without layers, containing less gibbsite but more quartz.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
236
Appendix 11.5 Detailed operation procedure of the sequential extraction method
Extraction WAE:
1. To 1 g of sample in a 50 mL screw-cap centrifuge tube, add 20 mL of 1.0 M
CH3COONa (at pH 5 with CH3COOH) and cap.
2. Vortex contents for 5-10 s and place in an end-to-end tumbler at 25 °C constant
temperature for 6 h.
3. Centrifuge for 15 min. at 3000 rpm and decant supernatant liquid into a labelled
test-tube. Rinse residue with 5 ml of MilliQ water, vortex and centrifuge again; repeat
and add supernatant rinses to the test-tube. Make up to the 30 mL mark with MilliQ
water and analyse.
4. Carry out a second 20 mL 1M CH3COONa leach of the residue, repeating steps 2
and 3.
Extraction Org:
5. To the residue, add 40 mL of 0.1 M Na4P2O7.
6. Vortex contents for 5-10 s and place the centrifuge tubes in an end-to-end tumbler
at 25 °C constant temperature for 1 h and then centrifuge.
7. Decant the supernatant into a new labeled test-tube.
8. Rinse the residue with 5 mL H2O, centrifuge; do this twice and add the supernatant
to the test-tube; mark up to 50 mL.
9. Repeat steps 5 to step 8.
Extraction FeAm:
10. To the residue from step 9, add 20 mL of 0.25 M NH2OH∙HCl in 0.25 M HCl, cap
and vortex for 5-10 s.
11. Place in a water bath at 60 °C for 2 h with cap loosened. Every 30 min., cap tightly
and vortex the contents.
12. Centrifuge for 15 min. and decant supernatant liquid into a labeled test-tube. Rinse
residue with 5 mL of water, vortex and centrifuge again; repeat and add supernatant
Chapter Eleven: Appendices
237
rinses to the test-tube. Make up to the 30 mL mark with MilliQ water and analyse.
13. Carry out a second 0.25 M NH2OH∙HCl leach of the residue but heat for only 30
min.
Extraction Fecry:
14. To the residue from step 13, add 30 mL of 1.0 M NH2OH∙HCl in 25% CH3COOH,
cap and vortex for 5-10 s.
15. Place in a water bath at 90 °C for 3 h with cap on tightly. Vortex contents every
30 min.
16. Centrifuge for 15 min. and decant supernatant liquid into a labeled test-tube. Rinse
residue with 10 mL of 25% CH3COOH, vortex and centrifuge again; repeat and add
supernatant rinses to the test-tube. Make up to the 50.0 mL mark with MilliQ water and
analyse.
17. Carry out a second 1.0 M NH2OH∙HCl leach of the residue but heat for only 1.5 h.
Extraction Res:
18. Wash the residues with MilliQ water and dry in the oven at 60 °C. Add 0.1000 g
dried residue sample and 0.7000 g 12:22 Norrish flux (Lithium metaborate/ Lithium
tetraborate) into a pure platinum crucible.
19. Fuse at 1050 °C for 40 min.
20. Remove crucible from furnace and allow cooling. Place crucible into a labelled
120 mL polypropylene screw cap vial.
21. Add 100 ml of 10% HCl using a calibrated dispenser.
22. Place on tumbler for ½ hour or until bead is fully dissolved. Occasionally the
ultrasonic bath may be required to speed up dissolution.
23. Sore the solutions in cool room at 5 ºC prior to analysis by ICP-MS the next day.
Total REE and Residue REE
24. Total and residual REE content was determined by sampling 0.1000 g of dried
residual soil with 0.7000 g flux fused at 1050 ºC in furnace.
Occurrence and behaviour and rare earth and associated elements in lateritic regolith profiles in Western Australia
238
Apparatus
All laboratory-ware should be of borosilicate glass and the centrifuge tubes of
polypropylene. Vessels in contact with samples or reagents should be cleaned by
soaking in 10% HCl (overnight) and rinsed repeatedly with distilled water and MilliQ
water before use.
For each batch of extractions, dry a separate 1 g sample in an oven (105±2 °C) until a
constant mass is achieved. From this, a correction ‘to dry mass’ is obtained which
should be applied to all analytical values reported (i.e., results should be quoted as
amount of metal per gram of dry sediment).
The tools include:
20 mL dispenser; 5 mL pipette; 50 mL dispenser; 30 mL dispenser; 10 mL pipette;
Reagents
All reagents should be of analytical-reagent grade or better. MilliQ water should be
used throughout.
Solution A (Sodium acetic, 1.0 M NaAc, buffer at pH5 with acetic acid HAc):
Dissolve 136.08 g NaAc∙3H2O (136.08 g/mol) in MilliQ water and dilute to 0.9 L in a
fume cupboard. Adjust to pH 5 by adding glacial acetic acid (HAc). Wash the
electrodes of pH meter thoroughly before placing it in the extracting solution. Make
the volume to 1 L with MilliQ water and store in sealed plastic containers.
Solution B (Sodium Pyrophosphate, 0.1 M Na4P2O7):
Dissolve 44.606 g Na4P2O7∙10H2O (446.06 g/mol) in MilliQ water and make the
volume to 1 L. Store in sealed plastic containers.
Solution C (hydroxylamine hydrochloride, 0.25 M NH2OH∙HCl, in 0.25 M HCl):
Dissolve 17.373 g NH2OH∙HCl (69.49 g/mol) in 200 mL MilliQ water and add
24.6 mL 32% HCl and make up to 1 L volume with distilled water. Prepare this
solution on the same day the extraction is carried out.
Chapter Eleven: Appendices
239
Solution D (hydroxylamine hydrochloride, 1.0 M NH2OH∙HCl, in 25% HAc):
Dissolve 69.49 g NH2OH∙HCl (69.49 g/mol) in 200 mL MilliQ water and add 250 mL
glacial acetic acid (HAc) and Make up to 1 L volume with distilled water. Prepare this
solution on the same day the extraction is carried out.
Blanks
Vessel blank. To one vessel from each batch, taken through the cleaning procedure, add
40 mL of solution A. Analyse this blank solution along with the sample solutions from
step 1.
Reagent blank. Analyse a sample of each batch of solutions A, B, C and D.
Procedural blank. With each batch of extractions, a blank sample (i.e., a vessel with no
sediment) should be carried through the complete procedure and analysed at the end of
each extraction step.
24
0
Appendix 11.6 ICP-MS analyses of the reference standards determined repeatedly with samples for each analysis
reference Trace element concentrations
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th
unit ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm
Batch 1
STSD-2 56.1 90.0 11.3 43.5 8.0 2.19 8.49 1.27 6.32 1.34 3.44 0.54 3.41 0.56 33.8 18.7
STSD-2 53.4 84.5 10.5 40.2 7.6 2.07 7.80 1.23 6.07 1.14 3.54 0.44 3.35 0.62 35.9 17.6
av 54.7 87.3 10.9 41.9 7.8 2.13 8.15 1.25 6.20 1.24 3.49 0.49 3.38 0.59 34.9 18.2
error% -7.2 -6.1 -2.5 -2.7 -2.8 6.4 -3.0 -7.6 -4.6 -8.3 -8.2 -18.4 -8.7 -15.7 -5.8 5.6
RSD 1.9 3.9 0.6 2.3 0.3 0.1 0.5 0.0 0.2 0.1 0.1 0.1 0.0 0.0 1.5 0.8
1Batch 2
STSD-2 55.9 94.9 11.6 44.9 8.72 2.20 9.05 1.35 7.10 1.45 3.91 0.57 3.61 0.62 38.3 19.4
STSD-2 54.9 92.4 11.7 45.1 8.66 2.06 7.72 1.17 5.87 1.31 3.54 0.51 3.34 0.53 34.4 16.5
av 55.4 93.6 11.7 45.0 8.69 2.13 8.38 1.26 6.49 1.38 3.72 0.54 3.48 0.58 36.4 17.9
error% -6.1 0.7 4.1 4.7 8.6 6.5 -0.2 -6.9 -0.2 2.5 -2.0 -10.3 -6.0 -17.4 -1.7 4.3
RSD 0.7 1.8 0.1 0.2 0.0 0.1 0.9 0.1 0.9 0.1 0.3 0.0 0.2 0.1 2.7 2.1
Batch 3
STSD-2 53.5 87.7 11.1 42.6 8.27 2.03 7.87 1.26 6.27 1.35 3.72 0.53 3.51 0.56 33.8 19.1
STSD-2 56.1 92.6 11.4 43.8 8.34 2.06 7.84 1.27 6.39 1.35 3.63 0.54 3.40 0.55 34.2 18.2
STSD-2 55.5 91.5 11.3 43.9 8.15 1.96 7.71 1.26 6.33 1.35 3.56 0.52 3.49 0.55 33.8 18.2
STSD-2 55.2 89.9 11.3 43.3 8.29 1.99 7.96 1.29 6.80 1.46 3.88 0.58 3.58 0.58 36.2 17.7
av 55.1 90.5 11.3 43.4 8.26 2.01 7.85 1.27 6.45 1.38 3.70 0.54 3.50 0.56 34.5 18.3
error% -6.7 -2.7 0.7 1.0 3.3 0.5 -6.6 -5.9 -0.8 2.0 -2.7 -9.7 -5.5 -19.9 -6.8 6.4
RSD 1.1 2.1 0.1 0.6 0.1 0.0 0.1 0.0 0.2 0.1 0.1 0.0 0.1 0.0 1.1 0.6
24
1
reference Trace element concentrations
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y Th
unit ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm 2Sequential
STSD-2 53.6 86.5 11.8 41.7 7.80 2.00 7.90 1.20 6.30 1.30 3.50 0.60 3.40 0.60 36.4 17.9
STSD-2 54.2 87.7 11.8 42.0 7.80 1.90 7.60 1.20 6.10 1.30 3.50 0.60 3.30 0.60 35.3 16.8
av 53.9 87.1 11.8 41.9 7.80 1.95 7.75 1.20 6.20 1.30 3.50 0.60 3.35 0.60 35.9 17.4
exp 59.0 93.0 11.2 43.0 8.00 2.00 8.45 1.30 6.50 1.35 3.78 0.62 3.70 0.70 37.0 17.2
error% -8.6 -6.3 5.4 -2.7 -2.5 -2.5 -8.3 -7.7 -4.6 -3.7 -7.4 -3.2 -9.5 -14.3 -3.1 0.9
RSD 0.4 0.8 0.0 0.2 0.0 0.1 0.2 0.0 0.1 0.0 0.0 0.0 0.1 0.0 0.8 0.8
exp STSD-2 59.0 93.0 11.2 43.0 8.00 2.00 8.40 1.35 6.50 1.35 3.80 0.60 3.70 0.70 37.0 17.2
1Batch 1 (GE profile) and Batch 2 (MQ profiles) were determined together by ICP-MS;
2Sequential refers to the determination of trace element concentrations in sequential extractions and particle size fractions by ICP-MS.
24
2
Appendix 11.7 EPMA detection limits of element concentrations in Ti-, Zr- and Th- bearing minerals in the JG profile
No. Min Element concentrations (wt%)
Si Zr Ti Pb Th U Al Y La Ce Pr Nd Sm Eu Gd Dy Yb Lu Fe Ca Sr K P F
1 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.04 0.12
2 Thr 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.12
3 Fc 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.07
4 Fc 0.01 0.03 0.01 0.02 0.01 0.02 0.01 0.01 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.08
5 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.02 0.03 0.01 0.02 0.04 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.11
6 Spn 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.02 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.14
7 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.03 0.04 0.01 0.02 0.04 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.12
8 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.03 0.03 0.01 0.02 0.04 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.11
9 Rt 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.03 0.02 0.01 0.02 0.02 0.01 0.02 0.01 0.00 0.01 0.01 0.01 0.15
Zrn-zircon; Thr-thorite; Fc-fluorocarbonate; Ilm-ilmenite; Spn-sphene; Rt-rutile.
Appendix 11.8 EPMA detection limits of element concentrations in minerals of parent granitoids and regolith samples from the GE and MQ profiles
No. Min Element concentrations (wt%)
Si Zr Ti Pb Th U Al Y La Ce Pr Nd Sm Eu Gd Tb Dy Er Tm Yb Lu Fe Mg Ca Sr Na K P S F
10 Aln 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.03 0.02 0.01 0.02 0.09 0.03 0.02 0.02 0.01 1.73 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.09
11 Aln 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.03 0.02 0.01 0.02 0.09 0.03 0.02 0.02 0.01 1.75 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.09
12 Aln 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.03 0.02 0.01 0.02 0.09 0.03 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.10
13 MnzI 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
14 MnzI 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
15 MnzI 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.10
16 MnzII 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.01 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.03 0.02 0.02 0.02 0.01 2.11 0.01 0.01 0.01 0.01 0.03 0.01 0.01 0.01 0.12
17 MnzII 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.01 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.03 0.02 0.02 0.02 0.02 2.11 0.01 0.01 0.01 0.01 0.03 0.01 0.01 0.01 0.12
18 MnzII 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.03 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.10
19 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.05 0.01 0.11
20 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.04 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.04 0.01 0.12
21 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.05 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.04 0.01 0.12
22 Thr 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.05 0.05 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.02 0.01 0.13
23 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.02 0.04 0.01 0.02 0.16 0.04 0.04 0.03 0.01 1.73 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.11
24 Fsp 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.04 0.03 0.03 0.02 0.02 0.01 0.02 0.09 0.03 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.13
25 Fc 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.05 0.02 0.01 0.02 0.07 0.03 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.07
26 Fc 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.05 0.02 0.01 0.02 0.07 0.03 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.08
27 Fc 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.06 0.03 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.07
28 Mnz 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
29 Mnz 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
30 Mnz 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
31 Aln 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.09 0.03 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.12
32 Aln 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.03 0.03 0.02 0.03 0.02 0.01 0.02 0.09 0.03 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.09
33 Aln 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.03 0.02 0.01 0.02 0.09 0.03 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.10
34 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.02 0.03 0.01 0.02 0.15 0.04 0.04 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.10
35 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.02 0.04 0.01 0.02 0.14 0.04 0.03 0.03 0.01 0.01 0.01 0.01 0.00 0.01 0.02 0.01 0.01 0.01 0.12
36 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.03 0.04 0.01 0.02 0.12 0.03 0.03 0.02 0.01 0.01 0.01 0.01 0.00 0.01 0.02 0.01 0.01 0.01 0.12
No. Min Element concentrations (wt%)
Si Zr Ti Pb Th U Al Y La Ce Pr Nd Sm Eu Gd Tb Dy Er Tm Yb Lu Fe Mg Ca Sr Na K P S F
37 Spn 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.02 0.02 0.01 0.02 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.13
38 Spn 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.13
39 Spn 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.02 0.02 0.01 0.02 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.07
40 Ap 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.14
41 Ap 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.03 0.02 0.01 0.02 0.03 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.14
42 Ap 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.03 0.02 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.13
43 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.03 0.02 0.01 0.02 0.03 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.05 0.01 0.12
44 Mnz 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
45 Mnz 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
46 Mnz 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.04 0.04 0.02 0.01 0.02 0.03 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.09
47 Rbp 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 2.14 0.01 0.02 0.01 0.01 0.04 0.01 0.01 0.01 0.10
48 Rbp 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.04 0.02 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.08
49 Rbp 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.02 0.02 2.14 0.01 0.02 0.01 0.01 0.04 0.01 0.01 0.01 0.09
50 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.02 0.03 0.01 0.02 0.16 0.04 0.04 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.10
51 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.03 0.03 0.01 0.02 0.17 0.04 0.04 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.10
52 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.04 0.03 0.03 0.01 0.02 0.18 0.04 0.04 0.03 0.01 1.75 0.01 0.01 0.01 0.01 0.03 0.01 0.01 0.01 0.11
53 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.03 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.04 0.01 0.11
54 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.03 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.04 0.01 0.11
55 Zrn 0.01 0.03 0.01 0.02 0.01 0.02 0.01 0.02 0.04 0.03 0.03 0.03 0.02 0.01 0.02 0.02 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.04 0.01 0.12
56 Thr 0.01 0.04 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.04 0.03 0.03 0.02 0.01 0.02 0.07 0.03 0.02 0.02 0.02 2.12 0.01 0.01 0.01 0.01 0.03 0.01 0.02 0.01 0.10
Aln: allanite; Ap: apatite; Fc: REE-rich fluorocarbonate; Fsp: feldspar;Ilm: ilmenite; Mnz: monazite, I and II refers to Type 1 or Type 2; Rbp: rhabdophane; Spn: sphene; Thr:
thorite; Zrn: zircon.
24
5
Appendix 11.9 Concentrations of REE in grain size fractions of the MQ II profile
Sample Element concentrations(ppm)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y
d.l. 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
MQ15sand 49.9 85.6 8.6 26.1 4.3 0.4 3.6 0.8 5.5 1.4 4.6 0.8 5.1 0.8 44.3
MQ15silt 54.1 107.8 9.7 30.4 5.5 1.1 4.5 0.8 5.0 1.1 3.2 0.5 3.1 0.5 31.0
MQ15clay 55.1 133.4 9.6 30.6 5.6 1.5 5.0 0.9 5.5 1.2 3.3 0.5 2.7 0.4 33.4
MQ15matrix 66.6 114.9 10.2 32.6 5.4 0.7 5.4 0.7 3.5 0.6 1.9 0.2 1.5 b.d. 19.0
MQ15gravel 35.9 66.3 6.3 18.2 3.8 0.8 3.2 0.5 2.9 0.7 1.8 0.3 1.8 0.3 13.9
MQ14sand 37.3 61.5 6.1 18.6 2.7 0.2 2.0 0.3 2.3 0.5 1.7 0.3 1.7 0.3 16.5
MQ14silt 53.4 103.7 10.3 32.4 5.7 1.1 4.5 0.8 5.2 1.2 3.4 0.6 3.6 0.6 29.2
MQ14clay 31.7 51.2 6.8 22.1 4.5 1.2 3.6 0.7 4.6 1.0 3.0 0.5 2.9 0.5 25.3
MQ14matrix 49.1 81.3 7.8 26.0 4.7 0.8 4.3 0.7 3.3 0.7 2.3 0.3 2.2 0.1 18.7
MQ14gravel 29.9 53.5 5.4 15.9 3.3 0.8 2.9 0.5 2.6 0.7 1.8 0.3 1.9 0.3 12.9
MQ13sand 10.3 17.8 1.7 5.4 0.9 b.d. 0.6 0.1 0.6 0.1 0.4 b.d. 0.4 b.d. 3.8
MQ13silt 26.1 63.3 5.1 16.6 3.0 0.6 2.3 0.4 2.9 0.6 2.0 0.4 2.3 0.4 15.9
MQ13clay 12.6 40.6 3.0 10.3 2.2 0.6 1.9 0.4 2.3 0.5 1.7 0.3 1.9 0.3 13.2
MQ13matrix 27.8 54.2 4.2 14.0 3.1 0.5 3.1 0.4 2.2 0.5 1.7 0.2 2.0 0.1 13.3
MQ13gravel 20.4 46.2 4.1 12.1 2.6 0.5 2.3 0.4 2.3 0.5 1.6 0.3 1.8 0.3 9.9
MQ12sand 1.6 3.0 0.2 0.7 0.2 b.d. 0.2 b.d. 0.3 b.d. 0.2 b.d. 0.3 b.d. 2.1
MQ12silt 20.6 40.0 3.6 11.2 1.9 0.3 1.5 0.3 1.9 0.5 1.6 0.3 1.9 0.3 13.8
MQ12clay 9.7 21.2 1.8 6.0 1.2 0.3 1.0 0.2 1.3 0.3 1.0 0.2 1.1 0.2 9.1
MQ12matrix 10.7 19.1 1.6 5.3 1.0 0.4 1.3 0.2 1.0 0.2 0.8 0.2 0.9 b.d. 7.4
MQ12gravel 7.5 14.5 1.3 4.0 1.0 0.2 0.8 0.2 1.1 0.3 0.8 0.2 1.0 0.2 5.6
MQ11sand 5.6 10.0 0.9 2.9 0.5 b.d. 0.4 b.d. 0.5 0.1 0.4 b.d. 0.6 0.1 3.9
MQ11silt 25.0 41.7 4.2 13.1 2.1 0.3 1.6 0.3 2.0 0.5 1.6 0.3 2.0 0.3 14.6
MQ11clay 11.2 15.2 1.8 5.8 1.1 0.3 1.0 0.2 1.3 0.3 1.0 0.2 1.0 0.2 8.9
MQ11matrix 7.6 12.9 1.1 4.1 1.2 0.3 0.7 0.1 0.9 0.2 0.6 0.1 1.0 b.d. 6.3
MQ11gravel 6.0 10.0 1.0 2.9 0.9 0.3 0.6 0.1 0.7 0.2 0.6 0.1 0.7 0.1 3.9
24
6
Sample Element concentrations(ppm)
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Y
d.l. 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
MQ10sand 6.2 10.5 1.0 3.2 0.6 b.d. 0.4 b.d. 0.4 0.1 0.3 b.d. 0.5 0.1 3.1
MQ10silt 41.1 69.6 6.7 20.7 3.2 0.3 2.1 0.4 2.4 0.6 1.9 0.3 2.3 0.4 17.1
MQ10clay 7.9 11.7 1.2 3.9 0.7 0.2 0.6 0.1 0.9 0.2 0.7 0.1 0.7 0.1 6.2
MQ10matrix 10.3 16.3 1.4 4.6 1.5 0.2 0.8 0.1 0.8 0.1 0.5 0.1 0.6 b.d. 5.6
MQ10gravel 5.3 9.4 0.9 2.5 0.7 0.3 0.5 0.1 0.5 0.1 0.4 0.1 0.5 0.1 2.7
24
7
Appendix 11.10 EPMA detection limits of element concentrations in REE-bearing minerals from the JG profile
No. Min Element concentrations (wt%)
Si Zr Ti Pb Th U Al Y Ce Pr Nd Sm Eu Gd Dy Yb Lu Fe Mg Ca Sr P S
57 Fc 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01
58 Fc 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.04 0.03 0.04 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01
59 Ap 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.02 0.02 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
60 Fsp 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.03 0.02 0.02 0.01 0.01 0.03 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
61 Mag 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.02 0.02 0.01 0.02 0.06 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01
62 Mag 0.01 0.03 0.01 0.02 0.01 0.02 0.01 0.01 0.03 0.02 0.04 0.03 0.03 0.05 0.07 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.01
63 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.03 0.03 0.03 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.04 0.01
80 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.04 0.03 0.03 0.01 0.02 0.01 0.01 1.69 0.01 0.01 0 0.01 0.01 0.01
81 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.04 0.02 0.02 0.01 0.02 0.04 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01
82 Ilm 0.01 0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.03 0.04 0.03 0.05 0.01 0.02 0.04 0.01 1.72 0.01 0.01 0.01 0.01 0.01 0.01
83 TiO 0.01 0.02 0.01 0.01 0.01 0.01 0 0.01 0.03 0.04 0.02 0.02 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0 0.01 0.01 0.01
84 Zrn 0.01 0.03 0.01 0.02 0.01 0.02 0.01 0.02 0.03 0.03 0.03 0.02 0.01 0.02 0.02 0.01 1.89 0.01 0.01 0.01 0.01 0.04 0.01
85 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.03 0.03 0.03 0.02 0.01 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.05 0.01
86 Zrn 0.01 0.03 0.01 0.02 0.01 0.01 0.01 0.02 0.03 0.03 0.03 0.02 0.01 0.02 0.03 0.01 0.02 0.01 0.01 0.01 0.01 0.04 0.01
87 Zrn 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.03 0.03 0.03 0.02 0.01 0.02 0.02 0.01 0.02 0.01 0.01 0.01 0.01 0.05 0.01
88 Thr 0.01 0.03 0.01 0.02 0.02 0.02 0.01 0.02 0.05 0.04 0.04 0.03 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.01 0.02
Ap: apatite; Fc: REE-rich fluorocarbonate; Fsp: feldspar; Ilm: ilmenite; Mag: magnetite; TiO: titanium oxides (rutile/anatase); Thr: thorite; Zrn: zircon.