Chapter 6 Magmatic inclusions in groundmass minerals of ... · Chapter 6 Magmatic inclusions in minerals 148 crystal is fractured in the presence of melt or fluid, then the fractures

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Chapter 6 Magmatic inclusions in minerals

147

Chapter 6 Magmatic inclusions in groundmass

minerals of the Udachnaya-East kimberlite

Research into identification of kimberlite liquids has been hampered by several

factors. Most significant among these is the presence of variable amounts of mineral

grains and mineral aggregates that are probably unrelated to the kimberlite

crystallisation (xenocrysts and xenoliths), but are hard to separate from true phenocrysts

and cognate inclusions especially, in altered rocks. A related problem is syn-

crystallisation degassing of volatiles (CO2 and H2O), which is assumed to be significant

in kimberlitic magmas. It is now well accepted that inclusions of melt and fluid trapped

in phenocrysts during crystallisation may represent true magmatic compositions.

Magmatic (crystal, melt and fluid) inclusions in phenocrysts provide a snapshot of a

particular evolutionary stage of the magma, as sampled at a particular time, place and

physical and chemical conditions (e.g., reviews in Frezzotti, 2001; Kamenetsky et al.,

2003; Lowenstern, 2003; Roedder, 1979; Roedder, 1984; Roedder, 1992; Sobolev,

1996).

Magmatic inclusions within phenocrysts are present in almost all types of

intrusive and volcanic rocks, and they have even been reported in mantle minerals.

When crystals grow or recrystallise in magma or a fluid, they trap small batches of melt

or fluid, or other crystals, preserving them as inclusions. The mechanisms and

conditions of trapping can be very different, even for inclusions within a single crystal.

One of the classification schemes describes the origin of inclusions and divides them

into three different types.

The most informative primary inclusions are trapped by a crystal from a

medium from which it forms. This kind of inclusion provides information on the

chemical and physical conditions at the moment of the crystal's formation. To recognise

them, one should look for individual inclusions, or for the clusters of two to several tens

of randomly distributed inclusions, or for a group of inclusions which form a regular

zoning pattern that follows relic crystal-melt, or crystal-fluid interfaces.

Another type is secondary inclusions. Secondary inclusions are those that form

by any process after crystallisation of the host mineral is essentially complete. If a


148

crystal is fractured in the presence of melt or fluid, then the fractures could be filled

with the material, which often has no genetic relation to the crystal. Secondary

inclusions are often aligned along fractures, and this helps to identify them.

Usually we think of primary inclusions as forming during the growth of the

crystal and of secondary inclusions as forming at some later time, from entirely different

magmas or fluids. Pseudosecondary inclusions are broadly intermediate between

primary and secondary. They look secondary, but are formed during the growth of the

host crystal, if the crystal fractures during formation.

Another inclusion classification scheme is based on phase composition and

appearance of inclusions. These subtypes are: crystal, melt, and fluid, but all three

phases may occur within one inclusion. In general, any given magmatic inclusion can

contain several different crystal phases, one or two different silicate glasses, immiscible

sulphide globules, aqueous saline liquid and one to several vapour bubbles.

From magmatic inclusion studies, combined with suitable theoretical,

experimental and numerical treatment, the composition, temperature and pressure of the

primary melt can be successfully reconstructed. Thus, the study of different types of

inclusions in the groundmass minerals, especially olivine, may overcome the problems

mentioned above and put constraints on the origin and evolution of kimberlitic magma.

Phenocryst-hosted melt and fluid inclusions in kimberlite offer a new window to

deciphering the geochemical budget of parental magmas, and therefore may refine our

understanding of the composition and evolution of this important magma type. The

great advantage of inclusion studies compared to an experimental and theoretical

approach is that they directly apply to natural melts and fluids, and the use of modern

microbeam analytical techniques, vital for success of this project, can put solid

constraints on element partitioning and the ultimate concentrations and fate of volatile

elements.

Only a few studies of melt and fluid inclusions in kimberlite groundmass olivine

have been published to date, and all of them concern the Udachnaya-East pipe. The first

study by Popivnyak and Laz'ko (1979) reported the discovery of extremely rare

primary, partly crystallized melt inclusions, fluid (“gas-glass” or “gas” inclusions) and


149

sporadic pseudosecondary inclusions in the groundmass olivine of the Udachnaya pipe.

The authors stated that most studied inclusions were small (less than 1 µm) and

homogenized at temperatures 780-1100oC. The reliability of these results is

questionable, because it is impossible to observe any changes in such small inclusions

during experiment. No chemical compositions of melt and fluid inclusions were

reported in this study.

Later, Sobolev et al. (1989) performed a more detailed study of magmatic

inclusions in the groundmass olivine, but mainly focused on melt and fluid inclusions in

lamproitic olivine. Lesser details were provided from the study of kimberlite olivine-

hosted melt inclusions. They showed that inclusions in olivine-II were represented by

different crystal phases (chromite, low-Ca pyroxene, olivine, phlogopite, ilmenite and

rutile), nearly pure CO2 fluid with density as high as 0.75 g/cm-3 (no H2O was detected

in fluid inclusions), multiphase inclusions consisting “mainly of high-density fluid with

minor melt”, and secondary melt inclusions. Heating stage experiments with secondary

melt inclusions demonstrated low temperatures of homogenization (600-650 oC) and

authors envisaged a non-silicate nature (concentrated brines) for these inclusions based

on their low viscosity. “Normal” silicate melt inclusions were not been found in this

study. The temperature (1100 ± 30oC) of the olivine-II crystallization was also

estimated using the olivine-spinel geothermometer of Fabries (1979) and the graphical

version of the two-pyroxene thermometer of Lindsley (1983). The pressure of

crystallization (4-5 kbar, probably the lowest limit) was deduced from the density of

CO2-rich fluid inclusions.

The most recent melt inclusion study of the Udachnaya-East kimberlite (Golovin

et al., 2003) was performed at the same time as my research and on the same samples.

The publication by Golovin et al. (2003) postdates the presentation of my melt inclusion

studies in 2002 at the Goldschmidt Geochemical Conference in Davos, Switzerland and

at the Workshop-Short Course on volcanic systems “Melt inclusions: methods,

applications and problems” in Napoli, Italy (Kamenetsky et al., 2002a; Kamenetsky et

al., 2002b). Several coauthors of my publication (N.P. Pokhilenko and N.V. Sobolev)

also participated in the paper by Golovin et al. (2003), without my consent. Melt

inclusions in both generations of olivine described by Golovin et al. (2003) were


150

interpreted to be secondary, and composed of finely crystallized aggregates of

carbonates (calcite, dolomite, siderite, and shortite-zemkorite), different sulphates, Na-

K chlorides, silicates (tetraferriphlogopite, phlogopite, olivine, humite-clinohumite,

diopside), oxides (magnetite series), and sulphides (Ni-pyrrhotite and djerfisherite). As

it shown below, many results of my study and those of Golovin et al. (2003) overlap,

but my study is considerably more detailed and supported by abundant high-quality

analytical data.

6.1 Inclusions in groundmass olivine

Magmatic inclusions in groundmass olivine are abundant in some grains, but

rare in others. Inclusion sizes are variable (<1 to ~300 µm) and the distribution of

inclusions within a single olivine crystal is very heterogeneous, with some parts totally

devoid of inclusions, and some parts so crowded with inclusions as to make olivine

almost opaque (Fig. 6.1A). The highest density of inclusions is observed along internal

fractures and growth planes (Fig. 6.1A, C). As a rule, the number of inclusions increases

from the cores to rims of olivine. Three main types of magmatic inclusions are

recognized in the studied samples: crystals, fluid and melt. Crystal inclusions are always

primary, whereas fluid and melt inclusions show features reminiscent of both primary

and secondary origin.

6.1.1 Crystal inclusions

The previous study Sobolev et al. (1989) has revealed a variety of crystal (low-

Ca pyroxene, phlogopite, ilmenite, olivine, Cr-spinel and rutile) and fluid (mainly CO2)

inclusions in groundmass olivine-II. Here I report discovery of previously unknown

inclusions of high-Ca pyroxene. Brief descriptions of other crystal inclusions – low-Ca

pyroxene, olivine, phlogopite, and two unidentified phases are also given.

Pyroxene inclusions are not common but are genetically important, and they

provide new information on the nature of the original kimberlitic melt. Pyroxene

inclusions are represented by both high-Ca and low-Ca varieties.

Chapter 6 Magmatic inclusions in minerals 151

Figure 6.1. Distribution of inclusions in groundmass olivine. A – inclusions spread throughout olivine-II grain; B – inclusion distribution in olivine-I; C – inclusions along healing fractures in olivine core; D – large multiphase inclusion in olivine core; E - large multiphase inclusion in olivine rims

D

EC

B

A


Figure 6.2. High-Ca pyroxene (cpx) inclusions in groundmass olivine. A –distribution and zoning of cpx inclusions in the groundmass olivine; B – cluster of three inclusions, single subhedral and rounded cpx inclusions; C – Pyroxene inclusion in association with phlogopite inclusion in olivine core, insert – melt inclusions hosted by pyroxene inclusion:

20 μm 50 μm

B

50 μm

host ol

cpx

phl

C

Ca Na20 μm

A

Ca

Ca Cr Na20 μm

10 μm

Figure 6.3. High-Ca pyroxene inclusions in the core of groundmass olivine. Element distribution maps show the presence of alkali-Cl-C rich melt inclusions, partly exposed at surface by the electron beam, and the rim around pyroxene

Na K

CCl


FeMg

Ca

50 μm Ca

min max


154

High-Ca pyroxene inclusions occur as a single crystal or as clusters of several

crystals, and are restricted to the olivine cores (Fig. 6.2A). In the present set of data

olivine Fo86.3-87 and Fo91-93 are found only as hosts to high-Ca pyroxene. Pyroxene

inclusions are in Fe-Mg equilibrium with the host olivine, which can be seen in a

positive co-variation of pyroxene Mg# and host olivine Fo compositions (Fig. 6.4).

These inclusions are represented by crystals of different size (25-200 µm) and colour

(emerald-green to greyish-green). Based on shape, two types of high-Ca pyroxene

inclusion can be recognised: 1 - rounded crystals with smooth edges, and 2 - euhedral-

subhedral grains (Fig. 6.2B). The latter, in most cases, have slightly resorbed edges and

contain a large amount of fluid and melt inclusions (Fig. 6.2B, C). All high-Ca pyroxene

inclusions are intimately associated with a vapour-rich substance that forms a coating

around them. X-ray mapping of the element distribution shows high concentrations of

chlorine, carbon, potassium and sodium in these coats and in pyroxene-hosted

inclusions of melt/fluid (Fig. 6.3). CaO content in olivine sharply increases from ~0.02

wt% to 0.05-0.08 wt% towards high-Ca pyroxene inclusions (Figs. 6.2A and 6.3).

Compositionally, high-Ca pyroxene inclusions are similar to Cr-diopside, and

are characterised by low Al2O3 (0.6-2.9 wt%), and high CaO (19.5-23.8 wt%), Na2O

(0.7-2.3 wt%) and Cr2O3 (0.9-2.7 wt%) contents (Table 6.1; Fig. 6.4). Individual

crystals show pronounced fine-scale compositional zoning, with a general pattern of

MgO and CaO increase, and Na2O, Cr2O3 and in some cases Al2O3, decrease towards

the rims (Figs. 6.3 and 6.5). Such zoning is atypical for pyroxenes from the Udachnaya-

pipe mantle and crustal xenoliths (Shimizu et al., 1997).

Trace element compositions of exposed high-Ca pyroxene inclusions in

groundmass olivine were determined by ion microprobe (SIMS) (Cameca ims-3f) at the

Max-Planck-Institut für Chemie, Mainz, Germany. Cores of the pyroxene inclusions are

highly enriched in most incompatible trace elements, but depleted in heavy rare-earth

elements. The general pattern of enriched trace element compositions of core regions

(Table 6.1 and Fig. 6.6A) is complicated by relative depletion of the most incompatible

elements (e.g., Ba), high-field strength elements (e.g., Nb, Zr, Ti) and moderately

compatible elements (e.g., Gd is strongly fractionated from Yb). Such trace element

compositions are different from those of pyroxenes in the Udachnaya mantle xenoliths


Figure 6.4. Composition of high-Ca pyroxene inclusions in groundmass olivine, shaded field shows the compositions of host olivine rims, (abundances in wt%).

53.9

54.1

54.3

54.5

54.7

54.9

87 89 91 93 95

SiO2

0.05

0.10

0.15

0.20

0.25

87 89 91 93 95

TiO2

0.5

1.0

1.5

2.0

2.5

3.0

87 89 91 93 95

Al2O3

CaO

0.5

1.0

1.5

2.0

2.5

87 89 91 93 95

Na2O

0.8

1.2

1.6

2.0

2.4

2.8

87 89 91 93 95

Cr2O3

Fo, host olivine

Mg#

, cpx

incl

usi

on

88

90

92

94

86 88 90 92 94

Mg#

19

20

21

22

23

24

87 89 91 93 95

MgO

14.6

14.8

15

15.2

15.4

15.6

15.8

16

CaO

19.2

19.4

19.6

19.8

20

20.2

20.4

20.6

Na2O

1.5

1.7

1.9

2.1

2.3

2.5

Cr2O3

0

0.5

1

1.5

2

2.5

MgO

14.815

15.215.415.615.8

16

CaO

19.419.619.8

2020.220.420.6

Na2O

1.5

2

2.5

Cr2O3

0.5

1

1.5

2

2.5


Figure 6.5. Profiles of element distributions within high-Ca pyroxene inclusions in groundmass olivine

BA


MgO

15.6

15.8

16

16.2

TiO2

0.1

0.2

0.3

K2O

0.015

0.02

0.025

0.03

0.035

0.04

0.045

Cr2O3

2.3

2.4

2.5

2.6

2.7

2.8

Al2O3

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

Na2O

0.6

0.7

0.8

0.9

1

1.1

1.2

Cr2O3

0.8

1

1.2

1.4

Figure 6.5 (cont). Profiles of element distributions within high-Ca pyroxene inclusions in the core of groundmass olivine

C D

Figure 6.6. Primitive/mantle (Sun & McDonough, 1989) - normalized compositions of (A) high-Ca pyroxene inclusions in cores of Udachnaya groundmass olivine (red lines) and clinopyroxenes from diamondites (blue lines with diamonds, Dobosi & Kurat, 2002 ), a field (ligh blue) of compositions of high-Ca pyroxenes from garnet peridotite xenoliths in the Udachnaya kimberlites (Shimizu et al, 1997); B -hypothetical melts (blue lines) in equilibrium with high-Ca pyroxene inclusions in the olivine and composition of the groundmass (red lines with circles).


B

sam

ple/

prim

itiv

e m

antl

e

A

sam

ple/

prim

itiv

e m

antl

e

0.01

0.10

1.00

10.00

100.00

Ba Nb La Ce Pr Sr Nd Sm Zr Eu Ti Gd Tb Dy Y Ho Er Yb

0.1

1.0

10.0

100.0

1000.0

Ba Nb La Ce Pr Sr Nd Sm Zr Eu Ti Gd Tb Dy Y Ho Er Yb


159

and xenocrysts in having higher concentrations of moderately incompatible elements

(Zr, Eu, Sm) (Fig. 6.5A). On the other hand, a compositional similarity between olivine-

hosted high-Ca pyroxene inclusions and clinopyroxenes from diamond-bearing rocks -

diamondites (Dobosi and Kurat, 2002) should be noted.

The composition of hypothetical melts parental to these clinopyroxenes was

calculated using Kd (distribution coefficients) for low–Al clinopyroxene and melt

(Sobolev et al., 1996) for rare-earth elements and Y, and Kd = 0.008, 0.13, and 0.12 for

Nb, Sr, and Zr, respectively, and a comparison with the trace element composition of

the Udachnaya-East kimberlite groundmass is shown in Figure 6.6B.

Carbonate- and chloride-rich material decorating inclusions of high-Ca pyroxene

is difficult to confidently ascribe to a particular origin (i.e. primary vs secondary);

however inclusions of the same material inside pyroxene grains are likely to have been

coeval with pyroxene crystallisation. Another phase, co-trapped with high-Ca pyroxene

in olivine cores, is phlogopite. Only one occurrence of attached grains of high-Ca

pyroxene and phlogopite is recorded (Fig. 6.2C), and phlogopite was studied for its

water content by SIMS (Cameca ims-3f, Max-Planck-Institut für Chemie, Mainz), using

the method described in (Sobolev and Chaussidon, 1996). Unlike phlogopite inclusions

in the rims of olivine-II, this phlogopite in the olivine core is water-saturated (4.3 wt%

H2O) (Table 6.2).

The rims and marginal zones of olivine-II grains show abundant inclusions of

different minerals that are never present in the cores. Among them, magnetite,

phlogopite and rutile are relatively abundant, whereas perovskite and spinel are less

common. Inclusions of low-Ca pyroxene and olivine can occur in both cores and rims.

Phlogopite inclusions usually occur in the rim regions of olivine grains and are

either brownish euhedral grains 30-200 µm in size, or clusters of a few inclusions.

Phlogopite inclusions frequently coexist with low-Ca pyroxene (Fig. 6.7A). Phlogopite

grains are compositionally homogeneous (Table 6.2) and resemble groundmass

phlogopite, although they have less TiO2 (2.2-2.6 wt%), Al2O3 (13.1-13.4 wt%) and

FeO (4.4-4.6 wt%). A specific feature of phlogopite inclusions from olivine rims is a


160

significant undersaturation in water (2.7 wt%) that distinguishes them from phlogopite

inclusions found in the core (Table 6.2).

Rutile occurs as euhedral needle-like crystals up to 100 µm long (Fig. 6.7C).

Commonly, one olivine grain can contain numerous randomly oriented rutile crystals,

often forming “L”-like intergrowths.

Olivine inclusions in olivine-II are widespread in this kimberlite. They usually

occur as single euhedral grains of variable size (30-200 µm). A coating of fluid-rich

material helps to optically distinguish olivine inclusions from the olivine-host (Fig.

6.7B). Compositions of olivine inclusions (Fo85-91) are almost identical to those of their

host olivine-II (Table 6.3; Fig. 6.7B).

Low-Ca pyroxene is typically present in clusters of several (10-30) euhedral

grains in olivine-II cores and rims (Fo86-91). A common association of low-Ca pyroxene

includes numerous melt and fluid inclusions, and a volatile-rich substance coating

surfaces of pyroxene crystals (Fig. 6.8). A positive correlation between Mg# of

pyroxene inclusions and host olivine Fo is indicative of their local equilibrium in terms

of Mg-Fe. The compositions of low-Ca pyroxene (Fig. 6.9 and Table 6.4) are distinctly

different from mantle orthopyroxene in having high SiO2 (53.3-58 wt%), Na2O (0.1-0.9

wt%), elevated TiO2 (0-0.5 wt%), and low Al2O3 (0.7-1.4 wt%), CaO (0.7-1.7 wt%) and

Cr2O3 (0.1-0.6 wt%), and thus resemble primitive orthopyroxene in Si-oversaturated

volcanic rocks (e.g., boninites). There is a tendency for Al2O3, CaO and Na2O to

increase and Cr2O3 decrease in the grains with Mg# <89 mol%.

84

86

88

90

92

84 86 88 90 92 94

C

Figure 6.7. Crystal inclusions in groundmass olivine. A – phlogopite inclusion, note the association of phlogopite inclusion with low-Ca pyroxene (opx) inclusion; B –olivine inclusions and Fo in host olivine vs Fo of olivine inclusions, (abundances in wt%), shaded field shows the compositions of host olivine rims; C – putile; D –perovskite.


phl

phlA

opx

phl

B

D

20 μm

Fo, host ol

Fo, ol inclusions

ol

ol

Figure 6.8. Low-Ca-pyroxene (opx) inclusions in the groundmass olivine. A –distributions of opx inclusions; B – unheated single opx inclusions; C– heated opxinclusions with large CO2 rich bubble and silicate glass


B

10 μm

C

10 μm

20 μm

A

opx

87

88

89

90

91

92

86 87 88 89 90 91

Al2O3

TiO2

CaO

Na2O

Cr2O3

SiO2


Fo, host olivine

Mg#

, opx

incl

usi

ons

Mg#

Figure 6.9. Composition of low-Ca pyroxene inclusions in groundmass olivine, shaded field shows compositions of host olivine rims, (abundances in wt%).

53

55

57

59

87 88 89 90 91 92

0.6

0.9

1.2

1.5

87 88 89 90 91 92

0.0

0.2

0.4

0.6

87 88 89 90 91 92

0.6

1.0

1.4

1.8

87 88 89 90 91 92

0.1

1.0

87 88 89 90 91 92

0.0

0.2

0.4

0.6

87 88 89 90 91 92

Ca Mg

KNa

opxcpx

glass

opx opx

Figure 6.10. Heated opx inclusions with large bubble and silicate glass, and associated cpx inclusions hosted by groundmass olivine.


cpx

cpx


165

Table 6.1 Compositions of high-Ca pyroxene inclusions and their host olivine

wt% Cpx 8a Cpx 2a Cpx 2b Cpx 2c Cpx 3a Cpx 3b Cpx 3c Cpx 4 Cpx 5a Cpx 5b SiO2 54.6 54.2 54.0 54.6 54.5 54.3 54.4 55.2 54.5 54.4 TiO2 0.04 0.21 0.19 0.23 0.12 0.17 0.16 0.06 0.23 0.24 Al2O3 2.0 1.9 1.9 1.9 1.5 1.5 1.4 2.9 2.3 2.2 FeOt 3.6 4.0 3.9 2.5 3.6 3.6 3.4 1.6 2.7 2.7 MgO 15.8 15.4 15.5 16.0 15.8 16.0 16.2 15.9 15.9 16.1 CaO 21.7 20.7 20.5 20.6 22.3 22.5 22.5 21.6 20.0 20.3 Na2O 1.7 1.9 2.0 2.0 1.5 1.5 1.4 1.9 2.3 2.1 Cr2O3 1.0 2.0 2.1 2.6 1.6 1.2 0.9 2.2 2.2 2.1 Total 100.6 100.4 100.2 100.5 100.9 100.8 100.5 101.4 100.2 100.2 Mg# 88.6 87.4 87.5 92.1 88.8 88.7 89.3 94.8 91.4 91.3

P kbar 40 41 32 35 36 39 25 40 39 ToC 847 846 644 638 592 641 715 862 875

ppm Cpx 8a Cpx 2a Cpx 2b Cpx 2c Cpx 3a Cpx 3b Cpx 3c Cpx 4 Cpx 5a Cpx 5b Ba 1.8 0.3 0.2 0.2 0.3 0.2 0.1 9.5 4.0 6.6 Nb 0.8 0.7 0.8 0.7 0.5 0.6 0.6 0.6 1.1 2.3 K 165.0 582.0 425.0 233.0 142.0 116.0 125.0 723.0 514.0 604.0 La 3.6 3.1 2.5 3.4 2.7 2.6 3.9 4.5 5.4 7.3 Ce 13.7 11.1 10.3 13.7 10.6 10.5 15.3 13.4 19.8 23.1 Pr 2.5 2.1 1.9 2.3 2.2 2.1 2.8 2.0 3.6 4.2 Sr 116.0 196.0 178.0 234.0 191.0 185.0 260.0 189.5 340.0 345.0 Nd 11.5 9.1 8.6 11.5 10.8 10.1 14.2 6.2 16.9 18.4 Sm 2.6 2.2 2.0 2.9 2.6 2.5 3.7 0.5 3.7 4.1 Zr 29.1 24.6 22.3 22.9 33.6 31.8 36.7 3.0 30.9 31.5 Eu 0.9 0.7 0.6 0.9 0.8 0.8 1.1 0.1 1.2 1.3 Ti 255.0 1210.0 1100.0 1270.0 861.0 832.0 864.0 379.5 1480.0 1460.0 Gd 1.7 1.4 1.8 2.1 2.0 2.1 2.4 0.0 2.7 2.2 Tb 0.2 0.2 0.2 0.2 0.3 0.2 0.3 0.0 0.1 0.3 Dy 1.3 1.0 0.8 1.0 1.1 1.0 1.3 0.1 1.1 0.7 Y 3.5 2.8 3.0 2.6 3.0 3.1 3.7 0.4 2.8 3.1

Ho 0.2 0.1 0.1 0.2 0.2 0.2 0.2 0.0 0.2 0.1 Er 0.4 0.3 0.3 0.3 0.3 0.2 0.3 0.1 0.3 0.1 Yb 0.3 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0

Host olivine Cpx 8a Cpx 2a Cpx 2b Cpx 2c Cpx 3a Cpx 3b Cpx 3c Cpx 4 Cpx 5a Cpx 5b

SiO2 40.2 40.3 40.2 41.6 41.1 41.1 FeOt 12.8 12.9 12.7 7.2 8.2 8.1 MnO 0.2 0.1 0.2 0.1 0.1 0.1 MgO 45.6 45.6 45.3 50.1 49.1 49.1 CaO 0.0 0.0 0.1 0.0 0.0 0.0 NiO 0.3 0.3 0.3 0.4 0.3 0.3

Cr2O3 0.0 0.0 0.0 0.0 0.0 0.0 Total 99.1 99.2 98.7 99.5 98.9 98.8

Fo 86.4 86.3 86.4 92.6 91.4 91.5


166

wt% Cpx 5c Cpx 6 Cpx 7 2 2a 2b 3 6 6a 33-2 SiO2 54.7 54.0 54.9 54.4 54.6 54.6 54.5 54.9 54.9 54.2 TiO2 0.22 0.15 0.15 0.14 0.19 0.19 0.12 0.19 0.19 0.16 Al2O3 2.4 1.0 2.0 1.6 0.6 0.6 1.6 2.2 2.2 0.6 FeOt 3.1 3.2 2.4 3.4 3.0 3.0 3.5 2.5 2.5 2.2 MgO 16.8 16.5 16.2 16.2 16.9 16.9 16.5 16.5 16.5 18.1 CaO 19.6 23.8 19.8 21.1 22.8 22.8 21.6 19.2 19.2 21.7 Na2O 2.2 0.8 2.2 1.5 0.7 0.7 1.4 2.3 2.3 1.1 Cr2O3 2.0 1.2 2.6 1.5 1.4 1.4 0.9 2.7 2.7 2.2 Total 101.0 100.8 100.4 100.0 100.3 100.3 100.2 100.6 100.6 100.3 Mg# 90.7 90.3 92.4 89.3 90.9 90.9 89.3 91.9 91.9 93.6

P kbar 43 29 42 41 44 44 43 37 37 44 ToC 998 487 938 906 877 877 880 1007 1007 977

ppm Cpx 5c Cpx 6 Cpx 7 2 2a 2b 3 6 6a 33-2 Ba 1.9 0.7 2.7 0.8 0.3 0.5 0.2 0.3 0.2 9.2 Nb 1.2 0.6 1.4 0.9 0.6 0.3 0.4 0.7 0.6 2.4 K 341.0 59.0 362.0 202.0 195.4 224.8 165.9 321.1 306.8 139.2 La 3.4 7.3 5.2 2.7 2.0 2.8 3.2 4.3 4.4 9.9 Ce 13.5 22.8 18.6 11.1 6.7 9.2 12.1 16.6 16.8 26.9 Pr 2.5 4.1 3.4 2.1 1.3 1.7 2.3 2.9 3.2 4.6 Sr 244.0 218.0 289.0 234.9 234.1 221.6 213.2 264.9 259.3 353.8 Nd 12.0 16.4 16.2 11.2 7.1 9.3 11.4 14.3 14.5 20.8 Sm 2.6 3.4 3.7 2.5 1.6 2.0 2.7 2.8 3.1 4.5 Zr 23.0 26.1 24.0 32.7 30.6 29.6 35.6 27.8 28.8 32.0 Eu 0.9 1.1 1.2 0.7 0.4 0.6 0.8 0.8 0.9 1.6 Ti 1190.0 916.0 795.0 859.4 862.6 892.8 833.0 1428.2 1383.2 974.8 Gd 2.1 2.1 2.6 2.0 1.3 1.2 1.5 2.5 2.2 3.3 Tb 0.2 0.2 0.2 0.2 0.1 0.1 0.2 0.3 0.2 0.2 Dy 0.7 1.2 0.8 1.2 0.5 0.6 0.9 0.8 0.9 1.4 Y 2.2 3.1 2.4 3.0 3.1 2.8 3.8 3.4 3.1 4.2

Ho 0.1 0.2 0.1 0.3 0.1 0.1 0.1 0.1 0.1 0.0 Er 0.2 0.1 0.2 0.3 0.0 0.4 0.4 0.2 0.3 0.2 Yb bdl 0.2 bdl 0.2 0.0 0.1 0.4 0.0 bdl 0.2

Host olivine SiO2 41.2 40.1 41.2 FeOt 8.2 12.5 7.0 MnO 0.1 0.2 0.1 MgO 49.1 45.5 49.8 CaO 0.0 0.1 0.0 NiO 0.4 0.3 0.3

Cr2O3 0.0 0.0 0.0 Total 99.0 98.6 98.6

Fo 91.4 86.7 92.7

Note to Table: Pressure and temperature calculated using single pyroxene geothermobarometry (Nimis and

Taylor, 2000)


167

Table 6.2 Representative analyses of phlogopite inclusions and their host olivine, phl* - phlogopite in association with high-Ca pyroxene inclusion.

phlogopite inclusions in olivine

1 2 3 4 7 8 9 10 11 12 13 14 phl*

SiO2 39.3 39.8 40.7 40.9 40.1 39.8 40.7 40.2 40.2 39.5 40.1 40.9 41.7

TiO2 2.2 2.5 2.5 2.5 2.4 2.4 2.3 2.4 2.6 2.5 2.2 2.2 0.6

Al2O3 13.2 13.3 13.3 13.3 13.4 13.4 13.1 13.4 13.2 13.2 13.4 13.3 12.2

FeOt 4.6 4.5 4.4 4.4 4.5 4.5 4.6 4.5 4.6 4.6 4.5 4.5 4.7

MgO 22.7 23.0 23.1 23.3 22.8 22.8 23.1 23.2 23.2 22.7 22.8 23.1 25.5

Na2O 0.19 0.19 0.22 0.28 0.22 0.22 0.21 0.19 0.25 0.11 0.19 0.22 0.10

K2O 10.2 10.0 9.7 9.9 9.9 9.9 9.9 9.8 9.9 10.0 10.1 9.9 10.6

Cr2O3 1.5 1.6 1.4 1.4 1.3 1.5 1.4 1.4 1.4 1.5 1.5 1.5

Cl 0.05 0.05 0.05 0.07 0.05 0.05 0.05 0.04 0.04 0.04 0.05 0.05 0.10

F 0.4 0.5 0.5 0.5 0.4 0.4 0.4 0.5 0.4 0.4 0.6 0.6

H2O 2.7 4.3

Total 94.4 95.4 95.8 96.4 95.0 94.8 95.5 95.4 95.8 94.4 95.4 99.0 99.8

host olivine 1 2 3 4 7 8 9 10 11 12 13 14

SiO2 40.2 40.1 40.4 39.9 40.5 40.4 39.7 40.0 40.0 40.4 40.9 40.2

Al2O3 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

FeOt 10.7 10.5 10.6 10.7 10.8 10.5 10.5 11.0 10.9 10.9 10.6 10.8

MnO 0.1 0.1 0.1 0.2 0.2 0.1 0.2 0.1 0.2 0.1 0.1 0.1

MgO 48.4 48.3 48.6 48.1 48.8 48.9 47.7 48.0 47.7 48.6 49.3 48.2

CaO 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.0

NiO 0.3 0.2 0.2 0.2 0.2 0.3 0.2 0.2 0.2 0.3 0.3 0.3

Cr2O3 0.1 0.0 0.1 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.1 0.1

Total 99.8 99.4 100.2 99.3 100.6 100.4 98.4 99.4 99.1 100.4 101.4 99.7

Fo 89.0 89.1 89.1 88.9 88.9 89.3 89.0 88.6 88.7 88.9 89.2 88.8

Heating of grains containing low-Ca pyroxene inclusions to 1250oC reveals

some changes occurring in the material around inclusions. After quenching, a film of

clear glass, containing CO2-rich bubbles (some with visible liquid-vapor boundary) is

always present around low-Ca pyroxene crystals (Figs. 6.8A, C and 6.10). The glass is

characterised by high SiO2 (60-70 wt%), moderately enriched alkalis (K2O+Na2O = 3-6

wt%), low CaO (3.5-6 wt%), and exceptionally low Al2O3 (2-4 wt%) (Table 6.4). In one

instance several grains of high-Ca pyroxene are present in association with a group of

low-Ca pyroxene crystals and the silicate glass (Fig. 6.10). This high-Ca pyroxene (Mg#


168

87.3-88 mol %) has higher TiO2 (0.92 wt%) and lower CaO (16.22 wt%) abundances in

comparison with individual Cr-diopside inclusions in the olivine cores.

Perovskite and Cr-spinel, together with magnetite (individual grains and rims

around Cr-spinel), occur at the very edge of olivine crystals and form encrustations

around the olivine grains. Perovskite often occurs as aggregates of euhedral zoned

crystals (Fig. 6.7D). Cr-spinel forms euhedral, often zoned crystals (Fig. 6.11B),

mantled by Ti-magnetite. Rarely, spinel grains contain melt inclusions (Fig. 6.11C). The

cores of Cr-spinel inclusions have compositions typical of kimberlitic spinel (Table 6.

5). These are most depleted in Al2O3 (9.4-12.3 wt%), and at the same time most

enriched in TiO2 (4.4-5 wt%) among magmatic Cr-spinels in the terrestrial rocks

(Kamenetsky et al., 2001a). They are more enriched in TiO2 (at similar Al2O3) than

spinel in subduction-related volcanics, and more depleted in Al2O3 (at similar TiO2)

than spinel from oceanic island basalts (Fig. 6.11A). Their overall resemblance to Cr-

spinel from primitive flood basalts and meimechites (highly magnesian enriched picrites

from Meimecha-Kotui province, northern Siberia) should be noted.

Figure 6.11. Spinel inclusions in groundmass olivine. A – composition of spinelinclusions compared to spinel from LIP, OIB, ARC and MORB (Kamenetsky et al., 2001), (abundances in wt%); B – zoned spinel inclusion; C – spinel inclusion, containing melt inclusion, .


0.01

0.10

1.00

10.00

0 10 20 30 40 50

LIP

MORB

OIB

ARC

Al2O3, spinel

TiO

2, s

pin

elA

CB


170

Table 6.3 Representative analyses of olivine inclusions and their host olivine

SiO2 FeOt MnO MgO CaO NiO Cr2O3 Fo N olivine inclusions in olivine 1 39.2 13.8 0.1 43.8 0.05 0.22 0.01 85.0 2 41.0 13.0 0.1 46.7 0.03 0.24 0.04 86.5 3 40.3 11.0 0.1 47.5 0.04 0.36 0.08 88.5 4 40.8 8.6 0.1 48.9 0.02 0.27 0.07 91.0 5 41.4 12.3 0.1 48.2 0.07 0.29 0.01 87.5 6 41.3 9.0 0.1 49.1 0.10 0.32 0.11 90.7 7 40.4 10.7 0.1 46.5 0.04 0.26 0.10 88.5 8 40.4 11.0 0.1 46.9 0.03 0.21 0.03 88.3 9 39.5 13.9 0.2 44.1 0.04 0.13 0.00 84.9

10 41.3 12.0 0.1 47.4 0.03 0.27 0.06 87.6 11 40.2 11.1 0.1 46.4 0.03 0.19 0.08 88.2 12 41.6 11.7 0.1 48.6 0.05 0.36 0.05 88.2 13 40.8 9.8 0.2 47.3 0.06 0.24 0.07 89.5

host olivine 1 40.1 10.9 0.2 46.6 0.1 0.3 0.1 88.4 2 40.6 14.1 0.1 45.5 0.1 0.2 0.0 85.2 3 40.3 11.1 0.1 47.5 0.0 0.2 0.0 88.4 4 41.0 10.1 0.1 47.7 0.0 0.3 0.0 89.4 5 41.4 11.0 0.1 48.1 0.1 0.3 0.0 88.7 6 40.9 9.2 0.2 48.5 0.1 0.3 0.1 90.3 7 39.9 11.6 0.2 45.9 0.2 0.1 0.0 87.6 8 41.3 7.2 0.1 50.1 0.0 0.3 0.1 92.6 9 39.4 14.5 0.2 44.6 0.0 0.2 0.0 84.6

10 40.9 12.7 0.1 46.9 0.0 0.2 0.0 86.9 11 40.1 11.4 0.1 46.7 0.0 0.2 0.1 88.0 12 41.5 9.5 0.1 49.3 0.1 0.3 0.0 90.2 13 40.5 10.4 0.2 46.9 0.0 0.3 0.1 88.9


171

Table 6.4 Representative analyses of low-Ca pyroxene (opx) inclusions, their host olivine, and associated high-Ca pyroxene (cpx*) and melt compositions

opx inclusions in olivine II cpx* cpx*

SiO2 57.9 57.8 53.3 58.0 57.2 56.5 56.8 56.8 56.4 57.1 56.7 53.5 55.4

TiO2 0.2 0.2 0.5 0.2 0.2 0.1 0.1 0.1 0.2 0.2 0.2 0.9 0.6

Al2O3 0.9 0.9 1.4 1.0 0.8 0.8 1.0 1.0 1.0 1.2 1.1 1.4 1.2

FeOt 6.2 7.5 7.5 6.6 7.6 5.7 7.7 7.7 7.7 7.8 7.6 4.9 4.9

MnO 0.1 0.1 0.2 0.2 0.1 0.1 0.2 0.2 0.2 0.2 0.1 0.1 0.2

MgO 33.2 32.0 35.7 34.3 33.4 34.2 31.4 31.7 32.0 31.8 31.5 18.8 20.2

CaO 1.3 1.7 1.1 1.3 1.1 1.3 1.5 1.4 1.3 1.7 1.6 17.2 16.2

Na2O 0.2 0.6 0.9 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.2 1.3 1.2

K2O 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.3 0.0

NiO 0.1 0.2 0.1 0.1 0.1 0.0 0.1 0.1

Cr2O3 0.6 0.1 0.3 0.5 0.2 0.5 0.1 0.1 0.1 0.2 0.1 1.4 1.4

Fo 90.6 88.3 89.4 90.3 88.7 91.4 88.0 88.1 88.1 87.9 88.0 87.3 88.0

host olivine

SiO2 40.6 39.9 41.5 41.4 40.5 41.2 39.8

FeOt 9.5 11.3 10.0 10.4 12.8 8.3 12.1

MnO 0.1 0.1 0.0 0.1 0.2 0.2 0.1

MgO 48.5 46.6 48.8 48.2 45.6 48.7 46.2

CaO 0.1 0.1 0.1 0.1 0.1 0.1 0.1

NiO 0.4 0.3 0.3 0.3 0.2 0.4 0.3

Cr2O3 0.1 0.1 0.1 0.0 0.0 0.1 0.0

Fo 90.1 88.0 89.7 89.2 86.4 91.3 87.2

melt associated with opx inclusions

SiO2 70.7 70.6 59.6 54.6 57.3 62.1 61.9 59.5 60.3

TiO2 0.4 0.4 0.8 1.0 1.5 1.3 1.1 1.0 1.3

Al2O3 2.0 2.1 1.6 1.6 3.5 3.6 3.6 3.0 3.5

FeOt 6.7 6.5 5.5 5.2 7.4 7.4 7.3 7.4 7.5

MnO 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1

MgO 11.9 11.6 16.7 15.1 10.1 13.0 15.7 17.1 12.4

CaO 3.5 3.6 5.4 10.6 6.1 5.4 4.2 4.5 5.2

Na2O 2.1 2.7 8.3 9.5 2.2 1.9 1.4 1.4 1.2

K2O 1.2 1.2 1.7 3.2 3.4 3.1 2.6 2.2 3.0

P2O5 0.0 0.0 0.1 0.2 0.3 0.3 0.3 0.3 0.3

Cr2O3 0.4 0.3 0.4 0.1 0.1 0.1 0.1 0.2 0.2

Cl 0.0 0.0 0.1 0.1 0.2 0.2 0.2 0.1 0.2

SO3 0.0 0.0 0.0 0.2 0.1 0.1 0.1 0.1 0.2

Total 99.0 99.0 100.3 101.5 92.3 98.7 98.7 96.9 95.4


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Table 6.5 Representative analyses of spinel inclusions in groundmass olivine

SiO2 0.2 0.2 0.2 0.3 0.2 0.2 0.4 0.2 0.2 0.3 0.2 0.5 0.2

TiO2 4.7 4.7 4.6 4.4 4.6 4.5 4.9 4.5 5.0 14.2 19.1 12.1 11.2

Al2O3 11.3 11.7 10.9 11.2 10.1 11.1 9.6 12.3 9.4 2.2 4.7 4.4 6.0

Cr2O3 45.1 44.6 43.4 44.0 45.9 44.4 45.3 44.2 45.4 3.8 3.4 1.3 30.4

Fe2O3 6.2 6.3 7.6 7.7 7.5 7.0 6.7 6.9 7.6 38.7 30.0 43.5 14.6

FeO 20.0 19.6 18.4 19.5 19.3 18.3 19.1 19.7 20.8 26.8 27.7 24.6 23.8

MnO 0.4 0.2 0.4 0.1 0.3 0.3 0.5 0.5 0.8 1.9 1.9 1.5 0.9

MgO 11.4 11.8 12.0 11.7 11.8 12.2 11.8 11.7 10.7 10.1 13.1 11.2 11.6

NiO 0.2 0.3 0.1 0.2 0.2 0.2 0.2 0.2 0.1 0.0 0.1 0.1 0.2

ZnO 0.2 0.1 0.1 0.0 0.1 0.0 0.1 0.1 0.1 0.2 0.2 0.0 0.0

Total 99.7 99.5 97.7 99.2 100.0 98.1 98.6 100.1 100.1 98.1 100.2 99.1 98.8

Mg# 0.46 0.52 0.54 0.52 0.52 0.54 0.52 0.51 0.48 0.40 0.56 0.45 0.50

Cr# 0.77 0.72 0.73 0.73 0.75 0.73 0.76 0.71 0.76 0.53 0.33 0.16 0.73

(Fe2+/Fe3+)sp 3.6 3.4 2.7 2.8 2.9 2.9 3.2 3.2 3.1 0.8 1.0 0.6 1.8 host olivine

SiO2 39.9 40.5 40.4 41.1 41.2 40.4 40.6 40.7 40.8 40.1 41.1 40.6 40.3

FeOt 11.6 11.4 11.4 10.9 11.3 10.9 11.5 12.6 11.2 11.1 11.2 10.7 11.5

MnO 0.2 0.2 0.1 0.3 0.2 0.2 0.2 0.3 0.2 0.2 0.3 0.2 0.3

MgO 45.9 47.4 47.0 47.5 47.9 47.1 47.3 46.2 47.0 46.7 47.6 47.5 46.8

CaO 0.2 0.0 0.1 0.2 0.0 0.1 0.1 0.3 0.1 0.1 0.2 0.2 0.1

NiO 0.1 0.4 0.2 0.2 0.3 0.1 0.2 0.2 0.2 0.2 0.2 0.1 0.2

Cr2O3 0.0 0.1 0.0 0.1 0.1 0.1 0.1 0.0 0.0 0.1 0.1 0.1 0.0

Fo 87.6 88.1 88.0 88.6 88.3 88.5 88.0 86.7 88.2 88.3 88.3 88.8 87.9 Note to Table: Mg# = Mg/(Mg + Fe2+) at%, Cr# = Cr/(Cr + Al) at%.

Two unidentified crystal inclusions have been found in olivine-II during this

study. They appear to be mineral inclusions under optical microscope in transmitted

light, but because of their highly heterogeneous compositions (Table 6.6), they may

well be multiphase crystalline assemblages.

Fig. 6.12 shows a euhedral, hexagonal crystal trapped in the rim of an olivine,

but “squeezed” in between two compositionally different cores. In transmitted light this

crystal has polysynthetic twinning, reminiscent of clinoenstatite (Dallwitz et al., 1966),

and the refractive index is also similar to that of pyroxene. The central exposed part of

this crystal is apparently homogeneous, but the rims are strongly different, being

enriched in Mg. Fe, Cr and Ca and depleted in Na and K. A patchy enrichment in Al, Ti,


173

Sr, P, F and Cl is noted. According to electron microprobe analysis, only the rims of this

crystal correspond to low-Ca pyroxene (Table 6.6 #1), whereas the rest of this grain,

althoudh still having compositional features of orthopyroxene, is significantly enriched

in Ca, alkalies (~8 wt%), F, P and Cl. Because of the heterogeneous element

distribution, confidant identification of phases present in this inclusion was not possible.

Table 6.6 Compositions of unidentified euhedral hexagonal crystal inclusion

No SiO2 TiO2 Al2O3 FeOt MgO CaO Na2O K2O SrO F P2O5 Cr2O3 Total

1 56.7 0.12 0.78 5.95 33.3 1.34 0.29 0.03 0.19 0.26 98.9

2 50.6 0.38 0.43 6.57 27 3.62 4.97 2.71 0.2 0.4 0.46 0.28 97.6 Note to Table: MnO, Cl and SO3 were analysed, but their concentrations are negligible

Fig. 6.13 shows another “odd” inclusion that is difficult to identify because of

very irregular distribution of elements on the exposed surface. It is located in the rim of

olivine; however, the fact that it is partly “embedded’ into the resorbed face of the core

suggests that its trapping was coeval with a drastic change in the growth history of the

groundmass olivine. A specific interest in this inclusion was prompted by its relatively

large size (~ 100 µm), jagged shape, high relief and high birefringence, as well as its

unusual sugary texture. Exposing at the surface by careful polishing revealed that the

inclusion had a fluid-rich substance attached (at present a hole on Fig. 6.13). A polished

surface has an uneven refractive index, lower than host olivine, and a negative

topography. The application of microbeam techniques (including laser Raman

spectroscopy that was unsuccessful in this case) significantly damaged the inclusion,

making much less translucent with “burn marks” throughout the volume. X-ray

mapping (Fig. 6.13) and several quantitative microprobe analyses (Table 6.7) show very

strong fine-scale heterogeneity in the concentration of most elements. However, the two

most abundant elements in this inclusion, silicon and carbon, are strongly decoupled in

the analyses (Table 6.7, Figs. 6.13 and 6.14, R = -0.96), and according to the element

and “topographic” maps they may belong to different phases. The Si-rich endmember is

oxygen-bearing (Table 6.7, Fig. 6.14, R = 0.88), but does not have much affinity with

other analysed elements, except Ti (Table 6.7, Figs. 6.13 and 6.14, R = 0.92). The

carbon-rich endmember is notably oxygen-poor, as carbon is inversely-correlated with

oxygen (Table 6.7, Fig. 6.14, R = - 0.96), and thus the carbon-rich phase is not a


174

carbonate. Volatile elements such as F, N, S and Cl are definitely present in this carbon-

rich phase (Fig. 6.13), although their total abundance is < 6 wt%. The presence of

hydrogen cannot be excluded. Sodium and potassium, although present in relatively

small quantities (total < 5.5 wt%), are largely correlated in the analyses (R = 0.94), but

according to the element maps neither the Si-rich, nor the C-rich phase host alkalies.

This enigmatic inclusion is tentatively interpreted to be originally moissanite (SiC),

which changed to a mixture of silica and carbon.

Table 6.7 Compositions of individual analytical spots of unidentified Si-C-rich crystalline inclusion. Analyses are recalculated to 100%

C Na S Fe Cl N Al K Ti O Si Ca F 51.94 0.30 0.06 6.50 0.22 0.00 0.26 0.19 0.01 25.68 14.78 0.06 0.00 72.87 0.33 0.27 0.66 0.60 0.00 2.81 0.47 0.01 20.26 1.64 0.08 0.00 25.18 0.89 0.03 0.27 0.40 0.00 0.06 0.93 0.42 41.34 30.38 0.07 0.02 67.02 0.77 0.03 0.20 0.42 0.00 0.08 0.87 0.13 19.34 11.10 0.04 0.00 54.53 0.63 0.12 0.23 0.44 0.99 0.03 0.80 0.18 25.98 15.88 0.19 0.00 64.58 1.27 0.26 0.32 0.94 5.17 0.32 1.29 0.04 22.29 3.37 0.15 0.00 75.25 2.58 0.66 0.89 3.34 0.00 1.47 2.19 0.03 10.26 2.59 0.73 0.00 77.22 1.23 0.42 0.52 2.05 0.00 2.57 1.32 0.03 13.48 0.67 0.49 0.00 62.79 1.39 0.12 0.23 0.81 0.00 1.56 0.83 0.02 26.18 5.85 0.22 0.00 26.21 0.77 0.22 1.58 0.22 0.00 2.20 0.97 0.14 37.97 25.24 4.47 0.00 51.83 0.79 0.05 0.18 0.49 2.07 0.16 0.94 0.17 28.50 14.68 0.14 0.00 14.61 0.79 0.00 0.26 0.58 0.00 0.05 0.80 0.41 48.76 33.68 0.08 0.00 51.84 0.70 0.02 0.18 0.51 0.61 0.04 0.79 0.22 25.59 19.43 0.08 0.00 70.82 0.58 0.11 0.65 0.62 0.00 0.17 0.77 0.05 22.46 3.61 0.16 0.00 63.36 3.32 0.03 0.40 1.20 2.26 0.07 2.17 0.00 26.57 0.61 0.00 0.00 29.52 0.92 0.04 0.28 0.61 0.00 0.34 0.99 0.31 43.59 23.25 0.15 0.00 43.45 0.76 0.02 0.20 0.55 0.00 0.10 0.95 0.21 36.89 16.78 0.09 0.00 18.08 0.77 0.02 0.27 0.56 0.00 0.06 0.92 0.37 46.04 31.30 1.62 0.00 36.91 0.79 0.06 0.23 0.47 0.47 0.16 0.87 0.29 34.41 25.05 0.28 0.02

Figure 6.12. Elemental maps, and optical and backscattered images of the unusual ‘orthopyroxene” inclusion in the groundmass olivine.


min max

Figure 6.13. Backscattered, optical and topographic images and elemental maps of the unusual high Si- C-F-N-bearing inclusion in groundmass olivine.


C

Al

Ti

Ca

Cl

F N

K

Fe

S

Na

Si 20 μm

100 μm

min max

05

10152025303540

10 20 30 40 50 60 70 80

Si

0.0

0.1

0.2

0.3

0.4

0 5 10 15 20 25 30 35

Ti


C

Figure 6.14. Composition of unusual high Si- C-F-N-bearing inclusion in groundmass olivine, (abundances in wt%).

0

10

20

30

40

50

60

10 20 30 40 50 60 70 80

O

Si

0

10

20

30

40

50

60

0 5 10 15 20 25 30 35

O

0.0

1.0

2.0

3.0

0.0 0.5 1.0 1.5 2.0 2.5K

Na


178

6.1.2 Fluid inclusions

Fluid inclusions are abundant and occur as clusters in all zones of olivine grains

(Fig.6.15A). They are usually very small (<20 µm) and have either spherical or

elongated shapes. Most inclusions are single phase CO2-bearing vapour, although some

inclusions show a boundary between liquid and gaseous CO2 (Fig. 6.15B). Two-phase

inclusions homogenise into liquid at temperatures as low as 14oC. H2O-bearing fluid

inclusions have not been detected in this and previous studies (Sobolev et al., 1989).

Single phase vapour inclusions are suspected to result from decrepitation of denser

inclusions, and thus any estimate of pressure of trapping cannot be reliable.

6.1.3 Melt inclusions

Melt inclusions are trapped either individually within olivine cores and rims

(Fig. 6.1D, E), or occur along healed fractures terminated within the inner rims of

olivine Fo89 (Fig. 6.1C). Only inclusions of these two types are considered truly

magmatic (i.e., primary and pseudosecondary in origin). But even these inclusions are

often interconnected by thin channels, and thus modifications of original melt

compositions by “necking down” cannot be ruled out. Abundant secondary melt

inclusions in fractures connected to the groundmass, and decrepitated inclusions, are

assumed to have experienced exchange and loss of material, respectively, after

entrapment.

“Necking down” can explain why the proportions of fluid and mineral phases in

the studied melt inclusions show considerable variability (Figs. 6.16, 17). Fluids are

represented by low-density CO2 bubbles, whereas solid phases are mainly olivine,

phlogopite, Na-K-Ca carbonates, halite and sylvite (Fig.6.18). Calcite, nahcolite

(NaHCO3), Na-K-Ca sulphates, Fe-Ni-K sulphides, Fe-Ti-Cr oxides, F-Cl apatite and

unidentified silicate and aluminosilicate minerals are less common.

Silicate minerals. Phlogopite, most abundant among silicate mineral inclusions,

forms brownish 5-20 µm long crystals, characterised by high TiO2 (3.26-3.64 wt%) and

elevated Na2O (0.41-0. 59 wt%) and Cl (0.14-1.59 wt%) contents (Table 6.8a #1-3).

Daughter olivine (Fo88.6-81.5) is characterised by higher CaO (up to 0.18 wt%) and lower

Figure 6.15. Fluid inclusions in groundmass olivine. A – distribution of the inclusions; B – partly decrepitated low-density CO2 fluid inclusions, rarely with boundary between liquid and gaseous CO2 (Th ~14oC).


5μm 10μm

1250 1300 1350 1400 1450 Wavenumber (cm-1)

CO2

CO2

10μm

10μm

B

A

50μm

50μm

50μm

20μm

50μm

Figure 6.16. Large multiphase melt inclusions in groundmass olivine. These inclusions usually have irregular shapes.


20μm20μm

20μm

20μm50μm

20μm

Figure 6.17. Relatively small, well-formed (some euhedral) multiphase melt inclusions in groundmass olivine. Dominant phases: low-density CO2 bubbles, Na-K-Ca carbonates and Na-K-chlorides


Figure 6.18. A-C - elemental maps of the exposed large multiphase melt inclusions in groundmass olivine., alk-carb – alkali-rich carbonate, phl – phlogopite; D –compositional variations of the alkali-rich carbonates


68

1012141618

6 8 10 12 14

K2O

Na2O

35

40

45

50

55

60

10 15 20 25 30

K2O+Na2O

CaO

2468

101214

60 65 70 75 80 85 90Total oxides

SO3

D

phl

alk-

carb

C

Al CaK

Ca

K Na

clc

alk-carb A

Ca

ClNa

alk-carb

NaCl

B


183

NiO (0.11 wt%) contents compared to host olivine. A number of other different

silicate phases (SiO2 25-38 wt%) are recorded by electron microprobe analisis (Table

6.8a #4-8), but cannot be confidentially identified. All but one are aluminosilicates. The

Al-free phase has a total ~ 100 wt%, and is characterised by the lowest SiO2, but highest

MgO (28 wt%), K2O (16.5 wt%), and SO3 (9 wt%) and very high Cl (6.8 wt%). Other

silicates have broadly similar abundances of Na2O (3-4.5 wt%), but widely variable

concentrations in Al2O3, CaO, MgO, K2O and volatile elements, and most likely belong

to the sodalite and cancrinite groups of minerals.

Carbonate daughter minerals are dominant and represented by calcite and alkali-

Ca-S rich species (Fig. 6.18 A-C and Table 6.8b). Alkali elements in these carbonates

are correlated with each other, and the total of alkalies shows antithetic correlation with

CaO (R= -0.89; Fig. 6.18D). Sulphur is very variable (3-12 wt% SO3), and its positive

correlation with the microprobe totals (R = 0.8; Fig. 6.18D) suggests that sulphur

substitutes for CO3, probably as a sulphate. Sulphur is very high in one analysis (27

wt% SO3, Table 6.8b, #30), which is also remarkable in having very high soda (32

wt%). A number of analyses of carbonate- or /sulphate-like phases show the presence of

SiO2 and MgO (up to 18 and 22 wt%, respectively, Table 6.8b, #32-38), and although

the incorporation of olivine in the analyses cannot be excluded, the Si/Mg ratios are

unlike those in olivine.

Chlorides are found as halite (NaCl) and less abundant sylvite (KCl). Because of

their high solubility in water, chlorides usually dissolve when melt inclusions are

exposed to the surface, even though precautions were taken. If the inclusions were

analysed immediately after exposure, then chlorides were noticed in melt inclusions as

fine-grained aggregates (Figs. 6.18B and 6.26). After several heating experiments,

chlorides become more evident in the inclusions, forming fields with orthogonal face

boundaries (Fig. 6.21D).

As kimberlite is a silicate-rich, although Si-undersaturated rock, a special effort

was made to locate silicate glasses among melt inclusions. However, despite a search in

several thousand grains, only one glass-like inclusion was found (Fig. 6.19, Table 6.9).

The abundances of most elements in this phase are basalt-like apart from negligible

Al2O3 (1.2 wt%) and exceptionally high Na2O (13 wt%) concentrations. In close


184

proximity to this inclusion, a film (or thin embayment in olivine), of what looks like a

silicate glass, was noted and analysed (Fig. 6.19, Table 6.9). Although very different in

many details to the inclusion, this film is indeed a silicate phase (35-37 wt% SiO2), but

unusual for a silicate glass in having low Al2O3 (3-4 wt%), and very high TiO2 (7-10

wt%) and Na2O (8-15 wt%). A few chemical components of this phase correlate

positively or negatively with SiO2 and each other (Fig. 6.19), but the nature of these

variations and the identity of this phase(s) remain unclear.

Inclusions of sulphide melt are very rare in the groundmass olivine (only three

such occurrences are recorded), but they are present in clusters of several tens (Fig.

6.20A). They are either spherical or cylindrical, confirming their origin as immiscible

liquids. The composition of sulphides is mainly pyrrhotite, which is rimmed by

djerfisherite where a film or a pool of chloride-rich material is present around sulphide

(Fig. 6.20B).

Figure 6.19 Glassy silicate melt inclusion and associated embayment of silicate melt. The diagrams show compositional variations of this embayment, abundances in wt%


SiO2

8

10

12

14

16

34 35 36 37 38

Na2O

6

7

8

9

10

11

34 35 36 37 38

TiO2

1.5

2.0

2.5

3.0

3.5

34 35 36 37 38

K2O

9

10

11

12

13

14

34 35 36 37 38

FeOt

9

10

11

12

13

14

34 35 36 37 38

CaO

10 μm

Figure 6.20. Sulphide inclusions in groundmass olivine.


Cl

Ni

KNa

S

NaCl

djer

10 μm

40 μm

B

A


187

Table 6.8 a Representative analyses of different silicate and phosphate minerals in

multiphase melt inclusions in groundmass olivine: 1-3 – phlogopite, 4-8 – unidentified

silicate minerals, 9-10 – apatite

1 2 3 4 5 6 7 8 9 10 SiO2 37.24 34.81 39.37 24.93 29.82 27.26 38.06 32.80 1.88 4.72 TiO2 3.26 3.50 3.64 0.05 0.28 1.58 0.02 0.03 0.03 0.06 Al2O3 14.78 14.66 12.67 0.03 8.13 10.36 31.07 27.42 1.18 0.80 FeOt 6.10 8.44 4.71 6.39 1.64 3.10 1.23 0.54 0.55 0.91 MnO 0.00 0.12 0.04 0.06 0.03 0.03 0.04 0.02 0.00 0.04 MgO 21.27 20.32 21.35 27.92 16.23 16.26 1.65 1.15 0.05 3.50 CaO 0.05 0.04 0.07 0.01 2.77 13.44 15.27 16.06 55.05 47.36 Na2O 0.41 0.59 0.48 6.77 4.55 3.14 4.10 4.03 0.61 3.05 K2O 10.49 10.08 10.23 16.53 14.15 7.86 0.72 0.80 0.09 0.74 P2O5 0.00 0.00 0.00 0.00 0.03 0.16 0.48 0.00 29.80 33.65

Cl 0.14 0.22 1.59 6.75 9.80 0.99 6.45 5.79 0.34 1.49 SO3 0.09 0.05 0.55 9.08 0.46 0.71 0.09 0.12 0.08 0.05

F 1.00 0.93 0.08 0.00 0.04 1.92 2.22 Sr 0.06 0.00 0.14

BaO 0.00 0.65 0.66 Total 94.83 93.77 94.79 98.58 88.55 85.68 99.18 88.80 91.60 98.59


188

Table 6.8 b Representative analyses of calcite (1-2), alkali-Ca-S-bearing carbonate (3-29) and unidentified minerals (30-36) in olivine-hosted multiphase melt inclusions

N 1 2 3 4 5 6 7 8 9 10 11 12 SiO2 0.12 0.10 0.72 0.23 0.04 0.46 0.00 0.16 0.24 0.05 0.43 0.09 TiO2 0.02 0.01 0.00 0.01 0.01 0.00 0.03 0.00 0.00 0.00 0.00 0.00 Al2O3 0.01 0.01 0.01 0.03 0.06 0.03 0.02 0.44 0.02 0.02 0.05 0.00 FeOt 0.44 0.39 0.85 0.38 0.45 0.46 0.29 0.53 0.48 0.46 0.45 0.39 MnO 0.04 0.00 0.06 0.04 0.00 0.00 0.00 0.01 0.00 0.04 0.02 0.03 MgO 0.11 0.09 1.64 0.94 1.08 2.98 0.16 1.29 0.86 0.93 1.48 0.92 CaO 63.57 63.95 36.25 42.77 37.48 42.93 46.21 39.05 45.46 49.49 48.74 53.14 Na2O 0.03 0.02 17.46 12.92 12.44 11.41 11.34 10.41 9.55 7.53 7.48 6.64 K2O 0.03 0.01 10.82 10.21 13.19 9.18 10.81 12.10 8.70 8.32 8.16 7.33 P2O5 0.01 0.05 0.28 0.26 0.15 0.27 0.50 0.18 0.25 0.46 0.38 0.53

Cl 0.00 0.00 0.07 0.03 0.04 0.05 0.04 0.08 0.09 0.04 0.07 0.11 SO3 0.02 0.04 4.08 4.26 3.53 4.45 5.28 3.93 3.43 4.87 5.00 5.62 Sr 0.58 0.60 0.54 0.46 0.62 0.48 0.53 0.65

BaO 0.04 0.00 0.12 0.10 0.11 0.12 0.18 0.16 Total 65.03 65.26 72.24 72.73 68.45 72.76 74.68 68.24 69.80 72.84 72.97 75.62

N 13 14 15 16 17 18 19 20 21 22 23 24 SiO2 0.37 0.10 0.28 0.25 0.21 0.81 2.07 0.34 1.18 0.14 0.11 0.09 TiO2 0.00 0.00 0.02 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Al2O3 0.03 0.02 0.00 0.00 0.03 0.01 1.12 0.01 0.00 0.01 0.00 0.00 FeOt 0.68 0.45 0.44 0.45 0.44 0.46 0.48 0.45 0.54 0.38 0.39 0.34 MnO 0.06 0.00 0.08 0.06 0.01 0.02 0.00 0.03 0.03 0.00 0.00 0.00 MgO 0.74 0.26 0.21 0.21 0.23 0.73 2.22 0.22 13.65 0.26 0.21 0.27 CaO 51.14 56.75 49.35 48.16 50.48 46.13 49.00 47.50 40.28 46.34 45.41 44.41 Na2O 4.73 7.81 8.39 6.47 7.74 9.19 9.05 7.87 8.99 12.23 11.26 11.56 K2O 6.54 6.30 6.76 6.57 7.11 7.94 7.22 7.21 7.20 7.59 7.71 7.70 P2O5 0.40 0.31 0.29 0.32 0.28 0.18 0.20 0.26 0.19 0.20 0.19 0.25

Cl 0.06 0.04 0.02 0.02 0.07 0.34 0.02 0.03 0.02 0.03 0.03 0.06 SO3 4.70 12.24 10.80 10.56 10.30 10.29 9.82 9.79 9.64 9.64 9.45 9.44 Sr 0.59 0.62 0.50 0.46 0.60 0.43 0.55 0.44 0.45 0.57 0.59 0.42

BaO 0.35 0.11 0.19 0.06 0.13 0.05 0.13 0.05 0.10 0.11 0.18 0.10 Total 70.38 85.02 77.32 73.59 77.65 76.58 81.90 74.20 82.28 77.50 75.54 74.64

N 25 26 27 28 29 30 31 32 33 34 35 36 SiO2 0.14 0.28 0.08 0.15 0.12 14.14 11.25 13.91 12.10 11.00 TiO2 0.00 0.01 0.00 0.01 0.00 0.04 0.05 0.05 0.13 0.19 0.01 0.23 Al2O3 0.00 0.00 0.03 0.00 0.02 0.01 0.01 0.18 3.90 2.90 0.03 0.30 FeOt 0.41 0.45 0.49 0.42 0.34 1.04 0.90 2.73 3.50 6.73 MnO 0.00 0.00 0.00 0.04 0.00 0.03 0.11 0.00 0.06 0.04 0.10 0.06 MgO 0.32 0.22 0.23 1.06 0.94 0.52 8.74 7.03 9.31 12.89 CaO 47.67 51.69 52.25 51.93 51.81 10.83 20.86 11.21 41.26 38.78 5.15 2.82 Na2O 9.70 9.67 6.13 6.19 6.81 32.18 11.51 22.76 3.67 5.04 18.05 16.70 K2O 7.56 7.28 7.49 7.11 6.95 2.48 15.42 11.04 7.52 8.40 5.18 9.19 P2O5 0.14 0.29 0.28 0.68 0.54 0.46 0.15 1.11 0.22 0.11 0.39 0.75

Cl 0.06 0.06 0.03 0.08 0.10 0.86 0.45 2.62 0.07 0.11 3.65 8.24 SO3 9.38 9.31 8.37 7.56 7.51 27.31 14.64 12.91 8.27 9.34 12.10 9.66 Sr 0.62 0.67 0.40 0.46 0.56 0.16 0.26 0.11 0.39 0.33 0.08 0.00

BaO 0.07 0.18 0.16 0.04 0.04 0.08 0.08 0.08 0.38 0.25 0.00 0.00 Total 76.07 80.12 75.94 75.74 75.73 74.43 63.52 77.78 86.76 89.17 69.65 78.58


189

Table 6.9 Compositions of “glassy” melt inclusions and associated embayment of melt

SiO2 TiO2 Al2O3 FeOt MnO MgO CaO Na2O K2O P2O5 Cl SO3 Cr2O3 total Glass (?) melt inclusion

49.76 1.39 1.18 6.20 0.30 12.52 12.95 13.04 2.30 0.15 0.00 0.08 0.01 99.87 embayment of melt

36.09 9.18 3.62 12.76 0.35 6.87 9.26 10.78 2.46 1.19 0.00 0.08 0.09 92.73 35.43 9.19 4.09 10.37 0.31 7.05 9.92 15.08 2.47 1.92 0.00 0.03 0.01 95.86 37.30 10.39 3.73 13.39 0.45 7.19 9.31 8.40 2.67 1.36 0.00 0.00 0.00 94.17 36.59 10.18 3.36 13.66 0.38 7.27 9.72 8.36 3.03 1.07 0.00 0.14 0.00 93.76 34.91 8.69 4.03 9.82 0.24 7.55 11.33 12.74 2.32 2.09 0.01 0.06 0.04 93.83 35.10 8.32 3.80 9.95 0.36 7.76 11.52 13.56 2.23 1.83 0.01 0.09 0.00 94.53 34.89 7.26 3.28 11.53 0.46 8.97 13.14 13.02 2.14 0.96 0.00 0.09 0.01 95.75 35.39 9.23 3.02 12.20 0.38 9.13 10.61 13.03 2.34 0.84 0.00 0.11 0.07 96.35 34.78 6.99 3.25 9.05 0.30 10.80 13.73 14.18 1.80 1.82 0.01 0.11 0.05 96.86 35.46 8.72 2.94 11.68 0.35 12.17 10.54 11.17 2.43 0.89 0.00 0.08 0.01 96.45

6.2 Inclusions in porphyroclastic olivine

Porphyroclastic olivine-I in most cases does not contain any crystal, melt or fluid

inclusions. Only a number of olivine-I grains host abundant melt inclusions (mostly

recrystallised) and associated with them fluid (usually represented by deformed vapour

bubbles). These inclusions are highly irregular in shape and variable in size, and are

always restricted to fractures and zones of decrepitation of precursor inclusions (Fig.

6.1B). They are undoubtedly secondary in origin, and as shown below, compositionally

similar to primary inclusions in olivine-II.

6.3 Heating experiments with multiphase melt inclusions

Rounded, relatively small (40-60 µm in diameter) inclusions were selected for

experimental tests (Fig. 6.17) because larger inclusions often leak or decrepitate during

heating. In some cases the grains were first bulk heated in a vertical furnace to 1250oC

and quenched, then decrepitated inclusions were eliminated. The olivine grains with

inclusions that survived heating to high temperature were doubly polished for individual

runs in the heating stages with optical control, using a petrographic microscope

(Olympus BX 60) fitted with a CCD camera (Olympus DP11).


190

Thermometric experiments were performed on olivine-hosted melt inclusions

using a Linkam TS1500 heating stage with visual control and the Vernadsky Institute

heating stage (Sobolev et al., 1980) in an inert atmosphere of ultra-high purity argon and

helium, respectively, to prevent olivine oxidation. The purpose of these experiments

with multiphase melt inclusions in olivine of both generations was to document phase

transformations (melting, crystallisation, bubble disappearance, homogenisation,

heterogenisation etc) and the temperature of the above mentioned events.

Melting in the inclusions is inferred to begin in the temperature interval of 160-

450oC, as indicated by jolting movements of either solid phases or vapour bubbles

inside inclusions. In fact, initial melting may have occurred at even lower temperature,

but the accurate determination of this exact point was impossible because of very small

“amplitude” of visible changes. Further heating enhances phase boundaries through the

increasing amount of interstitial liquid, which also causes bubbles to coalesce at 420-

580oC. At 540-600oC daughter phases experience some changes in their relative

position, shape and colour that are interpreted to be related to recrystallisation and

melting. At temperature above 600oC two distinct phases, apart from vapour bubbles

and opaque minerals, are clearly visible. One is pale-blue and constitutes the “matrix”

for a large number of pinkish globules that move freely and change shape continuously.

The shape of a single globule is always very smooth: it can instantaneously change from

perfectly spherical to cylindrical, embayed or lopsided, similar to an amoeba. With

further heating, the number and size of floating and shape-changing pinkish globules, as

well as the number and size of vapour bubbles, gradually decrease. Homogenisation of

the inclusions (except some opaque crystals) occurs when the pinkish globules and

vapour bubbles disappear almost simultaneously (within 20-30oC) at temperatures of

660-760oC (Figs. 6.21A, B).

These temperatures were achieved in eight experiments, and only in one run did

homogenisation take place at a much higher temperature (955oC). A number of

inclusions that did not homogenise below 760oC were further heated. Most decrepitated

(expansion of vapour phase filling the whole volume of inclusions) at some instant at

higher temperatures (770-1000oC), whereas in other inclusions decrepitation did not

show up as a dramatic event, and the bubble was still present even at 1250oC.

Figure 6.21 A. Heating stage experiments with multiphase melt inclusions in olivine-II. Run 14 – complete melting and bubble dissolution (homogenisation) occur almost simultaneously at temperature as low as 680oC. Carbonate-chloride immiscibility occurs at 580oC


629oC20oC

680oC463oC

618oC564oC

580oC614oC

1

2

3

4

5

6

7

8

Figure 6.21 B. Heating stage experiments with multiphase melt inclusions in olivine-II. Run 27B – homogenization occurs at 720oC, immiscibility at ~600oC


25oC

568oC

622oC

654oC

710oC

720oC

646oC

610oC

1

2

3

4

5

6

7

8

Figure 6.21. Heating stage experiments with multiphase melt inclusions in olivine-II. C – orientation of immiscible globules; D - chlorides and carbonates are more evident in the inclusions after several heating experiments


D

C

before experiment after experiment

after experiment


194

During slow cooling (5-20oC/min) cooling, a vapour bubble(s) nucleates at 690-

650oC and then progressively increases in size (Fig. 6.21A, B). Almost at the same time

(within 30-50oC), formation of a colourless (?), needle-like crystal(s) was noticed in

several runs. Nucleated on the inclusion walls, the growing crystal rapidly propagates

into the melt, and after reaching the opposite wall continues growing in width. Cooling

to 610-580°C results in a spontaneous process when for a split second inclusions

acquire a ‘foggy” appearance. This process can be best described as the formation of

emulsion, i.e, microglobules of liquid in another liquid (melt immiscibility).

Microglobules coalesce immediately into larger, elongate, sausage-like pinkish

globules. The neighbouring globules (“boudins”) are subparallel, and are grouped into

regularly aligned formations with a common angle of ~75-80o (Fig. 6.21C). A

resemblance to the skeletal or spinifex texture is evident for several seconds, after which

the original “pinch-and-swell structure” pulls apart giving rise to individual blebs of

melt. The latter coalesce and become more spherical with time or further cooling. They

continue floating that slows down with decreasing temperature and further coalescence

until two distinctive phases are forced (by slow cooling 3oC/min) to aggregate in

different parts of inclusions. The exact moment of crystallisation or complete

solidification is not recorded, but finally a fine-grained phase with high birefringence

and an isotropic phase with orthogonal grain boundaries form (Fig. 6.21D). These

phases can be recognized as the carbonate and chloride, respectively, on the basis that

these minerals are dominantly present in the unheated melt inclusions.

In every experiment, all inclusions present in a field of view show similar phase

transformations at nearly the same temperature. All successful experiments

demonstrated very similar pattern in inclusion behaviour. Importantly, the results can be

reproduced for a single inclusion many times, and a change in the heating/cooling rate

has no significant influence on the results.

Heating stage experiments with inclusions hosted by porphyroclastic olivine-I

show similar melting behaviour, similar temperatures of phase transformations and

homogenisation, as well as analogous carbonate-chloride melt unmixing during cooling

(Fig.6.22).

Figure 6.22. Heating stage experiments with multiphase melt inclusions in olivine-I. Run 2. Homogenization occurs at 660oC, recrystallisation at 560oC. Note similar behavior and temperatures of homogenization and heterogenizationfor inclusions in olivine of both generations.


475oC25oC 510oC

590oC 640oC 655oC

660oC 560oC

1 2 3

4 5 6

7 8


196

Overheating (to 1250oC) and fast quenching (>100oC/sec) of the inclusions

results in the formation of homogeneous “frosted glass” (Fig. 6.23), providing those

inclusions survived decrepitation (a large volume vapour bubble in these inclusions

indicates partial decrepitation). Unlike slowly cooled inclusions, quenched melt

inclusions that appear as “frosted glass” at room temperature are ideally suited for

analysis of bulk melt composition. However, inclusions of homogeneous “frosted glass”

decompose and recrystallise immediately after being exposed at the surface, even if

precautions are taken during lapidary work. This suggests that the “frosted glass” melt

inclusions are a highly metastable melt when fast quenched and then exposed at room

conditions.

The immiscible liquids formed in a fast quenching experiment were identified as

predominantly Ca-carbonate and Na-K chloride (Fig. 6.25) using PIXE imaging

(CSIRO-GEMOC Nuclear Microprobe, North Ryde, analysed by Dr Chris Ryan) and

laser Raman spectroscopy (Geoscience Australia, Canberra, analysed by Dr Terry

Mernagh). Laser Raman spectroscopy (GFZ Potsdam, analysed by Dr Rainer Thomas)

of a “frosted glass” inclusion at room temperature (Fig. 6.24) shows bands of hydrogen-

bearing carbonate and common carbonate mixtures. The band at 1046 cm-1 corresponds

to nahcolite (NaHCO3) and the broad band, centred at about 1077 cm-1, results from

different carbonate species. Quantitative deconvolution of the overlapping bands gives

three components centred at 1068, 1077, and 1087 cm-1, respectively. The first two

bands are typically for Na2CO3 with a weak band at 1067.5 and a strong band at 1077.6

cm-1. The weak band at 1087 cm-1 is probably from Mg-bearing calcite.

6.4 Compositions of melt inclusion in groundmass olivine

The bulk compositions of melt inclusions were recalculated using analyses of

individual daughter crystals and mineral aggregates within thoroughly exposed

inclusions and their estimated volume and weight ratios (Fig. 6.26 and Table 6.10). It

should be noted that there is a large uncertainty in the estimate of the olivine component

present, because olivine, crystallised on the inclusion walls (daughter olivine) is

optically and compositionally indistinguishable from the host olivine.

Figure 6.23. Dominantly carbonate-chloride melt inclusions in olivine-II from the Udachnaya kimberlite, quenched into “frosted glass” after heating to 1150oC. Scale bars are 20 μm.

Figure 6.24. Raman spectrum of a “frosted glass” inclusion at room temperature, showing bands of nahcolite (NaHCO3) and carbonate mixtures.

1020 1040 1060 1080 11000

20

40

60

80

100

120


1087

1077

1068

1046

Inte

nsity

(a.u

.)

Wavenumber (cm-1)

nahcolite

Na2CO3

Mg-bearing carbonate

K

Br

S

Ca

Sr

Rb

Cl

20μm

Figure 6.25. Optical images and PIXE (proton-induced X-ray emission) element maps of the “frosted glass” inclusion.


min max


199

Table 6.10 Representative compositions of daughter minerals in melt inclusions in

olivine-II.

wt.ratio SiO2 TiO2 Al2O3 FeO MgO CaO Na2O K2O Cl H2O S 1 34 0.2 0.0 0.4 0.7 0.0 0.0 19.5* 26.0* 54.5 n.d. 0.0 2 27 40.6 0.0 0.0 11.6 47.5 0.2 0.0 0.0 0.0 n.d. 0.0 3 17 36.0 3.4 14.7 7.3 20.8 0.0 0.5 10.3 0.2 4.0** 0.0 4 22 1.2 0.0 0.0 1.0 1.6 36.4 14.7 9.5 0.1 n.d. 1.5

Note to Table: Daughter minerals in the olivine-hosted melt inclusion: (1) - Na-K chloride; (2) – olivine; (3) – phlogopite; (4) - Na-K-Ca carbonate. * - elemental wt%; **- calculated on the basis of stoichiometry (OH+F+Cl=2.0 apfu); n.d. – not determined.

Although the analysed sections of individual inclusions are different in terms of

phase assemblage and mineral proportions, the calculated bulk melt compositions are

distinct from the kimberlite groundmass in having very high Na and K and Cl contents

(Table 6.11 #5). This is a consequence of the contribution from Na-K chlorides and

sulphur-bearing alkali carbonates present in melt inclusions. Similarities between melt

inclusions and the groundmass include relatively low Si and Al, enrichment in volatile

components and presence of water-soluble alkali chlorides, carbonates and sulphates.

The variability in the melt components is likely due to post-entrapment evolution of

melt inclusions (e.g., necking down). Table 6.11 shows that the estimates of the

kimberlitic melt are markedly different from the composition of the groundmass of

studied kimberlites and the typical composition of the type-I kimberlites in having a

significant amount of Na-K chloride component (>20 wt%). This previously unknown

component carries considerable amounts of Cl, Na, and K in the original kimberlite

magma.

Figure 6.26. Multiphase melt inclusion in transmitted (A) and reflected light (B), respectively. Principal daughter phases are indicated - alk-carb - Na-K-Ca carbonate; chl - K-Na chloride; phl – phlogopite; ol – olivine, and their average compositions are given in the text, Table 6.10.


chlol

50μmalk-carb

phl chlchl

olol

ol

phl

B

A

Ca Cl

Na+K 1

2

5

3

4

Figure 6.27. Compositions of olivine-hosted melt inclusions and estimated parental kimberlitic melts (1) - melt inclusions hosted in olivine-II, analysed by LA-ICPMS; (2) - calculated bulk composition of the melt inclusion (Fig. 6.19; Table 6.x); (3) - bulk composition of the melt inclusion calculated on the basis of estimated mass proportions of chloride and carbonate phases (Fig. 6.14); (4) -Udachnaya pipe kimberlite groundmass; (5) - diamond-hosted brine inclusions (Izraeli et al., 2001).

Figure 6.28. Primitive mantle normalised (Hofmann, 1988) compositions of olivine-hosted melt inclusions, analysed by LA-ICPMS and normalised to La in the Udachnaya kimberlite groundmass YBK-0 in comparison with average groundmass

1

10

100

1000

Nb La Ce Pr Sr Nd Zr Sm Eu Gd Tb Dy Y

groundmass

individual melt inclusionaverage melt inclusion



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Table 6.11 Compositions of group-I kimberlites and calculated parental melt of the Udachnaya pipe kimberlite

NN SiO2 TiO2 Al2O3 FeOt MgO CaO Na K P2O5 CO2 Cl H2O 1 27.6 1.6 3.2 7.6 24.3 14.1 0.1 0.7 0.5 10.8 n.d. 7.9 2 25.7 3.0 3.1 11.4 23.8 14.1 0.1 0.5 1.1 8.6 n.d. 7.2 3 28.1 0.7 1.6 6.6 23.1 16.7 0.1 0.4 0.8 12.1 n.d. 7.7 4 24.3 1.7 2.5 7.4 25.5 15.6 2.9 1.9 0.6 12.0 4.1 0.6 5 17.3 0.6 2.6 4.6 16.6 8.1 9.1 12.1 0.1 7.0 18.5 0.7

Note to Table: (1)- average Siberian kimberlite (Mitchell, 1989); (2)-average South African on-craton kimberlite (Price et al., 2000); (3)-Jericho pipe kimberlite JD82, Canada (Price et al., 2000); (4)-groundmass of studied Udachnaya-East pipe kimberlite YBK-0; (5)- bulk composition of the melt inclusion (Fig 6.26) calculated using relative abundances and average compositions of daughter phases. FeOt – total Fe; n.d. – not determined.

Unexposed melt inclusions were analysed by laser-ablation ICP-MS at CODES

ARC Special Research Centre, University of Tasmania. The setup includes a UP213 UV

laser with a custom-built small-volume (~4 cm3) ablation cell, coupled to a HP4500

ICP-MS. Samples were ablated using 30-60 µm spots, a repetition rate of 5 Hz and laser

energy of 14 J/cm2. The measurements were calibrated using the NIST612 standard.

Because the signal from the ablated melt inclusions included unknown contributions

from host olivine, major constituents of olivine (Si, Mg, and Fe) could not be estimated

in the melt inclusions. In the absence of an internal standard (independently determined

concentration of an element), the compositions of melt inclusions were recalculated

based on two assumptions. First, the major (Cl, Na, K, Ca, Al, and Ti) and lithophile

trace (e.g., REE) element contribution from the host olivine was taken to be negligible.

Second, the total of analysed non-olivine elements and CO2, related to Na-K-Ca

carbonate, should be 100 wt% (Table 6.12).

In an attempt to calculate a bulk kimberlite melt from the melt inclusions,

relative proportions of the observed daughter phases and immiscible liquids were

recombined, taking into account data from heating experiments and the element ratios

derived from the LA-ICPMS analyses of unexposed inclusions. When recast in terms of

Ca, Cl and (Na+K), the composition of the melt inclusions is enriched in Cl and alkalies

compared to the kimberlite groundmass (Fig. 6.27). As mentioned above, the observed

variability in the major melt components is likely due to post-entrapment evolution of


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melt inclusions (e.g., necking down), but nevertheless the melt inclusions have the

enriched trace element signature similar to that in the kimberlite groundmass (Fig.

6.28).

6.5 Inclusions in other groundmass minerals and carbonate-

chloride nodules

6.5.1 Inclusions in groundmass calcite, anhydrite and chlorides

Individual groundmass grains of calcite, as well as mineral segregations of

calcite contain crystal and fluid inclusions. Crystal inclusions are mainly represented by

perovskite and fine-grained magnetite aggregates (Fig. 6.29A #1,2). A number of

perovskite inclusions occur in association with sulphide blebs and a vapour-rich phase.

Fluid inclusions are relatively small (20-30 µm) with almost perfect negative crystal

shapes. Most are saline aqueous fluid with either a single chloride crystal or a mosaic-

textured aggregate of chloride grains and a vapour bubble (Fig. 6.A #4-6).

Abundant fluid inclusions were also found in groundmass anhydride and

chlorides. Numerous small (<10 µm) rounded fluid inclusions are trapped along the

growth planes and usually consist of single phase aqueous fluid. However, multiphase

saline fluid inclusions are also present. These inclusions have good hexagonal shapes

and consist of a perfectly shaped cubic chloride crystal, aqueous liquid and a vapour

bubble (Fig. 6.29B, C).

6.5.2 Inclusions in carbonate from carbonate-chloride nodules

The inclusion study reveals the presence of different mineral and multiphase

melt inclusions, trapped in Na-Ca carbonate. As mentioned in Chapter 3, mineral

inclusions are represented by abundant euhedral crystals of apatite, zoned crystals of

phlogopite, chlorides, Na-K-bearing sulphates and different carbonates (northupite, Ca-

Ba-Na carbonate and S-bearing alkali-Ca carbonate). Multiphase melt inclusions are

shown on Fig. 6.30-32. These inclusions occur in clusters along healed cracks and

growth planes, and thus are likely to be pseudo-secondary. Multiphase melt inclusions

have rounded elongated shapes, are 30-60 µm across, and consist of crystals, aqueous


Figure 6.29. Different types of inclusions in groundmass calcite and carbonate aggregates (A); in groundmass anhydrite (B) and chloride (C)

chloride

perovskiteperovskite

sulphide

15μm

15μm

15μm

C

B

Amagnetite

1 2 3

4 5 6

Figure 6.30. Multiphase (crystals, aqueous fluid, vapor bubble) inclusions in Na-Ca-carbonate from chloride-carbonate nodules


1

2

3

12 3

Figure 6.31. Exposed multiphase inclusions in Na-Ca-carbonate. Empty cavities were probably filled with fluid. ap – apatite, sy-sylvite, ha-halite, phl-phlogopite, phl*-tetraferriphlogopite (Fe-rich and Al-poor phlogopite), slf-sphalerite

ap

ap

ap

ha

sy sy

sy

phl

phl*

phl

phl*

phl*

phl

Fig. 6.32. Multiphase carbonate-sulphate inclusion in Na-Ca-carbonate. 1. Na-K-sulphate; 2. S-bearing alkali-Ca carbonate; 3. Na-K-Ca sulphate.


ap

slf

Na

S

K

Na S K

Ca

CaK

S

Na


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Table 6.12 Composition of unexposed melt inclusions (analysed by LA ICPMS)

1 2 3 4 5 6 7 8 Na (wt%) 16.0 20.8 18.6 13.0 19.9 22.1 15.6 23.3

Cl 57.3 50.6 50.2 52.5 44.4 46.6 38.8 46.3 K 21.8 16.0 22.0 30.4 16.2 16.3 20.2 17.7

CaO 2.1 4.9 4.2 1.7 7.5 5.4 10.0 3.5 Rb (ppm) 456.8 451.2 471.2 1010.8 480.1 530.4 635.7 607.8

Sr 238.3 813.3 646.2 167.6 808.9 839.2 1116.7 550.4 Y 1.8 2.3 0.6 1.1 5.5 2.9 3.2 2.0 Zr 22.4 81.2 5.3 2.9 95.2 81.7 37.0 11.4 Nb 4.4 20.4 1.1 2.9 39.0 33.7 37.3 7.0 Ba 1091.2 1503.1 307.5 225.6 851.9 1347.4 548.9 733.6 La 5.3 14.3 4.6 30.4 11.1 17.9 4.7 Ce 6.8 16.8 0.7 4.4 45.3 10.2 29.7 6.1 Nd 1.9 4.6 1.6 8.8 2.7 8.6 1.7 Sm 0.3 0.2 0.7 0.8 1.5 0.9 0.2 0.5 Eu 0.2 0.1 0.1 0.4 0.3 0.4 0.2 Gd 0.3 0.9 0.6 0.3 0.7 0.8 0.2 Dy 0.5 0.0 0.0 0.3 1.1 0.6 1.0 0.3 Er 0.2 0.1 0.1 0.6 0.4 0.4 0.1 Yb 0.1 0.4 0.3 0.5 1.3 0.5 Pb 21.7 66.9 111.8 25.4 76.3 170.7 75.5 91.9

9 10 11 12 13 14 15 16

Na (wt%) 20.6 15.3 20.7 22.5 13.5 17.0 23.7 10.3

Cl 38.1 32.2 36.4 38.2 21.2 27.6 47.1 4.9

K 12.4 15.8 14.8 16.5 9.6 15.9 16.3 8.8

CaO 9.9 14.3 6.4 4.7 12.7 10.5 4.1 28.6

Rb (ppm) 383.5 496.7 490.5 491.1 287.6 565.2 464.6 469.4

Sr 861.8 1624.4 761.4 636.6 1058.1 1126.6 580.5 3145.0

Y 11.0 6.7 52.7 4.0 40.9 6.0 3.3 64.4

Zr 204.1 297.5 360.0 99.2 226.9 126.8 82.6 435.2

Nb 231.7 395.7 557.0 28.5 304.1 145.7 15.6 1157.6

Ba 1396.6 1507.3 1730.7 539.2 726.6 2158.6 1046.4 1510.3

La 206.4 44.0 489.7 18.8 606.9 40.6 23.5 738.1

Ce 339.6 69.6 594.9 19.0 639.9 81.3 28.6 1236.2

Nd 108.2 29.3 170.6 5.9 175.3 32.4 7.6 396.1

Sm 15.1 6.3 29.2 1.3 27.7 4.9 1.0 54.0

Eu 3.2 0.5 9.3 0.7 8.3 0.5 0.6 9.3

Gd 9.7 26.7 1.4 23.2 1.1 1.3 33.3 Dy 4.5 0.6 17.1 0.5 12.5 0.8 0.9 19.9 Er 0.9 1.2 4.6 0.8 3.8 0.6 0.3 4.9 Yb 1.2 0.1 1.7 1.5 1.8 0.2 0.2 3.0 Pb 46.9 72.7 74.2 72.2 51.5 218.1 85.2 105.6


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fluid and a vapour bubble (Fig. 6.30). Daughter phases are represented mainly by

apatite, chlorides, phlogopite and tetraferriphlogopite (Fe-rich and Al-poor phlogopite).

Sulphides (Fig. 6.31) and sulphates (Fig. 6.32) are less common.

6.5.3 Heating experiment with melt inclusions in carbonate from

carbonate-chloride nodules

At room temperature the studied inclusion has an elongated euhedral

(hexagonal) shape and consists of several (3-4?) crystalline phases, interstitial liquid

(aqueous?) and a vapour bubble. The daughter crystals are euhedral and based on

optical and crystallographic features they are likely to be different phases. During the

heating stage experiment, upon heating, visible changes of crystal shape happen around

170oC. At this temperature, the crystals start melting slowly or possibly recrystallise.

Further heating from 240 to 340oC results in intensive melting and the disappearance of

some crystals at 348oC. At approximately 390oC the inclusion starts increasing in size,

and the remaining mineral phases melt. At ~ 400oC, it appears that at least two

immiscible substances have separated from each other. Based on the optical properties

one phase (A) occupies “left and right corners” of the inclusion, and another phase (B)

is in between (Fig. 6.33). At this particular moment the bubble was stretched between

these two phases (bottom right), and gradually poured to the right into phase A. The size

of the inclusion continues to grow significantly. At 415oC the bubble suddenly

disappears, and this coincides with drastic changes in the inclusion appearance. Melt or

fluid contained in the inclusion reacts intensively with the host mineral, causing a

further increase in the inclusion’s size (by the factor of ~4-5 compared to original

volume). The host mineral becomes unstable at >400oC (this temperature is consistent

with the upper termal stability of shortite and zemkorite ~427oC (Parthasarathy et al.,

2002), and the experiment was stopped. Cooling to room temperature causes inclusion

size to decrease and inclusion outlines to become jagged, possibly because of

crystallisation on the walls.

Figure 6.33. Heating stage experiment with inclusion in the alkali-bearing carbonate from chloride-carbonate nodules. Note, inclusion increases in size. Red outline shows the original size of inclusion before experiment


415oC

125oC 170oC 240oC

295oC 335oC 340oC

390oC 400oC

415oC

22oC – after experiment


210

6.6 Summary

The study of magmatic inclusions in kimberlite groundmass minerals provided

invaluable information about the crystallising mineral assemblage at different stages of

magma evolution, and the composition and temperature of the magma. The most

important observations and inferences are summarised below:

1. A variety of primary and pseudosecondary magmatic inclusions in

groundmass olivine is a clear indication of that olivine-II, at least their rims, crystallised

from the melt, and were not accidentally entrained during magma ascent and

emplacement;

2. The evolution of the magmatic system during ascent and cooling is recorded

in significant variability in the compositions and proportions of its constituents, such as

melt, crystals and fluid;

3. The early evolution of the kimberlite magma (protokimberlite) occurred at

deep mantle conditions (>150 km), possibly in the melting source regions, and resulted

in crystallisation of olivine cores and Na- and Cr-enriched high-Ca pyroxene;

4. The intermediate evolution of the kimberlite magma occurred during

emplacement in the crust, and was characterised by significant degassing of C-H-O

fluids and crystallisation of olivine-II rims, phlogopite, low-Ca pyroxene, perovskite,

Cr-spinel, rutile and magnetite. The absence of silicate melt inclusions in the studied

olivine indicates that in this case the crystallisation of Fe-Mg silicate minerals occurred

from an atypical melt. The evidence from the melt inclusions strongly implies that such

melt was particularly enriched in chloride and carbonate components, whereas the

amount of the aluminosilicate components was significantly reduced relatively to those

in common mafic magmas. This melt still crystallised olivine at temperatures as low as

~ 700oC;

5. A strong chloride-carbonate “flavour” of the kimberlite melt at the latest

stages of evolution is most pronounced in the compositions of melt inclusions in the

rims of olivine-II, segregations in the groundmass and nodule-like mineral aggregates.

The mafic silicate component is nearly exhausted from this melt following preceding


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crystallisation of abundant olivine, and the melt undergoes chloride-carbonate

immiscibility and crystallisation of chloride and carbonate minerals at <600oC. The

composition of fluids at this stage is likely to be brine.

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Chapter 6 Magmatic inclusions in groundmass minerals of ... · Chapter 6 Magmatic inclusions in minerals 148 crystal is fractured in the presence of melt or fluid, then the fractures