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Ž .Earth and Planetary Science Letters 146 1997 243–258
Basaltic liquids and harzburgitic residues in the GarrettTransform: a case study at fast-spreading ridges
Yaoling Niu a,), Roger Hekinian b´a Department of Earth Sciences, The UniÕersity of Queensland, Brisbane, Queensland 4072, Australia
b IFREMER, Centre de Brest, DROrGM, B.P. 37, 29287 Plouzane, France´
Received 9 May 1996; revised 28 October 1996; accepted 30 October 1996
Abstract
X Ž .The peridotite–basalt association in the Garrett Transform, ;13828 S, East Pacific Rise EPR , provides a primeopportunity for examining mantle melting and melt extraction processes from both melts and residues produced in acommon environment beneath fast-spreading ridges. The peridotites are highly depleted, clinopyroxene-poor, harzburgites.Residual spinel, orthopyroxene and clinopyroxene in these harzburgites are extremely depleted in Al O , and plot at the2 3
Žmost depleted end of the abyssal peridotite array defined by samples from slow-spreading ridges including samples from.hotspot-influenced ridges , suggesting that these harzburgites are residues of very high extents of melting. The residual
Ž .peridotites from elsewhere at the EPR i.e., Hess Deep and the Terevaka Transform also are similarly depleted. Thissuggests that the extent of melting beneath the EPR is similar to, or even higher than, beneath ridges influenced by hotspotsŽ .e.g., Azores hotspot in the Atlantic Ocean and Bouvet hotspot in the Indian Ocean , and is significantly higher than F10%,a value that has been advocated to be the average extent of melting beneath global ocean ridges. Many of these harzburgitesamples, however, show whole-rock incompatible element abundances higher than expected. These same samples also havevarious amounts of excess olivine with forsterite contents as low as Fo . The total olivine modes correlate inversely with85
olivine forsterite contents, and positively with whole-rock incompatible element abundances. These correlations suggest thatthe excess olivine and incompatible element enrichment are both the result of melt–solid re-equilibration. The buoyant meltsthat ascend through previously depleted residues crystallize olivine at shallow levels as a result of cooling. Entrapment ofthese melts leads to whole-rock incompatible element enrichment. These observations contrast with the notion that meltsformed at depth experience little low pressure equilibration during ascent.
Keywords: harzburgite; mid-ocean ridge basalts; East Pacific Rise; mantle; melts; mid-ocean ridges
1. Introduction
Abyssal peridotites and mid-ocean ridge basaltsŽ .MORB are complementary products of mantlemelting and melt extraction processes that create the
) Corresponding author. Tel.: q61 7 3365 2372. Fax: q61 73365 1277. E-mail: niu@earthsciences.uq.edu.au
w xocean crust. Studies of abyssal peridotites 1–4 andMORB showed that the extent of mantle melting ishigh beneath hotspot-influenced shallow ridges, andis low beneath deep ridges away from hotspots.These results have led to the recognition of globalcorrelation of MORB chemistry with ridge axial
w xdepth and crustal thickness 5 , and the notion thatmantle temperature variation exerts the primary con-trol on the extent of melting beneath global ocean
0012-821Xr97r$12.00 Copyright q 1997 Elsevier Science B.V. All rights reserved.Ž .PII S0012-821X 96 00218-X
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´244
Ž . X Ž .Fig. 1. a Location of the Garrett Transform offsetting the EPR at ;13828 S. b Morphotectonic representation of the Garrett TransformŽ . w x Ž . Ž .showing the volcanic ridges Alpha, Beta, and Gamma and transform troughs 17,18 . c A detailed bathymetry 100 m intervals of the
Ž .Garrett Deep and northern flank of the East Median Ridge showing the extent of the outcrop of peridotite–basalt association shaded area .Ž .The thick lines are diving tracks e.g., GN 01, GN 03, etc. . GS 03 is the track of a deep-towed bottom camera run. The samples studied are
Ž . Ž .from dives GN 03, GN 04, GN 13, GN 14 and GN 15. The observed lithologies are: fresh B and altered b basalt flows, consolidated andŽ . Ž . Ž .metamorphosed breccia Mbr , metabasalt Mb , and serpentinized peridotites P .
w xridges 2,5–8 . This conclusion is, however, basedlargely on data from slow-spreading ridges in theAtlantic and Indian Oceans. At the fast-spreading
Ž .East Pacific Rise EPR , there is little correlationw xbetween MORB chemistry and ridge depth 8–11 ,
and EPR MORB show chemical systematics thatdiffer from those at slow-spreading ridges on both
w xregional and ridge segment scales 8–12 . Clearly, an
improved understanding of EPR MORB genesis re-quires petrological data on both residual peridotitesand associated basalts. However, there had been no
w xdetailed sampling of peridotites from the EPR 13,14w xuntil very recent submersible 15–20 and drilling
w x21 investigations at Hess Deep, and the Garrett andTerevaka transforms.
In this paper, we present geochemical data on
Notes to Table 1:a Ims impregnation; total volume of impregnation veinsrveinlets were estimated on board. Thin-section chips and analyzed portions ofharzburgite samples are fresh cuttings away from impregnation veinlets and hydration haloes.b Ž .The reported mineral modes vol% were obtained by point-counting at 1 mm intervals based on one or two thin sections for each sample,and are partially reconstructed. We assigned all textural serpentines, whether or not they have olivine psedomorphs, and associated
Ž . Ž . Ž .magnetite abundant to the serpentine Serp category; optically fresh unaltered olivine, either as individual grainsraggregates or asŽ .‘‘meshes’’ surrounded or veined by serpentine, to olivine Ol ; unaltered orthopyroxene, spatially associated bastite pseudomorphs and
Ž .peripheral talc to primary orthopyroxene Opx ; unaltered clinopyroxene and associated tremolitic amphiboles to primary clinopyroxeneŽ . Ž .Cpx ; spinel with or without magnetite rims to primary spinel Sp .
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´ 245
Tab
le1
Gar
rett
tran
sfor
msa
mpl
ede
scri
ptio
nsa
bSa
mpl
eD
epth
Setti
ngSa
mpl
eSa
mpl
eIm
Mod
esof
sam
ples
anal
ysed
Cou
nts
size
desc
ript
ion
Ž.
Ž.
Ž.
Ž.
Ž.
mcm
On
boar
dvo
l%Se
rpO
lO
pxC
pxSp
tota
l
Serp
enti
nize
dha
rzbu
rgit
eG
N3-
943
24O
utcr
opon
25=
29=
13H
arzb
urgi
te;n
oim
preg
natio
n0
79.1
54.
4015
.00
nil
1.45
2000
asc
arp
Ž.
GN
14-1
5050
Tal
us20
=18
=18
Har
zbur
gite
;4ga
bbro
icve
inle
ts2
–5
mm
thic
k8
71.8
010
.16
15.2
40.
851.
9512
37Ž
.G
N14
-250
46T
alus
13=
15=
15H
arzb
urgi
te;1
gabb
roic
vein
let
-2
mm
thic
k10
83.8
98.
716.
620.
700.
0812
30G
N14
-549
08O
utcr
op12
=14
=12
Har
zbur
gite
;no
impr
egna
tion
049
.29
39.8
89.
470.
221.
1417
10Ž
.G
N14
-748
33T
alus
40=
40=
15H
arzb
urgi
te;1
gabb
roic
vein
let
;2
mm
thic
k1
73.9
27.
3117
.36
0.30
1.12
2007
GN
14-9
4660
Out
crop
16=
16=
5H
arzb
urgi
te;n
oim
preg
natio
n0
49.7
637
.18
11.6
90.
161.
2112
37Ž
.G
N14
-14
4244
Out
crop
30=
20=
9H
arzb
urgi
te;3
gabb
roic
vein
lets
2–
10m
mth
ick
2074
.01
11.6
513
.07
0.24
1.03
1263
Ž.
and
1di
kele
t3
mm
thic
kG
N15
-246
47T
alus
,fla
nk15
=17
=17
Dun
ite;n
oim
preg
natio
n0
68.2
722
.97
7.41
nil
1.35
1258
ofth
eri
dge
GN
15-5
4688
Out
crop
,17
=10
=10
Har
zbur
gite
;no
impr
egna
tion
083
.49
0.34
14.5
0ni
l1.
6812
05fl
ank
ofth
eri
dge
GN
15-1
042
17O
utcr
op21
=12
=8
Har
zbur
gite
;no
impr
egna
tion
051
.87
22.3
724
.27
0.08
1.41
1212
Dia
base
GN
4-3
4602
Tal
us,a
ctiv
e25
=20
=15
Plag
iocl
ase–
oliv
ine
phyr
ic,
;eq
uala
mou
nts
plag
iocl
ase
and
clin
opyr
oxen
e,pl
ustr
ace
ofol
ivin
ean
dm
agne
tite
tect
onic
zone
GN
4-9
4013
Out
crop
28=
20=
20M
etam
orph
osed
,aph
yric
,;
equa
lam
ount
spl
agio
clas
ean
dcl
inop
yrox
ene
ingr
ound
mas
sG
N13
-937
64T
alus
40=
20=
8A
phyr
ic,e
qual
amou
nts
plag
iocl
ase
and
clin
opyr
oxen
epl
ustr
ace
oliv
ine
and
opaq
ueG
N15
-446
97T
alus
,fla
nk26
=16
=10
Aph
yric
,equ
alam
ount
spl
agio
clas
ean
dcl
inop
yrox
ene
plus
;3%
opaq
ueof
the
ridg
e
Bas
alt
GN
4-1
5062
Tal
us,a
ctiv
e25
=20
=20
Pillo
wfr
agm
ent;
aphy
ric
glas
ste
cton
iczo
neG
N4-
1137
20O
utcr
op,
20=
10=
10Pi
llow
frag
men
t;ap
hyri
cgl
ass
dike
sG
N13
-145
28O
utcr
op20
=15
=15
Shee
tflo
w;p
lagi
ocla
se–
oliv
ine
basa
ltic
glas
sG
N13
-636
57O
utcr
op25
=25
=12
Pillo
wfr
agm
ent;
aphy
ric
glas
sG
N13
-837
64T
alus
40=
20=
15Pi
llow
frag
men
t;ap
hyri
cno
n-gl
assy
basa
ltG
N15
-148
01T
alus
,28
=14
=12
Pillo
wfr
agm
ent;
oliv
ine–
phyr
icgl
ass
Cen
tral
Bas
in
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´246
peridotites and spatially associated basaltic samplesfrom the Garrett Transform. This peridotite–basaltassociation allows, for the first time, a close exami-nation of mantle melting and melt extraction pro-cesses from both melts and residues produced in acommon environment at the EPR. Also, this study isthe first attempt at whole-rock trace element system-atics of abyssal peridotites, and will be a usefuladdition to the geochemical data base for modelsconcerning MORB genesis and chemical geodynam-ics in general.
2. Geological background
The Garrett Transform is associated with theŽ . w xfastest 145 mmryr 22 strike-slip motion on the
Earth, and offsets the EPR at 13828XS by 130 kmw x Ž .23 Fig. 1a . This transform is one of the fewtransforms in which active volcanism is observedw x17–20,24 , and mantle peridotites are recoveredw x13,14,17–20 at the EPR. Previous studies of some
w xdredged samples from this transform 25,26 war-ranted an in situ observation by the submersible
w xNautile 17 . In the Garrett Transform, active volcan-ism is primarily taking place along three parallelridges oblique to the transform strike, but fresh lavasalso exist at deep locations near the peridotite out-
Ž .crops Fig. 1b . Mantle peridotites are exposed as an;10 km long sliver in the transform valley and
Žalong the northern flank of the median ridge Fig..1c . Gabbros occur as a dike complex within the
peridotite domain, and diabasic dikes of variablethickness are common. The peridotites are variouslyserpentinized, and are locally impregnated with
Žbasaltic veinsrveinlets of variable thickness -1.mm to a few centimetres .
3. Samples and analytical techniques
Field occurrence, hand specimen characteristics,and petrography of the samples studied are summa-rized in Table 1. These include 10 serpentinizedharzburgites; 4 fresh diabases; 1 fresh non-glassybasalt; and 5 fresh glassy basalts. The analyzedportions of harzburgites are fresh cuttings away fromimpregnation veinlets and seafloor weathering haloes.
Major element compositions of residual minerals inharzburgites and basaltic glasses were analyzed us-
w xing a Camebax SX 50 microprobe at IFREMER 18 .Whole-rock major elements for non-glass basalts,diabases and harzburgites, and Cr and Ni forharzburgites were analyzed by XRF at The Univer-
w xsity of Queensland following 27,28 . Trace elementsŽ .except for Cr and Ni in harzburgites were analyzedusing a FISONS PQ inductively coupled plasmamass spectrometer at The University of Queensland.The whole-rock analytical data and uncertainties forrepeated analyses of reference standards PCC-1 andBIR-1 for trace elements are given in Table 2. Wehave determined 37 trace elements for all the sam-ples but we report here rare-earth elements, Y, Zr,Nb, Hf, Ta, and Th, because all other elements areobserved to be mobile during serpentinization. Sam-ple preparation and analytical procedure can be ob-tained from the authors and are described in anEPSL Online Background dataset 1.
4. Data and interpretations
4.1. Basaltic liquids
Fig. 2 shows that the large range of incompatibleelement abundances in basaltic samples can be ex-plained by various extents of fractional crystalliza-
Žtion from a compositionally similar parent Fig. 2a–. Žd . This is better described by glass samples liquid
.compositions . The inflected trend on CaOrAl O2 3Ž .vs. MgO plot Fig. 2c defined by glass samples
suggests that crystallization follows the sequence ofolivine ™ olivine q plagioclase ™ olivine qplagioclase q clinopyroxene. The different abun-dance levels on chondrite-normalized rare-earth ele-
Ž . Ž .ment REE diagrams Fig. 2e–f are consistent withŽ .varying extents of fractional crystallization Fig. 2d .
The overall parallel REE patterns suggest that theirparental melts may have formed by similar extents ofmelting of a similarly depleted fertile mantle. Byassuming a fertile mantle that is slightly more de-
Žpleted than source for average MORB see caption
1 Žhttp:rrwww.elsevier.nlrlocaterepsl mirror site USA,.http:rrwww.elsevier.comrlocaterepsl
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´ 247
.of Fig. 2 , the primary melts parental to these basalticliquids can be explained by G20% melting by either
Ž . Ž .the batch Fig. 2g or fractional Fig. 2h meltingmodel. While the extent of melting calculated usingincompatible trace elements alone is subject to largeerrors, due to source compositional uncertainties, thiscalculated value is nevertheless comparable with 23
Ž"2% melting calculated from major elements Na8. w xf2.23"0.09% and Ca rAl f0.87"0.01 7 .8 8
4.2. Harzburgitic residues
4.2.1. Mineral modes and whole-rock major elementcompositions
Fig. 3a shows that, compared with abyssal peri-w xdotites from slow-spreading ridges 4,29,30 , Garrett
peridotites are extremely depleted harzburgites withŽ .clinopyroxene cpx being essentially absent. This
suggests that these harzburgites are residues of highextents of melting. Also, these harzburgites havevariable and high abundances of ‘‘total modal
Ž .olivine’’ olivineqserpentine . The apparent higholivine modes could be over-estimated because or-
Ž .thopyroxene opx is often subject to serpentinizationw xas well 31 , and volume expansion is often associ-
ated with serpentinization. However, petrographyshows that serpentinization is primarily after olivine
Ž .in these harzburgites see note to Table 1 , which isalso evident from the inverse correlation between
Ž .olivine and serpentine modes Fig. 3b . Note that thecorrelation in Fig. 3b is not due to data closurebecause the opx mode is not constant. The observa-tion that olivine pseudomorphs are preserved in mostsamples suggests that the effect of volume expansionon the estimated olivine mode can be neglected forthis purpose. Importantly, the significant inverse cor-relation of ‘‘total olivine’’ modes with olivine
Ž . Ž .forsterite Fo contents Fig. 3c demonstrates thepresence of excess olivine in these harzburgites priorto serpentinization. The excess olivine, by its vari-able and low Fo contents, is inconsistent with beinga residual phase, but is consistent with being crystal-lized from a basaltic melt. In terms of whole-rockmajor element compositions, the high FeO contents
Ž .in these harzburgites Table 2 are largely controlledby the presence of excess olivine of low Fo contentŽ .Fig. 3d . Therefore, these harzburgites are not sim-ple residues.
4.2.2. Major element compositions of residual miner-als
a Ž wIt has been demonstrated that Cr sCrr Crqx.Al in spinel and Al O contents in opx and cpx of2 3
residual peridotites are sensitive indicators of thew xextent of melting 1,3,32,33 because Al is a moder-
ately incompatible element and its abundances inthese phases decrease with progressive melting. Fig.4 shows that these minerals in Garrett harzburgitesare extremely depleted in Al O , and plot at the2 3
most depleted end of the abyssal peridotite arrayŽdefined by samples from slow-spreading ridges i.e.,
the Mid-Atlantic Ridge and Southwest Indian Ocean.Ridge . Note that Garrett residual minerals are as
depleted as, or even more depleted than, peridotitesŽ .from hotspot-influenced ridges points 5–7 . Com-
parison with melting residues produced by peridotitew xmelting experiments 32 , the highly depleted resid-
ual mineral compositions and the point close to cpxŽ .exhaustion Fig. 3a suggests that these harzburgites
are residues of ;25% melting. This high extent ofmelting is comparable to that estimated from majorand REE abundances in the associated basaltic liq-
Ž .uids see above , but is significantly higher thanF10%, a value that has been advocated as theaverage extent of melting beneath global ocean ridgesw x34,35 .
4.2.3. Whole-rock minor and trace element systemat-ics
We have demonstrated that Garrett harzburgitesare not simple residues, but residues of very high
Ž .extents of melting Fig. 3aFig. 4 that have beenmodified with additional olivine with low Fo contentŽ .Fig. 3c . The low-Fo olivine can only be explainedby its crystallization from a cooling melt. The signa-ture of such a melt component is well preserved inthe whole-rock incompatible element compositionsŽ .Fig. 5 . The whole-rock incompatible element abun-dances correlate positively with ‘‘total olivine’’
Ž .modes Fig. 5a and inversely with olivine Fo con-Ž .tents Fig. 5b–d . These correlations, which are in-
consistent with melting, but consistent with refertil-ization of a melt, indicate that both excess olivineand elevated abundances of incompatible elements inthese harzburgites are the consequence of the samemelt–solid equilibration process. Buoyant melts thatascend through previously depleted melting residues
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´248
Tab
le2
Maj
oran
dtr
ace
elem
ent
anal
yses
ofha
rzbu
rgit
es,
diab
ases
and
basa
lts
from
the
Gar
rett
Tra
nsfo
rm
Sam
ple:
3-9
14-1
14-2
14-5
14-7
14-9
14-1
415
-215
-515
-10
PC
C-1
RS
D4-
34-
913
-813
-915
-44-
14-
1113
-113
-615
-1B
IR-1
RS
DŽ
.Ž
.H
arz
Har
zH
arz
Har
zH
arz
Har
zH
arz
Har
zH
arz
Har
z%
Dbs
Dbs
Bsl
tD
bsD
bsG
lass
Gla
ssG
lass
Gla
ssG
lass
%
Wei
ght
per
cen
tb
yX
RF
Wei
ght
per
cen
tb
ym
icro
pro
be
SiO
39.7
639
.54
43.8
540
.10
41.4
140
.64
39.9
038
.48
36.4
742
.43
51.0
050
.28
50.7
850
.83
49.0
750
.78
50.1
350
.58
50.4
750
.50
2
TiO
0.05
0.05
0.06
0.10
0.02
0.08
0.06
0.05
0.07
0.02
1.17
1.27
2.05
2.25
1.09
1.25
1.11
1.16
1.73
2.45
2
Al
O0.
600.
720.
750.
690.
580.
930.
690.
760.
450.
5314
.76
14.2
113
.48
12.6
915
.25
14.1
715
.40
15.5
914
.73
14.9
62
3
FeO
t8.
6410
.58
9.64
11.1
57.
9210
.36
10.1
512
.10
13.2
88.
309.
969.
8811
.76
12.6
38.
979.
819.
078.
9510
.15
10.0
7M
nO0.
060.
130.
160.
160.
110.
150.
130.
120.
130.
120.
180.
180.
200.
210.
190.
190.
190.
160.
140.
15M
gO39
.27
36.9
135
.37
36.3
039
.09
38.2
137
.25
38.0
538
.11
37.8
87.
668.
656.
876.
498.
528.
058.
958.
797.
576.
83C
aO0.
101.
243.
011.
230.
790.
780.
900.
200.
150.
212
.68
11.3
011
.31
11.5
112
.94
12.2
912
.24
12.3
411
.27
10.2
6N
aO
0.27
––
––
––
––
–2.
342.
482.
742.
701.
832.
232.
182.
372.
673.
712
KO
––
0.01
––
––
––
0.01
0.10
0.04
0.20
0.15
0.02
0.04
0.03
0.04
0.11
0.19
2
PO
0–
––
––
––
––
0.07
0.08
0.18
0.20
0.05
0.06
0.05
0.10
0.15
0.06
25
LO
I10
.87
10.0
46.
219.
809.
118.
1810
.86
9.53
10.5
410
.33
–0.
980.
09–
1.34
––
––
–T
otal
99.6
399
.20
99.0
699
.54
99.0
499
.33
99.9
499
.28
99.1
999
.82
99.9
199
.36
99.6
799
.66
99.2
610
0.24
99.3
510
0.09
98.9
999
.18
Mg
a89
.086
.186
.785
.389
.886
.886
.784
.983
.689
.160
.46
3.45
3.6
50.5
65.3
61.9
66.2
66.1
59.6
57.3
()
Par
tsp
erm
illi
onp
pm
by
XR
FC
r21
5421
6820
0324
3026
6440
3433
3823
2019
7519
62N
i18
4718
1616
3416
2622
1820
6821
1619
8217
7622
11
()
Par
tsp
erm
illi
onp
pm
by
ICP
–M
SS
c9.
6410
.511
.617
.89.
210
.39.
486.
917.
149.
298.
653.
936
.639
.839
.244
.837
.536
.426
.433
.336
.335
.943
.51.
6V
2428
3150
2332
2413
2421
23.3
20.
826
026
433
537
924
525
519
323
530
233
128
33.
5C
r–
––
––
––
––
––
–13
722
423
227
242
537
231
739
433
030
737
33.
1C
o10
711
311
111
710
911
511
014
013
311
412
73.
749
4946
4547
4840
4746
5155
7.1
Ni
––
––
––
––
––
––
4748
8966
102
6792
129
102
153
170
2.1
Ga
0.68
1.05
1.64
0.97
0.53
1.02
0.95
0.94
1.11
0.63
0.50
34.
116
16.2
17.8
18.1
15.7
15.7
15.4
15.8
17.5
18.9
15.4
2.3
Y0.
480.
991.
181.
850.
220.
881.
180.
681.
900.
120.
079
7.9
24.0
24.9
40.1
43.3
23.0
22.6
18.3
22.6
34.2
45.6
13.5
2.4
Zr
0.39
0.96
1.88
0.80
0.12
1.37
2.27
0.57
1.64
0.03
0.19
26.
548
.356
.411
712
542
.549
.136
.046
.398
.113
113
.12.
0
()
()
Par
tsp
erb
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onp
pb
by
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per
mil
lion
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Nb
118.
816
128.
214
127.
668
6.1
4217
.50.
921.
123.
133.
300.
860.
930.
570.
772.
442.
050.
541.
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a25
3772
7313
4540
538
5937
327.
61.
411.
813.
994.
141.
371.
420.
981.
243.
183.
610.
583.
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e29
106
169
218
1686
124
9327
811
057
3.3
5.02
6.33
13.0
113
.59
5.03
5.01
3.55
4.63
10.6
112
.89
1.84
1.7
Pr
4.7
2130
382.
819
2716
7017
814
.71.
001.
232.
402.
521.
020.
980.
720.
961.
972.
480.
381.
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d33
125
180
225
1511
816
677
465
7128
14.1
5.55
6.56
12.1
812
.98
5.64
5.36
4.10
5.39
10.1
113
.07
2.24
1.7
Sm
1753
7811
56.
455
7629
203
137
26.7
2.23
2.47
4.24
4.54
2.24
2.05
1.64
2.15
3.54
4.68
1.02
1.5
Eu
4026
4155
3.5
2328
8.3
1330
249
.50.
880.
951.
471.
570.
890.
820.
660.
871.
251.
550.
491.
7G
d31
9013
621
013
9312
655
287
148
34.1
3.22
3.51
5.72
6.27
3.17
2.94
2.39
3.09
4.81
6.39
1.67
3.2
Tb
7.8
1926
412.
819
2412
512.
32
28.8
0.59
0.63
1.02
1.11
0.57
0.55
0.44
0.57
0.85
1.15
0.32
2.6
Dy
6814
519
532
627
140
188
9634
416
127.
84.
204.
417.
127.
734.
033.
823.
123.
965.
958.
012.
382.
2H
o18
3444
718.
732
4025
724.
23
21.0
0.88
0.92
1.48
1.61
0.84
0.81
0.66
0.83
1.23
1.67
0.52
2.4
Er
6111
214
521
731
107
132
8422
115
1311
.32.
632.
714.
404.
742.
462.
401.
952.
423.
664.
911.
542.
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m10
1923
335.
417
2214
352.
63
20.5
0.40
0.41
0.66
0.72
0.37
0.36
0.30
0.37
0.56
0.75
0.22
2.1
Yb
8113
915
821
944
122
148
9723
026
227.
32.
522.
594.
164.
512.
312.
301.
862.
313.
514.
691.
492.
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u15
2427
358.
620
2517
395.
36
9.6
0.38
0.39
0.63
0.68
0.34
0.34
0.28
0.34
0.53
0.70
0.22
2.1
Hf
1630
6545
3.7
4979
1944
1.3
817
.11.
491.
703.
303.
561.
401.
431.
111.
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733.
700.
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82.
22.
81.
73.
23.
22.
12.
67.
12.
111
7.0
0.07
90.
097
0.24
70.
265
0.07
20.
077
0.05
90.
065
0.19
40.
177
0.04
91.
1T
h1.
31.
82.
51.
61.
41.
63.
81.
68.
11.
810
9.8
0.04
50.
057
0.18
90.
200
0.04
00.
044
0.03
00.
037
0.14
80.
111
0.03
06.
6
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´ 249
crystallize olivine at shallow levels as a result ofcooling. Entrapment of these melts raises the abun-dances of incompatible elements in the whole-rockharzburgites.
Fig. 6 shows chondrite-normalized REE abun-dances in the harzburgites. For comparison, model
Ž .melting residues by batch Fig. 6a and fractionalŽ .Fig. 6b melting also are shown. Obviously, boththe abundance levels and patterns are inconsistentwith their being simple melting residues. The abun-dance levels are variable and higher than expectedgiven the similarly depleted residual mineral compo-sitions in these harzburgites. The slopes for lightREEs are too shallow for melting residues. Also,batch melting cannot in any way explain samples of
Ž .low REE abundances i.e., GN14-7 and GN15-10 .Some samples show no obvious Eu anomalies,whereas others show either negative or positive Euanomalies. Note that GN15-2 has anomalously highLa, which, together with smaller La anomalies inseveral other samples, is interpreted as La beingmobile during serpentinization. Any process that in-volves a melt would have affected Ce and Pr, at thevery least, as well as La. Although the effects ofplagioclase may be invoked to explain the Euanomalies, the absence of plagioclase in the harzbur-gites analyzed does not support this interpretation.Given that Sr lacks any systematics in these rocksŽ .not shown , it is possible that Eu, like Sr, may alsobe quite mobile during serpentinization. In any case,the overall patterns and variable abundance levels
Ž .are consistent with refertilization of a melt Fig. 5c .
4.2.4. EÕaluation of the extent of refertilizationTo evaluate precisely the extent of refertilization
is not straightforward. However, the relative extentof refertilization can be estimated using REEs byadding a melt with composition similar to GN4-11 to
residues of 25% fractional melting. Such a melt isŽ .chosen because: 1 all the Garrett basaltic liquids
Ž . Ž .have similar REE patterns Fig. 2e ; 2 GN4-11 hasŽonly olivine on the liquidus see Fig. 2c; olivine is
the only phase observed to have crystallized from the. Ž . arefertilizing melt ; and 3 GN4-11 has an Mg of
Ž 3q w 3q 2qx;63.8–66.2 assuming Fe r Fe qFe f0–.0.1 in the melt , which would be in equilibrium with
olivine of Fo , comparable to the observed85.4 – 86.7
excess olivine. A residue of 25% fractional meltingŽ .is indicated by the near exhaustion of cpx Fig. 3a
and highly depleted residual mineral compositionsŽ .Fig. 4 , and batch melting cannot explain the low
Ž .REE abundance samples Fig. 6 . The results areŽ .shown in Fig. 7a–c for representative light Ce ,
Ž . Ž .intermediate Tb , and heavy Yb REEs. The amountŽ .of GN4-11 f required to match the data varies
from 0.1% to ;12%. Note that the f values derivedfrom REEs also explain the abundances of minorŽ . Ž .TiO and major FeO element variations, and2
Ž .olivine Fo contents in the harzburgites Fig. 7d–f .
( )4.2.5. Issues on high field strength elements HFSEsFig. 8a–c plots primitive mantle normalized in-
compatible element abundances in the harzburgites.The patterns display several anomalous features con-cerning HFSEs. Not shown, Garrett basaltic liquidsexhibit rather smooth patterns. Th is more depletedthan Nb in the basaltic liquids, yet the opposite isobserved in the harzburgites. Nb is more depletedthan Ta in both liquids and harzburgites. A positiveTi anomaly exists in all but GN15-5 samples. Whilepositive Zr–Hf anomalies are present in someharzburgites, the large negative anomalies in othersare striking. Since the refertilization process dis-cussed above affects Ti as well as the ‘‘adjacent’’
Ž .REEs Fig. 7 , positive Ti anomalies would havealready existed prior to refertilization, which seems
Note to Table 2:Harzsserpentinized harzburgite; Dbssdiabase; Bsltsnon-glassy basalt; Glasssbasaltic glass; FeOts total Fe expressed as FeO;
a w 2qx 2q 2qLOIs loss on ignition; Mg sMgr MgqFe , assuming total FeO as Fe for harzburgites, and 90% total FeO as Fe for diabasesand basalts. All the analyses are whole-rock compositions except for basaltic glasses. Major element data for glasses were analyzed at
w x w xIFREMER using a Camebax SX 50 18 . Whole-rock XRF analyses were done at the University of Queensland following 27,28 . Except forCr and Ni in harzburgites, which were analyzed by XRF, all the trace elements were analyzed at the University of Queensland using a
Ž .Plasma Quad Inductively coupled mass spectrometer ICP–MS . PCC-1saverage of 6 repeated analyses of 2 digestions. BIR-1saverageof 18 repeated analyses of 12 digestions. %RSD is the relative standard deviations about the averages for PCC-1 and BIR-1. Samplepreparation and analytical procedures can be obtained from the authors and are described in an EPSL Online Background dataset.
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´250
to be a common feature in highly depleted peri-w xdotites 36 . However, the anomalies associated with
Zr–Hf and Nb–Ta need attention.Zr–Hf anomalies: As Zr and Hf do not show
anomalies in basalts, these anomalies in residualharzburgites are thus unexpected. Rampone et al.w x37 interpreted the negative anomalies of Zr and Srin an N-MORB type ophiolite from Internal Lig-
Ž . Ž . Ž . Ž .Fig. 2. a – d MgO variation diagrams of representative major, minor, and trace elements for Garrett basaltic rocks. e and fw x Ž . Ž . Ž .Chondrite–normalized 42 REE abundances for these basaltic rocks. g and h Comparisons of model melts by batch melting g and
Ž . Ž .fractional melting h with a hypothetical primary melt parental to Garrett basaltic liquids thick shaded line . The primary melt isŽapproximated by adding 12% olivine to GN4-11 to be in equilibrium with mantle olivine Fo i.e., the primary melt would have90
Fe – Mg w x. Ž .Mgaf72, assuming olivinermelt K s0.3 56 . The preferred fertile mantle source thick solid line is the average of R717, thedw xleast depleted, and R238, the moderately depleted, Ronda spinel lherzolites 57 ; this choice was made because Garrett basaltic liquids arew xslightly more depleted than average N-type MORB, for which R717 is ideal 57 . This fertile mantle has subsolidus modes of 66% olivine
Ž . Ž . Ž . Ž .Ol , 24% orthopyroxene Opx , 9% clinopyroxene Cpx , and 2% spinel Spl , but we have assumed high pressure modes of 55% Ol, 30%w x w xOpx, 13% Cpx, and 2% Spl following 58 . The melting modes are taken from the polybaric melting reaction of 52 : 0.464 Cpxq0.681
Opxq0.048 Spl´0.193 Olq1 Melt. Mineralrliquid partition coefficients are given in Appendix A.
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´ 251
uride, North Italy, as reflecting ‘‘changes in therelative magnitude of Zr, Sr, and REE partitioncoefficients, depending on specific melting condi-tions’’. As Sr in Garrett harzburgites is quite mobile,whether Sr exhibits similar features prior to serpen-
Ž .tinization is unknown. Since both positive weakŽ .and negative strong Zr–Hf anomalies are present
Ž .Fig. 8a–c , variation in melting condition may notbe important. Both positive and negative Zr anoma-lies have been observed in melt inclusions in high-Mg
w xolivine phenocrysts in basalts 38,39 , and were in-terpreted as representing unmixed ‘‘true’’ instanta-neous melt fractions. If so, this implies that ‘‘micro-scale’’ heterogeneities exist with respect to Zr andHf in the mantle. The presence of a Zr–Hf richphase in the fertile mantle is possible, but it isunlikely for it to survive in residues after )20%melting. As Garrett harzburgites have undergone melt
refertilization, it is possible that such a Zr–Hf phaseŽ .ZrO –baddeleyite is a more likely candidate may2
have co-precipitated with the excess olivine. If so,Ž .two possibilities exist: 1 uneven distribution of this
phase in the harzburgites would lead to the observedŽ .Zr–Hf anomalies; 2 positive Zr–Hf anomalies may
w xbe expected in harzburgites 36 , but large negativeZr–Hf anomalies may be due to incomplete diges-tion of this phase during sample preparation. Acareful evaluation of these scenarios are warranted.
Nb–Ta fractionation, and the peculiar Nb–Threlationship: it has been widely accepted that Nb andTa are geochemically identical, and that the NbrTaratio is constant, and is close to chondritic valuesŽ . w x16–18 in all oceanic basalts 40–42 . Very recent
w xstudies 43,44 have shown that NbrTa ratios inMORB are not constant, but vary significantly, from18 in highly enriched lavas to as low as 9 in ex-
Ž . Ž . Ž .Fig. 3. a Orthopyroxene opx and clinopyroxene cpx modes are plotted against olivine modes to compare Garrett harzburgites withw xabyssal peridotites from slow-spreading ridges discussed by 4,29,30 . For Garrett harzburgites, the ‘‘total olivine’’ plotted is the sum of
Ž . Ž . Ž .olivine and serpentine see Table 1 modes. b Olivine modes correlate inversely with serpentine modes in Garrett harzburgites. c OlivineŽ . Ž .forsterite Fo contents inversely correlate with ‘‘total olivine’’ in Garrett harzburgites. d Relative to whole-rock analyses of abyssal
w x w xperidotites from the Southwest Indian Ocean Ridge 59 and to reconstructed whole-rock abyssal peridotite compositions 52 using thew x Žpublished modes and mineral analyses of Dick 4,29,30 , whole-rock Garrett harzburgites show low MgOrFeO and SiO rFeO ratios high2
.FeO; see Table 2 , which are consistent with the presence of low-Fo olivine. The diamonds symbols are olivine compositions plotted forŽ .reference. Note that Garrett whole-rock compositions show MgO deficiency relative to SiO Table 2 , perhaps due to MgO loss during2
serpentinization.
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´252
tremely depleted ones, indicating that Nb is moreincompatible than Ta during melting. If so, onewould expect even lower NbrTa ratios in residualharzburgites. This is indeed the case. Fig. 8f showsthat the NbrTa ratio increases with increasing extent
Fig. 4. In comparison, residual minerals in Garrett harzburgitesplot at the most depleted end of the data array defined by those in
w xabyssal peridotites from slow-spreading ridges 2,4,29,30,60,61 .These harzburgites are as depleted as, or even more depleted than,
Ž .peridotites from hotspot-affected ridges e.g., points 5–7 . TheGarrett mineral data are from samples with olivine Fo)89,
w xincluding new analyses of samples previously studied 25,26 .These indicate that, prior to olivine addition, Garrett harzburgites
Ž .were residues of very high extents ;25% of melting. Thenumbered symbols are regional averages: 1sAtlantis II faultzone; 2sVulcan fault zone; 3s Islas Orcadas fault zone; 4s
Ž .Bullard fault zone; 5sBouvet fault zone near Bouvet hotspot inŽ w x.the Indian Ocean; 6s158N axial valley near 148N hotspot 62 ;
Ž .7s438N fault zone in the Atlantic near Azores hotspot .
Ž .Fig. 5. a ‘‘Total olivine’’ modes correlate positively with incom-Ž .patible element e.g., Ga and Ti abundances in Garrett harzbur-
Ž . Ž . Ž .gites. b – d Olivine forsterite Fo contents in these harzburgitesŽcorrelate inversely with incompatible element e.g., TiO , Sm and2
.Nb abundances. The values in parentheses are confidence levelsof the correlation coefficients. Note that no olivine is analyzed
Žfrom sample GN15-2 due to almost complete serpentinization see.Table 1 .
of refertilization, indicating that the NbrTa ratio intruly depleted residues unaffected by melt refertiliza-tion would be as low as 2. Given that Nb and Ta are
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´ 253
w x Ž .Fig. 6. Chondrite 42 normalized rare earth element abundances of Garrett harzburgites. For comparison, calculated depletion curves by aŽ .batch and b fractional melting models are also plotted. The model parameters are as for Fig. 2g–h. Note that the abundance levels and
patterns are inconsistent with their being simple melting residues by either model, given their overall similarly depleted in residual mineralŽ .compositions Fig. 4 , but are consistent with melt refertilization. Also note that the anomalous La in GN3-9, 14-7, and 15-2 is interpreted as
resulting from relative mobility of La during serpentinization.
chemically quite similar, it is difficult to explain thevariable NbrTa ratios by chemical considerationalone. Because Nb and Ta have very different atomic
93 181 Žmasses, Nb vs. Ta the superscripts are atomic.masses , we propose that diffusion may control their
apparent incompatibility. Heavier elements woulddiffuse slower than lighter ones, and would be evenslower when the abundance level is extremely lowŽ .in residues due to reduced compositional gradient
across the mineral grains undergoing melting. Thisexplains the curved NbrTa vs. Nb and Th trendsŽ .Fig. 8d,e with large NbrTa changes in more de-
w x w x 90 – 92pleted rocks 43 . The observation 43 that Zr ismore incompatible than 177 – 180 Hf, and 85,87Rb ismore incompatible than 133Cs in highly depletedMORB can be explained likewise. The positive Th
Ž .anomaly relative to Nb in the harzburgites mayŽ232 93also be explained likewise Th vs. Nb; i.e., low
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´254
Ž . Ž . Žw x . Ž . Ž .Fig. 7. a – c Comparisons between the data and calculated abundances CN , chondrite-normalized of light Ce , intermediate Tb , andŽ . Ž . Ž .heavy Yb REE abundances. The calculation is done by adding various amounts f of a basaltic melt MsGN4-11 to a residue
Ž . Ž . Ž .Rs25% fractional melting by simultaneously solving the equation fs C yC r C yC for all, but La and Eu, REEs. Note thatData R M RŽ .the fits are less good for light REEs given their more variable patterns for some samples Fig. 6 , but are excellent for intermediate and
Ž . Ž .heavy REEs. d – f Plots of whole-rock TiO and FeO, and olivine forsterite contents with the calculated f to show that these significant2
correlations are consistent with melt refertilization.
.NbrTh ratios despite their different chemical prop-erties.
5. Implications
5.1. What causes the high extents of melting beneaththe EPR: a hot mantle or fast spreading rate?
We have shown that Garrett harzburgites are ex-tremely depleted, as depleted as, or even more de-pleted than, abyssal peridotites from hotspot-in-fluenced ridged in the Atlantic and Indian oceansŽ .Fig. 4 . In addition, recently drilled samples from
w xHess Deep 21 , and our unpublished data from bothw xHess Deep and the Terevaka Transform 19 also are
similarly depleted. One may interpret these highlydepleted harzburgites as resulting from a depletedfertile mantle, but this requires that the observationsat the three locations be fortuitous. Importantly,MORB from the broad EPR region are not highlydepleted. Therefore, the highly depleted harzburgitesfrom these locations are consistent with very high
Ž .extents of melting ;25% . If the extent of meltingrelates in a simple way to mantle temperature varia-
w xtion 5,8 , this would suggest a hotter EPR mantle.However, the EPR in this broad region has normaltopography and morphology thermally unaffected by
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´ 255
Ž . Ž . w xFig. 8. a – c Primitive mantle 42 normalized incompatible element abundances in Garrett harzburgites to show some anomalous featuresŽ .about Ti, Zr, Hf, Nb, Ta and Th relative to REEs. d Plot of NbrTa vs. Nb of all the lithologies analyzed from the Garrett Transform to
w x Ž .show that the curved trend is consistent with the observation that Nb is more incompatible than Ta 43,44 . e Plot of NbrTa vs. Th toŽ . Ž .show that NbrTa ratio in a rock increases with increasing incompatible element e.g., Th abundances. f The significant correlation
Ž .between NbrTa ratio and the calculated refertilization parameter Fig. 7 attests that the true melting residues would have NbrTa-2, andthe variably higher NbrTa ratios result from the refertilization process discussed in the text.
any known hotspots. Hence, factors other than man-tle temperature variation may play a role. We pro-pose that plate spreading rate variation may be asimportant as mantle temperature variation. The highextents of melting beneath the EPR may result froma fast spreading rate. Given the fact that mantleupwelling beneath ridges results from plate separa-tion, fast separation leads to fast mantle upwellingw x45–48 . Fast upwelling allows the adiabat to extend
Žto shallower levels i.e., against the conductive cool-.ing to the seafloor , thus decompression melting
continues up to a shallower level, and more meltforms from a given parcel of mantle.
That oceanic crustal thickness is generally con-stant, and is independent of plate spreading ratew x49,50 , has been the primary constraint leading tothe idea that the extent of melting beneath oceanridges is independent of spreading rate. However, thecrustal thickness is entirely inferred from seismicdata. The fact that thin crust and peridotite outcropshave been observed within axial zones at slow-spreading ridges but not at fast-spreading ridges
( )Y. Niu, R. HekinianrEarth and Planetary Science Letters 146 1997 243–258´256
suggests that crustal thickness derived from seismicw xdata must be used with caution 51 . The significant
Ž .implications are: 1 the notion that MORB representF10% melting beneath global ocean ridges needsrevision with the available peridotite data from the
Ž .EPR; and 2 models on MORB genesis that neglectspreading rate variation need re-evaluation.
5.2. Do MORB melts experience low pressure equili-bration during ascent?
Recent studies have led to the idea that meltsformed at depth must be extracted rapidly withoutexperiencing low-pressure equilibration during as-
w xcent 8 . The presence of excess olivine and incom-patible element refertilization seen in Garrettharzburgites attest the significant melt–solid equili-bration during melt ascent. As excess olivine isobserved on petrographic scales in abyssal peri-
w xdotites from slow-spreading ridges as well 52 ,porous flows may be as important as channel flowsduring melt migration. Therefore, melt–solid equili-bration during melt ascent at shallow levels is un-avoidable. Caution is thus necessary when inferringmelting pressures from basalt chemistry, as has been
w xpointed out in recent studies 53–55 .
Acknowledgements
YN acknowledges support from The University ofQueensland and from the Australian Research Coun-cil. RH acknowledges support from Ministere desAffairs Etrangeres in France as well as the Depart-ment of Marine Geosciences at IFREMER. Both YNand RH are grateful for the Bilateral Science andTechnology Program of Australia and the Ministerede l’Education de l’enseignement Superieur et de laRecherche of France, without which this work wouldnot have been completed. We thank Robyn Frank-land and Kim Baublys for helping with samplepreparations, Peter Colls for thin sections, and FrankAudsley for XRF analyses at the University ofQueensland, and Marcel Bohn for assistance with themicroprobe analysis at IFREMER. We are gratefulfor the constructive comments of Jon Snow and two
[ ]anonymous reviewers. FA
Appendix A. Rare earth element mineralrrrrr liquidpartition coefficients used in this study
Kd Clino- Ortho- Spinel Olivinevalues pyroxene pyroxene
La 0.0560 0.0053 0.0003 0.0067Ce 0.0920 0.0090 0.0005 0.0060Pr 0.1610 0.0127 0.0007 0.0060Nd 0.3030 0.0163 0.0008 0.0059Sm 0.4450 0.0200 0.0009 0.0067Eu 0.5005 0.0275 0.0010 0.0074Gd 0.5560 0.0350 0.0011 0.0095Tb 0.5700 0.0480 0.0013 0.0115Dy 0.5820 0.0610 0.0018 0.0154Ho 0.5825 0.0740 0.0023 0.0193Er 0.5830 0.0838 0.0030 0.0256Tm 0.5625 0.0935 0.0038 0.0374Yb 0.5420 0.1033 0.0045 0.0491Lu 0.5060 0.1130 0.0050 0.0600
w xData from 63–67 .
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