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Figures and Tables

The pyroxenite-diamond connection

E.S. Kiseeva1,

1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK

B.J. Wood1,

1Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK

S. Ghosh2,

2Institute of Geochemistry and Petrology Department of Earth Sciences, ETH Zürich, 8092 Zürich, Switzerland
Current address: Department of Geology & Geophysics, Indian Institute of Technology, 721302 Kharagpur, India

T. Stachel3

3Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3 Canada

Affiliations  |  Corresponding Author  |  Cite as

Kiseeva, E.S., Wood, B.J., Ghosh, S., Stachel, T. (2016) The pyroxenite-diamond connection. Geochem. Persp. Let. 2, 1-9.

Geochemical Perspectives Letters v2, n1  |  doi: 10.7185/geochemlet.1601
Received 8 July 2015  |  Accepted 17 September 2015  |  Published 15 October 2015
Copyright © 2016 European Association of Geochemistry



Figure 1 Compositions of 16 eclogitic, 75 pyroxenitic and 32 peridotitic majorite inclusions in natural diamonds as functions of (Si + Ti) pfu (per formula unit), showing (dashed lines) the theoretical “peridotitic” and “eclogitic” substitutions discussed in the text (after Kiseeva et al., 2013a).
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Figure 2 Bulk compositions of natural pyroxenites (grey circles) plotted in MgO-Al2O3 compositional space. Dashed red line shows the regression line for all the pyroxenites. (a) Compositions of majoritic inclusions in diamonds of eclogitic (red), pyroxenitic (blue) and peridotitic (light green) affinities. Fields of bulk rock peridotite and MORB are shown as blue ovals. (b) Tie line between a majoritic garnet and calculated coexisting clinopyroxene (Cpx; see text for details). A possible bulk rock composition for this pair was placed on the red regression line. (c) Population of natural majoritic garnets and reconstructed clinopyroxene compositions. Field of Cpx compositions is shown with the yellow oval. Note wide range of potential bulk (pyroxenitic) source rock compositions. Placement of the latter around the regression line is meant only to be indicative. (d) Majoritic inclusions in diamonds (green circles) which coexist with clinopyroxene(s) (green triangles) within the same diamond. The range of observed natural clinopyroxene compositions occupy the area predicted by our modelling. Orange circles show the bulk composition and the majorite-clinopyroxene pair which crystallised in an experimental run at 15 GPa and 2150 °C.
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Figure 3 Chemical potential diagram (at fixed pressure and temperature) showing stabilities of phases coexisting with clinopyroxene. Curvature of garnet field boundaries is due to changing composition on passing from eclogite to peridotite (see text). Interaction between eclogite and peridotite will lead to gradients in chemical potentials of MgO and CaO (red dashed line) and layers of orthopyroxene next to peridotite and garnet next to eclogite. Fo-forsterite, En-orthopyroxene, Q/Co – quartz/coesite, Grt – garnet, An – anorthite, Cpx – clinopyroxene.
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Figure 4 Back-scattered electron images of SA2-1 run products. The experimental conditions are 3 GPa and 1350 °C. Panel (b) shows an enlarged area from panel (a). Orthopyroxene (on the peridotite side) and garnet (on the eclogite-side) are formed within the reaction zone between carbonated eclogite and peridotite.
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Supplementary Figures and Tables


Figure S-1 Bulk rock compositions of natural pyroxenites (grey circles), majoritic inclusions in diamonds of eclogitic (orange triangles), pyroxenitic (purple triangles) and peridotitic (green triangles) affinities and reconstructed clinopyroxene compositions of eclogitic (orange diamonds) and pyroxenitic (purple diamonds) affinities. Blue lines represent selected tie lines between natural majoritic garnets and reconstructed clinopyroxenes.
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Figure S-2 Majoritic garnet LILE and HFSE abundances normalised to CI-chondrite (McDonough and Sun, 1995). Majorites of peridotitic affinity are significantly more depleted in Nb, Ti, Zr, Hf and Y than pyroxenitic majorites. Low Ba and Sr abundances in pyroxenitic majorite is possibly due to coexisting clinopyroxene. Red circles show abundances for primitive (>8.7 wt. % MgO) MORB glasses (Jenner and O'Neill, 2012).
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Figure S-3 Majoritic garnet REE abundances normalised to CI-chondrite (McDonough and Sun, 1995). Majorites of peridotitic affinity are significantly more depleted in Heavy REE. Red circles show abundances for primitive (>8.7 wt. % MgO) MORB glasses (Jenner and O'Neill, 2012).
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Figure S-4 Comparison of Mg#, CaO, Al2O3 and Na2O contents (in wt %) of natural clinopyroxene inclusions with those calculated to be in equilibrium with pyroxenitic and eclogitic majorite. Clinopyroxene compositions are taken from the following studies: Moore and Gurney, 1989; Wilding, 1990; Hutchinson, 1997; Stachel et al., 2000; Davies et al., 2004; Pokhilenko et al., 2004; Bulanova et al., 2010.
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Figure S-5 Estimated pressure of majoritic inclusions based on three different geobarometers.
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Table S-1 Experimental run products.
Exp №T, ºCP, GPaDuration, hCapsule materialCompositionEclogite layerPeridotite layerReaction zone
Sa2-1135038PtAl-rich sandwichGrt, Co, Cor, meltFo, Grt, Opx, CpxOpx, Grt, Cpx
15-12150152RePyroxenite


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Table S-2 Compositions of run products.

SiO2TiO2Al2O3Cr2O3FeOMgOCaOMnONa2OK2OCO2Sum
Sa2-1 eclogite











Bulk53.42
17.96

11.4214.62


2.58100.00
Garnet44.46
24.90

22.888.78



101.01
σ0.96
0.61

0.700.85




Melt59.49
17.49

4.5311.36


7.15100.01
σ3.63
2.06

1.941.18




 
Sa2-1 peridotite











Bulk45.90
4.97

41.994.88


2.26100.00
Fo42.94



58.260.28



101.48
σ0.33



0.290.14




Opx57.64
4.47

37.292.06



101.47
σ0.50
0.70

0.500.34




Garnet45.03
23.15

27.036.14



101.36
σ1.77
2.62

2.100.85




Cpx53.31
4.40

23.5818.39



99.68
σ2.22
0.83

1.932.72




 
Sa2-1 zone











Peridotite side











Opx57.07
4.16

37.172.27



100.67
σ1.28
0.48

0.630.58




 
Sa2-1 zone











Eclogite side











Garnet44.61
24.13

25.126.95



100.80
σ0.49
1.16

2.631.57




Cpx55.06
2.62

22.8419.41



99.93
σ1.53
1.02

1.290.88




 
15-1 Bulk48.990.0611.630.356.3020.0411.290.151.20

100.00
Garnet46.490.0418.840.493.6220.939.570.150.33

100.46
σ0.500.010.940.050.390.420.400.020.08


Cpx56.070.043.130.224.2516.3117.040.111.50

98.66
σ0.390.030.830.050.850.240.470.020.18


CO2 in melt estimated by difference.
Cpx – clinopyroxene, Opx – orthopyroxene, Grt – garnet, Fo - forsterite, Co - coesite, Cor - corundum.

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