Coesite eclogites from the Roberts Victor kimberlite, South Africa

Lithos 54 Ž2000. 23–32 www.elsevier.nlrlocaterlithos

Coesite eclogites from the Roberts Victor kimberlite, South Africa Daniel J. Schulze a,) , John W. Valley b, Michael J. Spicuzza b a

Department of Geology, Erindale College, UniÕersity of Toronto, Mississauga, Ontario, Canada L5L 1C6 b Department of Geology and Geophysics, UniÕersity of Wisconsin, Madison, WI 53706, USA Received 13 December 1999; accepted 19 May 2000

Abstract Coesite Žor pseudomorphic quartz. is more common than previously recognized in the eclogite xenolith suite from the Roberts Victor Mine, South Africa. All coesite eclogites in this study Ž18 samples. are classified as group I, and have mineral compositions that span virtually the entire compositional range of group I eclogites described from this locality. Most of the Roberts Victor group I eclogite suite may be in equilibrium with free silica, thus weakening the case for these rocks representing residues of partial melting. Oxygen isotope compositions of garnets from 15 samples are in the range 5.32–6.95‰, similar to other Roberts Victor group I eclogites. These observations are consistent with a model in which protoliths of these rocks were oceanic basalts and intrusive rocks that underwent exchange with seawater at relatively low temperatures, to variable extents, prior to subduction and metamorphism to eclogite facies, without suffering significant partial melting during or following subduction. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Eclogite; Coesite; Kimberlite; Subduction; Oxygen isotopes

1. Introduction Despite years of intensive study, the origin of mantle eclogites, brought to Earth’s surface as xenoliths in kimberlites, remains a hotly debated issue ŽJacob et al., 1998; Snyder et al., 1998.. Modern work on the problem began with interest in the Upper Mantle Project in the 1950s, and as an outgrowth of this interest, hypotheses emerged that focussed on igneous origins, involving crystal–liquid equilibria at mantle pressures ŽKushiro and Aoki, )

Corresponding author. E-mail address: [email protected] ŽD.J. Schulze..

1968; MacGregor and Carter, 1970; Hatton, 1978.. As plate tectonic theory became more widely accepted, comparisons of crustal examples of eclogites with those found as mantle xenoliths lead to hypotheses that some mantle eclogite xenoliths represent subducted oceanic basaltic crust ŽHelmstaedt and Doig, 1975.. Stable and radiogenic isotope and trace element studies of eclogites have confirmed that many mantle eclogite xenoliths are likely to be products of subduction and prograde metamorphism of altered ocean-floor mafic rocks ŽJagoutz et al., 1984; MacGregor and Manton, 1986; Shervais et al., 1986; Neal et al., 1990., whereas others may be the product of processes occurring entirely within the upper mantle ŽShervais et al., 1986; Neal et al.,

0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 0 0 . 0 0 0 3 1 - 1

D.J. Schulze et al.r Lithos 54 (2000) 23–32

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1990.. Recently, it has been suggested that many mantle eclogites represent residues of partial melting of amphibolite during production of trondjemites and tonalites in Archean times ŽIreland et al., 1994; Rapp and Watson, 1995; Rollinson, 1997.. Xenoliths of mantle eclogite occur in many kimberlites worldwide, but eclogite is unlikely to represent more than a few percent of the volume of the upper mantle ŽSchulze, 1989., and eclogite xenoliths are abundant in only a few kimberlites. Much information on their origins and bearing on the evolution of the mantle have therefore been gathered by study of material from a few key sites, such as the Roberts Victor and Bellsbank localities in South Africa, Orapa in Botswana, Colorado–Wyoming kimberlites in USA, and Udachnaya in Russia. Among these localities, the eclogite xenolith suite from the Roberts Victor Mine in South Africa is probably the best known, as eclogite xenoliths occur in particular abundance here Ž95–98% of the xenolith population — MacGregor and Carter, 1970; Hatton, 1978., and samples range in size up to several tens of centimetres in maximum dimension. Xenoliths from this locality have been the focus of many investigations ŽKushiro and Aoki, 1968; Mac-

Gregor and Carter, 1970; Hatton, 1978; Jagoutz et al., 1984; McCandless and Gurney, 1989; Caporuscio, 1990; Schulze et al., 1996.. Our investigation extends this earlier work by focussing on a particular group of Roberts Victor eclogites, those that contain the high-pressure silica polymorph coesite. Eclogites containing coesite, or quartz pseudomorphs after coesite, have been described previously from this xenolith locality ŽSmyth and Hatton, 1977; Smyth, 1977; Schulze and Helmstaedt, 1988., and from other kimberlites. A considerable number of coesite eclogites were discovered in a new collection from the Roberts Victor Mine, and the mineral chemical and oxygen isotope data we have collected for this suite place important constraints on models of eclogite genesis.

2. Analytical techniques Garnets and clinopyroxenes were analyzed for major and minor elements using a CAMECA SX-50 electron microprobe at the University of Toronto and standard wavelength dispersive methods. Counting times on peaks were in the range 30–60 s, with 60 s

Table 1 Compositions of garnets from Roberts Victor coesite eclogites, values in wt.% Sample a

R-71 13-64-1 13-64-3 13-64-6 13-64-100 b 13-64-101 13-64-102 c 13-64-103 13-64-104b 13-64-107 b 13-64-109 b 13-64-121 13-64-122 c 13-64-125 13-64-133 13-64-136 c 13-64-137 13-64-138 a

SiO 2

TiO 2

Al 2 O 3

Cr2 O 3

FeO

MnO

MgO

CaO

Na 2 O

Total

41.54 41.48 40.54 40.57 39.18 40.84 40.52 40.38 39.68 39.16 40.86 40.52 39.20 40.11 39.50 39.88 40.32 39.50

0.07 0.30 0.24 0.27 0.44 0.20 0.26 0.24 0.32 0.38 0.23 0.24 0.31 0.26 0.30 0.29 0.26 0.29

23.95 22.99 23.24 23.26 22.53 23.64 23.38 23.40 22.98 22.54 23.62 23.17 22.65 22.67 22.83 22.63 23.11 22.61

0.12 0.05 0.06 0.09 0.06 0.06 0.05 0.01 0.07 0.12 0.06 0.29 0.05 0.27 0.04 0.19 0.13 0.12

12.34 10.67 13.47 13.00 10.77 10.05 12.22 11.30 10.98 13.96 10.21 12.43 19.39 16.49 18.42 18.62 14.97 19.34

0.26 0.18 0.26 0.28 0.16 0.18 0.24 0.21 0.20 0.26 0.19 0.25 0.57 0.33 0.49 0.47 0.40 0.52

18.42 14.47 14.97 14.31 6.96 15.57 14.72 13.03 9.34 7.93 14.98 14.85 10.35 14.78 11.57 13.75 14.81 12.17

3.60 10.43 7.01 8.32 19.36 9.11 8.36 11.24 16.19 15.42 9.70 7.95 7.62 4.88 6.91 4.23 5.78 5.62

0.06 0.09 0.09 0.10 0.12 0.09 0.10 0.08 0.10 0.13 0.09 0.09 0.13 0.09 0.11 0.09 0.10 0.10

100.36 100.57 99.88 100.20 99.58 99.74 99.85 99.89 99.86 99.90 99.94 99.79 100.27 99.88 100.17 100.15 99.88 100.27

Contains accessory sanidine. Contains accessory kyanite. c Contains accessory rutile. b

D.J. Schulze et al.r Lithos 54 (2000) 23–32

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Table 2 Compositions of clinopyroxenes from Roberts Victor coesite eclogites, values in wt.% Sample

SiO 2

TiO 2

Al 2 O 3

Cr2 O 3

FeO

MnO

MgO

CaO

Na 2 O

K 2O

Total

R-71 13-64-100 13-64-101 13-64-102 13-64-103 13-64-104 13-64-107 13-64-109 13-64-121 13-64-122 13-64-125 13-64-133 13-64-137 13-64-138

55.35 56.73 57.39 55.86 57.09 55.31 55.61 56.01 55.38 56.58 54.51 55.62 56.19 55.81

0.16 0.39 0.24 0.34 0.21 0.30 0.36 0.28 0.34 0.42 0.33 0.40 0.33 0.36

8.63 17.21 14.31 13.27 15.72 16.39 15.92 16.01 13.46 16.13 6.46 13.48 10.08 7.30

0.21 0.02 0.09 0.06 0.05 0.08 0.15 0.07 0.25 0.05 0.24 0.03 0.18 0.13

2.86 1.86 1.65 2.16 1.89 1.75 2.58 1.49 2.23 3.53 5.83 3.99 3.90 6.59

0.04 0.00 0.03 0.02 0.02 0.04 0.02 0.05 0.01 0.07 0.10 0.07 0.07 0.12

12.45 6.24 9.20 9.16 7.60 6.96 6.67 7.54 8.68 6.22 12.98 8.02 10.79 11.71

15.86 11.68 12.48 12.61 11.34 12.07 11.41 11.13 12.22 9.08 14.72 10.66 13.63 14.35

4.45 6.21 5.89 5.69 6.49 6.01 6.38 6.32 5.92 7.49 3.53 6.53 5.06 4.09

0.11 0.18 0.10 0.10 0.10 0.15 0.12 0.10 0.11 0.07 0.16 0.11 0.14 0.15

100.12 100.52 101.39 99.27 100.51 99.06 99.22 99.00 98.60 99.64 98.86 98.89 100.37 100.61

used for Na in garnet and K in clinopyroxene to achieve detection limits of 0.01 wt.% Na 2 O and K 2 O. Each composition in Tables 1 and 2 is an average of three to eight individual analyses. Oxygen isotope ratios were determined at the University of Wisconsin by laser fluorination methods described by Valley et al. Ž1995. and are reported in standard d18 O notation relative to VSMOW. Reproducibility of garnet standard UWG-2 Ž d18 O s 5.80‰. is on the order of 0.07‰ Ž1 standard deviation.. Corrections for small daily shifts ŽF 0.09‰. in measured d18 O of the standard Ž5.79– 5.89‰, averaging 5.85‰ over the course of this study. have been applied to the data.

3. Eclogite classification MacGregor and Carter Ž1970. and Hatton Ž1978. divided the Roberts Victor eclogites into two groups on the basis of texture. These workers referred to eclogites in which coarse, rounded garnets are set in a clinopyroxene matrix as group I, and those in which both garnet and clinopyroxene occur as tightly interlocking irregular anhedral crystals as group II. Similar textural variations have been reported for other eclogite suites ŽSmyth and Caporuscio, 1984; Viljoen, 1995.. The textural criteria Žat Roberts Victor and elsewhere. can be ambiguous, however, and

are not completely consistent with minor element mineral chemical classification schemes that also divide eclogites into two groups. MacGregor and Manton Ž1986. and McCandless and Gurney Ž1986, 1989. showed that the eclogites from the two textural groups have different values of Na 2 O in garnet and K 2 O in clinopyroxene, and McCandless and Gurney Ž1989. suggested that group I eclogites be defined as those with G 0.09 wt.% Na 2 O in garnet or G 0.08 wt.% K 2 O in clinopyroxene, with group II eclogites having values below these. We adopt the somewhat lower limit of G 0.07 wt.% Na 2 O in garnet for group I eclogites based on suggestions of Gurney Ž1984. and Fipke et al. Ž1995.. This division is consistent with the distribution of Na 2 O values in the population of eclogite garnet xenocrysts from the Roberts Victor Mine ŽFig. 1., discussed below.

4. Classification of Roberts Victor garnet xenocrysts To assist in understanding the constitution of the mantle sampled by the Roberts Victor kimberlite, and to help constrain the proportion of group I to group II eclogites in the upper mantle in this region of southern Africa, we have analysed a suite of garnet xenocrysts Ž n s 196. selected at random from

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1–2 mm fraction of run-of-mine heavy mineral concentrate from the Roberts Victor Mine. This data set, together with the coesite eclogite mineral chemical data in this paper, can be downloaded from the Internet by connecting to the University of Toronto Department of Geology’s WWW server at http:rrwww.geology.utoronto.ca. Follow the links to ASoftware and Data SetsB. Eclogite garnet xenocrysts Ž- 2 wt.% Cr2 O 3 ; Gurney, 1984. constitute 74% of the Roberts Victor garnet xenocryst population in this study Ž146 of 196 garnets.. Seventy nine percent of the eclogitic garnet xenocrysts in this population are classified as belonging to group I ŽG 0.07 wt.% Na 2 O., consistent with the estimation of Hatton Ž1978. that approximately 3r4 of Roberts Victor eclogite xenoliths belong to group I. Note the clear division of Na 2 O contents of garnets at 0.07 wt.% in Fig. 1, suggesting that this may be a better discriminant value Žcf. Gurney, 1984 and Fipke et al., 1995. to divide the Roberts Victor eclogite population than G 0.09 wt.% Na 2 O in garnet ŽMcCandless and Gurney, 1989.. Using this Na 2 O discriminant, there is a distinctly bimodal distribution of group I and group II eclogite garnet xenocrysts in terms of CarŽCa q Mg q Fe. and MgrŽMg q Fe. values ŽFig. 2.. A division into two groupings near MgrŽMg q Fe. s 0.73 is clear,

Fig. 1. Distribution of Na 2 O contents of eclogitic garnets Ž - 2.0 wt.% Cr2 O 3 . in 146 Roberts Victor garnet xenocrysts in this study. Note the clear minimum at 0.07 wt.% Na 2 O that is used in this study Žand consistent with the suggestions by Gurney, 1984 and Fipke et al., 1995. to divide group I eclogite garnets Ž G 0.07 wt.% Na 2 O. from group II eclogite garnets Žlower Na 2 O contents. at Roberts Victor.

Fig. 2. Variation in MgrŽMgqFe. and CarŽCaqMgqFe. values of 146 eclogite garnet xenocrysts from this study Žillustrated in Fig. 1., with division into groups 1 and 2 made at 0.07 wt.% Na 2 O.

although there is some overlap. This distinction is discussed further below.

5. Coesite-bearing eclogites Eighteen eclogites that contain coesite or quartz pseudomorphs after coesite have been identified in this investigation, mostly in samples from coarse concentrate at the Roberts Victor Mine dumps Žmaterial dominantly in the size range 2–8 cm.. Three of these samples were previously described by Schulze and Helmstaedt Ž1988., including one also studied by Manton and Tatsumoto Ž1971. and MacGregor and Manton Ž1986., sample R-71. Fresh coesite occurs in two of these samples, and quartz pseudomorphs in the others were identified using the criteria of Smyth and Hatton Ž1977., Smyth Ž1977. and Schulze and Helmstaedt Ž1988.. The abundance of coesite ranges from trace quantities Žone or two grains per thin section. to approximately 20% by volume. Accessory minerals are listed in Table 1, and include kyanite in five samples, and sanidine in R-71. 5.1. Mineral chemistry All of the Roberts Victor coesite eclogites in this study are classified as group I eclogites, based on

D.J. Schulze et al.r Lithos 54 (2000) 23–32

elevated K 2 O in clinopyroxene Ž0.10–0.16 wt.%. and elevated Na 2 O in garnet Ž0.08–0.13 wt.%, except R-71 with 0.06 wt.%.. In several samples, clinopyroxene is completely altered Žsamples for which no analysis is given in Table 2.. Sodium contents are moderate to high Ž3.5–7.5 wt.% Na 2 O. in omphacites. Garnets have a large compositional range. Assuming all Fe in the ferrous state, values of MgrŽMg q Fe. range from 0.49 to 0.73 and CarŽCa q Mg q Fe., essentially mole percent grossular component, range from low Ž0.10. to high Ž0.52.. The most calcic sample, 13-64-100, with fresh coesite, also contains kyanite and is thus a grospydite. TiO 2 contents are low to moderate Ž0.07–0.44 wt.%. and Cr2 O 3 values are uniformly low Ž- 0.3 wt.%.. In Fig. 3, garnet MgrŽMg q Fe. and CarŽCa q Mg q Fe. values are shown for Roberts Victor coesite eclogites, compared with garnets from other Roberts Victor eclogites. There is virtually a complete overlap between the group I coesite eclogites and the other group I eclogites, and there is no overlap between garnets from coesite-bearing eclogites and garnets from corundum-bearing eclogites in the Roberts Victor suite Žalthough too few corundum eclogites have

Fig. 3. Variation of MgrŽMgqFe. and CarŽCaqMgqFe. for garnets from Roberts Victor eclogites. Coesite eclogite data from this paper and Smyth and Hatton Ž1977., and other eclogite data from MacGregor and Manton Ž1986. and McCandless and Gurney Ž1986.. Note that McCandless and Gurney Ž1986. re-analyzed a representative suite of Roberts Victor eclogites studied by Hatton Ž1978., with greater precision for minor elements, and thus we use the McCandless and Gurney Ž1986. Roberts Victor eclogite data for comparison in this paper.

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Table 3 Oxygen isotope compositions of garnets from Roberts Victor coesite eclogites analysed in this study, values in ‰ Sample

d18 O

13-64-1 13-64-3 13-64-6 13-64-100 13-64-103 13-64-104 13-64-107 13-64-109 13-64-121 13-64-122 13-64-125 13-64-133 13-64-136 13-64-137 13-64-138

5.91 6.12 5.92 5.32 5.90 5.88 5.55 5.73 6.01 6.95 5.90 6.84 6.34 6.08 6.69

been described from Roberts Victor to adequately define their compositional range.. Comparison of MgrŽMg q Fe. and CarŽCa q Mg q Fe. values of eclogitic garnets assigned to groups I and II in Figs. 2 Žxenocrysts. and 3 Žxenoliths. shows some significant differences between those from xenoliths and xenocryst populations. Specifically, there are many more Fe-rich and Ca-rich examples classified as group II in the eclogite xenolith data where textural criteria were used ŽFig. 3, data from MacGregor and Manton, 1986 and McCandless and Gurney, 1986. than in our new eclogite garnet xenocryst population ŽFig. 2.. As Hatton Ž1978. included many fresh eclogites in his group II, although they had a variety of textures, some samples may be incorrectly classified Žincluding samples subsequently analysed by McCandless and Gurney, 1986, 1989, and used in this paper.. This could account for the fact that many group II garnets in Fig. 3 are more iron-rich than shown in the distribution of group II garnet xenocrysts in Fig. 2 Žin which assignment to groups I and II is based on Na 2 O contents instead of texture.. Although, at present, the significance of variation of Na 2 O in garnet and K 2 O in clinopyroxene with texture is not understood, the classification of eclogites based on Na and K contents yields tighter groupings than classification based

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D.J. Schulze et al.r Lithos 54 (2000) 23–32

on texture alone, and mineral chemistry ultimately may be a more valuable classification tool. 5.2. Oxygen isotope composition Garnets from 15 of the coesite eclogites have been analysed for oxygen isotope composition ŽTable 3.. Values of d18 O are in the range 5.32–6.95‰. Although we have not analysed the oxygen isotope composition of coexisting clinopyroxenes, the fact that previous studies have shown that in most mantle eclogites garnet and clinopyroxene are in oxygen isotope equilibrium and that there is only a small Ž0.2–0.3‰. cpx–garnet fractionation effect ŽMattey et al., 1994; Jacob et al., 1994; Snyder et al., 1998. allows us to conclude that the oxygen isotope composition of garnet is likely to be close to that of the whole rock. Furthermore, oxygen diffusion in garnet is slow ŽCoghlan, 1990. and thus if oxygen isotope ratios have been disturbed by alteration processes, garnets are more likely to retain primary values than clinopyroxenes. Little correlation exists between oxygen isotope composition and garnet major element compositions in Figs. 4 and 5, in which comparisons are made of our data with oxygen isotope data for Roberts Victor eclogites from MacGregor and Manton Ž1986., Ongley et al. Ž1987. and Caporuscio Ž1990.. The slight

Fig. 4. Variations of molar MgrŽMgqFe. with d18 O Ž‰. for garnets from Roberts Victor coesite eclogites Žthis study with additional data from MacGregor and Manton, 1986 and Caporuscio, 1990., compared with group I and group II Roberts Victor eclogites lacking coesite Ždata from MacGregor and Manton, 1986; Ongley et al., 1987; Caporuscio, 1990..

Fig. 5. Variation of molar CarŽCaqMgqFe. with d18 O Ž‰. for garnets from coesite eclogites and eclogites lacking coesite Ždata sources as in Fig. 4..

negative correlation between CarŽCa q Mg q Fe. and d18 O ŽFig. 5. is discussed below.

6. Discussion Coesite eclogites are well known from several kimberlite xenolith localities, including Roberts Victor ŽSmyth and Hatton, 1977; Schulze and Helmstaedt, 1988.. Schulze and Helmstaedt Ž1988. showed that in eclogite suites from several different localities, the composition of coesite eclogites spanned much of the range of eclogites in which coesite had not been identified, and concluded that the presence, and particularly the distribution, of coesite Žand sanidine. could not be explained by fractional crystallization processes of mafic magmas in the mantle. Our new mineral chemical data for coesite eclogites from Roberts Victor confirm these findings, and greatly expand the number of examples, and the compositional range, of coesite eclogites at this locality ŽFig. 3.. The fact that the coesite eclogites cover the compositional range of virtually the entire group I eclogite population described previously from Roberts Victor ŽMacGregor and Manton, 1986; McCandless and Gurney, 1986. is consistent with the hypothesis that almost all Roberts Victor group I eclogites were in equilibrium with free silica Žcoesite. in the upper mantle. ŽThe lack of overlap of coesitebearing samples with the most magnesian ŽMgrŽMg

D.J. Schulze et al.r Lithos 54 (2000) 23–32

q Fe. ) 0.80. Roberts Victor group I eclogites implies that this interpretation may not extend to the very magnesian members of the group I population.. Furthermore, the few corundum-bearing eclogites described from Roberts Victor seem to occupy a different compositional range than the coesite varieties ŽFig. 3.. This suggests that major element composition of garnets may be useful as a guide to qualitative estimates of silica activity in the Roberts Victor eclogite suite, even in the apparent absence of the defining minerals coesite Žsilica over-saturated bulk compositions. and corundum Žsilica under-saturated bulk compositions. in most eclogites. Aside from the presence of coesite, there are no apparent differences Žin texture, modal mineralogy, or major and minor element chemistry of garnets and clinopyroxenes. between the coesite-bearing group I eclogites and group I eclogites in which coesite has not been identified, suggesting a common origin for both types of group I Roberts Victor eclogites. The abundance, and compositional range, of Roberts Victor coesite eclogites suggests that the hypothesis that mantle eclogites represent the residues of partial melting events that produced the Archean tonalite–trondjemite–granodiorite ŽTTG. suite of plutonic igneous rocks ŽIreland et al., 1994; Rapp and Watson, 1995; Rapp, 1995; Rollinson, 1997. does not apply to the well-characterised Roberts Victor eclogite suite. Although small quantities of free silica were reported on the liquidus in some of the experiments of Rapp and Watson Ž1995., the high degrees of partial melting postulated as a requirement for formation of TTG liquids Ž20–40%. would seem likely to eliminate silica Žand sanidine. from the residue. Although the TTG restite hypothesis may apply to other suites of eclogite xenoliths such as the Koidu suite ŽRollinson, 1997., or the Bellsbank eclogites Žwhere coesite has not been reported — Smyth and Caporuscio, 1984., our new data are consistent with the interpretation that the bulk of the Roberts Victor eclogites represent subducted ocean-floor and shallow hypabyssal mafic rocks subducted into the upper mantle without undergoing significant degrees of partial melting. Oxygen isotope data provide some of the most compelling evidence that many mantle eclogites are the products of subduction and prograde metamorphism of oceanic lithosphere ŽJacob et al., 1998..

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Although early studies recognized that some mantle eclogites xenoliths had anomalous values of d18 O for mantle material ŽGarlick et al., 1971., Jagoutz et al. Ž1984. were the first to propose a subduction origin for mantle eclogite xenoliths on the basis of d18 O values significantly different from those of fresh mid-ocean ridge basalts Ž d18 O of approximately 5.7‰ V-SMOW — Muehlenbachs and Clayton, 1972; Ito et al., 1987.. With expanded oxygen isotope data sets Žincluding those from studies of hydrothermally altered oceanic lithosphere — Gregory and Taylor, 1981., it has been suggested that mantle eclogite xenoliths with d18 O values below those expected from fresh MORB represent subducted and metamorphosed equivalents of mafic oceanic lithosphere altered to relatively high-temperature greenschists and amphibolites, whereas those with d18 O values elevated above MORB represent low-temperature alteration and sea-floor weathering products ŽMacGregor and Manton, 1986; Neal et al., 1990.. The oxygen isotope values for garnets from the Roberts Victor coesite eclogites in this study Ž5.32– 6.95‰. range from approximately those expected for typical mantle garnets Žnew laser fluorination data are 5.3 " 0.2, Mattey et al., 1994. to values significantly in excess of this. The small positive fractionation between clinopyroxene and garnet Žapproximately 0.2–0.3‰ — Mattey et al., 1994; Jacob et al., 1994. indicates that the isotopic composition of garnet is very close to that of a bimineralic whole rock. Our garnet data correspond to those of group I eclogites analysed by MacGregor and Manton Ž1986. from Roberts Victor Žalthough the coesite eclogite data do not extend to values as high as 8.0‰ found at Roberts Victor by MacGregor and Manton, 1986., and are interpreted as the products of subduction and prograde metamorphism of low-temperature altered sea-floor basalts and related rocks. If, indeed, group I eclogites represent low-temperature near sea-floor alteration products, and group 2 eclogites are the equivalents of deeper oceanic lithosphere that underwent oxygen isotope exchange with heated seawater ŽMacGregor and Manton, 1986., it is puzzling that coesite should be restricted to only group 1 eclogites, as appears to be the case in the large suite of coesite eclogite samples from Roberts Victor. Free silica should be expected in mantle eclogites derived from a range of rock types and, in

30

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fact, we have found coesite-bearing eclogites from elsewhere ŽBlaauwbosch and Lace Mines in South Africa. to belong to both groups I and II, and to have a wider range of d18 O values Ž3.30–7.27‰ in Blaauwbsoch coesite eclogite garnets — Schulze and Valley, unpublished data.. Ocean-floor tholeiitic basalts should form silica-bearing eclogites following subduction ŽGreen and Ringwood, 1967a,b., and this process may have produced many of the Roberts Victor coesite eclogites. Prograde eclogite-forming reactions for plagioclase-rich compositions, such as albite s jadeiteq silica and anorthites grossularq kyaniteq silica, indicate that plutonic mafic rocks from deeper oceanic crustal section Že.g., anorthosites. should also yield coesite eclogites, but if they have undergone substantial oxygen isotope exchange with heated seawater, they would be expected to have d18 O values below typical mantle. We have not recognized any such rocks in the Roberts Victor coesite eclogite suite, either by their oxygen isotope values or the sodium contents of their garnets Žfollowing MacGregor and Manton, 1986, one would predict that these would be group 2 eclogites, with low sodium contents in their garnets.. In this regard, however, it is important to note that diamond-bearing group I eclogites ŽG 0.07 wt.% Na 2 O in garnet. from the Mir kimberlite in Russia range in d18 O value down to 3.1‰ ŽBeard et al., 1996.. The one Roberts Victor coesite eclogite in our study that does have a d18 O value approximately that of Aordinary mantleB Žgrospydite a13-64-100, with d18 O s 5.32‰. may bear on this paradox. It may represent a deeper gabbroicranorthositic plagioclase-rich cumulate, in which the coesite, kyanite, and high grossular component in garnet originated by the anorthite breakdown reaction cited above. A similar coesite Žsanidine. grospydite from Roberts Victor was reported by Smyth and Hatton Ž1977., and it, too, has an oxygen isotope composition approximately that of Aordinary mantleB Ž d18 O Žgarnet. s 5.0‰ — Caporuscio, 1990., as does the most calcic Roberts Victor eclogite garnet documented to date, sample R-53 shown by MacGregor and Manton Ž1986. to have a d18 O value of 5.4‰ and CarŽCa q Mg q Fe. s 0.63. These three rocks may represent deeper cumulates that escaped significant exchange with seawater, thus preserving their primary oxygen isotope signatures. This may explain

the slight negative correlation between garnet d18 O values and CarŽCa q Mg q Fe. ratios in Fig. 4.

7. Conclusions The compositional range of coesite-bearing eclogites from the Roberts Victor kimberlite corresponds to that of group 1 eclogites in which coesite has not been described and are abundant at this locality. This is consistent with the hypothesis that the bulk of the eclogite suite at Roberts Victor is in equilibrium with free-silica Žcoesite., and thus is unlikely to represent the residues of partial melting events that yielded TTG rocks. This conclusion does not necessarily apply to other mantle eclogite xenolith suites. Oxygen isotope signatures of garnets from the Roberts Victor coesite eclogites are consistent with derivation by subduction and prograde metamorphism of ocean-floor basaltic rocks and plagioclase-rich cumulates, and the presence of coesite indicates that they did not undergo significant partial melting during, or following, subduction.

Acknowledgements We thank Barry Hawthorne, Roger Clement, Fanus Viljoen and other members of De Beers for their generous hospitality and assistance during several collecting visits to the Roberts Victor Mine and other locations in southern Africa. We also thank Claudio Cermignani, Dragan Andjelkovic, Lori-Ann Pizzolato and Jennifer Wizsniewski for technical help. NSERC, NSF and DOE are thanked for providing financial assistance.

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