oxygen isotope composition of syngenetic inclusions in diamond from the finsch mine, rsa

PII S0016-7037(99)00120-9

Oxygen isotope composition of syngenetic inclusions in diamond from theFinsch Mine, RSA

DAVID LOWRY, DAVID P. MATTEY, and JEFFREY W. HARRIS*Department of Geology, Royal Holloway University of London, Egham TW20 0EX, UK

*Department of Geology and Applied Geology, University of Glasgow, Glasgow G12 8QQ, UK

(Received March 11, 1998; accepted in revised form March 3, 1999)

Abstract—Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associatedminerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotopeanalyses of syngenetic inclusions weighing 20–400mg have been obtained by laser heating in the presenceof ClF3.

18O/16O ratios are determined on oxygen converted to CO2 over hot graphite and, for samplesweighing less than 750mg (producing,12mmoles O2) enhanced CO production in the graphite reactor causesa systematic shift in bothd13C andd18O that varies as a function of sample weight. A “pressure effect”correction procedure, based on the magnitude ofd13C (CO2) depletion relative tod13C (graphite), is used toobtain correctedd18O values for inclusions with an accuracy estimated to be60.3‰ for samples weighing40 mg.

Syngenetic inclusions in host diamonds with similard13C values (28.4‰ to22.7‰) have oxygen isotopecompositions that vary significantly, with a clear distinction between inclusions of peridotitic (14.6‰ to15.6‰) and eclogitic paragenesis (15.7‰ to18.0‰). The meand18O composition of olivine inclusions isindistinguishable from that of typical peridotitic mantle (5.256 0.22‰) whereas syngenetic purple garnetinclusions possess relatively lowd18O values (5.006 0.33‰). Reversed oxygen isotope fractionation betweenolivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnetpreserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with thediamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range ofd18O values from12.3‰to 17.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values.4.7‰. Copyright © 1999 Elsevier Science Ltd

1. INTRODUCTION

Diamonds contain rare inclusions of silicate minerals, oxidesand sulphides whose mineralogy and chemical compositionhave provided key information on the timing and environmentof diamond formation. The great chemical inertness and me-chanical stability of diamond, along with imperviousness tofluids has effectively armoured syngenetic inclusions againstany further exchange with the mantle. Thus, syngenetic inclu-sions have remained chemically isolated for long periods oftime (e.g. Richardson et al., 1984, 1994; Smith et al., 1985;Richardson, 1986) and are regarded as pristine samples ofancient mantle. Diamond inclusion assemblages broadly paral-lel the mineralogy of the two major categories of mantlexenoliths, namely a “peridotitic” (P) paragenesis (dominated byharzburgitic with rare lherzolitic associations) and an“eclogitic” (E) paragenesis (e.g. Meyer, 1987). Syngenetic in-clusions have also been dated, model ages of 3.3 Ga recordedfor subcalcic (purple) harzburgitic garnets from the Finsch andDe Beers Pool mines in South Africa (Richardson et al., 1984).Inclusions of eclogitic or lherzolitic affinity give significantlyyounger Proterozoic ages (Richardson, 1986; Richardson et al.,1984; 1990; 1994). Debate now centres around the age ofdiamond formation; the inclusion ages may represent entrap-ment ages during diamond formation (e.g. Richardson et al.,1984) or diamonds may be much younger and related closelywith kimberlite eruption (Shimizu and Sobolev, 1995).

Oxygen isotope studies have made important contributionsto understanding the evolution and origin of mantle eclogitesand peridotites. Although the fractionation of18O/16O at high

temperatures among mantle phases is significantly less than1‰, eclogites possessd18O values that vary from12‰ to18‰ and provide compelling evidence that some eclogites aresubducted hydrothermally altered crust (MacGregor and Carter,1970; Manton and Tatsumoto, 1971; Kramers, 1979; Jagoutz etal., 1984; MacGregor and Manton, 1986; Neal et al., 1990;Jacob et al., 1994). Eclogites probably have more than oneorigin, and, with the story further complicated by the effects ofubiquitous metasomatic hydration, the oxygen isotopic varia-tion in eclogites reflects several different processes which arenot at present readily distinguishable (e.g. Deines et al., 1991).Although mantle peridotites have more uniform oxygen isoto-pic compositions than eclogites, the publishedd18O data usingclassical techniques for spinel and garnet lherzolites, withd18Ovalues ranging from14‰ to 17‰ (e.g. Kyser et al., 1981,1982; Harmon et al., 1986/87; Kempton et al., 1988; Ionov etal., 1994), stimulated much debate concerning mantle hetero-geneity and the significance of apparent isotopic disequilibriumbetween olivine and clinopyroxene. New laser fluorination datafor olivine and pyroxenes (including re-analysis of pristine,unmetasomatised xenoliths with previously published data por-traying isotopic disequilibrium) shows that oxygen isotopicvariation in peridotitic mantle is less than 1‰ with a bulkd18Oclose to15.5‰ and that isotopic equilibrium prevails (Matteyet al., 1994).

Oxygen isotopes provide a powerful tool for the examinationof fluid-rock interactions in crustal environments. Whilst theoxygen isotope characteristics of the mantle obtained from thexenolith populations provide evidence of subduction and large

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Geochimica et Cosmochimica Acta, Vol. 63, No. 11/12, pp. 1825–1836, 1999Copyright © 1999 Elsevier Science LtdPrinted in the USA. All rights reserved

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scale heterogeneity, an isotopic record of fluid processes suchas hydration and diamond formation remains elusive. Suchevidence may simply be obscured by the combined effects oflow temperature alteration, serpentinisation or interaction withthe host magma. Oxygen isotope records of past metasomaticepisodes will be eliminated given sufficient time for re-equili-bration. Of particular interest is whether the early mantle is inany way different to younger mantle that has undergone meltextraction and recycling of subducted oceanic crust. Assumingthat syngenetic inclusions encapsulated in diamond have re-mained a closed system since entrapment they provide a meansof examining the isotopic characteristics of older mantle, but itremains unknown whether the inclusion represents a sample ofambient mantle that has been accidentally incorporated into thegrowing diamond, or fragments of fluid-buffered material thatpreserve an isotopic imprint of the fluid/melt responsible fordiamond growth.

Oxygen isotopic data for diamond inclusions has hithertobeen impossible to obtain because of their small size. Mostinclusions are in the order of 100–200mm in diameter (weigh-ing approx. 10–30mg) with inclusions exceeding 100mg inweight normally only occurring in diamonds of.0.25 carats.The classical method of determining18O/16O ratio in silicate oroxide minerals (e.g. Baertschi and Silverman, 1951; Taylor andEpstein, 1962; Clayton and Mayeda, 1963; Borthwick andHarmon, 1982) requires large samples (5–30 mg) to obtainreproducible results. Furthermore, many refractory minerals,particularly olivine and garnet are difficult to analyse by theclassical method. Recent advances in laser-based techniques forthe microanalysis of stable isotopes of oxygen in silicates andoxides have opened up new possibilities for the oxygen isotopeanalysis of samples that are orders of magnitude smaller thanpreviously attainable. These laser-fluorination techniques pro-vide opportunities of performing oxygen isotope analysis in-situ by laser-heating selected areas of thin section (e.g. Sharp,1990, 1992; Elsenheimer and Valley, 1992) or by laser heatingor ablation of single mineral grains (e.g. Mattey and Macpher-son, 1993; Eiler et al., 1996; Wiechert et al., 1995). The highperformance of this technique rests with the ability to meltsingle grains of known mass to obtain a quantitative oxygenyield, and coupled with the negligible blanks, 100% puremineral separates (the one perfect grain), and ability to melteven the most refractory mantle phases, the laser-fluorinationmethod providesd18O data capable of resolving mineral frac-tionations at the 0.1‰ level for all mantle phases. Mattey andMacpherson (1993) demonstrated the feasibility of oxygenisotope analysis of single grains weighing as little as 20mg bylaser fluorination and we have refined this technique in thisstudy to determine the oxygen isotope composition of synge-netic inclusions in diamond.

This study examines the oxygen isotope systematics of dia-mond inclusions and associated xenolith suites in kimberlitesfrom the Finsch mine, RSA. The Finsch basaltic kimberlite(Smith, 1983) is the best documented diamond locality insouthern Africa. Previous studies have concentrated on mineralchemistry of garnets in kimberlite (Gurney and Switzer, 1973),diamondiferous peridotite xenoliths (Shee et al., 1982; Viljoenet al., 1992), inclusion chemistry (Gurney et al., 1979; Tsai etal., 1979), and the diamonds themselves (Fesq et al., 1975;Deines et al., 1984, 1989). The last of these papers includes

d13C data for the diamonds, but there are no publishedd18Odata for the xenoliths or kimberlite. Sm-Nd and Rb-Sr modelages of 3.2 to 3.3 Ga were obtained for peridotitic (P-type)inclusion composites (Richardson et al., 1984). Richardson etal., (1984, 1990) have derived a model age of 15806 50 Mafor eclogitic (E-type) garnet inclusions from Finsch diamonds,compared to;118 Ma for the emplacement of the kimberlite(Smith et al., 1985).

Peridotitic inclusions, of which 10–20% are purple garnets,comprise 98% and 90% of the Finsch and Kimberley Poolinclusion populations, respectively (Harris and Gurney, 1979;Harris et al., 1983). Gurney et al., (1979) show that garnets,orthopyroxenes and olivine inclusions are highly magnesian,emphasising residual characteristics of the inclusion assem-blage. Eclogitic assemblages, consisting of sulphides, orangegarnet, pale green clinopyroxene, kyanite and rare coesite,although less common, tend to occur as large inclusions in thelarger diamonds. Relationships betweend13C and inclusionchemistry have been investigated by Deines et al. (1984). Theobjectives of this study are to compare the18O/16O composi-tion of P- and E-type syngenetic inclusions with their mantlecounterparts in an attempt to characterise the oxygen isotopecomposition of peridotitic mantle, and to determine the range ofoxygen isotopic compositions of pristine eclogite paragenesisminerals.

2. ANALYTICAL TECHNIQUES

2.1. Preparation of Syngenetic Inclusions and Mineral Separatesfrom Xenoliths

Forty peridotitic and forty-four eclogitic inclusion-bearing diamondswere made available for this study. Note that the even distribution ofperidotitic and eclogitic diamonds analysed in this study is not typicalof Finsch where only 1% of the inclusion-bearing diamonds areeclogitic (Gurney et al., 1979). Inclusions were released by splitting thediamonds using a steel diamond cracker and washed in deionized waterand dichloromethane. Inclusions weighed between 1mg and 380mg,with 2 of the 3 inclusions weighing.200 mg coming from diamondsof .1 carat. An increase in average inclusion weight with increasingdiamond sieve size was observed. Whenever possible, fragments ofinclusions were retained for microprobe analysis (Lowry et al., in prep)andd13C values were determined on the host diamond using conven-tional sealed-tube combustion techniques (Mattey and Lowry, in prep).

Xenolith samples were crushed under acetone and wet-sieved inde-ionised water to give fractions suitable for mineral separation.Mineral separation was carried out by hand picking under a binocularmicroscope to provide 1–2 mg of a high purity fraction consisting of asmall number of single grains for each analysis.

2.2. Oxygen Isotope Analysis by Laser-Fluorination andCalibration of the Pressure Effect Correction

The laser-fluorination system used in this study is described inMattey and Macpherson (1993). The amount of silicate material whichprovides optimum performance and precision is in the order of 1–2 mg,giving d18O data that are reproducible to better than60.1‰. For theanalysis of samples smaller than 750mg (equivalent to,12 mmoles ofoxygen) Mattey and Macpherson (1993) describe what they call a“pressure effect” whereby measuredd13C andd18O values are system-atically lower than expected. A study of replicate analyses of differentamounts of a garnet standard show systematic variations betweensample weight and apparent oxygen yield, and a quasi-linear isotopicfractionation of bothd13C andd18O of CO2 generated during conver-sion over hot graphite.

Other studies have now confirmed similar systematic variations ofmeasured oxygen isotope composition with sample weight (Wiechertand Hoefs, 1995; Yui et al., 1995; Sato et al., in press) although it is

1826 D. Lowry, D. P. Mattey, and J. W. Harris

clear that for some laser fluorination systems, the effect is minimal orabsent (e.g. Elsenheimer and Valley, 1992). Wiechert and Hoefs (1995)and Sato et al. (in press) report very similar decreases ind13C andd18Owith sample size and note that the pressure effect is minimised by usingnatural or CVD diamond rather than graphite in the converter. Othermechanisms that may also generate low measured isotope values forvery small samples include blank contributions, incomplete reaction ofvery small residual grains (due to inefficient thermal coupling of thelaser with very small cross sections of silicate) or oxygen loss viaoxidation of the metal line or within liquid nitrogen traps. However itis the covariance of bothd13C andd18O that point to CO formation atlow oxygen conversion pressures as the cause of the systematicallyshifted data obtained for very small samples on the Royal Hollowaysystem. Furthermore, that the pressure effect is also observed whenpure oxygen reference gas is introduced into the system (i.e. eliminat-ing the fluorination stage and gas clean-up stage (Mattey and Macpher-son, 1993), provides the most compelling support for this conclusion.

The magnitude of the shift of the measuredd13C (CO2) value awayfrom the knownd13C value of the graphite can be used to correct thefractionatedd18O values obtained for small samples. This correction isbased on the slope M of the best fit linear calibration curve fitted tod13C andd18O data obtained for small samples of known standards.The shift in thed13C value is used to recalculate thed18O of the silicate,and the relationship

d18Osilicate 5 d18Omeasured,CO21 M 3 (d13Cgraphite2 d13Cmeasured,CO2)

provides a means of extending the analytical range down to 20mg witha concomitant decrease in precision. Errors in the correction procedureare propagated from the precision of the value for the graphite rod, the

reproducibility of O2–CO2 conversion step, precision of measuredd13Candd18O values for CO2 and the quality of the fit to the calibration data.Thed13C composition of the graphite rod (Johnson Matthey SpecPurespectrograph rod) is well constrained: over a 2 year period, involvingat least 6 recharges of the converter with fresh graphite, the meand13Cof CO2 produced by conversion of oxygen from normal (.1 mg)samples is223.956 0.03‰ (1s, n ' 2000).

The reproducibility of the pressure effect was closely monitoredthroughout this study via the analysis of CO2 converted from oxygengas and of silicate standards analysed over a range of sample weights(20–750 mg). These experiments were carried out under differentreactor operating conditions and using different batches of graphite.The variation ind13C–d18O in CO2 produced from GP143 garnet overa range of sample weights from 20–750mg are plotted in Figure 1. Theslope of thed13C–d18O best-fit line varied very slightly after a newbatch of graphite was loaded but after a period of conditioning the slopeof the best fit line stabilised at 2.7 for graphite rods 1, 2 and 4 and 2.8for rod 3.

Raw and correctedd18O values for small samples of GP143 garnetare plotted in Figure 2 as a function of sample weight. Note that a fora given weight of sample thed18O (and d13C) value of the CO2produced varies significantly, implying that the degree of CO produc-tion also varies, probably as a result of small variations in gas handlingprocedures and the surface temperature of the graphite during theconversion process. However the striking correlation shown in Figure1 shows that, irrespective of the amount of isotopic fractionation, bothd13C and d18O co-vary systematically, so when the pressure effectcorrection is applied to samples of similar weight the correctedd18Ovalues converge. The overall reproducibility of corrected values for

Fig. 1. Relationship betweend13C andd18O of CO2 produced by conversion of sub 12mmole amounts of oxygen overhot graphite. Data for 4 graphite rods are shown: circles—rod 1; closed squares—rod 2; diamonds—rod 3; open squares—rod 4.Oxygen is produced by laser fluorination of grains of GP143 garnet weighing between 20 and 750mg. The best fit linethrough these data pinned to the meand13C andd18O value for CO2 produced from.1 mg of GP143 (open box) gives aslope of between 2.8 (rod 3) and 2.7 (rods 1, 2, 4) and is used for the pressure effect correction applied to small samples.

1827Oxygen isotope composition of syngenic inclusions in diamond

analyses of garnet GP 143 weighing between 20 and 750mg is 60.3‰(1s, n 5 110). Above 100mg the reproducibility markedly improvesand approaches60.15‰ but below'40 mg the reproducibility dete-riorates such that for practical purposes, a lower limit of 0.5mmoles isregarded as the minimum amount of oxygen that can be analysed toprovide corrected values of acceptable accuracy (nominally with 1sbetter than60.3‰).

2.3. Analytical Procedure

Analysis of mineral separates from xenoliths were performed fol-lowing the methods given in Mattey and Macpherson (1993). Thefollowing description applies to the analysis of syngenetic inclusions.

During the period of this study (;12 months) the graphite in theO2–CO2 reactor was recharged 3 times. After each change it wasnecessary to run batches of normal and small GP 143 samples until thenew graphite was producing CO2 of consistent isotopic composition forgrains of similar weight. At least 20 small standards (,750 mg) wereanalysed before commencing a period of inclusion analyses to checkthe slope of thed13C–d18O correlation.

Sample trays were loaded with normal size standards (SC olivine andGP143 garnet; 1–2 mg sample weight) and, for each syngenetic inclu-sion loaded, 2 additional small GP 143 garnet standards of matchingweight, to provide close control on the quality of the data correctionprocedure (see Table 1). Inclusions smaller than 40mg were compos-ited from diamonds with similar paragenesis andd13C values (normallywithin 0.2‰ of each other) to provide sufficient oxygen (.0.5mmoles)for analysis (see Table 2 for details). After loading the sample tray the

analytical procedure followed that described in Mattey and MacPher-son (1993). A normal run would begin with analyses of aliquots of O2

reference gas converted to CO2 followed by a fluorination proceduralblank (measured by heating an empty hole for 10 min at 50% laserpower). Procedural blanks were typically,4% of a 40mg sample gasequivalent (,0.02 mmoles O2). Additional large standards were anal-ysed at the end of the run. The laser-fluorination reaction times forsmall samples are typically,1 min compared to 3–10 min for samplesweighing.1 mg. After conversion to CO2 and determination of yields,d13C andd18O data were obtained using a VG PRISM mass spectrom-eter that is on-line from the laser extraction system.

2.4. Data normalisation and correction for the pressure effect

The data for large standards (San Carlos olivine and GP143 garnet)are used for normalising data obtained during an analytical session toknown values. The measuredd13C–d18O values for the large standardsnormally show small systematic shifts on a day-day basis, dependent online condition and other operating conditions. All the data for the daysrun are therefore normalised to thed13C value of the graphite rod(223.95‰) and the meand18O values obtained for SC olivine(14.88‰) and garnet GP143 (17.21‰). Analyses of all large SColivine and GP143 garnet standards obtained at the beginning and endof each run showed good reproducibility, averaging14.88 6 0.08‰(1s, n 5 14) and17.18 6 0.08‰ (1s, n 5 7), respectively. Themaximum amount of normalisation applied to each batch on a day-to-day basis is in the order of60.05‰.

Oxygen isotope data for small samples are corrected on the basis of

Fig. 2. Plot of rawd18O values (closed circles) and pressure-correctedd18O values (open circles) versus sample weightof GP 143 garnet. The known value of GP143 is represented as a solid line. The dashed lines represent61s of standarddeviation of the correctedd18O values (17.20 6 0.32‰,n 5 110).


the measuredd13C value using the equation given above. The offset ofraw data can be up to 2‰ ind13C for samples weighing;20 mg, witha concomitant change ind18O by ;5.4 to 5.6‰ (Figures 1 and 2). Forthe great majority of analyses the corrected analyses for the inclusionstand alone and the data for matching GP143 standards provide anindependent check of the quality of the inclusion analysis in providingan estimate of confidence limits. However the data for matching GP143standards also served as a back-up calibration for a number of instanceswhere for experimental reasons anomalous values were obtained usingthe pressure effect correction.

3. RESULTS

The measured and corrected oxygen isotope data for 31silicate inclusions from Finsch diamonds and 41 matched GP143 garnets are reported in the order of analysis in Table 1, asummary of correctedd18O values for inclusions are given inTable 2 andd18O data for Finch xenoliths are given in Table 3.

3.1. Pressure Correctedd18O Values

Raw isotope data given in Table 1 have been normalised tolarge standards prior to the pressure effect correction. Thequality of each diamond inclusion analysis can be gauged fromthe values obtained for matched GP143 garnets. The meancorrectedd18O value of all matched GP 143 garnet standardsrun in this study is17.186 0.27‰ (1s, n 5 41), compared to7.216 0.10‰ (1s, n 5 30) for large GP 143 garnets weighing.1 mg. For the majority of diamond inclusion-matched GP143garnet “triplets”, the correctedd18O values of the matchedGP143 analyses lie within60.2‰ of the known value, andprovide confidence limits that can be applied to thed18O valueobtained for each diamond inclusion analysis (see below).

Six of the 31 inclusiond18O values are subject to largeruncertainties and are identified in Table 1. In these cases thepressure effect corrections on the inclusion and matched stan-dards gave anomalous correctedd18O results using the slopevalue calibrated for the carbon rod in use over that period.Three samples (Finsch 7(1), Finsch 9(1) and Finsch composite{8(2) 1 31A(2)}) and their matched garnet standards havemeasuredd13C values appropriate for the sample size but witha smallerd18O fractionation than normal. Other analyses high-lighted in Table 1 are where the measuredd18O value wasappropriate for the sample size butd13C is too light, resultingin overcorrection of thed18O values. For these aberrant anal-yses the rawd18O values of matching garnet standards havebeen used to correct the inclusiond18O value using a modifiedslope value (Table 1).

The inclusion data are shown on a plot of inclusiond18Ovalue versus host diamondd13C value in Figure 3. Uncertain-ties in all the correctedd18O values are indicated by error barsbased on the deviation of the corrected matching garnet stan-dards from their known values and are plotted as6(maximumdifference from known value).

Seven of the 31 inclusion analyses are composites of inclu-sions based on paragenesis and similard13C of the host dia-monds, and therefore a total of 42 inclusions from 38 diamondsare represented in the results. Coexisting olivine and garnethave been analysed from 3 peridotitic diamonds, although onlydiamond 13A(1) contained inclusions of both minerals largeenough to analyse individually (Table 2). Garnet and an olive-green inclusion, probably amphibole, have been analysed from

an eclogitic diamond, but the garnet is also part of a compositedanalysis.

3.2. d18O Variation Among Silicate Inclusions inFinsch Diamonds

Peridotitic diamond inclusions (purple chrome pyrope gar-net, olivine, orthopyroxene and diopside) possess a restrictedrange ind18O from 14.1‰ to15.6‰ (n5 18, Table 2). Thelightest value is for a large diopside inclusion in a lherzoliticparagenesis host. Chrome pyrope inclusions give a range ofd18O from 14.6‰ to 15.6‰ with a mean of 5.006 0.37‰(1s, n 5 10). Data for olivine (and a single orthopyroxene)vary from 14.9‰ to 15.5‰, i.e. similar to the garnet inclu-sions, with a mean of 5.276 0.20‰ (1s, n 5 7).

Eclogitic diamond inclusions (orange almandine garnet, om-phacite and amphibole) show a wider range ind18O from14.7‰ to 18.8‰, with the majority of data falling between15.7‰ to 18.0‰ (Table 2). Orange garnets give a range ofvalues from 15.7‰ to 18.0‰. Two omphacitic clinopy-roxenes possess lower values of14.7‰ and15.9‰. The most18O enriched sample with a value of18.8‰ is the amphiboleinclusion. Although there is no evidence of a fracture in thisdiamond the inclusion may not be a primary phase.

3.3. d13C of Finsch Inclusion-Bearing Diamonds

The d13C of all 84 Finsch diamonds has been analysed,however only data associated withd18O analysis of inclusionsare shown in Table 2. A comprehensive discussion of thecarbon isotope data will be published elsewhere (Mattey andLowry, in prep). Thirty-eight diamonds used in the presentinclusion d18O study have a range ofd13C from 28.4‰ to22.7‰ (Table 2, Fig. 3). The eclogitic diamonds range from28.4‰ to24.0‰ (mean26.346 1.58‰, 1s, n 5 18) and theperidotitic diamonds from27.2‰ to24.6‰ (mean26.1160.72‰, 1s, n 5 19) with a single lherzolitic (diopside-bearing)diamond having a value of22.7‰. Deines et al. (1989) reportthe same range of28.5‰ to22.5‰ for 81 Finsch diamonds.Both eclogitic and peridotitic diamonds fall within the typicalperidotite facies range of;29‰ to 22‰ typically indicativeof a mantle source for the carbon (e.g. Kirkley et al., 1991).

4. DISCUSSION

Whereas peridotite and eclogite xenoliths occur in varyingproportions in kimberlites, diamondiferous eclogite xenolithsare significantly more abundant than diamondiferous peridotitexenoliths. As xenoliths are potential source rocks for the dia-monds hosted by kimberlites, and therefore provide informa-tion about the conditions of diamond formation, they have beenthe focus of numerous conventional oxygen isotope studies.Analyses of southern African eclogites predominate and in-clude studies on Roberts Victor (Garlick et al., 1971; Jagoutz etal., 1984; Ongley et al., 1987; Sharp et al., 1992), Bellsbank(Neal et al., 1990; Caporuscio, 1990), Orapa (Deines et al.,1991) and Southern Africa in general (Shervais et al., 1988).Oxygen isotope data for mantle peridotite xenoliths fromSouthern African kimberlites have been reported by Kyser et al.(1982).

New laser-fluorinationd18O data for constituent minerals


from more than 100 xenoliths in South African and Siberiankimberlites and in basalts have been reported in Mattey et al.,(1994) and Jacob et al., (1994). These data, comprising 94olivine and 44 garnet analyses from peridotite xenoliths (in-cluding 9 diamondiferous garnet lherzolites) and 61 garnetsfrom eclogite xenoliths (including 24 diamondiferouseclogites), are compared with the inclusion data in Figure 4.

The d18O composition of mantle olivine, determined bylaser-fluorination, has been shown to be almost constant inxenoliths from widespread localities, with an average value of'15.2‰ (Mattey et al., 1994). Thed18O composition ofolivine syngenetic inclusions in diamonds show very similarvalues with mean of15.256 0.22‰ (1s, n 5 6). Thed18Ocomposition of olivine in diamondiferous peridotites(5.16

Table 1. Raw and corrected oxygen isotope data for syngenetic inclusions from Finsch diamonds and adjacent GP 143 standards in order of analysisover the period 3/92 to 3/93.d13C values are adjusted by up to60.05‰ so that all are relative to a value of224‰ for the large silicate standardsanalysed with each inclusion batch. Figures highlighted in the slope column are those where a lower slope correction factor has been obtained frommatching garnet analyses. See main text for further details.

Sample no. Mineral Wt. (mg) mmoles O2 d13C d18Oraw Slope d18Ocorr

CARBON ROD 1

FL 1 Opx 350 4.86 224.22 4.76 2.70 5.35FL 2.1 Diopside 380 4.19 224.35 3.14 2.70 4.09

CARBON ROD 2

GP 143 51 0.73 225.40 4.64 1.95 7.377(1) Orange gt 48 0.63 225.28 4.92 1.95 7.40GP 143 50 0.72 225.09 4.97 1.95 7.09GP 143 78 1.08 224.91 5.00 2.70 7.447(7) Orange gt 81 1.06 224.46 6.72 2.70 7.96GP 143 80 1.12 224.81 5.02 2.70 7.22GP 143 70 0.98 224.83 4.86 2.70 7.107(11) Orange gt 71 0.95 224.83 3.62 2.70 5.76GP 143 74 1.05 224.94 4.81 2.70 7.34

CARBON ROD 3

GP 143 56 0.76 225.20 3.73 2.80 7.2512(2) Orange gt 58 0.70 225.11 2.41 2.80 5.67GP 143 93 1.22 224.83 4.80 2.80 7.2612(4) Orange gt 91 1.07 224.77 4.20 2.80 6.49GP 143 89 1.19 224.77 4.37 2.80 6.66GP 143 84 1.10 224.87 4.58 2.80 7.1412(6) Orange gt 79 1.00 224.80 3.57 2.80 5.93GP 143 77 1.00 224.89 4.11 2.80 6.72GP 143 53 0.69 225.22 3.77 2.80 7.2912(7) Orange gt 57 0.69 225.28 4.16 2.80 7.85GP 143 218 2.89 224.39 6.26 2.80 7.3513(1) Purple gt 210 2.80 224.12 4.63 2.80 4.98GP 143 196 2.59 224.14 6.55 2.80 6.94GP 143 54 0.73 225.23 3.78 2.80 7.2213(2) Purple gt 55 0.67 225.43 1.60 2.80 5.59GP 143 54 0.69 225.29 3.56 2.80 7.15GP 143 61 0.82 225.21 3.33 2.80 6.7213(3) Purple gt 51 0.60 225.32 1.29 2.80 4.99GP 143 76 0.99 225.09 4.15 2.80 7.2013(4) Purple gt 76 0.95 225.11 1.60 2.80 4.70GP 143 81 1.11 225.00 4.31 2.80 7.11GP 143 76 0.92 225.10 4.21 2.80 7.291(10) Green cpx 74 0.96 225.02 1.86 2.80 4.71GP 143 122 1.49 224.80 4.83 2.80 7.0713(5) Purple gt 125 1.68 224.69 2.62 2.80 4.62GP 143 61 0.71 225.50 3.53 2.80 7.8513(6) Purple gt 63 0.78 225.20 1.15 2.80 4.66GP 143 83 0.96 225.19 4.44 2.80 7.7613(11) Purple gt 83 1.09 224.95 2.72 2.80 5.38GP 143 73 0.82 225.03 4.35 2.80 7.2312(5) Orange gt 72 0.83 225.38 3.18 2.80 7.04GP 143 46 0.56 225.48 3.85 2.30 7.259(1) Olive amph 47 0.54 225.55 5.21 2.30 8.78GP 143 153 2.04 224.51 5.07 2.80 6.578(6) Olivine 157 2.15 224.49 3.46 2.80 4.90GP 143 165 2.21 224.44 5.84 2.80 7.17GP 143 47 0.61 225.22 3.26 2.80 6.8013A(1) Purple gt 49 0.65 225.28 1.26 2.80 4.99


6 0.07‰, 1s, n 5 7) is indistinguishable from the mean fornon-diamondiferous peridotites (5.196 0.13‰, 1s, n 5 76).The limited range in oxygen isotope variation shown by olivineis consistent with the predominance of the phase in mantlelithologies and its residual nature during melting processes.

In contrast to olivine, thed18O values for garnet in mantlexenoliths show subtle variations. The new data for diamondif-erous garnet lherzolites from Finsch (Table 3, Figure 4) suggestthat whereas garnet in typical non-diamondiferous lherzoliteshave meand18O values of15.42 6 0.12‰ (1s, n 5 29)garnet in diamondiferous peridotite xenoliths have slightlylower d18O values averaging15.186 0.20‰ (1s, n 5 11).This relationship is also reflected by the data for syngeneticgarnet inclusions which also possess relatively lowd18O valuesaveraging15.00 6 0.33‰ (1s, n 5 14).

Garnet in eclogite xenoliths in kimberlite show a range ofd18O values from12.3‰ to17.3‰ (Figure 4) with diamon-diferous eclogites all having values above14.7‰. Eclogiticgarnet inclusions in Finsch diamonds have values in the range15.7‰ to18.0‰ (Table 3), consistent with the range for alldiamondiferous lithologies. Xenoliths are rare in the Finschkimberlite (except for one peridotite-rich zone (Shee et al.,1982)) and data for 2 small non-diamondiferous eclogite xeno-liths are given in Table 3. These have strikingly differentd18Osignatures to the inclusions, the garnets having values of12.8‰ and13.4‰, and it is suggested by comparison withother eclogitic garnets that these 2 xenoliths, and possibly alleclogites withd18O of ,4.7‰, do not represent diamond hostrocks or a source of carbon for diamond formation.

Normal garnet peridotites have a positiveD18Ogarnet-olivine

fractionation of 0.2–0.4‰ (Mattey et al., 1994) consistent withempirical and experimentally determined fractionation at mantletemperatures (e.g. Bottinga and Javoy, 1975; Richter and Hoernes,1988; Zheng, 1993; Matthews, 1994). Non-diamondiferous

garnet lherzolites from Finsch possessD18Ogarnet-olivine frac-tionations of approximately10.2‰ but diamondiferous sam-ples haveD18Ogarnet-olivinefractionations that are much smalleror even reversed. Significantly, theD18Ogarnet-olivinefraction-ation between coexisting garnet and olivine inclusions in dia-mond 13A(1) is20.4‰, as are the composited analyses con-taining inclusions from the same diamond (Table 2). Theseobservations suggest that garnet, both as syngenetic inclusionsand as the macroscopic phase in diamondiferous peridotites,may preserve subtle isotopic disequilibrium.

There are several mechanisms that could create garnets withlow d18oxygen: these include high pressure/temperature isoto-pic reversals, mineral transformations or metasomatic reactionsinvolving a lowd18O phase such as spinel, or isotopic exchangewith the fluid during diamond growth (Mattey et al., in prep).For microgram amounts of a syngenetic inclusion it is easy toenvisage that trapped grains are locally buffered by the fluidresponsible for diamond precipitation. These transient isotopicsignatures are preserved once encapsulated in diamond.Whether loweredd18O values is a feature specific to garnet ora characteristic of all syngenetic inclusions is unclear from thepresent data; the only analysis of coexisting garnet and olivineinclusions in the same diamond 13A(1) show a lowd18Ogarnet–normald18O olivine reversed fractionation relationship(Table 2) but analyses of other phases in Table 2 do not all haveequivocally “normal” peridotiticd18O values (e.g. diopside FL2.1 with d18O 5 14.1‰). It remains possible therefore thatmany syngenetic inclusions are fluid-buffered during diamondgrowth and the anomalous lowd18O values preserved by garneteither implicates a lowd18O fluid or interaction between a fluidand cooler mantle (Mattey and Lowry, in prep). It is unlikelythat the oxygen isotopic composition of a small volume fluid ormelt associated with diamond formation will be significantlyremoved from what would be in equilibrium with ambient

Table 1. (Continued)

Sample no. Mineral Wt. (mg) mmoles O2 d13C d18Oraw Slope d18Ocorr

CARBON ROD 4

GP 143 63 0.97 225.11 4.41 2.70 7.418(4) Olivine 63 1.04 225.10 2.21 2.70 5.18GP 143 59 0.82 225.24 4.13 2.70 7.49GP 143 63 0.87 224.92 4.42 2.70 6.908(7) Olivine 62 1.03 225.12 2.48 2.70 5.49GP 143 67 0.89 225.02 4.26 2.70 7.00GP 143 57 0.82 225.37 3.53 2.70 7.2113A(1) Olivine 57 0.82 225.29 1.95 2.70 5.43GP 143 70 1.01 225.04 4.69 2.70 7.5113(5) 1 8(3) Olivine 65 0.92 225.11 2.37 2.70 5.36GP 143 57 0.81 225.07 4.08 2.70 6.961(7) 1 10(1) Green cpx 61 0.86 225.18 2.65 2.70 5.85GP 143 69 0.96 224.63 5.42 2.70 7.13GP 143 87 1.17 224.92 4.37 2.70 6.867(4) 1 12(1) 1 9(2) Orange gt 80 0.91 225.29 4.09 2.70 7.56GP 143 89 1.21 224.83 4.83 2.70 7.087(2) 1 7(6) 1 9(1) 1 12(3) Orange gt 87 1.12 224.94 5.28 2.70 7.82GP 143 49 0.56 225.40 3.70 2.70 7.4913(9) 1 13(7) Purple gt 50 0.65 225.39 0.87 2.70 4.61GP 143 52 0.71 225.46 4.05 2.20 7.268(2) 1 13A(2) Olivine 54 0.81 225.35 2.17 2.20 5.15GP 143 49 0.66 224.96 4.44 2.70 7.0413(10)1 13(8) 1 13A(2) Purple gt 49 0.64 225.30 1.99 2.70 5.49


peridotitic mantle and the slight shift in18O could be a result ofdecreasing temperature during diamond growth (Mattey andLowry, in prep).

The mechanism responsible for oxygen isotope disequilib-rium in macroscopic garnet and its relationship with the dia-mond growth event at 3.3 Ga (Richardson et al., 1984) is lessobvious. The prolonged time interval between diamond forma-tion and kimberlite emplacement provides ample opportunityfor oxygen isotope re-equilibration. If formation of lowd18O

garnet and diamond are closely related this requires that eitheroxygen diffusion in high Cr-garnet is extremely slow, or thatdiamond genesis is a relatively recent event. Alternatively, ifthe formation of lowd18O garnet and diamond genesis are notrelated, other mechanisms for creating Cr-rich lowd18O garnetmay be responsible. One interesting possibility is that Cr-richgarnet has formed from a lowd18O precursor such as Cr-spinel(Mattey et al., in prep), either during inversion of spinel faciesperidotite during subduction (e.g. Kesson and Ringwood, 1989)

Table 2. Weights and pressure-correctedd18O values for inclusions, andd13C of the host diamonds.

Paragenesis Mineral SampleInclusion

weight (mg)d18O

(inclusion)d13C

(diamond)

Eclogitic Garnet 7 (1) 48 7.4 24.57 (7) 81 8.0 25.97 (11) 71 5.8 24.9

12 (2) 58 5.7 24.612 (4) 91 6.5 24.712 (5) 72 7.0 25.412 (6) 79 5.9 26.012 (7) 57 7.9 28.4

7 (2) 35 27.517 (6) 15 7.8 27.519 (1) 12 27.8112 (3) 25 27.6

7 (4) 23 28.419 (2) 31 7.6 28.3112 (1) 26 28.3

Clinopyroxene 1 (10) 74 4.7 24.0

1 (7) 36 5.9 25.4110 (1) 25 25.3

Amphibole 9 (1) 47 8.8 27.8

Peridotitic Garnet 13 (1) 210 5.0 26.813 (2) 55 5.6 26.413 (3) 51 5.0 26.113 (4) 76 4.7 27.213 (5) 125 4.6b 25.513 (6) 63 4.7 26.913 (11) 83 5.4 27.1

13A (1) 49 5.0a 26.3

13 (7) 26 4.6 25.0113 (9) 24 25.3

13 (8) 13 26.1113 (10) 20 5.5c 26.5113A (2) 16 26.3

Olivine 8 (4) 63 5.2 26.58 (6) 157 4.9 26.58 (7) 62 5.5 24.6

13A (1) 57 5.4a 26.3

8 (2) 40 5.2c 26.3113A (2) 14 26.3

8 (3) 33 5.4b 25.6113 (5) 32 25.5

Orthopyroxene FL 1 350 5.4 25.4

Diopside FL 2.1 380 4.1 22.7

Data for coexisting inclusions:a d18O values given for coexisting garnet 13(A) and olivine 13(A) from same diamond;b d18O values given forgarnet 13(5) and composite analysis that includes olivine 13(5);c d18O values given for composite analyses that contain garnet and olivine fromdiamond 13A(2).

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or by a recent metasomatic reaction involving chromite (Pear-son et al., 1995). The origin and significance of lowd18Odiamond facies garnets are discussed elsewhere (Mattey et al.,in prep).

Evidence for fluid buffering of peridotitic syngenetic inclu-sions is clearly going to be more evident against the relativelyuniform background oxygen isotopic signature of peridotiticfacies rocks. In the case of eclogites, where significant oxygen

Table 3. Oxygen isotope composition of olivine (ol), orthopyroxene (opx), clinipyroxene (cpx) and garnet (gt) in peridotite and eclogite xenolithsfrom Finsch. Data are averages of replicate analyses (except F866 garnet) with a reproducibility better than60.1‰.

Sample Lithology Diamonds

d18O

Olv Opx Cpx Gt

XM 48 lherzolite Yes 5.15 5.07F 865 harzburgite Yes 5.07 5.63 5.02F 866 harzburgite Yes 5.08 5.76 4.93

JJG 147 lherzolite No 5.15 5.63 5.62 5.32JJG 1141 lherzolite No 5.94 5.69 5.24JJG 1142 lherzolite No 5.72 5.80 5.40

JJG 546 eclogite No 3.21 2.79JJG 1146 eclogite No 3.63 3.37

Fig. 3. Plot ford13C of host diamonds versusd18O of peridotitic inclusions (open symbols) and eclogitic inclusions (blacksymbols) from Finsch. Estimated uncertainties associated with the dataset are as follows:d13C values—range obtained forreplicate analyses of diamond fragments except where host diamond adjacent to inclusion has been analysed (in this caseanalytical error is60.05‰ and no error bar is drawn).d18O values—estimated from the values obtained for matched GP143 garnets analysed before and after the inclusion (see text for details).


isotope variation exists, evidence for buffering and the isotopiccomposition of the fluid is less clear. Eclogitic garnet inclusionsin Finsch diamonds average'7.0‰ and are enriched by'1‰relative to the averaged18O of garnets in diamondiferouseclogite xenoliths as a whole. These in turn are more enrichedthan the average composition of non-diamondiferous eclogites('5.5‰). It is likely that the limited data for diamonds andinclusions presents an incomplete picture of thed18O variationin Finsch eclogite facies rocks but it may be that melt or fluidthat is significantly18O enriched (.17‰) is associated withdiamond formation in eclogites.

5. CONCLUSIONS

1. The oxygen isotope composition of syngenetic inclusionsweighing,380 mg in diamonds has been determined by amodified laser-fluorination technique employing a correc-tion for the “pressure effect”. Using this correction, repro-

ducibility of 110 analyses of samples of a garnet standardweighing mostly ,250 mg is 60.32‰, compared to60.10‰ for normal samples weighing.1 mg.

2. Syngenetic inclusions in host diamonds with similard13Cvalues (28.4‰ to 22.7‰) have oxygen isotope composi-tions that vary significantly, with a clear distinction betweeninclusions of peridotitic and eclogitic paragenesis. Garnetinclusions of eclogitic affinity haved18O of 15.7‰ to18.0‰ whereas garnet and olivine inclusions of peridotiticaffinity which have values of14.6‰ to15.6‰. The meand18O composition of olivine syngenetic inclusions is indis-tinguishable from that of typical peridotitic mantle (15.256 0.22‰) whereas syngenetic purple garnet inclusions pos-sess relatively lowd18O values (15.006 0.33‰).

3. There is a striking correspondence between thed18O valuesof syngenetic inclusions and thed18O of diamondiferousperidotite and eclogite xenoliths also analysed by laser flu-

Fig. 4. Compilation ofd18O data for olivine and garnet in a) diamondiferous and non-diamondiferous xenoliths fromkimberlites, and b) syngenetic inclusions in diamond. Note the strong similarity betweend18O values of olivine and garnetsyngenetic inclusions and diamondiferous xenoliths.


orination. This implies that the diamondiferous xenolithshave not undergone any gross exchange ofd18O since thetime of diamond growth and inclusion entrapment.

4. Reversed oxygen isotope fractionation between olivine andgarnet in both diamond inclusions and diamondiferous pe-ridotite xenoliths may be evidence of subtle isotopic dis-equilibrium related to genesis of Cr-rich garnet and/or ex-change with the diamond-forming fluid. The existence of adistinctive oxygen isotopic imprint associated with diamondformation and in diamond facies peridotites has importantimplications on the mechanism and timing of diamondgrowth and will be discussed elsewhere (Mattey and Lowry,in prep; Mattey et al., in prep).

5. Garnet in eclogite xenoliths in kimberlite show a range ofd18O values from12.3‰ to17.3‰ but garnets in diamon-diferous eclogites and as inclusions in diamond all havevalues.14.7‰.

Acknowledgments—This work was supported by NERC grant 8272/GR3 awarded to D. Mattey. The authors wish to thank De BeersConsolidated Mines Ltd. for supplying inclusion-bearing diamonds.John Gurney, Graham Pearson, Melissa Kirkley, Dorrit Jacob, GailKiviets and Gilles Chazot are thanked for supplying mineral separatesand hand specimens of eclogite and peridotite xenoliths for laserfluorination analysis. Constructive comments from Dorrit Jacob, JohnValley and one anonymous reviewer are gratefully acknowledged.

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oxygen isotope composition of syngenetic inclusions in diamond from the finsch mine, rsa

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