Journal of Petrology | Volume 44 | Number 4 | Pages 629-657 | 2003
© Oxford University Press 2003
Garnet Lherzolites from the Kaapvaal Craton (South Africa): Trace Element Evidence for a Metasomatic History
1 DEPARTMENT OF GEOLOGICAL SCIENCES, UNIVERSITY OF CAPE TOWN, RONDEBOSCH 7701, SOUTH AFRICA
2 DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MA 02139, USA
Present address: UMR 5562, Observatoire Midi-Pyrénées, 14 Av. E. Belin, 31400, France. Telephone: 33 (0)561 33 29 77. Fax: 33 (0)5 61 33 29 00. E-mail: michel.gregoire{at}cnes.fr
RECEIVED JANUARY 7, 2002; ACCEPTED OCTOBER 8, 2002
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONANALYTICAL METHODSSAMPLE LOCATION AND PETROGRAPHYMINERAL COMPOSITIONSWHOLE-ROCK COMPOSITIONSTEMPERATURE AND PRESSURE...DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Kimberlites from the Kaapvaal craton have sampled numerous mantle garnet lherzolites in addition to garnet harzburgites. Trace element characteristics of constituent clinopyroxenes allow two groups of garnet lherzolites to be distinguished. Trace element compositions of all clinopyroxenes are characterized by enrichment in light rare earth elements (LREE) and large ion lithophile elements and by a relative depletion in Ti, Nb, Ta, and to a lesser extent Zr and Hf. However, the LREE enrichment and the depletion in Nb and Zr (Hf) are less in the Type 1 clinopyroxenes than in the Type 2 clinopyroxenes. Our study suggests that the two melts responsible for the metasomatic imprints observed in the two garnet lherzolite groups are highly alkaline mafic silicate melts. Type 1 clinopyroxenes that have trace element similarities to those of PIC (Phlogopite–Ilmenite–Clinopyroxene) rocks appear to have crystallized from, or been completely equilibrated with, the same melt related to Group I kimberlite magma. The Type 2 clinopyroxenes have trace element similarities to those of MARID (Mica– Amphibole–Rutile–Ilmenite–Diopside) rocks and are therefore probably linked to melt related to Group II kimberlite magma.
KEY WORDS: garnet lherzolites; Kaapvaal craton; mantle xenoliths; mantle metasomatism; trace elements
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONANALYTICAL METHODSSAMPLE LOCATION AND PETROGRAPHYMINERAL COMPOSITIONSWHOLE-ROCK COMPOSITIONSTEMPERATURE AND PRESSURE...DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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General features of mantle peridotite xenoliths of southern Africa
Peridotite xenoliths brought to the surface by kimberlites provide a valuable window to the composition of the continental upper mantle and are key witnesses of the processes responsible for the origin and evolution of this part of the Earth (e.g. Nixon & Boyd, 1973
; Gurney & Harte, 1980; Harte, 1983; Nixon, 1987; Gurney et al., 1991; Boyd et al., 1997). Geochemical studies of peridotites from southern African kimberlites indicate that, relative to typical upper mantle, the continental mantle in this region has been strongly depleted in basaltic components (O'Hara & Mercy, 1963; Boyd & Mertzman, 1987; McDonough & Sun, 1995). In contrast, most peridotites are variably enriched in incompatible trace elements (Shimizu, 1975; Hoal et al., 1994; McDonough & Sun, 1995). In some cases this enrichment is accompanied by the growth of new minerals (Erlank & Rickard, 1977; Erlank et al., 1987; Haggerty, 1987; Winterburn et al., 1990; Griffin et al., 1999b) and in others not (Shimizu, 1975; Richardson et al., 1985; Shimizu & Richardson, 1987; Hoal et al., 1994; Burgess & Harte, 1999). The enrichments are generally believed to be caused by the infiltration of mobile fluid phases into mantle peridotite at some time before entrainment in kimberlite and as such are manifestations of mantle metasomatism (Dawson, 1972; Erlank, 1973; Harte, 1987; Harte et al., 1993). Metasomatism of peridotite wall rocks can in some instances be related to veins defined by concentrations of hydrous minerals or clinopyroxene (e.g. Jones et al., 1982), whereas in others an obvious source is not apparent (e.g. Winterburn et al., 1990). One of the greatest drawbacks of xenolith-based studies of metasomatism is the lack of spatial information, so that it is difficult to distinguish, for example, between the distal metasomatic effects of veins and pervasive background events.
Manifestations of modal and cryptic metasomatism
A number of styles of mantle metasomatism have been identified on the basis of different geochemical and mineralogical characteristics (Harte, 1983, 1987; Menzies, 1983; Dawson, 1984; Harte et al., 1987; Harte & Hawkesworth, 1989) that are linked to differences in the character of the metasomatic agent. Detailed studies of individual suites of metasomatized xenoliths have contributed a great deal to understanding the relationship between trace element enrichment and mineral growth in peridotites and the veins rich in hydrous minerals, clinopyroxene, etc., that are inferred to mark zones of fluid passage through the lithosphere (Jones et al., 1982; Erlank et al., 1987; Harte et al., 1987), or the megacrysts that may indicate a more pervasive infiltration of melt into lithospheric mantle (Harte & Gurney, 1981; Hops et al., 1992; Burgess & Harte, 1999). These studies suggest that metasomatic agents include both silicate melts of broadly alkaline basaltic to kimberlitic character (Harte, 1983) as well as more hydrous, potassic fluids that may be similar to lamproite or its derivatives. Harte et al. (1993) have proposed that a continuum of metasomatic melt compositions may result from chemical differentiation associated with percolative flow and reaction in the upper mantle, and have shown that metasomatic mineral compositions are consistent with such a process.
Among Kaapvaal craton xenoliths, a geochemical and geochronological relationship exists between MARID rock (Mica–Amphibole–Rutile–Ilmenite–Diopside; Dawson & Smith, 1977), PKP (Phlogopite and K-richterite-bearing Peridotites; Erlank et al., 1987) type metasomatism and Group II kimberlite or lamproite magmatism (Dawson & Smith, 1977; Waters, 1987; Sweeney et al., 1993; Konzett et al., 1995, 1998; Hamilton et al., 1998; Grégoire et al., 2002), whereas Group I kimberlite magmatism has been linked to the clinopyroxene-glimmerite–wehrlite suite (Jones, 1984; Grégoire et al., 2002) as well as some MARID rocks (Konzett et al., 2000). In particular, on the basis of detailed major and trace element studies of clinopyroxene, phlogopite and ilmenite, Grégoire et al. (2002) defined and clarified the differences between the two main groups of phlogopite-rich mafic xenoliths: the MARID rocks and the cpx-glimmerite suite of rocks as defined by Jones (1984, 1987) and renamed PIC rocks in the study by Grégoire et al. (2002). In combination with the limited available isotopic data from the literature, Grégoire et al. (2002) proposed that PIC and MARID rocks are deep-seated segregations from highly alkaline melts genetically linked to Group I and Group II kimberlite magmas, respectively.
Incompatible element enrichment is a fundamental characteristic of the magnesian, low-temperature, commonly coarse-granular, garnet peridotite xenoliths thought to constitute the bulk of the Kaapvaal cratonic mantle (e.g. Nixon et al., 1981; McDonough & Sun, 1995). This enrichment occurs with or without the growth of primary textured phlogopite or edenitic amphibole, and without the apparent introduction of ‘basaltic’ components such as Al, Fe or Ti (Shimizu, 1975; Erlank et al., 1982, 1987; Winterburn et al., 1990). The metasomatism is manifested by light rare earth element (LREE)-enriched clinopyroxenes, and often ‘sinusoidal’ REE patterns in garnet, that have been subject to many different interpretations [see review by Stachel et al. (1999)]. This geochemical signature is also characterized by low Ti, Zr and sometimes high Sr contents (Shimizu, 1975; Shimizu & Richardson, 1987; Shimizu et al., 1999; Griffin et al., 1992, 1993). Winterburn et al. (1990) suggested that the metasomatic agent is a water-rich fluid. Other proposed agents include carbonatite (Griffin et al., 1992) and methane-rich fluids (Stachel et al., 1999). It is possible that a number of events with the same broad geochemical characteristics, but with potentially different origins, have affected the cratonic mantle over its long history.
The above review suggests that multiple enrichment events affected the cratonic mantle xenoliths of the Kaapvaal craton. The possibility that all types of metasomatism discussed above may be accompanied by cryptic enrichment of the mantle outside the zone of modal metasomatism poses serious problems for unravelling the origins of the chemical enrichment seen in common peridotite xenoliths that do not show gross mineralogical modification. It is, however, apparent that very few cratonic mantle peridotites from South Africa have escaped such enrichment. This is potentially a major barrier to understanding the chemical evolution of the cratonic lithosphere, particularly when the effects of these later events on isotopic systems such as Re–Os remain unknown. To evaluate the role and nature of the more ancient processes responsible for the formation and modification of the cratonic lithosphere, it is important that the chemical fingerprints of different metasomatic events be better understood.
Aims of the present study
This paper focuses on the major and trace element characteristics of the constituent minerals of garnet ± phlogopite-bearing lherzolite xenoliths from four well-known South African kimberlite localities of the Kaapvaal craton (Bultfontein, Jagersfontein, Monastery and Premier). The intent of this study was to conduct a broad-ranging investigation of the trace element characteristics of garnet lherzolite xenoliths lacking obvious evidence for modal metasomatism so as to assess the complexity of processes that may have affected typical cratonic mantle. In doing so, a number of geochemical similarities to suites of modally metasomatized rocks have emerged, prompting us to explore and highlight the relationships between metasomatic fluids reacting at depth with the peridotitic mantle of the Kaapvaal craton, the phlogopite-rich mafic xenoliths [MARID rocks and the cpx-glimmerite suite of xenoliths renamed PIC rocks by Grégoire et al. (2002)] and the two groups of kimberlites erupted at the surface (Group I and Group II). We mostly focus on the mineral trace element data, because the whole-rock trace element inventory is compromised through interaction with the host kimberlite. We discuss this latter topic at the end of the paper.
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ANALYTICAL METHODS |
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TOPABSTRACTINTRODUCTIONANALYTICAL METHODSSAMPLE LOCATION AND PETROGRAPHYMINERAL COMPOSITIONSWHOLE-ROCK COMPOSITIONSTEMPERATURE AND PRESSURE...DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Samples (50–100 g) from the central parts of xenoliths were ground in an agate mill. Major element compositions of these bulk-rock powders were determined by X-ray fluorescence spectrometry (XRF) at the University of Cape Town [see Duncan et al. (1984)
and the web page http://www.uct.ac.za/depts/geolsci/antech.html for descriptions of methods). The concentrations of 31 minor and trace elements (REE, Cs, Ba, Rb, Th, U, Nb, Ta, Pb, Sr, Zr, Y, Sc, V, Ni, Co, Cu and Zn) were analysed with a Perkin–Elmer Sciex ELAN 6000 ICP-MS instrument at the University of Cape Town. The sample preparation for ICP-MS has been described by le Roex et al. (2001) and Grégoire et al. (2002). Lower limits of detection were 100–500 ppb for Sc, V and Cr, 15–100 ppb for Ni, Cu, Rb and Sr, and 1–15 ppb for all other elements. Average within-run precision (%RSD) was better than 5% for Er, Tm, Yb and Lu, and better than 3% for all other elements. Multiple analyses of the reference standard BHVO-1 yielded results that agree well with recommended values (le Roex et al., 2001). Electron microprobe analyses of minerals were carried out using a fully automated CAMECA Camebax electron microprobe (University of Cape Town), operating at 15 kV accelerating voltage, 40 nA beam current and 10 s/peak, 10 s/background counting times, and natural and synthetic minerals as standards. Nominal concentrations were subsequently corrected using the PAP data reduction method. The complete dataset may be downloaded from the Journal of Petrology website at http://www.petrology.oupjournals.org/.
Concentrations of 29 trace elements (REE, Ba, Rb, Th, U, Nb, Ta, Pb, Sr, Zr, Hf, Ti, Y, Ga, Sc, V and Ni) in silicate minerals (olivine, garnet, orthopyroxene, clinopyroxene and phlogopite) were determined in situ on >120 µm thick polished sections by laser ablation (LA)-ICP-MS at the University of Cape Town. The Perkin–Elmer Elan 6000 ICP-MS instrument was coupled to a Cetac LSX-200 laser ablation module that uses a 266 nm frequency-quadrupled Nd–YAG laser. A typical analysis consisted of three replicates of 100 readings each, with each replicate representing one sweep of the mass range. The counting time for one sample was typically 160–170 s. Every hour three replicates of 100 readings were counted on the carrier gas (argon) alone to establish the background. The NIST 610 and 612 glass standards were used to calibrate relative element sensitivities for the analyses of the silicate minerals. Each analysis was normalized using either Si or Ca values determined by electron microprobe. Typical theoretical detection limits are in the range of 10–20 ppb for REE, Ba, Rb, Th, U, Nb, Ta, Pb, Sr, Zr, Hf, Y, Ga, 100 ppb for V and Sc, and 2 ppm for Ti, Ni and Cr. The typical precision and accuracy for a laser microprobe analysis range from 1 to 10%. Modal compositions were calculated by mass balance based on major element bulk-rock compositions and electron-microprobe analyses of constituent minerals.
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SAMPLE LOCATION AND PETROGRAPHY |
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The 24 studied xenoliths were selected from the collection of J. J. Gurney and the late A. J. Erlank housed at the University of Cape Town. They come from four well-known localities (Bultfontein, Jagersfontein, Monastery and Premier; Table 1) of the South African part of the Kaapvaal craton (Fig. 1). The xenoliths from these four localities were hosted in Group I kimberlites (Fesq et al., 1975
; Kramers, 1977; Kramers et al., 1981; Nixon et al., 1981; Smith et al., 1985). Three of the kimberlites display approximately the same age: Bultfontein 84 ± 0·9 Ma (Kramers et al., 1983), Jagersfontein 85·6 ± 1 Ma (Smith et al., 1985), Monastery 88 ± 4 Ma (Allsopp & Barrett, 1975). In contrast, the Premier kimberlite is much older (1179 ± 36 Ma; Smith, 1983). Some of the samples from Bultfontein and Monastery have previously been studied by Lawless (1978) and Moore (1986), respectively, but not in detail with regard to their whole-rock and mineral trace element compositions.
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The xenoliths are garnet-bearing lherzolites with variable garnet (1–14·5 wt %) and clinopyroxene (0·5–11·5 wt %) abundances (Table 1). Olivine (47–92·5 wt %) and orthopyroxene (5–38 wt %) are the two main mineral constituents of these rocks. Primary phlogopite (0·5–3·5 wt %) in the sense of Erlank et al. (1987)
(i.e. phlogopites not contained in kelyphitic selvages, in serpentinized veins or in melt pools and not appearing as overgrowths on other minerals such as garnet) appears in nine samples (BD2425, BD2379, BD2358, BD2421, rom194, rom302, rom210, rom68 and PR90-6). These samples therefore correspond to the type GPP xenoliths following the classification of Erlank et al. (1987), whereas the phlogopite-free garnet lherzolites correspond to the type GP. Chromite occurs in only two samples (BD2379 and rom377lh111).
Sixteen samples display typical coarse textures (Table 1), whereas the eight other samples show deformed textures corresponding, in the terminology of Harte (1977), to porphyroclastic (BD2308, BD2421, BD2426, JJG 1773, PR89-1 and PR90-9) and mosaic-porphyroclastic textures (JAG90-13 and JAG90-19). Olivine and orthopyroxene of the coarse rocks commonly occur as irregular grains with smoothly curved boundaries. Grain size predominantly ranges between 2 and 7 mm but may be over 1 cm. Garnet typically occurs as large (3–6 mm) rounded to irregular fractured crystals, whereas clinopyroxene tends to occur as discrete, irregularly shaped grains 1–3 mm in size. The latter are commonly associated with garnet, but interstitial crystals of clinopyroxene can also been observed far from any garnet. Primary phlogopite (<3 mm in size but can reach up to 1 cm) occurs as lath-shaped grains that are in textural equilibrium with olivine, pyroxenes and garnet. The deformed xenoliths show a wide range in grain size (0·5–6 mm), which is distinctly bimodal. Large irregularly shaped porphyroclasts of olivine and orthopyroxene (3–7 mm) occur within an assemblage of smaller (<2 mm) polygonal-shaped grains. Clinopyroxene and garnet are commonly found only as porphyroclasts, the latter mineral rounded and rimmed by kelyphite. Only one deformed xenolith contains primary phlogopite (BD2421) present as large, irregularly shaped and deformed crystals (>5 mm).
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MINERAL COMPOSITIONS |
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Clinopyroxene
Clinopyroxene is present in relatively low abundances in the analysed samples, but its composition allows the distinction of two types of garnet lherzolites. This classification is based solely on the clinopyroxene compositions, and the two types of garnet lherzolites include phlogopite-free and phlogopite-bearing samples as well as coarse and deformed samples. Clinopyroxenes of Type 1 garnet lherzolites (samples BD2421, BD2426, JAG90-10, JAG90-11, JAG90-12, JAG90-13, JAG90-19, JJG 1773, rom68, rom194, rom302, PR89-1, PR90-6, PR90-9 and PR90-57) are lower in CaO (mostly Mg-augite: En50·45–58·90 Fs3·40–6·35 Wo35·30–45·70) than those of Type 2 garnet lherzolites (samples BD2308, BD2358, BD2379, BD2425, JAG90-1, JAG90-8, rom198, rom210 and rom377lh111), which are mostly diopsides (Table 2: En48·40–51·15 Fs2·50–4·50 Wo45·40–48·20). Moreover, their Na2O and Cr2O3 contents display a positive correlation with Mg number [= 100 x Mg/(Mg + Fet)], whereas clinopyroxenes from Type 2 lherzolites do not display these correlations (Fig. 2).
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, Type 1 garnet lherzolites; 
, Type 2 garnet lherzolites.The trace element characteristics of clinopyroxene further emphasize the existence of two types of garnet lherzolites (Table 3). Clinopyroxenes from the Type 2 lherzolites display LREE-enriched patterns (Fig. 3e and f), whereas the clinopyroxenes from the Type 1 lherzolites are characterized by convex-upward REE patterns (Fig. 3a–d). Moreover, (Sm/Yb)N and (Ce/Yb)N ratios of the Type 1 lherzolite clinopyroxenes display a positive correlation not observed for the Type 2 clinopyroxenes. The latter are characterized by higher (Ce/Yb)N ratios than those of Type 1 (Fig. 4a). The Type 2 lherzolite clinopyroxenes also have higher (La/Nb)N ratio than clinopyroxenes from Type 1 garnet lherzolites (Fig. 4b and c). Finally, primitive-mantle-normalized trace element patterns of the Type 2 clinopyroxenes are distinctive in their deep negative Nb, Ta, Zr and Ti anomalies (Fig. 3). The Type 1 clinopyroxene trace element patterns also show the deep negative Ti anomalies, but many of them do not display negative Nb–Ta anomalies and have only a slight negative Zr anomaly (Fig. 3).
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Garnet
Garnets from both types of lherzolites are pyrope rich, displaying significant amounts of almandine (Alm8·5–16·5), grossular (Gr9·5–13) and uvarovite (Uv4·5–25·5) components (Table 2). In terms of Cr2O3 variation with CaO, their compositions fall within the field of garnet lherzolites of Boyd et al. (1993)
and are clearly different from those of subcalcic garnet-bearing harzburgites (Fig. 5). The Cr2O3 contents of garnets from both Type 1 and Type 2 lherzolites display a rough positive correlation when plotted against garnet Mg numbers (Fig. 5). Garnets of most Type 2 lherzolites display humped chondrite-normalized REE patterns (Fig. 6e and g), although samples BD2358 and BD2425 have normal REE patterns [low LREE and high middle REE (MREE) and heavy REE (HREE)]. The REE patterns of the Type 1 lherzolite garnets are more variable in shape, with samples displaying normal REE patterns, convex-upward REE patterns (samples BD2426, JAG90-12 and rom68), LREE- and MREE-depleted patterns (sample PR90-57), or slightly humped REE patterns (samples JAG90-11 and PR90-6) (Fig. 6a and c). Garnets from the two lherzolite types display large negative Sr and Ti anomalies when their trace element abundances are normalized to primitive mantle values (Fig. 6), with two of the Type 2 xenoliths containing garnets further characterized by large negative Zr and Hf anomalies (Fig. 6 h; Table 4).
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Orthopyroxene
Orthopyroxenes are Al- and Ti-poor enstatites (Al2O3<1·10 wt % and TiO2<0·30 wt %), with Mg numbers ranging from 91·7 to 93·7 in the Type 2 lherzolites and from 91·7 to 94·5 in the Type 1 lherzolites (Table 2). Orthopyroxenes in the Type 1 lherzolites commonly have higher Na2O abundances than those in the Type 2 (Fig. 5). There is no significant difference in the trace element contents of the orthopyroxene of the two lherzolite types (Table 5). They commonly display significant amounts of Ni (400–700 ppm), Co (30–45 ppm), V (20–55 ppm) and Ti (8–335 ppm), and are very low in other trace elements (Table 5).
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Olivine
The Mg numbers of olivines from Type 1 and Type 2 lherzolites range from 90·45 to 94·0 and from 91·25 to 93·1, respectively (Table 2). Olivines contain significant amounts of Ni (1740–2650 ppm) and Co (70–105 ppm), and low amounts of V (0·4–10 ppm), Sc (0·4–3·5 ppm) and Ti (up to 80 ppm). They also display sometimes very minor Nb (up to 1·25 ppm) and Zr (up to 1 ppm), which may be due to the occurrence of small inclusions. Other trace elements occur at levels near or below the detection limits (Table 6).
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Phlogopite
Phlogopites in the two lherzolite types are highly magnesian and low in TiO2 (Mg number 93·2–94·8 and TiO2 0·05–0·55 wt %; Table 2). They display high Ni contents (940–1495 ppm) and significant amounts of Co (30–40 ppm), V (45–135 ppm), Sr (14–175 ppm), Ga (18–100 ppm) and Zr (2–16 ppm). With the exception of the Premier sample, Rb ranges from 70 to 310 ppm, Ba from 700 to 3800 ppm, Nb from 9 to 35 ppm and Ta from 0·3 to 3·5 ppm. Other trace element abundances are low, with REE contents near or below detection limit (Table 7). The phlogopites of the Premier sample (PR90-6) are lower in Al2O3 (12·4 wt %) and Rb (45 ppm), and higher in Cr2O3 (1·15 wt %), Ba (11 000 ppm), Nb (145 ppm) and Ta (10 ppm) than those of the other samples (Al2O3 13·25–14·75 wt %, Cr2O3 0·65–0·95 wt %).
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WHOLE-ROCK COMPOSITIONS |
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Type 1 and Type 2 garnet lherzolites have similar bulk-rock major element compositions, with CaO/Al2O3 ratios and Mg numbers that range from 0·42 to 1·37 and from 90·4 to 93·6, respectively (Table 8). By comparison with the composition of the estimated primitive upper mantle (CaO 3·23–3·60 wt %; Al2O3 4·0–4·46 wt %; Na2O 0·33–0·66 wt %; Jagoutz et al., 1979
; McDonough & Sun, 1995) the xenoliths are depleted to slightly depleted in basaltic components. In terms of their trace element compositions, the two types do not show significant differences, and display primitive-mantle-normalized trace element patterns characterized by negative Zr and Hf anomalies (Fig. 7 and Table 8).
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TEMPERATURE AND PRESSURE CONSTRAINTS |
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Estimation of the P–T conditions of equilibration of mantle rocks requires equilibrium conditions between constituent mineral phases. In subsequent sections we demonstrate that most of the samples in this study show clear evidence for trace element disequilibrium. However, all the samples display textural equilibrium between garnet, orthopyroxene and clinopyroxene. We therefore assume that major element equilibrium exists between these mineral phases. The P–T estimates are based on the three recent integrated geothermobarometers of Brey & Kohler (1990)
, Taylor (1998) and Nimis & Taylor (2000), and the results are summarized in Table 9. The P–T estimations for the coarse garnet lherzolites range from 610 to 1190°C at 2·0–4·5 GPa, those for the deformed samples from Bultfontein (BD2421, BD2426 and BD2308) range from 920 to 1140°C at 3·5–4·5 GPa, and finally estimations for the other deformed garnet lherzolites from Jagersfontein and Premier range from 1225 to 1370°C at 4·0–6·0 GPa. Temperature calculations using the Brey & Kohler (1990) calibration are always higher than those using the Taylor (1998) and Nimis & Taylor (2000) calibrations, whereas the pressure calculations using the Taylor (1998) calibration give lower results than the Brey & Kohler (1990) and Nimis & Taylor (2000) barometers (Table 9). All the high-T deformed samples, i.e. those from Jagersfontein and Premier, are Type 1 garnet lherzolites. On the other hand, two low-T deformed samples from Bultfontein (BD2421 and BD2426) are Type 1 garnet lherzolites, whereas the third one (BD2308) is a Type 2 garnet lherzolite (Table 9).
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The large database of published values regarding the P–T conditions of equilibration of the garnet peridotites from the Kaapvaal craton clearly shows a distribution into two main P–T regions, representing a low-temperature and a high-temperature group (Boyd, 1973
; Whitelock, 1973; Lawless, 1978; Dawson, 1980; Harte, 1983; Boyd & Mertzman, 1987; Finnerty & Boyd, 1987; Gurney et al., 1991). The bulk of the low-temperature group range from 650 to 1100°C at 2·3–5·0 GPa, whereas the high-temperature group mostly range from 1200 to 1400°C at 5·5–6·5 GPa. The P–T estimations for the coarse garnet peridotites from the four studied localities mostly fall into the range proposed for the low-temperature group, with the deformed samples from Jagersfontein and Premier being more similar to the high-temperature group. In contrast, the deformed samples from Bultfontein fall into the low-temperature group. |
DISCUSSION |
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Evidence for metasomatism
The petrographic and chemical features noted in these samples are typical of previously studied peridotite xenoliths from the Kaapvaal craton in presenting abundant evidence for mantle metasomatism. The most direct evidence is the occurrence of primary textured phlogopite in many samples, a feature widely accepted to result from the infiltration of fluid or melt into solid mantle rock (i.e. mantle metasomatism) (Dawson, 1972
; Boyd, 1973; Cox et al., 1973; Aoki, 1974; Harte et al., 1975; Erlank et al., 1987; Harte, 1987). Further modal evidence for metasomatism is not apparent in the studied sample suite, although we consider below the possibility that clinopyroxene may also be of metasomatic origin.
The trace element compositions of all clinopyroxenes analysed are characterized by strong enrichment in LREE and other incompatible trace elements, and many garnets exhibit the sinusoidal REE patterns inferred to be related to reactions with metasomatic agents (Shimizu & Richardson, 1987; Hoal et al., 1994;Griffin et al., 1999a; Stachel et al., 1999; Van Achterbergh et al., 2001). These incompatible element enrichments occur in all rocks, including those where phlogopite was not noted, suggesting either that its presence has been overlooked in some samples or that trace element enrichment may proceed without the growth of mica. This latter case of ‘cryptic metasomatism’ (Dawson, 1984) may be of widespread occurrence in Kaapvaal xenoliths (Harte, 1987) and is analogous to similar occult enrichment in peridotite xenoliths from basalt (e.g. Frey & Prinz, 1978; Kempton, 1987; Kempton et al., 1999). Because of the ubiquitous trace element enrichment, the presence or absence of primary phlogopite in low-temperature cratonic garnet peridotite may in many cases reflect more the scale of sampling than the metasomatic history of the sample.
Metasomatic clinopyroxene
Texturally equilibrated clinopyroxene of primary appearance in cratonic garnet peridotite may be a constituent of the original solid assemblage residual from partial melting, be exsolved from it upon cooling (e.g. Cox et al., 1987) or of primary metasomatic origin (Erlank et al., 1987; Griffin et al., 1999a; Van Achterberg et al., 2001). If primary phlogopite is accepted as an indicator of modal metasomatism in the studied sample suite, the potential for a metasomatic origin of the clinopyroxene in our samples needs to be discussed. For example, it is evident from the study of Richardson et al. (1985) that mica-bearing garnet lherzolites are prone to garnet–clinopyroxene Nd isotope disequilibrium, suggesting later growth of clinopyroxene. Isotope disequilibrium in cratonic lherzolites was also noted by Günther & Jagoutz (1994).
In this study, many of the clinopyroxenes, notably Type 2, are high in Na2O and Cr2O3, a characteristic attributed to metasomatic origin in similar kimberlitic xenoliths and in numerous mantle xenoliths in basalts (Stiefenhofer, 1993; Yaxley et al., 1998; Grégoire et al., 2000b; Van Achterberg et al., 2001). Furthermore, as noticed by Van Achterberg et al. (2001) for the mantle xenoliths from the Letlhakane kimberlites, our trace element data suggest cases of disequilibrium between garnet and clinopyroxene. Indeed DZrcpx/garnet ranges from 0·1 to 125, whereas equilibrium cpx–garnet pairs (of similar major element composition and equilibrated at similar P–T conditions) have DZrcpx/garnet < 1·9 (Van Achterbergh et al., 1998). Five samples (Group 1: JAG90-11, PR90-6, PR90-57; Group 2: rom198, rom377lh111) have DZrcpx/garnet substantially greater than two and are probably disequilibrium assemblages.
Finally, clinopyroxenes from both Type 1 and Type 2 garnet lherzolites display REE and trace element patterns almost identical to those of clinopyroxenes from PIC and MARID rocks, respectively (Fig. 3). These patterns are repeated in clinopyroxenes from obviously modally metasomatized rocks of the phlogopite wehrlite and PP–PKP type associations (Grégoire et al., 2002). Their appearance in the studied suite of xenoliths is therefore a strong motivation for us to consider the possibility that these clinopyroxenes may also be metasomatic in origin.
Nature and affinities of the metasomatic agents
Both types of garnet lherzolite are rich in elements such as Sr, Ba, Rb, K, Na and LREE (Fig. 3). Of particular note is that they both appear to be depleted in Ti, Nb, Ta, and to a lesser extent Zr and Hf. However, based on the major and trace element compositions and patterns of the clinopyroxenes from the two types of garnet ± phlogopite lherzolites (Type 1 and Type 2), it seems that at least two metasomatic agents have affected the mantle beneath the Kaapvaal craton. The difference in the two types is highlighted in Fig. 8, which shows the magnitude of the Nb anomaly in clinopyroxene plotted as a function of degree of LREE enrichment. It is clear from Fig. 8 that Type 2 clinopyroxene is more depleted in Nb than Type 1 clinopyroxene. Furthermore, Fig. 3 clearly illustrates the similarity between the trace element patterns of the clinopyroxenes from Type 1 garnet ± phlogopite lherzolites and those of the PIC rocks, and between the trace element patterns of the clinopyroxenes from the Type 2 garnet ± phlogopite lherzolites and those of the MARID rocks (Dawson & Smith, 1977; Jones, 1984; Waters, 1987; Grégoire et al., 2002). These similarities suggest genetic links. It is proposed, therefore, that the melts responsible for modal metasomatism of the Type 1 and Type 2 garnet lherzolites have a genetic relationship to melts responsible for the formation of PIC and MARID rocks, respectively. The assemblage of metasomatic minerals crystallized in the two types of garnet lherzolites (clinopyroxene, phlogopite), together with the assemblage of minerals observed in PIC and MARID xenoliths (clinopyroxene, phlogopite, K-richterite, ilmenite, rutile, sulphide), argues for crystallization from silicate melts rather than fluids, as previously proposed by Harte et al. (1993) for a similar range of samples. We therefore propose that the two melt types responsible for the metasomatic imprints observed in the two types of garnet lherzolites studied here are highly alkaline mafic silicate melts.
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, Type 1 garnet lherzolites from Premier; To better assess the nature of these two metasomatic agents, we calculated melt compositions in equilibrium with clinopyroxene in the two types of garnet lherzolites. We used a set of clinopyroxene–alkaline mafic silicate melt partition coefficients, with most elements from the compilation of Chazot et al. (1996)
, the value for Ti from Hart & Dunn (1993) and the value for Ta from Chalot-Prat & Boullier (1997). The results are shown in Fig. 9. The melts in equilibrium with the two types of clinopyroxenes show a similar limited range of compositions. The two melt types display similar positive Ba anomalies and negative Ti anomalies. The only significant differences are the higher LREE contents and strong negative Nb–Ta anomalies of the melts in equilibrium with Type 2 clinopyroxene. The Type 2 melts also have lower HREE abundances than melts in equilibrium with Type 1 clinopyroxene and display negative Zr–Hf anomalies (Fig. 9). These contrasting characteristics indicate either (1) the action of two chemically distinct, genetically unrelated, metasomatic melts that have similar trace element contents, or (2) two metasomatic melts that are genetically related. In case (1) the differences between the two melts could be related to a higher carbonatitic component in the metasomatizing, highly alkaline mafic silicate melt responsible for Type 2 clinopyroxene, in that the higher LREE content and strong negative Nb–Ta–Ti anomaly are features typical of carbonatitic melts (e.g. Ionov, 1998; Yaxley et al., 1998). It should be noted that these features could also reflect the presence of a subduction-derived component in the source of this melt (e.g. Grégoire et al., 2001).
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In case (2) the two melts could evolve from one another by a differentiation process, such as a percolative fractional crystallization process (Harte et al., 1993
). In this case the negative Nb, Ta, Zr and Hf anomalies could be related to the significant crystallization of Ti oxides, such as rutile and ilmenite, during the evolution from melt 1 to melt 2. Both ilmenite and rutile are known to crystallize from highly alkaline mafic silicate systems and to be potential reservoirs for high field strength elements (HFSE) (e.g. Dawson & Smith, 1977; Erlank et al., 1987; Waters, 1987; Grégoire et al., 2000a, 2002). The higher LREE abundances (and slightly lower HREE abundances) of the Type 2 metasomatizing melt could result from a percolative fractional crystallization process (Harte et al., 1993). The variation in LREE/HREE ratio, resulting in crossovers in the MREE range, between the two melts could also be linked to the extent to which melt differentiation has occurred in the presence of garnet (Menzies et al., 1987; Harte et al., 1993). Grégoire et al. (2002) similarly recognized that the magmas parental to the MARID and PIC xenolith suites could relate to one another through a percolative fractional crystallization process. Ionov et al. (2002a) proposed a similar process, chromatographic fractionation during reactive porous melt flow, to explain the trace element characteristics of the two types of mantle spinel peridotite xenoliths from Spitsbergen. They argued that this process could produce a variety of enrichment patterns in a single event. Furthermore, in a complementary isotopic study of the Spitsbergen xenoliths (Ionov et al., 2002b), they showed that such a process could produce a Sr–Nd isotope decoupling.
On the basis of the trace element contents, Rb–Sr and Sm–Nd isotopic compositions of clinopyroxenes, a genetic link has been proposed between the cpx-glimmerite suite of xenoliths renamed PIC rocks by Grégoire et al. (2002) and the Group I kimberlites (Jones, 1984; Grégoire et al., 2002) and between the MARID rocks and the Group II kimberlites (Sweeney et al., 1993; Grégoire et al., 2002). Grégoire et al. (2002) argued that PIC rocks and MARID rocks are deep-seated segregations from highly alkaline melts genetically linked to Group I and Group II kimberlite magmas, respectively. We therefore propose that clinopyroxenes of Type 1 garnet lherzolites have crystallized from, or been completely equilibrated with, metasomatic agents related to PIC parental magmas and Group I kimberlite magmas. In many cases, where trace element disequilibrium between clinopyroxene and garnet is preserved, this clinopyroxene may be entirely new, i.e. have grown in a harzburgitic protolith, as proposed by Van Achterbergh et al. (2001) for the Letlhakane samples. The PIC rocks, like Group I kimberlites, are characterized by positive 
Nd values (Jones, 1984, 1987) and therefore clinopyroxenes that are demonstrably linked to this source but are LREE enriched (as all indeed are) must be substantially younger than their Archean peridotite host rocks.
Clinopyroxenes in Type 2 lherzolites have metasomatic trace element compositions that display similarities to clinopyroxene from MARID rocks. They have crystallized from, or been completely equilibrated with, metasomatic agents linked to MARID-parental magmas and Group II kimberlite magmas. The published ages for Group II kimberlite magmatic activity range from 110 to 156 Ma with one locality at 
200 Ma (Smith et al., 1985; Allsopp et al., 1989). It may, therefore, be significant for the proposed Type 2 garnet lherzolite clinopyroxenes–MARID rocks–Group II kimberlites link that Type 2 clinopyroxenes have not been identified in xenoliths from the mid-Proterozoic Premier kimberlite. However, this cannot be regarded as strong evidence, because of (1) the limited number of samples from Premier (four) and (2) indications that the mantle beneath Premier is compositionally anomalous (Harte, 1983; Carlson et al., 1999). The metasomatic process may give rise to local refertilization (harzburgite to lherzolite), but its importance cannot be assessed without specific indication of new mineral growth as discussed above. Whether new material is actually added to the continental lithosphere in this process is dependent on whether or not the MARID-type melts (and for that matter Group II kimberlites) derive from sources external to the lithosphere (e.g. le Roex, 1986).
Trace element residence sites
It is well known that kimberlite-hosted xenoliths contain higher levels of incompatible trace elements than their combined mineral inventories would suggest. It has been established that this material is situated along grain boundaries and in mineral fractures (Fraser et al., 1984; Richardson et al., 1985). The introduction of such material may occur at several stages, including interaction with kimberlite-derived fluids before, during, and after eruption, as well as in subsequent meteoric–hydrothermal serpentinization events. This poses particular problems for determining the pre-eruption whole-rock chemical and isotopic compositions of kimberlite-hosted xenoliths, and suggests that the most accurate method is recombination of mineral analyses according to modal proportions. For example, it has proved very difficult to ascertain, by direct measurement, the concentration of heat-producing elements (K, U, Th) in cratonic peridotite, thus rendering uncertain an important constraint on mantle thermal structure (Rudnick et al., 1998). In some cases this problem extends to major elements such as Ca and Fe, making it difficult to calculate mineral proportions from bulk analysis (Boyd et al., 1997). Our analysis of a large range of major and trace elements in both whole rock and all constituent primary minerals allows a quantitative assessment of these budgets.
Eight of the freshest samples, four of the Type 1 garnet lherzolites (BD2426, JJG 1773, rom68 and PR89-1) and four of the Type 2 garnet lherzolites (BD2308, BD2425, JAG90-1 and rom198) were compared in this way. Only those trace elements for which there were a maximum number of values above the detection limit for the bulk rock and the individual constituent minerals were considered, to minimize the possibility of analytical errors. All eight samples have bulk-rock concentrations of the first-row transition element (Ni, Sc) and HREE (from Ho to Lu) that can be readily explained by the trace element contents of their constituent minerals. All other trace elements display significant discrepancies between calculated and measured bulk-rock trace element compositions (Fig. 10). Discrepancies are especially evident for the most incompatible trace elements, with >90% of the Ba, Sr (except for samples BD2308 and BD2425), Nb (except for sample rom198) and LREE residing outside the primary minerals in most samples. It is noteworthy that samples containing phlogopite (BD2425 and rom68), which is a potential reservoir for Ba, Sr and Nb, display the same discrepancies as the phlogopite-free samples (Fig. 10).
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We conclude that a significant budget of the most highly incompatible trace elements in these garnet ± phlogopite lherzolites is located in cracks, along grain boundaries, in secondary minerals and/or in fluid inclusions, as suggested by previous studies (Ehrenberg, 1982
; Fraser et al., 1984; Richardson et al., 1985; Erlank et al., 1987). In contrast, the budget of the first-row transition elements and the HREE may be fully accounted for by the constituent minerals and may be more reliably used to constrain the petrogenetic processes that affected the garnet lherzolites before their entrapment by the host kimberlite. Establishing the origin and timing of grain boundary enrichment in the present samples is beyond the scope of the present study, but may be approachable with isotopic leaching experiments. Figure 11, however, demonstrates that, at least for two samples from Bultfontein (BD2308 and BD2421), the incompatible trace elements have been incorporated in a process related directly to the host kimberlite in that the shapes of the normalized trace element patterns of the two samples are virtually identical. Schmidberger & Francis (2001) have proposed the same contamination process by kimberlitic liquids to explain the excess LREE abundances observed in some garnet peridotites from the Nikos kimberlite pipe (Somerset Island, Canadian Arctic).
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONANALYTICAL METHODSSAMPLE LOCATION AND PETROGRAPHYMINERAL COMPOSITIONSWHOLE-ROCK COMPOSITIONSTEMPERATURE AND PRESSURE...DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Major and trace element characteristics of clinopyroxenes from garnet lherzolite xenoliths from the South African part of the Kaapvaal craton point to a metasomatic origin and allow the recognition of two distinct types. Clinopyroxenes of the Type 1 lherzolites have trace element similarities to PIC rocks (Grégoire et al., 2002
), and appear to have crystallized from, or been completely equilibrated with, a metasomatic melt with a genetic relationship to Group I kimberlites. Clinopyroxenes of Type 2 lherzolites have metasomatic trace element compositions that display similarities to clinopyroxene from MARID rocks. They are interpreted to have crystallized from, or have completely equilibrated with, a metasomatic agent genetically linked to Group II kimberlites. A significant budget of the most highly incompatible trace elements in the analysed bulk-rock garnet ± phlogopite lherzolites is located in cracks, along grain boundaries, in secondary minerals and/or in fluid inclusions, as suggested by previous studies. The recalculated trace element budget based on individual mineral analyses combined with their modal abundances is more reflective of the original composition.
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SUPPLEMENTARY DATA |
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TOPABSTRACTINTRODUCTIONANALYTICAL METHODSSAMPLE LOCATION AND PETROGRAPHYMINERAL COMPOSITIONSWHOLE-ROCK COMPOSITIONSTEMPERATURE AND PRESSURE...DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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Supplementary data for this paper are available on Journal of Petrology online.
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ACKNOWLEDGEMENTS |
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This work has been made possible by the assistance and expertise of A. Späth, F. Pocock, I. Wilson, E. Stout, D. Wilson and P. J. le Roux. John Gurney is gratefully thanked for making the samples available from his mantle collection. Reviews by D. Francis, M. Kopylova and T. Gasparik, and editorial comments by P. Kempton are highly appreciated. Financial support was provided by the South African National Research Foundation, THRIP and the University of Cape Town.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONANALYTICAL METHODSSAMPLE LOCATION AND PETROGRAPHYMINERAL COMPOSITIONSWHOLE-ROCK COMPOSITIONSTEMPERATURE AND PRESSURE...DISCUSSIONCONCLUSIONSSUPPLEMENTARY DATAREFERENCES |
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