Journal of Petrology | Volume 39 | Number 11-12 | Pages 1931-1941 | 1998
© Oxford University Press 1998
Trace Element Composition of Mantle-derived Carbonates and Coexisting Phasesin Peridotite Xenoliths from Alkali Basalts
Gemoc, School of Earth Sciences, Macquarie University Sydney, 2109 N.S.W., Australia
Received September 30, 1997; Revised typescript accepted May 21, 1998
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ABSTRACT |
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The trace element compositions of carbonate, clinopyroxene, amphibole and silicate glass were determined in four mantle lherzolite xenoliths in alkali basalts from Spitsbergen and Mongolia by laser ablation ICP-MS. Carbonates in the xenoliths occur in fine-grained pockets that appear to have been produced by reaction of carbonate-rich melts with the host peridotites. The carbonates are rich in Sr and Ba, but have low contents of rare earth elements. (Na,Al)-rich silicate glass commonly associated with the carbonates is a major host for many incompatible lithophile elements in the carbonate-bearing pockets. The carbonates in the xenoliths do not appear to represent quenched carbonatite liquids but probably are crystal cumulates from carbonate-rich melts. Trace element patterns estimated for liquids that may have produced the carbonate-bearing pockets are consistent with general characteristics of carbonate-related metasomatism (enrichments in light rare earth elements, Th, U, Ba and negative anomalies for high field strength elements). However, the absolute incompatible element abundances estimated for those liquids cannot provide the extremely strong enrichments invoked by some models of carbonate mantle metasomatism. Clinopyroxene and amphibole outside the carbonate-bearing pockets in the xenoliths from Spitsbergen have high contents of incompatible trace elements, indicating that the lherzolites also experienced metasomatic enrichment before the formation of the carbonates.
KEY WORDS: carbonate; clinopyroxene; LAM–ICP-MS; mantle metasomatism; (Na,Al)-rich silicate glass
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Introduction |
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TOPABSTRACTIntroductionAnalytical TechniquesSample DescriptionTrace Element DataDiscussionReferences |
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Carbonates may be an important accessory component in the mantle. They have been found in mantle peridotite, pyroxenite and eclogite xenoliths as texturally equilibrated or interstitial grains (Wass, 1979
), ‘globules’ in association with silicate glass (Amundsen, 1987; Ionov et al., 1993, 1996; Pyle & Haggerty, 1994; Kogarko et al., 1995; Norman, 1998) and components of fluid microinclusions (Frezzotti et al., 1994; Schiano & Clocchiatti, 1994). There is unambiguous experimental evidence that carbonates are stable in peridotitic assemblages at appropriate temperatures and pressures, and that small-degree partial melting of carbonated peridotite at pressures 
2 GPa produces carbonatitic liquids (Wyllie, 1987; Dalton & Wood, 1993a; Lee & Wyllie, 1998). Interaction of carbonate melts with the lithospheric mantle has been invoked to explain the formation and compositional characteristics of some metasomatized mantle peridotites (Wallace & Green, 1988; Yaxley et al., 1991; Dautria et al., 1992; Hauri et al., 1993; Ionov et al., 1993; Rudnick et al., 1993).
Little is known about trace element composition of carbonates in mantle rocks and of primary carbonate-rich liquids generated in the mantle. The models relating mantle metasomatism to carbonate-rich melts usually assume that these melts are similar in trace element composition to carbonatites exposed in the crust and, in particular, that such melts should be strongly enriched in incompatible trace elements. Unfortunately, crustal carbonatites show a wide range in chemical compositions and few workers believe that carbonatites reflect liquid compositions. Experimental data on element partitioning between immiscible silicate and carbonate liquids have also been used to assess the composition of mantle carbonates and primary carbonate-rich melts. It is not clear, however, whether such data are relevant in this case because liquid immiscibility may not have played an important role in their formation (Lee & Wyllie, 1998). Direct data on trace element composition of mantle carbonates and related minerals in natural mantle rocks may provide important evidence for models of trace element enrichment by ‘carbonate’ metasomatism and generation of parental carbonate liquids.
Ionov et al. (1993) provided analyses of whole-rock carbonate-bearing peridotite xenoliths from Spitsbergen and of acid leaches from these rocks done by solution inductively coupled plasma mass spectrometry (ICP-MS). They found enrichments in light rare earth elements (LREE) and Sr relative to heavy rare earth elements (HREE) and high field strength elements (HFSE) in plots normalized to their abundances in chondrites or primitive mantle (PM), and concluded that these trace element signatures are characteristic of the carbonate material in the xenoliths Ionov et al. (1996) reported high contents of Sr and Ba in carbonates and S in clinopyroxene in these xenoliths determined in situ by proton microprobe analyses. The present work is based on laser ablation ICP-MS analyses for a large number of trace elements in carbonates and coexisting silicate minerals in mantle peridotite xenoliths in alkali basaltic rocks from Spitsbergen and Mongolia. This study examines the trace element compositions of mantle carbonates and an attempt is made to constrain the composition of mantle-derived carbonate-rich liquids.
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Analytical Techniques |
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TOPABSTRACTIntroductionAnalytical TechniquesSample DescriptionTrace Element DataDiscussionReferences |
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All samples were analysed by laser ablation microprobe (LAM)–ICP-MS (Jackson et al., 1992
) in 200 µm thick polished rock sections. The ICP-MS instrument is a Perkin–Elmer Sciex ELAN 5100 coupled with a UV (266 nm) laser. The laser was operated with 1 mJ/pulse energy and 4 Hz frequency for silicate minerals, and 2 Hz frequency with the laser beam focused above the sample surface for carbonates and silicate glass. Spot diameter for these analyses i 30–50 µm. NIST 610 glass was used as a calibration standard for all samples, with 44Ca as an internal standard. Detection limits for most elements in clinopyroxene and amphibole were 
0.1 ppm. For carbonate and silicate glass, detection limits were higher when a low-energy, defocused beam was used. Analytical precision is 
5% at the ppm lev. Details of ICP-MS and laser operating conditions have been published by Norman et al. (1996) and Norman (1998). Nonmatrix-matched calibration standards have been successively used for quantitative carbonate analysis by Feng (1994), who employed the NIST 612 silicate glass as a calibration standard to determine Sr, Y, Ba and REE in calcite and dolomite.
In-run signal intensity for indicative major and minor elements was monitored during analysis of small mineral grains and glass pockets (Fig. 1a and b) to make sure that the laser beam stayed within the grain or phase selected. Ablation of carbonates was aborted when signal intensity for Al, Si, Cr or Ni increased well above background levels, and carbonate analyses with considerable levels of these elements were discarded. Rb and Ba were monitored for small clinopyroxene grains to check for contamination by silicate glass. Estimates of major element contents (Si, Al, Mg) obtained by LAM–ICP-MS agree well with electron microprobe (EMP) analyses of the same minerals. There is also a good agreement between LAM–ICP-MS data in this work and proton probe (PIXE) analyses for Sr, Ba, Y and Zr obtained earlier for Spitsbergen xenoliths 4-36-90 and 21–6 (Ionov et al., 1996). Average contents of Sr, Y and Zr determine by PIXE in primary and carbonate-related clinopyroxene in the same samples are largely within 15% of corresponding LAM–ICP-MS data. Some differences may be due to significant chemical zoning and grain-to-grain variations in carbonate and carbonate-related clinopyroxene (Ionov et al., 1996).
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Sample Description |
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Trace element data were obtained for three spinel lherzolite xenoliths from NW Spitsbergen an one spinel lherzolite xenolith from SE Mongolia (Dariganga). Table 1 gives a summar of petrography, mg–number [mg-number = Mg/(Mg + Fe)at] of olivine, Cr2O3 content of spinel and estimates of equilibration temperatures for these samples. Detailed data on petrography and major element composition of the Spitsbergen xenoliths (including the samples in this study) have already been published (Ionov et al., 1996
). The xenoliths are coarse- to medium-grained rocks with microstructures ranging from coarse protogranular to mosaic equigranular. Typical grain sizes range from 2–6 mm for olivine and orthopyroxene to 1–2 mm for clinopyroxene and spinel. Xenolith 4-90-9 contains rare, small (0.5–1 mm) amphibole grains. Amphibole is more abundant and its grains are larger in xenolith 21-6, which also contains rare equant grains of apatit 0.1–0.5 mm in size.
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Carbonates analysed in this work occur in fine-grained pockets replacing minerals of host peridotite, most commonly spinel, orthopyroxene and amphibole (Ionov et al., 1996
). These pockets also contain small (<100 µm) euhedral grains of clinopyroxene and olivine, Fe–Ni sulphides and (Na,Al)–rich silicate glass with microphenocrysts of Cr-rich spinel (Fig. 1). The pockets typically make up 2–3% of the rock and are 0.2–2 mm in size. Pyroxenes, spinel and amphibole commonly have embayed, resorbed outlines and spongy reaction zones where in contact with the fine-grained pockets (Fig 1c). The carbonates in the pockets occur as mosaic aggregates with smooth curvilinear boundaries made up of interlocking grains 20–50 µm in size; these aggregates contain no silicate or oxide minerals (Fig. 1b and c). Carbonate in xenolith BY-44 from SE Mongolia occurs in fine-grained pockets and also as interstitial grains texturally equilibrated with the host lherzolite.
Average or representative major element compositions of minerals and silicate glass analysed for trace elements are given in Table 2. The carbonates range from dolomite to more common Mg-bearing calcite and have high mg–numbers (0.90–0.99). The fine-grained clinopyroxene, olivine and spinel in the carbonate-bearing pockets differ in major element composition from the same minerals in the peridotite host. In particular, olivine in the pockets has higher mg-numbers and Ca contents, and clinopyroxene is typically ver rich in Al, Cr and Ti, and has low Na contents. These minerals are referred to here as second generation or carbonate related, in contrast to coarse, ‘primary’ (in the sense that they existed in the rock before the formation of the fine-grained pockets) olivine and clinopyroxen in the texturally and chemically equilibrated host lherzolite. The carbonate and glass pockets i the xenoliths are sharply terminated at contacts with host lava. The (Na,Al)-rich silicate glass in the xenoliths is very different from the glass in the host basaltic rocks in terms of colour, phenocryst assemblage and chemical composition (Ionov et al., 1993).
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Many carbonate aggregates are altered. They contain small euhedral magnesite crystals and dar cryptocrystalline material replacing the granular carbonates. Target areas for laser analyses were selected to avoid turbid areas near contacts of the carbonate aggregates; clear cores of large segregations (Fig. 1a) were analysed in most cases. Some xenoliths and their host lavas also contain globules or patches of ankerite–magnesite carbonates commonly associated with saponite clay filling fractures and vesicles (Treiman et al., 1998
). These ar rare in the xenoliths analysed in this work, but nevertheless their presence indicates interaction of CO2-bearing groundwater with the xenoliths. The preservation of fresh silicate glass and Fe–Ni sulphides in the carbonate-bearing pockets appears to indicate that such interaction did not significantly affect their composition. |
Trace Element Data |
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Results of the LAM–ICP-MS analyses are given in Table 3. The carbonates have low contents of heavy and intermediate REE and Y; these are much lower than in clinopyroxenes (Figs 2 and 3). The shapes of REE patterns in the carbonates cannot be fully established because HREE values in most cases are close to or below detection limits. However th values above detection limits suggest a general increase in PM-normalized values from HREE t LREE with conspicuous inflections for La. The carbonates in xenolith 4–36–90 (for which the largest number of individual analyses has been carried out) show a range of REE and Sr contents (Fig. 2). The majority of analyses have relatively low values of REE and Y and moderate Sr (<2000 ppm). Some analyses have yielded much higher contents of Sr reaching 2.4 wt % and noticeably higher levels of REE, with enrichment in LREE. Ba appears to show a positive correlation with Sr in the carbonates, but this is not the case for LREE in samples 4–36–90 and 21–6. The highest contents of La and Ce are not in analyses with highest Sr values; these high La and Ce contents cannot be due to contamination by silicat glass as Rb and Nb in these analyses are below detection limits or very low (Table 3).
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The data for samples 4–36–90 and 4–90–9, which host large and clear carbonate aggregates, indicate that Sr and Ba are the only elements among those analysed that are enriched in the carbonates relative to clinopyroxene (Figs 2 and 3). In most carbonate analyses Sr ranges from 1300 to 2600 ppm. Some carbonates, mostly in xenoliths 21–6 and BY–44, yielde moderate to high values of Pb and U, with very high U/Th ratios. The high Pb and U values are erratic and do not correlate with Sr, Ba or REE. Carbonate pockets in these xenoliths are relatively small, and selecting an unaltered spot for analyses was not always possible. These value should be regarded with caution because Pb and U are mobile at the Earth's surface and may be due to secondary alteration along cracks and grain boundaries.
Primary clinopyroxene in the Spitsbergen xenoliths is enriched in LREE relative to intermediate REE and HREE in PM-normalized plots (Figs 2 and 3). REE patterns o amphibole in xenoliths 4–90–9 (Fig. 3) and 21–6 are nearly parallel to those of coexisting primary clinopyroxene at slightly higher levels. Carbonate-related clinopyroxene in the pockets has higher contents of HREE and intermediate REE and Y, and lower contents of Ce, La, Th and U than primary clinopyroxene (Figs 2 and 3). Another significant difference is that primary clinopyroxene has strong negative Ti anomalies in PM-normalized abundance patterns whereas Ti contents in the carbonate-related clinopyroxene are higher and show no major anomalies relative to adjacent REE. On the other hand, both generations of clinopyroxene have negative anomalies of Zr, Hf and Nb. Silicate glass in carbonate-bearing pockets also has negative anomalies of Zr, Hf and Nb, but their magnitudes are much lower than for the clinopyroxenes. The contents of moderately incompatible REE and Y in the silicate glass are not very different from those in the clinopyroxenes, but the silicate glass is strongly enriched in highly incompatible elements (Figs 2 and 3).
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Discussion |
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TOPABSTRACTIntroductionAnalytical TechniquesSample DescriptionTrace Element DataDiscussionReferences |
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Two stages of metasomatic alteration in the xenoliths
High Sr contents (>200 ppm) and LREE enrichment in coarse clinopyroxene from amphibole-free xenolith 4-36-90 indicate cryptic metasomatism. Amphibole occurring in the two other xenoliths from Spitsbergen is texturally equilibrated with the coarse olivine and pyroxenes in the lherzolites. Roughly parallel REE patterns in amphibole coexisting with coarse clinopyroxene in xenoliths 4-90-9 (Fig. 3) and 21–6 as well as high content of Sr, Th and U in both minerals indicate that the lherzolites experienced metasomatic enrichment in which intermineral chemical equilibrium was attained (Ionov et al., 1997
). Enrichment took place before the formation of the carbonate-bearing pockets, because these pockets replace the coarse, texturally and chemically equilibrated minerals of the host peridotites (including amphibole and clinopyroxene). Chemical compositions of clinopyroxene in the fine-grained pockets and in host lherzolites are distinct. The trace element patterns of second-generation clinopyroxene seem to be consistent with its derivation from magmatic liquids controlled by cpx–liquid partition coefficients that are very low for highly incompatible elements. Major element zoning in carbonate-related clinopyroxene and significant grain-to-grain variations in major and trace element composition of carbonates and carbonate-related clinopyroxene may indicate lack of chemical equilibrium in the fine-grained pockets.
The nature of the carbonates and their parental liquids
Evidence for mantle origin of the carbonates
A major argument for the mantle origin of the carbonates in these xenoliths is their common association withsecond-generation clinopyroxene and olivine, and silicate glass that have distinctive textural characteristics and chemical compositions inconsistent with derivation either fro host basaltic magma or by post-eruptive alteration (Ionov et al., 1996). The mg-numbers and Ca/(Ca + Mg) ratios in the carbonates generally match those obtained experimentally for carbonates coexisting with peridotites in the P–T stability field of spinel lherzolites (Dalton & Wood, 1993b). Post-eruption alteration may precipitate carbonates in vesicles in basalts and also in xenoliths. However, the secondary carbonates (commonly pure calcite) are deposited in xenoliths mostly along grain boundaries and cracks in minerals, and are accompanied by products of secondary alteration of olivine and orthopyroxene. This is not the case for the xenoliths studied in this work. The carbonate-bearing pockets typically show fresh olivine, pristine silicate glass (which is particularly prone to secondary alteration) and sharp boundaries of carbonate aggregates (Fig. 1b and c). Fe–Ni sulphides commonly associated with the carbonates (Fig. 1c) show no signs of secondary alteration; their compositions are typical of primary sulphides in mantle xenoliths (Dromgoole & Pasteris, 1987). In some cases (Fig. 1d), second-generation olivine and clinopyroxene grains are not in direct contact with the wall of the pockets and ‘hang’ in the carbonate mass. These features (as well as optical continuity of adjacent clinopyroxene grains in the pockets) indicate metasomatic precipitation of carbonates and associated second-generation minerals, and are not consistent with secondary, post-eruption origin for carbonates filling empty vugs in the xenoliths. On the other hand, the extent and possible effects of post-eruption interaction of the carbonates with circulating groundwater is hard to assess from our data.
Fractionation crystallization vs liquid immiscibility
The low alkali contents, high mg-numbers and Ca/(Ca + Mg) ratios in the carbonate pockets from this study appear to be consistent with their origin as crystal cumulates from carbonate-rich liquids rather than as quenched melts. The origin of the textural relationships between the carbonate globules and silicate glass remains enigmatic. Round aggregates o calcite (ocelli or globules) associated with silicate glass were found in some mantle xenoliths and interpreted as representing immiscible carbonate liquids (Pyle & Haggerty, 1994; Kogarko et al., 1995). Carbonate-bearing pockets in Spitsbergen xenoliths show similar textures that are commonly taken as evidence for origin by liquid immiscibility: sharp curvilinear boundaries of the carbonate aggregates and round shapes of small carbonate ‘spherules’ in the silicate glass (Fig. 1b and c). However, recent experimental studies of relevant model systems have shown that the compositions of th calcite globules and silicate glass bear no relationship to the experimentally determined miscibility gap, and that silicate–carbonate liquid immiscibility is unlikely to be encountered durin mantle melting processes (Lee & Wyllie, 1997, 1998). Lee & Wyllie also noted that through a wide range of compositions, round calcite grains coexisting with silicate or silicate–carbonate liquids were in fact crystalline calcite at experimental conditions, and not immiscible liquids. Other experimental work has also suggested that carbonate-ric liquids generated in the mantle should have sodic dolomitic compositions with alkali contents varying according to the alkali content of the source material (Wallace & Green, 1988; Dalton & Wood, 1993a; Sweeney, 1994; Yaxley & Green, 1996).
Estimates of trace element composition of carbonate-rich melts
The textural, major element and trace element data suggest that the carbonates precipitated from carbonate-rich melts that intruded the lherzolites shortly before their transport to the surface as xenoliths (e.g. because they contain silicate glass). Reaction of the initial melts wit host lherzolites produced fine-grained pockets with wehrlitic mineral assemblages, interstitial silicate melt, and a range of carbonate compositions. In such a model, the carbonate pockets ma represent either quenched fractionated liquids or products of fractional crystallization from such liquids. We believe that the carbonates in the xenoliths crystallized from a coexisting carbonate-rich liquid, some of which might have escaped, although it is not known in which proportion.
The trace element composition of the residual liquid, which presumably escaped, could be estimated using mineral-melt partition coefficients (D), but these are poorly known for carbonates. Green et al. (1992) reported experimentally determined D values between dolomite and coexisting carbonatitic melt (Sr 0.37, Ba 0.02, Y 0.28, La 0.12). Estimates for a hypothetical liquid that could have coexisted with carb- and carb-2 compositions (Table 3) in sample 4-36-90 based on these D values are within the abundance range in carbonatites (Nelson et al., 1988; Simonetti et al., 1997) for Sr (0.5–2.2%), but appear to be too high for Ba (0.7–2.5%) and too low for Y (1–2 ppm) an La (3–21 ppm) (Fig. 4). The poor match for Ba, Y and La estimates may be due to uncertainties in the D values or the use of dolomite D values for magnesian calcites or may indicate that the assumption of mineral–melt equilibrium is not valid.
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Alternatively, one could assume that the carbonate-bearing pockets formed from batches of a silicate-bearing carbonate-rich liquid trapped in peridotite and that the trace element composition of that liquid can be roughly estimated by combining those of the carbonate (carb), fine-grained clinopyroxene (cpx) and silicate glass that make up the pockets. The estimates of the hypothetical trapped liquid compositions calculated for xenolith 4-36-90 are illustrated in Fig 4; these estimates are made for carb/cpx/glass ratios of 80:10:10, 60:20:20 and 40:20:40 that largely cover the range of their variations in that sample. Carbonate aggregates with little or no associated silicate glass and phenocrysts are rare and have not been analysed i this work because of their small size. The contents of REE, Sr, Ba, Th and alkalis in olivine and spinel are much lower than in coexisting silicate glass and clinopyroxene (Ionov et al., 1995
; Eggins et al., 1998; Norman, 1998) and can be neglected. All these estimates show moderate enrichments in LREE, Th, U and Rb, positive anomalies for Sr, Ba and Pb, and negative anomalies for Zr, Hf and Nb in PM-normalized plots. The abundances of all elements except Sr in the estimates are proportional to the weight fraction of the silicate glass. The lead anomaly is specific to this sample and does not show up in analogous estimates for xenolith 4-90-9. These estimates have very large uncertainties and cannot be taken as real compositions of parental liquids. The calculations assume that no residual liquid escaped, which probably is not realistic. Textural evidence shows that the carbonate-rich melt reacted with minerals of the host peridotite whose trace element inventory must have contributed to the composition of the carbonate-bearing pockets, but that contribution is hard to define quantitatively. In particular, the above calculations assume that the silicate glass in the pockets is a differentiation product of a parental carbonate-rich melt containing a silicate component, but it may also be produced by the reaction of trapped carbonatite melt with host peridotite. It cannot be ruled out as well that the trace element composition of the carbonates in the xenoliths has been modified by interaction with groundwater after the eruption.
It is not clear what relation these estimates illustrated in Fig. 4 may have to parental liquids of carbonatites. They show some features that are common in crustal carbonatites, such as enrichments in incompatible trace elements and negative HFSE anomalies, but the levels of LREE, Th and U in the carbonatites are at least an order of magnitude higher (Nelson et al., 1988; Bell, 1989). Experimental studies (e.g. Lee & Wyllie, 1997) suggest that parental melts for many carbonatites can be derived from the mantle. Sr–Nd–Pb isotope data (Bell & Simonetti, 1996; Kalt et al., 1997; Harmer & Gittins, 1997) indicate that parental magmas of carbonatites were produced from mantle sources, probably within heterogeneous lithospheric mantle. However, the compositions of these parental liquids and their evolution paths to produce carbonatites that occur in the crust have not yet been well defined.
Mantle carbonates and metasomatic trace element enrichment
Figures 2 and 3 show that carbonates are major hosts for Sr and Ba in th carbonate-bearing pockets, but most other lithophile elements largely reside in (Na,Al)-rich silicate glass and to a lesser extent in carbonate-related clinopyroxene. Because the carbonates probably are products of crystal fractionation from a parental carbonate-rich liquid that has not been preserved in the xenoliths and are not related to the silicate glass by liquid immiscibility our data cannot be used to estimate partition coefficients between immiscible carbonate and silicate liquids or between carbonate and silicate minerals. Experimentally determined partition coefficients between carbonatite and silicate minerals (see Hamilton et al., 1989; Green et al., 1992; Jones et al., 1995) may no be relevant to the trace element composition of the silicate minerals obtained in this work. Nevertheless, our data may indicate that a large proportion of incompatible trace elements in carbonate-bearing mantle peridotites in the P–T field where carbonates (magnesia calcite, dolomite, magnesite) are stable reside in related accessory silicate and other mineral rather than in the carbonates themselves.
Carbonate-rich mantle melts have been invoked to explain metasomatic enrichments observed in many mantle xenoliths. This study has demonstrated enrichments in incompatible trace elements in the carbonate-bearing pockets, but their net contribution to the trace element composition of the host peridotites is hard to examine because the peridotites experienced earlier enrichment events (which may have been related to carbonate-rich melts or fluids as well). Overall, the trace element patterns estimated for the parental carbonate-rich liquids that produced the carbonate-bearing pockets are consistent with general characteristics of carbonate-related metasomatism formulated earlier (Green & Wallace, 1988; Yaxley et al., 1991; Ionov et al., 1993; Rudnick et al., 1993). In particular, it shows relative enrichments in LREE, Th, U and Ba, and negative anomalies for HFSE in PM-normalized plots. However, the absolute contents of incompatible elements estimated for those liquids are hardly adequate to provide the extremely strong enrichments invoked by some models, in particular for LREE. Enrichment in Sr is present in many metasomatized xenoliths, but its magnitude usually is less than could be imposed by such liquids. We emphasize that the estimates o trace element abundances in hypothetical carbonate-rich liquids that may have produced the carbonate-bearing pockets in the limited number of xenoliths studied in this work have large uncertainties and their absolute values cannot be taken directly as model compositions for carbonate-rich metasomatic media in the mantle.
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Acknowledgements |
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We thank Yu. Genshaft and M. Kopylova for contributing samples for this study; N. Pearson, A Sharma and M. Norman for expert help with analytical work; and S. Jackson, W. L. Griffin and S Y. O'Reilly for comments. Constructive reviews by K. Bell, F. Frey, C. Petibon and an anonymous reviewer are appreciated. This research was supported by ARC Research Fellowship, ARC Large Grant and ARC Small Grant to D. Ionov. This is Publication 115 in the ARC National Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC).
* Telephone: (61-2) 98508378. Fax: (61-2) 98508428. e-mail: dmitri.ionov{at}mq.edu.au
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