Journal of Petrology | Volume 39 | Number 11-12 | Pages 1895-1903 | 1998
© Oxford University Press 1998
The Case for Primary, Mantle-derived Carbonatite Magma
1 Council for Geoscience Private Bag X112, Pretoria 0001, South Africa
2 Geology Department, University of Toronto Toronto, Ont., M5S 3B1, Canada
Received September 30, 1997; Revised typescript accepted June 16, 1998
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ABSTRACT |
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 TOP ABSTRACT IntroductionIsotopic EvidenceDiscussionConclusionsReferences |
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There is much debate about whether carbonatite magmas are derived in ‘secondary’ fashion through the advent of liquid immiscibility operating in the crust on evolved nephelinitic magma, or whether they are derived in the mantle by direct partial melting of a carbonated peridotite. This paper briefly summarizes the
Sr–Nd data fo carbonatites in general and evaluates the isotopic relationships between carbonatites and alkaline silicate rocks in several well-studied complexes from Africa. Available data for carbonatites younger than 200 Ma have a range in Sr–Nd that is less than that found in oceanic basalts despite the fact that carbonatites traverse lithospheres that ar much more complex than those in the oceans. By contrast, for the Napak, Kerimasi, Shombole, Dorowa, Shawa and Spitskop complexes the alkaline silicate rocks show greater variability and have more enriched Sr–Nd (higher Sr, lower Nd) values than their associated carbonatites. In general, the carbonatites hav isotopic compositions that are closer to the more primitive silicate rocks, such as melilitites a olivine nephelinites, than to more evolved nephelinites and phonolites. In the case of the Napak Complex the enriched component was introduced from the lower crust whereas for the Dorowa and Shawa complexes of SE Zimbabwe, the component was derived from the sub-continental lithospheric mantle. These relationships indicate that the carbonatites must have existed as discrete magmas i themantle and argue against a derivation by liquid immiscibility in the crust. Although a contrast i isotopic composition does not rule out an immiscibility relationship at mantle depths and early in the evolutionary history of a melilititic or nephelinitic magma, there is little experimental support for it. Existing experimental data indicate that immiscibility between carbonate an silicate liquids is favoured at low, crustal, pressures but that immiscibility is unlikely to occur in realistic mantle melts or their derivatives at mantle pressures. Many experimental data exist to show that magnesian carbonatite liquids form as the near-solidus melts of carbonated mantl peridotite at depths in excess of 75 km. We conclude that the calcitic and dolomitic carbonatite magma discussed in this paper are best considered as being derived from primary carbonatite magmas generated in the mantle by partial melting of carbonated peridotite. KEY WORDS: carbonatitec; liquid immiscibility; isotopes; petrogenesis
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Introduction |
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TOPABSTRACTIntroductionIsotopic EvidenceDiscussionConclusionsReferences |
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Models for carbonatite genesis essentially consider two options (e.g Gittins, 1989
): (1) that carbonatites evolve as a secondary magma during differentiation of a mantle-derive silicate parental melt; or (2) that carbonatites are derived directly by evolution of a mantle-derived primary carbonate magma. The first option has dominated debate during the past 20 years an has favoured the concept of carbonatite magma having separated immiscibly from evolved silicate magma (e.g. Le Bas, 1987).
Many publications describe experimental studies that define the compositions of conjugate carbonate and silicate liquids (e.g. Kjarsgaard & Hamilton, 1988, 1989; Kjarsgaard & Peterson, 1991; Kjarsgaard et al., 1995) but relatively few have tested the experimental data against field and petrological relationships of carbonatite-bearing alkaline complexes. Liquid immiscibility has often been assumed rather than deduced from strong geological evidence. There is no obvious reason why either one of the abov options must necessarily apply to all carbonatite complexes; petrological data from each complex require critical and objective evaluation.
Immiscibility that goes to completion destroys much of the petrographic evidence of its havin occurred but isotopic and geochemical clues should remain, however, in the conjugal liquids and the carbonate and silicate rocks that ultimately crystallize from them. In this paper we show how the isotopic compositions of Sr and Nd in associated silicate and carbonatite rocks can be used to test the possible involvement of liquid immiscibility.
In the following sections the Sr, Nd and Pb isotopic data for carbonatites are reviewed briefly. We then discuss isotopic contrasts between carbonatites and their associated silicate rocks from a selection of African complexes, which we believe argue against the carbonatites being derived by immiscibility.
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Isotopic Evidence |
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TOPABSTRACTIntroductionIsotopic EvidenceDiscussionConclusionsReferences |
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Sr–Nd and Pb isotopic variations in world carbonatitesWe have prepared a comprehensive data base of published 87Sr/86Sr–143Nd/144Nd measurements for carbonatites (218 values) ranging in age from 2700 Ma to the present. The isotopic characteristics of carbonatites have been reviewed before (Nelson et al., 1988
; Bell & Blenkinsop, 1989) and only a brief discussion will suffice here.
To limit the potential distortions caused by comparing epsilon values for carbonatites of different ages, we restrict our discussion to the 145 analyses of carbonatites younger than 200 Ma (shown in Fig. 1a). The range of Sr–Nd values is remarkably limited considering that they are from carbonatites in six continents and two Atlantic Ocean islands. A parallelogram field has been drawn in Fig. 1(a) to enclose the area of maximum sample density: 77% of the analyses fall within this ‘box’ an its extension encloses 81%. We attribute no special meaning to this field: it is constructed simply as a reference for later discussion. A group of 23 analyses cluster to the high Sr side of the box: of these ‘high Sr’ carbonatites, most are from the Jacupiranga (10) or Amba Dongar (9) complexes.
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In contrast to the
Sr–Nd data, Pb isotope composition vary substantially between individual complexes. The range is, however, closely comparable with the variation found in oceanic basalts, most carbonatites having radiogenic values similar to those of ocean island basalts. Nelson et al. (1988) first drew attention to th similarity between the Sr–Nd and Pb isotopic composition of carbonatites and ocean island basalts (OIB), and argued that these rocks had a common source.
In Fig. 1(b) all but five of the carbonatite analyses fall within the range of oceanic basalt compositions compiled by Hofmann (1997). Most data points concentrat between the groups of oceanic islands identified as being dominated by contributions from the ‘HIMU’ and ‘EM-I’ mantle reservoirs by Hofmann (1997). This relationship has been commented on previously by Simonetti & Bell (1994), Tilton & Bell (1994) and Bell & Simonetti (1996), who related the source of young carbonatites to mixtures between an EM-I type mantle reservoir and a plume-derived HIMU component. The ‘high–Sr’ group of carbonatites clearly also contain contributions from the ‘EM-II’ reservoir as well.
It is clear from Fig. 1(b) that young carbonatites contain no isotopic component not also present in oceanic basalts. This is extremely significant because, with the exception o the Atlantic island examples, the carbonatites represented in Fig. 1 were emplaced through mantle lithospheres and continental crusts of far greater complexity of age, thickness an composition than that encountered by the oceanic basalts.
Comparison between Sr–Nd in carbonatites and associated alkaline silicate rocks: a test for immiscibility
Immiscibility models assume that carbonatite magmas separate from evolved carbonate-rich silicate primary magmas. At the point of immiscibility, both carbonate and silicate liquids must be i chemical and isotopic equilibrium: if both silicate and carbonate liquids subsequently evolve an crystallize without significant contamination then the carbonatites would have similar Sr–Nd values to their conjugate alkaline silicate rocks. We have already shown that carbonatites have rather consistent Sr–Nd values; if carbonatites were produced principally by liquid immiscibility then their associate alkaline silicate rocks should have similar ranges in Sr–Nd compositions.
Published Sr–Nd data for six African carbonatites are now reviewed where contrasts in isotopic composition exist between carbonatite and silicate components: Napak, Shombole and Kerimasi in East Africa; Dorowa and Shawa in Zimbabwe; and Spitskop i South Africa. In the interests of brevity in the following discussion, radiogenic 87Sr/86Sr (positive Sr) and unradiogenic 143Nd/144Nd (negative Nd) values will be referred to as ‘enriched’ (i.e. reflecting time-integrated enrichments in Rb/Sr and Nd/Sm), and negative Sr and positive Nd as ‘depleted’ (i.e. reflecting time-integrated depletions in Rb/Sr and Nd/Sm).
Napak
Data for the Napak Complex of Uganda (Simonetti & Bell, 1994) are shown in Fig. 2(a). Calcite carbonatite, the latest intrusive phase in the complex, has Sr–Nd that plots within the carbonatite reference ‘box’ whereas the silicate rocks describe an array to progressively higher Sr an lower Nd with progressive differentiation. The carbonatite is isotopically similar to the melilitite nephelinites and some olivine nephelinites but distinct from the more evolved olivine-free rocks.
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Simonetti & Bell (1993)
demonstrated that clinopyroxene phenocrysts in the olivine nephelinites from Napak were out of isotopic equilibrium with their hosts, the phenocrysts consistently having lower 87Sr/86Sr and higher 143Nd/144Nd than their whole rocks. Separate measurements were performed for ‘light’ and ‘dark’ clinopyroxene phenocrysts from two of the samples: darker grains have higher FeOTotal, TiO2 and Na2O, and lower MgO, and clearly crystallized from a more evolved liquid than the lighter grains. ‘Light’ phenocrysts have lower Sr and Pb isotopic values and higher Nd values than th ‘dark’ clinopyroxenes, which shows that the Napak nephelinite magma developed progressively more negative Nd and more positive Sr values as it evolved.
Simonetti & Bell (1993, 1994) favoured open-system, AFC (assimilation–fractional crystallization), evolution of a mantle-derived nephelinitic magma interacting with lower-crustal granulites to explain these disequilibrium features. Significantly, the Napak carbonatites show no evidence of the high-Sr–low-Nd component assimilated by the nephelinites, proving that the carbonatites must have existed as a discrete magma before the contamination of the nephelinites, i.e. at sub-crustal depths.
Kerimasi
Kerimasi, a volcano close to Oldoinyo Lengai, has melilititic to nephelinitic lavas, calcite carbonatite lavas and tuffs, and subvolcanic uncompahgrite and ijolite. As shown in Fig. 2(b), the carbonatites are isotopically similar to melilitite lava and an ijolite intrusive but distinct from the nephelinite and phonolitic nephelinite lavas (Church, 1995; Paslick et al., 1995), which have significantly higher Sr and lower Nd than the carbonatites.
Shombole
Shombole is a Pliocene volcano in southern Kenya containing nephelinite, phonolite and carbonatite. The Sr–Nd data for the components of Shombole determined by Bell & Peterson (1991) are shown in Fig. 2(c): carbonatites have an identical range of Sr–Nd to the nephelinites whereas the phonolites have distinctly lower Nd.
These data suggest that the nephelinites and the carbonatites could represent conjugate liquids [as argued by Kjarsgaard & Peterson (1991)] but that the phonolites and carbonatites cannot.
Carbonatites of the Buhera District, SE Zimbabwe (Dorowa and Shawa)
The Dorowa and Shawa carbonatites are the time equivalents of the Mashikiri nephelinite volcanics, which initiated the Karoo magmatic cycle in the northern Lebombo and Nuanetsi regions (abbreviated to NNL) of Zimbabwe and South Africa related to the proto-rifting of eastern Gondwanaland (Bristow, 1984). Both the NNL nephelinites and overlying mafic volcanic rocks have anomalous isotopic signatures (Hawkesworth et al., 1984), characterized by highly negative Nd (–2.8 to –10; one nephelinite at –16 coupled to moderately enriched Sr (+4 to +32). Combined Sr–Nd–Pb (Ellam & Cox, 1989, 1991) and Os (Ellam et al., 1992) isotopic studies on picrite samples from this succession have demonstrated that the isotopically enriched component in the NNL volcanics was derived from the sub-continental lithospheric mantle (SCLM) and was not the result of crustal contamination.
Carbonatites in the Dorowa and Shawa complexes are associated with nephelinitic rocks as two plugs of intrusive olivine (Fo88–75) ijolite (Zwibe and Chikomo) and a swarm of nephelinitic dykes. The Sr–Nd results (from Harmer et al., 1998) on the Buhera intrusives are shown in Fig. 3. Data for the nephelinitic rocks extend the field of NNL mafic volcanics, indicating that the Buhera nephelinitic magmas were derived largely from the enriched SCLM reservoir beneath the Zimbabwean Craton By contrast, the Shawa carbonatites plot close to the carbonatite reference ‘box’ an show no evidence of the enriched SCLM component whatsoever. The Dorowa carbonatite has Nd intermediate between that of the nephelinites and the Shawa carbonatite. No contrast in Pb isotopic values is noted between the carbonatites and nephelinites: both have unradiogenic values similar to those of the NNL mafic volcanics (Harmer et al., 1998).
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Ellam et al. (1992)
interpeted the isotopic composition of the Nuanetsi picrites as the result of mixing between enriched SCLM (negative Nd values) and material from a sub-lithospheric plume (positive Nd). Using these constraints (annotated in Fig. 3), the new Sr–Nd data suggest that the Buhera nephelinites contain greater contributions from SCLM than the Nuanetsi picrites whereas the Shawa carbonatites approximate the isotopic characteristics of the implied sub-lithospheric plume material.
Spitskop Complex
The Proterozoic (1341 ± 30 Ma: Harmer, 1999) Spitskop Complex is one of suite of alkaline intrusions, many bearing carbonatites, which intruded the central Kaapvaal Crton of South Africa during the time interval 1450–1200 Ma (Harmer, 1992; Verwoerd, 1993). Spitskop is composed of silicate and carbonatitic rocks emplaced in the sequence pyroxenite–ijolite–nepheline syenite–carbonatite. The carbonatite is a composite heart-shaped plug comprising an incomplete outer rim of earlier calcite carbonatite with a larger volume of ferroan dolomite carbonatite at the centre.
Spitskop is a Proterozoic intrusion and so a direct comparison of its Sr–Nd characteristics with young carbonatites is potentially misleading. A compilation of 20 Proterozoic carbonatites from North America (from Bell & Blenkinsop, 1989) are plotted in Fig. 4 as reference. The Spitskop carbonatites have Sr–Nd values that are more enriched than those found in Proterozoic carbonatites. Although some overlap exists, the dolomitic carbonatites have less enriched Sr–Nd signatures than the calcite carbonatites. A clear contrast in Sr–Nd between the silicate and carbonate units is evident in Fig. 4: the distinction in Nd is particularly well defined in that the carbonatites all have Nd greater than –8 whereas the silicates have less radiogenic values down to –13. The positions of the different lithologies within the Sr–Nd ‘trend’ are unrelated to relative time of emplacement: Sr values increase progressively in the sequence pyroxenites–ijolites–nepheline syenites whereas Sr decreases and Nd increases with decreasing age of intrusion in the sequence silicates–calcite carbonatite–dolomite carbonatite.
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). The reference box for young carbonatites is indicated by a broken line. |
Discussion |
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TOPABSTRACTIntroductionIsotopic EvidenceDiscussionConclusionsReferences |
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Implications of the isotopic data
Isotopic data for associated silicate rocks and carbonatites in the Napak, Shombole and Kerimasi carbonatite complexes show that the silicate rocks have more variable, and enriched,
Sr–Nd than the carbonatites and that the degree of isotopic enrichment tends to correlate with degree of ‘differentiation’ of the silicate rocks. If immiscibility was involved in the generation of these carbonatites, it is constrained to have occurred at an early stage in the evolution of the parental magmas—from less rather than more evolved silicate magmas (i.e. melilitite or olivine nephelinite rather than phonolite).
It is important to emphasize that isotopic equivalence, as is found between the Shombole carbonatites and nephelinites, does not, on its own, demonstrate that immiscibility has occurred. It is simply the ‘first test’. The substantial differences in Sr–Nd compositions found in the Buhera and Spitskop complexes are rather more difficult to reconcile with liquid immiscibility.
At the point when discrete carbonate and silicate liquids are generated by immiscibility, bot liquids must have identical isotopic compositions. For carbonatites and associated silicate rock to have different isotopic compositions and yet be related by liquid immiscibility, the differences must result from different degrees of contamination of the silicate and carbonate liquids after the immiscibility occurred. Carbonatites characteristically contain elevated Sr and Nd concentrations, which are substantially higher than the levels found in common crustal rocks, and it has been argued that this concentration contrast will preferentially ‘buffer’ the carbonatite 87Sr–86Sr and 143Nd–144Nd values against the effects of contamination by crust to a greater degree than will be the case for any conjugate phonolitic or nephelinitic liquid. Obviously, this buffering effect can only operate once the carbonatite exists as a discrete liquid with elevated Sr and Nd concentrations. Data for element partitioning between conjugate silicate and carbonate liquids are sparse, but those of Hamilton et al. (1989) indicate that immiscibility alone will not produce the necessary Nd enrichment in the carbonate liquid. Carbonatites separating from silicate magmas will be enriched in Nd by at most 1.3 times that of a conjugate nephelinite liquid or 4.4 times that of a conjugate phonolitic liquid (both maxima achieved at 0.6 GPa) and for immiscibility at <0.3 GPa the carbonatite magma will contain lower light rare earth elements (LREE) than its conjugate silicate melt! Thus, the enrichment levels of 10-fold or higher that are commonly found in carbonatites are not attainable by immiscibility alone and thus must be acquired during subsequent evolution of the carbonatite magma. At the point of separation, then, carbonate liquids will not have sufficiently elevated LREE concentrations to protect their 143Nd/144Nd values from the effects of crustal contamination.
Crustal interaction may be excluded as a cause of the isotopic variation amongst the Buhera complexes. The isotopic effect of crustal assimilation may be assessed using picrite sample N296 (see Fig. 3), which was included in the Ellam & Cox (1989) study because it shows clear chemical signals of crustal contamination. Sample N296 has markedly elevated Sr, 207Pb/204Pb and 208Pb/204Pb values but similarNd values to the uncontaminated picrites; consequently, crustal assimilation does not adequately explain the isotopic variation noted in the Buhera rocks. The carbonatites and nephelinites have comparable concentrations of the LREE: Shawa carbonatite sample ZS-4 (identified in Fig. 3) has only 4.2 ppm Nd, which is much lower than either the Dorowa carbonatite (20 ppm) or the nephelinites (9–14 ppm). These concentrations are comparable with, or lower than, the average abundance in continental crust (20 ppm) reported by Taylor & McLennan (1985). Differences in Nd between the nephelinites and carbonatites are clearly not the result of ‘buffering’ of the 143Nd/144Nd ratios through elevated concentrations of Nd.
Identification of SCLM-derived isotopic components in the Buhera ijolites or nephelinites but not in the Shawa carbonatite requires that this carbonatite existed as a discrete magma at the time the nephelinites acquired the SCLM component. We believe that the nephelinites, like their counterparts in the NNL volcanics, were derived by melting of enriched SCLM (Harmer et al., 1998); by implication, then, the Buhera carbonatites existed as discrete magmas before this. Similarly, if contamination by lower-crustal granulites is responsible for the variable isotopic composition of the Napak nephelinites [as argued by Simonetti & Bell (1994)], then the Napak carbonatites must also have existed as discrete magmas by this stage, i.e. in the mantle and at P >1–1.5 GPa.
Most of the experimental support for liquid immiscibility is at low, crustal pressures of 0.5 GPa or less (e.g. Kjarsgaard & Peterson, 1991; Kjarsgaard et al., 1995). Although a miscibility gap between silicate and carbonate magmas exists at mantle pressures, it would not be intersected by realistic mantle melts or their derivatives (Lee & Wyllie, 1997). Brooker & Holloway (1997) argued that intersection of this miscibility gap is achievable under CO2 saturation but we consider the amount of CO2 required to be unrealistically high and agree with them that: ‘The general conclusion [is] that any immiscibility [from mantle-generated melts] will be a low pressure event.’
On these grounds we argue that immiscibility is not a viable mechanism for the generation of the Shawa, Dorowa, Spitskop, Kerimasi or Napak carbonatites. The available data suggest that these carbonatites crystallized from carbonatite parental magmas generated by melting of carbonated mantle peridotite.
Experimental evidence for primary carbonatite melts
Several experimental studies have shown that carbonatite liquids are produced as near-solidus melts of carbonated mantle peridotite (Wallace & Green, 1988; Thibault et al., 1992; Dalton & Wood, 1993; Sweeney, 1994). The experimental conditions at which carbonatite melts exist in equilibrium with appropriate mantle materials are summarized in Fig. 5. These carbonate liquids are magnesian (Ca/Mg <1), have high mg-number and contain 5–7 wt % total alkalis. Both the total alkali contents and the Na/K ratio are strongly dependent on the starting composition of the peridotite (Thibault et al., 1992; Sweeney, 1994): carbonatite melt coexists with pargasite ± phlogopite lherzolite where K/Na is low and with phlogopite lherzolite where K/Na is high (Sweeney, 1994). Interestingly, the pargasite megacryst from the carbonatite tuffs at Kerimasi (see Fig. 2b) has Sr–Nd that is identical to the values for the carbonatites in this complex. This is consistent with the megacrysts representing xenocrysts from mantle in equilibrium with the carbonate melt from which the Kerimasi carbonatite magma evolved.
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These primary carbonatite melts form on the high-pressure side of the thermal maximum, or ‘ledge’, in the solidus for fertile peridotite–CO2 ± H2O (see Fig. 5) and, on ascent these liquids will be eliminated by reaction if they remai in equilibrium with their mantle host according to the reaction
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). Suites of mantle xenoliths have now been identified as being the products of reaction between carbonatite melt and mantle lherzolite (Yaxley et al., 1991; Dautria et al., 1992; Hauri et al., 1993; Ionov et al., 1993; Rudnick et al., 1993). Furthermore, Yaxley & Green (1996) have effectively ‘reversed’ the above reaction by melting such xenoliths in the presence of CO2 at appropriate pressures to produce sodic dolomite melts.
It is clear from the melting experiments that primary carbonatitic melts are generated in the mantle. The fundamental question is whether they can avoid destruction at the solidus ‘ledge’, when rising through the mantle from the melting site, and escape from the mantle to crystallize as the carbonatite complexes that we see in the crust. Harmer & Gittins (1997) argued that this must be possible because dolomitic carbonatites are common in old shield areas such as the Kaapvaal and Zimbabwean cratons of southern Africa and the Canadian Shield: most of the carbonatites in the Spitskop and Buhera complexes discussed above are dolomitic.
Reaction (1) above produces metasomatic clinopyroxene at the expense of orthopyroxene, changing lherzolite to wehrlite. Equilibration of magnesian carbonatite melt with wehrlite at pressure lower than that of the solidus ‘ledge’ (i.e. <2.5 GPa) changes the composition of the carbonatitic melt to more calcic compositions (Dalton & Wood, 1993; Sweeney, 1994) reaching a maximum of 80% CaCO3 (Wyllie & Lee, 1998). Once channelways lined with wehrlite reaction products are established in the upper mantle, carbonate melts are able to survive and ascend into the crust [see discussions by Barker (1996) and Harmer & Gittins (1997)].
Experimental data confirm, then, that primary carbonatite melts are generated in the mantle and can ascend into the crust to form both calcite-rich and dolomite-rich carbonatites.
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Conclusions |
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TOPABSTRACTIntroductionIsotopic EvidenceDiscussionConclusionsReferences |
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The substantial isotopic differences between carbonatite and silicate rocks in the Buhera and Spitskop complexes are not consistent with these carbonatites being derived from secondary magma produced as immiscible liquids from a silicate magma. Similarly, the smaller, though significant isotopic differences encountered in the Napak and Kerimasi complexes suggest that these carbonatites, too, were not generated by liquid immiscibility. In all these cases we feel the available evidence favours the carbonatites being derived from primary, mantle-derived carbonate magmas, which ascend from mantle depths of 70–100 km (2–3 GPa) through conduits lined with orthopyroxene-free (wehrlitic) metasomatic assemblages.
Although isotopic differences between the silicate and carbonate components of carbonatite complexes argue against the operation of liquid immiscibility, it is important to emphasize again that the absence of isotopic contrast does not necessarily indicate that immiscibility has occurred. Only where isotopic contrasts exist in the mantle source of the carbonatite–alkaline magmatism will differences be manifest. We believe it is the well-documented presence of chemically and isotopically complex mantle lithosphere below the Kaapvaal (e.g. Richardson et al., 1984, 1985) and Zimbabwean cratons (Ellam et al., 1992) that provides the scale of isotopic contrast observed in the Buhera and Spitskop complexes, which in turn exposes processes that remain undetected in regions where little isotopic variation exists because of the relative immaturity of the lithosphere.
We acknowledge the possibility that carbonatite-bearing complexes may be generated according to different petrogenetic schemes but regard the consistent Sr–Nd isotopic variation in world carbonatites as persuasive evidence that most other carbonatites are also generated from primary, mantle-derived carbonate melts.
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Acknowledgements |
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We gratefully acknowledge the support of the Foundation for Research Development and the Council for Geoscience in South Africa, and the Natural Sciences and Engineering Research Council of Canada. We are somewhat in awe of the nearly 13 000 words of comment produced by six reviewers; thought-provoking questions and constructive suggestions by G. Tilton, F. O. Dudas and an anonymou reviewer led to a marked improvement of the manuscript. Additional stimulation was provided by the critical comments of A. Simonetti and K. Bell, and the numerous suggestions of Bruce Kjarsgaar are gratefully acknowledged. J.G. is particularly grateful to Professor Lord Lewis, Warden of Robinson College, Cambridge, and the College Council for the tenure of a Bye Fellowship on several occasions, and to the Department of Earth Sciences in the University of Cambridge for sabbatical leave facilities.
* Corresponding author. Telephone: +27-12-8411378. Fax: +27-12-8411278. e-mail: jock{at}geoscience.org.za
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