Journal of Petrology | Volume 39 | Number 11-12 | Pages 1987-1996 | 1998
© Oxford University Press 1998
Radiogenic Isotope Constraints on Relationships between Carbonatites and Associated Silicate Rocks—a Brief Review
Ottawa-Carleton Geoscience Centre, Department of Earth Sciences, Carleton University Ottawa, Ont., Canada, K1S 5B6
Received September 30, 1997; Revised typescript accepted June 12, 1998
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ABSTRACT |
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 TOP ABSTRACT IntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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The diversity of silicate rocks in alkaline–carbonatite complexes cannot be attributed to differentiation of parent magmas operating under closed chemical conditions. Constraints imposed by Nd, Pb and Sr isotope data require discrete partial melting events coupled, in some cases, with open-system behaviour that involves mixing either with other mantle melts or sources, or with lower continental crust. Patterns shown by isotope ratio diagrams for carbonatites and some nephelinites from East Africa indicate mixing dominated by two mantle end-members (broadly similar to HIMU and EMI), first recognized in oceanic basalts. Mixing is on a scale sufficient to generate: (1) coherent but variable ratios in carbonatites from much of East Africa, and (2) variable isotope ratios among some of the silicate rocks from the same eruptive centre (e.g. Oldoinyo Lengai, Shombole). Overlapping isotope ratios between carbonatites and some nephelinites from complexes from East Africa and elsewhere are consistent with magmatic differentiation (crystal fractionation, liquid immiscibility), or the melting of the same or isotopically similar sources. The wide isotopic variation shown by the ijolites, phonolites, syenites and even some of the nephelinites requires the involvement of other mantle components and/or continental crust.
KEY WORDS: isotopes; silicate rocks; carbonatites
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Introduction |
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TOPABSTRACTIntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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Isotope data have now firmly established that the ultimate source of carbonatites lies within the mantle. Nd, Pb, Sr and noble gas data indicate that carbonatites have many isotopic similarities to ocean island basalts (OIBs), and that parental melts to most carbonatites are generated from an isotopically depleted mantle. However, debate continues about the origin of carbonatites and their relationship to associated silicate rocks. Models proposed for the origin of carbonatites include: (1) melting of carbonate-bearing mantle to generate a primary carbonatitic melt (e.g. von Eckermann, 1948
; Sweeney, 1994), (2) fractional crystallization of a carbonated alkaline silicate liquid (e.g. King & Sutherland, 1960; Lee & Wyllie, 1994), and (3) immiscible separation from a carbonated silicate melt (e.g. Koster van Groos & Wyllie, 1963; Kjarsgaard & Hamilton, 1989). Although melting experiments have now shown that carbonatitic melts can be produced in different ways, what are lacking are robust criteria that can be used to separate primary carbonatitic melts from those produced by differentiation of a parent silicate melt. The occurrence of carbonatites and associated silicate rocks as flows, plugs, dykes, cone sheets, and sills, lends overall support to a magmatic origin for both groups of rocks. Although the volume of carbonatite in most complexes is relatively small, normally <10%, most of the >350 carbonatites now documented are associated with silicate rocks. Those carbonatites not associated with silicate rocks are few in number, and invariably rich in dolomite.
Because of the great diversity of the silicate rocks associated with carbonatites it is difficult to generalize about them, but certain groups are spatially, if not genetically, related. Associations include:
- carbonatite–nephelinite–phonolite, common in East Africa, e.g. Napak, Uganda; Oldoinyo Lengai, Tanzania; Shombole, Kenya;
- carbonatite–melilitolite, e.g. Oka, Canada; Kovdor, Russia; the Turiy Complex, Russia;
- carbonatite–pyroxenite, e.g. Phalaborwa, South Africa;
- carbonatite–syenite, e.g. Chilwa Island, Malawi; Khibina, Russia; Siilinjarvi, Finland;
- carbonatite–lamprophyre, e.g. Kandalaksha, Russia; northwest Namibia.
Further information about the silicate rocks fromselected complexes discussed in this paper is given in Table 1.
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A considerable amount of isotope data from carbonatites, especially Nd, Pb and Sr, has accumulated within the last 10 years. For a review of some of the appropriate references the reader is referred to Bell & Simonetti (1996)
. Data show that carbonatites are of mantle origin, they share isotope characteristics with some OIBs, and many young carbonatites, especially those from East Africa, correspond to mixtures of components that broadly correspond to HIMU and EMI, similar to the LoNd array of Hart et al. (1986). The story from the isotopic data from the silicate rocks associated with carbonatites, however, is not so straightforward. Models for the origin of the silicate rocks include those involving low degrees of partial melting of a metasomatized upper mantle (e.g. Varne, 1968; Le Bas, 1977; Olafsson & Eggler, 1983), and those that resort to rheomorphism of fenites at crustal levels, especially for rocks of ijolitic and syenitic composition (e.g. Woolley, 1987; Kramm, 1994). [The following abbreviations, perhaps not familiar to the general reader, are used throughout the text: HIMU, mantle material with high time-integrated U/Pb ratios; EMI, enriched mantle I; DMM, depleted mid-ocean ridge basalt (MORB) mantle; PREMA, prevalent mantle reservoir.]
Summarized in this paper are some of the isotope findings from both carbonatites and associated silicate rocks, and particular attention is paid to volcanic rocks, especially those from East Africa (see Fig. 1). Given that most plutonic rocks have been subjected to fluid migration and wall-rock reaction, features consistent with an evolving magma chamber, the closest approximation to liquid compositions is probably best represented by data from the volcanic rocks.
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Volcanic Rocks |
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TOPABSTRACTIntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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Continental setting
The coherency shown by the isotope data from young carbonatites, all associated with silicate rocks, from East Africa (<120 Ma) reflects mixing dominated by HIMU and EMI, two mantle components based on data from OIBs (Zindler & Hart, 1986
). The data form an array in a plot of 143Nd/144Nd vs 87Sr/86Sr, referred to as the East African Carbonatite Line (EACL) by Bell & Blenkinsop (1987). Isotope ratios from the carbonatitic flows from Oldoinyo Lengai (Keller & Krafft, 1990; Bell & Peterson, 1991; Bell & Simonetti, 1996), the only active carbonatite volcano, lie along this line even though the carbonatite flows from Oldoinyo Lengai are sodium rich (natrocarbonatites), and are chemically very different from other known carbonatites. The observation that their isotope ratios fall on the same trend as that defined by data from other carbonatites from East Africa is consistent with generation by the melting and the mixing of the same two mantle components (Bell & Simonetti, 1996). HIMU and EMI involvement is also indicated by many young carbonatites from other continents (Tilton & Bell, 1994).
Complexities within individual centres and the relationship between carbonatites and associated silicate rocks are assessed by comparing the isotopic data from two nephelinite–phonolite–carbonatite centres from East Africa, Oldoinyo Lengai and Shombole (see Fig. 1). Both volcanoes have been extensively studied, they are situated only 80 km apart, and the carbonatites from Oldoinyo Lengai are sodic (natrocarbonatites) unlike the calciocarbonatites from Shombole. For further details the reader is referred to the publications of Peterson (1989a, 1989b) and Bell & Keller (1995). Given in Table 2 are some new whole-rock Pb isotope data from Shombole with the result that all three isotope ratios are now known for both complexes. High abundance of Sr and Nd found in carbonatites and some nephelinites, well in excess of crustal abundances, means that their isotopic compositions are buffered so that ratios can be used to provide insights into their mantle sources (Bell & Blenkinsop, 1987). Pb isotopic ratios, on the other hand, can be used to monitor contamination, especially crustal, given the relatively low abundances of Pb in many mantle-derived melts.
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The distribution of the data in Figs 2 and 3 highlights the isotopic variability of the silicate rocks, shown especially by the phonolites, and also reveals the following features:
- the isotopic data for the carbonatites are fairly restricted, especially the data from the Oldoinyo Lengai natrocarbonatites. Their 143Nd/144Nd and 87Sr/86Sr ratios cluster close to the CHUR–bulk Earth intersection (Bell & Simonetti, 1996).
- The Shombole carbonatites have considerably higher Pb isotope ratios than those from Oldoinyo Lengai.
- The carbonatites from both centres have higher 207Pb/204Pb and 206Pb/204Pb ratios than most of the silicate rocks that are associated with them.
- Isotope data from carbonatites and some silicate rocks within each of the two centres are similar, and in some cases overlap.
- The nephelinites from Oldoinyo Lengai fall into two groups [designated Groups I and II by Bell & Dawson (1995)] in all but the Pb isotope ratio diagram.
- The Nd and Sr isotope data from the Group I nephelinites of Oldoinyo Lengai overlap with some of the nephelinites from Shombole.
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The isotope data for the silicate rocks are fairly variable, and contrast with the relatively tight groupings shown by the carbonatites. This indicates the involvement of isotopically heterogeneous sources, which for the nephelinites is mantle (Simonetti & Bell, 1994a
, 1994b, 1995), and for phonolites is probably mantle plus lower-crustal granulites (Bell & Peterson, 1991; Bell & Simonetti, 1996). Many of the ratios from Oldoinyo Lengai and Shombole seem to converge towards a common isotope ratio centred around the data from the carbonatites and some of the nephelinites (best shown in the Pb–Pb plot; see Fig. 3).
The overlap of isotope data from the carbonatites and some of the silicate rocks from each of the two centres is consistent with closed-system magma differentiation and hence could be used as supporting evidence to favour either liquid immiscibility or crystal fractionation. At Oldoinyo Lengai, the natrocarbonatite and the peralkaline nephelinite have similar isotope ratios (Bell & Dawson, 1995; Bell & Simonetti, 1996), and theShombole carbonatites and nephelinites share a similar range of values (see Figs 2 and 3). The two groups of nephelinites from Oldoinyo Lengai are clearly demarcated in all but the Pb–Pb isotope ratio diagram, suggesting that their Pb isotopes are decoupled from those of Nd and Sr, perhaps during the melting process. Similar findings from some East African carbonatites and some nephelinites were attributed to melting of heterogeneous lithospheric mantle (Paslick et al., 1995; Kalt et al., 1997).
Of all of the silicate rocks, the data from the phonolites show the greatest scatter (see Figs 2 and 3). The distribution of the phonolitic data cannot be attributed solely to binary mixing but requires a third component such as lower-crustal granulites, DMM or PREMA (Bell & Simonetti, 1996). Assuming that the phonolites and the carbonatites are derived from a common parental magma, one of the constraints imposed by the isotope data is that contamination of the phonolitic melt must have occurred after the two melts had separated from one another.
The isotope features so far discussed are not unique to Oldoinyo Lengai and Shombole. Other complexes, such as the 
30 Ma Napak complex in Uganda (Simonetti & Bell, 1994a) and the 130 Ma Chilwa Island carbonatite complex in Malawi (Simonetti & Bell, 1994b), contain hypabyssal nephelinites, and carbonatites with similar Nd, Pb and Sr isotopic ratios. The considerable variation in isotopic composition of the Napak nephelinites is worth emphasizing, because most of the data fall in the lower, left-hand quadrant in the Nd vs Sr isotope ratio diagram, an area generally considered to be ‘out-of-bounds’ for most igneous rocks. On the basis of the isotope data, it seems unlikely that most of the nephelinites from Napak were derived from parental or conjugate melts to the carbonatites. However, there is still some degree of isotopic overlap between the Napak carbonatites and some of the nephelinites (Simonetti & Bell, 1994a).
Other complexes outside Africa also indicate an isotopically heterogeneous mantle, and mixing as an important process in generating many silicate magmas. Large variations in Nd, Pb and Sr isotope compositions from the Kaiserstuhl complex, Germany, a Miocene alkaline volcanic complex in the Rhine Graben (Schleicher et al., 1990, 1991) can be correlated with rock types. Rocks from Kaiserstuhl consist of a sodic (olivine nephelinites, olivine melilitites, limburgites, bergalites, and hauynophyres) and a potassic silicate series (tephrites, essexites, phonolites and tinguaites). Three distinct isotopic groups can be correlated with different magma series. The first, a group of unfractionated primary asthenospheric melts (olivine nephelinites, olivine melilitites), has lower 87Sr/86Sr and 206Pb/204Pb ratios, and higher 143Nd/144Nd ratios than the more evolved rocks represented by tephrites and phonolites, and a third group distinct from the others consists of carbonatites, bergalites and haunophyres. Schleicher et al. (1990, 1991) attributed the differences in isotopic compositions to source variations and interaction during magma ascent between asthenospherically derived carbonatitic melts and an enriched lithospheric reservoir. Only the tephritic magma series and some phonolites were considered crustally contaminated. Overlapping ratios between carbonatites and bergalites, a highly evolved group of melilitic rocks considered to be a ‘missing link’ between silicate melts and carbonatites, favour generation of the Kaiserstuhl carbonatites by protracted magmatic evolution rather than a model involving generation of a primary carbonatitic melt (Schleicher et al., 1990).
‘Oceanic’ setting
Mixing has also played a major role in the genesis of carbonates and silicate rocks observed in complexes associated with transitional oceanic–crustal environments such as the Canary Islands and the Cape Verde Islands. Unaltered carbonatites from Fuerteventura, Canary Islands, and nephelinites, syenites and ijolites in the so-called ‘basal complex’ have similar Nd, Pb and Sr isotope signatures, suggesting that they may be genetically related (Hoernle & Tilton, 1991). Hoernle & Tilton (1991) noted the comparable isotope compositions among the carbonatites from Fuerteventura, Canary Islands, and carbonatites from Oka, Kaiserstuhl and Magnet Cove, and argued that this was consistent with interaction between an asthenopheric plume (marked by HIMU) and lithosphere (marked by EMI). In the case of the Cape Verdes, the only other example of an ‘oceanic’ carbonatite–alkaline silicate rock association, isotope ratios are large when compared with other oceanic islands (Gerlach et al., 1988). The Nd and Sr isotope compositions of the Cape Verde carbonatites overlap with those from some of the silicate rocks (Gerlach et al., 1988), but the variation requires at least three, isotopically distinct components in the Cape Verde mantle sources. A HIMU component contributed mainly to the islands in the northern part of the archipelago, and both EM and DMM components contributed to the southern islands. The most Si-undersaturated samples from the northern and southern Cape Verde Islands, many larnite normative, have the highest Nd and Pb isotopic ratios, suggesting a greater contribution from the HIMU component.
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Plutonic Silicate Rocks |
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TOPABSTRACTIntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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The open-system behaviour that has been documented from many plutonic alkaline silicate rocks makes it unlikely that such rocks reflect liquid compositions. Ijolites from the type locality at Iivaara, Finland, confirm these findings (Kramm 1994
); the data are scattered, and fall in the enriched segment of the Nd vs Sr isotope ratio diagram. On the basis of these observations along with trace element data, Kramm (1994) proposed that the ijolites were generated by melting of high-grade, fenitized country rocks produced by incursions of fluids from carbonatites. Even some of the syenitic and ijolitic blocks from Oldoinyo Lengai have been attributed to interactive processes between a depleted mantle and fenitized continental crust (Kramm & Sindern, 1997) involving multiple stages of metasomatism, coupled with the mixing of palingenetic and mantle-formed melts. Kramm et al. (1997) also attributed the origin of cancrinite syenites at Lueshe to similar processes.
Of the ijolites from Oldoinyo Lengai (Bell & Simonetti, 1996), Nd and Sr isotope ratios fall along a linear array that lies close to, but above the EACL, and most of the 87Sr/86Sr and 143Nd/144Nd ratios overlap with data from the more depleted Group I nephelinites of Bell & Dawson (1995). Although the relatively well-behaved nature of the Oldoinyo Lengai ijolites seemingly contrasts with the data from Iivaara and appears to be more consistent, isotopically, with the behaviour of mantle-derived melts, their Pb isotopic signatures and those from some of the phonolites, require a third component, in addition to HIMU and EMI. Bell & Simonetti (1996) suggested lower-crustal granulites, DMM or PREMA as possible candidates.
Plutonic silicate rocks from other centres also reflect complex histories. Included among these are the central ijolite plug at Napak, and intrusive silicate rocks from Chilwa (Simonetti & Bell, 1994a, 1994b). The complicated evolution for the Chilwa carbonatite complex involves magma differentiation, crustal contamination, and groundwater interaction. Radiogenic and stable isotope as well as major and trace element data from the carbonatites and nephelinites are consistent with the differentiation of a carbonated nephelinitic magma. On the other hand, data from the non-nephelinitic, intrusive silicate rocks (mainly nepheline syenite, ijolite, and camptonite) reflect an evolution that is much more complicated, involving either selective binary mixing between a nephelinitic melt and lower-crustal granulites, or incremental batch melting of a depleted source coupled with subsequent crustal contamination. The model favoured by Simonetti & Bell, (1994b) for the non-nephelinitic intrusive silicate rocks is one in which each intrusive event involved distinct mantle-derived melts that subsequently underwent interaction with lower-crustal granulites.
The data from Spitskop, discussed by Harmer & Gittins (1997), are among the first to indicate the absence of overlap between isotope ratios from carbonatites and their associated intrusive silicate rocks that include both ijolite and nepheline syenite. The Nd and Sr isotope data were considered by them to be consistent with a primary origin for the carbonatites. The more enriched isotope signatures for all of the silicate rocks might reflect involvement with, or generation from, either enriched mantle or even continental crust.
In summary, the isotope data from most plutonic alkaline silicate rocks reflect complicated histories, involving interaction between melts or fluids generated at either mantle and/or crustal levels. Given such complexities, the isotope data from silicate plutonic rocks should be used cautiously when evaluating carbonatite–silicate rock relationships.
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Isotopic Disequilibrium |
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TOPABSTRACTIntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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Further complications arise in interpreting the isotopic data by the finding that some alkaline silicate rocks need not be in isotopic equilibrium. Examples include phenocryst-rich nephelinites from Mt Elgon and Napak (Simonetti & Bell, 1993
, 1995), and the type ijolite from Iivaara (Kramm, 1994). Although the incomplete isotopic equilibration from Iivaara can be attributed to remobilization of fenites (Kramm, 1994), it is more difficult to explain such findings from porphyritic nephelinites from Mt Elgon and Napak. Isotopic disequilibrium among clinopyroxene phenocrysts from nephelinites from both of these centres rules out simple magmatic differentiation, and argues for crystal fractionation in a magma chamber that was undergoing continuous isotopic change, either by reaction with lower crust or renewed inputs of melt from a heterogeneous mantle source (Simonetti & Bell, 1993, 1995). That the Pb isotopic data from diopside phenocrysts from Mt Elgon and Napak fall between HIMU and EMI prompted Simonetti & Bell (1993, 1995) to argue for the mixing of isotopically different nephelinite magmas along with suspended pyroxene crystals within a ponded magma chamber. |
Discussion |
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TOPABSTRACTIntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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The significant isotopic variations of many of the silicate rocks show that they cannot be produced by either simple melting of an isotopically homogeneous mantle source or differentiation of a single parent magma operating under closed-system conditions. Any appeal to periodic tapping of a closed magma chamber is inconsistent with the isotopic data. Instead, the isotopic variations from many of the silicate rocks from individual centres are the result of discrete, small volume, mantle melts, involving isotopically heterogeneous sources.
The patterns shown in the isotope ratio diagrams given in this paper are best attributed to mixing of sources with very different isotope signatures. In East Africa, such sources include HIMU, EMI, DMM, PREMA or lower-crustal granulites (e.g. Simonetti & Bell, 1994a, 1995; Bell & Dawson, 1995; Bell & Simonetti, 1996). In addition, all of the isotopic data from nephelinites from Oldoinyo Lengai, Napak and Shombole reflect mixing, be it HIMU–EMI for Oldoinyo Lengai and Shombole, or HIMU plus other components for Napak (Simonetti & Bell, 1994a).
The isotopic variations shown by the nephelinites from individual centres such as Mt Elgon and Oldoinyo Lengai are considerable, and can cover almost the entire range found in all of the East African carbonatites (Simonetti & Bell, 1994a, 1995). Thus the scale and the degree of mixing reflected in the isotope ratios from these two centres suggest that mixing was episodic, was marked by gross differences in the contributions from each of the two end members, and that the degree of partial melting was insufficient to isotopically homogenize the source. The amplitude of isotopic mantle heterogeneity below a large part of East Africa and even below an individual centre is therefore large, but the physical scale involved in the melting has to be relatively small and perhaps in the case of centres such as Oldoinyo Lengai, Napak and Shombole, less than the size of an individual volcano. Both the HIMU and EMI components might reflect interaction between veins and mantle host in metasomatized lithosphere (e.g. Bell & Simonetti, 1996) or might even be contained within a ‘streaky’ mantle, their isotopic identities retained by insufficient stirring and mixing within the asthenophere (e.g. Allègre & Turcotte, 1986).
The large isotopic variations shown by the silicate lavas from Oldoinyo Lengai contrast to the relatively uniform 143Nd/144Nd and 87Sr/86Sr ratios from the natrocarbonatites that lie close to the intersection of present-day bulk Earth and CHUR and away from either extreme of the near-linear array of data from the silicate rocks. It seems unlikely that interaction between carbonatite and the mantle wall rocks could have produced the silicate rocks at Oldoinyo Lengai, given the tight clustering of the natrocarbonatite data close to the bulk Earth–CHUR intersection, and with the data from temporally associated nephelinites lying close to, but on either side of, the carbonatite data (Bell & Simonetti, 1996).
In spite of the fairly close spatial relationship between Oldoinyo Lengai and Shombole, the carbonatite data from the two centres differ in most of the isotope ratio diagrams. Although melt generation below Oldoinyo Lengai and Shombole probably involved the interaction of the same mantle components, at least during the formation of the nephelinitic melts, it appears that they must have contributed to the melts in very different proportions, with the HIMU component playing a more important role in melt generation below Shombole.
Because the isotopic signatures from both silicate rocks and carbonatites from many of the East African centres, along with some oceanic island volcanics, fall between HIMU and EMI, it would seem that sources with similar isotopic signatures are capable of generating silicate melts of very different compositions. The mantle component HIMU (low Rb/Sr, high Sm/Nd, high U/Pb) is based on data from St Helena and Tubuaii, whereas EMI (enriched mantle I; high Rb/Sr, low Sm/Nd, low U/Pb), is characterized by data from the Walvis Ridge (Zindler & Hart, 1986; Hart, 1988). In the case of Tubuaii, which carries the HIMU signature, basanites, oceanites, mugearites, and nephelinites predominate and on the basis of their mg-numbers, few represent primary mantle melts. Volcanics from St Helena share many of these features. In the case of Walvis Ridge, the EMI signature is contained in aphyric quartz-normative tholeiites from ridge crests, similar to those of alkaline basalts on Tristan da Cunha. In their preferred model, Richardson et al. (1982) attributed the Walvis Ridge basalts to partial melting of an E-type MORB source that became heterogeneous, on a small scale, during introduction of small-volume melts and metasomatic fluids.
The ranges of Pb isotope data for MORBs and the approximate values for HIMU and EMI, along with those from the East African carbonatites and elsewhere, are given in Fig. 4. Similarities are shown between carbonatites with the lowest Pb isotope ratios (Jacupiranga, Brazil) and EMI, whereas those carbonatites with the most radiogenic signatures from East Africa have ratios similar to HIMU. Figure 4 emphasizes the wide range of Pb isotopic ratios for carbonatites from continental settings, a range now seen to be just as great as that of most oceanic basalts. Such similarities imply that sources with approximate HIMU and EMI signatures, or their mixtures, are capable of generating a wide range of melts such as quartz tholeiites, alkali basalts, basanites and mugearites, in addition to nephelinites and carbonatites. The isotopic variations, to a first approximation, seem to be independent of the degree of partial melting. Nephelinites, their carbonated equivalents or perhaps even primary carbonatites are generated by degrees of partial melting that are generally too low to produce most basaltic liquids, and at higher CO2/H2O ratios. The addition of CO2 to mantle with OIB-like signatures, along with mixing, could result in the wide range of isotopic signatures observed in most carbonatites. Control of CO2 by lithospheric thickness might help explain the sparsity of carbonatites in oceanic environments.
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Returning to the individual complexes, isotopic similarities have now been recognized between natrocarbonatites and peralkaline nephelinites from Oldoinyo Lengai, between calciocarbonatites and nephelinites from Chilwa and Shombole, and between calciocarbonatites and bergalites from the Kaiserstuhl complex. Although such similarities can be interpreted in several different ways, perhaps the simplest explanation of the isotopic data is one involving magma differentiation. Other explanations include generation from the same source by different melting events, or generation from different sources which have undergone similar differentiation histories.
To distinguish primary carbonatites from those generated by magmatic differentiation of a carbonated melt is difficult, as isotopic signatures primarily reflect sources rather than processes. Isotope similarities can thus be interpreted in more than one way. However, if some of the East African carbonatites are considered primary, then the similarities in isotope ratios between carbonatites and some of their associated silicate rocks would imply that both were produced by melting of similar end members, such that similar proportions of each contributed to the parental liquids. Such a scenario seems unlikely.
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Conclusions |
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TOPABSTRACTIntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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In spite of the complexities shown by the isotope ratios from alkaline silicate rocks, the following broad generalizations can be made:
- Many silicate melts were generated by discrete partial melting of an isotopically heterogeneous mantle. The notion of large-volume, closed magma chambers undergoing differentiation cannot be reconciled with the isotopic data.
- In some complexes, similarities in isotopic data between some of the nephelinites and carbonatites are consistent with models involving magma differentiation. On the other hand, there are cases where the isotopic signatures of the carbonatites and associated silicate rocks are very different (e.g. Spitskop), and these require models that involve either open-system behaviour or melt generation from isotopically different sources.
- The silicate rocks, especially phonolites and most plutonic rocks, consistently show a much greater variation in isotopic ratios than their associated carbonatites. Such variations suggest a more complex evolution for some of the silicate melts, perhaps involving continental crust or hetereogeneous mantle.
Although isotope ratios cannot, on their own, be used to distinguish primary from secondary carbonatites, they can be used to identify sources and to monitor open-system behaviour in mantle-generated melts.
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Acknowledgements |
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This work was partly supported by funds from the Natural Sciences and Engineering Research Council operating grant A7813. Shombole samples analysed for Pb isotope ratios were kindly provided by T. D. Peterson. The author has benefited greatly from discussions and reviews by fellow editors, A. Simonetti and B. Kjarsgaard. An earlier version of this manuscript was much improved by the comments of L. A. Dunworth, M. Hamilton, G. Jenner, H. Mirnejad, and H. Schleicher.
* Telephone: 613-520-2600 (ext. 4419). Fax: 613-520-4900. e-mail: kbell{at}ccs.carleton.ca
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References |
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TOPABSTRACTIntroductionVolcanic RocksPlutonic Silicate RocksIsotopic DisequilibriumDiscussionConclusionsReferences |
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