ORIGINAL PAPER

Carbon and oxygen isotopic compositions of NewaniaDolomite Carbonatites, Rajasthan, India: implicationsfor source of carbonatites

Jyotiranjan S. Ray & Anil D. Shukla &

Lokesh K. Dewangan

Received: 5 January 2009 /Accepted: 17 July 2009 /Published online: 31 July 2009# Springer-Verlag 2009

Abstract The Newania carbonatite complex of Rajasthan,India is one of the few dolomite carbonatites of the world,and oddly, does not contain alkaline silicate rocks thusproviding a unique opportunity to study the origin andevolution of a primary carbonatite magma. In an attempt tocharacterize the mantle source, the source of carbon, andthe magmatic and post-magmatic evolution of Newaniacarbonatites, we have carried out a detailed stable carbonand oxygen isotopic study of the complex. Our resultsreveal that, in spite of being located in a metamorphicterrain, these rocks remarkably have preserved theirmagmatic signatures in stable C and O isotopic composi-tions. The δ13C and δ18O variations in the complex arefound to be results of fractional crystallization and lowtemperature post-magmatic alteration suggesting that likeother carbonatites, dolomite carbonatites too fractionateisotopes of both elements in a similar fashion. The majordifference is that the fractional crystallization of dolomitecarbonatites fractionates oxygen isotopes to a largerextent. The modes of δ13C and δ18O variations in thecomplex, −4.5±1‰ and 7±1‰, respectively, clearlyindicate its mantle origin. Application of a multi-component Rayleigh isotopic fractionation model to thecorrelated δ13C versus δ18O variations in unalteredcarbonatites suggests that these rocks have crystallizedfrom a CO2 + H2O fluid rich magma, and that the primarymagma comes from a mantle source that had isotopiccompositions of δ13C ~ −4.6‰ and δ18O ~ 6.3‰. Such a

mantle source appears to be a common peridotite mantle(δ13C = −5.0±1‰) whose carbon reservoir has insignificantcontribution from recycled crustal carbon. Other Indiancarbonatites, except for Amba Dongar and Sung Valley thatare genetically linked to Reunion and Kerguelen plumesrespectively, also appear to have been derived from similarmantle sources. Through this study we establish thatdolomite carbonatites are generated from similar mantlesource like other carbonatites, have comparable evolutionaryhistory irrespective of their association with alkaline silicaterocks, and may remain resistant to metamorphism.

Introduction

Carbonatites are magmatic rocks that are highly enriched incarbon compared to their source regions in the mantle. Beingthe largest extractors of carbon, carbonatites hold keys to ourunderstanding of the nature of carbon in the mantle, and thelong-term global carbon cycle. By definition, carbonatitescontain more than 50 modal % of carbonate minerals likecalcite, dolomite, ankerite and siderite, and less than 10% ofsilicate minerals (Wooley and Kempe 1989 and IUGSrecommendations). Because of their very low viscosity(e.g., Dobson et al. 1996) and short residence time in thecrust (Pyle et al. 1991) carbonate magmas are believed topass through the crust without significant contamination. Itis also believed that much higher contents of many traceelements in these melts are likely to buffer any minorcontamination, leading to a good preservation of sourcesignatures (e.g., Bell 1998). Carbonatites span in age fromArchean to Recent, and are known from both oceanic andcontinental settings. These aspects make carbonatites one ofthe best samples to study the chemical evolution of themantle through time (Bell 1989, 1998).

Editorial handling: L.G. Gwalani

J. S. Ray (*) :A. D. Shukla : L. K. DewanganPhysical Research Laboratory,Navrangpura, Ahmedabad 380009, Indiae-mail: jsray@prl.res.in

Miner Petrol (2010) 98:269–282DOI 10.1007/s00710-009-0073-2

Every aspect of carbonatites is unusual, be it theircompositional variability, associations with other magmaticrocks, and occurrence in various tectonic settings. The onlyactive carbonatite volcano of the world, Oldoinyo Lengai ofTanzania, emanates natrocarbonatite, whereas all othercarbonatites are calciocarbonatite or magnesiocarbonatite(with minor ferrocarbonatite). While the origin of natrocar-bonatite magma still remains elusive, multiple theories havebeen proposed for the origin and evolution of calcio/magnesiocarbonatites. Many workers have used the com-positional duality observed in carbonatites (calcitic versusdolomitic/ankeritic) as an evidence for the bimodal natureof primary carbonatite magma (e.g., Bailey 1993; Baileyand Kearns 2003), however such a hypothesis has beenquestioned (e.g., Gittins et al. 2005). The issue is furthercomplicated by the fact that a majority of carbonatites areemplaced together with a diverse group of alkaline silicaterocks. While it is sometimes difficult to petrogeneticallyrelate carbonatites with the associated alkaline silicaterocks, their coeval emplacement and geochemical affinitysuggest a common parentage (e.g., Bell 1998; Ray 1998).Experimental petrological studies do not provide anunequivocal answer to the carbonatite genesis since theysupport all the three proposed scenarios for carbonatitegeneration: (i) by fractional crystallization of silicatemagmas (e.g., Lee and Wyllie 1994), (ii) by silicate-carbonate melt immiscibility (e.g., Kjarsgaard and Hamilton1989), and (iii) by direct partial melting of a carbonate-bearing mantle peridotite (e.g., Gittins 1989; Sweeney 1994).Considering the diversity of carbonatites and lack of auniversal theory for their origin and evolution someresearchers have even proposed that there may actually be“carbonatite and carbonatites and carbonatites” (Mitchell2005). Most carbonatites occur in stable intra-plate settingsand at continental margins. A few have been reported fromsubduction zones as well (D’Orazio et al. 2007, 2008). Thefact that all carbonatites, except for a couple (Cape VerdeIslands and Canary Islands), occur in continental settingssuggests that the source for these melts might lie in thesubcontinental lithospheric mantle. However, the observationthat a large number of young carbonatites (younger than200 Ma) occur in continental flood basalt provinces (CFB)have led some researchers to advocate that like the CFBs thecarbonatites too have been derived from lower mantle plumes(e.g., Bell and Tilton 2002). Unfortunately, geochemical andradiogenic isotope tracers have not yet been able to resolvethe issue on the depth of derivation, since radiogenic isotopicratios of carbonatites can be argued in favour of theirderivation from either of the two sources.

One of the most important aspects of carbonatite magma-tism that has not been fully understood is the nature and sourceof carbon. It is now generally accepted that carbonatiteprimary magmas are generated from carbonated periodotites

or eclogites by small degrees of partial melting (e.g., Wallaceand Green 1988; Hammouda 2003; Dasgupta et al. 2004;Dasgupta and Hirschmann 2006). Carbon being a traceelement in the mantle rocks it would have required constantreplenishment to produce carbonatite magmas continuouslythroughout the geologic time. This would imply that thecarbonatite sources could receive crustal carbon throughsubduction. Determining the timing of such incorporationsand understanding these mechanisms have been the primaryobjectives of our research. We attempt to trace the carbonpathways in carbonatite systems by studying these rocks in atemporal scale using carbon isotopic ratio as a proxy. Wewere one of the firsts to provide evidence showing thatcarbonatites incorporate recycled crustal carbon at the source(Ray et al. 1999). This work was done in three carbonatites ofnortheastern India, where the plutonic complexes did notshow effects of fractional crystallization in their C and Oisotopic compositions. Unfortunately, this is not true in mostcarbonatites where the carbon isotopic composition is stronglyaffected by crystallization and secondary processes making itdifficult to understand the nature of carbon. However, this canbe overcome with the combined use of C and O isotope ratios,which has remained one of the valuable means for trackingcarbonatite evolutionary pathways.

Indian carbonatites spanning in age from Precambrian toLate Cretaceous offer a unique opportunity to study thetemporal evolution of carbonatite sources in a single litho-sphere. Of the twenty major Indian complexes, Newania ofRajasthan is the only known “dolomite carbonatite” in India.The most intriguing feature of Newania is that unlike mostcarbonatites it is not associated with alkaline silicate rocks.The carbonatite complex is plutonic and present within theoldest Archean basement complex of the region that containsgneisses, granitoids and granulites representing middle tolower crustal levels. Therefore, it is unlikely that associatedalkaline rocks of the Newania carbonatites, derived throughcarbonate-silicate melt immiscibility, could be located belowthe current erosional surface. Interestingly, it is believed thatunlike calcite carbonatites, dolomite carbonatites cannot beproduced by melt immiscibility from alkaline silicate magmas(Lee andWyllie 1998), suggesting that they could be primary.If true, Newania carbonatites could represent a primarycarbonatite magma and study of these rocks might help usunderstand the origin and evolution of such magmas. It wouldalso be interesting to understand how would the stable carbonand oxygen isotope systematics behave in such melts thatmight not have undergone immiscible separation. Newania isPrecambrian in age; hence understanding the nature of itscarbon source could shed some light on the nature of mantlecarbon then. To address these issues and to understand thefractionation of carbon and oxygen isotopes during crystalli-zation of dolomite carbonatites we studied samples from thecomplex for their chemical and isotopic signatures.

270 J.S. Ray et al.

Geology and petrography

The Newania carbonatite complex (N24o38′, E74o03′) islocated about 40 km NE of the western Indian city ofUdaipur in the state of Rajasthan. The complex is about3 km long (at its present exposure limits) and trendsnorthwest (Fig. 1). It intrudes into 2.95 Ga old Untalagranite gneisses (Choudhary et al. 1984) of the BandedGneissic Complex, one of the oldest cratonic blocks of theIndian subcontinent. Our field and petrographic studies(using thin sections and X-ray diffraction methods) revealthat the complex is predominantly dolomite carbonatiteswith minor ankerite carbonatites that mostly occur as thinbands (50–150 m) or as veins (2–10 cm) within thedolomite carbonatites (Fig. 1). These ankerite carbonatitebands are present mainly within the marked region on themap (Fig. 1). Dolomite carbonatites show flow bandingswith parallel alignment of accessory minerals like amphi-bole, phlogopite, and apatite. Although earlier workers havereported presence of calcite carbonatites as one of thecarbonatite units in the complex (e.g., Schleicher et al.1997; Viladkar 1998; Pandit and Golani 2001), we foundnone. Calcite occurs mostly as a minor mineral in dolomitecarbonatites. Lenses/bands of cumulate apatite of varyingthickness and length, from a few centimetres to a meter, arepresent at the northwest corner of the complex within thedolomite carbonatites (Fig. 1). Pure dolomite carbonatites,devoid of ankerite or apatite bands, occur in the southeasternpart of the complex. Fenitization of the country rock (Fig. 1)is evident from the presence of sodic amphiboles anddolomites within the Untala granite along the contact

region. As discussed later, the stable isotopic compositionsof these dolomites are akin to those of the carbonatites.

The dolomite carbonatites of Newania are medium tocoarse-grained rocks that contain predominantly dolomites,generally more than 95 modal % as determined by X-raydiffractometry (XRD). The red and brown colouration inmost of these rocks appear to be due to higher Fe contentsthan the rest, as confirmed from their major element data(Ray et al. in preparation); however, XRD results do notindicate presence of large quantities of ankerites. Thisindicates that dolomites of Newania generally are high Febearing. Apatite is the major accessory mineral in allcarbonatites of Newania. Other accessory minerals includemagnetite, phlogopite, alkali amphibole, zircon and pyro-chlore. Megacrystic dolomite carbonatites with dolomitecrystals of several centimetres are also observed in somedeeper layers, exposed by mining. We interpret these to becarbonatite cumulates. The ankerite carbonatites, composedmainly of ankerites, are invariably altered to iron oxides.These also contain abundant phlogopite.

The chemical classification (Fig. 2), using the majoroxide criteria of Wooley and Kempe (1989), suggests thatthe Newania carbonatites can be classified as magnesiocar-bonatites and ferrocarbonatites. Many of the dolomitecarbonatites are classified chemically as ferrocarbonatites,due to their higher Fe contents. Interestingly, P2O5 contentprogressively increases from magnesiocarbonatites to ferro-carbonatites owing to higher abundance of apatite in thelatter. A detailed discussion on various chemical aspects ofthese rocks is beyond the scope of this article. One point,however, clearly emerges from this dataset is that there is a

Fig. 1 A geological map ofNewania carbonatite complex,Rajasthan, India at the presentexposure level. The inset showsa schematic map of westernIndia with the present day limitsof the Deccan Traps and theapproximate location ofNewania

Stable isotopic compositions of Newania dolomite carbonatites 271

clear trend of fractional crystallization in Newania carbo-natites from dolomite rich magnesiocarbonatites throughapatite rich carbonatites and ferrocarbonatites.

The age of intrusion of Newania is yet to be determinedunambiguously. The earliest efforts using K-Ar method onalkali amphiboles from fenites had yielded an age of 959 Ma(Deans and Powell 1968), which has now been deemed torepresent a high temperature metamorphic event in this region(e.g., Schleicher et al. 1997; Buik et al. 2006). Gruau et al.(1995) attempted to date the carbonatites using Sm-Nd and U-Pb methods on whole-rocks and mineral separates, andreported ages varying from 1,200–1,400 Ma. They haveinterpreted this as the age of emplacement, however, in theabsence a detailed peer-reviewed publication makes thesenumbers unusable. Subsequently, Schleicher et al. (1997)reported whole-rock Pb-Pb ages for the complex andsuggested that the dolomite carbonatites of Newania wereemplaced at 2.27 Ga and the ankerite carbonatites at1,551 Ma. However, very high MSDW (Mean SquaredWeighted Deviates) values of these isochrons, cast uncer-tainty on the robustness of these ages. The isochron for thedolomite carbonatite (their Fig. 5) may also be interpreted asa two component mixing line, since the data show twoclusters of points. The authors too consider this possibility.If these ages were accepted then it would mean thatNewania had two major episodes of carbonatite activitywith a hiatus of 719 Ma, which creates severe problems fordevelopment of a general petrogenetic model for thecomplex. Existing geochemical studies on these carbona-tites reveal that although the general geochemistry ofNewania carbonatites tend to follow other carbonatites,they lack a simple fractional crystallization trend and theirrare earth element (REE) patterns are not as enriched ascarbonatites elsewhere (with lower total REE contents)(Schleicher et al. 1997; Pandit et al. 2000; Pandit and Golani2001; Viladkar and Pawaskar, 1989; Viladkar 1998). Stableisotope data appear to suggest a magmatic origin for thecomplex (Viladkar 1998); whereas, Pandit et al. (2000)

believe that a large portion of the complex could benonmagmatic.

Samples and analytical procedure

Stable carbon isotopic composition of carbonatites reflects thatof the carbonate minerals, whereas carbonate, phosphate,silicate and oxide minerals together control the oxygen isotopiccomposition. However, considering that Newania carbonatitescontain less than 5 modal % of non-carbonate minerals the Cand O isotopic compositions of carbonates can be safelyassumed to represent that of the whole rocks. Therefore, thecarbonate minerals, identified by means of XRD, wereanalyzed for their C and O isotopic compositions using thestandard analytical techniques. CO2 from pure carbonatemineral powders was extracted following a modified methodof McCrea (1950), and from samples containing physicallyinseparable carbonates by a selective extraction technique(Ray and Ramesh 1998). Some of the samples in whichconcentration of one or more of the phases was too low (<10modal %) to be chemically separated for its CO2, wereconsidered to be made up entirely of the largest carbonatephase. Samples were weighed (~10 mg) into side-arm (35 cc)vessels with the arm filled with ~2 cc of 100% H3PO4. Afterevacuation, vessels were kept in a constant-temperature waterbath at 50°C for a couple of hours and then reacted. CO2

from pure dolomite or ankerite was extracted after 12 h ofreaction, while CO2 from mixtures was collected after 15 minfor calcite and after 12 h for dolomite or ankerite with an in-between pumping step. During the extraction the vesselswere kept at 50°C using small water baths. The δ13C andδ18O of CO2 were measured on a Europa-2020 massspectrometer against a pre-calibrated laboratory standardCO2 (prepared from foraminifers) and then converted toPDB and SMOW, respectively. The 50°C acid fractionationfactors used for δ18O of calcite/dolomite/ankerite were fromSwart et al. (1991) and Rosenbaum and Sheppard (1986).Reproducibility measured as 1σ for eight analyses of NBS-19 (δ13CPDB = 1.95 and δ18OPDB = −2.3‰) and a localdolomite standard (δ13CPDB = 0.7 and δ18OPDB = −9.2‰),during the course of the measurements, was better than 0.1‰for both δ13C and δ18O.

Results and discussion

Extent of variations and evidence for mantle origin

The isotopic compositions of Newania carbonatite aresummarized in Table 1. In Fig. 3, we present these data ina histogram format depicting the observed variations in thecomplex. As can be seen, the carbonatites show large

Fig. 2 Classification of Newania carbonatites according to the schemeof Wooley and Kempe (1989). Data source: Ray et al. (in preparation)

272 J.S. Ray et al.

Table 1 Stable carbon and oxygen isotopic composition of Newania carbonatites

Sample δ13CPDB (‰) δ18OSMOW (‰) δ13CPDB (‰) δ18OSMOW (‰) δ13CPDB (‰) δ18OSMOW (‰)dolomite dolomite calcite calcite ankerite ankerite

NEWA-1a −1.9 29.1

NEWA-2a −4.3 12.1

NEWA-3 −2.1 14.5

NEWA-4 −4.8 6.8

NEWA-5(80:20) −5.0 7.0 −4.9 9.8

NEWA-5/C −5.1 6.7

NEWA-6 −4.9 6.7

NEWA-7 −5.0 7.3

NEWA-8 −4.7 7.1

NEWA-9/A −4.5 9.5

NEWA-10 −3.5 9.9

NEWA-11/A −3.3 13.8

NEWA-12 −0.5 22.2

NEWA-13 −3.6 13.6

NEWA-14 −4.3 9.6

NEWA-15/1 −4.7 7.6

NEWA-15/2 −4.7 7.9

NEWA-17 −5.0 6.3

NEWA-18 −4.8 6.9

NEWA-19 −4.8 7.5

NEWA-20 −3.8 12.1

NEWA-21/A −5.0 7.1

NEWA-21/B −4.9 7.4

NEWA-21/C −4.9 6.5

NEWA-21/D −4.8 8.0

NEWA-21/E −4.7 7.6

NEWA-21/F −5.6 5.8

NEWA-21/G −2.2 26.8

NEWA-22/A −4.9 6.9

NEWA-22/B −4.9 7.0

NEWA-23/A −4.9 7.7

NEWA-23/B −4.8 6.6

NEWA-23/C −4.9 6.4

NEWA-24/A-Ib −0.5 31.8

NEWA-24/A-IIb −2.1 26.8

NEWA-24/A-IIIb −0.9 29.0

NEWA-24/A-IVb −1.1 29.8

NEWA-24/A-Vb −0.8 30.2

NEWA-24/B-I −4.9 6.5

NEWA-24/B-II −1.5 30.7

NEWA-24/B-III −0.1 26.9

NEWA-25/A −4.8 6.9

NEWA-25/B −5.0 24.8

NEWA-25/C −4.3 23.9

NEWA-25/D −4.2 12.2

NEWA-26/A −0.9 25.5

NEWA-26/B −1.4 25.4

NEWA-27/A −5.1 7.2

Stable isotopic compositions of Newania dolomite carbonatites 273

variations in their δ13C and δ18O values. The most commonranges for δ13C and δ18O, based on the modes (~50% oftotal samples) of the distributions are −4.5±1‰ and 7±1‰,respectively. The range of δ13C is narrower (−6 to 1‰) thanthat of δ18O (4 to 36‰), and the values of dolomites aremore restricted than those for other carbonate minerals. Thelowest values observed in dolomites, δ13C = −5.6‰ andδ18O = 5.8‰, are within the estimated range of mantlecompositions (e.g., Cartigny et al. 1998; Deines 2002;Kyser 1990). In addition, more than 60% of the sampleshave δ13C composition within the range of −4 to −6‰(Fig. 3), the most common carbon isotopic composition forthe mantle (e.g., Cartigny 2005; Deines 2002), which atteststo the mantle derivation of carbon for Newania carbona-tites. Results of chemical and isotopic studies on samplesfrom the Newania complex reveal that the minimum initial87Sr/86Sr ratio (at 2,273 Ma) for the complex is 0.701902,and their chondrite-normalized REE patterns show enrich-ments in lighter REEs similar to carbonatites fromelsewhere (Ray et al. in preparation). Newania carbonatitesalso show, like most carbonatites worldwide, depletions inZr and Hf contents with respect to the primitive mantle(Ray et al. in preparation). These evidences combined withfield and petrographic inferences, such as flow bandings inmeso and micro scale, presence of cumulates, fenitizationof country rocks, and presence of numerous magmatic

minerals, unequivocally indicate that the carbonatites ofNewania are mantle derived, and negates the conclusionsmade by earlier workers that these rocks are nonmagmatic(Pandit et al. 2000; Pandit and Golani 2001).

One of the most interesting observations in Newania isthat, despite its presence in a terrain that has undergoneregional metamorphism at least twice during the last 3billion years (Buick et al. 2006), the δ13C and δ18O of alarge portion of the complex have preserved their magmaticcompositions. Lack of key silicate minerals (e.g., diopside,tremolite, wollastonite, talc etc.) makes it difficult to gaugethe effect of metamorphism on these carbonatites, however,the presence of disseminated graphite probably attests tosuch a history. Metamorphism tends to lower δ13C andδ18O values of marbles (Valley 1986); carbonatites beingmineralogically and texturally similar to marbles shouldbehave in similar fashion during metamorphism. However,the observations that many of the samples fall well withinthe mantle compositions suggests that these rocks might nothave been subjected to high temperature alteration bymetamorphic fluids. Such a scenario could be attributed toclosed-system metamorphism of carbonate minerals withno associated silicate minerals. This could be the reasonwhy many of the Precambrian carbonatites worldwideappear to have preserved their magmatic signatures in spiteof their presence in metamorphic belts.

Table 1 (continued)

Sample δ13CPDB (‰) δ18OSMOW (‰) δ13CPDB (‰) δ18OSMOW (‰) δ13CPDB (‰) δ18OSMOW (‰)dolomite dolomite calcite calcite ankerite ankerite

NEWA-27/B −5.2 7.3

NEWA-28/A −1.3 25.1

NEWA-28/B −0.9 24.6

NEWA-29 −4.9 7.6

NEWA-30/A −4.8 7.9

NEWA-30/B −4.7 6.8

NEWA-31 −4.1 24.7

NEWA-32 −4.7 7.8

NEWA-33 −4.7 7.0

NEWA-34/A −4.8 6.8

NEWA-34/B −5.4 7.3

NEWA-34/C −4.7 6.9

NEWA-35 −4.9 7.5

NEWA-36/A 0.6 33.1

NEWA-36/B 0.9 35.0

The value in paranthesis is dolomite to calcite ratio, determined by XRD

The measured values of δ13C and δ18O for the international standard NBS-19 are 1.95±0.04 ‰ and −2.30±0.10‰(n=20) with respect to PDB, respectively which agree with the values (1.95 and −2.20, respectively) recommended

by the International Atomic Energy Agency (1993)a carbonate from the country rocksb calcite with inseparable minor ankerite

274 J.S. Ray et al.

Understanding the δ13C and δ18O variations

The δ13C and δ18O values of Newania carbonatite carbonatesshow a predominant mantle derivation; however, the largerspreads observed in both hint at involvement of non-magmatic processes in their evolution (Fig. 3). In a δ18Oversus δ13C co-variation diagram data for dolomites, a fewcalcite and ankerite data plot well within the field for“primary carbonatite” (Ray and Ramesh 1999a) suggestingtheir crystallization from a magma, whereas data for a largenumber of calcites and ankerites fall well outside this fieldwith very high δ18O values, showing the effects of alterationby secondary processes (Fig. 4). To decode the nature of themantle source of Newania carbonatites and the nature of theprocesses that control the evolutionary pathways we need torecognize the factors that control the δ13C and δ18Ovariations in the carbonatites. Stable isotopic variations incarbonatite minerals, in general, can be attributed to isotopicheterogeneity in the mantle source, fractionation processes

that occur during the carbonatite generation and evolutionincluding retrograde isotopic exchange, and post-magmaticalteration processes (e.g., Deines 1989; Ray and Ramesh1999a). Magmatic processes that control isotopic fractiona-tions include extraction of carbonatite melt from the mantle,carbonate-silicate melt-immiscibility, crustal contamination,and fractional crystallization.

The extraction of carbon from the mantle in form ofcarbonatite magma would require contribution from a largemantle domain, which is likely to cause a completehomogenization of effects of δ13C heterogeneity (if any)in the source. Therefore, the carbon isotopic composition ofthe primary carbonate magma should reflect that of itssource—a large mantle domain. Although carbonate-silicateliquid immiscibility is believed to be a major carbonatiteforming process and can fractionate stable isotopes betweenthe two immiscible melts, absence of associated alkalinesilicate rocks rules out this mode of genesis in Newania.Assimilation of carbonate rocks (of marine origin) duringcrustal emplacement can affect the carbon isotopic compo-sition of the carbonatites (e.g., Demény et al. 1998;Demény et al. 1999); however, in the present study werule out such a possibility as there are no older carbonate

Fig. 4 Plot of δ13C versus δ18O of carbonate minerals from Newaniacarbonatites. The dark dashed line represents RIFMS model curvecompared with the data for starting parental magma compositions ofδ13C = −4.6‰ and δ18O = 8.3‰, temperature of crystallization of700°C and rH2O-CO2 value of 0.9. The dark solid lines are modelcurves that represent ± 1.5‰ variation in the δ18O of the magma.Fractionation factors are from Richet et al. (1977), Scheele and Hoefs(1992), Sheppard and Schwarcz (1970), and Zheng (1999). Mantleenvelope is defined based on data from Deines (1992); Cartigny et al.(1998); Cartigny (2005), and Kyser (1990). The field of primarycarbonatite is from Ray and Ramesh (1999a). Also plotted are thefluid-rock interaction model curves (dashed lines), for low temperaturealteration (30–50°C) by a CO2 bearing hydrothermal fluids, fordifferent CO2/H2O and Fc/Rc ratios. The starting compositions usedin this model for rock and fluid are: δ13Crock = −5‰ and δ18Orock =9.8‰, and δ13Cfluid = −1.0‰ and δ18Ofuid = −1.0‰

Fig. 3 Stacked histogram of δ13C (a) and δ18O (b) of variouscarbonate minerals from Newania carbonatites

Stable isotopic compositions of Newania dolomite carbonatites 275

formations in the vicinity. Therefore, the entire variationobserved in Newania carbonatites is likely to be the resultof fractional crystallization of carbonates, and post-magmatic alteration processes involving carbon-bearingfluids.

Unlike the carbon isotopic composition, the oxygenisotopic composition of a mantle derived carbonate magmaor a carbonated silicate magma is expected to be differentfrom its source because of isotopic fractionation duringmelting. Calculations by Deines (1989) and Rosenbaum etal. (1994) based on known oxygen isotope fractionationbetween mantle minerals and carbonates reveal that δ18O ofa carbonate magma generated at about 1,000°C byequilibrium melting would be ~2‰ higher compared tothat of its source. Considering that the δ18O of a peridotitemantle lies in the range of 4–7‰ (Kyser 1990), we shouldexpect carbonatite magma to have values between 6 and9‰. If, these magma are derived from eclogitic lithosphericmantle (δ18O = 6 −8‰; Kyser 1990) as suggested by some(e.g., Hammouda 2003; Yaxley and Brey 2004) then theirδ18O would be higher (8–10‰) than that of carbonatemagma derived from peridotitic mantle. Therefore, theoxygen isotopic composition of primary carbonatite magmashould fall in the range of 6–10‰. Low temperature natureof carbonate magma would likely to buffer it against largeamounts of assimilation of crustal silicate rocks, therefore,any detectable change in its δ18O due to such a process canbe ruled out. Assimilation of limestone or dolostone cancause significant change in δ18O (Demény et al. 1998),however, as discussed above there is no evidence that sucha process could have occurred during the emplacement ofNewania carbonatites. Consequently, fractional crystalliza-tion and secondary alteration are the only two mechanismsthat are likely to be responsible for fractionating oxygenisotopic composition of carbonatites within and beyond theδ18O range of 6–10‰.

Evidences for fractional crystallization in dolomitecarbonatites in Newania come from a variety of tracers,one of these is a robust linear correlation between chondritenormalized La/Yb and La content in unaltered dolomitecarbonatites (Fig. 5a). The lighter REEs get fractionatedfrom heavier REEs with increasing amount of fractionationcrystallization, and therefore, a positive correlation isexpected in a plot like above. Presence of numerous lowtemperature secondary minerals, quartz and calcite veins,and weathered horizons attests to the fact that the complexhas undergone several cycles of alteration events during itsentire history. From the plot of δ18O versus δ13C ofNewania data (Fig. 4) it appears that dolomites fromdolomite carbonatites of the complex have preserved theirprimordial composition, whereas other minerals haveundergone variable amounts of post-magmatic, and lowtemperature alteration. The correlated variation of δ18O and

δ13C seen in dolomites is similar to trends observedelsewhere that happen to be trends of fractional crystalliza-tion (e.g., Ray and Ramesh 2006). However, it is fairlypossible that the observed co-variation trend in Newania isfortuitous, and that both isotopic systems have evolvedindependently. In the following section prospect of such ascenario is assessed.

Carbon isotopic fractionation during crystallization

To determine whether carbon isotopic composition haveevolved independent of oxygen isotopes, we used simpleRayleigh crystallization to model the δ13C distribution ofNewania carbonatites that show evidence for fractionalcrystallization. Using the method devised by Deines (1989)

Fig. 5 (a) Plot of chondrite-normalized La/Yb ratios versus Laconcentrations of dolomite carbonatites from Newania. The dashedline represents a linear regression to the data. (b) Correlation betweenthe amount of dolomite crystallized (in %) and carbon isotopiccomposition for dolomite in Newania carbonatites presented in alog10(10

3 + δ13C) versus log10f plot, where f is the fraction ofremaining source. The line represents a linear regression for the data,and the values for α and δ13Csource

i (initial isotopic compositions ofthe source) are derived from the regression equation. See text fordiscussion

276 J.S. Ray et al.

we assume that the observed frequency of a certain δ13Cvalue is proportional to the volume of carbonatite of thatisotopic composition. In a single component Rayleighfractionation process, the δ13C of the product (i.e.,carbonatite) is a function of the initial isotopic compositionof the source reservoir (δ13CS

i), the fraction of remainingreservoir (f), and the temperature-dependent carbon isotopicfractionation factor between the product and the source(α13Cproduct-source). The relation is as follows:

d13CP ¼ a d13CiS þ 1000

� �f a�1ð Þ � 1000 ð1Þ

This relation is an equation of a straight line in thecoordinates of log10 (1,000 + δ13CP) and log10f. Wedetermined f from the cumulative frequency for a givenisotopic composition and plotted the data in Fig. 5b. Here, fis expressed in percentage (%) for convenience. The linearregression to the data has a good r2 value of 0.90. Therelationship although yields an acceptable initial compositionof −3.3‰ for the parent magma, the estimated value forα13Cdolomite-CO2 of 0.9996 corresponds to an unreasonablylow temperature of carbonatite crystallization of ~245°C(determined using the temperature dependent fractionationrelation of Ohmoto and Rye 1979). This would mean that thecarbon isotope fractionation in carbonatite system may be aRayleigh type, but it unlikely to be the only melt-carbonatefractionation process that controls the isotopic compositionof the final product. There could be other source compo-nents, such as CO2 and H2O (magmatic fluids), involved inthe carbonatite formation. However, such fluids would likelycause fractionation in both C and O isotopes in the melt-fluid-carbonates system. Therefore, we need to consider aprocess that fractionates both carbon and oxygen isotopessimultaneously, and generates the observed crystallizationtrends in δ13C versus δ18O plots.

Correlated δ13C versus δ18O variations and compositionsof the mantle source

Several models have been proposed to explain thecorrelated variations observed in the δ13C versus δ18O dataof carbonatites (e.g., Deines 1989; Pineau et al. 1973; Rayand Ramesh 2000) using primary magmatic processes.Model proposed by Pineau et al. (1973) is an approximatemodel, since it does not take into account the presence ofsilicate minerals and large amounts of water in thecarbonate magma. Similarly, many of their assumptionssuch as very high molar ratio of carbon between carbo-natites and associated silicate rocks (~1,000), the fraction ofremaining carbon in the source is equal to the fraction ofremaining oxygen, and identical amounts of fractionationsduring liquid immiscibility and fractional crystallizationhave been deemed to be unrealistic (e.g., Ray and Ramesh

2006). The model proposed by Ray and Ramesh (2000) is amore realistic approach and have been successfully used toexplain the correlated δ13C versus δ18O variations in manyIndian carbonatites (Ray and Ramesh 2006). According tothis model, the initial isotopic compositions of the magma,initial molar ratio of H2O and CO2, and the temperature ofcrystallization control the primary isotopic variations incarbonatites. Although the original model, called theRayleigh isotopic fractionation from a multi-componentsource (RIFMS), was developed assuming that calcite is themajor carbonate phase that precipitate from a carbonatemagma-fluid system, it can easily be modified for dolomiteprecipitation as in the present case. The details of thederivation of the original model are given in Ray andRamesh (2000). Here we present the main equations,assumptions involved and other important features of themodel that are directly applicable to Newania.

In the RIFMS model we can rewrite the calcite formingreaction as a dolomite forming reaction: Ca2+ + Mg2+ +2CO2 + 2H2O → CaMg(CO3)2 + 4H+. In which, thecations are supplied by the magma, whereas the fluids thatare present in isotopic equilibrium with the magma supplythe C and O atoms to the product. Since dolomite is themajor, and in most samples the only carbonate phase, theisotopic composition of dolomite and carbonatite wholerock may be considered equivalent. The C (and O) isotopicfractionation behaviours of a carbonate magma and car-bonate minerals are probably very similar (e.g., Deines1989). Hence, it is reasonable to assume that thefractionation factors of C (and O) between dolomiteand carbonate magma are equal to unity, implying thatthe crystallization of dolomite from the liquid magmamay not cause measurable isotopic variations. In con-trast, if the dolomite crystallizes in isotopic equilibriumwith the associated magmatic fluids (CO2 + H2O), thensignificant fractionation could occur. The crystallizationprocess is fractional, so is the isotopic fractionationprocess, hence the model proposed is a Rayleigh fraction-ation model. In the above reaction, CO2 is the only sourceof carbon for dolomite, while the oxygen is derived fromboth CO2 and H2O. As a result, the carbon isotopicevolution of carbonatite will follow a single-componentRayleigh fractionation process, whereas that of oxygen, atwo-component Rayleigh process. The model equationsare:

d13Cdolomite ¼ ac 1000þ d13Csi

� �f cÞ½ � ac�1ð Þ � 1000 ð2Þ

and

d18Odolomite ¼ 103aodol�1a�b=f o

� 1� �

þ aodol�1a�b=f o

� �d18Os ð3Þ

Stable isotopic compositions of Newania dolomite carbonatites 277

Where αc is the fractionation factor of carbon betweendolomite and CO2, fc is the fraction of remaining carbon inthe source and, δ13Cs

i is the initial carbon isotopiccomposition of the source. Similarly, αdol−1

o is thefractionation factor of oxygen between dolomite and thelargest (and first) source component (CO2 or H2O), and fois the fraction of remaining oxygen in the reservoir. Todetermine the parameters a and b we need the number ofatoms contributed by various source components to theproduct, total number of atoms contributed, fractionationfactors between various source components and the first/largest component, r (ratio of the initial number of molesof a component and the first component) and fj (j

th criticalvalue of f, when the jth reservoir gets exhausted). In thecase of carbonate magmas, we believe that CO2 is thelargest source component, hence, the values of critical f (i.e., fH2O), a, and b will be:

fH2O ¼ 1� 3rH2O�CO2

2þ rH2O�CO2

ð4Þ

a ¼ 1

32þ aH2O�CO2Þð ð5Þ

b ¼ 2

3aH2O�CO2 � 1ð Þ 1� rH2O�CO2

2þ rH2O�CO2

� �ð6Þ

The δ18Os in the Eq. 3 is determined from the generalequation of the RIFMS model:

d18Os � d18Ois

� � ¼ 103aodol�1

a

� �ln afo�b

a�b

� �� ln f on o

ð7Þ

To plot δ13C versus δ18O of carbonatite at a given timeduring the crystallization process we need to correlate themthrough time, which is achieved by relating fc and fo:

f c ¼ 1� rH2O�CO2

3

� �þ f o

2þ rH2O�CO2

3

� �ð8Þ

The RIFMS model equations have four unknowns:initial isotopic compositions of the source (δ13Cs

i andδ18Os

i), initial molar ratio of H2O and CO2 in the sourcemelt rH2O�CO2ð Þ, and temperature of crystallization (toC)that controls the fractionation factors ac, ao

dolomite�CO2, and

aoH2O�CO2

. In this forward model, three of the aboveparameters can be assumed based on results from other tracerstudies on carbonatites, and a successful model application tothe observed data could reveal the fourth. The parameters thatcan be suitably assumed include, the temperature of crystal-lization, which is expected be in the range of ~800–550°C(e.g., Haynes et al. 2003), and the δ18OS

i, which is believedto fall in the range of 6–10‰ (as discussed earlier). The fluid

associated with carbonate magma is generally CO2 rich(CO2 > H2O), the evidence for this in Newania comes fromthe carbonation of the country rocks by the magma intrusion.The δ13C and δ18O of carbonates present in basementgneisses are found to be similar to that of the carbonatites(Table 1). Since carbonate minerals are not the only oxygenbearing phases in the product it is highly likely that otherminerals, such as apatite, magnetite, phlogopite, monazite,pyroxene, olivine, and quartz, would also account foradditional oxygen isotopic fractionation in the source.Considering the observations made by various workers(e.g., Zheng 1993, 1996, 1999; Haynes et al. 2003), webelieve that a maximum of ±2‰ variation in the δ18OS

i

during the model evolution can account for isotopicfractionation caused by precipitation of non-carbonateminerals. With the help of above parameters one cangenerate series of RIFMS model curves for various initialδ13C values to fit the observed data in a given complex, andestimate the best initial δ13C for the magma. As the δ13C of acarbonate magma is believed to reflect that of its sourceregion (Deines 1989), the model can then provide estimatesof the δ13C of the mantle source regions of carbonatites.

Series of RIFMS model curves are generated for differentsets of δ18OS

i (6 to 10‰), temperature of crystallization (550to 800°C), and rH2O-CO2 values, and are compared with thetrends shown by the “primary” dolomite carbonatites in theδ13C versus δ18O plot (Fig. 4). The temperature dependentfractionation factors ao

dolomite�CO2, and ao

H2O�CO2are from

Richet et al. (1977) and Zheng (1999), whereas acdolomite�CO2

is derived from the relations given by Scheele and Hoefs(1992) and Sheppard and Schwarcz (1970) for carbonisotope fractionation between CO2 and calcite, and dolomiteand calcite, respectively. Of all the RIFMS model curves, theevolution curve for δ18OS

i = 8.3‰, rH2O-CO2 = 0.9, andtemperature = 700°C appears to explain the observed trendin our data for a δ13CS

i value of −4.6±0.2‰ (thick dashedcurve in Fig. 4). However, to explain the entire spread weneed a ±1.5‰ variation in the initial oxygen isotopiccomposition of the parental magma (solid curve on eitherside of the dashed curves in Fig. 4). The isotopic composi-tions of the mantle source of Newania carbonatites, therefore,are δ13C = −4.6‰ and δ18O = 6.3‰ (2‰ lower than that ofthe magma—see discussions above). These values lie wellwithin the normal ranges of isotopic compositions for aperidotite mantle (Cartigny 2005; Deines 2002; Kyser 1990).As for stable C and O isotopic fractionations, the onlydifference between the crystallization of dolomite carbona-tites, as in Newania, and the crystallization of calcitecarbonatites else where, appears to be the lower slope ofprimary evolution trend in the δ13C versus δ18O plot. Thismeans that dolomite carbonatites tend to fractionate oxygenisotopes to a higher degree compared to calcite carbonatites,at given ranges of δ13C and temperature.

278 J.S. Ray et al.

Extreme isotopic compositions and secondary alteration

As mentioned above, the data for Newania carbonatites thatdo not fall within the primary carbonatite polygon in Fig. 4,and have very high δ13C and δ18O values, can be termed asaltered carbonatites. To explain such extreme compositionsby fluid-rock interaction processes various models havebeen proposed (e.g., Deines 1989; Santos and Clayton1995; Ray and Ramesh 1999b). The enrichment only in 18Ocan be explained simply by interaction of carbonateminerals with pure meteoric water (e.g., Deines 1989),however, Newania samples appear to have been altered bya CO2 bearing hydrothermal fluid since the changes areobserved both in δ13C and δ18O (Fig. 4). To understand thefluid-rock interaction process and to explain the observedextreme values of δ13C and δ18O in Newania carbonatiteswe made use of a simple closed system CO2-bearing fluid-rock interaction model, proposed earlier by Ray andRamesh (1999b). The model equations are functions ofinitial isotopic compositions of the rock and the fluid, CO2/H2O ratio in the fluid, fluid-rock fractionation factor for Cand O (and hence, temperature of interaction), and fluid torock ratio by mass.

Fluid-rock interaction model curves are generated forNewania data for various temperatures and CO2/H2O ratiosassuming the following starting compositions: δ13Crock =−5‰ and δ18Orock = 9.8‰, and δ13Cfluid = −1.0‰ andδ18Ofuid = −1.0‰, and temperature dependent fractionationfactors from Richet et al. (1977) and Chacko et al. (1991).The starting compositions of the rock basically represent aprimary carbonatite, whereas the fluid is a CO2 bearingmeteoric water. The models curves explain our data foraltered carbonatites for Fc/Rc ≤ 1.0, CO2/H2O ratio in therange of 0.001–0.1, and temperature <50°C (Fig. 4). Thisimplies that the C and O isotopic compositions of Newaniacarbonatites have been affected by near-surface secondaryalteration by fluids of meteoric origin, possibly after theexhumation of the complex.

Evolution of mantle sources for Indian carbonatitesand implication for carbon in the mantle

From the above discussion it is now clear that the stable Cand O isotope systematics of dolomite carbonatites, notassociated with alkaline silicate rocks, are similar to thatobserved in calcite carbonatites worldwide, except for thesole difference of somewhat higher oxygen isotopicfractionation during crystallization of dolomite carbonatites.Successful application of the RIFMS model also suggeststhat the magmatic fluids associated with the carbonatemagma play a greater role than the magma itself in shapingthe stable isotopic variations in carbonatites, which subse-quently get modified by crustal fluids at lower temper-

atures. The isotopic compositions of the mantle source forNewania dolomite carbonatites, determined by the RIFMSmodel (δ13C ~ −4.6‰ and δ18O ~ 6.3‰), suggest that thecarbonatite source regions during Precambrian were notdifferent from those of Phanerozoic. In particular, thecarbon isotopic composition of the source lies well withinthe range of mantle values that is little affected by recycledcrustal inorganic carbon. Irrespective of the composition ofthe primary magma for carbonatites (e.g., dolomitic,calcitic, sodic, carbonated-silicate etc.) the source of carbonappears to have remained the same.

Fig. 6 (a) Minimum initial 87Sr/86Sr versus age of emplacement forIndian carbonatites. The solid line depicts the temporal evolution of87Sr/86Sr of the Bulk Earth (Bell and Tilton 2002). The dashed linesrepresent the possible evolutionary pathways for the mantles sourcesfor southern Indian carbonatites and the carbonatites from Rajasthan.Both the lines intersect the evolution line for the Bulk Earth at2,410 Ma. Data source: Basu et al. (1993); Kumar et al. (1998);Miyazaki et al. (2000); Ray et al. (2000); Simonetti et al. (1995);Schleicher et al. (1998) (b) Plot of model estimated δ13C compositionof mantle sources versus age of emplacement for Indian carbonatites.Also shown is a range for a common peridotitic mantle (horizontalshaded band), which is estimated from the modes of δ13C distributionin MORB, peridotite diamonds, kimberlites and carbonatites (Cartigny2005; Coltice et al. 2004; Deines 2002 and references therein). Datasource: Ray and Ramesh (2006)

Stable isotopic compositions of Newania dolomite carbonatites 279

A comparison of 87Sr/86Sr ratios for the mantle sourcesof Indian carbonatites, varying in age from ~2.41 Ga to65 Ma (Fig. 6a), suggests that Newania carbonatites werederived from a mantle reservoir that had slightly higher87Sr/86Sr than that of the Bulk-Earth. All the Indiancarbonatites appear to have been derived from long-livedRb/Sr enriched mantle reservoirs, however, the enrichmentis particularly larger in mantle sources for southern Indiancarbonatites compared to that for carbonatites of Rajasthan(Fig. 6a). 87Sr/86Sr evolutionary trend lines drawn throughthese two groups of carbonatite complexes intersect theBulk-Earth at 2.41 Ga, which could represent timing of amajor change in the mantle differentiation. Amba Dongarand Sung Valley carbonatites that are located neither in theRajasthan craton nor in southern Indian craton showdifferent source characteristics. Interestingly, Amba Dongarand Sung Valley are present within the Deccan Flood BasaltProvince and Rajmahal-Syhlet Flood Basalt Province,respectively, and are believed to have genetic links to thedeep mantle plumes-Reunion and Kerguelen, respectively,which were responsible for the generation the flood basalts(Ray et al. 1999, 2000). A similar comparison of carbonisotopic compositions of mantle sources for Indian carbo-natites (Fig. 6b) suggests that all the carbonatites, except forSung Valley, lie within the range observed in a commonperidotitic mantle (between −4 and −6‰; Cartigny et al.1998; Coltice et al. 2004). Mantle source for Sung Valleyhas a much higher δ13C than the rest, and that for AmbaDongar lies at the upper limit of the common peridotitemantle range. This could mean that the mantle sources forIndian carbonatites, except for these two, are located in theasthenospheric region that have remained unaffected by therecycled crustal inorganic carbon. Sung Valley and AmbaDongar carbonatites, owing to their genetic links to theflood basalts, could have sampled 13C enriched carbon fromthe deep mantle plumes. These plume probably carried 13Cenriched carbon derived from slab-carbonates that hadsurvived subduction and carried deeper into the mantle(e.g., Dasgupta et al. 2004; Zhu and Ogasawara 2002).Alternatively, their sources could be located in thecontinental lithosphere that had already been enriched withheavy carbon isotope, derived from slabs, through mantlemetasomatism.

Conclusions

We report new stable carbon and oxygen isotopic compo-sition data for Newania dolomite carbonatite complex ofRajasthan, India after a comprehensive study of thecomplex and analysis of all carbonate phases present.Although the δ13C and δ18O of carbonatite carbonatesshow large variations, their most common ranges of −4.5±

1‰ and 7±1‰, respectively clearly indicate that theserocks are mantle-derived, consistent with their radiogenicisotopic ratios, and chemical behaviour similar to mostcarbonatites worldwide. The remarkable preservation ofprimary isotopic compositions in Newania suggests that thestable isotope systematics of carbonatites is probablyresistant to metamorphism. The variations of δ13C andδ18O in Newania have been explained here by fractionalcrystallization of carbonate minerals, and postmagmaticlow temperature alteration processes. Theoretical treatmentof the data from unaltered carbonatites (primary) using amulti-component Rayleigh model suggests that the New-ania carbonatites have likely crystallized from a CO2 + H2Ofluid rich magma, and that the primary magma would haveoriginated from a source having isotopic compositions ofδ13C ~ −4.6‰ and δ18O ~ 6.3‰. The extreme isotopiccompositions observed in the complex have been attributedto near-surface low temperature alteration. Stable isotopicfractionation mechanism in dolomite carbonatites appear tobe similar to that of other varieties, except that themagnitude of oxygen isotopic fractionation is somewhathigher in the former. A comparison of these results withthose from other Indian carbonatites reveals that the mantlesource domain for Newania and other carbonatites ofRajasthan has evolved independently from that of carbo-natites of southern India, and that the latter is more enrichedin terms of large ion lithophile elements. Except for AmbaDongar and Sung Valley carbonatites that appear to haveincorporated recycled inorganic carbon, most Indian carbo-natites are derived from an asthenospheric mantle whosecarbon reservoir has remained unaffected by recycledcrustal carbon.

Acknowledgements We thank L.G. Gwalani for inviting us tocontribute to this special issue that is being brought out in the honourof Keith Bell. We are indebted to Keith Bell for his constantencouragement, and constructive criticism. We are extremely happyto be part of this special issue. We thank C. Creixell and ananonymous reviewer for their comments and suggestions. We thankR. Ramesh for his inputs.

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