Journal of Asian Earth Sciences 36 (2009) 252–260

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Journal of Asian Earth Sciences

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Origin of graphite, and temperature of metamorphism in Precambrian EasternGhats Mobile Belt, Orissa, India: A carbon isotope approach

Prasanta Sanyal a,*, B.C. Acharya b, S.K. Bhattacharya c, A. Sarkar a, S. Agrawal a, M.K. Bera a

a Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, Indiab Institute of Minerals and Materials Technology, Bhubaneswar 751 013, Indiac Physical Research Laboratory, Ahmadabad 380009, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 31 December 2008Received in revised form 15 June 2009Accepted 18 June 2009

Keywords:GraphiteCarbon isotope ratioEastern Ghats Mobile BeltOrissaCalc-silicate granuliteMetamorphic temperature

1367-9120/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.jseaes.2009.06.008

* Corresponding author. Tel.: +91 322283378.E-mail address: psanyal@gg.iitkgp.ernet.in (P. Sany

The carbon isotope composition of graphite and carbon and oxygen isotope composition of associatedcalcite from different locations of the Eastern Ghats Mobile Belt (EGMB) of Orissa have been measuredin order to understand the origin of graphite. The d13C values of graphite range from �2.4‰ to�26.6‰. Forty-four of sixty-one samples have d13C values less than �20‰. Most of these low d13C valuesgraphite corresponds to schists and disseminations in khondalite and calc-silicate granulites, thus indi-cating graphitization of organic matter. The remaining light-carbon–graphite occurs as veins which isthe result of graphitization of transported organic matter. The graphite with intermediate d13C value(�13‰ to �19‰) indicates carbon contributions from both organic and carbonates sources and/or man-tle sources. The higher d13C values graphite (�2.4‰ to �8.8‰) represent mantle carbon and/or carbonatesources without significant contribution from organic carbon.

The temperatures of metamorphism have been estimated using carbon isotope ratios of graphite andassociated calcite of calc-silicate granulites, where typical cation exchange thermometer assemblagesare lacking and significant mineral reaction textures used to calculate pressure–temperature of metamor-phic events are absent. Metamorphic temperatures obtained 945 �C are close to the ultrahigh-tempera-ture reported from the EGMB. The minimum temperature estimated using the graphite–carbonate carbonisotope ratio is 90 �C. The lower estimates of temperatures probably indicate changes in the carbon iso-tope ratio of calcite by decarbonation reaction or armoring of carbonaceous matter in silicates duringmetamorphism preventing continuous exchange with calcite.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

Graphite deposits in the Indian state of Orissa are restricted tothe Eastern Ghats Mobile Belt (EGMB) and constitute the mainsource of this important mineral in India (Indian Minerals YearBook, 2003). The graphite in the EGMB is associated with differenttypes of host rocks, which have been affected by multiple episodesof deformation and metamorphism (Chetty, 2001; Dasgupta andSengupta, 2003) suggesting possible diverse genesis of thesegraphite occurrences.

Previous workers have proposed that the Eastern Ghats (Fig. 1)graphite are abiogenic and probably formed by the dissociation ofmethane produced during decarbonation of calc-silicate granulites(Acharya and Dash, 1984). However, a large number of graphitebodies are present in migmatised khondalite and acid gneiss with-out associated calc-silicate granulite. As graphite, is the end prod-uct of metamorphism of either carbon bearing fluids or

ll rights reserved.

al).

carbonaceous matter, its d13C values reflect the sources of carbon:carbonate with d13C near 0‰, reduced organic matter near �25‰

and mantle derived carbon with d13C value near �7‰ (Schidlowskiet al., 1983; Kehelpannala, 1999). Thus distinct d13C values canprovide an insight into the source, and mechanism of graphite pre-cipitation in the EGMB.

When graphite forms in the presence of carbonate, as can hap-pen in calc-silicate granulites, carbon isotope exchange occurs be-tween these two phases during metamorphism (Valley and O’Neil,1981). Given that exchange of carbon isotopes is dependent ontemperature, the difference in carbon isotope ratio between thetwo phases can be used to determine the metamorphic tempera-ture (Dunn and Valley, 1992). However, for accurate interpreta-tions of temperature, the carbon isotopic composition of bothphases must remain constant during cooling, a condition that isnot always met. For example, in the case of wollastonite-bearingcalc-silicate granulites, CO2 produced during isobaric cooling canchange the pristine isotopic character of carbonate through ex-change. Consequently, the estimated temperature using carbonisotope thermometry will provide retrograde metamorphic

Fig. 1. Geographical distributions of rock types in Eastern Ghats of Orissa and locations of graphite mines from where samples were collected (after Acharya and Dash,(1984)).

P. Sanyal et al. / Journal of Asian Earth Sciences 36 (2009) 252–260 253

temperature. Temperature and pressure of calc-silicate granuliterocks from the Eastern Ghats where three phases of metamor-phism are recorded in the time range 1.6 Ga to �550 Ma have beendetermined by intersection of fluid-absent reaction equilibria forspecific phase compositions (Dasgupta, 1993; Bhowmik et al.,1995; Shaw and Arima, 1996). However, determination of meta-morphic temperature where metamorphism has occurred in pres-ence of fluids is always challenging as the composition of the fluidsare not always known. In such situation calcite–graphite carbonisotope thermometry can be used to determine metamorphic tem-perature as well as evolution of the metamorphic fluids.

In this study, we identify the source of carbon in graphite andthe mechanisms of graphite formation in the EGMB using carbonisotopes. The temperature of metamorphism of calc-silicate granu-lite has also been calculated using carbon isotope ratios of graph-ite–calcite pairs.

2. Geological setting

The Eastern Ghats of Orissa form part of the Eastern Ghats Mo-bile Belt which extends from Brahmani River in Orissa to Ongole inthe state of Andhra Pradesh over a stretch of 900 km with a max-imum width of 300 km in the northern part of Orissa. It tapersdown to <10 km in width to the southern part of Andhra Pradesh(Chetty and Murthy, 1994; Ramakrishnan et al., 1998). The EGMBis a poly-deformed and poly-metamorphic terrain (Halden et al.,1982; Dash et al., 1987; Bhattacharya et al., 1994; Shaw and Arima,

1996). Studies of metamorphism from different areas suggest atleast three phases of metamorphism with ultrahigh-temperatureof �1000 �C and pressure of �9 kbar (Shaw and Arima, 1996; Seng-upta et al., 1999). The oldest tectono-thermal event in the westernpart of the EGMB is a granulite-facies metamorphism at 1.6–1.4 Ga(Mezger and Cosca, 1999). The second stage of pervasive metamor-phism is recorded during the Grenvillian orogeny around 1000–960 Ma (Simmat and Raith, 1998; Mezger and Cosca, 1999). Intru-sion of S-type granite marks the closing phase of this orogeny (Paulet al., 1990; Kovach et al., 1997; Krause et al., 2001) at �940 Ma.Pan-African thermal overprinting around 516 Ma marks the finaldeformation/metamorphism in the EGMB (Upadhyay and Raith,2006). The protoliths of the EGMB are derived mostly from re-worked Archean and early to middle Proterozoic crustal material(Rickers et al., 2001). Based on Sm–Nd, Rb–Sr and Pb isotope data,Rickers et al. (2001) identified four crustal domains (Fig. 2) of dif-ferent protolith ages. The rocks of the present study, composed ofmacroscopically banded khondalite, calc-silicate granulite, acidgneiss, charnockite and basic granulite (Halden et al., 1982; Parkand Dash, 1984; Acharya and Dash, 1984), are mainly from DomainII having protolith ages ranging from 2.5 to 2.2 Ga (Rickers et al.,2001).

In the present study area, major structural trends (e.g. antiform-al and synformal axes) are NE–SW to NNE–SSW (Fig. 1) and similartrends are also observed from the western part of Koraput throughBhawanipatna and Bolangir to Sambalpur. Structural analysis re-vealed four successive phases of folding, that is isoclinal to tight

Paderu-Araku-Anantagiri

Rayagada

Rajamundry

Chilka

AngulPhulbani

IV

MahanadiRift

III

Visakhapatnam

Riamal

Godavari Rift

IA

IB

II

920ºC

945ºC Eastern Ghats Mobile Belt

Nd-Model Ages(After Rickers et al., 2001)Domain I: 2.9 -3.9 GaDomain II: 2.2-2.5 GaDomain III: 1.8- 2.2 GaDomain IV: 2.5-2.9 Ga

0 100km

N

Present Study

(Shaw and Arima, 1996)

1000ºC(Sengupta et al., 1999)

Bolangir

Khariar

Fig. 2. Different crustal domains based on Nd-isotopic ratio in the EGMB. Samplelocations for present study are in Domain II. Temperature estimates by previousworkers from Domain II showed maximum temperature up to 920 �C whereas usingcarbon isotope thermometry the estimated peak temperature is 945 �C which isclose to the ultrahigh-temperature (�1000 �C) reported from the EGMB.

Fig. 3. Mode of graphite occurrences: (a) Disseminated graphite (silky black) inKhondalite. (b) Graphite schist (black) with sharp margin at the contact ofpegmatite (white). The migmatised khondalite host rock (light grey) is seen attop right of the photograph, Beniamal. (c) Vein graphite as large tablet, Ganjaudarvein.

254 P. Sanyal et al. / Journal of Asian Earth Sciences 36 (2009) 252–260

intrafolial folds (F1), asymmetrical to isoclinal intrafolial folds (F2),open folds (F3) and warp folds (F4). Graphite is mostly concen-trated at the hinges of F2, F3 and F4 folds, and in fractures andjoints. The lithologic units in the study area have undergone gran-ulite-facies metamorphism and were finally affected by granitisa-tion and migmatisation.

3. Modes of occurrence of graphite

Graphite occurs as disseminations (Fig. 3a), schists (Fig. 3b) andveins (Fig. 3c). The disseminated graphite scales and flakes arefound in almost all the rock types and are wide-spread. Dissemi-nated flakes are oriented parallel to gneissosity, or occur alongthe grain boundaries of different gangue minerals. The size of thegraphite flakes increase in the migmatised portions of the hostrock. Graphite schists show parallel arrangement of flaky graphiteof varying proportions alternating with silicate minerals inkhondalite, calc-silicate granulite, biotite schist and acid gneiss.The contact between graphite schist and the host rocks are gener-ally sharp but occasionally gradational. Some of the migmatisedcalc-silicate granulites have calcareous graphite schist (e.g. Dudk-amal, Beniamal graphite mines). The veins predominantly consistof graphite and have more or less distinct boundaries against thecountry rocks and are observed in khondalite, calc-silicate granu-lite, acid gneiss and pegmatite, e.g. in mines at Beniamal, Dudk-amal and Temerimal. The fixed carbon content reaches 60–70% inrich lodes but are never free from gangue minerals.

4. Sampling strategy and methodology

The number of samples from a particular graphite deposit de-pended based on the mode of occurrence and the association ofgraphite with different rock types. For each deposit when graphiteis present in different modes or lithotypes, samples were collectedfrom each one of them.

Carbonate phase was indentified using Energy-Dispersive X-ray(EDX) and X-ray diffraction (XRD). EDX analysis of carbonate-bear-

ing graphite was performed using a JEOL JSM 5800 SEM instrumentconfigured with an OXFORD ISI 300 EDS Microanalysis system atDepartment of Metallurgical and Materials Engineering at IndianInstitute of Technology, Kharagpur. XRD was performed using aRigaku Miniflex II instrument at Department of Physics and Mete-orology at Indian Institute of Technology, Kharagpur. For stable iso-topic analysis of calcite, powdered samples were obtained fromrock chips using a hand drill under a binocular microscope to avoidfracture-filling carbonate. The powdered samples were reactedwith 100% H3PO4 in vacuum at 80 �C with in an online carbonatepreparation system (CAPS). The evolved CO2 was purified cryogen-ically and introduced into a Europa GEO 20–20 stable isotope ratiomass spectrometer for measurement of the d13C and d18O values.

2-theta28 30 32 34 36 38 40

Inte

nsity

0

500

1000

8000

Graphite

Calcite

a

b

Fig. 4. (a) Energy dispersive X-ray image of sample Du-2. Numbers (1–9) represent analysis point. Results are given in Table 1. Absence of magnesium confirms carbonatephase is exclusively calcite. (b) X-ray diffraction analysis (target-Co) of sample Du-2. The position of the graphite peak indicate well crystalline and absence of peak at 36.14(2h) confirms the carbonate phase is devoid of dolomite.

Table 1Chemical compositions (Sample Du-2) at different points. The abundances of theelements are given in weight percent. Absence of Mg indicates the carbonate phase ispure calcite.

Point C Ca O Si Mg Cl

1 100 � � � � �2 96.07 0.40 3.23 � � 0.403 10.65 39.97 49.38 � � �4 6.46 63.23 30.30 � � �5 9.82 0.36 45.05 44.77 � �6 12.47 36.36 51.17 � � �7 76.91 � 19.11 3.43 � 0.558 100 � � � � �9 100 � � � � �

P. Sanyal et al. / Journal of Asian Earth Sciences 36 (2009) 252–260 255

Samples were also analyzed in continuous flow DeltaPLUSXP stableisotope ratio mass spectrometer equipped with a Gas Bench II andElemental analyzer Flash EA 1112 at Indian Institute of Technology,Kharagpur, India. For carbonate analysis, 200–300 lg of samplewere reacted with H3PO4 at 72 �C. The evolved CO2 was analyzedfor its isotopic composition in the mass spectrometer. Sampleswere analyzed along with an internal laboratory standard (ZC-2002 with d13C: 2.1‰ d18O: �2.1‰ VPDB) which has been cali-brated to the VPDB scale by NBS 19. For analysis of graphite, sam-ples were first crushed and treated with 0.5 N HCl to removecarbonate materials. The calcite-free residue was rinsed 5 or 6times with distilled water and dried at 100 �C. Subsequently, afew milligrams of the samples were combusted in sealed silicatubes in the presence of CuO at 1000 �C for 6 h. The evolved CO2

was purified cryogenically and analyzed for isotopes in the massspectrometer. Graphite samples were loaded in tin capsules withvanadium pentoxide and combusted in Flash EA 1112 where thereactor was kept at 1100 �C. Analyses were made using a Delta-PLUSXP isotope ratio mass spectrometer. To check the reproducibil-ity of measurements, the internationally recognized graphitestandard NBS-21 (d13C: �28.18‰) was analyzed along with eachset of samples. Many of the samples were re-analyzed to check

for homogeneity. Isotopic ratios of carbon and oxygen are pre-sented in the d notation (‰) with respect to VPDB. Estimated 1-r uncertainties in d13C and d18O values of calcite are about±0.05‰ in Europa Geo 20–20 and in DeltaPLUSXP ±0.10‰ and forthe d13C value of graphite it is about ±0.1‰ based on both samplesand standards.

Table 2d13C values of graphites; and d13C and d18O values of calcite from the Eastern Ghats Mobile Belt, Orissa.

Mode of occurrences Sample no. Sample description d13C(graphite) d13C(calcite) d18J(calcite) DCal–Grap

DisseminationsT-14 Temerimal, graphite dissemination in calc-silicate granulite �24.3T-30(1) Temerimal, graphite dissemination in khondalite migmatite �23.7T-30(2) Temerimal, graphite dissemination in khondalite migmatite �22.6Dj-13 Dudkijharia, graphite dissemination in calc-silicate granulite �24.8G-17 Ganjaudar, graphite dissemination in calc-silicate granulite �21.3 �18.1 �12.0 3.3GR-17-1 Ganjaudar, graphite dissemination in khondalite �24.6R-17(2) Raju Nagfena, graphite dissemination in khondalite �16.6D-3 Dudkamal, graphite dissemination in calc-silicate granulite �24.3 �12.1 �6.0 12.2D-3� Dudkamal, graphite dissemination in calc-silicate granulite �24.3Bab-3 Babja, graphite dissimination in Mn-ore �23.3Thk-3 Thakurpali, graphite dissimination in Mn-ore �22.1U-6 Uchhabpali, graphite dissimination in Mn-ore �19.0U-8 Uchhabpali, graphite dissimination in Mn-ore �19.0G-15 Ganjaudar, graphite dissemination in calc-silicate granulite �20.4 �17.8 �15.8 2.6

SchistsDa-9 Dandatapa, graphite schist associated with pegmatite �18.9T-10 Temerimal, graphite schist on both margins of pegmatite �26.6T-16 Temerimal, Biotite-graphite schist in calc-silicate granulite �24.1 �3.7 �12.3 20.4T-19 Temerimal, graphite schist associated with pegmatite �24.7P-7 Pondkimal, graphite schist associated with pegmatite and calc-silicate granulite �14.3G-11 Ganjaudar, graphite schist associated with pegmatite and calcite �23.1 �10.8 �3.9 12.3G-18 Ganjaudar, graphite schist associated with pegmatite �16.1G-7 Ganjaudar, graphite schist associated with pegmatite �23.4G-16-1 Ganjaudar, graphite schist associated with pegmatite and contain calcite crystals �22.4 �11.2 �4.9 11.2G-16-2 Ganjaudar, graphite schist associated with pegmatite and contain calcite crystals �22.5GR-9 Ganjaudar, graphite schist in quartzite �23.4GR-14 Ganjaudar, graphite schist associated with pegmatite �13.0B-2 Beniamal, graphite schist in khondalite �22.1R-6 Raju Nagfena, graphite schist associated with pegmatite in khondalite �21.4R-12-1 Raju Nagfena, graphite schist associated with pegmatite in khondalite �21.4R-12-2a Raju Nagfena, graphite schist associated with pegmatite in khondalite �21.5R-12-2 Raju Nagfena, graphite schist associated with pegmatite in khondalite �21.4R-18 Raju Nagfena, graphite schist associated with Garnet-quartz-feldspar pegmatite �21.5Du-2 (R) Dudkamal, Cacareous graphite schist �25.2Du-10 Dudkamal, calcareous graphite schist in calc-silicate granulite �25.6DU-2 Dudkamal, calcareous graphite schist with secondary calcite veins �25.2 �13.5 �4.7 11.7DU-3 Dudkamal, calcareous graphite schist �24.6 �9.8 �5.9 14.8Du-6a Dudkamal, graphite lode with secondary calcite on surface �24.3 �7.7 �3.9 16.6D-2 Dudkamal, graphite schist along margins of pegmatite �26.0U-7 Uchhabpali, calcareous graphite schist �19.4 �6.9 �2.5 12.6U-9 Uchhabpali, graphite schist in khondalite �20.7D-1 Dudkamal, calc-silicate granulite converted to calcareous graphite schist �24.5 �8.0 �3.9 16.5L-12 Dumerpara, calcareous graphite schist �5.0 �2.1 �12 2.9L-12-1 Dumerpara, calcareous graphite schist �7.1 �2.6 �13 4.5L-19 Dumerpara, calcareous graphite schist �7.9 �5.5 �10.9 2.4L-19-1 Dumerpara, calcareous graphite schist �2.4 1.8 �10.6 4.2

VeinsA-1 Adeswar, graphite lode in khondalite �20.0Tu-8 Tumudibandh, graphite lode in khondalite �18.5S-10 Sapmund, graphite lode associated with pegmatite �16.3Sar-1 Sargipalli, graphite lode with pegmatite in khondalite & calc-silicate granulite �14.8Sgm-2 Sargimunda, graphite lode associated with pegmatite in khondalite �8.8Sgb-5 Sargibahal, graphite lode in Khondalite �17.1Be-5 Beherapani, graphite lode in khondalite �14.5G-4 Ganjaudar, graphite lode containing large flake �22.5GR-12-1 Ganjaudar, graphite lode with pegmatite �22.2GR-12-2 Ganjaudar, graphite lode with pegmatite �24.4GR-13 Ganjaudar, graphite lode �20.8B-1 Beniamal, graphite lode in khondalite & calc-silicate granulite �23.2DU-6b-1 Dudkamal, graphite lode associated with pegmatite and secondary calcite �24.2 -6.0 -3.0 18.2DU-6b-2 Dudkamal, graphite lode associated with pegmatite and secondary calcite �24.2L2-2v Chilipaka, graphite at contact of calcite and calcsilicate granulite �21.3 3.2 �10.4 24.5L2-6v Chilipaka, graphite at contact of calcite and calcsilicate granulite �23.8 3.4 �10.2 27.2

Calcite without graphiteT-13 Temerimal, calc-silicate granulite with out graphite �4.0 �13.6T-17 Temerimal, calc-silicate granulite with out graphite �5.2 �15.3T-18 Temerimal, calc-silicate granulite with out graphite �1.3 �15.4G-13 Ganjaudar, calc-silicate granulite with out graphite �5.6 �14.4B-10 Beniamal, calc-silicate granulite with out graphite �4.0 �16.4N-3 Nagfena, calc-silicate granulite without graphite �6.4 �17.4DU-2(s) Dudkamal, secondary calcite vein in DU-2 �10.1 �4.3DU-6a(s) Dudkamal, secondary calcite vein on Du-6a �6.8 �3.1

256 P. Sanyal et al. / Journal of Asian Earth Sciences 36 (2009) 252–260

P. Sanyal et al. / Journal of Asian Earth Sciences 36 (2009) 252–260 257

5. Results

EDX and XRD data show that calcite is the only carbonate phase,and that graphite is present as crystalline graphite (Fig. 4a and b;Table 1). Additionally, well developed graphite flakes with brightluster showing strong reflection, bireflectance and anisotropy un-der reflected light also indicates crystalline graphite.

The d13C values of graphite from the EGMB of Orissa vary from�2.4‰ to �26.6‰ (Table 2). The majority of samples (44 of the 61analyzed samples) have d13C values less than �20‰ (Fig. 5). Thed13C values of disseminated graphite range from �16.6‰ to�24.6‰ with an average of �22.2 ± 2.4‰ (n = 14; 1r), schistgraphite has average d13C value –20.0 ± 6.5‰ (n=31) with spreadof �2.4‰ to �26.0‰. The d13C values of vein graphite range from�8.8‰ to �24.4‰ with an average of �19.8 ± 4.5‰ (n = 16). Thed13C values of graphite have a wide range corresponding to differ-ent lithologies. The d13C values of graphite-associated with calc-sil-

δ13CGraphite

-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0

Num

ber o

f sam

ples

0

2

4

6

8

10

12

14

16

18Organic Carbon

Variable mixing of organic and carbonate and/or mantle sources Carbonate and/

or mantle sources

Fig. 5. Frequency distribution of d13C values of graphite from Eastern Ghats ofOrissa. Forty-four out of sixty-one samples have d13C values less than �20‰

indicating graphite is dominantly organic in origin. The intermediate d13C values(between �13‰ and �19‰) are a result of precipitation of graphite from a mixtureof fluids derived from both organic and carbonate and/or mantle sources. Thesource of carbon for higher d13C values is carbonate and/or mantle carbon.

-20 -15 -10 -5 0 5 10

δ18O

Cal

cite

(‰)

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

δ13C Calcite (‰)

Calcite associate with organic graphiteCalcite without graphiteCalcite associate with carbonate graphiteCalcite without graphite (From Bhowmik et al., 1995)

Fig. 6. Plot of d13C and d18O values of calcite. Calcite associated with organicgraphite has lower d13C values compared to calcite in rocks without graphite orcalcite associated with high 13C graphite.

icate granulites range from �2.4‰ to �25.6‰, in khondalite from�16.6‰ to �24.6‰ and in pegmatite from �8.8 to �26.6‰.

The d13C and d18O values of graphite-associated calcite are dis-tinct from those of graphite-free calcite (Fig. 6). Usually, calcite de-void of graphite is characterized by higher d13C values. However,calcite associated with higher d13C value graphite also has higherd13C values. It is also noteworthy that the spread in d13C andd18O values of the calcites associated with lower d13C value ofgraphite are larger compared to calcite devoid of graphite. Thed18O values of the former are lower compared to the latter. Thed13C values of calcites associated with lower d13C value graphitevary from �3.7‰ to �18.1‰ with average �10.4 ± 4.3‰ and thecorresponding d18O values vary from �2.5‰ to �15.8‰ with anaverage of �6.2 ± 4.1‰ (n = 14; 1r). The d13C values of graphite-free calcite vary from �1.3‰ to �6.4‰ with an average of�4.4 ± 1.8‰ and the corresponding d18O values vary from�13.7‰ to �17.4‰ with an average of �15.4 ± 1.3‰ (n = 6; 1r).

6. Discussion

6.1. Isotopic evidence of graphite carbon source

The carbon isotope ratio has long been used as a tool for iden-tifying the sources of carbon in graphite as the three major reser-voirs of carbon are characterized by distinctly different d13Cvalues. The d13C value of organic carbon varies from about �20‰

to �30‰ in contrast to carbonate sources with d13C value as highas �0‰ (Schidlowski et al., 1983). The mantle derived carbon fluidhas value around �7‰ (Kehelpannala, 1999). However, dependingon the mixing of these three sources d13C value of graphite can varywidely (Rumble and Hoering, 1986). In the Eastern Ghats, d13C val-ues of graphite from important deposits like Temerimal, Ganjau-dar, Dudkamal, Raju Nagfena, Adeswar, Dudkijharia, Chilipakaand Beherapani (Table 2) suggests that the source of carbon ismostly organic carbon. Most of the organic-carbon-derived graph-ite in these localities occurs as schist or disseminated in metasedi-ments like khondalite and calc-silicate granulite in the EGMB.These graphite formed during metamorphism of the organic mat-ter within the protolith. Precambrian sedimentary basins hostedabundant microorganisms mostly in the form of bacteria (Schid-lowski, 1995) and during progressive metamorphism, gradualgraphitization of bacteria cells could have occurred (Tazaki et al.,1992).

Graphite samples with intermediate d13C values (�13‰ to�19‰) could result from mixing of carbon derived from organicand carbonate and/or mantle carbon sources (Rumble and Hoering,1986; Rumble et al., 1986). The mixing of these two end-membersis possible by fluids derived from organic matter and carbonate.Metamorphic fluids are generally dominated by H2O with varyingamounts of CO2, CH4, O2 etc. and can be represented by the C–O–Hternary system. Metamorphism of organic matter at low fo2 pro-duces CH4-bearing fluids which are 13C depleted. In contrast, CO2

produced during metamorphism of argillaceous limestone is en-riched in 13C relative to organic matter and carbonate carbon attemperatures above about 200 �C (Ferry and Burt, 1982; Chackoet al., 2001). Provided that CO2/CH4 ratio are sufficiently low, suchfluids, will precipitate graphite of intermediate d13C values. Graph-ite occurring mostly in veins, within pegmatite and associated withpegmatite show the intermediate d13C values. These graphite sam-ples (e.g., Dandatopa Da-9, Tumudibandh T-8, Sapmund S-10, Sar-gipalli Sar-1, Sargimunda Sgm-2, Sargibahal Sgb-5, Pondkimal P-7,Beherapani Be-5, Ganjaudar GR-14) are interpreted to signifymixed fluids of organic and carbonate carbon origin. The requiredfluid could have precipitated graphite through reaction such asCO2 + CH4 = 2H2O + 2C (Deines, 1980) leading to a fluid rich in

)

25

30

Δ13CCalcite–Graphite(‰)= δ13CCalcite

_δ13CGraphite

258 P. Sanyal et al. / Journal of Asian Earth Sciences 36 (2009) 252–260

H2O. This mechanism is supported by the presence of hydrationreactions, for example sericitisation and chlorite-graphite inter-growth association with graphite. It is also possible that the graph-ite in these places represent multiple precipitation eventsdepending on the pegmatite intrusion during D2 or D3 deformationperiod (Acharya and Dash, 1984). Although multiple episodes ofgraphite precipitation can be documented by isotopic analysis ofgraphite flakes in micro scale (Satish-Kumar, 2000), such high res-olution sampling was not possible in the current study, and thenumber of events cannot be resolved.

Graphite can also precipitate from carbon bearing fluids duringcooling (Dubessy, 1984; Mathez et al., 1989). Heated C–O–H fluidscould rise up and precipitate graphite (Luque et al., 1998; Luqueand Rodas, 1999). Lamb and Valley (1984) showed that graphitecan precipitate from CO2 rich fluids under granulite-facies condi-tions when fo2 is sufficiently low. The d13C value of graphite occu-pying the fractures in e.g., Dumerpara and Sargimunda deposits,suggests precipitation from C-rich fluids during cooling, derivedfrom mantle or devolatilization reactions. However, if C–O–H fluidsexperiences decrease in pressure, even drastic decrease in temper-ature can not keep the fluid in the stability field of graphite (Paster-is, 1999).

Some of the foliation-controlled pegmatites could have playedan important role in the localization of graphite bodies. The origi-nally dispersed organic matter in the country rocks was mobilizedduring emplacement of the pegmatites related to D2 and D3 defor-mations, and precipitated as graphite along the margins. Graphitehaving d13C values of around �22‰ to �26‰, e.g., Temerimal T-10and T-16, Ganjaudar G-12 and G-16 dominantly represent organicmatter transported to veins along the margins of pegmatite as asuspension in a fluid phase (Dissanayake, 1981). Movement ofgraphite at solid state is another mechanism for localization of thisgraphite (Acharya and Dash, 1984; Crespo et al., 2005).

6.2. Graphite–Calcite d13C values and carbon isotope thermometry

When graphite forms in the presence of carbonate during meta-morphism, the partition of carbon isotopes between the twophases is strongly dependent on temperature. The d13C value ofboth the phases will be modified depending on the relative amountof the phases along with the magnitude of the isotopic fraction-ation and temperature of metamorphism (Valley and O’Neil,1981; Dunn and Valley, 1992; Kitchen and Valley, 1995; Dunn,2005). Optical microscopic study of calcite-bearing graphite sam-ples from the EGMB show that specks, scales and flakes of graphiteare intergrown with calcite (Fig. 7), which indicate that they haveformed together (Acharya and Dash, 1984). As noted before, rela-

Fig. 7. Graphite flakes (G) are intergrown with calcite (C). Disseminated flakes arealso present along silicate (S) grain boundaries. Locality: Dudkamal.

tively lower d13C value of calcite associated with graphite is indic-ative of change in carbon isotope ratio of calcite duringgraphitization through isotopic exchange. Relatively higher d13Cvalues (Fig. 6) of calcite in samples that are free of graphite prob-ably indicate little or no exchange with carbon bearing fluid. Ab-sence of graphite in these samples can be due to a loss of organicmatter related to an influx of oxidizing fluids during metamor-phism. Fluid interaction is also supported by the lower d18O valuesof the same samples. CO2 released by devolatilization reactions andinteraction with carbonate can also lower the d18O value of carbon-ate (Bottinga, 1968; Valley, 1986). Similar depletion in oxygen iso-tope ratio of carbonate was also observed by Bhowmik et al. (1995)from the EGMB. It is also possible that the lower d18O values of cal-cite are result of interaction with an aqueous fluid lower in d18Ovalue.

In general it is assumed that the carbon isotope ratio of graphitepreserves its composition obtained at peak metamorphism due toits refractory nature (Scheele and Hoefs, 1992; Chacko et al., 2001).Therefore, the carbon isotope ratio of graphite and calcite togethercan be used to estimate maximum temperature achieved duringmetamorphism provided equilibrium was attained during ex-change and the graphite is fully crystalline (Dunn and Valley,1992). The temperature estimate from carbon isotope ratios ofgraphite and calcite could also be erroneous if calcite exchangesduring retrograde metamorphism and d13C of graphite remains un-changed (Satish-Kumar et al., 2002) or the exchange does not at-tain equilibrium as happens in graphite at lower metamorphicgrade (Valley, 2001).

Eighteen samples contained both graphite and calcite in suffi-cient concentrations to allow 13C-based estimates of metamorphictemperatures. Of these 18 samples, approximately 12 samplesexhibited large D13CCalcite–Graphite values (12 to 27 ‰) and 6 exhib-ited smaller D13CCalcite–Graphite values (2.3 to 4.8 ‰) (Fig. 8). Tem-peratures calculated using the Kitchen and Valley (1995)equation, best suited for temperatures above 650 �C (Dunn,2005), correspond to a temperature range of 620–950 �C in onegroup of samples while a range of 90–290 �C is obtained for an-other group (Fig. 8). The large D13CCalcite–Graphite values could thusindicate that either calcite or graphite has not attained isotopicequilibrium (Chacko et al., 1991), or the pristine isotopic ratio ofone or both minerals was altered by a later generation of

Temperature (t0C)

0 200 400 600 800 1000

Δ13C

calc

ite-g

raph

ite(‰

0

5

10

15

20

Equilibrium Exchange

Inequilibrium exchangeor change of carbon isotope ratio subsequent toattaining the highest temperature

Fig. 8. Relation between temperature of metamorphism and DCalcite–Graphite values(d13C value of calcite minus d13C value of graphite). Temperature was estimatedusing the equation D13C(Calcite–Graphite) = 3.56 � 106 T�2 (K), (Kitchen and Valley,1995). The lower DCalcite–Graphite values show temperature of metamorphismbetween 620 and 945 �C. The highest temperature obtained from carbon ther-mometry is close to the ultrahigh-temperature reported from the EGMB.

P. Sanyal et al. / Journal of Asian Earth Sciences 36 (2009) 252–260 259

carbon-rich fluid infiltration or armoring of carbonaceous matterwithin silicates preventing exchange with calcite (Bergfeld et al.,1996).

At equilibrium, the magnitude of carbon isotope fractionationbetween calcite and graphite decreases with increasing tempera-ture (Valley and O’Neil, 1981; Wada and Suzuki, 1983; Morikiyo,1984). However, at temperatures below 600 �C equilibrium be-tween calcite and graphite may not attained resulting in unusuallylarge fractionation (Valley and O’Neil, 1981; Valley, 1986). Duringisobaric cooling decarbonation is most common reaction in wollas-tonite-bearing calc-silicate granulite rocks (Bergfeld et al., 1996).The CO2 derived from decarbonation reaction can alter the carbonisotope ratio of calcite (Valley, 2001), while leaving that of graphiteunaffected. This mineral-specific exchange during retrogrademetamorphism would result in the lower estimates of metamor-phic temperatures.

The highest metamorphic temperature obtained by the graph-ite–calcite 13C thermometer (sample L-19) is 945 �C. On the otherhand, samples showing temperature less than 945 �C but morethan 600 �C have suffered the effect of decarbonation reactionsbut as the temperature was more than 600 �C equilibrium betweengraphite and calcite was maintained. Therefore, except the temper-ature 945 �C which is close to the ultrahigh-temperature reportedfrom the EGMB, the estimated other temperatures (<600 �C) do notcorrespond to the peak metamorphic temperature; instead it maycorrespond to the temperature of decarbonation reactions duringthe retrograde reactions (Bergfeld et al., 1996).

6.3. Comparison of temperatures estimated from different crustaldomains of the EGMB and implications thereof

As mentioned, EGMB has been divided into four crustal Do-mains (Fig. 2); (Rickers et al., 2001). Domain II, where the majorityof samples for the present study were collected from, has experi-enced three stages of metamorphism between 1.6 Ga and 550 Ma(Mezger and Cosca, 1999). In this domain the highest temperatureestimated using carbon isotope ratios of graphite–calcite is about30 �C higher than to the temperature estimated from reaction gridsusing calc-silicate granulite rocks of other areas (Dasgupta, 1993;Bhowmik et al., 1995; Shaw and Arima, 1996). The temperatureand pressure estimated from Domain II (Fig. 2) using the intersec-tion of the following reactions: (i) meionite = 3anorthite + calciteand (ii) anorthite + 2wollastonite = grossular + quartz, in a P–T gridis about 920 �C and 9 kbar, respectively, which reflects the retro-grade temperature (Shaw and Arima, 1996). Sengupta et al.(1999) documented ultrahigh-temperatures for the EGMB fromDomain IA (Fig. 2) P1000 �C from high Mg–Al granulites, whichis close to the highest temperature estimated by us supportingthe usefulness of calcite–graphite system in estimating ultrahigh-temperature.

7. Conclusions

Graphite in the EGMB of Orissa, India has possibly three sourcesof carbon but d13C values suggest that a major part of the graphitebodies formed by conversion of organic matter during metamor-phism. Most of the graphite which occurs as schist and dissemina-tions in metasediments, khondalite and calc-silicate granulite,derive from organic carbon sources. Other graphite deposits alsoformed by mixing of organic carbon and carbon derived from car-bonate and/or mantle sources. Some of the vein graphite and somewithin pegmatites fall in this category. Temperature estimationusing carbon isotope ratio of graphite and carbonate showed meta-morphic temperature up to 945 �C which is close to the ultrahigh-temperature reported from the EGMB. Relatively lower tempera-

ture (<600 �C) estimates indicate change in the carbon isotope val-ues of calcite during retrograde metamorphism.

Acknowledgements

One of the authors (BCA) thanks the Director, IMMT, Bhubane-swar for necessary permission to publish this paper. Variousgraphite mine owners provided necessary help in field studiesand sample collection. PS thanks Dr. S.K. Bhowmik for his adviceduring the initial writing of the manuscript. Thanks to Dr. SumanDas for assisting PS during field work. PS also thanks Dr. PeterSauer and Souvik Mukherjee for reading out the final manuscript.We are thankful to Prof. Pulak Sengupta and Prof. F.J. Luque fortheir critical comments and suggestions which improved thispaper significantly.

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