Geoscience Frontiers 10 (2019) 1167e1186

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Research Paper

Petrology and Sr-Nd isotope systematics of the Ahobil kimberlite (Pipe-16)from the Wajrakarur field, Eastern Dharwar craton, southern India

Abhinay Sharma a, Alok Kumar a, Praveer Pankaj a, Dinesh Pandit a, Ramananda Chakrabarti b,N.V. Chalapathi Rao a,*a EPMA and SEM Laboratories, Department of Geology, Banaras Hindu University, Varanasi 221005, IndiabCentre for Earth Sciences, Indian Institute of Science, Bangalore 560012, India

a r t i c l e i n f o

Article history:Received 25 March 2018Received in revised form4 July 2018Accepted 6 August 2018Available online 30 August 2018Handling Editor: Vinod Oommen Samuel

Keywords:PetrologyIsotopesKimberliteWajrakarurDharwar cratonIndia

* Corresponding author.E-mail address: nvcr100@gmail.com (N.V.C. Rao).Peer-review under responsibility of China University

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a b s t r a c t

Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobilkimberlite (Pipe-16) from the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC),southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopiteshowing orangeitic evolutionary trends, spinel displaying magmatic trend-1, abundant perovskite, Ti-rich hydrogarnet, calcite and serpentine are the various mineral constituents. On the basis of (i) liq-uidus mineral composition, (ii) bulk-rock chemistry, and (iii) Sr-Nd isotopic composition, we show thatAhobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites andlamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degreemelting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to thoseof orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. The TDM Nd model age (w2.0Ga) of the Ahobil kimberlite is (i) significantly older than those (1.5e1.3 Ga) reported for Wajrakarur andNarayanpet kimberlites of EDC, (ii) indistinguishable from those of the Mesoproterozoic EDC lamproites,and (iii) strikingly coincides with the timing of the amalgamation of the Columbia supercontinent. Highbulk-rock Fe-Ti contents and wide variation in oxygen fugacity fO2, as inferred from perovskite oxy-barometry, suggest non-prospective nature of the Ahobil kimberlite for diamond.

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1. Introduction

Kimberlites are small-volume and unusual ultramafic rockswhich are extremely enriched in incompatible trace elements as wellas in volatiles. Kimberlites are of economic as well as scientific valueowing to the following reasons: (i) they are major primary hosts todiamonds, (ii) entrain abundant mantle and crustal xenoliths, (iii)links to mantle metasomatism and (iii) constitute the deepestmagmas produced in the mantle which we may observe at the sur-face (e.g., Sparks, 2013; Aulbach et al., 2017;Tappe et al., 2018).Investigation of the kimberlite entrained xenoliths and diamondinclusions have significantly enhanced a better understanding of theevolution of the earth. However, despite decades of researche thereare several unresolved and contentious issues regarding the (i) origin

of Geosciences (Beijing).

eijing) and Peking University. Produc-nd/4.0/).

of kimberlite magma (see Le Roex, 1986; Heaman and Kjasgaard,2000; Heaman et al., 2003; Torsvik et al., 2010, 2016a, b; Currieand Beamount, 2011) (ii) composition of kimberlite magma(includingwall-rock assimilation) (e.g., Donnelly et al., 2012; Pilbeamet al., 2013; Kamenetsky and Yaxley, 2015) and (iii) the extent towhich kimberlites are modified by syn- and post emplacementprocesses including alteration by groundwaters (e.g., see contrastingviews of Stripp et al., 2006; Mitchell, 2013; Sparks, 2013; Afanasyevet al., 2014; Giuliani et al., 2014). Regardless of the generalcomplexity of kimberlite magma formation and evolution, a generalconsensus is that fresh magmatic bonafide kimberlites are strikinglysimilar in terms of mineralogy (Mitchell, 2008), major- and trace-element geochemistry (Khazan and Fialko, 2005; Kjarsgaard et al.,2009), as well as in radiogenic isotope compositions (Griffin et al.,2014; Sun et al., 2014) on a global scale.

Based on their mineralogical, geochemical and isotopic charac-teristics kimberlites have been conventionally divided into Group-1(archetypal kimberlite) and Group-2 (orangeite) types (Smith et al.,1983, 1985; Mitchell, 1995). It was initially thought that orangeites

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are confined only to the Kaapvaal Craton of southern Africa, buttheir subsequent reports from other localities like Dronning MaudLand, Antarctica (ca. 159 Ma; Romu et al., 2008), the Mainpur areaof the Bastar Craton, central India (ca. 65 Ma; Lehmann et al., 2010),the Timmasamudram area, Eastern Dharwar craton, southern India(ca. 90 Ma to ca. 1100 Ma; Chalapathi Rao et al., 2016; Dongre et al.,2017) and West Karelia (ca. 1.2 Ga; Kargin et al., 2014) firmlyestablished that they also occur outside the Kaapvaal craton.

The Eastern Dharwar craton (EDC) of southern India is theworld’s largest known repository of Proterozoic kimberlitesnumbering more than a hundred. In view of overlapping mineral-ogical and geochemical aspects of some of these occurrences withthose from the Group I and II kimberlites as well as lamproites andultramafic lamprophyres (aillikites) their precise nomenclature hasbeen a subject of contention (see Haggerty and Birkett, 2004; Kaurand Mitchell, 2013, 2016; Smith et al., 2013; Shaikh et al., 2017). Onthe basis of petrological and geochemical characteristics ChalapathiRao and Dongre (2009) and Chalapathi Rao et al. (2012) classifiedmany of the NKF (Narayanpet kimberlite field) kimberlites to betransitional between Group-I and -II variants with a strong affinitytowards the Group-I type. Interestingly, a Late Cretaceous kimber-lite from Timmasamudram cluster in WKF (Chalapathi Rao et al.,2016) has recently been demonstrated to be of Group-II (orange-ite) variety. Thus, the presence of Group I, II as well as their tran-sitional variants of kimberlites is indicated in the eastern Dharwarcraton. In this study, we present the petrology and geochemistry(including Sr and Nd isotopic composition) of a newly discoveredPipe-16 kimberlite pipe from theWajrakarureLattavaram cluster ofWKF in the EDC. The pipe is to referred as Ahobil kimberlite in thispaper because of its spatial proximity close to Penna Ahobilam, afamous temple of Lord Narashima. The objectives of this study areto (i) understand the petrological and mineralogical characteristicsof the Ahobil kimberlite, (ii) characterize the occurrence withrespect to global as well as Indian kimberlites and orangeites, (iii)constrain its petrogenesis and (iv) provide insights into its diamondprospectivity.

2. Geological setting

The ArchaeanDharwar Craton of the Indian shield is bounded byBastar Craton towards north east, by polymetamorphic ProterozoicEastern Ghats granulite facies mobile belt towards the east, lavasflows of Deccan large Igneous province in the north west and thesouthern granulite terrain in the south (Ramakrishnan andVaidyanadhan, 2008; Fig. 1). The Dharwar Craton is dominated bythe granite-green stone belts as well as gneissic basement oftonalite-trondhjemite-granidiorite (TTG) composition. Theseinturn are intruded by north-south trending granitic plutonscollectively known as Closepet Granite of 2510 Ma (Friend andNutman, 1991). A number of Paleo-Mesoproterozoic intracratonicsedimentary basins overlie the granite-greenstone terrain towardsits eastern and northern margins. The Chitradurga schist belt di-vides the Dharwar craton into two distinct groups called as EasternDharwar Craton (EDC) and Western Dharwar Craton (WDC)(Jayananda et al., 2006).

In the Dharwar Craton, kimberlites are virtually restricted to theEDC and are distributed over four distinct fields viz., (i) the Waj-rakarur field (WKF), (ii) the Tungabhadra field (TKF), (iii) the Rai-chur field (RKF) and (iv) the Narayanpet field (NKF) (see Nayak andKudari, 1999; Neelakantam, 2001; Paton et al., 2009) towards thewestern margin of the Paleo-to Mesoproterozoic Cuddapah sedi-mentary basin. The WKF is the largest of them and comprises fourclusters viz., (i) the WajrakarureLattavaram, (ii) the Chigicherla,(iii) the Timmasamudram and (iv) the Kalyandurg. Availableemplacement ages of the kimberlites in the EDC reveals two

distinct age groups of (i) Mesoproterozoic at w1100 Ma (Gopalanand Kumar, 2008; Osborne et al., 2011; Chalapathi Rao et al.,2013a,b) and (ii) Late Cretaceous at w90 Ma (Chalapathi Raoet al., 2016).

The Ahobil kimberlite under study was discovered by thegeologists of GSI during their regular field work and reported itas Pipe-16 (Fig. 1 for location) in their annual progress report(Geological Survey of India, 2018). Preliminary field andgeochemical studies on this kimberlite were also reported byPhani and Raju (2017). The body exposed at the confluence of astream (Balkamthota vanka) with the Penner river bed (Fig. 2),has an irregular outline, and most of it lies submerged underwater cover barring some portions which have protruded aboveflowing water of river and are exposed along the river channelmargin.

3. Sampling and analytical techniques

The freshest possible samples were collected from the exposedparts of the outcrop that too during the dry season when thewater content of the river channel was low. Special care was takento remove all visible crustal and mantle-derived contaminantsbefore subjecting the samples to geochemical analysis. Petro-graphic study of the Ahobil kimberlite was carried out by com-bined optical microscopy and EPMA based back scattered electron(BSE) imaging. Mineral chemistry was carried out on CAMECA SXFive model EPMA at the Mantle Petrology lab, Department ofGeology, Institute of Science, Banaras Hindu University, Varanasi.Wavelength Dispersive Spectrometry (WDS) with TAP, LIF, LLIFand PET were employed. A number of in house standards (Pandeyet al., 2018) were used for calibration using an accelerationvoltage of 15 kV, beam current of 10 nA, 1 mm beam diameter wereused. The accuracy of analysis is 3s error and confidence level of95.5% and precision of (0.1 wt.%). Major and trace elements inolivine were analysed together at an acceleration voltage of 25 kV,beam current of 40 nA and beam size of 1 mm and calibrationsettings are given in the Supplementary Table S1. Mineral chem-istry data obtained for various phases are presented in Tables 1e6.

Whole rock major and trace elements of 6 samples werecarried out at Activation laboratories, Ancaster Canada. Multiacid digestion ICP-OES Model: (Thermo-JarretAsh ENVIRO II)was used to analyse the major elements, while ICP-MS (Perki-nElmer Sciex ELAN 6000) was used to analyse the trace and rareearth elements (REE). STM1, MRG1, DNC1, W2, SY3 were used asinternal standards and precision is approximately 5% and 5%e10% for major oxides and trace elements respectively at100 � detection limit. The bulk rock geochemical data is pro-vided in Table 7. The analytical procedure is detailed by Galeet al. (1997) and is available at the Activation Laboratories Ltdwebsite (www.actlabs.com).

For the determination of Sr and Nd isotope compositions, threesamples were dissolved using ultra-pure HF, HNO3 and HCl acidmixtures. Strontium and neodymiumwere separated from the rockmatrix using ion exchange column chromatography and thedetailed procedure is described in Banerjee et al. (2016). The Sr andNd isotope ratio measurements were carried out using a ThermoScientific Triton Plus Thermal Ionization Mass Spectrometer (TIMS)at the Centre for Earth Sciences, Indian Institute of Science using aprotocol described in Banerjee et al. (2016). Measured 87Sr/86Sr and143Nd/144Nd ratios were normalized to 86Sr/88Sr ¼ 0.1194 and146Nd/144Nd ¼ 0.7219, respectively to correct for the instrumentalmass fractionation. JNdi-1 Nd isotopic-standard and SRM-987 Srisotopic standard were used during the analyses. The results ob-tained are provided in Table 7.

http://www.actlabs.com

Figure 1. Location map of various kimberlites in Wajrakarur kimberlite field (modified after Nayak and Kudari, 1999). Red asterisk denoted as P16 is the kimberlite of this study.

A. Sharma et al. / Geoscience Frontiers 10 (2019) 1167e1186 1169

4. Petrography and mineral chemistry

4.1. Petrography

Petrographic studies show that the studied samples belong tocoherent facies of kimberlite volcanism (Cas et al., 2008) andpossess the characteristic inequigranular texture typical of kim-berlites (Fig. 3A) (see Mitchell, 1997) imparted by different sizedpopulations of olivine viz., (i) euhedral macrocrystal (>0.5mm) and(ii) subhedral microphenocrystal (

Figure 2. Field view of Ahobil kimberlite emplaced within the basement gneiss and exposed in Penner river bed.

A. Sharma et al. / Geoscience Frontiers 10 (2019) 1167e11861170

4.2. Mineral chemistry

4.2.1. OlivineOlivine shows a range in composition (Fo: 83e93 and NiO:

0.44e0.15; Tables 1 and 2) and NiO varies with Fo content

Table 1Mineral chemistry (oxide in wt.%) of olivine phenocrysts in the Ahobil kimberlite sampl

Oxide (wt.%) 1 2 3 4 5

SiO2 40.17 40.36 40.19 40.36 40.27TiO2 0.04 0.04 0.04 0.04 0.04Al2O3 0.02 0.04 0.02 0.01 0.04Cr2O3 0.05 0.05 0.04 0.04 0.04FeO 11.38 9.49 12.43 11.96 11.01MnO 0.13 0.13 0.15 0.14 0.14MgO 48.07 49.69 46.86 47.51 48.50NiO 0.38 0.39 0.39 0.39 0.39CaO 0.00 0.00 0.00 0.00 0.00Na2O 0.04 0.08 0.03 0.03 0.05V2O3 0.01 0.01 0.02 0.02 0.02Total 100.29 100.28 100.17 100.5 100.5Cations based on 4 oxygenSi 0.991 0.978 0.992 0.990 0.980Ti 0.001 0.001 0.001 0.001 0.001Al 0.001 0.001 0.001 0.000 0.001Cr 0.001 0.001 0.001 0.001 0.001Fe(II) 0.235 0.233 0.277 0.266 0.265Mn 0.003 0.003 0.003 0.003 0.003Mg 1.768 1.795 1.724 1.738 1.759Ni 0.008 0.008 0.008 0.008 0.008Ca 0.000 0.000 0.000 0.000 0.000Na 0.002 0.004 0.002 0.001 0.002V 0.000 0.000 0.000 0.000 0.000Total 3.008 3.022 3.008 3.009 3.020Fo 88.16 88.41 86.01 86.60 86.80Fa 11.71 11.46 13.83 13.25 13.06Tp 0.14 0.13 0.16 0.15 0.15Trace elements (ppm)Ti 214 223 254 235 236Al 130 231 104 79 215Cr 342 350 291 286 289Mn 1005 1024 1154 1118 1121Ni 3020 3033 3086 3039 3044Na 310 608 259 228 390V 91 99 133 130 111T (�C) 1257 1335 1221 1188 1314

(Supplementary Fig. 1). Olivine macrocrysts and microphenocrystshave distinct compositions. The macrocrystic olivines have higherFo and lower Ti concentrations which imply that they are of foreignorigin unlike the phenocryst olivines which have crystalliseddirectly from the kimberlite magma. Al concentration is below

es. Temperature (T) is calculated using the method of De Hoog et al. (2010).

6 7 8 9 10 11

39.75 39.64 41.02 39.54 39.37 39.790.05 0.06 0.01 0.07 0.06 0.030.01 0.01 0.02 0.02 0.06 0.040.07 0.08 0.12 0.06 0.08 0.0212.97 14.33 7.45 14.08 14.27 12.820.15 0.16 0.11 0.16 0.16 0.1846.73 45.60 51.76 45.84 46.29 46.190.37 0.38 0.40 0.33 0.38 0.210.00 0.00 0.00 0.00 0.00 0.000.03 0.02 0.01 0.03 0.03 0.010.00 0.02 0.01 0.02 0.02 0.01100.13 100.3 100.91 100.15 100.72 99.3

0.985 0.991 0.985 0.984 0.981 0.9970.001 0.001 0.000 0.001 0.001 0.0010.000 0.000 0.001 0.001 0.002 0.0010.001 0.002 0.002 0.001 0.001 0.0000.289 0.300 0.162 0.314 0.297 0.2690.003 0.003 0.002 0.003 0.003 0.0041.726 1.700 1.853 1.701 1.720 1.7250.007 0.008 0.008 0.007 0.008 0.0040.000 0.000 0.000 0.000 0.000 0.0000.001 0.001 0.001 0.001 0.001 0.0000.000 0.000 0.000 0.000 0.000 0.0003.014 3.007 3.014 3.014 3.016 3.00285.51 84.87 91.88 84.28 85.11 86.3614.34 14.96 8.01 15.55 14.72 13.440.15 0.17 0.11 0.17 0.17 0.20

321 378 62 418 385 20161 44 102 105 303 215455 525 818 419 520 1551135 1267 882 1275 1258 14302924 3013 3111 2569 3014 1657192 138 102 195 187 410 147 48 152 141 991176 1139 1263 1240 1399 1282

Table 2Mineral chemistry (oxide in wt.%) of olivine macrocrysts in the Ahobil kimberlite samples.

Oxide 1 2 3 4 5 6 7 8 9 10

SiO2 41.29 40.70 40.70 40.72 41.04 40.90 40.92 40.69 41.02 41.70TiO2 0.02 0.04 0.04 0.02 0.00 0.04 0.04 0.00 0.00 0.00Al2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Cr2O3 0.09 0.03 0.03 0.02 0.01 0.04 0.04 0.01 0.02 0.02FeO 7.16 8.42 8.42 9.82 6.53 9.06 9.05 6.61 6.59 6.56MnO 0.11 0.12 0.12 0.19 0.09 0.13 0.13 0.09 0.09 0.09MgO 52.44 50.91 50.91 49.56 52.27 50.82 51.20 53.81 52.95 52.98NiO 0.44 0.30 0.30 0.27 0.39 0.36 0.35 0.39 0.39 0.39CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Na2O 0.02 0.01 0.01 0.00 0.01 0.01 0.02 0.01 0.01 0.01V2O3 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.00 0.01 0.01Total 100.58 100.54 100.54 100.61 100.34 100.37 100.76 100.61 100.08 100.76Cations based on 4 oxygenSi 0.987 0.983 0.983 0.988 0.990 0.988 0.985 0.972 0.983 0.991Ti 0.000 0.001 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000Al 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Cr 0.002 0.001 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000Fe(II) 0.143 0.190 0.190 0.220 0.132 0.183 0.182 0.132 0.132 0.130Mn 0.002 0.002 0.002 0.004 0.002 0.003 0.003 0.002 0.002 0.002Mg 1.869 1.833 1.833 1.793 1.879 1.830 1.837 1.915 1.892 1.877Ni 0.008 0.006 0.006 0.005 0.008 0.007 0.007 0.007 0.008 0.007Ca 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Na 0.001 0.000 0.000 0.000 0.000 0.001 0.001 0.000 0.000 0.000V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total 3.01 3.02 3.02 3.01 3.01 3.01 3.02 3.03 3.02 3.01Fo 92.78 90.49 90.49 88.92 93.37 90.79 90.86 93.47 93.40 93.42Fa 7.11 9.40 9.40 10.89 6.54 9.08 9.00 6.44 6.52 6.49Tp 0.11 0.12 0.12 0.19 0.09 0.13 0.13 0.09 0.09 0.09Trace elements (ppm)Ti 112 232 232 141 28 210 220 23 24 27Al 0 0 0 0 0 0 0 0 0 0Cr 594 228 228 105 101 241 242 100 105 103Mn 844 916 916 1453 676 1021 1009 691 675 685Ni 3469 2388 2388 2094 3050 2793 2727 3036 3071 3050Na 114 77 77 23 41 85 119 53 57 39V 46 67 67 85 30 82 73 24 35 37

Table 3Mineral chemistry (oxide in wt.%) of ground mass phlogopite from the Ahobil kimberlite.

Oxide 1 2 3core

4rim

5rim

6core

7 8 9 10 11 12

SiO2 42.00 42.21 41.28 40.33 40.01 39.43 40.72 43.73 39.81 41.36 45.30 45.73TiO2 2.03 2.11 2.43 2.87 2.64 2.64 2.71 1.52 2.63 1.74 2.03 1.57Al2O3 6.08 5.95 5.91 7.21 6.33 6.84 6.77 6.18 6.71 5.60 6.00 6.11Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00FeOT 9.25 9.26 9.03 10.32 11.27 10.81 9.38 9.95 10.55 11.13 10.53 10.58MnO 0.00 0.04 0.00 0.09 0.11 0.00 0.00 0.00 0.00 0.03 0.00 0.00MgO 23.56 23.79 22.42 21.14 20.64 21.35 23.16 25.33 21.44 23.76 24.39 24.82CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00BaO 0.08 0.12 0.27 0.61 0.53 0.34 0.88 0.23 0.61 0.23 0.00 0.15Na2O 0.73 0.66 0.73 0.73 0.49 0.50 0.49 0.19 0.52 0.44 0.15 0.14K2O 8.76 9.33 9.16 8.45 8.50 8.39 8.88 7.59 8.81 6.92 4.04 4.36Cl 0.02 0.04 0.01 0.02 0.02 0.03 0.00 0.01 0.04 0.00 0.00 0.00F 1.83 2.95 3.74 1.55 3.06 2.84 3.34 0.60 2.81 3.73 4.32 2.69Total 94.34 96.46 94.97 93.33 93.60 93.18 96.32 95.34 93.92 94.94 96.74 96.14Cations for 22 oxygenSi 6.29 6.27 6.29 6.15 6.22 6.12 6.12 6.33 6.14 6.28 6.55 6.56Al 1.07 1.04 1.06 1.30 1.16 1.25 1.20 1.06 1.22 1.00 1.02 1.03Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ti 0.23 0.24 0.28 0.33 0.31 0.31 0.31 0.17 0.30 0.20 0.22 0.17Fe(II) 1.16 1.15 1.15 1.32 1.46 1.40 1.18 1.21 1.36 1.41 1.27 1.27Mn 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ba 0.00 0.01 0.02 0.04 0.03 0.02 0.05 0.01 0.04 0.01 0.00 0.01Mg 5.26 5.27 5.10 4.80 4.78 4.94 5.19 5.47 4.93 5.38 5.26 5.31Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Na 0.21 0.19 0.22 0.22 0.15 0.15 0.14 0.05 0.15 0.13 0.04 0.04K 1.67 1.77 1.78 1.64 1.68 1.66 1.70 1.40 1.73 1.34 0.75 0.80Cl 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00F 0.86 1.39 1.80 0.75 1.50 1.40 1.59 0.27 1.37 1.79 1.97 1.22Total 15.89 15.94 15.89 15.80 15.81 15.85 15.90 15.70 15.88 15.76 15.11 15.18

A. Sharma et al. / Geoscience Frontiers 10 (2019) 1167e1186 1171

Table 4Mineral chemistry (oxide in wt.%) of ground mass spinel from Ahobil kimberlites of this study.

Oxide 1 2 3 4 5 6 7 8 9 10 11

SiO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00TiO2 10.77 16.32 11.48 11.67 14.79 14.79 13.48 8.55 12.57 14.99 11.27Al2O3 4.65 3.76 5.41 4.98 3.51 3.51 3.88 7.54 5.16 3.52 5.50Cr2O3 17.46 5.56 24.62 22.12 8.37 8.37 9.48 34.61 16.54 3.56 16.07V2O3 0.51 0.66 0.59 0.57 0.61 0.61 0.58 0.58 0.57 0.59 0.53Fe2O3 27.78 28.88 22.15 24.09 31.69 31.69 31.78 13.75 27.37 33.49 28.91FeO 29.27 37.48 21.92 23.53 32.72 32.72 34.87 22.67 28.15 37.10 28.55MnO 1.29 2.20 1.06 1.06 1.77 1.77 1.99 1.27 1.21 2.07 1.35MgO 7.22 4.23 12.98 11.94 7.16 7.16 4.89 10.85 9.55 3.92 8.34CaO 0.00 0.31 0.00 0.00 0.00 0.00 0.04 0.00 0.02 0.00 0.00ZnO 0.24 0.35 0.23 0.22 0.34 0.34 0.42 0.25 0.24 0.34 0.26Total 99.19 99.76 100.44 100.19 100.97 100.97 101.42 100.07 101.39 99.57 100.78Cations for 32 oxygen atomsSi 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Ti 2.283 3.530 2.293 2.361 3.103 3.103 2.859 1.717 2.558 3.269 2.326Al 1.542 1.275 1.693 1.579 1.156 1.156 1.290 2.374 1.647 1.203 1.778Cr 3.889 1.264 5.169 4.703 1.847 1.847 2.115 7.306 3.539 0.816 3.485V 0.116 0.153 0.126 0.124 0.137 0.137 0.130 0.123 0.123 0.138 0.117Fe(III) 5.888 6.248 4.426 4.874 6.653 6.653 6.746 2.762 5.575 7.306 5.968Fe(II) 6.893 9.011 4.869 5.289 7.636 7.636 8.226 5.063 6.370 8.995 6.549Mn 0.307 0.535 0.238 0.241 0.419 0.419 0.476 0.286 0.277 0.508 0.314Mg 3.033 1.814 5.140 4.788 2.979 2.979 2.057 4.319 3.855 1.694 3.409Ca 0.000 0.096 0.000 0.000 0.000 0.000 0.013 0.000 0.007 0.000 0.000Zn 0.049 0.074 0.046 0.043 0.070 0.070 0.088 0.050 0.049 0.072 0.053Total 24 24 24 24 24 24 24 24 24 24 24Ti/(Ti þ Cr þ Al) 0.30 0.58 0.25 0.27 0.51 0.51 0.46 0.15 0.33 0.62 0.31Fe2þ/(Fe2þ þ Mg) 0.69 0.83 0.49 0.52 0.72 0.72 0.80 0.54 0.62 0.84 0.66Oxide 12 13 14 15 16 17 18 19 20 21 22 23 24 25

SiO2 0.00 0.27 0.00 0.18 0.00 0.00 0.00 0.00 0.32 0.00 0.00 0.00 0.00 0.00TiO2 11.57 12.94 14.49 11.82 10.37 8.42 7.66 11.35 8.36 15.11 13.31 11.93 8.63 8.19Al2O3 5.14 4.15 2.92 3.68 6.32 8.51 7.00 5.84 6.03 4.34 4.97 4.53 7.59 6.56Cr2O3 19.13 11.83 3.58 12.11 25.81 36.33 35.58 22.75 30.90 9.53 14.48 16.71 34.74 35.63V2O3 0.55 0.59 0.61 0.53 0.57 0.63 0.56 0.61 0.53 0.70 0.65 0.60 0.61 0.61Fe2O3 25.89 29.68 33.32 32.05 20.24 11.41 15.11 22.69 18.58 27.70 27.23 27.91 13.23 13.95FeO 27.77 33.20 40.48 32.47 25.35 21.75 22.53 26.65 23.61 34.20 28.83 27.24 22.25 22.32MnO 1.24 1.86 2.30 1.67 1.27 1.21 1.22 1.08 1.18 1.99 1.19 1.10 1.15 1.17MgO 9.05 5.90 1.05 5.71 10.01 11.46 10.33 10.00 10.23 6.10 9.31 9.43 11.15 10.69CaO 0.00 0.13 0.00 0.00 0.00 0.12 0.17 0.00 0.00 0.10 0.00 0.01 0.04 0.00ZnO 0.25 0.36 0.47 0.37 0.25 0.22 0.23 0.24 0.26 0.37 0.24 0.25 0.22 0.24Total 100.59 100.92 99.23 100.58 100.20 100.07 100.40 101.22 100.00 100.14 100.20 99.71 99.61 99.36Cations for 32 oxygen atomsSi 0.000 0.077 0.000 0.050 0.000 0.000 0.000 0.000 0.086 0.000 0.000 0.000 0.000 0.000Ti 2.380 2.729 3.251 2.513 2.111 1.675 1.544 2.295 1.700 3.203 2.745 2.476 1.735 1.666Al 1.655 1.372 1.028 1.227 2.016 2.652 2.209 1.850 1.923 1.440 1.607 1.474 2.393 2.089Cr 4.136 2.622 0.843 2.707 5.519 7.595 7.536 4.837 6.608 2.124 3.140 3.645 7.345 7.612V 0.120 0.133 0.146 0.119 0.123 0.133 0.120 0.132 0.115 0.157 0.142 0.133 0.131 0.131Fe(III) 5.329 6.262 7.480 6.820 4.120 2.271 3.047 4.590 3.781 5.874 5.620 5.795 2.662 2.837Fe(II) 6.351 7.785 10.097 7.678 5.735 4.809 5.047 5.993 5.340 8.059 6.614 6.286 4.976 5.045Mn 0.286 0.441 0.582 0.399 0.290 0.271 0.277 0.246 0.271 0.474 0.276 0.258 0.260 0.268Mg 3.691 2.466 0.468 2.409 4.036 4.519 4.127 4.010 4.124 2.561 3.807 3.878 4.444 4.305Ca 0.000 0.039 0.000 0.000 0.000 0.033 0.047 0.000 0.000 0.032 0.000 0.004 0.010 0.000Zn 0.051 0.075 0.104 0.078 0.050 0.043 0.046 0.047 0.051 0.076 0.049 0.051 0.044 0.048Total 24 24 24 24 24 24 24 24 24 24 24 24 24 24Ti/(Ti þ Cr þ Al) 0.29 0.41 0.63 0.39 0.22 0.14 0.14 0.26 0.17 0.47 0.37 0.33 0.15 0.15Fe2þ/(Fe2þ þ Mg) 0.63 0.76 0.96 0.76 0.59 0.52 0.55 0.60 0.56 0.76 0.63 0.62 0.53 0.54

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detection limit for the macrocrysts but ranges of 44e303 ppm inmicrophenocrysts. Temperature for the phenocrystal olivine hasbeen calculated using Al in olivine geothermometry of De Hooget al. (2010) at an assumed pressure of 50 kbar based on the xe-noliths from kimberlites of Wajrakarur-Lattavaram cluster (seeGanguly and Bhattacharya, 1987; Nehru and Reddy, 1989). Thecalculated temperatures (Table 1) are within the range of crystal-lisation temperature of olivine and absence of complex zoningsuggests that they were crystallised from the kimberlite magma.BSE images reveal little composition variation apart from serpen-tinisation along the rims. Composition of the olivines of this studyis indistinguishable from the data reported for macrocrysts andmicrophenocrysts from world-wide kimberlites and orangeite (Fo81.7e91.5 and up to 0.42 wt.% of NiO; Arndt et al., 2010).

4.2.2. PhlogopiteThe chemistry of phlogopite from ultramafic alkaline rocks has

often been used for their nomenclature aswell as for understandingthe evolution of their magmas (Mitchell, 1995; Brod et al., 2001;Reguir et al., 2009; Lepore et al., 2017). In the Ahobil phlogopite,TiO2 ranges from 1.52 wt.% to 2.87 wt.% whereas Al2O3 varies from5.60 wt.% to 7.21 wt.% (Table 3). On the other hand, the Cr2O3 con-tents are too low (

Table 5Mineral chemistry (oxide in wt.%) of perovskite from Ahobil kimberlite samples under study.

Oxide 1 2 3 4 5 6 7 8 9 10 11 12

Na2O 0.20 0.20 0.40 0.35 0.40 0.28 0.31 0.24 0.24 0.23 0.25 0.24Al2O3 0.13 0.15 0.13 0.15 0.15 0.16 0.16 0.13 0.14 0.13 0.14 0.13SiO2 0.02 0.04 0.05 0.03 0.00 0.01 0.01 0.04 0.04 0.05 0.03 0.05CaO 40.02 40.47 39.05 39.06 38.56 39.80 39.50 40.31 40.08 40.40 40.15 40.04TiO2 57.31 56.74 56.79 56.71 57.29 56.11 56.27 56.18 56.49 56.31 56.68 56.58FeOT 1.30 1.16 0.87 1.16 1.14 1.03 1.12 1.05 0.97 0.97 1.01 1.02SrO 0.29 0.32 0.29 0.25 0.22 0.24 0.23 0.37 0.32 0.34 0.35 0.28Nb2O5 0.23 0.44 0.30 0.21 0.14 0.24 0.38 0.39 0.38 0.34 0.31 0.35La2O3 0.15 0.14 0.47 0.41 0.34 0.33 0.33 0.21 0.18 0.19 0.18 0.27Ce2O3 0.34 0.27 1.12 1.09 1.19 0.62 1.06 0.40 0.44 0.40 0.41 0.48Pr2O3 0.00 0.02 0.18 0.12 0.10 0.01 0.07 0.05 �0.02 0.01 0.04 0.00Nd2O3 0.06 0.04 0.42 0.39 0.41 0.17 0.38 0.05 0.11 0.04 0.14 0.13Sm2O3 0.00 0.00 0.00 0.10 0.03 0.01 0.01 0.00 0.00 0.00 0.00 0.00Ta2O5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00ThO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Total 100.04 100.00 100.10 100.03 99.97 99.02 99.83 99.43 99.38 99.41 99.68 99.57Cations for 3 oxygen atomsNa 0.009 0.009 0.018 0.015 0.018 0.012 0.014 0.011 0.011 0.010 0.011 0.011Al 0.003 0.004 0.004 0.004 0.004 0.004 0.004 0.003 0.004 0.004 0.004 0.003Si 0.000 0.001 0.001 0.001 0.000 0.000 0.000 0.001 0.001 0.001 0.001 0.001Ca 0.964 0.979 0.953 0.950 0.936 0.975 0.963 0.983 0.978 0.985 0.976 0.975Ti 0.969 0.963 0.973 0.968 0.977 0.965 0.964 0.962 0.967 0.964 0.968 0.967Fe 0.049 0.044 0.033 0.044 0.043 0.039 0.043 0.040 0.037 0.037 0.038 0.039Sr 0.004 0.004 0.004 0.003 0.003 0.003 0.003 0.005 0.004 0.004 0.005 0.004Nb 0.002 0.004 0.003 0.002 0.001 0.003 0.004 0.004 0.004 0.003 0.003 0.004La 0.001 0.001 0.004 0.003 0.003 0.003 0.003 0.002 0.002 0.002 0.002 0.002Ce 0.003 0.002 0.009 0.009 0.010 0.005 0.009 0.003 0.004 0.003 0.003 0.004Pr 0.000 0.000 0.002 0.001 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000Sm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Nd 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Ta 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Th 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total 2.00 2.01 2.00 2.00 2.00 2.01 2.01 2.02 2.01 2.01 2.01 2.01DNNO �4.38 �2.86 �0.43 �3.23 �3.08 �2.05 �2.65 �2.02 �1.29 �1.32 �1.64 �1.72

FeOT is total iron. DNNO is the log fO2 relative to the NNO (nickel-nickel oxide) buffer.

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kimberlites and orangeites than to those of lamproites. However, inFeO vs. Al2O3 compositional diagram (after Mitchell, 1995) phlog-opites display an evolutionary trend of orangeites (Fig. 5B) similar tothose displayed by NKF and WKF phlogopites as well as those fromBehradih orangeite, Bastar craton, Central India (Chalapathi Raoet al., 2011). Low Al concentration in the liquid and high fO2 couldbe the factors for the low Ti tetra-ferriphlogopite character dis-played by themica under study (Table 3; Heathcote andMcCormick,1989; Brigatti et al., 1996). Moreover, as phlogopites are the latestage phases, small-scale variation or heterogeneities in the magmacomposition may also have been responsible for displaying thisaspect (Reguir et al., 2009) and likely to be a characteristic feature ofthe Ahobil kimberlite magma (Guarino et al., 2013).

4.2.3. SpinelSpinel is ubiquitous and their representative composition is

presented in Table 4. Spinel from the present study shows acompositional range from magnesian titanian magnetite totitanian-magnesiochromite. Their MgO content has a wide range(1.05e12.98 wt.%) and are conspicuously Mn-rich (up to 2.30 wt.%).Mitchell (1986) delineated two trends amongst kimberlitesgroundmass spinel a magmatic trend-I (magnesian ulvospinel-magnetite trend) and magmatic trend-II (titanian magnetitetrend). Where magmatic trend-I is characteristic of kimberlites, themagmatic trend-II is well known from orangeites, basalts andlamprophyres (Tappe et al., 2004, 2005; Roeder and Schulze, 2008).Significant population of spinels from the Ahobil kimberlite displaytrend-I in contrast to the evolutionary trend-II displayed by TK-1and TK-4 orangeites of Timmasamudram cluster (Fig. 6A). Spinelsof this study also preferentially show Ti enrichment with

decreasing Cr content (Fig. 6B) at a constant or slight increase in Fecontent which is also considered to be a characteristic of magmatictrend-I (Mitchell, 1995).

4.2.4. PerovskitePerovskite is paragenetically a well characterised common

groundmass phase in kimberlites and accommodates a broad rangeof elements (mostly rare earth elements) in its crystal structure(Chakhmourdian et al., 2000, 2013). Volumetric abundance (w5e7vol.%) of perovskite is one of the key features of Ahobil kimberlite ascompared to many other Eastern Dharwar Craton kimberlites andits representative composition is presented in Table 5. At places,perovskite occur as garlands around olivine and to better representthis feature, X-ray elemental maps of Si, Mg, Fe, Ca, and Ti areprovided (Fig. 4BeF). Perovskites from the Ahobil kimberlite have arestricted range of CaO (38.49e40.82 wt.%) and TiO2(55.9e57.3 wt.%). Their FeOT content ranges of 0.81e1.44 wt.% andis comparable to that reported in perovskite from other globalarchetypal kimberlite (1e2 wt.%; Mitchell, 1995). On the contrary,perovskite from orangeite reportedly have higher TiO2, and lowerFeO (Mitchell, 1995). The SrO content (0.37e0.16 wt.%) in the pe-rovskites under study is similar to that reported from archetypalsouthern African kimberlites and thus differentiates it from theperovskite found in southern African orangeites. The Nb2O5(0.12e0.44 wt.%) content of perovskites is also lower than thosefound in NKF (0.22e2.33 wt.%) andWKF (0.34e2.33 wt.%). LREE2O3of perovskites of this study range from (0.34e1.91 wt %) and in bi-variate plot (Fig. 7) involving TiO2 and REE2O3, the perovskites ofthis study are confined to the kimberlite field unlike those from theTimmasamudram orangeite (Dongre et al., 2017).

Table 6Mineral chemistry (oxide in wt.%) of hydrogarnet cores from the groundmass of Ahobil kimberlite.

Oxide 1 2 3 4 5 6 7 8 9 10

SiO2 30.77 31.35 31.46 31.98 31.70 31.01 31.25 31.27 31.67 30.57TiO2 26.08 24.56 25.47 24.89 24.69 25.97 25.34 23.15 23.44 24.20Al2O3 2.83 2.79 3.03 3.74 3.45 3.42 2.95 3.36 2.66 2.98Cr2O3 0.92 1.23 0.34 0.09 0.11 0.18 0.21 0.18 0.33 0.77FeO 4.44 5.80 5.30 4.01 4.40 4.92 5.93 9.07 7.96 8.51MnO 0.06 0.11 0.07 0.04 0.04 0.08 0.06 0.14 0.10 0.15MgO 8.27 9.09 8.88 9.97 10.33 9.61 9.07 10.02 10.10 9.68CaO 21.46 19.93 20.60 19.66 19.54 19.41 20.04 17.79 19.01 18.72Total 94.83 94.86 95.15 94.39 94.26 94.60 94.86 94.97 95.28 95.58Cations calculated for 12 oxygen atomsSi 2.632 2.670 2.670 2.707 2.685 2.639 2.661 2.648 2.671 2.584Ti 1.677 1.573 1.626 1.585 1.572 1.662 1.623 1.474 1.487 1.539Al 0.285 0.280 0.303 0.373 0.344 0.343 0.296 0.335 0.264 0.297Cr 0.062 0.083 0.023 0.006 0.008 0.012 0.014 0.012 0.022 0.052Fe3þ 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe2þ 0.318 0.413 0.377 0.284 0.311 0.350 0.423 0.643 0.562 0.602Mn 0.005 0.008 0.005 0.003 0.003 0.006 0.005 0.010 0.007 0.011Mg 1.054 1.154 1.124 1.259 1.304 1.219 1.151 1.264 1.270 1.220Ca 1.967 1.819 1.873 1.784 1.773 1.769 1.828 1.614 1.718 1.696Total 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00Almandine 9.50 12.17 11.15 8.52 9.18 10.47 12.40 18.20 15.79 17.05Pyrope 31.53 34.01 33.27 37.81 38.46 36.46 33.80 35.81 35.71 34.59Grossular 8.29 7.74 8.62 10.19 9.35 9.00 8.21 8.41 7.19 7.56Spessartine 0.14 0.24 0.14 0.08 0.09 0.17 0.13 0.28 0.21 0.30Uvarovite 1.80 2.29 0.64 0.17 0.21 0.32 0.40 0.30 0.59 1.31Andradite 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Schorlomite 48.73 43.56 46.18 43.23 42.71 43.59 45.06 37.00 40.51 39.19

Oxide 11 12 13 14 15 16 17 18 19

SiO2 32.26 32.02 31.69 31.51 30.63 29.95 31.63 30.87 29.57TiO2 24.19 24.18 24.49 24.34 24.00 26.65 24.42 23.92 24.51Al2O3 3.16 2.73 3.21 2.77 3.30 3.52 3.09 3.46 3.97Cr2O3 0.18 0.66 0.12 0.16 0.15 0.13 0.43 1.37 0.77FeO 5.10 4.32 6.83 6.83 7.84 4.97 4.03 5.90 8.73MnO 0.05 0.06 0.07 0.08 0.12 0.05 0.05 0.10 0.11MgO 10.20 10.25 9.80 9.23 10.16 8.90 11.39 10.25 8.44CaO 19.59 19.93 19.28 19.48 18.41 19.88 19.04 18.74 18.50Total 94.73 94.14 95.49 94.39 94.60 94.05 94.07 94.63 94.60

Si 2.722 2.718 2.668 2.693 2.601 2.576 2.671 2.616 2.541Ti 1.535 1.544 1.551 1.565 1.533 1.724 1.550 1.524 1.584Al 0.315 0.273 0.319 0.279 0.330 0.356 0.307 0.345 0.402Cr 0.012 0.044 0.008 0.011 0.010 0.009 0.029 0.092 0.052Fe3þ 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe2þ 0.360 0.307 0.481 0.488 0.557 0.357 0.284 0.418 0.628Mn 0.004 0.004 0.005 0.006 0.009 0.004 0.003 0.007 0.008Mg 1.283 1.297 1.230 1.176 1.286 1.141 1.433 1.295 1.082Ca 1.771 1.813 1.739 1.783 1.675 1.832 1.723 1.702 1.703Total 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00 8.00Almandine 10.54 8.97 13.91 14.13 15.78 10.72 8.26 12.22 18.34Pyrope 37.53 37.92 35.61 34.06 36.47 34.22 41.62 37.85 31.62Grossular 8.76 7.77 8.55 7.76 8.37 9.37 8.14 8.76 9.82Spessartine 0.11 0.13 0.15 0.16 0.25 0.11 0.09 0.22 0.24Uvarovite 0.33 1.26 0.21 0.30 0.26 0.24 0.76 2.33 1.27Andradite 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Schorlomite 42.73 43.96 41.57 43.58 38.86 45.33 41.12 38.63 38.70

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4.2.5. GarnetGarnet is a characteristic xenocrystic mineral in kimberlite

where its abundance and size is highly variable. Garnet is derivedthrough the disaggregation of the mantle and lower crustal rockssuch as peridotites, pyroxenites, and eclogites during the rapidascent of kimberlite magma to the surface (Boyd et al., 2004;Lazarov et al., 2009). It is commonly present as pyrope, whereasandradite, schorlomite (Ti rich) and kimzeyite type are rare; how-ever such Ti-Ca rich garnets have been reported from WKF (Daset al., 2013; Smith et al., 2013; Dongre et al., 2016). Garnets fromthe present study are Ti-Ca rich varieties and their representativechemical composition is given in Table 6. The Ahobil garents havehigh contents of TiO2 (up to 26.65 wt.%) and CaO (up to 21.46 wt.%)

and lower contents of Al2O3 (

Table 7Bulk-rock geochemistry of Ahobil kimberlite samples under study (major oxide in wt.% and trace element in ppm).

Sample N/AHBK/1 N/AHBK/2 AK/AHBK 1/1 AK/AHBK 1/2 AK/AHBK 2/2 AK/AHBK 2/1

SiO2 30.38 32.24 38.37 31.63 30.34 39Al2O3 2.49 2.73 6.12 3.12 2.84 5.6Fe2O3

T 13.09 14.29 12.7 11.46 12.68 11.39MnO 0.195 0.209 0.146 0.143 0.186 0.144MgO 25.34 24.99 18.42 23 24.46 18.11CaO 10.57 5.97 11.88 10.65 8.63 13.38Na2O 0.09 0.08 0.46 0.08 0.08 0.56K2O 1.22 1.45 0.31 0.85 1.16 0.34TiO2 4.327 4.526 3.121 3.658 4.276 3.56P2O5 0.85 0.83 0.32 0.8 0.8 0.8LOI 11.15 11.35 7.63 14.15 13.74 7.04Total 99.7 98.66 99.48 99.53 99.18 99.46Mg# 0.79 0.78 0.74 0.80 0.79 0.76Sc 18 19 13 15 17 16Be 3 3 2 2 2 2V 178 188 206 348 265 212Ba 1870 1085 683 1058 1297 608Sr 1257 879 834 736 744 912Y 17 14 19 17 17 19Zr 431 457 338 384 430 402Cr 1130 1200 560 930 1180 670Co 87 90 68 63 88 78Ni 910 970 650 710 920 690Cu 120 120 120 80 110 120Zn 100 80 90 60 90 100Ga 12 12 13 9 11 14Ge 1 1

Figure 3. Photomicrographs depicting various aspects of Ahobil kimberlite. (A) Inequigranular texture imparted by two generation of olivine larger phenocrysts smaller ones withinthe ground mass (N/AHBK/1; XPL). (B) A sub-rounded large olivine macrocryst (N/AHBK/2; XPL). (C) Association between perovskite, spinel and phlogopite in the groundmass (N/AHBK/2; PPL). (D) Back scattered electron (BSE) image showing presence of unaltered and subrounded perovskite and garnet (N/AHBK/1). (E) BSE showing stubbed laths ofphlogopite in the groundmass (N/AHBK/1). (F) BSE depicting cores of garnet being surrounded by rims of spinel (N/AHBK/1).

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values < 1.2 and the relatively higher Si/Mg contents (1.61e1.67) ofthe above two samples further suggests their contaminated nature.Relatively lower Rb concentration of the above two samples is alsolikely to be a consequence of alteration. All the Ahobil samplesshow positive Pb spikes (Pb/Pb* ¼ 0.94e3.31) with highest valuesin AK/AHBK 1/1 (Pb/Pb* ¼ 3.31) in their multi-element primitivenormalized plots (Fig. 10B). As crustal rocks generally have higherPb contents, such positive Pb spikes can be generated throughcrustal contamination if they have an accompanying increment inSiO2 content (Le Roex et al., 2003). With the exception of AK/AHBK1/1 and AK/AHBK 2/1 samples, others have minor positive Pbanomalies without any obvious disturbance of their SiO2 content.The reason for such Pb anomalies can be attributed to late-stage,low temperature alteration. In any case, the geochemical data ofthe two samples (AK/AHBK 1/1 and AK/AHBK 2/1) has beenexcluded in the ensuing discussion.

5.2. Major element geochemistry

The Ahobil kimberlite samples are silica undersaturated (SiO2 <40 wt.%) whereas their Mg contents (Mg# ¼ 74e80) are highattesting an ultramafic character. SiO2 and MgO contents display abroad positive relation (Supplementary Fig. 3). SiO2 content variesof 30.34e39 wt.%, MgO varies of 18.11e25.34 wt.% and Al2O3 variesof 2.49e6.12 wt.% (Table 7). When compared to the orangeites fromBehradih and Timmasamudram, the samples of present study havelower SiO2 and Al2O3 but elevated Fe2O3T (11.39e14.29 wt.%) andCaO (5.97e13.38 wt.%) contents and are identical to many of thekimberlites from WKF and NKF (Supplementary Fig. 3). TiO2 andK2O show a broad positive correlation with the MgO content(Supplementary Fig. 3). The Ahobil kimberlite samples have lowerK2O (0.31e1.45 wt.%) and higher TiO2 (3.12e4.52 wt.%) whencompared to those from the Behradih, Timmasamudram and

Figure 4. (A) BSE displaying garlanding texture where central olivine grain is circumscribed by pervoskites with spinel overgrowth; (B) Si elemental X-ray map showing that majorsilicates are olivine and phlogopite; (C) Fe elemental X-ray map displaying spinel is the main iron bearing phase; (D) Ca elemental X-ray map reflecting perovskite is the main carrierof calcium; (E) Mg elemental X-ray map demonstrating that olivine is the major Mg-bearing minerals and (F) Ti elemental X-ray map indicating perovskite as a major Ti hostingphase.

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worldwide orangeites which is endorsed by their lower abundanceof phlogopite and higher modal perovskite and spinel. However,higher TiO2 content can also result from accumulation and/orassimilation of ilmenite megacryst and xenocrysts so we haveemployed Ilmenite Index (Ilm. I) of Taylor et al. (1994) to assesssuch a possibility. Samples having Ilm. I < 0.52 are generallyregarded not to have accumulated or dissolved ilmenite

megacrysts. With the exception of AK/AHBK 1/1 and AK/AHBK 2/1,which have Ilm. I close of 0.52, rest others depict little or noentrainment of ilmenite. Na2O content is too low (

Figure 5. (A) TiO2 vs. Al2O3 and (B) FeOT vs. Al2O3 of groundmass phlogopite of this study. Data sources: Cuddapah lamproites from Chalapathi Rao et al. (2004); Wajrakarurkimberlites (WKF) and Narayanpet kimberlites (NKF) from Chalapathi Rao et al. (2012); Behradih orangeite from Chalapathi Rao et al. (2011); TK-1 kimberlite and TK-4 orangeite ofTimmasamudram from Dongre et al. (2017); Krishna lamproites from Paul et al. (2007) and Chalapathi Rao et al. (2010). Fields of kimberlites, Leucite Hills madupite, West Kimberlyand Smoky Butte lamproite are taken from Dawson and Smith (1977) and Gibson et al. (1995). Arrows (indicate evolutionary trends of mica composition) in (B) are from Mitchell(1995).

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(Supplementary Fig. 3) it can be inferred that the Abohil kimberlitesamples have higher CaO, Fe2O3T, and TiO2 compared to Behradihorangeite and are similar to those of EDC kimberlites in their majoroxide chemistry. Furthermore as Ti is an immobile high fieldstrength element and K is the most mobile large ion lithophileelement, their co-variance is known to be a good discriminatorbetween kimberlites and orangeites (Smith et al., 1985) and in sucha plot the samples under study are clearly confined to thekimberlite field (Fig. 8).

5.3. Trace element geochemistry

The Ahobil samples display minor variation in concentration ofcompatible trace elements (Ni ¼ 650e970 ppm,Co ¼ 560e1200 ppm, V ¼ 178e348 ppm, and Sc ¼ 13e19 ppm;Table 7). Ni and Cr in the studied samples also display good

Figure 6. (A) Fe2þ/(Fe2þ þ Mg) vs. Ti/(Ti þ Cr þ Al) (molar fraction) of groundmass spinels of(1984); Wajrakarur kimberlites from Chalapathi Rao et al. (2004); Behradih orangeites from Cfrom Chalapathi Rao et al. (2014). (B) Cr/(Cr þ Al) vs. Ti/(Ti þ Cr þ Al) for groundmass spinelfrom Mitchell (1995).

correlation with the MgO (Supplementary Fig. 4). In comparison toorangeites, the Ahobil samples are vanadium rich and correspondsmore to those found in EDC kimberlites (75e355 ppm). Thisabundance can be attributed to higher modal spinel and lowerproportion of phlogopite as vanadium is hosted primarily by theseminerals. Zr and Nb are the High-field strength elements (HFSE)which are well known to be immobile during alteration and/orweathering. These HFSE show good correlation with other incom-patible trace element (e.g., Hf; Supplementary Fig. 4C) but corre-lates poorly with fluid mobile large ion lithophile elements such asBa (Supplementary Fig. 4D). This reflects that weathering; deutericalteration and/or post emplacement hydrothermal activity mayhave affected the large ion lithophile element (LILE) distribution, atleast to some extent, in the studied samples (Paton et al., 2009). Thekimberlite samples of present study have elevated concentration ofZr and Hf compared to other kimberlites and orangeites

this study Data sources: south African kimberlite spinels from Scott-Smith and Skinnerhalapathi Rao et al. (2011). Spinel from crater facies Tokapal kimberlite, Central India, iss of this study. Data sources remain the same as in Fig. 6A. Magmatic trend 1 and 2 are

Figure 7. REE2O3 involving La, Ce, Sm, Pr vs. TiO2 of pervoskites from this kimberlites.Data for Timmasamudram samples from Dongre et al., (2017); and the fields ofkimberlite and orangeite are from Donelly et al. (2011).

Figure 9. (A) Nb vs. Ba and (B) La vs. Rb variation for kimberlite of this study, fields forkimberlite and orangeites are from Becker and Le. Roex (2006).

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(Supplementary Fig. 4C and D); but Zr is still low enough todistinguish them from lamproites which are highly enriched in Zr(typically 500e1800 ppm; Taylor et al., 1994). Large ion lithophileelement (LILE) concentrations are highly variable (e.g.Rb ¼ 25e149 ppm and Ba ¼ 608e1870 ppm). Bivariate plots be-tween various LILE pairs such as Nb vs. Ba (Fig. 9A) and La vs. Rb(Fig. 9B) are widely used to distinguish kimberlites and orangeitesand the samples under study displaymarked geochemical affinitiesto kimberlites.

The Ahobil samples display highly fractionated chondritenormalized REE distribution patterns (La/Yb ¼ 79e133; Fig. 10A)similar to those of Wajrakarur (73e145) (Chalapathi Rao et al.,2004; Chalapathi Rao and Srivastava, 2009; Paton et al., 2009)and Narayanpet (La/Yb ¼ 72e156; Chalapathi Rao et al., 2012)kimberlites and characteristic of potassic and ultra potassic

Figure 8. K2O versus TiO2 bulk-rock bivariate plot for Ahobil kimberlite samplescompared with those of WKF, NKF, Bastar and Timmasamudram orangeites. Datasources for Narayanpet kimberlites from Chalapathi Rao et al. (2004, 2012); Bastarorangeites from Chalapathi Rao et al. (2011). Wajrakarur kimberlites from ChalapathiRao et al. (2004) and Timmasamudram orangeites from Chalalapathi Rao et al.(2016). Various kimberlite and orangeite fields are after Smith et al. (1985) andMitchell (1995); aillikite and olivine lamproite fields are from Taylor et al. (1994) andSmith et al. (2013). Sample symbols remain the same for rest of the figures.

magmas reflecting that they are derived from very small degreesof partial melting of phlogopite bearing garnet lherzolite sources(e.g., Mitchell, 1995; Le. Roex et al., 2003; Bailey and Lupulescu,2015). Normalized multi element plot (Fig. 10B) reveals thattrace element content of the Ahobil samples are considerablyenriched compared to that in primitive mantle. Troughs found atK, Sr, P and Hf in all the samples (Fig. 10B) either reflects hy-drothermal alteration and/or presence of residual phases whichare characteristic of source region. Such negative anomalies arewell known from Group-I, II and transitional kimberlites fromsouthern Africa. Scatter observed at Rb and Ba in the multielement plot can be attributed to fluid-mobile behaviour of theseelements. Potassium bearing residual phase such as phlogopite isthought to be the most likely region for K-depletion of melt (Foleyet al., 1999). Sr depletion can be attributed to residual clinopyr-oxene and carbonate (Gibson et al., 1995; Tappe et al., 2006) andcan even be due to a source that had experienced Sr depletiondue melt extraction during an earlier event (Mitchell, 1995). Onthe other hand, Becker and Le. Roex (2006) suggested that suchHf depletions are characteristic of primary kimberlite magmas.The Ahobil samples also exclude large-scale subduction signaturesas assured by the lack of Nb and Ta negative anomalies in Fig. 10B.It should be pointed out here that positive Pb anomalies arecharacteristic of uncontaminated orangeites (Coe et al., 2008). Inthe absence of significant phlogopite, which is the likely re-pository of Pb, the spikes at Pb can be ascribed to leaching of Pbfrom granitoid basement rocks by volatile (CO2) rich magma(Tappe et al., 2017) rather than by processes involving exclusivelycrustal contamination.

Figure 10. (A) Chondrite-normalised (after Evenssen et al., 1978) REE distributionpattern for Ahobil kimberlite samples. (B) Primitive mantle normalized (after Sun andMcDonough, 1989) multi element plot for the samples under study.

Figure 11. (A) Whole-rock 87Rb/86Sr vs. 87Sr/86Sr isochron plot for samples under study;(B) whole rock εNd(t) values versus (87Sr/86Sr)i ratios (adopted from Tappe et al., 2011) ofthe Ahobil kimberlite compared with those for the whole-rock samples (data fromChalapathi Rao et al., 2004; Paton et al., 2007, 2009) and for perovskites (Chalapathi Raoet al., 2013a,b) from WKF and NKF. Data sources for other fields: Mesozoic southern Af-rican kimberlites and orangeites (Nowell et al., 2004); Mesoproterozoic Labrador lamp-roites (Tappe et al., 2007); Mesoproterozoic Greenland lamproites (Nelson, 1989; Tappeet al., 2007). Behradih orangeite, Central India (Chalapathi Rao et al., 2011) and Neo-

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5.4. Sr and Nd isotopes

Three minimally crustal contaminated and relatively fresh bulkrock samples were selected for Sr and Nd isotope measurementsand the data is presented in Table 8. Recently Chalapathi Rao et al.(2013a, b) suggested a wide spread 1.1 Ga tectono-magmatic eventfor the EDC kimberlites on basis of their perovskite U-Pb ages. Themeasured 87Sr/86Sr of Ahobil kimberlite samples is correlated with

Table 8Sr and Nd isotopic data for Ahobil kimberlite samples of this study.

Sample N/AHBK/1 N/AHBK/2 AK/AHBK2/2

Age (Ga) 1.0 1.0 1.0Rb 128 149 121Sr 1257 879 744(87Sr/86Sr)m 0.707241 0.709739 0.71076587Rb/86Sr 0.293940 0.489427 0.469620(87Sr/86Sr)i 0.703037 0.702740 0.7040493Sr �4.20 �8.44 10.18Sm 14.80 14.60 15.30Nd 94.80 98.10 98.20(143Nd/144Nd)m 0.51203 0.51199 0.51202147Sm/144Nd 0.09484 0.09041 0.09465(143Nd/144Nd)i 0.51141 0.51140 0.511403Nd(t) 1.19 1.01 0.95TDM (Ma) 2037 1979 2048

Slope of 87Rb/86Sr vs. 87Sr/86Sr isochron is 0.015 and intercept is 0.702, and theirisochron age was calculated using the formula t ¼ 1/l*ln(slope þ 1).(87Sr/86Sr)i ¼ (87Sr/86Sr)S � (87Rb/86Sr)S (eRbt�1).εNd(t) ¼ {[(143Nd/144Nd)S � (147Sm/144Nd)S (eSmt�1)]/(143Nd/144Nd)CHUR�1} � 10,000.TDM ¼ 1/lSm ln {[(143Nd/144Nd)S � 0.51315]/[(147Sm/144Nd)S � 0.2137]}, whereS ¼ sample, (143Nd/144Nd)CHUR ¼ 0.512638, and (147Sm/144Nd)CHUR ¼ 0.1967.

proterozoic Greenland kimberlites (Tappe et al., 2011).

the 87Rb/86Sr of these rocks and yields an approximate age of 1.05Ga (Fig. 11A) which is close to the emplacement age of 1.1 Ga of theMesoproterozoic kimberlites from the EDC. (87Sr/86Sr)i in thestudied samples range from 0.702740 to 0.704049 and(143Nd/144Nd)i range from 0.511397 to 0.511400; likewise, theircorresponding 3Sr ranges from �4.2 to 10.2 and 3Nd(t) from 0.95 to1.19. All these values are closer to those of the perovskite and wholerock Sr and Nd isotopic ratios available for the kimberlites fromWKF and NKF (Fig.11B). In the (87Sr/86Sr)i versus 3Nd(t) plot which isconventionally employed to distinguish the various isotopic reser-voirs of the sources of world-wide kimberlites and lamproites (e.g.,Mitchell, 2006; Paton et al., 2009; Tappe et al., 2011), the samplesunder study are clearly confined to the ‘depleted’ quadrant andresemble that of Group-1 kimberlites of South Africa and Greenland(Fig. 11B). The Ahobil samples are different from orangeites fromSouth Africa, Bastar Craton (India), and lamproites e all of whichare confined to the ‘enriched’ quadrant. Thus, the mantle sources ofthe Ahobil samples suggest a time-integrated LREE depleted sour-ces relative to present day bulk earth.

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6. Discussion

Petrography and mineral chemistry reveals that the Ahobilkimberlite samples share several characteristic features ofarchetypal kimberlites as well as the WKF kimberlites. Clinopyr-oxene is absent in the Ahobil kimberlite which is otherwiseknown to be a ubiquitous phase in orangeites and lamproites. Lackof Ti-enrichment in phlogopites and high modal abundance ofperovskite also distinguishes it from orangeites. However,phlogopite shows evolution towards orangeite compositionwhich could be due to low Al content and high fO2. Paucity ofperovskite is a characteristic feature of the Mesoproterozoiclamproites from the EDC (Chalapathi Rao et al., 2010, 2014, 2016)whereas perovskite is an abundant phase in the Ahobil samples.The major element concentrations of the Ahobil kimberliteresemble the range reported for global archetypal kimberlites (forcompilations see Kjasgaard et al., 2009) and in the bi-variate ratioplot of bulk SiO2/MgO vs. MgO/CaO (Supplementary Fig. 5), thestudied samples are confined to the field of average kimberlitecomposition. The bi-variate plot of K2O vs. TiO2 (Fig. 8) clearlydistinguishes the Ahobil kimberlite from (i) orangeites and olivinelamproites-both of which have enriched K2O content and (ii) ul-tramafic lamprophyres (aillikites) which are clearly moreenriched in TiO2. The strong bonafide kimberlite affinity of theAhobil kimberlite is additionally demonstrated by their over-lapping Ba/Nb (4.94e13.85) and La/Nb (0.72e0.93) ratios (Fig. 12),which are within the range of archetypal south African kimberliteand distinct from those of orangeites (Ba/Nb ¼ 10e40; La/Nb ¼ 1.2e2.2; Becker and Le Roex, 2006), Greenland-Labradorultramafic lamprohyres (Ba/Nb ¼ 2e14; La/Nb ¼ 2.7; Tappeet al., 2011) and transitional ultramafic lamprophyre-kimberliteof Khaderpet, WKF (Ba/Nb ¼ 5.49e28.67; La/Nb ¼ 1.2e2.7;Smith et al., 2013).

The ‘depleted’ 3Nd isotopic composition further attest to itskimberlitic character and distinguish them from isotopically‘enriched’ orangeites from Timmasamudram (TK-1: �5.31to �8.12 and TK-4: �10.67 to �12.63) (Chalapathi Rao et al., 2016)and Bastar craton (�6.30 to �10.53) (Chalapathi Rao et al., 2011).In summary, mineral chemistry suggests affinity towards

Figure 12. Ratio plots of Ba/Nb vs. La/Nb for the samples of this study. The fields for SouthLabrador UML are from Tappe et al. (2006, 2008, 2017) and Nielsen et al. (2009); Khaderpe

kimberlite as well as to some extent that of orangeite, whereasgeochemical and isotopic evidence favours a kimberlite affinity. Infact, such decoupling between mineralogy and geochemistry(unlike that found in southern African Group I and II kimberlites)is common in the EDC kimberlites wherein various factors suchdegree of melting, depth of melting, the nature and content of thesources, residual mineralogy, digestion or fractional crystal-lisation, degree of metasomatism of mantle source, and timing ofmetasomatism may have influenced (e.g. Chalapathi Rao andDongre, 2009; Chalapathi Rao et al., 2011). Thus, based onmineralogy, chemistry and isotope systematics we classify theAhobil body as an archetypal kimberlite in contrast to its ‘tran-sitional’ variety as recently suggested by Phani and Raju (2017). Itshould be pointed out here that some kimberlites from the WKFhave been recently re-classified as lamproites solely on the basisof mineral chemistry (Kaur and Mitchell, 2013, 2016; Sheikh et al.,2017). However, their geochemical and isotopic characteristics areclearly distinct from the well-studied Mesoproterozoic Dharwarcraton lamproites (see Chalapathi Rao et al., 2004, 2010, 2014,2016). In fact, Taylor and Kingdom (1999) argued that more reli-ance should be placed on trace element and radiogenic isotopiccomposition as geochemical discriminators, as opposed to min-eral chemistry, whilst distinguishing between kimberlite andorangeite. Likewise, Francis and Patterson (2010) also highlightedthe utility of bulk-rock geochemistry in distinguishing betweenkimberlites, lamproites and aillikites.

6.1. Nature of source region and genesis

High concentration of compatible trace elements (Ni, Cr, andCo), high Mg# (up to 80), low Al2O3 as well as low concentration ofHREE indicate a highly refractory source such as depleted perido-tite, that experienced previous melt extraction, in the generation ofAhobil kimberlite magma (Beard et al., 2000; Chalapathi Rao et al.,2004). Likewise, in order to account for high abundance ofincompatible elements, low degrees of partial melting of a meta-somatised source is also required. Therefore a previously depletedand subsequently enriched mantle source is inferred for the Ahobilkimberlite similar to that invoked for the generation of other

African kimberlites and orangeites are from Becker and Le Roex (2006); Greenland-t UML are from Smith et al. (2013).

Figure 14. MgO/CaO versus SiO2/Al2O3 for Ahobil kimberlite samples of this study. ExperimGPa and 8 GPa is from Gudfinnsson and Presnall (2005). The compositional field of re-constet al. (2014) and the references therein. The experimentally produced melt compositions fet al., 2009) is represented in dotted lines and phlogopite absent 6 and 10 GPa (Brey et al.

Figure 13. (A) Nb versus Nb/U (B) Ce versus Ce/Pb incompatible trace element plots forAhobil kimberlite (Various fields are taken from Le Roex et al., 2003).

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kimberlites from EDC (Chalapathi Rao et al., 2004; Chalapathi Raoand Srivastava, 2009). Source region of kimberlites remainscontroversial and various competing models are in vogue involving(i) sub-continental lithospheric mantle (SCLM; Heaman, 1989;Skinner, 1989; Tainton McKenzie, 1994; Le. Roex et al., 2003;Chalapathi Rao et al., 2004; Donnelly et al., 2011), (ii) astheno-spheric convecting mantle (Mitchell, 1995, 2006; Paton et al., 2007;Wu et al., 2010; Tappe et al., 2012), (iii) transition zone (Ringwoodet al., 1992; Nowell et al., 2004; Paton et al., 2009; Tappe et al.,2013a,b), (iv) core-mantle boundary (Haggerty, 1999; Collersonet al., 2010; Torsvik et al., 2010) and (v) even from the sourcesinvolving cratonic lithosphere and asthenosphere (Griffin et al.,2000; Tappe et al., 2011).

In order to constrain the source region of the Ahobil kimberlite,we have used various incompatible element ratios as Nb/U and Ce/Pb (Fig. 13A and B) to distinguish between magmas derived fromthe SCLM and from the asthenosphere such MORB and OIB (Le.Roex et al., 2003; Paton et al., 2009). The Ahobil kimberlite sam-ples display signatures of both SCLM as well as asthenosphere.Involvement of a cratonic lithosphere in the genesis of EDC kim-berlites is also affirmed by their Re-Os data (Chalapathi Rao et al.,2013b). Experimental studies have shown that kimberlite magmascould be generated from a carbonated mantle lithologies at up to8 GPa (Gudfinnsson and Presnall, 2005). In the MgO/CaO vs. SiO2/Al2O3 diagram (Fig. 14) the Ahobil kimberlite samples plots withinor close to the composition field of experimentally derived nearsolidus partial melts of carbonated peridotite above 5 GPa (Breyet al., 2008) and displays striking similarities to global kimber-lite parental melt estimates. On the other hand, the orangeitesfrom the Bastar and EDC fall within the compositional field forexperimentally produced near-solidus partial melts of phlogopite-rich carbonated peridotite (Foley et al., 2009). This clearly restrictsderivation of Ahobil kimberlite melt from potassic metasomesbeneath the EDC. In order to impose better constraints on magmaformation incompatible fluid immobile trace elements arecommonly deployed as they are known to serve as good indices of

entally produced melt compositions from synthetic carbonated peridotite between 3ructed parental kimberlite melts from various cratons world-wide is taken from Tapperom synthetic carbonated peridotite between 4 and 6 GPa (phlogopite present) (Foley, 2008) in solid lines.

Figure 15. Gd/Yb vs. La/Sm for the samples under study compared with those from kimberlites of WKF and NKF and orangeites from the Bastar craton. Ahobil and Wajrakarur samplesshow an affinity to kimberlites unlike the Timmasamudram and Behradih samples which exhibits an orangeite affinity, while Narayanpet exhibits a transitional character. Here the curvesrepresent melting trajectories of kimberlite and orangeites source regions. Numbers represent degree of partial melting. Melting curves in continuous line were obtained using partitioncoefficients fromBecker and Le Roex (2006), whereasmelting curves in dashed line represent experimentally determined bulk peridotite/melt partition coefficients at 8.6 GPa and 1470 �C(fromDasgupta et al., 2009). Source region compositions and residualmodalmineralogy data is from Becker and Le Roex (2006). Data sources: Narayanpet kimberlites are fromChalapathiRao et al. (2004, 2012); Bastar orangeites are fromChalapathi Rao et al. (2011).Wajrakarur kimberlites fromChalapathi Rao et al. (2004), Chalapathi Rao and Srivastava (2009). The legendsare same as in Fig. 8.

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degree of melting and source regions. By assuming the sourcecomposition inferred for southern African kimberlites andorangeites from the Kaapaval Craton (Becker and Le Roex, 2006),we have calculated the trace element abundance for the partialmelts utilizing a batch partial melting model. Experimentalstudies have shown that kimberlitic melts can be generated by lowdegrees of partial melting of carbonated peridotite at pressureranging from 6 GPa to 10 GPa (Gudffinnson and Presnell, 2005;Brey et al., 2008; Dasgupta et al., 2009; Foley et al., 2009). In thepartial melting model, carbonated peridotite/melt partition coef-ficient at 8.6 GPa and 1470 �C given by Dasgupta et al. (2009) wereutilized to obtain the melting trajectories of the kimberlites andorangeite source regions (Fig. 15). The melting trajectories ofBecker and Le Roex (2006) are also provided in Fig. 15 for acomparison. This plot demonstrates that the Ahobil kimberlitemagma was derived from 2%e3% of melting of a source havinghigher Gd/Yb and lower La/Sm, in contrast to orangeites whichwere sourced from altogether distinct source regions having alower Gd/Yb and higher La/Sm.

Nd TDM model age of w2.0 Ga (Table 8) of the Ahobil kimberliteis significantly older than those of the bulk-rock as well as perov-skite Nd TDM model ages (1.5e1.3 Ga) available for WKF and NKFkimberlites (see Chalapathi Rao et al., 2013a) but strikingly similarto those (2.0 Ga) of the Mesoproterozoic EDC lamproites (seeChalapathi Rao et al., 2004; Chakrabarti et al., 2007). This impliesthat the regional metasomatic event during Paleoproterozoic,recorded in the SCLM beneath the Cuddapah basin and its environs(see Chalapathi Rao et al., 2010), was far more widespread and alsoinfluenced the Wajrakarur domain. The Paleoproterozoic modelages of 2 Ga are also consistent with the timing of amalgamation ofthe supercontinent of Columbia (Meert and Santosh, 2017 andreferences therein) and provide links to its tectonics.

6.2. Diamond prospectivity

A number of criteria are used for evaluating the diamond pro-spectivity of kimberlites and include (i) Indicator mineral compo-sition (e.g., Grutter, 2004), (ii) usage of bulk rock first row transitionelements (Sc to Zn) (e.g., Birkett, 2008), (iii) estimation of oxygenfugacity fO2 of perovskites (e.g., Canil and Bellis, 2007), and (iv)from the bulk rock Fe and Ti contents (e.g., Francis and Patterson,2009). In this study, we investigate the diamond prospectivity ofthe Ahobil kimberlite by utilising its bulk-rock major and trace el-ements, as well as by estimating oxygen fugacity (fO2) fromperovskite Fe-Nb oxybarometer. Francis and Patterson (2009)suggested that best diamond grades are found in kimberlites thathave least bulk rock Ti and Fe contents (lesser titano-magnetite). Bythis analogy, the Fe and Ti contents of the Ahobil kimberlite arehigh (TiO2 3.12e4.5 wt.%) and (Fe2O3 11.46e14.29 wt.%) point outits non-prospective nature.

To estimate the possible role of oxygen fugacity (fO2) in influ-encing diamond preservation we have used Bellis and Canil (2007)empirical oxygen barometer based on Fe-Nb content of perovskitebased on the below equation:

DNNO ¼ [0.50(�0.021 � NbeFe (�0.031)þ 0.030(�0.001)] / 0.004 (�0.0002)

Bellis and Canil (2007) suggested that kimberlites having lowestestimated fO2 values relative to the NNO (nickel-nickel oxide)buffer will, have the least number of reabsorbed diamonds andcould be most prospective relative to kimberlites having interme-diate and highest fO2 values. Perovskites from the Ahobil kimberlitehave their DNNO estimates ranging from �5.74 to 0.29 with anaverage value of 2.37. Thus they show a very wide range of fO2 and

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such wide range (Table 5; Supplementary Fig. 6) may not allowpreservation of diamonds in the kimberlite.

7. Conclusions

We document the mineralogy, bulk-rock geochemistry, Sr-Ndisotopic composition and assess the diamond prospectivity of thenewly reported Ahobil kimberlite from the Wajrakarur field,southern India. Modal abundance of perovskite and presence ofgarnet cores rimmed by spinel are found to be its characteristicfeatures. Mineral chemistry of various liquidus phases (spinel,phlogopite and perovskite) display several characteristics consid-ered to be typical of archetypal kimberlite. Bulk-rock geochemistryand Sr and Nd isotopic composition of the Ahobil kimberlite areindistinguishable from those of world-wide kimberlites andexclude its melt origin from highly potassic metasomes beneath theEDC. The Ahobil kimberlite samples shares the compositional fieldof experimentally derived near solidus partial melts of carbonatedperidotite above 5 GPa and displays striking similarities to globalkimberlite parental melt estimates. Geochemical modelling indi-cate derivation of Ahobil kimberlite magma from 2%e3% of meltingof a source having higher Gd/Yb and lower La/Sm in contrast toorangeites from the Dharwar and Bastar cratons which weresourced from altogether distinct source regions having a lower Gd/Yb and higher La/Sm. The TDM Ndmodel age ofw2 Ga of the Ahobilkimberlite is considerably older by 500 Ma than those reported forWKF and NKF kimberlites but indistinguishable from those of theMesoproterozoic EDC lamproites. The Paleoproterozoic TDM Ndmodel age ofw2 Ga of the Ahobil kimberlite also coincides with theamalgamation of the Columbia supercontinent and impliestectono-magmatic links to it. High bulk-rock Fe-Ti contents andwide variation in oxygen fugacity fO2, as inferred from perovskiteoxybarometry, suggest Ahobil kimberlite to be non-prospective fordiamond.

Acknowledgements

The Head, Department of Geology, BHU, Varanasi is thanked forextending the facilities. DST-SERB, New Delhi sanctioned a majorresearch project (IR/S4/ESF-18/2011 dated 12.11.2013) to NVCRwhich made this research possible. DP thanks DST-SERB forfinancial assistance in the form of a research scientist. AS ac-knowledges CSIR for awarding JRF (NET). Anupam Banerjee isthanked for his help with the Sr and Nd isotopic analyses at IISc(Bengaluru). Two anonymous referees are thanked for the helpfulcomments.

Appendix A. Supplementary data

Supplementary data related to this article can be found athttps://doi.org/10.1016/j.gsf.2018.08.004.

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