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Research Paper

Petrology and Sr-Nd isotope systematics of the Ahobil kimberlite (Pipe-16) from the Wajrakarur fi eld, 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,*

aEPMA and SEM Laboratories, Department of Geology, Banaras Hindu University, Varanasi 221005, India

bCentre 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 2018 Received in revised form 4 July 2018

Accepted 6 August 2018 Available online 30 August 2018 Handling Editor: Vinod Oommen Samuel

Keywords:

Petrology Isotopes Kimberlite Wajrakarur Dharwar craton India

a b s t r a c t

Detailed mineralogical, bulk-rock geochemical and Sr-Nd isotopic data for the recently discovered Ahobil kimberlite (Pipe-16) from the Wajrakarur kimberlite field (WKF), Eastern Dharwar craton (EDC), southern India, are presented. Two generations of compositionally distinct olivine, Ti-poor phlogopite showing 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 that Ahobil kimberlite shares several characteristic features of archetypal kimberlites than orangeites and lamproites. Geochemical modelling indicate Ahobil kimberlite magma derivation from small-degree melting of a carbonated peridotite source having higher Gd/Yb and lower La/Sm in contrast to those of orangeites from the Eastern Dharwar and Bastar cratons of Indian shield. TheTDMNd model age (w2.0 Ga) of the Ahobil kimberlite is (i) significantly older than those (1.5e1.3 Ga) reported for Wajrakarur and Narayanpet 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. High bulk-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.

Ó2018, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

1. Introduction

Kimberlites are small-volume and unusual ultramafic rocks which are extremely enriched in incompatible trace elements as well as in volatiles. Kimberlites are of economic as well as scientific value owing to the following reasons: (i) they are major primary hosts to diamonds, (ii) entrain abundant mantle and crustal xenoliths, (iii) links to mantle metasomatism and (iii) constitute the deepest magmas 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 diamond inclusions have significantly enhanced a better understanding of the evolution of the earth. However, despite decades of researche there are several unresolved and contentious issues regarding the (i) origin

of kimberlite magma (see Le Roex, 1986; Heaman and Kjasgaard, 2000; Heaman et al., 2003; Torsvik et al., 2010, 2016a, b; Currie and Beamount, 2011) (ii) composition of kimberlite magma (including wall-rock assimilation) (e.g.,Donnelly et al., 2012; Pilbeam et al., 2013; Kamenetsky and Yaxley, 2015) and (iii) the extent to which kimberlites are modified by syn- and post emplacement processes including alteration by ground waters (e.g., see contrasting views ofStripp et al., 2006; Mitchell, 2013; Sparks, 2013; Afanasyev et al., 2014; Giuliani et al., 2014). Regardless of the general complexity of kimberlite magma formation and evolution, a general consensus is that fresh magmatic bonafide kimberlites are strikingly similar 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

*Corresponding author.

E-mail address:nvcr100@gmail.com(N.V.C. Rao).

Peer-review under responsibility of China University of Geosciences (Beijing).

H O S T E D BY Contents lists available atScienceDirect

China University of Geosciences (Beijing)

Geoscience Frontiers

j o u r n a l h o m e p a g e : w w w . e l s e v i e r. c o m / l o c a t e / g s f

https://doi.org/10.1016/j.gsf.2018.08.004

1674-9871/Ó2018, China University of Geosciences (Beijing) and Peking University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC- ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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are confined only to the Kaapvaal Craton of southern Africa, but their subsequent reports from other localities like Dronning Maud Land, Antarctica (ca. 159 Ma;Romu et al., 2008), the Mainpur area of 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) firmly established that they also occur outside the Kaapvaal craton.

The Eastern Dharwar craton (EDC) of southern India is the world’s largest known repository of Proterozoic kimberlites numbering more than a hundred. In view of overlapping mineral- ogical and geochemical aspects of some of these occurrences with those from the Group I and II kimberlites as well as lamproites and ultramafic lamprophyres (aillikites) their precise nomenclature has been a subject of contention (seeHaggerty and Birkett, 2004; Kaur and Mitchell, 2013, 2016; Smith et al., 2013; Shaikh et al., 2017). On the basis of petrological and geochemical characteristicsChalapathi Rao and Dongre (2009)andChalapathi Rao et al. (2012)classified many of the NKF (Narayanpet kimberlitefield) kimberlites to be transitional between Group-I and -II variants with a strong affinity towards 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 Dharwar craton. In this study, we present the petrology and geochemistry (including Sr and Nd isotopic composition) of a newly discovered Pipe-16 kimberlite pipe from the WajrakarureLattavaram cluster of WKF in the EDC. The pipe is to referred as Ahobil kimberlite in this paper because of its spatial proximity close to Penna Ahobilam, a famous temple of Lord Narashima. The objectives of this study are to (i) understand the petrological and mineralogical characteristics of the Ahobil kimberlite, (ii) characterize the occurrence with respect to global as well as Indian kimberlites and orangeites, (iii) constrain its petrogenesis and (iv) provide insights into its diamond prospectivity.

2. Geological setting

The Archaean Dharwar Craton of the Indian shield is bounded by Bastar Craton towards north east, by polymetamorphic Proterozoic Eastern Ghats granulite facies mobile belt towards the east, lavas flows of Deccan large Igneous province in the north west and the southern granulite terrain in the south (Ramakrishnan and Vaidyanadhan, 2008;Fig. 1). The Dharwar Craton is dominated by the granite-green stone belts as well as gneissic basement of tonalite-trondhjemite-granidiorite (TTG) composition. These inturn are intruded by north-south trending granitic plutons collectively known as Closepet Granite of 2510 Ma (Friend and Nutman, 1991). A number of Paleo-Mesoproterozoic intracratonic sedimentary basins overlie the granite-greenstone terrain towards its eastern and northern margins. The Chitradurga schist belt di- vides the Dharwar craton into two distinct groups called as Eastern Dharwar Craton (EDC) and Western Dharwar Craton (WDC) (Jayananda et al., 2006).

In the Dharwar Craton, kimberlites are virtually restricted to the EDC and are distributed over four distinctfields viz., (i) the Waj- rakarurfield (WKF), (ii) the Tungabhadrafield (TKF), (iii) the Rai- churfield (RKF) and (iv) the Narayanpetfield (NKF) (seeNayak and Kudari, 1999; Neelakantam, 2001; Paton et al., 2009) towards the western margin of the Paleo-to Mesoproterozoic Cuddapah sedi- mentary basin. The WKF is the largest of them and comprises four clusters viz., (i) the WajrakarureLattavaram, (ii) the Chigicherla, (iii) the Timmasamudram and (iv) the Kalyandurg. Available emplacement ages of the kimberlites in the EDC reveals two

distinct age groups of (i) Mesoproterozoic atw1100 Ma (Gopalan and Kumar, 2008; Osborne et al., 2011; Chalapathi Rao et al., 2013a,b) and (ii) Late Cretaceous at w90 Ma (Chalapathi Rao et al., 2016).

The Ahobil kimberlite under study was discovered by the geologists of GSI during their regularfield work and reported it as Pipe-16 (Fig. 1for location) in their annual progress report (Geological Survey of India, 2018). Preliminary field and geochemical studies on this kimberlite were also reported by Phani and Raju (2017). The body exposed at the confluence of a stream (Balkamthota vanka) with the Penner river bed (Fig. 2), has an irregular outline, and most of it lies submerged under water cover barring some portions which have protruded above flowing water of river and are exposed along the river channel margin.

3. Sampling and analytical techniques

The freshest possible samples were collected from the exposed parts of the outcrop that too during the dry season when the water content of the river channel was low. Special care was taken to remove all visible crustal and mantle-derived contaminants before 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 SX Five model EPMA at the Mantle Petrology lab, Department of Geology, Institute of Science, Banaras Hindu University, Varanasi.

Wavelength Dispersive Spectrometry (WDS) with TAP, LIF, LLIF and PET were employed. A number of in house standards (Pandey et al., 2018) were used for calibration using an acceleration voltage of 15 kV, beam current of 10 nA, 1mm beam diameter were used. The accuracy of analysis is 3serror and confidence level of 95.5% and precision of (0.1 wt.%). Major and trace elements in olivine were analysed together at an acceleration voltage of 25 kV, beam current of 40 nA and beam size of 1mm and calibration settings are given in theSupplementary Table S1. Mineral chem- istry data obtained for various phases are presented inTables 1e6.

Whole rock major and trace elements of 6 samples were carried out at Activation laboratories, Ancaster Canada. Multi acid 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 rare earth elements (REE). STM1, MRG1, DNC1, W2, SY3were used as internal standards and precision is approximately 5% and 5%e 10% for major oxides and trace elements respectively at 100 detection limit. The bulk rock geochemical data is pro- vided inTable 7. The analytical procedure is detailed byGale et al. (1997)and is available at the Activation Laboratories Ltd website (www.actlabs.com).

For the determination of Sr and Nd isotope compositions, three samples were dissolved using ultra-pure HF, HNO3and HCl acid mixtures. Strontium and neodymium were separated from the rock matrix using ion exchange column chromatography and the detailed procedure is described inBanerjee et al. (2016). The Sr and Nd isotope ratio measurements were carried out using a Thermo Scientific Triton Plus Thermal Ionization Mass Spectrometer (TIMS) at the Centre for Earth Sciences, Indian Institute of Science using a protocol described inBanerjee et al. (2016). Measured87Sr/86Sr and

143Nd/144Nd ratios were normalized to 86Sr/88Sr ¼ 0.1194 and

146Nd/144Nd¼0.7219, respectively to correct for the instrumental mass fractionation. JNdi-1 Nd isotopic-standard and SRM-987 Sr isotopic standard were used during the analyses. The results ob- tained are provided inTable 7.

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4. Petrography and mineral chemistry

4.1. Petrography

Petrographic studies show that the studied samples belong to coherent facies of kimberlite volcanism (Cas et al., 2008) and possess the characteristic inequigranular texture typical of kim- berlites (Fig. 3A) (seeMitchell, 1997) imparted by different sized populations of olivine viz., (i) euhedral macrocrystal (>0.5 mm) and (ii) subhedral microphenocrystal (<0.5 mm). A number of olivine macrocrysts of size ranging from 2 mm to 11 mm (Fig. 3B) are also occasionally present. Whereas most of the olivines are fresh, a few of them have been altered to serpentine mostly along rims and fractures. Apart from the two generation of olivines, phlogopite,

serpentine, spinel, calcite, perovskite and garnet are also present (Fig. 3C). One of the peculiar traits of Ahobil kimberlite, when compared to rest of the WKF occurrences, is the high modal abundance of perovskite throughout the ground mass (Fig. 3D).

Scattered, stubby-shaped and highly pleochroic phlogopites are present in the groundmass and are sometimes altered to chlorite and serpentine (Fig. 3E). Perovskite also occurs as necklace around olivines displaying a garlanding texture (Fig. 4A). Spinel is present as euhedral crystals in size range of 150e100mm and is found as clusters throughout the ground mass and even as rims around cores of garnet (Fig. 3F). Carbonate is present throughout the ground- mass. Garnets are restricted to the groundmass and are intimately associated with spinels, suggesting that they may be formed by replacement of spinel (Fig. 3F).

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

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4.2. Mineral chemistry 4.2.1. Olivine

Olivine shows a range in composition (Fo: 83e93 and NiO:

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

(Supplementary Fig. 1). Olivine macrocrysts and microphenocrysts have distinct compositions. The macrocrystic olivines have higher Fo and lower Ti concentrations which imply that they are of foreign origin unlike the phenocryst olivines which have crystallised directly from the kimberlite magma. Al concentration is below

Table 1

Mineral chemistry (oxide in wt.%) of olivine phenocrysts in the Ahobil kimberlite samples. Temperature (T) is calculated using the method ofDe Hoog et al. (2010).

Oxide (wt.%) 1 2 3 4 5 6 7 8 9 10 11

SiO2 40.17 40.36 40.19 40.36 40.27 39.75 39.64 41.02 39.54 39.37 39.79

TiO2 0.04 0.04 0.04 0.04 0.04 0.05 0.06 0.01 0.07 0.06 0.03

Al2O3 0.02 0.04 0.02 0.01 0.04 0.01 0.01 0.02 0.02 0.06 0.04

Cr2O3 0.05 0.05 0.04 0.04 0.04 0.07 0.08 0.12 0.06 0.08 0.02

FeO 11.38 9.49 12.43 11.96 11.01 12.97 14.33 7.45 14.08 14.27 12.82

MnO 0.13 0.13 0.15 0.14 0.14 0.15 0.16 0.11 0.16 0.16 0.18

MgO 48.07 49.69 46.86 47.51 48.50 46.73 45.60 51.76 45.84 46.29 46.19

NiO 0.38 0.39 0.39 0.39 0.39 0.37 0.38 0.40 0.33 0.38 0.21

CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na2O 0.04 0.08 0.03 0.03 0.05 0.03 0.02 0.01 0.03 0.03 0.01

V2O3 0.01 0.01 0.02 0.02 0.02 0.00 0.02 0.01 0.02 0.02 0.01

Total 100.29 100.28 100.17 100.5 100.5 100.13 100.3 100.91 100.15 100.72 99.3

Cations based on 4 oxygen

Si 0.991 0.978 0.992 0.990 0.980 0.985 0.991 0.985 0.984 0.981 0.997

Ti 0.001 0.001 0.001 0.001 0.001 0.001 0.001 0.000 0.001 0.001 0.001

Al 0.001 0.001 0.001 0.000 0.001 0.000 0.000 0.001 0.001 0.002 0.001

Cr 0.001 0.001 0.001 0.001 0.001 0.001 0.002 0.002 0.001 0.001 0.000

Fe(II) 0.235 0.233 0.277 0.266 0.265 0.289 0.300 0.162 0.314 0.297 0.269

Mn 0.003 0.003 0.003 0.003 0.003 0.003 0.003 0.002 0.003 0.003 0.004

Mg 1.768 1.795 1.724 1.738 1.759 1.726 1.700 1.853 1.701 1.720 1.725

Ni 0.008 0.008 0.008 0.008 0.008 0.007 0.008 0.008 0.007 0.008 0.004

Ca 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Na 0.002 0.004 0.002 0.001 0.002 0.001 0.001 0.001 0.001 0.001 0.000

V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Total 3.008 3.022 3.008 3.009 3.020 3.014 3.007 3.014 3.014 3.016 3.002

Fo 88.16 88.41 86.01 86.60 86.80 85.51 84.87 91.88 84.28 85.11 86.36

Fa 11.71 11.46 13.83 13.25 13.06 14.34 14.96 8.01 15.55 14.72 13.44

Tp 0.14 0.13 0.16 0.15 0.15 0.15 0.17 0.11 0.17 0.17 0.20

Trace elements (ppm)

Ti 214 223 254 235 236 321 378 62 418 385 201

Al 130 231 104 79 215 61 44 102 105 303 215

Cr 342 350 291 286 289 455 525 818 419 520 155

Mn 1005 1024 1154 1118 1121 1135 1267 882 1275 1258 1430

Ni 3020 3033 3086 3039 3044 2924 3013 3111 2569 3014 1657

Na 310 608 259 228 390 192 138 102 195 187 41

V 91 99 133 130 111 0 147 48 152 141 99

T(C) 1257 1335 1221 1188 1314 1176 1139 1263 1240 1399 1282

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

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Table 2

Mineral 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 41.70

TiO2 0.02 0.04 0.04 0.02 0.00 0.04 0.04 0.00 0.00 0.00

Al2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Cr2O3 0.09 0.03 0.03 0.02 0.01 0.04 0.04 0.01 0.02 0.02

FeO 7.16 8.42 8.42 9.82 6.53 9.06 9.05 6.61 6.59 6.56

MnO 0.11 0.12 0.12 0.19 0.09 0.13 0.13 0.09 0.09 0.09

MgO 52.44 50.91 50.91 49.56 52.27 50.82 51.20 53.81 52.95 52.98

NiO 0.44 0.30 0.30 0.27 0.39 0.36 0.35 0.39 0.39 0.39

CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na2O 0.02 0.01 0.01 0.00 0.01 0.01 0.02 0.01 0.01 0.01

V2O3 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.00 0.01 0.01

Total 100.58 100.54 100.54 100.61 100.34 100.37 100.76 100.61 100.08 100.76

Cations based on 4 oxygen

Si 0.987 0.983 0.983 0.988 0.990 0.988 0.985 0.972 0.983 0.991

Ti 0.000 0.001 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000

Al 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Cr 0.002 0.001 0.001 0.000 0.000 0.001 0.001 0.000 0.000 0.000

Fe(II) 0.143 0.190 0.190 0.220 0.132 0.183 0.182 0.132 0.132 0.130

Mn 0.002 0.002 0.002 0.004 0.002 0.003 0.003 0.002 0.002 0.002

Mg 1.869 1.833 1.833 1.793 1.879 1.830 1.837 1.915 1.892 1.877

Ni 0.008 0.006 0.006 0.005 0.008 0.007 0.007 0.007 0.008 0.007

Ca 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Na 0.001 0.000 0.000 0.000 0.000 0.001 0.001 0.000 0.000 0.000

V 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Total 3.01 3.02 3.02 3.01 3.01 3.01 3.02 3.03 3.02 3.01

Fo 92.78 90.49 90.49 88.92 93.37 90.79 90.86 93.47 93.40 93.42

Fa 7.11 9.40 9.40 10.89 6.54 9.08 9.00 6.44 6.52 6.49

Tp 0.11 0.12 0.12 0.19 0.09 0.13 0.13 0.09 0.09 0.09

Trace elements (ppm)

Ti 112 232 232 141 28 210 220 23 24 27

Al 0 0 0 0 0 0 0 0 0 0

Cr 594 228 228 105 101 241 242 100 105 103

Mn 844 916 916 1453 676 1021 1009 691 675 685

Ni 3469 2388 2388 2094 3050 2793 2727 3036 3071 3050

Na 114 77 77 23 41 85 119 53 57 39

V 46 67 67 85 30 82 73 24 35 37

Table 3

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

Oxide 1 2 3

core 4 rim

5 rim

6 core

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 45.73

TiO2 2.03 2.11 2.43 2.87 2.64 2.64 2.71 1.52 2.63 1.74 2.03 1.57

Al2O3 6.08 5.95 5.91 7.21 6.33 6.84 6.77 6.18 6.71 5.60 6.00 6.11

Cr2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

FeOT 9.25 9.26 9.03 10.32 11.27 10.81 9.38 9.95 10.55 11.13 10.53 10.58

MnO 0.00 0.04 0.00 0.09 0.11 0.00 0.00 0.00 0.00 0.03 0.00 0.00

MgO 23.56 23.79 22.42 21.14 20.64 21.35 23.16 25.33 21.44 23.76 24.39 24.82

CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

BaO 0.08 0.12 0.27 0.61 0.53 0.34 0.88 0.23 0.61 0.23 0.00 0.15

Na2O 0.73 0.66 0.73 0.73 0.49 0.50 0.49 0.19 0.52 0.44 0.15 0.14

K2O 8.76 9.33 9.16 8.45 8.50 8.39 8.88 7.59 8.81 6.92 4.04 4.36

Cl 0.02 0.04 0.01 0.02 0.02 0.03 0.00 0.01 0.04 0.00 0.00 0.00

F 1.83 2.95 3.74 1.55 3.06 2.84 3.34 0.60 2.81 3.73 4.32 2.69

Total 94.34 96.46 94.97 93.33 93.60 93.18 96.32 95.34 93.92 94.94 96.74 96.14

Cations for 22 oxygen

Si 6.29 6.27 6.29 6.15 6.22 6.12 6.12 6.33 6.14 6.28 6.55 6.56

Al 1.07 1.04 1.06 1.30 1.16 1.25 1.20 1.06 1.22 1.00 1.02 1.03

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Al 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ti 0.23 0.24 0.28 0.33 0.31 0.31 0.31 0.17 0.30 0.20 0.22 0.17

Fe(II) 1.16 1.15 1.15 1.32 1.46 1.40 1.18 1.21 1.36 1.41 1.27 1.27

Mn 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ba 0.00 0.01 0.02 0.04 0.03 0.02 0.05 0.01 0.04 0.01 0.00 0.01

Mg 5.26 5.27 5.10 4.80 4.78 4.94 5.19 5.47 4.93 5.38 5.26 5.31

Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na 0.21 0.19 0.22 0.22 0.15 0.15 0.14 0.05 0.15 0.13 0.04 0.04

K 1.67 1.77 1.78 1.64 1.68 1.66 1.70 1.40 1.73 1.34 0.75 0.80

Cl 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.01 0.00 0.00 0.00

F 0.86 1.39 1.80 0.75 1.50 1.40 1.59 0.27 1.37 1.79 1.97 1.22

Total 15.89 15.94 15.89 15.80 15.81 15.85 15.90 15.70 15.88 15.76 15.11 15.18

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detection limit for the macrocrysts but ranges of 44e303 ppm in microphenocrysts. Temperature for the phenocrystal olivine has been calculated using Al in olivine geothermometry ofDe Hoog et al. (2010)at an assumed pressure of 50 kbar based on the xe- noliths from kimberlites of Wajrakarur-Lattavaram cluster (see Ganguly and Bhattacharya, 1987; Nehru and Reddy, 1989). The calculated temperatures (Table 1) are within the range of crystal- lisation temperature of olivine and absence of complex zoning suggests 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 study is indistinguishable from the data reported for macrocrysts and microphenocrysts from world-wide kimberlites and orangeite (Fo 81.7e91.5 and up to 0.42 wt.% of NiO;Arndt et al., 2010).

4.2.2. Phlogopite

The chemistry of phlogopite from ultramafic alkaline rocks has often been used for their nomenclature as well as for understanding the evolution of their magmas (Mitchell, 1995; Brod et al., 2001;

Reguir et al., 2009; Lepore et al., 2017). In the Ahobil phlogopite, TiO2ranges from 1.52 wt.% to 2.87 wt.% whereas Al2O3varies from 5.60 wt.% to 7.21 wt.% (Table 3). On the other hand, the Cr2O3con- tents are too low (<0.1 wt.%) compared to those reported from phlogopites in Canadian and Russian kimberlites (Cr2O3¼0.12e1.72 wt.%;Reguir et al., 2009). Their FeO content ranges from 9.03 wt.% to 11.27 wt.%. In the bivariate plot of TiO2vs.

Al2O3(Fig. 5A), the phlogopites of this study show compositional overlaps with those from the NKF and WKF pipes (Chalapathi Rao et al., 2010, 2011) and display an affinity to world-wide Table 4

Mineral 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 0.00

TiO2 10.77 16.32 11.48 11.67 14.79 14.79 13.48 8.55 12.57 14.99 11.27

Al2O3 4.65 3.76 5.41 4.98 3.51 3.51 3.88 7.54 5.16 3.52 5.50

Cr2O3 17.46 5.56 24.62 22.12 8.37 8.37 9.48 34.61 16.54 3.56 16.07

V2O3 0.51 0.66 0.59 0.57 0.61 0.61 0.58 0.58 0.57 0.59 0.53

Fe2O3 27.78 28.88 22.15 24.09 31.69 31.69 31.78 13.75 27.37 33.49 28.91

FeO 29.27 37.48 21.92 23.53 32.72 32.72 34.87 22.67 28.15 37.10 28.55

MnO 1.29 2.20 1.06 1.06 1.77 1.77 1.99 1.27 1.21 2.07 1.35

MgO 7.22 4.23 12.98 11.94 7.16 7.16 4.89 10.85 9.55 3.92 8.34

CaO 0.00 0.31 0.00 0.00 0.00 0.00 0.04 0.00 0.02 0.00 0.00

ZnO 0.24 0.35 0.23 0.22 0.34 0.34 0.42 0.25 0.24 0.34 0.26

Total 99.19 99.76 100.44 100.19 100.97 100.97 101.42 100.07 101.39 99.57 100.78

Cations for 32 oxygen atoms

Si 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Ti 2.283 3.530 2.293 2.361 3.103 3.103 2.859 1.717 2.558 3.269 2.326

Al 1.542 1.275 1.693 1.579 1.156 1.156 1.290 2.374 1.647 1.203 1.778

Cr 3.889 1.264 5.169 4.703 1.847 1.847 2.115 7.306 3.539 0.816 3.485

V 0.116 0.153 0.126 0.124 0.137 0.137 0.130 0.123 0.123 0.138 0.117

Fe(III) 5.888 6.248 4.426 4.874 6.653 6.653 6.746 2.762 5.575 7.306 5.968

Fe(II) 6.893 9.011 4.869 5.289 7.636 7.636 8.226 5.063 6.370 8.995 6.549

Mn 0.307 0.535 0.238 0.241 0.419 0.419 0.476 0.286 0.277 0.508 0.314

Mg 3.033 1.814 5.140 4.788 2.979 2.979 2.057 4.319 3.855 1.694 3.409

Ca 0.000 0.096 0.000 0.000 0.000 0.000 0.013 0.000 0.007 0.000 0.000

Zn 0.049 0.074 0.046 0.043 0.070 0.070 0.088 0.050 0.049 0.072 0.053

Total 24 24 24 24 24 24 24 24 24 24 24

Ti/(TiþCrþAl) 0.30 0.58 0.25 0.27 0.51 0.51 0.46 0.15 0.33 0.62 0.31

Fe/(FeþMg) 0.69 0.83 0.49 0.52 0.72 0.72 0.80 0.54 0.62 0.84 0.66

Oxide 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 0.00

TiO2 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 8.19

Al2O3 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 6.56

Cr2O3 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 35.63

V2O3 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 0.61

Fe2O3 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 13.95

FeO 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 22.32

MnO 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 1.17

MgO 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 10.69

CaO 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 0.00

ZnO 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 0.24

Total 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 99.36

Cations for 32 oxygen atoms

Si 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 0.000

Ti 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 1.666

Al 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 2.089

Cr 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 7.612

V 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 0.131

Fe(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 2.837

Fe(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 5.045

Mn 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 0.268

Mg 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 4.305

Ca 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 0.000

Zn 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 0.048

Total 24 24 24 24 24 24 24 24 24 24 24 24 24 24

Ti/(TiþCrþAl) 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 0.15

Fe/(FeþMg) 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

(7)

kimberlites and orangeites than to those of lamproites. However, in FeO vs. Al2O3compositional diagram (afterMitchell, 1995) phlog- opites display an evolutionary trend of orangeites (Fig. 5B) similar to those displayed by NKF and WKF phlogopites as well as those from Behradih orangeite, Bastar craton, Central India (Chalapathi Rao et al., 2011). Low Al concentration in the liquid and highfO2could be the factors for the low Ti tetra-ferriphlogopite character dis- played by the mica under study (Table 3;Heathcote and McCormick, 1989; Brigatti et al., 1996). Moreover, as phlogopites are the late stage phases, small-scale variation or heterogeneities in the magma composition may also have been responsible for displaying this aspect (Reguir et al., 2009) and likely to be a characteristic feature of the Ahobil kimberlite magma (Guarino et al., 2013).

4.2.3. Spinel

Spinel is ubiquitous and their representative composition is presented in Table 4. Spinel from the present study shows a compositional range from magnesian titanian magnetite to titanian-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 kimberlites groundmass spinel a magmatic trend-I (magnesian ulvospinel- magnetite trend) and magmatic trend-II (titanian magnetite trend). Where magmatic trend-I is characteristic of kimberlites, the magmatic trend-II is well known from orangeites, basalts and lamprophyres (Tappe et al., 2004, 2005; Roeder and Schulze, 2008).

Significant population of spinels from the Ahobil kimberlite display trend-I in contrast to the evolutionary trend-II displayed by TK-1 and TK-4 orangeites of Timmasamudram cluster (Fig. 6A). Spinels of this study also preferentially show Ti enrichment with

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

4.2.4. Perovskite

Perovskite is paragenetically a well characterised common groundmass phase in kimberlites and accommodates a broad range of elements (mostly rare earth elements) in its crystal structure (Chakhmourdian et al., 2000, 2013). Volumetric abundance (w5e7 vol.%) of perovskite is one of the key features of Ahobil kimberlite as compared to many other Eastern Dharwar Craton kimberlites and its representative composition is presented inTable 5. At places, perovskite occur as garlands around olivine and to better represent this feature, X-ray elemental maps of Si, Mg, Fe, Ca, and Ti are provided (Fig. 4BeF). Perovskites from the Ahobil kimberlite have a restricted range of CaO (38.49e40.82 wt.%) and TiO2

(55.9e57.3 wt.%). Their FeOTcontent ranges of 0.81e1.44 wt.% and is comparable to that reported in perovskite from other global archetypal kimberlite (1e2 wt.%;Mitchell, 1995). On the contrary, perovskite from orangeite reportedly have higher TiO2, and lower FeO (Mitchell, 1995). The SrO content (0.37e0.16 wt.%) in the pe- rovskites under study is similar to that reported from archetypal southern African kimberlites and thus differentiates it from the perovskite found in southern African orangeites. The Nb2O5

(0.12e0.44 wt.%) content of perovskites is also lower than those found in NKF (0.22e2.33 wt.%) and WKF (0.34e2.33 wt.%). LREE2O3

of perovskites of this study range from (0.34e1.91 wt %) and in bi- variate plot (Fig. 7) involving TiO2and REE2O3,the perovskites of this study are confined to the kimberlitefield unlike those from the Timmasamudram orangeite (Dongre et al., 2017).

Table 5

Mineral 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 0.24

Al2O3 0.13 0.15 0.13 0.15 0.15 0.16 0.16 0.13 0.14 0.13 0.14 0.13

SiO2 0.02 0.04 0.05 0.03 0.00 0.01 0.01 0.04 0.04 0.05 0.03 0.05

CaO 40.02 40.47 39.05 39.06 38.56 39.80 39.50 40.31 40.08 40.40 40.15 40.04

TiO2 57.31 56.74 56.79 56.71 57.29 56.11 56.27 56.18 56.49 56.31 56.68 56.58

FeOT 1.30 1.16 0.87 1.16 1.14 1.03 1.12 1.05 0.97 0.97 1.01 1.02

SrO 0.29 0.32 0.29 0.25 0.22 0.24 0.23 0.37 0.32 0.34 0.35 0.28

Nb2O5 0.23 0.44 0.30 0.21 0.14 0.24 0.38 0.39 0.38 0.34 0.31 0.35

La2O3 0.15 0.14 0.47 0.41 0.34 0.33 0.33 0.21 0.18 0.19 0.18 0.27

Ce2O3 0.34 0.27 1.12 1.09 1.19 0.62 1.06 0.40 0.44 0.40 0.41 0.48

Pr2O3 0.00 0.02 0.18 0.12 0.10 0.01 0.07 0.05 0.02 0.01 0.04 0.00

Nd2O3 0.06 0.04 0.42 0.39 0.41 0.17 0.38 0.05 0.11 0.04 0.14 0.13

Sm2O3 0.00 0.00 0.00 0.10 0.03 0.01 0.01 0.00 0.00 0.00 0.00 0.00

Ta2O5 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

ThO2 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 100.04 100.00 100.10 100.03 99.97 99.02 99.83 99.43 99.38 99.41 99.68 99.57

Cations for 3 oxygen atoms

Na 0.009 0.009 0.018 0.015 0.018 0.012 0.014 0.011 0.011 0.010 0.011 0.011

Al 0.003 0.004 0.004 0.004 0.004 0.004 0.004 0.003 0.004 0.004 0.004 0.003

Si 0.000 0.001 0.001 0.001 0.000 0.000 0.000 0.001 0.001 0.001 0.001 0.001

Ca 0.964 0.979 0.953 0.950 0.936 0.975 0.963 0.983 0.978 0.985 0.976 0.975

Ti 0.969 0.963 0.973 0.968 0.977 0.965 0.964 0.962 0.967 0.964 0.968 0.967

Fe 0.049 0.044 0.033 0.044 0.043 0.039 0.043 0.040 0.037 0.037 0.038 0.039

Sr 0.004 0.004 0.004 0.003 0.003 0.003 0.003 0.005 0.004 0.004 0.005 0.004

Nb 0.002 0.004 0.003 0.002 0.001 0.003 0.004 0.004 0.004 0.003 0.003 0.004

La 0.001 0.001 0.004 0.003 0.003 0.003 0.003 0.002 0.002 0.002 0.002 0.002

Ce 0.003 0.002 0.009 0.009 0.010 0.005 0.009 0.003 0.004 0.003 0.003 0.004

Pr 0.000 0.000 0.002 0.001 0.001 0.000 0.001 0.000 0.000 0.000 0.000 0.000

Sm 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Nd 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Ta 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Th 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

Total 2.00 2.01 2.00 2.00 2.00 2.01 2.01 2.02 2.01 2.01 2.01 2.01

DNNO 4.38 2.86 0.43 3.23 3.08 2.05 2.65 2.02 1.29 1.32 1.64 1.72

FeOTis total iron.DNNO is the logfO2relative to the NNO (nickel-nickel oxide) buffer.

References

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