Platinum-group element (PGE) geochemistry of Deccan orangeites, Bastar craton, central India:...

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Platinum-group element (PGE) geochemistry of Deccan orangeites,Bastar craton, central India: Implication for a non-terrestrial originfor iridium enrichment at the K–Pg boundary

N.V. Chalapathi Rao a,⇑, B. Lehmann b, V. Balaram c

a Department of Geology, Centre of Advanced Study, Banaras Hindu University, Varanasi 221 005, Indiab Mineral Resources, Technical University of Clausthal, 38678 Clausthal-Zellerfeld, Germanyc CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India

a r t i c l e i n f o

Article history:Available online 5 July 2013

Keywords:IridiumPGEKimberliteDeccanMass extinctionMainpurIndia

a b s t r a c t

We report platinum group element (PGE) concentrations of twelve bulk-rock samples from the Behradihand Kodomali orangeite intrusions in the Mainpur field, Bastar craton, central India, which are emplacedsynchronously with the Deccan flood basalts. Their palladium-group PGE (PPGE) (1.8–5.2 ppb Pt, 1.2–6.4 ppb Pd) contents are distinctly higher compared to their iridium-group PGE (IPGE) concentrations(0.8–2 ppb Os, 0.8–1.2 ppb Ir, 3.2–4.2 ppb Ru, and 0.2–0.8 ppb Rh). Their PGE contents as well as Pd/Irratios are either similar or even lower than those from the Mesoproterozoic and Cretaceous kimberlitesand orangeites from the Kaapvaal craton (southern Africa), Cretaceous kimberlites from the Sao Fransiscocraton (Brazil), Ordovician kimberlites from the North China craton and the Mesoproterozoic southernIndian kimberlites from the Eastern Dharwar craton. Anomalously elevated iridium (and other PGE) con-tents in sediments at the Cretaceous–Paleogene (K–Pg) boundary are commonly attributed either to alarge bolide impact triggering the K–Pg mass extinction or to terrestrial causes such as volcanic eruptions(Deccan flood basalts) or even to mantle-plume derived lithospheric gaseous explosions (Verneshots).Lack of unusually high abundances of PGE in the Mainpur orangeties as well as in the co-eval Deccanflood basalts and associated alkaline rocks implies that the anomalous iridium enrichment reported atthe K–Pg boundary sections was not sourced from the mantle and likely originated from an extraterres-trial source.

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1. Introduction

The K–Pg mass extinction at the Cretaceous–Paleogene bound-ary (formerly known as K–T, i.e. Cretaceous–Tertiary boundary) isthe youngest as well as most extensively studied of the massextinctions in Earth history. However, its causal mechanism stillremains widely debated (see Schulte et al., 2010; Archibald et al.,2010 and references therein). The discovery of unusually high con-tents of >100 ppb Ir in global sedimentary sections at the K–Pgboundary has led Alvarez et al. (1980) to propose an extraterres-trial (astrobleme) impact leading to mass extinction (see also Gra-up et al., 1986; Bhandari et al., 1993, 1994; Stüben et al., 2005;Parthasarathy et al., 2008). However, terrestrial causes, such asenrichment of PGE during marine-sedimentary or diagenetic pro-cesses may also account for high Ir at the K–Pg boundary (seeEvans and Chai, 1997; Hou et al., 1998; Shrivastava and Ahmad,2008). The K–Pg boundary coincides with the main phase of erup-

tion of the Deccan flood basalts (India), one of the largest LIP (largeigneous province) on Earth. The amount of volatiles released (CO2,SO2, halogens and sulphuric acid aerosols) in a very short timecould also be responsible for a global modification of the biosphere(e.g., Officer and Drake, 1985; McLean, 1985; Courtillot et al., 1988;Sahni, 1990; Self et al., 2006, 2008; Keller et al., 2009, 2011). LargeCO2-rich explosive events termed Verneshots (after Jules Verne,who first visualised them), triggered by the incubation of a mantleplume beneath cratonic lithosphere and typically associated withthe onset of continental rifting, have been proposed by PhippsMorgan et al. (2004) as causative factor for continental flood basaltvolcanism, associated kimberlite activity, induced impact signals(such as shocked quartz), iridium enrichment and also massextinctions.

Kimberlites are currently defined (e.g., Woolley et al., 1996;Mitchell, 2008) as volatile-rich (dominantly CO2), potassic,ultrabasic and ultramafic rocks possessing a distinctive inequi-granular texture resulting mostly from the presence of two gener-ations of olivine viz. (i) Macrocrysts (0.5–10 mm) and (ii)groundmass phenocrysts (<0.5 mm). Other minerals comprising

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⇑ Corresponding author. Tel.: +91 9935647365.E-mail address: [email protected] (N.V. Chalapathi Rao).

Journal of Asian Earth Sciences 84 (2014) 24–33

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the megacryst/macrocryst assemblage include magnesian ilmenite,pyrope, diopside (subcalcic), phlogopite, enstatite and Ti-poorchromite. Late-stage poikilitic laths of phlogopite are common.The groundmass constituents commonly comprise one or moreof the following: monticellite, phlogopite, perovskite, spinel, apa-tite, carbonate and serpentine. Two groups of kimberlite (termedas Group I and II) are recognized in the Kaapvaal craton of southernAfrica primarily on the basis of their mineralogy (mica propor-tions) and Sr, Nd and Pb isotopic ratios (Smith, 1983). Group II kim-berlites (also termed as micaceous kimberlites or orangeites) havehigher 87Sr/86Sr ratios and lower 143Nd/144Nd and 206Pb/204Pb ra-tios than Group I (mica-poor) kimberlites. For many years, orange-ites were widely considered to be restricted only to southern Africa(e.g. Skinner, 1989; Mitchell, 2006) but recently occurrence of suchrocks is also recognized from the Bastar craton of central India(Lehmann et al. 2010; Chalapathi Rao et al. 2011a). The end-Creta-ceous (65 Ma) diamondiferous orangeites from the Mainpur area ofthe Bastar craton, Central India, constitute excellent candidates totest the terrestrial model for iridium enrichment at the K–Pgboundary. This is because they are (i) synchronous with the maineruption event of the Deccan Traps as well as with the mass extinc-tions at the K–Pg boundary (see Chenet et al., 2007; Lehmann et al.,2010), (ii) the deepest (>160 km) as well as the least fractionatedmantle-derived magmas yet known that erupted at the K–Pgboundary, (iii) demonstrated to be products of interaction betweenthe Reunion hot spot-Indian lithosphere (Chalapathi Rao et al.,2011a; Chalapathi Rao and Lehmann, 2011), and (iv) the closestanalogues of the Verneshot explosive events suggested by PhippsMorgan et al. (2004). Cratonic kimberlites from southern Africaare known to have far more primitive PGE signatures and higherconcentrations of Os, Ir, and Ru than other volatile-poor mantle-derived melts such as MORB, OIB and komatiites (McDonaldet al., 1995) thereby attaching significance to the PGE content ofMainpur orangeites.

In this paper, we report the PGE concentrations of two of thewell-characterized orangeites of the Mainpur field at Behradihand Kodomali, Bastar craton, Central India, with the objective toinvestigate (i) whether the anomalous Ir enrichment at the K–Pgboundary could indeed have been sourced from the deeper mantle,and (ii) as to how their PGE concentrations compare with those re-ported from global kimberlites and orangeites.

2. Geological setting of the Bastar craton and the Mainpurorangeite field

The Bastar craton is one of the oldest nuclei in the Central In-dian shield and consists largely of granitic rocks and mafic dykeswarms of Meso-Neoarchaean age (Crookshank, 1963; Sarkaret al., 1993; Ramakrishnan and Vaidyanadhan, 2008 and referencestherein). A number of supracrustal, intracratonic Meso- to Neopro-terozoic sedimentary sequences unconformably overlie the base-ment, i.e., form south to north, the Sabri Group (Sukma basin),the Indravati Group (Indravati basin), Pairi Group (Khariar basin)and Chhattisgarh Supergroup (Chhattisgarh basin) (see Ramakrish-nan and Vaidyanadhan, 2008 (Fig. 1). Recently, mafic dykes of Dec-can age encountered in boreholes have also been reported(Chalapathi Rao et al., 2011b) from within the Chhattisgarh basin,near Raipur (Fig. 1). Geophysical and geochronological studies re-veal a thickened continental crust (35–40 km) in the Bastar cratonsince at least 3.6 Ga (e.g., Jagadeesh and Rai, 2008; Rajesh et al.,2009). Three kimberlite/orangeite fields are hosted in the Bastarcraton: (i) the southern Indravati kimberlite field (IKF) representedby the �620-Ma non-diamondiferous kimberlite pipes intrusiveinto the sedimentary rocks of the Indravati basin at Tokapal, Bejr-ipadhar and Dunganpal (e.g. Mainkar et al., 2004; Lehmann et al.,

2006, 2007); (ii) the Nawapada lamproite field of 1.1 Ga in theNawapada area at the NE part of the Bastar craton (Patnaik et al.,2002; Sahu et al., 2012), and (iii) the 65 Ma Mainpur orangeite fieldcomprising the six diamondiferous pipes at Behradih, Kodomali,Payalikhand, Jangra, Bajaghati and Kosumbara at the NE part ofthe Bastar craton (e.g. Chatterjee et al., 1995; Mainkar and Leh-mann, 2007; Lehmann et al., 2010). The present study concernsthe orangeites from the Mainpur field.

The Bundeli granitoids (equivalents of nearby 2300 Ma Dongar-garh granites) together with gabbroic rocks and dolerites consti-tute the oldest outcropping litho-units in the Mainpur orangeitefield (Mishra et al., 1988). Meso-Neoproterozoic platformal sedi-mentary rocks of the Khariar and Pairi basins in close westernvicinity of the thrust fault between the Bastar craton and the East-ern Ghats Mobile belt (EGMB) overlie the Bundeli granitoids(Fig. 1). The orangeite pipes, together with minor basalts of DeccanTrap geochemistry and age, represent the youngest igneous activ-ity and are associated with the Bundeli granites (Mainkar and Leh-mann, 2007; Chalapathi Rao et al., 2011b). With the exception ofthe Behradih and the Kodomali pipes, the remainder of the Main-pur orangeites are not suitable for petrological and geochemicalstudies owing to their highly silicified and weathered nature. Abrief description of the geology of the Behradih and Kodomaliorangeites is provided below.

Behradih orangeite (82�1206.300; 20�12054.500): This is the first dis-covered diamondiferous pipe from the Bastar craton located�15 km south of Mainpur town (Newlay and Pashine, 1993). Itconsists of a deeply-weathered, greenish clayey ‘yellow ground’covered by a thin veneer of bluish-black gritty eluvial soil. Drillingof the pipe was carried out to a depth of 188 m via five bore holesby the Directorate of Mines and Geology, Chhattisgarh. Relativelyunaltered bluish-gray to black, hard and compact pipe rock wasencountered beyond 65 m in all of them. The Behradih pipe is car-rot shaped, filled by tuffisitic breccia and contains both pyroclasticas well as diatreme facies rocks. It has a dimension of�300 � 300 m and is the largest orangeite occurrence in the Main-pur field. Macrodiamonds up to 200 ct as well as �486 microdia-monds have been recovered from treatment of pit material(Newlay and Pashine, 1993; Verma and Saxena, 1997). Diamondsoccur mostly as octahedrons and dodecahedrons with a few ofthem having eclogitic and peridotitic inclusions (Jha et al., 1995).

Kodomali orangeite (82�14025.600; 20�11018.200): This dyke-likepipe is located �4.8 km SSE of the Behradih pipe and is the only‘harde bank’ outcrop in the Mainpur field (Fig. 1). It has dimensionsof 80 � 30 m and strikes in a E–W direction. The outcrop is darkgrayish green, hard, compact and massive in nature. One surfacesample weighing 45 kg collected during prospecting yielded 6macro and 1 micro diamond (ORAPA, 2000). Autoliths of earliererupted pipe rock are also reported (Chatterjee et al., 1994)

3. Petrography

Detailed petrological as well as geochemical studies of the Beh-radih and Kodomali orangeites have been provided by previousstudies (e.g., Fareeduddin et al., 2006; Paul et al., 2006; Mainkarand Lehmann, 2007; Chalapathi Rao et al., 2011a). The Behradihorangeite is characterised by rounded to sub-rounded olivine mac-rocrysts and pelletal lapilli set in a very fine grained glassy to cryp-tocrystalline serpentine-chlorite matrix dominated bymicrophenocrysts of olivine. The groundmass is made of phlogo-pite, clinopyroxene, spinel, perovskite, apatite, carbonate and ser-pentine (Fig. 2A). The macrocrysts as well as the matrix phasesare overprinted by pervasive carbonate-talc-serpentine alteration.Fresh and unaltered olivine has been reported only from autoliths(Chalapathi Rao et al., 2011a). The Kodomali orangeite, on the

N.V. Chalapathi Rao et al. / Journal of Asian Earth Sciences 84 (2014) 24–33 25


other hand, has a distinct inequigranular texture dominated by mac-rocrysts (up to 1 mm) and microphenocrysts of fresh and unalteredolivine (Fig. 2B), set in a fine grained groundmass dominated byphlogopite, clinopyroxene, spinel, perovskite and apatite. Microlitesof clinopyroxene, present as acicular laths, are a characteristic fea-ture of the pipe along with clusters of spinel and perovskite. Serpen-tinisation of olivine and chloritisation of phlogopite is minimal.Magmatic flow layering is conspicuously noticed in the groundmass.

4. Sampling and analytical techniques

Petrologically and geochemically well-characterised drill-core(Behradih) and surface samples (Kodomali), free of alteration andlack of visible crustal xenoliths/xenocrysts (Fig. 2), were chosen forPGE analysis. The PGE concentrations of the samples and the refer-ence material were determined at the geochemical laboratories ofthe National Geophysical Research Institute (NGRI) in Hyderabadusing the NiS fire assay technique followed by Te co-precipitationand inductively coupled plasma mass spectrometry (ICP-MS). APerkinElmer SCIEX Model ELAN� DRC™ II ICP mass spectrometer(Concord, Ontario, Canada) was used throughout this work. Thesample introduction system consisted of a standard Meinhard� neb-ulizer with a cyclonic spray chamber. An external calibration proce-dure was adopted for the analysis, and Cd and Tl were used as

internal standards because of the close proximity of these massesto light (Ru, Rh, and Pd) and heavy (Ir and Pt) PGE, respectively.Though there are two international kimberlitic reference materials(SARM-39, MINTEK, RSA and MY-4, IGEM, Russia) available with cer-tified values for rare earth elements (REE) and several trace elements(Roy et al., 2007), unfortunately no certified data are available forPGE. Hence, reference materials, WGB-1 (Gabbro Rock, CCRMP, Can-ada) and WMG-1 (Mineralised Gabbro, CCRMP, Canada) were uti-lised for calibrating the instrument and to monitor the accuracy ofthe data generated. The precisions obtained were found to be betterthan 8% RSD in general for all PGE with comparable accuracies. Thereis also a good agreement between the certified values for PGE andthe values obtained in this study for WMG-1 (Table 1). The detectionlimits were in the range of 0.004–0.015 ng/g with blank values rang-ing from 0.001 to 0.002 ng/g for different PGE. More details on theanalytical methodology followed in this study including samplepreparation, NiS fire-assay separation and Te-coprecipitation pre-concentration procedures were provided elsewhere by Balaramet al. (2006) and Roy et al. (2010).

5. Results and discussion

The total PGE content in the Kodomali orangeite (14.5 ± 2.9 ppb,with a range of 11.8–18.0 ppb) is slightly higher than in the

Fig. 1. Generalised geological map of the Bastar craton, central India (after Chaudhuri et al., 2002). The location of the end-Cretaceous Mainpur orangeites (Behradih andKodomali) of this study as well as sub-surface mafic dykes of Deccan-age near Raipur within the Mesoproterozoic Chhattisgarh basin are also shown. The inset map showsmain outcrop area of the Deccan Traps and location of the study area.

26 N.V. Chalapathi Rao et al. / Journal of Asian Earth Sciences 84 (2014) 24–33


Behradih pipe (11.2 ± 1.3 ppb, with a range of 9.2–13.6 ppb). Theconcentrations of the individual PGE are indistinguishable in bothpipes (0.8–2 ppb Os; 0.8–1.6 ppb Ir; 3.2–4 ppb Ru; 0.2–0.8 ppb Rh).However, Pt and Pd contents are distinctly higher in Kodomali

(4.2 ± 0.9 ppb Pt, with a range of 3.2–5.2 ppb, and 3.3 ± 0.5 ppb Pd,with a range of 1.4–7.4 ppb Pd) compared to Behradih (2.5 ±0.8 ppb Pt, with a range of 1.8–4.4 ppb Pt, and 2.4 ± 0.7 ppb Pd, witha range of 1.2–3.6 ppb Pd) (Table 1). The total PGE contents in theBehradih and Kodomali pipes are, however, distinctly lower thanthose estimated for the primitive mantle (24 ppb; Palme and O’Neill,2003). It should be mentioned here that the Os content measured bythermal mass spectrometry on a sample from the Kodomali orange-ite (Os = 0.69 ppb; see Table 1 in Chalapathi Rao et al., 2013) is strik-ingly similar to the values reported in this study and provides furtherconfidence in the results obtained here.

Based on their differential solubility in silicate magmas andtheir overall geochemical behaviour PGE are conventionally classi-fied into (i) the Ir Group (IPGE: Ir, Os, and Ru) which are refractoryelements that are least soluble in silicate melts and display a sider-ophile nature, and (ii) the Pd Group (PPGE: Pd, Pt and Rh) whichare elements that are more soluble in silicate melts and of chalco-phile character (Barnes et al., 1988; Lorand et al., 2008). Concentra-tions of PGE as well as Cr and Ni from the Behradih and Kodomaliorangeite samples are plotted versus their MgO contents inFig. 3A–I. Whereas Cr shows very good positive correlation, Nishows a moderate correlation and most of the PGE display a lackof correlation with Pd in Behradih and Rh in Kodomali being excep-tions. This is in turn also reflected in their Pd/Ir plot (Fig. 3I) wherethe samples under study share similar Pd/Ir ratios as those of theprimitive mantle.

An overall lack of correlation of PGE with MgO excludes significantCu-Ni sulphide fractionation during the evolution of the orangeitemagma whereas good correlation portrayed by Cr, and to some extentNi, with respect to MgO demonstrates that chromite and olivine frac-tionation exercised a dominant control (see Zhang et al., 2005). The lackof correlation for elemental pairs (above) also indicates that the PGEwere not hosted by a single mineral/phase during the evolution ofthese magmas. There is also a poor correlation between Ir and otherPGE (Fig. 4A–F) with better correlations being displayed only by (i)Rh for both Behradih and Kodomali samples, (ii) Os in case of Kodo-mali, and (iii) Ru in case of Behradih samples.

Primitive-mantle normalised PGE distribution patterns of theMainpur orangeites indicate a strong similarity in their patterns(Fig. 5A). They have broadly flat profiles from Os to Pd, similar tothose displayed by the Mengyin kimberlites of China, and lackthe relatively steeper slope displayed for these elements by thekimberlites and orangeites from southern Africa and Brazil(Fig. 5A). In general, abundances of Os, Ru, Pt and Pd to Ir and Rhare similar or lower than those from southern Africa, Brazil andChina. Positive anomalies in Ru are a characteristic feature of the

Fig. 2. Photomicrographs (plane polarised transmitted light) of the Behradih (A:DB-5-111) and Kodomali (B Kodo-A). Abbreviations: Ol = olivine; Ph = phlogopite;P = pelletal lapilli. A lack of significant alteration and contamination is a charac-teristic feature of the samples and for details of description please refer to theSection 3 in the text.

Table 1Platinum Group of Element (PGE) concentrations in the Behradih and Kodomali pipes of this study. WMG-1 is the standard run during analysis.

WMG-1 WMG-1 Behradih pipe

PGE (ppb) Certified During analysis DEB-5-116 DEB3-90 DEB-5-106 DEB4-49 DEB-5-99 DEB-4-56 DEB-5-111 DEB-39-79

Os 24 ± 6.7 24 ± 0.71 0.8 ± 0.02 0.8 ± 0.02 1.2 ± 0.04 1.5 ± 0.04 1.2 ± 0.04 2.0 ± 0.06 1.2 ± 0.04 1.8 ± 0.05Ir 46 ± 2 45 ± 1.41 0.8 ± 0.03 0.8 ± 0.03 1.2 ± 0.04 1.2 ± 0.04 1.2 ± 0.04 0.8 ± 0.03 0.8 ± 0.03 1.0 ± 0.03Ru 35 ± 1 34 ± 0.71 4.2 ± 0.09 3.2 ± 0.07 3.2 ± 0.07 3.6 ± 0.07 3.4 ± 0.07 3.8 ± 0.08 3.8 ± 0.08 3.4 ± 0.07Rh 26.00 25 ± 2.12 0.2 ± 0.02 0.6 ± 0.05 0.6 ± 0.05 0.4 ± 0.03 0.8 ± 0.07 0.2 ± 0.02 0.3 ± 2.02 0.2 ± 0.02Pt 731 ± 25 716 ± 13.46 2.4 ± 0.05 1.8 ± 0.03 2.0 ± 0.04 2.4 ± 0.05 1.86 ± 0.35 2.6 ± 0.05 2.2 ± 0.04 4.4 ± 0.08Pd 382 ± 10 364 ± 2.84 2.6 ± 0.02 2.0 ± 0.02 2.6 ± 0.02 1.8 ± 0.01 3.6 ± 0.03 1.2 ± 0.01 2.8 ± 0.02 2.8 ± 0.02

Kodomali pipe

PGE (ppb) K-8 KM-1 KM-2 Kodo-A

Os 1.0 ± 0.03 1.4 ± .04 0.8 ± .02 1.0 ± .03Ir 1.6 ± 0.05 1.4 ± .04 0.8 ± .03 1.0 ± .03Ru 3.0 ± 0.06 3.6 ± .07 3.0 ± .06 4.0 ± .08Rh 0.4 ± 0.03 0.2 ± .02 0.2 ± .02 0.2 ± .02Pt 4.6 ± 0.09 3.8 ± .64 5.2 ± .10 3.2 ± .06Pd 7.4 ± 0.06 1.4 ±.01 2.4 ± .02 6.4 ± .05



Fig. 3. (A–I) Bi-viariate plots of MgO (wt%) versus various trace elements and platinum group elements and their ratios of the Behradih and Kodomali orangeite. The solid linein (I) represents the composition of the primitive mantle and is taken from Palme and O’Neill (2003). Composition of MgO (wt%), Ni (ppm) and Cr (ppm) in the figures is takenfromMainkar and Lehmann (2007) and Chalapathi Rao et al. (2011a). Triangles represent samples from the Behradih orangeite and squares represent samples from theKodomali orangeite.



Mainpur orangeites. The Pd/Ir ratios as well as transition metalcontents involving Ni/Cu (Fig. 5B), Cr (Fig. 5C) and Ni (Fig. 5D) ofthe Mainpur orangeites are strikingly similar to those from kimber-lites and orangeites from elsewhere and imply a broad similarity inthe petrogenesis of PGE in their magmas. However, the Pd/Ir ratioof the Mainpur orangeites is less primitive than in mantle xenolithsand chromitites shown for comparison (Fig. 5B–D).

It has been widely suggested that the PGE are hosted in themantle by sulphides or by PGE alloys (e.g., Rehkämper et al.,1999; Lorand and Alard, 2001; Maier et al., 2003). As PGE areknown to behave compatibly during partial melting of the mantle,they tend to remain in the residuum (source) in low-degree meltproducts such as kimberlites, orangeites and other alkaline rocks,and lower overall PGE contents and Pd/Ir ratios are expected inthese rocks (see Barnes et al., 1988; Maier and Barnes, 2004; Liet al., 2012). Therefore, the minor compositional differences be-tween the Mainpur orangeites and global kimberlites may reflecteither heterogeneity in the composition of the lithospheric keelsof respective cratons (Maier and Barnes, 2004; Zhang et al.,

2010), or the degree of partial melting undergone by these mag-mas, with consequences on sulphide retention at the source (seeMaier et al., 2003), or a combination of any of these factors. It isof interest to note here that Crocket et al. (in press) have recentlyshown that Deccan basalts in the western Deccan Province arecharacterised by relatively high Ni–Ir–Ru–Pt values whereas thosein the eastern Deccan Province are typified by dominance in Au, Ni,and particularly Pd. The clear dominance of Pd (and to a lesser ex-tent Pt) over the other PGE in both Behradih and Kodomali orange-ites, also located in the eastern Deccan Province, implies lesscompatible behavior of Pd and Pt relative to IPGE, even at low de-gree of partial melting.

Initial 87Sr/87Sr ratios in five samples from the Behradih andKodomali orangeites range from 0.707666 to 0.708690 and theirNdi values range from �4.7 to �9. 7 and imply derivation domi-nantly from melt source regions with lower time-integrated Sm/Nd ratios than Bulk Earth such as the metasomatised lithosphericmantle (Lehmann et al., 2010; Chalapathi Rao et al., 2011a). There-fore it is important to assess the influence of mantle metasomatism

Fig. 4. Bi-variate plots of Ir versus other PGE (A–E) and between Pt and Pd (F). Symbols are the same as in Fig. 3.



on PGE distribution patterns. Fig. 6A and B depicts a lack of positivecorrelation of Pd/Ir and Cu/Pd with Nb/La, which is considered as akey index of monitoring carbonatitic metasomatism (Lorand andAlard, 2001), and suggests (i) the relative immobility of PGE duringequilibration of orangeite magmas and fluids with mantle sulp-hides/alloys (see also Mitchell and Keays, 1981; Rehkämper et al.,1999; Maier and Barnes, 2004), and (ii) lack of sulphide segregationtriggered by assimilation of the crust (Li et al., 2012). Experimentalstudies have shown that PGE can be soluble over a wide range oftemperatures (Xiong and Wood, 2000) and as the Behradih orange-ite was subjected to carbonate-talc-serpentine alteration (above)this might have influenced its PGE concentrations. However, weopine that this mobility is probably not of significance since (i)the samples chosen have minimal visible and chemical alterationand/or contamination, (ii) their major, trace element (compatibleand incompatible) and radiogenic isotope (Sr and Nd) contentsand their ratios demonstrate the negligible role of crustal contam-ination and alteration (Lehmann et al., 2010; Chalapathi Rao et al.,2011a, 2013), and (iii) the PGE patterns of both the Behradih andKodomali orangeites show strong similarity (Fig. 5A). As the latterare free of any alteration/low temperature overprint this suggeststhat alteration did not influence PGE concentrations.

One final aspect concerns the possible degassing of iridium (andother PGE) which could have occurred in the Behradih and Kodo-mali orangeites leading to a deficiency in Ir as observed in the caseof basalts (McLean, 1985) and some hotspot volcanoes (Zoller et al.,1983; Rocchia et al., 1988; Toutain and Meyer, 1989; Day, 2013).However, as the Kodomali orangeite is emplaced as a dyke escapeof PGE and other volatiles is considered insignificant and IPGEabundances can be regarded as pristine. However, the relatively

Fig. 5. (A) Primitive-mantle normalised PGE plot for Behradih and Kodomali orangeites. Primitive mantle values are from Palme and O’Neill (2003). Data for on-cratonkimberlites and orangeites from southern Africa and Brazil is from McDonald et al. (1995) and for the kimberlites (Mengyin and Fuxian) from North China craton is fromZhang et al. (2010). Note that Os values are not reported by Zhang et al. (op.cit). Pd/Ir versus Ni/Cu (B), Cr (C) and Ni (D) plots for Behradih and Kodomali orangeites.Comparative data for kimberlites and orangeites from southern Africa and Brazil as well as for mantle xenoliths and podiform chromitites is from McDonald et al. (1995); datafor the kimberlites (Mengyin and Fuxian) from North China craton is from Zhang et al. (2010); and the data for Wajrakarur kimberlites, Dharwar craton, southern India, isfrom Roy et al. (2009). Symbols are the same as in Fig. 3. Note the similarity in metal content of the Mainpur orangeites compared to global occurrences. They have, however,lower Pd/Ir contents than those of the Wajrakarur field, Dharwar craton, southern India.

Fig. 6. NbN/LaN versus and Pd/Ir (A) and Cu/Pd (B) of the orangeites of this study.Symbols are the same as in Fig. 3. Nb, La and Cu contents are taken from Mainkarand Lehmann (2007) and Chalapathi Rao et al. (2011a) and their primitive-mantlenormalisation values are from McDonough and Sun (1995). Overall lack of positivecorrelation with Nb/LaN,which is a key metasomatic index, displays relativeimmobility of PGE.



lower concentration of some PGE in the Behradih orangeite (above)can be accounted for by this process.

Some of the first PGE studies on the Deccan basalts and theirintertrappean bole (clay) beds suggested Ir concentrations at or be-low the analytical detection limits (0.04 to 0.3 ppb) of neutron acti-vation analysis (Rocchia et al., 1988). The Ir content in the DeccanTraps is fairly well established and varies from 0.06 to 0.15 ppb intholeiites and intertrappean bole beds of the Western Ghats (Keaysand Lightfoot, 2010; Crocket and Paul, 2004); 0.11 ppb in the Kutchhalkaline basalts (Crocket and Paul, 2008) and 0.06 ppb in the easternDeccan province (at Amarkantak, Umaria, Shadol and Chirimiri;Crocket et al., in press). The Ir contents from the alkaline flows atthe Anjar alkaline flows (0.178 ppb) and the carbonatite (0.08 ppb)at Amba Dongar, NW India, were found to be more than an orderof magnitude higher than those from Deccan tholeiitic basalts buttheir overall contribution is insignificant in terms of the global irid-ium budget at the K-T boundary (Shukla et al., 2001a,b).

Our PGE measurements from the Mainpur orangeites revealhigher abundances compared to any of those documented fromthe Deccan basalts and alkaline rocks. However, their overall rela-tively low PGE concentrations together with the small volume ofthese rocks are grossly inadequate to account for the recordedanomalous Ir contents at the K–Pg boundaries such as 1.2 ppb Irat the Anjar inter-trappeans (Bhandari et al., 1995) and 12.1 ppbIr at Meghalaya (Bhandari et al., 1994), both in India, as well asthe global inventory of Ir estimated as up to 250,000 tonnes Ir inK–Pg boundary clays (Orth et al., 1990). Therefore, we infer thatthe anomalous iridium – enrichment reported at the global K–Pgboundary sections was not contributed from the mantle and anextraterrestrial origin seems a very strong cause.

6. Conclusions

The major conclusions of this study are summarised below:

� Palladium-group platinum group element (PPGE) contents (1.8–5.2 ppb Pt, 1.2–6.4 ppb Pd) of the end-Cretaceous orangeites(Behradih and Kodomali) from the Mainpur field, Bastar craton,central India, are distinctly higher compared to their iridium-group platinum group element (IPGE) concentrations (0.8–2 ppb Os, 0.8–1.2 ppb Ir, 3.2–4.2 ppb Ru, and 0.2–0.8 ppb Rh).� PGE abundances of Mainpur orangeites are higher than those

documented from Deccan Traps or associated alkaline rocks.� PGE contents as well as Pd/Ir ratios of the studied samples are

either similar or lower than those of the Mesoproterozoic andCretaceous kimberlites and orangeites from the Kaapvaal craton(southern Africa), Cretaceous kimberlites from the Sao Fransiscocraton (Brazil), Ordovician kimberlites from the North Chinacraton and the Mesoproterozoic southern Indian kimberlitesfrom the Dharwar craton.� An overall lack of correlation of PGE with MgO excludes signif-

icant Cu–Ni sulphide fractionation whereas a good to moderatecorrelation portrayed by Cr and Ni respectively, with MgO dem-onstrates the dominant role of olivine and chromite fraction-ation in the studied orangeites.� Primitive-mantle normalised PGE distribution patterns of the

Mainpur orangeites have broad flat profiles similar to those dis-played by the Mengyin kimberlites of China, and lack the rela-tively steeper slope displayed for these elements by thekimberlites and orangeites from southern Africa and Brazil.Positive Ru anomalies are a characteristic feature of the Main-pur orangeites.� Recent studies have shown that basalts from the eastern Deccan

Province are typified by dominance in Pd, compared to thosefrom the western Deccan Province (Crocket et al., in press).

Predominance of Pd (and rarely Pt) over the other PGE in boththe Behradih and Kodomali orangeites appears be a characteris-tic feature of the sub-continental lithospheric mantle in theeastern Deccan province.� PGE contents of the Mainpur orangeites are higher compared to

any of those documented from the Deccan basalts and associ-ated alkaline rocks. Together with the small-volume of theorangeites they are, however, insufficient to account for theanomalous Ir contents at the K–Pg boundary. Therefore, theiridium enrichment reported at the global K–Pg boundary sec-tions could not be traced to the mantle and an extraterrestrialsource is strongly implied.

Acknowledgements

We thank Hetu C. Sheth for his kind invitation to contribute tothis special issue on the Flood basalts of Asia in the memory of JohnMahoney whose contributions on Deccan flood basalts are indeedmonumental. We are grateful to Ashok Sahni (Lucknow) for hissuggestion to take up this interesting project of looking into theiridium content of the Deccan orangeites. Authors also expresstheir grateful thanks to Mrinal Sen (Director, NGRI) for his enthu-siastic support and encouragement for this inter-institutional col-laboration. NVCR thanks the Head of the Department of Geology,BHU, the Department of Science and Technology (ESS/16/336/2007 dt.6/11/2008 under DCS scheme) and Alexander von Hum-boldt Foundation for support. Datta Mainkar of Directorate ofMines and Geology, Chhattisgarh, Raipur is thanked for extendinghis help in sample collection. Gregory Shellnutt and two anony-mous reviewers provided critical comments and Hetu Sheth madeseveral useful editorial suggestions. Our sincere thanks to all ofthem.

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