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Geochimica et Cosmochimica Acta 127 (2014) 10–24

Hafnium–neodymium isotope systematics of the 2.7 Ga Gadwalgreenstone terrane, Eastern Dharwar craton, India:

Implications for the evolution of the Archean depleted mantle

Tarun C. Khanna a,b, Michael Bizimis b,⇑, Gene M. Yogodzinski b, Soumen Mallick b,c

a National Geophysical Research Institute, Council of Scientific and Industrial Research, Hyderabad 500 007, Indiab Department of Earth and Ocean Sciences, University of South Carolina, Columbia, SC 29208, USA

c Department of Geological Sciences, Brown University, Providence, RI 02912, USA

Received 16 January 2013; accepted in revised form 17 November 2013; available online 25 November 2013

Abstract

The Neoarchean Gadwal greenstone belt in the eastern Dharwar craton, India, hosts a well preserved metavolcanicsequence that is dominated by tholeiitic and calc-alkaline basalt-andesite-dacite-rhyolite series, which includes boninitic geo-chemical varieties. Bulk-rock Lu–Hf and Sm–Nd isotope systematics of these apparently arc-related volcanic rocks yieldindistinguishable ages of 2.701 ± 0.024 Ga and 2.702 ± 0.026 Ga, respectively. On the basis of the close spatial associationand identical ages of the different rock types we suggest 2.70 ± 0.03 Ga as the age of crystallization of the different rock typeswithin the Gadwal metavolcanic sequence. In contrast, bulk-rock Pb–Pb isotope systematics of the same samples yield a sig-nificantly younger and less precise age of 2.466 Ga (+0.068/�0.110 Ga). We tentatively interpret this younger age to representa metallogenic and crustal reworking event in the Dharwar craton, which disturbed the U–Pb system but not the Lu–Hf orSm–Nd systems. The Gadwal metavolcanic rocks have positive initial eHf(2.70Ga) = + 1.6 to + 8.7 and slightly negative topositive eNd(2.70Ga) = �0.1 to + 3.0 values, consistent with an origin from a long term depleted source relative to a chondriticreservoir at �2.7 Ga. Lack of correlation between initial isotopic compositions and major or trace element indices of fraction-ation and alteration suggest that the observed isotope variability probably reflects compositional variation in the Gadwalsource, similar to that observed in modern day island arcs. Two boninitic samples of the Gadwal sequence have eHf � 8.3and 8.7, and are more radiogenic than average depleted mantle for the time period 3.2 to 2.5 Ga (eHf = 4 to 6). Early (perhapsHadean) differentiation events that led to a depleted and heterogeneous mantle are apparent in the Nd and Hf isotope sys-tematics of 3.7–3.8 Ga Isua supracrustal rocks. The radiogenic Hf isotopes of the Gadwal boninites and the Hf, Nd isotopesystematics of rocks from other locations in the 3.4 to 2.5 Ga time period are consistent with the survival of fragments of anearly depleted mantle later in the Archean. From �2.0 Ga to present, the time-integrated 176Lu/177Hf and 147Sm/144Nd of thedepleted mantle appears nearly constant and similar to the present day average MORB source. These data indicate that pro-gressive elimination of early (>4.5 Ga) formed heterogeneities in the depleted mantle dominated the history of the Archeanmantle, and that portions of early depleted reservoirs survived through the Mesoarchean. These results have implications forthe mixing scales for the early terrestrial mantle and the timing of the initiation of present day plate tectonics.� 2013 Elsevier Ltd. All rights reserved.

0016-7037/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.gca.2013.11.024

⇑ Corresponding author. Tel.: +1 803 777 5565; fax: +1 803 7776610.

E-mail address: mbizimis@geol.sc.edu (M. Bizimis).

1. INTRODUCTION

Archean volcanic terranes provide important constraintson the nature of crustal growth and the evolution of themantle-crust system (Blichert-Toft and Arndt, 1999;

T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24 11

Frei et al., 2002; Smithies et al., 2005; Polat and Kerrich,2006; Guitreau et al., 2012; Jayananda et al., 2013). Geo-chemical studies of juvenile volcanic rocks (those producedpredominantly by partial melting of mantle peridotite) thaterupted throughout the Earth’s history, suggest that the de-pleted mantle underwent early differentiation with relativelyhigh Lu/Hf and Sm/Nd in the Hadean (Vervoort and Blic-hert-Toft, 1999; Boyet and Carlson, 2005; Bennett et al.,2007; Hoffmann et al., 2010; Rizo et al., 2012), followedby more modest and nearly constant fractionations laterin Earth’s history (Blichert-toft and Puchtel, 2010). Azircon-based Hf-isotope evolution for the depleted mantle(Hawkesworth et al., 2010) is also consistent with an earlydepletion and little subsequent fractionation in uppermantle Lu/Hf. The near total absence of 142Nd excesses(Bennett et al., 2007; Rizo et al., 2013) or deficits (O’Neilet al., 2008) in terrestrial rocks formed after �3.6 Ga sug-gest that any early depleted and/or enriched reservoirs weredestroyed within the first billion years of Earth’s history byrecycling or convective mixing (Murphy et al., 2010). This iscorroborated by the highly radiogenic Hf isotopes of the3.72 Ga Isua boninitic metabasalts (Hoffmann et al.,2010) which require an early depleted mantle source withhigher time-intergrated Lu/Hf than observed in later volca-nic sources. These data raise some important questionsabout the evolution of the crust and mantle system. Forexample, how extensive were the early depleted and en-riched reservoirs in the Hadean, and how long did they sur-vive mixing? Examining these questions can provideimportant clues about changes in the tectonic style in theearly Earth, such as the onset of Wilson cycle and plate tec-tonics (e.g. Shirey and Richardson, 2011).

The validity and details of these models critically dependon the availability of isotope data from samples of the Ar-chean depleted mantle. In this paper we present the firstcombined Hf–Nd–Pb isotope investigation of the Archeanintraoceanic volcanic arc sequence from Gadwal, in theeastern Dharwar craton, India. Our results show that theGadwal volcanic association has concordant Lu–Hf andSm–Nd ages of �2.70 ± 0.03 Ga and boninitic lavas thatextend to Hf isotope compositions (eHf(2.7 Ga) � 8.7) thatare relatively more radiogenic than mantle-derived rocksfrom elsewhere in that time period. We use these data alongwith published data to explore the time-integrated Lu/Hfand Sm/Nd evolution of the Earth’s depleted mantle fromthe Early Archean to present day and discuss the possibilityfor the survival of early (Hadean) depleted mantle reser-voirs through the Archean.

2. REGIONAL GEOLOGY

The Archean Dharwar craton of Peninsular India isdominated by composite granite-greenstone terranes thatare intruded by mafic dikes and surrounded by youngergranitoid batholiths (Moyen et al., 2003). Contacts betweendifferent greenstone belts and adjacent granites are inferredto be either tectonic or intrusive (Naqvi and Rogers, 1987).The volcanic, plutonic and meta-sedimentary assemblagesin the Dharwar greenstone belts vary in age from �3.4 to2.5 Ga and are formed by diverse tectonic processes. The

Dharwar craton is divided into eastern and western regionsby a NNW-SSE trending shear zone, which extends contin-uously along the eastern margin of the Chitradurga green-stone belt (Jayananda et al., 2000). The eastern Dharwarcraton is composed of linear, more or less north–southtrending greenstone terranes, which include a small quan-tity of metasedimentary rock, but are dominated by volca-nic assemblages that define both tholeiitic and calc-alkalineigneous series (Naqvi et al., 2006; Manikyamba and Khan-na, 2007). The Gadwal greenstone belt, which is the subjectof this study, is located in the eastern Dharwar craton(Fig. 1a). Within the Gadwal terrane there is a diverse suiteof high-Mg boninitic rocks and an associated basalt-andes-ite-dacite-rhyolite suite. The petrogenesis of Gadwal volca-nic rocks, and their geochemical similarity to modern daysubduction-related volcanic rocks, has been discussed in de-tail by Manikyamba et al. (2005, 2007), Manikyamba andKhanna (2007) and Khanna (2013).

The Gadwal belt follows a N–S trend in the southernpart and NNW-SSE in the north, imparting an arcuateshape. Three generations of folding have been recognizedwithin the belt and the rocks have been metamorphosedto greenschist and lower amphibolite facies (Matin, 2001).The north-central part of the belt is composed primarilyof basalts of both tholeiitic and calc-alkaline affinity (Khan-na, 2013), and a geochemically distinctive group of boninit-ic basalts, which appear geochemically similar to thosefound among the Isua metavolcanic rocks (Polat et al.,2002; Frei et al., 2004). Some metabasalts show well-pre-served pillow structures. Dacites and rhyolites are also pres-ent in the north-central belt. Some of these havefractionated (adakitic) trace element patterns (Mani-kyamba et al., 2007), and on this basis appear to be differ-ent from andesites, dacites, rhyolites and minor tuffs andagglomerate that dominate the southern part of the belt(Fig. 1b). Mafic alkaline dikes occur in the northeast, eastand southeast parts of the belt. Some of these crosscutthe metabasalts in Gadwal terrane (Khanna et al., 2013).Recent work suggests an age of �2.5 Ga for the granitoidsin the eastern Dharwar craton (Jayananda et al., 2000;Moyen et al., 2003), but neither the granites nor the dikesin the vicinity of the Gadwal belt have been dated by isoto-pic methods. Volcanic rocks in the adjacent Penakacherlagreenstone belt yield a 2.775 + 0.050/�0.1 Ga age bybulk-rock Pb–Pb isotope method (Khanna et al., 2011).The felsic volcanic rocks of Hutti greenstone belt in theeastern Dharwar craton were dated at 2.587 ± 0.007 Maby U–Pb in zircon (Sarma et al., 2008). The metabasaltsfrom Nellore greenstone belt, abutting the eastern marginof Proterozoic Cuddapah Basin in the eastern Dharwar cra-ton, were dated at 2.693 ± 0.094 Ma by bulk-rock Sm–Nd(Ravikant, 2010). The ages summarized here for adjacentformations potentially imply a Neoarchean age for theGadwal belt.

3. ANALYTICAL TECHNIQUES

After petrographic screening, subsets of undeformedand minimally altered samples that display relict igneoustextures (Manikyamba and Khanna, 2007) were selected

(A)(B)

Fig. 1. (A) Simplified geological map of southern peninsular India comprising of three major tectonic blocks: western Dharwar craton(WDC), eastern Dharwar craton (EDC) and southern granulite terrane (SGT). Also shown in the box is the location of Gadwal greenstonebelt in the eastern Dharwar craton. (B) Generalized geological map of Gadwal greenstone belt, modified after Srinivasan (1990).

12 T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24

for detailed isotope study. Rocks were powdered manuallyusing an agate mortar and pestle, and analyzed for Pb, Ndand Hf isotope compositions at the Center for ElementalMass spectrometry, Department of Earth and Ocean Sci-ences, University of South Carolina.

About 100–200 mg of sample powders were digested insteel jacketed Teflon bombs using HF–HNO3 mixtures to en-sure complete digestion of possible zircon phases. The pow-ders were not leached before digestion. Lead was extractedfirst using conventional HNO3-HBr extraction techniquesin anion resin (e.g. Abouchami et al., 1999), while Hf andNd extracted from the washes of the Pb chemistry. Hafniumwas separated following the Munker et al. (2001) technique.Neodymium was first separated from the bulk rock usingcation exchange resin (e.g., Hart and Brooks, 1977) and sub-sequently purified using micro-columns with Ln Resin (Ei-chrom, USA) (Pin and Zalduegui, 1997).

Lead isotope compositions where determined on theThermo Finnigan NEPTUNE MC-ICP-MS using theTl-addition technique (White et al., 2000). The sample

solutions were introduced with a 50 ll self-aspirating Teflonnebulizer (ESI, USA) coupled to an APEX-Q (ESI, USA)system using an H-sampler cone and X-skimmer coneconfiguration. The Pb isotope ratios were corrected formass fractionation using 203Tl/205Tl = 0.418911 and theexponential law. The average fractionation-corrected ratiosfor the NBS-981 standard were: 206Pb/204Pb = 16.936 ±0.0021, 207Pb/204Pb = 15.4905 ± 0.0010, 208Pb/204Pb =36.6942 ± 0.0029 (2 standard deviations, n = 17, for a65 ppb Pb solution, consuming �45 ng of Pb per analysis).The Pb/Tl ratios in the samples were kept near identical tothe standard by first checking the Pb signal intensities andthen adding Tl at the appropriate levels. The fraction-ation-corrected Pb isotope ratios were then corrected forinstrumental bias using the fractionation corrected averageratios of the NBS-981 and the values reported by Todt et al.(1996).

Hafnium and Nd isotopes were also determined with thesame instrument configuration. Instrumental mass fraction-ation was corrected using 179Hf/177Hf = 0.7325 and

T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24 13

146Nd/144Nd = 0.7219. During the course of the measure-ments, the JMC-475 standard was determined at176Hf/177Hf = 0.282143 ± 0.000008 (2 standard deviations,n = 9, consuming 36 ng per analysis) identical to the176Hf/177Hf = 0.282145 value recently reported for an ali-quot of this solution by Salters et al. (2011). An in-houseHf standard prepared from Hf oxide powder (Alfa Aesar)was measured at 0.282118 ± 0.000008 (2 standard devia-tions, n = 9, also consuming 36 ng per analysis), at identicalreproducibility to the JMC-475. The masses of 172Yb and175Lu were monitored and the 176Hf signal was correctedonline using the instrument software. The La Jolla Ndstandard was determined at 143Nd/144Nd = 0.511840 ±0.000018 (2 standard deviations, n = 32, 36 ng runs), andthe JNdi-1 standard at 0.512107 ± 0.000012 (2 standarddeviations, n = 13, 36 ng runs), both values being identicalto literature values (e.g. Tanaka et al., 2000; Salters et al.,2011). Full procedural blanks were Nd < 20 pg,Hf < 100 pg, Pb < 100 pg, and generally insignificant rela-tive to the amount of Hf, Nd and Pb recovered from thesamples. The measured Hf, Nd and Pb isotope ratios ofthe USGS reference materials BHVO-1, AGV-2 andBCR-2 are reported in Tables 1 and 2, and are identicalto literature values, when normalized to the same standardisotope compositions (e.g. Weis et al., 2005; Weis et al.,2007; Salters et al., 2011).

The 147Sm/144Nd and 176Lu/177Hf ratios were deter-mined by isotope dilution on a separate digestion of50 mg of rock powder spiked with a mixed 145Nd–149Smand 176Lu–179Hf enriched isotope solution. The particularenriched isotope spikes do not permit the determinationof the isotope composition and concentration by isotopedilution from the same digestion. We therefore opted forpreparing the mixed isotope spikes so that they minimize er-ror propagation in the isotope dilution concentration deter-mination of the samples. Two different mixed Sm–Nd andLu–Hf spikes were calibrated against gravimetric standardsand specifically optimized for the range of Nd, Sm, Lu, HfNd-Sm-Lu-Hf concentrations of these samples. As parent/daughter ratios and isotope compositions were determinedon different sample aliquots, it is theoretically possible thatsample powder heterogeneity may result in slight decou-pling between isotope compositions and parent/daughterratios. Extra care were taken to assure that powders arewell mixed prior to subsampling.

The spiked samples were dissolved in steel jacketed Tef-lon bombs using HF-HNO3 mixtures and separated withthe column chemistry described above. The Nd, Sm, Lu,Hf concentrations were determined on a Thermo FinniganELEMENT 2 HR-ICP-MS, using a 100 ll self-aspiratingTeflon nebulizer and a cyclonic spray chamber (ESI,USA), in low resolution mode. The ion beams were re-stricted to the analog counting mode to minimize possibleinstability of the analog-to-counting factor and kept to atleast 106 cps (except for the blanks) for enhanced precision.The isotope ratios where determined separately for each ele-ment with 300 ratios (30 scans, 10 passes) with routine in-run precision of 0.1–0.2% (2se). Potential interferences werealso monitored and corrected off line (e.g., 152Gd on 152Smby monitoring 155Gd, 156Gd), but these were always insig-

nificant. Fractionation correction for Nd, Hf, and Sm wasperformed offline using the exponential law.

For 176Lu/175Lu determination we followed the methodoutlined by Vervoort et al. (2004), by testing the reproduc-ibility of the natural 176Lu/175Lu ratio (0.02655) in differentLu/Yb mixtures. We determined 176Lu/175Lu =0.026533 ± 0.7% (n = 5) for Yb/Lu ratios from 0.25 to 1.For the fractionation correction, we used173Yb/171Yb = 1.129197 (Vervoort et al., 2004; normalizedto 174Yb/171Yb = 2.2326) and we calculated the 176Ybcontribution to 176Lu signal using 176Yb/173Yb(natural) =0.793045. On the basis of a plot of 176Yb-interference cor-rected and fractionation corrected 176Lu/177Lu vs. Yb/Lu,we determined an instrumental mass bias factor of 0.995on the 176Yb/173Yb ratio so that the corrected 176Lu/177Luratios remain constant regardless of added Yb (i.e.Vervoort et al., 2004). This mass bias factor is instrumentdependent and relates to the transmission of each isotopebeam through the instrument. As the samples are spikedwith 176Lu, the measured ratios are far higher than the nat-ural (we determined ratios from 176Lu/175Lu = 0.1 to 1),and the 176Yb correction is even less significant than inthe natural isotope ratio case.

The measured 147Sm/144Nd and 176Lu/177Hf ratios forthe USGS standards AGV-2, BHVO-2, and BIR-1 are gi-ven in Table 1. The reproducibility of the ratios is betterthan 0.8% and well within the range reported in the GEO-REM database. For the isochron and initial isotope com-position determinations, we conservatively assign a 1%error on the measured 147Sm/144Nd and 176Lu/177Hf ratios.Isochrons were calculated with the ISOPLOT program(Ludwig, 2001), using the decay constants k147Sm = 6.54 �10�12, k176Lu = 1.867 � 10�11. The initial isotope com-positions were calculated using the chondritic values:143Nd/144Nd = 0.512630, 147Sm/144Nd = 0.1960, 176Hf/177Hf =0.282785, 176Lu/177Hf = 0.0336 (Bouvier et al., 2008).

4. RESULTS

4.1. Lu–Hf and Sm–Nd ages of the Gadwal metavolcanic

rocks

The entire Gadwal metavolcanic rocks suite yields abulk-rock Lu–Hf isochron age of 2.700 ± 0.024 Ga(MSWD = 40) with an initial 176Hf/177Hf = 0.281197 ±0.000024 and eHf(2.700 Ga) = + 5.3 ± 0.7 (Fig. 2a). Thesesamples define a nearly identical bulk-rock Sm–Nd iso-chron age of 2.701 ± 0.028 Ma (MSWD = 7.8) with initial143Nd/144Nd = 0.509231 ± 0.000037, or eNd(2.701 Ga) =+ 1.85 ± 0.6 (Fig. 2b). The essentially identical ages pro-duced by the two isotope systems suggest a �2.7 Ga ageof crystallization for the Gadwal metavolcanic rocks.

The large spread in the Lu/Hf and Sm/Nd ratios andcorresponding extreme range in isotope compositions(e.g., eHf(present day) = �45 to 167, eNd(present day) = �32 to94) may obscure subtle differences in eruption ages and/orinitial isotopic compositions of the different metavolcanicrocks in Gadwal belt. To evaluate these possible effects,we calculated Lu–Hf and Sm–Nd isochrons for theboninitic and basalt-andesite-dacite-rhyolite sample suites

Table 1Bulk-rock Lu–Hf and Sm–Nd isotope data for the Gadwal metavolcanic rocks.

176Lu/177Hf 176Hf/177Hf ± (2r) Lu (ppm) Hf (ppm) eHf (t=2700 ± 30 My)147Sm/144Nd 143Nd/144Nd ± (2r) Sm (ppm) Nd (ppm) eNd (t=2700 ± 30 My)

Boninite

TCK-36 0.1126 0.287112 ± 08 0.185 0.234 8.69 ± 1.65 0.3920 0.516274 ± 24 0.271 0.418 3.00 ± 0.77TCK-43 0.1098 0.286957 ± 08 0.184 0.239 8.30 ± 1.60 0.4127 0.516617 ± 29 0.243 0.356 2.48 ± 0.85G-21 0.0414 0.283345 ± 08 0.204 0.701 5.49 ± 0.16 0.1645 0.512134 ± 08 0.756 2.78 1.28 ± 0.12G-22 0.0597 0.284277 ± 06 0.201 0.478 5.06 ± 0.55 0.1728 0.512281 ± 11 0.602 2.11 1.27 ± 0.09TCK-35 0.1100 0.286863 ± 10 0.167 0.216 4.61 ± 1.60 0.4205 0.516665 ± 26 0.264 0.380 0.69 ± 0.88TCK-44 0.0973 0.286207 ± 12 0.125 0.182 4.57 ± 1.33 0.2889 0.514414 ± 23 0.238 0.498 2.52 ± 0.36G-23 0.0521 0.283842 ± 08 0.241 0.657 3.63 ± 0.39 0.1748 0.512249 ± 08 0.853 2.95 -0.07 + 0.08TCK-40 0.1240 0.287505 ± 13 0.084 0.096 1.59 ± 1.89 0.4624 0.517463 ± 37 0.239 0.312 1.71 ± 1.04

Basalt

GWL-48 0.0310 0.282835 ± 06 0.147 0.673 6.56 ± 0.05 0.1960 0.512765 ± 05 1.87 5.77 2.65 ± 0.01G-38 0.0273 0.282639 ± 06 0.374 1.94 6.47 ± 0.13 0.1944 0.512698 ± 06 2.43 7.56 1.90 ± 0.01G-33 0.0284 0.282665 ± 06 0.369 1.84 5.34 ± 0.11 0.1836 0.512518 ± 06 2.84 9.35 2.14 ± 0.05G-58 0.0325 0.282874 ± 05 0.582 2.54 5.11 ± 0.02 0.1777 0.512359 ± 07 2.84 9.67 1.06 ± 0.07G-35 0.0308 0.282771 ± 08 0.399 1.84 4.65 ± 0.06 0.1864 0.512568 ± 07 2.55 8.26 2.15 ± 0.04G-46 0.0356 0.283010 ± 05 0.439 1.75 4.25 ± 0.04 0.1790 0.512395 ± 10 2.34 7.92 1.35 ± 0.07

Adakite

G-69 0.0068 0.281521 ± 04 0.161 3.35 4.31 ± 0.56 0.1171 0.511269 ± 04 2.88 14.88 0.89 ± 0.31G-72 0.0054 0.281433 ± 04 0.158 4.15 3.78 ± 0.59 0.0985 0.510959 ± 05 2.83 17.37 1.29 ± 0.38

Andesite

AD-4 0.0075 0.281610 ± 06 0.321 6.09 6.22 ± 0.55 0.1095 0.511200 ± 03 5.96 32.88 2.16 ± 0.34TM-15 0.0079 0.281617 ± 05 0.310 5.54 5.67 ± 0.54 0.1079 0.511212 ± 14 6.12 34.33 2.99 ± 0.35GS-32 0.0087 0.281650 ± 05 0.275 4.50 5.48 ± 0.52 0.1167 0.511338 ± 03 4.67 24.21 2.38 ± 0.31GS-25 0.0085 0.281633 ± 04 0.263 4.41 5.23 ± 0.53 0.1161 0.511318 ± 03 3.78 19.68 2.20 ± 0.31

Rhyolite

TM-26 0.0050 0.281508 ± 06 0.291 8.31 7.21 ± 0.60 0.1067 0.511133 ± 04 5.98 33.91 1.84 ± 0.35G-88 0.0061 0.281509 ± 06 0.392 9.16 5.25 ± 0.58 0.0936 0.510949 ± 03 8.18 52.88 2.81 ± 0.40G-91 0.0082 0.281612 ± 04 0.343 5.93 4.96 ± 0.53 0.1086 0.511159 ± 04 5.25 29.22 1.68 ± 0.34

Standards

BCR-2 0.282871 ± 03 0.512626 ± 02AGV-2 0.0059 0.282959 ± 02 0.1086 0.512783 ± 01BHVO-1 0.283103 ± 02 0.512974 ± 01BIR-1 0.0595 0.283269 ± 05 0.2767 0.513085 ± 03BIR-1d* 0.0588 0.283271 ± 05 0.2799BHVO-2 0.0087 0.1494BHVO-2d* 0.0086 0.1491

* indicates duplicate analysis. Uncertainty in the eHf and eNd is calculated for 30My emplacement age uncertainty.

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Table 2Bulk-rock Pb isotope data for Gadwal metavolcanic rocks.

206Pb/204Pb ±(2r) 207Pb/204Pb ±(2r) 208Pb/204Pb ±(2r)

Boninite

G-21 21.932 0.0119 16.259 0.0028 43.038 0.0172G-22 16.117 0.0009 15.441 0.0006 36.069 0.0021TCK-44 16.346 0.0013 15.461 0.0007 35.597 0.0025G-23 19.883 0.0028 16.067 0.0011 39.939 0.0049TCK-40 16.123 0.0014 15.517 0.0009 34.987 0.0029

Basalt

GWL-48 15.482 0.0003 15.283 0.0003 35.202 0.0008G-38 18.616 0.0006 15.804 0.0005 38.524 0.0014G-33 19.415 0.0008 15.908 0.0006 39.570 0.0018G-58 17.120 0.0004 15.502 0.0004 37.989 0.0011G-35 18.605 0.0007 15.807 0.0006 38.991 0.0018G-46 16.373 0.0003 15.397 0.0003 36.542 0.0009

Adakite

G-69 27.617 0.0040 17.307 0.0009 49.585 0.0054G-72 26.438 0.0032 17.069 0.0007 48.906 0.0052

Andesite

AD-4 20.938 0.0005 16.195 0.0003 40.367 0.0011GS-32 21.299 0.0007 16.230 0.0003 40.969 0.0013GS-25 23.694 0.0009 16.660 0.0004 43.706 0.0015

Rhyolite

TM-26 21.729 0.0006 16.275 0.0003 42.018 0.0016G-88 20.496 0.0006 16.119 0.0003 42.809 0.0022G-91 18.995 0.0003 15.873 0.0003 39.301 0.0011

Standards

BCR-2 18.750 0.0005 15.618 0.0005 38.712 0.0015AGV-2 18.859 0.0004 15.611 0.0004 38.519 0.0011BHVO-1 18.681 0.0012 15.566 0.0010 38.323 0.0029

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separately, on the basis of their distinct Lu–Hf andSm–Nd ratios (Fig. 2a–f). Further division of the samplesuite is not warranted because of the relatively small spreadin Lu–Hf, Sm–Nd, and/or the limited number of samples.For the boninitic samples, the Lu–Hf isochron gives anage of 2.712 ± 0.099 Ga (MSWD = 6.6), with an initial176Hf/177Hf = 0.28117 ± 0.00018 (eHf(2712 Ma) = + 4.63 ±6.5). The Sm–Nd isochron gives a similar age of 2.734± 0.028 Ma (MSWD = 3.7) with an initial 143Nd/144Nd =0.50915 ± 0.00011 (eNd(2734 Ma) = + 1.1 ± 2.0) (Fig. 2cand d). The Lu–Hf isochron for the basalt-andesite-dacite-rhyolite suite gives an age of 2.697 ± 0.067 Ga(MSWD = 6.1) with an initial 176Hf/177Hf = 0.281200 ±0.000026 (eHf(2697 Ma) = + 5.3 ± 0.9). The Sm–Nd isochrongives again a similar age of 2.688 ± 0.072 Ma (MSWD =8.4) for these samples, with an initial 143Nd/144Nd =0.509249 ± 0.000068, (eNd(2688 Ma) = + 1.9 ± 1.2).

The calculated isochron ages for the separate subsets ofsamples (Fig. 2) produce Lu–Hf and Sm–Nd ages that agreewell with each other at the 95% confidence level, and withthe ages determined for the entire sample suite. Age differ-ences of �15 My for the Lu–Hf system and a slightly largerspread of 46 My for the Sm–Nd system at �2.7 Ga aresmall compared to the age of the rocks. The calculatedinitial 176Hf/177Hf and 143Nd/144Nd ratios of the subsetisochrons also agree well within error. Some of the uncer-tainty in the ages and variability in the initial ratios

undoubtedly reflect heterogeneity in the initial source com-positions of these different types of metavolcanic rocks,combined with potentially small age differences amongthe different samples. This is also reflected in the relativelylarge MSWDs (Fig. 2). Nevertheless, the concordant Lu–Hf and Sm–Nd ages defined by this diverse rock suite indi-cate that the Lu–Hf and Sm–Nd systems were not signifi-cantly disrupted by post-magmatic processes. Based onthe concordant age information summarized here, and inthe absence of other absolute age determinations for thesesamples (e.g., zircon U–Pb ages), we propose an age of�2.70 ± 0.03 Ga as the mean age of formation for the Gad-wal volcanic sequence. The 30 My uncertainty is the aver-age deviation of the Lu–Hf and Sm–Nd ages between theboninitic and basalt-andesite-dacite-rhyolite metavolcanicsuites. This uncertainty is slightly larger than the error atthe 95% confidence limit of the full suite Lu–Hf and Sm–Nd ages. This age also agrees with ages in the adjacentareas as discussed above.

4.2. Hf and Nd isotope compositions

Initial eHf and eNd values for the Gadwal metavolcanicrocks calculated for 2.70 ± 0.03 Ga are given in Table 1.The eHf(2.70Ga) values are variable from +1.6 to +8.7 andthe eNd(2.70Ga) from �0 to +3 (Table 1). The positive initialeNd and eHf values calculated from the isochrons (Fig. 2)

0.281

0.282

0.283

0.284

0.285

0.286

0.287

0.288

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

176 H

f/177 H

f

boninite adakite

basalt andesite

rhyolite

Age = 2700 ± 24 MaInitial 176Hf/177Hf =0.281197 ± 0.000024

MSWD = 40; εHf (2700) = 5.31

a

0.509

0.510

0.511

0.512

0.513

0.514

0.515

0.516

0.517

0.518

0.00 0.10 0.20 0.30 0.40 0.50

143 N

d/14

4 Nd

boninite adakite

basalt andesite

rhyolite

Age = 2701 ± 28 MaInitial 143Nd/144Nd=0.509231 ± 0.000037

MSWD = 7.8; εNd (2701) = 1.85

b

0.283

0.284

0.285

0.286

0.287

0.288

0.289

0.02 0.04 0.06 0.08 0.10 0.12 0.14

176 H

f/177 H

f

boninite

Age = 2712 ± 99 MaInitial 176Hf/177Hf =0.28117 ± 0.000018

MSWD = 6.6; εHf (2712) = 4.63

c

0.511

0.512

0.513

0.514

0.515

0.516

0.517

0.518

0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50

143 N

d/14

4 Nd

boninite

Age = 2734 ± 28 MaInitial 143Nd/144Nd=0.50915 ± 0.00011

MSWD = 3.7; εNd (2734) = 1.10

d

0.2810

0.2814

0.2818

0.2822

0.2826

0.2830

0.2834

0.00 0.01 0.02 0.03 0.04

176Lu/177Hf

176 H

f/177 H

f

adakite basalt

andesite rhyolite

e

Age = 2697 ± 67 MaInitial 176Hf/177Hf =0.281200 ± 0.000026

MSWD = 6.1; εHf (2697) = 5.34

0.5104

0.5108

0.5112

0.5116

0.5120

0.5124

0.5128

0.5132

0.07 0.09 0.11 0.13 0.15 0.17 0.19 0.21147Sm/144Nd

143 N

d/14

4 Nd

adakite basalt

andesite rhyolite

Age = 2688 ± 72 MaInitial 143Nd/144Nd=0.509249 ± 0.000068

MSWD = 8.4; εNd(2688) = 1.87

f

Fig. 2. Plot of 176Hf/177Hf versus 176Lu/177Hf and 143Nd/144Nd versus 147Sm/144Nd bulk-rock isochrons, respectively, regressed for entireGadwal arc volcanic suite (a & b); for eight Gadwal boninites (c & d); and remaining basalt-andesite-rhyolite-adakite suite, excludingboninitic rocks (e & f); using the ISOPLOT program of Ludwig (2001).

16 T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24

and from the individual samples at 2.7 Ga indicate that theGadwal samples were derived primarily from a long-termdepleted source, relative to chondritic reservoir. On a eHf(i)

versus eNd(i) plot (Fig. 3), the Gadwal samples display a

broadly positive trend that is qualitatively similar to thepresent-day terrestrial array (Chauvel et al., 2008; Vervoortet al., 2011). This indicates that the source of the Gadwalmetavolcanic rocks at 2.7 Ga records time-integrated pro-

14.6

15.0

15.4

15.8

16.2

16.6

17.0

17.4

17.8

12 16 20 24 28 32

207 Pb

/204 Pb

boninite adakite

basalt andesite

rhyolite

a

Age = 2466 +68/-110 Ma

15.2

15.4

15.6

15.8

16.0

16.2

16.4

207 Pb

/204 Pb

boninite

Age = 2488 +320/-2200 Ma

b

T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24 17

cesses similar to those that have controlled the Hf–Nd iso-topic variability of the crust-mantle system, throughoutEarth’s history. The basalts, andesites, dacites and rhyolitesdisplay a narrower range of initial isotopic compositionsthan the boninitic samples (Fig. 3). Given their much largerLu/Hf and Sm/Nd ratios, the calculated initial Hf and Ndisotope ratios for the boninitic samples are more sensitive toage uncertainties than the other rock types. Taking the30 Ma age uncertainty into consideration, the initial Hf,Nd isotope ratios of the boninitic samples overlap the otherrock types but also extend toward the most radiogenic ini-tial Hf and Nd isotope ratios. This is discussed furtherbelow.

4.3. Pb Isotopes

A bulk-rock 206Pb/204Pb–207Pb/204Pb isochron for theGadwal metavolcanic suite (Table 2; Fig. 4a) gives an ageof 2.466 Ga (+0.068/�0.110 Ga), significantly youngerthan the Lu–Hf and Sm–Nd ages discussed above and witha larger uncertainty. When the boninitic samples are con-sidered separately as above, they display more scatter rela-tive to the basalt, andesite, rhyolite and adakitic samples(Fig. 4b) and give a highly imprecise age of 2.48 (+0.32/�2.2) Ga. The basalt-andesite-dacite-rhyolite suite definesan age of 2.528 (+0.035/�0.044) Ga (Fig. 4c), significantlyyounger than the Lu–Hf and Sm–Nd age of �2.7 Ga. Thelarge uncertainties and very high MSWDs (>1000) associ-ated with these Pb-ages suggest a regional disturbance ofthe Pb-isotope system and at a younger age than the2.7 Ga age derived from the Lu–Hf and Sm–Nd systems.

-2

0

2

4

6

8

10

12

14

-1 0 1 2 3 4 5

εNd(initial)

εHf (i

nitia

l)

Wawa, Abitibi

mean Gadwal EDR, SCHEM @ 2.7 Ga

Superior @ 2.7 Ga

Abitibi @ 2.72 Ga

depleted Gadwal @ 2.7 Ga

Fig. 3. Initial eNd vs. eHf plot of the Gadwal metavolcanic rocks.Symbols as in Fig. 2. Field outlined with dashed line for the�2.7 Ga Wawa and Abitibi volcanics (Vervoort and Blichert-Toft,1999; Blichert-Toft and Arndt, 1999; Polat and Munker, 2004) isplotted for comparison. The mean Gadwal source isotope compo-sition, represented by the initial of the isochrons, is compared tothe estimated Superior Province depleted mantle at 2.7 Ga (Polatand Munker, 2004), the Abitibi Komatiite source at 2.725 Ga(Blichert-Toft and Puchtel, 2010) and the composition of the EarlyDepleted Reservoir (EDR), Superchondritic Earth Mantle(SCHEM) at 2.7 Ga, derived from the 142Nd isotope systematicsof terrestrial rocks (Boyet and Carlson, 2006; Caro and Bourdon,2010). The Hf and Nd isotope composition of the depleted Gadwalsource is taken as the most radiogenic sample (TCK-36).

14 16 18 20 22 24

14.8

15.2

15.6

16.0

16.4

16.8

17.2

17.6

12 16 20 24 28 32206Pb/204Pb

207 Pb

/204 Pb

adakite basalt

andesite rhyolite

Age = 2528 +35/-44Ma

c

Fig. 4. Plot of 207Pb/204Pb versus 206Pb/204Pb isotope systematicsfor the entire Gadwal sample suite (a); the five boninitic rocks (b);basalt-andesite-rhyolite-adakite sample suite (c). Ages are calcu-lated using the ISOPLOT program of Ludwig (2001).

5. DISCUSSION

5.1. Crustal contamination and post-magmatic alteration

Although there is compelling evidence for the presenceof Archean continental crust in the eastern Dharwar craton

0

2

4

6

8

10

12

14

16

0 0.02 0.04 0.06 0.08 0.1 0.12 0.14176Lu/177Hf

ε H

f (in

itial

)

b

Zunhua

Isua

Wawa

Noranda

0

1

2

3

4

5

0 0.1 0.2 0.3 0.4 0.5147Sm/144Nd

ε N

d (in

itial

)

boninite adakitebasalt andesitedacite/rhyolite Wawa arc volcanics

Isua

Noranda

Whundo

Wawa

a

Fig. 5. (a) 147Sm/144Nd vs. eNd(2.70Ga) and (b) 176Lu/177Hf vs.eHf(2.70Ga). The lack of correlation between initial isotope compo-sitions and parent-daughter ratios implies that the Gadwalmetavolcanic rocks did not receive any appreciable contributionfrom mature crustal material either in their source region, or byassimilation en route to the surface (e.g. Vervoort and Blichert-Toft, 1999). Other Archean metavolcanic rocks are plotted forcomparison: �2.7 Ga Noranda metavolcanic rocks (Vervoort andBlichert-Toft, 1999); �2.7 Ga Wawa arc metavolcanic rocks (Polatand Kerrich, 2002; Polat and Munker, 2004); �3.12 Ga Whundoarc metavolcanic rocks (Smithies et al., 2005); �2.5 Ga Zunhuaperidotites, north China craton (Polat et al., 2006b); and �3.72 Gaboninitic metabasalts from Isua (Hoffmann et al., 2010). Note thelarge Sm/Nd and Lu/Hf fractionations displayed by Gadwalboninites compared to other Archean arc volcanic rocks.

18 T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24

(Jayananda et al., 2000; Moyen et al., 2003; Maibam et al.,2011), the relatively high MgO, Mg#, Cr and Ni abun-dances of the Gadwal basalts and boninitic rocks (Mani-kyamba et al., 2005), indicate relatively primitivecompositions that are unlikely to have been significantlymodified by extensive fractionation and assimilation ofcrustal rocks in shallow magmatic plumbing systems. Rela-tively low abundances of rare-earth and high field strengthelements (REEs, HFSEs) combined with relatively unfrac-tionated trace element patterns (low Zr/Y and Ce/Yb),indicate limited enrichments in the source of Gadwal bas-alts and boninites (Manikyamba and Khanna, 2007; Khan-na, 2013). Positive Eu and Zr anomalies in many of theandesitic samples (Manikyamba et al., 2007) are also incon-sistent with assimilation of, or interaction with, upper crus-tal rocks. Across all Gadwal rock types, we find norelationships between initial Nd or Hf isotope ratios and147Sm/144Nd or 176Lu/177Hf ratios (Fig. 5) or other geo-chemical indices of fractional crystallization (e.g., MgO,Mg#, SiO2, Cr, Ni, Ce, Th/Ce, etc.). Such relationshipsshould be present if, as expected, assimilation and frac-tional crystallization processes are linked (DePaolo,1981). In the absence of such evidence, we conclude thatthe assimilation of continental crust played no more thana minor role in controlling the compositions of the Gadwalmetavolcanic rocks.

Concordant age results for the Lu–Hf and Sm–Nd sys-tems (Fig. 2) also suggest that post-magmatic alterationhas not significantly affected the abundances of the REEand HFSE in the Gadwal metavolcanic rocks. The absenceof Ce anomalies (see Fig. A1, Appendix), such as those seenin some Archean metavolcanic rocks from southwestGreenland (Polat et al., 2002), is particularly important be-cause several studies have shown that Ce is more mobilethan Lu–Hf or Sm–Nd during weathering and metamor-phism (e.g. Rubatto et al., 2001; Patino et al., 2003). Theexcellent correlation between Nd and Zr (Fig. A1b, Appen-dix) is also taken as evidence that the light-REEs were notsignificantly mobilized during post magmatic alteration,deformation and metamorphism (Polat et al., 2006a).

The large scatter of the Pb–Pb isotope trends for theboninitic samples (Fig. 4b), and the relatively youngPb–Pb age calculated for the basalt-andesite-dacite-rhyolitesuite (Fig. 4c) probably indicate some post-magmaticdisruption of the U–Pb system in the Gadwal samples.Interestingly, the �2.53 Ga Pb–Pb age recorded by thebasalt-andesite-dacite-rhyolite suite coincides with most ofthe syntectonic granitoid intrusions in the eastern Dharwarcraton (Jayananda et al., 2000; Vasudev et al., 2000; Rogerset al., 2007) and with mineralization events in the adjacentgreenstone terrane (Sarma et al., 2008). This youngerPb–Pb age of �2.53 Ga may then represent a basin-scalehydrothermal event, possibly related to syntectonic orpost-tectonic granitic intrusions (Rogers et al., 2007) inthe eastern Dharwar craton. This postulated hydrothermalevent does not appear to have significantly affected theSm–Nd and Lu–Hf isotope systems or the relativeabundances of the REEs and HFSEs, but it probably didaffect the relative abundances of many of the low-charge,large-ion lithophile elements, such as Rb, K, Cs, Ba and

Sr, which can therefore not be used to interpret the mag-matic history of these rocks.

5.2. Hf and Nd isotope variability in the Gadwal

metavolcanic rocks source

In eHf(i)–eNd(i) space (Fig. 3) the Gadwal metavolcanicrocks show a significant overlap with the �2.7 Ga Wawametavolcanic rocks and Abitibi komatiites from the Supe-rior Province (Blichert-Toft and Arndt, 1999; Polat andMunker, 2004). The initial eHf(2.70Ga) (5.3 ± 0.7) andeNd(2.70Ga) (1.9 ± 0.6) derived from the isochrons of allGadwal samples, which represents a mean initial

T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24 19

composition for the Gadwal source, is similar to those cal-culated for the Superior Province mantle at 2.7 Ga(4.5 ± 0.5, 2.5 ± 0.5, respectively; Polat and Munker,2004), and Abitibi komatiites at 2.725 Ga (5.7 ± 0.8,3.4 ± 0.8 respectively; Blichert-Toft and Arndt, 1999; Blic-hert-Toft and Puchtel, 2010) (Fig. 3). The large overlap be-tween these datasets suggests that the global depletedmantle at 2.7 Ga may have been, on average, relativelyhomogeneous.

In detail, however, there is also significant variability inthe initial Hf and Nd isotopic compositions of the differentGadwal and Wawa rock types. The Wawa basalts havebeen interpeted to contain a recycled component in theirsource, which produced less radiogenic Nd isotopes in theMg-andesites compared to the adakites and Abitibi komat-iites (Polat and Kerrich, 2002; Polat and Munker, 2004).The Gadwal metavolcanic rocks show similar variabilityin initial Hf and Nd isotope ratios, but this variation doesnot appear related to the major element characteristics thatdefine the different rock types. We interpret these results toindicate that variation in the initial isotopic compositions ofthe Gadwal metavolcanic rocks is inherited primarily fromtheir source, and that major element compositions are con-trolled by processes that are active during magma genesisand transport. This interpretation stems from our under-standing of modern island-arc rocks, which have widelyvariable isotopic compositions that are primarily inter-preted to reflect recycling and mixing in their source (e.g.,Davidson, 1987; White and Patchett, 1984; Morris et al.,1990; Plank and Langmuir, 1993). Isotopic variation of5–6 epsilon units for Hf and Nd is observed in zero-ageAleutian lavas (Yogodzinski et al., 2010), and this variationincreases to more than 12 epsilon units in arcs such as theLesser Antilles, where the trench is sediment-choked andrecycling rates are high (Woodhead et al., 2001). It is there-fore not surprising to find significant initial isotopic varia-tion in the source of Archean metavolcanic rocks fromGadwal, which have a variety of major and trace elementcharacteristics like those of modern arc rocks ranging fromboninitic to tholeiitic, and calc-alkaline compositions(Manikyamba and Khanna, 2007; Khanna, 2013).

Furthermore, major element variation in modern arc la-vas is commonly controlled by fractional crystallization(Gill, 1981), but in the Archean, andesites dacites and rhy-olites, such as those found in the Gadwal belt, may also beproduced by remelting of basaltic source rocks in a presum-ably hotter Archean tectonic regime (e.g. Herzberg et al.,2010). In many respects the Gadwal Hf–Nd isotope system-atics resemble those of western Aleutian arc lavas today,which display a wide range of arc basalts, andesites and da-cites that are all dominated by depleted Hf and Nd isotopecompositions with little addition of recycled, geochemicallymature continental crust (Yogodzinski et al., 1995, 2010).We therefore contend that the variable initial isotope com-positions of the Gadwal metavolcanic rocks also containinformation about the compositional heterogeneity of theirmantle source. Two of the boninitic rocks (TCK-36 andTCK-43) have more radiogenic Hf isotope compositionsthan the other Gadwal metavolcanic rocks, and the Wawaand Abitibi 2.7 Ga counterparts. Because sediment recy-

cling and crustal contamination can only decrease theHf–Nd isotope ratios, we consider the most depletedisotope composition (sample TCK-36, eHf(2.70Ga) � 8.7and eNd(2.70Ga) � 3, Table 1) to represent the minimumHf and Nd isotopic composition of a depleted mantlereservoir that was present in Gadwal at 2.7 Ga (Fig. 3).

5.3. Lu/Hf and Sm/Nd evolution of the depleted mantle

throughout the Archean

Following Blichert-Toft and Puchtel (2010) we use theinitial Hf and Nd isotope ratios of the Gadwal samples tocalculate the time-integrated 176Lu/177Hf and 147Sm/144Ndrange of the Gadwal source at 2.7 Ga, assuming derivationfrom a chondritic Earth at 4.558 Ga, approximately 10 Myafter solar system formation. Of course, the precise extrac-tion age of the Gadwal source from the primitive mantle isunknown. Nevertheless, comparison of the time-integratedparent/daughter fractionation of the Gadwal source withother mantle sources from the Eoarchean to present-daycan place first order constraints on the changes of theLu/Hf and Sm/Nd ratios of the depleted mantle throughtime (Fig. 6). For these calculations, we use thepresent-day isotope composition of CHUR: 176Hf/177Hf =0.282785, 176Lu/177Hf = 0.0336 (Bouvier et al., 2008),143Nd/144Nd = 0.512638 and 147Sm/144Nd = 0.1967 (Jacob-sen and Wasserburg, 1980), the decay constants given ear-lier, and the equations given in Bennett et al. (2007). Onthe basis of these parameters, the time-integrated parent/daughter ratios of the present day average MORB sourcewith 176Hf/177Hf = 0.28328 and 143Nd/144Nd = 0.5131(Salters and Stracke, 2004) are 176Lu/177Hf = 0.03917 and147Sm/144Nd = 0.2119, respectively (Fig. 6). In comparison,the Gadwal depleted source has time-integrated superchon-dritic 176Lu/177Hf = 0.0402 ± 0.0013 and 147Sm/144Nd =0.209 ± 0.003 (Fig. 6), that is within-error identical tothat of the MORB source. In turn, the mean Gadwalsource, based on the initial Hf and Nd isotope ratiosfrom the isochrons, has 176Lu/177Hf = 0.0376 ± 0.0005and 147Sm/144Nd = 0.2042 ± 0.0024, about 4% lower thanthe contemporary depleted mantle.

The mean, time-integrated 176Lu/177Hf for the Gadwalsource is also identical to the mean source compositionscalculated for other �2.7–2.8 Ga datasets (Wawa: Polatand Munker, 2004; Abitibi, Belingwe, Kostomuksha:Blichert-Toft and Puchtel, 2010), whereas the 147Sm/144Ndis somewhat lower. These data suggest that the terrestrialdepleted mantle already by 2.7 Ga had, on average, time-integrated Lu/Hf and Sm/Nd ratios similar (within 4%) tothose of the present-day MORB source.

The higher-than chondritic 142Nd/144Nd ratios in terres-trial rocks have been explained by bulk Earth with a higherSm/Nd ratio than chondrite meteorites (Brandon et al.,2009; Caro and Bourdon, 2010). Alternatively, the terres-trial 142Nd-excesses have been explained by a chondriticearth where an early (>4.53 Ga) enriched reservoir (perhapsa primordial crust) separated and remains hidden from oursampling ability, leaving behind an early depleted reservoir(EDR) as the source of all observed terrestrial rocks (Boyetand Carlson, 2005). Both a non-chondritic Earth and an

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Age (Ma)

-5

0

5

10

15

20

ε Hf

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Age (Ma)

0.025

0.030

0.035

0.040

0.045

0.050

0.055

0.060

time-

inte

grat

ed 17

6 Lu/

177 H

f

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Age (Ma)

-5

-3

-1

1

3

5

7

9

11

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Age (Ma)

0.170

0.180

0.190

0.200

0.210

0.220

0.230

0.240

0.250

MORB Source

EDR, BSE

CHUR

MORB Source

EDR, BSE

CHUR

Isua, B

Barberton

GadwalZunhua

Isua

Frederikshåb

Isua, B

Barberton

Isua

Frederikshåb

a

dIsua, B

Barberton

Isua

Frederikshåb

Barberton

Gadwal

Isua

Frederikshåb

Abitibi

Abitibi

MORB Source

EDR, BSE

CHUR

MORB Source

EDR, BSE

CHUR

Zunhua

GadwalGadwal

b

c

ε Nd

time-

inte

grat

ed 14

7 Sm

/144

Nd

Isua, B

Fig. 6. Lu/Hf and Sm/Nd evolution of the depleted mantle since the early Archean (�3.8 Ga). (a) eHf and (b) eNd isotope compositions ofArchean to present day juvenile rocks, calculated relative to CHUR. Key rock formations discussed in the text are labeled for clarity. Alsoshown are the CHUR evolution (red line) and a hypothetical single stage Depleted Mantle evolution (green line) that originates from CHURat 4.558 Ga to the present day MORB source (Salters and Stracke, 2004). (c) Calculated time-integrated 176Lu/177Hf, and d) 147Sm/144Nd ofthe Archean sources shown in panels a) and b), respectively. All data have been recalculated using k147Sm = 6.54 � 10�12, k176Lu = 1.867 -� 10�11, and the CHUR values 143Nd/144Nd = 0.512638, 147Sm/144Nd = 0.1967, 176Hf/177Hf = 0.282785, 176Lu/177Hf = 0.0336. EDR, BSE isthe present day “visible” Bulk Silicate Earth composition based on the 142Nd systematics of terrestrial rocks (Boyet and Carlson, 2006; Caroand Bourdon, 2010). From the Early Archean to about 2 to 2.5 Ga, there appears to be a progressive elimination of early-formedheterogeneities and extreme Sm/Nd and Lu/Hf ratios produced during early differentiation of the Earth. “Isua, B” are the Isua Boniniticrocks. Data sources from: White and Patchett (1984), Jacobsen and Dymek (1988), Shirey (1991), Bennet et al. (1993), Bowring and Housh(1995), Vervoort and Blichert-Toft (1999), Blichert-Toft and Arndt (1999), Blichert-Toft and Puchtel (2010), Blichert-Toft et al. (2004),Smithies et al. (2005), Polat and Munker (2004), Polat et al. (2006b), Hoffmann et al. (2010, 2012), Szilas et al. (2012). (For interpretation ofthe references to color in this figure legend, the reader is referred to the web version of this article.)

20 T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24

EDR models result in a present-day bulk silicate Earth witheNd � 7, eHf = 12 ± 3, and 147Sm/144Nd = 0.2082,176Lu/177Hf = 0.0375 (Boyet and Carlson, 2006; Caro andBourdon, 2010). These time-integrated 176Lu/177Hf and147Sm/144Nd ratios are slightly lower than the MORBsource estimates but similar to Archean depleted mantlesources (Fig. 6). It may then be argued that the Lu/Hfand Sm/Nd fractionations in the Archean mantle are notthe result of common magmatic processes (depletion, recy-cling, etc.) from a chondritic mantle. Rather, they representundifferentiated sources from a non-chondritic Earth (Caroand Bourdon, 2010) or from the EDR (Boyet and Carlson,2006). However, the 176Lu/177Hf and 147Sm/144Nd ratios ofdepleted Gadwal, Frederikshab, Barberton and Isuasources (Fig. 6) extend to higher values than the estimatesfor a non-chondritic Earth or EDR. Therefore, an earlydepletion event or events, are still required regardless ofwhether the Earth is chondritic or not.

The time-integrated Lu/Hf and Sm/Nd ratios of Arche-an sources show evidence for the presence of more de-

pleted reservoirs within the 2.7 Ga depleted mantle thatvary up to 176Lu/177Hf = 0.0402 (Gadwal) and147Sm/144Nd = 0.2158 (Abitibi komatiite sample KAL-1;Blichert-Toft and Arndt, 1999). This is shown in Fig. 6,where the time-integrated 176Lu/177Hf and 147Sm/144Ndfor the source of juvenile magmas from the Early Archeanto Phanerozoic are compared to the time-integrated176Lu/177Hf and 147Sm/144Nd of the MORB source.Fig. 6 further shows that the Early Archean depleted man-tle, as sampled by the 3.7–3.8 Isua supracrustals and the3.45–3.55 Ga Barberton komatiites and tholeiites, includesmany samples with highly superchondritic time-integrated176Lu/177Hf and 147Sm/144Nd ratios, and appears moreheterogeneous than the later Archean to present-day man-tle. It can also be seen that from about 2.0 Ga to the pres-ent, the time-integrated 176Lu/177Hf and 147Sm/144Nd ofthe depleted mantle appears to be relatively constantand similar to the present day average MORB source.Thus, the emerging picture is that the isotopic evolutionof the depleted mantle throughout the Archean reflects

T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24 21

the reduction and loss of heterogeneities produced duringthe early differentiation of the Earth (e.g. Bowring andHoush, 1995; Rizo et al., 2012).

The combined 142Nd excesses and high 143Nd/144Nd ra-tios of the Isua amphibolites and tonalites (Boyet et al.,2003; Caro et al., 2003; Bennett et al., 2007; Rizo et al.,2011) place the age of depletion of the Isua source at�4.45 ± 0.1 Ga, with a time-integrated 147Sm/144Nd =0.22–0.236 (Bennett et al., 2007; Rizo et al., 2011; Kinoshitaet al., 2012), which is higher than the present-day MORBsource or other Archean reservoirs (Fig. 6). The high,time-integrated 176Lu/177Hf ratios calculated for the Isuaboninite source (>0.055, Fig. 6) also imply that a Hadeandepleted mantle source with higher Lu/Hf ratio than pres-ent day MORB or other Archean mantle sources, survivedmixing in the mantle for some 800 Ma. Such an earlydepleted mantle reservoir (or reservoirs) is probably theresult of early planetary differentiation (Bowring andHoush, 1995), perhaps in the presence of a magma ocean(e.g. Bennett et al., 2007; Rizo et al., 2011). The survivalof such a reservoir for at least 800 Ma in a hotter andpresumably more vigorously convecting mantle can beexplained by the lack of sustained deep-reaching subduc-tion zones, which would otherwise mix the upper and lowermantle, and the dominance of “stagnant lid” style tectonics(O’Neill et al., 2007; Shirey and Richardson, 2011; Debailleet al., 2013). The decoupling of the Hf from Nd isotopes insome Isua amphibolites notwithstanding (Rizo et al., 2011),both Hf and Nd isotopes provide evidence for the survivalof an early (�4.4- ± 0.1 Ga) depleted mantle to the EarlyArchean (�3.7 Ga), albeit from different sets of rocks(e.g. Hf from boninites, Nd from tonalites). Therefore, animportant question is, how long did this early depletedmantle survived mixing in the terrestrial mantle?.

With one exception (Upadhyay et al., 2009), the absenceof resolvable 142Nd excesses in terrestrial rocks since 3.5 Gasuggests that the early depleted source identified in the Ndand Hf isotope systematics of Isua rocks was efficientlymixed back into the mantle (Bennett et al., 2007). However,the relatively high Lu/Hf of some Barberton (3.55–3.45 Ga), Frederikshab (2.97 Ga), Gadwal (2.7 Ga) andZunhua (2.57 Ga) sources (Fig. 6c) could also be explainedby the survival of fragments of an early depleted mantlesource through the Mesoarchean. A similar conclusioncan be drawn from the Sm–Nd system, with high Sm/Ndratios in the Isua rocks and gradual decrease from 3.7 to�2.7 Ga (Fig. 6d). The high, time-integrated Lu/Hf andSm/Nd ratios in the mantle sources of some Archean rocks,including the source of the Gadwal boninitic lavas, are con-sistent with portions of an early depleted mantle, currentlybest recognized in the 3.7–3.8 Isua supracrustals, that sur-vived mixing and homogenization for another billion years,up to around 2.7 Ga. Recent reports of 142Nd excesses inthe 3.07 Ga Ivisaartoq greenstone belt (Bennet et al.,2010) and tholeiites from the 2.7 Ga Abitibi greenstone belt(Debaille et al., 2013), as well as 142Nd deficits in some3.4 Ga Isua mafic dikes (Rizo et al., 2012) further supporta prolonged survival of early depleted and enriched reser-voirs, perhaps through the Mesoarchean (Rizo et al.,2012; Debaille et al., 2013). Such proposed survival of early

formed depleted and enriched reservoirs has implicationsfor how quickly the Earth transitioned from stagnant lidto subduction style tectonics (Shirey and Richardson,2011; N�raa et al., 2012; Debaille et al., 2013). Finally,the approximately constant Lu/Hf, and the subtle increasein Sm/Nd with time of the depleted mantle after about�2.7 Ga are consistent with a nearly constant volume ofcontinental crust since 2–3 Ga (Armstrong, 1981; Hawkes-worth et al., 2010).

6. CONCLUSIONS

The concordant bulk-rock Lu–Hf and Sm–Nd ages ofthe Gadwal basalt-andesite-dacite-rhyolite-boninite volca-nic suite are consistent with a mean age of formation at2.70 ± 0.03 Ga. In contrast, the younger �2.53 Ga Pb–Pbage is interpreted to reflect a wide scale crustal reworkingand mineralization event (or events) in the Dharwar craton,which did not disturb the Sm–Nd and Lu–Hf isotope sys-tems. The eHf(2.70Ga) values for the Gadwal metavolcanicrocks range from +1.6 to +8.7, and the eNd(2.70Ga) from0 to +3, suggesting an origin from a long lived depletedmantle reservoir relative to chondrites. The data shows littleevidence for crustal contamination or post magmatic alter-ation of the Sm–Nd and Lu–Hf isotope systems. Therefore,we suggest that the most radiogenic Hf and Nd isotope ra-tios recognized in the boninitic samples represent the mini-mum isotopic composition of the Gadwal depleted mantleat 2.7 Ga.

Comparison of time-integrated 176Lu/177Hf and147Sm/144Nd ratios of juvenile volcanic sources throughoutthe Earth’s history show that the isotopic evolution of thedepleted mantle in the Archean primarily reflects the reduc-tion and loss of heterogeneities produced during early dif-ferentiation events. From �2.0 Ga to the present, time-integrated 176Lu/177Hf and 147Sm/144Nd in the depletedmantle appears to have been nearly constant and similarto the present day average MORB source. Heterogeneitiesin the depleted mantle established prior to 4.4 Ga, basedon 142Nd isotope systematics, are readily recognized inthe Nd and Hf isotope systematics of 3.7–3.8 Ga Isuasupracrustal rocks, but appear also present in the Gadwalboninitic samples at 2.7 Ga, and in samples from otherlocations in the time period 3.0–2.5 Ga. Additional com-bined Hf and Nd isotope studies of juvenile Archean rockswill elucidate the fate of early differentiated reservoirs,which has implications on the mixing time scales of theearly terrestrial mantle and the onset of present day styleplate tectonics.

ACKNOWLEDGEMENTS

We sincerely thank the three anonymous reviewers and theAssociate Editor, Janne Blichert-Toft, for their very detailed com-ments that resulted in a much more focused presentation. Thiswork was carried out at the University of South Carolina underthe BOYSCAST fellowship (SR/BY/A-03/09) awarded to TarunC. Khanna for which the Department of Science and Technology,Government of India, is thankfully acknowledged. ProfessorMrinal K. Sen, Director NGRI, is thankfully acknowledgedfor his permission to publish this work. Drs. D.V. Subba Rao,

22 T.C. Khanna et al. / Geochimica et Cosmochimica Acta 127 (2014) 10–24

C. Manikyamba and M. Ram Mohan are thanked for their encour-agement and supportive discussions in the field. Professor K.Gopalan and Dr. Anil Kumar are thankfully acknowledged fortheir constructive discussion in the initial stages of the manuscript.Many thanks to Igor Puchtel for the modeling discussions. Eliza-beth Bair, Carl Frisby, Courtney Douglas and Mark Wieland arethankfully appreciated for their assistance in the clean chemistrylab at the University of South Carolina. Parts of this work wheresupported by NSF grants OCE-0928280 and OCE-1129280 to Mi-chael Bizimis. Gene M. Yogodzinski acknowledges the NationalScience Foundation grant OCE-0728077.

APPENDIX A. SUPPLEMENTARY DATA

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.gca.2013.11.024.

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Associate editor: Janne Blichert-Toft