Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland

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Geochimica et Cosmochimica Acta 74 (2010) 7236–7260

Highly depleted Hadean mantle reservoirs in the sources ofearly Archean arc-like rocks, Isua supracrustal belt, southern

West Greenland

J. Elis Hoffmann a,b,c,⇑, Carsten Munker a,b,c, Ali Polat d, Stephan Konig a,c,Klaus Mezger b,1, Minik T. Rosing e

a Rheinische Friedrich Wilhelms-Universitat Bonn, Steinmann Institut fur Geologie, Mineralogie und Palaontologie, Abteilung Endogene

Prozesse, Poppelsdorfer Schloss, 53115 Bonn, Germanyb Westfalische Wilhelms-Universitat Munster, Institut fur Mineralogie, Corrensstr. 24, 48149 Munster, Germany

c Universitat zu Koln, Geologisch-Mineralogisches Institut, Zulpicher Strasse 49a, 50674 Koln, Germanyd University of Windsor, Department of Earth and Environmental Sciences, Windsor, ON, Canada N9B 3P4

e Nordic Center for Earth Evolution, Natural History Museum of Denmark, Øster Voldgade 5-7, DK-1350 Copenhagen K., Denmark

Received 9 February 2010; accepted in revised form 17 September 2010; available online 1 October 2010

Abstract

Growing evidence from the accessible geological record reveals that crust–mantle differentiation on Earth started as earlyas 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained 176Lu–176Hf, 147Sm–143Nd, and highfield strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and asso-ciated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initialeHf(3720) values from �0.7 to +1.5 and initial eNd(3720) values from +1.6 to +2.1. Initial eHf(3720) values of the least alteredboninite-like metabasalts span a range from +3.5 to +12.9 and initial eNd(3720) values from �0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO2, Al2O3/TiO2 and other relatively immobile elements, indicating contamina-tion via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model isalso consistent with previously reported initial cOs(3720) values for some of the samples. In addition to the positive eHf(3720)

values, the least disturbed samples exhibit positive eNd(3720) values and a co-variation of eHf(3720) and eNd(3720) values. Basedon these observations, it is argued, that the most depleted samples with initial eHf(3720) values of up to +12.9 and high176Lu/177Hf of �0.05 to �0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stabilityfield. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend.Even the most primitive samples display negative Nb–Ta anomalies and elevated Nb/Ta, indicating a subduction zone settingand overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data forboninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotopedata for Hadean zircons and combined 142Nd–143Nd isotope data for early Archean rocks.� 2010 Elsevier Ltd. All rights reserved.

0016-7037/$ - see front matter � 2010 Elsevier Ltd. All rights reserved.

doi:10.1016/j.gca.2010.09.027

⇑ Corresponding author at: Rheinische Friedrich Wilhelms-Uni-versitat Bonn, Steinmann Institut fur Geologie, Mineralogie undPalaontologie, Abteilung Endogene Prozesse, Poppelsdorfer Sch-loss, 53115 Bonn, Germany. Tel.: +49 0 228 737967; fax: +49 0 228732763.

E-mail address: [email protected] (J.E. Hoffmann).1 Present address: Institute of Geological Sciences, Universitat

Bern, Baltzerstrasse 1-3, 3012, Bern, Switzerland.

1. INTRODUCTION

The Earth’s continental crust formed by distinct mag-matic processes that depleted the mantle through geologicaltime. However, there is an ongoing debate as to when thecrust–mantle system first differentiated and when modernplate tectonic processes started to operate. Zircons have

http://dx.doi.org/10.1016/j.gca.2010.09.027

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Highly depleted Hadean mantle sources of Isua arc-like metabasalts 7237

played a key role in unravelling the early history of Earth’scrust and mantle, as they represent the only unambiguouslyknown Hadean material preserved on Earth (e.g., Wildeet al., 2001; Harrison et al., 2005; Kemp et al., 2010).Isotope compositions of zircons from the Jack Hills andMt. Narryer (Australia) provide the framework for a con-cept that suggests the existence of some form of differenti-ated (or enriched) crust possibly as early as 4.4 Ga (e.g.,Harrison et al., 2005; Scherer et al., 2007; Blichert-Toftand Albarede, 2008; Kemp et al., 2010). In addition, initialPb isotope compositions of early Archean rocks from theItsaq Gneiss Complex (Nutman et al., 1996) of southernWest Greenland point to crust forming events in theHadean (Kamber et al., 2003). Recently, mineral and fluidinclusions in Hadean zircons have provided independentmineralogical evidence for the presence of felsic crust(e.g., Valley et al., 2005).

Due to the paucity of well preserved mafic rocks in theearly Archean record, the extent of Hadean mantle deple-tion is poorly constrained. It is possible that only small vol-umes of crust were generated, without having a significanteffect on the upper mantle composition (e.g., Schereret al., 2001). Broadly coherent depleted mantle evolutiontrends for 143Nd/144Nd and 176Hf/177Hf for post-4.0 Gamafic rocks were for a long time interpreted as indicatingcontinuous crustal growth, not having started until 4.0 Gaago (e.g., Veizer and Jansen, 1979; Taylor and McLennon,1985; Vervoort and Blichert-Toft, 1999 and referencestherein). However, positive 142Nd/144Nd anomalies of ca.+10 to +20 ppm in 3.8 Ga old metasedimentary rocksand metabasalts from the Isua supracrustal belt and meta-tonalites from the Isua region support the existence of a de-pleted mantle reservoir that may have formed as early as4.4–4.5 Ga (e.g., Boyet et al., 2003; Caro et al., 2003,2006; Bennett et al., 2007). However, it is not known if thisdepletion was directly related to the formation of the firstcrust or, alternatively, to magma ocean formation. Likethe 142Nd–143Nd evidence (Bennett et al., 2007), the pres-ence of early mantle heterogeneities and strongly depletedmantle reservoirs was also previously proposed on the basisof elevated 143Nd/144Nd isotope compositions in early Ar-chean rocks (e.g., Hamilton et al., 1978, 1983; Jacobsenand Dymek, 1988; Collerson et al., 1991; Bennett et al.,1993, 2007; Bowring and Housh, 1995; Nutman et al.,2007a). However, it is strongly debated if the 147Sm–143Ndisotope system is sufficiently robust during metamorphismto preserve reliable evidence for strong depletion events(Moorbath et al., 1997; Vervoort et al., 1996; Nutman,2001). Nevertheless, the initially reported radiogenic eNdvalues for samples from Greenland (eNd(3850) up to +4.5;Bennett et al., 1993) were recently confirmed for well pre-served single phase tonalites from the Itsaq Gneiss Complex(Nutman et al., 2007a), which is the world’s most extensivearea of early Archean crust (Nutman et al., 1996).

As for 147Sm–143Nd, it was also argued in early176Lu–176Hf studies on early Archean samples from westernGreenland (Vervoort et al., 1996; Blichert-Toft et al., 1999;Vervoort and Blichert-Toft, 1999; Amelin et al., 2000) thatthese rocks were derived from a long-term depleted uppermantle reservoir, and a possible heterogeneous source

region (Blichert-Toft et al., 1999), leading to a controversialdebate about the results and interpretations (Villa et al.,2001; Albarede et al., 2001). However, these original con-clusions were based on using the ca. 4% higher 176Lu decayconstants of Patchett and Tatsumoto (1980), Tatsumotoet al. (1981) and Sguigna et al. (1982). The revision of the176Lu decay constant (Nir-El and Lavi, 1998; Schereret al., 2001; Soderlund et al., 2004) prompted a significantre-interpretation of these Hf isotope data (e.g., Albaredeet al., 2001; Scherer et al., 2001; Bennett, 2003). Whenapplying the new decay constant, recalculated initial Hf iso-tope compositions of many early Archean rocks from wes-tern Greenland are close to the chondritic value. Thispattern is also confirmed by recent studies on Isua tholeiites(Polat et al., 2003) and tonalite-tronhjemite-granodiorite(TTG) rocks from the Itsaq Gneiss Complex (Hiess et al.,2009), all finding no evidence for the presence of long-termHf-depleted mantle domains. There are, however, notableexceptions, with some samples still displaying non-chon-dritic Hf isotope compositions (Blichert-Toft et al., 1999,recalculated; Boyet et al., 2003).

For this study, we analyzed 176Lu–176Hf and147Sm–143Nd isotope compositions of unusually trace ele-ment depleted boninite-like metabasalts from the Isuasupracrustal belt, southwest Greenland, comprising 14 ofthe most pristine samples found so far. For the same sam-ples we obtained highly precise high-field-strength element(HFSE) ratios by isotope dilution. In previous studies theseboninite-like metabasalts were found to have partly pre-served primary geochemical signatures of their early Arche-an mantle sources (Polat et al., 2002; Frei et al., 2004).However, their primary mineralogy has been changed dur-ing several metamorphic episodes, and now comprisesamphibole, plagioclase, chlorite and quartz as the dominantmineral phases (Polat et al., 2002). Based on a detailed eval-uation of alteration features, Polat et al. (2002) interpretedthese samples as being emplaced in an island arc-like set-ting, similar to present-day settings where boninites form.To discriminate between the effects of alteration and pris-tine magmatic features, the new data are evaluated in com-bination with published datasets including major and traceelements (Polat et al., 2002) and 147Sm–143Nd, 187Re–187Osand Pb isotope data (Frei et al., 2004).

2. GEOLOGICAL SETTING AND PETROGRAPHY

The Eoarchean (3.8–3.7 Ga) Isua Greenstone belt is lo-cated in the Isukasia terrane, southern West Greenland(Friend and Nutman, 2005). The belt is about 30 km longand up to 4 km wide and is embedded into a terrane of quar-tzo-feldspathic orthogneisses (Fig. 1; Bridgwater andMcGregor, 1974; Allaart, 1976; Compston et al., 1986;Nutman, 1986; Nutman et al., 2002, 2007b; Appel et al.,1998; Myers, 2001). The belt contains the oldest knownrocks deposited on the surface of the Earth, comprisingvolcanic rocks, volcaniclastic, clastic and chemical sediments(Fig. 2; Bridgwater and McGregor, 1974; Gill et al., 1981;Nutman et al., 1984; Appel et al., 1998; Fedo, 2000; Myers,2001; Polat and Hofmann, 2003; Friend et al., 2008; Nutmanand Friend, 2009). The belt is dated at 3.70 to >3.80 Ga

Fig. 1. Simplified geologic map of the eastern part of the Isua supracrustal belt (ISB), modified after Appel et al. (1998), Myers (2001) andNutman and Friend (2007b). CD (central tectonic domain), NWD (northwestern tectonic domain), SED (southeastern tectonic domain).Sample locations for the boninite-like metabasalts, for tholeiitic basalts and for the garnet-micaschists are shown as black, white and greycircles, respectively. Sample localities for the compositionally layered metasediments from the western part of the Isua supracrustal belt arenot shown.

7238 J.E. Hoffmann et al. / Geochimica et Cosmochimica Acta 74 (2010) 7236–7260

(Moorbath et al., 1973; Baadsgaard et al., 1986a; Kamberet al., 1998; Nutman et al., 1997, 2002, 2009; Crowley,2003; Frei et al., 2004). Based on recent mapping on the scaleof 1:20,000 and new geochronological data, Nutman andFriend (2009) and Nutman et al. (2009) divided the belt intotwo tectonically juxtaposed Eoarchean terranes: a southernca. 3.80 Ga old terrane and a northern ca. 3.70 Ma oldterrane. In a previous study, Appel et al. (1998) divided thenorth-eastern part of the Isua Greenstone belt in threelitho-tectonic domains: the Northwestern, Central andSoutheastern Domain (Fig. 1). Each domain exhibits differ-ent lithological associations and styles of deformation. TheNorthwestern and Central Domain in the eastern belt liewithin the northern terrane of Nutman and Friend (2009);the Southeastern Domain in the southern terrane.

Multiple phases of deformation and amphibolite faciesmetamorphism have largely obliterated primary structures,

textures and minerals (Nutman et al., 1984, 2002; Myers,2001; Hanmer et al., 2002; Rollinson, 2003), but somelow-strain zones still display a wealth of well-preserved pri-mary volcanic features (Fig. 2; Appel et al., 1998, 2009;Komiya et al., 1999; Myers, 2001; Polat et al., 2003; Friendet al., 2008). Paleoarchean mafic dykes (Ameralik dykes)cut the foliation of the rock units (Nutman et al., 1984,2009). Volcanic rocks within the Isua Greenstone beltmainly comprise deformed mafic pillow lavas (Maruyamaet al., 1992; Appel et al., 1998; Komiya et al., 1999; Myers,2001; Polat et al., 2002, 2003; Nutman and Friend, 2009),with minor debris flows and pillow breccias (Appel et al.,1998). Low-strain domains also preserve intercalations ofultramafic units with pillow basalt flows, whereas originalstratigraphic relationships between pillow flows and ultra-mafic units (serpentinites) have been disrupted elsewhere,and it is not clear whether the ultramafic units were

Fig. 2. Field photographs of the Isua Greenstone belt showing the preservation of primary textures. (A) Boninitic pillow basalts from thecentral tectonic domain (see Appel et al., 1998). (B) Tholeiitic pillow basalts from the central tectonic domain (see Polat and Frei, 2005). (C)Pillow breccia from the central tectonic domain. (D) Amphibolites from the central tectonic domain. (E) Conglomerate from the southeasterntectonic domain. (F) Banded iron formation from central tectonic domain. The coin used for scale is ca. 1.5 cm in diameter. Photos A, C, D,E, F were taken by P.W.U. Appel.


originally emplaced as extrusive flows or intrusive sills(Myers, 2001).

Metasedimentary assemblages are rare in the Isua Green-stone belt, and different depositional types have been found(e.g., Nutman et al., 1984; Rosing et al., 1996; Bolhar et al.,2005), including quartzite (Nutman et al., 1997), garnet-micaschists interpreted as metapelites (e.g., Kamber et al.,1998; Fedo, 2000), banded iron formations (e.g., Appelet al., 1999), and graphite-bearing, sandstone-dominatedlayered metasediments interpreted as a turbiditic stack(e.g., Nutman et al., 1984; Rosing, 1999). In some localitiescarbonate alteration has completely obliterated all primarystructures, and severely modified bulk rock compositions(Rose et al., 1996; Polat and Hofmann, 2003).

The geochemical characteristics of the least altered volca-nic rocks in both the southern and northern terranes are sim-ilar to those of Cenozoic subduction related rocks (Gill et al.,1981; Polat et al., 2002; Polat and Hofmann, 2003; Jenneret al., 2009, and references therein). A sheeted dyke complexwith supra-subduction zone geochemical characteristics hasbeen reported from the northern terrane in the western partof the Isua belt, interpreted in terms of sea-floor spreadingprocesses (Furnes et al., 2007). However, the recognition ofthese lithologies has been questioned by Nutman and Friend

(2007) and Hamilton (2007). Volcanic rocks in the ca.3.70 Ga old northern terrane have boninite-like (Polatet al., 2002) and arc tholeiitic to picritic compositions (Polatand Hofmann, 2003; Furnes et al., 2009; Appel et al., 2009).The boninite-like metabasalt samples used in this study werecollected from the northern terrane in the eastern Isua Green-stone belt (Central domain of Appel et al., 1998; formerlyalso known as part of the “Garbenschiefer unit”). In severallocalities, volcanic rocks within the Central Domain still ex-hibit primary magmatic features (Fig. 2). However, on themicroscopic scale igneous textures and minerals are not pre-served, and major mineral phases present include amphibole(hornblende, anthophyllite), plagioclase, chlorite, epidote,quartz, calcite, and titanite (Fig. 3A–F, see also Polat et al.,2002). Based on their petrography the boninite samples canbe classified as belonging to the least altered (Fig. 3A–C)and variably altered groups (Fig. 3D–F). Samples classifiedas variably altered contain more calcite, epidote and quartzthan the least altered counterparts (Fig. 3D–F).

In addition to the boninite-like metabasalts, typicalmetasedimentary rocks from a compositionally layeredsediment unit from the western part of the Isua supracrustalbelt (locality of Rosing, 1999) and from the B2 micas-chist unit (Nutman et al., 1984) were analyzed. Detailed

Fig. 3. Photomicrographs of the least altered (A–D) and variably altered (E and F) volcanic rocks from the central tectonic domain. amp:amphibole, chl: chlorite, plg: plagioclase, epi: epidote, and cal: calcite.


mineralogical descriptions of these metasedimentary unitsare given in Nutman et al. (1984), Rosing (1999) and Polatand Frei (2005).

A minimum age for the boninite-like metabasalts is con-strained by a 3.71–3.72 Ga crosscutting tonalite sheet fromthe western part of the Isua Greenstone belt (Nutman andFriend, 2009). The age of the compositionally layeredmetasedimentary rocks was estimated to be equal to the3.72 Ga age of the boninite-like metabasalts since they areinterlayered with these sediments in the western part ofthe belt (Rosing, 1999). The garnet-micaschists yielded ananalytically indistinguishable whole rock Sm–Nd age ofca. 3742 ±49 Ma (MSWD 13.7; Kamber et al., 1998). Forcalculations of initial isotope ratios an age of 3.72 Ga hasbeen used for all rock units in the present study.

3. PREVIOUS GEOCHEMICAL CONSTRAINTS ON

THE ISUA BONINITE-LIKE METABASALTS

Gill et al. (1981), Hamilton et al. (1983), Gruau et al.(1996), Nutman et al. (1996, 1997) and Furnes et al.(2009) studied the boninite-like metabasalts from both the

western and eastern Isua Greenstone belt (“Garbenschieferamphibolites”). Detailed geochemical characterisations ofboninite-like metabasalts and meta-tholeiites from the east-ern belt were carried out by Polat et al. (2002, 2003), Polatand Hofmann (2003) and Frei et al. (2004). A subset ofboninite-like samples that were previously analyzed byPolat et al. (2002) for major and trace elements and by Freiet al. (2004) for Nd–Pb–Os isotope compositions was se-lected for this study. Polat et al. (2002) confirmed the pris-tine character of many samples by systematic correlationsof selected major and trace elements with immobile ele-ments such as Zr and by a comparison of incompatibletrace element patterns in primitive-mantle normalized traceelement diagrams (see also Figs. 4 and 5). Based on theseobservations, Polat et al. (2002) identified a subset of “leastaltered” samples that were preferentially selected for analy-sis in the present study. Between these “least altered” sam-ples, primary fractionation trends are preserved for the leastimmobile elements (REE, Th, Ti, and Nb–Ta, Zr–Hf).Hence, the contents of these elements may mirror primarycharacteristics of the early Archean mantle sources. Someselected co-variation diagrams that display primary igneous

Fig. 4. SiO2 and MgO co-variation diagrams for immobile major and trace element data for the boninite-like metabasalts, exhibiting pristinemagmatic fractionation trends. Data for “least-altered” boninite-like metabasalts include data from Polat et al. (2002) and Furnes et al.(2009).

Fig. 5. Primitive mantle normalized trace element patterns for the boninite-like metabasalts (this study, Polat et al., 2002; Frei et al., 2004)and for tholeiitic metabasalts from the CD (Polat et al., 2003). Values for primitive mantle are taken from Hofmann (1988).


fractionation trends are shown in Fig. 4 (see also Polatet al., 2002; Furnes et al., 2009).

The subset of samples used for our study is from Polatet al. (2002) and characterized by high MgO contents (6.8–16.1 wt.%) and variable Al2O3 contents (14.9–19.1 wt.%).Abundances in SiO2 are also variable (46–54 wt.%) andTiO2 contents are generally low (0.2–0.4 wt.%; Polat et al.,

2002). Representative trace element and REE patterns com-bining data from Polat et al. (2002), Frei et al. (2004) andhigh-precision HFSE data from this study are shown inFig. 5 where they are compared to patterns obtained for coresof tholeiitic pillow basalts that are also found in the CentralDomain (Polat et al., 2003). As previously noted by Polatet al. (2002), light REE (LREE) abundances are overall


depleted, and display a much larger variability than heavyREE (HREE) abundances. These features were regardedby Polat et al. (2002) as largely reflecting LREE mobility dur-ing secondary alteration or variable overprint by subductionfluids in the source region. As an alternative model for theLREE variability, Nutman et al. (1996) suggested contami-nation by crustal components. All samples are characterizedby negative Nb and Ta anomalies, which were interpreted byPolat et al. (2002) as a pristine geochemical feature mostlikely indicating a subduction zone setting. Positive Zr andTi anomalies and high MgO contents were taken as evidencethat the samples from the Isua supracrustal belt resembleanalogues of Phanerozoic boninites (Polat et al., 2002). TheeNd(3720) values reported for the least altered boninite-likemetabasalts by Frei et al. (2004), including samples analyzedin this study, are +0.5 to +3.6, tentatively suggesting a de-pleted source region. However, samples with disturbed Sm–Nd isotope systematics were also found (eNd(3720) between�49 and +33; Frei et al., 2004; recalculated to 3720 Ma).The influence of enriched components on the budget of sometrace elements was inferred from cOs(3720) values that show aspread from +3.4 and +5.0 (Frei et al., 2004; recalculated to3720 Ma) and Pb–Pb leaching isotope data (206Pb/204Pb11.8–20.2; 207Pb/204Pb 13.4–16.2; Frei et al., 2004). Someclear outliers in the initial Nd and Os isotope compositionsappear to indicate disturbance of both isotope systems insome samples (Frei et al., 2004). Hence, the original criteriaused by Polat et al. (2002) to discriminate between “variablyaltered” and “least altered” samples need to be refined.

4. ANALYTICAL METHODS

All samples were powdered in an agate mill. Two differ-ent techniques for sample digestion were employed: highpressure acid digestion in Parr� bombs and flux fusionusing Li2B4O7 and Li2B2O4. The abundances of Lu, Hf,Sm, Nd, Nb, Ta, Hf and Zr were determined by isotopedilution employing mixed 149Sm–150Nd and 180Ta–180H-f–176Lu–94Zr tracers (e.g., Weyer et al., 2002). Using theParr� bomb technique, Hf–Nd isotope and Lu–Hf–Sm–Nd–Zr–Nb–Ta concentration measurements were carriedout on the same sample split (�100–200 mg) following theanalytical technique for combined HFSE and Lu measure-ments described in Munker et al. (2001) and Weyer et al.(2002). All samples were spiked using the mixed HFSE tra-cer and digested in a 3:2:1 HF–HNO3–HClO4 acid mixturein Savillex� vials placed inside Parr� bombs at 180 �C for4–5 days. During the course of our study it was found thatbombing for less than four days failed in some cases toachieve full sample-spike equilibration with respect toLu–Hf. These results are not reported in the tables. Follow-ing digestion, the samples were evaporated to dryness andthe residues were treated once with concentrated HNO3

and a trace of HF (<0.05N), followed by an evaporationstep. Samples were then equilibrated in 6N HCl–0.06 HFand dried down again. To verify complete digestion of thesamples, which is especially required to obtain sufficientlyaccurate initial eHf(3720) values, the same whole-rock pow-ders used for bomb digestion were fused with both Li2B4O7

and LiB2O4 as flux agents (one part whole-rock powder to

four parts Li2B4O7 or LiB2O4; see Table 1 for method usedfor individual samples) in graphite crucibles at 1150 �C(Lagos et al., 2007; slightly modified after the procedure de-scribed in Bizzarro et al., 2003a). Subsequently, the sampleswere quenched and dissolved in 3N HCl, spiked with180Ta–180Hf–176Lu–94Zr tracers, and equilibrated in closedSavillex� vials for 24 h. The solutions were then dried andthe residues were treated four times with concentratedHNO3 and a trace of HF (<0.05N) and subsequently evap-orated to dryness. These additional steps ensured full sam-ple-spike equilibration for all HFSE. Afterwards, thesamples were equilibrated in 6 N HCl and a trace HF to en-sure sample equilibration.

Following a three stage ion exchange column chemistry(Munker et al., 2001), Hf isotope compositions were mea-sured using the Isoprobe MC-ICP-MS at Munster andthe Neptune MC-ICP-MS in Bonn; all data were correctedto a 179Hf/177Hf of 0.7235 using the exponential law. Alldata are given relative to a 176Hf/177Hf of 0.282160 forthe Munster AMES and JMC 475 standards at a typicalexternal reproducibility of ±50 ppm. To estimate the exter-nal reproducibility for samples with low Hf abundances, anempirical relationship was used where the 2r externalreproducibility equals 4� the 1r internal standard error(Bizzarro et al., 2003b). For Lu measurements, the mea-sured and interference corrected 176Lu/175Lu was normal-ized to the measured 173Yb/171Yb. For interferencecorrection of 176Yb, the measured 176Yb/171Yb of pureYb was used that was measured in the same run(Blichert-Toft et al., 2002; Vervoort et al., 2004; Lagoset al., 2007). The external reproducibilities reported inTable 2 include effects of error magnification due to non-ideal spike-sample ratios as well as uncertainties impartedby corrections for interferences with Yb. The calculated ini-tial Hf isotope compositions include both the propagatedexternal errors from the measured Hf isotope compositionsand Lu/Hf ratios. Due to the unusually high 176Lu/177Hfratios of some samples the error propagation on the calcu-lated initial isotope ratios is substantial, despite an externalLu/Hf reproducibility of ±0.2% for ideally spiked samples.

Samarium and Nd were retrieved from the separated sam-ple matrix, employing conventional cation and HDEHP ionexchange techniques. Samarium and Nd isotope composi-tions were determined using the Finnigan Triton TIMS(Nd) and the VG Sector 54 (Sm) at Munster and the NeptuneMC-ICP-MS at Bonn (Sm and Nd). Mass fractionation cor-rections were carried out using a 146Nd/144Nd of 0.7219 andthe exponential law. During the TIMS measurements,143Nd/144Nd values of 0.511840 ± 14 (2r, n = 4) were ob-tained for the La Jolla standard. During MC-ICPMS mea-surements 143Nd/144Nd from 0.511759 to 0.511786 wereobtained for La Jolla, and all data are given relative to a143Nd/144Nd of 0.511859. The typical external reproducibil-ity for 143Nd/144Nd measurements was ±30 ppm for TIMSand ±40 ppm for MC-ICP-MS measurements. The externalreproducibility for 147Sm/144Nd of TIMS and MC-ICP-MSmeasurements is ±0.2%. External reproducibilities are ca.±0.6% for Zr/Hf, ±4% for Nb/Ta and ±4% for Zr/Nb (all2r r.s.d.). HFSE data for BIR-1 obtained during the courseof our measurements at Bonn are listed in Tables 1 and 2,

Table 1Trace element data determined by isotope dilution for Isua boninite-like metabasalts and Isua metasediments.

Sample Digestiontechnique

Lu [ppm] Hf [ppm] Zr [ppm] Nb [ppm] Ta [ppm] Sm[ppm]

Nd[ppm]

Lu/Hf Zr/Hf Zr/Nb Nb/Ta

Least altered boninite-like metabasalts

462901 B 0.2335 0.4115 11.75 0.246 0.0179 0.40$ 1.06$ 0.5674 28.55 47.76 13.8FFT 0.2293 0.4078 11.50 0.5623 28.20FFT 0.2292 0.4094 0.5706

462902 B 0.2979 0.4522 12.88 0.355 0.0209 0.401 1.15 0.6589 28.49 36.3 17.0B 0.2680 0.4327 0.6194FFT 0.2560 0.4214 0.6075FFT 0.2682 0.4414 0.6076

462903 B 0.2533 0.7126 21.27 0.554 0.0436 0.60$ 1.88$ 0.3555 29.85 38.4 12.7FFT 0.2328 0.6676 20.43 0.3487 30.60FFT 0.2411 0.6891 0.3498

462904 B 0.2385 0.4260 12.84 0.268 0.0199 0.575 1.70 0.5599 30.14 47.9 13.5BFFT 0.2309 0.4245 0.5439

462905 B 0.2188 0.3753 10.44 0.230 0.0133 0.357 0.944 0.5831 27.81 45.3 17.4B 0.2186 0.3778 10.50 0.213 0.0126 0.5788 27.80 49.3 17.0FFT 0.2112 0.3604 10.44 0.5860 28.96

462944 B 0.2348 0.6216 18.43 0.572 1.73 0.3778 29.65FFT 0.2037 0.5720 17.88 0.3561 31.26

462945 B 0.2442 0.6133 17.65 0.489 0.0360 0.503 1.43 0.3981 28.78 36.1 13.6BFFT 0.2382 0.6029 18.14 0.3951 30.09

462946 B 0.2670 0.6726 0.550 0.0354 0.613 1.75 0.3970 35.2 15.5FFT 0.2521 0.6644 17.69 0.3794

462948 B 0.2598 0.7065 21.05 0.578 0.0406 0.588 1.71 0.3678 29.80 36.4 14.2B 0.2651 0.6931 21.00 0.3825 30.29FFT 0.2418 0.6619 19.98 0.3653 30.19FFT 0.2462 0.6897 0.3570

462949 B 0.2648 0.6997 20.55 0.543 0.0416 0.579 1.68 0.3784 29.37 37.9 13.0FFT 0.2456 0.6885 0.3568

462965 B 0.2251 0.3803 10.59 0.197 0.0142 0.290 0.731 0.5918 27.84 53.9 13.8B 0.2312 0.3784 10.53 0.300 0.757 0.6110 27.84B 0.2089 0.3563 0.5863FFT 0.1917 0.3237 9.329 0.5923 28.82FFT 0.2198 0.3737 0.5881

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Table 1 (continued)

Sample Digestiontechnique

Lu [ppm] Hf [ppm] Zr [ppm] Nb [ppm] Ta [ppm] Sm[ppm]

Nd[ppm]

Lu/Hf Zr/Hf Zr/Nb Nb/Ta

Least altered Isua boninite-like metabasalts after Polat et al., 2002; indicated as variably altered in this study

462906 B 0.2189 0.3568 9.162 0.156 0.330 0.984 0.6136 25.68 58.7FFT 0.1943 0.3476 0.5589

462966 B 0.2172 0.3812 10.81 0.249 0.0164 0.32$ 0.82$ 0.5698 28.37 43.3 15.3FFT 0.2075 0.3647 0.5689FFT 0.2018 0.3779 0.5339FFM 0.1993 0.3486 9.909 0.5717 28.42

462968 B 0.3393 0.5041 15.58 0.359 0.0210 0.483 1.37 0.6731 30.90 43.4 17.1B 0.3161 0.4925 14.24 0.339 0.0221 0.6418 28.91 42.0 15.3FFT 0.3029 0.5027 0.6025FFM 0.2940 0.4375 12.79 0.6721 29.24

Metasediments from the type locality of Rosing (1999) interlayered with boninite-like metabasalts from the Western Isua supracrustal belt

465438 B 0.4274 6.498 201.2 5.80 0.511 6.34 31.51 0.0658 30.97 34.7 11.4465439 B 0.1700 3.952 131.3 3.82 0.297 3.07 15.02 0.0430 33.23 34.4 12.9465440 B 0.5041 8.042 266.9 8.31 0.614 6.41 31.80 0.0627 33.18 32.1 13.5465441 B 0.1947 3.488 118.5 3.59 0.278 2.72 12.93 0.0558 33.99 33.0 12.9465442 B 0.4039 5.968 194.0 6.70 0.444 5.06 25.05 0.0677 32.51 29.0 15.1465443 B 0.2101 3.520 119.4 3.84 0.268 2.35 11.57 0.0597 33.91 31.1 14.3

Garnet-micaschists (B2 unit; Nutman et al., 1984; Polat and Frei, 2005)

462912 B 0.2981 1.738 58.41 2.08 0.130 0.1716 33.61 28.1 16.0B 0.2983 1.714 2.49 7.825 0.1740

462919 B 0.4141 5.651 207.3 6.73 0.482 0.07328 36.68 30.8 14.0B 0.3677 5.657 5.29 27.55 0.06500FFM 0.3524 5.689 207.1 0.06194 36.40

Reference material

BIR-1 B 0.2461 0.5808 13.9 0.558 0.0360 0.4236 23.93 24.9 15.5BIR-1 (n = 6) (Munkeret al., 2004)

0.244 ± 1.0% 0.581 ± 1.5% 14.0 ± 1.3% 0.549 ± 3.6% 0.0350 ± 3% n.a. n.a. 0.4200 24.1 ± 0.7% 25.2 ± 2.2% 15.8 ± 2.8%

Digestion techniques: B = 4–5 days in Parr� bomb, FFT = flux fusion Li-tetraborate, FFM = flux fusion Li-metaborate.$ Sm–Nd data from Frei et al. (2004).

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Table 2176Lu–176Hf and 147Sm–143Nd data for Isua boninite-like metabasalts and Isua metasedimentary rocks.

Sample Digestion technique 176Lu/177Hf (±2r) 176Hf/177Hf (±2r) eYf(3720) (±2r) 147Sm/144Nda 143Nd/144Nd (±2r) eNd(3720)

Least altered boninite-like metabasalts

462901 B 0.225$ 0.513386$ ±6 +1.0 ± 0.3$

FFT 0.07986 ±19 0.286205 ±4 +3.3 ±0.7FFT 0.07950 ±18 0.286203 ±16 +4.2 ±1.2w.avg. +3.5 ±0.6

462902 B 0.2118 0.513135 ±13 +2.5 ± 0.5FFT 0.08630 ±20 0.286957 ±5 +13.6 ±0.7FFT 0.08631 ±19 0.286894 ±14 +11.3 ±1.1w.avg. +12.9 ±0.6

462903 B 0.194$ 0.512683$ ±16 +2.2 ±0.4$

FFT 0.04951 ±11 0.284149 ±4 +7.8 ±0.6FFT 0.04967 ±11 0.284183 ±3 +8.6 ±0.6w.avg. +8.2 ±0.4

462904 B 0.2041 0.512854 ±40 +0.7 ±0.8FFT 0.07725 ±18 0.286097 ±14 +6.2 ±1.1

462905 B 0.08283 ±20 0.286542 ±7 +7.7 ±0.7 0.2282 0.513573 ±14 +3.2 ±0.5B 0.08234 ±19 0.286508 ±4 +7.8 ±0.7FFT 0.08314 ±19 0.286609 ±5 +9.3 ±0.7

462944 FFT 0.05056 ±11 0.284192 ±3 +6.7 ±0.6 0.1923 0.512510 ±8 -0.3 ±0.4462945 B 0.2125 0.513120 ±28 +1.9 ±0.6

FFT 0.05611 ±13 0.284582 ±5 +6.4 ±0.6462946 B 0.2119 0.513073 ±42 +1.2 ±1.0

FFT 0.05387 ±12 0.284425 ±10 +6.5 ±0.8462948 B 0.2079 0.512912 ±43 0± 0.9

FFT 0.05182 ±12 0.284216 ±5 +4.3 ±0.6FFT 0.05068 ±11 0.284147 ±3 +4.8 ±0.6w.avg. +4.6 ±0.4

462949 B 0.2083 0.512986 ±8 +1.2 ±0.4FFT 0.05065 ±11 0.284213 ±5 +7.2 ±0.6

462965 B 0.08405 ±21 0.286698 ±3 +10.2 ±0.7 0.2399 0.513819 ±10 +2.3±0.5B 0.08330 ±21 0.286630 ±8 +9.7 ±0.8 0.2397 0.513786 ±19 +1.8 ±0.5FFT 0.08403 ±19 0.286680 ±8 +9.6 ±0.8FFT 0.08355 ±20 0.286678 ±17 +10.7 ±1.3w.avg. +9.9 ±0.7 +2.1 ±0.4

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Table 2 (continued)

Sample Digestion technique 176Lu/177Hf (±2r) 176Hf/177Hf (±2r) eYf(3720) (±2r) 14 m/144Nda 143Nd/144Nd (±2r) eNd(3720)

Least altered Isua boninite-like metabasalts after Polat et al., 2002; indicated as variably altered in this study

462906 B 0 026 0.512952 ±14 +3.4 ±0.4FFT 0.07938 ±18 0.286255 ±3 +6.3 ±0.7

462966 B 0 35$ 0.513078$ ±6 -9.9 ±0.4$

FFT 0.08080 ±19 0.286231 ±4 +1.8 ±0.7FFT 0.07582 ±18 0.285931 ±17 +3.9 ±1.3w.avg. +2.3 ±0.6

462968 B 0 129 0.513014 ±41 -0.4 ±0.8FFT 0.08557 ±20 0.286549 ±3 +0.9 ±0.7

Metasediments from the type locality of Rosing (1999) interlayered with boninite-like metabasalts from the Western Isua supracrustal belt

465438 B 0.009333 ±3 0.281031 ±6 -0.3 ±0.5 0 216 0.510869 ±12 +1.6 ±0.5465439 B 0.006104 ±3 0.280850 ±6 +1.5 ±0.5 0 235 0.510940 ±7 +2.1 ±0.3465440 B 0.008894 ±3 0.280989 ±7 -0.7 ±0.5 0 218 0.510883 ±7 +1.8 ±0.3465441 B 0.007920 ±3 0.280940 ±5 +0.1 ±0.5 0 271 0.511025 ±8 +2.0 ±0.3465442 B 0.009604 ±3 0.281068 ±5 +0.3 ±0.5 0 220 0.510887 ±11 +1.7 ±0.4465443 B 0.008468 ±3 0.281001 ±5 +0.8 ±0.5 0 229 0.510928 ±11 +2.1 ±0.4

Garnet-micaschists (B2 unit; Nutman et al., 1984; Polat and Frei, 2005)

462912 B 0.02435 ±6 0.282152 ±3 +1.2 ±0.5B 0.02471 ±6 0.282174 ±7 +1.0 ±0.5 0 923 0.512510 ±8 -0.4 ±0.4w.avg. +1.1 ±0.4

462919 B 0.009223 ±3 0.281049 ±5 +0.6 ±0.5 0 160 0.510761 ±9 +2.2 ±0.4FFM 0.008720 ±2 0.281010 ±5 +0.5 ±0.7w.avg. +0.6 ±0.4

Reference material

eHfBIR-1 B 0.06014 ±1 0.283293 ±4 +18.4 ±0.5BIR-1 (n = 6) (Munker et al., 2004) 0.0600 ± 1%

CHUR values: 176Lu/177Hf = 0.03360, 176Hf/177Hf = 0.282785 (Bouvier et al., 2008). 147Sm/144Nd and 143Nd/144Nd data for samples not m asured in this study are from Frei et al. (2004). eNdwere calculated using the CHUR values 147Sm/144Nd = 0.1967, 143Nd/144Nd = 0.512638 and a decay constant for 147Sm of 6.54 � 10�12 a� Minimum ages for the recalculation of initial isotopevalues for the boninite-like metabasalts and for them metasedimentary units are based on U–Pb zircon ages of crosscutting tonalites (Nu an and Friend, 2009).w.avg. = 2r weighted average of fluxed samples.

$ Sm–Nd isotope data from Frei et al. (2004) are recalculated to 3720 Ma. GPS coordinates for sample localities are found in the Appe dix.a External error on the ratio is 0.2%.

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together with a long term average published in Munker et al.(2004).

Typical blank contributions from the Li2B4O7 flux diges-tion were 160–180 pg for Hf, 12–24 pg for Lu, and 66–85 ngfor Zr. Typical blank contributions from LiB2O4 flux diges-tions were ca. 80 pg for Hf and ca. 40 pg for Lu. Typicalblank contributions from the Parr� bomb digestion were40–50 pg for Hf, 10–30 pg for Lu, <25 pg for Sm and<30 pg for Nd. Other HFSE blanks were 0.7–3.2 ng forNb, 160 pg for Ta, and 1.5–2.6 ng for Zr.

5. RESULTS

5.1. Lu–Hf and Sm–Nd systematics

Fourteen boninite-like metabasalts were analyzed for Hfand Nd isotopes and their HFSE/REE concentrations. Allthese samples were characterized by Polat et al. (2002) asbeing the least altered of the available samples. In addition,six Isua metasediments collected at the type locality ofRosing (1999) and two garnet-micaschists of Polat and Frei(2005) collected from the B2 micaschist unit of Nutmanet al. (1984) were analyzed. Results are given in Tables 1and 2. The flux fusion results are used for interpretationof Lu–Hf results of the boninite-like metabasalt samples re-ported here.

The measured Lu/Hf of the least altered boninite-likemetabasalt samples range from 0.35 to 0.67. Such Lu/Hf ra-tios are all much higher than found in most Phanerozoicbasalts and in many tholeiitic metabasalts from Isua. Thedistribution of the samples is bimodal (Fig. 6A): one grouphas Lu/Hf ranging from 0.36 to 0.40 and the other grouphas Lu/Hf ranging from 0.54 to 0.67. Except for one sample(462903), all other samples with low Lu/Hf occur in thesouth-western segment of the CD whereas the samples withhigh Lu/Hf originate from the north-eastern part of theCD. Notably Al2O3/TiO2 increase with Lu/Hf (Fig. 6A).Three samples belonging to the high Lu/Hf group are iden-tified in this study as altered and therefore not plotted inFig. 6A. In contrast to 176Lu/177Hf, the initial eHf valuesoverlap between both groups, ranging from +0.9 to+12.9, using the recently updated 176Lu/177Hf and176Hf/177Hf CHUR parameters of 0.3360 and 0.282785,respectively (Bouvier et al., 2008). Where replicates areavailable, the weighted mean of the initial values deter-mined for fluxed samples are used.

The 147Sm/144Nd ratios determined for the metabasaltsin this study (0.192–0.240, Table 2) are all higher than theprimitive mantle value (0.1967, Table 2), except for onesample (462944), and are within the range of previouslypublished 147Sm/144Nd for these samples (0.19–0.31, Freiet al., 2004). The measured 147Sm/144Nd can differ by sev-eral percent between the Frei et al. (2004) study and thisstudy. However, these differences are not systematic and,most importantly, the initial eNd(3720) of most samples re-ported by Frei et al. (2004) overlap within analytical uncer-tainty with values reported here. Hence, these differencesbetween the two studies can be explained by sample heter-ogeneity. The small scale sample heterogeneities in the dif-ferent powder aliquots measured might be caused by the

presence of accessory allanite that has been found in smallveins and has formed during early/late Archean metaso-matic events (Frei et al., 2002, 2004; Frei and Kastbjerg-Jernsen, 2003). The initial eNd(3720) values obtained fromour measurements range from �0.4 to +3.4.

The 176Lu/177Hf ratios of the compositionally layeredmetasedimentary samples ranges from 0.00610 to 0.00960,and yield initial eHf(3720) of �0.7 to +1.5 (Table 2). Thisrange is consistent with values determined for two samplescollected at the same outcrop by Blichert-Toft et al. (1999),that yielded initial eHf(3720) of �0.7 and +1.5. The147Sm/144Nd ratio of the compositionally layered metasedi-mentary rock samples range from 0.1216 to 0.1271 witheNd(3720) ranging from +1.6 to +2.1, also consistent withthe samples analyzed by Blichert-Toft et al. (1999;eNd(3720) = +1.7 and +2.2). The two garnet-micaschist sam-ples yielded 176Lu/177Hf of 0.02435 and 0.09223, with initialeHf(3720) of +0.6 and +1.1, respectively. The 147Sm/144Nd ra-tio of 0.1923 and 0.1160 are in the same range as reported byprevious studies (Hamilton et al., 1983; Moorbath et al.,1997; Kamber et al., 1998). The samples yield initialeNd(3720) values of �0.4 and +2.2, respectively.

5.2. High-field strength element (Zr, Hf, Nb, Ta) systematics

Concentrations of HFSE obtained by isotope dilutionare listed in Table 1. The boninite-like metabasalts analyzedin this study are depleted in Zr and Hf relative to Sm andNd and display overall sub-chondritic and variable Zr/Hfranging from 25.7 to 31.3, in accord with the depletedSm/Nd and Lu/Hf ratios. The measured Nb/Ta ratios forthe boninite-like rocks are all sub-chondritic and rangefrom 12.7 to 17.4, covering the entire known range of pres-ent-day mafic volcanic rocks (Munker et al., 2003, 2004;Pfander et al., 2007 and references therein). Notably, forthe boninite-like metabasalts, Zr/Nb ratios decrease withincreasing SiO2 (Fig. 6B). There are no coupled variationsbetween Nb/Ta and Zr/Hf as expected from experimentalevidence and trends defined by Phanerozoic rocks (McDadeet al., 2003; Munker et al., 2003).

6. DISCUSSION

6.1. Metamorphic disturbance of major and trace element

systematics

Rocks of the Isua supracrustal belt were affected by mul-tiple metamorphic episodes between ca. 3.8 and 1.8 Ga,including seafloor hydrothermal alteration, regional meta-morphism, carbonatization and K-metasomatism, allpotentially modifying primary features (e.g., Baadsgaardet al., 1986b; Gruau et al., 1996; Rosing et al., 1996;Nutman et al., 1997; Frei et al., 2002, 2004). However, asPolat et al. (2002) showed, for many major and trace ele-ments primary fractionation trends are still preserved inthe sample suite studied here (see also Fig. 4). Notably,there are systematic co-variations of Lu/Hf vs. Al2O3/TiO2 (Fig. 6A), indicating the preservation of pristine mag-matic Lu/Hf in the boninite-like metabasalts. As indicatedby the preservation of primary fractionation trends, the

Fig. 6. Major and trace element co-variation diagrams of the least altered boninite-like metabasalts evaluating the effects of contamination byan enriched component. Shallow level contamination of primitive melts with ocean floor sediments appears to be the most plausiblemechanism. (A) Lu/Hfflux fusion vs. Al2O3/TiO2, (B) Zr/Nb vs. SiO2, (C) 176Lu/177Hf vs. 147Sm/144Nd, (D) Gd/Yb vs. Zr/Hf, (E) La/Yb vs. Nb/Th, (F) Nb/Nb* vs. Th/Yb, (G) Th/Yb vs. Nb/Yb. (MORB-OIB field is taken from Pearce, 2008), (H) Nb/Ta vs. Zr/Hf. Data for Isuametasediments are taken from Bolhar et al. (2005) and Polat and Frei (2005); data for TTGs are taken from Nutman et al. (1999, 2007). Majorand trace element data are from Polat et al. (2002) and this study. The field for recent volcanic rocks in (C) is taken from Albarede et al. (2000)extended with data for Phanerozoic boninites from Cyprus (Konig et al., 2008), the fields for a variety of terrestrial rocks in (H) are fromMunker et al. (2003); CC, continental crust; OIB, ocean island basalts; MORB, mid ocean ridge basalt; CB, continental basalts. Blackdiamonds are the revised group of least altered boninite-like metabasalts, light grey squares are compositionally layered metasedimentaryrocks and dark grey squares are garnet-micaschists.


elements Mg, Ni, Cr, Si, Al, Ti, Zr–Hf, Nb–Ta and Th canbe regarded as relatively immobile during metamorphism.

However, some of the samples characterized as “least alter-ed” by Polat et al. (2002) show evidence for SiO2 loss or


enrichment during later silicification. Correlations betweenvariable LREE abundances with differentiation parameterslike Zr were taken by Polat et al. (2002) as an argumentagainst REE mobility. Rather, these authors interpretedthe variable LREE as a primary feature, most likely relatedto variable subduction zone enrichment. Alternatively,Nutman et al. (1996) and Polat et al. (2002) proposed thatthe LREE variations could also reflect variable amounts ofcrustal contamination, which was also suggested on the ba-sis of Nd isotopes (Hamilton et al., 1983). The differentmodels are evaluated below on the basis of new Lu–Hfand Sm–Nd data.

6.2. Metamorphic disturbance of radiogenic isotope systems

Based on a comparison with modern mafic rocks, previ-ous studies argued for a decoupling of the 147Sm–143Nd and176Lu–177Hf isotope systems between Isua supracrustal beltsamples and the boninite-like metabasalts (Fig. 6C;Blichert-Toft et al., 1999; Albarede et al., 2000). Becauseof low Hf abundances and the likely absence of zircon asa host for Hf, the high 176Lu/177Hf have been originallyinterpreted as a result of Hf mobility during later metamor-phic events, whereas the Sm–Nd system appears to havebeen disturbed to a lesser extent (e.g., Blichert-Toft et al.,1999; Boyet et al., 2003). However, by applying the revised176Lu decay constant of Scherer et al. (2001), this picturehas changed. The initial eNd values of these samples aremostly positive ranging from ca. 0 to +5, and most of theinitial eHf values are near chondritic. However, some sam-ples analyzed by Blichert-Toft et al. (1999) and Boyet et al.(2003) still exhibit pronounced deviations in their initial eHffrom the chondritic value. In a combined Hf–Nd isotopestudy on tholeiitic pillow basalts sampled from outcrops

Fig. 7. Hafnium–Nd isotope diagram for boninite-like metabasalts, illuseHf(3720) vs. eNd(3720). Within this group of undisturbed samples, the sprethe array defined by the boninite-like metabasalts from Isua yields an eH1999). For samples with initial eNd(3720) that overlap with results in Frmetasediments samples show a decoupling of eHf and eNd(t). Thescontaminant for the boninite-like metabasalts.

in the central portion of the CD, close to the sample local-ities of this study (see Fig. 1), Polat et al. (2003) showed thatthe Lu–Hf system in some samples was more robust duringlater tectonothermal events than the Sm–Nd system.

Disturbances of the 147Sm–143Nd and 187Re/187Os iso-tope systems in some samples used for the present studyhave been reported by Frei et al. (2004). However, as theynoted there is good agreement of early Archean Sm–Nd(3792 ± 60 Ma; Frei et al., 2004) and Re–Os isochron ages(3762 ± 90 Ma, Frei et al., 2004) obtained for the “least al-tered” group of boninite-like metabasalts with previouslyproposed emplacement age of 3.72 Ga based on zirconU–Pb geochronology (Nutman et al., 1997; Nutman andFriend, 2009). These overlapping ages indicate only minordisturbance on the whole-rock scale that was more or lesscoeval with emplacement, of the metabasalts. Hydrother-mal overprint immediately after eruption of the metabasaltsor during tonalite emplacement are both viable mechanismsthat may explain these patterns (e.g., Frei et al., 2002). Forthe Pb–Pb isotope system, however, Frei et al. (2004) arguefor a disturbance by a significantly younger tectono-meta-morphic event at 3510 ± 65 Ma.

For a closer evaluation of Hf–Nd mobility, the initialeHf(3720) and eNd(3720) compositions of the boninite-likemetabasalts are shown in an eHf–eNd co-variation diagram(Fig. 7). The initial eHf(3720) values that were calculatedfrom the data obtained by Li2B4O7 flux fusion show a rangefrom +0.9 to +12.9. Except for sample 462944(eNd(3720) = �0.4), the initial eNd(3720) values are all posi-tive (up to +3.2) and eHf(3720)–eNd(3720) exhibit a clear co-variation. Notably, the array defined by the boninite-likemetabasalts from Isua yield eHf(3720) between ca. +2 and+4 at eNd(3720) of 0 (Fig. 7), similar to Phanerzoic rocks(Vervoort et al., 1999). The metasediment samples exhibit

trating that most analyzed samples exhibit a co-variation betweenad in eHf is about a factor of 2–3 larger than that of eNd. Notably,f = +3 for eNd = 0, similar to Phanerozoic rocks (Vervoort et al.,

ei et al. (2004), the weighted means are used. The majority of thee metasediments analyzed are therefore not a suitable enriched


a more restricted range in their initial eNd values as the bon-inite-like metabasalts whereas the initial eHf(t) values aresignificantly lower, showing near chondritic values.

Two samples (462966, 462968), originally classified as“least altered” by Polat et al. (2002), are shifted towards sig-

Fig. 8. Variation diagrams illustrating systematic co-variations of initialdata for 462901 and 462903 from Frei et al., 2004; white diamonds: litera2004; light grey squares: compositionally layered metasedimentary rocksgarnet-micaschists from this study) with SiO2 and Al2O3/TiO2 and oconcentration (major element data from Polat et al., 2002 and Furnes et aet al. (1993), Vervoort et al. (1996), Nutman et al. (2007a) and Hiess et aet al., 1984) are plotted as dark gray squares. Major element data for thlayered metasedimentary samples from the type locality of Rosing (1999)the Table A1 of the Appendix. If replicates were indistinguishable within e

nificantly lower initial eHf(3720) compared to the other “leastaltered” boninite-like metabasalts (Table 2). A closer inspec-tion of the two samples reveals alteration features. Sample462968 displays evidence for strong K-metasomatism, ascorroborated by high K2O, Ba and Rb concentrations and

eHf(3720) and eNd(3720) values (black diamonds: this study, Sm–Ndture data from Hamilton et al., 1983; Gruau et al., 1996; Frei et al.,

from this study and Blichert-Toft et al., 1999; dark grey squares:f initial cOs(t)values (from Frei et al., 2004) with SiO2 and Osl., 2009). The fields for early Archean TTGs are taken from Bennettl. (2009). Garnet-micaschists from the B2 micaschist unit (Nutmanese two samples are from Polat and Frei (2005). Compositionallyare light gray squares. Major element data for these samples are inrror, the weighted mean (2r external r.s.d.) of both analyses is used.


abundant alteration veins on the hand specimen scale. Sam-ple 462966 displays the most anomalous initial eNd(3720) of�9.9 and is therefore interpreted to be altered. The majorand trace element composition of sample 462906 was alteredand it plots away from the differentiation trends defined bythe “least altered” samples (e.g., Cr, Ni vs. MgO and Gd/Ybvs. Zr/Hf; Figs. 4 and 6). Hence, these three samples areexcluded from the group of “least altered” samples. Limit-ing the dataset to this revised group of least altered samplesconfines the range of initial eHf(3720) to values between +3.5and +12.9 and this group of samples is discussed in detailbelow. Frei et al. (2004) further recognized the disturbanceof samples 462901 and 462902 with respect to 187Re/187Os.However, these samples are still included in the leastaltered sample set of this study because both show a coupleddepletion of their initial eHf(3720) and eNd(3720) values(Fig. 7).

The robustness of the 176Lu–176Hf and 147Sm–143Nd sys-tems can be further evaluated in isochron plots. The176Lu–176Hf data of the revised “least altered” group of11 metabasalts define an errorchron with an age of 3825± 160 Ma (MSWD = 56; initial eHf = +5.1 ± 7.5, Fig. 9).Taking all 176Lu–176Hf data into account, the data definean errorchron with an age of 3723 ± 170 Ma (MSWD = 94;initial eHf = +6.6 ± 8.2, Fig. 9). Within analytical uncer-tainty, both ages correspond to the minimum age for theboninite-like metabasalts as defined by a crosscutting tona-litic vein (3.71–3.72 Ga, sample G07/25; Nutman andFriend, 2009). A compilation of published Sm–Nd data

Fig. 9. Lutetium–Hf isochron diagram for the Isua boninite-like metaerrorchron for the 11 least altered samples yields an age of 3825 ± 160 Mage of 3723 ± 170 Ma (MSWD = 94). For isochron calculations, exterreplicates, those with the lowest errors were used. (k176Lu = 1.867 � 10�

(excluding outliers with initial eNd < �1) for boninite-likemetabasalts from the Isua supracrustal belt (Hamiltonet al., 1983; Gruau et al., 1996; Blichert-Toft et al., 1999;Frei and Kastbjerg-Jernsen, 2003; Frei et al., 2004) com-bined with data obtained during the course of our studyyields an errorchron defining an age of 3890 ±130 Ma(MSWD = 99) and an initial eNd(t) value of +2.4 ± 3.4(Fig. 10). Only taking into account the samples discussedin our study, a Sm–Nd age of 4017 ±340 Ma is calculated.The Sm–Nd age obtained from the compiled data set isslightly but significantly older than the age of 3720 definedby crosscutting relationships, and this most likely reflectsassimilation of enriched material as discussed below. ForSm–Nd the observed scatter has been recognized previously(e.g., Gruau et al., 1996; Hamilton et al., 1983; Nutmanet al., 1996) and it has been proposed that the scattermay reflect LREE mobility during early Archean tectono-metamorphic events (Gruau et al., 1996), crustal contami-nation (Hamilton et al., 1983; Nutman et al., 1996) orsource heterogeneity (Polat et al., 2002; Frei et al., 2004).As both errochrons still yield early Archean ages, a youngerdisturbance as shown for nearby CD tholeiites (Polat et al.,2003) can be ruled out, and any disturbance of the Sm–Ndand the Lu–Hf system must have been close in time to theemplacement age. This is also confirmed by the initial eNdand eHf of the errorchrons that yield realistic values. How-ever, the values are not precise enough to provide any ro-bust information on the depletion state of the mantlesource.

basalts calculated with ISOPLOT ver. 2.49 (Ludwig, 2001). Ana (MSWD = 56). Including all 14 samples, the errorchron yields annal reproducibilities were estimated as described in the text. For11, Scherer et al., 2001; Soderlund et al. 2004).

Fig. 10. Samarium–Nd isochron diagram including all published data for boninite-like metabasalts with eNd(3720) > �1 (Hamilton et al.,1983; Gruau et al., 1996; Blichert-Toft et al., 1999; Frei and Kastbjerg-Jernsen, 2003; Frei et al., 2004; this study). Literature data are plottedas grey circles; data acquired during this study and additional data for samples used in this study from Frei et al. (2004) are plotted as blackdiamonds. The slightly older age for the compiled data most likely reflects assimilation of an enriched component. A 147Sm decay constant of6.54 � 10�11 (Lugmair and Marti, 1978) and ISOPLOT ver. 2.49 (Ludwig, 2001) have been used for age calculation.


Collectively, for the majority of least altered samples,both the Lu–Hf and Sm–Nd systems appear to be little dis-turbed, and most samples exhibit consistently positive eHfand eNd values (Fig. 7). As predicted from the observedco-variations of both isotope systems in Phanerozoic rocks(Vervoort et al., 1999) the spread in eHf values should beabout a factor of 2 larger than that of eNd. However, thisis not the case for the boninite-like metabasalts analyzedhere, where the initial eHf are comparatively more depleted(ca. +13 at eNd of ca. +3 to +4). A possible mechanism toexplain this decoupling towards higher initial eHf valuescould be a previous depletion of the source in the garnet-stability field (e.g., Pearce, 2008).

The initial eHf(3720) of the revised group of “least alter-ed” samples shows systematic variations with several ratiosof relatively immobile major and trace elements (e.g., withAl2O3/TiO2, Zr/Hf, Zr/Nb, and SiO2; Fig. 8A and B). Ex-cept for the plot of eNd(3720) vs. SiO2 (Fig. 8C), such vari-ations are not clearly visible most likely because the spreadin eNd(3720) is much smaller than for eHf(3720). Therefore,the co-variations of both eHf(3720) and eNd(3720) values withrelatively immobile major and trace elements suggest asomewhat robust behaviour of both isotope systems. Theoutliers could either reflect primary source heterogeneityor disturbance in some samples. Some previous studies(e.g., Bennett et al., 2007; Nutman et al., 2007a) haveargued that the Sm–Nd system is not disturbed in rocks

selected on textural and structural criteria (e.g., Bennettet al., 2007; Nutman et al., 2007a).

In addition to the observations for Hf–Nd isotope com-positions, Hf abundances and Lu/Hf ratios also exhibit sys-tematic co-variations with other immobile elements andelement ratios (e.g., Zr/Hf, Gd/Yb, La/Yb, Fig. 6; and Zr,Nb, Ta, Th (not shown)). As expected from present daymagmatic systems, Lu/Hf and Hf concentrations are inver-sely correlated. We therefore, interpret the high Lu/Hf in the“least altered” sample suite as a primary magmatic feature,possibly mirroring igneous differentiation, contaminationby enriched components or the heterogeneity of the earlyArchean mantle sources (see also Blichert-Toft et al., 1999;Albarede et al., 2000). These patterns imply that the176Lu–176Hf and 147Sm–143Nd errorchron plots shown inFigs. 9 and 10 do not necessarily have age significance, butmight represent mixing arrays, e.g., between depleted man-tle melts and enriched components. The effects of contami-nation by enriched components would be strongest in thesamples displaying the lowest 176Lu/177Hf and 147Sm/144Nd,leading to an overestimation of the emplacement ages. How-ever, as the difference in initial Hf–Nd isotope compositionbetween the boninites and the enriched contaminants wasmost likely small in the early Archean, the effect on the iso-chron is probably not very strong and the ages obtained bythe errorchrons might still lie within uncertainty of the trueemplacement age of the boninites.

Fig. 11. eHf(3720) versus age diagram, illustrating mantle–crust evolution of important crustal blocks in comparison to compositions measuredin metabasalts from the Isua supracrustal belt (this study and Polat et al., 2003). All 176Hf/177Hf data were recalculated to CHUR parametersof Bouvier et al. (2008) and a k176Lu = 1.867 � 10�11 (Scherer et al., 2001; Soderlund et al., 2004). Initial eHf(t) values obtained for Archeanand Hadean zircons are taken from (1) Amelin et al. (2000), (2) Harrison et al. (2005); (3) Albarede and Blichert-Toft (2008); (4) Harrisonet al. (2008), (5) Kemp et al. (2010), (6) Choi et al. (2006), (7) Vervoort et al. (1996), (8) Vervoort and Blichert-Toft (1999), (9) Hiess et al.(2009), (10) Kemp et al. (2009) and (11) Zeh et al. (2009). Grey fields represent initial eHf(t) obtained for mafic and ultramafic sample suites(data sources: Blichert-Toft and Arndt, 1999; Polat et al., 2003, 2006; Blichert-Toft et al., 2004; Polat and Munker, 2004).


6.3. Shallow level contamination versus source contamination

Based on the lack of correlations between Nb/Nb* andSiO2, MgO, Th, Ni, Cr, Co, and the high eNd(3720) (ca. +2,Gruau et al., 1996), Polat et al. (2002) ruled out crustal con-tamination and argued in favour of a source contaminationmodel. Increasing MREE–HREE ratios with decreasingAl2O3/TiO2 were also interpreted as a source feature (Polatet al., 2002; Frei and Kastbjerg-Jernsen, 2003). However,using the high precision HFSE data, new inferences aboutthe contamination process can be made by comparison withHf–Nd–Os isotope data. Notably, the metabasalt sampleswith the least radiogenic eHf(3720) and eNd(3720) have thelowest Al2O3/TiO2 and highest SiO2 contents (Fig. 8Cand D), the most radiogenic samples exhibit the highestAl2O3/TiO2 and lowest SiO2 contents. Whereas theAl2O3/TiO2 ratios could be explained as source features,this would be difficult for SiO2 contents that are intimatelyrelated by fractional crystallisation processes. In the case ofsource contamination, most of the co-variations between

major elements and isotope ratios would be obscured.Hence, most element distribution patterns observed in theIsua boninites rather resemble typical assimilation-frac-tional-crystallisation trends (e.g., DePaolo, 1981), foundin many present day igneous systems undergoing differenti-ation and shallow level crustal assimilation. Co-variationsof Zr/Nb with SiO2 (Fig. 6B) and of 176Lu/177Hf withAl2O3/TiO2, are also consistent with a modification of thesetrace element ratios during fractionation and assimilation.Samples close to the low SiO2 end member also exhibit highMgO, Mg# and thus constitute the most primitive melts.The high SiO2 contaminant must have been more felsicand must have had a less radiogenic Hf composition. Geo-logically feasible felsic contaminants include metasedimen-tary rocks or TTGs.

Rhenium–Os systematics are also consistent with igne-ous fractionation and assimilation. High Os concentrations(1.6–3.7 ppb) reported by Frei et al. (2004) confirm theprimitive nature of the SiO2-poor magmas. Although thereis some scatter in cOs(3720) vs. SiO2 space, there is some


increase in cOs(3720) with increasing SiO2 and with decreas-ing Os abundances (Fig. 8E and F), both being consistentwith fractional crystallisation and assimilation of an Os-poor enriched component. The enriched Os isotope compo-sitions in the boninite-like metabasalts were originally inter-preted by Frei et al. (2004) as a source signature, possiblyreflecting core contribution to the early Archean mantleor source enrichment. However, the apparent increase ofOs isotope compositions and decrease of Os concentrationswith SiO2 does rule out assimilation of an enriched compo-nent as suggested from Hf–Nd isotopes.

Systematics of the Lu–Hf, Sm–Nd, and Re–Os isotopesystems and immobile trace element abundances rather ar-gue for shallow level contamination of the boninite-likemetabasalts with an enriched component. Two scenarioscould account for the observed assimilation trends: (1)assimilation of TTG crust during magma ascent, and (2)assimilation of ocean floor sediments during ascent anderuption. Compared to the most primitive metabasalts,the contaminant is characterized by lower Lu/Hf, Zr/Nb,Nb/Nb*, and Nb/Ta. Conversely, Th/Yb, La/Yb, Gd/Yband Zr/Hf are higher in the contaminant (Fig. 6B, D, E,G and H). Hence, major and trace element compositionsof Isua metasedimentary rocks and TTGs of the ItsaqGneiss Complex are plotted in Fig. 6B and D (Bolharet al., 2005; Polat and Frei, 2005; Nutman et al., 1999,2007a). The relationships of eHf(3720) and eNd(3720) vs.Al2O3/TiO2 (Fig. 8B and D), eNd(3720) vs. SiO2 (Fig. C),eNd(3720) vs. Al2O3/TiO2 (Fig. 8C and D) and Zr/Nb vs.SiO2 (Fig. 6B) are in favour of assimilation of ocean floorsediments and can rule out contamination with TTG-likecrust. Assimilation of sediments is also more consistentwith the geological setting of the boninite-like metabasalts.These basalts erupted in an intra-oceanic setting and thereis neither geochemical nor geological evidence for thickTTG-like rock suites underlying these units (e.g., Polatand Hofmann, 2003; Nutman and Friend, 2009).

There is some geological evidence for clastic metasedi-mentary rocks being intercalated with boninite-like metaba-salts of the Isua supracrustal belt. In the western part of thebelt, compositionally layered sandstone dominated sedi-ments, interpreted as turbidites (Nutman et al., 1984;Rosing, 1999), are directly intercalated with boninite-likemetabasalts. Similar sediments occur in the eastern partof the Isua supracrustal belt (Nutman et al., 1997, 2009).However, the western belt clastic metasedimentary rockshave near chondritic eHf(3720) of �0.8 to +1.5 and positiveeNd(3720) values of +1.6 to +2.3 (Blichert-Toft et al., 1999,confirmed in this study). Such compositions are not consis-tent with these sediments being a suitable contaminant be-cause initial eNd compositions are too radiogenic at giveneHf compositions (see eHf(3720)–eNd(3720) co-variation dia-gram, Fig. 7). Both sediment units are also slightly tooyoung to be the contaminant (Nutman and Friend, 2009).Given the eHf(3720)–eNd(3720) trend defined by the boni-nite-like metabasalts, it appears more likely that the con-taminant had negative eHf(t) and eNd(t) values. Such acontaminant has not been found yet in the Isua region.Only one of the two garnet-micaschist samples analyzedhere could constitute a suitable contaminant. However,

based on U–Pb zircon geochronology this unit is suppos-edly ca. 10 Ma younger than the boninite-like metabasalts(e.g., Kamber et al., 2005; Nutman et al., 2009). Followinga geochemical study on Isua metasedimentary rocks byBolhar et al. (2005), Kamber et al. (2005) proposed a modelin which older metasedimentary rocks in the region werecovered in a stack of thickened mafic volcanic rocks. Assim-ilation of such covered, ancient sediments during magmaascent would indeed be the best mechanism to accountfor the observed eHf(3720)–eNd(3720) trends. In support ofthe Kamber et al. (2005) model, rare detrital zircons inca. 3.70 to >3.85 Ga old Isua sediments and inheritedzircon grains in granites (Nutman et al., 1997, 2009; Friendet al., 2008), point towards the presence of >3.85 Ga oldcontinental crust in the Isua region, that could have sup-plied isotopically enriched ocean floor sediments. Theserare detrital zircons with ages of 3.85–3.94 Ga are foundin metasedimentary rocks associated with the southern�3.8 Ga old terrane in southern Isua (Nutman et al., 2009).

Collectively, there is strong support from the isotopeand trace element data for combined fractional crystallisa-tion and shallow level contamination with an “enriched”

component, most likely ocean floor sediments, embeddedin a stack of thickened mafic oceanic crust.

6.4. Mantle sources

Given the contamination model illustrated above, onlythe compositions of the least contaminated samples maybe regarded as good representatives of their mantle sources.It is evident from the high Nb/Nb* and Nb/Th in the mostprimitive samples (Fig. 6E and F) that the subduction-related signature is not entirely inherited from thecontaminant. Rather, the depleted mantle sources of theboninite-like metabasalts were already overprinted by sub-duction components (see Fig. 6H), in accord with earliermodels by Polat et al. (2002) and Frei et al. (2004). Notably,some samples display rather high Nb/Ta (ca. 17) despitehaving very low Zr/Hf (ca. 26–31). This feature is surpris-ing, given that experimental constraints suggest the cou-pling of both element pairs during mantle melting (e.g.,McDade et al., 2003). Extremely low Nb/Ta (ca. 10–12)would be expected based on the observed variations of bothelement ratios in Phanerozoic oceanic basalts (Fig. 6H,Munker et al., 2003). However, a similar decoupling ofNb/Ta and Zr/Hf was recently reported for highly depletedarc basalts and boninites from Cyprus and the Solomon Is-lands (Konig et al., 2008) that display similarly low Zr/Hf(c. 20–30) and similarly variable Nb/Ta (c. 11–26). Forthese two suites, the decoupling was explained by selectiveNb–Ta enrichment via slab components. For the Isua meta-basalts, the decoupling may therefore, be the result of theinteraction between high-Nb/Ta felsic slab melts withhighly depleted mantle domains, only affecting Nb/Ta butnot Zr/Hf. The high Nb/Ta in tonalites typically originatesfrom melting leaving behind an eclogitic residue, where Nb/Ta can be significantly fractionated in the presence of rutilethat is the major carrier of both elements. Experimentaldata imply DNb/Ta <1 for rutile–melt systems (e.g., Rappet al., 2003; Schmidt et al., 2004; Klemme et al., 2005;


Xiong et al., 2005), arguing for high Nb/Ta in such slab-de-rived melts. In marked contrast to Nb/Ta, the preservationof extremely low Zr/Hf in the most primitive metabasaltsindicates that the budget of these two elements was mostlikely not affected by subduction components, thus mirror-ing the original degree of mantle depletion. This observa-tion has important implications for the interpretation ofthe high Lu/Hf and highly radiogenic initial Hf isotopecompositions in the boninite-like metabasalts.

In 176Lu/177Hf vs. 147Sm/144Nd space, all boninite-likemetabasalts plot above the field for present day volcanicrocks (Fig. 6C), confirming previous isotope studies onboninite-like metabasalts from the western and central partof the Isua supracrustal belt by Blichert-Toft et al. (1999)and Albarede et al. (2000). These authors originally re-ported a strong decoupling of 176Lu/177Hf (0.02 to 0.10)from 147Sm/144Nd (0.16 to 0.25). As the major element vari-ations and the low Zr/Hf and Gd/Yb in the samples ana-lyzed here indicate that neither alteration nor sourcecontamination affected the Lu–Hf budget, these featurescan now be confirmed as being the vestige of ancient mantledepletion. By analogy with compositions of young mantleperidotites (e.g., Weyer et al., 2003) the decoupling of Lu/Hf and Sm/Nd was most likely generated by re-melting ofa mantle residue with elevated Lu/Hf that had been de-pleted while in the garnet stability field. Such a decouplingcan also account for the much larger spread of eHf(3720) val-ues when compared to the spread in eNd(3720) values(Fig. 7).

A garnet-field depletion signature as proposed above hasbeen previously suggested for several other Archeankomatiite and boninite suites (e.g., Wilson, 2003; Smithieset al., 2004). Possibly, the occurrence of such signatures inArchean rocks reflects a steeper thermal gradient than atpresent day and greater depths of melting. However, the vol-ume of such depleted mantle domains beneath the Isuasupracrustal belt remains uncertain. Published results fortholeiitic metabasalts from nearby localities in the easternIsua supracrustal belt (Fig. 1, Polat et al., 2003) yieldedmuch lower 176Lu/177Hf and close to chondritic initialeHf(3720), consistent with near primitive mantle sources.However, the stratigraphic and petrologic relationshipsbetween the tholeiitic and the boninite-like suites are notcertain. Like for the Isua tholeiites, initial Hf isotope com-positions of early Archean TTGs that now surround theIsua belt scatter around the chondritic value (Hiess et al.,2009). In any case, the depleted trace element signature ofthe boninite-like metabasalts together with their radiogenicinitial eHf(3720) and eNd(3720) clearly indicate the presence ofmantle sources with a long term depletion history. The sam-ples with the most radiogenic eHf and eNd (462902, 462905,462965) provide the best estimate for the elemental andisotopic composition of these mantle sources.

6.5. Implications for mantle–crust evolution

A selection of published Hf isotope data for Archeanand Hadean zircons is shown in Fig. 11 and compared withdata from Isua. Hadean detrital zircons provide evidencefor crust formation events before 3.8 Ga, also implying

the complementary evolution of depleted mantle reservoirs(Fig. 11; e.g., Harrison et al., 2005, 2008; Blichert-Toft andAlbarede, 2008; Kemp et al. 2010). Yet, Lu–Hf isotopestudies on post-Hadean zircons and on Archean maficrocks lack evidence for the long-term persistence of such de-pleted mantle reservoirs (Fig. 11; Amelin et al., 2000; Hiesset al., 2009; Kemp et al., 2009). Possible causes for this dis-parity are that the crustal volumes extracted are either vol-umetrically small or, alternatively, that ancient mantleheterogeneities have been efficiently homogenised withprimitive mantle domains. The Lu–Hf and Sm–Nd isotopedata for the boninite-like metabasalts from Isua are amongthe first datasets that unambiguously reveal the preserva-tion of such ancient depleted mantle domains in the earlyArchean mafic rock record.

The highly radiogenic initial eHf(3720) obtained for theboninite-like metabasalts from Isua clearly plot above theevolution curve of the present day depleted mantle. As illus-trated in Fig. 11, a time integrated 176Lu/177Hf of ca. 0.05would be required to account for such depleted mantlesources, assuming mantle depletion occurred as early as4.5 Ga. Such an early depletion age is in accord with142Nd studies on Isua rocks which argue generally for man-tle depletion to have occurred as early as 4.4–4.5 Ga (Caroet al., 2003, 2006; Bennett et al., 2007). However, for themost depleted samples, the strong Lu–Hf source depletionof the mantle would then result in an even stronger depleted142Nd signature than previously observed in other samplesfrom Isua, provided that the 176Lu–176Hf and the147Sm–143Nd decay systems were indeed coupled. Of allthe boninite-like samples from the CD only one has beenanalyzed so far for 142Nd abundance (Caro et al., 2006,their sample 98/21). This sample was collected close tothe sites of other samples discussed here (462944–49) andyielded a l142Nd value of +10.5, indistinguishable fromor even slightly lower than l142Nd compositions of othermetabasalts from Isua (e.g., Caro et al., 2003; Bennettet al., 2007). If sample 98/21 of Caro et al. (2006) is indeedrepresentative of the Isua boninite-like rocks, the strongdepletion event recognized here by 176Hf and 143Nd isotopeabundances would consequently represent a younger event,when the 146Sm–142Nd chronometer was extinct and a gen-eral 142Nd anomaly of ca. +10 to +20 ppm (e.g., Boyetet al., 2003; Caro et al., 2003; Bennett et al., 2007) had al-ready been established in the source region. A “late” deple-tion event at <4.5 Ga would require an even higher Lu/Hfratio for the mantle source of the boninite-like metabasalts.Such a scenario is very likely, as the 176Lu/177Hf in themetabasalts (0.049–0.086) define the minimum values fortheir mantle sources, given that DLu/DHf is >1 duringmantle melting (e.g., McDade et al., 2003).

Compared to the present day mantle evolution line,younger rocks such as �3.5 Ga old komatiites from theBarberton Greenstone belt, (Fig. 11; Blichert-Toft andArndt, 1999; Blichert-Toft et al., 2004), Hf isotope compo-sitions in zircons of ca. 3.85 Ga old orthogneisses from east-ern Antarctica (Choi et al., 2006), and 2.5 Ga oldperidotites from China (Fig. 11; Polat et al., 2006) also dis-play more radiogenic eHf(t), although the magnitude of themantle depletion for the mafic samples of these suites is


much smaller than for the boninite-like metabasalts fromIsua. Collectively, these features could indicate the presenceof ultra depleted mantle domains of currently unknown sizethat persisted throughout the Archean.

7. CONCLUSIONS

New 176Lu–176Hf, 147Sm–143Nd and high precisionHFSE data for boninite-like metabasalts from the easternportion of the Isua supracrustal belt, the oldest known ma-fic subduction-related volcanic rocks on Earth, permit thefollowing new insights into their petrogenesis as well asthe composition and history of their mantle sources:

(1) The highly depleted trace element signatures andstrong isotopic heterogeneities of boninite-likemetabasalts from the Isua supracrustal belt rocks arepristine magmatic features, as indicated by systematicco-variations between relatively immobile major andtrace elements. The 176Lu–176Hf and the 147Sm–143Ndsystems appear to be little affected by secondary over-print, and the majority of samples exhibits systematicco-variations of eHf(3720) and eNd(3720).

(2) The boninite-like metabasalts preserve some co-vari-ations between isotope rations and major/trace ele-ments. Assimilation of enriched material combinedwith fractional crystallisation appears to be the mostlikely mechanism to account for these co-variations.Based on combined geochemical and geological evi-dence, ocean floor sediments and not TTG-like crustconstitutes the most likely enriched contaminant.Metasedimentary rocks presently recognized in theIsua supracrustal belt, however, do not constitute asuitable contaminant, as they are too radiogenic intheir initial eNd.

(3) The decoupling of Lu/Hf and Sm/Nd element ratiosin the boninite-like metabasalts, together with theirlow Zr/Hf, is the consequence of re-melting a refrac-tory mantle source that was previously depleted inthe garnet stability field. The magnitude of this deple-tion signature was reduced by contamination with anenriched component.

(4) The spread in the Nb depletion in the boninite-likemetabasalts partially reflects contamination. Yet,the most primitive samples still have a pronouncedNb anomaly, thus supporting a subduction zone set-ting for this suite

(5) The Nb/Ta in the boninite-like metabasalts is highlyelevated compared to extremely low Zr/Hf. Such adecoupling indicates that the Nb–Ta inventory ofthe depleted mantle source had been overprinted bysubduction components. The high Nb/Ta in the sub-duction components argue for source overprint byfelsic slab melts generated in the eclogite stabilityfield, by analogy with some present day boninites.

(6) The Hf–Nd isotope signature of the mantle source isbest mirrored by the radiogenic initial eHf(3720) andeNd(3720) values in the most primitive metabasalts(ca. +8 to +13 and +3 to +4, respectively), indicatinga long term convective isolation of this mantle

domain. This finding supports previous evidencefrom radiogenic eNd values (P+3) in Isua rocks ofsimilar age (e.g., Bennett et al., 1993, 2007). Assum-ing a depletion age as old as 4.50 Ga, a time inte-grated 176Lu/177Hf of at least 0.05 is required forthe mantle source, which is significantly higher thanthe value proposed for average present day depletedmantle. There is no difference between the 142Nd sig-nature of the boninite-like metabasalts and otherrocks from Isua, thus the depletion age is probablymuch younger than 4.50 Ga and requires an evenhigher 176Lu/177Hf in the mantle source. In any case,the depleted Hf isotope compositions of boninite-likemetabasalts from Isua provide evidence for the per-sistence of Hadean mantle heterogeneities into theArchean. At present, however, the volumetric signif-icance of such heterogeneities remains unconstrained.

ACKNOWLEDGMENTS

This research was in part supported by the DFG (German Re-search Foundation), Grant No Mu 1406/8, by GEUS (GeologicalSurvey of Denmark and Greenland) and the Geological Museumof Copenhagen. We thank Erik Scherer, Dewashish Upadhyay,Markus Lagos, Stephan Schuth, Maria Kirchenbaur and RaulO.C. Fonseca for discussions. Comments by Jan Kramers, SimonWilde, Alan Nutman, Balz Kamber and two anonymous reviewersas well as editorial comments by Peter Ulmer and Frank Podosekhelped to improve the manuscript.

APPENDIX A

GPS coordinates for boninite-like metabasalts, composi-tionally layered metasedimentary rocks and garnet-micas-chists used for this study taken from Polat et al. (2002),Rosing (1999) and Polat and Frei (2005).

Boninite-like metabasalts:

462901 65�10.6500 N �49�47.4950 W462902 65�10.7260 N �49�47.4740 W492903 65�10.8060 N �49�47.3470 W492904 65�10.8020 N �49�47.2590 W462905 65�10.9240 N �49�47.0380 W462906 65�11.0340 N �49�46.8780 W462944 65�10.3680 N �49�49.3640 W462945 65�10.3680 N �49�49.3640 W462946 65�10.3680 N �49�49.3640 W462948 65�10.3680 N �49�49.3640 W492949 65�10.3680 N �49�49.3640 W462965 65�11.1390 N �49�47.3730 W462966 65�11.3110 N �49�47.6680 W462968 65�10.9350 N �49�47.8770 W

Compositionally layered Metasedimentary rocks (typelocality of Rosing, 1999):

465438, 465439, 465440, 465441, 465442, 465443

65�08.850 N �50�09.970 W

Table A1Major element data for Isua compositionally layered Metasedimentary rocks (type locality of Rosing, 1999).

465438 465439 465440 465441 465442 465443

SiO2 72.8 65.3 62.1 66.6 65.3 56.1TiO2 0.26 0.6 0.42 0.54 0.38 0.64Al2O3 11.9 18.0 15.1 15.5 14.5 17.7Fe2O3 5.13 8.18 10.4 4.81 6.53 11.4MnO 0.02 0.03 0.06 0.04 0.06 0.06MgO 1.39 2.01 2.55 1.89 2.14 2.98CaO 2.15 0.27 1.07 2.58 2.36 1.49K2O 1.55 4.48 1.73 1.89 2.17 0.19Na2O 2.59 0.31 3.67 4.05 2.43 5.66P2O5 0.04 0.10 0.07 0.10 0.06 0.11LOI 1.52 3.2 1.89 1.08 3.3 2.98Sum 99.4 102.4 99.0 99.1 99.3 99.3


Garnet-micaschists:

462912 65�11.9140 N �49� 47.2650 W462919 65�11.9100 N �49�47.2750 W

REFERENCES

Albarede F., Blichert-Toft J., Vervoort J. D., Gleason J. D. andRosing M. (2000) Hf–Nd isotope evidence for a transientdynamic regime in the early terrestrial mantle. Nature 404, 488–

490.

Albarede F., Blichert-Toft J., Frei R. and Rosing M. (2001) Replyto the Comment by Villa I. M., Kamber, B. S., Nagler T. F. on“The Nd and Hf isotopic evolution of the mantle through theArchean. Results from the Isua supracrustals, West Greenland,and from the Birimian terranes of West Africa”. Geochim.

Cosmochim. Acta 65, 2023–2025.

Allaart J. H. (1976) The pre-3760 m.y. old supracrustal rocks of theIsua Area, Central West Greenland, and associated occurrenceof quartz-banded ironstone. In The Early History of Earth (ed.B. F. Windley). John Wiley and Sons, London, pp. 177–189.

Amelin Y., Lee D.-C. and Halliday A. N. (2000) Early-middleArchaean crustal evolution deduced from Lu–Hf and U–Pbisotopic studies of single zircon grains. Geochim. Cosmochim.

Acta 64, 4205–4225.

Appel P. W. U., Fedo C. M., Moorbath S. and Myers J. S. (1998)Recognizable primary volcanic and sedimentary features in alow-strain domain of the highly deformed, oldest known (3.7–3.8 Gyr) Greenstone Belt, Isua, West Greenland. Terra Nova

10, 57–62.

Appel P. W. U., Polat A. and Frei R. (2009) Dacitic ocelli in maficlavas, 3.8–3.7 Ga Isua greenstone belt, West Greenland: Geo-chemical evidence for partial melting of oceanic crust andmagma mixing. Chem. Geol. 258, 105–124.

Baadsgaard H., Nutman A. P. and Bridgwater D. (1986a)Geochronology and isotopic variation of the early ArcheanAmitsoq gneisses of the Isukasia area, southern West Green-land. Geochim. Cosmochim. Acta 50, 2173–2183.

Baadsgaard H., Nutman A. P., Rosing M., Bridgewater D. andLongstaffe F. J. (1986b) Alteration and metamorphism ofAmitsoq gneisses from the Isukasia, West Greenland: recom-mendations for isotope studies of the early crust. Geochim.

Cosmochim. Acta 50, 2165–2172.

Bennett V. (2003) Compositional evolution of the mantle. Treat.

Geochem. 2, 493–519.

Bennett V. C., Nutman A. P. and McCulloch M. T. (1993) Ndisotopic evidence for transient, highly depleted mantle

reservoirs in the early history of the Earth. Earth Planet. Sci.

Lett. 119, 299–317.

Bennett V. C., Brandon A. and Nutman A. P. (2007) Coupled142Nd–143Nd isotopic evidence for Hadean mantle dynamics.Science 318, 1907–1910.

Bizzarro M., Baker J. A. and Ulfbeck D. (2003a) A new digestionand chemical separation technique for rapid and highlyreproducible determination of Lu/Hf and Hf isotope ratios ingeological materials by MC-ICP-MS. Geostandards Newsl. 27,

133–145.

Bizzarro M., Baker J. A., Haack H., Ulfbeck D. and Rosing M.(2003b) Early history of Earth’s crust–mantle system inferredfrom Hf isotopes in chondrites. Nature 421, 931–933.

Blichert-Toft J. and Albarede F. (2008) Hafnium isotopes in JackHills zircons and the formation of the Hadean crust. Earth

Planet. Sci. Lett. 265, 686–702.

Blichert-Toft J. and Arndt N. T. (1999) Hf isotope compositions ofkomatiites. Earth Planet. Sci. Lett. 171, 439–451.

Blichert-Toft J., Albarede F., Rosing M., Frei R. and BridgewaterD. (1999) The Nd and Hf isotopic evolution of the mantlethrough the Archean. Results from the Isua supracrustals, WestGreenland, and from the Birimian terranes of West Africa.Geochim. Cosmochim. Acta 63, 3901–3914.

Blichert-Toft J., Boyet M., Telouk P. and Albarede F. (2002) Sm-147–Nd-143 and Lu-176–Hf-176 in eucrites and the differenti-ation of the HED parent body. Earth Planet. Sci. Lett. 204,

167–181.

Blichert-Toft J., Arndt N. T. and Gruau G. (2004) Hf isotopicmeasurements on the Barberton komatiites: effects of incom-plete sample dissolution and importance for primary andsecondary magmatic signatures. Chem. Geol. 207, 261–275.

Bolhar R., Kamber B. S., Moorbath S., Whitehouse M. J. andCollerson K. D. (2005) Chemical characterisation of Earth’smost ancient clastic metasediments from the Isua GreenstoneBelt, southern West Greenland. Geochim. Cosmochim. Acta 69,

1555–1573.

Bowring S. A. and Housh T. (1995) The Earth’s early evolution.Science 269, 1535–1540.

Bouvier A., Vervoort J. D. and Patchett P. J. (2008) The Lu–Hfand Sm–Nd isotopic composition of CHUR: constraints fromunequilibrated chondrites and implications for the bulk com-position of terrestrial planets. Earth Planet. Sci. Lett. 273, 48–

57.

Boyet M., Blichert-Toft J., Rosing M., Story M., Telouk P. andAlbarede F. (2003) 142Nd evidence for early Earth differentia-tion. Earth Planet. Sci. Lett. 214, 427–442.

Bridgwater D. and McGregor V. R. (1974) Field work on the veryearly Precambrian rocks of the Isua area, Southern WestGreenland. Rapp. Greenland’s Geol. Under. 65, 49–54.


Caro G., Bourdon B., Birck J. L. and Moorbath S. (2003)146Sm–142Nd evidence from Isua metamorphosed sediments forearly differentiation of the Earth’s mantle. Nature 423, 428–432.

Caro G., Bourdon B., Birck J.-L. and Moorbath S. (2006) High-precision 142Nd/144Nd measurements in terrestrial rocks: con-straints on the early differentiation of the Earth’s mantle.Geochim. Cosmochim. Acta 70, 164–191.

Choi S. H., Mukasa S. B., Andronikov A. V., Osanai Y., Harley S.L. and Kelly N. M. (2006) Lu–Hf systematics of the ultra-hightemperature Napier Metamorphic Complex in Antarctica:evidence for the early Archean differentiation of Earth’s mantle.Earth Planet. Sci. Lett. 246, 305–316.

Collerson K. D., Campbell L. M., Weaver B. L. and Palacz Z. A.(1991) Evidence for extreme mantle fractionation in earlyArchean ultramafic rocks from northern Labrador. Nature 349,

209–214.

Compston W., Kinny P. D., Williams I. S. and Foster J. J. (1986)The age and Pb loss behaviour of zircons from the Isuasupracrustal belt as determined by ion microprobe. Earth


Crowley J. L. (2003) U–Pb geochronology of 3810–3630 Magranitoid rocks south of the Isua greenstone belt, southernWest Greenland. Precamb. Res. 126, 235–257.

DePaolo D. J. (1981) Trace element and isotopic effects ofcombined wallrock assimilation and fractional crystallization.Earth Planet. Sci. Lett. 53, 189–202.

Fedo C. M. (2000) Setting and origin for problematic rocks fromthe >3.7 Ga Isua greenstone belt, southern west Greenland:Earth’s oldest coarse clastic sediments. Precamb. Res. 101, 69–

78.

Frei R., Rosing M. T., Waight T. E. and Ulfbeck D. G. (2002)Hydrothermal-metasomatic and tectono-metamorphic pro-cesses in the Isua supracrustal belt (West Greenland): a multi-isotopic investigation of their effects on the Earth’s oldestoceanic crustal sequence. Geochim. Cosmochim. Acta 66, 467–

486.

Frei R. and Kastbjerg-Jernsen B. (2003) Re–Os, Sm–Nd isotope-and REE systematics on ultramafic rocks and Kissen basaltsfrom Earth’s oldest oceanic fragments (Isua Supracrustal Beltand Ujarassuit nunat area, W Greenland). Chem. Geol. 196,

163–191.

Frei R., Polat A. and Meibom A. (2004) The Hadean upper mantleconundrum: evidence for source depletion and enrichment fromSm–Nd, Re–Os, and Pb isotopic compositions in 3.71 Gyboninite-like metabasalts from the Isua Supracrustal Belt,Greenland. Geochim. Cosmochim. Acta 68, 1645–1660.

Friend C. R. L. and Nutman A. P. (2005) New pieces to theArchean jigsaw puzzle in the Nuuk region, southern WestGreenland: steps in transforming a simple insight into acomplex regional tectonothermal model. J. Geol. Soc. London

162, 147–162.

Friend C. R. L., Nutman A. P., Bennett V. C. and Norman M. D.(2008) Seawater-like trace element signatures (REE + Y) ofEoarchaean chemical sedimentary rocks from southern WestGreenland, and their corruption during high-grade metamor-phism. Contrib. Mineral. Petrol. 155, 229–246.

Furnes H., de Wit M., Staudigel H., Rosing M. and MuehlenbachsK. (2007) A vestige of Earth’s oldest ophiolite. Science 315,

1704–1707.

Furnes H., Rosing M., Dilek Y. and de Wit M. (2009) Isuasupracrustal belt (Greenland) – a vestige of a 3.8 Ga suprasub-duction zone ophiolite, and the implications for Archeangeology, Lithos, doi: 10.1016/j.lithos.2009.03.043.

Gill R. C. O., Bridgwater D. and Allaart J. H. (1981) Thegeochemistry of the earliest known basic metavolcanic rocks,

West Greenland: a preliminary investigation. Geol. Soc. Aust.

Spec. Publ. 7, 313–325.

Gruau G., Rosing M., Bridgewater D. and Gill R. C. O. (1996)Resetting of Sm–Nd systematics during metamorphism of>3.7 Ga rocks: implications for isotopic models of early Earthdifferentiation. Chem. Geol. 133, 225–240.

Hamilton P. J., O’Nions R. K., Evensen N. M., Bridgewater D.and Allaart H. (1978) Sm–Nd isotopic investigations of Isuasupracrustals and implications for mantle evolution. Nature

272, 41–43.

Hamilton P. J., O’Nions R. K., Bridgewater D. and Nutman A. P.(1983) Sm–Nd studies of Archaean metasediments and meta-volcanics from West Greenland and their implications for theEarth’s early history. Earth Planet. Sci. Lett. 62, 263–272.

Hamilton W. B. (2007) Comment “A vestige of Earth’s oldestophiolite”. Science 318, 746d.

Hanmer S., Hamilton M. A. and Crowley J. L. (2002) Geochro-nological constraints on Paleoarchean thrust-nappe and Neo-archean accretionary tectonics in southern West Greenland.Tectonophysics 350, 255–271.

Harrison T. M., Blichert-Toft J., Muller W., Albarede F., HoldenP. and Mojzsis S. (2005) Heterogeneous Hadean hafnium:evidence of continental crust at 4.4 to 4.5 Ga. Science 310,

1947–1950.

Harrison T. M., Schmitt A. K., McCulloch M. T. and Lovera O.M. (2008) Early (P4.5 Ga) formation of terrestrial crust: Lu–Hf, d18O, and Ti thermometry results for Hadean zircons.Earth Planet. Sci. Lett. 268, 476–486.

Hofmann A. W. (1988) Chemical differentiation of the Earth: therelationship between mantle, continental crust, and oceaniccrust. Chem. Geol. 90, 297–314.

Hiess J., Bennett V. C., Nutman A. P. and Williams I. S. (2009) Insitu U–Pb, O and Hf isotopic compositions of zircon andolivine from Eoarchaean rocks, West Greenland: New insightsto making old crust. Geochim. Cosmochim. Acta 73, 4489–4516.

Jacobsen S. B. and Dymek R. F. (1988) Nd and Sr isotopesystematics of clastic metasediments from Isua, West Green-land: identification of pre-3.8 Ga differentiated crustal compo-nents. J. Geophys. Res. 93, 338–354.

Jenner F. E., Bennett V. C., Nutman A. P., Friend C. R. L.,Norman M. D. and Yaxley G. (2009) Evidence for subductionat 3.8 Ga: geochemistry of arc-like metabasalts from thesouthern edge of the Isua Supracrustal Belt. Chem. Geol. 261,

83–98.

Kamber B. S., Moorbath S. and Whitehouse M. J. (1998) ExtremeNd-isotope heterogeneity in the early Archaean – fact orfiction? Case histories from northern Canada and WestGreenland – Reply. Chem. Geol. 148, 219–224.

Kamber B. S., Collerson K. D., Moorbath S. and Whitehouse M.J. (2003) Inheritance of early Archaean Pb-isotope variabilityfrom long-lived Hadean protocrust. Contrib. Mineral. Petrol.

145, 25–46.

Kamber B. S., Whitehouse M. J., Bolhar R. and Moorbath S.(2005) Volcanic resurfacing and the early terrestrial crust:zircon U–Pb and REE constraints from the Isua GreenstoneBelt, southern West Greenland. Earth Planet. Sci. Lett. 240,

276–290.

Kemp A. I. S., Foster G. L., Schersten A., Whitehouse M. J.,Darling J. and Storey C. (2009) Concurrent Pb–Hf isotopeanalysis of zircon by laser ablation multi-collector ICP-MS,with implications for the crustal evolution of Greenland and theHimalayas. Chem. Geol. 261, 244–260.

Kemp A. I. S., Wilde S. A., Hawkesworth C. J., Coath C. D.,Nemchin A., Pidgeon R. T., Vervoort J. D. and DuFrane S. A.(2010) Hadean crustal evolution revisted: new constraints from

http://dx.doi.org/10.1016/j.lithos.2009.03.043


Pb–Hf isotope systematics of the Jack Hills zircons. Earth


Klemme S., Prowatke S., Hametner K. and Gunther D. (2005)Partitioning of trace elements between rutile and silicate melts:implications for subduction zones. Geochim. Cosmochim. Acta

69, 2361–2371.

Komiya T., Maruyama S., Masuda T., Nohda S., Hyashi M. andOkamoto K. (1999) Plate tectonics at 3.8–3.7 Ga: field evidencefrom the Isua accretionary complex, Southern West Greenland.J. Geol. 107, 515–554.

Konig S., Schuth S., Munker C. and Garbe-Schonberg D. (2008)Mobility of tungsten in subduction zones. Earth Planet. Sci.

Lett. 274, 82–92.

Lagos M., Scherer E. E., Tomaschek F., Munker C., Keiter M.,Berndt J. and Ballhaus C. (2007) High precision Lu–Hfgeochronology of Eocene eclogite-facies rocks from Syros,Cyclades, Greece. Chem. Geol. 243, 16–35.

Ludwig K. R. (2001) Users manual for Isoplot/Ex version 2.49.Berkeley Geochronology Center, Special Publication No 1a,2455 Ridge Road, Berkeley, CA, 48pp.

Lugmair G. W. and Marti K. (1978) Lunar initial 143Nd/144Nd:differential evolution of the lunar crust and mantle. Earth


Maruyama S., Masuda T., Nohda S., Appel P., Otofuji Y., Miki M.,Shibata T. and Hagiya H. (1992) The 3.9–3.8 Ga plate tectonicson the Earth: evidence from Isua, Greenland. Evolving EarthSymposium, Okazaki, Japan Program and Abstracts, pp. 113.

McDade P., Blundy J. and Wood B. J. (2003) Trace elementpartitioning on the Tinaquillo lherzolite solidus at 1.5 GPa.Phys. Earth Planet. Inter. 139, 129–147.

Moorbath S., O’Nions R. K. and Pankhurst R. J. (1973) EarlyArchean age for the Isua iron formation, West Greenland.Nature 245, 138–139.

Moorbath S., Whitehouse M. J. and Kamber B. S. (1997) ExtremeNd-isotope heterogeneity in the early Archean – fact or fiction?Case histories from northern Canada and West Greenland.Chem. Geol. 135, 213–231.

Munker C., Weyer S., Scherer E. and Mezger K. (2001) Separationof high field strength elements (Nb, Ta, Zr, Hf) and Lu fromrock samples for MC-ICPMS measurements. Geochem. Geo-

phys. Geosys. 2 (G3) paper number 10.1029/2001GC000183.Munker C., Pfander J. A., Weyer S., Buchl A., Kleine T. and

Mezger K. (2003) Evolution of planetary cores and the Earth–Moon system from Nb/Ta systematics. Science 301, 84–87.

Munker C., Worner G., Yogodzinski G. and Churikova T. (2004)Behaviour of high field strength elements in subduction zones:constraints from Kamchatka-Aleutian arc lavas. Earth Planet.

Sci. Lett. 224, 275–293.

Myers J. S. (2001) Protoliths of the 3.8–3.7 Ga Isua greenstone beltWest Greenland – reply. Precamb. Res. 117, 151–156.

Nir-El Y. and Lavi N. (1998) Measurement of half-life of 176Lu.Appl. Radiat. Isot. 49, 1653–1655.

Nutman A. P. (1986) The early Archean to Proterozoic history ofthe Isukasia area, southern West Greenland. Geol. Survey

Greenland Bull. 154, 80.

Nutman A. P. (2001) On the scarcity of >3900 Ma detrital zirconsin >=3500 Ma metasediments. Precamb. Res. 105, 93–114.

Nutman A. P. and Friend C. R. L. (2007) Comment “A vestige ofEarth’s oldest ophiolite”. Science 318, 746c.

Nutman A. P. and Friend C. R. L. (2009) A glimpse of Eoarchaeancrust formation and orogeny: a new synthesis and 1:20,000geological maps of the Isua supracrustal belt and adjacentorthogneisses, Nuuk region, southern West Greenland. Pre-

camb. Res. 172, 189–211.

Nutman A. P., Allaart J. N., Bridgwater D., Dimroth E. andRosing M. (1984) Stratigraphic and geochemical evidence for

the depositional environment of early Archean Isua supracru-stal belt, southern west Greenland. Precamb. Res. 25, 365–

396.

Nutman A. P., McGregor V. R., Friend C. R. L., Bennett V. C.and Kinny P. D. (1996) The Itsaq Gneiss Complex of southernwest Greenland; the world’s most extensive record of earlycrustal evolution (3900–3600). Precamb. Res. 78, 1–39.

Nutman A. P., Bennett V. C., Friend C. R. L. and Rosing M. T.(1997) 3710 and >3790 Ma volcanic sequences in the Isua(Greenland): supracrustal belt; structural and Nd isotopeimplications. Chem. Geol. 141, 271–287.

Nutman A. P., Bennett V. C., Friend C. R. L. and Norman M. D.(1999) Meta-igneous (non-gneissic) tonalites and quartz-dior-ites from an extensive ca. 3800 Ma terrain south of the Isuasupracrustal belt, southern West Greenland: constraints onearly crust formation. Contrib. Mineral. Petrol. 137, 364–388.

Nutman A.P., Friend C.R.L. and Bennett V.C. (2002) Evidence for3650–3600 Ma assembly of the northern end of the Itsaq GneissComplex, Greenland: Implication for early Archean tectonics.Tectonics 21, 1. doi:10.1029/2000TC001203.

Nutman A. P., Bennett V. C., Friend C. R. L., Horie K. andHidaka H. (2007a) 3850 Ma tonalites in the Nuuk region,Greenland: geochemistry and their reworking within an Eoar-chaean gneiss complex. Contrib. Mineral. Petrol. 154, 385–408.

Nutman A. P., Friend C. R. L., Horie K. and Hidaka H. (2007b)The Itsaq Gneiss complex of southern West Greenland and theconstruction of Eoarchean crust at convergent plate bound-aries. In Earth’s Oldest Rocks. Developments in Precambrian

Geology, vol. 15 (eds. H. Van Kranedonk, H. Smithies and V.C. Bennett). Elsevier, Amsterdam, pp. 187–218.

Nutman A. P., Friend C. R. L. and Paxton S. (2009) Detrital zirconsedimentary provenance ages for the Eoarchaean Isua supra-crustal belt southern West Greenland: Juxtaposition of a ca.3700 Ma juvenile arc assemblage against an older complex with3920–3800 Ma components. Precamb. Res. 172, 212–233.

Patchett P. J. and Tatsumoto M. (1980) Lu–Hf whole rockisochron for the eucrite meteorites. Nature 288, 571–574.

Pearce J. A. (2008) Geochemical fingerprinting of oceanic basaltswith applications to ophiolite classification and the search forArchean oceanic crust. Lithos 100, 14–48.

Pfander J. A., Munker C., Stracke A. and Mezger K. (2007) Nb/Taand Zr/Hf in ocean island basalts – Implications for crust–mantle differentiation and the fate of Niobium. Earth Planet.

Sci. Lett. 254, 158–172.

Polat A. and Hofmann A. W. (2003) Alteration and geochemicalpatterns in the 3.7–3.8 Ga Isua greenstone belt, West Green-land. Precamb. Res. 126, 197–218.

Polat A. and Munker C. (2004) Hf–Nd isotope evidence forcontemporaneous subduction processes in the source of lateArchean arc lavas from the Superior Province, Canada. Chem.

Geol. 213, 403–429.

Polat A. and Frei R. (2005) The origin of early Archean bandediron formation and of continental crust, Isua, Greenland.Precamb. Res. 138, 151–175.

Polat A., Hofmann A. W. and Rosing M. T. (2002) Boninite-likevolcanic rocks in the 3.7–3.8 Ga Isua Greenstone Belt, WestGreenland: geochemical evidence for intra-oceanic subductionzone processes in the early Earth. Chem. Geol. 184, 231–254.

Polat A., Hofmann A. W., Munker C., Regelous M. and Appel P.W. U. (2003) Contrasting geochemical patterns in the 3.7–3.8 Ga pillow basalt cores and rims, Isua Greenstone Belt,southwest Greenland: Implications for postmagmatic alterationprocesses. Geochim. Cosmochim. Acta 67, 441–457.

Polat A., Herzberg C., Munker C., Rodgers R., Kusky T., Li J.,Fryer B. and Delaney J. (2006) Geochemical and petrologicalevidence for a supra-subduction zone origin of Neoarchean (ca.

http://dx.doi.org/10.1029/2000TC001203


2.5 Ga) peridotites, central orogenic belt, North China craton.Geol. Soc. Am. Bull. 118, 771–784.

Rapp R. P., Shimizu N. and Norman M. D. (2003) Growth ofcontinental crust by partial melting of eclogite. Nature 425,

605–609.

Rollinson H. (2003) Metamorphic history suggested by garnet-growth chronologies in the Isua greenstone belt, West Green-land. Precamb. Res. 126, 181–196.

Rose N. M., Rosing M. T. and Bridgewater D. (1996) The origin ofmetacarbonate rocks in the Isua Archean Supracrustal belt,West Greenland. Am. J. Sci. 296, 1004–1044.

Rosing M. T. (1999) 13C-depleted carbon microparticles in >3700-Ma sea-floor sedimentary rocks from West Greenland. Science

283, 674–676.

Rosing M. T., Rose N. M., Bridgwater D. and Thomsen H. S.(1996) Earliest part of Earth’s stratigraphic record: a reap-praisal of the >3.7 Ga Isua (Greenland) supracrustal sequence.Geology 24, 43–46.

Scherer E., Munker C. and Mezger K. (2001) Calibration of thelutetium–hafnium clock. Science 293, 683–687.

Scherer E., Whithehouse M. J. and Munker C. (2007) Zircon asmonitor for crustal growth. Elements 3, 19–24.

Schmidt M. W., Dardon A., Chazot G. and Vanucci R. (2004) Thedependence of Nb and Ta rutile–melt partitioning on meltcomposition and Nb/Ta fractionation during subduction pro-cess. Earth Planet. Sci. Lett. 226, 415–432.

Sguigna A. P., Larabee A. J. and Waddington J. C. (1982) Thehalf-life of 176Lu by a c–c coincidence measurement. Can. J.

Phys. 60, 361–364.

Smithies H., Champion D. C. and Sun S.-S. (2004) The case ofArchean boninites. Contrib. Mineral. Petrol. 147, 705–721.

Soderlund U., Patchett P. J., Vervoort J. D. and Isachsen C. E.(2004) The 176Lu decay constant determined by Lu–Hf and U–Pb isotope systematics of Precambrian mafic intrusions. Earth


Tatsumoto M., Unruh D. and Patchett P. J. (1981) U–Pb and Lu–Hf systematics of Antarctic meteorites. In Proc. Sixth Sympo-

sium on Antarctic Meteorites: Memoirs of National Institute of

Polar Research (Tokyo), Special Issue 20, 237–249.

Taylor S. R. and McLennon S. M. (1985) The Continental Crust:

Its Composition and Evolution. Blackwell Scientific Publications,Oxford.

Valley J. W., Lackey J. S., Cavosie A. J., Clenchenko C. C.,Spicuzza M. J., Basei M. A. S., Bindemann I. N., Ferreira V. P.,Sial A. N., King E. M., Peck W. H., Sinha A. K. and Wei C. S.(2005) 4.4 billion years of crustal maturation: oxygen isotoperatios of magmatic zircon. Contrib. Min. Petrol. 150, 561–580.

Veizer J. and Jansen S. L. (1979) Basement and sedimentaryrecycling and continental evolution. J. Geol. 87, 341–370.

Vervoort J. D. and Blichert-Toft J. (1999) Evolution of thedepleted mantle: Hf isotope evidence from juvenile rocksthrough time. Geochim. Cosmochim. Acta 63, 533–556.

Vervoort J. D., Patchett P. J., Gehrels G. E. and Nutman A. P.(1996) Constraints on early Earth differentiation from hafniumand neodymium isotopes. Nature 379, 624–627.

Vervoort J.D., Patchett P.J., Soderlund U. and Baker M. (2004)Isotopic composition of Yb and the determination of Luconcentrations and Lu/Hf ratios by isotope dilution using MC-ICPMS. Geochem. Geophys. Geosys. 5, Q11002, doi:10.1029/2004GC000721.

Vervoort J. D., Patchett P. J. and Blichert-Toft J. (1999)Relationships between Lu–Hf and Sm–Nd isotopic systems inthe global sedimentary system. Earth Planet. Sci. Lett. 168, 79–

99.

Villa I. M., Kamber B. S. and Nagler T. (2001) Comment on “TheNd and Hf isotopic evolution of the mantle through theArchean. Results from the Isua supracrustals, West Greenland,and the Birimian terranes of West Africa” by Blichert-Toftet al. (1999). Geochim. Cosmochim. Acta 65, 2017–2021.

Weyer S., Munker C., Rehkamper M. and Mezger K. (2002)Determination of ultra-low Nb, Ta, Zr and Hf concentrationsand the chondritic Zr/Hf and Nb/Ta ratios by isotope dilutionanalyses with multiple collector ICP-MS. Chem. Geol. 187, 295–

313.

Weyer S., Munker C. and Mezger K. (2003) Nb/Ta, Zr/Hf andREE in the depleted mantle: implications for the differentiationhistory of the crust mantle system. Earth Planet. Sci. Lett. 205,

309–324.

Wilde S. A., Valley J. W., Peck W. H. and Graham C. M. (2001)Evidence from detrital zircons for the existence of continentalcrust and oceans on earth 4.4 Gyr ago. Nature 409, 175–178.

Wilson A. H. (2003) A new class of silica enriched, highly depletedkomatiites in the southern Kaapvaal Craton, South Africa.Precamb. Res. 127, 125–141.

Xiong X. L., Adam J. and Green T. H. (2005) Rutile stability andrutile/melt HFSE partitioning during partial melting of hydrousbasalt: implications for TTG genesis. Chem. Geol. 218, 339–359.

Zeh A., Gerdes A. and Barton, Jr., J. M. (2009) Archean accretionand crustal evolution of the Kalahari Craton – the zircon ageand Hf isotope record of granitic rocks from Barberton/Swaziland to the Francistown Arc. J. Petrol. 50, 933–966.

Associate editor: Peter Ulmer

http://dx.doi.org/10.1029/2004GC000721

http://dx.doi.org/10.1029/2004GC000721

Highly depleted Hadean mantle reservoirs in the sources of early Archean arc-like rocks, Isua supracrustal belt, southern West Greenland

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