A rare earth element study of complex zircons from early Archaean Amı̂tsoq gneisses,...

Precambrian Research 126 (2003) 363–377

A rare earth element study of complex zircons from earlyArchaean Am̂ıtsoq gneisses, Godthåbsfjord,

south-west Greenland

Martin J. Whitehousea,∗, Balz S. Kamberba Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden

b Department of Earth Sciences, University of Queensland, St. Lucia, Qld 4072, Australia

Accepted 5 February 2003

Abstract

We present high spatial resolution ion-microprobe rare earth element (REE) data for discrete growth phases of complexpolyphase zircons from early Archaean Amı̂tsoq gneisses, outer Godthåbsfjord, SW Greenland. In Matsuda diagrams, the twomajor growth phases, >3.8 Ga cores and ca. 3.65 Ga rims, have steep positive slopes from La to Lu, prominent positive Ce anoma-lies and negative Eu anomalies that are consistent with growth in a melt. Exceptions to this are non-cathodoluminescent zircondeveloped between the cores and rims, sometimes truncating zoning in the cores, and late Archaean prismatic tip overgrowths,both of which exhibit flatter light REE (LREE) patterns and have small or no Eu anomaly, which we interpret as the result ofmetamorphism and/or small-degree, isolated partial melting. Our data support previous interpretations that the ca. 3.65 Ga zirconphase was generated in a melt, with the >3.8 Ga phase representing either original protolith zircons in a large degree partial meltor inherited zircons in an introduced magma. Regardless which of these two interpretations is correct for these, and similar, rocksin the outer Godthåbsfjord, the 3.65 Ga event will have profoundly affected isotopic systems and obscured beyond recognitionany earlier igneous features such as cross-cutting relationships, which may only be assigned a minimum 3.65 Ga age.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Lanthanides; Zircon; Petrogenesis; Archaean; Greenland; Godthåbsfjord

1. Introduction

Of the many controversies surrounding the absoluteage of geological features and events, the interpreta-tion of U–Pb zircon data from gneisses exposed onthe tiny island of Akilia (SW Greenland) is a text-book example. It has been claimed repeatedly thatgraphite inclusions in apatite crystals from Akiliacontain evidence for biologic C-processing more than

∗ Corresponding author. Tel.:+46-8-519-551-69;fax: +46-8-519-540-31.

E-mail address: [email protected] (M.J. Whitehouse).

ca. 3.85 Ga ago (Mojzsis et al., 1996; Nutman et al.,1997; Mojzsis and Harrison, 2002). The implicationsfor establishment of life on Earth at this unexpectedlyearly stage, when the Moon and the Earth were stillsubject to heavy meteorite bombardment (Schoenberget al., 2002), are evident and have been discussed atlength (e.g.Sleep et al., 1989; Chyba, 1993). Recentstudies of feldspar Pb-isotope systematics (Kamberand Moorbath, 1998), U–Pb apatite dating (Sanoet al., 1999), detailed field mapping of relative agerelationships (Myers and Crowley, 2000; Whitehouseand Fedo, 2003), U–Pb dating of better characterisedzircon (Whitehouse et al., 1999), and chemical and

0301-9268/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0301-9268(03)00105-0

364 M.J. Whitehouse, B.S. Kamber / Precambrian Research 126 (2003) 363–377

petrographical study of the apatite host rock (Fedo andWhitehouse, 2002a), have all documented that the evi-dence for life on Earth at or before ca. 3.85 Ga remainsat best inconclusive, if not fundamentally flawed.

There is wide agreement that much of the basicinvestigation, such as documentation of relative agerelationships, definition of the nature of host rocksand study of the role of multiple deformation andmetamorphism events should ideally have precededpublication of technically challenging C-isotope data.A further complicating factor was that the claim forlife before 3.85 Ga was made without supporting agedata (Mojzsis et al., 1996), which were publishedseparately at a later stage (Nutman et al., 1997).Surprising also was the fact that in the latter study,the dated zircons had not been imaged to documentinternal complexities and that the crucial claimed ig-neous cross-cutting relationship was only publishedas a low-quality field sketch which upon closer com-parison with the actual outcrop, transpired to bemirror-imaged (Myers and Crowley, 2000).

Even if disagreement persists about the significanceof C-isotope data as a biomarker (e.g.van Zuilenet al., 2002, who have re-evaluated such evidence fromthe Isua Greenstone Belt), the young U–Pb apatitedates, the more-radiogenic-than-expected feldsparPb-isotope ratios, the ambivalent cross-cutting rela-tionship and the inappropriate whole-rock chemistryof the claimed metasediment, the debate centred onAkilia should still be able to resolve the dispute aboutthe true age of the tonalitic gneisses that containzircons ranging in age from ca. 3.6 to >3.85 Ga.

There is strong evidence that tonalitic gneisses>3.8 Ga do exist in SW Greenland and that thebest-preserved examples occur in an area ca. 160 kminland from Akilia, to the immediate south of the IsuaGreenstone Belt (Nutman et al., 1999). It is also clearthat the type Am̂ıtsoq gneisses of the Godthåbsfjordarea are 3.65 Ga old (Nutman et al., 1996; Whitehouseet al., 1999) and that they belong to the youngest,most voluminous pulse of granitoid magmatismrecorded along the coast. In comparison with thearea south of the IGB, however, the coastal gneisseshave been much more strongly deformed and meta-morphosed during multiple tectonic events, the mostprominent of which occurred at ca. 3.6 and 2.7 Ga.The late Archaean event has clearly resulted in localanatexis (Friend et al., 1996) and juxtaposed pre-

viously unrelated terranes (Nutman and Collerson,1991).

Here we present new rare earth element (REE)abundance data for zircons from two tonalitic gneissesfor which emplacement ages in excess of 3.8 Ga havebeen claimed (Kinny, 1986; Nutman et al., 1996,1997) and contested (Whitehouse et al., 1999). Theaim of this study is to provide additional informationabout the growth histories of these complex zirconsin the hope of arriving at a more conclusive interpre-tation of the U–Pb dates.

2. Sample selection and geology

Since the Akilia debate is littered with claims andcounterclaims about the precise equivalence of stud-ied rocks, even when these come from the same gen-eral outcrop (e.g.Nutman et al., 2001; Whitehouseet al., 2001; Friend et al., 2002; Fedo and Whitehouse,2002b; Whitehouse and Fedo, 2003), we focus hereon two samples where all parties agree about the exactlocation, a fact that is confirmed by the very compa-rable U–Pb age data obtained by three sets of authors.

The first sample (GGU 110999) was collected bythe late V.R. McGregor in 1968 from the east coastof the Island of Angissorssuaq (2 km west of thesouthern tip of Akilia). The rock is a weakly banded,relatively homogeneous, grey tonalitic gneiss. Bycomparison with other tonalitic gneisses, GGU110999 is less strongly banded but none of the rockforming minerals (quartz, feldspar, biotite) retain anyoriginal magmatic features, being completely recrys-tallised. Splits of GGU 110999 were widely distributedand have been used to determine a large number ofisotopic and chemical parameters, both from wholerock and mineral separates. Results of three indepen-dent U–Pb zircon studies are available (Baadsgaard,1973; Kinny, 1986; Whitehouse et al., 1999).

The second sample (SM/GR/97/2) is from Akiliaitself and was collected in 1997 by S. Moorbath,(in the company of V.R. McGregor and S.J. Mojz-sis) from the southern contact of felsic gneiss withsupracrustal rock of the Akilia association. The exactsampling locality, at the extreme south-western endof the island, was documented byWhitehouse et al.(1999). This sample is considered to be equivalent tosample G88-66 ofNutman et al. (1996, 1997)who

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described it to be ‘slightly discordant’ with the ad-joining supracrustal rocks. Because evidence for theclaimed discordance at this locality is so weak andbecause a tectonic juxtaposition cannot be ruled out(Nutman et al., 1997; Myers and Crowley, 2000) fur-ther work on Akilia association chronology (Mojzsisand Harrison, 2002) has focused on samples fromthe northern contact, which we have deliberatelyavoided here in order to establish the basic in-sight from REE patterns in zircon from rocks sur-rounded by less controversy but of comparable bulkcomposition, mineralogy, metamorphic grade andantiquity.

3. Review of zircon structures and publishedU–Pb data

Zircons from GGU 110999 were originally dated bythe conventional U–Pb method byBaadsgaard (1973)and subsequently the subject of two ion-microprobestudies (Kinny, 1986; Whitehouse et al., 1999).Ion-microprobe U–Pb zircon data for SM/GR/97/2were obtained byWhitehouse et al. (1999)and furtherdata exist from what we believe to be the identicalrock (G88-66;Nutman et al., 1996, 1997). In all thesestudies, onlyWhitehouse et al. (1999)documentedinternal structures of the dated zircons prior to anal-ysis using an appropriate surface imaging technique(i.e. cathodoluminescence, CL). These structures arebriefly reviewed here.

3.1. Structure

Zircons from the two gneisses share many similari-ties. Remembering that in both cases, the zircons cho-sen for analytical work were naturally biased towardsthe largest grains, the observation is that all contain rel-atively large (generally rounded) cores with fine-scalegrowth banding (Fig. 1; interested readers are alsoreferred to Figs. 2 and 6 ofWhitehouse et al., 1999).

In GGU 110999 the cores are immediately sur-rounded by thin (<15�m) rims that show no cathodo-luminescence. In many cases this CL-dark phaseis seen to truncate banding in the cores (e.g. grain16, Fig. 1) and was termed a “corrosive rim” byWhitehouse et al. (1999). The narrow corrosive rimsare encased in much more substantial overgrowths

(termed “inner” rims byWhitehouse et al., 1999, anomenclature we retain here to differentiate themfrom a later overgrowth) that we estimate to makeup 30–50% of the total zircon mass. These rimsshow growth banding comparable to that of the cores.The outermost overgrowths consist of structurelessCL-bright zircon, in some cases forming euhedral tips.

In SM/GR/97/2 the cores are overgrown by moreheterogeneous material than in GGU 110999. How-ever, truncation and other mechanical processing ofthe cores prior to growth of the enveloping mantles(both CL-bright and CL-dark) are better developed(Fig. 1). The material surrounding the cores showsgrowth banding unlike the very thin, poorly devel-oped terminal tips that are less common than in GGU110999.

3.2. U–Pb data

The first attempt at dating zircon from GGU110999 pre-dated the recognition of the importance ofimaging (e.g.Grauert, 1974; surprisingly CL imagingwas not commonly applied until the mid-1990s) andwas therefore understandably unaware of the struc-tural complexities of the grains. A near-concordant207Pb/206Pb date of 3630± 2 Ma (recalculated usingpresently accepted decay constants) was obtained byBaadsgaard (1973)from a bulk fraction of 0.5–0.8 g.The ion-microprobe study byKinny (1986) later re-vealed a large spread in concordant zircon dates rang-ing from 3.58 to 3.83 Ga.Whitehouse et al. (1999)confirmed this spread in ages, but with the benefit ofCL imaging to reveal distinct zircon growth phases,they were able to show that cores yielded the oldestdate of 3801±20 Ma, while the inner rims clustered at3668± 17 Ma. In addition, the outermost tips yieldedlate Archaean ages.

Nutman et al. (1996)reported U–Pb data for zirconsfrom sample G88-66, described by them as “mostlyoptically homogeneous”, but whichWhitehouseet al. (1999)later showed to have a structural com-plexity comparable to GGU 110999. Both groupsobtained concordant dates for the cores in excessof 3.8 Ga, approaching 3.84 Ga (Whitehouse et al.,1999) and 3.87 Ga (Nutman et al., 1996). Many of thecores, however, yield younger apparent ages (downto 3.64 Ga). BecauseNutman et al. (1996)workedfrom the assumption of homogeneous grains, they


Fig. 1. Cathodoluminescence images of selected grains from samples GGU 110999 and SM/GR/97/2. Analytical site labels corresponddirectly to those used in the U–Th–Pb study ofWhitehouse et al. (1999); unlabelled ellipses show the location of U–Th–Pb analyseswhich have not been analysed for REE, Y and Hf in this study. Complete CL documentation may be found inWhitehouse et al.(1999).

did not differentiate between rims and cores whilstWhitehouse et al. (1999)determined a mean age of3648± 9 Ma for the substantial rims that they docu-mented as enveloping the cores. In this latter study,

a single analysis of an outermost tip yielded a lateArchaean age of 2749± 9 Ma.

In summary, zircons from GGU 110999 andSM/GR/97/2 show remarkably similar structures and


age distribution which, however, have been subjectto two very different interpretations.Kinny (1986)andNutman et al. (1996, 1997)claimed that the old-est dates of 3.82 (GGU 110999) and 3.87 (G88-66)most likely correspond to the age of igneous crys-tallisation and that the spread in dates towards 3.6 Gawas caused by Pb-loss during one or more thermalevents.Whitehouse et al. (1999)on the other handused CL imaging to reveal distinct zircon growthphases and, while each of these phases itself showeda range of ages attributed to Pb-loss, the oldest age(s)from each phase were considered to represent thetime of a distinct zircon growth episode. Truncationof fine-scale oscillatory zoning of the cores by therims as well as the fine-scale zoning of the latterfurther suggested that 3.65–3.67 Ga rim growth oc-curred in a melt.Whitehouse et al. (1999)arguedthat this could be explained either by entrainmentof xenocrystic zircons (from country rock) into a3.65–3.67 Ga pluton, or (Whitehouse et al., 2001) that>3.8 Ga protoliths were subjected to metamorphictemperatures that permit large-scale in situ melting at3.65–3.67 Ga.

If the rocks are only 3.65–3.67 Ga, claims forigneous discordances at >3.8 Ga cannot be sup-ported and back-correction of radioactive decaybeyond 3.65–3.67 Ga yields spurious information(Moorbath et al., 1997). Even if the protoliths areindeed >3.8 Ga, the severity of metamorphic over-print and remobilisation as a melt-crystal mushwould obscure all original igneous features, includingcross-cutting relationships, and isotopic clocks wouldcertainly have been disturbed at 3.65–3.67 Ga beyondrecovery.

However, if the rocks are indeed >3.8 Ga and theca. 3.65 Ga rims grew from an in situ melt, an out-standing geochronological conundrum is that duringthe well-documented late Archaean event (e.g.Friendet al., 1996), which also reached partial melting con-ditions, the zircons seem to have responded in an en-tirely different way. In other words, why was theresuch strong response of the zircons to the proposed3.65–3.67 Ga overprint both in terms of new zircongrowth and/or Pb-loss, when the 2.7 Ga event left thezircons largely unaffected? Below we test whetherthe distribution of REE in the various portions ofthese zircons could help to explain this unresolvedissue.

4. Rare earth element investigation

4.1. REE incorporation into zircon

Zircon most commonly occurs either as a precip-itate from a melt (“igneous” zircon) or as a result ofmetamorphism. As pointed out byHoskin and Black(2000), strictly “metamorphic” zircon might resultfrom a number of processes including precipitationfrom metamorphic fluids, sub-solidus nucleation andcrystallisation utilising Zr and Si released by thebreakdown of other minerals, and recrystallisationof original igneous protolith zircon. These authorsconsidered that zircon precipitated from an anatec-tic melt, while the direct result of metamorphism, isstrictly “igneous” in definition, a point with whichwe are in general agreement. The goal of the REEstudy of zircon reported here, is to use REE pro-files, supported by CL imaging, to distinguish be-tween these different origins for specific U–Pb-datedgrowth/recystallisation phases within polyphase zir-con grains. In order to achieve this goal, it is necessaryto understand the controls on REE partitioning intozircon in different environments.

Igneous zircons are typified by a steep positiveslope from La–Lu in a Matsuda diagram together withprominent positive Ce and negative Eu anomalies (e.g.Hinton and Upton, 1991; Maas et al., 1992; Barbeyet al., 1995; Hoskin and Ireland, 2000; Whitehouseand Kamber, 2002). Since zircon preferentially incor-porates the smaller ionic radius heavy REE (HREE)into its structure, the observed distribution of trivalentREE between zircon and whole rock in many casesfollows that predicted by crystal chemical proper-ties (i.e. lattice strain induced by incorporation of amis-fitting ion; Whitehouse and Kamber, 2002). Thepositive Ce anomaly may be attributed to the presencein a melt of small amounts of Ce4+ (Schreiber et al.,1980), a strongly compatible ion in zircon; the neg-ative Eu anomaly probably records overall depletionof a melt in Eu by plagioclase fractionation. Some-times, however, igneous zircon has light REE (LREE)concentrations at higher than expected levels, a fea-ture which may be accounted for by several factorsincluding disequilibrium crystallisation and/or laterdisturbance. Such unsystematic and over-proportionalincorporation of LREE has been documented recentlyin pristine, undisturbed zircons from a 3.81 Ga tonalite


from south of the Isua Greenstone Belt (Whitehouseand Kamber, 2002).

REE contents of zircons crystallised from an ana-tectic melt will be controlled by similar factors to thosedescribed above. Differences from the typical igneouszircon REE pattern are likely, however, because thecomposition of the partial melt itself, particularly forsmall-degree partial melts, may be quite distinct fromthe melted-protolith.

A number of recent studies (Schaltegger et al.,1999; Bea and Montero, 1999; Rubatto, 2002;Whitehouse and Platt, 2003) have documented theREE pattern of zircons that have grown in equilibriumwith garnet during metamorphism. These are char-acterised most notably by strong depletion in HREEwhich are preferentially partitioned into garnet. Therole of other REE-bearing minerals, in particularLREE-rich phases such as monazite and allanite re-mains to be investigated in detail, but it seems likelythat the involvement of such minerals in metamor-phic reactions during which zircon is simultaneouslygrowing will have a strong influence on the resultingzircon REE pattern.

Lastly, in the case of sub-solidus recrystallisation ofprotolith zircon,Hoskin and Black (2000)have doc-umented changes in REE (and other trace elements)that occur as a result of a recrystallisation front pro-gressing through zircon. In this case, it appears thatthe more incompatible trace elements (i.e. the LREEin zircon) are initially displaced from the protolith zir-con, being concentrated in zircon formed behind therecrystallisation front.

4.2. Analytical details

Determination of the REEs together with Y and Hfwere made using a Cameca IMS1270 ion-microprobeat the Swedish Museum of Natural History (Nordsimfacility). Where possible, sites previously analysedfor U–Th–Pb geochronology were re-occupied inthis study. Primary beam size, shape and ion currentare the same as those used for U–Th–Pb analysis ofzircons (ca. 30 mm slightly elliptical flat-bottomedcrater, 5 nA O2

− current). Analyses were performedat low mass resolution using energy filtering toremove molecular interferences and a single ioncounting electron multiplier was used for detection.Concentrations were normalised to NIST SRM610

glass using the working values ofPearce et al. (1996)and chondrite normalisation uses the values ofAndersand Grevasse (1989). Basic analytical procedures aregiven inWhitehouse and Platt (2003).

Detection limits were approached for some of thelight REE (especially La, Pr, Nd) in the original set ofanalyses, often manifested by over-correction of in-terfering ZrSiO species on La and Pr (Ce is also inter-fered but is generally enriched in zircons) resulting innegative concentrations. Since the MREE–HREE ofthese analyses are reliable, a series of replicate anal-yses were performed using simultaneous detection insix electron multipliers (five adjustable EMs and thefixed “axial” EM) positioned to measure139La, 140Ce,141Pr, 144Nd (in the axial EM), 152Sm and 153Eu(Whitehouse and Ilyinsky, 2002). High mass resolu-tion (ca. 4200 MRP) was used on the five adjustableEMs, effectively separating La, Ce and Pr from theinterfering ZrSiO species, together with moderateenergy filtering (−70 V offset from 10% low energyedge) to remove molecular interferences on the otherspecies. In this mode of operation, all ion counts onLa, Ce and Pr represent the pure element and so arenot subject to any correction. Inter-multiplier gains,measured by sequentially putting a30Si beam of ca.105 cps into each EM, are<±1% during the analyt-ical session, introducing an error that is considerablylower than the counting statistic error on the smallLREE peaks. Ratios of each element were made to152Sm and the original concentration of Sm was thenused to calculate LREE concentrations. The accuracyof this technique was confirmed by comparing LREEpatterns obtained on the Geostandards 91500 zirconwith those obtained from a complete analysis of allREE. Precisions using this combined multicollector—dynamic monocollector routine remain similar tothose reported byWhitehouse and Kamber (2002),ranging from a few percent for the enriched HREE to10–20% for the least enriched LREE.

4.3. Results

4.3.1. GGU 110999REE data are listed inTable 1and are plotted as

Matsuda diagrams inFig. 2. Cores (interpreted to be>3.8 Ga), enveloping rims (∼3.67 Ga) together withan analysis of a corrosive rim that truncates zoning ina core, and late Archaean tips are plotted separately

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Table 1Zircon REE, Y and Hf concentrations (ppm)

Grain ID Y Hf La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

GGU 110999Cores

1b 239 13110 0.003 4.55 0.015 0.28 0.76 0.17 5.30 1.73 23.1 8.23 40.9 8.99 90.7 21.83c 340 10510 0.005 5.33 0.038 0.63 1.4 0.41 5.05 2.10 23.0 9.12 4.00 9.11 79.3 18.94a 2260 9624 0.042 15.6 0.34 5.7 12 3.0 65.6 20.7 228 75.7 306 58.6 442 92.55a 626 10730 0.008 5.53 0.082 1.5 3.2 0.73 20.8 5.99 77.3 24.6 103 22.3 174 40.27a 801 10090 0.014 6.69 0.072 1.3 3.0 0.71 12.2 5.22 59.5 22.7 99.1 20.2 170 39.58b 469 9195 0.002 4.81 0.027 0.61 1.7 0.4 9.83 3.39 40.1 15.5 69.2 15.4 127 29.89b 539 10480 0.031 7.17 0.048 0.83 2.0 0.54 11.4 4.25 56.0 18.3 89.0 17.6 141 33.910a 620 8865 0.055 5.56 0.049 0.64 1.7 0.49 9.80 4.11 49.8 20.1 93.0 21.1 192 43.412a 899 9545 0.057 6.19 0.088 1.3 3.0 0.76 18.2 6.06 85.4 31.2 147 29.5 252 57.613a 455 11100 0.007 6.30 0.033 0.67 1.8 0.36 14.4 4.37 51.9 16.8 79.3 17.0 142 31.816b 450 9050 0.014 8.42 0.055 0.65 1.6 0.45 5.46 2.85 34.7 14.4 62.5 14.0 114 27.4

Core corrosion16d 86.6 27960 0.37 5.69 0.35 1.7 0.8 0.26 1.02 0.362 5.05 2.06 13.7 3.79 45.7 14.1

Inner rims3b,2 115 12630 0.005 3.32 0.012 0.15 0.26 0.071 1.31 0.748 7.03 2.96 18.3 5.00 47.9 12.64b 175 13980 0.005 8.49 0.020 0.28 0.55 0.097 2.46 0.978 17.5 5.86 29.3 8.77 76.2 20.55b 195 11710 0.004 7.91 0.020 0.32 0.60 0.10 3.29 1.27 18.7 6.58 29.6 7.65 71.5 18.17b 311 14280 0.006 12.4 0.029 0.43 0.87 0.17 3.51 1.35 24.7 7.62 41.4 9.99 98.0 24.58a 251 15210 0.006 9.65 0.023 0.27 0.68 0.15 3.85 1.51 21.5 7.42 41.4 10.4 100 25.810b 171 11480 0.005 7.50 0.015 0.26 0.51 0.097 3.61 0.985 13.0 5.63 27.9 7.08 70.0 16.612e 317 13140 0.035 13.7 0.050 0.58 1.0 0.2 6.00 1.94 25.6 9.76 50.6 13.2 123 31.313b 397 13210 0.008 12.7 0.036 0.54 1.1 0.19 4.33 1.85 29.0 9.57 48.9 12.3 116 29.416a,2 218 12560 0.006 9.49 0.026 0.32 0.72 0.12 3.39 1.30 17.5 6.93 36.4 9.57 91.2 22.0

Late-Archaean tips10c 64.7 12240 0.079 1.04 0.033 0.071 0.10 0.075 0.725 0.290 6.21 1.97 12.7 3.44 38.2 10.813c 78.5 12160 0.013 0.955 0.008 0.028 0.12 0.10 0.375 0.312 3.97 1.72 10.3 2.75 29.1 8.7116c 53.1 10970 0.001 0.74 0.005 0.021 0.08 0.056 0.428 0.231 3.49 1.53 8.70 1.97 20.5 6.25

SM/GR/97/2Cores

2d 441 9705 0.023 5.57 0.047 0.71 1.5 0.46 9.85 2.69 33.5 14.2 66.2 15.0 134 33.43a 330 9974 0.027 11.3 0.082 1.1 2.1 0.72 4.14 2.21 32.7 9.96 47.2 9.98 99.7 24.35b 1380 11890 1.9 34.6 2.3 17.7 9.5 4.4 30.9 10.8 128 47.5 222 50.0 474 1006a 536 9472 0.036 4.32 0.11 1.5 2.6 0.69 6.63 3.60 45.1 16.6 69.0 17.3 155 35.86a2 552 10590 n/d 8.27 0.063 1.3 2.6 0.53 6.44 3.65 45.4 17.2 78.4 18.3 156 39.2

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Table 1 (Continued )

Grain ID Y Hf La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

8c 520 9862 0.026 4.67 0.085 1.1 1.9 0.63 9.90 3.65 49.0 17.2 81.3 17.1 165 37.39b 913 9763 0.050 8.02 0.27 3.4 5.8 1.7 19.2 6.80 64.7 25.6 108 23.3 220 48.710b 546 10920 0.014 7.08 0.057 0.77 1.8 0.61 10.5 3.73 49.2 19.0 86.6 20.0 190 45.211a2 236 8373 0.32 7.30 0.28 2.3 1.7 0.52 6.29 1.84 23.6 8.70 37.5 8.23 72.1 16.711a3 311 9100 0.10 7.58 0.10 0.87 1.8 0.48 7.31 2.30 30.7 10.6 48.0 11.0 101 23.212b 368 10640 0.02 7.01 0.044 0.58 1.3 0.40 4.97 2.05 22.2 9.56 42.5 9.92 95.5 22.513b 760 11790 0.10 9.83 0.12 1.1 2.2 0.64 16.5 5.70 77.0 30.5 139 30.2 273 65.113b3 409 12180 0.13 8.03 0.051 0.58 1.2 0.36 9.01 3.07 41.8 14.3 70.1 17.1 148 35.214a 466 11930 0.023 6.54 0.065 0.89 1.8 0.54 9.50 3.06 37.2 14.6 67.8 16.0 150 36.515a 772 8153 0.054 7.45 0.24 3.4 5.5 1.7 22.1 6.56 71.0 24.4 103 22.1 186 40.916a 424 10590 0.020 5.09 0.05 0.57 1.5 0.51 9.87 3.26 41.1 14.3 68.6 15.6 144 33.2

CL-dark rims3b 268 13520 0.010 6.89 0.031 0.33 0.61 0.11 2.69 1.44 19.9 7.87 38.5 9.66 91.6 23.86b 170 11550 0.097 1.77 0.061 0.24 0.36 0.27 1.23 0.666 9.30 4.30 20.8 5.77 52.6 14.36b3 139 10800 0.12 1.46 0.030 0.10 0.22 0.18 1.27 0.676 9.75 3.79 19.3 5.07 43.3 10.113a 92.6 17350 0.60 3.81 0.33 1.8 0.56 0.25 2.66 0.568 7.53 3.32 19.3 5.43 65.6 20.515b 218 11640 0.10 5.83 0.040 0.34 0.44 0.12 3.95 1.28 19.8 7.04 36.1 9.62 93.4 22.516b 180 11600 0.017 6.78 0.035 0.23 0.52 0.14 2.59 0.959 12.1 5.04 24.7 6.08 61.8 16.3

CL-bright rims11b 487 13730 0.018 9.87 0.050 0.60 1.4 0.14 6.12 3.07 51.3 15.1 72.2 15.8 137 29.914b 425 13180 0.021 12.3 0.055 0.60 1.5 0.12 9.55 3.55 46.3 14.9 65.5 16.5 130 27.315c 212 14420 0.041 9.08 0.088 0.74 1.0 0.20 4.2 1.56 21.4 7.07 34.8 8.30 75.8 17.617a 355 15600 0.021 12.2 0.049 0.57 1.4 0.18 7.67 2.94 37.8 13.4 60.4 14.6 115 29.1

Italicised numbers for La, Ce, Pr, Nd and Eu are older analyses that did not use the multicollector method for LREE determination. Grain ID nomenclature correspondsdirectly to that used byWhitehouse et al. (1999)with the exception of GGU 110999 analysis 12e which refers to a different analytical site within the same growth phase asdated location 12b. Number suffixes refer to analyses repeated in a particular location, n/d: not determined.


Fig. 2. Chondrite-normalised REE patterns for zircons from sample GGU 110999. REE, ordered according to increasing atomic number(from La–Lu) with spacing corresponding to their ionic radius in eight-fold co-ordination (Shannon, 1976; indicated in picometres on theupperx-axis). (a) Zircon cores; (b) inner rims and corrosive rim (analysis 16d); (c) late-Archaean tip overgrowths. In (b) and (c) the coreanalyses from (a) are reproduced for reference as grey lines without symbols.


(Figs. 2a–c, respectively). The first observation is thatthe late Archaean tips are readily identified on accountof the absence of a negative Eu anomaly. The otherfeature of the tips is that their abundance of La and Prare highly unsystematic while the patterns resemblethose of the rims and cores for REE heavier than Sm.

The cores display parallel patterns with coherentHREE over LREE enrichment and comparable neg-ative Eu and positive Ce anomalies, confirming theinterpretation of an original magmatic origin. Thereis variability in the total abundance but patterns aresub-parallel (Fig. 2a), unlike in some disturbed casessuch as that demonstrated recently byWhitehouseand Kamber (2002). The ca. 3.67 Ga rims (Fig. 2b)also show very coherent patterns that are sub-parallelto those of the cores. Eu and Ce anomalies are alsocomparable but there is a clear distinction in over-all abundance. The rims are, on average, depleted intotal REE by a factor of three (in strictly trivalentREEs) compared to the cores. The one analysis inFig. 2bthat is different (spot 16d) is clearly identifiedas a CL-dark corrosive rim that truncates zoning in a>3.8 Ga core and is quite distinct from the ca. 3.67 Garims, in that it resembles much more closely the lateArchaean tips (i.e. absence of a Eu anomaly andflatter LREE patterns). The coherence in the degreeof LREE discrimination is more clearly illustratedin Fig. 3, where LREE/MREE and MREE/HREEratios are plotted. Both the >3.8 Ga cores and the ca.3.67 Ga rims define tight clusters that are distinctivebut overlap at the extreme values. Late Archaean tipsplot separately (at the high LREE/MREE end) as doesthe corrosive rim (analysis 16d).

4.3.2. SM/GR/97/2REE data are listed inTable 1and shown as Mat-

suda diagrams inFig. 4. Compared to cores of GGU110999, those of the Akilia gneiss sample are less co-herent. They share the general features of igneous zir-con (steep HREE enrichment, negative Eu and posi-tive Ce anomalies) but there is considerably more scat-ter. This is also reflected in plots of LREE/MREE andMREE/HREE ratios (Fig. 5) where it becomes obvi-ous that the rims have more coherent patterns than thecores.

A feature that the SM/GR/87/2 zircons share withthose of GGU 110999 is that the rims (both CL-brightand CL-dark;Fig. 4b) have on average two to three

Fig. 3. Plots of LREE/MREE ((La/Gd)CN and (Pr/Gd)CN) againstMREE/HREE ((Gd/Yb)CN) for zircons from sample GGU 110999.

times lower REE abundances than the cores (Fig. 4a).With two exceptions, the patterns of the rims are morecoherent and show a higher degree of parallelism thanthose of the cores. The two anomalous patterns (spots13a and 6b3) have much lower Th/U ratios (0.04 and<0.01, respectively) than the rest of the spots (aver-age Th/U of 0.55). These patterns show weaker Euanomalies and a preferential incorporation of LREE,features that characterise the late Archaean zircon tipsas well as the >3.67 Ga CL-dark embayment zirconrepresented by analysis 16d of GGU 110999.

5. Discussion

Combined with the CL image information, theREE patterns help to further refine interpretation ofthe previously obtained U–Pb age data for zirconsof these two samples. In both samples,Whitehouseet al. (1999)discovered clear discordances, truncations


Fig. 4. Chondrite-normalised REE patterns for zircons from sample SM/GR/97/2. (a) Zircon cores and (b) rims, with core analyses from(a) reproduced for reference as grey lines without symbols.

and embayments of structurally younger zircongrowth phases into older cores. This observation alonestrongly argues against an originally homogenous>3.8 Ga zircon population, whose U–Pb integrity wasdisturbed during one or several thermal events (at ca.3.6 Ga). The REE data fully support the structuralevidence. In both rocks, the 3.65–3.67 Ga rims havesubstantially lower average REE abundance than thecores, implying that these portions of zircon crys-tallised from a compositionally different reservoir,specifically one that was more depleted in total REE.

The late Archaean metamorphic tips in sampleGGU 110999 have clearly distinctive REE patterns,which lack a strong negative Eu anomaly and showover-proportional LREE abundance. A comparablepattern from sample GGU 110999 (analysis 16d,

Figs. 1 and 2) deserves special attention. This wasobtained from a CL-dark embayment into a >3.8 Gacore, clearly overgrown by a 3.6 Ga finely zoned sub-stantial rim that makes up more than 40% of the esti-mated volume of the entire grain.Hoskin and Black(2000) have attributed similar REE patterns to thoseof analysis 16d, and other chemical (U-enrichment,low Th/U, possibly higher Hf) and morphologicalcharacteristics to sub-solidus recrystallisation. In thisprocess, a recrystallisation front proceeds throughthe zircon erasing original igneous oscillatory zoningand mobilising more incompatible trace elements (i.e.LREE, Th, U) into the recrystallised zones behindthe front. The observation that a 3.67 Ga zoned rimsurrounds both the core and its recrystallised regionsin zircons from GGU 110999 strongly suggests that


Fig. 5. Plots of LREE/MREE ((La/Gd)CN and (Pr/Gd)CN) againstMREE/HREE ((Gd/Yb)CN) for zircons from sample SM/GR/97/2.

the >3.8 Ga core experienced a metamorphic over-print before 3.67 Ga. Analysis 16d yielded discordantU–Pb data (with a207Pb/206Pb age of 3.74 Ga) whichcannot be used to directly constrain the age of thismetamorphic event. However, the important impli-cation is that zircon dissolution, truncation, and re-crystallisation occurred before 3.67 Ga (the mean ageof the faintly zoned rims that surround the CL-darkembayments found in other grains as well). Anal-ysis 16d is also characterised by a very low Th/U(0.03), as well as high Hf (Table 1). By analogy, weinterpret the two anomalous patterns from rims inSM/GR/97/2 zircons (analyses 13a and 6b3) to reflecta similar sub-solidus recrystallisation process. Bothspots were obtained from CL-dark areas that corrodedthe old cores and both gave discordant U–Pb dates.The observation of a likely metamorphic overprint ofthe zircon cores before growth of the 3.65–3.67 Garims is also in conflict with the original interpretationof <3.8 Ga dates as simply reflecting Pb-loss fromoriginally older zircon.

In both samples, but particularly 110999, the coreshave parallel REE patterns with coherent HREE/MREE/LREE discrimination trends (Figs. 3 and 5).This certainly argues against inheritance of geneti-cally unrelated zircons from pre-existing sources suchas sandstone or arkose. Core and rim patterns in bothsamples are sub-parallel. This largely reflects the zir-con REE distribution coefficients and not the REEpatterns of the melts with which they were in equilib-rium with (Hoskin and Ireland, 2000). Nevertheless,there is no obvious reason why the cores and rimscould not have grown from similar material.

The key observations for a refined interpretation ofthe 3.65–3.67 Ga rims in both samples are that theserims make up a substantial portion of the entire grains,that they show fine-scale zoning, and that their REEpatterns are distinct from the clearly metamorphic lateArchaean tips. The implications are that the rims arenot simply solid-state recrystallised portions of oldergrains but that they have grown from a melt. Whetherthis melt, which crystallised between 3.65 and3.67 Ga, originated from partial melting of a >3.8 Gaprotolith and remained largely in situ, or whether thenewly grown material requires input of new batchesof unrelated melt is difficult to establish. In the caseof in situ melting, a major issue is that of Zr mass bal-ance. Recalling that the 3.65–3.67 Ga rims make upsubstantial portions of the grains as they are preservedtoday and that zircon dissolution occurred prior to3.67 Ga along the cores, it would have to be arguedthat the 3.65–3.67 Ga event liberated an additional Zrreservoir that was not available to zircon crystallisa-tion before 3.67 Ga. Additional zirconium could havebeen derived from dissolution of small zircons or bymelting of a relatively Zr-rich rock forming mineralsuch as amphibole of clinopyroxene. However,Griffinet al. (1980)have argued that the >3.67–3.8 Ga meta-morphic event (which we believe to be responsiblefor the CL-dark corrosive rims) reached granulitefacies and stabilised pyroxene in the gneisses. Fur-thermore, the similarity in REE pattern between theca. 3.65 Ga rims and >3.8 Ga cores except for loweroverall abundances in the rims, argues against insitu partial melting since this process, except at veryhigh percentages of melting, would likely generatea melt, and hence any zircon crystallised from it, ofquite distinct composition from that of the protolith.The likely mobility of an extensively melted rock


has important implications for the interpretation offield relationships that are proposed to pre-date themelting, as discussed previously byWhitehouse et al.(2001).

The alternative explanation is that >3.8 Ga(tonalitic?) gneisses that had already experienced ametamorphic event became entrained in the muchmore voluminous batholiths of 3.65 Ga-type Amı̂tsoqtonalites which dominate the coastal outcrops in theouter Godthåbsfjord. In this scenario, host gneissesto >3.8 Ga zircons were disaggregated and almostcompletely assimilated in the younger batholiths andthe majority of (smaller) zircons were dissolved intothe hybrid melt. Only some of the largest zirconsavoided complete dissolution and eventually wereovergrown by new zircon that crystallised upon cool-ing of the hybrid melt. This scenario offers the mostplausible explanation for the feldspar Pb-isotope con-straints from GGU 110999 (Kamber and Moorbath,1998) since the Pb-isotopic signature, while hybrid,would be dominated by the newly introduced melt.(Despite persistent claims to the contrary by work-ers who have yet to provide a quantitative treatment,e.g. Mojzsis and Harrison, 2002, a metamorphicoverprint at 3.65 Ga cannot simply shift a >3.8 Gadatapoint to the position of GGU 110999 in commonPb-isotope space unless metamorphism was accom-panied by >90% U-loss.) Similarly, such a model ofwholesale assimilation by a voluminously dominantca. 3.65 Ga melt can readily account for ca. 3.65 Gaconcordant whole-rock Rb–Sr, Sm–Nd and Pb–Pbage regressions reported from the Amı̂tsoq gneisses(Kamber and Moorbath, 1998), with at least some ofthe greater-than-analytical scatter in these regressionsreflecting incomplete assimilation of the older compo-nent. In addition to implications for the interpretationof geochemical and isotopic data, a model involvingsuch widespread assimilation and generation of aca. 3.65 Ga hybrid melt, containing greater or lesseramounts of older material, renders meaningless anyinterpretation of field relationships predicated on theassumption that the oldest (in our model, inherited)zircons date the most recent magmatic and/or intru-sive phase (Nutman et al., 1997, 2002, cf. Whitehouseet al., 2001; Whitehouse and Fedo, 2003).

A potential problem with the partial dissolutionhypothesis forwarded above is that according to ex-perimental constraints ofWatson and Harrison (1983),

tonalites should be so undersaturated in Zr at 950◦Cthat no xenocrystic zircon is expected to survive. Onthis basis, it has therefore been claimed (Mojzsis andHarrison, 2002) that widespread zircon inheritance inthe Godthåbsfjord Am̂ıtsoq gneisses is unlikely. How-ever, the fact remains that in many examples whereindependent age estimates exist for Zr-undersaturatedtonalites, appreciable amounts of inherited zirconsare nevertheless present (e.g.Miller et al., 2000),indicating that the experimental finding does notstrictly apply in nature. Specifically for the Amı̂tsoqgneisses, we envisage assimilation of older gneissesnot from the melt source but along the walls and roofof the magma chamber where lower melt tempera-tures imply lower Zr solubility. Considerations of Zrsaturation provide a general measure of likelihood ofinherited zircon survival and we do not contest thatmany, perhaps the majority, of zircons would havebeen dissolved into the 3.65–3.67 Ga melt, but be-cause mineral separation for ion-microprobe datingheavily biases towards the rare large grains, the pre-ponderance of cores in dated crystals is not surprising.A mineral separate dominated by smaller zircons,with much smaller proportions (if any) of older cores,could explain why the bulk zircon date for GGU110999 ofBaadsgaard (1973)is ca. 3.63 Ga. From awhole-rock isotopic point of view, the possibility re-mains that many of the 3.65 Ga Amı̂tsoq gneisses thatapparently lack inherited zircon cores may also haveentrained and assimilated older gneisses. Therefore,the relatively few gneisses with zircon cores in ex-cess of 3.8 Ga might indeed represent the exceptionalcases in which portions of some pre-existing zirconssurvived in a Zr-undersaturated crystal-melt mush.

6. Summary

REE patterns of zircons provide additional infor-mation for interpretation of U–Pb data from some ofthe Earth’s oldest gneisses. They rule out the possibil-ity that the mass of zircon that we now see was all of>3.8 Ga igneous origin and suffered Pb-loss during a3.6 Ga thermal event. The 3.65–3.67 Ga zircon man-tles which envelope the old cores share similar REEpatterns with undisputed igneous cores and are like-wise interpreted to have grown from a melt. Whetherthis melt represents injection of widely documented


new pulses of 3.65 Ga-type Amı̂tsoq gneisses orwhether it formed in situ remains unresolved. In thelatter case however, the similarity of REE patternsbetween the two zircon growth phases would haverequired high degrees of partial melting, resulting ina mobile rock that generated cross-cutting relation-ships dating from the time of melting. We consider itinadvisable, if not impossible, to use age data fromsuch complex zircons, in combination with purportedcross-cutting igneous relationships, to constrain a>3.65 Ga age for rocks claimed to host evidence forEarth’s earliest life. Indeed it is probable that the3.65 Ga event will have profoundly affected the chem-ical and isotopic integrity of, and obscured beyondrecognition, any original cross-cutting relationshipsassociated with the studied samples and with similargneisses in the outer Godthåbsfjord region.

Acknowledgements

We thank Stephen Moorbath for providing sam-ples and for comments on an earlier draft of thismanuscript, as well as John Hanchar, Paul Hoskinand Roland Maas for their thorough and constructivereviews. Thanks are also expressed to the Danish Nat-ural Science Research Council, the Commission forScientific Research in Greenland and the Bureau ofMinerals and Petroleum, Nuuk, Greenland, for gen-erous support to the Isua Multidisciplinary ResearchProject, from which this is a contribution. BSK issupported by the DVC Research at the University ofQueensland. Nordsim is operated and financed underan arrangement with the joint Nordic research coun-cils (NOS-N) and the Swedish Museum of NaturalHistory; this is Nordsim publication #81.

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