Assigning Dates to Thin Gneissic Veins in High-Grade Metamorphic Terranes: A Cautionary Tale from...

Assigning Dates to Thin Gneissic Veins inHigh-Grade Metamorphic Terranes: ACautionary Tale from Akilia, SouthwestGreenland

MARTIN J. WHITEHOUSE1* AND BALZ S. KAMBER2

1SWEDISH MUSEUM OF NATURAL HISTORY, BOX 50007, SE-104 05 STOCKHOLM, SWEDEN

2ADVANCED CENTRE FOR QUEENSLAND UNIVERSITY ISOTOPE RESEARCH EXCELLENCE (ACQUIRE),

UNIVERSITY OF QUEENSLAND, ST LUCIA, QLD 4072, AUSTRALIA

RECEIVED MAY 7, 2004; ACCEPTED SEPTEMBER 6, 2004ADVANCE ACCESS PUBLICATION NOVEMBER 19, 2004

A granodiorite from Akilia, southwest Greenland, previously

suggested to date putative life-bearing rocks to �3�84 Ga, is re-investigated using whole-rock major and trace-element geochemistry,

and detailed cathodoluminescence image-guided secondary ion mass

spectrometer analyses of zircon U–Th–Pb and rare earth elements.

Complex zircon internal structure reveals three episodes of zircon

growth and/or recrystallization dated to c. 3�84 Ga, 3�62 Ga and2�71 Ga. Rare earth element abundances imply a significant role forgarnet in zircon generation at 3�62Ga and 2�71Ga. The 3�62Gaevent is interpreted as partial melting of a c. 3�84Ga grey gneiss

precursor at granulite facies with residual garnet. Migration of this

3�62 Ga magma (or melt–crystal mush) away from the melt source

places a maximum age limit on any intrusive relationship. These

early Archaean relationships have been complicated further by iso-

topic reworking in the 2�71Ga event, which could have included afurther episode of partial melting. This study highlights a general

problem associated with dating thin gneissic veins in polyphase

metamorphic terranes, where field relationships may be ambiguous

and zircon inheritance can be expected.

KEY WORDS: Archaean; geochronology; Greenland; secondary ion mass

spectrometry; zircon

INTRODUCTION

A major source of controversy in the debate about theorigin and evolution of life on Earth is the assignment of

both relative and absolute ages to key lithologies. Indeed,the significance of actual evidence for biogenicity or forkey steps of biological evolution is impossible to evaluatein cases where no consensus can be reached about relat-ive and absolute age. Nowhere is this problem morepronounced than on the island of Akilia, southwestGreenland, where evidence for Earth’s earliest life hasbeen claimed on the basis of isotopically light carbon,occurring as graphite inclusions hosted by apatite, in arock originally interpreted as a banded iron formation(Mojzsis et al., 1996). Since this claim was first published,several lines of evidence have emerged that question boththe biogenicity and antiquity of the Akilia example, to theextent that most specialists now prefer to focus the dis-cussion about earliest life on the much better preservedand unambiguous volcano-sedimentary sequences in the(near-) contemporaneous Isua Greenstone Belt and thesomewhat younger Warrawoona Group in the EastPilbara Craton. Despite this sobering outcome of theAkilia debate, many lessons can still be learnt, e.g.regarding establishment of relative and absolute agerelationships of key lithologies.

All lithologies on Akilia have experienced two majorepisodes of metamorphism at 3�60–3�65 and 2�71 Ga,reaching granulite and upper amphibolite facies respect-ively, as well as later thermal and minor structural over-prints. In cases where age relationships are determinedwith granitoid rocks, such high-T metamorphism andhigh-strain deformation conspire to inextricably link

*Corresponding author. Telephone: þ46 8 519 551 69. Fax: þ46 8

519 540 31. E-mail: [email protected]

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relative and absolute ages. First, quartz and feldspar, themain constituents of granitoids, deform plastically undersuch conditions (e.g. Pryer, 1993) and, in the presence ofan aqueous fluid, have potential to melt at temperaturesexceeding 670�C (e.g. Clemens & Vielzeuf, 1987). Oncean anatectic melt is present, the crust is further weakened(Hollister & Crawford, 1986), which facilitates andenhances deformation. As a result, partially moltenrocks can easily lose their original relative field contextand assume discordant relationships that preferentiallyreflect the latest preserved episode of deformation ratherthan original relationships. Second, although the U–Pbzircon geochronometer is very resilient, Pb-loss, recrys-tallization or even new growth occur typically underamphibolite- and granulite-facies metamorphism (e.g.Fraser et al., 1997; Vavra et al., 1999; Hoskin & Black,2000; Hoskin & Schaltegger, 2003; Whitehouse & Platt,2003). Memory loss and recrystallization are certainlyaided by the presence of an aqueous fluid (Villa, 1998;Tomaschek et al., 2003) but crystallization of overgrowthson pre-existing grains or even entirely new grains is morelikely in a partial melt (Roberts & Finger, 1997).

The combination of disruption of original relative agerelationships with variable extent of zircon (re-)crystal-lization under high-grade tectonometamorphism inex-tricably links the tasks of establishing the relative andabsolute ages of highly deformed and migmatized litho-logies. In the Akilia example, the validity of cross-cuttingfield relationships is ambiguous at best. Here, we presentnew high-spatial resolution (c. 10–30mm) U–Th–Pbisotopic and rare earth element (REE) abundance datafor carefully characterized growth phases in polyphasezircon crystals from a granitoid whose intrusion age hasbeen claimed, by some, to provide a minimum age for lifeon Earth. The complexity of field, structural, textural,chemical and isotopic relationships of these outcropsreflects the fact that the southwestern tip of the island isessentially a highly metamorphosed shear zone. The keyto finding more convincing evidence for the establish-ment of life is to concentrate on low-strain outcrops,where unequivocal relative field relationships can beestablished and where absolute dates and isotopic integ-rity are preserved to the extent that the null hypothesiscan be applied.

REVIEW OF AKILIA GEOLOGY AND

GEOCHRONOLOGY

The lithologies on southwest Akilia that are key toinferences for early terrestrial life occur as enclaves ofmafic–ultramafic rocks and putative metasediments[collectively defined as the Akilia association byMcGregor & Mason (1977)] within the regional earlyArchean Am��tsoq gneisses. The rock types in these

enclaves are not amenable to precise dating with methodsavailable at present; hence, the debate about their abso-lute age has revolved around the nature of their relativeage relationship to their host gneisses and, on Akilia itself,focuses on a single, small outcrop where highly strainedgranitoid gneisses occur in contact with the putative life-bearing enclave. Despite their obviously complex struc-tural context, these granitoid gneisses have, none the less,repeatedly been interpreted to cross-cut and thereforeprovide a minimum age for the mafic–ultramafic enclave(Nutman et al., 1997, 2000, 2002; Mojzsis & Harrison,2000, 2002). In their comprehensive treatment of fieldrelationships on southwest Akilia, Myers & Crowley(2000) concluded that the high-strain nature of the out-crop made it impossible to determine with any degree ofconfidence the relative age relationships between gneissicprotoliths. Whitehouse & Fedo (2003) have presentedfurther structural data from this specific outcrop andconclude that, with regard to relative dating constraints,the critical contact is not only tectonized at the point atwhich discordance has previously been claimed, but alsodocuments a truncation of structures in the gneiss sheetby the mafic–ultramafic enclave, yielding the completeopposite of previously claimed relative age relationships.Based on these studies, there is clearly little point inobtaining absolute age data from the gneisses in orderto constrain the age of the mafic–ultramafic enclave.Indeed, more recent studies of the putative metasedi-ments (Fedo & Whitehouse, 2002; Bolhar et al., 2004)that have been claimed to be dated by the supposedlydiscordant gneiss sheet leave, in our view at least, littledoubt that the original evidence for traces of life itselfmust be in error. This view is further reinforced by areported absence of apatite-hosted graphite particles in areinvestigation (Lepland et al., 2005) of the original sam-ple. None the less, for scientists who remain persuaded bythe original geological and geochemical evidence (Mojz-sis et al., 1996), it is important to test whether a coherentabsolute age interpretation of the quartz diorite sheet isattainable. As a prelude to our own study, we reviewbelow all previous geochronological data obtained fromthis sheet.

The only data of relevance are for rocks from thecontact between the Akilia association enclave andAm��tsoq gneisses (Fig. 1) on the northern side of thesmall peninsula that forms the southwestern tip of Akilia.This so-called ‘northern contact’ is where Nutman et al.(1997) collected their sample, G93-05, which theydescribed as a ‘quartz-diorite sheet’. The rock apparentlyyielded plenty of zircon (Nutman et al., 1997, p. 2478)‘which range from approximately 100 to 300 mm in lengthwith most of them prismatic and the remainder beingstubby-prismatic to ovoid-equant in habit’. These workersfurther claimed (p. 2478) that ‘structurally older inheritedcores were not discerned in the photomicrographs of the

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analysed zircons’. As will become apparent when discuss-ing our new data, it is important to recall Nutman et al.’s(1997) statement (p. 2478) that ‘in selecting grains of themain population for analysis, the clearest (generally low-est U) grains were predominantly selected’. The clear,low-U grains analysed by Nutman et al. (1997) define astatistically relevant population with an average age of3865 � 11 Ma (2s), whereas the deeper-coloured, mostlyovoid grains yielded ages between 3630 and 3530 Ma.On the basis of the their data and observations regardingzircon morphology, size and colour, Nutman et al. (1997)concluded that the igneous protolith of the quartz-dioritegneiss sheet intruded before 3850 Ma.

Whitehouse et al. (1999) also published ion-probe U–Pbzircon data for a so-called quartz-diorite gneiss from thesame general outcrop (their sample SM/GR/97/7).They presented cathodoluminescence (CL) images ofthe prismatic, elongate zircons selected for analysis,which showed oscillatory banding and a paucity of corestructures. Ion-probe data for SM/GR/97/7 zircons didnot directly correspond to Nutman et al.’s (1997) results,either in age distribution (younger) or U-content (higher).Whitehouse et al. (1999) concluded based on their own

data and CL imaging that the bulk of the zircon consistedof igneous growth at 3650 Ma, with minor inherited coresin excess of 3�8 Ga. Not surprisingly, Nutman et al. (2001)commented on the new results and inferred thatWhitehouse et al. (1999) had either mislabelled or other-wise confused a sample, as the differences between datafor G93-05 and SM/GR/97/7 appeared too substantialto permit a common origin of both samples. In theirresponse, Whitehouse et al. (2001) confirmed that theirsampling site of SM/GR/97/7 was no more than 25 cmfrom where Nutman et al. (1997) claimed to have takenG93-05. Whitehouse et al. (2001) acknowledged the sub-stantial difference in data for zircon extracted from sam-ples in such close proximity but reiterated their originalexplanation that this could have been because of a bias inselecting grains for mounting and/or that there could betrue sample heterogeneity on a dm scale—something thatis indeed evident from the now numerous detailed mapsand photographs of the outcrop (Myers & Crowley, 2000;Nutman et al., 2000, 2002; Whitehouse & Fedo, 2003).

Subsequently, Nutman et al. (2000) published retrospec-tive CL images for zircons of G93-05, which confirmedthe paucity of the 3�6–3�7 Ga CL-zoned thick rims that

Fig. 1. Location maps of the Archaean craton in southern Greenland and the location of Akilia in the Godthabsfjord. Detailed map of southwestAkilia (main panel), modified after Myers & Crowley (2000), shows the location of sample SM/GR/97/8 (GPS coordinates 63�55�8530N,51�41�0180W, WGS84 datum) close to the eastern margin of the folded Akilia mafic–ultramafic enclave.

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Whitehouse et al. (1999) described from their sampleSM/GR/97/7. Nutman et al. (2000) also provided newU–Pb ion-probe dates for additional zircon grains fromG93-05. Their new data confirmed that among the grainsselected by Nutman et al. (1997), biased to large, low-U,clear, prismatic material, the pre-3�8 Ga oscillatory grainsclearly dominate. The only exception is their grain 16[shown in fig. 10 of Nutman et al. (2000)], which is astrongly zoned, but equant grain, that is clearly3�65 Ga old.

The most recently published dataset (Mojzsis &Harrison, 2002) for zircon from another sample(GR9716) from this locality, claimed to be equivalent toG93-05, has added considerably to the complexity.Unfortunately, Mojzsis & Harrison (2002) did not pro-vide CL images of their zircon. Instead, they obtainedtwo datasets using two different experimentalapproaches. One dataset was obtained by depth profilinginto the centre of a single, clear (presumably low-U)euhedral, prismatic zircon (grain GR9716_69) similar tothe majority of zircons analysed by Nutman et al. (1997,2000) from their sample G93-05. The profiling techniqueyielded clear evidence for this single grain of a 3828 �8 Ma core, mantled by a c. 3�6–3�65 Ga inner rim,which itself is mantled by a c. 2�7 Ga outer rim. Thisobservation is entirely consistent with the general picturecommonly obtained from tonalite–trondhjemite–grano-diorite (TTG) suite gneisses in the area (e.g. Whitehouseet al., 1999; Nutman et al., 2002) when dates are obtainedfrom suites of handpicked zircon grains selected for CL-image-guided U–Pb ion-probe dating. The second data-set presented by Mojzsis & Harrison (2002), however,is potentially more insightful than the depth profilingbecause it was obtained in situ in petrographic thin sec-tions. Of the 71 grains analysed, only a single grain,embedded in biotite, yielded an age in excess of 3�8 Ga.This grain (g1-1) yielded a near-concordant 207Pb/206Pbage of 4079 � 18 Ma and was interpreted by Mojzsis &Harrison (2002) as an inherited xenocryst at a relativelylate stage of protolith emplacement. This is by far theoldest zircon yet reported from the North Atlantic Cratonand has no correspondence to any other previouslystudied sample. However, the important observationfrom the in situ study is that almost all the grains analysedyielded ages between 3�5 and 3�7 Ga. This promptedMojzsis & Harrison (2002) to propose that in a �3�6 Gamigmatization event, a significant fraction ‘(�50%?)’ ofthe protolith zircon dissolved and re-precipitated via par-tial melting, dissolution–reprecipitation or Ostwaldripening. However, provided that the dated zircons inthis section are representative of the rock’s entire popula-tion (i.e. only one grain in 71 analysed is older than3�8 Ga), the estimate of the fraction of reworked zirconsought to be moved upward from ‘(�50%?)’ to �98%.Although the in situ method has obvious advantages for

preserving the petrographic enviroment of the analysedzircon, it should be realized that unlike separated zircongrain mounts, where polishing is optimized to reveal asmuch internal structure as possible, the zircon surfaceexposed in a thin section is essentially random. Withoutadditional control of CL (or BSE) imaging, application ofthis technique to potentially complex zircons will inevi-tably yield ambiguous results owing to mixing of differentgrowth phases. Indeed, a plot of Th/U against apparentage [fig. 7 of Mojzsis & Harrison (2002)] shows poorcorrespondence between in situ analyses and results of anearlier (Mojzsis & Harrison, 2000) grain mount study onthe very same sample. The apparent paucity of >3�8 Gazircon growth zones in the grains exposed in the thinsection studied by Mojzsis & Harrison (2002) could thuspartly stem from overlapping the primary ion beam ontoubiquitous younger mantles around old cores.

When the entire zircon evidence from this locality isconsidered, two obvious implications for further studies(such as ours) emerge. First, there appears to be verysignificant sample heterogeneity on the dm-scale as faras the distribution of zircon with different age, structureand chemistry is concerned. Only this can explain whyNutman et al. (1997) obtained a maximum age of 3865 �11 Ma [later revised to 3841 � 6 Ma by Nutman et al.(2000)], and Mojzsis & Harrison (2002) determined (froma single grain) a slightly younger age of 3828 � 8 Ma,whereas Whitehouse et al. (1999) found only a single> 3�8 Ga age from the core of a c. 3�65 Ga grain. As itis physically impossible to sample exactly the same mate-rial as previous workers (unless a concerted effort is madeto obtain a single large sample that is later split intoaliquots and distributed to different laboratories), we havereprocessed one of our samples (SM/GR/97/8, takenimmediately adjacent to SM/GR/97/7) which we nowbelieve corresponds to the original G93-05 [see discus-sion also by Whitehouse & Fedo (2003)] in an effort toobtain a higher yield of zircon for a new imaging anddating session. The second implication is that irrespectiveof the heterogeneity in zircon distribution, it is obviousthat the selection of rare, clear (low-U), prismatic grainsfor ion-probe analysis very strongly biases to the oldestgrains. The picture gained from such grains is not atall representative of the entire zircon population.Although this bias may be logical and intentional whendirected towards estimating the potential age of a pre-metamorphic protolith, it can be misleading with regardto the geological history experienced by a sample. In thecase of the much-discussed Akilia locality, the question isclearly that of how old the relative field relationships are(which some interpret as igneous cross-cutting), and it is,therefore, critically important to test whether these rocksunderwent significant, pervasive partial melting thatwould have obliterated older structural relationships per-taining to their original, magmatic emplacement. For this

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reason, we have tried to select the complete variety ofzircon present in these rocks, and to analyse the variousgrowth and recrystallization domains observed in the CLimages. Furthermore, building on our previous work(Whitehouse & Kamber, 2002, 2003), we have obtainedREE compositional data for the dated zircon spots.

SAMPLING

The sample studied here, SM/GR/97/8, was collectedfrom the northern contact locality (Fig. 1) by S. Moorbathin the company of the late V. R. McGregor and S. J.Mojzsis. It is closely related spatially to sample SM/GR/97/7 of Whitehouse et al. (1999), coming from <25 cmwest across regional strike of that sample. Whitehouse &Fedo (2003) noted that the latter sample preserves gneis-sic banding in thin section, whereas SM/GR/97/8 isessentially homogeneous (Fig. 2), with equigranularquartz and feldspar mosaic texture implying largely sta-tic, post-deformational recrystallization. This led them tosuggest that SM/GR/97/8 is the most likely equivalentof the so-called ‘homogeneous tonalite’ sheet worked onby Nutman et al. (1997, 2000) and Mojzsis & Harrison(2000, 2002), with SM/GR/97/7 representing thebanded gneiss immediately adjacent to the east, which,they noted, in places, appears to be cross-cut by the homo-geneous sheet [see fig. 4d of Whitehouse & Fedo (2003)].

ANALYTICAL METHODS

High spatial resolution U–Th–Pb and REE data weregenerated using a Cameca IMS1270 large-format ionmicroprobe at the Nordsim facility, Swedish Museum ofNatural History. Detailed analytical methods have beendescribed previously for U–Th–Pb (Whitehouse et al.,1997, 1999) and REE (Whitehouse & Kamber, 2003;Whitehouse & Platt, 2003). In all cases, a defocused O2-primary beam was used to project the image of either a100mm or a 50 mm aperture onto the sample, generatingelliptical, flat-bottomed craters of nominal c. 30mm andc. 15mm (long axis), respectively. For some analyses, thespatial resolution was further enhanced by use of the fieldaperture in the secondary ion optics, restricting the actualanalysed region admitted to the mass spectrometer to ac. 11mm square on the illuminated object, regardless ofthe size of primary probe. Complete U–Th–Pb analyses ata mass resolution (M/DM) of c. 5000 were performedusing a peak switching routine, with a single ion-countingelectron multiplier (EM) as the detection device. Anenergy window of 60 eV was used throughout, withenergy adjustments made using the 90Zr2

16O peak. Pre-cise mass calibration was maintained by using an auto-matic routine in the Cameca CIPS software to scan overlarge peaks and extrapolate the mass to B-field curve for

peaks between these reference points (e.g. Pb-isotopeswere calibrated by centring the 94Zr2

16O peak at nominalmass 204 and the 177Hf16O2 peak at nominal mass 209).Pb/U ratios, elemental concentrations and Th/U ratioswere calibrated relative to the Geostandards zircon91500 reference, which has an age of 1065 Ma(Wiedenbeck et al., 1995). In some cases, particularly thehighest spatial resolution analyses of very small zircondomains, only Pb-isotopic ratios were measured usingeither a short mono-collection peak-switching routine,or a multi-collection routine using four ion-countingEMs positioned at the appropriate mass spacing andoperating at a mass resolution of c. 8000 (determinedby the fixed exit slit width of the multicollector system).For the multicollector measurements, the relativeefficiencies of the EMs were first matched as closely as

(a)

hbl

hbl

bi

bi

qtz

qtz

pl

pl

(b)

(c)

Fig. 2. Thin section images comparing texture of (a) granodiorite,SM/GR/97/8 (scale bar represents 4 mm) and (b) adjacent gneiss,SM/GR/97/7 (scale bar represents 2 mm). Note strong millimetre-scale banding in (b) defined primarily by hornblende. (c) Photomicro-graph (plane-polarized light) of SM/GR/97/8 showing equigranular,mosaic recrytstallization texture of quartz (qtz) and feldspar (pl),together with hornblende (hbl) and biotite (bi) laths, the latter defininga slight foliation; field of view is c. 2 mm � 3 mm.

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possible using pulse height analysis to set appropriatehigh voltage and thresholds, and then calibrated by directmeasurement on the NIST SRM610 standard, assumingthe composition determined by TIMS (Belshaw et al.,1994). Decay constants follow the recommendations ofSteiger & J€aager (1977).

Simultaneous high mass resolution ion counting wasalso used for most of the analyses of Light REE (LREE)(La–Eu) coupled with low mass resolution, energy filteredmono-collection measurements of the middle to heavyREE (MREE to HREE) following the method detailed byWhitehouse & Kamber (2003) and Whitehouse (2004).For a few analyses, selected high-abundance HREE (i.e.Sm, Gd, Dy, Er, Yb) were also analysed at high massresolution using simultaneous ion counting, with a singlepeak switch to measure 92Zr28Si16O2 at nominal mass152 as a matrix reference peak. REE abundances werenormalized relative to NIST SRM610 [working values ofPearce et al. (1996)] with the exception of the multi-collector HREE measurements where the Geostandards91500 zircon was used, assuming the concentrationsdetermined by Whitehouse & Platt [2003; these werethemselves calibrated against NIST SRM610 in thisearlier study, ensuring compatibility of the datasetsgenerated by the different methods; furthermore, theseconcentrations are in good agreement with the ‘workingvalues’ of Wiedenbeck et al. (2004)]. Tables 2 and 3 pre-sent the U–Th–Pb and REE analytical data, respectively,and also document the spatial resolution and methodused for each analysis.

Cathodoluminescence images of zircon were obtainedusing a Hitachi S4300 scanning electron microscope atthe Swedish Museum of Natural History.

Whole-rock major and trace-element analyses of SM/GR/97/8 were obtained from an aliquot that was milledin tungsten carbide at Oxford University. No data for Wand Ta are reported because of contamination from themill. Major element compositions were determined at theDepartment of Earth Sciences (UQ) by ICP–OES from asolution obtained by dissolving a fused bead. Results arereported in Table 1. Trace-element analysis was per-formed at the ACQUIRE laboratory (UQ) by ICP–MSfrom a solution obtained by digesting the sample in asteel-jacketed PTFE (Teflon), vessel. Data are reported inTable 1. Analytical details were the same as thosereported by Kamber et al. (2003).

RESULTS

Whole-rock major and trace-elementgeochemistry

At least a part of the confusion regarding equivalence ofsamples from this outcrop stems from the disparatepetrographic names assigned by various workers. The

original sample, G93-05, was called a ‘quartz-diorite’by Nutman et al. (1997), a term also adopted byWhitehouse et al. (1999) for their sample, SM/GR/97/7,but was later referred to as a homogeneous ‘tonalite’ by

Table 1: Major and selected trace-element data from

Akilia samples

Sample SM/GR/97/8 SM/GR/97/8 G93/051 GR97-162

analysis 1 analysis 2

SiO2 65.74 65.49 67.61 65.50

TiO2 0.39 0.39 0.41 0.38

Al2O3 16.32 16.20 16.00 16.90

Fe2O3 3.95 3.91 3.76 3.91

MnO 0.07 0.07 0.06 0.06

MgO 2.36 2.33 2.39 2.26

CaO 4.86 4.79 4.53 4.65

Na2O 3.89 3.86 3.72 3.75

K2O 1.34 1.30 1.45 1.31

P2O5 0.09 0.09 0.11 0.29

LOI 0.43 0.43 0.54

Total 99.45 98.87 100.04 99.55

Cs 2.4

Rb 73.1

Ba 84.1

Th 1.9 1.8

U 0.47 0.5

Nb 5.4

La 11.7

Ce 28.4

Pr 3.6

Pb 8.9 8

Sr 129

Nd 14.8

Be 1.4

Zr 128 144

Hf 3.6

Sm 3.2

Eu 0.72

Gd 2.9

Tb 0.39

Dy 2.1

Ho 0.41

Y 10.9

Er 1.1

Tm 0.16

Yb 0.97

Lu 0.14

Li 35.3

Major elements in wt %; trace elements in mg/g.1Data reported by Nutman et al. (1997).2Data reported by Mojzsis & Harrison (2002).

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Nutman et al. (2000). Mojzsis & Harrison (2002) referredto their supposedly equivalent sample GR9716 also astonalite. Major and selected trace-element (Zr, Pb, U,Th) compositions of these samples are listed in Table 1alongside that of our sample, SM/GR/97/8.

We note the close correspondence of all analyses fromthese samples, in accord with our belief that they doindeed originate from the same, homogeneous part ofthe gneiss sheet. The normative mineralogy calculatedfrom bulk-rock composition also indicates that all threesamples are in fact granodiorites according to thewidely accepted IUGS classification of plutonic rocks(Streckeisen, 1976; le Maitre, 2002), richer in quartz(c. 27%) than quartz-diorite (5–20%) and lower inplagioclase as a proportion of total feldspar (c. 85%)than tonalite (90–100%). On this basis, we use grano-diorite throughout this paper. We note, however, thatalthough quartz-diorite is definitely an inappropriatename, use of other plutonic rock-classification schemesadds ambiguity to the distinction between granodioriteand tonalite. In an ab–an–or diagram (Barker, 1979), thepercentage of normative orthoclase (c. 14%) means thatall three analyses plot within the tonalite field. The use ofnormative mineralogy in classification schemes such asQAP or ab–an–or that are intended principally for modalmineralogy can introduce additional ambiguity whendealing with hydrated rocks (SM/GR/97/8 containsabundant modal biotite) because of the tendency ofnorms to assign K entirely to orthoclase and to consideronly orthoclase as an alkali feldspar. In the only com-monly used classification scheme that utilizes elementalabundances instead of normative or modal mineralogy,the R1–R2 cation proportion scheme of de la Roche et al.(1980), these analyses fall on the boundary between thegranodiorite and tonalite field.

This confusion over precise petrographic name is farfrom trivial when set against the backdrop of argumentsabout zircon saturation and the ability of melts to retaininherited zircon that has long been a feature of the Akiliageochronology debate. As noted recently by Hanchar &Watson (2003), the actual Zr-concentration and cationratio (Na þ K þ 2Ca)/(AlSi) value (M) have only a smallinfluence on zircon saturation temperature (� a few tensof �C), the most significance factor being the assumedtemperature of crystallization. The higher magmatictemperature expected for a tonalite implies a greaterability to dissolve xenocrystic zircon compared with alower-temperature granodiorite, although, as we shalldemonstrate below, several additional factors need to betaken into account in discussing zircon solubility, not theleast of which is the presence of water, which willsignificantly lower the solidus of any granitic composition.

With regard to incompatible trace elements, sampleSM/GR/97/8 has general characteristics similar tothose of arc (sensu lato) magmas (Fig. 3a). The key

features are over-enrichment (relative to similarly com-patible elements) in Cs, Pb, Be, Li and relative depletionin Nb (note that the reported value is an overestimate as aresult of likely contamination from the tungsten–carbidemill). Although these features do not necessarily imply anorigin of this chemistry from supra-subduction zone melt-ing, we emphasize that enrichment in Cs, Pb, Be and Li isa very clear indication that source melting was triggeredby aqueous fluids. Nutman et al. (1999) and Kamber et al.(2002) have reported similar incompatible trace-elementpatterns for a variety of early Archaean TTGs fromsouthwest Greenland. The former researchers commen-ted on the fact that trace-element systematics such asthose of sample SM/GR/97/8 cannot be generatedwithout preferential element enrichment from aqueousfluids. The latter workers provided additional evidence

10-1

100

101

102

103

Cs

Rb

Ba

Th U Nb

Ta La Ce

Pr

Pb

Sr

Nd

Be Zr

Hf

Sm Eu

Gd

Tb

Dy

Ho Y Er

Tm Yb

Lu Li

N-M

OR

Bn

orm

alis

ed

con

cen

tra

tion

Pb

Nb

LiBe

B

0 5 10 15 20 25 300

20

40

60

80

100

120

140

160

Archaean TTG field

Post-Archaeangranitoid field

A

(a)

(b)

(La

/Yb

) N

101

100

10-1

102

103

Cs CeRb Pr

Ba PbTh Sr

U NdNb Be

Ta ZrLa Hf

SmEu

GdTb

Dy TmHo Yb

Y LuEr Li

YbN

Fig. 3. (a) N-MORB normalized (Sun & McDonough, 1989) trace-element pattern of sample SM/GR/97/8 (solid circles) compared withaverage continental crust estimate (open circles) of Rudnick &Fountain (1995). Beryllium concentration estimate for N-MORBexplained by Kamber et al. (2002). Note coherence in preferential Cs,Pb, Be and Li enrichment. (b) Chondrite-normalized La–Yb vs Ybdiagram after Martin (1986). Note that there is considerable variationin La–Yb for >3�8 Ga tonalites and quartz-diorites shown as opencircles [data from Nutman et al. (1999) and Kamber et al. (2000)] butwith one exception, they all plot into the field of Archaean TTG. Themore potassic 3�65 Ga Am��tsoq gneisses [shown as solid circles; datafrom Kamber et al. (2002)] show even more variability in LREE/HREE fractionation, with several samples plotting outside theArchaean TTG field. Sample SM/GR/97/8 (large open cross) plotsat the lowermost limit of Archaean TTG.

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for the role of aqueous fluids in generating the earliestpreserved continental crust. In the context of the Akiliadebate, it is very significant that trace-element distribu-tion strongly argues in favour of hydrous rather thananhydrous melting because previous consideration ofzircon solubility (Nutman et al., 1997; Mojzsis & Harrison,2002) has assumed that the igneous precursor of the stu-died rock crystallized from a ‘dry’ melt which, togetherwith inappropriate petrographic identification discussedabove, implies a much higher magmatic temperatureand, hence, ability to dissolve inherited zircon. Thispoint will be discussed in more detail after presentationof the zircon data.

A final aspect of the whole-rock geochemistry that is ofrelevance to the present study is the observation that thestudied granodiorite does not show the strong LREE overHREE enrichment that is so characteristic of typicalArchaean TTGs. In fact, the chondrite-normalized La/Yb of our sample is only 9�0, whereas the average ofreported >3�8 Ga tonalite and quartz-diorite datafrom south of the Isua Greenstone Belt is 18—almosttwice as fractionated as sample SM/GR/98/8. For thisreason, this sample only just plots into the ArchaeanTTG field (Fig 3b) defined by Martin (1986). The morepotassic 3�65 Ga Am��tsoq gneisses, reported by Kamberet al. (2002), show a bimodal distribution of chondrite-normalized La/Yb: one group with very high values ofc. 50, the other with much lower values of 6–17. Thisimplies that at least in terms of REE systematics, thestudied sample from Akilia is rather atypical for anArchaean TTG but akin to more potassic 3�65 Ga gran-itoids in the area. The origin of extreme HREE depletionin the more typical TTG may be a function of fractionalcrystallization processes rather than direct source pro-cesses as suggested by Kleinhanns et al. (2003), possiblydue in part to differing H2O-contents (Muentener et al.,2001).

Zircon populations in SM/GR/97/8

CL imaging reveals complex, polyphase internal struc-tures in most zircon grains from sample SM/GR/97/8(Fig. 4). On the basis of this imaging, we identify threeprincipal types of zircon growth, commonly more thanone of which is represented within a single grain. Group 1acomprises oscillatory-zoned cores occurring in euhe-dral to slightly rounded prismatic grains with length/width ratio of c. 3–4. The oscillatory zoned part of thegrain is generally CL-bright (subgroup 1a) and its zoningis commonly sharply truncated, in some cases (e.g. grain2–34) showing evidence for fracturing. Several cores(subgroup 1b) exhibit considerably darker CL andless distinct zoning (e.g. grains 1–4, 1–9 and 1–16).Group 2 ‘soccer ball’ zircon grains are generally equantand multifaceted, showing very low cathodoluminescence,

although vaguely zoned internal structures are occas-ionally visible (e.g. grain 2–13, Fig. 4). Group 3 zircongrains comprise CL-dark, sometimes zoned, rims orembayments that occur on both group 1a zircon grains(in which case, they are classified as subgroup 3a) andgroup 1b and 2 zircon grains (classified as subgroup 3b).This type of zircon varies from substantial tip overgrowthsof prismatic grains (e.g. grain 2–4, Fig. 4) to thin rimsdeveloped on typical group 2 grains (e.g. grain 2–14,Fig. 4). A fourth group of zircon is defined as outer rimsthat are found on grains with two earlier phases of growth.

Zircon U–Th–Pb geochronology

SIMS U–Th–Pb analytical data for zircons from SM/GR/97/8 are presented in Table 2 and in a plot of207Pb/206Pb dates in increasing order (Fig. 5). These arediscussed below in the context of the grouping of zircongrowth phases based on CL characteristics.

Group 1 zircon cores exhibit a wide but bimodalrange of 207Pb/206Pb dates from c. 2�7 to >3�8 Ga.The CL bright, oscillatory-zoned majority of thisgroup (subgroup 1a) show a more restricted range of207Pb/206Pb dates from c. 3�57 to 3�88 Ga. These ana-lyses are concordant to slightly discordant with low-Uconcentrations (c. 70–220 ppm) and moderate Th/Uratios (0�14–0�70). The similar CL characteristics andTh/U systematics suggest that these grains could belongto a single igneous protolith with an age in excess of3�8 Ga which has experienced variable amounts ofancient Pb-loss, probably during the high-grade eventsrecorded in the Godthabsfjord at c. 3�65 and c. 2�7 Ga,resulting in a spread of ages along concordia. In thiscontext, the oldest coherent group of 207Pb/206Pb datesthat define a prominent plateau on the cumulative agediagram (Fig. 5) yields a weighted average of 3842 �5 Ma (n ¼ 7, MSWD ¼ 0�62, 95% confidence). Thisinterpretation is in accord with previous interpretationsof similarly spread dates from other gneisses in this region(e.g. Nutman et al., 1996, 2000; Whitehouse et al., 1999),but the possibility that younger magmatic zircon is repre-sented in this population cannot be entirely ruled out. Asingle analysis yielding a 207Pb/206Pb age of 3876 �26 Ma (2s) is slightly older than the main c. 3�84 Ga groupand might represent a genuinely older grain. Four ana-lyses of CL-dark cores of subgroup 1b have high-U con-centrations (1800–8300 ppm), and low Th/U ratios(<0�04). Only one concordant date has been obtainedat c. 2�72 Ga, with the other analyses plotting between27 and 47% discordant. One of the discordant analyseshas a 207Pb/207Pb age of c. 2�78 Ga, suggesting that itcannot simply represent an equivalent of the concordant2�72 Ga analysis that has undergone later Pb-loss butprobably contains an inherited component or is a much

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2-1

2-3

2-2

2-6

2-9

2-10

2-12

2-23

2-21

2-20

2-24

2-22

2-31

2-7 2-8

2-5

2-4

2-13

2-30

2-15

2-33

2-32

2-36

2-35

2-37

1-12

2-34

2-14

2-11

a

a

a

a

a

a

a

a

a

b

a

a

a

b

c

b

b

b

b

b

b

b

b

dc

b

b

b

b

b

b

c

b

db

b

b

ba

a

b

a

a

a

a

a

a

a

c

aa

a

a

a

a

a

100 mµ

Fig. 4. Cathodoluminesence images of all zircon grains investigated in this study. Labelling nomenclature 1–12b, for example, indicates samplemount 1, grain number 12, analysis b. Ellipses show location of analysed region (U–Th–Pb or Pb-isotopes for geochronology and REEdeterminations). Where spatial resolution was further enhanced using field aperture blanking, this is indicated by square outline, which representsthe region of the grain that was actually analysed; in this case, the primary beam size is indicated by a dashed ellipse. Note that dark areas in someof the analysed regions correspond to residual gold coating from later analytical sessions that was not entirely removed prior to post-analytical CLimaging (e.g. grain 2–2, 2–8, 2–10).

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older grain (i.e. early-Archaean) that has experiencedextreme, possibly multiple, Pb-loss.

Group 2 ‘soccer ball’ zircon grains mostly yield a smallrange of 207Pb/206Pb dates from c. 3�57 to c. 3�62 Ga.Th/U ratios are very low (<0�03) and U-concentrationsfall in the range c. 500–800 ppm. The spread to youngerdates is attributed to minor Pb-loss from originally�3�62 Ga zircons, probably during the c. 2�71 Garegional high-grade event (although still concordant atthe 2s error level, the two analyses with youngest207Pb/206Pb dates are displaced to the right of the inverseconcordia). One zircon core (1–15a) that is provisionallyclassified into group 2 on the basis of its CL-dark,rounded appearance yields a concordant age ofc. 2�71 Ga, with low Th/U (0�025) and high U-concen-tration (1100 ppm).

Group 3a zircon phases comprise distinct, rather thinrim, overgrowths or embayments developed on oscilla-tory zoned CL-bright zircon cores of group 1a. Pb-isotope ratios only were obtained from this group usinga small primary beam spot (c. 15mm), together withfurther enhancement of spatial resolution of c. 11 mmusing a field aperture in the secondary ion beam (seeFig. 4). Despite the overall similarity of appearance inCL as well as consistently low Th/U ratios (<0�06), thisgroup yields a distinctly bimodal set of 207Pb/206Pb datesin the range c. 3�70–3�50 and 2�72–2�58 Ga. In line withour earlier interpretations, we attribute this limitedspread in zircon dates to Pb-loss affecting zircon grownoriginally at c. >3�60 and c. 2�71 Ga (Fig. 5). Group 3bzircon (rims on group 1b and group 2 zircon), togetherwith group 4 zircon (rims on polyphase grains) show

Fig. 4. Continued.

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Table 2: SIMS U–Th–Pb data from Akilia sample SM/GR/97/8

Analysis1 Analysis U4 (mg/g) Pb (mg/g) Th/U5 f 206Pb6 (%) Ratio � s Age � s

Mode2 Area3 207Pb/206Pb 206Pb/238U 207Pb/206Pb 206Pb/238U

Group 1a——cores——oscillatory zoned, CL bright

2—5a 1 30 270 250 0.33 0.22 0.3198 � 0.0012 0.6693 � 0.0114 3569 � 6 3303 � 44

2—11a 1 30 210 210 0.13 0.16 0.3250 � 0.0008 0.7363 � 0.0117 3594 � 4 3557 � 44

2—9a 1 30 130 120 0.092 0.28 0.3250 � 0.0010 0.7184 � 0.0114 3594 � 5 3490 � 43

2—2a 1 30 120 120 0.25 0.33 0.3407 � 0.0014 0.7464 � 0.0118 3666 � 6 3594 � 44

2—21a 3 [11] (119) 0.31 0.36 0.3462 � 0.0015 3690 � 6

2—36b 3 [11] (40) 0.59 0.07 0.3515 � 0.0022 3714 � 9

2—36d 3 [11] (13) 0.46 0.69 0.3559 � 0.0046 3732 � 19

2—6a 1 30 150 170 0.38 0.23 0.3569 � 0.0009 0.7671 � 0.0123 3737 � 4 3670 � 45

2—23a 3 15 (147) 0.39 0.57 0.3585 � 0.0012 3743 � 5

2—33a 3 15 (26) 0.67 0.28 0.3606 � 0.0018 3752 � 8

2—10a 1 30 110 130 0.52 0.26 0.3651 � 0.0016 0.7574 � 0.0120 3771 � 7 3635 � 44

2—7a 1 30 220 290 0.74 0.16 0.3691 � 0.0007 0.8273 � 0.0133 3788 � 3 3886 � 47

2—30a 3 [11] (28) 0.76 0.07 0.3721 � 0.0018 3800 � 7

2—35b 3 [11] (42) 0.85 0.01 0.3723 � 0.0019 3801 � 8

2—31a 3 [11] (32) 0.27 0.01 0.3760 � 0.0016 3816 � 7

2—4a 1 30 68 85 0.63 0.33 0.3779 � 0.0016 0.8187 � 0.0129 3823 � 6 3856 � 46

2—22a 3 15 (60) 0.70 0.10 0.3802 � 0.0021 3833 � 8

2—32b 3 [11] (17) 0.60 <0.01 0.3806 � 0.0026 3834 � 10

2—8a 1 30 97 120 0.37 0.16 0.3815 � 0.0017 0.8468 � 0.0134 3838 � 7 3955 � 47

2—34a 3 [11] (11) 0.63 0.03 0.3819 � 0.0032 3839 � 13

2—1a 1 30 73 89 0.52 0.27 0.3828 � 0.0013 0.8039 � 0.0128 3843 � 5 3803 � 46

2—24a 3 [11] (81) 0.88 0.01 0.3833 � 0.0019 3845 � 7

2—20a 3 15 (176) 1.00 0.05 0.3836 � 0.0009 3846 � 3

2—37a 3 [11] (16) 0.59 <0.01 0.3913 � 0.0033 3876 � 13

Group 1b——cores——oscillatory zoned, CL dark

1—9a 1 30 8300 2400 0.018 0.11 0.1648 � 0.0003 0.2512 � 0.0028 2505 � 3 1445 � 14

1—8a 1 30 4100 1400 0.019 0.11 0.1794 � 0.0005 0.2948 � 0.0030 2647 � 4 1665 � 15

1—16b 1 30 1800 870 0.042 0.05 0.1940 � 0.0005 0.3949 � 0.0042 2776 � 4 2146 � 19

1—4b 1 30 1800 1100 0.007 0.07 0.1870 � 0.0006 0.5201 � 0.0067 2716 � 5 2700 � 28

Group 2——structureless ‘soccer balls’

1—15a 1 30 1100 700 0.025 0.01 0.1870 � 0.0003 0.5221 � 0.0058 2716 � 2 2708 � 25

2—13a 1 30 810 770 0.008 0.06 0.3195 � 0.0004 0.7107 � 0.0113 3567 � 2 3461 � 43

1—12b 1 30 650 620 0.005 0.07 0.3238 � 0.0010 0.7227 � 0.0079 3588 � 5 3506 � 30

2—3a 1 30 600 600 0.006 0.06 0.3274 � 0.0007 0.7529 � 0.0120 3605 � 3 3618 � 44

2—14a 1 30 510 510 0.008 0.07 0.3301 � 0.0006 0.7442 � 0.0118 3618 � 3 3586 � 44

2—12a 1 30 560 570 0.007 0.08 0.3317 � 0.0009 0.7658 � 0.0121 3625 � 4 3666 � 44

Group 3a——rims/embayments on group 1a

2—5�2 2 [11] 0.016 0.10 0.1727 � 0.0014 2584 � 13

2—22b 3 15 (838) 0.020 0.29 0.1739 � 0.0004 2595 � 4

2—7b 3 15 (950) 0.057 0.66 0.1807 � 0.0007 2659 � 7

2—33b 3 15 (306) 0.013 0.06 0.1820 � 0.0005 2671 � 4

2—31c 3 [11] (98) 0.046 0.21 0.1844 � 0.0008 2693 � 7

2—31b 3 [11] (80) 0.030 0.02 0.1873 � 0.0009 2719 � 7

2—20b 3 15 (500) 0.010 0.09 0.3061 � 0.0005 3501 � 2

2—32a 3 15 (373) 0.011 0.08 0.3130 � 0.0006 3536 � 3

2—23b 3 15 (224) 0.016 0.87 0.3149 � 0.0013 3545 � 6

2—35a 3 [11] (52) 0.021 0.01 0.3149 � 0.0018 3545 � 9

2—36a 3 [11] (85) 0.019 0.06 0.3167 � 0.0011 3554 � 5

2—34b 3 [11] (64) 0.014 0.03 0.3199 � 0.0014 3569 � 7

2—4b 3 [11] 0.012 0.01 0.3294 � 0.0008 3614 � 4

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207Pb/206Pb dates around 2�7 Ga or lower and consist-ently low Th/U ratios (<0�1). Six of the seven analyses ingroup 3b for which full U–Pb data are available lie on aregression with an upper intercept date of 2709 � 13 Ma(MSWD ¼ 2�2, 95% confidence); a subset of four of theseanalyses define a concordia age of 2709 � 7 Ma(MSWD ¼ 0�73, 2s; decay constant errors included). Asomewhat older, discordant age of c. 2�95 Ga fromanalysis 1–12a probably represents a mixed analysisbetween a c. 2�7 Ga rim and older core, as a207Pb/206Pb age of 2741 � 26 Ma (2s) was obtainedfrom a subsequent higher spatial resolution analysis.

The U–Th–Pb data that we obtain from sample SM/GR/98/8 are in good agreement, both in maximum ageand age range, with data reported by Nutman et al. (1997)and Mojzsis & Harrison (2002) from their respectivesamples of the homogeneous tonalite from this locality,

again suggesting that SM/GR/97/8 is the equivalent ofthese previously investigated samples.

Zircon REE geochemistry

REE concentration data are presented in Table 3, in aseries of normalized [to C1 chondrite values of Anders &Grevasse (1989)] abundance diagrams (Figs 6–9), and inplots showing the degree of LREE and HREE fractiona-tion against age (Fig. 10a and b). Zircon samples fromgroup 1a, i.e. oscillatory zoned c. 3�6 to >3�8 Ga cores ofgrains, show a limited range of compositions in Fig. 6.Most analyses show typical ‘igneous’ zircon REE profileswith closely parallel steep slopes from La to Lu, markedpositive-Ce and small negative-Eu anomalies that arevery similar to the profiles reported from otherearly-Archaean zircon grains in the Godthabsfjord

Table 2: Continued

Analysis1 Analysis U4 (mg/g) Pb (mg/g) Th/U5 f 206Pb6 (%) Ratio � s Age � s

Mode2 Area3 207Pb/206Pb 206Pb/238U 207Pb/206Pb 206Pb/238U

Group 3a——rims/embayments on group 1a, contd.

2—21b 3 15 (508) 0.023 0.11 0.3302 � 0.0007 3618 � 3

2—37b 3 [11] (102) 0.039 <0.01 0.3306 � 0.0012 3620 � 6

2—8b 3 [11] (121) 0.013 0.06 0.3317 � 0.0017 3625 � 8

2—24b 3 [11] (127) 0.024 0.32 0.3404 � 0.0018 3664 � 8

2—30b 3 [11] (71) 0.013 0.02 0.3478 � 0.0012 3698 � 5

Group 3b——rims/embayments on group 1b and group 2

2—14d 2 [11] 0.019 0.01 0.1830 � 0.0012 2680 � 11

1—8b 1 30 530 320 0.065 0.61 0.1835 � 0.0007 0.4925 � 0.0099 2685 � 6 2581 � 43

1—15b 1 30 750 460 0.035 0.03 0.1860 � 0.0004 0.5150 � 0.0057 2707 � 3 2678 � 24

2—15a 1 30 790 480 0.008 0.15 0.1863 � 0.0004 0.5105 � 0.0081 2710 � 3 2659 � 35

1—14a 1 30 580 360 0.022 0.04 0.1864 � 0.0005 0.5241 � 0.0086 2710 � 4 2717 � 36

1—16a 1 30 890 560 0.027 0.12 0.1865 � 0.0004 0.5260 � 0.0068 2712 � 4 2725 � 29

2—14c3� 2 [11] 0.021 0.03 0.1866 � 0.0009 2713 � 8

1—12a2 2 [11] 0.1899 � 0.0015 2741 � 13

1—12a 1 30 390 260 0.012 0.86 0.2193 � 0.0017 0.5494 � 0.0069 2976 � 12 2823 � 29

1—4a 1 30 3000 1500 0.014 0.10 0.1830 � 0.0002 0.4082 � 0.0043 2680 � 2 2207 � 20

Group 4——outer rims on polyphase grains

2—23c 3 15 (1354) 0.007 0.11 0.1866 � 0.0004 2713 � 3

2—36c 3 [11] (242) 0.010 0.05 0.1854 � 0.0007 2702 � 6

1Italics denote samples for which no REE analyses have been performed.2Analysis mode——1: monocollection, full U—Pb; 2: monocollection, Pb—Pb only; 3: multicollection Pb—Pb only.3Area refers either to primary beam spot size in micrometres (long axis of elliptical area) or, when given in parentheses, thefield of view defined by a square field aperture in the secondary beam which has been used in these cases, together with a15mm primary spot to increase spatial resolution.4U-concentrations in parentheses have been estimated using measured primary beam, instrument sensitivity and knownconcentrations in the Geostandards 91500 reference zircon.5Th/U ratios presented are those calculated from common Pb-corrected 208Pb/206Pb ratios, assuming the 207Pb/206Pb ageand closed system behaviour and are essentially the same as those calculated from measured Th- and U-signals, calibratedto the 91500 zircon reference.6Percentage of common 206Pb in measured 206Pb, calculated from the 204Pb signal assuming a present-day Stacey &Kramers (1975) model terrestrial Pb-isotope composition.

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(e.g. Whitehouse & Kamber, 2002, 2003). Two of thesefive profiles (2a–x and 10a–x) represent analyses of thesame growth phase (based on CL imaging) that wasanalysed for U–Th–Pb but in a slightly different location.Analyses from the original dated location show markedlyincreased LREE concentrations (omitted from Fig. 6 butdata are retained in Table 3) that might be related to laterLREE mineralization along cracks, perturbing the origi-nal profile. The exact nature of this mineralization isunclear but, given the clearly LREE-depleted nature ofthe original zircon, only a very small amount of a stronglyLREE-enriched phase (e.g. monazite or allanite) wouldbe required to influence the REE profiles, whereas othercharacteristic enrichments (e.g. higher levels of Th inmonazite) remain undetected. Analysis 2–5a exhibits ahighly unusual REE profile that is enriched in bothLREE and MREE. Because it is unlikely that zirconcan incorporate REE in this pattern [by see discussionin Whitehouse & Kamber (2002)], we conclude that theREE profile of this particular grain must be controlled byinclusions (and/or microcrack fillings) of both LREE-and MREE-enriched minerals, such as monazite,allanite or apatite. This analysis is not considered furtherfor the purposes of protolith characterization. In general,the REE profiles from group 1a, together with Th/Uratios, are typical of zircon crystallized in equilibriumwith a primary melt.

Group 1b zircon samples also represent cores, but theseshow markedly different characteristics from those ofgroup 1a, particularly with regard to the LREE, whichare consistently flat at chondrite-normalized valuesbetween c. 40 and 1000, with absent to small positiveCe-anomalies (Fig. 6). The HREE profiles of this groupare either highly fractionated, similar to those of group 1a,or in one case (analysis 1–4b), flat at c. 100� chondritic.

Group 2 zircon grains exhibit an internally coherent setof REE profiles (Fig. 7) that are notably different fromthose of either group 1a or group 1b. The most promin-ent difference is the marked depletion of the HREE withflat or even concave-up profiles from Dy to Lu, irrespect-ive of whether the analysed grain is early or lateArchaean. These zircon grains show more variablefractionation of LREE relative to group 1a, with theearly Archaean grains showing the steepest profiles. Ingeneral, these analyses show smaller-magnitude Ce-anomalies and larger negative Eu-anomalies. Interest-ingly, the least suppressed positive Ce-anomaly (analysis14a2) is accompanied by the least enhanced negative Eu-anomaly—a feature that would be consistent with vari-able redox control of the behaviour of these two elementsduring the event generating the c. 3�6 Ga zircon. Thecharacteristic HREE depletion of this group (andpossibly in analysis 1–4b of group 1b) is the most impor-tant observation, as it suggests crystallization in an

2400

2800

3200

3600

4000

0 10 20 30

207

206

Pb

/P

ba

ge

(Ma

)

# of analyses probability

{

{

{

Pb-loss

Pb-loss

Pb-loss

mixedanalyses?

3842 ± 5(95% conf., n = 7,

MSWD = 0.62)3619 ± 3

(95% conf., n = 6,MSWD = 0.91)

2712 ± 3(95% conf., n = 10,

MSWD = 0.95)

1a1b

2

3a3b

4

Group

Fig. 5. Cumulative plot of 207Pb/206Pb ages, arranged in order of consecutive age and differentiated into different zircon groups, combined with aprobability plot. Flat regions in the cumulative plot, corresponding to significant peaks in the probability plot, are interpreted to result fromspecific geological events for which weighted average ages have been calculated, in some cases including analyses from zircon belonging todifferent groups. Sloping regions in the cumulative plot and smaller peaks to the low-age side of the main peaks in the probability plot areinterpreted to result from non-recent Pb-loss.

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Table3:SIM

SREE,YandHfdatafrom

AkiliasampleSM/GR/97/8

Analysis

Age1

Analysis

YHf

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Mode2

Area3

Group1a——

cores——oscillatory

zoned

,CLbright

2—5a

3569

230

599

8990

14107

16119

75. 8

9.9

81. 3

13. 5

69. 3

17. 4

71. 9

14. 3

136

30. 2

2—2a

3666

230

744

8560

5.6

16. 9

1.9

7.1

2.3

0.5

15. 2

4.9

62. 0

24. 1

113

24. 6

206

51. 8

2—2a-x

3666

230

839

9680

0.1

13. 8

0.3

4.8

7.0

1.3

26. 4

7.1

85. 9

27. 6

119

25. 5

217

51. 0

2—21a

3690

3[11]

0.007

3.7

0.08

1.3

2.5

0.5

11. 3

48. 6

76. 4

149

2—36d

3732

4[11]

2.0

9.3

41. 7

72. 4

151

2—23a

3743

3[11]

0.002

5.3

0.04

0.8

1.6

0.5

9.1

44. 9

89. 1

230

2—33a

3752

4[11]

4.7

19. 1

90. 3

131

216

2—10a

3771

230

974

9760

4.9

292.0

11. 3

6.2

1.5

25. 2

7.1

90. 4

31. 7

144

31. 3

272

62. 4

2—10a2

3771

530

5.2

29. 5

2.2

11. 7

6.2

1.6

2—10a-x

3771

230

1100

7460

0.01

6.8

0.1

2.4

5.4

2.1

26. 0

8.4

101

34. 9

151

33. 3

281

68. 1

2—30a

3800

4[11]

10. 6

11. 9

42. 1

62. 2

117

2—35b

3801

4[11]

5.9

22. 5

116

203

436

2—31a

3816

4[11]

1.2

5.9

26. 7

47. 6

96. 6

2—4a

3823

230

691

7840

0.01

4.8

0.1

1.7

3.2

0.9

16. 4

5.5

64. 6

23. 7

101

22. 0

190

45. 7

2—22a

3833

3[11]

0.002

6.9

0.2

4.1

7.3

1.1

24. 5

85. 3

123

219

2—32b

3834

4[11]

10. 7

40. 2

192

334

606

2—8a

3838

230

392

12000

0.002

4.4

0.02

0.3

1.0

0.3

7.2

2.5

35. 3

12. 4

60. 3

14. 6

132

34. 7

2—34a

3839

4[11]

6.4

20. 8

75. 3

114

205

2—1a

3843

230

1420

8020

0.02

5.8

0.1

2.4

5.1

1.9

32. 6

10. 8

133

48. 9

219

46. 8

391

91. 0

2—24a

3845

3[11]

0.03

21. 4

0.2

3.4

8.0

3.0

28. 2

113

170

320

2—20a

3846

3[11]

0.02

17. 5

0.5

8.4

15. 8

1.4

54. 5

176

205

321

2—37a

3876

4[11]

3.1

15. 1

74. 4

119

227

Group1b

——co

res——oscillatory

zoned

,CLdark

1—9a

2505

230

2640

13800

8.6

39. 3

4.4

34. 5

10. 5

3.4

39. 4

15. 6

224

83. 0

374

82. 3

705

145

1—8a

2647

230

1880

15800

204

499

62. 4

388

120

17. 6

183

28. 9

202

60. 5

277

60. 7

539

115

1—16bx

2776

230

3110

16200

14. 7

63. 9

6.8

41. 5

6.1

4.2

39. 9

17. 2

259

101

476

108

937

201

Group2——structureless

‘soccer

balls’

1—15a

2716

130

145

12800

0.3

3.8

0.2

1.4

1.7

0.4

7.6

2.2

22. 0

4.5

13. 0

1.8

11. 5

2.9

1—15a2

2716

230

284

9960

0.3

3.2

0.3

2.2

2.7

0.5

13. 4

4.2

34. 8

7.2

19. 6

2.9

18. 0

2.9

2—13a

3567

230

253

11100

0.7

4.1

0.6

4.4

7.3

0.9

25. 7

7.4

41. 2

6.9

17. 3

3.4

35. 1

8.7

1—12b

3588

230

481

10000

0.05

0.9

0.07

0.7

3.1

0.3

17. 6

6.4

51. 6

12. 3

41. 9

6.9

52. 1

10. 4

2—3a

3605

230

240

13200

0.008

0.6

0.04

0.6

2.5

0.3

20. 1

6.1

44. 2

7.4

17. 7

2.6

19. 4

3.9

2—14a

3618

230

384

12800

0.03

9.8

0.08

1.1

3.0

1.0

23. 5

6.8

60. 6

12. 1

33. 9

5.9

39. 1

7.8

2—12a

3625

230

624

11800

0.1

1.0

0.08

0.9

3.6

0.4

24. 3

8.8

77. 6

18. 3

50. 1

7.6

44. 9

7.9

304



Dow

nloaded from


Analysis

Age1

Analysis

YHf

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

Mode2

Area3

Group3a——

rims/em

bayments

ongroup1a

2—5�

2584

2[11],30

206

9710

29. 3

132

20129

58. 6

10. 9

106

11. 6

54. 2

7.8

24. 3

3.6

39. 7

9.1

2—22b

2595

3[11]

1.7

9.7

1.6

12. 6

8.0

1.2

7.7

10. 5

7.8

9.7

2—7b

2659

3[11]

6.7

365.0

35. 4

14. 9

4.3

17. 9

50. 0

101

214

2—33b

2671

4[11]

4.6

4.4

3.0

2.7

4.1

2—31b

2719

4[11]

1.0

3.9

20. 0

31. 4

58. 0

2—20b

3501

3[11]

0.7

0.03

0.4

0.8

0.08

4.1

20. 2

39. 6

110

2—32a

3536

4[11]

0.2

1.0

8.6

22. 3

69. 2

2—23b

3545

3[11]

0.4

0.01

0.2

0.9

0.1

4.1

13. 2

9.0

11. 9

2—35a

3545

4[11]

31. 6

86. 7

321

482

859

2—36a

3554

4[11]

1.2

7.0

41. 8

70. 6

169

2—34b

3569

4[11]

4.3

18. 3

78. 8

104

209

2—4b

3614

4[11]

1.9

8.1

28. 2

25. 6

41. 1

2—21b

3618

3[11]

0.2

5.7

0.4

4.1

3.6

2.2

9.3

42. 7

125

484

2—37b

3620

4[11]

2.0

8.5

27. 1

34. 4

70. 1

2—8b

3625

3[11]

0.5

0.02

0.1

0.8

0.2

5.1

12. 6

6.5

10. 6

2—24b

3664

3[11]

0.8

9.6

0.9

5.2

2.7

1.0

6.9

14. 6

6.8

9.2

2—30b

3698

4[11]

1.7

4.5

12. 9

6.5

8.6

Group3b

——rims/em

baymen

tsongroup1b

andgroup2

1—8b

2685

130

161

13400

201

376

51. 6

203

36. 7

4.7

62. 8

9.1

37. 3

5.4

23. 8

2.3

17. 2

2.6

1—15b

2707

230

133

9310

0.2

2.8

0.3

1.8

1.6

0.3

6.4

2.0

15. 0

3.2

8.9

1.3

9.1

1.5

2—15a

2710

230

566

13500

3.4

11. 7

1.7

10. 7

2.8

0.8

9.0

3.1

42. 5

16. 2

79. 1

17. 6

150

37. 3

1—14a

2710

130

900

14500

2.4

15. 6

2.3

17. 5

7.8

2.1

17. 8

5.8

77. 6

30. 1

145

32. 5

282

67. 3

1—16a

2712

130

713

14100

374

641

83. 4

320

504.5

74. 4

13. 4

85. 3

24. 8

126

25. 2

218

50. 0

2—14b

2713

2[11],30

138

10100

0.9

7.3

0.8

5.1

3.2

0.9

7.1

1.7

10. 0

3.8

9.8

1.7

16. 5

4.0

1—12a

2976

230

191

11400

0.01

0.7

0.01

0.07

0.2

0.05

1.2

0.8

10. 1

4.1

23. 1

5.5

46. 1

12. 6

Group4——outerrimsonpolyphasegrains

2—36c

2702

4[11]

0.2

1.3

9.9

16. 9

37. 2

2—23c

2713

3[11]

32. 1

116

16. 4

96. 8

40. 5

11. 2

61295

692

2020

Allco

ncentrationsaregiven

inmg

/g.

1Agederived

from

207Pb/2

06Pb(inMa).

2Analysismode——1:

monoco

llection,allR

EE;2:

monoco

llectionMREE—HREE,multicollectionLREE;3:

multicollectionallR

EE;4:

multicollectionMREE—HREEonly;

5:multicollectionLREEonly.

3Areareferseither

toprimarybeam

spotsize

inmicrometres(longaxisofellipticalarea)

or,when

given

inparen

theses,thefieldofview

defined

byasquarefieldap

erture

intheseco

ndarybeam

whichhas

beenusedin

thesecases,

together

witha15

mmprimaryspot(unless

indicated

otherwise),to

increase

spatialresolution.

305



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environment depleted in HREE, probably because ofthe role of garnet.

Group 3a zircon grains show a wide range of REEprofiles (Fig. 8) that probably reflect different originswithin this group. The similarities of individual analysesto those of either group 1a and 1b or group 2 are appar-ent in comparison of profiles in chondrite-normalizedabundance diagrams and are particularly well defined inplots of (Pr/Sm)N and (Dy/Yb)N against age. The chara-cteristic HREE depletion of group 2 zircon is evident insome of both the early and late Archaean analyses of this

group, suggesting crystallization from a garnet-influencedmedium, although fractionated HREE profiles are alsoseen in both age groups. LREE profiles show a range offractionation in the early Archaean analyses from steepprofiles similar to those of group 1a through to somewhatflatter (but still fractionated) profiles. The late Archaeananalyses show only flatter LREE profiles.

Analyses of zircon in group 3b, all of which arelate Archaean in age, exhibit a wide range of fractionationin both HREE and LREE (Fig. 9a). In two cases, the cor-responding core (group 1b) and rim (group 3b) analyses

115 110 105 100104

103

102

101

100

10-1

10-2

10-3

norm

alis

ed

abundance

Group 1a - >3.8 Ga osc. zoned zircon

2-1a

2-2a-x

2-4a

2-8a

2-10a-x

2-05a

2-20a

2-21a

2-22a

2-23a

2-24a

2-35b

2-30a

2-31a

2-32b

2-33a

2-34a

2-36d

2-37a

1-16bx1-8a1-4b 1-9a

104

103

102

101

Group 1b - CL-dark zircon cores

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm YbLu

Fig. 6. Chondrite-normalized (C1, Anders & Grevasse, 1989) abundances of REE from zircon growth belonging to group 1a (oscillatory zonedcores) and group 1b (CL-dark cores). Dashed lines connect Sm and Gd for HREE-only analyses in which Eu was not determined. Horizontalspacing corresponds to effective ionic radius in eightfold coordination (Shannon, 1976; indicated in pm on the upper x-axis).

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may be compared. For zircon grain 1–4, the core (1–4b)and rim (1–4a) REE systematics are very similar, whereasfor grain 1–16, the rim (1–16a) shows a similar degree offractionation of HREE at lower concentrations relativeto the core (1–16b), but a strongly reverse fractionatedLREE profile.

Group 4 zircon analyses both show fractionatedHREE, in one case (2–36c) at a low overall concentrationlevel, whereas the LREE of analysis 2–23c show a flatprofile (Fig. 9b). It is interesting to note that the HREE ofanalysis 2–23c contrasts with the flat profile seen in theearly Archaean group 3a core overgrowth in this samezircon grain (analysis 2–23bx).

DISCUSSION

Equivalence of studied samples

The first point to note about our new zircon geochrono-logical data from SM/GR/97/8 is that they exhibit thesame range of ages and, within specific age groups, Th/Uratios as data previously presented (Nutman et al., 1997;Mojzsis & Harrison, 2002) from a homogeneous grani-toid sheet within the much debated c. 0�7 m wide compo-site gneiss layer on Akilia. Taken together with ourwhole-rock major- and trace-element data, which alsoshow strong similarity to these earlier studies, we are,therefore, convinced that this sample is an appropriate

representative of the homogeneous granodiorite sheetfrom which claims for the >3�83 Ga age for the mafic–ultramafic enclave on southwest Akilia have beenderived. A previous claim (Whitehouse et al., 1999) thatanother sample from the composite gneiss sheet (SM/GR/97/7) was the equivalent, despite its youngerc. 3�65 Ga zircons, was based on an incomplete under-standing of the complexity of the gneiss sheet (Nutmanet al., 1997), that was subsequently revealed by Myers &Crowley (2000), Nutman et al. (2002) and Whitehouse &Fedo (2003); this claim is now considered invalid,although data from this sample remain valid.

In this paper, we also reiterate the detailed fieldobservations of Whitehouse & Fedo (2003), which indi-cate (1) that the homogeneous granodiorite sheet cannotbe demonstrated to cross-cut the mafic–ultramaficenclave and hence cannot provide an age constraint,and (2) that the homogeneous granodiorite itself appearsto cross-cut a banded gneiss (SM/GR/97/7) that con-tains typically igneous oscillatory-zoned zircons with anage of c. 3�65 Ga (Whitehouse et al., 1999). Thus, thepossibility needs to be considered that the preserveddiscordance could indicate intrusive remobilization ofthe granodiorite �3�65 Ga. Against this background,>3�65 Ga oscillatory zoned zircons in the granodioritecould be remnants from a c. 3�84 Ga protolith that werenot digested during the c. 3�62 and 2�71 Ga remelting andremobilization events.

Group 2 - ca. 3.65 Ga & 2.7 Ga"soccer ball" zircon

115 110 105 100104

103

10-2

102

101

100

10-1

no

rma

lise

da

bu

nd

an

ce

2-3a

2-12a

2-13a

2-14a2

1-12b

1-15a

1-15a2

2.7 Ga

3.65 Ga


Fig. 7. Chondrite-normalized abundances of REE from zircon belonging to group 2 (‘soccer-ball’ zircon). Normalization and axis spacingas in Fig. 6.

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Although it is beyond any doubt that 3�75–3�84 Gagranitoids existed in the Godthabsfjord region, thecritical issue on Akilia Island is not to provide evidencefor relict original >3�8 Ga features (such as zircon cores),but to establish which aspects of the gneiss as it presentsitself today can be attributed to known stages of theregional geology. The key question in this regard is towhat extent the gneiss was remelted and deformedduring the 3�6 and 2�71 Ga events. Only then can relativeage relationships seen in the outcrop be dated. In thefollowing sections, we discuss our new data together withpreviously published observations in the context of threepossible emplacement scenarios.

Emplacement models consistent withzircon data and field observations

Model 1: 3�60–3�65Ga emplacement of thegranodiorite with inheritance of older zircon

As previously discussed for other TTG gneisses from theGodthabsfjord, the distribution of zircon ages in SM/GR/97/8 and equivalent samples is formally consistentwith a magmatic age of 3�60–3�65 Ga. This age is withinerror of that of the immediately adjacent sample SM/GR/97/7, which appears structurally older (Myers &Crowley, 2000; Whitehouse & Fedo, 2003). There isthus no conflict between relative and absolute age datesin this interpretation, in which >3�65 Ga zircon is

115 110 105 100104

103

102

101

100

10-1

10-2

no

rma

lise

da

bu

nd

an

ce

Group 3a - rims or embaymentson >3.8 Ga zoned cores(a) early Archean

2-30b

2-32a

2-34b

2-35a

2-36a

2-37b2-24b

2-23bx

2-8b

2-20b

2-21b

104

103

102

101

100

(b) late Archean

2-7b

2-22b

2-31b

2-33b

2-05x


Fig. 8. Chondrite-normalized abundances of REE from zircon growth belonging to group 3a (overgrowths or embayments on oscillatory zonedcores of group 1a), differentiated into (a) early Archaean and (b) late Archaean. Dashed lines connect Sm and Gd for HREE-only analyses inwhich Eu was not determined. Normalization and axis spacing as in Fig. 6.

308



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viewed as inherited. The possibility that melts of tonalitic–granodioritic composition can entrain older zirconhas long been a vexed issue in discussion of the protolithages of rocks from the Gothabsfjord region. Severalworkers, applying the zircon solubility arguments ofWatson & Harrison (1983), considered that the assumedhigh temperature and low-Zr concentration of mostTTG melts a priori preclude survival of older zircon(Nutman et al., 1997; Mojzsis & Harrison, 2002). Wenote that these studies universally ignore the factorsconsidered by Watson (1996) to favour the survivabilityof inherited zircon, namely (1) presence of large zirconin the source rock or occlusion of zircon grains withinmajor silicates; (2) a brief or cool magmatic event; and(3) failure of the melt to coalesce into a large body. In ourview, two additional important issues that seriously ques-tion the validity of rejecting the hypothesis of inheritedzircon concern the assumed solidus temperature for thestudied rocks and the estimated fraction of zircon whichactually survived as xenocrystic grains.

With regard to the homogeneous granodiorite sheet onAkilia, no evidence has ever been presented that it repres-ents, as claimed (e.g. Mojzsis & Harrison, 2000), ahigh-temperature (c. 900�C) dry melt. Rather, the new

trace-element data presented here for SM/GR/97/8,in accordance with published data for similar rocks(Nutman et al., 1999; Kamber et al., 2002), stronglyargue against a dry (and hence hot) solidus. Experimentsof wet melting of tonalite compositions have shown thatvery significant melt fractions (30–70%) are still present attemperatures below 750�C (e.g. Peters & Kamber, 1994;Sawyer, 1994). As pointed out by Kamber & Moorbath(1998), a large proportion of typical Am��tsoq gneisses willretain a ‘critical melt fraction’, sufficient still to be mobile,at c. 775�C and at this temperature, they would beZr-oversaturated and hence capable of preserving inher-ited zircon. Under these conditions, the criteria noted byWatson (1996) of cool magmatic events and failure ofmelts to coalesce into large bodies would be met. Becausetemperature is the single most important parameterdetermining zircon saturation (Hanchar & Watson,2003), advocates of a c. 900�C solidus for the studiedrock have to produce conclusive supporting evidence,before discarding the possibility of inheritance.

Inheritance of xenocrystic zircon in exactly theserocks has been demonstrated with the >4�0 Ga grainanalysed in situ by Mojzsis & Harrison (2002). Theseworkers suggested, however, that survival of a significant

115 110 105 100105

104

103

102

101

100

10-1

10-2

no

rma

lise

da

bu

nd

an

ce

Group 3b - ca. 2.7 Ga rims on Group 1bGroup 4 - ca. 2. 7 Ga rims on three-phase zircon

2- 23c

2-36c 2-15a

2-14b

1-12a

1-4a

1-15b

1-16a

1-14a


Group 4

Group 3b

Fig. 9. Chondrite-normalized abundances of REE from zircon growth belonging to group 3b (late Archaean rims on group 1b zircon) and group4 (outer late Archaean rims on polyphase zircon). Dashed lines connect Sm and Gd for HREE-only analyses in which Eu was not determined.Normalization and axis spacing as in Fig. 6.

309



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proportion of inherited zircon was unlikely (assuming, ofcourse, that their solidus temperature estimate of 900�Cis correct). The question, then, is what should beregarded as a significant proportion of inherited grains?As discussed earlier, for understandable reasons, zirconseparation, grain selection and ion-probe dating all bias

towards large grains of high-quality (low-U) zircon. Inour study, we have strived to date a representativeselection of grains, although even with these efforts, themajority of zircon grains (many of which are too small forroutine handling) have remained untouched. Although itis impossible to estimate reliably the true proportion of>3�8 Ga zircon in this rock, it is sufficient to note fromthe CL images (Fig. 4) that group 1a zircon grains are notnecessarily a predominant component of the total zirconfraction.

We thus maintain that if the homogeneous granitoidsheet was emplaced as a crystal-rich magma, instead of asa completely liquid melt, the observation by Mojzsis &Harrison (2002) that the oldest zircon grains are found inthe middle of feldspars and must therefore represent theprotolith age is irrelevant, because they would merelyrepresent the age of the source rock from which thehomogeneous granodiorite sheet was ultimately derivedand have no bearing on the emplacement age of thegranodiorite sheet itself, which has been the singularimportant goal of these studies.

Although a magmatic age of 3�6–3�65 Ga appearsplausible from the data discussed thus far, our new REEdata for group 2 zircon grains (which best representgrowth of zircon in this time period) suggest thatthis event more probably represented lower-crustalremelting of an older protolith (in the presence of garnetor another HREE-retaining phase). We investigate this inthe following two models.

Model 2: remelting of 3�84Ga gneiss at 3�62GaStructural and geochronological constraints discussed byWhitehouse & Fedo (2003) preclude a >3�65 Ga mag-matic age for the homogeneous granitoid sheet. It istherefore likely that the actual magmatic age is repres-ented by one of the two remaining age groups identified:either c. 3�62 or c. 2�7 Ga. In this section, we investigatethe constraints imposed by our REE data on these ages.

The distinctive HREE-depleted profiles seen in thegroup 2 zircon grains, as well as c. 3�62 Ga overgrowthson older cores (group 3a), are typical of zircon that hascrystallized and/or recrystallized in equilbrium with astrongly HREE-enriched phase of which garnet is mostlikely (e.g. Rubatto, 2002; Whitehouse & Platt, 2003). Asgarnet is not present in this granodiorite, we propose thatit was a residual phase during melting of an older proto-lith. Melting of a garnet-bearing mafic source, althoughcapable of producing a quartzofeldspathic melt, is con-sidered unlikely because a much steeper whole-rock REEprofile similar to typical TTGs (Fig. 3a) would beexpected (e.g. Martin, 1986). Furthermore, a non zircon-bearing mafic precursor would not yield older zircon,as seen in the cores of many of the SM/GR/97/8grains. Instead, we propose melting of a tonalite or

101

100

10-1

10-2

101

100

10-1

10-2

(Pr/

Sm

) N(D

y/Y

b) N

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0207 206Pb/ Pb age (Ga)

1a1b

2

3a3b

4

Zircon group

100

10-1

10-2

10-3

Th/U

(a)

(b)

(c)

2.7

1G

a

3.6

2G

a

3.8

4G

a

flat LREE

steep LREE

steep HREE

flat HREE

Fig. 10. Temporal evolution of (a) extent of LREE depletion repres-ented by (Pr/Sm)N, (b) HREE enrichment expressed as (Dy/Yb)N, and(c) Th/U ratios. Dashed vertical lines represent the age (in Ma) ofdefined events in the Godthabsfjord region, as labelled in (a).

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quartz–diorite protolith during granulite-facies meta-morphism at c. 3�62 Ga. Several experimental studieshave documented garnet as a reaction product formedby dehydration melting of biotite-bearing quartzofelds-pathic rocks (e.g. Rutter & Wyllie, 1988; Skjerlie & John-ston, 1993, 1996; Pati~nno-Douce & Beard, 1996; Singh &Johannes, 1996a, 1996b). The study of Singh & Johannes(1996a, 1996b) specifically investigated melting of a tona-lite and showed that garnet appeared in the melt residuesat pressures above 10 kbar and temperatures exceedingc. 750�C, whereas the melts themselves were of grano-dioritic composition.

An intriguing aspect of the REE composition of�3�62 Ga zircon in this sample is that it appears to recorddifferent growth conditions, with the group 2 and some ofthe group 3a analyses recording the garnet influence onHREE described above, whereas others show fraction-ated HREE that appear to be unaffected by garnet. Thisis further emphasized in Fig. 11a, which plots a vector forpairs of analyses from the same grain, showing thechange in degree of HREE fractionation (representedby (Dy/Yb)N) and HREE concentration (represented by[Dy]). Correlation of REE profiles with CL images shows

that within group 3a, there is no direct correspondencebetween morphological character and REE composition,as both obvious embayments (e.g. 2–8b and 2–36a) andmore continuous, enveloping overgrowths (e.g. 2–23band 2–20b) show both types of REE composition. A pos-sible explanation for this could be that the zircon grainsrecord prograde development from early metamorphic–anatectic conditions in which garnet was not presentthrough to later stages when melt was produced inequilibrium with a garnet residue to produce both rimsand entire grains (group 2) that are HREE-depleted.

Model 3: remelting of early Archaean gneissat 2�71GaAn interesting and potentially important observationfrom the complex polyphase zircon crystals in SM/GR/97/8 is that key features of the c. 3�6 Ga growthphases, namely low Th/U ratios and depleted HREE,are shared by some of the 2�7 Ga growth phases (Fig. 10).Similar Th/U characteristics at both 3�6 and 2�7 Ga wereconsidered by Mojzsis & Harrison (2002) to indicatethat similar, specifically metamorphic, processes were

101 102 103 104

101

100

10-1

10-2

[Dy]

(Dy/

Yb) N

2-82-202-212-232-24

2-302-322-34

2-372-36

Group 3a

Group 2

(a) early Archean rims

101 102 103 104

2-52-222-312-33

1-121-4

1-15

2-141-16

2-232-36

Group 2

Group 3a Group 3b Group 4

(b) late Archean rims

Fig. 11. HREE vectors for pairs of analyses from individual zircon grains in (Dy/Yb)N vs Dy concentration (ppm) space for: (a) early Archaeanrims of group c. 3a (i.e. developed on c. 3�8 Ga cores) and (b) late Archaean rims of group 3a (developed on c. 3�8 Ga cores), group 3b (developedon late Archaean group 1b cores), and group 4 (outer rims on polyphase grains). Symbol represents composition of zircon on which the rim isdeveloped and the position of the arrowhead represents the composition of the rim itself (note that for grain 2–34 in (a), the change in compositionis smaller than the size of the symbol and so the arrowhead has been omitted). Shaded area represents the total compositional range of group 2zircon.

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responsible for these two episodes of zircon growth and/or reworking. It is clear, however, from a plot of Thagainst U concentrations from our study (Fig. 12) thatTh/U ratios of the 3�6 Ga overgrowths (group 3a) andsoccer-ball grains (group 2) are, on average, low becauseof relatively low Th-concentrations rather than U-enrich-ment. Increased U-contents, on the other hand, are themain reason for low Th/U ratios in 2�7 Ga overgrowths(groups 3a, 3b and 4) as well as late Archaean cores(group 1b). Low Th-concentrations are not unknownfrom magmatic or migmatitic zircons and, on the basisof Th/U ratios alone, it cannot be inferred that suchzircon grew from a metamorphic fluid. The possibilityremains, however, that at least some of the 2�7 Ga zirconsgrew from a U-rich but Th-poor metamorphic fluid.

Our new REE data provide additional constraints onthe growth environment for the 2�7 Ga zircon over-growths. In common with the c. 3�6 Ga overgrowths, awide range of HREE profiles are seen, ranging fromsteeply fractionated, with low (Dy/Yb)N, to flat, with(Dy/Yb)N � 1 (Fig. 10b), whereas the LREE are similarlyvariable and, on average, more enriched (Fig. 10a). Asdiscussed for Model 2, above, the HREE depletion maybe consistent with melting of a tonalitic precursor undergranulite-facies conditions in which garnet is residual,but, in this case, it would necessarily be remelting of a

rock that had already been through such a process. In theearlier proposed melting episode at c. 3�6 Ga, only zirconwith typically igneous, fractionated REE from thec. 3�84 Ga precursor was originally present, whereas at2�7 Ga, previously generated HREE-depleted zircon inthe form of group 2 soccer balls and group 3a over-growths would be present. In this case, mechanismsother than melting may be invoked in order to explainthe HREE-depleted rims of groups 3a, 3b and 4, such asdissolution–reprecipitation of existing zircon or Ostwaldripening. If one of these mechanisms is indeed respons-ible for the c. 2�7 Ga rims, some transport of the REE isrequired because, as shown in Fig. 11b, there are com-monly significant differences between the REE pattern ofthe 2�7 Ga growth phase and the substrate upon which itis developed. For example, grains 1–12 and 2–23 haveHREE-fractionated rims developed on zircon with flatHREE, whereas grains 2–22 and 2–5 have flat HREErims developed on HREE-fractionated zircon.

Grains 1–4 and 1–15 are of particular significance ininterpreting the nature of the 2�7 Ga event. These aregrains in which the HREE-depleted phase actually con-stitutes the core rather than an overgrowth. Superficially,these grains resemble the c. 3�62 Ga group 2 soccer-ballgrains (1–4 is assigned to that group on the basis ofmorphology, despite its 2�7 Ga age) and they appear to

103

102

101

100

10-1

103102101100 104

[Th

]

[U]

U-enrichment

Th-d

eple

tion

1a1b

2

3a (ca. 3.6 Ga)3a (ca. 2.7 Ga)

3b4

Zircon group

Th/U

=1

0.1

10

0.01

0.00

1

I

II

III

IV

Fig. 12. Plot of Th-concentration (ppm) vs U-concentration (ppm) contoured for Th/U ratios from 0�001 to 10. Group symbols follow otherdiagrams, except that group 3a is further subdivided into c. 3�6 and c. 2�7 Ga groups. Large symbols labelled with Roman numerals representaverage compositions of: I, – >3�8 Ga cores (group 1a); II, – c. 2�7 Ga cores (group 1b); III, – c. 3�6 Ga overgrowths (group 3a); IV, c. 2�7 Gaovergrowths (groups 3a, 3b and 4). Vectors labelled Th-depletion and U-enrichment indicate that lowering of Th/U ratios may be resolved into acombination of these two components.

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indicate that entirely new zircon grains might have beengrowing at 2�7 Ga—a process that would be consistentwith their origin in a partial melt. An alternative explana-tion, in which the cores of these grains simply reflect totalPb-loss from early Archaean group 2 grains, seemsunlikely, given that these latter grains with very similarU-concentration show limited Pb-loss, with 207Pb/206Pbages slipping only to c. 3�57 Ga. A partial melt may alsoaccount better for the contemporaneous feldspar–leachate–whole-rock Pb–Pb date of 2746 � 150 Ma(MSWD ¼ 1�3) reported from this sample by Kamber &Moorbath (1998) that clearly points to profound dis-turbance and re-equilibration of the whole-rock U–Pbsystematics.

The increasing complexity of zircon growthmorphology and REE chemistry by the time of the lateArchaean event serves to obscure its true nature.Although we may speculate that a second, granulite-facies partial melting event was responsible for thedistinctive HREE-depleted nature of some of the over-growths and whole grains that yield c. 2�7 Ga ages, wecannot entirely dismiss mechanisms such as dissolution–reprecipitation or Ostwald ripening that might have beenresponsible for redistributing Zr, Si and REE at this time.

Recognition of magmatic vs metamorphiczircon in quartzofeldspathic gneisses

The debate over the ‘age’ of the granodiorite gneiss sheeton Akilia serves to highlight a semantic but non-trivialissue of recognizing ‘metamorphic’ versus ‘igneous’zircon. In the present context, proponents of a >3�8 Gaage for the mafic–ultramafic enclave on southwest Akilia(and any possible early terrestrial life relicts) require (1) anunambiguous cross-cutting field relationship with thegranodiorite [recently discredited by Whitehouse &Fedo (2003)], and (2) evidence for a purely subsolidusprocess to explain all post-3�8 Ga zircon in the granodior-ite, in order that original igneous field relationshipsremained undisrupted.

In accord with Hoskin & Black (2000), we consider‘metamorphic’ zircon to be that generated strictly bysubsolidus processes, namely (1) diffusion of Zr and Sireleased by metamorphic breakdown reactions leading tonucleation and crystallization of zircon (e.g. Fraser et al.,1997; Bingen et al., 2001) and (2) recrystallization ofprotolith zircon (e.g. Friend & Kinny, 2001). Zirconprecipitated from a partial melt, although technically‘magmatic’, is ultimately a product of metamorphism(e.g. Roberts & Finger, 1997). Under such circumstances,the distinction between metamorphic and magmatic zir-con becomes blurred and herein lies the difficulty ofdefining the actual ‘emplacement’ age of a rock such asSM/GR/97/8. Specifically, in cases where the zirconis simply classified as ‘metamorphic’ on the basis of

geochemistry, the possibility remains that it could haveformed from sufficient partial melt for its host to bemobile and therefore disrupt and obliterate originalfield relationships. Using geochemical features of zircon,as well as textural relationships exposed by CL imaging, itis possible to distinguish between zircon crystallized from‘primary melts’ (see Mojzsis & Harrison, 2002) and zirconthat formed from a partial melt during a metamorphicevent in which the solidus was reached and metamorphiczircon generated by subsolidus processes. In the con-cluding section of this paper, we discuss the importanceof correctly applying criteria to discriminate betweensuch zircon origin.

Interpretation of zircon Th/U ratios

In their depth-profiling study of a single zircon from anequivalent sample to our SM/GR/97/8 at this locality,Mojzsis & Harrison (2002) assigned a ‘metamorphic’origin to c. 3�6 and c. 2�7 Ga outer regions of their studiedzircon on the basis of low Th/U ratios. The only reason-ing forwarded for this assignment was that the expectedincrease of Th/U ratios in orthogneisses during progrademetamorphism implied that associated metamorphicfluids should be characterized by correspondinglylow Th/U. An ad hoc assumption was made by theseworkers that zircon precipitated from just such a meta-morphic fluid, but no suggestion was made regarding thestage at which during prograde metamorphism thisprecipitation took place and, most importantly, whetherZr (and Si) would have been available to facilitate zirconcrystallization.

It is not uncommon for low Th/U ratios to be used todiscriminate zircon resulting from metamorphic pro-cesses from magmatic zircon, with competing effects ofTh-rich minerals such as monazite and allanite typicallybeing invoked (e.g. Williams et al., 1996; Rubatto et al.,2001). This discriminant cannot be applied universally,however, not least because ‘metamorphic’ zircon fromhigh-metamorphic-grade orthogneisses can exhibit highTh/U ratios (e.g. Friend & Kinny, 2001; M€ooller et al.,2003), whereas other studies have documented low Th/U ratios in unambiguously magmatic zircon (e.g. Zeck &Whitehouse, 1999). In other cases, Th/U ratios appear tobe robust through high-grade metamorphism, preservingprotolith values (M€ooller et al., 2002).

Evolution of Th/U ratios during prograde metamor-phism of orthogneisses is undoubtedly more complexthan envisaged by Mojzsis & Harrison (2002) anddepends largely on the location of each element and itsability to be released to a fluid at different times. Geo-chemical and Pb-isotope studies of metamorphism in theLewisian Complex of NW Scotland (Fowler, 1986;Whitehouse, 1989) have shown that grain boundaryhosted U, which can account for >50% of total U in

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amphibolite-facies rocks (Dostal & Capedri, 1978), islargely removed by the time hornblende–granulite faciesis attained, resulting in higher Th/U ratios in the residualrock. Higher-grade pyroxene–granulite-facies rocks inthe Lewisian, however, return to modest and ratherhomogeneous Th/U ratios, presumably as a result ofthe onset of breakdown of Th-bearing accessory phase(s)(e.g. allanite, apatite). The low Th/U fluid envisaged byMojzsis & Harrison (2002) is therefore likely to be presentonly during the earliest stages of prograde metamorph-ism, as the labile grain boundary U is lost. The availabil-ity of Zr at this stage will depend on whether any earlyreacting phases release sufficient Zr to facilitate zirconcrystallization. Fraser et al. (1997) suggested that Zr ismost likely to be released during hornblende and/orgarnet (the latter unlikely in the protolith to this particularlithology) breakdown, because these minerals host thehighest Zr-concentration, but their breakdown willoccur at higher grade than the loss of labile grain bound-ary U and possible release of a low Th/U fluid. Further-more, our new REE data require that such a fluid wouldalso have a highly specific (HREE-depleted) compositionwhich is in fact the opposite of that expected to be releasedby breakdown of HREE-enriched hornblende andgarnet. Hence, interpretation of Th/U systematics inzircon needs to consider the metamorphic conditionsunder which new zircon has grown and the likely miner-alogy of the rock from which metamorphic fluids werereleased.

Mojzsis & Harrison (2002) also discussed the implica-tions of Th/U ratios from their in situ analyses of 71zircon grains. They attached particular relevance to theobserved variation in Th/U ratio in c. 3�6 Ga zircongrains from values consistent with primary magmas tovalues that they considered to result from fluid precipita-tion. It is instructive to compare their fig. 7, which sum-marizes data from their own in situ study and two earlierstudies of separated grains with our Fig. 10c, from whichit can be seen that neither our data nor those of Nutmanet al. (1997) and Mojzsis & Harrison (2000) show elevatedTh/U in the 3�6 Ga population. Only the in situ data ofMojzsis & Harrison (2002) show a spread towards elev-ated Th/U ratios at c. 3�6 Ga. In our view, this spreadis probably an artefact of their analytical procedure,which did not utilize any pre-analytical imaging to revealinternal morphology. Given the complexity of zirconinternal morphology documented herein (Fig. 4) andelsewhere (Nutman et al., 2000), we consider it likelythat many of the in situ analyses of Mojzsis & Harrison(2002) sample mixed zircon growth phases, resulting inboth the aforementioned mixed ages as well as mixedTh/U signatures. Hence, although we agree with theinterpretation that a partial melt was present in thisrock at 3�6 Ga, in our view, as argued above, this was ofstrictly low Th/U character.

Textural observations

Minimization of analytical artefacts derived from in situ

ion-probe data from mixed zircon growth phases as dis-cussed above is not the only benefit of comprehensivemapping of zircon internal structure prior to analysis.Independent of geochemical observations, the internalmorphology of complex zircon revealed by CL (as usedin this paper) or BSE imaging provides critical informa-tion on whether purely subsolidus processes can beinvoked to explain later phases of zircon growth orwhether a (partial) melt is required. Several features ofzircon morphology (Fig. 4) suggest that the >3�8 Gazoned cores were at some stage in contact with a meltphase. Corrosive features, in particular embayments thatcut across the earlier growth zoning, are particularly wellillustrated by grains 2–4, 2–21, 2–32 and 2–36. In allcases, there is no evidence of ghost zoning as might beexpected for solid-state recrystallization fronts (e.g.Hoskin & Black, 2000) and so we prefer to assign theseembayments to the action of a corrosive melt. In addi-tion, several of the zircon grains shown in Fig. 4 showclear evidence for fracturing. Most commonly, this ismanifested as truncated zoning where an expectedpyramidal termination that would preserve the symmetryof the original >3�8 Ga zircon, e.g. in grains 2–1, 2–10,2–20 and 2–34, is missing. Grain 2–34 is particularlyinformative because, in addition to truncation, the grainhas cracked down the centre and the internal zoningclearly reveals relative rotation of both parts prior toannealing of the crack with later zircon—a processwhich simply cannot be achieved in a subsolidus envir-onment.

REE patterns from dated and texturally mapped zircongrowth phases

Proponents of a 3�84 Ga intrusion age for the Akiliagranodiorite would probably agree with us that the rockexperienced partial melting at 3�6 Ga and that at leastsome of the concordant zircon of that age grew from apartial melt. In the absence of REE data, it could beargued that the extent of partial melting was relativelyminor and remained insufficient to remobilize the rockper se, thus favouring preservation of original 3�84 Gacross-cutting relationships. However, the REE patternsof type 2 zircon are very difficult to explain by ‘minor’melting because the granodiorite gneiss itself is devoidof garnet and textures that could suggest the formerpresence of garnet. In our view, this implies that thegranodiorite melt formed during prograde upper amphi-bolite- to peak granulite-facies metamorphism of aquartzofeldspathic, lower-crustal protolith from wherethe melt migrated to its present location. We envisagethat once sufficient melt and strain had accumulated,possibly because of melting reactions involving biotite

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breakdown, the granodiorite (as we analyse it today)became sufficiently mobile to intrude along older litholo-gical contacts. Judging from the textural and geochemicalcomplexity of the studied zircon, it is plausible thatzircon now present in the granodiorite sheet originatedas: (1) entrained original igneous grains from the protolithpossibly shielded from fluids and melts in silicatephases; (2) original igneous grains that experiencedsubstantial Pb-loss in the 3�6 Ga melt; (3) original3�84 Ga material that reprecipitated or recrystallized dur-ing high-grade metamorphism either during progradefluid-dominated phases of growth or during melting;(4) freshly crystallized, new igneous 3�6 Ga zircons fromZr that was liberated during amphibole (� garnet) break-down. To what extent these processes were repeatedduring the high-grade 2�71 Ga event is difficult toestimate as a result of the generally thin zircon growthphases of that age, which make even ion-probe analysesdifficult.

The complexity of relative and absolute age relation-ships of lithologies on Akilia Island demonstrates that newattempts to find evidence for early life on Earth must bebased on a sound understanding of the field relationshipsand the protoliths of the studied lithologies, as well asrobust absolute chronology. Elucidation of the geo-chronology of such target areas should ideally becarried out prior to publication of spectacular claimsregarding the origin of life. In any case, establishment ofa sound geochronological framework requires carefulapplication of the full spectrum of tools available to thegeologist.

CONCLUSIONS

Whole-rock major-element composition and U/Pb zirconage distribution leave no doubt that the granodioritesample analysed in this study is equivalent to the samplesanalysed by Nutman et al. (1997) and Mojzsis & Harrison(2002). Data for the three samples can therefore becombined for a complete interpretation.

Three age clusters determined here at 3�83, 3�65–3�60and 2�71 Ga correspond to distinct zircon growth phasesidentified with CL imaging. Our preferred interpretationis that the >3�8 Ga grains (and portions of grains) arefrom the melt source of the granodiorite, which weenvisage to be a >3�8 Ga lower-crustal TTG.

The granodiorite in its present composition separatedfrom its lower-crustal TTG source during a pervasivegranulite-facies event in which crustal melts (such as thestudied rock) intruded along zones of melt-enhancedstructural weakness. The maximum age for the claimeddiscordance of this rock with mafic gneisses is thus 3�6–3�65 Ga, in agreement with the fact that it cuts ac. 3�65 Ga foliated gneiss (SM/GR/97/7) previouslyanalysed by Whitehouse et al. (1999).

REE abundance patterns for the dated zircon grainsare crucial for interpretation of the 3�60–3�65 Ga zircongrowth phases, which were previously interpreted to haveformed in equilibrium with a metamorphic fluid (Mojzsis& Harrison, 2002) rather than a melt. However, theHREE-depleted nature of many of these growth phasesrequires the presence of garnet—an expected residualmineral during granulite-facies melting of tonalites. Weagree with previous workers that this growth phase haslow Th/U but this feature is not uncommon for zirconcrystallizing from partial crustal melts.

The youngest zircon growth phase formed at 2�71 Gaand manifests in a variety of textural contexts. In agree-ment with the textural observation, 2�71 Ga growthphases show several types of REE patterns, but they aregenerally richer in U than the older growth phases. It islikely that these growth phases formed from morethan one process. Fluid precipitation is certainly apossibility but renewed (partial) melting cannot bedismissed. Hence, the claimed discordance of the rockas it is preserved today could even be as young as2�71 Ga.

The combined data (field relationships, zircon U–Th–Pb, zircon internal structure, zircon REE, whole-rock major elements, whole-rock trace elements, mineraland whole-rock Pb-isotopes) make this one of the mostthoroughly studied rocks ever. Because of the great com-plexity of the zircon data alone, it is not unexpected thatvarious researchers will regard one or the other inter-pretation as more likely. This, however, is not the issue atstake. Rather, what matters is the realization that irre-spective of the sophistication of analysis, there may neverbe broad agreement about whether this rock indeedcross-cuts potentially life-bearing lithologies, and, if so,how old the discordance truly is. Therefore, this rockcannot serve to date something as important as the oldestevidence for life on Earth. Attempts to date this milestonein planetary history need to work with samples fromlow-strain areas, in which the zircon systematics can beinterpreted unequivocally.

In general, great caution must be applied to interpret-ing zircon U–Pb geochronological data from thin gneissicveins in polyphase high-grade metamorphic terranes. Inparticular, application of zircon solubility argumentsrequire realistic estimates of melt temperature and shouldtake into account factors that apply to melts that do notbehave as ‘infinite reservoirs’ (Watson, 1996).

ACKNOWLEDGEMENTS

We are grateful to Stephen Moorbath for providing thesample investigated here, as well as invaluable discus-sions, as well as Chris Fedo and John Hanchar for valu-able discussions on aspects of Akilia geology and zircon

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geochemistry, respectively. We also thank Roland Maasand John Myers for their detailed and constructivereviews. The ion microprobe facility in Stockholm(Nordsims) is operated under an agreement between thejoint Nordic research councils (NOS-N), the GeologicalSurvey of Finland and the Swedish Museum of NaturalHistory. This paper is Nordsims publication 106.

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