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1 Chemical Geology 259 (2009) Contents lists available at ScienceDirect Chemical Geology journal homepage: Reworking of Hadean crust in the Acasta gneisses, northwestern Canada: Evidence from in-situ Lu Hf isotope analysis of zircon Tsuyoshi Iizuka a,, Tsuyoshi Komiya a, Simon P. Johnson b, Yoshiaki Kon a, Shigenori Maruyama a, Takafumi Hirata a a Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama Meguro-ku, Tokyo , Japan b Geological Survey of Western Australia, Mineral House, 100 Plain Street, East Perth, WA 6004, Australia article info abstract Article history: Received 9 June 2008 Received in revised form 31 October 2008 Accepted 10 November 2008 Editor: R.L. Rudnick Keywords: Ancient zircon Crustal evolution Hadean Lu Hf isotope LA-MC-ICPMS Crustal reworking The Acasta Gneiss Complex of northwestern Canada comprises early Archean orthogneisses and includes the oldest known terrestrial rocks ( Ga). Previous zircon geochronological studies revealed the presence of zircon xenocrysts with ages up to 4.2 Ga in the oldest rocks, indicating that the source of early Archean granitoids contained Hadean (N4.03 Ga) crust. In this study, we have determined the Lu Hf isotopic compositions of zircon grains extracted from the early Archean Acasta gneisses to evaluate the extent of the Hadean crust contribution to the formation of these granitoids. Analyses of the Lu Hf isotopes were carried out using in-situ laser ablation inductively coupled plasma mass spectrometry combined with cathodoluminescence images of the internal structure of the grains. Two ca Ga granitoids have ε Hf (T) of 2.4±2.2 and 3.2±2.5, suggesting that the source of the granitoids was extracted from the mantle as far back as 3.8 Ga. This is consistent with the presence of abundant zircon xenocrysts with ages up to 3.9 Ga. The Hf isotopic composition of a 3.72 Ga granitoid is significantly less radiogenic, with an ε Hf (T) of 6.1±2.5. This indicates that the granitoid formed by remelting of very old crust. The ε Hf (T) values for the two oldest rocks, the ca Ga tonalites, are 1.2±3.3 and 3.3±1.7, respectively, indicating that even the oldest known granitoids contain reworked older crustal materials. These results suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean Acasta granitoids Elsevier B.V. All rights reserved. 1. Introduction Knowledge of early crustal growth history is critical for understanding the evolution of the early Earth. The presence of a 4.4 Ga detrital zircon in a metasediment from the Yilgarn Craton of Western Australia provides evidence for crust formation only ~170 Myr after the beginning of the Solar System (Wilde et al., 2001). However, no crustal rocks have yet been found from the first 500 Myr of Earth's history. The oldest crustal rocks identified are the Ga magmatic protoliths of the Acasta gneisses in the Slave Province, northwestern Canada (Bowring et al., 1989a; Bowring and Housh, 1995; Bleeker and Stern, 1997; Stern and Bleeker, 1998; Bowring and Williams, 1999; Sano et al., 1999; Iizuka et al., 2006; Iizuka et al., 2007). The Acasta Gneiss Complex comprises a heterogeneous assemblage of early Archean foliated to gneissic tonalite, granodiorite and granite, together with minor quartz diorite, diorite, gabbro and ultramafic rocks. Elucidating the tectonomagmatic origin of these early Archean rocks and constraining the relationships between them Corresponding author. Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Fax: address: tsuyoshi.iizuka@anu.edu.au (T. Iizuka). and the Hadean (N4.03 Ga) mantle crust system will yield valuable insights into early crustal evolution. Previous zircon geochronological studies on the Acasta Gneiss Complex (Bowring and Williams, 1999; Iizuka et al., 2006; Iizuka et al., 2007) have demonstrated that some early Archean Acasta granitoids (even the oldest known rocks) contain zircon xenocrysts with ages up to 4.2 Ga. This indicates the early Archean Acasta granitoids were derived from Hadean crustal materials. It has yet to be established, however, whether their protoliths were sourced exclusively from Hadean crust, or whether they represent juvenile melts that interacted with, and entrained a limited amount of Hadean crust. Better constraints on these differing models can be provided by the radiogenic isotope systematics. The first radiogenic isotope study of the Acasta Gneiss Complex (Bowring et al., 1989b) reported a whole-rock Nd chondritic model age of ~4.1 Ga for a tonalitic gneiss. Bowring and coworkers (Bowring et al., 1990; Bowring and Housh, 1995) later reported a wide range of initial 143 Nd/ 144 Nd, when calculated back to the zircon U Pb ages, from markedly radiogenic to unradiogenic relative to the chondritic uniform reservoir (CHUR), leading to an inference that the silicate Earth was highly differentiated into a geochemically depleted reservoir and its complementary enriched reservoir by the early Archean. Additionally, the 142 Nd/ 144 Nd of a few Acasta gneisses have been shown to be indistinguishable from those of the modern accessible mantle(mcculloch and Bennett, /$ see front matter 2008 Elsevier B.V. All rights reserved. doi: /j.chemgeo

2 T. Iizuka et al. / Chemical Geology 259 (2009) ; Jacobsen and Harper, 1996; Caro et al., 2006), and are more radiogenic relative to CHUR (Boyet and Carlson, 2005). The superchondritic 142 Nd/ 144 NdmaybesuggestiveofthederivationoftheAcasta rocks from a mantle reservoir that was depleted during the extant of 146 Sm (in the first ~400 Myr of the Solar System). Since the Acasta gneisses have been subject to several periods of intense high-grade metamorphism and ductile deformation, however, it is unclear as to whether the whole-rock Sm Nd isotope systems have remained closed during these events. Indeed, 147 Sm 143 Nd whole-rock data on 34 Acasta samples yielded a regression age of 3.37 Ga (Moorbath et al., 1997), corresponding to the age of an important metamorphic event in the complex (Bleeker and Stern, 1997; Stern and Bleeker, 1998). This suggests that the whole-rock Sm Nd isotope systematics of the Acasta gneisses likely do not yield valid information on crust mantle evolution at the time of their magmatic zircon crystallization (Kamber et al., 2001; Whitehouse et al., 2001). Zircon can retain its primary Lu Hf isotopic signature through highgrade metamorphism due to its ability to remain a closed system during thermal resetting. This, in conjunction with the use of zircon as a U Pb geochronometer, makes the Lu Hf isotope system in zircon ideal to study the radiogenic isotopic evolution of the early Archean gneiss (e.g., Patchett et al., 1981; Vervoort et al., 1996). Limited Lu Hf isotopic Fig. 1. Geological map of the Acasta Gneiss Complex, Slave Province, northwestern Canada after Iizuka et al. (2006) and Iizuka et al. (2007). Location of samples is indicated.

3 232 T. Iizuka et al. / Chemical Geology 259 (2009) analyses of ca Ga to 3.57 Ga Acasta zircon by solution multicollectorinductively coupled plasma-mass spectrometry (MC-ICPMS) have demonstrated that these zircon grains were derived from significantly older crustal materials (Amelin et al., 2000). These data are difficult to interpret because it is not clear if the Lu Hf isotopic results are analyses of magmatic zircon, or mixture with altered domains, xenocrystic cores and/or metamorphic overgrowths, all of which are common Acasta gneisses (Stern and Bleeker, 1998; Bowring and Williams, 1999; Rayner et al., 2005; Iizuka et al., 2006; Iizuka et al., 2007). In this study, we extend the application of the zircon Lu Hf isotope systematics to the older Acasta granitoids, including the oldest known rocks, but here we utilize a procedure of combining in-situ laser ablation (LA)-MC-ICPMS Lu Hf isotope analysis with cathodoluminescence (CL) imagery to try to minimize the effects of mixed analyses and to obtain potentially less ambiguous isotopic information on the early Archean crust. 2. Regional geology The Acasta Gneiss Complex is exposed along the western margin of the Slave Province, northwestern Canada. Fig. 1 shows a detailed geological map of the main area of the complex based on Iizuka et al. (2006) and Iizuka et al. (2007). The main area is subdivided into two blocks by a major crustal structure that trends northeast. The eastern block comprises tonalitic to granitic gneiss (felsic gneiss series) and quartz dioritic gneiss (mafic intermediate gneiss series). The general gneissic foliation of the felsic gneiss series strikes NW to N and dips W. Its magmatic protoliths were interpreted as emplaced at Ga (Bowring et al., 1989a; Bowring and Housh, 1995; Iizuka et al., 2006; Iizuka et al., 2007), Ga (Bowring and Housh, 1995; Iizuka et al., 2007), 3.66 Ga (Iizuka et al., 2007) and Ga (Bowring and Housh,1995; Iizuka et al., 2007). The mafic intermediate gneiss series occurs mainly as 3 km 1 km to 10 cm 10 cm enclaves within the felsic gneiss. The ages of their protoliths are interpreted to be 4.0 Ga and 3.6 Ga (Bowring and Housh, 1995). The western block is dominated by a layered gneiss sequence that comprises tonalitic- to granitic- and dioritic- to quartz dioritic suites (layered gneiss series) and is intruded by N-trending foliated granite bodies up to 100 m wide (foliated granite). The gneissic foliation generally strikes N and dips W. The granitic intrusions are cut by the central fault, whereas NW-trending mafic dikes transect the central fault (Fig. 1). At least three magmatic events have been recognized in this block: formation of the layered gneiss granitoid protoliths that are interpreted to have been emplaced at Ga (Bowring et al., 1989a; Bowring and Housh, 1995; Iizuka et al., 2007) and 3.74 Ga (Bowring and Housh, 1995), and intrusion of the granitic protolith to the foliated granite at 3.58 Ga (Iizuka et al., 2007). The protoliths of the Acasta gneisses were subjected to intense metamorphism at ca Ga, 3.6 Ga and 3.4 Ga, coincident with the intrusion of the younger granitoid suites (Bleeker and Stern, 1997; Stern and Bleeker, 1998; Iizuka et al., 2006; Iizuka et al., 2007). The youngest event appears to have been responsible for the resetting of whole rock Sm Nd isotopic data (Moorbath et al., 1997). Subsequently, the complex was subjected to numerous minor magmatic and metamorphic episodes including the intrusion of granitic and syneitic sheets at 2.88 Ga, 2.6 Ga, and 1.8 Ga (Bleeker and Stern, 1997) and thermal events relating to the ca. 1.9 Ga Wopmay Orogeny (Hodges et al., 1995; Sano et al., 1999). The northwest-tending mafic dikes that crosscut all fabrics and faults in the complex may be part of the ca Ga Mackenzie dike swarm, which intruded the Slave Province (LeCheminant and Heaman, 1989), and could also potentially contribute to system complexity. 3. Analytical methods Zircon grains were separated from rock samples using standard crushing, magnetic separation and heavy-liquid techniques. The grains Fig. 2. Cathodoluminescence images of grain #21 and #09 from tonalitic gneiss AC478. Circles show positions for U Pb dating (D) and Lu Hf isotope analyses (Hf). Scale bars are 50 µm. were mounted in epoxy and were polished to expose their mid sections. Before isotopic analyses, we checked the external and internal structure of the zircon using transmitted/reflected light microscopy and cathodoluminescence (CL) imaging (Fig. 2 and Appendix A). We obtained CL images using a JEOL JSM-5310 scanning electron microprobe combined with an Oxford CL system at the Tokyo Institute of Technology. The Lu Hf isotope analyses were performed using a MicroLas GeoLas 200CQ ArF excimer laser ablation system attached to a Nu Plasma 500 MC-ICPMS at the Tokyo Institute of Technology. Helium gas ( l/min) was flushed into the ablation cell, minimizing aerosol deposition around the ablation pit and improving transport efficiency (Eggins et al., 1998). Furthermore, 4 ml/min N 2 were mixed into the Ar sample carrier gas (0.8 l/min) to enhance the signal intensity. Analyses were carried out with beam diameters of 35 or 63 µm, 6 15 Hz repetition rates, and ~45 s ablation times, resulting in pits µm deep. The Nu Plasma 500 MC-ICPMS contains 12 Faraday collectors. The dispersion of the spectrometer is adjusted by using a pair of quadrupole lenses that act as zoom lenses, focusing the ion beams into the Faraday collectors. The lenses were set to detect 171 Yb, 173 Yb, 175 Lu, 176 (Hf+Yb+Lu), 177 Hf, 178 Hf, 179 Hf and 180 Hf simultaneously. The analyses were carried out using a time resolved analytical (TRA) procedure, in which signal intensities for each mass and isotope ratios, including the initial 176 Hf/ 177 Hf are displayed as a function of time during the analysis. This procedure allows one to select a stable interval of the ablation for analysis and to evaluate the depth profile isotopic homogeneity of the analysis, whereas the CL images only present a two-dimensional view of the zircon grain surface. Mass discrimination effects were corrected using an exponential law (Russell et al., 1978; Albarède et al., 2004) defined by R meas i=j = R true i=j m β i ð1þ m j where m i is mass of isotope i, R meas and R true are true and measured isotope ratios, respectively. Thus mass bias factor ß is β = ln! Rmeas i=j R true =ln m i i=j m j Mass bias factor for Hf, ß(Hf), was calculated by normalizing 179 Hf/ 177 Hf to (Patchett et al., 1981). To obtain accurate 176 Hf/ 177 Hf ratios for zircon, the contribution of isobaric interferences by 176 Lu and 176 Yb on the 176 Hf signal must be carefully corrected. The interferences ð2þ

4 T. Iizuka et al. / Chemical Geology 259 (2009) Table 1 Results from in-situ Lu Hf isotope analysis of zircon standards Zircon Age (Ma) a 176 Lu/ 177 Hf (2 S.D.) 176 Yb/ 177 Hf (2 S.D.) 176 Hf/ 177 Hf (2 S.D.) Initial 176 Hf/ 177 Hf (2 S.D.) ±3 Long term averages (n=108) ± ± ± ±47 [ ] b [ ] This study (n=12) ± ± ± ±41 QGNG (n=10) ± ± ± ± ±37 [ ] [ ] TEMORA (n=12) 416.8± ± ± ± ±34 [ ] [ ] a b Age data for 91500, QGNG, and TEMORA are from Wiedenbeck et al. (1995), Black et al. (2003a), and Black et al. (2003b), respectively. The numbers in parentheses are ranges. were corrected by measuring 175 Lu and 173 Yb. The signal intensity of 176 Hf was calculated by I 176Hf = I 176ðLu " + Yb + HfÞ I 175Lu R true 176Lu=175Lu m βðluþ 176Lu + I m 173Yb R true 176Yb=173Yb m # βðybþ 176Yb 175Lu m 173Yb ð3þ where I i =signal intensity of isotope i, and ß(Lu) and ß(Yb) are mass bias factors for Lu and Yb, respectively. The true 176 Lu/ 175 Lu value of (Chu et al., 2002) and 176 Yb/ 173 Yb value of (Thirlwall and Anczkiewicz, 2004) were employed for the calculation. The ß(Yb) was calculated by normalizing the measured 173 Yb/ 171 Yb to (Thirlwall and Anczkiewicz, 2004), whereas the ß(Lu) was assumed to be identical to the ß(Hf). More detailed instrumental setting and analytical procedures are described in Iizuka and Hirata (2005). We have accumulated Lu Hf isotopic data for the zircon standard 91500, which is one of the most widely distributed and therefore extensively investigated standards for Lu Hf isotope systematics by TIMS (Wiedenbeck et al., 1995) and solution MC-ICPMS (Amelin et al., 2000; Goolaerts et al., 2004; Woodhead et al., 2004; Davis et al., 2005; Nebel-Jacobsen et al., 2005; Qi et al., 2005; Richards et al., 2005; Wu et al., 2006; Blichert-Toft, 2008). Our long term repeated analysis yields an initial 176 Hf/ 177 Hf value of ±47 (2 S.D.) (Table 1). The initial 176 Hf/ 177 Hf value is somewhat higher than a reference value of ±6, which is calculated from the present-day 176 Hf/ 177 Hf value of ±6 (Blichert-Toft, 2008: normalized to 176 Hf/ 177 Hf = for JMC-Hf 475) and 176 Lu/ 177 Hf value of ±3 obtained from the mean values of solution chemistry studies, even though the relative difference is smaller than the error. To allow comparison with literature values, our Hf isotopic data were normalized to a value of for the standard, using the mean initial 176 Hf/ 177 Hf values obtained for this standard on any given day. To evaluate the accuracy and precision of data obtained by the present technique, we have analyzed the zircon standards QGNG and TEMORA. Our determinations of the initial 176 Hf/ 177 Hf for QGNG ( ±37, 2 S.D.) and for TEMORA ( ±34, 2 S.D.) compare well with the solution-mc-icpms results for QGNG ( ±4, calculated from 176 Lu/ 177 Hf = and present-day 176 Hf/ 177 Hf= ±4 reported by Woodhead and Hergt, 2005) and TEMORA ( ±8, calculated from 176 Lu/ 177 Hf= and present-day 176 Hf/ 177 Hf= ±8 reported by Woodhead and Hergt, 2005), respectively. This clearly indicates that the present analytical protocols are robust under in-situ analysis of moderate to high HREE/Hf zircon. We report analytical errors on the initial 176 Hf/ 177 Hf, rather than the present-day 176 Hf/ 177 Hf, for single spot measurements (Table 3), because resolvable variations in the present-day 176 Hf/ 177 Hf due to radiogenic ingrowth of 176 Hf may exist within early Archean zircon grains having heterogeneous Lu/Hf. The analytical errors combine the internal run errors (2 S.E.) and the reproducibility of the standard analyses (2 S.D.), added in quadrature. The Lu Hf isotopic data for the standard during the period of this study is shown in Appendix B. Calculation of initial 176 Hf/ 177 Hf use the 176 Lu decay constant of yr 1 (Söderlund et al., 2004). The present-day chondritic parameters reported by Bouvier et al. (2008) are used to calculate ε Hf (T) values and model ages. Note that using the chondritic parameters of Blichert- Toft and Albarède (1997) would result in a change in ε Hf (T) value of 0.6, which is smaller than the analytical uncertainty. 4. Samples and results The zircons analyzed in this study were extracted from four samples of the felsic gneiss series (granitic gneiss AY066, AC458, and AY120 and tonalitic gneiss AC012) and one sample of a tonalitic layer of the layered gneiss series (AC478) (Fig. 1). We performed U Pb dating of the zircons previously (AC012 Iizuka et al., 2006, others Iizuka et al., 2007). The U Pb data are given in the supplementary material (Appendix C), and summarized in Table 2. As observed within other early Archean gneisses, the zircon grains from most of the Acasta Table 2 Summary of U Pb ages of zircons from the Acasta gneisses Sample Sample locality Rock type Zoning type N of spots analyzed N of grains analyzed 207 Pb/ 206 Pb age range (Ga) Inferred crystallization a age (Ma) AY N, W Granitic gneiss Oscillatory zoned ca ±26 12 AC N, W Granitic gneiss Oscillatory zoned ±70 11 Xenocrystic core AY N, W Granitic gneiss Oscillatory zoned ±40 5 AC N, W Tonalitic gneiss Oscillatory zoned ± Xenocrystic core 2 1 ca. 4.2 Altered AC N, W Tonalitic gneiss Oscillatory zoned ±26 8 Recrystallized 2 2 ca. 3.5 AY N, W Granodioritic gneiss Oscillatory zoned ±41 7 Altered Data sources for AC012 and others are from Iizuka et al. (2006) and Iizuka et al. (2007), respectively. The U Pb isotopic data are given in Appendix C. a The protolith crystallization ages are calculated from the mean (2 S.D.) of the highest identical 207 Pb/ 206 Pb of oscillatory-zoned zircon. N of 207 Pb/ 206 Pb used for the age calculation

5 234 T. Iizuka et al. / Chemical Geology 259 (2009) samples have complex U Pb isotope systematics (Appendix C). The complexity of the U Pb data, which can be caused by many factors such as multiple Pb-loss events, zircon inheritance, recrystallization, and new (secondary) growth, makes it difficult to determine precise crystallization ages for their magmatic protoliths. However, if magmatic zircon, that has lost Pb to various degrees, can be distinguished from metamorphic and xenocrystic zircon with the aid of a detailed imaging study, then a crystallization age of the magmatic protoliths can be estimated from the magmatic zircon with the highest radiogenic 207 Pb/ 206 Pb that are equal within analytical uncertainty of each other (see also discussions in Nutman et al., 1997; Bowring and Williams, 1999). Given that oscillatory zoning is very common in magmatic zircon (Rubatto and Gebauer, 1998; Corfu et al., 2003), we calculated the protolith crystallization ages from the mean (2 S.D.) of the highest identical 207 Pb/ 206 Pb of multiple oscillatory-zoned zircon grains. The Lu Hf isotope analysis pits were placed within similar internal structures and close to the original pits produced during the U Pb isotopic analyses (Fig. 2 and Appendix A). To determine the primary Lu Hf isotopic compositions of the Acasta gneiss protoliths, we specifically targeted the concordant oscillatory-zoned zircon grains. Since the Table 3 Lu Hf isotopic data for zircons from Acasta gneisses Grain-spot Internal structure a Age (Ma) 206 Pb/ 238 U (2) b 207 Pb/ 206 Pb (2σ) b 176 Lu/ 177 Hf (2 S.E.) 176 Yb/ 177 Hf (2 S.E.) 176 Hf/ 177 Hf Initial 176 Hf/ 177 Hf (2σ) c AY066 (T=3586±26 Ma) 01 2 osc 3639 ± ± ± ± ± osc 3569± ± ± ± ± osc 3600± ± ± ± ± osc 3613± ± ± ± ± osc 3596± ± ± ± ± osc 3577± ± ± ± ± osc 3586± ± ± ± ± osc 3602 ± ± ± ± ± osc 3556± ± ± ± ±45 AC458 (T=3585±70 Ma) 05 3 osc 3395± ± ± ± ± osc 3395± ± ± ± ± xenocrystic 3889 ± ± ± ± ± xenocrystic 3889 ± ± ± ± ± osc 3569± ± ± ± ± osc 3592± ± ± ± ± osc 3586± ± ± ± ± osc 3586± ± ± ± ±61 AY120 (T=3728±40 Ma) 03 2 osc 3370± ± ± ± ± osc 3833± ± ± ± ± osc 4094 ± ± ± ± ± osc 3533± ± ± ± ± osc 3994 ± ± ± ± ±56 AC012 (T=3942±100 Ma) 10 2 osc 3933± ± ± ± osc 3900± ± ± ± osc 3876± ± ± ± osc 3978± ± ± ± osc 3956± ± ± ± 58 AC478 (T=3974±26 Ma) 01 2 osc 3919± ± ± ± ± osc 3760 ± ± ± ± ± osc 4023 ± ± ± ± ± osc 3691± ± ± ± ± osc 3967± ± ± ± ± osc 3828 ± ± ± ± ± osc 4043 ± ± ± ± ± recryst 2961± ± ± ± ± osc 3990 ± ± ± ± ± osc 3863 ± ± ± ± ± osc 3933± ± ± ± ± osc 4024± ± ± ± ± osc 3859 ± ± ± ± ± osc 3941± ± ± ± ± osc 4012± ± ± ± ± osc 3867± ± ± ± ± osc 3914± ± ± ± ± recryst 3343± ± ± ± ± osc 3940 ± ± ± ± ±51 U Pb age data sources for AC012 and others are from Iizuka et al. (2006) and Iizuka et al. (2007), respectively. a Internal structures in cathodoluminescence images osc oscillatory zoning; recryst recrystallized; xenocrystic xenocrystic core. b Errors include the counting statistics and the reproducibility of the standard analyses (2 S.D.). c Initial 176 Hf/ 177 Hf ratios are calculated using the λ l76 Lu value of Söderlund et al. (2004) and the protolith ages (T) with exception of recrystallized and xenocrystic zircons, for which zircon 207 Pb/ 206 Pb ages were used. Errors combine the internal errors and the reproducibility of the standard analyses (2 S.D.) in quadrature.

6 T. Iizuka et al. / Chemical Geology 259 (2009) diffusion of Lu and Hf in crystalline zircon is slower than that of Pb (Cherniak et al., 1997a,b), it is expected that the Lu Hf isotope systematics of such zircon grains have not been disturbed during later metamorphic events. Given the slower diffusion rate of Hf relative to rare earth elements that are major contributors to CL intensity (Mariano, 1989; Marshall, 1988), the preservation of magmatic oscillatory zoning structures in CL images also suggests that the Lu Hf isotope systematics have been preserved. For sample AC478 we also analyzed oscillatoryzoned zircon grains that have undergone demonstrable Pb-loss and recrystallized parts to evaluate the relationship between the U Pb and Lu Hf isotope systems and the effect of zircon alteration on its Lu Hf isotopic composition. Furthermore, for sample AC458 we analyzed a xenocrystic core to compare Hf isotopic compositions between the xenocrystic core and magmatic zircon grains. The Lu Hf isotopic data for all zircon grains are summarized in Table 3 and graphically presented on Fig. 3 as plots of initial 176 Hf/ 177 Hf against the 207 Pb/ 206 Pb age of individual zircon grains (in the case where a single oscillatory-zoned zircon has multiple 207 Pb/ 206 Pb ages, the oldest age is plotted). The initial Hf isotope ratios of oscillatoryzoned zircon domains are calculated using the crystallization ages of the protoliths of their host rocks, with exception of the recrystallized zircon suites from sample AC478 and the xenocrystic zircon core from sample AC458 for which their individual 207 Pb/ 206 Pb ages were used. The difference between initial 176 Hf/ 177 Hf values calculated using the protolith crystallization ages and individual zircon 207 Pb/ 206 Pb ages are significantly smaller than the level of analytical uncertainty of the present technique. To test the reliability of the correction method for isobaric interferences on 176 Hf in the unknown sample analyses, we plot initial 176 Hf/ 177 Hf versus 176 Lu/ 177 Hf (Fig. 4a) and 176 Yb/ 177 Hf (Fig. 4b) for all zircon analyzed here. The figure illustrates that while the zircons exhibit awiderangeof 176 Lu/ 177 Hf and 176 Yb/ 177 Hf, no correlation was observed between these and calculated initial 176 Hf/ 177 Hf values, demonstrating the accuracy of the interference correction method Sample AY066 (granitic gneiss, N, W) Granitic gneiss sample AY066 typically yielded euhedral to subhedral, 300 to 400 µm long, and oscillatory-zoned zircon grains (Appendix A, AY066 #01). All twelve U Pb isotope analyses on oscillatory-zoned grains are concordant with a mean 207 Pb/ 206 Pb value of ±55 (2 S.D.), giving an age of 3586±26 Ma (Table 2 and Appendix C). The age is considered the crystallization age of the granite (protolith of AY066). The Lu Hf isotopic data were obtained from nine out of dated zircon grains. All ratios lie slightly below the CHUR evolution curve (Table 3 Fig. 3. Plots of 207 Pb/ 206 Pb age versus initial 176 Hf/ 177 Hf value for zircon from (a) AY066, (b) AC458, (c) AY120, (d) AC012, and (e) AC478. Error bars represent errors quoted in Table 2. CHUR evolution curve is based on parameters proposed by Bouvier et al. (2008).

7 236 T. Iizuka et al. / Chemical Geology 259 (2009) Fig. 4. Plots of initial 176 Hf/ 177 Hf ratio versus (a) 176 Lu/ 177 Hf and (b) 176 Yb/ 177 Hf for zircons analyzed in this study. The lack of correlation suggests that the isobaric interference correction method used here produced accurate Hf isotopic data. and Fig. 3a) and yield a mean initial 176 Hf/ 177 Hf of ±58 (2 S.D.), corresponding to ε Hf (T) value of 2.4±2.2 at 3586 Ma (Table 4) Sample AC458 (granitic gneiss, N, W) Zircons from granitic gneiss AC458 were generally euhedral to subhedral, and 150 to 300 µm long. The oscillatory-zoned domains (Appendix A, AC458 #13) range in 207 Pb/ 206 Pb age from ca. 3.6 Ga to 3.3 Ga (Table 2 and Appendix C), suggesting that granitic magma (protolith of AC458) was emplaced at ca. 3.6 Ga, and subsequently underwent metamorphism. The eleven highest 207 Pb/ 206 Pb measurements on oscillatory-zoned zircon are equal within analytical uncertainty and yielded nearly concordant ages. Their mean 207 Pb/ 206 Pb value of ±148 (2 S.D.) gives an age of 3585±70 Ma, was interpreted as the best estimate of the crystallization age of the granite. Importantly, the CL images reveal that some magmatic zircons contained non-zoned xenocrystic cores (Appendix A, AC458 #03), which have 207 Pb/ 206 Pb ages up to 3.9 Ga (Table 2 and Appendix C). Six spot analyses for Lu Hf isotopic composition were made on four of the most concordant grains of oscillatory-zoned igneous zircon. All initial 176 Hf/ 177 Hf values are equal within analytical uncertainty (Table 3 and Fig. 3b), and yield a mean value of ±49 (2 S.D.), which corresponds to an ε Hf (T) of 3.2±2.5 at 3585 Ma (Table 4). We also determined the Lu Hf isotopic compositions of two sites in a xenocrystic core that gave 207 Pb/ 206 Pb ages of ca. 3.9 Ga, and obtained initial 176 Hf/ 177 Hf values of and (Table 3 and Fig. 3b). Table 4 Lu Hf isotopic data for the Acasta gneisses Rock Age (Ma) Initial 176 Hf/ 177 Hf (2 S.D.) ε Hf (T) a T PM (Ga) b AY ± ±58 2.4± AC ± ±49 3.2± AY ± ±63 6.1± AC ± ±61 1.2± AC ± ± ± AY199 c 3744± ±52 6.7± ε Hf (T) and T PM are calculated with chondritic parameters of Bouvier et al. (2008) and the λ 76 Lu of Söderlund et al. (2004). a Errors are propagated to include uncertainties in age, initial 176 Hf/ 177 Hf and chondritic parameters. b Calculated using the ratio of average Precambrian granitoid crust ( 176 Lu/ 177 Hf=0.0093) (Vervoort and Patchett, 1996). c Age and Lu Hf isotopic data are from Iizuka et al. (2007) and Iizuka and Hirata (2005), respectively Sample AY120 (granitic gneiss, N, W) The zircon grains separated from granitic gneiss sample AY120 are euhedral to subhedral and 100 to 200 µm long. The U Pb data from oscillatory-zoned domains (Appendix A, AY120 #06) spread from concordia with ages of ca Ga to discordia with 207 Pb/ 206 Pb ages of ca Ga (Table 2 and Appendix C). We interpret a mean age of 3728±40 Ma, which is calculated from the six highest 207 Pb/ 206 Pb producing a mean of ±94 (2 S.D.), to indicate the crystallization age of the protolith. We determined the Lu Hf isotope compositions of five oscillatoryzoned zircon grains having 207 Pb/ 206 Pb ages older than 3.66 Ga. The initial 176 Hf/ 177 Hf values of the grains are uniform with a mean value of ±63 (2 S.D.), equivalent to an ε Hf (T) of 6.1±2.5 (Tables 3 and 4 and Fig. 3c) Sample AC012 (tonalitic gneiss, N, W) The zircon grains separated from tonalitic gneiss sample AC012 are typically µm long, and are euhedral to subhedral. The analyses of oscillatory-zoned zircon (Appendix A, AC012 #47) range in 207 Pb/ 206 Pb age from 4.0 to 3.4 Ga (Table 2 and Appendix C), suggesting that crystallization of the protolith took place at around 4.0 Ga. We estimated the (minimum) protolith crystallization age from the ten highest 207 Pb/ 206 Pb values of oscillatory-zoned zircon, which are identical within analytical uncertainty and yield a mean value of ±272 (2 S.D.), and obtained an age of 3942±100 Ma. In addition, the CL and backscattered electron images (Fig. 1 in Iizuka et al., 2006) showed that a single magmatic zircon contains a xenocrystic core. LA-ICPMS and SHRIMP dating defined its 207 Pb/ 206 Pb age as ca. 4.2 Ga. We analyzed five of the oldest 207 Pb/ 206 Pb aged oscillatory-zoned grains for their Lu Hf isotopic compositions. The data plot on or slightly below the CHUR evolution curve (Fig. 3d), with initial 176 Hf/ 177 Hf values ranging from to (Table 3). We obtained a mean initial 176 Hf/ 177 Hf value of ±61 (2 S.D.) and ε Hf (T) of 1.2±3.3 at 3942 Ma (Table 4) Sample AC478 (tonalitic gneiss, N, W) The tonalitic gneiss sample AC478 yielded euhedral to subhedral and µm long zircon grains that reveal oscillatory zoning under CL images (Appendix A, AC478 #12). However, in some grains

8 T. Iizuka et al. / Chemical Geology 259 (2009) transgressive recrystallization structures (luminous and homogeneous under CL, e.g., Corfu et al., 2003) cut the oscillatory zoning or mosaic structures (Fig. 2). Analyses of oscillatory-zoned sites plot close to concordia with a spread of 207 Pb/ 206 Pb ages from ca. 4.0 Ga to 3.75 Ga (Table 2 and Appendix C). In contrast, analyses of the recrystallized sites yielded 207 Pb/ 206 Pb ages at ca Ga (Table 2 and Appendix C). These data suggest that the tonalite crystallized at around 4.0 Ga, and underwent metamorphism at ca Ga. We estimated the crystallization age from the eight oldest 207 Pb/ 206 Pb ages of oscillatory-zoned zircons that are identical within analytical uncertainty. A mean value of ±72 (2 S.D.) is equivalent to 3974±26 Ma. We measured the Lu Hf isotopes for two recrystallized sites with 207 Pb/ 206 Pb ages of ca Ga (Fig. 2) and seventeen oscillatory-zoned sites with 207 Pb/ 206 Pb ages ranging from ca. ~4.0 Ga to 3.76 Ga. While the oscillatory-zoned sites have initial 176 Hf/ 177 Hf values ranging from to , the two recrystallized sites have ratios of and respectively (Table 3 and Fig. 3e). The best estimate of a mean initial 176 Hf/ 177 Hf of ±42 (2 S.D.) for the tonalitic protolith of AC478 is given by the zircon that appear to preserve primary U Pb isotope systematics (i.e. the oscillatory-zoned zircon with the oldest concordant 207 Pb/ 206 Pb ages). This is equivalent to an ε Hf (T) value of 3.3±1.7 at 3974 Ma (Table 4). 5. Discussion 5.1. Zircon geochronology of Archean gneisses: The Hf isotope perspective As observed in this study, zircon populations from Archean gneisses often show complex age patterns (Appendix C). The crystallization ages of the gneiss protoliths have been estimated from the oldest 207 Pb/ 206 Pb ages of oscillatory-zoned zircon, assuming that the age variation in oscillatory-zoned zircons was caused by Pb-loss events that affected a single generation of magmatic zircon, rather than by mixing of multiple generation of magmatic zircon (e.g., Nutman et al., 1997; Bowring and Williams, 1999: Iizuka et al., 2007). However, since xenocrystic zircons could preserve their primary oscillatory zoning structures (e.g., Corfu et al., 2003), it is possible that the oldest oscillatory-zoned zircon ages within Archean gneisses, including the Acasta samples, represent the ages of xenocrysts that were caught in the protoliths (Moorbath et al., 1997; Whitehouse et al., 1999). Although the main intent of this paper is to use zircon U Pb isotopic data to determine accurate initial 176 Hf/ 177 Hf, it is worthwhile exploring this issue in light of the new Hf isotopic data presented here. Since diffusion studies (Cherniak et al., 1997a,b) suggest that zircon grains preserving oscillatory zoning in CL images would retain their primary Hf isotopic signatures, it would be anticipated that an oscillatory-zoned zircon population exhibiting various 207 Pb/ 206 Pb ages owing to ancient Pb-loss within a single magmatic population would yield a unique initial 176 Hf/ 177 Hf value. However, if the zircon population includes xenocrysts, Lu Hf isotope systematics for each zircon would likely yield distinct initial 176 Hf/ 177 Hf values rather than a single uniform isotopic population. Indeed, the xenocrystic core from the granitic gneiss (AC458) has markedly lower initial 176 Hf/ 177 Hf value relative to magmatic zircon grains (Fig. 3b). Thus, integration of imaging and Lu Hf isotope studies into U Pb dating of zircon can potentially allow one to resolve zircon inheritance from ancient Pb-loss (see also discussion in Amelin et al., 2000 and Gerdes and Zeh, 2008). Fig. 3e illustrates that oscillatoryzoned zircon grains with various 207 Pb/ 206 Pb ages extracted from the tonalitic gneiss (sample AC478) have the same initial 176 Hf/ 177 Hf value within analytical uncertainty. Even though the analytical uncertainty of the present technique is relatively large, this observation suggests that the 207 Pb/ 206 Pb age variation is not due to zircon inheritance, but more likely to ancient Pb-loss, consistent with our interpretation that the magmatic protolith of the tonalitic gneiss was emplaced by 3.97 Ga. In Fig. 3e, note that whereas one recrystallized zircon domain has a Hf isotopic composition similar to those of the oscillatory-zoned zircon Fig. 5. Plot of age versus ε Hf (T) for in-situ Lu Hf isotopic studies of the early Archean Acasta meta-granitoids (this study and Iizuka and Hirata, 2005) together with data for Acasta zircon reported by Amelin et al. (2000). Error bars represent errors quoted in Table 3. The dotted line show ε Hf evolution of average granitoid crust with 176 Lu/ 177 Hf= (Vervoort and Patchett,1996) with the assumption that it originated from undifferentiated mantle (CHUR) at 4.2 Ga. The CHUR evolution line is from Bouvier et al. (2008). sites, the other one yields a significantly higher initial 176 Hf/ 177 Hf. Since recrystallization of zircon is commonly promoted by the presence of aqueous fluids (Pidgeon, 1992; Nemchin and Pidgeon, 1997; Schaltegger et al., 1999), the higher initial 176 Hf/ 177 Hf value of the recrystallized zircon can be interpreted as reflecting the Hf isotopic signature of the aqueous fluid (Zheng et al., 2005). Alternatively, breakdown of minerals having high Lu/Hf during zircon recrystallization could also lead to more radiogenic 176 Hf/ 177 Hf compositions. In contrast, the other recrystallized zircon appears to have inherited the primary magmatic Hf isotopic signature through the recrystallization process (Choi et al., 2006). These observations clearly indicate the importance of imaging study for determining accurate Hf isotopic compositions of magmatic zircon grains from Archean gneisses Insights into the sources of the early Archean granitoids The protolith crystallization ages and Hf isotopic compositions of the five Acasta gneisses of this study, and those of Iizuka and Hirata (2005) (granodioritic gneiss AY199, N, W) are summarized in Table 4 and graphically presented on Fig. 5. The U Pb ages and Hf isotopic compositions of the zircon grains extracted from the Acasta gneisses by Amelin et al. (2000) are also plotted on Fig. 5 (recalculated with the 176 Lu decay constant of yr 1, and normalized to a value of for JMC-475 Hf standard). The 3.59 Ga protoliths of granitic gneisses (AY066 and AC458) have slightly negative ε Hf (T) values ( 3.2 to 2.4). These are comparable to the Hf isotopic data obtained by Amelin et al. (2000) from ca Ga Acasta rocks (Fig. 5) and are in the same range as Ga detrital zircon from Jack Hills in Western Australia (Amelin et al., 1999). The negative ε Hf (T) values of the protoliths suggest that they were formed by remelting of older crustal material, consistent with the presence of xenocrystic zircon cores within the protolith of sample AC458. The ε Hf (T) values for the ca Ga granitoids AY120 and AY199 are more highly negative ( 6.7 to 6.1), suggesting reworking of significantly older crustal material relative to the granitoids. These ε Hf (T) values are similar to that of a 3.69 Ga xenocrystic zircon extracted from a 3.59 Ga Acasta tonalite (Fig. 5; Amelin et al., 2000) as well as a 3.72 Ga detrital zircon from Jack Hills (Amelin et al., 1999). The oldest tonalitic gneisses (ca Ga; AC012 and AC478) exhibit nearly chondritic to moderately negative ε Hf (T) values, which are close to the median value obtained for the ca. 4.0 Ga detrital zircon from Jack Hills (Amelin et al., 1999; Harrison et al., 2005; Blichert-Toft and Albarède, 2008; Harrison et al., 2008). These Hf isotopic signatures suggest that even the oldest known Acasta rocks were derived by older crustal material,

9 238 T. Iizuka et al. / Chemical Geology 259 (2009) in agreement with the presence of significantly older zircon xenocrysts in the oldest known rocks (Bowring and Williams, 1999; Iizuka et al., 2006). Importantly, the Acasta zircons of this study and Amelin et al. (2000) contain no Hf isotopic evidence for a depleted mantle component in the source of the Acasta rocks, despite superchondritic initial 143 Nd/ 144 Nd values (calculated back to the zircon crystallization ages) in the whole-rock samples. Given that the Lu/Hf and Sm/Nd fractionation by magmatic processes are generally well correlated (Vervoort and Patchett, 1996), the discrepancy suggests the disturbance of the whole-rock Sm Nd isotope systems during later metamorphism, consistent with the younger whole-rock 147 Sm 143 Nd isotopic regression age of 3.37 Ga (Moorbath et al., 1997). To estimate the approximate age of the reworked crust within the Acasta gneiss protoliths, we determined the model age T PM for the early Archean granitoids, under the assumption that the parental magmas of the protoliths were produced from crustal materials that were originally extracted from undifferentiated primitive mantle (CHUR) and had 176 Lu/ 177 Hf of (Vervoort and Patchett, 1996)(Table 4). Note that if (1) the parental magmas were mixtures of juvenile magmas and reworked crustal materials, rather than consisting entirely of reworked crustal materials; or (2) early Earth's upper mantle was depleted in Hf relative to Lu (as suggested by Harrison et al., 2005; Blichert-Toft and Albarède, 2008 based on Hf isotopic composition of Hadean detrital zircons), the extraction ages of the crustal materials will be older than the T PM.TheT PM obtained for the 3.59 Ga granitoids (AY066 and AC458) are ca Ga. These ages are consistent with the age of the zircon xenocrysts within granitoid sample AC458. On the other hand, the T PM obtained for the ca Ga (AY120 and AY199) and ca Ga granitoids (AC012 and AC478) are ca Ga. These are consistent with the ages up to 4.2 Ga of the zircon xenocrysts within the oldest granitoids (Bowring and Williams, 1999; Iizuka et al., 2006) and strongly suggest that Hadean crust had significantly contributed to the genesis of some of the early Archean granitoids. 6. Conclusions We have studied the Lu Hf isotopic compositions of zircon grains extracted from various rocks of the Acasta Gneiss Complex using LA- MC-ICPMS coupled with detailed CL images of the internal structure of the grains. A comparison of the data from oscillatory-zoned zircon domains and from regions of recrystallized zircon indicates that metamorphism-induced alteration has significantly affected the Hf isotopic composition. This indicates that detailed imaging studies and careful placing of the analytical pits are critical for interpreting and understanding the Lu Hf isotopic data of zircon. The Lu Hf isotope systematics of magmatic zircon grains indicate that the Ga granitoids have near-chondritic to sub-chondritic initial 176 Hf/ 177 Hf values. This is consistent with the presence of zircon xenocrysts in these rocks (Iizuka et al., 2006; Iizuka et al., 2007), and suggests that some of them were formed from Hadean crustal materials that were extensively reworked during the early Archean. Acknowledgements We wish to thank Mr. M. Senkiw, Drs. W. Humphries, D. Baldwin and K. Gochnauer for assistance during our field seasons within the Acasta Gneiss Complex. Drs. Y. Ueno, I. Katayama, A. Motoki and S. Rino joined our geological survey. We are grateful to Dr. G. Mortimer for generously providing precious zircon standards, and two anonymous reviewers for critical and thorough reviews of various versions of this paper. T. Iizuka thanks the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi: /j.chemgeo References Albarède, F., Telouk, P., Blichert-Toft, J., Boyet, M., Agranier, A., Nelson, B., Precise and accurate isotopic measurements using multiple-collector ICPMS. Geochimica et Cosmochimica Acta 68, Amelin, Y., Lee, D.C., Halliday, A.N., Early middle Archaean crustal evolution deduced from Lu Hf and U Pb isotopic studies of single zircon grains. Geochimica et Cosmochimica Acta 64, Amelin, Y., Lee, D.C., Halliday, A.N., Pidgeon, R.T., Nature of the Earth's earliest crust from hafnium isotopes in single detrital zircons. 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