Excess hafnium-176 in meteorites and the early Earth zircon record

Transcription

1 Article Volume 13, Number 3 3 March 2012 Q03002, doi: /2011gc ISSN: Excess hafnium-176 in meteorites and the early Earth zircon record Martin Bizzarro and James N. Connelly Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, DK-1350 Copenhagen, Denmark (bizzarro@snm.ku.dk) Kristine Thrane Geological Survey of Denmark and Greenland, DK-1350 Copenhagen, Denmark Lars E. Borg Lawrence Livermore National Laboratory, Livermore, California 94550, USA [1] The long-lived 176 Lu-to- 176 Hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation. Detrital Hadean zircons (>3.8 Gyr) from the Jack Hills metasedimentary belt of Western Australia record extremely enriched Hf-isotope signals suggesting early extraction of a continental crust (>4.5 Gyr) but fail to identify a prevalent complementary depleted mantle reservoir, suggesting that crust formation processes in the early Earth were fundamentally distinct from today. However, this conclusion assumes that the Hf-isotope composition of bulk chondrite meteorites can be used to estimate the composition of Earth prior to its differentiation into major silicate reservoirs, namely the bulk silicate Earth (BSE). We report a 176 Lu- 176 Hf internal mineral isochron age of Myr for the pristine SAH99555 angrite meteorite. This age is 300 Myr older than the age of the Solar System, confirming the existence of an energetic process yielding excess 176 Hf in affected early formed Solar System objects through the production of the 176 Lu isomer (t 1/2 3.9 hours). Thus, chondrite meteorites contain excess 176 Hf and their present-day composition cannot be used to infer the Lu-Hf parameters of BSE. Using a revised BSE estimate based on the SAH99555 isochron, we show that Earth s oldest zircons preserve a record of coexisting enriched and depleted hafnium reservoirs as early as 4.3 Gyr in Earth s history, with little evidence for the existence of continental crust prior to 4.4 Gyr. This new view suggests continuous juvenile crustal growth and recycling throughout the Hadean and Archean eras, perhaps analogous to modern plate tectonics. Components: 5100 words, 2 figures, 4 tables. Keywords: Bulk Silicate Earth; Lu-Hf; angrites; meteorites. Index Terms: 1040 : Radiogenic isotope geochemistry. Received 14 December 2011; Revised 19 January 2012; Accepted 23 January 2012; Published 3 March Bizzarro, M., J. N. Connelly, K. Thrane, and L. E. Borg (2012), Excess hafnium-176 in meteorites and the early Earth zircon record, Geochem. Geophys. Geosyst., 13, Q03002, doi: /2011gc Copyright 2012 by the American Geophysical Union 1 of 10

2 1. Introduction [2] Zircon is a ubiquitous accessory mineral of Earth s continental crust and, because of its durability and antiquity, is unique in preserving a record of the evolution of the terrestrial crust-mantle system in deep time. In addition to its standard utility for U-Pb geochronology, zircon is also ideally suited for the application of the 176 Lu-to- 176 Hf decay system (half-life of 37 Gyr), a powerful isotopic clock and tracer that allows identification of ancient chemical fractionation associated with planetary differentiation and crust formation [Patchett et al.,1981; Stevenson and Patchett, 1990; Amelin et al., 1999, 2000; Harrison et al., 2005; Kemp et al., 2006, 2009; Blichert-Toft and Albarède, 2008; Harrison et al., 2008; Pietranic et al., 2008; Kemp et al., 2010; Amelin et al., 2011]. Using this approach, concurrent U-Pb and Lu-Hf studies in Archean and Hadean zircons including the oldest remnants of Earth s earliest crust have identified unradiogenic Hf isotope signals [Harrison et al., 2008; Kemp et al., 2010] with little evidence for a complementary depleted reservoir. An implication of these data is that Earth s first crust was extracted from a primitive mantle as early as 4.5 Gyr, and extensively reprocessed throughout the Hadean era without additional generation of juvenile crust [Kemp et al., 2010]. This observation is difficult to reconcile with a modern-style plate tectonic regime, implying that crust formation in the Archean and Hadean epochs occurred by fundamentally distinct processes. However, these conclusions, based on the 176 Lu- 176 Hf system, presume accurate knowledge of the initial Hf isotope composition of the Solar System, which is used to infer the Hf isotope composition of Earth prior to its differentiation into major silicate reservoirs, that is, the Bulk Silicate Earth (BSE). Previous attempts to directly define the initial 176 Hf/ 177 Hf ratio of the Solar System were based on whole-rock measurements of sample suites with complex chemical and/ or thermal histories such as chondrite and eucrite meteorites [Patchett and Tatsumoto, 1980; Blichert- Toft and Albarède, 1997; Bizzarro et al., 2003; Patchett et al., 2004; Bouvier et al., 2008; Blichert- Toft et al., 2002], which fail to define isochronous relationships [Thrane et al., 2010]. Thus, estimates of the initial 176 Hf/ 177 Hf ratio of the Solar System are now based on the present-day Hf isotope composition of least metamorphosed chondrite meteorites [Bouvier et al., 2008], but this necessitates knowledge of the 176 Lu decay rate (l 176 Lu) as well as 176 Lu/ 177 Hf ratio of the chondritic reservoir to back-calculate the 176 Hf/ 177 Hf value to the initial value of BSE. Although recent calibrations of the l 176 Lu by geological comparison based on both terrestrial and extraterrestrial materials are now in excellent agreement [Scherer et al., 2001; Amelin, 2005; Söderlund et al., 2004], the 176 Hf/ 177 Hf and 176 Lu/ 177 Hf ratios of chondrite meteorites are sufficiently variable to make the choice for the correct value for BSE difficult. Additionally, the 176 Hf/ 177 Hf ratios of chondrite and eucrite meteorites were apparently affected by irradiation processes in the early Solar System resulting in accelerated 176 Lu decay through the formation of the 176 Lu isomer ( 176m Lu) at 123 kev excitation energy above the ground state, which decays to 176 Hf in 3.7 hours [Albarède et al., 2006].Ifcorrect,thismodelseriously undermines the assumption of a Lu-Hf chondritic Earth, because it requires that the material that accreted to form the terrestrial planets and chondrite meteorites were identically irradiated. [3] In light of these uncertainties, we have investigated the 176 Lu- 176 Hf systematics of the SAH99555 angrite to provide the first determination of the initial 176 Hf/ 177 Hf ratio of the Solar System by the internal isochron approach. Angrites are rapidly cooled and un-metamorphosed igneous meteorites of basaltic composition believed to have erupted at the surface of the angrite parent body in the first 10 Myr of the Solar System [Amelin, 2008]. They are characterized by an unusual mineralogy and chemistry mainly composed of Ca-rich olivine, fassaitic clinopyroxene, and anorthite [Mittlefehldt et al., 2002; Floss et al., 2003]. Some angrites display evident quenching textures due to rapid cooling from magmas. In these quenched angrites, ulvöspinel, troilite, and silico-phosphates are minor, but common late-stage crystallization products [Mikouchi and Barrat, 2009]. With an absolute Pb-Pb age of Myr [Connelly et al., 2008] and evidence for live 26 Al at the time of its crystallization [Spivak-Birndorf et al., 2009; Schiller et al., 2010], the SAH99555 quenched angrite is one of the oldest and most pristine basaltic meteorites. Moreover, trace element variations in SAH99555 indicate rapid crystallization under near closed system conditions, consistent with its mineralogical and textural features [Floss et al., 2003]. Thus, the SAH99555 angrite is ideally suited for high-resolution 176 Lu- 176 Hf systematics. 2. Methodology and Results [4] A three gram piece of SAH99555 acquired from Labenne Meteorites was lightly crushed with an 2 of 10

3 Table Lu- 176 Hf Data for the SAH99555 Angrite a Fraction Weight (mg) Lu (ppm) Hf (ppm) 176 Lu/ 177 Hf 176 Hf/ 177 Hf Bulk-rock (1) Bulk-rock (2) Pyroxene (1) Pyroxene (2) Olivine (1) Olivine (2) a Analytical uncertainties for the 176 Lu/ 177 Hf values refer to the last digits and represent the external reproducibility (0.47%, 2sd) estimated from the repeated analyses of the BHVO-1 standard. Analytical uncertainties for the 176 Hf/ 177 Hf values refer to the last digits and represent the internal precision (2se) of individual measurements. The external reproducibility of the 176 Hf/ 177 Hf values is 55 ppm (2sd) and was estimated by repeated analyses of the BHVO-1 standard. agate mortar and pestle and sieved to approximately 100 mm grain size. Following gentle cleaning in Milli-Q water and acetone, this material was passed through a Frantz magnetic separater to create concentrates of olivine and pyroxene that were subsequently purified by hand picking using a binocular microscope. An uncrushed piece of SAH99555 was broken into 2 pieces of approximately 100 mg each for duplicate whole rock analyses. The whole rock pieces were not cleaned or leached prior to dissolution to avoid dissolving soluble accessory phases such as phosphate minerals. Mineral fractions were pre-cleaned in the clean laboratory in several short cycles of warm distilled water and 0.1M HNO 3 before they were spiked with 176 Lu and 180 Hf tracer solutions and dissolved using a 5:1 mixture of 28M HF and 14M HNO 3 on a 130 C hotplate for 5 days. Whole rock fragments were spiked and digested in the same fashion as the mineral fractions, but without any pre-cleaning. The samples were then dried down and fluxed in 14M HNO 3 in as many cycles as was necessary to fully dissolve the sample. Purification of Lu and Hf utilized a first stage cation column and a second stage TODGA (Eichrom Industries) following the methods outlined by Connelly et al. [2006]. Lu and Hf were analyzed by MC-ICPMS following methods outlined by Bizzarro et al. [2003]. The concentration and isotopic data are presented in Table 1. Following Lu and Hf recovery, the remaining REE were purified using RE-spec resin in 0.05N and 1N HNO 3 prior to loading on pressurized 2-hydroxyisobutyric acid columns. The 2-hydroxyisobutyric acid was separated from the Sm and Nd using 2 ml cation cleanup columns. Total procedural blanks (N = 5) include contributions associated with the tracers and were: Sm = 6 3 picograms (pg) and Nd = 7 2 pg (1 sd). Isotopic measurements were done in the Radiogenic Isotope Laboratory at the University of New Mexico on a Micromass (VG) Sector 54 mass spectrometer. Neodymium and Sm were run as oxides on single Re filaments using an oxygen bleed system at torr. Mass fractionation of Nd was corrected using 146 Nd/ 144 Nd = , whereas Sm interference was corrected using 147 Sm/ 152 Sm = Measured 143 Nd/ 144 Nd ratios were normalized to the LaJolla Nd standard value of The Sm-Nd results are compiled in Table 2. [5] Two pyroxene, two olivine and two total rock fractions define an isochron in 176 Lu/ 177 Hf and 176 Hf/ 177 Hf space with slope and intercept of and , respectively (Figure 1). Using an average l 176 Lu of yr 1 determined by recent geological calibrations [Söderlund et al., 2004], we calculate a 176 Lu- 176 Hf age of Myr. This age is approximately 300 Myr older than the currently accepted age of the Solar System defined by condensation of calcium-aluminum-rich inclusions at Myr [Amelin et al., 2010] Table Sm- 143 Nd Fata for the SAH99555 Angrite a Fraction Weight (mg) Sm (ppm) Nd (ppm) 147 Sm/ 144 Nd 143 Nd/ 144 Nd Bulk-rock (1) Bulk-rock (2) La Jolla Nd (N = 11) a All samples and standards run as NdO +. Error limits apply to last digits and include a minimum uncertainty of 0.5% plus 50% of the blank correction for Sm and Nd added quadratically. Normalized to 146 Nd/ 144 Nd = Uncertainties refer to last digits and are 2s m calculated from the measured isotopic ratios. 2s m =[S(m i m) 2 /(n(n 1))] 0.5 for n ratio measurements m i with mean value m. Error limits refer to last digits and are 2s p.2s p =[S(M i p) 2 /(N 1)] 0.5 for N measurements M i with mean value p. 3 of 10

4 Figure 1. Lu-Hf isochron diagram for the SAH9995 angrite. We interpret the intercept of the regression [( 176 Hf/ 177 Hf) 0 ] to represent the initial 176 Hf/ 177 Hf ratio of the Solar System. MSWD, mean square of weighted deviations. Errors on the 176 Lu/ 177 Hf and 176 Hf/ 177 Hf are smaller than symbols. The external reproducibility of the 176 Hf/ 177 Hf and 176 Lu/ 177 Hf values, determined by repeated analysis of 36 sample digestions of the BHVO-1 basalt, are 0.47% and %, respectively. For the isochron calculations, we used the external reproducibility or the internal precision of the individual measurements, whichever is larger. and, thus, cannot represent the true crystallization age of SAH Discussion [6] The accuracy of the analytical protocol employed here for the Lu-Hf measurements has been evaluated by analysis of matrix-matched terrestrial rock standards processed though our chemical purification scheme in an identical manner as the extraterrestrial samples. The reproducibility and accuracy of the 176 Lu/ 177 Hf and 176 Hf/ 177 Hf measurements was estimated by repeated analysis of a reference rock standard. During the course of this study, 36 sample digestions of the Hawaiian basalt BHVO-1 were processed through the chemical purification procedure and analyzed by MC-ICPMS, yielding average values of and for the 176 Lu/ 177 Hf and 176 Hf/ 177 Hf ratios, respectively. These values are identical, within analytical uncertainties, to that obtained in previous studies [Blichert-Toft, 2001; Münker et al., 2001; Bizzarro et al., 2003; Vervoort et al., 2004; Connelly et al., 2006]. Isotope anomalies of nucleosynthetic origin affecting the hafnium mass array could, in principle, compromise the accuracy of the 176 Hf/ 177 Hf and 176 Lu/ 177 Hf isotope ratios presented here for SAH However, the analysis of an un-spiked total rock aliquot indicates that the Hf stable isotopes ratios of SAH99555 are identical, within analytical uncertainty, to the terrestrial values (Table 3). This result is consistent with a recent study suggesting the absence of nucleosynthetic Hf-isotope heterogeneity in chondrite meteorites [Sprung et al., 2010]. Quenched angrites such as SAH99555 typically contain minor Table 3. Stable Hf-Isotope Composition of an Unspiked Aliquot of the SAH Angrite a Fraction 178 Hf/ 177 Hf 179 Hf/ 177 Hf 180 Hf/ 177 Hf Bulk-rock (1) Accepted values a Analytical uncertainties for all isotope ratios refer to the last digits and represent the internal precision (2se) of individual measurements. The accepted Hf stable isotope ratios are based on Stevenson and Patchett [1990] and Blichert-Toft et al. [1997]. 4 of 10

5 accessory phases such as ulvöspinel and silicophosphates, which can potentially strongly fractionate rare earth and high-field strength elements. Although we have not analyzed the 176 Lu- 176 Hf systematics of these phases separately, we emphasize that the two whole-rock fractions lie on the mineral isochron defined by the olivine and pyroxene fractions. This observation is consistent with a closed system behavior of the 176 Lu- 176 Hf systematics in SAH99555, suggesting that the Lu-Hf regression line presented in Figure 1 represents a meaningful isochron. Finally, given that the U-Pb system and short-lived radioisotope chronometers ( 26 Al- 26 Mg, 53 Mn- 53 Cr and 182 Hf- 182 W) define crystallizations ages for SAH99555 that are within 5 Myr of Solar System formation [Spivak-Birndorf et al., 2006,2009;Markowski et al., 2007;Connelly et al., 2008; Schiller et al., 2010], it is unlikely that the apparent old 176 Lu- 176 Hf crystallization age for this meteorite results from a complex postcrystallization history. Thus, we conclude that the Hf isotope data and Lu/Hf ratios presented here for SAH99555 are accurate within the stated uncertainties, free of nucleosynthetic effects within the resolution of our analyses, and not affected by postcrystallization disturbance(s). [7] The apparent old 176 Lu- 176 Hf isochron age defined by minerals of the SAH99555 angrite is similar to that obtained for bulk chondrite and eucrite meteorites [Patchett and Tatsumoto, 1980; Blichert-Toft and Albarède, 1997; Bizzarro et al., 2003; Patchett et al., 2004; Bouvier et al., 2008; Blichert-Toft et al., 2002], suggesting the presence of variable excess 176 Hf correlated to the 176 Lu/ 177 Hf ratios in all these extraterrestrial samples. Thus, our results confirm the existence of an energetic process yielding excess 176 Hf in early formed Solar System solids and planetesimals through the production of 176m Lu, thereby resulting in 176 Lu- 176 Hf isochron ages that are consistently too old [Albarède et al., 2006]. This process has been attributed to the influx of solar or supernova(e)-generated g-radiation of early Solar System solids, an energy source with a limited penetration of a few centimeters into silicate and metal materials [Albarède et al., 2006]. Petrographic features of quenched angrites such as SAH99555 suggest cooling rates of C/hour [Mikouchi et al., 2001], indicating that these basaltic meteorites crystallized at depths m (calculated with a thermal diffusivity of cm 2 /s, a typical value for solid rock [Myamoto et al., 1986]). Thus, solar or supernova(e)- generated g-radiation cannot account for the excess 176 Hf in SAH While our results do not necessarily rule-out the g-radiation model, they suggest the existence of an additional more penetrative energy source(s) affecting early Solar System objects. [8] Fully penetrative supernova-derived neutrinos could, in principle, provide sufficient energy for the production of the 176 Lu isomer and associated excesses 176 Hf. But the required neutrino flux for adequate production of 176m Lu would place the supernova within one astronomical unit of the angrite parent body making this scenario implausible [Thrane et al., 2010]. Alternatively, excitation of 176 Lu to its isomeric state can occur from the energy released through the impact of cosmic rays penetrating the surface of accreted planetesimals, via Coulomb excitation and Bremsstrahlung radiation. Cosmic rays are high-energy particles pervading the galaxy commonly believed to be accelerated by supernova shock waves [Uchiyama et al., 2007]. Thus, Solar System formation in association with massive stars [Hester et al., 2004] would have exposed early formed solids and planetesimals to a high cosmic ray flux. Recent numerical models [Thrane et al., 2010] indicate that the energy released from one proximal supernova is more than adequate to enhance the production of 176m Lu within accreted planetesimals to a maximum depth of 200 m and, hence, account for the presence of 176 Hf excesses in meteorites. If correct, this implies that Solar System solids were exposed to a significant flux of cosmic rays after the crystallization of SAH99555 at Myr but before formation of the 4557 Myr old phosphates from the Acapulco meteorite [Amelin, 2005], if these represent samples that existed within the upper 200 m of a planetesimal. Despite uncertainties regarding the relative contributions of the g-radiation and cosmic ray models to the irradiation history of early Solar System solids, the overriding result is the limited penetration depth (<200 m) of these energy sources within accreted planetesimals. [9] The Solar System s initial 176 Hf/ 177 Hf ratio determined here by the internal isochron approach is lower by 4 to 5 -units compared to that backcalculated from the present-day Lu-Hf parameters of bulk chondrite meteorites based on Blichert-Toft and Albarède [1997] and Bouvier et al. [2008], respectively, and the l 176 Lu value of yr 1 [Söderlund et al., 2004]. However, it is identical within analytical uncertainty to less precise estimates based on linear arrays (errorchrons) defined by bulk samples of basaltic eucrites and chondrites [Patchett and Tatsumoto, 1980; Blichert-Toft and Albarède, 1997; Bizzarro et al. 2003; Patchett et al., 2004; Bouvier et al., 2008; 5 of 10

6 Table 4. Lu-Hf BSE Parameters Based on the SAH99555 Angrite a 176 Hf/ 177 Hf ( 176 Hf/ 177 Hf) T =0 176 Lu/ 177 Hf a The estimate of the initial Hf-isotope composition of the Solar System [( 176 Hf/ 177 Hf ) T =0 ] is based on the intercept of the 176 Lu- 176 Hf isochron of the SAH99555 angrite and is believed to representan irradiation-free composition ( 176 Lu/ 177 Hf = 0). The present-day Hf-isotope composition of the BSE was calculated by forward projection of the Solar System initial 176 Hf/ 177 Hf using the l 176 Lu of Söderlund et al. [2004] and a 176 Lu/ 177 Hf of , which represents an estimate based on chondrite meteorites [Bouvier et al., 2008] and adjusted for a 0.3% 176 Lu burnout inferred from the excess 176 Hf in SAH Blichert-Toft et al., 2002]. Blichert-Toft et al. [2002] reported a Lu-Hf errorchron for bulk basaltic eucrites with the following regression parameters: slope = and intercept = (MSWD = 4.5). The precision of these regression parameters appears similar to that presented here for the SAH99555 internal isochron. However, we were not able to reproduce the uncertainties quoted by these authors using the widely available Isoplot regression software [Ludwig, 1991], which is the standard in the geoscience community. Indeed, when using the exact same dataset as Blichert-Toft et al. [2002], we obtained the following regression parameters: slope = and intercept = (MSWD = 4.5). The uncertainty on the initial 176 Hf/ 177 Hf defined by the basaltic eucrite isochron corresponds to nearly 4 -units and, therefore, cannot be used to define the Lu-Hf parameters of the bulk silicate Earth. [10] The two whole rock fractions of SAH99555 that, together with pyroxene and olivine mineral separates, define the 176 Lu- 176 Hf isochron in Figure 1 were also studied for 147 Sm- 143 Nd systematics (Table 2). Using the Pb-Pbage of SAH99555 and a l 147 Sm values of y 1 [Begemann et al., 2001], we calculate initial 143 Nd/ 144 Nd ratios of and at the time of solidification of SAH99555 for these two whole rock aliquots, consistent with previous Solar System initial 143 Nd/ 144 Nd estimates based on differentiated and chondrite meteorites [Bouvier et al., 2008; Jacobsen and Wasserburg, 1984]. These results support our interpretation that the initial 176 Hf/ 177 Hf value defined by the SAH99555 angrite internal isochron is primary and that its initial ratio represents the best estimate of the irradiation-free initial Hf-isotope composition of the Solar System. [11] Whether this initial 176 Hf/ 177 Hf value defines BSE depends on the irradiation history of the material that accreted to form the Earth. Although the mechanisms responsible for the initial growth of dust-like particles into km-size objects are still poorly understood, this process must have occurred over timescales of 10 4 to 10 5 years after formation of the solar protoplanetary disk well before the irradiation event that produced excess 176 Hf in SAH99555 to avoid Earth s precursor material spiraling into the Sun [Youdin and Shu, 2002; Chambers, 2004). Indeed, Mg- and W-isotope data of differentiated meteorites indicate that accretion and differentiation of planetesimals >100 km in diameter also occurred within the first few million years of the Solar System [Scherstén et al., 2006; Schiller et al., 2011]. As such, the bulk of Earth s precursor material most likely consisted of planetesimals >10 km that completed their accretions over timescales comparable to that of the angrite parent body and, hence, were mostly shielded from galactic cosmic rays and/or g-radiation given the limited penetration depth (<200 m) of these energy sources. We calculate that the effect of the accelerated decay of 176 Lu on the 176 Hf/ 177 Hf of BSE are negligible (<30 ppm) if Earth formed from planetesimals at least 10 km in diameter. Therefore, we conclude that Earth s precursor material consisted predominantly of un-irradiated material such that the present-day Hf isotopic composition of the irradiated chondrite meteorites cannot be used to estimate the Hf isotopic composition of BSE. [12] Using our newly defined BSE Lu-Hf parameters based on the internal isochron of the SAH99555 angrite (Table 4), we re-explore the depletion of Earth s mantle at the time of formation of the Jack Hills zircons (Figure 2). We note that Harrison et al. [2005] and Blichert-Toft and Albarède [2008] reported heterogeneous Hf values for >4000 Myr zircons from the Jack Hills metasedimentary belt of Western Australia, including highly radiogenic signals suggesting the existence of short-lived, ultra-depleted Hadean mantle domains. The Hf isotope compositions in these previous studies were obtained either by laser ablation using large beam sizes (62 81 mm [Harrison et al., 2005]), or by whole grain dissolution [Harrison et al., 2005; Blichert-Toft and Albarède, 2008]. However, the highly radiogenic signatures reported by Harrison et al. [2005] and Blichert-Toft and Albarède [2008] were not confirmed by later studies of Jack Hills zircons, where the Hf and Pb isotope data were obtained concurrently on the 6 of 10

7 Figure 2. Hf isotope evolution diagrams for Archaean and Hadean igneous and detrital zircons. Data taken from Amelin et al. [2000, 2001], Harrison et al. [2008], Kemp et al. [2009], Pietranic et al. [2008], and Kemp et al. [2010]. (a) Calculated with the old BSE parameters based on chondrite meteorites [Blichert-Toft and Albarède, 1997; Bouvier et al., 2008]. (b) Calculated using the BSE parameters based on the initial Hf-isotope composition of the Solar System defined by the SAH99555 mineral isochron (Table 4). The l 176 Lu of Söderlund et al. [2004] was used in all calculations. The depleted mantle growth line depicts the evolution of a reservoir extracted from a primitive mantle at 4530 Myr [Boyet and Carlson, 2005] to a modern mid-oceanic ridge basalt (MORB) 176 Hf/ 177 Hf value of [Chauvel and Blichert-Toft, 2001]. The evolution of the putative felsic crust reservoir was calculated with a 176 Lu/ 177 Hf of Harrison et al. [2005] and Blichert-Toft and Albarède [2008] reported heterogeneous Hf values for >4000 Myr Jack Hills zircons, including highly radiogenic signals. However, these highly radiogenic signals were not confirmed by recent studies where Hf and Pb data were obtained by concurrent analysis of carefully characterized zircons, thereby affirming concerns [Harrison et al., 2008; Kemp et al., 2010] that the earlier results may be compromised by sampling domains with disparate age and 176 Hf/ 177 Hf values. Therefore, we have omitted these data from our compilation. Using the BSE parameters based on chondrite meteorites (Figure 2a), some of the most unradiogenic Jack Hills zircons plot in an impossible region of the Hf isotope diagram, which requires their formation prior to the accretion of the Earth if these minerals were derived from a source lithology with a Lu/Hf However, using the revised BSE parameters determined in our study (Figure 2b), we note that all data can be explained if crustal reservoirs of felsic composition existed on Earth from 4400 Myr. 7 of 10

8 same spatial domain of well-characterized zircons [Harrison et al., 2008; Kemp et al., 2010]. For example, Harrison et al. [2008] reported concurrent Hf and U-Pb data for 68 Jack Hills zircons with ages greater than 3.9 Gyr, yielding Hf T values ranging from 11 to 1. Similarly, Kemp et al. [2010] recently reported data for 96 Jack Hills zircons for which the Hf and Pb isotope ratios were measured concurrently, and obtained Hf T values ranging from 11 to 1. We concur with Harrison et al. [2008] and Kemp et al. [2010], who concluded that the highly radiogenic Hf signals reported by Harrison et al. [2005] and Blichert-Toft and Albarède [2008] are spurious, caused by the analysis of complexly zoned zircons resulting in mixed sampling of domains with disparate age and 176 Hf/ 177 Hf ratios. This feature has been clearly demonstrated by Kemp et al. [2010, Figure 2a], where a range of up to 15 -units is observed within a single complexly zoned grain, with the most radiogenic Hf T values associated with the younger domains of the zircon. Similarly, Bizzarro et al. [2002] showed that whole grain dissolution of 3 Gyr cogenetic zircons and baddeleyites from a single carbonatite analyzed by solution mode MC-ICPMS (the same technique used by Blichert- Toft and Albarède [2008]) have a range of up to 6 -units, although these grains have similar 207 Pb/ 206 Pb ages. However, the Hf T values are correlated to the 206 Pb/ 238 U ratios, with the most discordant grains showing the more radiogenic Hf T values [see Bizzarro et al., 2002, inset in Figure 1]. Given these concerns, we have not included the earlier data of Harrison et al. [2005] and Blichert- Toft and Albarède [2008] in our compilation of Figure 2. [13] The most important result emerging from our study is that, contrary to current thinking, the terrestrial Hadean zircon record supports coexistence of both enriched and depleted hafnium reservoirs as early as 4.3 Gyr in Earth s history. Approximately 50% of the zircons older than 4 Gyr have Hf >0, indicating that these grains were derived from a source reservoir with a time-integrated record of depletion. Equally conspicuous is that a significant portion of the younger zircons (<3.8 Gyr) from Greenland, Canada, Australia and South Africa were extracted from depleted source regions compatible with the extent of depletion recorded by the >4-Gyr grains. Moreover, the extent of depletion defined by the most radiogenic Archean and Hadean zircons is broadly consistent with a source reservoir that evolves to the modern depleted mantle. This indicates that the crustal rocks from which these zircons crystallized were ultimately derived from a mantle source region with a 176 Lu/ 177 Hf ratio similar to that of the present-day mantle as sampled by mid-ocean ridge basalts. [14] The genesis of silicic magmas required to stabilize zircon crystallization did not occur via direct melting of a mantle source, but involved further differentiation processes and/or re-melting of older likely more mafic juvenile lithologies. In previous studies [Kemp et al., 2006; Pietranic et al., 2008; Kemp et al., 2010], the lack of zircons with Hfisotope signatures matching that of the depleted mantle evolution curve was interpreted to reflect a temporal decoupling between juvenile crust generation and zircon crystallization, suggesting that the genesis of silicic magmas may have been delayed by 100 to 400 million years following crust extraction. However, the new view presented here establishes that, compatible with modern plate tectonics [Tamura and Tasumi, 2002; Vogel et al., 2004], juvenile crust generation and subsequent differentiation in the Hadean epoch were nearly contemporaneous in some cases and, therefore, intrinsically linked. The smear of zircon Hf-isotope compositions along and below the depleted mantle curve is indicative of continuous juvenile crust formation and reprocessing throughout the Hadean and Archean eras. Taking the Hf-isotope signal of the >4.1 Gyr zircons at face value, a significant result of our new BSE reference framework is the lack of evidence for the existence of continental crust prior to 4.4 Gyr, a time that may coincide with the establishment of a plate tectonic regime similar to that operating today. [15] The present-day Hf-Nd BSE isotopic value, based on chondrite meteorites, is slightly displaced toward lower 176 Hf/ 177 Hf ratios (3 -unit) compared to the modern mantle array, which apparently requires the presence of an unidentified reservoir in Earth s mantle [Blichert-Toft and Albarède, 1997]. Our revised Lu-Hf parameters for the BSE lowers the estimated Hf isotopic composition of the BSE through time by 5 -unit, which exacerbates the observed offset between BSE and the modern mantle array and, therefore, increases the need for a missing reservoir enriched in incompatible elements. The striking consistency in the timeintegrated Hf-isotope composition of the presentday, Archean and Hadean depleted mantle source reservoirs is consistent with the idea that depletion of the terrestrial mantle resulted from the extraction of this early formed complementary enriched reservoir, perhaps related to a global differentiation event that occurred by 4.53 Gyr [Boyet and Carlson, 8 of 10

9 2005]. Apart from an ambiguous Hf-isotope signal in deep-seated magmas such as carbonatites and kimberlites [Bizzarro et al., 2002], there is little evidence that the unidentified enriched reservoir has contributed to surface magmatism, requiring its isolation in the lower mantle. If correct, this model implies that continuous extraction and recycling of crust throughout Earth s history has had limited impact on the chemical budget of the mantle sampled by mid-ocean ridge basalts through geologic time. Acknowledgments [16] The Centre for Star and Planet Formation is financed by the Danish National Research Foundation and the University of Copenhagen s Programme of Excellence. We thank Yuri Amelin for comments on an earlier version of this paper. References Albarède, F., E. E. Scherer, J. Blichert-Toft, M. Rosing, A. Simionovici, and M. Bizzarro (2006), g-ray irradiation in the early solar system and the conundrum of the 176 Lu decay constant, Geochim. Cosmochim. Acta, 70, Amelin, Y. (2005), Meteorite phosphates show constant 176 Lu decay rate since 4557 million years ago, Science, 310, Amelin, Y. (2008), U-Pb ages of angrites, Geochim. Cosmochim. Acta, 72, Amelin, Y., D.-C. Lee, A. N. Halliday, and R. T. Pidgeon (1999), Nature of the Earth s earliest crust from hafnium isotopes in single detrital zircons, Nature, 399, Amelin, Y., D.-C. Lee, and A. N. Halliday (2000), Earlymiddle Archaean crustal evolution deduced from Lu-Hf and U-Pb isotopic studies of single grain zircons, Geochim. Cosmochim. Acta, 64, Amelin, Y., A. Kaltenbach, T. Iizuka, C. H. Stirling, T. R. Ireland, M. Petaev, and S. B. Jacobsen (2010), U-Pb chronology of the solar system s oldest solids with variable 238 U/ 235 U, Earth Planet. Sci. Lett., 300, Amelin, Y., S. L. Kamo, and D. C. Lee (2011), Evolution of early crust in chondritic or non-chondritic Earth inferred from U-Pb and Lu-Hf data for chemically abraded zircon from the Itsaq Gneiss Complex, West Greenland, Can. J. Earth Sci., 48, Begemann, F., K. R. Ludwig, G. W. Lugmair, K. Min, L. E. Nyquist, P. J. Patchett, P. R. Renne, C.-Y. Shih, I. M. Villa, and R. J. Walker (2001), Call for an improved set of decay constants for geochronological use, Geochim. Cosmochim. Acta, 65, Bizzarro, M., A. Simonetti, R. K. Stevenson, and J. David (2002), Hf isotope evidence for a hidden mantle reservoir, Geology, 30, Bizzarro, M., J. A. Baker, H. Haack, D. Ulfbeck, and M. Rosing (2003), Early history of Earth s crust-mantle system inferred from hafnium isotopes in chondrites, Nature, 421, Blichert-Toft, J. (2001), On the Lu-Hf geochemistry for silicate rocks, Geostand. Newsl., 25, Blichert-Toft, J., and F. Albarède (1997), The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantlecrust system, Earth Planet. Sci. Lett., 148, Blichert-Toft, J., and F. Albarède (2008), Hafnium isotopes in Jack Hills zircons and the formation of the Hadean crust, Earth Planet. Sci. Lett., 265, Blichert-Toft, J., C. Chauvel, and F. Albarède (1997), Separation of Hf and Lu for high-precision isotope analysis of rock samples by magnetic sector-multiple collector ICP-MS, Contrib. Mineral. Petrol, 127, Blichert-Toft, J., M. Boyet, P. Télouk, and F. Albarède (2002), 147 Sm- 143 Nd and 176 Lu- 176 Hf in eucrites and the differentiation of the HED parent body, Earth Planet. Sci. Lett., 204, Bouvier, A., J. D. Vervoort, and P. J. Patchett (2008), The Lu-Hf and Sm-Nd isotopic composition of CHUR: Constraints from unequilibrated chondrites and implications for the bulk composition of terrestrial planets, Earth Planet. Sci. Lett., 273, Boyet, M., and R. Carlson (2005), 142 Nd evidence for early (>4.53 Ga) global differentiation of the silicate Earth, Science, 309, Chambers, J. E. (2004), Planetary accretion in the inner Solar System, Earth Planet. Sci. Lett., 224, Chauvel, C., and J. Blichert-Toft (2001), A hafnium isotope and trace element perspective on melting of the depleted mantle, Earth Planet. Sci. Lett., 190, Connelly, J. N., D. Ulfbeck, K. Thrane, M. Bizzarro, and T. Housh (2006), A method for purifying Lu and Hf for analyses by MC-ICP-MS using TODGA resin, Chem. Geol., 233, Connelly, J. N., M. Bizzarro, K. Thrane, and J. A. Baker (2008), The Pb-Pb age of angrite SAH99555 revisited, Geochim. Cosmochim. Acta, 72, Floss, C., G. Crozaz, G. McKay, T. Mikouchi, and M. Killgore (2003), Petrogenesis of angrites, Geochim. Cosmochim. Acta, 67, Harrison, T. M., J. Blichert-Toft, W. Müller, F. Albarède, P. Holden, and S. J. Mojzsis (2005), Heterogeneous Hadean hafnium: Evidence of continental crust at 4.4 to 4.5 Ga, Science, 310, Harrison, T. M., A. K. Schmitt, M. T. McCulloch, and O. M. Lovera (2008), Early (4.5 Ga) formation of terrestrial crust: Lu-Hf, d 18 O, and Ti thermometry results for Hadean zircons, Earth Planet. Sci. Lett., 268, Hester, J. J., S. J. Desch, K. R. Healy, and L. A. Leshin (2004), The cradle of the solar system, Science, 304, Jacobsen, S. B., and G. J. Wasserburg (1984), Sm-Nd isotopic evolution of chondrites and achondrites, II, Earth Planet. Sci. Lett., 67, Kemp, A. I. S., C. J. Hawkesworth, B. A. Paterson, and P. D. Kinny (2006), Episodic growth of the Gondwana supercontinent from hafnium and oxygen isotope ratios, Nature, 439, Kemp, A. I. S., G. L. Foster, A. Scherstén, M. J. Withehouse, J. Darling, and C. Storey (2009), Concurrent Pb-Hf isotope analysis of zircon by laser ablation multi-collector ICP-MS, with implications for the crustal evolution of Greenland and the Himalayas, Chem. Geol., 261, Kemp, A. I. S., S. A. Wilde, C. J. Hawkesworth, C. D. Coath, A. Nemchin, R. T. Pidgeon, J. D. Vervoort, and S. A. Dufrane (2010), Hadean crustal evolution revisited: New constraints from Pb-Hf isotope systematics of the Jack Hills zircons, Earth Planet. Sci. Lett., 296, of 10

10 Ludwig, K. R. (1991), ISOPLOT: A plotting and regression program for radiogenic-isotope data, U.S. Geol. Surv. Open File Rep., Markowski, A., G. Quitté, T. Kleine, A. N. Halliday, M. Bizzarro, and A. J. Irving (2007), Hafnium-tungsten chronometry of angrites and the earliest evolution of planetary objects, Earth Planet. Sci. Lett., 262, Mikouchi, T., and J. A. Barrat (2009), NWA 5029 basaltic shergottite: A clone of NWA 480/1460?, Meteorit. Planet. Sci., 44, A143. Mikouchi, T., M. Miyamoto, G. McKay, and L. Le (2001), Cooling rate estimates of quenched angrites: Approaches by crystallization experiments and cooling rate calculations of olivine xenocryst, Meteorit. Planet. Sci., 36, A134. Mittlefehldt, D. W., M. Killgore, and M. T. Lee (2002), Petrology and geochemistry of D Orbigny, geochemistry of Sahara 99555, and the origin of angrites, Meteorit. Planet. Sci., 37, Münker, C., S. Weyer, E. Scherer, and K. Mezger (2001), Separation of high field strength elements (Nb, Ta, Zr, Hf) and Lu from rock samples for MC-ICPMS measurements, Geochem. Geophys. Geosyst., 2, 1064, doi: / 2001GC Myamoto, M., D. S. McKay, G. A. McKay, and M. P. Duke (1986), Chemical zoning and homogenization of olivines in ordinary chondrites and implications for thermal histories of chondrules, J. Geophys. Res., 91, 12,804 12,816. Patchett, P. J., and M. Tatsumoto (1980), Lu-Hf total-rock isochron for the eucrite meteorites, Nature, 288, Patchett, P. J., O. Kouvo, C. E. Hedge, and M. Tatsumoto (1981), Evolution of continental crust and mantle heterogeneity: Evidence from Hf isotopes, Contrib. Mineral. Petrol., 78, Patchett, P. J., J. D. Vervoort, U. Söderlund, and V. J. M. Salters (2004), Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth, Earth Planet. Sci. Lett., 222, Pietranic, A. B., C. J. Hawkesworth, C. D. Storey, A. I. S. Kemp, K. N. Sircombe, M. J. Withehouse, and W. Bleeker (2008), Episodic, mafic crust formation from 4.5 to 2.8 Ga: New evidence from detrital zircons, Slave craton, Canada, Geology, 36, Scherer, E. E., C. Münker, and K. Mezger (2001), Calibrating the Lu-Hf clock, Science, 293, Scherstén, A., T. Elliott, C. Hawkesworth, S. Russell, and J. Mazarik (2006), Hf-W evidence for rapid differentiation of iron meteorite parent bodies, Earth Planet. Sci. Lett., 241, Schiller, M., J. A. Baker, and M. Bizzarro (2010), 26 Al- 26 Mg dating of asteroidal magmatism in the young solar system, Geochim. Cosmochim. Acta, 74, Schiller, M., J. A. Baker, J. Creech, C. Paton, M. A. Millet, A. Irving, and M. Bizzarro (2011), Rapid timescales for magma ocean crystallization on the howardite-eucritediogenite parent body, Astrophys. J., 740, L22. Söderlund, U., P. J. Patchett, J. D. Vervoort, and C. E. Isachsen (2004), The 176 Lu decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions, Earth Planet. Sci. Lett., 219, Spivak-Birndorf, L., M. Wadhwa, and P. E. Janney (2006), 26 Al and 53 Mn chronology of the D Orbigny and Sahara angrites, Meteorit. Planet. Sci., 36, Spivak-Birndorf, L., M. Wadhwa, and P. E. Janney (2009), 26 Al- 26 Mg systematics in D Orbigny and Sahara angrites: Implications for high-resolution chronology using extinct chronometers, Geochim. Cosmochim. Acta, 73, Sprung, P., E. E. Scherer, D. Upadhyay, I. Leya, and K. Mezger (2010), Non-nucleosynthetic heterogeneity in non-radiogenic stable Hf isotopes: Implications for early solar system chronology, Earth Planet. Sci. Lett., 295, Stevenson, R. K., and P. J. Patchett (1990), Implications for the evolution of continental-crust from Hf-isotope systematics of Archean detrital zircons, Geochim. Cosmochim. Acta, 54, Tamura, Y., and Y. Tasumi (2002), Remelting of an andesitic crust as a possible origin for rhyolitic magma in oceanic arcs: An example from the Izu-Bonin arc, J. Petrol., 43, Thrane, K., J. N. Connelly, M. Bizzarro, B. S. Meyer, and L.-S. The (2010), Origin of excess 176 Hf in meteorites, Astrophys. J., 717, Uchiyama, Y., F. A. Aharonian, T. Tanaka, T. Takahashi, and Y. Meada (2007), Extremely fast acceleration of cosmic rays in a supernova remnant, Nature, 449, Vervoort, J. D., P. J. Patchett, U. Söderlund, and M. Baker (2004), Isotopic composition of Yb and the determination of Lu concentrations and Lu/Hf ratios by isotope dilution using MC-ICPMS, Geochem. Geophys. Geosyst., 5, Q11002, doi: /2004gc Vogel, T. A., L. C. Patino, G. E. Alvarado, and P. B. Gans (2004), Silicic ignimbrites within the Costa Rican volcanic front: Evidence for the formation of continental crust, Earth Planet. Sci. Lett., 226, Youdin, A. N., and F. H. Shu (2002), Planetesimal formation by gravitational instability, Astrophys. J., 580, of 10