Meteorites and the Timing, Mechanisms, and Conditions of Terrestrial Planet Accretion and Early Differentiation

Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 775

Alex N. Halliday University of Oxford

Thorsten Kleine Eidgenössische Technische Hochschule (ETH) Zentrum

Isotopic studies of meteorites provide the fundamental data for determining how the terrestrial planets, including Earth, accreted. Over the past few years there have been major advances in our understanding of both the timescales and processes of terrestrial planet accretion, largely as a result of better definition of initial solar system abundances of short-lived nuclides and their daughters, as determined from meteorites. Cosmogenic effects, cross calibrations with other isotopic systems, and decay constant uncertainties are also critical in many instances. Recent improvements to 182Hf-182W chronology in all these areas have been particularly noteworthy. Uncertainty has surrounded the initial Hf- and W-isotopic composition of the solar system and the meaning of the unradiogenic W-isotopic compositions of iron meteorite data, rendered com- plicated by cosmogenic effects. However, the timescales for the formation of certain iron meteorite parent bodies would appear to be very fast (<1 m.y.). Similarly, the accretion of Mars would appear to have been very fast, consistent with rapid accretion via runaway growth. Pre- cise quantification is difficult, however, because martian meteorites display variable W-isotopic compositions that relate in part to the levels of depletion in siderophile elements. This is as expected from a planet that never achieved a well-mixed silicate reservoir characterized by uniform siderophile-element depletion, as is found on Earth. Therefore, attempts to apply W- isotopic models to martian meteorites need to be treated with caution because of this demon- strable variability in early source Hf/W presumably resulting from partial metal or core segregation. The current best estimates for martian reservoirs as represented by Zagami would imply formation within the first 1 m.y. of the solar system. The modeled timescales for metal segregation from the source of other meteorites, Nakhla, for example, would appear to be more like 10 m.y. These estimates are based on trace-element and isotopic data obtained from different and probably unrepresentative aliquots. Further high-quality combined trace-element and isotopic studies are needed to confirm this. Nevertheless, the chondritic 142Nd abundance for Zagami provides powerful supporting evidence that the W-isotopic effects record extremely rapid (<1 m.y.) accretion and core formation on Mars. The timescales for Earth accretion are significantly more protracted. The last major stage of accretion is thought to be the Moon-forming giant impact, the most recent Hf-W age estimates for which are in the range 40–50 m.y. after the start of the solar system. Applying this to accretion models for Earth provides evidence that some of the accreted metal did not fully equilibrate with silicate reservoirs. This cannot explain the very late apparent accretion ages deduced from other chronometers, in particular U-Pb. Either all the estimates for the Pb-isotopic composition of the bulk silicate Earth are in error or there was some additional late-stage U/Pb fractionation that removed Pb from the silicate Earth. If the latter was the case there was either late segregation of Pb to the core after W removal, or removal of Pb via atmospheric escape following the “giant impact.” Changes in the mechanisms and partitioning associated with core formation are indeed predicted from the stability in the mantle of S-rich metal before, and sulfide after, the giant impact. However, losses from Earth also need to be evaluated. Strontium-isotopic data provide evidence of major late (>10 m.y.) losses of moderately volatile elements from the material that formed the Moon and probably Earth. The Earth’s nonchondritic Mg/Fe may similarly reflect silicate losses during growth of Earth itself or the protoplanets that accreted to Earth. The budgets for plutonogenic Xe provide evidence that some erosion was extremely late (>100 m.y.), clearly postdating the giant impact and presumably related to irradiation and bombardment during the Hadean.

775 776 Meteorites and the Early Solar System II

1. INTRODUCTION detection methods to Jupiter-sized objects orbiting with a very short period. Young stars that are the most interesting For the past 50 years it has been clear that meteorites from the point of view of planet formation are sufficiently provide the crucial evidence that permits determination of energetic and internally unstable that they are largely un- the formation times of the terrestrial planets and tests of dy- suitable for detecting the small Doppler shift effects used namic models for their growth and early differentiation. The so far to identify most extrasolar planets. most remarkable example of this was the classic experiment 3. Isotopic measurements have long provided key evi- by Patterson (1956), in which the age of Earth was deter- dence for the processes, rates, and timing of formation of mined by measuring the Pb-isotopic compositions of iron inner solar system objects. Most of this work has been re- meteorites and achondrites. Prior to this time, Houtermans stricted to studies of planetesimals. However, with the de- (1946) and Holmes (1946) had both independently shown velopment of multiple-collector inductively coupled plasma that Pb isotopes could determine the age of Earth. How- mass spectrometry (MC-ICPMS) (Halliday et al., 1995), ever, at that time they had to make their best estimates of this field has taken off in several new directions involving the minimum age (roughly 3 Ga) on the basis of the Pb- short-lived isotopic chronometers, including the determina- isotopic compositions of terrestrial samples measured by tion of accretion rates for the terrestrial planets. It now ap- Nier et al. (1941). Although it was Houtermans who in this pears that calculated timescales differ depending on the context first coined the term “isochron,” it was Patterson chronometer. These differences may be providing us with who, by measuring meteorites, got the right points and slope important new information about how planets are built, as that defined the age of the solar system and Earth. discussed below. In this review the latest pertinent information on the various isotopic chronometers as obtained from meteorites 3. THE MAIN ISOTOPIC SYSTEMS is summarized and it is explained how this yields constraints RELEVANT TO PLANET FORMATION on the age and early evolution of the terrestrial planets. Some of the ways in which various isotopic systems can be The kinds of isotopic systems that could conceivably be jointly utilized are discussed. The chapter finishes with the used for studying how planets form can be crudely catego- best current estimates of the timescales for the formation rized as follows: of the terrestrial planets and asteroid belt, closing with some 1. Long-lived radioactive decay systems have mainly speculations on how the use of these isotopic systems may been used to provide information on the absolute age (Pat- develop in the future. terson, 1956; Hanan and Tilton, 1987; Wasserburg et al., 1977a; Lugmair and Galer, 1992; Carlson and Lugmair, 2. CONSTRAINTS ON PLANET FORMATION 2000; Amelin et al., 2002). Such systems also can provide PROCESSES AND TIMESCALES constraints on the time-averaged chemical composition of the materials from which an object formed by defining the The timescales and processes associated with planetary radioactive parent element/radiogenic daughter element ra- accretion can be studied with three complementary ap- tio (Gray et al., 1973; Wasserburg et al., 1977b; Hanan and proaches. Tilton, 1987; Halliday and Porcelli, 2001). 1. Theoretical calculations and dynamic simulations pro- 2. Short-lived nuclides have been extremely successful vide clear indications of what is expected in terms of the as high-resolution chronometers of events in the early so- timescales and mechanisms of accretion and core formation lar system, in particular 26Al (Lee et al., 1977), 53Mn (Birck given certain assumptions about the prevailing conditions and Allègre, 1988; Lugmair and Shukolyukov, 1998), 107Pd (see Chambers, 2004, for a recent review). However, the (Chen and Wasserburg, 1996), 129I (Reynolds, 1960), and dependence on the amount of gas present, the potential for 182Hf (Harper et al., 1991a; Lee and Halliday, 1995). How- planet migration since accretion, and the difficulty in deriv- ever, they need to be “mapped” onto an absolute timescale. ing a clearly workable mechanism to build objects that are They too can provide time-averaged chemical compositions of sufficient size for gravity to play a dominant role leave (Halliday et al., 1996; Halliday, 2004; Jacobsen and Har- considerable uncertainty about the degree to which these per, 1996). models represent a real solar system. There is still no un- 3. Nucleosynthetic isotopic heterogeneity in the disk derstanding of how circumstellar dust sticks and eventually from which the planets form has been proposed on the basis becomes kilometer-scale objects that can accrete with one of O-isotopic data (Clayton, 1986, 1993; Clayton and May- another gravitationally (Blum, 2000; Benz, 2000), and/or eda, 1975, 1996), although the currently favored mecha- whether gravitational instability may play a role (Ward, nism for production of mass-independent anomalies in O 2000). isotopes is by photochemical means either in the solar 2. Observations can now be made of circumstellar disks nebula or in the precursor molecular cloud (Clayton, 2002; (McCaughrean and O’Dell, 1996) as well as extrasolar Yurimoto and Kuramoto, 2004). Whatever their cause, O- planets (Mayor and Queloz, 1995). However, the extrasolar isotopic data provide a tool for studying the provenance of planets found so far are almost exclusively limited by the early solar system materials and the precision on these Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 777

measurements, and hence the ability to resolve small dif- lized, the precision and accuracy that can be achieved with ferences in average provenance has moved to new levels 238/235U-207/206Pb is so much greater that the results of every (Wiechert et al., 2001, 2004). Mass-independent isotopic other kind of chronometry, short- or long-lived, are refer- effects of plausible nucleosynthetic origin increasingly are enced to the data derived from this system. being demonstrated as very high precision measurements The 238/235U-207/206Pb system is very reliable and well of a wide range of elements have become possible using understood. The uncertainties on the decay constants are MC-ICPMS and thermal ionization mass spectrometry of potential importance in defining an absolute age (Schön (TIMS) (Dauphas et al., 2002a,b, 2004; Yin et al., 2002a; et al., 2004) but are small and insignificant when it comes Schönbächler et al., 2003; Podosek et al., 1997; Trinquier to resolving time differences at the start of the solar sys- et al., 2005). tem. There does exist uncertainty concerning the amount 4. Isotopic heterogeneity in highly volatile elements like of 247Cm in the early solar system (Stirling et al., 2005), the noble gases provides clues to the mixture of nebular and and because this decays to 235U with a long characteristic dust-derived volatile phases (Wieler et al., 1992) and have half-life of 12 m.y. it could potentially change early solar been crucial for understanding the timescales of volatile system 238/235U-207/206Pb chronometry. However, the 247Cm accretion and loss (Porcelli et al., 2001; Porcelli and Pepin, abundance is probably very small (Stirling et al., 2005) and 2000). so at present there exists no case for a correction. 5. Mass-dependent stable-isotope fractionation of light Other long-lived isotopic systems have, in general terms, elements such as H, C, N, O, Mg, Si, S, and K has been been more problematic. The greatest difficulties have been thoroughly characterized in recent years. Such fraction- with 87Rb and 176Lu. In both cases it is found that to achieve ations or lack thereof provide powerful constraints on the concordance with isotopic ages obtained by 238/235U-207/206Pb conditions and processes associated with planetary accre- or 147Sm-143Nd in meteorites, a different decay constantfrom tion and development (Young, 2000; Humayun and Clayton, that obtained by counting experiments or by comparing the 1995). Evidence for large isotopic fractionations associated ages of terrestrial samples is required. The reasons for this with volatile loss in the early solar system has been found are unclear at present and seem to be different in the two in the gases Ne and Xe (Hunten et al., 1987; Pepin, 1997, cases. One should also take note of the recommendations 2000). This field is likely to expand rapidly and become of Steiger and Jäger (1977). Some of the recent measure- increasingly interesting as a wide range of elements from ments relevant to 176Lu are found in Scherer et al. (2001), Li to U become amenable to study at high precision with Blichert-Toft et al. (2002), Bizzarro et al. (2003), and Söder- new MC-ICPMS techniques (Galy et al., 2000; Poitrasson lund et al. (2004). A recent compilation and discussion of et al., 2004). this very topical issue can be found in Scherer et al. (2003). 6. Cosmic-ray exposure ages do not provide much di- These decay constant uncertainties of a few percent ren- rect evidence for how the planets formed. They do provide der as limited the usefulness of the decay schemes for de- evidence of transit times of material delivered to Earth (e.g., fining a precise absolute timescale. However, such problems Heck et al., 2004) and it has occasionally been proposed are of lesser consequence when it comes to determining that some isotopic effects in chondrites reflect primordial small time differences between early objects and events. A preexposure on a meteorite parent-body regolith. decay constant error of 1% produces a huge uncertainty in 7. Local irradiation phenomena in the early Sun also absolute time at the start of the solar system at 4.5 Ga that provide a mechanism for generating isotopic heterogene- renders it of little use given that the solar system was prob- ity and a local source of short-lived nuclides in the early ably completed in <50 m.y. However, this same percentage solar system (Lee et al., 1998; Gounelle et al., 2001; Leya uncertainty also translates into a similar fractional error on et al., 2000, 2003). Studies of some of the isotopic varia- time differences, where it is of negligible importance. The tions predicted should shed new light on how material that simplest and most relevant example is for long-lived chro- formed close to the Sun was scattered across the disk (for nometers where the time difference Dt between t1 and t2 is example, by X-winds). << 1/λ (the inverse of the decay constant or mean life) and This paper mainly focuses on the conclusions one de- can therefore be simplified to rives from the first two of these: long- and short-lived (extinct) nuclides. ∆t = λ × (D – D )/P t1 t2

4. CALIBRATION OF CHRONOMETERS where D is the atomic ratio of a radiogenic daughter to a FROM THE START OF THE SOLAR SYSTEM nonradiogenic stable isotope of the same element and P is the ratio of the radioactive parent nuclide to the same stable 4.1. Long-lived Chronometers denominator isotope of the daughter element. It can be seen that the fractional error on λ translates linearly to a fractional The definition of events in absolute time at the start of error on ∆t. As such, Sr-isotopic compositions, for example, the solar system is accomplished using long-lived radionu- can be used to infer time differences of less than a million clides (Table 1). Although several of these have been uti- years at the start of the solar system for which a decay con- 778 Meteorites and the Early Solar System II

TABLE 1. Main isotopic decay systems in use in studying the origin and early evolution of the terrestrial planets.

Parent Daughter(s) Half-Life (yr) Status* Principal Applications and Comments Long-lived 40K (10.7%) 40Ar 1.25 × 109 ✓ Chronology, especially lunar bombardment 40K (89.3%) 40Ca 1.25 × 109 ✓ Not used greatly 87Rb 87Sr 4.75 × 1010 ✓ Chronology, refractory/volatile fractionations 138La (66.4%) 138Ba 1.05 × 1011 ✓ Not used greatly 138La (33.6%) 138Ce 1.05 × 1011 ✓ Not used greatly 147Sm 143Nd 1.06 × 1011 ✓ Chronology and early silicate differentiation 176Lu 176Hf 3.57 × 1010 ✓ Chronology and early silicate differentiation 187Re 187Os 4.23 × 1010 ✓ Chronology of metals 190Pt 186Os 6.5 × 1011 ✓ Not used greatly 232Th 208Pb 1.40 × 1010 ✓ Time-averaged Th/U 235U 207Pb 7.04 × 108 ✓ Chronology, refractory/volatile fractionations 238U 206Pb 4.47 × 109 ✓ Chronology, refractory/volatile fractionations

Extinct 7Be 7Li 1.459 × 10–1 ? CAI formation 10Be 10B 1.5 × 106 ✓ CAI formation 26Al 26Mg 7.3 × 105 ✓ CAIs, chondrules, early silicate melting, heat 41Ca 41K 1.04 × 105 ✓ CAI formation 53Mn 53Cr 3.7 × 106 ✓ Early silicate melting and provenance 60Fe 60Ni 1.49 × 106 ✓ Massive star signature, early metals, heat 92Nb 92Zr 3.6 × 107 ✓ p-process signature, disk heterogeneity 93Zr 93Nb 1.53 × 106 ? Daughter is monoisotopic 97Tc 97Mo 2.6 × 106 ? p-process signature, disk heterogeneity 98Tc 98Mo 4.2 × 106 ? s-process signature, disk heterogeneity 99Tc 99Ru 2.13 × 105 ? s-process signature, disk heterogeneity 107Pd 107Ag 6.5 × 106 ✓ Metal and refractory/volatile chronology 126Sn 126Te 2.35 × 105 ? r-process signature 129I 129Xe 1.57 × 107 ✓ CAI, chondrule chronology, degassing 135Cs 135Ba 2.3 × 106 ? Refractory/volatile fractionations 146Sm 142Nd 1.03 × 108 ✓ Early solar system, crustal evolution 182Hf 182W 8.9 × 106 ✓ r-process, metal, accretion chronology 205Pb 205Tl 1.5 × 107 ✓ s-process signature 244Pu 136Xe† 8 × 107 ✓ CAI, chondrule chronology, degassing 247Cm 235U 1.6 × 107 ? Supernova r-process *A check mark means that the nuclide’s presence or former presence has been established. Nuclides with a question mark are still not convincingly demonstrated. † Spontaneous fissionogenic product. stant error of 1% is completely negligible (Gray et al., 1973; started to become significant when resolving time differ- Papanastassiou and Wasserburg, 1976; Wasserburg et al., ences in the early solar system and comparing results be- 1977b; Lugmair and Galer, 1992; Halliday and Porcelli, tween chronometers (Halliday, 2003, 2004). Of greater con- 2001). cern perhaps is that some half-life determinations have been inaccurate by far outside their stated uncertainties, as was 4.2. Uncertainties in Short-lived Systems — the case with 60Fe until the work of Kutschera et al. (1984). Decay Constants The new study by Vockenhuber et al. (2004) addresses the 182Hf issue and has produced a new, precise, and highly re- The relative importance of uncertainties for short-lived liable value of 8.904 ± 0.088 m.y. for the 182Hf decay con- systems is somewhat different. The errors on the decay constant. Therefore, this issue can now be considered closed. stants are generally insignificant but with increasing precision and greater opportunity for cross-calibration these be- 4.3. Uncertainties in Short-lived Systems — come critical. The 182Hf decay constant is a particularly Initial Abundances good (or bad) case in point and has been the most problematic in recent years when it comes to early solar system The uncertainties in the bulk solar system initial (BSSI) timescales. Unfortunately, the artificial production of sig- isotopic composition of radioactive parent or radiogenic nificant 182Hf for activity measurements requires very large daughter elements introduce further and frequently the larg- neutron fluxes. As a consequence, little work has been done est error in precise calibration of the timescale. All the short- until recently. The half-life of 9 m.y. has long been limited lived nuclide systems suffer from this problem but it has by an uncertainty of more than ±20% (Wing et al., 1961). been particularly apparent for 26Al-26Mg (Bizzarro et al., This stated uncertainty does not affect first-order conclu- 2004; Young et al., 2005), 53Mn-53Cr (Lugmair and Shukol- sions regarding the accretion rates of the planets but has yukov, 1998; Birck et al., 1999; Nyquist et al., 1997, 2001), Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 779

60Fe-60Ni (Shukolyukov and Lugmair, 1993; Mostefaoui et esses following different laws. With only three isotopes, one al., 2004), 92Nb-92Zr (Harper et al. 1991b; Hirata, 2001; of which is radiogenic, a judgment has to be made about Münker et al., 2000; Sanloup et al., 2000; Yin et al., 2000; how best to determine the radiogenic difference. This be- Schönbächler et al., 2002, 2003), 107Pd-107Ag (Chen and comes important in strongly fractionated residues like bulk Wasserburg, 1996; Carlson and Hauri, 2001; Woodland et CAIs and has the potential to leave an apparent residual iso- al., 2005), 182Hf-182W (Harper and Jacobsen, 1996; Jacob- topic anomaly that is significant relative to the small effect sen and Harper, 1996; Lee and Halliday, 1995, 1996,2000a; being expected from low Al/Mg CAIs. Davis et al. (2005) Kleine et al., 2002; Quitté et al., 2000; Quitté and Birck, argue that the high 26Al/27Al reported by Young et al. (2005) 2004; Schönberg et al., 2002; Yin et al., 2002b), and 247Cm- could reflect inaccurate characterization of this natural frac- 235U (Arden, 1977; Blake and Schramm, 1973; Chen and tionation. Young et al. claim that on its own, this is insuffi- Wasserburg, 1980; Tatsumoto and Shimamura, 1980; Stir- cient to explain the total amount of the apparent discrep- ling et al., 2005). In the first three cases (26Al-26Mg, 53Mn- ancy with the results reported by Bizzarro et al. (2004). 53Cr, and 60Fe-60Ni) the uncertainty is largely dominated by However, Bizzarro et al. (2005) have now reported an error studying different archives and issues having to do with the in their treatment of their data resulting in a corrected 26Al/ handling and interpretation of the data. In the case of 92Nb- 27Al = (5.83 ± 0.11) × 10–5. This is consistent with the con- 92Zr and 107Pd-107Ag there are apparent discrepancies in the ventional bulk data in Young et al. (2005). In fact, regres- data produced from different laboratories that have not been sion of all data from Young et al. gives an identical, although satisfactorily explained. In the case of 247Cm-235U there is less-precise, 26Al/27Al of 5.9 ± 0.3 × 10–5. However, the 26 27 –5 a clear indication that the U-isotopic variations reported in very high “supracanonical” ( Al/ Al)BSSI value of 7 × 10 some studies were probably analytical artifacts (Chen and is determined as an upper limit to the scatter of the data Wasserburg, 1980; Stirling et al., 2005). In the case of 182Hf- obtained by laser ablation and has not yet been ratified by 182W there are both discrepancies in the data and differences any conventional measurements. in interpretation. However, it is now known that certain 4.3.2. Manganese-53–chromium-53. The former pres- early W-isotopic data for carbonaceous chondrites (Lee and ence of live 53Mn in the early solar system is now well es- Halliday, 1995, 1996) are definitely incorrect by about 180– tablished (Birck and Allègre, 1988; Lugmair and Shukol- 200 ppm. The discrepancies over 107Pd-107Ag and 247Cm- yukov, 1998). However, the study of different meteorite 235U are of little relevance to planetary timescales at the archives has yielded different values for the initial abun- present time and will not be discussed further here. In the dance and this has a dramatic effect on the timescales de- following the uncertainties over the other systems are deduced for Mn/Cr fractionation in planetary bodies. The Mn- scribed briefly. Cr-isotopic data for CAIs (Birck and Allègre, 1988) are 4.3.1. Aluminum-26–magnesium-26. The canonical consistent with a solar system initial 53Mn/55Mn that is sig- 26 27 –5 value of ( Al/ Al)BSSI is 5 × 10 (Lee et al., 1977). How- nificantly higher than that implied from the Mn-Cr system- ever, new Mg-isotopic data for bulk CAIs and minerals have atics of angrites and eucrites (Lugmair and Shukolyukov, led Young et al. (2005) to propose that this only represents 1998, 2001). The data for CAIs could conceivably reflect the isotopic abundance at the time of recrystallization of the nucleosynthetic effects that are well established in chon- CAIs. Young et al. present a large amount of new data for drites (Podosek et al., 1997) and/or disturbance of Mn-Cr- CAIs measured by laser ablation MC-ICPMS. From the isotopic systematics (Papanastassiou et al., 2005). Manga- scatter of data for minerals and bulk CAIs they extract a nese-chromium chronometry is left in a somewhat equivo- 26 27 –5 minimum apparent ( Al/ Al)BSSI ~ 7 × 10 using the cal status until this is clarified. The ramifications for early upper portion of the scatter to the Al-Mg isochron. The planetesimal accretion are discussed further below. difficulty with the approach is that nonsystematic and ma- 4.3.3. Iron-60–nickel-60. The demonstration of for- trix-sensitive errors in element ratios can be quite signifi- merly live 60Fe in the early solar system (Shukolyukov and cant using laser ablation. These are hard to quantify in fine- Lugmair, 1993) was accomplished by measuring the Ni-iso- grained materials, particularly in the absence of detailed topic compositions of basaltic achondrites and in particular matrix matching and laser artifact studies. As a result the eucrites. Recently other groups have demonstrated very high meaning of the upper limit of such a scattered dataset be- 60Fe/56Fe in some sulfides from chondrites (e.g., Mostefaoui comes ambiguous. A few bulk CAIs that were analyzed et al., 2004). The recently reported evidence for very high conventionally in the same study lie within the range of the 60Fe/56Fe (Moynier et al., 2004) in iron meteorites has not laser ablation measurements but do not extend to such high been confirmed by more recent measurements (Cook et al., 26Al/27Al. The Young et al. study appears to be in conflict 2005; Quitté et al., 2006). Exactly how high is the value of 26 27 –5 60 56 –6 with the very precise ( Al/ Al)BSSI = (5.25 ± 0.10) × 10 ( Fe/ Fe)BSSI is now unclear but it could be as high as10 obtained for bulk Allende CAIs by Bizzarro et al. (2004). in which case early solar system events recorded in eucrites The reasons for the apparent difference are complex. The are more protracted than previously thought or the Ni-iso- two groups have analyzed different CAIs, in some cases topic systems have been partially reset at some late stage. from different meteorites. However, they also treat the cor- 4.3.4. Niobium-92–zirconium-92. In the case of 92Nb 92 92 rection for isotopic fractionation differently. The raw data there exist basically two values. The first is ( Nb/ Nb)BSSI ~ are distorted by both natural and instrument-induced proc- 10–3 (Münker et al., 2000; Sanloup et al., 2000; Yin et al., 780 Meteorites and the Early Solar System II

92 92 –5 2000). The second is ( Nb/ Nb)BSSI ~ 10 (Harper et al., 1991b; Hirata, 2001; Schönbächler et al., 2002, 2003, 2005). These two very distinct estimates result in radically different constraints on the early differentiation history of the terrestrial planets. If the former is correct the Zr-isotopic compositions for the silicate Earth imply late formation of Earth’s core and continental crust (Münker et al., 2000; Jacobsen and Yin, 2001). The reasons for the differences are unclear. Schönbächler et al. (2003) were unable to re- produce the Zr-isotopic variations reported by Münker et al. (2000) and Sanloup et al. (2000) when similar materials (the same bulk chondrites and similar refractory low Nb/ Zr Allende CAIs) were analyzed. Similarly, the results for iron meteorite rutiles reported by Yin et al. (2000) differ from those reported by Harper et al. (1991b) and the results for an early (meteorite) zircon with Nb/Zr = 0 reported by Yin et al. (2000) are different from the results for similar phases studied by Hirata (2001). These apparent discrepancies warrant further studies to settle the issue definitively. 4.3.5. Hafnium-182–tungsten-182. In the case of the 182 180 value of ( Hf/ Hf)BSSI the initial estimate of Harper and Jacobsen (1996) and Jacobsen and Harper (1996) of ~10–5 was low because of a paucity of data (Ireland, 1991) and Fig. 1. The W-isotopic compositions of carbonaceous chondrites, the modeling available from comparisons with other isoto- as determined by Kleine et al. (2002, 2004a), shown here, but also pic systems (Wasserburg et al., 1994). Lee and Halliday by Yin et al. (2002b) and Schoenberg et al. (2002), demonstrate (1995, 1996) presented the first W-isotopic data for carbo- that the earlier measurements of Allende and Murchison (Lee and 182 180 –4 Halliday, 1995, 1996) are inaccurate by roughly 180 to 200 ppm. naceous chondrites and inferred a ( Hf/ Hf)BSSI > 10 . They then appeared to confirm this with a value of 182Hf/ The new values agree within error with the values determined for enstatite chondrites (Lee and Halliday, 2000a) shown here but pre- 180Hf ~ 2 × 10–4 for internal Hf-W isochrons of rather vari- viously considered anomalous and hard to explain. The average able quality for the ordinary chondrites Forest Vale and Ste. for carbonaceous chondrites (vertical line) is ε182W = –1.9 ± 0.1. Marguerite (Lee and Halliday, 2000b). Three groups have independently shown that some of these University of Michigan data are incorrect (Kleine et al., 2002; Schönberg et al., 2002; Yin et al., 2002b). They The W-isotopic composition of iron meteorites may, 182 180 –4 have instead demonstrated a ( Hf/ Hf)BSSI ~ 1 × 10 . however, provide evidence that the initial abundance lies The most clear-cut error is in the compositions of the car- between these two values. Iron meteorites sample early solar bonaceous chondrites Allende and Murchison. These, and system W without Hf and therefore provide an indication all carbonaceous chondrites measured thus far (Kleine et of the W-isotopic composition at the start of the solar sys- al., 2002, 2004a), have a 182W/184W that is 180–200 ppm tem in an analogous fashion to the way in which the initial lower than that originally determined by Lee and Halliday Pb-isotopic composition at the start of the solar system has (1995, 1996) (Fig. 1). been defined by Canyon Diablo troilite. The most recent 182 180 –4 The reason for the error in the Michigan measurements estimate of ( Hf/ Hf)BSSI is (1.7 ± 0.3) × 10 based on is unclear. The MC-ICPMS instrument used was the first the least-radiogenic W-isotopic composition of solar sys- built and had relatively poor sensitivity compared with more tem materials yet determined, as measured in the iron me- recent instruments. This should not matter directly unless teorite Tlacotopec (Quitté and Birck, 2004). This particular it resulted in a background problem. meteorite has a very long cosmic-ray exposure age and the The original internal isochrons for Forest Vale and Ste. effects on the W-isotopic composition (Leya et al., 2000, Marguerite (Lee and Halliday, 2000b) also differ in part 2003) could be significant. This has been confirmed by from the results presented by Kleine et al. (2002). The ex- more detailed high-precision W-isotopic measurements of planation could again be analytical error. There may also iron meteorites (Markowski et al., 2006). Although the dif- have been differences between the particular phases ana- ferences over the BSSI abundance of Hf and W isotopes lyzed. Clearly, the age or time of closure of a particular would appear to be minor compared with the uncertainties phase is critical to the 182Hf/180Hf determined. However, and disputes over other isotopic systems, in particular 53Mn- the data from CAIs and other ordinary chondrite isochrons 53Cr, 60Fe-60Ni, and 92Nb-92Zr, they do have a nonnegligible (Yin et al., 2002b) and the data obtained on bulk meteor- effect on calculated early solar system timescales and their 182 180 ites all appear to confirm a ( Hf/ Hf)BSSI of about 1 × interpretation, and are particularly important because of the 10–4, rather than 2 × 10–4. wider utility of 182Hf-182W. Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 781

TABLE 2. Isotopic ages of early objects and best estimates of early solar system timescales.

Type of Event Object or Model Isotopic System Reference Age (Ga) Time (m.y.) Start of solar system Efremovka CAIs 235/238U-207/206Pb Amelin et al. (2002) 4.5672 ± 0.0006 0.0 ± 0.6 Start of solar system Allende CAIs 235/238U-207/206Pb Göpel et al. (1991) 4.566 ± 0.002 1 ± 2 Start of solar system Allende CAIs 26Al-26Mg Bizzarro et al. (2004) 4.567 0.00 ± 0.03 Chondrule formation Acfer chondrules 235/238U-207/206Pb Amelin et al. (2002) 4.5647 ± 0.0006 2.5 ± 1.2 Chondrule formation UOC chondrules 26Al-26Mg Russell et al. (1996) <4.566 to 4.565 >1 to 2 Chondrule formation Allende chondrule 26Al-26Mg Galy et al. (2000) <4.5658 ± 0.0007 >1.4 ± 0.7 Chondrule formation Allende chondrules 26Al-26Mg Bizzarro et al. (2004) 4.567 to <4.565 0 to ≥1.4 H chondrite parent body Ste. Marguerite phosphate 235/238U-207/206Pb Göpel et al. (1994) 4.5627 ± 0.0006 4.5 ± 1.2 metamorphism Asteroidal core formation Magmatic irons 182Hf-182W Markowski et al. (2006) >4.566 <1 Vesta differentiation Silicate-metal 182Hf-182W Kleine et al. (2002) 4.563 ± 0.001 4 ± 1 Vesta accretion Earliest age 87Rb-87Sr Halliday and Porcelli (2001) <4.563 ± 0.002 >4 ± 2 Vesta differentiation Silicate-metal 182Hf-182W Lee and Halliday (1997) 4.56* 10* Vesta differentiation Silicate-silicate 53Mn-53Cr Lugmair and Shukulyokov (1998) 4.5648 ± 0.0009 1 ± 2 Vesta differentiation Silicate-metal 182Hf-182W Quitté et al. (2000) 4.550 ± 0.001* 16 ± 1* Vesta differentiation Silicate-metal 182Hf-182W Kleine et al. (2002) 4.563 ± 0.001 4 ± 1 Vesta differentiation Silicate-metal 182Hf-182W Yin et al. (2002b) 4.564 3 Early eucrites Asuka 881394 235/238U-207/206Pb Wadhwa et al. (2005), M. Wadhwa 4.5665 ± 0.0009 0.7 ± 1.1 (personal communication, 2005) Early eucrites Noncumulate eucrites 182Hf-182W Quitté and Birck (2004) 4.558 ± 0.003 9 ± 3 Early eucrites Chervony Kut 53Mn-53Cr Lugmair and Shukulyokov (1998) 4.563 ± 0.001 4 ± 1 Angrite formation Angra dos Reis and 235/238U-207/206Pb Lugmair and Galer (1992) 4.5578 ± 0.0005 9 ± 1 LEW 86010 Mars accretion Youngest age 146Sm-142Nd Harper et al. (1995) ≥4.54 ≤30 Mars accretion Mean age 182Hf-182W Lee and Halliday (1997) 4.560* 6* Mars accretion Youngest age 182Hf-182W Lee and Halliday (1997) ≥4.54* ≤30* Mars accretion Youngest age 182Hf-182W Halliday et al. (2001b) ≥4.55* ≤20* Mars accretion Youngest age 182Hf-182W Kleine et al. (2002) ≥4.55 <13 ± 2 Mars accretion Youngest age 182Hf-182W This study >4.566 <1 Earth accretion Mean age 235/238U-207/206Pb Halliday (2000) 4.527 to 4.562 15 to 40 Earth accretion Mean age 182Hf-182W Yin et al. (2002b) 4.556 ± 0.001 11 ± 1 Earth accretion Mean age 235/238U-207/206Pb Halliday (2004) 4.550 ± 0.003 17 ± 3 Moon formation Best estimate of age 235/238U-207/206Pb Tera et al. (1973) 4.47 ± 0.02 100 ± 20 Moon formation Best estimate of age 235/238U-207/206Pb Carlson and Lugmair (1988) 4.44 to 4.51 60 to 130 and 147Sm-143Nd Moon formation Best estimate of age 182Hf-182W Halliday et al. (1996) 4.47 ± 0.04* 100 ± 40* Moon formation Best estimate of age 182Hf-182W Lee et al. (1997) 4.51 ± 0.01* 55 ± 10* Moon formation Earliest age 182Hf-182W Halliday (2000) ≤4.52* ≥45* Moon formation Earliest age 87Rb-87Sr Halliday and Porcelli (2001) <4.556 ± 0.001 >11±1 Moon formation Best estimate of age 182Hf-182W Lee et al. (2002) 4.51 ± 0.01* 55 ± 10* Moon formation Best estimate of age 182Hf-182W Kleine et al. (2002) 4.54 ± 0.01 30 ± 10 Moon formation Best estimate of age 182Hf-182W Yin et al. (2002b) 4.546 29 Moon formation Best estimate of age 182Hf-182W Halliday (2004) 4.52 ± 0.01 45 ± 10 Lunar highlands Ferroan anorthosite 60025 235/238U-207/206Pb Hanan and Tilton (1987) 4.50 ± 0.01 70±10 Lunar highlands Ferroan anorthosite 60025 147Sm-143Nd Carlson and Lugmair (1988) 4.44 ± 0.02 130 ± 20 Lunar highlands Norite from breccia 15445 147Sm-143Nd Shih et al. (1993) 4.46 ± 0.07 110 ± 70 Lunar highlands Ferroan noritic anorthosite 67016 147Sm-143Nd Alibert et.al. (1994) 4.56 ± 0.07 10 ± 70 Earliest crust Jack Hills zircons 235/238U-207/206Pb Wilde et al. (2001) 4.44 ± 0.01 130 ± 10 *Based on early solar system initial abundances now thought incorrect. CAIs = calcium-aluminum-rich refractory inclusions. UOC = unequilibrated ordinary chondrite.

4.4. Uncertainties in Short-lived Systems — 2. The very precise 235/238U-207/206Pb data for angrites Conversion to Absolute Time (Lugmair and Galer, 1992) provides a powerful means of mapping 53Mn-53Cr onto an absolute timescale (Lugmair To convert the results from short-lived decay systems like and Shukolyukov, 1998, 2001). Hf-W into an absolute timescale one needs cross-calibration 3. Similarly, the 235/238U-207/206Pb data for ordinary chon- points. In principle, cross-calibrations with precise 235/238U- drites, in particular Ste. Marguerite (Göpel et al., 1994), 207/206Pb ages should allow one to determine the initial abun- allow the 182Hf-182W chronometer to be cross-calibrated dances and the decay constants of the short-lived nuclides onto an absolute timescale (Kleine et al., 2002, 2005b) if independently. There now exist at least four of these, all of they date the same event. In fact, the U-Pb closure tempera- which can be tied in to absolute timescales with the very ture in phosphates is 400°–500°C whereas Hf-W closure precise 235/238U-207/206Pb ages shown in Table 2. between metal and silicates takes place at temperatures in 1. The 26Al-26Mg chronometer yields time differences excess of 600°C (Kleine et al., 2005b). Using the 53Mn-53Cr (Russell et al., 1996) that are fully consistent with differ- chronometry (Polnau and Lugmair, 2001), which itself is ences in absolute time deduced from 235/238U-207/206Pb data mapped onto an absolute timescale by cross calibration with for CAIs and chondrules (Amelin et al., 2002). 235/238U-207/206Pb (Lugmair and Shukolyukov, 1998, 2001), 782 Meteorites and the Early Solar System II results in better agreement between the Hf-W and Mn-Cr 26Al/ 27Al for bulk CAIs than the canonical value based on whole-rock ages of the eucrites and also between the Hf- minerals. It is currently unclear whether this difference is W and 235/238U-207/206Pb age for CAIs (Kleine et al., 2005a). real. Either way, the resolution of these chronometers is get- 4. Most recently, the 182Hf-182W chronometer has been ting to the point at which incredible detail should be re- cross-calibrated onto an absolute timescale by determining vealed about the earliest solar system over the coming years an internal isochron for CAIs (Kleine et al., 2005a). This as these problems are resolved. permits direct comparison with 235/238U-207/206Pb and 26Al- How chondrules form is also unknown and highly con- 26Mg data. troversial. The most widely accepted theory is that they These calibrations are tremendously important as the form within the dusty circumstellar disk from which the field develops in the direction of increasing age resolution. planetesimals formed. A variety of studies have now shown For example, for many years there was a debate about that chondrule formation overlapped with CAI production, whether 26Al/27Al variations in early solar system objects but was protracted, extending to roughly 2 m.y. after the reflected time differences, variable resetting, or differences start of the solar system (Russell et al., 1996; Galy et al., in the abundance of 26Al in different settings. The latter 2000; Bizarro et al., 2004). Whether all these objects were model was fueled by more recent models of 26Al production formed in a similar manner is unclear. Some may have been close to the Sun (Lee et al., 1998) and the detection of for- produced as a result of molten planetesimal impacts (Zook, merly live 10Be in CAIs (McKeegan et al., 2000). However, 1981; Sanders, 1996). Another “planetary” model for chon- since 10Be is clearly spallogenic and excess 10B (from the drule formation involves bow shocks around eccentric as- decay of 10Be) has been detected in CAIs lacking evidence teroids driven by jovian resonances (Weidenschilling et al., of excess 26Mg, the vast majority of 26Al cannot be spallo- 1998). Similarly, Lugmair and Shukolyukov (2001) argue genic (Marhas et al., 2002). The close agreement between that early planetesimals would melt rapidly and that colli- 26Al-26Mg and 235/238U-207/206Pb chronometry, as well as the sions among them would release melt that ultimately forms 26 27 very precise initial abundance of ( Al/ Al)BSSI recently chondrules. determined for bulk CAIs (Bizzarro et al., 2004, 2005), ap- The question then arises as to how well we know how pears to demonstrate that any variation in initial abundance quickly the first planetesimals formed. From dynamic con- is very small. siderations it is thought that planetesimals formed within a Although such issues mandate further effort to achieve few times 105 yr (e.g., Kortenkamp et al., 2000). Optically clarity, there is no question that we now know the absolute dense circumstellar disks appear to form very early and last age of certain key objects extremely well and can get a good for about a million years or more. Some dusty disks like idea of how quickly they formed. HR 4796A and Beta Pictoris are an order of magnitude older (Schneider et al., 1999). The presence of these disks does 5. CHONDRITES, ACHONDRITES, IRONS, not imply that planetesimals or planets have not yet formed. AND EARLIEST PLANETESIMALS It is assumed that they are already formed and are present in the midplane. The most precise and accurate current estimate of the Chondrites themselves, while judged to be undifferenti- absolute age of the solar system is taken to be the 235/238U- ated, cannot have formed very early because they contain 207/206Pb age of Efremovka CAIs of 4.5672 ± 0.0006 Ga a variety of CAIs, chondrules, and presolar grains that must (Amelin et al., 2002). The most widely held view is that have become mixed together some time after chondrule CAIs did form within the earliest solar system. The reason formation. Certain kinds of presolar grains could not have for believing this is that they are the earliest objects yet survived the temperatures and conditions of chondrule for- dated, have the highest abundances of short-lived nuclides, mation. Therefore, a scenario for chondrite-parent-body for- and have roughly chondritic proportions of highly refractory mation must involve mixing of CAIs, chondrules, and pre- elements and isotopic compositions that are, broadly speak- solar grains into a particular region where chondrite parent ing, similar to those of average solar system. bodies were able to form by sticking or local gravitational When and how CAIs formed relative to the collapse of collapse of these small objects. The 26Al/27Al (Bizzarro et the portion of the molecular cloud that formed the Sun is al., 2004, 2005; Young et al., 2005) and high 60Fe/56Fe unknown. The most precise estimate of the duration of CAI (Mostefaoui et al., 2004) now thought to have character- formation is provided by the new high-precision 26Al-26Mg ized the earliest solar system should have produced molten data produced by Bizzarro et al. (2004, 2005), who obtained and differentiated planetesimals at an early stage. The very an identical apparent initial 26Al abundance for all CAIs existence of chondrites provides evidence of small-body measured, implying a time interval of perhaps as little as accretion of undifferentiated material millions of years after 50,000 years for Allende CAI formation. These measure- the start of the solar system. As such, their existence in the ments need to be extended to CAIs from other meteorites inner solar system is dynamically problematic. but the preliminary data are consistent with a very short dur- Isotopic data for differentiated meteorites provide clear ation of CAI production. As discussed above, there are ap- evidence that some of the earliest planetesimals formed parent discrepancies between this result and the Mg-isoto- within the first 10 m.y. of the solar system. Exactly how pic data of Young et al. (2005), who found a higher apparent early is a matter of some uncertainty. The primary lines of Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 783

evidence come from short-lived nuclides and in particular formation is 182Hf-182W. The timing of metal segregation 26Al-26Mg, 53Mn-53Cr, and 182Hf-182W. For example, Cher- is well defined by the W-isotopic composition because Hf vony Kut is an unbrecciated eucrite with relatively high 60Fe and W are both refractory and early solar system parent (Shukolyukov and Lugmair, 1993), lending confidence to the bodies can be safely assumed to have chondritic Hf/W (Lee view that it is a less-disturbed sample well suited for defin- and Halliday, 1996; Horan et al., 1998) (Fig. 2). As such ing early solar system timescales. The 53Mn-53Cr data for the parent-body isotopic evolution is predictable and the W- Chervony Kut can be compared with the data for the an- isotopic composition of the iron meteorite can define the grites Angra dos Reis and LEW 86010 (Table 2). These ob- time of metal separation to extremely high precision using jects are very precisely dated by 235/238U-207/206Pb and, al- modern MC-ICPMS techniques (Halliday, 2003, Halliday though relatively young, they permit mapping of the Mn-Cr et al., 2003; Lee and Halliday, 2003). The W-isotopic data timescale for eucrites to an absolute age. On this basis Cher- for iron meteorites are strikingly unradiogenic, indicating vony Kut formed within 5 m.y. of the start of the solar sys- that they formed within a few million years of each other at tem, allowing for the uncertainties in Pb-Pb chronometry the start of the solar system (Lee and Halliday, 1996; Horan of the angrites and CAIs (Lugmair and Shukolyukov, 1998). The eucrite whole-rock Mn-Cr isochron, thought to reflect planetary differentiation, defines an earlier age no more than 4 m.y. after the start of the solar system (Table 2). Despite these lines of evidence that differentiated objects formed early, the Sr-isotopic data for eucrites are difficult to explain as the primary planetesimals that supposedly formed within the first few hundred thousand years of the solar system. There is a difference between the initial Sr- isotopic compositions of CAIs (Gray et al., 1973; Podosek et al., 1991) on the one hand and eucrites and angrites (Wasserburg et al., 1977; Lugmair and Galer, 1992) on the other. This cannot be readily explained unless the eucrite parent body formed more than 2 m.y. after the start of the solar system (Halliday and Porcelli, 2001). The basis for this is as follows. All these objects are strongly depleted in moderately volatile Rb relative to refractory Sr and, as such, there is no possibility of the Sr-isotopic composition of the parent body or its mantle changing significantly by decay of 87Rb. Yet the Sr-isotopic difference requires a significant period of time in a high Rb/Sr environment. The highest Rb/Sr environment possible is the Rb/Sr of the solar nebula (0.3). The minimum time required to generate the difference in Sr-isotopic composition then defines the earliest time these objects could have formed. This time difference is at least 2 m.y. (Halliday and Porcelli, 2001). Basaltic achondrites are relatively rare and it could well be that in the harsh energetic processes associated with accretion and destruction of asteroids the silicate portions of many primary planetesimals have been destroyed. A better sample may be found among the many iron meteorites (Hal- liday, 2003). It has long been known from Pb-isotopic data as well as 107Pd-107Ag chronometry (Chen and Wasserburg, 1996) that iron meteorites are early objects. However, the Pb-isotopic data for iron meteorites appear to be offset from the initial compositions defined by CAIs for reasons that are not well understood (Tera and Carlson, 1999; Carlson and Lugmair, 2000) and the 107Pd-107Ag system is not well Fig. 2. The W-isotopic compositions of iron meteorites provide unequivocal evidence of early metal segregation. Assuming the calibrated against other chronometers (Chen and Wasser- parent bodies had chondritic relative proportions of refractory Hf 129 129 burg, 1996). Alternative chronometers such as I- Xe and W, one can calculate a model time difference between the 53 53 and Mn- Cr probably date cooling rather than the pri- times of formation of different metals. The most comprehensive mary formation of these objects. dataset published so far is that of Horan et al. (1998) shown here. The most powerful chronometer of the accretion and The vertical line is the best estimate of average solar system based metal segregation processes associated with iron meteorite on data for carbonaceous chondrites (Kleine et al., 2002, 2004a). 784 Meteorites and the Early Solar System II et al., 1998). The absolute age can be calculated from cross- tical to the U-Pb age for Efremovka CAIs (Amelin et al., calibrations of the Hf-W-isotopic system such as that pro- 2002). vided by Ste. Marguerite, discussed above. New studies now Recently, very precise W-isotopic data have been ob- show that the W-isotopic compositions of iron meteorites tained for a large range of magmatic and nonmagmatic yield resolvable differences at very high precision (Lee and irons. By correcting for the maximum possible cosmogenic Halliday, 2003; Markowski et al., 2004, 2006; Schersten et effect, it can be demonstrated that some magmatic irons seg- al., 2004; Kleine et al., 2005a; Lee, 2005). regated within <0.5 m.y. of the start of the solar system, as The least-radiogenic 182W/184W reported to high precision defined by W-isotopic data for Allende CAIs (Markowski is ε182W = –4.4 ± 0.2 (the deviation in parts per 104 rela- et al., 2006). tive to bulk silicate Earth), obtained for Tlacotopec (Quitté and Birck, 2004). However, this low value may in part re- 6. MARTIAN METEORITES AND flect burnout of W isotopes caused by the interaction with EARLIEST PLANETARY thermal neutrons produced during a prolonged exposure to DIFFERENTIATION PROCESSES cosmic rays (Masarik, 1997; Leya et al., 2003). Several iron meteorites, including Tlacotopec, have very long cosmic- Martian meteorites provide the most powerful constraints ray exposure ages of hundreds of millions of years. As such, on the timescales of formation of another terrestrial planet. their W-isotopic compositions are not expected to be pris- This is not just of interest to comparative planetology. In tine. The production rate of thermal neutrons and, hence, many respects, Mars represents an example of how Earth the cosmogenic effects on W isotopes vary with depth in a and other terrestrial planets may have first started. There is meteorite. Therefore, different W-isotopic data may be ob- evidence that a Mars-sized object sometimes referred to as tained from different aliquots of the same meteorite. To “Theia” (the Greek goddess who was the mother of Selene, obtain really precise estimates of the timescales of metal the goddess of the Moon) was responsible for the Moon- segregation it may be necessary to measure exposure ages forming giant impact (Canup and Asphaug, 2001). There and W-isotopic compositions on the same aliquots and then also is evidence that certain features of the composition of to apply a correction (Markowski et al., 2006). Theia were similar to those of Mars (Halliday, 2004; Halli- Even if the least-radiogenic W in iron meteorites is only day and Porcelli, 2001). as low as εW = –3.7, other problems are apparent. The Hf- The constraints from short-lived nuclide data for mar- W data for Allende CAIs (Kleine et al., 2005a) define a tian meteorites are dominated by the data for four isotopic 182 184 ε182 129 129 146 142 182 182 244 ( W/ W)BSSI that is higher than W = –3.7, apparently systems: I- Xe, Sm- Nd, Hf- W, and Pu- suggesting that some iron meteorites formed at least a few 136Xe (Table 1). Although all these data provide a similar hundred thousand years before CAIs. This is contrary to the picture of rapid development of Mars, the comparisons reigning paradigm that CAIs are the first solids that formed between W and Nd have proved most interesting. The lat- in the solar nebula. Challenging this paradigm will require est compilation of W-isotopic data is shown in Fig. 3. It can a precise quantification of the 182W-burnout effects in iron be seen that all W-isotopic compositions are now resolv- meteorites (Markowski et al., 2006). It will also require able from that of the silicate Earth, with radiogenic values demonstration that the Hf-W systematics of CAIs have not in the range ε182W = 0–3. In general terms the shergottites been preferentially reset. Given the presence of W com- tend to be closer to 0 (Kleine et al., 2004a; Foley et al., pounds as Fremdlinge in Allende CAIs and the demonstra- 2005) whereas the nahklites are closer to 3 (Lee and Halli- tion that these are secondary features (Armstrong et al., day, 1997). The original W-isotopic data (Lee and Halliday, 1985, 1987; Hutcheon et al., 1987), one is left with some 1997; Kleine et al., 2004a) show a weakly defined relation- level of uncertainty as to whether there has been preferen- ship with the original ε142Nd (Harper et al., 1995; Jagoutz tial disturbance of the W isotopes. To counter this, it seems and Jotter, 2000; Jagoutz et al., 2003) that provided evi- unlikely that the CAI isochron is reset for the following dence that the early processes that affected each system reasons [a detailed discussion of this issue can be found in were somehow correlated. Recently this has been ques- Kleine et al. (2005a)]: (1) The isochron is based on mineral tioned (Foley et al., 2005) on the basis of new data, as dis- separates from one CAI and bulk analyses from two other cussed in detail below. The processes fractionating Hf/W CAIs. (2) Resetting of the Hf-W system would require mo- are partial melting and core formation (Lee and Halliday, bilization of radiogenic W from silicates into metal. The 1997; Halliday and Lee, 1999), whereas Sm/Nd responds diffusion of W from silicates into metal, however, appears only to partial melting because both parent and daughter to require temperatures in excess of 600°C (Kleine et al., elements are lithophile. Lee and Halliday (1997) proposed 2005b), which were not achieved on the Allende parent as- that this reflects contemporaneous partial melting and core teroid. The formation of scheelite and powellite as second- formation. However, this was based on the assumption that ary products from Fremdlinge as well as the mobilization the least-radiogenic W was chondritic, as was the least-ra- of W from Fremdlinge to surrounding silicates would not diogenic Nd. Now it is known that chondrites do not have cause resetting of the isochron. (3) The 182Hf-182W age de- ε182W = 0 (Lee and Halliday, 1995, 1996) but rather rived from the CAI isochron (calculated relative to the Ste. ε182W ~ –2 (Lee and Halliday, 2000a; Kleine et al., 2002; Marguerite H chondrite) is 4568.0 ± 1.7 Ma, which is iden- Schoenberg et al., 2002; Yin et al., 2002b). It has been pro- Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 785

W-isotopic data available as of August 2005 are plotted in Fig. 4. The nakhlites form a group with relatively uniform and radiogenic Nd and W (ε182W ~ 3). Chassigny is slightly less radiogenic. The shergottites are less radiogenic in W, as already noted. However, with the inclusion of the latest data for the two Saharan shergottites DaG 476 and SAU 051 (Foley et al., 2005), there is a broad spread in ε142Nd from –0.3 to radiogenic values of ~+1.0, like those of the nakhlites, and offset from the main correlation. The addition of the new data for the Saharan meteorites therefore changes the apparent distribution somewhat. In Fig. 4 both the new Foley data and the Jagoutz ε142Nd data are plotted to illustrate the effect. The reason for this apparent discrepancy is not certain although, as discussed by Foley et al., the lower ε142Nd value reported by Jagoutz et al. (2003) in the Sa- haran martian meteorites could result from the presence of a terrestrial weathering component. Until this discrepancy is clarified it is perhaps premature to venture an explanation Fig. 3. Tungsten-isotopic data for martian meteorites reveal a sig- for the apparent offset of these two Saharan Nd data from nificant offset relative to the newly defined average solar system at εW = –1.9. Data from Lee and Halliday (1997), Kleine et al. (2004a), and Foley et al. (2005). In general terms the sequence of papers is accompanied by a dramatic improvement in precision. Where the same meteorite has been analyzed in more than one study, only the most precise measurements are shown here and in the following figures. The three studies show generally good agreement but the imprecise value for EETA 79001 in Lee and Halliday (1997) differs significantly from the more precise measurements reported by Kleine et al. (2004a) and Foley et al. (2005). Assum- ing that these more precise recently determined values are likely to be correct, the data obtained for shergottites are relatively uniform and distinct from nakhlites, as pointed out by Kleine et al. and Foley et al.

posed that there is a small (~20 ppm) offset in chondritic ε142Nd as well (Boyet and Carlson, 2005). However, this is of second-order importance for the comparison with W isotopes. The least-radiogenic W-isotopic composition measured thus far for Mars is significantly offset to higher than chondritic values (Fig. 3). Kleine et al. (2002, 2004a) have Fig. 4. The W-isotopic compositions of martian meteorites dis- explained this in terms of a period of core formation prior play a broad relationship with their corresponding ε142Nd result- ε142 to silicate melting, whereas the correlation with Nd re- ing from decay of formerly live 146Sm. Data from Harper et al. flects the effects of silicate melting only. (1995), Jagoutz et al. (2003), and Foley et al. (2005). The latest More precise 182W and 142Nd data on previously stud- 142Nd data for the two Saharan meteorites (Foley et al., 2005) ied and additional martian meteorites have now been used differ from those reported by Jagoutz et al. (2003). The reasons to argue that no such overall Nd-W correlation exists (Foley for these discrepancies are unclear and both sets of values are et al., 2005). The analytical uncertainties on ε182W are shown here. Foley et al. (2005) point out that inclusion of their considerably improved in the Foley et al. study. Most pre- new Saharan data seriously reduces the evidence for a relation- viously published W data agree with the new data within ships between Nd- and W-isotopic effects and the proposal that the stated uncertainties except for the early report of ε182W ~ metal segregation and silicate melting overlapped in time (Lee and Halliday, 1997). The fact that some samples have W that is more 2 for EETA 79001 (Lee and Halliday, 1997), which is not radiogenic than chondritic, while still possessing nearly chondritic replicated by both of the more recent studies (Kleine et al., Nd, provides evidence that a component of core formation pre- 2004; Foley et al., 2005). There are also apparent discrepan- ceded large-scale silicate differentiation (Kleine et al., 2002, cies between the ε142Nd of two Saharan martian meteorites, 2004a). Note the new values for chondritic W (Kleine et al., DaG 476 and SAU 051 (Jagoutz and Jotter, 2000; Jagoutz 2004a) and chondritic Nd (Boyet and Carlson, 2005). The concen- et al., 2003; Foley et al., 2005). The most precise Nd- and trations of W (in ppb) are shown in parentheses. 786 Meteorites and the Early Solar System II

Fig. 5. Tungsten-isotopic compositions of martian meteorites do not correlate with measured Hf/W but do correlate with model source Hf/W determined from the chondritic value of (a) Hf/Ba and the sample’s Ba/W and (b) Hf/Th and the sample’s Th/W. Assuming Hf/ W is only fractionated by core formation and not silicate partial melting, these data can be modeled in terms of a progressive core formation model. However, it is very well established that W is much more incompatible than Hf during partial melting of the mantle (Newsom et al., 1986, 1996; Halliday and Lee, 1999). Note therefore that metal segregation probably only accounts for a fraction of the W-isotopic heterogeneity. The present-day W-isotopic compositions are shown for a range of martian reservoirs that have suffered different degrees of W depletion by core formation over different timescales. Each model curve deploys exponentially increasing Hf/ W with time and a time constant (m.y.–1) as shown. There is no particular basis for assuming that W depletion did change exponentially with time. Other models show a similar general curvature and the timescales calculated (see Fig. 6) are similar. However, a portion of the apparent rapidity of differentiation could reflect the unknown magnitude of increase in Hf/W resulting from silicate partial melting. See text for details.

the main trend. Foley et al. (2005) note that there still exists tion (Halliday and Lee, 1999). This is not unexpected be- a correlation between the ε182W and ε142Nd values for the cause martian meteorites are mainly young and their sources shergottites, as can be seen from Fig. 4, but it has a very dif- have experienced several billion years of evolution since the ferent (shallower) slope. very early processes that produced the W-isotopic effects. A concern with some of the Saharan samples is with However, the first, imprecise, W-isotopic compositions do mobility of certain trace elements. Some such samples have show a trend with Ba/W (Halliday et al., 2001b). This is anomalous trace-element compositions, thought to reflect less convincing with more precise measurements (Fig. 5a), partial terrestrial contamination or leaching. As shown in as discussed below. Barium and W are equally incompatible Fig. 4, the meteorites DaG 476 and SAU 051 have extremely in mantle melting and their relative proportions in terres- depleted W concentrations (Kleine et al., 2004) — among trial basalts are more or less uniform (Newsom et al., 1986, the most depleted yet recorded from shergottites and nahk- 1996). The Ba/W ratios of terrestrial basalts dominantly re- lites (Lodders, 1998). The Nd/W ratios are perfectly nor- flect the amount of W depletion in the mantle caused by core mal (Kleine et al., 2004; Dreibus et al., 2003). Therefore, formation. Assuming this is also true of silicate partitioning whether this depletion reflects leaching/dissolution of in- on Mars, one can use the variation in Ba/W as a proxy for compatible matrix elements or is a primary igneous feature different degrees of metal segregation. The implication is is unclear. There is no question from the bulk compositions that the incompatible trace elements in martian basalts carry that there has been enrichment in some fluid mobile ele- a record of very early metal segregation processes on the ments (e.g., Ba) and so leaching and exchange of either Nd planet. This at first seems quite remarkable. On Earth such or W may have also occurred. Whether this has anything effects have largely been homogenized by billions of years to do with the isotopic data distribution or the interlabora- of mantle convection. The oldest incompatible-element iso- tory differences is also unclear at present. topic heterogeneities in the present-day convecting mantle, The W-isotopic compositions of martian meteorites are as preserved in modern basalt magmas, are <2 b.y. old. Even not correlated with chemical indices of magmatic fractiona- the subcratonic lithospheric mantle is overwhelmingly dom- Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 787

inated by heterogeneity that is ≤3 b.y. Traces of very early that could be distributed heterogeneously on the scale of (>4.4 Ga) ε142Nd heterogeneity have been found in the an- the typical sample amounts used by various workers. cient sources of early Archean rocks from West Greenland 3. Partial melting. A second effect may have fraction- (Boyet et al., 2003; Caro et al., 2003). Therefore, Earth had ated the source Hf/W, resulting in changes in W-isotopic small mantle isotopic heterogeneities in the early Archean composition. The variability in ε142Nd (Fig. 4) indicates that that have since been homogenized. It has recently been some silicate partial melting was consanguineous with core proposed that the silicate Earth, as sampled by magmas, is formation. This would have produced source variations in offset from chondrites in ε142Nd by 20 ± 14 ppm (Boyet and Hf/W that are not determinable with the approach utilized Carlson, 2005). Whether this reflects a complementary hid- here. den reservoir or a slightly nonchondritic silicate Earth is 4. Variable siderophile-element depletion in time and unclear. None of these effects compare with the large het- space. Given the preservation of heterogeneity in the W- erogeneities preserved within the martian mantle from the isotopic composition and degree of W depletion, there is no earliest solar system. basis for assuming that metal segregation proceeded at a The fact that the martian W-isotopic compositions relate constant rate in all portions of Mars. to Ba/W provides evidence that the isotopic variations do This last feature can be modeled quite easily. This is define differences in the degree of W depletion related to important because clearly the W-isotopic compositions were core formation and not just partial melting. The samples not generated from a single primitive mantle reservoir with carry a chemical as well as isotopic record of variations in uniform Hf/W as is commonly deployed to calculate a the degree of siderophile depletion. One can simply convert model age of core formation. The data distributions shown the Ba/W to a model source Hf/W that would have been in Fig. 5a,b are plotted along with model curves, each of produced as a result of metal-silicate fractionation by mul- which defines the predicted present-day W-isotopic com- tiplying by the chondritic Hf/Ba ratio (0.044). This is il- positions of a reservoir that became isotopically heteroge- lustrated (Fig. 5a) for all the most precise W-isotopic data neous by undergoing variable degrees of metal segregation, for which Ba/W ratios are available. Saharan meteorites are hence W depletion, with a particular time constant, λ, i.e., strongly altered and as such it is not possible to use Hf/Ba 180 184 180 184 λt as a refractory lithophile multiplier in such samples. Un- Hf/ W = ( Hf/ W)BSS × e fortunately, U, the commonly used alternative to Ba, is also 180 184 highly mobile in Saharan meteorites. Thorium is probably where ( Hf/ W)BSS is the parent/daughter ratio of the the best, although also imperfect, alternative and is used bulk solar system (or chondrites), λ is the time constant, and here to calculate a second set of model source Hf/W values t is time. The model assumes that Mars was a chondritic from the measured Th/W and chondritic Hf/Th (4.0). When object that developed various such heterogeneous reservoirs, the W-isotopic data are compared to these model source Hf/ which differed in the rate at which they underwent W deple- W ratios, a trend is evident, whichever multiplier is deployed tion because of core formation. If the W depletion develops (Fig. 5a,b). This cannot be explained by silicate partial melt- rapidly the Hf/W ratio increases early and the reservoir ing. It indicates that core formation was a primary factor develops W that is relatively radiogenic for a given Hf/W. in generating the variations in W-isotopic composition. The longevity defined by the locus of each of these dif- The scatter to the data distribution in Fig. 5 could be ferent time constant curves is shown in Fig. 6. It can be seen caused by one or more of four factors: that the timescales that are required to generate martian 1. Alteration. The different symbols in Fig. 5 refer to meteorite compositions like that of Nakhla could in fact be Saharan (diamonds), Antarctic (squares), and other (circles) quite long (>10 m.y.) because, despite having radiogenic W, martian meteorites. The model Hf/W values calculated for the model Hf/W is also very high. Unless other effects (1 the two Saharan meteorites, using Th/W, define the full through 3 above) are responsible, the nakhlite source ap- spread in shergottite values (Fig. 5b) and should therefore pears to have carried on segregating metal over long (107 yr) be treated with some caution. timescales. The steepest line in Fig. 5 defines a very fast, 2. Analytical uncertainties and sample heterogeneity effectively instantaneous differentiation rate (λ = 1 m.y.–1) issues. The source Hf/W calculated either way has been and several of the martian meteorites plot along this line assigned a somewhat arbitrary uncertainty of ±20%. How- or slightly to the left with low Hf/W for a given W-isoto- ever, the trace-element compositions are generally deter- pic composition. The differentiation timescales for gener- mined on different aliquots from those used for measuring ating the W-isotopic compositions along this low Hf/W line isotopic compositions. Given the variability in trace-element are incredibly fast — around 1 m.y., as shown for Zagami concentrations reported by different workers, this may be (Fig. 6). an underestimate of the real uncertainties. (See, for example, Coeval silicate partial melting effects mean that these Hf/ C. Meyer’s Mars Meteorite Compendium of available data W values could both over- and underestimate the real source at http://www-curator.jsc.nasa.gov/curator/antmet/mmc/ Hf/W values. This could explain why some samples have mmc.htm.) Furthermore, some of the apparent variability in slightly radiogenic W for a given model Hf/W and plot to Ba/W for the same sample could be related to these ele- the left in Fig. 5, for example. That is, the source was also ments being predominantly cited in different minor phases depleted by partial melting generating higher Hf/W than 788 Meteorites and the Early Solar System II

similar to those predicted from runaway growth (Weiden- schilling et al., 1997). There is little evidence for late-stage planetary-scale collisions affecting and effecting the growth of Mars.

7. THE AGE OF THE MOON

The discovery of 182W variability in lunar samples has provided the most powerful constraint on the age of the Moon (Lee et al., 1997, 2002; Leya et al. 2000; Kleine et al., 2005c). The W-isotopic compositions of lunar samples are offset to radiogenic values relative to chondrites. At first, it appeared these effects were relatively large such that a rather simple model age calculation could be applied. How- ε ever, the most enhanced W values are now known to be the product of cosmogenic effects on 181Ta, which produces 182Ta, the intermediate decay product of 182Hf decay (Leya et al., 2000; Lee et al., 2002). The isotopic variations appear to be at most 1 or 2 ε units (Lee et al., 2002). However, variations within the lunar mantle, even of this magnitude, restrict the age of the Moon to the first 30 to 55 m.y. of the Fig. 6. Timescales implied by the model development of Hf/W solar system (Kleine et al., 2005c; Yin et al., 2002b; Halli- with time in the martian mantle assuming all the W-isotopic ef- day, 2003, 2004), in stark contrast to the limited constraints fects are the result of siderophile depletion associated with core based on the oldest dated rocks (Wasserburg et al., 1977b) formation. Two extreme examples are highlighted. The W deple- (Table 2). tion of the Nakhla source would have taken more than 10 m.y. to The determination of the Hf-W age of the Moon depends develop, whereas the time necessary to form the source of Zagami on knowing the magnitude of the W-isotopic effect pro- is <1 m.y. duced by decay of 182Hf within the Moon itself (Halliday, 2004). For this it is necessary to know (1) the initial Hf- and W-isotopic compositions of the solar system, (2) the accounted for with the approach adopted here that assumes initial W-isotopic composition of the Moon, (3) the present- that all the W-isotopic effects result from core formation. day W-isotopic composition of the lunar mantle, and (4) the However, this does not appear to apply to Zagami and Sher- Hf/W of the lunar mantle. gotty (Fig. 4). These meteorites yield chondritic Nd-isoto- The initial 182Hf/180Hf of the solar system is now fairly pic compositions; there is no evidence of fractionation as a well established (Kleine et al., 2002, 2005a; Schoenberg et result of silicate partial melting. Further combined W- and al., 2002; Yin et al., 2002b). The initial ε182W of the solar Nd-isotopic data and high-quality trace-element data on the system based on the W-isotopic compositions of CAIs and same and representative sample aliquots are needed to chondrites is established to be –3.5 (Kleine et al., 2002, evaluate this more generally. Model attempts based on the 2005a; Yin et al., 2002b); nevertheless, as discussed earlier, lithophile behavior of Nd have been utilized by Kleine et there is still some degree of uncertainty because of the ap- al. (2004a) and Foley et al. (2005). The deduced timescales parently less-radiogenic W-isotopic compositions of some for martian mantle reservoirs tend to be similar. Brandon iron meteorites. et al. (2000) have also presented a broadly similar story The initial W-isotopic composition of the Moon is at from Re-Os systematics. Marty and Marti (2002) similarly present unknown. The argument has been made (Halliday, have presented the case for very rapid (<35 m.y.) differenti- 2003, 2004) that the large number of samples with W-iso- ation based on Xe-isotopic systematics but with a more pro- topic compositions close to ε182W = 0 most likely means tracted degassing history for the nakhlite source. All these that this approximates a common composition from which approaches yield approximately the same view of Mars, that the Moon started. This could well be wrong; however, it is the accretion and primary differentiation were exceedingly unlikely to be far wrong. Clearly the initial composition of rapid. the Moon cannot be higher than its least-radiogenic value. Nevertheless, the Hf-W data provide unique evidence It is unlikely to be much lower either since the current the- that core formation proceeded over a range of timescales ory for the origin of the Moon involves a giant impact. The that in some cases were quite long (107 yr). Other meteor- Moon is thought to have been derived from the silicate ites such as Zagami appear to record more rapid timescales mantles of the proto-Earth and the impacting planet Theia. (106 yr). This in turn implies that accretion of Mars was All such silicate reservoirs studied thus far have ε182W ≥ early and rapid, on the order of 1 m.y. Such timescales are –0.5 (Quitté et al., 2000) and unless the Moon formed very Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 789

early (<20 m.y.), there is no reason why it would have a lower value. Most of the present-day W-isotopic compositions of lunar samples corrected for cosmogenic effects lie in the range ε182W = 0–1. Most cosmogenic corrections are very approximate. Some values are unequivocally higher (Lee et al., 2002). Lunar sample 15555 with no cosmogenic effect yields a value of 1.30 ± 0.39. From the current data- base it would appear that the average composition of the lunar mantle is unlikely to be greater than ε182W = 1, but it could conceivably be this high if 15555 is especially representative of the lunar mantle. Therefore, a likely scenario is an average radiogenic increase of about 0.5–1.0 (Halli- day, 2004). A more conservative estimate of the amount of radiogenic increase would be that it is less than 2.0 ε182W Fig. 7. The age information that can be derived from current W- units. isotopic data for the Moon at the present time is strongly model The fact that there are endemic, indigenous W-isotopic dependent. The effects on the calculated age of the Moon of not variations at all provides clear evidence that they were pro- knowing the average amount of radiogenic, as opposed to inherited or cosmogenic, 182W are shown. The increase in εW within duced by radioactive decay within the lunar mantle; they the Moon as a function of decay of primordial 182Hf, as can be cannot be residual from Earth or Theia given the inevitable deduced from currently available data, is probably ≤1 ε unit. The mixing associated with the high temperatures of accretion data are therefore consistent with an age of the Moon (hence gi- and early convective overturn. The initial W-isotopic com- ant impact) of about 40 to 50 m.y. after the start of the solar sys- position of the Moon does reflect the mix of components tem. The different curves are based on differing values of the ε182 182 180 from these parent planets, however, and provides informa- WBSSI, which directly affects the calculated ( Hf/ Hf)BSSI. λ182 –6 –1 tion on their history, as described below. All calculations assume Hf = 0.077 × 10 yr , Hf/WMOON = ε182 The Hf/W ratio of the lunar mantle has been estimated 22, and WBSS = –1.9. most recently and comprehensively by Jones and Palme (2000) to be 25.2. (Note that this value changes to 26.5 when using more up-to-date CI abundances; the estimate is based on U/W = 1.93 for the lunar mantle and depends useful upper limit, that is, the earliest likely time that the on the Hf/U used for chondrites.) This is similar to previ- Moon can have formed, given its W-isotopic composition. ously used estimates (Halliday, 2003, 2004). Recently, Kleine et al. (2005c) have reported W-isoto- The age of the Moon can be deduced from these com- pic data for metals separated from lunar samples that pro- bined estimates as shown in Fig. 7 and would appear to lie in vide what appears to be the best constraint on the exact age the range 40–50 m.y. (Halliday, 2004; Kleine et al., 2004b). of the Moon. The significance of these measurements lies The age can be refined more closely once more precise in the fact that the metal has very low Hf/W and Ta/W such estimates of the amount of radiogenic growth on the Moon that the W-isotopic composition is a real initial ratio for the are known. This estimate is later than the 29 m.y. proposed igneous rocks studied whatever their age and exposure. By by Kleine et al. (2002) and the value of 30 m.y. calculated comparing these W-isotopic compositions with independent by Yin et al. (2002b). These are model ages relative to a estimates of the Hf/W ratio of the magma sources, one can chondritic reservoir and are the equivalent of assuming that derive a differentiation age for the Moon. On this basis the Moon formed in a single step from an undifferentiated Kleine et al. derive an age for lunar differentiation of 40 ± chondritic reservoir. We now know the W-isotopic compo- 10 m.y. This is the most precise estimate yet obtained for sition of such a reservoir very well (Kleine et al., 2002; the age of crystallization of the lunar magma ocean. Schoenberg et al., 2002; Yin et al., 2002b). However, the Whatever the exact age of the Moon, there are clear im- Moon has a relatively low uncompressed density compared plications for these results: with that of the terrestrial planets, and this has long been 1. First, there is no question that the Moon formed late taken as evidence of an origin from a silicate-rich precursor, relative to other small objects. Therefore, models for its for- i.e., a planetary mantle. Therefore, current models of lunar mation must explain this. The fission and late impact theo- formation (e.g., Canup and Asphaug, 2001) involve a colli- ries would seem best suited. Capture and coaccretion do not sion between Earth and a Mars-sized planet that is already predict a late formation. differentiated into silicate and metal. As such, the prior W- 2. A significant time gap can now be identified between isotopic evolution of these parent planetary mantles will have the age of crystallization of the lunar magma ocean deduced contributed to the W-isotopic composition recorded in lunar from 182Hf-182W and the precise ages deduced from 235/238U- basalts. A model age relative to chondrites is the equivalent 207/206Pb and 147Sm-143Nd chronology for early lunar rocks of assuming that none of this had happened. It provides a and lunar differentiation events, previously thought to re- 790 Meteorites and the Early Solar System II late to the formation and crystallization of the lunar magma (2004) reveal differences relative to previously published ocean. results. They confirm only some of the nucleosynthetic ef- 3. Assuming the Moon was formed in a giant impact fects and find a decoupling between p- and r-process com- when the proto-Earth was ~90% formed, its age provides ponents. The results no longer correlate with Ru so well. the single most important piece of independent evidence The apparent consistency between 235/238U-207/206Pb, that can be used with the meteorite reference frame to cali- 244Pu/129I-136/129Xe, and 182Hf-182W chronometry may also brate Earth’s growth history, as explained next. be incorrect. The early W-isotopic data for chondrites (Lee and Halliday, 1995, 1996) are wrong by about 180–200 ppm 8. METEORITES AND THE (Kleine et al., 2002; Schönberg et al., 2002; Yin et al., GROWTH OF EARTH 2002b). The new 182Hf-182W timescales are shorter and appear to be inconsistent with 235/238U-207/206Pb timescales As explained above, the W-isotopic data for iron mete- (Halliday, 2003, 2004; Wood and Halliday, 2005). Even the orites provide evidence of very rapid accretion and core latest 244Pu/129I-136/129Xe timescales appear to be too long formation of earliest planetesimals in the inner solar sys- (Porcelli et al., 2001). In detail, therefore, it now appears tem. Similarly, the data for Mars are hard to explain unless as though the chronometers are reflecting different chemi- accretion and earliest core formation were extremely rapid cal fractionations that took place over distinct timescales, (<1 m.y.). The isotopic data for Earth, however, provide or that the degree to which the real processes are accurately strong support for protracted accretion over tens of millions simulated by the isotopic models differ between elements of years. This is consistent with dynamic simulations that (Halliday, 2003, 2004; Wood and Halliday, 2005). predict timescales for terrestrial planet accretion that are on The 235/238U-207/206Pb and 182Hf-182W chronometers both the order of a few tens of millions of years (Safronov, 1954; rely on determining the timing of fractionation during core Wetherill, 1980, 1986; Agnor et al., 1999; Canup and Agnor, formation. Both are usually utilized in one of two ways: 2000; Chambers, 2001a,b, 2004). These simulations can be (1) A model age can be calculated as the time that the sili- extremely sensitive to the amount of nebular gas present cate Earth was last in total isotopic equilibrium, since the (Agnor and Ward, 2002; Kominami and Ida, 2002). Their time when its silicate (high U/Pb and Hf/W) and metal (low accuracy for describing the real Earth needs to be tested U/Pb and Hf/W) reservoirs developed distinct compositions. rigorously, hence the central importance of isotopic ap- This can be thought of as defining an age of instantaneous proaches. core formation in a completely formed planet. Alternatively, The three most powerful and effective techniques for de- it can be thought of as catastrophic reequilibration of sili- termining the growth rate of Earth are 235/238U-207/206Pb, cate and metal reservoirs during some major overturn mix- 244Pu/129I-136/129Xe, and 182Hf-182W. These yield differing ing event. Finally, it can be thought of as simply the age of age estimates, and this in turn may provide insights into the the planet itself assuming the core formed simultaneously. processes that are likely to have accompanied Earth accre- There is considerable evidence against any of these repre- tion. As the first 182Hf-182W data became available there ap- senting the way Earth formed and developed in its earliest peared to be excellent agreement with the protracted time- stages. (2) A mean age for accretion can be calculated as scales for planetary accretion and atmospheric loss as de- the inverse of the time constant for exponentially decreas- duced from 92Nb-92Zr (Münker et al., 2000; Jacobsen and ing growth of Earth, assuming that, as it grew, the accreted Yin, 2001), 129I/244Pu-129/136Xe (Porcelli and Pepin, 2000), material isotopically equilibrated with the primitive mantle and 235/238U-207/206Pb chronometry (Halliday, 2000). The and continuously segregated further core material in pres- timescales deduced from 97Tc-97Mo (Yin and Jacobsen, ent-day proportion to Earth’s mass (Jacobsen and Harper, 1998) and 107Pd-107Ag (Carlson and Hauri, 2001) chro- 1996). There seems little question that this is more realis- nometry were shorter. tic (Halliday, 2004). Now it is clear that some of these constraints are not very The first of these models represents the standard proce- strong because some of the critical Mo, Zr, and Ag data dure adopted for dating Earth or core formation (Allègre appear to be incorrect or misinterpreted (Dauphas et al., et al., 1995; Galer and Goldstein, 1996; Lee and Halliday, 2002a,b, 2004; Chen et al., 2004; Lee and Halliday, 2003; 1995, 1996; Halliday and Lee, 1999). However, the “event” Schönbächler et al., 2002, 2003; Woodland et al., 2004). defined probably never occurred as such. The second of For example, the anomalies in Mo are hard to distinguish these models is more sophisticated and useful and is based from nucleosynthetic effects and different groups have re- on the formalism first proposed by Jacobsen and Wasser- ported significantly different results for some of the isotopes burg 1979). It was explored for 182Hf-182W by Harper and even when the same normalization procedure is deployed Jacobsen (1996) and Jacobsen and Harper (1996) and fur- (Yin et al., 2002a; Becker and Walker, 2003; Dauphas et al., ther explored in a series of models by Halliday et al. (1996, 2002a,b, 2004; Chen et al., 2004; Lee and Halliday, 2003). 2000), Halliday and Lee (1999), and Halliday (2000). These The reasons for these discrepancies are unclear. Dauphas all predate the correct determination of the isotopic com- et al. (2004) reported that his Mo-isotopic data correlated positions of chondrites (Kleine et al., 2002, 2004a; Lee and with effects for Ru reported by Chen et al. (2003), appar- Halliday, 2000a; Schoenberg et al., 2002; Yin et al., 2002b). ently confirming their validity as nucleosynthetic hetero- Recent studies have made extensive use of this approach geneities. The latest results for Mo reported by Chen et al. (and the correct reference compositions) to obtain an ac- Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 791 curate 182Hf-182W accretion timescale for Earth (Yin et al., The age of the Moon provides the only firm independent 2002b; Halliday, 2003, 2004; Kleine et al., 2004b). The constraint on the accretion rate because it is thought to be same idea was developed and exploited for 235/238U-207/206Pb the byproduct of the last major growth phase of Earth by Halliday (2000, 2003, 2004). (Canup and Asphaug, 2001). The later the age of the Moon, When the results for 182Hf-182W and 235/238U-207/206Pb are the more disequilibrium is needed to explain the radiogenic compared there appears to be a discrepancy (Halliday, 2003, W-isotopic composition of the silicate Earth. Using a win- 2004; Wood and Halliday, 2005). The 235/238U-207/206Pb ac- dow of 45 ± 5 m.y. (Fig. 7) as the most realistic for the giant cretion rates appear to be more protracted than those de- impact (Halliday, 2003, 2004; Kleine et al., 2004b, 2005c) termined from 182Hf-182W (Fig. 8). One possible explana- allows a model accretion curve to be constructed using tion to consider is that it relates to the differences in the rates larger-sized impacts with time, culminating in the giant im- of metal-silicate equilibration of refractory W vs. volatile Pb pact (Fig. 9). From this accretion curve one can deduce the during the accretion process itself (Halliday, 2003, 2004). level of W-isotopic equilibration that would be required The possibility that there had been a lack of equilibration assuming a general level of disequilibrium throughout the during accretion was discussed by Halliday (2001), but at accretion history or disequilibrium during the giant impact that stage it appeared that the W-isotopic composition of alone (Fig. 10). Assuming the latter case it can be seen that the silicate Earth was identical to that of chondrites such that equilibration had to be the general rule (Halliday, 2001). Now that a resolvable difference has been established between the silicate Earth and chondrites, the question arises as to whether this is caused by faster accretion, or lack of isotopic equilibration, or both (Halliday, 2004). The only way to test this is with some independent assessment of the age of the Moon, or the giant impact.

Fig. 9. Change in mass fraction of Earth as a function of time in the model used in Halliday (2004) illustrated with an accretion scenario calculated from a giant impact at 50 m.y. after the start of the solar system. The approximated mean life of accretion (τ) is the time taken to achieve 63% growth. Both this and the timing of each increment are calculated from the timing of

the giant impact (tGI) to achieve an overall exponentially decreasing rate of growth for Earth broadly consistent with dynamic simu- Fig. 8. Calculated values for Earth’s mean life of accretion (τ) lations. The smooth curves show the corresponding exponentially and time of the giant impact (TGI) given in million years as de- decreasing rates of growth. The step function curves define the duced from the different estimates of the Pb-isotopic composition growth used in the isotopic calculations. Growth of Earth is mod- of the bulk silicate Earth (BSE) (Doe and Zartman, 1979; Davies, eled as a series of collisions between differentiated objects. The 1984; Allègre et al., 1988; Zartman and Haines, 1988; Allègre and overall rate of accretionary growth of Earth may have decreased Lewin, 1989; Kwon et al., 1989; Galer and Goldstein, 1991; Liew in some predictable fashion with time, but the growth events would et al., 1991; Kramers and Tolstikhin, 1997; Kamber and Coller- have become more widely interspersed and larger. Therefore, the son, 1999; Murphy et al., 2003) using the type of accretion model model simulates further growth by successive additions of 1% M shown in Fig. 9 (see Halliday, 2004). Note that, while all calcu- objects from 1% to 10% of the current mass, then by 2% objects lations assume continuous core formation and total equilibration to 30%, and then by 4% objects to 90%. The Moon-forming gi- between accreted material and the BSE, the accretion is punctu- ant impact is modeled to take place when Earth was ~90% of its ated, as predicted from the planetesimal theory of planetary ac- current mass and involved an impactor planet Theia that was ~10% cretion. This generates more protracted calculated timescales than of the (then) mass of Earth. Therefore, in the model the giant those that assumer smooth accretion (Halliday, 2004). The 238U/ impact at 45 m.y. after the start of the solar system contributes a 204Pb values assumed for Earth = 0.7 (Allègre et al., 1995). Even further 9% of the current Earth mass. There is evidence against the Hf-W timescales using the exact same style of model are sig- large amounts of accretion after the giant impact. Although epi- nificantly shorter. The mean life model ages given in Table 2 are sodic relative to more conventional continuous core formation for a smooth accretion model that yields less-protracted results models (Jacobsen and Harper, 1996; Halliday et al., 1996, 2000; (Halliday, 2000) and are for the average (Halliday, 2004) of the Halliday and Lee, 1999; Halliday, 2000), the model is still smooth most recent (Kramers and Tolstikhin, 1997; Kamber and Coller- relative to accretion simulations (e.g., Agnor et al., 1999; Cham- son, 1999; Murphy et al., 2003) of these Pb-isotopic estimates. bers, 2001a,b, 2004). 792 Meteorites and the Early Solar System II the amount of equilibration of Theia’s core with the silicate with dynamic simulations and experimental and theoretical Earth required to explain its W-isotopic composition was mineral physics should help in this respect. limited to <60% during a giant impact at 45 m.y. and could Another variable that is becoming critical is the exact have been as little as 40% (cf. Halliday, 2004). composition of Earth. We now have excellent W-isotopic This point is made primarily to illustrate the importance data for chondrites (Kleine et al., 2004a), assumed to rep- of considering equilibration when determining model iso- resent the bulk Earth. This is unlikely to be far wrong. How- topic age constraints such as the accretionary mean life. If ever, it now becomes important to know the exact Hf/W of the giant impact involved a larger impactor, for example, a the total Earth. If Earth has a Mg/Fe ratio that is lower than 0.15 instead of 0.10 M object, then a greater proportion chondritic (Halliday et al., 2001a; H. Palme, personal com- of Earth would have been accreted at a later time. There- munication, 2003), then it probably has a Hf/W ratio that is fore, the radiogenic W in the silicate Earth would imply also subchondritic. The effect on W-isotopic models depends greater amounts of disequilibrium during accretion. The size on when this Hf/W depletion occurred. If it is an early fea- of the impactor affects the energy that is released. As such, ture it requires that the amount of disequilibrium during ac- it can be tuned in dynamic simulations to produce an Fe- cretion was greater. Even the chondritic Hf/W ratios are not depleted Moon at the right distance with the correct angu- so well known. New high-precision isotope-dilution data by lar momentum. However, a critical issue is the equation of Kleine et al. (2004a) are, on average, slightly lower than state, which defines the amount of energy released for a the previously utilized “standard” data (Newsom, 1995). Al- given sized impactor, and which is poorly known for high- though the data agree within error, these uncertainties are pressure materials such as perovskite, even though advances of sufficient magnitude that they now significantly limit are being made. A closer integration of isotopic modeling attempts to produce high-resolution model age and accretion rate calculations. Sample size effects have plagued Lu- Hf and Sm-Nd studies of chondrites and new Hf-W studies of large representative samples would be worthwhile. A Moon-forming impact at 45 m.y. does little to explain the discrepancy with the rates determined from the Pb-isotopic composition of Earth (Fig. 8). Assuming any one of these 11 estimates is approximately correct, the most likely explanation is that there was a change in U/Pb in the bulk silicate Earth during accretion. This might have been caused by changes in partitioning or volatile loss (Halliday, 2004). A recent study by Wood and Halliday (2005) proposes a new version of the former explanation. Tungsten and Pb may well segregate into planetary cores at different stages. Tung- sten is more siderophile than chalcophile, whereas Pb dis- plays the opposite behavior. During planetary accretion and differentiation a point will be reached where metal segre- gates as a result of the removal of ferric iron into perovskite. The metallic iron removes W at an early stage. However, Pb remains in the silicate Earth until the upper mantle of Earth becomes oxidized as a result of the giant impact dis- rupting perovskite in the lower mantle. The S added from the impactor, which appears to have been rich in chalcophile elements (Yi et al., 2000), results in the formation of sulfides as Earth cools. These will strip the silicate Earth of its Pb and, if sufficiently abundant, will accumulate at the Fig. 10. An illustration of the effect on calculated Hf-W time- base of the magma ocean, eventually sinking to the core scales for Earth’s formation of incomplete mixing and equilibra- (Wood and Halliday, 2005). Therefore, there appears to be tion of impacting core material. This plot shows the true time of a logical explanation for why the Pb-isotopic system records ε182 the giant impact that generates WBSE of zero as a function of a more protracted timescale than the Hf-W system. Cooling various levels of incomplete mixing of the impacting core mate- of Earth’s lower mantle should have been very fast after rial with the BSE. The lower curves are for disequilibrium during the giant impact (Solomatov, 2000). However, the upper the giant impact alone. The upper curves correspond to disequi- mantle from which the sulfide segregated could have cooled librium during the entire accretion process up to and including much more slowly (up to 108 yr). The earliest calculated the giant impact. The different curves for different Hf/WBSI (the Hf/W in the silicate portion of the impactor) are also shown. All timescales for removing Pb in this fashion are about 30 m.y. ε182 after a 45 m.y. giant impact based on the most recent esti- curves are calculated with WBSSI = –3.5. Assuming an age for the Moon in the range 40–50 m.y. after the start of the solar sys- mate for the Pb-isotopic composition of the BSE (Wood and tem, it would seem likely that there was significant isotopic dis- Halliday, 2005). Temperatures of roughly 3000 K are re- equilibrium during the giant impact (Halliday, 2004). quired to segregate sulfide in this fashion. The latest mod- Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 793 els of the giant impact (Canup, 2004) raise the tempera- angrites, and lunar rocks relative to CAIs provides strong ture of Earth to about 7000 K. Therefore, broadly speak- evidence that some process causes loss of Rb during accre- ing, Earth cooled at more than 100 K per m.y. after the giant tion (Halliday and Porcelli, 2001). This process cannot frac- impact. This would correspond to the time required to cool tionate K isotopes (Humayun and Clayton, 1995), and par- the upper portion of the mantle to the temperature at which tial evaporation would therefore appear unlikely. However, sulfide stripped Pb from the mantle. elemental fractionations unaccompanied by isotopic effects can in fact occur as a result of partial evaporation or con- 9. TIME-INTEGRATED CHEMICAL densation, as long as environmental conditions change suf- COMPOSITIONS AND THE VOLATILE ficiently slowly to allow for thermodynamic equilibrium BUDGETS OF EARLY PLANETESIMALS (Richter, 2003). In the case of early planetesimals like the AND TERRESTRIAL PLANETS angrite and eucrite parent bodies, it is thought that there was very early melting, differentiation, and volcanism predating The radiogenic isotopic compositions of early solar sys- the formation of the rocks sampled as achondrites (Lugmair tem objects can be used to define time-averaged parent/ and Shukolyukov, 1998). It is conceivable that such an early daughter ratios of precursor materials. This can be explored global differentiation was accompanied by extensive out- for any isotopic system and provides useful insights into gassing and almost complete loss of moderately volatile ele- the paleocosmochemistry of the early solar system. There ments. The volatile budgets would then be added by sub- are four systems to which this can be very usefully applied sequent accretion of new material. In the case of the Moon, to constrain early chemical evolution during accretion. however, it would appear more likely that the major loss (>90%) of Rb was associated with the giant impact (O’Neill, 9.1. Time-integrated Uranium/Lead 1991a,b).

Lead-isotopic compositions provide simultaneous con- 9.3. Time-integrated Hafnium/Tungsten straints on the age of an object and the parent/daughter ratios (238U/204Pb or primary “µ”) of the precursor materi- The Hf/W of a silicate reservoir is a function of the par- als. For objects like Earth and Mars the present-day Pb-iso- titioning of W during core formation but may also be ren- topic compositions of the silicate reservoirs are overwhelm- dered heterogeneous by silicate partial melting (Halliday ingly dominated by the time-integrated µ since accretion and Lee, 1999). The metal/silicate partition coefficient for and core formation. As such, these data say little about any W is strongly dependent on oxygen fugacity (Schmitt et al., earlier history. However, several lunar rocks formed early 1989; Walter et al., 2000). It presumably is also affected by and sampled the Pb-isotopic compositions of their precur- volatile contents and the pressure and temperature of core sor reservoirs. Lead is moderately volatile, and most lunar formation (Righter and Drake, 1996, 1999; Righter et al., rocks contain so little inherited Pb that the initial composi- 1997; Walter et al., 2000). The lack of depletion of W in tion is hard to resolve (Tera et al., 1973). It has been pro- the martian mantle has long been taken as providing evi- posed that this may represent the early silicate Earth (Galer, dence of more oxidizing conditions thought to be linked to 1993). However, an early high U/Pb silicate Earth is not the greater abundances of moderately volatile elements (for easy to reconcile with the present Pb-isotopic composition a review, see Halliday et al., 2001b). The fact that the W- of the silicate Earth (Halliday, 2004). Furthermore, some isotopic compositions recorded in martian meteorites are lunar rocks carry initial Pb-isotopic compositions that are only 200–500 ppm more radiogenic than chondritic despite relatively unradiogenic (Meyer et al., 1975; Hanan and Til- being generated very early is entirely consistent with this ton, 1987; Torigoye-Kita et al., 1995) and provide evidence lack of depletion. In the case of the silicate Earth the fact that that the Moon formed from material that was not so de- the W-isotopic composition also is within 200 ppm of chon- pleted in Pb. In fact, the primary µ values are broadly simi- dritic despite a Hf/W that is an order of magnitude greater lar to that of the bulk silicate Earth (Halliday et al., 1996). than chondritic is explained by protracted accretion of chon- Assuming that the Moon sampled a mixture of material dritic material equilibrating with the silicate Earth. How- from the proto-Earth and Theia, there is no clear evidence ever, the fact that the initial W-isotopic composition of the for any great difference in the magnitude of volatile deple- Moon is close to chondritic is more problematic (Halliday, tion in these precursor materials compared with the present 2004). The Moon has a high Hf/W, and it is thought that it Earth. This raises the issue of how the strong depletion in was mainly formed from the silicate portions of Theia, with 204Pb in the Moon was introduced. Presumably it was a subordinate contributions from the proto-Earth. If Theia was product of the giant impact. only a Mars-sized object it should have formed relatively fast like Mars itself, as discussed above. However, with such 9.2. Time-integrated Rubidium/Strontium a high Hf/W it would then be expected to have produced highly radiogenic W by the time of the giant impact. Instead, The Rb-Sr system also provides an indication of the the Moon started with εW ~ 0 (Halliday, 2004; Kleine et al., magnitude of volatile-element depletion (Halliday and Por- 2005c), within ~200 ppm of chondritic values. Assuming celli, 2001) of precursor materials. The differences between the Moon was mainly derived from Theia, there are four the Sr-isotopic compositions of early objects like eucrites, possible explanations: (1) The bulk silicate Theia had its 794 Meteorites and the Early Solar System II composition modified by large impacts shortly before the giant impact (Halliday et al., 2000), which is somewhat un- testable but not impossible. (2) Theia formed very slowly, and would need to be accreting at a rate much slower than Earth, Mars, or Vesta to have such unradiogenic W. (3) The material that formed the Moon equilibrated with quite large amounts of metal; to achieve an initial W-isotopic composition of εW ~ 0 in the Moon the amount of metal required is roughly 10% of the total material, which is well in excess of that predicted in giant impact simulations; (4) The silicate portions of Theia and/or the proto-Earth had a Hf/W that was close to chondritic; W partitioning changes as a function of oxygen fugacity (Walter et al., 2000), and the implication would be that the combination of silicate material that formed the Moon was significantly more oxidizing than the present silicate Earth. The last of these scenarios seems the most likely. When Fig. 11. Hafnium/tungsten appears to be negatively correlated the time-integrated Hf/W for the source of the Moon’s con- with Rb/Sr in the primitive mantles of inner solar system plan- stituents (generally thought to be dominated by the bulk etesimals and planets. A possible explanation for this is that the silicate Theia) is compared with the time-integrated Rb/Sr loss of moderately volatile elements was linked to loss of other (Halliday and Porcelli, 2001), the combined composition volatiles during planetary collisions such that the mantles changed is strikingly similar to that of the martian mantle (Halliday, from more oxidizing to more reducing. The Moon is an extreme example of this. The fact that the mixture of material from the 2004), lending support for the idea that Mars represents a proto-Earth and Theia that is calculated to have formed the Moon type of normal protoplanet that was an essential building is so like Mars provides evidence that such volatile-rich objects block in the formation of larger terrestrial planets. In fact, may have been common in the inner solar system during the early the composition plots at the extreme end of an array of stages of planetary accretion. See Halliday and Porcelli (2001) known compositions for terrestrial planets and differenti- and Halliday (2004) for further details. ated planetesimals (Fig. 11), providing evidence that loss of moderately volatile elements is somehow linked to the metal/silicate partitioning of W. Mars has a higher FeO/Fe ratio than Earth, which means giant impact and probably predates planetary formation. that the average oxidation state of Mars is higher, so this Models for the collapse of the solar nebula and accretion would explain why W was more lithophile (Walter et al., of a planetary disk predict high temperatures (1500 K) in 2000). The Earth’s upper mantle has a higher ferric iron the inner solar system (Boss, 1990). Therefore, it is likely content and thus oxygen fugacity than Mars. However, this that the inner solar system became depleted in volatile el- is thought to be due to the self-oxidation process of Earth’s ements before accretion of sizable bodies. The Earth and mantle associated with growth of the core. Once Earth had Moon share similar Cr-isotopic compositions, distinct from established a certain mass, greater than that of Mars, per- those of chondrites (Lugmair and Shukolyukov, 1998), con- ovskite would have become stable in the lower mantle. sistent with an early depletion in (more volatile) Mn (Halli- Therefore, during accretion and despite its low FeO/Fe, day et al., 1996; Cassen and Woolum, 1997). The magni- Earth started producing ferric iron that pushed up the oxy- tude of the difference in Cr-isotopic composition between gen fugacity (Wood and Halliday, 2005). This would not the BSE and some chondritic meteorites (about 0.5 ε53Cr/ have happened on Mars because perovskite is only just sta- 52Cr units) would be consistent with the depletion occur- ble at the base of the martian mantle. ring within about 3.5 m.y. after the formation of the mate- The striking differences between the present composi- rial in Allende, given the differences between the estimated tion of the Moon and the time-integrated Rb/Sr, U/Pb, and Mn/Cr ratios of the total Earth (~0.18) and chondrites Hf/W of Theia provide evidence that a major compositional (~0.40) and assuming an initial 53Mn/55Mn ratio of 4.4 × change was effected by the giant impact. In some respects 10–5. It should be emphasized that the interpretation of Mn- this is no surprise because the energy was enormous. How- Cr data is controversial and complex (Lugmair and Shukol- ever, little has been established about the physical chemis- yukov, 1998; Birck et al., 1989). The primary hypothesis try of such accretion processes and remarkably little is proposed by Lugmair and Shukolyukov (1998) is that the known about the feasibility of losing volatile elements from variations in Cr-isotopic composition simply reflect a ra- the debris of a giant impact. dial gradient in the 53Mn distribution resulting from incomplete mixing of material injected into the disk from a nearby 9.4. Time-integrated Manganese/Chromium stellar source that synthesized this and other nuclides. Isotope geochemistry therefore provides limited evidence The depletion of Earth’s and the Moon’s inventories of to support the widely accepted and almost certainly correct some volatile elements must have occurred earlier than the view that some volatile-element depletion in the planets was Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 795 very early. Other lines of isotopic evidence indicate that some changes in volatile abundance were late and some may have been related to energetic accretion processes like the Moon-forming giant impact.

10. CONCLUSIONS AND OUTLOOK

Isotope geochemistry of meteorites has been central to the determination of the ages, rates, and mechanisms of accretion of the terrestrial planets. Not only do isotopic studies of meteorites provide information on the average solar system and hence a reference for models of accretion and differentiation, meteorites provide important clues about first planetesimals and the formation of the earliest planetary embryos from which the rest of the solar system was built. The development of the Hf-W chronometer has had Fig. 12. The current best estimates for the timescales over which a bigger effect on this area of science than any other as- very early inner solar system objects and the terrestrial planets pect of isotope geochemistry. With the excellent reference formed. The approximated mean life of accretion (τ) is the time dataset now available for carbonaceous chondrites and the taken to achieve 63% growth at exponentially decreasing rates of newly determined highly precise value for the 182Hf decay growth. The dashed lines indicate the mean life for accretion de- constant (Vockenhuber et al., 2004), the precision and ac- duced for Earth based on W and Pb isotopes (Halliday, 2000, 2003, 2004; Kleine et al., 2002; Yin et al., 2002b). The earliest curacy of Hf-W chronometry has taken major steps forward. age of the Moon assumes separation from a reservoir with chon- At the time of this writing, several important conclusions dritic Hf/W (Kleine et al., 2002; Yin et al., 2002b). The best esti- can be drawn from Hf-W and other isotopic data about how mates are based on the radiogenic ingrowth deduced for the the inner solar system was built (Fig. 12): interior of the Moon (Halliday, 2003, 2004; Kleine et al., 2005c). 1. Iron meteorites appear to provide the best bet for sam- See Table 2 for details of other sources. ples of the first planetesimals. The ubiquitous unradiogenic W, sometimes lower in 182W than any other solar system sample measured so far, provides strong evidence that they formed early. 6. The age of the Moon is best defined by Hf-W data 2. The exact timing of metal segregation as represented that provide strong evidence of an origin from a giant im- by iron meteorites requires improved understanding of cos- pact, in the time range 30–50 m.y. after the start of the solar mogenic effects, which tend to decrease 182W and will be system, the exact age depending on the model deployed. present in many iron meteorites with long exposure ages. 7. The age of the Moon provides an important, indepen- By applying a maximum correction for cosmogenic effects dent, and unique constraint on the accretion rate of Earth it appears that some iron meteorites formed within <1 m.y. assuming it defines the last major phase adding roughly of the start of the solar system, as defined by the W-isoto- 10% of the present mass of Earth. pic composition of Allende CAIs (Markowski et al., 2006). 8. The W-isotopic composition of the silicate Earth is 3. It is also essential to provide better cross calibration consistent with this timescale for accretion. The small ex- of chronometers. These will be important as improvements cess of 182W in the bulk silicate Earth relative to chondrites in mass spectrometric techniques will provide increasing is explicable if the initial W-isotopic composition of the so- age resolution. lar system is ε182W < –3.5, or if there was some level of 4. There exist conflicts between the interpretations of disequilibrium between incoming metal and the BSE dur- different isotopic data for angrites and eucrites. The Sr-iso- ing accretion. topic data are hard to explain unless the parent body formed 9. The difference between W- and Pb-isotopic estimates >2 m.y. after the start of the solar system. for the rates of accretion of Earth cannot be explained by 5. The W-isotopic data for Mars are best explained if disequilibrium during accretion. Either these estimates for parts of the martian mantle differentiated very rapidly. The the average Pb-isotopic composition of the bulk silicate current data are consistent with accretion and differentiation Earth are in error or there has been an additional process that of some reservoirs within less than 1 m.y. Other reservoirs has fractionated U/Pb at a relatively late stage during Earth appear to have developed over longer timescales (>10 m.y.). accretion. However, these timescales assume that other factors such 10. Time-integrated parent/daughter ratios provide lim- as alteration, analytical and sampling issues, and silicate ited evidence that some volatile-element depletion in the differentiation have not affected the inferred Hf/W ratios planets was very early. Other lines of isotopic evidence indi- in the source reservoirs of the martian meteotites. Further cate that some changes in volatile abundance were late and studies that combine high-quality trace-element determina- possibly related to energetic accretion processes like the tion with new W and Nd measurements are needed to con- Moon-forming giant impact. It is essential to understand firm this. how planetary chemical compositions are achieved given 796 Meteorites and the Early Solar System II the evidence from isotopic compositions that these have Bizzarro M., Baker J. A., Haack H., Ulfbeck D., and Rosing M. changed over time. (2003) Early history of Earth’s crust-mantle system inferred Although the development of Hf-W has resulted in major from hafnium isotopes in chondrites. Nature, 421, 931–933. progress in quantifying the accretion of the terrestrial plan- Bizzarro M., Baker J. A., and Haack H. (2004) Mg isotope evi- ets, further progress is essential in certain critical areas. dence for contemporaneous formation of chondrules and refractory inclusions. Nature, 431, 275–278. Acknowledgments. We are very grateful to S. Jacobsen, C. Bizzarro M., Baker J. A., and Haack H. (2005) Mg isotope evi- Münker, and an anonymous reviewer for their comments on an dence for contemporaneous formation of chondrules and re- earlier version of this chapter. M. Wadhwa kindly provided access fractory inclusions — correction. Nature, 435, 1280. to the unpublished martian meteorite data of N. Foley and co- Blake J. D. and Schramm D. N. (1973) 247Cm as a short-lived r- workers. B. Wood is thanked for discussions on mantle oxidation process chronometer. Nature, 243, 138–140. and siderophile-element partitioning during core formation. We Blichert-Toft J., Boyet M., Télouk P., and Albarède F. (2002) are deeply indebted to D. Lauretta and M. Wadhwa for editorial 147Sm-143Nd and 176Lu-176Hf in eucrites and the differentiation advice and for their patience in giving us time to produce and re- of the HED parent body. Earth Planet. Sci. Lett., 204, 167– vise this paper in the midst of many complications that intervened. 181. Blum J. (2000) Laboratory experiments on preplanetary dust ag- gregation. In From Dust to Terrestrial Planets (W. Benz et al., REFERENCES eds.). pp. 265–278. Space Sci. Reviews, Vol. 92. Boss A. P. (1990) 3D solar nebula models: Implications for Earth Agnor C. B. and Ward W. R. (2002) Damping of terrestrial-planet origin. In Origin of the Earth (H. E. Newsom and J. H. Jones, eccentricities by density-wave interactions with a remnant gas eds.), pp. 3–15. Oxford Univ., Oxford. disk. Astrophys. J., 567, 579–586. Boyet M. and Carlson R. W. (2005) 142Nd evidence for early Agnor C. B., Canup R. M., and Levison H. F. (1999) On the (>4.53 Ga) global differentiation of the silicate Earth. Science, character and consequences of large impacts in the late stage 309, 576–581. of terrestrial planet formation. Icarus, 142, 219–237. Boyet M., Blichert-Toft J., Rosing M., Storey M., Telouk P., and Alibert C., Norman M. D., and McCulloch M. T. (1994) An ancient Albarede F. (2003) 142Nd evidence for early Earth differentia- Sm-Nd age for a ferroan noritic anorthosite clast from lunar tion. Earth Planet. Sci. Lett., 214, 427. breccia 67016. Geochim. Cosmochim. Acta, 58, 2921–2926. Brandon A. D., Walker R. J., Morgan J., and Goles C. (2000) Re- Allègre C. J. and Lewin E. (1989) Chemical structure and history Os isotopic evidence for early differentiation of the Martian of the Earth: Evidence from global non-linear inversion of iso- mantle. Geochim. Cosmochim. Acta, 64, 4083–4095. topic data in a three box model. Earth Planet. Sci. Lett., 96, Canup R. M. (2004) Simulation of a late lunar forming impact. 61–88. Icarus, 168, 433–456. Allègre C. J., Lewin E., and Dupré B. (1988) A coherent crust- Canup R. M. and Agnor C. B. (2000) Accretion of the terrestrial mantle model for the uranium-thorium-lead isotopic system. planets and the Earth-Moon system. In Origin of the Earth and Chem. Geol., 70, 211–234. Moon (R. M. Canup and K. Righter, eds.), pp. 113–129. Univ. Allègre C. J., Manhès G., and Göpel C. (1995) The age of the of Arizona, Tucson. Earth. Geochim. Cosmochim. Acta, 59, 1445–1456. Canup R. M. and Asphaug E. (2001) Origin of the moon in a giant Amelin Y., Krot A. N., Hutcheon I. D., and Ulyanov A. A. (2002) impact near the end of the Earth’s formation. Nature, 412, 708– Lead isotopic ages of chondrules and calcium-aluminum-rich 712. inclusions. Science, 297, 1678–1683. Carlson R. W. and Hauri E. H. (2001) Extending the 107Pd-107Ag Arden J. W. (1977) Isotopic composition of uranium in chondritic chronometer to low Pd/Ag meteorites with the MC-ICPMS. meteorites. Nature, 269, 788–789. Geochim. Cosmochim. Acta, 65, 1839–1848. Armstrong J. T., El Goresy A., and Wasserburg G. J. (1985) Willy: Carlson R. W. and Lugmair G. W. (1988) The age of ferroan anor- A prize noble Ur-Fremdling — Its history and implications for thosite 60025: Oldest crust on a young Moon? Earth Planet. the formation of Fremdlinge and CAI. Geochim. Cosmochim. Sci. Lett., 90, 119–130. Acta, 49, 1001–1022. Carlson R. W. and Lugmair G. W. (2000) Timescales of planetesi- Armstrong J. T., Hutcheon I. D., and Wasserburg G. J. (1987) mal formation and differentiation based on extinct and extant Zelda and company: Petrogenesis of sulfide-rich Fremdlinge radioisotopes. In Origin of the Earth and Moon (R. M. Canup and constraints on solar nebular processes. Geochim. Cosmo- and K. Righter, eds.), pp. 25–44. Univ. of Arizona, Tucson. chim. Acta, 51, 3155–3173. Caro G., Bourdon B., Birck J. L., and Moorbath S. (2003) Sm- Becker H. and Walker R. J. (2003) Efficient mixing of the solar 146-Nd-142 evidence from Isua metamorphosed sediments for nebula from uniform Mo isotopic composition of meteorites. early differentiation of the Earth’s mantle. Nature, 423, 428– Nature, 425, 152–155. 432. Benz W. (2000) Low velocity collisions and the growth of plan- Cassen P. and Woolum D. S. (1997) Nebular fractionations and etesimals. In From Dust to Terrestrial Planets (W. Benz et al., Mn-Cr systematics (abstract). In Lunar and Planetary Science eds.), pp. 279–294. Space Sci. Reviews, Vol. 92. XXVIII, pp. 211–212. Lunar and Planetary Institute, Houston. Birck J.-L. and Allègre C. J. (1988). Manganese-chromium iso- Chambers J. E. (2001a) Comparing planetary accretion in two and tope systematics and the developement of the early solar sys- three dimensions. Icarus, 149, 262–276. tem. Nature, 331, 579–584. Chambers J. E. (2001b) Making more terrestrial planets. Icarus, Birck J. L., Rotaru M., and Allègre C. J. (1999) 53Mn-53Cr evolu- 152, 205–224. tion of the early solar system. Geochim. Cosmochim. Acta, 63, Chambers J. E. (2004) Planetary accretion in the inner solar sys- 4111–4117. tem. Earth Planet. Sci. Lett., 223, 241–252. Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 797

Chen J. H. and Wasserburg G. J. (1996) Live 107Pd in the early Foley C. N., Wadhwa M., Borg L. E., Janney P. E., Hines R., and solar system and implications for planetary evolution. In Earth Grove T. L. (2005) The early differentiation history of Mars Processes: Reading the Isotopic Code (A. Basu and S. R. Hart, from 182W-142Nd isotope systematics in the SNC meteorites. eds.), pp. 1–20. Geophysics Monograph 95, American Geo- Geochim. Cosmochim. Acta, 69, 4557–4571. physical Union, Washington, DC. Galer S. J. G. (1993) Significance of lead isotope similarities be- Chen J. H. and Wasserburg G. J. (1980) A search for isotopic tween the early Earth and the Moon. Eos Trans. AGU, 74, 655. anomalies in uranium Geophys. Res. Lett., 7, 275–278. Galer S. J. G. and Goldstein S. L. (1991) Depleted mantle Pb iso- Chen J. H., Papanastassiou D. A., and Wasserburg G. J. (2003) topic evolution using conformable ore leads. Terra Abstracts Endemic Ru isotopic anomalies in meteorites (abstract). In (EUG VI), 3, 485–486. Lunar and Planetary Science XXXIV, Abstract #1789. Lunar Galer S. J. G. and Goldstein S. L. (1996) Influence of accretion and Planetary Institute, Houston (CD-ROM). on lead in the Earth. In Isotopic Studies of Crust-Mantle Evo- Chen J. H., Papanastassiou D. A., Wasserburg G. J., and Ngo H. H. lution (A. R. Basu and S. R. Hart, eds.), pp. 75–98. American (2004) Endemic Mo isotopic anomalies in iron and carbon- Geophysical Union, Washington, DC. aceous meteorites (abstract). In Lunar and Planetary Science Galy A., Young E. D., Ash R. D., and O’Nions R. K. (2000) High XXXV, Abstract #1431. Lunar and Planetary Institute, Hous- precision magnesium isotopic composition of Allende mate- ton (CD-ROM). rial: A multiple collector inductively coupled mass spectrom- Clayton R. N. (1986) High temperature isotope effects in the early etry study (abstract). In Lunar and Planetary Science XXXI, solar system. In Stable Isotopes in High Temperature Geologi- Abstract #1193. Lunar and Planetary Institute, Houston (CD- cal Processes (J. W. Valley et al., eds.), pp. 129–140. Miner- ROM). alogical Society of America, Washington, DC. Göpel C., Manhès G., and Allègre C. J. (1991) Constraints on the Clayton R. N. (1993) Oxygen isotopes in meteorites. Annu. Rev. time of accretion and thermal evolution of chondrite parent Earth Planet. Sci., 21, 115–149. bodies by precise U-Pb dating of phosphates. Meteoritics, 26, Clayton R. N. (2002) Self-shielding in the solar nebula. Nature, 73. 415, 860–861. Göpel C., Manhès G., and Allègre C. J. (1994) U-Pb systematics Clayton R. N. and Mayeda T. K. (1975) Genetic relations between of phosphates from equilibrated ordinary chondrites. Earth the moon and meteorites. Proc. Lunar Planet. Sci. Conf. 11th, Planet. Sci. Lett., 121, 153–171. pp. 1761–1769. Gounelle M., Shu F. H., Shang H., Glassgold A. E., Rehm K. E., Clayton R. N. and Mayeda T. K. (1996) Oxygen isotope studies and Lee T. (2001) Extinct radioactivities and protosolar cos- of achondrites. Geochim. Cosmochim. Acta, 60, 1999–2017. mic-rays: Self-shielding and light elements. Astrophys. J., 548, Clayton R. N., Grossman L., and Mayeda T. K. (1973) A compo- 1051. nent of primitive nuclear composition in carbonaceous mete- Gray C. M., Papanastassiou D. A., and Wasserburg G. J. (1973) orites. Science, 182, 485–487. The identification of early condensates from the solar nebula. Cook D. L., Wadhwa M., Clayton R. N., Janney P. E., Dauphas Icarus, 20, 213–239. N., and Davis A. M. (2005) Nickel isotopic composition of Halliday A. N. (2000) Terrestrial accretion rates and the origin of meteoritic metal: Implications for the initial 60Fe/56Fe ratio in the Moon. Earth Planet. Sci. Lett., 176, 17–30. the early solar system (abstract). Meteoritics & Planet. Sci., Halliday A. N. (2003) The origin and earliest history of the Earth. Abstract #5136. In Treatise on Geochemistry, Vol. 1: Meteorites, Comets and Dauphas N., Marty B., and Reisberg L. (2002a) Molybdenum Planets (A. M. Davis, eds.), pp. 509–557. Elsevier, Oxford. evidence for inherited planetary scale isotope heterogeneity of Halliday A. N. (2004) Mixing, volatile loss and compositional the protosolar nebula. Astrophys. J., 565, 640–644. change during impact-driven accretion of the Earth. Nature, Dauphas N., Marty B., and Reisberg L. (2002b) Molybdenum 427, 505–509. nucleosynthetic dichotomy revealed in primitive meteorites. Halliday A. N. and Lee D-C. (1999) Tungsten isotopes and the Astrophys. J. Lett., 569, L139–L142. early development of the Earth and Moon. Geochim. Cosmo- Dauphas N., Marty B., Davis A. M., Reisberg L., and Gallino R. chim. Acta, 63, 4157–4179. (2004) The cosmic molybdenum-ruthenium isotope correlation. Halliday A. N. and Porcelli D. (2001) In search of lost planets — Earth Planet. Sci. Lett., 226, 465–476. The paleocosmochemistry of the inner solar system. Earth Davies G. F. (1984) Geophysical and isotopic constraints on Planet. Sci. Lett., 192, 545–559. mantle convection: An interim synthesis. J. Geophys. Res., 89, Halliday A. N., Lee D-C., Christensen J. N., Walder A. J., Freed- 6017–6040. man P. A., Jones C. E., Hall C. M., Yi W., and Teagle D. (1995) Davis A. M., Richter F. M., Mendybaev R. A., Janney P. E., Recent developments in inductively coupled plasma magnetic Wadhwa M., and McKeegan K. D. (2005) Isotopic mass frac- sector multiple collector mass spectrometry. Intl. J. Mass Spec. tionation laws and the initial solar system 26Al/27Al ratio (ab- Ion Processes, 146/147, 21–33. stract). In Lunar and Planetary Science XXXVI, Abstract Halliday A. N., Rehkämper M., Lee D-C., and Yi W. (1996) Early #2334. Lunar and Planetary Institute, Houston (CD-ROM). evolution of the Earth and Moon: New constraints from Hf-W Doe B. R. and Zartman R. E. (1979) Plumbotectonics I: The isotope geochemistry. Earth Planet. Sci. Lett., 142, 75–89. Phanerozoic. In Geochemistry of Hydrothermal Ore Deposits Halliday A. N., Lee D-C., and Jacobsen S. B. (2000) Tungsten (H. L. Barnes, ed.), pp. 22–70. Wiley, New York. isotopes, the timing of metal-silicate fractionation and the ori- Dreibus G., Haubold R., Huisl W., and Spettel B. (2003). Com- gin of the Earth and Moon. In Origin of the Earth and Moon parison of the chemistry of Yamato 980459 with DaG 476 and (R. M. Canup and K. Righter, eds.), pp. 45–62. Univ. of Ari- SaU 005 (abstract). In Intl. Symp. on Evolution of Solar Sys- zona, Tucson. tem Materials: A New Perspective from Antarctic Meteorites, Halliday A. N., Lee D-C., Porcelli D., Wiechert U., Schönbächler pp. 19–20. National Institute of Polar Research, Tokyo. M., and Rehkämper M. (2001a) The rates of accretion, core 798 Meteorites and the Early Solar System II

formation and volatile loss in the early solar system. Phil. Trans. and S. Hart, eds.), pp. 47–74. American Geophysical Union, R. Soc. London, 359, 2111–2135. Washington, DC. Halliday A. N., Wänke H., Birck J-L., and Clayton R. N. (2001b) Jacobsen S. B. and Wasserburg G. J. (1979). The mean age of The accretion, bulk composition and early differentiation of mantle and crust reservoirs. J. Geophys. Res., 84, 7411–7427. Mars. Space Sci. Rev., 96, 197–230. Jacobsen S. B. and Yin Q. Z. (2001) Core formation models and Halliday A., Quitté G., and Lee D.-C. (2003) Tungsten isotopes and extinct nuclides (abstract). In Lunar and Planetary Science the time-scales of planetary accretion. Meteoritics & Planet. XXXII, Abstract #1961. Lunar and Planetary Institute, Hous- Sci., 38, A133. ton (CD-ROM). Hanan B. B. and Tilton G. R. (1987) 60025: Relict of primitive Jagoutz E. and Jotter R. (2000) New Sm-Nd isotope data on lunar crust? Earth Planet. Sci. Lett., 84, 15–21. Nakhla minerals (abstract). In Lunar and Planetary Science Harper C. L. and Jacobsen S. B. (1996) Evidence for 182Hf in the XXXI, p. 1609. Lunar and Planetary Institute, Houston. early solar system and constraints on the timescale of terrestrial Jagoutz E., Dreibus G., and Jotter R. (2003) New 142Nd data on core formation. Geochim. Cosmochim. Acta, 60, 1131–1153. SNC meteorites. Geochim. Cosmochim. Acta, 67, A184. Harper C. L., Nyquist L. E., Bansal B., Wiesmann H., and Shih Jones J. H. and Palme H. (2000) Geochemical constraints on the C.-Y. (1995) Rapid accretion and early differentiation of Mars origin of the Earth and Moon. In Origin of the Earth and Moon indicated by 142Nd/144Nd in SNC meteorites. Science, 267, (R. M. Canup and K. Righter, eds.), pp. 197–216. Univ. of 213–217. Arizona, Tucson. Harper C. L., Völkening J., Heumann K. G., Shih C.-Y., and Kamber B. S. and Collerson K. D. (1999) Origin of ocean-island Wiesmann H. (1991a) 182Hf-182W: New cosmochronometric basalts: A new model based on lead and helium isotope sys- constraints on terrestrial accretion, core formation, the astro- tematics. J. Geophys. Res., 104, 25479–25491. physical site of the r-process, and the origin of the solar system Kleine T., Münker C., Mezger K., and Palme H. (2002) Rapid (abstract). In Lunar and Planetary Science XXII, pp. 515–516. accretion and early core formation on asteroids and the terres- Lunar and Planetary Institute, Houston. trial planets from Hf-W chronometry. Nature, 418, 952–955. Harper C. L., Wiesmann H., and Nyquist L. E., Howard W. M., Kleine T., Mezger K., Munker C., Palme H., and Bischoff A. Meyer B., Yokoyama Y., Rayet M., Arnould M., Palme H., (2004a) 182Hf-182W isotope systematics of chondrites, eucrites Spettel B., and Jochum K. P. (1991b) 92Nb/93Nb and 92Nb/ and martian meteorites: Chronology of core formation and 146Sm ratios of the early solar system: Observations and com- early mantle differentiation in Vesta and Mars. Geochim. Cos- parison of p-process and spallation models (abstract). In Lunar mochim. Acta, 68, 2935–2946. and Planetary Science XXII, pp. 519–520. Lunar and Planetary Kleine T., Mezger K., Palme H., and Münker C. (2004b) The W Institute, Houston. isotope evolution of the bulk silicate Earth: Constraints on the Heck P. R., Schmitz B., Baur H., Halliday A. N., and Wieler R. timing and mechanisms of core formation and accretion. Earth (2004) Fast delivery of meteorites from a major asteroid colli- Planet. Sci. Lett., 228, 109. sion. Nature, 430, 323–325. Kleine T., Mezger K., Palme H., Scherer E., and Münker C. Hirata T. (2001) Determinations of Zr isotopic composition and (2005a) Early core formation in asteroids and late accretion of U-Pb ages for terrestrial and extraterrestrial Zr-bearing miner- chondrite parent bodies: Evidence from 182Hf-182W in CAIs, als using laser ablation-inductively coupled plasma mass spec- metal-rich chondrites and iron meteorites. Geochim. Cosmo- trometry: Implications for Nb-Zr isotopic systematics. Chem. chim. Acta, 69, 5805–5818. Geol., 176, 323–342. Kleine T., Mezger K., Palme H., Scherer E., and Münker C. Holmes A. (1946) An estimate of the age of the Earth. Nature, (2005b) The W isotope composition of eucrites metal: Con- 157, 680–684. straints on the timing and cause of the thermal metamorphism Horan M. F., Smoliar M. I., and Walker R. J. (1998) 182W and of basaltic eucrites. Earth Planet. Sci. Lett., 231, 41. 187Re-187Os systematics of iron meteorites: Chronology for Kleine T., Palme H., Mezger K., and Halliday A. N. (2005c) Hf-W melting, differentiation, and crystallization in asteroids. Geo- chronometry of lunar metals and the age and early differentia- chim. Cosmochim. Acta, 62, 545–554. tion of the Moon. Science, 310, 1671–1674. Houtermans F. G. (1946) Die Isotopenhäufigkeitem in natürlichen Kominami J. and Ida S. (2002) The effect of tidal interaction with a Blei und das Alter des Urans. Naturwissenschaften, 33, 185– gas disk on formation of terrestrial planets. Icarus, 157, 43–56. 186. Kortenkamp S. J., Kokubo E., and Weidenschilling S. J. (2000) Humayun M. and Clayton R. N. (1995) Potassium isotope cosmo- Formation of planetary embryos. In Origin of the Earth and chemistry: Genetic implications of volatile element depletion. Moon (R. M. Canup and K. Righter, eds.), pp. 85–100. Univ.of Geochim. Cosmochim. Acta, 59, 2131–2151. Arizona, Tucson. Hunten D. M., Pepin R. O., and Walker J. C. G. (1987) Mass frac- Kramers J. D. and Tolstikhin I. N. (1997) Two terrestrial lead iso- tionation in hydrodynamic escape. Icarus, 69, 532–549. tope paradoxes, forward transport modeling, core formation and Hutcheon I. D., Armstrong J. T., and Wasserburg G. J. (1987) the history of the continental crust. Chem. Geol., 139, 75–110. Isotopic studies of Mg, Fe, Mo, Ru, and W in Fremdlinge from Kutschera W. and 10 colleagues (1984) Half-life of Fe-60. Nucl. Allende refractory inclusions. Geochim. Cosmochim. Acta, 51, Instr. Meth. Phys. Res., B5, 430–435. 3175–3192. Kwon S.-T., Tilton G. R., and Grünenfelder M. H. (1989) Lead Ireland T. R. (1991). The abundance of 182Hf in the early solar isotope relationships in carbonatites and alkaline complexes: system (abstract). In Lunar and Planetary Science XXII, An overview. In Carbonatites — Genesis and Evolution (K. pp. 609–610. Lunar and Planetary Institute, Houston. Bell, ed.), pp. 360–387. Unwin-Hyman, London. Jacobsen S. B. and Harper C. L. Jr. (1996) Accretion and early Lee D-C. (2005) Protracted core formation in asteroids: Evidence differentiation history of the Earth based on extinct radionu- from high precision W isotopic data. Earth Planet. Sci. Lett., clides. In Earth Processes: Reading the Isotope Code (A. Basu 198, 267–274. Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 799

Lee D.-C. and Halliday A. N. (1995) Hafnium-tungsten chronom- Marty B. and Marti K. (2002) Signatures of early differentiation etry and the timing of terrestrial core formation. Nature, 378, of Mars. Earth Planet. Sci. Lett., 196, 251–263. 771–774. Masarik J. (1997) Contribution of neutron-capture reactions to Lee D.-C. and Halliday A. N. (1996) Hf-W isotopic evidence for observed tungsten isotopic ratios. Earth Planet. Sci. Lett., 152, rapid accretion and differentiation in the early solar system. 181–185. Science, 274, 1876–1879. Mayor M. and Queloz D. (1995) A Jupiter-mass companion to a Lee D.-C. and Halliday A. N. (1997) Core formation on Mars and solar-type star. Nature, 378, 355–359. differentiated asteroids. Nature, 388, 854–857. McCaughrean M. J. and O’Dell C. R. (1996) Direct imaging of Lee D-C. and Halliday A. N. (2000a) Accretion of primitive plan- circumstellar disks in the Orion nebula. Astronom. J., 111, etesimals: Hf-W isotopic evidence from enstatite chondrites. 1977–1986. Science, 288, 1629–1631. McKeegan K. D., Chaussidon M., and Robert F. (2000) Incorpo- Lee D-C. and Halliday A. N. (2000b) Hf-W isotopic systematics ration of short-lived Be-10 in a calcium-aluminum-rich inclu- of ordinary chondrites and the initial 182Hf/180Hf of the solar sion from the Allende meteorite. Science, 289, 1334–1337. system. Chem. Geol., 169, 35–43. Meyer J. C., McKay D. C., Anderson D. H., and Butler P. Jr. Lee D.-C. and Halliday A. N. (2003) High precision W and Mo (1975) The source of sublimates on the Apollo 15 green and isotope compositions for iron meteorites. Geochim. Cosmo- Apollo 17 orange glass samples. Proc. Lunar Sci. Conf. 6th, chim. Acta, 67, A246. pp. 1637–1699. Lee D.-C., Halliday A. N., Snyder G. A., and Taylor L. A. (1997) Mostefaoui S, Lugmair G. W., Hoppe P., and El Goresy A. (2004) Age and origin of the Moon. Science, 278, 1098–1103. Evidence for live Fe-60 in meteorites. New Astron. Rev., 48, Lee D-C., Halliday A. N., Leya I., Wieler R., and Wiechert U. 155–159. (2002) Cosmogenic tungsten and the origin and earliest differ- Moynier F., Blichert-Toft J., Telouk P., and Albarede F. (2004) entiation of the Moon. Earth Planet. Sci. Lett., 198, 267–274. The Ni isotope geochemistry of chondrites and iron meteorites. Lee T., Papanastassiou D. A., and Wasserburg G. J. (1977) 26Al Geochim. Cosmochim. Acta, 68, A757–A757. in the early solar system: Fossil or fuel? Astrophys. J. Lett., Münker C., Weyer S., Mezger K., Rehkämper M., Wombacher F., 211, L107–L110. and Bischoff A. (2000) 92Nb-92Zr and the early differentiation Lee T., Shu F. H., Shang H., Glassgold A. E., and Rehm K. E. history of planetary bodies. Science, 289, 1538–1542. (1998) Protostellar cosmic rays and extinct radioactivities in Murphy D. T., Kamber B. S., and Collerson K. D. (2003) A refined meteorites. Astrophys. J., 506, 898–912. solution to the first terrestrial Pb-isotope paradox. J. Petrol., Leya I., Wieler R., and Halliday A. N. (2000) Cosmic-ray pro- 44, 39–53. duction of tungsten isotopes in lunar samples and meteorites Newsom H. E. (1995) Composition of the solar system, planets, and its implications for Hf-W cosmochemistry. Earth Planet. meteorites, and major terrestrial reservoirs. In Global Earth Sci. Lett., 175, 1–12. Physics, A Handbook of Physical Constants (T. J. Ahrens, ed.), Leya I., Wieler R., and Halliday A. N. (2003) The influence of pp. 159–189. AGU Reference Shelf 1, American Geophysical cosmic-ray production on extinct nuclide systems. Geochim. Union, Washington, DC. Cosmochim. Acta, 67, 527–541. Newsom H. E., White W. M., Jochum K. P., and Hofmann A. W. Liew T. C., Milisenda C. C., and Hofmann A. W. (1991) Isotopic (1986) Siderophile and chalcophile element abundances in constrasts, chronology of element transfers and high-grade oceanic basalts, Pb isotope evolution and growth of the earth’s metamorphism: The Sri Lanka Highland granulites, and the core. Earth Planet. Sci. Lett., 80, 299–313. Lewisian (Scotland) and Nuk (S. W. Greenland) gneisses. Geol. Newsom H. E., Sims K. W. W., Noll P. D. Jr., Jaeger W. L., Maehr Rundsch., 80, 279–288. S. A., and Bessera T. B. (1996) The depletion of W in the bulk Lodders K. (1998) A survey of shergottite, nakhlite and chassigny silicate Earth. Geochim. Cosmochim. Acta, 60, 1155–1169. meteorites whole-rock compositions. Meteoritics & Planet. Nier A. O., Thompson R. W., and Murphey B. F. (1941) The iso- Sci., 33, A183–A190. topic composition of lead and the measurement of geological Lugmair G. W. and Galer S. J. G. (1992) Age and isotopic rela- time. III. Phys. Rev., 60, 112–116. tionships between the angrites Lewis Cliff 86010 and Angra Nyquist L. E., Lindstrom D., Shih C-Y., Weismann H., Mittlfelhdt dos Reis. Geochim. Cosmochim. Acta, 56, 1673–1694. D., Wentworth S., and Martinez R. (1997) Mn-Cr systematics Lugmair G. W. and Shukolyukov A. (1998) Early solar system of chondrules from the Bishunpur and Chainpur meteorites timescales according to Mn-53-Cr-53 systematics. Geochim. (abstract). In Lunar and Planetary Science XXVIII, pp. 1033– Cosmochim. Acta, 62, 2863–2886. 1034. Lunar and Planetary Institute, Houston. Lugmair G. W. and Shukolyukov A. (2001) Early solar system Nyquist L. E., Lindstrom D., Mittlefehldt D., Shih C. Y., Wies- events and timescales. Meteoritics & Planet. Sci., 36, 1017– mann H., Wentworth S., and Martinez R. (2001) Manganese- 1026. chromium formation intervals for chondrules from the Bishun- Marhas K. K., Goswami J. N., and Davis A. M. (2002) Short-lived pur and Chainpur meteorites. Meteoritics & Planet. Sci., 36, nuclides in hibonite grains from Murchison: Evidence for solar 911. system evolution. Science, 298, 2182–2185. O’Neill H. St. C. (1991a) The origin of the Moon and the early Markowski A., Quitté G., and Halliday A. N. (2004) Systematic history of the Earth — A chemical model; Part I: The Moon. differences in 182W/184W between iron meteorite groups. Mete- Geochim. Cosmochim. Acta, 55, 1135–158. oritics & Planet. Sci., 39, 5075. O’Neill H. St. C. (1991b) The origin of the Moon and the early Markowski A., Quitté G., Halliday A. N., and Kleine T. (2006) history of the Earth — A chemical model; Part II: The Earth. Tungsten isotopic compositions of iron meteorites: Chronologi- Geochim. Cosmochim. Acta, 55, 1159–1172. cal constraints vs. cosmogenic effects. Earth Planet. Sci. Lett., Papanastassiou D. A. and Wasserburg G. J. (1976) Early lunar dif- in press. ferentiates and lunar initial 87Sr/86Sr (abstract). In Lunar Sci- 800 Meteorites and the Early Solar System II

ence VII, pp. 665–667. Lunar Science Institute, Houston. Planet. Inter., 100, 115–142. Papanastassiou D. A., Wasserburg G. J., and Bogdanovski O. Russell S. S., Srinivasan G., Huss G. R., Wasserburg G. J., and (2005) The 53Mn-53Cr system in CAIs: An update (abstract). MacPherson G. J. (1996) Evidence for widespread 26Al in the In Lunar and Planetary Science XXXVI, Abstract #2198. Lu- solar nebula and constraints for nebula time scales. Science, nar and Planetary Institute, Houston (CD-ROM). 273, 757–762. Patterson C. C. (1956) Age of meteorites and the Earth. Geochim. Safronov V. S. (1954) On the growth of planets in the protoplane- Cosmochim. Acta, 10, 230–237. tary cloud. Astron. Zh., 31, 499–510. Pepin R. O. (1997) Evolution of Earth’s noble gases: Conse- Sanders I. S. (1996) A chondrule-forming scenario involving quences of assuming hydrodynamic loss driven by giant im- molten planetesimals. In Chondrules and the Protoplanetary pact. Icarus, 126, 148–156. Disk (R. Hewins et al., eds.), pp. 327–334. Cambridge Univ., Pepin R. O. (2000) On the isotopic composition of primordial Cambridge. xenon in terrestrial planet atmosopheres. Space Sci. Rev., 92, Sanloup C., Blichert-Toft J., Télouk P., Gillet P., and Albarède F. 371–395. (2000) Zr isotope anomalies in chondrites and the presence of Podosek F. A., Zinner E. K., MacPherson G. J., Lundberg L. L., live 92Nb in the early solar system. Earth Planet. Sci. Lett., Brannon J. C., and Fahey A. J. (1991) Correlated study of 184, 75–81. initial Sr-87/Sr-86 and Al-Mg isotopic systematics and petro- Scherer E. E., Münker C., and Mezger K. (2001) Calibration of logic properties in a suite of refractory inclusions from the the lutetium-hafnium clock. Science, 293, 683–687. Allende meteorite. Geochim. Cosmochim. Acta, 55, 1083– Scherer E. E., Mezger K., and Münker C. (2003) The 176Lu decay 1110. constant discrepancy: Terrestrial samples versus meteorites. Podosek F. A., Ott U., Brannon J. C., Neal C. R., Bernatowicz Meteoritics & Planet. Sci., 38, A136. T. J., Swan P., and Mahan S. E. (1997) Thoroughly anomalous Scherstén A., Elliott T., Russell S. S., and Hawkesworth C. (2004) chromium in Orgueil. Meteoritics & Planet. Sci., 32, 617–627. W isotope chronology of asteroidal core formation. Geochim. Poitrasson F., Halliday A. N., Lee D-C., Levasseur S., and Teutsch Cosmochim. Acta, 68, Abstract 1240. N. (2004) Iron isotope differences between Earth, Moon, Mars Schmitt W., Palme H., and Wänke H. (1989) Experimental deter- and Vesta as possible records of contrasted accretion mecha- mination of metal/silicate partition coefficients for P, Co, Ni, nisms. Earth Planet. Sci. Lett., 223, 253–266. Cu, Ga, Ge, Mo, and W and some implications for the early Polnau E. and Lugmair G. W. (2001) Mn-Cr isotope systematics evolution of the Earth. Geochim. Cosmochim. Acta, 53, 173– in the two ordinary chondrites Richardton (H5) and Ste. Mar- 185. guerite (H4) (abstract). In Lunar and Planetary Science XXXII, Schneider G., Smith B. A., Becklin E. E., Koerner D. W., Meier Abstract #1527. Lunar and Planetary Institute, Houston (CD- R., Hines D. C., Lowrance P. J., Terrile R. I., and Rieke M. ROM). (1999) Nicmos imaging of the HR 4796A circumstellar disk. Porcelli D. and Pepin R. O. (2000) Rare gas constraints on early Astrophys. J. Lett., 513, L127–L130. earth history. In Origin of the Earth and Moon (R. M. Canup Schön R., Winkler G., and Kutschera W. (2004) A critical review and K. Righter, eds.), pp. 435–458. Univ. of Arizona, Tucson. of experimental data for the half-lives of the uranium isotopes Porcelli D., Cassen P., and Woolum D. (2001) Deep Earth rare 238U and 235U. Appl. Radiation Isotopes, 60, 263–273. gases: Initial inventories, capture from the solar nebula and Schönbächler M., Rehkämper M., Halliday A. N., Lee D. C., losses during Moon formation. Earth Planet. Sci. Lett, 193, Bourot-Denise M., Zanda B., Hattendorf B., and Günther D. 237–251. (2002) Niobium-zirconium chronometry and early solar sys- Quitté G. and Birck J-L. (2004) Tungsten isotopes in eucrites re- tem development. Science, 295, 1705–1708. visited and the initial 182Hf/180Hf of the solar system based on Schönbächler M., Lee D.-C., Rehkämper M., Halliday A. N., Fehr iron meteorite data. Earth Planet. Sci. Lett., 219, 201–207. M. A., Hattendorf B., and Günther D. (2003) Zirconium iso- Quitté G., Birck J-L., and Allègre C. J. (2000) 182Hf-182W sys- tope evidence for incomplete admixing of r-process compo- tematics in eucrites: The puzzle of iron segregation in the early nents in the solar nebula. Earth Planet. Sci. Lett., 216, 467– solar system. Earth Planet. Sci. Lett., 184, 83–94. 481. Quitté G., Meier M., Latkoczy C., Halliday A. N., and Günther D. Schönbächler M., Lee D.-C., Rehkämper M., Halliday A. N., (2006) Nickel isotopes in iron meteorites — nucleosynthetic Hattendorf B., and Günther D. (2005) Nb/Zr fractionation on

anomalies in sulphides with no effects in metals and no trace the Moon and the search for extinct 92Nb. Geochim. Cosmo- of 60Fe. Earth Planet. Sci. Lett., in press. chim. Acta, 69, 775–785. Reynolds J. H. (1960) Determination of the age of the elements. Schönberg R., Kamber B. S., Collerson K. D., and Eugster O. Phys. Rev. Lett., 4, 8. (2002) New W isotope evidence for rapid terrestrial accretion Richter F. M. (2003) Time scales for elemental and isotopic frac- and very early core formation. Geochim. Cosmochim. Acta, 66, tionation by evaporation and condensation (abstract). In Lu- 3151–3160. nar and Planetary Science XXXVI, Abstract #1231. Lunar and Shih C.-Y., Nyquist L. E., Dasch E. J., Bogard D. D., Bansal Planetary Institute, Houston (CD-ROM). B. M., and Wiesmann H. (1993) Age of pristine noritic clasts Righter K. and Drake M. J. (1996) Core formation in Earth’s from lunar breccias 15445 and 15455. Geochim. Cosmochim. Moon, Mars, and Vesta. Icarus, 124, 513–529. Acta, 57, 915–931. Righter K. and Drake M. J. (1999) Effect of water on metal-sili- Shukolyukov A. and Lugmair G. W. (1993a) 60Fe in eucrites. cate partitioning of siderophile elements: A high pressure and Earth Planet Sci. Lett., 119, 159–166. temperature terrestrial magma ocean and core formation. Earth Söderlund U., Patchett J. P., Vervoort J. D., and Isachsen C. E. Planet. Sci. Lett., 171, 383–399. (2004) The 176Lu decay constant determined by Lu-Hf and U- Righter K., Drake M. J., and Yaxley G. (1997) Prediction of sid- Pb isotope systematics of Precambrian mafic intrusions. Earth erophile element metal/silicate partition coefficients to 20 GPa Planet. Sci. Lett., 219, 311–324 and 2800°C: The effects of pressure, temperature, oxygen fu- Solomatov V. S. (2000) Fluid dynamics of a terrestrial magma gacity, and silicate and metallic melt compositions. Phys. Earth ocean. In Origin of the Earth and Moon (R. M. Canup and K. Halliday and Kleine: Meteorites and Planetary Accretion and Differentiation 801

Righter, eds.), pp. 323–338. Univ. of Arizona, Tucson. (W. K. Hartmann et al., eds.), pp. 519–550. Lunar and Plane- Steiger R. H. and Jäger E. (1977) Subcommission on geochronol- tary Institute, Houston. ogy: Convention on the use of decay constants in geo- and Wiechert U., Halliday A. N., Lee D.-C., Snyder G. A., Taylor cosmochronology. Earth Planet. Sci. Lett., 36, 359–362. L. A., and Rumble D. A. (2001) Oxygen isotopes and the Stirling C. H., Halliday A. N., and Porcelli D. (2005) In search Moon-forming giant impact. Science, 294, 345–348. of live 247Cm in the early solar system. Geochim. Cosmochim. Wiechert U., Halliday A. N., Palme H., and Rumble D. (2004) Acta, 69, 1059–1071. Oxygen isotopes and the differentiation of planetary embryos. Tatsumoto M. and Shimamura T. (1980) Evidence for live Cm- Earth Planet. Sci. Lett., 221, 373–382. 247 in the early solar system. Nature, 286, 118–122. Wieler R., Anders E., Baur H., Lewis R. S., and Signer P. (1992) Tera F. and Carlson R. W. (1999) Assessment of the Pb-Pb and Characterisation of Q-gases and other noble gas components U-Pb chronometry of the early solar system. Geochim. Cosmo- in the Murchison meteorite. Geochim. Cosmochim. Acta, 56, chim. Acta, 63, 1877–1889. 2907–2921. Tera F., Papanastassiou D. A., and Wasserburg G. J. (1973) A lunar Wilde S. A., Valley J. W., Peck W. H., and Graham C. M. (2001) cataclysm at ~3.95 AE and the structure of the lunar crust (ab- Evidence from detrital zircons for the existence of continental stract). In Lunar Science IV, pp. 723–725. Lunar Science In- crust and oceans on the Earth 4.4 G.y. ago. Nature, 409, 175– stitute, Houston. 178. Torigoye-Kita N., Misawa K., Dalrymple G. B., and Tatsumoto Wing J., Schwartz B., and Huizenga J. (1961) New hafnium iso- M. (1995) Further evidence for a low U/Pb source in the Moon: tope, 182Hf. Phys. Rev., 123, 1354–1355. U-Th-Pb, Sm-Nd, and Ar-Ar isotopic systematics of lunar Wood B. J. and Halliday A. N. (2005) Kelvin revisited: Cooling meteorite Yamato-793169. Geochim. Cosmochim. Acta, 59, and core formation after the giant impact. Nature, 437, 1345– 2621–2625. 1348. Trinquier A., Birck J-L., and Allègre C. J. (2005) 54Cr anomalies Woodland S. J., Rehkämper M., Halliday A. N., Lee D-C., Hatten- in the solar system: Their extent and origin (abstract). In Lu- dorf B., and Günther D. (2005) Accurate measurement of silver nar and Planetary Science XXXVI, Abstract #1259. Lunar and isotopic compositions in geological materials including low Pd/ Planetary Institute, Houston (CD-ROM). Ag meteorites. Geochim. Cosmochim. Acta, 69, 2153–2163. Vockenhuber C., Oberli F., Bichler M., Ahmad I., Quitté G., Meier Yi W., Halliday A. N., Alt J. C., Lee D.-C., Rehkämper M., Garcia M., Halliday A. N., Lee D-C., Kutschera W., Steier P., Gehrke M., Langmuir C., and Su Y. (2000) Cadmium, indium, tin, R. J., and Helmer R. G. (2004) A new half-life measurement of tellurium and sulfur in oceanic basalts: Implications for chal- 182Hf — Sharpening a tool for the chronology of the early solar cophile element fractionation in the earth. J. Geophys. Res., system. Phys. Rev. Lett., 93, Article No. 172501. 105, 18927–18948. Wadhwa M., Amelin Y., Bogdanovski O., Lugmair G. W., and Yin Q. Z. and Jacobsen S. B. (1998) The 97Tc-97Mo chronometer Janney P. E. (2005) High precision relative and absolute ages and its implications for timing of terrestrial accretion and core for Asuka 881394, a unique and ancient basalt (abstract). In formation (abstract). In Lunar and Planetary Science XXIX, Lunar and Planetary Science XXXVI, Abstract #2126. Lunar Abstract #1802. Lunar and Planetary Institute, Houston (CD- and Planetary Institute, Houston (CD-ROM). ROM). Walter M. J., Newsom H. E., Ertel W., and Holzheid A. (2000) Yin Q. Z., Jacobsen S. B., McDonough W. F., Horn I., Petaev Siderophile elements in the Earth and Moon: Metal/silicate M. I., and Zipfel J. (2000) Supernova sources and the 92Nb- partitioning and implications for core formation. In Origin 92Zr p-process chronometer. Astrophys. J. Lett., 535, L49–L53. of the Earth and Moon (R. M. Canup and K. Righter, eds.), Yin Q., Jacobsen S. B., and Yamashita K. (2002a) Diverse super- pp. 265–289. Univ.of Arizona, Tucson. nova sources of pre-solar material inferred from molybdenum Ward W. R. (2000) On planetesimal formation: The role of col- isotopes in meteorites. Nature, 415, 881–883. lective behavior. In Origin of the Earth and Moon (R. M. Yin Q. Z., Jacobsen S. B., Yamashita K., Blichert-Toft J., Télouk P., Canup and K. Righter, eds.), pp. 75–84. Univ.of Arizona, Tuc- and Albarède F. (2002b) A short time scale for terrestrial planet son. formation from Hf-W chronometry of meteorites. Nature, 418, Wasserburg G. J., Busso M., Gallino R., and Raiteri C. M. (1994) 949–952. Asymptotic giant branch stars as a source of short-lived ra- Young E. D. (2000) Assessing the implications of K isotope cos- dioactive nuclei in the solar nebula. Astrophys. J., 424, 412– mochemistry for evaporation in the preplanetary solar nebula. 428. Earth Planet. Sci. Lett., 183, 321–333. Wasserburg G. J., Papanastassiou D. A., Tera F., and Huneke J. C. Young E. D., Simon J. I., Galy A., Russell S. S., Tonui E., and (1977a) Outline of a lunar chronology. Philos. Trans. R. Soc. Lovera O. (2005) Supra-canonical 26Al/27Al and the residence London, A285, 7–22. time of CAIs in the solar protoplanetary disk. Science, 308, Wasserburg G. J., Tera F., Papanastassiou D. A., and Huneke J. C. 223–227. (1977b) Isotopic and chemical investigations on Angra dos Yurimoto H. and Kuramoto K. (2004) Molecular cloud origin for Reis. Earth Planet. Sci. Lett., 35, 294–316. the oxygen isotope heterogeneity in the solar system. Science, Weidenschilling S. J., Spaute D., Davis D. R., Marzari F., and 305, 1763–1766. Ohtsuki K. (1997) Accretional evolution of a planetesimal Zartman R. E. and Haines S. M. (1988) The plumbotectonic model swarm. 2. The terrestrial zone. Icarus, 128, 429–455. for Pb isotopic systematics among major terrestrial reservoirs — Weidenschilling S. J., Marzari F., and Hood L. L. (1998) The A case for bi-directional transport. Geochim. Cosmochim. Acta, origin of chondrules and jovian resonances. Science, 279, 681– 52, 1327–1339. 684. Zook H. A. (1981) On a new model for the generation of chon- Wetherill G. W. (1980) Formation of the terrestrial planets. Annu. drules (abstract). In Lunar and Planetary Science XII, pp.1242– Rev. Astron. Astrophys., 18, 77–113. 1244. Lunar and Planetary Institute, Houston. Wetherill G. W. (1986) Accmulation of the terrestrial planets and implications concerning lunar origin. In Origin of the Moon