Wadhwa et al.: Timescales of Planetesimal Differentiation 715

Timescales of Planetesimal Differentiation in the Early Solar System

M. Wadhwa The Field Museum

G. Srinivasan University of Toronto

R. W. Carlson Carnegie Institution of Washington

Chronological investigations of a variety of meteoritic materials indicate that planetesimal differentiation occurred rapidly in the early solar system. Some planetesimals only experienced the earliest stage of the onset of melting and were subsequently arrested in that state. On some of these bodies, such incipient melting began ~3–5 m.y. after the formation of calcium-alumi- num-rich inclusions (CAI), but possibly extended to ≥10 m.y. on others. Extensive differentia- tion took place on other planetesimals to form silicate crust-mantle reservoirs and metallic cores. Crust formation began within ~2 m.y. of CAI formation, and likely extended to at least ~10 m.y. Global mantle differentiation, which established the source reservoirs of the crustal materials, occurred contemporaneously for the angrites and eucrites, within ~2–3 m.y. of CAI formation. Metal segregation on different planetesimals occurred within a narrow time interval of ≤5 m.y. In the specific case of the howardite-eucrite-diogenite (HED) parent body, the process of core formation is estimated to have occurred ~1 m.y. before global mantle differentiation. Follow- ing metal segregation, core crystallization is likely to have occurred over a time interval of 10 m.y. or less.

1. INTRODUCTION drites that have near-chondritic bulk compositions (e.g., brachinites, acapulcoites, lodranites, and winonaites) and Within the last decade, significant advances have been formed during the earliest stages of melting on planetesi- made in our understanding of the timescales involved in the mals (which were subsequently arrested at this stage and differentiation history of planetesimals in the early solar did not differentiate further); and (2) meteorites that resulted system. In particular, the application of several chronom- from more extensive differentiation such that their bulk eters based on short-lived radionuclides (i.e., with half-lives compositions are significantly different from those of chon- less than ~100 m.y.) has allowed unprecedented time res- drites. The latter are in turn comprised of three classes of olution (often ≤1 m.y. for events that occurred close to materials: (1) achondrites that formed within the silicate ~4.6 b.y. ago). crust of differentiated planetesimals, such as eucrites, diog- The earliest differentiation events in solar system history enites, and howardites (or HED meteorites), angrites, and were those that occurred between nebular gas and the ear- HED-like silicate clasts in mesosiderites; (2) metal-silicate liest condensates that are represented by components within meteorites (pallasites) that may have been formed near the chondritic meteorites. Timescales involved in such events, core-mantle boundary; and (3) metal-rich meteorites that are which likely occurred during the accretion disk phase of the products of metal segregation from essentially chondritic the nebula, are discussed elsewhere in this book (Russell precursors and some of which may represent the cores of et al., 2006). In this chapter, we will focus on the timescales extensively differentiated planetesimals. involved in the differentiation events that occurred on plan- Age determinations on these meteoritic materials with etesimals, defined here as small asteroidal-sized bodies (up the appropriate chronometers then make it possible to esti- to hundreds of kilometers in diameter) that accreted from mate the timing of planetesimal differentiation events rang- nebular dust and the earliest condensates. The processes ing from incipient melting to more extensive processing likely to be involved in planetesimal differentiation are dis- involving silicate differentiation (including crust formation cussed in more detail in McCoy et al. (2006). and fractionation within the mantle), metal segregation, and The meteoritic record includes a variety of materials re- core crystallization. Thus far, chronometers based on both sulting from planetesimal differentiation. These materials long- and short-lived radionuclides have been applied ex- are primarily composed of two types: (1) primitive achon- tensively for age-dating such events. Long-lived chronom-

715 716 Meteorites and the Early Solar System II

TABLE 1. Selected short-lived radioisotopes utilized so far in constraining the timescales of planetesimal differentiation.

Radioisotope Half-life Daughter Reference Solar System Time Anchor*

(R*) (m.y.) Isotope (D*) Isotope (R) Initial Ratio (R*/R)0 (if any) 26Al 0.72 26Mg 27Al ~5 × 10–5 CAIs –5 (R*/R)T = 5 × 10 at 4.567 Ga 60Fe 1.5 60Ni 56Fe ~3–10 × 10–7 53Mn 3.7 53Cr 55Mn ~10–5 LEW 86010 Angrite –6 (R*/R)T = 1.25 × 10 at 4.558 Ga 107Pd 6.5 107Ag 108Pd ~5 × 10–5 Gibeon (IVA) Iron –5 (R*/R)T = 2.4 × 10 at ~4.56 Ga 182Hf 9 182W 180Hf 1.0–1.6 × 10–4 CAIs and Chondrites –4 (R*/R)T = 1 × 10 at ~4.56 Ga 146Sm 103 142Nd 144Sm ~7 × 10–3 Angrites –3 (R*/R)T = 7 × 10 at 4.558 Ga *References: 26Al: Lee et al. (1976), Amelin et al. (2002), and references therein; 60Fe: Tachibana and Huss (2003), Mostefaoui et al. (2004); 53Mn: Lugmair and Galer (1992), Lugmair and Shukolyukov (1998); 107Pd: Chen and Wasserburg (1996); 182Hf: Kleine et al. (2002), Yin et al. (2002b), Quitté and Birck (2004); 146Sm: Lugmair and Galer (1992) and references therein. We note that some recent investigations indicate that the initial 26Al/27Al ratio at the time of CAI formation was possibly as high as ~7 × 10–5 (e.g., Young et al., 2005). Since Amelin et al. (2002) determined a 26Al/27Al ratio of 4.63 ± 0.44 in the same Efremovka CAI (E60) that had a Pb-Pb age of 4567.4 ± 1.1 Ma, we will assume here that the initial 26Al/27Al ratio at the time of CAI formation was near-canonical (i.e., ~5 × 10–5). eters, however, typically do not have the time resolution afforded by the U-Pb chronometer compared to chronom- required to resolve events occurring within the first tens of eters based on other long-lived radioisotopes results from millions of years of solar system history. In contrast, short- the rapid evolution of the radiogenic Pb isotopic composi- lived chronometers can have high time resolution (i.e., a tion (i.e., 207Pb/206Pb) due to the relatively short half-life million years or less) and are thus capable of precisely age of 235U and the occasionally very high U/Pb ratios in plan- dating the earliest solar system events. Nevertheless, the etesimals and planets affected by volatile loss and/or core application of chronometers based on short-lived radionu- formation since Pb is both volatile and chalcophile, but U clides is not without its challenges. In particular (and pro- is neither. vided there are no other complications such as heterogeneity in the initial distribution of the parent radionuclide and/or 2. INITIAL CONDITIONS later disturbance of the isotope systematics), isochrons based on short-lived radionuclides can provide only relative ages Prior to a discussion of the timescales of planetesimal since the slope of such an isochron reflects the abundance differentiation, we briefly summarize our current under- of the now-extinct radionuclide at the time of isotopic clo- standing of the initial conditions prevalent in the solar neb- sure following a parent-daughter fractionation event. There- ula immediately prior to the formation of these planetesi- fore, since the high-resolution chronometers based on short- mals. More detailed discussions of nebular conditions and lived radionuclides can provide only relative time differ- processes may be found elsewhere in this book (e.g., the ences between early solar system events, it is essential to chapters in Part III). We note that an understanding of con- have a “time anchor” [i.e., a sample that allows one to de- ditions and processes in the solar nebula (especially the de- termine the value of the parameter (R*/R)T at a precisely gree to which it was homogenized) is particularly important defined absolute time T, where R* is the abundance of the with regard to the application of the high-resolution chro- radioisotope and R is that of a stable reference isotope of nometers based on short-lived radionuclides, since factors the same element] so as to map the relative ages derived such as the abundance and distribution of the parent radio- from such a chronometer onto an absolute timescale. The nuclides are critical in determining the feasibility of age- only absolute chronometer capable of providing time reso- dating events in the early solar system. lution comparable to that of chronometers based on short- Table 1 provides a listing of selected short-lived radio- lived radionuclides, and thus providing an appropriate time nuclides that have so far been applied toward determining anchor, is the U-Pb chronometer. A unique attribute of this the timescales involved in planetesimal differentiation. This chronometer is that it is based on the decay of two long- table includes current estimates of the solar system initial 235 207 lived radioisotopes, i.e., U that decays to Pb with a abundance ratios (R*/R)0, which are estimates of the ini- half-life of 703.8 m.y., and 238U that decays to 206Pb with tial abundances of these radionuclides in the meteorite- a half-life of 4469 m.y. The extraordinarily high precision forming region of the solar nebula at the time of formation Wadhwa et al.: Timescales of Planetesimal Differentiation 717

of the earliest condensates, and the refractory CAIs found neities were present in the nebula on the small scale, i.e., on in primitive chondrites, the most precise age estimate for the scale of micrometer-sized presolar grains and millime- which is currently 4567.2 ± 0.6 Ma (Amelin et al., 2002). ter- to centimeter-sized inclusions (CAIs) within primitive In Table 1, the solar system initial abundance ratios (R*/ meteorites (e.g., Loss et al., 1994; Zinner, 1996). However, ≥ 146 R)0 of the radionuclides with half-lives 10 m.y. (i.e., Sm there are only a few instances where isotopic heterogene- and 182Hf), but perhaps also some shorter-lived radionuclides ities have been documented on the planetesimal scale. Spe- (such as 107Pd and 53Mn), may be accounted for by con- cifically, mass-independent variations in the isotopic com- tinuous galactic nucleosynthesis (e.g., Meyer and Clayton, positions of O (Clayton et al., 1973; Clayton, 1993) and Cr 2000). Radionuclides whose abundances may be accounted (Lugmair and Shukolyukov, 1998; Shukolyukov et al., 2003) for by this means are generally assumed to be homogene- have been documented in bulk samples of primitive and dif- ously distributed in the solar nebula. ferentiated meteorites. Although disputed by some (Becker The initial abundance ratios of several of the radionu- and Walker, 2003a,b), such variations have also been noted clides with half-lives ≤10 m.y. (e.g., 26Al and possibly also for Mo and Ru (Dauphas et al., 2002, 2004a; Yin et al., 53Mn) are too high to be accounted for by continuous ga- 2002a; Chen et al., 2003, 2004; Papanastassiou et al., lactic nucleosynthesis alone. Alternatives for the predomi- 2004). By and large, however, most other isotopic systems nant production sites of these shorter-lived radionuclides are investigated so far in meteorites indicate homogeneity on that either (1) they were synthesized in a stellar environ- the planetesimal scale at the level of precision achievable ment and injected into the molecular cloud just prior to its by current state-of-the-art instrumentation (e.g., Zhu et al., collapse and solar nebula formation (e.g., Cameron et al., 2001; Dauphas et al., 2004bc). 1995; Cameron, 2001; Gallino et al., 2004) or (2) they were produced by energetic particle irradiation within the nebula 3. TIMESCALES OF PLANETESIMAL during an early active phase of the Sun (Shu et al., 1996; DIFFERENTIATION Gounelle et al., 2001; Leya et al., 2003a). Additionally, in 10 the specific case of Be (t1/2 ~ 1.5 m.y.), which is produced 3.1. Incipient Melting only by spallation reactions and not by stellar nucleosyn- thesis, it is suggested that this radionuclide may have been Primitive achondrites, such as acapulcoites, lodranites, incorporated into the solar nebula by trapping of galactic winonaites, and brachinites, have igneous textures but their cosmic rays in the molecular cloud as it collapsed (Desch bulk compositions, although showing some variations, are et al., 2004). Whether the short-lived radioisotopes that may relatively close to those of chondrites. Therefore, these be produced in any of the above scenarios were effectively meteorites are considered to be the products of the earliest homogenized prior to the formation of solids in the nebula stages of melting and igneous processing on planetesimals is presently unresolved. Numerical simulations of processes (Mittlefehldt et al., 1998, and references therein). associated with molecular cloud core collapse triggered by The acapulcoites and lodranites are thought to be the a nearby explosive stellar event indicate that spatial hetero- residual products of partial melting of chondritic precur- geneities in the distributions of shock-wave-injected short- sors (Mittlefehldt et al., 1996; McCoy et al., 1997a,b). Chro- lived radionuclides may survive on short timescales (Van- nological constraints obtained so far indicate that these me- hala and Boss, 2002). Nevertheless, in the case of an exter- teorites formed within ~10 m.y. of the formation of the ear- nal seeding source for these short-lived radionuclides, dif- liest solids in the nebula. Evidence for the presence of live 244 ferential rotation (and possibly also turbulence) in the neb- Pu (t1/2 ~ 82 m.y.) in Acapulco phosphates supports early ula is generally anticipated to result in radial and vertical formation of this meteorite (Pellas et al., 1997). Samari- mixing on the timescale of significantly less than a million um-147–neodymium-143 systematics determined by Prinz- years subsequent to initiation of collapse (which is assumed hofer et al. (1992) for Acapulco gave a very old age (4.60 ± to occur immediately prior to formation of the first solids 0.03 Ga), which the authors interpreted as the time of re- in the nebula). If, however, the short-lived radionuclides crystallization immediately following its formation event. were predominantly produced by local irradiation within the Given this old 147Sm-143Nd age, one would expect a 146Sm/ early solar system, they are expected to be distributed het- 144Sm ratio significantly higher than in angrites at 4558 Ma erogeneously within the nebular disk at least over the dura- (i.e., the time anchor for the 146Sm-142Nd system; Table 1). tion of the active phase of the early Sun (e.g., Gounelle et However, the 146Sm/144Sm ratio of 0.0067 ± 0.0019 deter- al., 2001). mined for this sample (Prinzhofer et al., 1992) is within The application of a chronometer based on a short-lived error of that in angrites at their time of formation. More- radionuclide rests critically on the assumption that the ini- over, U-Pb systematics in phosphates from Acapulco give tial abundance ratio of the radioactive to the stable isotope, an age of 4557 ± 2 Ma (Göpel et al., 1992, 1994), margin- 147 143 i.e., (R*/R)0, is uniform in the meteorite-forming region. In ally younger, but much more precise, than the Sm- Nd the specific case of determining the timing of planetesimal age. The older 147Sm-143Nd age is therefore questionable differentiation using such a chronometer, the scale at which and could be due to disturbance during extensive later meta- the isotopic heterogeneity may be present is also important. morphism experienced by this meteorite (McCoy et al., Numerous studies have shown that large isotopic heteroge- 1996). Furthermore, Acapulco has an inferred 53Mn/55Mn 718 Meteorites and the Early Solar System II ratio of (7.5 ± 1.4) × 10–7; compared to the LEW 86010 146Sm/144Sm ratio of 0.007 in angrite LEW 86010 (Table 1) angrite (the time anchor for the 53Mn-53Cr system; Table 1); provides an age of 4.633 ± 0.022 Ga, consistent with its old this translates to a 53Mn-53Cr age of 4555.1 ± 1.2 Ma (Zipfel 147Sm-143Nd age of 4.62 ± 0.07 Ga, although uncertainties et al., 1996), consistent with its Pb-Pb and 146Sm-142Nd sys- are large (Bogdanovski and Jagoutz, 1996). These ages are tematics. Iodine-129–xenon-129 (t1/2 ~ 17 m.y.) systematics problematic since they are older than the age inferred for in Acapulco phosphates indicate Xe closure ~9 m.y. after CAI formation, which is generally assumed to reflect the the Bjurbole L4 chondrite, which also appears to be con- time of formation of the first solids in the solar nebula. sistent with Pb-Pb systematics (Nichols et al., 1994; Brazzle Manganese-53–chromium-53 systematics in this achondrite, et al., 1999). on the other hand, indicate that Cr isotopes were equili- Brachinites are olivine-rich primitive achondrites gener- brated at ≤4542 Ma when the 53Mn/55Mn ratio was ≤6 × 10–8 ally considered to be either partial melt residues (e.g., Nehru (Bogdanovski et al., 1997). et al., 1983, 1992, 1996; Goodrich, 1998) or igneous cu- Despite some discrepancies, the above discussion shows mulates (e.g., Warren and Kallemeyn, 1989). Although pre- that the onset of melting on some planetesimals began as cise absolute formation ages for these primitive achondrites early as ~3–5 m.y. after the formation of the first solids and are not yet available, there are indications that they formed possibly extended to ≥10 m.y. on others. While the most close to ~4.56 Ga, soon after the beginning of the solar plausible heat source for early incipient melting on some system. These include the presence of fission tracks from planetesimals is the decay of short-lived radionuclides such the decay of live 244Pu (Crozaz and Pellas, 1983) and 129Xe as 26Al and 60Fe, such melting episodes at significantly later excesses due to the decay of 129I (Bogard et al., 1983; Ott times on other planetesimals are likely to be the result of et al., 1987; Swindle et al., 1998). Wadhwa et al. (1998) impact heating. reported a 53Mn-53Cr isochron for Brachina that indicated a 53Mn/55Mn ratio of (3.8 ± 0.4) × 10–6 at the time of last 3.2. Crust Formation equilibration of Cr isotopes (Fig. 1). Comparison of this value with the 53Mn/55Mn ratio of (1.25 ± 0.07) × 10–6 in Primary crystallization ages, ideally based on internal the angrite LEW 86010 at 4558 Ma (Fig. 1; Table 1) im- mineral isochrons obtained from individual members of plies a Mn-Cr age of 4563.7 ± 0.9 Ma for this meteorite, achondrite groups such as the angrites and noncumulate which is only 3.5 ± 1.1 m.y. after the time of CAI forma- eucrites that represent basaltic rocks that formed in aster- tion (i.e., 4567.2 ± 0.6 Ma) (Amelin et al., 2002). oidal near-surface environments, offer the best means of However, not all primitive achondrites investigated so far assessing the timescales of crust formation on planetesimals. have consistent long- and short-lived radioisotope systemat- 3.2.1. Angrites. Angrites are a small group of min- ics. The Divnoe meteorite is an ultramafic primitive achon- eralogically unique basalts composed mostly of Ca-Al-Ti- drite whose relationship to other primitive achondrite groups rich pyroxenes (fassaite), olivine, and anorthitic plagioclase is as yet unclear (Petaev et al., 1994; Weigel et al., 1996). (Mittlefehldt et al., 1998, and references therein). Early evi- This meteorite is inferred to have an extremely high 146Sm/ dence for the presence of fission Xe (from the decay of 144 244 Sm ratio of 0.0116 ± 0.0016 that in comparison to the Pu; t1/2 ~ 82 m.y.) in Angra dos Reis, the type meteorite of the angrites, established its antiquity (Hohenberg, 1970; Lugmair and Marti, 1977; Wasserburg et al., 1977). Sub- sequently, evidence for the former presence of 244Pu has also been reported in two other angrites (Eugster et al., 1991; Hohenberg et al., 1991). Samarium-147–neodymium-143 systematics in Angra dos Reis (ADOR) and LEW 86010 (LEW) are well behaved and give old crystallization ages between 4.53 ± 0.04 and 4.56 ± 0.04 Ga (Lugmair and Marti, 1977; Wasserburg et al., 1977; Jacobsen and Wasser- burg, 1984; Lugmair and Galer, 1992; Nyquist et al., 1994). Samarium-147–neodymium-143 systematics have also been determined in the more recently discovered angrite D’Or- bigny, and, despite some disturbance evident in the plagio- clase, possibly due to late metamorphism and/or terrestrial weathering, are generally consistent with earlier results for ADOR and LEW (Nyquist et al., 2003a; Tonui et al., 2003). Samarium-146–neodymium-142 systematics in ADOR and LEW (146Sm/144Sm ~0.007) are concordant with their 147Sm- Fig. 1. Excesses in the 53Cr/52Cr ratio in parts per 104, or ε(53), 143 relative to a terrestrial standard vs. 55Mn/52Cr ratios in the primitive Nd systematics, although preliminary data for D’Orbigny 146 142 achondrite Brachina (CHR = chromite; TR = whole rock; SIL = suggest disturbance of the Sm- Nd system (Tonui et al., silicates); data from Wadhwa et al. (1998). The dashed line is the 2003). isochron for the LEW 86010 angrite (Lugmair and Shukolyukov, The most precise estimate of the crystallization age ofthe 1998). angrites is offered by their Pb-Pb systematics. An internal Wadhwa et al.: Timescales of Planetesimal Differentiation 719

Pb-Pb isochron defined by LEW minerals gives an age of significantly greater numbers of known noncumulate eu- 4558.2 ± 3.4 Ma, concordant with the highly precise U-Pb crites than there are angrites. Recent high-precision O-iso- model age of 4557.8 ± 0.5 Ma obtained from the extremely topic data of Wiechert et al. (2004) demonstrate that most radiogenic Pb compositions in the pyroxenes of ADOR and noncumulate eucrites along with the cumulate eucrites, diog- LEW (Lugmair and Galer, 1992). The highly precise Pb-Pb enites, and howardites have uniform ∆17O (within ± 0.02‰) age of the LEW angrite has made it possible to use the for- and thus lie on a single mass fractionation consistent with mation time of this sample as a precise time anchor for the their origin on a single parent body. Therefore, these basalts application of chronometers based on short-lived radionu- are the most numerous crustal rocks available from any sin- 53 146 clides, particularly Mn (t1/2 ~ 3.7 m.y.) and Sm (t1/2 = gle solar system body other than Earth and the Moon. A 103 m.y.) (Table 1). Preliminary U-Pb model ages derived handful of the noncumulate eucrites (in particular Ibitira, but from D’Orbigny pyroxenes appeared to be in agreement possibly also Caldera, Pasamonte, and ALHA 78132) have with those derived from ADOR and LEW pyroxenes (Ja- O-isotopic compositions distinct from the others, implying goutz et al., 2003). However, a recent reevaluation of these either that these samples originated on different parent bod- data by Jagoutz and colleagues has resulted in a somewhat ies or that isotopic heterogeneity was preserved on the HED older age of 4563 ± 1 Ma for D’Orbigny (G. W. Lugmair, parent body (Wiechert et al., 2004). personal communication, 2005). Finally, Baker et al. (2005) Unlike angrites (which did not undergo any significant have reported an even older Pb-Pb age for the Sahara 99555 degree of recrystallization or metamorphism), the noncumu- angrite of 4566.2 ± 0.1 Ma. late eucrites appear to record a protracted history of exten- Evidence for the presence of live 53Mn at the time of sive thermal processing on their parent body subsequent to their formation is found in ADOR and LEW. Nyquist et al. their original crystallization. As a result, many of the long- (1994) reported a 53Mn/55Mn ratio of (1.44 ± 0.07) × 10–6 and short-lived chronometers investigated in these samples for the LEW angrite. Subsequently, a study by Lugmair and appear to record secondary thermal events rather than their Shukolyukov (1998) gave a 53Mn/55Mn ratio of (1.25 ± original crystallization. Nevertheless, there are several lines 0.07) × 10–6 for LEW (with the data point for ADOR olivine of evidence that suggest that these basalts crystallized very being consistent with this value). These values are in rea- early in the history of the solar system. Although typically sonable agreement, especially if the Cr-isotopic composi- characterized by large uncertainties, the 147Sm-143Nd ages tion of olivine reported by Nyquist et al. (1994) is corrected of several of these samples such as Chervony Kut (4580 ± for contribution from spallogenic Cr. No detectable evi- 30 Ma) (Wadhwa and Lugmair, 1995), Juvinas (4560 ± dence for the presence of live 26Al has been found in these 80 Ma) (Lugmair, 1974), Pasamonte (4580 ± 120 Ma) (Un- samples (26Al/27Al < 2 × 10–7) (Lugmair and Galer, 1992). ruh et al., 1977), Piplia Kalan (4570 ± 23 Ma) (Kumar et However, such a result is not unexpected given that, at the al., 1999), and Yamato 792510 (4570 ± 90 Ma) (Nyquist et time these angrites formed (4558 Ma), ~12 half-lives of 26Al al., 1997a) are close to ~4.56 Ga. In some cases where had elapsed since the time of CAI formation (4567 Ma) 147Sm-143Nd systematics appear to record ages younger than (Amelin et al., 2002) when the 26Al/27Al ratio was ~5 × 10–5. ~4.56 Ga, the 146Sm-142Nd systematics still provide evi- The 53Mn/55Mn ratio at the time of formation of D’Or- dence for early crystallization of basaltic eucrites such as bigny and Sahara 99555 angrites (~3 × 10–6) (Nyquist et Caldera (146Sm/144Sm = 0.0073 ± 0.0011) (Wadhwa and al., 2003a; Glavin et al., 2004) is significantly higher than Lugmair, 1996) and Ibitira (146Sm/144Sm ~ 0.009 ± 0.001) in ADOR and LEW, implying that D’Orbigny and Sahara (Prinzhofer et al., 1992). This may be explained by a model 99555 formed ~4–5 m.y. prior to ADOR and LEW. This is proposed by Prinzhofer et al. (1992), according to which consistent with the Pb-Pb systematics in D’Orbigny, which the apparent discrepancy between the long-lived 147Sm- indicate that this angrite is 5 ± 1 m.y. older than ADOR and 143Nd and the short-lived 146Sm-142Nd systems can be in- LEW (Lugmair and Galer, 1992; G. W. Lugmair, personal terpreted in terms of a short episodic disturbance resulting communication, 2005). There is also evidence for the pres- in partial reequilibration of the rare earth elements (REEs), ence of live 26Al in the D’Orbigny and Sahara 99555 an- predominantly between plagioclase (which has very low grites (26Al/27Al ~5 × 10–7) (Spivak-Birndorf et al., 2005), REE abundances) and phosphates (which are the primary which is consistent with the higher 53Mn/55Mn ratio and the REE carriers). As shown by the modeling results of these older Pb-Pb age of D’Orbigny compared to ADOR and authors, such a disturbance could partially reset the 147Sm- LEW. However, the Pb-Pb age of Sahara 99555 (Baker et 144Nd isochron, although the 146Sm-142Nd system would not al., 2005) is not concordant with the ages based on 26Al- be resolvably affected. 26Mg and 53Mn-53Cr internal isochrons (Spivak-Birndorf et There is at least one instance (i.e., the basaltic eucrite al., 2005). Finally, high-precision Mg-isotopic analyses of EET 90020) where the 147Sm-143Nd age (4430 ± 30 Ma) and bulk samples of the Sahara 99555 and NWA 1296 angrites 146Sm-142Nd systematics (146Sm/144Sm = 0.0048 ± 0.0020) show small but resolvable excesses in 26Mg, which may be are indeed concordant, although within rather large uncer- indicative of early Al/Mg fractionation in their source re- tainties. As will be discussed below, both the 147Sm-144Nd sulting from crust formation on their parent body while 26Al age and initial 146Sm/144Sm ratio for EET 90020 are simi- was still extant (Bizzarro et al., 2005). lar to values obtained for the Moore County and Moama 3.2.2. Eucrites. Like the angrites, the noncumulate eu- cumulate eucrites (Jacobsen and Wasserburg, 1984; Tera et crites are pyroxene-plagioclase rocks. However, there are al., 1997). The young 147,146Sm-143,142Nd age for this non- 720 Meteorites and the Early Solar System II cumulate eucrite has been interpreted to possibly record late magmatism on the parent body (Nyquist et al., 1997b). Therefore, EET 90020 could be the crystallization product of a younger melting event that may also be responsible for the formation of the cumulate eucrites. However, the 39Ar- 40Ar chronometer, which is more easily reset by shock re- heating than the Sm-Nd system, records a slightly older age of ~4.48–4.49 Ga for EET 90020 (Bogard and Garrison, 1997). Therefore, it is possible that this basalt underwent a complex petrogenetic history as suggested by Yamaguchi et al. (2001), such that its Sm-Nd systematics may not neces- sarily record its primary igneous formation. Although the Rb-Sr chronometer tends to be more sus- ceptible to resetting than the Sm-Nd system, it also gener- ally points to an ancient age of formation for the noncumu- late eucrites (e.g., Allègre et al., 1975; Nyquist et al., 1986). Fig. 2. Excesses in the 26Mg/24Mg ratio vs. 27Al/24Mg ratios in The most precise of the absolute chronometers, i.e., the U- the noncumulate eucrite Piplia Kalan (Px = pyroxene; Pl = plagio- Pb system, appears to have been affected by postcrystalli- clase). Figure from Srinivasan et al. (1999). zation events and terrestrial Pb contamination in most non- cumulate eucrites, recording mineral isochron ages in the range of 4128 Ma to 4530 Ma (Tatsumoto et al., 1973; Unruh et al., 1977; Galer and Lugmair, 1996; Tera et al., 1997). However, Ibitira whole-rock samples with radiogenic Pb-isotopic compositions gave old Pb-Pb model ages of 4556 ± 6 Ma (Chen and Wasserburg, 1985) and 4560 ± 3 Ma (Manhés et al., 1987). Furthermore, a recent determi- nation of the Pb-Pb mineral isochron for the Asuka 881394 eucrite yielded a precise and ancient age of 4566.5 ± 0.9 Ma (Wadhwa et al., 2005; Y. Amelin, personal communication, 2005). This is only 0.7 ± 1.1 m.y. younger than the time of CAI formation (4567.2 ± 0.6 Ma) (Amelin et al., 2002) and indicates that crust formation on differentiated planetesimals occurred within ~2 m.y. of the formation of the first solids in the early solar system. In recent years there has been increasing evidence for the presence of various short-lived radionuclides (particu- larly 26Al, 53Mn, and 60Fe) in the noncumulate eucrites at their time of crystallization, which further supports the early Fig. 3. Excesses in the 26Mg/24Mg ratio in per mil, or ∆26Mg, formation of these crustal basalts. The first evidence for live relative to a terrestrial standard plotted vs. 27Al/24Mg ratios in the 26Al in an achondrite was reported by Srinivasan et al. Asuka 881394 eucrite. Solid circles show the data of Wadhwa et (1999) (Fig. 2). These authors demonstrated the presence al. (2005) and the solid line is the isochron defined by these data, 26 27 of excess 26Mg from the decay of 26Al in plagioclase of the the slope of which corresponds to a Al/ Al ratio of (1.34 ± –6 Piplia Kalan basaltic eucrite, from which they inferred a 0.04) × 10 ; low Al/Mg data points near the origin are for py- roxene and whole rock, and high Al/Mg data points are for pla- 26Al/27Al ratio of (7.5 ± 0.9) × 10–7 in this sample at the time gioclase. Solid squares are the data of Nyquist et al. (2003b) and of its formation. However, this value is more likely to be a the dashed line is the isochron defined by these data, the slope of 26 27 lower limit on the actual Al/ Al ratio at the time of crys- which corresponds to a 26Al/27Al ratio of (1.18 ± 0.14) × 10–6; tallization of this eucrite, since the plagioclase appears to low Al/Mg data points near the origin are for pyroxene, and high have undergone intragrain equilibration (as is evident from Al/Mg data points are for plagioclase. the near-constant 26Mg excesses determined in various spot analyses of plagioclase with different Al/Mg ratios; Fig. 2). Compared to the 26Al/27Al ratio of ~5 × 10–5 in CAIs, the 26Al/27Al ratio inferred for Piplia Kalan suggests that crust ratio is most precisely determined in the Asuka 881394 eu- formation on the parent planetesimal of this eucrite occurred crite, i.e., (1.34 ± 0.04) × 10–6 (Fig. 3), and translates to an at most ~5 m.y. after the beginning of the solar system. Sub- Al-Mg age of 4563.5 ± 0.9 Ma, slightly younger than its sequently, evidence for the presence of live 26Al has been ancient Pb-Pb mineral isochron age, possibly indicative of found in additional basaltic eucrites (26Al/27Al ratios in the slow subsolidus cooling of this eucrite (Wadhwa et al., 2005; range of ~6 × 10–7 to ~2 × 10–6) (Srinivasan, 2002; Nyquist Y. Amelin, personal communication, 2005). Additionally, et al., 2003b; Wadhwa et al., 2005). Of these, the 26Al/27Al Bizzarro et al. (2005) have recently reported high-precision Wadhwa et al.: Timescales of Planetesimal Differentiation 721

Mg-isotopic analyses of bulk samples of several noncumu- for the early and extensive differentiation experienced by late eucrites, which show that these samples have small ex- their parent planetesimal. Energy sources that can account cesses in 26Mg, possibly indicative of early Al/Mg fractiona- for later igneous activity (i.e., tens of millions of years after tion in their source resulting from basaltic volcanism and CAI formation) on small planetesimals are not obvious crust formation on their parent body while 26Al was extant. unless the cumulate eucrites are the crystallization products Evidence for the presence of live 53Mn at the time of of impact melting on the eucrite parent body. Alternatively eucrite formation has been documented in several noncumu- (and perhaps more likely), since the cumulate eucrites are late eucrites including Chervony Kut, Juvinas, Ibitira (Lug- slowly cooled rocks that possibly formed deeper within the mair and Shukolyukov, 1998), and Asuka 881394 (Nyquist crust of their parent body than noncumulate eucrites, the et al., 2003b; Wadhwa et al., 2005). By comparison with long-lived chronometers could be recording the long cool- the LEW angrite (Table 1), Mn-Cr ages for these eucrites ing times required to achieve subsolidus temperatures. This were determined to be 4563.6 ± 0.9 Ma, 4562.5 ± 1.0 Ma, is supported by the modeling results of Ghosh and Mc- 4557 +2/–4 Ma, and 4563.8 ± 0.7 Ma, respectively (Lug- Sween (1998), which show that, assuming reasonable pa- mair and Shukolyukov, 1998; Wadhwa et al., 2005). These rameters, it is possible to maintain temperatures in excess of ages indicate that crust formation on the parent planetesimal the solidus temperature for basalt at a depth of ~100 km for of these basalts began possibly within ~3 m.y., and may have over ~100 m.y. in a Vesta-sized planetesimal. continued up to ~10 m.y., after the formation of the first 3.2.3. Other achondrites. Besides the angrites and the solids (additionally, the Mn-Cr age for the Asuka 881394 eucrites, there are other types of achondrites, such as au- eucrite is consistent with its Al-Mg age). Several other ba- brites and ureilites, whose origins are somewhat enigmatic saltic eucrites in which Mn-Cr systematics have been in- but that were nevertheless formed in the crusts of exten- vestigated do not show any evidence for live 53Mn (Lugmair sively differentiated asteroidal bodies. There are very few and Shukolyukov, 1998; Nyquist et al., 1996, 1997a,b). In chronological investigations of the aubrites, which are es- these cases, it is likely that secondary thermal events have sentially monomineralic rocks composed of coarse-grained resulted in later resetting of the Mn-Cr system, as appears enstatite. Brazzle et al. (1999) estimated an I-Xe age of to be the case for the U-Pb system in most noncumulate 4566 ± 2 Ma for the Shallowater aubrite, using the I-Xe and eucrites as well. Finally, there is unambiguous evidence for Pb-Pb systematics in phosphate of the Acapulco primitive the former presence of live 60Fe in three noncumulate eu- achondrite as an anchor. More recently, however, based on crites, Chervony Kut, Juvinas, and Bouvante (Shukolyukov correlations of relative I-Xe ages with absolute Pb-Pb ages and Lugmair, 1993a,b; Quitté et al., 2005), also attesting of various meteorites and their components, it has been to early formation of these basalts. suggested that enstatite in the Shallowater aubrite crystal- Basaltic noncumulate eucrites thus show clear evidence lized at 4563.5 ± 1.0 Ma (Pravdivtseva et al., 2005). Early of having formed close to ~4.56 Ga in the crust of their formation (i.e., within a few million years of CAIs) of the parent planetesimal. In contrast, cumulate eucrites, which aubrites is further supported by evidence for live 53Mn in formed in the crust of the same parent planetesimal as the Peña Blanca Spring (Shukolyukov and Lugmair, 2004). noncumulate eucrites (Clayton and Mayeda, 1996; Wiechert Uranium-thorium-lead and Sm-Nd isotope systematics et al., 2004), have significantly younger concordant Sm-Nd in the ureilites generally indicate early formation, although and Pb-Pb ages, ranging from the oldest of 4456 ± 25 Ma there are apparent complications resulting from later distur- (Sm-Nd) and 4484 ± 19 Ma (Pb-Pb) for Moore County (Tera bance, i.e., during a metasomatic event on the ureilite parent et al., 1997) to the youngest of 4410 ± 20 Ma (Sm-Nd) body and/or by recent terrestrial contamination (Goodrich (Lugmair et al., 1977) and 4399 ± 35 Ma (Pb-Pb) (Tera and Lugmair, 1995; Torigoye-Kita et al., 1995a,b). More et al., 1997) for Serra de Magé. Thus, Sm-Nd and Pb-Pb recent studies have demonstrated evidence for live 53Mn and systematics in the cumulate eucrites indicate that isotopic 26Al at the time of ureilite formation. Specifically, it has been closure occurred up to ~150 m.y. after the noncumulate shown that a feldspathic clast from the Dar al Gani 165 ureil- eucrites (Lugmair et al., 1977; Jacobsen and Wasserburg, ite had a 53Mn/55Mn ratio of (2.91 ± 0.16) × 10–6 at the time 1984; Lugmair et al., 1991; Tera et al., 1997), possibly im- of last equilibration of Cr isotopes (Goodrich et al., 2002). plying that active magmatism persisted on the eucrite par- Compared to the LEW angrite (Table 1), a Mn-Cr age of ent body for this extended period (Tera et al., 1997). This 4562.3 ± 0.6 Ma is determined for this sample. Kita et al. may be supported by the 4430 ± 30 Ma 147Sm-143Nd age of (2003) have reported Al-Mg data for a plagioclase-bearing the noncumulate eucrite EET 90020 (Nyquist et al., 1997b) clast from the Dar al Gani 319 ureilite that indicate a 26Al/ and the overlapping, relatively low, initial 146Sm/144Sm ra- 27Al ratio of (3.95 ± 0.59) × 10–7 at the time of its crystal- tios of Moore County, Moama, and EET 90020 (Jacobsen lization. Comparison with the canonical value at the time and Wasserburg, 1984; Nyquist et al., 1997b; Tera et al., of CAI formation (i.e., 4567.2 ± 0.6 Ma) (Amelin et al., 1997). If this is so, it further implies that the process of crust 2002) gives an Al-Mg age of 4562.2 ± 0.6 Ma, which agrees formation on the eucrite parent body possibly extended well with the Mn-Cr age discussed above. Finally, a Hf-W- to ~150 m.y. after solar system formation. Since, as dis- isotopic investigation of bulk samples of several ureilites cussed above, the oldest noncumulate eucrites formed has yielded subchondritic values for ε182W (i.e., the 182W/ within ~3 m.y. of CAI formation, the decay of short-lived 184W or 182W/183W ratio relative to the terrestrial standard in radionuclides such as 26Al and 60Fe is the likely heat source parts per 104) and Hf/W ratios, indicating that the differen- 722 Meteorites and the Early Solar System II tiation process that resulted in the formation of these ureil- parison with the LEW angrite (Table 1) gives a Mn-Cr age ites occurred while 182Hf was still extant (Lee et al., 2005). of 4564.8 ± 0.9 Ma for global silicate fractionation on the HED parent body, which is 2.4 ± 1.1 m.y. after the formation 3.3. Global Silicate (Mantle) Differentiation of CAIs (Amelin et al., 2002). This age is, within error, simi- lar to the Al-Mg and Mn-Cr ages of eucrite Asuka 881394 3.3.1. Whole-rock isochrons. While an internal mineral (Nyquist et al., 2003b; Wadhwa et al., 2005) and the Mn- isochron can provide constraints on the timing of formation Cr age of eucrite Chervony Kut (Lugmair and Shukolyukov, of an individual achondrite in the crust of a planetesimal, a 1998). The near agreement of the ages provided by the whole-rock isochron (based on a long- or a short-lived chro- whole-rock isochrons and internal isochrons of these eu- nometer) of a particular achondrite group can provide limits crites indicates not only that planetesimal differentiation on the timing of parent-daughter element fractionation in started soon after solar system formation, but that the dura- the source (mantle) reservoir of their parent planetesimal, tion of the initial HED crust-mantle differentiation may have which in turn is a reflection of the timescale involved in glo- been as short as ~2 m.y. or less. bal silicate fractionation (possibly associated with crystalli- Trinquier et al. (2005a,b) have recently suggested that zation of a magma ocean). Whole-rock Rb-Sr isochrons for HED meteorites have a deficit in the 54Cr/52Cr ratio of 0.72 ± the basaltic eucrites established early on that Rb-Sr fraction- 0.02 ε compared to the terrestrial standard. This is in contrast ation in the mantle source reservoir of these achondrites oc- to previous work on Cr-isotopic compositions of achondrites, curred close to ~4.6 Ga (Papanastassiou and Wasserburg, including eucrites, which indicates that the 54Cr/52Cr ratio 1969; Birck and Allègre, 1978). Smoliar (1993) evaluated all in these samples is the same as the terrestrial value to within available Rb-Sr data for the eucrites and obtained a whole- ±0.25 ε (Lugmair and Shukolyukov, 1998; Wadhwa et al., rock isochron age of 4.55 ± 0.06 Ga for the noncumulate 2003). A potentially anomalous 54Cr/52Cr ratio in bulk eu- eucrites. crites is relevant here since many previous Cr-isotopic stud- Achondrite whole-rock isochrons based on short-lived ies use a second-order fractionation correction that assumes chronometers have the potential for more precisely con- a normal (i.e., terrestrial) 54Cr/52Cr ratio. The reason for this straining the timing of global silicate differentiation on plan- apparent discrepancy is not evident at this time. Neverthe- etesimals. Such isochrons have recently been reported for the less, in the case of eucrites and other achondrites that have 53Mn-53Cr and the 182Hf-182W systems in the HED group undergone complete equilibration of Cr-isotopic composi- of achondrites (Lugmair and Shukolyukov, 1998; Quitté et tion between their constituent minerals because they crys- al., 2000; Kleine et al., 2004). The HED whole-rock 53Mn- tallized from a melt, a second-order correction such as used 53Cr isochron corresponds to a 53Mn/55Mn ratio of (4.7 ± by Lugmair and Shukolyukov (1998) will not alter the slope 0.5) × 10–6 (Fig. 4) (Lugmair and Shukolyukov, 1998). Com- of the 53Mn-53Cr isochron or the relative age based on this slope, which is derived from comparison with another mag- matic sample, the LEW angrite. A whole-rock 182Hf-182W isochron for the HEDs was ini- tially reported by Quitté et al. (2000). Subsequently, the data of Quitté et al. (2000) were combined with additional data by Kleine et al. (2004), who reported a 182Hf/180Hf ratio of (7.25 ± 0.50) × 10–5 at the time of crust-mantle differentia- tion on the eucrite parent body (Fig. 5a). Currently there is no consensus on the initial 182Hf/180Hf ratio of the solar sys- tem, with proposed values ranging from ~1 × 10–4 (Kleine et al., 2002, 2005; Yin et al., 2002b) to ~1.6 × 10–4 (Quitté and Birck, 2004). An initial 182Hf/180Hf ratio of ~1 × 10–4 would imply that global Hf-W fractionation in the HED mantle source occurred ~4 m.y. after the beginning of the solar system, which is consistent with the HED whole-rock 53Mn-53Cr systematics discussed above. However, if an ini- tial 182Hf/180Hf ratio of ~1.6 × 10–4 is assumed, then Hf-W fractionation in the mantle of the HED parent body occurred 53 53 Fig. 4. Mn- Cr systematics in the HED parent body. Data ~9 m.y. after the beginning of the solar system. This is in- points are whole rocks of noncumulate eucrites (CAL = Caldera; consistent not only with the HED whole-rock 53Mn-53Cr CK = Chervony Kut; IB = Ibitira; JUV = Juvinas; POM = Pomoz- systematics but also with the recent evidence for live 26Al in dino), cumulate eucrites (MC = Moore County; SM = Serra de Magé), and diogenites (JT = Johnstown; SHA = Shalka). ε(53) is several eucrites (Srinivasan et al., 1999; Srinivasan, 2002; as defined in the caption for Fig. 1. The slope of the HED whole- Nyquist et al., 2003b; Wadhwa et al., 2005). In this regard, rock isochron corresponds to a 53Mn/55Mn ratio of (4.7 ± 0.5) × it is worth noting that the initial 182Hf/180Hf ratio of 1.6 × 10–6 at the time of global silicate differentiation. Figure from Lug- 10–4 is based on the reanalysis of the W-isotopic composi- mair and Shukolyukov (1998). tion of the Tlacotepec (IVB) iron meteorite, which has an Wadhwa et al.: Timescales of Planetesimal Differentiation 723

undergone different histories and on Allende CAIs that have may have undergone some degree of alteration. Therefore, although a value close to 1 × 10–4 appears to be more plau- sible at this time, the question regarding the initial 182Hf/ 180Hf ratio for the solar system remains to be resolved. Recently, Wadhwa et al. (2003) reported a whole-rock 53Mn-53Cr isochron for eucritic and diogenitic clasts from the Vaca Muerta mesosiderite that corresponded to a 53Mn/ 55Mn ratio of (3.3 ± 0.6) × 10–6. These authors concluded that the lower slope of the isochron defined by these clasts implies that these materials originated on a parent planetesi- mal distinct from that of the HEDs, and underwent global (mantle) differentiation ~2 m.y. after the HED parent body (i.e., at ~4563 Ma compared to the LEW angrite anchor). Finally, bulk samples of three enstatite achondrites (au- brites) also define an isochron that indicates that the last global Mn/Cr fractionation on their parent body occurred at ~4563 Ma (Shukolyukov and Lugmair, 2004). 3.3.2. Initial strontium-isotopic composition. The antiq- uity of the highly volatile-depleted parent planetesimals of the angrites and the eucrites can also be inferred from their initial 87Sr/86Sr ratios. Assuming that the initial 87Sr/86Sr ratio at the beginning of the solar system is represented by the average initial 87Sr/86Sr ratio measured in Allende CAIs (Gray et al., 1973; Podosek et al., 1991), and thatsubsequent evolution of radiogenic Sr occurred in an environment with solar Rb/Sr ratios, the initial 87Sr/86Sr ratios of the angrites (Lugmair and Galer, 1992; Nyquist et al., 1994, 2003a; Tonui et al., 2003) translate to an age difference of ~4 m.y. between CAI formation and the timing of Rb/Sr depletion event that established the angrite source characteristics. The very low initial 87Sr/86Sr ratios of the eucrites simi- larly indicate that the volatile depletion characterizing the eucrite parent body may have occurred early in the history 182 182 ε Fig. 5. Hf- W systematics in the HED parent body. W is of the solar system (Carlson and Lugmair, 2000, and ref- the 182W/184W relative to a terrestrial standard in parts per 104. erences therein). A reevaluation of the Sr-isotopic data for (a) Data points are whole rocks of several noncumulate eucrites the eucrites by Smoliar (1993) shows that whole-rock Rb- (Ber = Bereba; Bouv = Bouvante; Juv = Juvinas; Jon = Jonzac; Sr isochrons for the noncumulate and the cumulate eucrites Mil = Millbillillie; Pas = Pasamonte; Sta = Stannern) and a cu- define slightly, but resolvably, different initial 87Sr/86Sr ra- mulate eucrite (SM = Serra de Magé). Solid symbols are data from tios [which are both distinctly lower than the eucrite initial, Kleine et al. (2005); open symbols: Quitté et al. (2000); open dia- BABI, previously defined by Papanastassiou and Wasser- mond: Yin et al. (2002b). The slope of the HED whole-rock iso- 87 86 chron corresponds to a 182Hf/180Hf ratio of (7.25 ± 0.50) × 10–5 burg (1969)]. In fact, the initial Sr/ Sr ratio for the cumu- at the time of global silicate differentiation. (b) Comparison of the late eucrites proposed by Smoliar (1993) is, within errors, slope of the eucrite whole-rock isochron from (a) with that of the similar to that for the angrites (e.g., Lugmair and Galer, bulk mantle-chondrite model isochron suggests that core forma- 1992), suggesting that the volatile depletion in their sources tion on the HED parent body occurred 0.9 ± 0.3 m.y. before glo- was established at similar times (possibly ~4 m.y. after CAI bal silicate differentiation. The W-isotopic composition of the formation; see above). This timescale for the fractionation eucrite bulk mantle is estimated from the whole-rock isochron, event that established the low Rb/Sr source characteristics using a 180Hf/184W ratio of ~40. Figure from Kleine et al. (2004). of the cumulate eucrites is essentially consistent with the timing of global mantle differentiation defined by the HED whole-rock 53Mn-53Cr and 182Hf-182W isochrons. However, extremely low ε182W value of –4.5 (Quitté and Birck, 2004); the slighter higher initial 87Sr/86Sr ratio defined by the non- similarly low ε182W values (i.e., lower than ~–3.5) for sev- cumulate eucrites is potentially problematic since the sim- eral iron meteorites have recently been called into question plest interpretation would be that their source evolved with (Yin and Jacobsen, 2003; Scherstén et al., 2004). However, a chondritic Rb/Sr ratio for ~4 m.y. longer (and is thus the proposed initial 182Hf/180Hf ratio of 1 × 10–4 is based younger) than that of the cumulate eucrites, further implying on the 182Hf-182W systematics in bulk chondrites that have that these two types of eucrites originated on distinct par- 724 Meteorites and the Early Solar System II ent bodies (Smoliar, 1993). This is inconsistent with recent ues. These authors reported an ε182W value of –3.5 ± 0.3 high-precision O-isotopic data (Wiechert et al., 2004) that for Duel Hill, significantly higher than that previously re- suggest that the cumulate and noncumulate eucrites (with ported for this meteorite by Horan et al. (1998) (i.e., –5.1 ± the possible exception of Ibitira) originated on a common 1.1). However, for Tlacotepec, they obtained an ε182W value parent planetesimal. As discussed by Carlson and Lugmair of –4.4 ± 0.3, which agrees well with the previous value (2000), a possible explanation could be that the severe vola- (–4.5 ± 0.4) (Horan et al., 1998). Therefore, it appears that tile depletion on the eucrite parent body did not occur in a the W-isotopic composition of the iron meteorites ranges single-step process, but rather took place over the course down to at least this value. However, only 4 out of 28 iron of its accretionary and early differentiation history. Subse- meteorites measured by Lee and Halliday (1996) and Horan quently, the process of magma ocean crystallization may et al. (1998) have ε182W values below the chondritic initial have resulted in a slighter higher Rb/Sr ratio in the source value, outside of uncertainty, and the mean ε182W value for of the noncumulate eucrites compared to that of the cumu- all these samples is –3.8 ± 0.5 (1σ). Therefore, the results late eucrites, thereby resulting in the higher initial 87Sr/86Sr from these studies are consistent with most iron meteorites ratio indicated by the whole-rock isochron for the noncumu- having ε182W values that are not resolvably below the ini- late eucrites. tial chondritic value. This is further supported by several recent investigations that have remeasured the W-isotopic 3.4. Metal Segregation and Core Crystallization compositions of iron meteorites for which previous work had indicated ε182W values lower than –3.5 (e.g., Yin and Limits on the timescales involved in metal segregation Jacobsen, 2003; Scherstén et al., 2004). As to the ε182W and core crystallization on planetesimals may be obtained values of some few iron meteorites lying resolvably and from chronological investigations of iron-rich meteorites, reproducibly below the initial chondritic value, one possi- such as magmatic irons (which represent the metallic cores bility is that the W-isotopic compositions of the components of differentiated asteroidal bodies) and pallasites (consid- (mainly metal and silicates) of the chondrites analyzed so ered to have formed near the core-mantle boundary). De- far, and used to defined the initial chondritic ε182W value, pending on the geochemical affinities of the parent and may have been equilibrated to some degree. Another pos- daughter elements, different chronometers applied to such sibility is that exposure to cosmic radiation could result in meteorite types can provide constraints on either the pro- spallation and neutron capture reactions on W isotopes that cess of metal segregation (e.g., the 182Hf-182W chronometer, could result in apparently lower ε182W values in some irons where Hf is lithophile and W is siderophile such that ma- with long exposure ages (Markowski et al., 2005; Leya et jor fractionation occurs during separation of the metallic al., 2000, 2003a,b; Masarik, 1997). These possibilities have melt from silicates) or of core crystallization (e.g., 187Re- implications for the initial 182Hf/180Hf ratio of the solar sys- 187Os, 107Pd-107Ag, and 53Mn-53Cr, where the parent-daugh- tem, but remain to be rigorously evaluated. ter elements are fractionated during crystallization of Fe-Ni The timing of metal segregation (core formation) on the phases, sulfides, and phosphates from the metallic melt). HED parent body has been estimated from 182Hf-182W sys- 3.4.1. Metal segregation. The W-isotopic compositions tematics in bulk samples of eucrites and diogenites (Kleine et of a variety of magmatic and nonmagmatic iron meteorites al., 2004; Quitté and Birck, 2004). Assuming that the HED have been reported within the last decade (e.g., Lee and parent body had a bulk chondritic Hf/W composition and Halliday, 1995, 1996; Horan et al., 1998) and have the that core formation resulted in a 180Hf/184W ratio in theHED lowest 182W/184W ratios of any solar system material, with mantle of ~40 (i.e., close to the highest Hf/W ratio meas- ε182W values ranging from ~–3 to –5 (with most values ured in eucrites that are the products of partial melting of between –3.5 and –4.1). As emphasized by Horan et al. this mantle), Kleine et al. (2004) estimated that core forma- (1998), these ε182W values suggest that metal segregation on tion on the HED parent planetesimal occurred 0.9 ± 0.3 m.y. different planetesimals occurred within a period of ~5 m.y. prior to global mantle (silicate) differentiation (Fig 5b). in the early history of the solar system. As mentioned ear- 3.4.2. Core crystallization. Once the metal has segre- lier, however, the lower end of the range of ε182W values gated into the core of a planetesimal, the timescales in- in the iron meteorites have been called into question, since volved in the crystallization of this metal may be constrained such values have the improbable implication that some iron by isochrons based on bulk samples and mineral phases of meteorites predate CAIs and chondrites (e.g., Kleine et al., magmatic iron meteorites and pallasites. In recent years, pre- 2005). Yin and Jacobsen (2003) have measured the W-iso- cise Re-Os isochrons have been obtained for various groups topic compositions of several iron meteorites, including of the iron meteorites. Most iron meteorite groups give rela- some of those previously analyzed by Lee and Halliday tively old Re-Os ages that range from 4557 ± 12 Ma (IIIAB (1996) and Horan et al. (1998), and reported that none had magmatic irons) to 4526 ± 27 Ma (IVB magmatic irons) ε182W values lower than the chondritic initial value of (Shen et al., 1996; Smoliar et al., 1996; Horan et al., 1998; –3.45 ± 0.25. Recently, Quitté and Birck (2004) reanalyzed Cook et al., 2004). There is a discrepancy, however, in the the W-isotopic compositions of two iron meteorites, Duel Re-Os systematics in the IVA magmatic irons. While Shen et Hill (IVA) and Tlacotepec (IVB), which were reported by al. (1996) report a Re-Os age for the IVA irons that is 60 ± Horan et al. (1998) to have among the lowest ε182W val- 45 m.y. older than for other iron meteorite groups, the data Wadhwa et al.: Timescales of Planetesimal Differentiation 725

of Smoliar et al. (1996) give an age of 4456 ± 25 Ma, signi- (Chen and Wasserburg, 1996; Chen et al., 2002), and indi- ficantly younger than other iron meteorite groups, which cates that closure of the Pd-Ag system occurred ~3.5 m.y. these authors attributed to later disturbance of the Re-Os after Gibeon and Canyon Diablo. The recently acquired system. Horan et al. (1998) subsequently reported the Re- high-precision Pd-Ag-isotopic data for the Canyon Diablo Os-isotopic compositions of several IVA irons and showed (IA) and Grant (IIIB) iron meteorites and the Brenham that while some lie on the 4456 ± 25 Ma isochron of Smo- pallasite have additionally demonstrated that ε-level differ- liar et al. (1996), others were consistent with an older age. ences in the initial Ag-isotopic compositions of these me- This suggests that the Re-Os-isotopic systematics in the IVA teorites can be resolved, and this has important implications are indeed disturbed, perhaps by processes such as breakup for their formation histories (Carlson and Hauri, 2001). The and reassembly of the parent planetesimal, which have been initial Ag-isotopic composition of Brenham indicates that invoked to explain the range of metallographic cooling rates during the 3.5-m.y. time interval between closure of its Pd- of the IVA irons (e.g., Haack et al., 1996). Ag system and that of Canyon Diablo, the parent planetesi- Rhenium-osmium ages reported so far use a 187Re decay mal of Brenham evolved with a nearly chondritic Pd/Ag constant that was determined by assuming that the IIIAB ratio, unlike the high Pd/Ag ratio characterizing the bulk isochron should give the same age as the U-Pb age of the composition of this pallasite. This may suggest that metal- angrites (Smoliar et al., 1996), so the accuracy of these ages silicate separation on the Brenham parent body did not is only as good as the validity of this assumption. Never- occur until shortly before its crystallization (Carlson and theless, the age range indicated by Re-Os isochrons is inde- Hauri, 2001). This result is distinct from that of the IIIB pendent of the half-life, so the results for various iron mete- iron Grant. Grant has a similar inferred initial 107Pd/108Pd as orite groups, with the exception of the IVA irons, suggest does Brenham, but a more radiogenic initial Ag-isotopic that core crystallization (or more specifically, Re-Os isoto- composition, which indicates that the high bulk Pd/Ag of pic closure) in planetesimals spanned a period of ~30 m.y. Grant was a characteristic of its source (Carlson and Hauri, (although the relatively large errors certainly allow this time 2001). With the exception of the disturbed Re-Os system- interval to be significantly narrower than this). Rhenium- atics in the IVA irons, the Re-Os and Pd-Ag ages provide osmium systematics in pallasites indicate that they may be a broadly similar temporal sequence of formation of the iron younger than iron meteorites by ~60 m.y.; however, this meteorites and pallasites. apparently young age may be due to later reequilibration Manganese-53–chromium-53 systematics in IIIAB mag- of the Re-Os system (Chen et al., 2002). Cook et al. (2004) matic irons and pallasites also indicate early fractionation recently reported the first high-precision 190Pt-186Os iso- and closure of the Mn-Cr system in these metal-rich mete- chrons for the IIAB and IIIAB magmatic irons, and esti- orites. The first evidence of live 53Mn in IIIAB irons was mated ages for these meteorite groups of 4323 ± 80 Ma provided by ion microprobe (SIMS) studies of phosphates and 4325 ± 26 Ma, respectively. These ages are somewhat that showed a wide range of 53Mn/55Mn ratios from ~1 × younger than Re-Os ages for iron meteorites, and these 10–6 to ~2 × 10–5 (Davis and Olsen, 1991; Hutcheon and authors suggested that this discrepancy could reflect an error Olsen, 1991; Hutcheon et al., 1992). Taken at face value, in the decay constant for 190Pt. this range of 53Mn/55Mn ratios translates to a time interval In contrast to the more leisurely pace of core crystalli- of ~16 m.y., which is inconsistent with the Pd-Ag system- zation suggested by Re-Os systematics (although as indi- atics that indicate that most of these meteorites formed con- cated above, the errors are rather large and could accommo- temporaneously. A more recent SIMS study of 53Mn-53Cr date a much narrower time interval for this process), evi- systematics in phosphates in several IIIAB iron meteorites dence for the former presence of live 107Pd in a variety of has demonstrated that the Mn-Cr ages based on inferred metal-rich meteorites, including irons and pallasites, indi- 53Mn/55Mn ratios are different for the different phosphate cates that this process occurred rapidly, well within the life- minerals, most likely due to the slow cooling rate of the time of this short-lived radionuclide. For most iron meteor- IIIAB iron meteorites combined with the difference in the ites, closure of the 107Pd-107Ag-isotopic system occurred diffusion behaviors of Mn and Cr in the different phosphates within a narrow time interval of only ~4 m.y. The highest (Sugiura and Hoshino, 2003); the apparently wide range inferred 107Pd/108Pd ratio of ~2.4 × 10–5 is in samples such of inferred 53Mn/55Mn ratios in the previous studies of as Gibeon (IVA) and Canyon Diablo (IA), for which well- IIIAB irons may be similarly explained. Sugiura and Ho- defined 107Pd-107Ag isochrons have been obtained (Chen shino (2003) also showed that one of the phosphates (i.e., and Wasserburg, 1996; Carlson and Hauri, 2001). Pallasites the Fe-Mn phosphate sarcopside) consistently recorded the have somewhat younger Pd-Ag ages, most of which indi- same 53Mn/55Mn ratio of ~3.4 × 10–6 in all the IIIAB iron cate isotopic closure ≤10 m.y. after Gibeon and Canyon meteorites they investigated. These authors reasoned that Diablo. Of the pallasites investigated so far, Brenham has the closure temperature of the Pd-Ag system in iron mete- the highest 107Pd/108Pd ratio of (1.65 ± 0.05) × 10–5, defined orites may be similar to that of the Mn-Cr system in by a four-point internal isochron comprised of silicates and sarcopside, and that the 107Pd/108Pd ratio of ~2 × 10–5 in metal (Carlson and Hauri, 2001). This more precisely de- these meteorites corresponds to a 53Mn/55Mn ratio of ~3.4 × fined value is only marginally higher than the 107Pd/108Pd 10–6. This would imply that, compared to the 53Mn/55Mn ratio of ~1.1 × 10–5 inferred from a single bulk data point ratio of 1.25 × 10–6 in the LEW angrite (Lugmair and Shu- 726 Meteorites and the Early Solar System II kolyukov, 1998), isotope closure of the Mn-Cr (and Pd-Ag) 4. SUMMARY AND CONCLUSIONS systems in the IIIAB irons occurred ~5 m.y. before angrite crystallization (i.e., at ~4563 Ma). Furthermore, it was sug- Figure 6 summarizes the timescales discussed here for gested by these authors that in pallasites, the 107Pd/108Pd the processes involved in planetesimal differentiation. Based ratio of ~1 × 10–5 (inferred from Glorieta Mountain and on the constraints available from long- and short-lived chro- Brenham pallasites) (Chen and Wasserburg, 1996; Carlson nometers, differentiation of planetesimals in the early his- and Hauri, 2001; Chen et al., 2002) corresponded to a tory of the solar system occurred very rapidly. Melting was 53Mn/55Mn ratio of ~1 × 10–6 (based on data for the Omolon initiated on some of the planetesimals, which were subse- pallasite) (Lugmair and Shukolyukov, 1998), implying broad quently arrested in this state and did not differentiate fully. consistency between Pd-Ag and Mn-Cr systematics (both reflecting a time interval of ~7 m.y.) between the iron me- teorites and pallasite. However, when all the available data for Pd-Ag and Mn- Cr systematics in various iron meteorites and pallasites are examined in detail, there are evident discrepancies between these two systems. For example, the Eagle Station pallasite has an inferred 53Mn/55Mn ratio of ~2.3 × 10–6 (Birck and Allègre, 1988), but there is no detectable evidence of live 107Pd in this meteorite (i.e., 107Pd/108Pd < ~7 × 10–6) (Chen and Wasserburg, 1996). Eagle Station is an anomalous pal- lasite not belonging to the “main group” (Mittlefehldt et al., 1998, and references therein) and, as discussed by Lugmair and Shukolyukov (1998), has a Cr-isotopic composition with an anomalously high 54Cr/52Cr ratio of ~1.5 ε. Therefore, this meteorite may contain an anomalous (most likely of presolar nucleosynthetic origin) Cr component that could complicate Mn-Cr systematics such that the 53Mn/55Mn Fig. 6. Timescales of planetesimal differentiation as inferred from ratio inferred from the measured 53Cr/52Cr ratios would not chronological investigations of differentiated meteorites (see text for details). The time of formation of the refractory calcium-alu- reflect the true value. Among the main-group pallasites, a minum-rich inclusions (the earliest solids to form in the protosolar 53 55 –5 Mn/ Mn ratio of (1.4 ± 0.4) × 10 has been inferred from nebula) at 4567 Ma is assumed to be coincident with the formation a SIMS study of Springwater (Hutcheon and Olsen, 1991); of the solar system (dotted-dashed line). Based on the ages of non- this value was since confirmed by another ion microprobe cumulate eucrites and angrites, it is inferred that crust formation on study (Hsu et al., 1997). However, based on thermal ion- planetesimals began within ~2 m.y. and continued until ~10 m.y. ization mass spectrometry (TIMS) analysis of a separated after CAI formation; Pb-Pb ages of cumulate eucrites may imply olivine fraction from Springwater, Lugmair and Shukol- that igneous activity continued on the howardite-eucrite-diogenite yukov (1998) inferred a 53Mn/55Mn ratio for this pallasite parent body (HED PB) for another ~100–150 m.y. On the HED more than an order of magnitude lower (~1 × 10–6). While PB, global silicate (mantle) differentiation occurred within ~2– TIMS studies of Eagle Station, Omolon, and Springwater 3 m.y. of CAI formation, and was preceded by core formation by (Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998) ~1 m.y. (indicated by the two crossed circles). Global silicate (mantle) differentiation in the parent bodies of the eucritic clasts have yielded 53Mn/55Mn ratios of ~1–2 × 10–6, the SIMS in Vaca Muerta and the aubrite parent body occurred ~2 m.y. after investigations of Albin, Brenham, and Springwater (Hutch- this same process in the HED PB. Metal segregation in the parent eon and Olsen, 1991; Hsu et al., 1997) all give values of bodies of magmatic iron meteorites and pallasites may have oc- ~1–4 × 10–5. The fact that analyses of olivine from the same curred over a period of ~5 m.y. (possibly less); assuming that the pallasite sample (i.e., Springwater) by bulk (TIMS) or micro- timing of this process was earliest in the HED PB (inferred to have analytical (SIMS) techniques yield such different inferred occurred only ~1–2 m.y. after CAI formation based on Hf-W sys- 53Mn/55Mn ratios could be indicative of redistribution of Mn tematics in the HEDs), this process probably lasted at most until and/or Cr within these olivines. If this is the case, then the ~4560 Ma. Pd-Ag and Mn-Cr systematics indicate that crystalli- bulk technique (which would “average” over any redistribu- zation of the metallic cores of the extensively differentiated par- tion of the Mn and/or Cr) may be more reliable in terms of ent bodies of the magmatic irons and pallasites is likely to have ≤ yielding the true 53Mn/55Mn ratio. As such, given the above occurred over a time interval of 10 m.y. (Re-Os systematics sug- gest that the duration of this process may have been longer, but discussion, the suggestion of Sugiura and Hoshino (2003) the large uncertainties in these ages allow for the short time in- that pallasites (at least those belonging to the “main group”) terval inferred from the Pd-Ag and Mn-Cr systematics). This time 53 55 –6 are characterized by a Mn/ Mn ratio of ~1 × 10 may interval is anchored to the oldest absolute Re-Os age for a mag- be a reasonable one. Nevertheless, some outstanding prob- matic iron meteorite, i.e., 4557 Ma (although, in fact, the uncer- lems with the consistency of the Mn-Cr and Pd-Ag ages in tainty of ±12 m.y. in this age is comparable to the time interval pallasites remain. over which this process is inferred to occur). Wadhwa et al.: Timescales of Planetesimal Differentiation 727

On some of these bodies, such incipient melting began as systems can indeed provide a robust chronology for early early as ~3–5 m.y. after the formation of the first solids in solar system events. the solar nebula (i.e., CAIs), but possibly extended to more The main challenge for future studies seeking to better than ~10 m.y. on others. Other planetesimals underwent clarify the reasons for discrepancies between various radio- more extensive melting and differentiation and acquired the genic isotope systematics will be to extend chronological layered structure comprised of a silicate crust and mantle investigations to a greater variety of meteoritic materials, and a metallic core. such that there are many more instances than there are at Crystallization ages of the basaltic angrites and noncu- present of multiple chronometers being applied to the same mulate eucrites provide the best means of constraining the objects. Such studies have been inhibited in the past by the timing of crust formation. Chronological evidence for these limited fractionation in meteoritic materials of various par- achondrites suggests that the earliest basalts were emplaced ent-daughter element pairs, such that it has been difficult to (and formed the crusts of planetesimals) within ~3 m.y. of apply multiple chronometers to a particular meteorite. These the formation of CAIs. Primary basalt formation is likely types of investigations should become increasingly feasible to have continued until ~10 m.y. after CAI formation. with future advancements in analytical capabilities that will Global silicate differentiation (i.e., the process that es- allow high-precision isotopic analyses on meteoritic com- tablished the mantle source reservoirs of the crustal mate- ponents having relatively low parent/daughter ratios. rials) on the parent planetesimals of the angrites and eucrites is likely to have occurred contemporaneously, ~2–3 m.y. Acknowledgments. We are grateful to Q. Yin and an anony- after CAI formation. However, this process occurred ~2 m.y. mous reviewer for their constructive and thorough reviews that later on the parent body of the HED-like clasts from the helped to improve the content and to K. McKeegan for the edito- Vaca Muerta mesosiderite and on the aubrite parent body. rial handling of this manuscript. 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