timescales of planetesimal differentiation in the early solar system

Wadhwa et al.: Timescales of Planetesimal Differentiation 715

715

Timescales of Planetesimal Differentiationin the Early Solar System

M. WadhwaThe Field Museum

G. SrinivasanUniversity of Toronto

R. W. CarlsonCarnegie Institution of Washington

Chronological investigations of a variety of meteoritic materials indicate that planetesimaldifferentiation occurred rapidly in the early solar system. Some planetesimals only experiencedthe earliest stage of the onset of melting and were subsequently arrested in that state. On someof 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 coreformation 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 of10 m.y. or less.

1. INTRODUCTION

Within the last decade, significant advances have beenmade in our understanding of the timescales involved in thedifferentiation history of planetesimals in the early solarsystem. In particular, the application of several chronom-eters based on short-lived radionuclides (i.e., with half-livesless than ~100 m.y.) has allowed unprecedented time res-olution (often ≤1 m.y. for events that occurred close to~4.6 b.y. ago).

The earliest differentiation events in solar system historywere those that occurred between nebular gas and the ear-liest condensates that are represented by components withinchondritic meteorites. Timescales involved in such events,which likely occurred during the accretion disk phase ofthe nebula, are discussed elsewhere in this book (Russellet al., 2006). In this chapter, we will focus on the timescalesinvolved in the differentiation events that occurred on plan-etesimals, defined here as small asteroidal-sized bodies (upto hundreds of kilometers in diameter) that accreted fromnebular dust and the earliest condensates. The processeslikely to be involved in planetesimal differentiation are dis-cussed in more detail in McCoy et al. (2006).

The meteoritic record includes a variety of materials re-sulting from planetesimal differentiation. These materialsare primarily composed of two types: (1) primitive achon-

drites that have near-chondritic bulk compositions (e.g.,brachinites, acapulcoites, lodranites, and winonaites) andformed during the earliest stages of melting on planetesi-mals (which were subsequently arrested at this stage anddid not differentiate further); and (2) meteorites that resultedfrom more extensive differentiation such that their bulkcompositions are significantly different from those of chon-drites. The latter are in turn comprised of three classes ofmaterials: (1) achondrites that formed within the silicatecrust of differentiated planetesimals, such as eucrites, diog-enites, and howardites (or HED meteorites), angrites, andHED-like silicate clasts in mesosiderites; (2) metal-silicatemeteorites (pallasites) that may have been formed near thecore-mantle boundary; and (3) metal-rich meteorites that arethe products of metal segregation from essentially chondriticprecursors and some of which may represent the cores ofextensively differentiated planetesimals.

Age determinations on these meteoritic materials withthe appropriate chronometers then make it possible to esti-mate the timing of planetesimal differentiation events rang-ing from incipient melting to more extensive processinginvolving silicate differentiation (including crust formationand fractionation within the mantle), metal segregation, andcore crystallization. Thus far, chronometers based on bothlong- and short-lived radionuclides have been applied ex-tensively for age-dating such events. Long-lived chronom-

716 Meteorites and the Early Solar System II

eters, however, typically do not have the time resolutionrequired to resolve events occurring within the first tens ofmillions of years of solar system history. In contrast, short-lived chronometers can have high time resolution (i.e., amillion years or less) and are thus capable of precisely agedating the earliest solar system events. Nevertheless, theapplication of chronometers based on short-lived radionu-clides is not without its challenges. In particular (and pro-vided there are no other complications such as heterogeneityin the initial distribution of the parent radionuclide and/orlater disturbance of the isotope systematics), isochrons basedon short-lived radionuclides can provide only relative agessince the slope of such an isochron reflects the abundanceof the now-extinct radionuclide at the time of isotopic clo-sure following a parent-daughter fractionation event. There-fore, since the high-resolution chronometers based on short-lived radionuclides can provide only relative time differ-ences between early solar system events, it is essential tohave a “time anchor” [i.e., a sample that allows one to de-termine the value of the parameter (R*/R)T at a preciselydefined absolute time T, where R* is the abundance of theradioisotope and R is that of a stable reference isotope ofthe same element] so as to map the relative ages derivedfrom such a chronometer onto an absolute timescale. Theonly absolute chronometer capable of providing time reso-lution comparable to that of chronometers based on short-lived radionuclides, and thus providing an appropriate timeanchor, is the U-Pb chronometer. A unique attribute of thischronometer is that it is based on the decay of two long-lived radioisotopes, i.e., 235U that decays to 207Pb with ahalf-life of 703.8 m.y., and 238U that decays to 206Pb witha half-life of 4469 m.y. The extraordinarily high precision

afforded by the U-Pb chronometer compared to chronom-eters based on other long-lived radioisotopes results fromthe rapid evolution of the radiogenic Pb isotopic composi-tion (i.e., 207Pb/206Pb) due to the relatively short half-lifeof 235U and the occasionally very high U/Pb ratios in plan-etesimals and planets affected by volatile loss and/or coreformation since Pb is both volatile and chalcophile, but Uis neither.

2. INITIAL CONDITIONS

Prior to a discussion of the timescales of planetesimaldifferentiation, we briefly summarize our current under-standing of the initial conditions prevalent in the solar neb-ula immediately prior to the formation of these planetesi-mals. More detailed discussions of nebular conditions andprocesses may be found elsewhere in this book (e.g., thechapters in Part III). We note that an understanding of con-ditions and processes in the solar nebula (especially the de-gree to which it was homogenized) is particularly importantwith regard to the application of the high-resolution chro-nometers based on short-lived radionuclides, since factorssuch as the abundance and distribution of the parent radio-nuclides are critical in determining the feasibility of age-dating events in the early solar system.

Table 1 provides a listing of selected short-lived radio-nuclides that have so far been applied toward determiningthe timescales involved in planetesimal differentiation. Thistable includes current estimates of the solar system initialabundance ratios (R*/R)0, which are estimates of the ini-tial abundances of these radionuclides in the meteorite-forming region of the solar nebula at the time of formation

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(R*/R)T = 5 × 10–5 at 4.567 Ga

60Fe 1.5 60Ni 56Fe ~3–10 × 10–7

53Mn 3.7 53Cr 55Mn ~10–5 LEW 86010 Angrite(R*/R)T = 1.25 × 10–6 at 4.558 Ga

107Pd 6.5 107Ag 108Pd ~5 × 10–5 Gibeon (IVA) Iron(R*/R)T = 2.4 × 10–5 at ~4.56 Ga

182Hf 9 182W 180Hf 1.0–1.6 × 10–4 CAIs and Chondrites(R*/R)T = 1 × 10–4 at ~4.56 Ga

146Sm 103 142Nd 144Sm ~7 × 10–3 Angrites(R*/R)T = 7 × 10–3 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 somerecent 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 hada 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).


of the earliest condensates, and the refractory CAIs foundin primitive chondrites, the most precise age estimate forwhich is currently 4567.2 ± 0.6 Ma (Amelin et al., 2002).

In Table 1, the solar system initial abundance ratios (R*/R)0 of the radionuclides with half-lives ≥10 m.y. (i.e., 146Smand 182Hf), but perhaps also some shorter-lived radionuclides(such as 107Pd and 53Mn), may be accounted for by con-tinuous galactic nucleosynthesis (e.g., Meyer and Clayton,2000). Radionuclides whose abundances may be accountedfor by this means are generally assumed to be homogene-ously distributed in the solar nebula.

The initial abundance ratios of several of the radionu-clides with half-lives ≤10 m.y. (e.g., 26Al and possibly also53Mn) are too high to be accounted for by continuous ga-lactic nucleosynthesis alone. Alternatives for the predomi-nant production sites of these shorter-lived radionuclides arethat either (1) they were synthesized in a stellar environ-ment and injected into the molecular cloud just prior to itscollapse and solar nebula formation (e.g., Cameron et al.,1995; Cameron, 2001; Gallino et al., 2004) or (2) they wereproduced by energetic particle irradiation within the nebuladuring an early active phase of the Sun (Shu et al., 1996;Gounelle et al., 2001; Leya et al., 2003a). Additionally, inthe specific case of 10Be (t1/2 ~ 1.5 m.y.), which is producedonly by spallation reactions and not by stellar nucleosyn-thesis, it is suggested that this radionuclide may have beenincorporated into the solar nebula by trapping of galacticcosmic rays in the molecular cloud as it collapsed (Deschet al., 2004). Whether the short-lived radioisotopes that maybe produced in any of the above scenarios were effectivelyhomogenized prior to the formation of solids in the nebulais presently unresolved. Numerical simulations of processesassociated with molecular cloud core collapse triggered bya nearby explosive stellar event indicate that spatial hetero-geneities in the distributions of shock-wave-injected short-lived radionuclides may survive on short timescales (Van-hala and Boss, 2002). Nevertheless, in the case of an exter-nal seeding source for these short-lived radionuclides, dif-ferential rotation (and possibly also turbulence) in the neb-ula is generally anticipated to result in radial and verticalmixing on the timescale of significantly less than a millionyears subsequent to initiation of collapse (which is assumedto occur immediately prior to formation of the first solidsin the nebula). If, however, the short-lived radionuclideswere predominantly produced by local irradiation within theearly solar system, they are expected to be distributed het-erogeneously within the nebular disk at least over the dura-tion of the active phase of the early Sun (e.g., Gounelle etal., 2001).

The application of a chronometer based on a short-livedradionuclide rests critically on the assumption that the ini-tial abundance ratio of the radioactive to the stable isotope,i.e., (R*/R)0, is uniform in the meteorite-forming region. Inthe specific case of determining the timing of planetesimaldifferentiation using such a chronometer, the scale at whichthe isotopic heterogeneity may be present is also important.Numerous studies have shown that large isotopic heteroge-

neities were present in the nebula on the small scale, i.e., onthe scale of micrometer-sized presolar grains and millime-ter- to centimeter-sized inclusions (CAIs) within primitivemeteorites (e.g., Loss et al., 1994; Zinner, 1996). However,there are only a few instances where isotopic heterogene-ities have been documented on the planetesimal scale. Spe-cifically, mass-independent variations in the isotopic com-positions of O (Clayton et al., 1973; Clayton, 1993) and Cr(Lugmair and Shukolyukov, 1998; Shukolyukov et al., 2003)have been documented in bulk samples of primitive and dif-ferentiated meteorites. Although disputed by some (Beckerand Walker, 2003a,b), such variations have also been notedfor Mo and Ru (Dauphas et al., 2002, 2004a; Yin et al.,2002a; Chen et al., 2003, 2004; Papanastassiou et al.,2004). By and large, however, most other isotopic systemsinvestigated so far in meteorites indicate homogeneity onthe planetesimal scale at the level of precision achievableby current state-of-the-art instrumentation (e.g., Zhu et al.,2001; Dauphas et al., 2004bc).

3. TIMESCALES OF PLANETESIMALDIFFERENTIATION

3.1. Incipient Melting

Primitive achondrites, such as acapulcoites, lodranites,winonaites, and brachinites, have igneous textures but theirbulk compositions, although showing some variations, arerelatively close to those of chondrites. Therefore, thesemeteorites are considered to be the products of the earlieststages of melting and igneous processing on planetesimals(Mittlefehldt et al., 1998, and references therein).

The acapulcoites and lodranites are thought to be theresidual products of partial melting of chondritic precur-sors (Mittlefehldt et al., 1996; McCoy et al., 1997a,b). Chro-nological constraints obtained so far indicate that these me-teorites formed within ~10 m.y. of the formation of the ear-liest solids in the nebula. Evidence for the presence of live244Pu (t1/2 ~ 82 m.y.) in Acapulco phosphates supports earlyformation of this meteorite (Pellas et al., 1997). Samari-um-147–neodymium-143 systematics determined by Prinz-hofer et al. (1992) for Acapulco gave a very old age (4.60 ±0.03 Ga), which the authors interpreted as the time of re-crystallization immediately following its formation event.Given this old 147Sm-143Nd age, one would expect a 146Sm/144Sm ratio significantly higher than in angrites at 4558 Ma(i.e., the time anchor for the 146Sm-142Nd system; Table 1).However, the 146Sm/144Sm ratio of 0.0067 ± 0.0019 deter-mined for this sample (Prinzhofer et al., 1992) is withinerror of that in angrites at their time of formation. More-over, U-Pb systematics in phosphates from Acapulco givean age of 4557 ± 2 Ma (Göpel et al., 1992, 1994), margin-ally younger, but much more precise, than the 147Sm-143Ndage. The older 147Sm-143Nd age is therefore questionableand could be due to disturbance during extensive later meta-morphism experienced by this meteorite (McCoy et al.,1996). Furthermore, Acapulco has an inferred 53Mn/55Mn


ratio of (7.5 ± 1.4) × 10–7; compared to the LEW 86010angrite (the time anchor for the 53Mn-53Cr system; Table 1);this translates to a 53Mn-53Cr age of 4555.1 ± 1.2 Ma (Zipfelet al., 1996), consistent with its Pb-Pb and 146Sm-142Nd sys-tematics. Iodine-129–xenon-129 (t1/2 ~ 17 m.y.) systematicsin Acapulco phosphates indicate Xe closure ~9 m.y. afterthe Bjurbole L4 chondrite, which also appears to be con-sistent with Pb-Pb systematics (Nichols et al., 1994; Brazzleet al., 1999).

Brachinites are olivine-rich primitive achondrites gener-ally considered to be either partial melt residues (e.g., Nehruet al., 1983, 1992, 1996; Goodrich, 1998) or igneous cu-mulates (e.g., Warren and Kallemeyn, 1989). Although pre-cise absolute formation ages for these primitive achondritesare not yet available, there are indications that they formedclose to ~4.56 Ga, soon after the beginning of the solarsystem. These include the presence of fission tracks fromthe decay of live 244Pu (Crozaz and Pellas, 1983) and 129Xeexcesses due to the decay of 129I (Bogard et al., 1983; Ottet al., 1987; Swindle et al., 1998). Wadhwa et al. (1998)reported a 53Mn-53Cr isochron for Brachina that indicateda 53Mn/55Mn ratio of (3.8 ± 0.4) × 10–6 at the time of lastequilibration of Cr isotopes (Fig. 1). Comparison of thisvalue with the 53Mn/55Mn ratio of (1.25 ± 0.07) × 10–6 inthe angrite LEW 86010 at 4558 Ma (Fig. 1; Table 1) im-plies a Mn-Cr age of 4563.7 ± 0.9 Ma for this meteorite,which is only 3.5 ± 1.1 m.y. after the time of CAI forma-tion (i.e., 4567.2 ± 0.6 Ma) (Amelin et al., 2002).

However, not all primitive achondrites investigated so farhave consistent long- and short-lived radioisotope systemat-ics. The Divnoe meteorite is an ultramafic primitive achon-drite whose relationship to other primitive achondrite groupsis as yet unclear (Petaev et al., 1994; Weigel et al., 1996).This meteorite is inferred to have an extremely high 146Sm/144Sm ratio of 0.0116 ± 0.0016 that in comparison to the

146Sm/144Sm ratio of 0.007 in angrite LEW 86010 (Table 1)provides an age of 4.633 ± 0.022 Ga, consistent with its old147Sm-143Nd age of 4.62 ± 0.07 Ga, although uncertaintiesare large (Bogdanovski and Jagoutz, 1996). These ages areproblematic since they are older than the age inferred forCAI formation, which is generally assumed to reflect thetime of formation of the first solids in the solar nebula.Manganese-53–chromium-53 systematics in this achondrite,on the other hand, indicate that Cr isotopes were equili-brated at ≤4542 Ma when the 53Mn/55Mn ratio was ≤6 × 10–8

(Bogdanovski et al., 1997).Despite some discrepancies, the above discussion shows

that the onset of melting on some planetesimals began asearly as ~3–5 m.y. after the formation of the first solids andpossibly extended to ≥10 m.y. on others. While the mostplausible heat source for early incipient melting on someplanetesimals is the decay of short-lived radionuclides suchas 26Al and 60Fe, such melting episodes at significantly latertimes on other planetesimals are likely to be the result ofimpact heating.

3.2. Crust Formation

Primary crystallization ages, ideally based on internalmineral isochrons obtained from individual members ofachondrite groups such as the angrites and noncumulateeucrites that represent basaltic rocks that formed in aster-oidal near-surface environments, offer the best means ofassessing the timescales of crust formation on planetesimals.

3.2.1. Angrites. Angrites are a small group of min-eralogically unique basalts composed mostly of Ca-Al-Ti-rich pyroxenes (fassaite), olivine, and anorthitic plagioclase(Mittlefehldt et al., 1998, and references therein). Early evi-dence for the presence of fission Xe (from the decay of244Pu; t1/2 ~ 82 m.y.) in Angra dos Reis, the type meteoriteof the angrites, established its antiquity (Hohenberg, 1970;Lugmair and Marti, 1977; Wasserburg et al., 1977). Sub-sequently, evidence for the former presence of 244Pu hasalso been reported in two other angrites (Eugster et al., 1991;Hohenberg et al., 1991). Samarium-147–neodymium-143systematics in Angra dos Reis (ADOR) and LEW 86010(LEW) are well behaved and give old crystallization agesbetween 4.53 ± 0.04 and 4.56 ± 0.04 Ga (Lugmair andMarti, 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 beendetermined 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 terrestrialweathering, are generally consistent with earlier results forADOR and LEW (Nyquist et al., 2003a; Tonui et al., 2003).Samarium-146–neodymium-142 systematics in ADOR andLEW (146Sm/144Sm ~0.007) are concordant with their 147Sm-143Nd systematics, although preliminary data for D’Orbignysuggest disturbance of the 146Sm-142Nd system (Tonui et al.,2003).

The most precise estimate of the crystallization age oftheangrites is offered by their Pb-Pb systematics. An internal

Fig. 1. Excesses in the 53Cr/52Cr ratio in parts per 104, or ε(53),relative to a terrestrial standard vs. 55Mn/52Cr ratios in the primitiveachondrite Brachina (CHR = chromite; TR = whole rock; SIL =silicates); data from Wadhwa et al. (1998). The dashed line is theisochron for the LEW 86010 angrite (Lugmair and Shukolyukov,1998).


Pb-Pb isochron defined by LEW minerals gives an age of4558.2 ± 3.4 Ma, concordant with the highly precise U-Pbmodel age of 4557.8 ± 0.5 Ma obtained from the extremelyradiogenic Pb compositions in the pyroxenes of ADOR andLEW (Lugmair and Galer, 1992). The highly precise Pb-Pbage of the LEW angrite has made it possible to use the for-mation time of this sample as a precise time anchor for theapplication of chronometers based on short-lived radionu-clides, particularly 53Mn (t1/2 ~ 3.7 m.y.) and 146Sm (t1/2 =103 m.y.) (Table 1). Preliminary U-Pb model ages derivedfrom D’Orbigny pyroxenes appeared to be in agreementwith those derived from ADOR and LEW pyroxenes (Ja-goutz et al., 2003). However, a recent reevaluation of thesedata by Jagoutz and colleagues has resulted in a somewhatolder age of 4563 ± 1 Ma for D’Orbigny (G. W. Lugmair,personal communication, 2005). Finally, Baker et al. (2005)have reported an even older Pb-Pb age for the Sahara 99555angrite of 4566.2 ± 0.1 Ma.

Evidence for the presence of live 53Mn at the time oftheir formation is found in ADOR and LEW. Nyquist et al.(1994) reported a 53Mn/55Mn ratio of (1.44 ± 0.07) × 10–6

for the LEW angrite. Subsequently, a study by Lugmair andShukolyukov (1998) gave a 53Mn/55Mn ratio of (1.25 ±0.07) × 10–6 for LEW (with the data point for ADOR olivinebeing consistent with this value). These values are in rea-sonable agreement, especially if the Cr-isotopic composi-tion of olivine reported by Nyquist et al. (1994) is correctedfor contribution from spallogenic Cr. No detectable evi-dence for the presence of live 26Al has been found in thesesamples (26Al/27Al < 2 × 10–7) (Lugmair and Galer, 1992).However, such a result is not unexpected given that, at thetime these angrites formed (4558 Ma), ~12 half-lives of 26Alhad elapsed since the time of CAI formation (4567 Ma)(Amelin et al., 2002) when the 26Al/27Al ratio was ~5 × 10–5.The 53Mn/55Mn ratio at the time of formation of D’Or-bigny and Sahara 99555 angrites (~3 × 10–6) (Nyquist etal., 2003a; Glavin et al., 2004) is significantly higher thanin ADOR and LEW, implying that D’Orbigny and Sahara99555 formed ~4–5 m.y. prior to ADOR and LEW. This isconsistent with the Pb-Pb systematics in D’Orbigny, whichindicate that this angrite is 5 ± 1 m.y. older than ADOR andLEW (Lugmair and Galer, 1992; G. W. Lugmair, personalcommunication, 2005). There is also evidence for the pres-ence of live 26Al in the D’Orbigny and Sahara 99555 an-grites (26Al/27Al ~5 × 10–7) (Spivak-Birndorf et al., 2005),which is consistent with the higher 53Mn/55Mn ratio and theolder Pb-Pb age of D’Orbigny compared to ADOR andLEW. However, the Pb-Pb age of Sahara 99555 (Baker etal., 2005) is not concordant with the ages based on 26Al-26Mg and 53Mn-53Cr internal isochrons (Spivak-Birndorf etal., 2005). Finally, high-precision Mg-isotopic analyses ofbulk samples of the Sahara 99555 and NWA 1296 angritesshow small but resolvable excesses in 26Mg, which may beindicative of early Al/Mg fractionation in their source re-sulting from crust formation on their parent body while 26Alwas still extant (Bizzarro et al., 2005).

3.2.2. Eucrites. Like the angrites, the noncumulate eu-crites are pyroxene-plagioclase rocks. However, there are

significantly greater numbers of known noncumulate eu-crites than there are angrites. Recent high-precision O-iso-topic data of Wiechert et al. (2004) demonstrate that mostnoncumulate eucrites along with the cumulate eucrites, diog-enites, and howardites have uniform ∆17O (within ± 0.02‰)and thus lie on a single mass fractionation consistent withtheir origin on a single parent body. Therefore, these basaltsare the most numerous crustal rocks available from any sin-gle solar system body other than Earth and the Moon. Ahandful of the noncumulate eucrites (in particular Ibitira, butpossibly also Caldera, Pasamonte, and ALHA 78132) haveO-isotopic compositions distinct from the others, implyingeither that these samples originated on different parent bod-ies or that isotopic heterogeneity was preserved on the HEDparent body (Wiechert et al., 2004).

Unlike angrites (which did not undergo any significantdegree of recrystallization or metamorphism), the noncumu-late eucrites appear to record a protracted history of exten-sive thermal processing on their parent body subsequent totheir original crystallization. As a result, many of the long-and short-lived chronometers investigated in these samplesappear to record secondary thermal events rather than theiroriginal crystallization. Nevertheless, there are several linesof evidence that suggest that these basalts crystallized veryearly in the history of the solar system. Although typicallycharacterized by large uncertainties, the 147Sm-143Nd agesof several of these samples such as Chervony Kut (4580 ±30 Ma) (Wadhwa and Lugmair, 1995), Juvinas (4560 ±80 Ma) (Lugmair, 1974), Pasamonte (4580 ± 120 Ma) (Un-ruh et al., 1977), Piplia Kalan (4570 ± 23 Ma) (Kumar etal., 1999), and Yamato 792510 (4570 ± 90 Ma) (Nyquist etal., 1997a) are close to ~4.56 Ga. In some cases where147Sm-143Nd systematics appear to record ages younger than~4.56 Ga, the 146Sm-142Nd systematics still provide evi-dence for early crystallization of basaltic eucrites such asCaldera (146Sm/144Sm = 0.0073 ± 0.0011) (Wadhwa andLugmair, 1996) and Ibitira (146Sm/144Sm ~ 0.009 ± 0.001)(Prinzhofer et al., 1992). This may be explained by a modelproposed by Prinzhofer et al. (1992), according to whichthe apparent discrepancy between the long-lived 147Sm-143Nd and the short-lived 146Sm-142Nd systems can be in-terpreted in terms of a short episodic disturbance resultingin partial reequilibration of the rare earth elements (REEs),predominantly between plagioclase (which has very lowREE abundances) and phosphates (which are the primaryREE carriers). As shown by the modeling results of theseauthors, such a disturbance could partially reset the 147Sm-144Nd isochron, although the 146Sm-142Nd system would notbe resolvably affected.

There is at least one instance (i.e., the basaltic eucriteEET 90020) where the 147Sm-143Nd age (4430 ± 30 Ma) and146Sm-142Nd systematics (146Sm/144Sm = 0.0048 ± 0.0020)are indeed concordant, although within rather large uncer-tainties. As will be discussed below, both the 147Sm-144Ndage and initial 146Sm/144Sm ratio for EET 90020 are simi-lar to values obtained for the Moore County and Moamacumulate eucrites (Jacobsen and Wasserburg, 1984; Tera etal., 1997). The young 147,146Sm-143,142Nd age for this non-


cumulate eucrite has been interpreted to possibly record latemagmatism on the parent body (Nyquist et al., 1997b).Therefore, EET 90020 could be the crystallization productof a younger melting event that may also be responsible forthe 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 ageof ~4.48–4.49 Ga for EET 90020 (Bogard and Garrison,1997). Therefore, it is possible that this basalt underwent acomplex petrogenetic history as suggested by Yamaguchi etal. (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).The most precise of the absolute chronometers, i.e., the U-Pb system, appears to have been affected by postcrystalli-zation events and terrestrial Pb contamination in most non-cumulate eucrites, recording mineral isochron ages in therange 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 radiogenicPb-isotopic compositions gave old Pb-Pb model ages of4556 ± 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 881394eucrite 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 ofCAI formation (4567.2 ± 0.6 Ma) (Amelin et al., 2002) andindicates that crust formation on differentiated planetesimalsoccurred within ~2 m.y. of the formation of the first solidsin the early solar system.

In recent years there has been increasing evidence forthe presence of various short-lived radionuclides (particu-larly 26Al, 53Mn, and 60Fe) in the noncumulate eucrites attheir time of crystallization, which further supports the earlyformation of these crustal basalts. The first evidence for live26Al in an achondrite was reported by Srinivasan et al.(1999) (Fig. 2). These authors demonstrated the presenceof excess 26Mg from the decay of 26Al in plagioclase of thePiplia Kalan basaltic eucrite, from which they inferred a26Al/27Al ratio of (7.5 ± 0.9) × 10–7 in this sample at the timeof its formation. However, this value is more likely to be alower limit on the actual 26Al/27Al ratio at the time of crys-tallization of this eucrite, since the plagioclase appears tohave undergone intragrain equilibration (as is evident fromthe near-constant 26Mg excesses determined in various spotanalyses of plagioclase with different Al/Mg ratios; Fig. 2).Compared to the 26Al/27Al ratio of ~5 × 10–5 in CAIs, the26Al/27Al ratio inferred for Piplia Kalan suggests that crustformation on the parent planetesimal of this eucrite occurredat most ~5 m.y. after the beginning of the solar system. Sub-sequently, evidence for the presence of live 26Al has beenfound in additional basaltic eucrites (26Al/27Al ratios in therange of ~6 × 10–7 to ~2 × 10–6) (Srinivasan, 2002; Nyquistet al., 2003b; Wadhwa et al., 2005). Of these, the 26Al/27Al

ratio is most precisely determined in the Asuka 881394 eu-crite, i.e., (1.34 ± 0.04) × 10–6 (Fig. 3), and translates to anAl-Mg age of 4563.5 ± 0.9 Ma, slightly younger than itsancient Pb-Pb mineral isochron age, possibly indicative ofslow subsolidus cooling of this eucrite (Wadhwa et al., 2005;Y. Amelin, personal communication, 2005). Additionally,Bizzarro et al. (2005) have recently reported high-precision

Fig. 2. Excesses in the 26Mg/24Mg ratio vs. 27Al/24Mg ratios inthe noncumulate eucrite Piplia Kalan (Px = pyroxene; Pl = plagio-clase). Figure from Srinivasan et al. (1999).

Fig. 3. Excesses in the 26Mg/24Mg ratio in per mil, or ∆26Mg,relative to a terrestrial standard plotted vs. 27Al/24Mg ratios in theAsuka 881394 eucrite. Solid circles show the data of Wadhwa etal. (2005) and the solid line is the isochron defined by these data,the slope of which corresponds to a 26Al/27Al ratio of (1.34 ±0.04) × 10–6; low Al/Mg data points near the origin are for py-roxene and whole rock, and high Al/Mg data points are for pla-gioclase. Solid squares are the data of Nyquist et al. (2003b) andthe dashed line is the isochron defined by these data, the slope ofwhich corresponds to a 26Al/27Al ratio of (1.18 ± 0.14) × 10–6;low Al/Mg data points near the origin are for pyroxene, and highAl/Mg data points are for plagioclase.


Mg-isotopic analyses of bulk samples of several noncumu-late eucrites, which show that these samples have small ex-cesses in 26Mg, possibly indicative of early Al/Mg fractiona-tion in their source resulting from basaltic volcanism andcrust formation on their parent body while 26Al was extant.

Evidence for the presence of live 53Mn at the time ofeucrite formation has been documented in several noncumu-late eucrites including Chervony Kut, Juvinas, Ibitira (Lug-mair and Shukolyukov, 1998), and Asuka 881394 (Nyquistet al., 2003b; Wadhwa et al., 2005). By comparison withthe LEW angrite (Table 1), Mn-Cr ages for these eucriteswere determined to be 4563.6 ± 0.9 Ma, 4562.5 ± 1.0 Ma,4557 +2/–4 Ma, and 4563.8 ± 0.7 Ma, respectively (Lug-mair and Shukolyukov, 1998; Wadhwa et al., 2005). Theseages indicate that crust formation on the parent planetesimalof these basalts began possibly within ~3 m.y., and may havecontinued up to ~10 m.y., after the formation of the firstsolids (additionally, the Mn-Cr age for the Asuka 881394eucrite is consistent with its Al-Mg age). Several other ba-saltic eucrites in which Mn-Cr systematics have been in-vestigated do not show any evidence for live 53Mn (Lugmairand Shukolyukov, 1998; Nyquist et al., 1996, 1997a,b). Inthese cases, it is likely that secondary thermal events haveresulted in later resetting of the Mn-Cr system, as appearsto be the case for the U-Pb system in most noncumulateeucrites as well. Finally, there is unambiguous evidence forthe former presence of live 60Fe in three noncumulate eu-crites, Chervony Kut, Juvinas, and Bouvante (Shukolyukovand Lugmair, 1993a,b; Quitté et al., 2005), also attestingto early formation of these basalts.

Basaltic noncumulate eucrites thus show clear evidenceof having formed close to ~4.56 Ga in the crust of theirparent planetesimal. In contrast, cumulate eucrites, whichformed in the crust of the same parent planetesimal as thenoncumulate eucrites (Clayton and Mayeda, 1996; Wiechertet al., 2004), have significantly younger concordant Sm-Ndand Pb-Pb ages, ranging from the oldest of 4456 ± 25 Ma(Sm-Nd) and 4484 ± 19 Ma (Pb-Pb) for Moore County (Teraet al., 1997) to the youngest of 4410 ± 20 Ma (Sm-Nd)(Lugmair et al., 1977) and 4399 ± 35 Ma (Pb-Pb) (Teraet al., 1997) for Serra de Magé. Thus, Sm-Nd and Pb-Pbsystematics in the cumulate eucrites indicate that isotopicclosure occurred up to ~150 m.y. after the noncumulateeucrites (Lugmair et al., 1977; Jacobsen and Wasserburg,1984; Lugmair et al., 1991; Tera et al., 1997), possibly im-plying that active magmatism persisted on the eucrite par-ent body for this extended period (Tera et al., 1997). Thismay be supported by the 4430 ± 30 Ma 147Sm-143Nd age ofthe noncumulate eucrite EET 90020 (Nyquist et al., 1997b)and the overlapping, relatively low, initial 146Sm/144Sm ra-tios of Moore County, Moama, and EET 90020 (Jacobsenand Wasserburg, 1984; Nyquist et al., 1997b; Tera et al.,1997). If this is so, it further implies that the process of crustformation on the eucrite parent body possibly extendedto ~150 m.y. after solar system formation. Since, as dis-cussed above, the oldest noncumulate eucrites formedwithin ~3 m.y. of CAI formation, the decay of short-livedradionuclides such as 26Al and 60Fe is the likely heat source

for the early and extensive differentiation experienced bytheir parent planetesimal. Energy sources that can accountfor later igneous activity (i.e., tens of millions of years afterCAI formation) on small planetesimals are not obviousunless the cumulate eucrites are the crystallization productsof impact melting on the eucrite parent body. Alternatively(and perhaps more likely), since the cumulate eucrites areslowly cooled rocks that possibly formed deeper within thecrust of their parent body than noncumulate eucrites, thelong-lived chronometers could be recording the long cool-ing times required to achieve subsolidus temperatures. Thisis supported by the modeling results of Ghosh and Mc-Sween (1998), which show that, assuming reasonable pa-rameters, it is possible to maintain temperatures in excess ofthe solidus temperature for basalt at a depth of ~100 km forover ~100 m.y. in a Vesta-sized planetesimal.

3.2.3. Other achondrites. Besides the angrites and theeucrites, there are other types of achondrites, such as au-brites and ureilites, whose origins are somewhat enigmaticbut that were nevertheless formed in the crusts of exten-sively differentiated asteroidal bodies. There are very fewchronological investigations of the aubrites, which are es-sentially monomineralic rocks composed of coarse-grainedenstatite. Brazzle et al. (1999) estimated an I-Xe age of4566 ± 2 Ma for the Shallowater aubrite, using the I-Xe andPb-Pb systematics in phosphate of the Acapulco primitiveachondrite as an anchor. More recently, however, based oncorrelations of relative I-Xe ages with absolute Pb-Pb agesof various meteorites and their components, it has beensuggested that enstatite in the Shallowater aubrite crystal-lized at 4563.5 ± 1.0 Ma (Pravdivtseva et al., 2005). Earlyformation (i.e., within a few million years of CAIs) of theaubrites is further supported by evidence for live 53Mn inPeña Blanca Spring (Shukolyukov and Lugmair, 2004).

Uranium-thorium-lead and Sm-Nd isotope systematicsin the ureilites generally indicate early formation, althoughthere are apparent complications resulting from later distur-bance, i.e., during a metasomatic event on the ureilite parentbody and/or by recent terrestrial contamination (Goodrichand Lugmair, 1995; Torigoye-Kita et al., 1995a,b). Morerecent studies have demonstrated evidence for live 53Mn and26Al at the time of ureilite formation. Specifically, it has beenshown that a feldspathic clast from the Dar al Gani 165 ureil-ite had a 53Mn/55Mn ratio of (2.91 ± 0.16) × 10–6 at the timeof last equilibration of Cr isotopes (Goodrich et al., 2002).Compared to the LEW angrite (Table 1), a Mn-Cr age of4562.3 ± 0.6 Ma is determined for this sample. Kita et al.(2003) have reported Al-Mg data for a plagioclase-bearingclast from the Dar al Gani 319 ureilite that indicate a 26Al/27Al ratio of (3.95 ± 0.59) × 10–7 at the time of its crystal-lization. Comparison with the canonical value at the timeof CAI formation (i.e., 4567.2 ± 0.6 Ma) (Amelin et al.,2002) gives an Al-Mg age of 4562.2 ± 0.6 Ma, which agreeswell with the Mn-Cr age discussed above. Finally, a Hf-W-isotopic investigation of bulk samples of several ureiliteshas yielded subchondritic values for ε182W (i.e., the 182W/184W or 182W/183W ratio relative to the terrestrial standard inparts per 104) and Hf/W ratios, indicating that the differen-


tiation process that resulted in the formation of these ureil-ites occurred while 182Hf was still extant (Lee et al., 2005).

3.3. Global Silicate (Mantle) Differentiation

3.3.1. Whole-rock isochrons. While an internal mineralisochron can provide constraints on the timing of formationof an individual achondrite in the crust of a planetesimal, awhole-rock isochron (based on a long- or a short-lived chro-nometer) of a particular achondrite group can provide limitson the timing of parent-daughter element fractionation inthe source (mantle) reservoir of their parent planetesimal,which in turn is a reflection of the timescale involved in glo-bal silicate fractionation (possibly associated with crystalli-zation of a magma ocean). Whole-rock Rb-Sr isochrons forthe basaltic eucrites established early on that Rb-Sr fraction-ation in the mantle source reservoir of these achondrites oc-curred close to ~4.6 Ga (Papanastassiou and Wasserburg,1969; Birck and Allègre, 1978). Smoliar (1993) evaluated allavailable Rb-Sr data for the eucrites and obtained a whole-rock isochron age of 4.55 ± 0.06 Ga for the noncumulateeucrites.

Achondrite whole-rock isochrons based on short-livedchronometers have the potential for more precisely con-straining the timing of global silicate differentiation on plan-etesimals. Such isochrons have recently been reported for the53Mn-53Cr and the 182Hf-182W systems in the HED groupof achondrites (Lugmair and Shukolyukov, 1998; Quitté etal., 2000; Kleine et al., 2004). The HED whole-rock 53Mn-53Cr isochron corresponds to a 53Mn/55Mn ratio of (4.7 ±0.5) × 10–6 (Fig. 4) (Lugmair and Shukolyukov, 1998). Com-

parison with the LEW angrite (Table 1) gives a Mn-Cr ageof 4564.8 ± 0.9 Ma for global silicate fractionation on theHED parent body, which is 2.4 ± 1.1 m.y. after the formationof CAIs (Amelin et al., 2002). This age is, within error, simi-lar to the Al-Mg and Mn-Cr ages of eucrite Asuka 881394(Nyquist et al., 2003b; Wadhwa et al., 2005) and the Mn-Cr age of eucrite Chervony Kut (Lugmair and Shukolyukov,1998). The near agreement of the ages provided by thewhole-rock isochrons and internal isochrons of these eu-crites indicates not only that planetesimal differentiationstarted soon after solar system formation, but that the dura-tion of the initial HED crust-mantle differentiation may havebeen as short as ~2 m.y. or less.

Trinquier et al. (2005a,b) have recently suggested thatHED meteorites have a deficit in the 54Cr/52Cr ratio of 0.72 ±0.02 ε compared to the terrestrial standard. This is in contrastto previous work on Cr-isotopic compositions of achondrites,including eucrites, which indicates that the 54Cr/52Cr ratioin these samples is the same as the terrestrial value to within±0.25 ε (Lugmair and Shukolyukov, 1998; Wadhwa et al.,2003). A potentially anomalous 54Cr/52Cr ratio in bulk eu-crites is relevant here since many previous Cr-isotopic stud-ies use a second-order fractionation correction that assumesa normal (i.e., terrestrial) 54Cr/52Cr ratio. The reason for thisapparent discrepancy is not evident at this time. Neverthe-less, in the case of eucrites and other achondrites that haveundergone complete equilibration of Cr-isotopic composi-tion between their constituent minerals because they crys-tallized from a melt, a second-order correction such as usedby Lugmair and Shukolyukov (1998) will not alter the slopeof the 53Mn-53Cr isochron or the relative age based on thisslope, 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 dataof Quitté et al. (2000) were combined with additional databy 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 isno consensus on the initial 182Hf/180Hf ratio of the solar sys-tem, with proposed values ranging from ~1 × 10–4 (Kleineet al., 2002, 2005; Yin et al., 2002b) to ~1.6 × 10–4 (Quittéand Birck, 2004). An initial 182Hf/180Hf ratio of ~1 × 10–4

would imply that global Hf-W fractionation in the HEDmantle source occurred ~4 m.y. after the beginning of thesolar system, which is consistent with the HED whole-rock53Mn-53Cr systematics discussed above. However, if an ini-tial 182Hf/180Hf ratio of ~1.6 × 10–4 is assumed, then Hf-Wfractionation in the mantle of the HED parent body occurred~9 m.y. after the beginning of the solar system. This is in-consistent not only with the HED whole-rock 53Mn-53Crsystematics but also with the recent evidence for live 26Al inseveral eucrites (Srinivasan et al., 1999; Srinivasan, 2002;Nyquist et al., 2003b; Wadhwa et al., 2005). In this regard,it is worth noting that the initial 182Hf/180Hf ratio of 1.6 ×10–4 is based on the reanalysis of the W-isotopic composi-tion of the Tlacotepec (IVB) iron meteorite, which has an

Fig. 4. 53Mn-53Cr systematics in the HED parent body. Datapoints are whole rocks of noncumulate eucrites (CAL = Caldera;CK = Chervony Kut; IB = Ibitira; JUV = Juvinas; POM = Pomoz-dino), cumulate eucrites (MC = Moore County; SM = Serra deMagé), and diogenites (JT = Johnstown; SHA = Shalka). ε(53) isas defined in the caption for Fig. 1. The slope of the HED whole-rock isochron corresponds to a 53Mn/55Mn ratio of (4.7 ± 0.5) ×10–6 at the time of global silicate differentiation. Figure from Lug-mair and Shukolyukov (1998).


extremely low ε182W value of –4.5 (Quitté and Birck, 2004);similarly low ε182W values (i.e., lower than ~–3.5) for sev-eral iron meteorites have recently been called into question(Yin and Jacobsen, 2003; Scherstén et al., 2004). However,the proposed initial 182Hf/180Hf ratio of 1 × 10–4 is basedon the 182Hf-182W systematics in bulk chondrites that have

undergone different histories and on Allende CAIs that havemay 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-rock53Mn-53Cr isochron for eucritic and diogenitic clasts fromthe Vaca Muerta mesosiderite that corresponded to a 53Mn/55Mn ratio of (3.3 ± 0.6) × 10–6. These authors concludedthat the lower slope of the isochron defined by these clastsimplies 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 lastglobal Mn/Cr fractionation on their parent body occurredat ~4563 Ma (Shukolyukov and Lugmair, 2004).

3.3.2. Initial strontium-isotopic composition. The antiq-uity of the highly volatile-depleted parent planetesimals ofthe angrites and the eucrites can also be inferred from theirinitial 87Sr/86Sr ratios. Assuming that the initial 87Sr/86Srratio at the beginning of the solar system is represented bythe average initial 87Sr/86Sr ratio measured in Allende CAIs(Gray et al., 1973; Podosek et al., 1991), and thatsubsequentevolution of radiogenic Sr occurred in an environment withsolar 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 depletionevent that established the angrite source characteristics.

The very low initial 87Sr/86Sr ratios of the eucrites simi-larly indicate that the volatile depletion characterizing theeucrite parent body may have occurred early in the historyof the solar system (Carlson and Lugmair, 2000, and ref-erences therein). A reevaluation of the Sr-isotopic data forthe eucrites by Smoliar (1993) shows that whole-rock Rb-Sr isochrons for the noncumulate and the cumulate eucritesdefine slightly, but resolvably, different initial 87Sr/86Sr ra-tios [which are both distinctly lower than the eucrite initial,BABI, previously defined by Papanastassiou and Wasser-burg (1969)]. In fact, the initial 87Sr/86Sr ratio for the cumu-late eucrites proposed by Smoliar (1993) is, within errors,similar to that for the angrites (e.g., Lugmair and Galer,1992), suggesting that the volatile depletion in their sourceswas established at similar times (possibly ~4 m.y. after CAIformation; see above). This timescale for the fractionationevent that established the low Rb/Sr source characteristicsof the cumulate eucrites is essentially consistent with thetiming of global mantle differentiation defined by the HEDwhole-rock 53Mn-53Cr and 182Hf-182W isochrons. However,the slighter higher initial 87Sr/86Sr ratio defined by the non-cumulate eucrites is potentially problematic since the sim-plest interpretation would be that their source evolved witha chondritic Rb/Sr ratio for ~4 m.y. longer (and is thusyounger) than that of the cumulate eucrites, further implyingthat these two types of eucrites originated on distinct par-

Fig. 5. 182Hf-182W systematics in the HED parent body. εW isthe 182W/184W relative to a terrestrial standard in parts per 104.(a) Data points are whole rocks of several noncumulate eucrites(Ber = Bereba; Bouv = Bouvante; Juv = Juvinas; Jon = Jonzac;Mil = Millbillillie; Pas = Pasamonte; Sta = Stannern) and a cu-mulate eucrite (SM = Serra de Magé). Solid symbols are data fromKleine et al. (2005); open symbols: Quitté et al. (2000); open dia-mond: Yin et al. (2002b). The slope of the HED whole-rock iso-chron corresponds to a 182Hf/180Hf ratio of (7.25 ± 0.50) × 10–5

at the time of global silicate differentiation. (b) Comparison of theslope of the eucrite whole-rock isochron from (a) with that of thebulk mantle-chondrite model isochron suggests that core forma-tion on the HED parent body occurred 0.9 ± 0.3 m.y. before glo-bal silicate differentiation. The W-isotopic composition of theeucrite bulk mantle is estimated from the whole-rock isochron,using a 180Hf/184W ratio of ~40. Figure from Kleine et al. (2004).


ent bodies (Smoliar, 1993). This is inconsistent with recenthigh-precision O-isotopic data (Wiechert et al., 2004) thatsuggest that the cumulate and noncumulate eucrites (withthe possible exception of Ibitira) originated on a commonparent planetesimal. As discussed by Carlson and Lugmair(2000), a possible explanation could be that the severe vola-tile depletion on the eucrite parent body did not occur in asingle-step process, but rather took place over the courseof its accretionary and early differentiation history. Subse-quently, the process of magma ocean crystallization mayhave resulted in a slighter higher Rb/Sr ratio in the sourceof the noncumulate eucrites compared to that of the cumu-late eucrites, thereby resulting in the higher initial 87Sr/86Srratio indicated by the whole-rock isochron for the noncumu-late eucrites.

3.4. Metal Segregation and Core Crystallization

Limits on the timescales involved in metal segregationand core crystallization on planetesimals may be obtainedfrom chronological investigations of iron-rich meteorites,such as magmatic irons (which represent the metallic coresof differentiated asteroidal bodies) and pallasites (consid-ered to have formed near the core-mantle boundary). De-pending on the geochemical affinities of the parent anddaughter elements, different chronometers applied to suchmeteorite types can provide constraints on either the pro-cess of metal segregation (e.g., the 182Hf-182W chronometer,where Hf is lithophile and W is siderophile such that ma-jor fractionation occurs during separation of the metallicmelt from silicates) or of core crystallization (e.g., 187Re-187Os, 107Pd-107Ag, and 53Mn-53Cr, where the parent-daugh-ter elements are fractionated during crystallization of Fe-Niphases, sulfides, and phosphates from the metallic melt).

3.4.1. Metal segregation. The W-isotopic compositionsof a variety of magmatic and nonmagmatic iron meteoriteshave been reported within the last decade (e.g., Lee andHalliday, 1995, 1996; Horan et al., 1998) and have thelowest 182W/184W ratios of any solar system material, withε182W values ranging from ~–3 to –5 (with most valuesbetween –3.5 and –4.1). As emphasized by Horan et al.(1998), these ε182W values suggest that metal segregation ondifferent planetesimals occurred within a period of ~5 m.y.in the early history of the solar system. As mentioned ear-lier, however, the lower end of the range of ε182W valuesin the iron meteorites have been called into question, sincesuch values have the improbable implication that some ironmeteorites predate CAIs and chondrites (e.g., Kleine et al.,2005). Yin and Jacobsen (2003) have measured the W-iso-topic compositions of several iron meteorites, includingsome of those previously analyzed by Lee and Halliday(1996) and Horan et al. (1998), and reported that none hadε182W values lower than the chondritic initial value of–3.45 ± 0.25. Recently, Quitté and Birck (2004) reanalyzedthe W-isotopic compositions of two iron meteorites, DuelHill (IVA) and Tlacotepec (IVB), which were reported byHoran et al. (1998) to have among the lowest ε182W val-

ues. These authors reported an ε182W value of –3.5 ± 0.3for Duel Hill, significantly higher than that previously re-ported for this meteorite by Horan et al. (1998) (i.e., –5.1 ±1.1). However, for Tlacotepec, they obtained an ε182W valueof –4.4 ± 0.3, which agrees well with the previous value(–4.5 ± 0.4) (Horan et al., 1998). Therefore, it appears thatthe W-isotopic composition of the iron meteorites rangesdown to at least this value. However, only 4 out of 28 ironmeteorites measured by Lee and Halliday (1996) and Horanet al. (1998) have ε182W values below the chondritic initialvalue, outside of uncertainty, and the mean ε182W value forall these samples is –3.8 ± 0.5 (1σ). Therefore, the resultsfrom these studies are consistent with most iron meteoriteshaving ε182W values that are not resolvably below the ini-tial chondritic value. This is further supported by severalrecent investigations that have remeasured the W-isotopiccompositions of iron meteorites for which previous workhad indicated ε182W values lower than –3.5 (e.g., Yin andJacobsen, 2003; Scherstén et al., 2004). As to the ε182Wvalues of some few iron meteorites lying resolvably andreproducibly below the initial chondritic value, one possi-bility is that the W-isotopic compositions of the components(mainly metal and silicates) of the chondrites analyzed sofar, and used to defined the initial chondritic ε182W value,may have been equilibrated to some degree. Another pos-sibility is that exposure to cosmic radiation could result inspallation and neutron capture reactions on W isotopes thatcould result in apparently lower ε182W values in some ironswith long exposure ages (Markowski et al., 2005; Leya etal., 2000, 2003a,b; Masarik, 1997). These possibilities haveimplications for the initial 182Hf/180Hf ratio of the solar sys-tem, but remain to be rigorously evaluated.

The timing of metal segregation (core formation) on theHED parent body has been estimated from 182Hf-182W sys-tematics in bulk samples of eucrites and diogenites (Kleine etal., 2004; Quitté and Birck, 2004). Assuming that the HEDparent body had a bulk chondritic Hf/W composition andthat core formation resulted in a 180Hf/184W ratio in theHEDmantle of ~40 (i.e., close to the highest Hf/W ratio meas-ured in eucrites that are the products of partial melting ofthis mantle), Kleine et al. (2004) estimated that core forma-tion on the HED parent planetesimal occurred 0.9 ± 0.3 m.y.prior to global mantle (silicate) differentiation (Fig 5b).

3.4.2. Core crystallization. Once the metal has segre-gated into the core of a planetesimal, the timescales in-volved in the crystallization of this metal may be constrainedby isochrons based on bulk samples and mineral phases ofmagmatic iron meteorites and pallasites. In recent years, pre-cise Re-Os isochrons have been obtained for various groupsof the iron meteorites. Most iron meteorite groups give rela-tively old Re-Os ages that range from 4557 ± 12 Ma (IIIABmagmatic irons) to 4526 ± 27 Ma (IVB magmatic irons)(Shen et al., 1996; Smoliar et al., 1996; Horan et al., 1998;Cook et al., 2004). There is a discrepancy, however, in theRe-Os systematics in the IVA magmatic irons. While Shen etal. (1996) report a Re-Os age for the IVA irons that is 60 ±45 m.y. older than for other iron meteorite groups, the data


of Smoliar et al. (1996) give an age of 4456 ± 25 Ma, signi-ficantly younger than other iron meteorite groups, whichthese authors attributed to later disturbance of the Re-Ossystem. Horan et al. (1998) subsequently reported the Re-Os-isotopic compositions of several IVA irons and showedthat while some lie on the 4456 ± 25 Ma isochron of Smo-liar et al. (1996), others were consistent with an older age.This suggests that the Re-Os-isotopic systematics in the IVAare indeed disturbed, perhaps by processes such as breakupand reassembly of the parent planetesimal, which have beeninvoked to explain the range of metallographic cooling ratesof the IVA irons (e.g., Haack et al., 1996).

Rhenium-osmium ages reported so far use a 187Re decayconstant that was determined by assuming that the IIIABisochron should give the same age as the U-Pb age of theangrites (Smoliar et al., 1996), so the accuracy of these agesis only as good as the validity of this assumption. Never-theless, the age range indicated by Re-Os isochrons is inde-pendent of the half-life, so the results for various iron mete-orite groups, with the exception of the IVA irons, suggestthat core crystallization (or more specifically, Re-Os isoto-pic closure) in planetesimals spanned a period of ~30 m.y.(although the relatively large errors certainly allow this timeinterval to be significantly narrower than this). Rhenium-osmium systematics in pallasites indicate that they may beyounger than iron meteorites by ~60 m.y.; however, thisapparently young age may be due to later reequilibrationof the Re-Os system (Chen et al., 2002). Cook et al. (2004)recently reported the first high-precision 190Pt-186Os iso-chrons for the IIAB and IIIAB magmatic irons, and esti-mated ages for these meteorite groups of 4323 ± 80 Maand 4325 ± 26 Ma, respectively. These ages are somewhatyounger than Re-Os ages for iron meteorites, and theseauthors suggested that this discrepancy could reflect an errorin the decay constant for 190Pt.

In contrast to the more leisurely pace of core crystalli-zation suggested by Re-Os systematics (although as indi-cated above, the errors are rather large and could accommo-date a much narrower time interval for this process), evi-dence for the former presence of live 107Pd in a variety ofmetal-rich meteorites, including irons and pallasites, indi-cates that this process occurred rapidly, well within the life-time of this short-lived radionuclide. For most iron meteor-ites, closure of the 107Pd-107Ag-isotopic system occurredwithin a narrow time interval of only ~4 m.y. The highestinferred 107Pd/108Pd ratio of ~2.4 × 10–5 is in samples suchas Gibeon (IVA) and Canyon Diablo (IA), for which well-defined 107Pd-107Ag isochrons have been obtained (Chenand Wasserburg, 1996; Carlson and Hauri, 2001). Pallasiteshave somewhat younger Pd-Ag ages, most of which indi-cate isotopic closure ≤10 m.y. after Gibeon and CanyonDiablo. Of the pallasites investigated so far, Brenham hasthe highest 107Pd/108Pd ratio of (1.65 ± 0.05) × 10–5, definedby a four-point internal isochron comprised of silicates andmetal (Carlson and Hauri, 2001). This more precisely de-fined value is only marginally higher than the 107Pd/108Pdratio of ~1.1 × 10–5 inferred from a single bulk data point

(Chen and Wasserburg, 1996; Chen et al., 2002), and indi-cates that closure of the Pd-Ag system occurred ~3.5 m.y.after Gibeon and Canyon Diablo. The recently acquiredhigh-precision Pd-Ag-isotopic data for the Canyon Diablo(IA) and Grant (IIIB) iron meteorites and the Brenhampallasite have additionally demonstrated that ε-level differ-ences in the initial Ag-isotopic compositions of these me-teorites can be resolved, and this has important implicationsfor their formation histories (Carlson and Hauri, 2001). Theinitial Ag-isotopic composition of Brenham indicates thatduring the 3.5-m.y. time interval between closure of its Pd-Ag system and that of Canyon Diablo, the parent planetesi-mal of Brenham evolved with a nearly chondritic Pd/Agratio, unlike the high Pd/Ag ratio characterizing the bulkcomposition of this pallasite. This may suggest that metal-silicate separation on the Brenham parent body did notoccur until shortly before its crystallization (Carlson andHauri, 2001). This result is distinct from that of the IIIBiron Grant. Grant has a similar inferred initial 107Pd/108Pd asdoes Brenham, but a more radiogenic initial Ag-isotopiccomposition, which indicates that the high bulk Pd/Ag ofGrant was a characteristic of its source (Carlson and Hauri,2001). With the exception of the disturbed Re-Os system-atics in the IVA irons, the Re-Os and Pd-Ag ages providea broadly similar temporal sequence of formation of the ironmeteorites and pallasites.

Manganese-53–chromium-53 systematics in IIIAB mag-matic irons and pallasites also indicate early fractionationand closure of the Mn-Cr system in these metal-rich mete-orites. The first evidence of live 53Mn in IIIAB irons wasprovided by ion microprobe (SIMS) studies of phosphatesthat showed a wide range of 53Mn/55Mn ratios from ~1 ×10–6 to ~2 × 10–5 (Davis and Olsen, 1991; Hutcheon andOlsen, 1991; Hutcheon et al., 1992). Taken at face value,this range of 53Mn/55Mn ratios translates to a time intervalof ~16 m.y., which is inconsistent with the Pd-Ag system-atics that indicate that most of these meteorites formed con-temporaneously. A more recent SIMS study of 53Mn-53Crsystematics in phosphates in several IIIAB iron meteoriteshas demonstrated that the Mn-Cr ages based on inferred53Mn/55Mn ratios are different for the different phosphateminerals, most likely due to the slow cooling rate of theIIIAB iron meteorites combined with the difference in thediffusion behaviors of Mn and Cr in the different phosphates(Sugiura and Hoshino, 2003); the apparently wide rangeof inferred 53Mn/55Mn ratios in the previous studies ofIIIAB irons may be similarly explained. Sugiura and Ho-shino (2003) also showed that one of the phosphates (i.e.,the Fe-Mn phosphate sarcopside) consistently recorded thesame 53Mn/55Mn ratio of ~3.4 × 10–6 in all the IIIAB ironmeteorites they investigated. These authors reasoned thatthe closure temperature of the Pd-Ag system in iron mete-orites may be similar to that of the Mn-Cr system insarcopside, and that the 107Pd/108Pd ratio of ~2 × 10–5 inthese meteorites corresponds to a 53Mn/55Mn ratio of ~3.4 ×10–6. This would imply that, compared to the 53Mn/55Mnratio of 1.25 × 10–6 in the LEW angrite (Lugmair and Shu-


kolyukov, 1998), isotope closure of the Mn-Cr (and Pd-Ag)systems in the IIIAB irons occurred ~5 m.y. before angritecrystallization (i.e., at ~4563 Ma). Furthermore, it was sug-gested by these authors that in pallasites, the 107Pd/108Pdratio of ~1 × 10–5 (inferred from Glorieta Mountain andBrenham pallasites) (Chen and Wasserburg, 1996; Carlsonand Hauri, 2001; Chen et al., 2002) corresponded to a53Mn/55Mn ratio of ~1 × 10–6 (based on data for the Omolonpallasite) (Lugmair and Shukolyukov, 1998), implying broadconsistency between Pd-Ag and Mn-Cr systematics (bothreflecting 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 areexamined in detail, there are evident discrepancies betweenthese two systems. For example, the Eagle Station pallasitehas an inferred 53Mn/55Mn ratio of ~2.3 × 10–6 (Birck andAllègre, 1988), but there is no detectable evidence of live107Pd in this meteorite (i.e., 107Pd/108Pd < ~7 × 10–6) (Chenand 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 Lugmairand Shukolyukov (1998), has a Cr-isotopic composition withan anomalously high 54Cr/52Cr ratio of ~1.5 ε. Therefore,this meteorite may contain an anomalous (most likely ofpresolar nucleosynthetic origin) Cr component that couldcomplicate Mn-Cr systematics such that the 53Mn/55Mnratio inferred from the measured 53Cr/52Cr ratios would notreflect the true value. Among the main-group pallasites, a53Mn/55Mn ratio of (1.4 ± 0.4) × 10–5 has been inferred froma SIMS study of Springwater (Hutcheon and Olsen, 1991);this value was since confirmed by another ion microprobestudy (Hsu et al., 1997). However, based on thermal ion-ization mass spectrometry (TIMS) analysis of a separatedolivine fraction from Springwater, Lugmair and Shukol-yukov (1998) inferred a 53Mn/55Mn ratio for this pallasitemore than an order of magnitude lower (~1 × 10–6). WhileTIMS studies of Eagle Station, Omolon, and Springwater(Birck and Allègre, 1988; Lugmair and Shukolyukov, 1998)have yielded 53Mn/55Mn ratios of ~1–2 × 10–6, the SIMSinvestigations of Albin, Brenham, and Springwater (Hutch-eon and Olsen, 1991; Hsu et al., 1997) all give values of~1–4 × 10–5. The fact that analyses of olivine from the samepallasite sample (i.e., Springwater) by bulk (TIMS) or micro-analytical (SIMS) techniques yield such different inferred53Mn/55Mn ratios could be indicative of redistribution of Mnand/or Cr within these olivines. If this is the case, then thebulk technique (which would “average” over any redistribu-tion of the Mn and/or Cr) may be more reliable in terms ofyielding the true 53Mn/55Mn ratio. As such, given the abovediscussion, the suggestion of Sugiura and Hoshino (2003)that pallasites (at least those belonging to the “main group”)are characterized by a 53Mn/55Mn ratio of ~1 × 10–6 maybe a reasonable one. Nevertheless, some outstanding prob-lems with the consistency of the Mn-Cr and Pd-Ag ages inpallasites remain.

4. SUMMARY AND CONCLUSIONS

Figure 6 summarizes the timescales discussed here forthe processes involved in planetesimal differentiation. Basedon the constraints available from long- and short-lived chro-nometers, differentiation of planetesimals in the early his-tory of the solar system occurred very rapidly. Melting wasinitiated on some of the planetesimals, which were subse-quently arrested in this state and did not differentiate fully.

Fig. 6. Timescales of planetesimal differentiation as inferred fromchronological investigations of differentiated meteorites (see textfor details). The time of formation of the refractory calcium-alu-minum-rich inclusions (the earliest solids to form in the protosolarnebula) at 4567 Ma is assumed to be coincident with the formationof the solar system (dotted-dashed line). Based on the ages of non-cumulate eucrites and angrites, it is inferred that crust formation onplanetesimals began within ~2 m.y. and continued until ~10 m.y.after CAI formation; Pb-Pb ages of cumulate eucrites may implythat igneous activity continued on the howardite-eucrite-diogeniteparent body (HED PB) for another ~100–150 m.y. On the HEDPB, global silicate (mantle) differentiation occurred within ~2–3 m.y. of CAI formation, and was preceded by core formation by~1 m.y. (indicated by the two crossed circles). Global silicate(mantle) differentiation in the parent bodies of the eucritic clastsin Vaca Muerta and the aubrite parent body occurred ~2 m.y. afterthis same process in the HED PB. Metal segregation in the parentbodies of magmatic iron meteorites and pallasites may have oc-curred over a period of ~5 m.y. (possibly less); assuming that thetiming of this process was earliest in the HED PB (inferred to haveoccurred only ~1–2 m.y. after CAI formation based on Hf-W sys-tematics in the HEDs), this process probably lasted at most until~4560 Ma. Pd-Ag and Mn-Cr systematics indicate that crystalli-zation of the metallic cores of the extensively differentiated par-ent bodies of the magmatic irons and pallasites is likely to haveoccurred over a time interval of ≤10 m.y. (Re-Os systematics sug-gest that the duration of this process may have been longer, butthe large uncertainties in these ages allow for the short time in-terval inferred from the Pd-Ag and Mn-Cr systematics). This timeinterval is anchored to the oldest absolute Re-Os age for a mag-matic iron meteorite, i.e., 4557 Ma (although, in fact, the uncer-tainty of ±12 m.y. in this age is comparable to the time intervalover which this process is inferred to occur).


On some of these bodies, such incipient melting began asearly as ~3–5 m.y. after the formation of the first solids inthe solar nebula (i.e., CAIs), but possibly extended to morethan ~10 m.y. on others. Other planetesimals underwentmore extensive melting and differentiation and acquired thelayered structure comprised of a silicate crust and mantleand a metallic core.

Crystallization ages of the basaltic angrites and noncu-mulate eucrites provide the best means of constraining thetiming of crust formation. Chronological evidence for theseachondrites suggests that the earliest basalts were emplaced(and formed the crusts of planetesimals) within ~3 m.y. ofthe formation of CAIs. Primary basalt formation is likelyto have continued until ~10 m.y. after CAI formation.

Global silicate differentiation (i.e., the process that es-tablished the mantle source reservoirs of the crustal mate-rials) on the parent planetesimals of the angrites and eucritesis likely to have occurred contemporaneously, ~2–3 m.y.after CAI formation. However, this process occurred ~2 m.y.later on the parent body of the HED-like clasts from theVaca Muerta mesosiderite and on the aubrite parent body.Therefore, the process of global mantle differentiation ofplanetesimals began early but possibly extended over a timeinterval of a few million years for different parent bodies.

Metal segregation on different planetesimals, representedby different groups of iron meteorites, is likely to have oc-curred over a short time interval of ~5 m.y. (and possiblyeven shorter). In the particular case of the HED parent body,metal segregation may have occurred ~1 m.y. prior to theglobal mantle differentiation as reflected in the Hf-W sys-tematics of the HEDs. Subsequent core crystallization isthought to have occurred over a somewhat longer period oftime. Rhenium-osmium systematics suggest that core crys-tallization could have continued for ~30 m.y. (although thelarger errors in absolute Re-Os ages could accommodate asignificantly narrower time interval for this process), butshort-lived chronometers (Pd-Ag and Mn-Cr) in most ironmeteorites and pallasites indicate that core crystallizationon their parent planetesimals spanned a shorter time inter-val of ≤10 m.y.

The various chronometers (long- and short-lived) appliedtoward obtaining the timescales discussed here are not al-ways consistent with each other. In some cases, this is tobe expected since these chronometers may be dating dif-ferent events owing to differences in their closure tempera-tures. However, in other cases, discrepancies may result fromdisturbance of isotope systematics by secondary events. Inthe specific case of chronometers based on the short-livedradionuclides (particularly those with half-lives ≤10 m.y.),there is the additional assumption of homogeneous distribu-tion in the meteorite-forming region of the early solar sys-tem. This assumption still remains to be rigorously evalu-ated for several of the short-lived chronometers consideredhere. Nevertheless, the apparent convergence of the relativetimescales indicated by Al-Mg and Mn-Cr with the abso-lute timescales provided by U-Pb dating suggests that these

systems can indeed provide a robust chronology for earlysolar system events.

The main challenge for future studies seeking to betterclarify the reasons for discrepancies between various radio-genic isotope systematics will be to extend chronologicalinvestigations to a greater variety of meteoritic materials,such that there are many more instances than there are atpresent of multiple chronometers being applied to the sameobjects. Such studies have been inhibited in the past by thelimited fractionation in meteoritic materials of various par-ent-daughter element pairs, such that it has been difficult toapply multiple chronometers to a particular meteorite. Thesetypes of investigations should become increasingly feasiblewith future advancements in analytical capabilities that willallow high-precision isotopic analyses on meteoritic com-ponents having relatively low parent/daughter ratios.

Acknowledgments. We are grateful to Q. Yin and an anony-mous reviewer for their constructive and thorough reviews thathelped to improve the content and to K. McKeegan for the edito-rial handling of this manuscript. This work was supported NASAgrant NAG5-7196 to M.W.

REFERENCES

Allègre C. J., Birck J. L., Fourcade S., and Semet M. P. (1975)Rubidium-87/strontium-87 age of Juvinas basaltic achondriteand early igneous activity in the solar system. Science, 187,436–438.

Amelin Y., Krot A. N., Hutcheon I. D., and Ulyanov A. A. (2002)Lead isotopic ages of chondrules and calcium-aluminum-richinclusions. Science, 297, 1678–1683.

Baker J., Bizzarro M., Wittig M., Connelly J., and Haack H. (2005)Early planetesimal melting from an age of 4.5662 Gyr for dif-ferentiated meteorites. Nature, 436, 1127–1131.

Becker H. and Walker R. J. (2003a) In search of extant Tc in theearly solar system: 98Ru and 99Ru abundances in iron meteor-ites and chondrites. Chem. Geol., 196, 43–56.

Becker H. and Walker R. J. (2003b) Efficient mixing of the solarnebula from uniform Mo isotopic composition of meteorites.Nature, 425, 152–155.

Birck J.-L. and Allègre C. J. (1978) Chronology and chemicalhistory of the parent body of the basaltic achondrites studiedby the 87Rb-87Sr method. Earth Planet. Sci. Lett., 39, 37–51.

Birck J.-L. and Allègre C. J. (1988) Manganese-chromium iso-tope systematics and development of the early solar system.Nature, 331, 579–584.

Bizzarro M., Baker J., and Haack H. (2004) Mg isotope evidencefor contemporaneous formation of chondrules and refractoryinclusions. Nature, 431, 275–278.

Bizzarro M., Baker J., and Haack H. (2005) Timing of crust for-mation on differentiated asteroids inferred from Al-Mg chro-nometry (abstract). In Lunar and Planetary Science XXXVI,Abstract #1312. Lunar and Planetary Institute, Houston (CD-ROM).

Bogard D. D. and Garrison D. H. (1997) 39Ar-40Ar ages of igne-ous non-brecciated eucrites (abstract). In Lunar and PlanetaryScience XXVIII, pp. 127–128. Lunar and Planetary Institute,Houston.


Bogard D. D., Nyquist L. E., Johnson P., Wooden J., and BansalB. (1983) Chronology of Brachina (abstract). Meteoritics, 18,269–270.

Bogdanovski O. and Jagoutz E. (1996) Sm-Nd system in theDivnoe meteorite (abstract). In Lunar and Planetary ScienceXXVII, pp. 129–130. Lunar and Planetary Institute, Houston.

Bogdanovski O., Shukolyukov A., and Lugmair G. W. (1997)53Mn-53Cr isotope system in the Divnoe meteorite (abstract).Meteoritics & Planet. Sci., 32, A16–A17.

Brazzle R. H., Pravdivtseva O., Meshik A. P., and HohenbergC. M. (1999) Verification and interpretation of the I-Xe chro-nometer. Geochim. Cosmochim. Acta, 63, 739–760.

Cameron A. G. W. (2001) From interstellar gas to the Earth-Moonsystem. Meteoritics & Planet. Sci., 36, 9–22.

Cameron A. G. W., Hoeflich P., Myers P. C., and Clayton D. D.(1995) Massive supernovae, Orion gamma rays, and the forma-tion of the solar system. Astrophys. J. Lett., 447, L53.

Carlson R. W. and Hauri E. H. (2001) Extending the 107Pd-107Agchronometer to low Pd/Ag meteorites with multicollectorplasma-ionization mass spectrometry. Geochim. Cosmochim.Acta, 65, 1839–1848.

Carlson R. W. and Lugmair G. W. (2000) Timescales of planetesi-mal formation and differentiation based on extinct and extantradionuclides. In The Origin of the Earth and Moon (R. M.Canup and K. Righter, eds.), pp. 25–44. Univ. of Arizona,Tucson.

Clayton R. N. (1993) Oxygen isotopes in meteorites. Annu. Rev.Earth Planet. Sci., 21, 115–149.

Clayton R. N. and Mayeda T. K. (1996) Oxygen isotope studiesof achondrites. Geochim. Cosmochim. Acta, 60, 1999–2017.

Clayton R. N., Grossman L., and Mayeda T. K. (1973) A compo-nent of primitive nuclear composition in carbonaceous meteor-ites. Science, 182, 485–488.

Chen J. H. and Wasserburg G. J. (1985) U-Th-Pb isotopic studieson meteorite ALHA 81005 and Ibitira (abstract). In Lunar andPlanetary Science XVI, pp. 119–120. Lunar and Planetary Insti-tute, Houston.

Chen J. H. and Wasserburg G. J. (1996) Live 107Pd in the earlysolar system and implications for planetary evolution. In EarthProcesses: Reading the Isotope Code (A. Basu and S. R. Hart,eds.), pp. 1–20. AGU Monograph 95, American GeophysicalUnion, Washington, DC.

Chen J. H., Papanastassiou D. A., and Wasserburg G. J. (2002)Re-Os and Pd-Ag systematics in Group IIAB irons and in pal-lasites. Geochim. Cosmochim. Acta, 66, 3793–3810.

Chen J. H., Papanastassiou D. A., and Wasserburg G. J. (2003)Endemic Ru isotopic anomalies in iron meteorites and inAllende (abstract). In Lunar and Planetary Science XXXIV, Ab-stract #1789. Lunar and Planetary Institute, Houston (CD-ROM).

Chen J. H., Papanastassiou D. A., Wasserburg G. J., and Ngo H. H.(2004) Endemic Mo isotopic anomalies in iron and carbona-ceous meteorites (abstract). In Lunar and Planetary ScienceXXXV, Abstract #1431. Lunar and Planetary Institute, Houston(CD-ROM).

Cook D. L., Walker R. J., Horan M. F., Wasson J. T., and MorganJ. W. (2004) Pt-Re-Os systematics of group IIAB and IIIABiron meteorites. Geochim. Cosmochim. Acta, 68, 1413–1431.

Crozaz G. and Pellas P. (1983) Where does Brachina come from?(abstract). In Lunar and Planetary Science XXIV, pp. 142–143.Lunar and Planetary Institute, Houston.

Dauphas N., Marty B., and Reisberg L. (2002) Molybdenum evi-dence for inherited planetary scale isotope heterogeneity of the

protosolar nebula. Astrophys. J., 565, 640–644.Dauphas N., Davis A. M., Marty B., and Reisberg L. (2004a) The

cosmic molybdenum-ruthenium isotope correlation. EarthPlanet Sci. Lett., 226, 465–475.

Dauphas N., Foley N., Wadhwa M., Davis A. M., Gopel C., BirckJ.-L., Janney P. E., and Gallino R. (2004b) Testing the homo-geneity of the solar system for iron (54, 56, 57, and 58) andtungsten (182, 183, 184, and 186) isotope abundances (ab-stract). In Lunar and Planetary Science XXXV, Abstract #1498.Lunar and Planetary Institute, Houston (CD-ROM).

Dauphas N., Janney P. E., Mendybaev R. A., Wadhwa M., Rich-ter F. M., Davis A. M., Zuilen M., Hines R., and Foley C. N.(2004c) Chromatographic separation and MC-ICPMS analy-sis of iron, investigating mass dependent and independent iso-tope effects. Anal. Chem., 76, 5855–5863.

Davis A. M. and Olsen E. J. (1991) Phosphates in pallasite mete-orites as probes of mantle processes in small planetary bodies.Nature, 353, 637–640.

Desch S. J., Connolly H. C. Jr., and Srinivasan G. (2004) An inter-stellar origin for the beryllium-10 in calcium-rich, aluminum-rich inclusions. Astrophys. J., 602, 528–542.

Eugster O., Michel T., and Niedermann S. (1991) 244Pu-Xe for-mation and gas retention age, exposure history and terrestrialage of angrites LEW 86010 and LEW 87051. Geochim. Cos-mochim. Acta, 55, 2957–2964.

Galer S. J. G. and Lugmair G. W. (1996) Lead isotope systemat-ics of noncumulate eucrites (abstract). Meteoritics & Planet.Sci., 31, A47–A48.

Gallino R., Busso M., Wasserburg G. J., and Straniero O. (2004)Early solar system radioactivity and AGB stars. New Astron.Rev., 48, 133–138.

Ghosh A. and McSween H. Y. Jr. (1998) A thermal model for thedifferentiation of asteroid 4 Vesta, based on radiogenic heating.Icarus, 134, 187–206.

Glavin D. P., Kubny A., Jagoutz E., and Lugmair G. W. (2004)Mn-Cr isotope systematics of the D’Orbigny angrite. Meteor-itics & Planet. Sci., 39, 693–700.

Goodrich C. A. (1998) Residues from low degrees of melting of aheterogeneous parent body (abstract). Meteoritics & Planet.Sci., 33, A60.

Goodrich C. A. and Lugmair G. W. (1995) Stalking the LREE-enriched component in ureilites. Geochim. Cosmochim. Acta,59, 2609–2620.

Goodrich C. A., Hutcheon I. D., and Keil K. (2002) 53Mn-53Crage of a highly-evolved igneous lithology in polymict ureiliteDaG 165 (abstract). Meteoritics & Planet. Sci., 37, A54.

Göpel C., Manhès G., and Allègre C. J. (1992) U-Pb study of theAcapulco meteorite (abstract). Meteoritics, 27, 226.

Göpel C., Manhès G., and Allègre C. J. (1994) U-Pb systematicsof phosphates from equilibrated ordinary chondrites. EarthPlanet. Sci. Lett., 121, 153–171.

Gounelle M., Shu F. H., Shang H., Glassgold A. E., Rehm E. K.,and Lee T. (2001) Extinct radioactivities and protosolar cosmic-rays: Self-shielding and light elements. Astrophys. J., 548,1051–1070.

Gray C. M., Papanastassiou D. A., and Wasserburg G. J. (1973)The identification of early condensates from the solar nebula.Icarus, 20, 213–239.

Haack H., Scott E. R. D., Love S. G., Brearley A. J., and McCoyT. J. (1996) Thermal histories of IVA stony-iron meteorites:Evidence for asteroid fragmentation and reaccretion. Geochim.Cosmochim. Acta, 60, 3103–3113.

Hohenberg C. M. (1970) Xenon from the Angra dos Reis mete-


orite. Geochim. Cosmochim. Acta, 34, 185–191.Hohenberg C. M., Bernatowicz T. J., and Podosek F. A. (1991)

Comparative xenology of two angrites. Earth Planet. Sci. Lett.,102, 167–177.

Horan M. F., Smoliar M. I., and Walker R. J. (1998) 182W and187Re-187Os systematics of iron meteorites: Chronology formelting, differentiation, and crystallization in asteroids. Geo-chim. Cosmochim. Acta, 62, 545–554.

Hsu W., Huss G. R., and Wasserburg G. J. (1997) Mn-Cr systemat-ics of differentiated meteorites (abstract). In Lunar and Plane-tary Science XXVIII, Abstract #1783. Lunar and Planetary Insti-tute, Houston (CD-ROM).

Hutcheon I. D. and Olsen E. J. (1991) Cr isotopic composition ofdifferentiated meteorites: A search for 53Mn (abstract). In Lu-nar and Planetary Science XXII, pp. 605–606. Lunar and Plan-etary Institute, Houston.

Hutcheon I. D., Olsen E., Zipfel J., and Wasserburg G. J. (1992)Chromium isotopes in differentiated meteorites: Evidence for53Mn (abstract). In Lunar and Planetary Science XXIII, pp. 565–566. Lunar and Planetary Institute, Houston.

Jacobsen S. B. and Wasserburg G. J. (1984) Sm-Nd isotopic evolu-tion of chondrites and achondrites, II. Earth Planet. Sci. Lett.,67, 137–150.

Jagoutz E., Jotter R., Kubny A., Varela M. E, Zartman R., KuratG. and Lugmair G. W. (2003) Cm?-U-Th-Pb isotopic evolu-tion of the D’Orbigny angrite (abstract). Meteoritics & Planet.Sci., 38, Abstract #5048.

Kita N., Ikeda Y., Shimoda H., Morishita Y., and Togashi S. (2003)Timing of basaltic volcanism in ureilite parent body inferredfrom the 26Al ages of plagioclase-bearing clasts in DaG-319polymict ureilite (abstract). In Lunar and Planetary ScienceXXXIV, Abstract #1557. Lunar and Planetary Institute, Houston(CD-ROM).

Kleine T., Münker C., Mezger K., and Palme H. (2002) Rapidaccretion and early core formation on asteroids and the ter-restrial planets from Hf-W chronometry. Nature, 418, 952–955.

Kleine T., Mezger K., Münker C., Palme H., and Bischoff A.(2004) 182Hf-182W isotope systematics of chondrites, eucrites,and martian meteorites: Chronology of core formation andearly mantle differentiation in Vesta and Mars. Geochim. Cos-mochim. Acta, 68, 2935–2946.

Kleine T., Mezger K., Palme H., and Scherer E. (2005) Tungstenisotopes provide evidence that core formation in some aster-oids predates the accretion of chondrite parent bodies (ab-stract). In Lunar and Planetary Science XXXVI, Abstract#1431. Lunar and Planetary Institute, Houston (CD-ROM).

Kumar A., Gopalan K., and Bhandari N. (1999) 147Sm-143Nd and87Rb-87Sr ages of the eucrite Piplia Kalan. Geochim. Cosmo-chim. Acta, 63, 3997–4001.

Lee D.-C. and Halliday A. N. (1995) Hafnium-tungsten chronom-etry and the timing of terrestrial core formation. Nature, 378,771–774.

Lee D.-C. and Halliday A. N. (1996) Hf-W isotopic evidence forrapid accretion and differentiation in the early solar system.Science, 274, 1876–1879.

Lee D.-C., Halliday A. N., Singletary S. J., and Grove T. L. (2005)182Hf-182W chronometry and an early differentiation on the par-ent body of urelilites (abstract). In Lunar and Planetary Sci-ence XXXVI, Abstract #1638. Lunar and Planetary Institute,Houston (CD-ROM).

Lee T., Papanastassiou D. A., and Wasserburg G. J. (1976) Dem-onstration of 26Mg excess in Allende and evidence for 26Al.Geophys. Res. Lett., 3, 109–112.

Leya I., Weiler R., and Halliday A. N. (2000) Cosmic-ray pro-duction of tungsten isotopes in lunar samples and meteoritesand its implications for Hf-W chronochemistry. Earth Planet.Sci. Lett., 175, 1–12.

Leya I., Halliday A. N., and Wieler R. (2003a) The predictablecollateral consequences of nucleosynthesis by spallation reac-tions in the early Solar System. Astrophys. J., 594, 605–616.

Leya I., Weiler R., and Halliday A. N. (2003b) The influence ofcosmic-ray production on extinct radionuclide systems. Geo-chim. Cosmochim. Acta, 67, 529–541.

Loss R. D., Lugmair G. W., Davis A. M., and MacPherson G. J.(1994) Isotopically distinct reservoirs in the solar nebula: Iso-topic anomalies in Vigarano meteorite inclusions. Astrophys.J. Lett., 436, L193–L196.

Lugmair G. W. (1974) Sm-Nd ages: A new dating method. Mete-oritics, 9, 369.

Lugmair G. W. and Galer S. J. G. (1992) Age and isotopic relation-ships among angrites Lewis Cliff 86010 and Angra dos Reis.Geochim. Cosmochim. Acta, 56, 1673–1694.

Lugmair G. W. and Marti K. (1977) Sm-Nd-Pu timepieces in theAngra dos Reis meteorite. Earth Planet. Sci. Lett., 35, 273–284.

Lugmair G. W. and Shukolyukov A. (1998) Early solar systemtimescales according to 53Mn-53Cr systematics. Geochim. Cos-mochim. Acta, 62, 2863–2886.

Lugmair G. W., Scheinin N. B., and Carlson R. W. (1977) Sm-Ndsystematics of the Serra de Magé eucrite (abstract). Meteoritics,10, 300–301.

Lugmair G. W., Galer S. J. G., and Carlson R. W. (1991) Isotopesystematics of cumulate eucrite EET-87520 (abstract). Mete-oritics, 26, 368.

Manhès G., Göpel C., and Allègre C. J. (1987) High resolutionchronology of the early solar system based on lead isotopes(abstract). Meteoritics, 22, 453–454.

Markowski A., Quitté G., Kleine T., and Halliday A. N. (2005)Tungsten isotopic constraints on the formation and evolutionof iron meteorite parent bodies (abstract). In Lunar and Plan-etary Science XXXVI, Abstract #1308. Lunar and PlanetaryInstitute, Houston (CD-ROM).

Masarik J. (1997) Contribution of neutron-capture reactions toobserved tungsten isotopic ratios. Earth Planet Sci. Lett., 152,181–185.

McCoy T. J., Keil K., Clayton R. N., Mayeda T. K., Bogard D. D.,Garrison D. H., Huss G. R., Hutcheon I. D., and Wieler R.(1996) A petrologic, chemical, and isotopic study of MonumentDraw and comparison with other acapulcoites: Evidence forformation by incipient partial melting. Geochim. Cosmochim.Acta, 60, 2681–2708.

McCoy T. J., Keil K., Clayton R. N., Mayeda T. K., Bogard D. D.,Garrison D. H., and Wieler R. (1997a) A petrologic and iso-topic study of lodranites: Evidence of early formation as par-tial melt residues from heterogeneous precursors. Geochim.Cosmochim. Acta, 61, 623–637.

McCoy T. J., Keil K., Muenow D. W., and Wilson L. (1997b)Partial melting and melt migration in the acapulcoite-lodraniteparent body. Geochim. Cosmochim. Acta, 61, 639–650.

McCoy T. J., Mittlefehldt D. W., and Wilson L. (2006) Asteroiddifferentiation. In Meteorites in the Early Solar System II (D. S.Lauretta and H. Y. McSween Jr., eds.), this volume. Univ. ofArizona, Tucson.

Meyer B. S. and Clayton D. D. (2000) Short-lived radioactivitiesand the birth of the sun. Space Sci. Rev., 92, 133–152.

Mittlefehldt D. W., Lindstrom M. M., Bogard D. D., Garrison


D. H., and Field S. W. (1996) Acapulco- and Lodran-likeachondrites: Petrology, geochemistry, chronology and origin.Geochim. Cosmochim. Acta, 60, 867–882.

Mittlefehldt D. W., McCoy T. J., Goodrich C. A., and KracherA. (1998) Non-chondritic meteorites from asteroidal bodies.In Planetary Materials (J. J. Papike, ed.), pp. 4-1 to 4-195.Reviews in Mineralogy, Vol. 36, Mineralogical Society ofAmerica.

Mostefaoui S., Lugmair G. W., and Hoppe P. (2004) In-situ evi-dence for live iron-60 in the early solar system: A potentialheat source for planetary differentiation from a nearby super-nova explosion (abstract). In Lunar and Planetary ScienceXXXV, Abstract #1271. Lunar and Planetary Institute, Houston(CD-ROM).

Nehru C. E., Prinz M., Delaney J. S., Dreibus G., Palme H., SpettelB., and Wänke H. (1983) Brachina: A new type of meteorite,not a Chassignite. Proc. Lunar Planet. Sci. Conf. 13th, in J.Geophys. Res., 87, A365–A373.

Nehru C. E., Prinz M., Weisberg M. K., Ebihara M. E., ClaytonR. N., and Mayeda T. K. (1992) Brachinites: A new primitiveachondrites group (abstract). Meteoritics, 27, 267.

Nehru C. E., Prinz M., Weisberg M. K., Ebihara M. E., ClaytonR. N., and Mayeda T. K. (1996) A new brachinites and petro-genesis of the group (abstract). In Lunar and Planetary ScienceXXVII, pp. 943–944. Lunar and Planetary Institute, Houston.

Nichols R. H., Hohenberg C. M., Kehm K., Kim Y., and MartiK. (1994) I-Xe studies of the Acapulco meteorite: AbsoluteI-Xe ages of individual phosphate grains and the Bjurböle stan-dard. Geochim. Cosmochim. Acta, 58, 2553–2561.

Nyquist L. E., Takeda H., Bansal B. M., Shih C.-Y., Wiesmann H.,and Wooden J. L. (1986) Rb-Sr and Sm-Nd internal isochronages of a subophitic basalt clast and a matrix sample from theY75011 eucrite. J. Geophys. Res., 91, 8137–8150.

Nyquist L. E., Bansal B., Wiesmann H., and Shih C.-Y. (1994)Neodymium, strontium and chromium isotopic studies of theLEW 86010 and Angra dos Reis meteorites and the chronologyof the angrite parent body. Meteoritics, 29, 872–885.

Nyquist L. E., Bogard D., Wiesmann H., Shih C.-Y., YamaguchiA., and Takeda H. (1996) Early history of the Padvarninkaieucrite. Meteoritics, 31, A101–A102.

Nyquist L., Bogard D., Takeda H., Bansal B., Wiesmann H., andShih C.-Y. (1997a) Crystallization, recrystallization, and im-pact-metamorphic ages of eucrites Y792510 and Y791186.Geochim. Cosmochim. Acta, 61, 2119–2138.

Nyquist L. E., Wiesmann H., Reese Y., Shih C.-Y., and Borg L. E.(1997b) Samarium-neodymium age and manganese-chromitesystematics of eucrite Elephant Moraine 90020. Meteoritics,32, A101–A102.

Nyquist L. E., Shih C.-Y., Wiesmann H., and Mikouchi T. (2003a)Fossil 26Al and 53Mn in D’Orbigny and Sahara 99555 and thetimescale for angrite magmatism (abstract). In Lunar and Plan-etary Science XXXIV, Abstract #1388. Lunar and PlanetaryInstitute, Houston (CD-ROM).

Nyquist L. E., Reese Y., Wiesmann H., Shih C.-Y., and TakedaH. (2003b) Fossil 26Al and 53Mn in the Asuka 881394 eucrite:Evidence of the earliest crust on asteroid 4 Vesta. Earth Planet.Sci. Lett., 214, 11–25.

Ott U., Begemann F., and Löhr H. P. (1987) Noble gases in ALH84025: Like Brachina, unlike Chassigny (abstract). Meteorit-ics, 22, 476–477.

Papanastassiou D. A. and Wasserburg G. J. (1969) Initial stron-tium isotopic abundances and the resolution of small time dif-ferences in the formation of planetary objects. Earth Planet.Sci. Lett., 5, 361–376.

Papanastassiou D. A., Chen J. H., and Wasserburg G. J. (2004)More on Ru endemic isotope anomalies in meteorites (ab-stract). In Lunar and Planetary Science XXXV, Abstract #1828.Lunar and Planetary Institute, Houston (CD-ROM).

Pellas P., Fieni C., Trieloff M., and Jessberger E. K. (1997) Thecooling history of the Acapulco meteorite as recorded by the244Pu and 40Ar-39Ar chronometers. Geochim. Cosmochim.Acta, 61, 3477–3501.

Petaev M. I., Baruskova L. D., Lipschutz M. E., Wang M.-S.,Arsikan A. A., Clayton R. N., and Mayeda T. K. (1994) TheDivnoe meteorite: Petrology, chemistry, oxygen isotopes andorigin. Meteoritics, 29, 182–199.

Podosek F. A., Zinner E. K., MacPherson G. J., Lundberg L. L.,Brannon J. C. and Fahey A. J. (1991) Correlated study of initial87Sr/86Sr and Al/Mg isotopic systematics and petrologic proper-ties in a suite of refractory inclusions from the Allende meteor-ite. Geochim. Cosmochim. Acta, 55, 1083–1110.

Pravdivtseva O. V., Hohenberg C. M., and Meshik A. P. (2005)I-Xe dating: The time line of chondrule formation and meta-morphism in LL chondrites (abstract). In Lunar and PlanetaryScience XXXVI, Abstract #2354. Lunar and Planetary Institute,Houston (CD-ROM).

Prinzhofer A., Papanastassiou D. A., and Wasserburg G. J. (1992)Samarium-neodymium evolution of meteorites. Geochim. Cos-mochim. Acta, 56, 797–815.

Quitté G. and Birck J. L. (2004) Tungsten isotopes in eucrites re-visited and the initial 182Hf/180Hf of the solar system based oniron meteorite data. Earth Planet. Sci. Lett., 219, 201–207.

Quitté G., Birck J. L., and Allègre C. J. (2000) 182Hf-182W sys-tematics in eucrites: The puzzle of iron segregation in the earlysolar system. Earth Planet. Sci. Lett. 184, 83–94.

Quitté G., Latkoczy C., Halliday A. N., Schönbächler M., andGünther D. (2005) Iron-60 in the eucrite parent body and theinitial 60Fe/56Fe of the solar system (abstract). In Lunar andPlanetary Science XXXVI, Abstract #1827. Lunar and PlanetaryInstitute, Houston (CD-ROM).

Russell S. S., Hartmann L., Cuzzi J., Krot A. N., Gounelle M.,and Weidenschilling S. (2006) Timescales of the solar proto-planetary disk. In Meteorites in the Early Solar System II (D. S.Lauretta and H. Y. McSween Jr., eds.), this volume. Univ. ofArizona, Tucson.

Scherstén A., Elliott T., Russell S., and Hawkesworth C. J. (2004)W isotope chronology of asteroidal core formation (abstract).Geochim. Cosmochim. Acta, 68, A729.

Shen J. J., Papanastassiou D. A., and Wasserburg G. J. (1996)Precise Re-Os determinations and systematics of iron meteor-ites. Geochim. Cosmochim. Acta, 60, 2887–2900.

Shu F. H., Shang H., and Lee T. (1996) Toward an astrophysicaltheory of chondrites. Science, 271, 1545–1552.

Shukolyukov A. and Lugmair G. W. (1993a) Live iron-60 in theearly solar system. Science, 259, 1138–1142.

Shukolyukov A. and Lugmair G. W. (1993b) 60Fe in eucrites.Earth Planet. Sci. Lett., 119, 159–166.

Shukolyukov A. and Lugmair G. W. (2004) Manganese-chromiumisotope systematics of enstatite chondrites. Geochim. Cosmo-chim. Acta, 68, 2875–2888.

Shukolyukov A., Lugmair G. W., and Bogdanovski O. (2003)Manganese-chromium isotope systematics of Ivuna, Kainsaz,and other carbonaceous chondrites (abstract). In Lunar andPlanetary Science XXXIV, Abstract #1279. Lunar and PlanetaryInstitute, Houston (CD-ROM).

Smoliar M. I. (1993) A survey of Rb-Sr systematics of eucrites.Meteoritics, 28, 105–113.

Smoliar M. I., Walker R. J., and Morgan J. W. (1996) Re-Os ages


of group IIA, IIIA, IVA, and IVB iron meteorites. Science, 271,1099–1102.

Spivak-Birndorf L., Wadhwa M., and Janney P. E. (2005) 26Al-26Mg chronology of the D’Orbigny and Sahara 99555 angrites(abstract). Meteoritics & Planet. Sci., 40, A145.

Srinivasan G. (2002) 26Al-26Mg systematics in eucrites A881394,A87122 and Vissannapeta (abstract). Meteoritics & Planet.Sci., 37, A135.

Srinivasan G., Goswami J. N., and Bhandari N. (1999) 26Al ineucrite Piplia Kalan: Plausible heat source and formation chro-nology. Science, 284, 1348–1350.

Sugiura N. and Hoshino H. (2003) Mn-Cr chronology of fiveIIIAB iron meteorites. Meteoritics & Planet. Sci., 38, 117–143.

Swindle T. D., Kring D. A., Burkland M. K., Hill D. H., andBoynton W. V. (1998) Noble gases, bulk chemistry, and pe-trography of olivine-rich achondrites Eagles Nest and LewisCliff 88763: Comparison to brachinites. Meteoritics & Planet.Sci., 33, 31–48.

Tachibana S. and Huss G. R. (2003) The initial abundance of 60Fein the solar system. Astrophys. J. Lett., 588, L41–L44.

Tatsumoto M., Knight R. J., and Allègre C. J. (1973) Time dif-ferences in the formation of meteorites as determined from theratio of lead-207 to lead-206. Science, 180, 1279–1283.

Tera F., Carlson R. W., and Boctor N. Z. (1997) Radiometric agesof basaltic achondrites and their relation to the early history ofthe solar system. Geochim. Cosmochim. Acta, 61, 1713–1731.

Tonui E. K., Ngo H. H., and Papanastassiou D. A. (2003) Rb-Srand Sm-Nd study of the D’Orbigny angrite (abstract). In Lunarand Planetary Science XXXIV, Abstract #1812. Lunar and Plan-etary Institute, Houston (CD-ROM).

Torigoye-Kita N., Misawa K., and Tatsumoto M. (1995a) U-Th-Pb and Sm-Nd isotopic systematics of the Goalpara ureilites:Resolution of terrestrial contamination. Geochim. Cosmochim.Acta, 59, 381–390.

Torigoye-Kita N., Tatsumoto M., Meeker G. P., and Yanai K.(1995b) The 4.56 Ga age of the MET 78008 ureilite. Geochim.Cosmochim. Acta, 59, 2319–2329.

Trinquier A., Birck J. L., and Allègre C. J. (2005a) 54Cr anomaliesin the solar system: Their extent and origin (abstract). In Lunarand Planetary Science XXXVI, Abstract #1259. Lunar andPlanetary Institute, Houston (CD-ROM).

Trinquier A., Birck J. L., and Allègre C. J. (2005b) Reevaluationof the 53Mn-53Cr systematic in the basaltic achondrites (ab-stract). In Lunar and Planetary Science XXXVI, Abstract#1946. Lunar and Planetary Institute, Houston (CD-ROM).

Unruh D. M., Nakamura N., and Tatsumoto M. (1977) Historyof the Pasamonte achondrite: Relative susceptibility of the Sm-Nd, Rb-Sr, and U-Pb systems to metamorphic events. EarthPlanet. Sci. Lett., 37, 1–12.

Vanhala H. A. T. and Boss A. P. (2002) Injection of radioactivitiesinto the forming solar system. Astrophys. J., 575, 1144–1150.

Wadhwa M. and Lugmair G. W. (1995) Sm-Nd systematics of theeucrite Chervony Kut (abstract). In Lunar and Planetary Sci-ence XXVI, pp. 1453–1454. Lunar and Planetary Institute,Houston.

Wadhwa M. and Lugmair G. W. (1996) Age of the eucrite “Cal-dera” from convergence of long-lived and short-lived chronom-eters. Geochim. Cosmochim. Acta, 60, 4889–4893.

Wadhwa M., Shukolyukov A., and Lugmair G. W. (1998) 53Mn-53Cr systematics in Brachina: A record of one of the earliestphases of igneous activity on an asteroid (abstract). Meteoritics& Planet. Sci., 29, 1480.

Wadhwa M., Shukolyukov A., Davis A. M., Lugmair G. W., andMittlefehldt D. W. (2003) Differentiation history of the meso-

siderite parent body: Constraints from trace elements and man-ganese-chromium isotopic systematics of Vaca Muerta silicateclasts. Geochim. Cosmochim. Acta, 67, 5047–5069.

Wadhwa M., Amelin Y., Bogdanovski O., Shukolyukov A., Lug-mair G. W. and Janney P. E. (2005) High precision relative andabsolute ages for Asuka 881394, a unique and ancient basalt(abstract). In Lunar and Planetary Science XXXVI, Abstract#2126. Lunar and Planetary Institute, Houston (CD-ROM).

Warren P. H. and Kallemeyn G. W. (1989) Allan Hills 84025: Thesecond brachinites, far more differentiated than Brachina, andan ultramafic achondritic clast from the L chondrite Yamato75097. Proc. Lunar Planet. Sci. Conf. 19th, pp. 475–486.

Wasserburg G. J., Tera F., Papanastassiou D. A., and Huneke J. C.(1977) Isotopic and chemical investigations on Angra dos Reis.Earth Planet. Sci. Lett., 35, 294–316.

Weigel A., Eugster O., Koeberl C., and Krähenbühl U. (1996)Primitive differentiated achondrite Divnoe and its relationshipto brachinites (abstract). In Lunar and Planetary ScienceXXVII, pp. 1403–1404. Lunar and Planetary Institute, Houston.

Wiechert U. H., Halliday A. N., Palme H., and Rumble D. (2004)Oxygen isotope evidence for rapid mixing of the HED meteor-ite parent body. Earth Planet. Sci. Lett., 221, 373–382.

Yamaguchi A., Taylor G. J., Keil K., Floss C., Crozaz G., NyquistL. E., Bogard D. D., Garrison D., Reese Y., Wiesman H. andShih C.-Y. (2001) Post-crystallization reheating and partialmelting of eucrite EET 90020 by impact into the hot crust ofasteroid 4 Vesta ~4.50 Ga ago. Geochim. Cosmochim. Acta, 65,3577–3599.

Yin Q. and Jacobsen S. B. (2003) The initial 182W/183W and 182Hf/180Hf of the solar system and a consistent chronology with Pb-Pb ages (abstract). In Lunar and Planetary Science XXXIV,Abstract #1857. Lunar and Planetary Institute, Houston (CD-ROM).

Yin Q., Jacobsen S. B., and Yamashita K. (2002a) Diverse super-nova sources of pre-solar material inferred from molybdenumisotopes in meteorites. Nature, 415, 881–883.

Yin Q., Jacobsen S. B., Yamashita K., Blichert-Toft J., Télouk P.,and Albarède A. (2002b) A short time scale for terrestrial planetformation from Hf-W chronometry of meteorites. Nature, 418,949–952.

Young E., Simon J., Galy A., Russell S. S., Tonui E., and LoveraO. (2005) Supra-canonical 26Al/27Al and the residence time ofCAIs in the solar protoplanetary disk. Science, 308, 223–227.

Zinner E. (1996) Presolar material in meteorites: An overview.In Astrophysical Implications of the Laboratory Study of Pre-solar Materials (T. J. Bernatowicz and E. K. Zinner, eds.),pp. 3–26. AIP Conference Proceedings 402, American Insti-tute of Physics, New York.

Zipfel J., Shukolyukov A., and Lugmair G. W. (1996) Manganese-chromium systematics in the Acapulco meteorite (abstract).Meteoritics & Planet. Sci., 31, A160.

Zhu X. K., Guo Y., O’Nions R. K., Young E. D., and Ash R. D.(2001) Isotopic homogeneity of iron in the early solar nebula.Nature, 412, 311–313.

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