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Timescales of Planetesimal Differentiation in the Early Solar System Wadhwa et al.: Timescales of Planetesimal Differentiation 715 Timescales of Planetesimal Differentiation in the Early Solar System M. Wadhwa The Field Museum G. Srinivasan University of Toronto R. W. Carlson Carnegie Institution of Washington Chronological investigations of a variety of meteoritic materials indicate that planetesimal differentiation occurred rapidly in the early solar system. Some planetesimals only experienced the earliest stage of the onset of melting and were subsequently arrested in that state. On some of these bodies, such incipient melting began ~3–5 m.y. after the formation of calcium-alumi- num-rich inclusions (CAI), but possibly extended to ≥10 m.y. on others. Extensive differentia- tion took place on other planetesimals to form silicate crust-mantle reservoirs and metallic cores. Crust formation began within ~2 m.y. of CAI formation, and likely extended to at least ~10 m.y. Global mantle differentiation, which established the source reservoirs of the crustal materials, occurred contemporaneously for the angrites and eucrites, within ~2–3 m.y. of CAI formation. Metal segregation on different planetesimals occurred within a narrow time interval of ≤5 m.y. In the specific case of the howardite-eucrite-diogenite (HED) parent body, the process of core formation is estimated to have occurred ~1 m.y. before global mantle differentiation. Follow- ing metal segregation, core crystallization is likely to have occurred over a time interval of 10 m.y. or less. 1. INTRODUCTION drites that have near-chondritic bulk compositions (e.g., brachinites, acapulcoites, lodranites, and winonaites) and Within the last decade, significant advances have been formed during the earliest stages of melting on planetesi- made in our understanding of the timescales involved in the mals (which were subsequently arrested at this stage and differentiation history of planetesimals in the early solar did not differentiate further); and (2) meteorites that resulted system. In particular, the application of several chronom- from more extensive differentiation such that their bulk eters based on short-lived radionuclides (i.e., with half-lives compositions are significantly different from those of chon- less than ~100 m.y.) has allowed unprecedented time res- drites. The latter are in turn comprised of three classes of olution (often ≤1 m.y. for events that occurred close to materials: (1) achondrites that formed within the silicate ~4.6 b.y. ago). crust of differentiated planetesimals, such as eucrites, diog- The earliest differentiation events in solar system history enites, and howardites (or HED meteorites), angrites, and were those that occurred between nebular gas and the ear- HED-like silicate clasts in mesosiderites; (2) metal-silicate liest condensates that are represented by components within meteorites (pallasites) that may have been formed near the chondritic meteorites. Timescales involved in such events, core-mantle boundary; and (3) metal-rich meteorites that are which likely occurred during the accretion disk phase of the products of metal segregation from essentially chondritic the nebula, are discussed elsewhere in this book (Russell precursors and some of which may represent the cores of et al., 2006). In this chapter, we will focus on the timescales extensively differentiated planetesimals. involved in the differentiation events that occurred on plan- Age determinations on these meteoritic materials with etesimals, defined here as small asteroidal-sized bodies (up the appropriate chronometers then make it possible to esti- to hundreds of kilometers in diameter) that accreted from mate the timing of planetesimal differentiation events rang- nebular dust and the earliest condensates. The processes ing from incipient melting to more extensive processing likely to be involved in planetesimal differentiation are dis- involving silicate differentiation (including crust formation cussed in more detail in McCoy et al. (2006). and fractionation within the mantle), metal segregation, and The meteoritic record includes a variety of materials re- core crystallization. Thus far, chronometers based on both sulting from planetesimal differentiation. These materials long- and short-lived radionuclides have been applied ex- are primarily composed of two types: (1) primitive achon- tensively for age-dating such events. Long-lived chronom- 715 716 Meteorites and the Early Solar System II TABLE 1. Selected short-lived radioisotopes utilized so far in constraining the timescales of planetesimal differentiation. Radioisotope Half-life Daughter Reference Solar System Time Anchor* (R*) (m.y.) Isotope (D*) Isotope (R) Initial Ratio (R*/R)0 (if any) 26Al 0.72 26Mg 27Al ~5 × 10–5 CAIs –5 (R*/R)T = 5 × 10 at 4.567 Ga 60Fe 1.5 60Ni 56Fe ~3–10 × 10–7 53Mn 3.7 53Cr 55Mn ~10–5 LEW 86010 Angrite –6 (R*/R)T = 1.25 × 10 at 4.558 Ga 107Pd 6.5 107Ag 108Pd ~5 × 10–5 Gibeon (IVA) Iron –5 (R*/R)T = 2.4 × 10 at ~4.56 Ga 182Hf 9 182W 180Hf 1.0–1.6 × 10–4 CAIs and Chondrites –4 (R*/R)T = 1 × 10 at ~4.56 Ga 146Sm 103 142Nd 144Sm ~7 × 10–3 Angrites –3 (R*/R)T = 7 × 10 at 4.558 Ga *References: 26Al: Lee et al. (1976), Amelin et al. (2002), and references therein; 60Fe: Tachibana and Huss (2003), Mostefaoui et al. (2004); 53Mn: Lugmair and Galer (1992), Lugmair and Shukolyukov (1998); 107Pd: Chen and Wasserburg (1996); 182Hf: Kleine et al. (2002), Yin et al. (2002b), Quitté and Birck (2004); 146Sm: Lugmair and Galer (1992) and references therein. We note that some recent investigations indicate that the initial 26Al/27Al ratio at the time of CAI formation was possibly as high as ~7 × 10–5 (e.g., Young et al., 2005). Since Amelin et al. (2002) determined a 26Al/27Al ratio of 4.63 ± 0.44 in the same Efremovka CAI (E60) that had a Pb-Pb age of 4567.4 ± 1.1 Ma, we will assume here that the initial 26Al/27Al ratio at the time of CAI formation was near-canonical (i.e., ~5 × 10–5). eters, however, typically do not have the time resolution afforded by the U-Pb chronometer compared to chronom- required to resolve events occurring within the first tens of eters based on other long-lived radioisotopes results from millions of years of solar system history. In contrast, short- the rapid evolution of the radiogenic Pb isotopic composi- lived chronometers can have high time resolution (i.e., a tion (i.e., 207Pb/206Pb) due to the relatively short half-life million years or less) and are thus capable of precisely age of 235U and the occasionally very high U/Pb ratios in plan- dating the earliest solar system events. Nevertheless, the etesimals and planets affected by volatile loss and/or core application of chronometers based on short-lived radionu- formation since Pb is both volatile and chalcophile, but U clides is not without its challenges. In particular (and pro- is neither. vided there are no other complications such as heterogeneity in the initial distribution of the parent radionuclide and/or 2. INITIAL CONDITIONS later disturbance of the isotope systematics), isochrons based on short-lived radionuclides can provide only relative ages Prior to a discussion of the timescales of planetesimal since the slope of such an isochron reflects the abundance differentiation, we briefly summarize our current under- of the now-extinct radionuclide at the time of isotopic clo- standing of the initial conditions prevalent in the solar neb- sure following a parent-daughter fractionation event. There- ula immediately prior to the formation of these planetesi- fore, since the high-resolution chronometers based on short- mals. More detailed discussions of nebular conditions and lived radionuclides can provide only relative time differ- processes may be found elsewhere in this book (e.g., the ences between early solar system events, it is essential to chapters in Part III). We note that an understanding of con- have a “time anchor” [i.e., a sample that allows one to de- ditions and processes in the solar nebula (especially the de- termine the value of the parameter (R*/R)T at a precisely gree to which it was homogenized) is particularly important defined absolute time T, where R* is the abundance of the with regard to the application of the high-resolution chro- radioisotope and R is that of a stable reference isotope of nometers based on short-lived radionuclides, since factors the same element] so as to map the relative ages derived such as the abundance and distribution of the parent radio- from such a chronometer onto an absolute timescale. The nuclides are critical in determining the feasibility of age- only absolute chronometer capable of providing time reso- dating events in the early solar system. lution comparable to that of chronometers based on short- Table 1 provides a listing of selected short-lived radio- lived radionuclides, and thus providing an appropriate time nuclides that have so far been applied toward determining anchor, is the U-Pb chronometer. A unique attribute of this the timescales involved in planetesimal differentiation. This chronometer is that it is based on the decay of two long- table includes current estimates of the solar system initial 235 207 lived radioisotopes, i.e., U that decays to Pb with a abundance ratios (R*/R)0, which are estimates of the ini- half-life of 703.8 m.y., and 238U that decays to 206Pb with tial abundances of these radionuclides in the meteorite- a half-life of 4469 m.y. The extraordinarily high precision forming region of the solar nebula at the time of formation Wadhwa et al.: Timescales of Planetesimal Differentiation 717 of the earliest condensates, and the refractory CAIs found neities were present in the nebula on the small scale, i.e., on in primitive chondrites, the most precise age estimate for the scale of micrometer-sized presolar grains and millime- which is currently 4567.2 ± 0.6 Ma (Amelin et al., 2002).
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