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Timing of accretion of the primary planetesimals in the early solar system has been investigated using short-lived (e.g., 26Al, 53Mn, 182Hf and 129I) and long-lived (e.g., 238U, 235U, 232Th and 147Sm) radionuclides. The first condensed objects in the solar system are known to be the calcium-aluminum-rich inclusions (CAIs) in chondrites (undifferentiated meteorites mainly composed of Fe- and Mg- bearing silicates), the most abundant meteorites found on Earth. The formation age of the CAIs was precisely determined by 207Pb/206Pb dating at ~4.56–4.57 Ga (Tilton 1988; Allègre et al. 1995; Amelin et al. 2002). Following the formation of the CAIs, more solids were rapidly accreted. Bodies grown to relatively large size (e.g., large terrestrial planets) experienced differentiation forming metallic cores, whereas smaller bodies of asteroids (e.g., parent bodies of chondritic meteorites) underwent relatively mild thermal metamorphism over the next ~15–20 Myr (summarized in Gilmour and Saxton 2001), followed by cooling to the their present thermal status.
Thermal histories of asteroids and planets after primary crystallization or metamorphism have been the subject of considerable attention. One reason for this attention is because the deduced thermal histories provide key information on accretion processes, physical dimensions and heat budget of the early solar bodies. Using Ni distribution between two metallic phases, kamacite and taenite, Wood (1967) deduced cooling rates of ~2–10 °C/Ma at ~500 °C for ordinary chondrites, and showed how this information can be used to estimate the size of the parent body and original depth of individual meteorites in the parent body. Turner et al. (1978) deduced cooling rates at ~240 ± 120 °C by studying 40Ar/39Ar system in 16 unshocked chondrites, and suggested that these are from parent bodies with radii less than 30 km, or …