Low-Temperature Thermochronometry of Meteorites

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 fi rst condensed objects in the solar system are known to be the calcium-aluminum-rich inclusions (CAIs) in chondrites (undifferentiated meteorites mainly composed of Feand Mgbearing 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.