PRECISE U-PB GEOCHRONOLOGY AND THERMOCHRONOLOGY OF METEORITES USING U-RICH ACCESSORY MINERALS

Recent developments in U-Pb chronology of meteorites - ultramafic or mafic rocks with low concentration of U about 0.01-0.1 ppm - focused on dating rock-forming minerals such as pyroxene, and whole rocks, and on improvement of leaching methods aimed at complete removal of non-radiogenic Pb. But some meteorites also contain relatively U-rich accessory minerals such as zircon, baddeleyite, perovskite and Ca-phosphates. Meteoritic perovskites contain 3-23 ppm U, whereas meteoritic zircons contain 30-133 ppm U. These minerals are, however, very rare, and their grain sizes are small, so their successful U-Pb dating was performed only by ion microprobe with the best precision of ²⁰⁷Pb/²⁰⁶Pb dates achieved so far of about 5 Ma. In contrast to zircon and perovskite, Ca-phosphates apatite and merrillite are minor minerals with abundances of 0.1-1% in many chondrites and achondrites. Uranium contents in these minerals typically range from ca. 0.03 ppm to 3 ppm, and the precision of ²⁰⁷Pb/²⁰⁶Pb dates reaches ± 0.5-1.0 Ma in favorable cases (Chen and Wasserburg 1981, Göpel et al. 1994). U-Pb dating of meteoritic phosphates coupled with U-Pb dating of silicate minerals makes it possible to calculate cooling rates of meteorite parent bodies and precisely link these cooling rates to the absolute time scale. Previous determinations of the cooling rates of equilibrated (metamorphosed) chondrites of ca. 3-40°C/Ma (Amelin et al. 2005, Blinova et al. 2007) were based on the age differences between phosphates and silicate minerals of more than 10 Ma. An increase in precision of the 207Pb/206Pb dates to ±0.1-0.3 Ma due to use of the ²⁰²Pb-²⁰⁵Pb double spike (Amelin and Davis 2006) allows expanding the application of pyroxene-phosphate thermochronology to faster cooling meteorites, such as plutonic angrites. The ²⁰⁷Pb/²⁰⁶Pb isochron dating of multiple pyroxene and Ca-phosphate fractions from the plutonic angrite NWA 4590 yielded the ages of 4558.69±0.28 Ma and 4558.14±0.07 Ma, respectively, and cooling rate of ca. 500±300°C/Ma (the uncertainty of the cooling rates is mainly controlled by uncertainty in the closure temperatures of Pb diffusion).