ZAOYANG CHONDRITE COOLING HISTORY FROM FE2+-MG INTRACRYSTALLINE ORDERING IN PYROXENES

The crystal chemistry of clinopyroxene, orthopyroxene. and olivine from a crushed fragment of the H5 Zaoyang chondrite has been investigated by X-ray structure refinement and detailed microprobe analysis. The meteoritic pyroxenes have cell and polyhedral volumes which compare well with such data from terrestrial pyroxenes that typically crystallize at low-pressure, Fe2+ and Mg are rather disordered in M1 and M2 sites of clino- and orthopyroxenes; the closure temperatures of the exchange reaction are 600 and 512-degrees-C respectively, which is consistent with a reasonably fast cooling rate, estimated to be of the order of 1-degrees-C/day. The closure temperature for the intercrystalline Ca-Mg exchange reaction for clino- and orthopyroxenes is 900-degrees-C, as calculated from clino- and orthopyroxene intergrowth. The cooling rates obtained from Fe2+-Mg intracrystalline partitioning suggest a cooling of the order of degrees per day at temperatures of 600-500-degrees-C due to a strong loss of heat by irradiation.