Early Chemical History of the Solar System

The extreme antiquity and lack of evidence for significant chemical processing of the chondritic meteorites since they were formed suggest he possibility that their chemistry and mineralogy may have been established uring the condensation of the solar system. By using equilibrium thermodynamics, the sequence of condensation of mineral phases from a cooling nebula of solar composition has been calculated. Applying the predictions of these theoretical models suggests that (1) the chemistry and mineralogy of Ca-Al-rich inclusions in C2 and C3 chondrites were established during condensation at temperatures >1300øK; (2) fractionation of such inclusions is necessary to account for the refractory element depletions of ordinary and enstatite chondrites relative to the carbonaceous chondrites; (3) the metal-silicate fractionation in ordinary chondrites took place in the nebula at T < 1000øK and Ptot • 10 -• arm; (4) the volatile element depletion of C2 and C3 chondrites relative to C1 chondrites took place during chondrule formation; (5) the most volatile elements are depleted in ordinary chondrites because they accreted before these elements were totally condensed; and (6) many chemical features of planetaw rare gases and organic material in carbona.ceous chondrites could have been established during condensation. Chemical fractionation during condensation may also be responsible for the heterogeneous accumulation of the earth, the refractory element enrichment of the moon, and the varying Fe/Si ratios of the terrestrial planets.