Major element chemical and isotopic compositions of refractory inclusions in C3 chondrites: The separate roles of condensation and evaporation

Literature data for major element oxide compositions of most coarse-grained Types A and B inclusions in CV3 chondrites may be in error due to non-representative sampling of spinel relative to other phases because of small sample sizes. When reported compositions are corrected to the solar CaO/Al2O3 ratio by addition or subtraction of spinel, distinct trends result on oxide-oxide plots. These trends lie close to trajectories of bulk compositions of equilibrium condensates calculated for solar or dust-enriched gases under various conditions, except on a plot of MgO vs. SiO2 contents, where there is considerable scatter of the data points to the MgO-poor side of the condensation trajectory. The irreversible process of evaporative mass loss from a liquid droplet into an unsaturated H-2 gas is modeled as a series of small equilibrium steps. This model is used to show that evolutionary paths of CMAS liquid compositions are identical for evaporation at all P-H2 from 1 x 10(-15) to 1 bar, with the ratio of the fraction of the SiO2 evaporated to that for MgO increasing both with increasing temperature from 1700 to 2000 It and with increasing SiO2 content of the starting composition. Such calculations show that compositions of most Type B inclusions can be explained by nonequilibrium evaporation of 10 to 30% of the MgO and 0 to 15% of the SiO2 into an H-2 gas at 1700 K from liquid droplets whose compositions originated on any one of many possible equilibrium condensation trajectories. Some Type As may have suffered similar evaporative losses of MgO and SiO2 but at higher temperature. This degree of evaporation is consistent with the amount of Mg and Si isotopic mass fractionation observed in Types A and B inclusions. Evaporation probably happened after most Mg and Si were removed from the nebular gas into lower-temperature condensates. Copyright (C) 2000 Elsevier Science Ltd.