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/Al 2O 3 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. SiO 2 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 H 2 from 1 × 10 −15 to 1 bar, with the ratio of the fraction of the SiO 2 evaporated to that for MgO increasing both with increasing temperature from 1700 to 2000 K and with increasing SiO 2 content of the starting composition. Such calculations show that compositions of most Type B inclusions can be explained by non-equilibrium evaporation of 10 to 30% of the MgO and 0 to 15% of the SiO 2 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 SiO 2 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.