Vapor-condensed phase processes in the early solar system

Abstract

Abstract– Equilibrium thermodynamic calculations of the sequence of condensation of phases from a cooling gas of solar composition at total pressures thought to have prevailed in the inner part of the solar nebula successfully predict the primary mineral assemblages of refractory inclusions in CM2 and CV3 chondrites. Many refractory inclusions in CM2 chondrites contain a relatively SiO2-poor assemblage (spinel, hibonite, grossite, perovskite, corundum) that represents a high-temperature stage of condensation, and some may be pristine condensates that escaped later melting. Compact Type A and Type B refractory inclusions, consisting of spinel, melilite, perovskite, Ca-rich clinopyroxene ± anorthite, in CV3 chondrites are more SiO2-rich and equilibrated with the solar nebular gas at a slightly lower temperature. Textures of many of these objects indicate that they underwent melting after condensation, crystallizing into the same phase assemblage as their precursors. The Ti3+/Ti4+ ratio of their pyroxene indicates that this process occurred in a gas whose oxygen fugacity () was approximately 8.5 log units below that of the iron-wüstite buffer, making them the only objects in chondrites known to have formed in a system whose composition was close to that of the sun. Relative to CI chondrites, these inclusions are uniformly enriched in a group of elements (e.g., Ca, REE, Zr, Ta, Ir) that are chemically diverse except for their high condensation temperatures in a system of solar composition. The enrichment factor, 17.5, can be interpreted to mean that these objects represent either the first 5.7 wt% of the condensable matter to condense during nebular cooling or the residue after vaporization of 94.3% of a CI chondrite precursor. The Mg and Si isotopic compositions of Types A and B inclusions are mass-fractionated by up to 10 and 4 ‰/amu, respectively. When interpreted in terms of Rayleigh fractionation during evaporation of Mg and Si from the inclusions while they were molten, the isotopic compositions imply that up to 60% of the Mg and up to 25% of the Si were evaporated, and that approximately 80% of the enrichment in refractory (CaO+Al2O3) relative to more volatile (MgO+SiO2) in the average inclusion is due to initial condensation and approximately 20% due to subsequent evaporation. The mineralogical composition, including the Ti3+/Ti4+ ratio of the pyroxene, in Inti, a particle sampled from Comet Wild 2 by the Stardust spacecraft, is nearly identical to that of a Type B inclusion, indicating that comets contain not only the lowest-temperature condensates in the form of ices but the highest-temperature condensates as well. The FeO/(FeO+MgO) ratios of olivine and pyroxene in the matrix and chondrules of carbonaceous and ordinary chondrites are too high to be made in a system of solar composition, requiring s only 1 or 2 log units below iron-wüstite, more than 105 times higher than that of a solar gas. Various ways have been devised to generate cosmic gases sufficiently oxidizing to stabilize significant FeO in olivine at temperatures above those where Fe-Mg interdiffusion in olivine ceases. One is by vertical settling of dust toward the nebular midplane, enriching a region in dust relative to gas. Because dust is enriched in oxygen compared to carbon and hydrogen relative to solar composition, a higher results from total vaporization of the region, but the factor by which theoretical models have so far enriched the dust is 10 times too low. Another is by transporting icy bodies from the outer part of the nebula into the hot, inner part where vaporization of water ice occurs. Not only does this method fail to make the needed by a factor of 30–1000 but it also ignores simultaneous evaporation of carbon-bearing ices that would make the even lower.