Evaluation of petrologic evidence for high partial pressures of SiO(g) in the solar nebula
Alan E. Rubin
Meteorit Planet Sci. . doi:10.1111/maps.13180
“Although they cannot be ruled out, high partial pressures of SiO(g) in the solar nebula are not required to produce various petrologic features of chondrules. These features include the low‐Ca‐pyroxene‐rich margins of type‐IAB chondrules, high concentrations of SiO2 and Na2O in mesostasis near the surface of some chondrules in LL3.0 Semarkona, and silica‐rich igneous rims around many type‐I chondrules in CR chondrites. It is a misconception that low‐Ca pyroxene phenocrysts are restricted mainly to the periphery of type‐IAB chondrules; instead, they are widely distributed in most of these chondrules. Type‐IAB chondrules can be modeled as having formed by remelting of type‐IB chondrules that contained relict olivine grains derived from disrupted type‐IA chondrules. Semarkona chondrule mesostases that show increased concentrations of SiO2 and Na2O from the core to the rim were probably affected by parent‐body aqueous alteration. The silica‐rich igneous rims around many type‐I chondrules in CR chondrites may have formed by melting of pyroxene‐rich dust aggregates that had been previously enriched in Na, K, and Mn derived from volatilized chondrules. “
