Equilibrium condensation of a solar composition gas, revisited: The anorthite effect
Glenn J. MacPherson, Michail I. Petaev
Geochimica et Cosmochimica Acta
Available online 31 December 2024
“New full equilibrium condensation calculations for a hot gas of solar composition show that anorthite condenses prior to forsterite at nebular pressures of 10−6, 10−5, 10−4, and 10−3 bars. Because of this difference relative to most previous condensation calculations, the predicted bulk composition trend for total condensed solids now more closely matches the trend defined by natural refractory inclusion bulk compositions. Especially this is true for Type B, Type C, and fine-grained spinel-rich inclusions. Some mismatch exists between our (and others’) calculations with respect to the MgO and SiO2 compositions of natural inclusions. This is likely due to the kinetically-controlled condensation of spinel prior to melilite. We also explored the effects of pyroxene solid solution models and small degrees of fractional condensation, and found no significant effects on the condensation sequence. Although fractional condensation certainly occurred in the pre-solar nebula, our calculations require the degree of such fractionation to have been less than ∼1 %. Finally, although mass-dependent isotopic fractionation in Type B inclusions indicates some evaporative loss of magnesium and silicon during the molten stage of Type B inclusions, our results remove the necessity that such evaporation occurred in order to explain the bulk compositions of Type Bs. Nevertheless, our results are not incompatible with such evaporative loss.”
