Condensation of CaO [sbnd]MgO [sbnd]Al 2O 3[sbnd]SiO 2 liquids from cosmic gases

Abstract

The Berman (1983) activity-composition model for non-ideal liquid solutions in the CaO MgO Al 2O 3 SiO 2 (CMAS) system is incorporated into equilibrium condensation calculations which are used to explore the vapor-solid-liquid stability relations as a function of temperature and total pressure ( P tot) in a gas of solar composition, and as a function of temperature and dust/gas ratio at P tot = 1 × 10 −3 atm in gases produced by total vaporization of systems enriched in interstellar dust relative to the gas compared to solar abundances. Condensate liquids are very non-ideal, suggesting that results of previous attempts to model their formation using ideal solution models are highly inaccurate. As is the case for the Berman (1983) liquid model itself, results of the present calculations are in very good agreement with experimentally determined liquid-crystal phase relations except where intermediate members of solid solution series, such as melilite and fassaite, are predicted to be in equilibrium with liquid, in which cases liquid-crystal equilibration temperatures are overestimated by 50 to 100 K. CMAS liquids are stable in a solar gas at a P tot at least as low as 5 × 10 −2 atm and perhaps as low as 1 × 10 −2 atnt, much lower than previous estimates for liquids of pure forsterite composition, due to the colligative effects of CaO and Al 20 3. CMAS liquids are stable at P tot = 1 × 10 −3 atm in systems with dust/gas enrichment factors at least as low as 16 and perhaps as low as 5 relative to solar abundances. Results of these calculations suggest that, upon cooling, a solid melilite + spinel condensate assemblage, comparable to a Type A refractory inclusion, would react with the vapor to produce a liquid much richer in MgO and SiO 2 than the starting material, at either elevated P tot or enhanced dust/gas ratio. If this partial melt were isolated from further reaction with the nebular gas, it would solidify into a spinel + melilite + fassaite + anorthite assemblage, similar in chemical and mineralogical composition to a Type B refractory inclusion. Forsterite coexists stably with CMAS condensate liquids over wide ranges of P tot and dust/gas ratio, extending to the lowest P tot and dust/gas ratio at which liquids are stable. If the compositions of glass inclusions inside isolated forsterite crystals in the Murchison CM2 chondrite have been modified by precipitation of 25 wt% forsterite as a daughter mineral from the liquid precursors of those glasses, the inclusions could represent condensate liquids that were in equilibrium with forsterite at P tot = 0.3 atm or at dust/gas enrichment factors of ∼70 at P tot = 1 × 10 −3 atm.