Mathematics – Logic
Scientific paper
Aug 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.3413y&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 59, Issue 16, p. 3413-3444.
Mathematics
Logic
88
Scientific paper
The Berman (1983) activity-composition model for non-ideal liquid solutions in the CaOMgOAl2O3SiO2 (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 (Ptot) in a gas of solar composition, and as a function of temperature and dust/gas ratio at Ptot = 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 Ptot 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 Al203. CMAS liquids are stable at Ptot = 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 SiO2 than the starting material, at either elevated Ptot 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 Ptot and dust/gas ratio, extending to the lowest Ptot 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 Ptot = 0.3 atm or at dust/gas enrichment factors of ˜70 at Ptot = 1 × 10-3 atm.
Grossman Lawrence
Yoneda Shigekazu
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