Effects of Pressure Dust Enrichment on Silicate Liquid Stability

D.S. Ebel and L. Grossman
Meteoritical Society Annual Meeting: 1997 (Hawaii; presented by L.G.)

D. Ebel1and L. Grossman1,2, 1Department of the Geophysical Sciences, 5734 South Ellis Ave., 2Enrico Fermi Institute, University of Chicago, Chicago, IL 60637.

Several lines of evidence suggest that most chondrites formed at oxygen fugacities significantly higher than that of a solar gas [1-4]. Enrichment of previously condensed dust relative to gas, followed by its vaporization, emerges as the only reasonable mechanism for such an enhanced oxidation state [1-3]. Although enrichment of dust relative to gas stabilizes FeO in olivine, such enrichment is even more conducive to silicate liquid stability. We have begun a systematic study of the temperature-pressure-composition conditions at which silicate liquids are stable condensates from fully speciated 23-element gases. Details of our method were reported in [5]. Here, we compare results for solar gas enriched 200 and 500 times in dust of C1 chondrite composition, at Ptot=10-3 and 10-6 bar.

Results at 200x dust enrichment and Ptot<=10-3 are nearly identical to those reported for T>1590K in [5] for an 18-element system. Liquid appears at 2000K, olivine(ol) at 1850K, metal alloy at 1730K, orthopyroxene(opx) at 1670K, spinel(sp) at 1660K. Once metal condenses, the proportion of the total Fe condensing as FeO increases less rapidly with falling temperature, continued condensation of SiO2 into the liquid causes its FeO and MgO concentrations to fall with decreasing temperature, and XFa and XFs of co-condensing ol and opx level off. This assemblage persists until feldspar(fsp) appears at 1450K and clinopyroxene(cpx) at 1430K. At 1500K, the liquid is 36 mol% of the oxide condensates, with 59.4 wt% SiO2, 0.72% TiO2, 15.2% Al2O3, 10.9% MgO, 12.7% CaO, 0.80% FeO, 0.024% Na2O, 0.064% K2O. Olivine is fo95, opx is en97. By 1420K, the liquid is 6.6 mol% of the oxide condensates, with 0.74% FeO, 0.19% Na2O, and 0.23% K2 >O. Olivine is fo94, opx is en96, fsp is an92. The solidus is just above 1390K, as in all cases investigated here.

Decreasing Ptot to 10-6 decreases the temperature of liquid appearance to 1550K, that of opx to 1450K, and that of metal to below the solidus. At 1500K, the liquid has 0.15 wt% FeO, and ol is fo100. By 1420K, the liquid has 0.15% FeO, ol is fo98, and opx is en98. Virtually no K2O or Na2O condense above 1400K.

Increasing the dust enrichment to 500x, at Ptot=10-3, causes most phases to appear at higher temperatures. Liquid, in at 2130K, and ol, at 1940K, are joined by metal at 1780K, sp at 1760K, and opx at 1700K. Feldspar and cpx appear at 1430K, and pyrrhotite (Fe0.877S, forming at the expense of metal) at 1420K. At 1500K, the liquid is 35 mol% of the oxide condensates, with 4.49% FeO, 0.32% Na2O, and 0.17% K2O; ol is fo87, opx is en91. At 1420K, the liquid is ~1.5 mol% of the oxides, with 3.42% FeO, 2.16% Na2O, and 0.5% K2O. Olivine is fo85, opx en89, and fsp an69.

At Ptot=10-6 bar, in the 500x case, liquid appears after sp, at 1640K, followed by ol at 1610K. At 1500K, the liquid has only 0.14% FeO and ol is fo98. Opx appears at 1490K. By 1420K, neither metal nor sulfide have condensed, and the liquid has 3.0% FeO and virtually no alkalies. Olivine is fo89, opx is en92, and fsp is an100.

In all four cases, silicate liquid is stable over a wide temperature range, with a solidus just above 1390K. Both increasing Ptot at constant dust enrichment and increasing dust enrichment at constant Ptot cause increases in the condensation temperatures of metal and silicate melt, the temperature where significant FeO, Na2O and K2O enter the liquid, and the maximum FeO content of the liquid.

References: [1] Fegley B. Jr. and Palme H. (1985) EPSL, 72, 311-326. [2] Wood J.A. (1967) GCA, 31, 2095-2108. [3] Rubin A. et al. (1988) in Meteorites and the Early Solar System , 488-511, U. of Arizona, Tucson. [4] Weinbruch S. et al. (1990) Meteoritics, 25, 115-125. [5] Ebel D.S. and Grossman L. (1997) LPS XXVIII, 317-318.

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