Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.437a&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 437
Computer Science
Abundance, Chondrule, Meteoritic Composition, Temperature Effects, Volatility, Chemical Equilibrium, Evolution, Gas Pressure, Liquidus
Scientific paper
Recently there has been renewed interest in the role of volatility in chondrule formation. Estimated peak temperatures chondrules experienced, based on their liquidus temperatures, range from 1500 to 2000 K. At these temperatures and nebular pressures most elements are volatile, but generally the alkali metals and S have been the only major elements considered as such. Correlated variations in Mg and Al abundances, if they are not due to precursor compositions, require the loss of up to 50% of the SiO2 from some chondrules. Appeals to the loss of Si to explain the range of chondrule compositions is problematical because experiments suggest that Mg and Si evaporation rates are, at least above 2000 K, comparable. Equilibrium gas pressures can be used to predict the degree of Si and Mg loss at equilibrium and even, to first order, in a kinetic regime. The ratio of SiO/Mg, the major Mg- and Si-bearing gas species, in equilibrium with a chondritic melt ranges from approximately 10 at 1500 K to 1 at 2000 K. Here it is suggested that chondrules formed by heating of initially FeO-rich dust (low liquidus T) to moderate temperatures of 1500-1800 K when the partial pressures of Si-bearing gas species are significantly higher than the Mg-bearing ones. Loss of Si and the more volatile Fe drives the melt from near its liquidus to subliquidus conditions, producing barred/excentroradial or porphyritic textures depending on whether the original material was entirely molten or not. The degree of volatile loss will depend on both temperature and either time (kinetic) or the density/composition of material in the formation region (equilibrium). Given sufficient time and no reconsideration, chondrules could have approached quasi-equilibrium with the ambient gas prior to cooling, in which case their compositions may be used to set limits on absolute nebular densities and gas/dust ratios. More speculatively, it is possible that chondrules and Ca-Al rich inclusions (CAIs) formed by the same basic process but that the difference in their relative abundances among meteorite classes reflects differences in the dust/gas ratio and absolute density in their formation regions.
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