Primordial compositions of refractory inclusions

Grossman, L. and Simon, S. B. and Rai, V. K. and Thiemens, M. H. and Hutcheon, I. D. and Williams, R. W. and Galy, A. and Ding, T. and Fedkin, A. V. and Clayton, R. N. and Mayeda, T. K. (2008) Primordial compositions of refractory inclusions. Geochimica et Cosmochimica Acta, 72 (12). pp. 3001-3021. DOI 10.1016/j.gca.2008.04.002

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Abstract

Bulk chemical and O-, Mg- and Si-isotopic compositions were measured for each of 17 Types A and B refractory inclusions from CV3 chondrites. After bulk chemical compositions were corrected for non-representative sampling in the laboratory, the Mg- and Si-isotopic compositions of each inclusion were used to calculate its original chemical composition assuming that the heavy-isotope enrichments of these elements are due to Rayleigh fractionation that accompanied their evaporation from CMAS liquids. The resulting pre-evaporation chemical compositions are consistent with those predicted by equilibrium thermodynamic calculations for high-temperature nebular condensates, but only if different inclusions condensed from nebular regions that ranged in total pressure from 10−6 to 10−1 bar, regardless of whether they formed in a system of solar composition or in one enriched in dust of ordinary chondrite composition relative to gas by a factor of 10 compared to solar composition. This is similar to the range of total pressures predicted by dynamic models of the solar nebula for regions whose temperatures are in the range of silicate condensation temperatures. Alternatively, if departure from equilibrium condensation and/or non-representative sampling of condensates in the nebula occurred, the inferred range of total pressure could be smaller. Simple kinetic modeling of evaporation successfully reproduces observed chemical compositions of most inclusions from their inferred pre-evaporation compositions, suggesting that closed-system isotopic exchange processes did not have a significant effect on their isotopic compositions. Comparison of pre-evaporation compositions with observed ones indicates that 80% of the enrichment in refractory CaO + Al2O3 relative to more volatile MgO + SiO2 is due to initial condensation and 20% due to subsequent evaporation for both Types A and B inclusions.

Item Type:	Article
Uncontrolled Keywords:	08AREP 2008 P IA56
Subjects:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Divisions:	05 - Petrology - Igneous, Metamorphic and Volcanic Studies
Journal or Publication Title:	Geochimica et Cosmochimica Acta
Volume:	72
Page Range:	pp. 3001-3021
Identification Number:	10.1016/j.gca.2008.04.002
Depositing User:	Sarah Humbert
Date Deposited:	16 Feb 2009 13:02
Last Modified:	23 Jul 2013 10:07
URI:	http://eprints.esc.cam.ac.uk/id/eprint/341

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