Allende inclusions: volatile-element distribution and evidence for incomplete volatilization of presolar solids |
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| Authors: | Philip A Baedecker John T Wasson |
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| Institution: | Institute of Geophysics and Planetary Physics, Departments of Chemistry and Geophysics and Space Physics, University of California, Los Angeles, California 90024, U.S.A. |
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| Abstract: | Neutron activation data on 14 trace elements in Allende bulk samples and in fractions of spheroidal Ca-Al-rich inclusions show several distinct distribution patterns. Refractories Ir and Sc have high inclusion/bulk ratios and show little variation with depth. Manganese, Fe, Co, Ni, Ga, Cd and In have low inclusion/bulk ratios and decrease with increasing depth; their presence in the inclusions reflects matrix contamination. Sodium and other alkalies have high inclusion/bulk ratios (near 0.5) and decrease with increasing depth; their high concentration despite moderate volatility seems related to condensation reactions in which refractory Al-bearing minerals are reactants. Chromium, Zn, Ge and Au show patterns similar to those of the alkalies; this seems to indicate that refractory minerals are reactants in their condensation reactions, but thermodynamic support for this hypothesis has not been found.We propose that the large size of Allende spheroidal inclusions indicates an origin by incomplete vaporization of presolar solid matter followed by recondensation of refractories on a limited number of condensation nuclei. The low abundance of large refractory inclusions in ordinary and enstatite chondrites reflects complete vaporization of presolar solids at their formation locations; constraints on homogeneous nucleation resulted in the simultaneous condensation of refractories and olivine at these locations.Quadruplicate analyses of the Orgueil chondrite are in good agreement with previous determinations with the exception of small systematic differences in Au and Ir. |
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