Trace elements in the Allende meteorite—III. Coarse-grained inclusions revisited

Abstract

New RNAA determinations of Ba, Sr, Zr, U, Re, Pd, Ag, Zn and Se and INAA measurements of Lu are added to published data for 21 other elements in the same suite of ten samples. On the average, 21 refractory elements are not significantly fractionated from one another. The mean of their enrichment factors relative to Cl chondrites is 17.5 ± 0.4, indicating that the high-temperature condensate inclusions represent 5.7 wt% of the total condensable matter. Os, Ir, Ru, Re and most of the W condensed in one or more refractory siderophile element alloys along with small fractions of the Pd, Co, Au and Ag. The bulk of the Eu and Sr condensed in solid solution in melilite. Sc, Zr, Hf, Ta, U and the remaining REE condensed in a phase whose abundance in the inclusions is negatively correlated with that of melilite, either diopside or one or more minor or trace phases, including perovskite. Ba condensed in a different phase, separately from all these elements. In individual inclusions, fractionations are common between elements which were carried in by different condensate phases. Smaller fractionations are also observed for elements which condensed together. These may be due to variable proportions of them in a common condensate phase in response to different nebular equilibration temperatures or to multiple condensate phases containing different proportions of these elements. Available evidence indicates that some trace elements no longer reside in the phases which carried them into the inclusions, indicating a post-accretion thermal event which redistributed some of them. From the minimal variation of the Zr/Hf ratio in the inclusions, the solar system ratio is estimated to be 29.6 ± 1.8. From the mean U content of the inclusions and estimates of the bulk terrestrial and lunar U abundances, the Earth and Moon are estimated to contain 21% and 22–30% high-temperature condensates, respectively.