Noble gases, bulk chemistry, and petrography of olivine-rich achondrites Eagles Nest and Lewis Cliff 88763: Comparison to brachinites

Abstract

Eagles Nest and Lewis Cliff (LEW) 88763 are olivine-rich achondrites that have been suggested to be related to Brachina and ALH 84025. Both are ultramafic, although LEW 88763 has ∼10% plagioclase, which Eagles Nest lacks. Olivine in Eagles Nest (Fo68) is virtually identical in composition to that in Brachina (Fo68-70), and that in LEW 88763 is only slightly different (Fo63-64). The meteorites' cosmic-ray exposure ages differ from each other and from Brachina and ALH 84025. Like Brachina, Eagles Nest has a substantial amount of radiogenic 129Xe and an apparent K-Ar age of <4.5 Ga. Lewis Cliff 88763, on the other hand, has no detectable radiogenic 129Xe, an apparent K-Ar age of ∼4.5 Ga, and a substantial amount of trapped Xe. The chemical composition of LEW 88763 is within a factor of two of chondritic for virtually every element analyzed, including siderophiles, which suggests that it was melted (or at least heated and recrystallized) in a closed system. Analyses of the chemical composition of Eagles Nest are hampered by extensive terrestrial weathering; but it appears to be broadly chondritic also, although it, like Brachina, is depleted in siderophiles. Our data suggest that Eagles Nest is genetically related to Brachina but that LEW 88763 is not, which is a conclusion that is consistent with the O isotopic data of Clayton and Mayeda (1996).