Oxygen isotopic constraints on the origin and parent bodies of eucrites, diogenites, and howardites

摘要

A few eucrites have anomalous oxygen isotopic compositions. To help understand their origin and identify additional samples,we have analyzed the oxygen isotopic compositions of 18 eucrites and four diogenites. Except for five eucrites, these meteorites have D17O values that lie within 2r of their mean value viz., 0.242 ± 0.016&, consistent with igneous isotopic homogenization of Vesta. The five exceptional eucrites—NWA 1240, Pasamonte (both clast and matrix samples), PCA 91007, A-881394, and Ibitira—have D17O values that lie, respectively, 4r, 5r, 5r, 15r, and 21r away from this mean value.NWA 1240 has a d18O value that is 5r below the mean eucrite value. Four of the five outliers are unbrecciated and unshocked basaltic eucrites, like NWA 011, the first eucrite found to have an anomalous oxygen isotopic composition. The fifth outlier,Pasamonte, is composed almost entirely of unequilibrated basaltic clasts. Published chemical data for the six eucrites with anomalous oxygen isotopic compositions (including NWA 011) exclude contamination by chondritic projectiles as a source of the oxygen anomalies. Only NWA 011 has an anomalous Fe/Mn ratio, but several anomalous eucrites have exceptional Na, Ti, or Cr concentrations. We infer that the six anomalous eucrites are probably derived from five distinct Vesta-like parent bodies (Pasamonte and PCA 91007 could come from one body). These anomalous eucrites, like the isotopically normal,unbrecciated eucrites with 4.48 Gyr Ar-Ar ages, are probably deficient in brecciation and shock effects because they were sequestered in small asteroids (10 km diameter) during the Late Heavy Bombardment following ejection from Vesta-like bodies. The preservation of Vesta’s crust and the lack of deeply buried samples from the hypothesized Vesta-like bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets,rather than by collisional grinding.