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

Abstract

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 ∆ 17 O values that lie within 2σ 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 ∆ 17 O values that lie respectively 4σ, 5σ, 5σ, 15σ, and 21σ away from this mean value. NWA 1240 has a δ 18 O value that is 5σ 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 many unbrecciated eucrites from Vesta, 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 Vestalike bodies are consistent with the removal of these bodies from the asteroid belt by gravitational perturbations from planets and protoplanets, rather than by collisonal grinding.

Iron isotope fractionation in planetary crusts

“…Although its texture indicates a rock formed from a rapidly-cooled melt, its composition is more akin to cumulate eucrites and led Barrat et al (2003) to propose an impact-melt origin. Nevertheless, these ungrouped eucrites are probably not from asteroid 4-Vesta, but from Vesta-like asteroid parent-bodies (Scott et al, 2009).…”

“…Howardites are polymict breccias and are mixtures primarily of diogenite and eucrite clasts. In addition, we have analyzed Pasamonte and NWA1240, two ungrouped eucrites recently found to have oxygen isotope compositions distinct from all other HEDs (Scott et al, 2009). Pasamonte is a polymict breccia that displays highly unequilibrated basaltic clasts (Takeda et al, 1978), and possibly records fluid-rock interactions (Schwartz and McCallum, 2005).…”

Zinc isotopes in HEDs: Clues to the formation of 4-Vesta, and the unique composition of Pecora Escarpment 82502

“…With a few exceptions (Scott et al, 2009a), they share the same D 17 O ratios that distinguish them from the Earth or the Moon (Clayton and Mayeda, 1996;Greenwood et al, 2005). Most eucrites are pigeonite-plagioclase basalts, most diogenites are orthopyroxenites, and howardites are mechanical mixtures of these (with some exceptions; see Warren et al (2009)).…”

40Ar/39Ar impact ages and time–temperature argon diffusion history of the Bunburra Rockhole anomalous basaltic achondrite

“…A distinct Δ 17 O value suggests that the Bunburra achondrite either belongs to a unique parent body not previously sampled before, or originated from an isotopically unequilibrated part of 4 Vesta. Yamaguchi et al (2002) and others (e.g., Scott et al, 2009), suggested that the latter hypothesis is less likely as the complete melting and differentiation of the parent body should have resulted in complete oxygen homogenisation.…”

“…Due to their distinct Δ 17 O signatures, these anomalous basaltic achondrites have been proposed to: (1) belong to distinct parent bodies, similar in nature, but unrelated to 4 Vesta, (2) originate from 4 Vesta but from a region where the parent body did not undergo complete oxygen homogenisation (Scott et al, 2009;Yamaguchi et al, 2002), and (3) originate from Vesta, but prior to or during impact ejection had their oxygen composition modified by an impactor 'contaminant' or late veneers (Greenwood et al, 2013).…”