Highly siderophile element and osmium isotope evidence for postcore formation magmatic and impact processes on the aubrite parent body

Abstract

Abstract-Aubrites exhibit a wide range of highly siderophile element (HSE-Re, Os, Ir, Ru, Rh, Pt, Pd, Au) Os ratios of 0.127 to 0.130, which is in the range of enstatite chondrites. Along with the brecciated nature of aubrites, the HSE and Re-Os isotope systematics support a history of extensive postaccretion processing, including core formation, late addition of chondritic material and ⁄ or core material and potential breakup and reassembly. Highly siderophile element signatures for some aubrites are consistent with a mixing of HSE-rich chondritic fragments with a HSE-free aubrite matrix. The enrichments in incompatible HSE such as Pd and Re observed in some aubrites, reminiscent of terrestrial basalts, suggest an extensive magmatic and impact history, which is supported by both the 187 Re-187 Os isotope system and silicate-hosted isotope systems (Rb-Sr, K-Ar) yielding young formation ages of 1.3-3.9 Ga for a subset of samples. Compared with other differentiated achondrites derived from small planetary bodies, aubrites show a wide range in HSE concentrations and 187 Os ⁄ 188Os, most similar to angrites. While similarities exist between the diverse groups of achondrites formed early in solar system history, the aubrite parent body(ies) clearly underwent a distinct evolution, different from angrites, brachinites, ureilites, howardites, eucrites, and diogenites.

Highly Siderophile Elements in Earth, Mars, the Moon, and Asteroids

“…It is more challenging to provide similar estimates for other planetary bodies, given either limited available datasets, or evidence for heterogeneity. This is exemplified by HSE abundance and Os isotope datasets obtained for diogenites (Day et al 2012a;Dale et al 2012), aubrites (van Acken et al 2012), and angrites (Riches et al 2012), which show up to five orders of magnitude HSE abundance variation, but in broadly chondritic-relative proportions. For these reasons, we do not report estimated mantle or crustal compositions for asteroidal parent bodies due to the large uncertainties associated with making these calculations.…”

“…Aubrites, or enstatite achondrites, are generally brecciated pyroxenites that consist primarily of FeO-free enstatite and formed under highly reducing conditions (IW-6 to IW-8; Mittlefehldt et al 1998). Aubrites are interpreted to have formed on one or more parent body(ies) after core formation (Casanova et al 1993), as the low HSE concentrations (Wolf et al 1983;van Acken et al 2012) are consistent with removal of metal to form a core (Righter 2003). Aubrites span a wide range in HSE concentrations, from one to three orders of magnitude lower than in chondrites, with the exception of the Shallowater and Mt.…”

A partial melting study of an ordinary (H) chondrite composition with application to the unique achondrite Graves Nunataks 06128 and 06129

“…All of these suites (HED, angrite, and aubrite) are depleted in not only volatile and moderately volatile elements but also siderophile elements (e.g., Mittlefehldt 2003). This suggests that, although many contrasting models have been proposed for the petrogenesis of these suites, all of the models should require extensive asteroidal differentiation that could segregate metallic cores (e.g., Stolper 1977;Righter and Drake 1997;Greenwood et al 2005;Markowski et al 2007;Van Acken et al 2012;Day et al 2012b;Riches et al 2012).…”

Use of Hydrofluoric Acid Desilicification in the Determination of Highly Siderophile Element Abundances and Re‐Pt‐Os Isotope Systematics in Mafic‐Ultramafic Rocks

“…, van Acken et al . , Day and Walker ). In particular, HSE abundances, and the 187 Re‐ 187 Os and 190 Pt‐ 186 Os systems that are embedded within them, have been used to study key processes in planetary and terrestrial mantle evolution, including the relative contributions and timing of post‐core formation late accretion, chronology of mantle and crustal melting events, compositions of mantle sources, and the fractionation behaviour of the HSE during partial melting and melt‐depletion.…”