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

Abstract

The δ66Zn (permil deviation of the 66Zn/64Zn ratio from a terrestrial standard) values for a suite of 20 non-Antarctic HED (howardite–eucrite–diogenite) meteorites and one mesosiderite, and for eight Antarctic eucrites and diogenites, were measured in order to determine the role of volatization in the formation of their presumed parent body, the asteroid 4-Vesta. The 20 non-Antarctic HEDs had δ66Zn values that ranged from −2.0‰ to +1.67‰, with a mean value of −0.01±0.39‰ (2se); this range likely represents a small-scale heterogeneity due to brecciation induced by multiple impacts. The non-Antarctic eucrites (δ66Zn=+0.00±0.58‰ (2se), n=12) were isotopically the same as the diogenites (δ66Zn=−0.31±0.80‰ (2se), n=4), and the howardites (δ66Zn=+0.26±0.37‰ (2se), n=4). On average, non-Antarctic eucrite falls were isotopically heavier (+0.50‰) than non-Antarctic finds (−1.00‰). The Antarctic finds studied were all unbrecciated samples, and they were significantly heavier than the non-Antarctic samples with a δ66Zn range of +1.63‰ to +6.22‰ for four eucrites (mean, +4.32‰) and +0.94‰ to +1.60‰ for three diogenites (mean+1.23‰), excluding one anomalous sample, while their Zn concentration is significantly lower than the brecciated samples. These data suggest that the unbrecciated eucrites probably represent the eucritic crust shortly after differentiation and cooling of the parent asteroid, at which time volatization of lighter zinc isotopes led to an isotopically heavy crust. Early impact events caused the ejection of these unbrecciated meteorites, which were subsequently spared from brecciation caused by multiple additional impacts on the much larger Vesta. The range of δ66Zn values and Zn concentration for the brecciated HEDs in this study supports a major contribution to the Vestan surface by chondritic impactors (−1.30<δ66Zn<+0.76‰ for ordinary and carbonaceous chondrites). The anomalous eucrite PCA 82502 (δ66Zn=−7.75‰) is significantly isotopically lighter than the other HEDs and is the natural sample with the lightest Zn isotopic composition reported in the solar system to date. This meteorite most likely originated from a distinct parent body.