Ureilite parent body evolution from the perspective of noble gases and oxygen in samples from the Almahata Sitta strewn field

Abstract

Most ureilites are melt residues from the partially melted Ureilite Parent Body. The Ureilite Parent Body was catastrophically disrupted at ∼ 5 Ma after Calcium-Aluminum rich Inclusions (CAI) while it was still hot and the ureilites provide a unique window into early solar system magmatic processing. One ureilitic trachyandesite, one cumulate, and 16 melt residue ureilites, all from the Almahata Sitta meteorite strewn field, were analyzed for their noble gas compositions and, when such data was unavailable, for oxygen isotopes and petrology. Additionally, ureilite noble gas data from the literature was compiled together with petrology and oxygen isotope data of the same samples, this data is available in the supplementary materials. The compositions of noble gases and oxygen, as well as petrological characteristics, are similar to previously analyzed ureilites. This includes variable ³⁶Ar_tr/¹³²Xe ratios of ∼ 20–1000 correlated with variable ⁸⁴Kr/¹³²Xe ratios of ∼ 0.15–2.5 and Xe isotopic compositions similar to the Q gases but with somewhat lower ^134,136Xe/¹³²Xe ratios. The well-established correlation between Mg-Fe olivine core composition and Δ’¹⁷O, interpreted as material mixing, is corroborated. There is no correlation between noble gas compositions and petrology or Δ’¹⁷O. Therefore, it is unlikely that the variable noble gas elemental ratios are due to mixing of noble gases from different sources, as previously suggested. We suggest that compositional variability was established during implantation of noble gases into disordered carbon prior to accretion and possibly during later processing. We discuss that partial graphitization resulted in noble gas loss, with noble gases remaining in un-graphitized organics, which were converted to diamond during the catastrophic disruption. Noble gases released during graphitization may have entered the melt. Isotopic compositions of trapped noble gases in the cumulate and trachyandesitic rocks, which crystallized from the melt are similar to those in the melt residue ureilites. The elemental noble gas composition of the cumulate shows evidence of a degassing stage and that the concentrations of noble gases in the ureilites were higher before melting. The noble gases in the trachyandesite contains radiogenic noble gases from decay of K, I, Th, and U, which were not enriched in the cumulate, showing that the trachyandesite crystallized from a more evolved melt. The cosmic-ray exposure ages of 15–22 Ma, with mostly overlapping uncertainties, are similar to those previously determined for ureilites from the Almahata Sitta strewn field and display a limited spread in contrast to ages previously detected in Almahata Sitta chondrites.