New constraints on the formation of lunar mafic impact melt breccias from S-Se-Te and highly siderophile elements

Abstract

Mass fractions of the siderophile volatile elements S, Se, and Te were determined together with highly siderophile elements (HSE) and osmium isotope ratios in multiple aliquots of five lunar mafic impact melt breccias. The impactites were sampled from presumably Imbrium-related ejecta deposits at the Apollo 14, 15, and 16 landing sites. As in many mafic impact melt breccias, all studied impactites display fractionated siderophile element patterns characteristic of differentiated metal, interpreted to reflect metal-rich impactor material from the core of a differentiated planetesimal. The compositional record of Fra Mauro crystalline matrix breccias and recent constraints on the time of their formation suggest that the admixture of this differentiated metal component to the Procellarum KREEP Terrane occurred before the Imbrium basin was formed. The impact melt rock portion of dimict breccia 61015 displays fractionations of HSE like other mafic impact melt breccias, but CI chondrite-like ratios of S, Se, Te, and Ir. Preservation of these contrasting impactor signatures in a single impactite sample demonstrates mixing of differentiated metal and CI chondrite-like impactor material and their homogenization in an impact melt sheet. Correlations of highly siderophile element ratios between impactites from different Apollo landing sites suggest that siderophile element inventories of many lunar impactites were affected by similar mixing processes. Mass fractions and ratios of S, Se, and Te in other mafic impact melt breccias closely resemble those of pristine mafic target rocks.