Re-Os geochronology and abundances of highly siderophile elements in ancient lunar impact rocks
Final Report Abstract
Non-chondritic Apollo 16 impact melt rocks display characteristic features like high Re/Os (and corresponding moderately suprachondritic 187Os/188Os ratios), increasingly suprachondritic HSE/Ir ratios from refractory to moderately volatile elements and suprachondritic Ru/Pt (the “iron meteorite signature”). These characteristics are interpreted to reflect large-scale fractional crystallization of solid metal from metal melt during core solidification in the parent body of one or several putative impactors. The observed HSE fractionation and the characteristic enrichment of Ru over Pt when normalized to CI chondrite values strongly indicates a light element-rich metal melt with a P/S ratio higher than in most iron meteorite parent bodies. The goal to determine the age of the Imbrium impact and the age of impact which delivered the differentiated HSE signature (not necessarily the same) could not be achieved. Disturbance and sometimes complete internal resetting of the Re-Os system is observed instead. The necessary heating most likely occurred during later impact events. Fe-Ni metal-schreibersite-troilite intergrowths which were observed in Apollo 16 impact melt rocks crystallized from immiscible liquid metal droplets in silicate impact melts. These intergrowths underwent fractional crystallization at the subcentimeter-scale which resulted in compositional variations and strong differences in absolute and relative HSE abundances of the different phases. This process is the main reason for variations in HSE/Ir ratios observed in mgsized aliquots of lunar impact melt rocks. Bulk HSE/Ir ratios derived from regression of multiple aliquot data are therefore prone to larger errors and might lead to inaccurate results. Weighted averages of concentration data of aliquots account for the full element budget of all analyzed aliquots and yield more accurate HSE/Ir ratios of the bulk sample. Modeling of solid metal-liquid metal partitioning in the Fe-Ni-S-P system reveals that fractionated HSE compositions as observed in the samples studied, are representative of liquid metal compositions in the impact melts and that fractional crystallization on the scale of large impact melt pools most likely did not occur. The HSE compositions are therefore in principle useful for impactor discrimination and study of impact and mixing processes. Whereas the HSE record of many studied samples is dominated by broadly chondritic or the differentiated impactor component, the observed range in Se/Te and S/Se ratios require a mixture of different meteoritic components and/or planetary processes that fractionated these elements. Se/Te ratios of three samples fall in the range of chondritic meteorites and are consistent with accretion of differentiated core metal along with chondrite-like material, including volatile-rich carbonaceous chondrite-like material. Se/Te ratios of two samples are strongly suprachondritic and thus, suggesting that ratios in these samples were fractionated due to fractionation processes in the lunar crust or mantle. The compositional record of Apollo 16 impactites shows a mixture of chondritic and a non-chondritic impactor signatures, represented by granulitic impactites and fragmental matrix breccias and KREEP-rich impact melt rocks with strongly fractionated HSE. This observation and the broadly linear correlation between 187Os/188Os ratios and HSE/Ir ratios in impactites from all Apollo landing sites, supports the interpretation of variable mixing of chondritelike impactor material with suprachondritic iron meteorite-like impactor material, and subsequently local homogenization by younger impacts. Although, similar differentiated compositions were only sampled in some main group pallasites, the occurrence of a similar Ru (and Pd) anomaly in the BSE suggests that the material accreted late to Earth possibly contained a considerable fraction of material of similar origin.
Publications
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(2015) Highly siderophile and chalcophile elements in lunar impact melt rocks: evidence for mixing of impactor compositions. GeoBerlin Conference / Annual Meeting of the DMG
Gleißner, P. and Becker, H.
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(2015) Highly siderophile element fractionations in Apollo 16 impact rocks: Origin by small-scale or large-scale processes? 25th Goldschmidt Conference
Gleißner, P. and Becker, H.
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(2016) Highly siderophile element fractionations in Apollo 16 impact melt rocks: Large-scale fractionation processes. 47th Lunar and Planetary Science Conference
Gleißner, P. and Becker, H.
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Formation of Apollo 16 impactites and the composition of late accreted material: constraints from Os isotopes, highly siderophile elements and sulphur abundances. Geochimica et Cosmochimica Acta
Gleißner, P. and Becker, H.
