Osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks

摘要

The concentrations of highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au) and ~(187)Os/ ~(188)Os isotope compositions have been determined for 67 subsamples of six lunar impact rocks from the Apollo 14, 16 and 17 landing sites, and the lunar meteorite Dar al Gani (DaG) 400 using inductively coupled plasma mass spectrometry (ICP-MS) and negative thermal ionization mass spectrometry (N-TIMS).We report the first Re-Os isochron age on a lunar impact melt rock. ~(187)Re- ~(187)Os isotope systematics for Apollo 16 sample 67935 define an isochron age of 4.21±0.13Ga (MSWD=1.5), which is interpreted to reflect localized partitioning processes between solid metal-liquid metal as this rock melted. The new age adds further constraints on the significance of pre-4.0Ga basin forming impacts on the Moon and possible mixing of ancient impactor compositions in lunar impact rocks.Linear correlations displayed by subsamples of a given impact rock in plots of HSE versus Ir concentrations are explained by dilution processes through essentially HSE-free anorthositic lunar crustal target rocks or binary mixing between a high HSE meteoritic end-member and a low HSE end-member composition. Slope-derived HSE ratios and ~(187)Os/ ~(188)Os of the meteoritic component in granulitic impactites 67915, 67955 and 79215 are similar to slightly volatile element depleted carbonaceous chondrites. Suprachondritic ratios of Ru/Ir, Pt/Ir, Rh/Ir, and Pd/Ir for Apollo 14 impact melt rock 14310 are similar to ratios observed for other Apollo 14 samples and Apollo 17 poikilitic impact melt rocks. Apollo 16 poikilitic and subophitic impact melt rocks 60315 and 67935 show slightly subchondritic Os/Ir and suprachondritic ratios of ~(187)Os/ ~(188)Os, Ru/Ir, Pt/Ir, Rh/Ir, Pd/Ir and Au/Ir. Their strongly fractionated HSE compositions are similar to some members of the IVA iron meteorite group and provide further evidence for an iron meteorite impactor component in Apollo 16 impact melt rocks.The range of chondritic to suprachondritic ratios of ~(187)Os/ ~(188)Os, Ru/Ir, Pt/Ir, Pd/Ir, and chondritic to slightly subchondritic Os/Ir of Apollo 17 and 14 impact melt rocks may be explained by variable mixing of a carbonaceous chondrite-like HSE composition (as in granulitic impactites) with a suprachondritic iron meteorite-like impactor composition as inferred from 67935 and 60315, and subsequent homogenization by younger impact events. The inferred dispersal of these two components across the lunar nearside and their antiquity (4.2Ga) suggest at least two basin-forming impacts that predate the 3.9-3.8Ga basin forming era. The HSE composition of the silicate Earth can be matched by a late veneer comprised of ~80% volatile depleted carbonaceous chondrite-like material and ~20% differentiated meteoritic metal, similar to what has been identified in ancient lunar impact rocks.