Diverse impactors in Apollo 15 and 16 impact melt rocks: Evidence from osmium isotopes and highly siderophile elements

Concentrations of highly siderophile elements (HSE) and 187Os/188Os isotopic compositions for eleven impact related rocks from the Apollo 15 and 16 landing sites are reported and combined with existing geochronological data to investigate the chemical nature and temporal changes in the large impactors implicated in the formation of the lunar basins. Data for the samples all define linear trends on plots of HSE versus Ir concentrations, whose slopes likely reflect the relative HSE compositions of the dominant impactors that formed the rocks. The inferred Imbrium basin impactor that generated Apollo 15 impact melt rocks 15445 and 15455 was characterized by modestly suprachondritic 187Os/188Os, Ru/Ir, Pt/Ir and Pd/Ir ratios. Diverse impactor components are revealed in the Apollo 16 impact melt rocks. The 187Os/188Os and HSE/Ir ratios of the impactor components in melt rocks 60635, 63595 and 68416, with reported ages 4.0Ga. By contrast, the impactor components in melt rocks 60235, 62295 and 67095, with reported ages of ∼3.9Ga, are characterized by suprachondritic 187Os/188Os and HSE/Ir ratios similar to the Apollo 15 impact melt rocks, and may also sample the Imbrium impactor. Three lithic clasts from regolith breccias 60016 and 65095, also with ∼3.9Ga ages, contain multiple impactor components, of which the dominant composition is considerably more suprachondritic than those implicated for Imbrium and Serenitatis (Apollo 17) impactors. The dominant composition recorded in these rocks was most likely inherited from a pre-Imbrium impactor. Consideration of composition versus age relations among lunar impact melt rocks reveals no discernable trend. Virtually all lunar impact melt rocks sampled by the Apollo missions, as well as meteorites, are characterized by 187Os/188Os and HSE/Ir ratios that, when collectively plotted, define linear trends ranging from chondritic to fractionated compositions. The impact melt rocks with HSE signatures within the range of chondritic meteorites are interpreted to have been derived from impactors that had HSE compositions similar to known chondrite groups. By contrast, the impact melt rocks with non-chondritic relative HSE concentrations could not have been made by mixing of known chondritic impactors. These signatures may instead reflect contributions from early solar