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

Abstract

Concentrations of highly siderophile elements (HSE) and 187 Os/ 188 Os 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 187 Os/ 188 Os, Ru/Ir, Pt/Ir and Pd/Ir ratios. Diverse impactor components are revealed in the Apollo 16 impact melt rocks. The 187 Os/ 188 Os and HSE/Ir ratios of the impactor components in melt rocks 60635, 63595 and 68416, with reported ages < 3.84 Ga, are within the range of chondritic meteorites, but slightly higher than ratios characterizing previously studied granulitic impactites with reported ages > 4.0 Ga. By contrast, the impactor components in melt rocks 60235, 62295 and 67095, with reported ages of ~3.9 Ga, are characterized by suprachondritic 187 Os/ 188 Os 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 ~3.9 Ga 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.

In search of late-stage planetary building blocks

“…Puchtel et al (2008) and Sharp et al (2014) (2012), thus, strengthening their observation. In the compilation of data reported by Liu et al (2015), nearly all data for lunar impact melt rocks plot along a continuous linear trend ranging from a HSE composition that is broadly chondritic, to an endmember with 187 Os/ 188 Os, Ru/Ir and Pd/Ir ratios far above those of known chondrites (Fig. 13).…”

“…To circumvent this problem, Norman et al (2002) Gödde and Becker, 2012;Sharp et al, 2014;Liu et al, 2015). A major difference between these studies and the study of Norman et al (2002) is that they examined multiple pieces of each rock studied.…”

“…The other component resembles a chemically evolved group IVA iron meteorite. Consequently, they proposed that both components became variably mixed during basin-forming impacts, but were not substantially modified by the inclusion of HSE derived from the basin-forming impactors.Most recently, data fromLiu et al (2015) for Apollo 15 and 16 melt rocks filled in the gaps in the apparent linear trend recognized by Fischer-Gödde and Becker…”

Geochemical evidence of an extraterrestrial component in impact melt breccia from the Paleoproterozoic Dhala impact structure, India

“…Nearly all impact melt rocks sampled from Apollo missions have traces of impactors with chondritic to fractionated compositions (e.g., Liu et al. ). Of the 10 oldest terrestrial impact structures listed in the Earth Impact Database (Earth Impact Database ), the Vredefort structure (2.02 Ga; Kamo et al.…”

Siderophile Elements in Tracing Planetary Formation and Evolution

“…Thus, it is possible that some primitive materials which existed at that time, or that may even exist today but have not been sampled, match the HSE characteristics of the BSE estimate. Indeed, the Ru/ Ir and Pd/Ir of some lunar impact melt rocks appear to record the HSE signatures of impactors with HSE ratios somewhat outside of the range of ratios observed in bulk chondrites currently falling to Earth (e.g., Fischer-Gödde and Becker, 2012;Liu et al, 2015). Histogram plot of 187 Os/ 188 Os data for bulk chondrites compared to the minimum BSE value estimated by Meisel et al (2001).…”