Targeting the impactors: siderophile element signatures of lunar impact melts from Serenitatis

Abstract

Highly siderophile element compositions of lunar impact melt breccias provide a unique record of the asteroid population responsible for large cratering events in the inner Solar System. Melt breccias associated with the 3.89 Ga Serenitatis impact basin resolve at least two separate impact events. KREEP-rich melt breccias representing the Apollo 17 poikilitic suite are enriched in highly siderophile elements. (3.6-15.8 ppb Ir) with Cl-normalized patterns that are elevated in Re, Ru and Pd relative to Ir and Pt. The restricted range of lithophile element compositions combined with the coherent siderophile element signatures indicate formation of these breccias in a single impact event involving an EH chondrite asteroids, probably as melt sheet deposits from the Serenitatis Basin. One exceptional sample, a split from melt breccia 77035, has a distinctive lithophile element composition and a siderophile element signature more like that of ordinary chondrites, indicating a discrete impact event. The recognition of multiple impact events, and the clear signatures of specific types of meteoritic impactors in the Apollo 17 melt breccias, shows that the lunar crust was not comprehensively reworked by prior impacts from 3.9 to 4.5 Ga, an observation more consistent with a late cataclysm than a smoothly declining accretionary flux. Late accretion of enstatite chondrites during a 3.8-4.0 Ga cataclysm may have contributed to siderophile element heterogeneity on the Earth, but would not have made a significant contribution to the volatile budget of the Earth or oxidation of the terrestrial mantle. Siderophile element patterns of Apollo 17 poikilitic breccias become more fractionated with decreasing concentrations, trending away from known meteorite compositions to higher Re/Ir and Pd/Pt ratios. The compositions of these breccias may be explained by a two-stage impact melting process involving: (1) deep penetration of the Serenitatis impactor into meteorite-free lower crust, followed by (2) incorporation of upper crustal lithologies moderately contaminated by prior meteoritic infall into the melt sheet. Trends to higher Re/Ir with decreasing siderophile element concentrations may indicate an endogenous lunar crustal component, or a non-chondritic late accretionary veneer in the pre-Serenitatis upper crust.