Highly Siderophile Element Characteristics of Lunar Impact Melt Breccias: a Picture Begins to Emerge

Introduction: It has long been hypothesized that the Earth-Moon system, and likely the entire inner solar system, underwent a phase of late accretion, termed late heavy bombardment (LHB), within the interval of time from ~4.1 to ~3.8 Ga [1-3]. Although the putative LHB had a major effect on shaping the surface of the Moon, it likely involved much less mass than is envisioned for late accretion as a whole. Even generous estimates for the mass of the LHB place the mass of materials involved as no more than about 10% of estimates for the overall mass of late accretionary additions. Nevertheless, the LHB may have delivered substantial water, and other volatile species, including organic molecules, to the Earth and Moon, so it is important to characterize the chemical nature of the materials involved. The primary means to examine the chemical characteristics of materials from the LHB has been to analyze lunar impact melt rocks that were created as a result of the basin-forming impacts. The bulk of the highly siderophile elements (HSE: including Re, Os, Ir, Ru, Pt, and Pd) present in lunar impact-melt rocks were derived from meteoritic materials incorporated in the melt fraction. The relative abundances of the HSE in these rocks can provide diagnostic chemical fingerprints of the impactors because pristine lunar crustal rocks have extremely low concentrations of these elements [4], and possible impactors, such as chondrites and iron meorites have comparatively high abundances of these elements [5]. Several recent studies have utilized isotope dilution techniques to measure HSE concentrations and have also reported Os/Os ratios, in order to characterize multiple pieces of a given melt rock. When the data for a given sample are collectively plotted, the slopes of the linear trends generated from plots of Ir versus other HSE, and Os/Os, define the relative abundances of HSE in the impactors associated with lunar basin formation [6-9]. There is now a sizable database for HSE present in impact melt rocks from Apollo 14, 15, 16 and 17 landing sites, as well as for several lunar meteorites. Comparison of data from the diverse locations provides a new way of considering the nature of late heavy bombardment to the Moon and Earth.