Low Abundances of Highly Siderophile Elements in the Lunar Mantle: Evidence for Prolonged Late Accretion

The highly siderophile elements (HSE: including Re, Au, Ir, Os, Ru, Pt, Pd, Rh) are strongly partitioned into metal relative to silicates. In the terrestrial planets these elements are concentrated in metallic cores. Earth s mantle has sufficiently high abundances of the HSE (~0.008 times CI abundances) that it has been hypothesized approximately 0.1-0.5% of the mass of the Earth was added following the last major interaction between the core and mantle [e.g. 1]. The additional material added to the Earth and Moon has been termed a late veneer , and the process has often been termed late accretion [2]. The timing of the dominant late accretionary period of the Earth and Moon is still poorly known. The abundances of HSE in the lunar mantle could provide important constraints on when the late veneer was added. The material that ultimately became the silicate portion of the Moon was likely stripped of most of its HSE prior to and during coalescence of the Moon. Consequently the initial lunar mantle likely had very low concentrations of the HSE. Unlike Earth, the generation of permanent lunar crust by ~4.4 Ga prevented subsequent additions of HSE to the lunar mantle via continued accretion. Thus, if a substantial portion of the late veneer was added after 4.4 Ga, the lunar mantle should have retained very low HSE concentrations. Conversely, if the late veneer was mostly added prior to 4.4 Ga, HSE abundances in the lunar mantle may be roughly similar to abundances in the terrestrial mantle.