Temporally limited late accretion after core formation in the Moon

Abstract

Highly siderophile element (HSE: Au, Re, Pd, Pt, Rh, Ru, Ir, Os) abundances in planetary silicate mantles provide constraints on accretion and differentiation, as well as late accretion additions after core formation. The first in situ analyses of the HSE in mare basalt sulfide and metal phases enable the determination of the distribution of these elements during fractional crystallization processes. Metals have low Ni/Co (<8) and strong HSE inter-element fractionation (Pd/Ir = 18-100) and, with troilite (Ni/Co < 4.2, Pd/Ir = 14-74), host 75-100% of the HSE in mare basalts. The compositions of these metal and sulfide grains are inconsistent with assimilation of impact-contaminated HSE-rich regolith materials. Furthermore, regolith contamination cannot explain the threshold of abundances of iridium (~50 ppt) in mare basalt bulk rock compositions. Instead, bulk rock mare basalts have HSE patterns consistent with inheritance from partial melting of mantle sources with no residual metal or sulfide. Mare basalt HSE and siderophile element abundances can be accounted for by their mantle sources recording prior separation of a small lunar core, fractionating Ni/Co and Hf/W and removing~99% of the HSE after lunar formation. Following this event, late accretion of~0.02% of lunar mass led to limited enrichment of the HSE in chondritic-relative abundances. Signatures of core formation preclude elevated HSE abundances in the lunar silicate interior and indicate that disproportional late accretion in the Earth-Moon system may, in part, be due to later core formation in the Moon relative to Earth.

A Review of the Lunar 182Hf-182W Isotope System Research

“…Data of the BSE and BSM were collected from [54,60]. Dho 287A, LAP 02205, MIL 05035, and LMB 15556 are lunar samples reported by [64]. Chondrite normalization follows [65].…”

“…The behaviors of HSEs are mainly controlled by sulfide and metals during mare basalt crystallization [64,72]. Thus, knowledge of phase relations during partial melting of their mantle sources is required to assess the HSEs abundances in the BSM from lunar mare basalts.…”

“…Thus, knowledge of phase relations during partial melting of their mantle sources is required to assess the HSEs abundances in the BSM from lunar mare basalts. If the sources of lunar mare basalt had retained residual metal or sulfide, the estimates of the HSEs abundances in the BSM would be essentially unconstrained [64]. However, whether metals and sulfides are residual phases in the mantle sources or not has long been debated [52,60,64,72,73].…”

“…If the sources of lunar mare basalt had retained residual metal or sulfide, the estimates of the HSEs abundances in the BSM would be essentially unconstrained [64]. However, whether metals and sulfides are residual phases in the mantle sources or not has long been debated [52,60,64,72,73]. From the recent research on metals in Apollo 12 lunar mare basalts [52] and in situ determinations of the HSEs in lunar metals and sulfides [64], at least some sources of Apollo 12 lunar basalt were found to contain no metals and sulfides.…”

“…However, whether metals and sulfides are residual phases in the mantle sources or not has long been debated [52,60,64,72,73]. From the recent research on metals in Apollo 12 lunar mare basalts [52] and in situ determinations of the HSEs in lunar metals and sulfides [64], at least some sources of Apollo 12 lunar basalt were found to contain no metals and sulfides. In the end, this issue requires further investment.…”