Siderophile Elements in Lunar Granulitic Impactites: Constraints on the Composition of pre 4 Ga Late Accretion.

Ancient lunar impactites provide constraints on the timing and composition of material accreted to the Moon prior to 3.8 Ga. Previous research suggests that the highly siderophile element (HSE) composition of many impactites from the Apollo sites is biased by a minor proportion of metal from a differentiated impactor component. Its association with KREEP-rich compositions was interpreted to reflect impact(s) of differentiated planetesimal core fragments onto KREEP-rich target regions, followed by mixing with older lithologies (including older impactites). Granulitic impactites are important components of the lunar crust which are characterized by complex histories, including contamination with siderophile elements from impactors and prolonged heating at temperatures >1000°C. In contrast to KREEP-rich impact melt rocks and breccias, most granulitic impactites have been derived from KREEP-poor feldspatic precursor rocks and they recrystallized prior to 4 Ga. Hence, the granulitic impactites resemble early lunar crust, which either formed before the lunar dichotomy or was derived from outside the Procellarum KREEP Terrane.

In order to gain insights into the composition of material accreted shortly after lunar differentiation and crust formation we determined abundances of highly siderophile and siderophile volatile elements in multiple aliquots of several granulitic impactites. All analyzed samples display refractory HSE in broadly chondritic relative abundances, but depletions towards moderately volatile Pd which exceed the range observed in primitive meteorites. ¹⁸⁷Os/¹⁸⁸Os ratios reveal long term chondritic to subchondritic Re/Os ratios. Together with available literature data, the HSE record of granulitic impactites is clearly distinct from more mafic and often KREEP-rich impact melt breccias of Imbrium age, but show similarities to some fragmental matrix breccias. The HSE signatures of the latter are dominated by older lithic clast, mainly granulitic and minor KREEP-rich impactites. Siderophile volatile elements in granulitic impactites are variably depleted and overlap with abundances in low-HSE crustal lithologies. The combined granulite data reveal late accretion of mainly volatile depleted primitive material and early large-scale reworking and mixing of the lunar crust.