Compositional heterogeneity of basin forming impactors and large-scale impact gardening in the lunar highlands.

Material that accreted after planetary core formation may have influenced the composition (e.g., availability of volatiles like H, C, S) and later evolution of the terrestrial planets. Central questions are: Was the accreted material similar to known solar system objects like the meteorites in our collections? Was it rich or poor in volatile components? When was the material accreted and can we relate impactite compositions to specific impact events and/or basins?

Lunar impact basins and ancient impactites provide a valuable record of late accretion, starting from the formation of a solid crust until the onset of mare volcanism (~4.4 to ~3.6 Ga). However, the origin of variably fractionated HSE patterns in different impact breccia lithologies and at landing sites is still debated [1-10]. Here we discuss all available data on highly siderophile element (HSE) abundances and ¹⁸⁷Os/¹⁸⁸Os ratios in lunar impactites. The current dataset comprises a variety of impact lithologies from different Apollo landing sites and a few lunar meteorites. A compilation of all data reveals that impactites from different landing sites often form clusters that together define a broadly linear correlation of ¹⁸⁷Os/¹⁸⁸Os ratios (as a long-term measure of the Re/Os ratio) and HSE/Ir ratios, which range from sub- to suprachondritic. The compositional range was interpreted to either reflect signatures of compositionally distinct basin forming primitive impactors, some of them outside the range of known chondrites [1, 2, 4, 5] or to result from variable mixing of several ancient impactor compositions, including differentiated metal [3, 6, 7, 8]. In order to better constrain the composition of accreted material we will discuss the different impact lithologies and their HSE patterns with respect to their crustal provenance and constrain their redistribution due to impact gardening.

The majority of HSE data was obtained on KREEP-rich mafic impact melt rocks and breccias. Samples from four different landing sites display variably fractionated HSE patterns with suprachondritic Re/Os and HSE/Ir ratios increasing towards moderately volatile HSE like Pd and Au. Collectively these impactites display suprachondritic Ru/Pt ratios, a feature which is observed only in a limited number of differentiated metal-bearing meteorites. The widespread occurrence of this rather uncommon composition was interpreted as the result of accretion of larger fragments of differentiated planetesimal core material to a KREEP-rich target region [6, 8]. Available data on nucleosynthetic isotope anomalies in Ru and Mo suggest that the differentiated signature might have originated from material of the inner solar system [10].

Granulitic impactites are KREEP-poor feldspathic impactites which display metamorphic texture and equilibrated mineral compositions, indicative of sub-solidus recrystallization. Hence, their HSE inventory is interpreted to reflect early accretion of material to a KREEP-poor target region prior to formation of the younger impact basins, which apparently dominate the accessible ejecta deposits. The relative HSE abundances in Apollo 16 and 17 granulites are similar and strongly suggest accretion of volatile-depleted impactor material (possibly volatile depleted carbonaceous chondrite or primitive achondrite-like).

Fragmental matrix breccias are dominant at the North Ray Crater of the Apollo 16 landing site and were interpreted as representative of the Descartes formation. The HSE inventory of these breccias is characterized by diverse impactite clasts, including KREEP-rich mafic melt breccias and granulitic impactites. In contrast to other impact lithologies, fragmental matrix breccias preserved an impactor signature different from known primitive meteorites. The HSE inventory is characterized by moderate depletions in Re and Os when compared to Ir, Ru and Pt, but chondritic Re/Os and a gradual depletion towards moderately volatile Pd and Au. The latter signature stems most likely from unknown primitive impactors with fractionations caused by nebular processes, like incomplete condensation or evaporation [7].

HSE and lithophile element compositions of granulitic impactites and fragmental matrix breccias reveal that material similar to carbonaceous chondrites and acapulcoite-lodranite primitive achondrites was accreted early onto KREEP-poor highland regions. However, most studied KREEP-poor impactites follow a linear mixing trend from slightly subchondritic HSE ratios towards the composition of HSE-rich and fractionated KREEP-rich impactites. This, together with the presence of KREEP-rich impactite clasts, comprising characteristically fractionated HSE, in breccias of different landing sites constrains physical mixing processes ranging from the scale of g-sized samples to the area covered by the Apollo missions. In addition, comparison of Pd/Ir ratios (as measure of HSE fractionation) with abundances of incompatible trace elements like U (i.e., the fraction of KREEP component) reveal preservation of distinct compositional clusters in impactites at different landing sites. Such systematic differences between landing sites are best explained by early accretion of a core fragment onto the area of the Procellarum KREEP Terrane, variable mixing with KREEP-rich highland rocks and subsequent distribution of this combined signature and mixing with more primitive impactor material from KREEP poor sites. This interpretation is consistent with recent results from simulations of impact driven megaregolith evolution [11, 12].

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