Evidence of impact melt sheet differentiation of the lunar South Pole-Aitken basin

The South Pole-Aitken (SPA) basin is the largest basin on the Moon. The basin-forming impact likely melted the upper part of the mantle and formed an impact melt sheet. Impact melt of large terrestrial craters differentiated following a general magmatic differentiation sequence. However, it is still debated whether or not the SPA melt sheet underwent differentiation. To address this, we investigated the vertical and lateral variations in mineral composition of the SPA impact melt sheet area by analyzing the surface mineralogy including 277 post-SPA craters using remote sensing reflectance data. We identified a 7km thick, high-Ca pyroxene-rich layer and below that, a>8km thick, low-Ca pyroxene-rich layer. Both filled the entire impact melt sheet area (630km in diameter). Considering the vast distribution of these two layers, their thicknesses, their FeO abundances (around 13.5wt%), which are lower than most of the lunar mare basalt but within the estimated range of the differentiated SPA impact melt, and that the observed lithologies are consistent with a differentiation model, we conclude that the SPA impact melt was differentiated. Moreover, based on the lunar magma ocean differentiation model, Al2O3 and CaO abundances of the upper part of the mantle would be much lower after the mantle overturn. If the SPA impact occurred after overturn, these elemental abundances of the initial impact melt would be much lower than that of the before-overturn impact case and result in different solidified layers. Our observation matches only the after-overturn case, which may imply that the SPA impact occurred after overturn.