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

Abstract

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 7 km thick, high‐Ca pyroxene‐rich layer and below that, a >8 km thick, low‐Ca pyroxene‐rich layer. Both filled the entire impact melt sheet area (630 km in diameter). Considering the vast distribution of these two layers, their thicknesses, their FeO abundances (around 13.5 wt %), 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.

Mafic Minerals in the South Pole‐Aitken Basin

“…For the Cpx‐dominated materials around the Bose crater (Figure a), named as the SPACA zone by Moriarty and Pieters (), previous studies suggest that they are either the materials representing cryptomare or a mixture of old mare/extrusive magma and some low‐mafic materials (Moriarty & Pieters, ; Ohtake et al, ; Pieters et al, ; Uemoto et al, ; Whitten & Head, ) or the differentiated SPA impact melt sheet (Ohtake et al, ; Uemoto et al, ). For the Cpx‐dominated materials in the northwestern SPA basin, they are interpreted as thin ejecta by Ohtake et al ().…”

Geologic History of the Northern Portion of the South Pole‐Aitken Basin on the Moon

“…To restrict the emplacement of mantle materials within the SPA floor may require a much smaller transient crater, which would allow formation of a melt pool within a roughly circular (~400‐ to 500‐km diameter) area near the apparent center of the SPA basin (Ohtake et al, ; Uemoto et al, ). This area approximately corresponds to the SPA compositional anomaly (Moriarty & Pieters, ) and is characterized by abundant Ca, Fe‐rich pyroxenes (Moriarty & Pieters, , ; Ohtake et al, ).…”

“…The compositional maps of FeO and TiO 2 concentrations derived from the Clementine mission spectral data (Lucey et al, , ) have resolutions comparable to the WAC mosaics and, in contrast to Kaguya compositional maps (Lemelin et al, ; Ohtake et al, ; Uemoto et al, ), cover the entire area of our study. The Clementine compositional maps provide additional and independent information, which in combination with the photogeological analysis, helps to interpret the nature of the mapped units and address several important problems of the regional geology of the SPA basin.…”

Deep Structure of the Lunar South Pole‐Aitken Basin

“…The total excess mass under the SPA basin is less dependent on model assumptions, though with a peak anomaly of at least 6 × 10 6 kg/m 2 and an integrated total mass excess of at least 2.18 × 10 18 kg, or approximately 0.003% of the Moon's total mass. The anomaly has no apparent correlation with surface mineralogy as detected by remote sensing (Moriarty & Pieters, 2018;Uemoto et al, 2017) or mare volcanism (Wilhelms et al, 1987; see Figure S5). The mass anomaly does coincide with the basin's central topographic depression (Figure 1), previously interpreted to result from impact melt sheet contraction (Moriarty & Pieters, 2016;Ohtake et al, 2014).…”