Geology and petrology of enormous volumes of impact melt on the Moon: A case study of the Orientale basin impact melt sea

Lunar basin-forming impacts produce enormous volumes (>105 km3) of impact melt. All known basin-forming impacts combined may produce ∼108 km3 of impact melt, ∼1/20th the volume of the lunar crust. Despite their volumetric importance, the geology and petrology of massive deposits of impact melt on the Moon have been little studied, in part because most basin impact melt deposits are old and have been obscured or buried by subsequent impact cratering and mare infill. We investigate the geology and model the petrology of fresh massive impact melt deposits in the relatively young 930 km diameter Orientale basin. Models of impact melt production combined with geologic analyses based on new LOLA topographic data suggest that most of the impact melt (∼2/3) produced by the Orientale-forming impact occurs in a ∼15 km thick impact melt sheet (better described as an impact melt sea) ∼350 km in diameter with a volume of ∼106 km3. We anticipate that the Orientale melt sea has undergone large-scale igneous differentiation, since terrestrial impact melt sheets (such as Manicouagan, Sudbury, and Morokweng) less than a tenth of the thickness and a hundredth of the volume of the Orientale melt sea have differentiated. We develop a model for the cumulate stratigraphy of the solidified Orientale impact melt sea. A modeled cumulate stratigraphy (occurring below a quench crust and anorthositic fallback breccia) with an ∼8 km thick layer of norite overlying a ∼4 km layer of pyroxenite and a basal ∼2 km thick layer of dunite produced by equilibrium crystallization of a homogenized melt sea, consistent with vigorous convection in that melt sea, is supported by remotely-sensed norite excavated by the central peak of Maunder crater from ∼4 km depth. Generally, we predict that very large basin-forming impacts, including the South Pole-Aitken (SPA) basin-forming impact, produce melt seas with a cumulate stratigraphy similar to that of the Orientale melt sea. Impact melt differentiation may explain apparently anomalous lithologies excavated in the SPA basin interior. We note that impact melt differentiates are slow-cooled (the Orientale melt sea took on the order of 105 years to solidify) and, if meteoritic siderophiles fractionate into metal or sulfide layers, may not be siderophile-enriched; therefore, impact melt differentiates may pass for pristine highland plutonic rocks in the lunar sample suite. These predictions can be tested with current and future mission data.