GEOLOGY AND PETROLOGY OF ENORMOUS VOLUMES OF IMPACT MELT: A CASE STUDY OF THE ORIENTALE MELT SEA

The huge volumes of impact melt (10⁶-10⁷ km³) produced by lunar basin-forming impacts are poorly understood. We investigate the geology and model the petrology of massive impact melt deposits occurring in the young 930 km diameter Orientale basin. Impact melt scaling relations predict that the Orientale-forming impact produced around 1.5 x 10⁶ km³ of impact melt. We account for this melt geologically and find that most impact melt occurs in a massive ~15 km thick melt sheet (better described as a melt lake or sea) in the central depression of the Orientale basin. Recent studies show that terrestrial impact melt sheets a thousandth of the Orientale melt lake's volume and a tenth of its thickness have undergone igneous differentiation. We therefore suggest that the Orientale melt lake too has differentiated and use petrologic models to investigate the course of its differentiation. A modeled cumulate stratigraphy with a ~8 km thick layer of norite overlying a ~4 km layer of pyroxenite and a basal ~2 km layer of norite produced by equilibrium crystallization of a homogenous melt sea is consistent with remotely-sensed norite excavated by the central peak of Maunder crater from ~4 km depth. Unusual lithologies and density structures modeled in the Orientale melt lake should be general features of basin impact melt deposits; differentiation of massive impact melt deposits may have important implications for the petrology and geophysics of the lunar highlands crust.