Impact melt differentiation in the South Pole-Aitken basin: Some observations and speculations

The stratigraphy of the South-Pole Aitken basin (SPA) interior is consistent with that of a massive impact melt sheet that differentiated to form cumulates. Spectroscopic and geophysical constraints on the stratigraphy of SPA suggest a ~12.5 km thick layer of norite above ultramafic pyroxenite and dunite layers. A similar stratigraphy is produced from differentiation by crystal settling of a ~50 km thick impact melt sheet (lunar impact melt sheets >10 km thick likely undergo differentiation by crystal settling) formed by an oblique impact (and thus containing ~20 vol. % crustal material). We propose that impact melt differentiation can account for geophysical (nonzero crustal thickness) and geochemical (~2 ppm Th) anomalies in SPA.