Subsurface Chemistry of the Imbrium Basin Inferred from Clementine UVVIS Spectroscopy

Since ejecta around an impact crater is excavated from a depth, its mineralogy and chemistry will provide us with information on the composition of the pre-impact subsurface. The depth from which crater ejecta were excavated was determined from laboratory experiments, field studies, and a simplified quantitative model (Z-model and the scaling law of ejection velocity). Based on the results of these studies, it is believed that surface material of an ejecta blanket between 1.1 and 1.5 radii from the crater was excavated from a depth of 0.13 to 0.15 radii. The following results were obtained from combining the surface and subsurface basalt distributions with crater-counting ages for the mare basalt, we obtained the following results: (1) The averages of TiO₂ and FeO increased with time from the Imbrian to the Eratosthenian periods, which is represented by a continuous trend curve on the TiO₂-FeO graph: (2) volcanic activities in Mare Imbrium drastically decreased and basalts changed from a low-Ti to high-Ti content around the transition of the Imbrian to Eratosthenian period: (3) basalts with less than 3 wt% TiO₂ erupted in succession mainly in the Imbrian period.