Evidence for early volcanism in mare Smythii

The major geologic units in and around the Smythii basin are reflected in the Al2O3 and MgO values obtained using improved orbital X-ray fluorescence data from Apollo 15. One of these units is a high albedo plains-forming unit that has Al2O3 and MgO concentrations different from those of similar plains-forming units located in the terra west of Smythii and south of Crisium and also different from those of adjacent terra and mare materials. The chemical composition of the plainsforming unit suggests a volcanic origin; the inferred Al2O3 and MgO-rich nature of this presumed early volcanic material is supported by the fact that even the later, low albedo, lightly cratered mare fill in Mare Smythii has a high MgO value, and its Al2O3 concentration is higher than that of similar basaltic material in other maria.