Are early magnesium-rich basalts widespread on the moon

Abstract

Orbital X-ray data indicate that high-magnesium material systematically occurs along mare/terra boundaries in all the lunar nearside basins for which there is orbital X-ray coverage. These peripheral units appear to represent the earliest stages of volcanism within each of the basins, although they were not necessarily emplaced at the same time in lunar history. High-magnesium subsurface layers excavated by post-mare impact craters in central Mare Crisium and Mare Fecunditatis suggest that the peripheral units are the only visible edges of a basin-wide basalt layer, largely buried by younger less-magnesian flows. This evidence suggests that early magnesium-rich mare basalts are widespread on the moon and that their volumes may be greatly underestimated. Abundant early mare deposits of this type would place severe constraints on models of mare basalt petrogenesis.