The titanium contents of lunar mare basalts

Lunar mare basalt sample data suggest that there is a bimodal distribution of TiO(2) concentrations. Using a refined technique for remote determination of TiO(2), we find that the maria actually vary continuously from low to high values. The reason for the discrepancy is that the nine lunar sample return missions were not situated near intermediate basalt regions. Moreover, maria with 2-4 wt% TiO(2) are most abundant, and abundance decreases with increasing TiO(2). Maria surfaces with TiO(2) >5 wt% constitute only 20% Of the maria. Although impact mixing of basalts with differing Ti concentrations may smear out the distribution and decrease the abundance of high-Ti basalts, the distribution of basalt Ti contents probably reflects both the relative abundances of ilmenite-free and ilmenite-bearing mantle sources. This distribution is consistent with models of the formation of mare source regions as cumulates from the lunar magma ocean.