On the Origin of Silicic Magmas on the Moon

The origins of lunar silicic magmas are not well understood. While clast to grain sizes of silicic materials have been returned, none of the Apollo missions visited a silicic target. Despite the availability of a wealth of remote data sets, their low spectral resolution has limited our ability to constrain the bulk composition of lunar silicic volcanic landforms and their pressure–temperature–composition ( P – T – X ) evolution. Here we test two proposed formation mechanisms for the formation of silicic melts using rhyolite-MELTS models. Models of crustal melting show that only 10%–20% partial melting of two likely starting compositions produces melts with >60 wt% SiO 2 and <3 wt% total alkali compared to >65 wt% SiO 2 and >5 wt% for returned samples. Alternatively, models of fractional crystallization of KREEP basalt residual liquids can produce silicic melt compositions with >68 wt% SiO 2 and >7 wt% total alkali, consistent with returned silicic samples. Furthermore, thorium partitioning calculations show that the fractionated silicic melts contain >60 ppm thorium, consistent with returned samples. We argue that fractional crystallization of KREEP basaltic magmas at shallow depths along a relatively high geotherm at low f O 2 is the most likely formation mechanism for lunar silicic magmas.