Experimental Petrology of Lunar Highland Basalt Composition and Applications to Models for the Lunar Interior

Abstract

Lunar Highland Basalt (gabbroic anorthosite) composition transforms with increasing pressure through assemblages of spinel + clinopyroxene + anorthite, and garnet + clinopyroxene + anorthite ± quartz to eclogitic assemblages of garnet + clinopyroxene + kyanite + quartz (p = 3.51 - 3.52). Estimates of the lunar thermal gradient allow evaluation of the mineralogical variation with depth in the moon and thus allow calculation of mean lunar density and coefficient of moment of inertia for several models for the moon. These calculations show that the simplest models of Highland Basalt as the mean lunar composition and Highland Basalt as comprising the outer 250-270 km (formed as a partial melt from a Ca, Al-rich lunar interior) fail to meet the lunar density constraint but could do so if the hypotheses were modified towards more Fe-rich compositions. The geochemical constraints are much stronger and show that it is not possible for Highland Basalt to act as a source rock for mare basalt magmas under any conditions of melting in the lunar interior. It is further shown that Highland Basalt cannot be a partial melt from a mean lunar composition matching the Ca, Al-rich inclusions of the Allende meteorite (Anderson 1973) leaving the residual deep lunar interior of diopside + merwinite + spinel mineralogy.