Mare basalt petrogenesis

Abstract Melting experiments in vacuum at Fe-saturation for high-Ti basalt compositions 70215 and 15318 Red Glass, and low-Ti basalt composition 15555-15016, have defined equilibrium crystallization sequences and the liquid-lines-of-descent, and show that much of the chemical variety in the natural samples is due to near-surface fractionation processes. However, the various groups within the high-Ti suite (Apollo 11 low-K, Apollo 11 high-K and Apollo 17 basalts; Apollo 17 Orange Glass) cannot be interrelated by such processes, nor can the high-Ti suite be a near-surface derivative of a low-Ti parent magma. Likewise, the low-Ti Apollo 12 basalts and Apollo 15 basalts and Green Glass cannot be interrelated by simple fractionation, although much of the chemical variety within each group is explained by olivine fractionation. On the basis of high-pressure studies, low-Ti basalts are believed to originate as a spectrum of partial melts from a previously differentiated olivine-pyroxenite lunar mantle, at depths ranging from approximately 200-500 km. High-Ti basalts are interpreted as hybrid liquids resulting from the assimilation of subcrustal Ti-rich material by a parental magma of low-Ti type.