Crystallization of Some Lunar Mafic Magmas and Generation of Rhyolitic Liquid

Abstract

During crystallization of lunar crystalline rocks 10022 and 10024, augite changes in composition almost continuously from titanaugite (Ca₃₆Mg₄₇ Fe, ₁₇ with TiO₂ 3 percent) to a very iron-rich variety (Ca₉Mg₅Fe₈₆), pigeonite changes from Ca₉Mg₆₆Fe₂₅ to Ca_1OMg₅₁Fe₃₉, and olivine changes (in 10022) from Mg₇₁Fe₂₉ to Mg₄₁Fe₅₉, whereas plagioclase stays as bytownite. These compositional variations, as well as the textural relations, may be explained by rapid crystallization of undercooled magmas. The residual liquids found as mesostasis are rhyolitic, which suggests that fractional crystallization of some lunar mafic magmas can generate rhyolitic magmas. Melting experiments were made on crystalline rocks to determine liquidus temperatures and crystallizing phases.