Petrology of the Apollo 11 ilmenite basalts

Abstract

Most of the crystalline rock fragments in the sample of lunar regolith returned by Apollo 11 are ilmenite basalts. The mineralogy of all these rocks is similar; major minerals are clinopyroxene, calcic plagioclase, and ilmenite, and olivine is a common minor constituent. But they differ considerably in textures, bulk chemical compositions, sequence of crystallization, and mineral compositions. Two textural variants make up the bulk of the fragments: intersertal basalts (most type A rocks); and ophitic basalts (all type B rocks). Ophitic basalts may be subdivided into fine- and medium-grained types, and their textures, wholerock compositions, mineral proportions, and mineral compositions vary systematically with grain size. The textural variations and compositional trends are similar to those of terrestrial basalts from partly differentiated sills or thick lava flows; the ophitic basalts are probably derived from one or more thick lunar lava flows or sills that fractionated slightly on cooling. Intersertal basalts have more uniform whole-rock compositions, mineral proportions, and mineral compositions than do ophitic basalts, and it appears that the samples are derived from the same or closely related magmas that did not fractionate appreciably during crystallization. These rocks have textures indicating that their parent magma was extruded onto the lunar surface and cooled rapidly. Hornfels textures in many samples suggest that they have been thermally metamorphosed, but this process did not greatly modify vesicularity, mineral proportions, whole-rock compositions, or mineral compositions. Textures of both primary volcanic and annealed basalts are identical to those of basalts from terrestrial lava lakes; it seems likely that the intersertal basalts crystallized in one or more lunar lava lakes. No reasonable process of fractional crystallization can link the parent magmas of the intersertal and ophitic ilmenite basalts, and it is likely that both formed independently by fractional melting in the lunar interior. Other types of basalt occur as sparse small fragments in the regolith and in breccias. Most common among these minor types are ilmenite basalt vitrophyres that have the same mineral assemblage as the two major types but have much more variable textures and mineral proportions. The general geology of the landing site suggests that the intersertal and ophitic rocks are fragments of local bedrock, and that the mare filling beneath the landing site consists largely of basaltic lava flows. Age-dating studies of the surface debris indicate that the regolith has an apparent age at least a half billion years older than the ilmenite basalt fragments it contains. There is some evidence to suggest that the regolith, in addition to fragmented basalt, contains 15–30% of very fine plagioclase-rich material that has a very old apparent age, and that may be derived from the lunar highlands.