The crystallisation of lunar basalts

Abstract

The crystallisation of olivine (O) and silica (S) normative varieties of Apollo 11 crystalline rocks has been followed at 1 atm. The sequence of phases precipitating is (S): spinel, olivine, ilmenite, clinopyroxene plus plagioclase; and (O): spinel, olivine, ilmenite plus plagioclase, clinopyroxene. The last 50% (O) to 75% (S) of the liquid crystallises as a pyroxene-plagioclase-ilmenite cotectic over a narrow temperature range, approximately 1095 to 1125° C, leaving a small silica-rich residue. Analyses of the important magnetic minerals shows i.a. that iron crystallises as 10 μm octahedra, which take up any nickel present in the melt. Spinel has MCr 2O 4:MAl 2O 4:M 2TiO 4 ≈ 1:1:3, with M = Fe+Mg ranging from 66 to 79% Fe. Ilmenite contains approximately 1% Al 2O 3, 1% Cr 2O 3 and a few mole percent Ti 2O 3, and has M = Fe+Mg ranging from 63 to 82% Fe. Partial analyses were made of the important silicate phases. Lunar basalts have viscosities from one to two orders of magnitude less than terrestrial basalts, and this permits faster flow, more crystal fractionation within individual flows, and (in intrusions) rapid crystal sinking and convection, and extreme rhythmic layering. The analytical data have been used to calculate the probable composition of the source region of the lunar basalts. The lunar mantle is thought to consist of major ortho- and clino-pyroxenes, with minor spinel, ilmenite, metal, sulphides, olivine, plagioclase and liquid, plus rutile and garnet at depth. The europium fractionation found in our experiments is consistent with the idea that the lunar highlands are anorthositic.