The significance of fractional crystallization in the petrogenesis of Apollo 17 Type A and B high-Ti basalts

Whole-rock and mineral analyses of 26 “new” Type A and B Apollo 17 basalts are reported. The petrography and mineral chemistry of these basalts are similar to previously reported Apollo 17 basalts. However, these “new” whole-rock data extend the compositional ranges of previously reported Type A and B basalts and require the division of the Type B basalts into Type B1 and B2 varieties. These three types display similar trends when both major and trace elements are plotted against a fractionation index of Cr/La ratio. Major element compositions of basalts from all three types fall on olivine + Ti oxide control lines. The compositional ranges of all three groups can be effectively generated by 40–50% fractional crystallization of observed phenocryst phases. The compositions of three Type A basalts are only generated after 80% fractional crystallization. Previous models of different degrees of partial melting and KREEP assimilation were developed in order to explain the range in La/Sm ratio of the Type B basalts. With the recognition of two types of “B” basalts, these models are no longer valid. This study demonstrates that Apollo 17 Type A, B1, and B2 basalts have a relatively simple petrogenesis, with the only post-magma-generation process being fractional crystallization.