Compositional characteristics of olivines from Apollo 12 samples

Abstract

The olivine phenocrysts of four basalts (12004, 12008, 12009 and 12022) are concentrically zoned and have core compositions about as magnesian as experimentally produced liquidas olivines, features which suggest fractional crystallization and absence of Fe-Mg reequilibration. In the magnesium- and olivine-rich granular basalt 12035, the olivines are either unzoned or are zoned toward adjacent grains and have compositions more iron-rich than either cumulus olivines or liquidus olivines (should the rock represent the composition of a melt), features which suggest extensive Fe-Mg re-equilibration. If one assumes that concentrations of minor elements at crystallization have also been preserved in the Mg-rich centers of olivines in the basalt porphyries, identification of two possible comagmatic pairs can be made: 12004 and 12009 (lower Ti and higher Cr in olivines), and 12008 and 12022 (higher Ti and lower Cr in olivines). Correlations are observed between the Fe-Mg proportions and the concentrations of minor elements in olivine (Ti, Mn and Ca all follow Fe, Cr follows Mg).