Compositional variability of San Carlos olivine

Abstract

Forsterite (Fo)-rich olivine compositions from San Carlos (Arizona, SW USA) are commonly used as starting material in experimental petrology. In comparison to the San Carlos reference material USNM 111312/444, it has been shown that the major element variability of non-USNM San Carlos olivine is significant. We complement the characterization of the compositional variability of the non-USNM San Carlos olivine with new data, including minor and trace element analyses. High precision analyses reveal that selected minor elements (e.g., ~6% NiO, ~10% MnO, ~16% CaO, relative) and trace elements (e.g., ~75% Cr, ~120% Cu, ~160% P and Ti, relative) present significant concentration variations between grains. At the scale of the individual grain, however, San Carlos Fo-rich olivines appear homogeneous with no systematic core-rim variations. We also discuss the origin of olivine pyroxenites associated with the peridotite xenoliths and argue that they are derived by melt-rock reaction resulting in olivine dissolution and pyroxene precipitation from the peridotite host. Further segregation and in situ crystallization of the hybrid residual melt can produce Fe-rich olivine-poor pyroxenites. Finally, we discuss the origin of the P variability in San Carlos mantle olivine and suggest that P enrichment by metasomatism implies a highly reactive process with fast dissolution-reprecipitation of solid phases.