Mg, Fe2+ site occupancies in coexisting pyroxenes

Abstract

The separate distributions for MgSiO3 and FeSiO3 in coexisting pyroxenes from the Skaergaard and Bushveld intrusions and charnockites, which were introduced in an earlier communication, indicate directly that significant amounts of both Fe2+ and Mg were present in the M(2) site of the Ca-rich pyroxene at the temperature of final intercrystalline equilibration. The calculated Fe2+ M(2) site occupancy in the Ca-rich pyroxene increases markedly with decrease in total MgSiO3 content but the corresponding Mg site occupancy appears largely independent of MgSiO3. The mean value of the distribution constant for intracrystalline exchange in the Ca-rich pyroxene decreases, away from unity, with decreasing temperature of equilibration. Occupancy of Mg and Fe2+ in the M (2) site of the Ca-rich pyroxene effectively compensates for the expected variation in KD with composition resulting from intracrystalline partition in Ca-poor pyroxene, and this largely accounts for the difference in KD between igneous and metamorphic pyroxenes.