An experimental study on partition of magnesium and manganese between olivine and orthopyroxene

Abstract

The partition of Mn and Mg between coexisting synthetic olivine, orthopyroxene and Ca-rich clinopyroxene was studied. The apparent equilibrium constant for the partition of Mn and Mg between olivine and orthopyroxene, K′ (=(Mn/Mg) Ol (Mg/Mn) Opx), was measured under the conditions of 1200 °C, 30 kb; 1000 °C, 30 kb; and 1050 °C, 0 kb. The pressure dependence of ln K′ was found to be −0.0040/kb (±0.0010) at 1000 °C in the composition range around Mn/(Mn+Mg) = 0.05. This value agrees quite well with the theoretical value, −0.0043/kb, based on unit cell parameter data. Composition and temperature dependences of K′ were also observed. In the systems containing Ca-rich clinopyroxene, the reaction rate was so small that no equilibrated samples were obtained. The formation condition of a garnet peridotite xenolith in South African kimberlite was estimated by thermodynamic analysis of the partition of MnMg and CoMg between olivine and orthopyroxene, assuming the binary regular solution model. It was concluded that the garnet peridotite xenolith had been formed at a temperature of about 100 °C lower and at a pressure of about 60 kb higher than the Itinomegata spinel lherzolite nodule. Unit cell parameters of synthetic MnMg olivine and orthopyroxene solid solutions are reported in the Appendix.