Effects of Pressure and Ca²⁺ on the Fe-Mg Partitioning between Olivine and Clinopyroxene

Abstract

The exchange reaction of Fe²⁺ and Mg²⁺ between olivine and clinopyroxene has been experimentally determined at 1150°C, and at pressures 1 and 1. 5 GPa in the Ca-poor and Ca-enriched systems, the bulk ratio Fe/ (Fe+Mg) which was fixed as about 0. 5. The CaO contents of olivine and clinopyroxene seriously affect on the Fe-Mg partition coefficient K_D [= (X^Cpx_Mg X^Ol_Fe) / (X^Cpx_Fe X^Ol_Mg) ], which increases with increasing X^Ol_Ca. The K_D was found as 1. 50 at 1 GPa for the assemblage of olivine (X^Ol_Ca = 0. 006) and subcalcic clinopyroxene (X^Cpx_Ca = 0. 2898) ; 2. 90 at 1. 5 GPa for the assemblage of diopside-hedenbergite clinopyroxene and olivine (X^Ol_Ca = 0. 1048), in contrast to 3. 00 at 1 GPa for the assemblage of diopside-hedenbergite clinopyroxene, olivine with X^Ol_Ca = 0. 1464 and ferroan monticellite. On the other hand, the distribution coefficient K_D was found to be insensitive to pressure comparing with the previous data. The olivine-ferroan monticellite solvus becomes narrow with increasing pressure, but this pressure effect on the solvus is very small.