Mg-Fe partitioning between plagioclase and liquid in basalts of Hole 504B, ODP Leg 111: a study of melting at 1 atm

Abstract

The effect of oxygen fugacity (fO2) on the partition relationship was investigated. Samples 111-504B-143R-2 and 111-504B-169R-1 have plagioclase as the liquidus phase. The distribution coefficient of Mg between plagioclase and melt is constant at about 0.04 against the variation of fO2, whereas that of Fe varies from 0.3 at fO2=0.2 atm to 0.03 at fO2=10-11.5 at 1200°C. The distribution coefficient of Mg suggests a kinetic disequilibrium effect on the distribution of Mg in plagioclase. Because Mg, Fe, and Fe3+ have similar diffusion coefficients in silicate melt, the disequilibrium effect is greatly reduced for the exchange reaction of Mg and total Fe between plagioclase and liquid. The exchange partition coefficient is highly dependent on fO2, with log fO2 ranging from -0.7 to -11.5 at approximately 1200°C. Using this relationship, the fO2, with log fO2 ranging from -0.7 to -11.5 at approximately 1200°C. Using this relationship, the fO2 of crystallization of the magmas is estimated to be near the one defined by the fayalite-quartz-magnetite assemblage. -from Author