Oxygen-Isotope Fractionation in the Mantle

A three-component, diffusion-limited, open-system exchange model for pyroxene, olivine, and fluid is presented that reproduces the range of oxygen isotope compositions of coexisting silicates from mantle-derived samples. Closed-system exchange cannot reproduce observed disequilibrium fractionations. Measured oxygen self-diffusion data were used to constrain fluid-silicate reaction rates. The magnitudes of fluid-mineral fractionations for typical mantle fluids are presented and show that equilibrated fluid will always be more O-18-rich than coexisting silicates. Three conditions are necessary to generate the disequilibrium pyroxene-olivine oxygen isotope fractionations observed in the mantle dataset. There must be at least an order of magnitude difference between the fluid-pyroxene and fluid-olivine reaction rates, fluid and solid phases must be out of equilibrium prior to reaction, and fluid must be moving several orders of magnitude faster than it is reacting with the silicates.