Isotope fractionation by diffusion in silicate melts: Insights from molecular dynamics simulations

Molecular dynamics simulations were performed to determine the isotope effect on diffusion in SiO2 and MgSiO3 liquids. The influence of an element’s atomic mass on its diffusivity can be expressed in terms of the empirical relation D1D2=m2m1β. For Si, β has a value of ∼0.05 in both SiO2 and MgSiO3 liquids, and is independent of pressure. The exponent β for Mg in MgSiO3 is larger, 0.135 at 1 atm, and decreases with pressure, to 0.084 at 50 GPa. Varying the mass and concentration of an isotope of one element is also found to have a significant influence on the diffusivity of other elements, due to the cooperative motions of the many atoms that are involved in diffusion. Interdiffusion between basaltic and rhyolitic magmas is inferred to be capable of producing isotope fractionations of tenths of per mil in Si, and tens of per mil in Mg. Significant diffusive fractionation of Si and Mg isotopes is also possible during the growth of olivine phenocrysts, if the growth rate is on the order of cm/yr or faster.