Simulations were used to determine the isotope effect upon diffusion in SiO2 and MgSiO3 melts. The influence of the atomic mass upon its diffusivity could be expressed in terms of the empirical relationship, D1/D2 = (m2-m1)β. For Si, β had a value of about 0.05 for SiO2 and MgSiO3 melts, and was independent of pressure. The exponent for Mg in MgSiO3 was larger (0.135 at 1atm) and decreased with pressure to 0.084 at 50GPa. Varying the mass and concentration of an isotope of one element was also found to have a significant influence upon the diffusivity of other elements; due to the cooperative motions of the many atoms that were involved in diffusion.

Isotope Fractionation by Diffusion in Silicate Melts: Insights from Molecular Dynamics Simulations. Goel, G., Zhang, L., Lacks, D.J., Van Orman, J.A.: Geochimica et Cosmochimica Acta, 2012, 93, 205-13