Diffusion coefficients of Mg isotopes in MgSiO3 and Mg2SiO4 melts calculated by first-principles molecular dynamics simulations

XH Liu and YH Qi and DY Zheng and C Zhou and LX He and F Huang, GEOCHIMICA ET COSMOCHIMICA ACTA, 223, 364-376 (2018).

DOI: 10.1016/j.gca.2017.12.007

The mass dependence of diffusion coefficient (D) can be described in the form of D-i/D-j = (m(j)/m(i))(beta) where m denotes masses of isotope i and j, and beta is an empirical parameter as used to quantify the diffusive transport of isotopes. Recent advances in computation techniques allow theoretically calculation of b values. Here, we apply first-principles Born-Oppenheimer molecular dynamics (MD) and pseudo- isotope method (taking m(j)/m(i) = 1/24, 6/24, 48/24, 120/24) to estimate beta for MgSiO3 and Mg2SiO4 melts. Our calculation shows that b values for Mg calculated with Mg-24 and different pseudo Mg isotopes are identical, indicating the reliability of the pseudo-isotope method. For MgSiO3 melt, beta is 0.272 +/- 0.005 at 4000 K and 0 GPa, higher than the value calculated using classical MD simulations (0.135). For Mg2SiO4 melt, beta is 0.184 +/- 0.006 at 2300 K, 0.245 +/- 0.007 at 3000 K, and 0.257 +/- 0.012 at 4000 K. Notably, beta values of MgSiO3 and Mg2SiO4 melts are significantly higher than the value in basalt-rhyolite melts determined by chemical diffusion experiments (0.05). Our results suggest that beta values are not sensitive to the temperature if it is well above the liquidus, but can be significantly smaller when the temperature is close to the liquidus. The small difference of beta between silicate liquids with simple compositions of MgSiO3 and Mg2SiO4 suggests that the beta value may depend on the chemical composition of the melts. This study shows that first-principles MD provide a promising tool to estimate beta of silicate melts. (C) 2017 Elsevier Ltd. All rights reserved.

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