Isotopic effects on diffusion in MgO melt simulated by the molecular dynamics (MD) method and implications for isotopic mass fractionation in magmatic systems

Abstract

Mass dependence of diffusion in MgO melt has been determined by means of molecular dynamics (MD) simulation. Self-diffusion coefficients of Mg and O with hypothetical masses in the ranges 1.6–360 and 1.0667–240 amu, respectively, are approximately proportional to the atomic mass to the −0.1 power for Mg in the temperature range 3000 to 6000 K and to the −0.091 power for O at 6000 K. Diffusivity mass dependence in the melt is smaller than in gas phase (i.e., [m] − 1 2 ) and is consistent with previous calculations for melts of rare gases and alkali halides. These results together with theoretical consideration may suggest that diffusivity mass dependence in a melt is small (roughly [ m] −0.1) probably in a silicate melt too. Based on the present results, isotopic mass fractionation in geological processes controlled by diffusion in a melt is discussed. Isotopic mass fractionation could be smaller than previously assumed because of the calculated [ m] −0.1 dependence vs. the assumed [m] − 1 2 in previous work. Isotopic mass fractionation due to diffusion in a magmatic melt is generally negligible in geological systems. However, a detectable amount of isotopic mass fractionation could be possible for light elements (e.g., >10%. for 26Mg 24Mg ) in a specific geological setting, if an element diffuses into a region where the element is initially at zero concentration. Models for crystal growth from a solution predict that negligible fractionation will occur (e.g., < 10%. for 26Mg 24Mg ) at small supersaturations of less than about 0.3–0.4 even if growth is diffusion controlled.