ISOTOPIC EFFECTS ON DIFFUSION IN MGO MELT SIMULATED BY THE MOLECULAR-DYNAMICS (MD) METHOD AND IMPLICATIONS FOR ISOTOPIC MASS FRACTIONATION IN MAGMATIC SYSTEMS

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 parts per thousand for Mg-26/Mg-24) 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 parts per thousand for Mg-26/Mg-24) at small supersaturations of less than about 0.3-0.4 even if growth is diffusion controlled.