Lawrence Berkeley National Laboratory

Noble gas isotopes are used extensively as tracers in hydrologic and paleoclimatic studies. These applications require knowledge of the isotopic mass (m) dependence of noble gas diffusion coefficients in water (D), which has not been measured but is estimated using experimental D-values for the major isotopes along with an untested relationship from kinetic theory, D prop m-0.5. We applied molecular dynamics methods to determine the mass dependence of D for four noble gases at 298 K, finding that D prop m-beta with beta < 0.2, which refutes the kinetic theory model underlying all current applications.