ACTINIDE DIFFUSION IN A HAPLOGRANITIC MELT: EFFECTS OF TEMPERATURE, WATER CONTENT, AND PRESSURE

Abstract

We have measured diffusion coefficients of U (DU) and Th (DTh) in melts of haplogranitic composition containing <1000 ppm each of u and th, at pressures ranging from 0.1 mpa to 3.5 gpa temperatures 1000°C to 1600°C. The values of DU and DTh in anhydrous haplogranitic melt at 0.1 MPa can be described by the following Arrhenius equations:DTh = 8.71+7.89-4.14 cm2 s-1 exp -368.6+45.6-45.6 KJ mol-1/RTDU = 12.27+5.06-3.58 cm2 s-1 exp -363.8+24.0-24.0 KJ mol-1/RTover the temperature range 1200-1600°C. The activation volumes for DTh and DU in haplogranite melt with water contents between 0.02 and 0.5 wt% are -5.86 +/- 3.06 cm3 mol-1 and -4.22 +/- 0.89 cm3 mol-1, respectively. Diffusivities of Th and U are very strong functions of water content over the range 0.02-5.05 wt% water. Diffusion of U and Th is shown to be intimately linked with the self-diffusion of network-forming molecules, and we find that the Eyring equation relating viscosity and diffusion can predict DTh and DU within experimental error in all of our experiments, when water content and pressure effects on viscosity are taken into account, except in those runs containing less than 0.3 wt% of water. We attribute the observed failure of the Eyring equation at these very low water contents to the fact that there are virtually no nonbridging oxygens in such highly polymerized melts to facilitate viscous flow. Because nonbridging oxygens are concentrated around impurities like the actinides used in our experiments, the approach to full polymerization will have less effect on diffusivities of the impurities than on the bulk viscosity, leading to a decoupling of the mechanisms and rates of viscous flow and trace element diffusivity.