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

We have measured diffusion coefficients of U (D-U) and Th (D-Th) in me lts of haplogranitic composition containing <1000 ppm each of U and Th , at pressures ranging from 0.1 MPa to 3.5 GPa and temperatures rangin g from 1000 degrees C to 1600 degrees C. The values of D-U and D-Th in anhydrous haplogranitic melt at 0.1 MPa can be described by the follo wing Arrhenius equations: D-Th = (-4.14)(+7.89)cm(2)s(-1)exp(-368.6(-4 5.6)(+45.6)KJ mol-1/RT) D-U = (-3.58)(+5.06)cm(2)s(-1)exp(-363.8(-24.0 )(+24.0)KJ mol(-1)/RT) over the temperature range 1200-1600 degrees C. The activation volumes for D-Th and D-U in haplogranite melt with wat er contents between 0.02 and 0.5 wt% are -5.86 +/- 3.06 cm(3) mol(-1) and -4.22 +/- 0.89 cm(3) mol(-1), respectively. Diffusivities of Th an d U are very strong functions of water content over the range 0.02-5.0 5 wt% water. Diffusion of U and Th is shown to be intimately linked wi th the self-diffusion of network-forming molecules, and we find that t he Eyring equation relating viscosity and diffusion can predict D-Th a nd D-U within experimental error in all of our experiments, when water content and pressure effects on viscosity are taken into account, exc ept 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 co ntents to the fact that there are virtually no nonbridging oxygens in such highly polymerized melts to facilitate viscous flow. Because nonb ridging 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 visco sity, leading to a decoupling of the mechanisms and rates of viscous f low and trace element diffusivity. Copyright (C) 1997 Elsevier Science Ltd.