SILICON AND OXYGEN SELF-DIFFUSIVITIES IN SILICATE LIQUIDS MEASURED TO15 GIGAPASCALS AND 2800-KELVIN

Mass transport properties of silicate liquids exhibit complex behavior as a function of pressure, as the tetrahedral framework structure of the liquid shifts to a more compact arrangement oi atoms. For highly p olymerized aluminosilicate liquids, oxygen diffusivities pass through a maximum at pressures below 10 gigapascals, whereas up to 15 gigapasc als diffusivities continue to increase for sodium tetrasilicate liquid . A diffusivity. maximum indicates a change in the mechanism of format ion of 5-coordinated silicon or aluminum in the liquid. In the case of aluminosilicate liquids, this mechanism is restricted to aluminum sit es in the network, suggesting that not only degree of polymerization, but also the ratio of aluminum to aluminum plus silicon strongly influ ences the behavior of magmatic processes at depth.