| Three experimental studies were conducted to determine the pressure dependence of silicon (Si) and oxygen (O) self-diffusion and the pressure dependence of viscous flow in relatively-polymerized silicate melts. Self-diffusion coefficients for Si and O self-diffusion in dacite (67 wt. % SiO2) melt increase from 1 to 4 GPa, reach maximum values at 5 GPa, then decrease to 5.7 GPa. At 1461°C, neutral SiO2 species are activated for self-diffusion. At 1561 and 1662°C, Si and O diffuse through the formation and disassociation of five-coordinated Si or A1 species. In contrast, Si and O self-diffusion coefficients have very weak pressure dependence in the 1 atm diopside-anorthite eutectic composition between 1 and 4 GPa at 1510 to 1764°C. This result suggests that this composition is near a transition from positive to negative pressure dependence, and Adam-Gibbs theory suggests that this transition occurs when 62–73% of O in the melt are bridging O. Finally, the viscosity of dacite melt was measured between 1.6 and 7 GPa, using an in situ falling sphere technique at the Advanced Photon Source, Argonne National Lab. In general, there is a strong negative pressure dependence of the viscosity of dacite melt. A direct comparison of O self-diffusion and viscosity shows that the Eyring equation predicts viscosities that are 30–50% greater than measured viscosities. The Eyring also predicts a minimum in the viscosity of dacite melt at 5 GPa that is not observed. An alternative relationship is required, possibly involving the decoupling of the motion for self-diffusion and viscosity as accommodated by Adam-Gibbs theory. |
