The self-diffusivity of O has been measured in Na2Si4O9 liquid at 1625 and 1825 °C and pressures in the range 2.5–10 GPa using a 1200-ton multianvil apparatus. Diffusion couples were prepared from glass starting materials with one-half of the sample enriched in 18O and the other half containing the natural abundance (~0.2 wt%). Following exchange of O across this diffusion couple at high temperature and pressure, 18O concentration profiles were analyzed using an ion microprobe. O diffusivities were obtained by fitting the equation for diffusion between two semi-infinite bodies to the concentration profiles. At 1825 °C, O diffusivity increases continuously from 1 × 10−10 m2/s at 2.5 GPa to 4 × 10−10 m2/s at 10 GPa, yielding an activation volume of −3.3 ± 0.4 cm3/mol. This trend is consistent with the results of molecular dynamics calculations and may be related to an increase in fivefold-coordinated Si with increasing pressure. The viscosity of Na2Si4O9 liquid at high pressure is estimated from the O diffusivity data using the Eyring relationship, which has been shown to be applicable to polymerized silicate liquids up to at least 2GPa. Estimated viscosities at 1825 °C and high pressure are consistent with an extrapolation of viscosity data obtained at 1 bar by other techniques and suggest a viscosity decrease of ~0.8 log units from 1 bar to 10 GPa. The results also suggest that the activation energies for viscous flow and O diffusivity decrease significantly in the pressure range 1 bar to 4 GPa. Provided the validity of the Eyring relationship can be established, these results demonstrate that melt viscosities can be estimated from O-diffusion experiments up to at least 10 GPa. In contrast, measurements using falling-sphere viscometry are extremely difficult at such conditions because of low viscosities and small sample size.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.