Diffusivity of oxygen in jadeite and diopside melts at high pressures |
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| Authors: | N Shimizu I Kushiro |
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| Institution: | 1. Center for Geoalchemy, Dept. of Earth and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 USA;2. Geophysical Laboratory, Carnegie Inst. Washington, Washington, D.C. 20008 USA |
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| Abstract: | The diffusivity of oxygen was determined in melts of Jadeite (NaAlSi2O6) and diopside (CaMgSi2O6) compositions using diffusion couples with 18O as a tracer. In the Jadeite melt, the diffusivity of oxygen increases from at 5 Kb to at 20 Kb at constant temperature (1400°C), whereas in the diopside melt at 1650°C, the diffusivity decreases from at 10 Kb to at 17 Kb. These results demonstrate that the diffusivity is inversely correlated with the viscosity of the melt. For the jadeite melt, in particular, the inverse correlation is very well approximated by the Eyring equation using the diameter of oxygen ions as a unit distance of translation, suggesting that the viscous flow is rate-limited by the diffusion of individual oxygen ions. In the diopside melt, the activation volume is slightly greater than the molar volume of oxygen ion, indicating that the individual oxygen ion is the diffusion unit. The negative activation volume obtained for the jadeite melt is interpreted as the volume decrease associated with a diffusive jump of an oxygen ion due to local collapse of the network structure. |
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