Effect of pressure on the diffusivity of network-forming cations in melts of jadeitic compositions

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The diffusivities of network-forming cations (Si 4+, Al 3+, Ge 4+ and Ga 3+) in melts of the jadeitic composition NaAl(Si, Ge) 2O 6 and Na(Al, Ga)Si 2O 6 have been measured at pressures between 6 and 20 kbar at 1400°C. The rates of interdiffusion of Si 4+-Ge 4+ and Al 3+-Ge 3+ increase with increasing pressure at constant temperature. The results are consistent with the ion-dynamics computer simulations of Jadeite melt by Angell et al. (1982, 1983). The coefficient measured for the Si 4+-Ge 4+ interdiffusion is between 8 × 10 −10 and 2.5 × 10 −8 cm 2 sec at 6 kbar, depending on the composition of the melt, whereas at 20 kbar it is between 7 × 10 −9 and 2 × 10 −7 cm 2 sec . The effect of pressure is greater for more Si-rich compositions ( i.e., closer to NaAlSi 2O 6 composition). The coefficient measured for the Al 3+-Ga 3+ inter- diffusion is between 9 × 10 −10 and 3 × 10 −9 cm 2/sec at 6 kbar and between 3 × 10 −9 and 1 × 10 −8 cm 2 sec at 20 kbar. The rate of increase in diffusivity with pressure of Al 3+-Ga 3+ (a factor of 3–4) is smaller than that of Si 4+-Ge 4+ (a factor of 7–17). The Si 4+-Ge 4+ interdiffusion in melts of Na 2O · 4(Si, Ge)O 2 composition has also been measured at 8 and 15 kbar for comparison. The effect of pressure on the diffusivity in this melt is significantly smaller than that for the jadeitic melts. The increase in diffusivity of the network-forming cations in jadeitic melts with increasing pressure may be related to the decrease in viscosity of the same melt. The present results, as well as the ion-dynamics simulations, suggest that the homogenization of partial melts and mixing of magmas would be more efficient at greater depths.