Diffusion in silicate melts

To better understand diffusion in silicate melts, we first briefly review silicate melt structure. In natural silicate melts from basalt (about 50% Si02) to rhyolite... 

It has been observed that water dramatically enhances oxygen diffusion in silicate melts as well as in many crystalline silicates. Furthermore, in systems studied in more detail, it has been observed that the diffusivity is roughly proportional to H2O pressure (Farver and Yund, 1990), or H2O content in the phase. This has been... 

The accuracy of the above relation can be addressed using Figures 3-34a and 3-34b. For diffusion in water, in A —20.12+ 0.40372E. Hence, at T= 297.9 K, all species would have diffusivity of 1.8 x 10 m /s in water. Examination of Table l-3a shows that this is only approximately so. For diffusion in silicate melts. In A = 19.29 + 0.04526E. Hence, at T = 2657 K, all species would have a diffusivity of 4.2 x 10 m /s in silicate melts. There are no data at such a high temperature to test this prediction. 

The dependence of diffusivity in silicate melts on composition is related to how melt structure (including degree of polymerization and ionic porosity) depends on composition. One the one hand, as Si02 concentration increases, the melt becomes more polymerized and the viscosity increases. Hence, diffusivity of most structural components, such as Si02 and AI2O3, decreases from basalt to rhyolite. On the other hand, as Si02 content increases, the ionic porosity increases. The increasing He diffusivity from basalt to rhyolite to silica, opposite to the viscosity... 

Liang Y., Richter F.M., Davis A.M., and Watson E.B. (1996a) Diffusion in silicate melts, I self diffusion in Ca0-Al203-Si02 at 1500°C and 1 GPa. Geochim. Cosmochim. Acta 60, 4353-4367. 

There is now a large amount of noble gas diffusion data obtained for rocks or minerals. However, very few studies have been done on noble gas diffusion in silicate melts. The latter bears a central importance in understanding the noble gas evolution in the mantle. Figure 2.11 shows one such scarce example (Lux, 1987), where the diffusion coefficients obtained for a tholeiite basalt melt at 1350°C are plotted as a function of noble gas radius. Diffusion obeys more or less the same linear relationship with r as does the solubility. As Lux (1987) noted, it is remarkable that the... 

Kubicki, J.D. and Lasaga, A.C., Molecular dynamics and diffusion in silicate melts, in Diffusion, Atomic Ordering, and Mass Transport Selected Problems in Geochemistry, Ganguly, J., Ed., Springer-Verlag, New York, 1991, P- L... 

Dunn T (1986) Diffusion in silicate melts An introduction and literature review. In Scarfe C, (ed) Silicate Melts Short Course Handbook 12 57-92. Mineral Assoc Canada, Ottawa, Ontario Dyer A, Molyneux A (1968) The mobility of water in zeohtes—I. Self-diffusion of water in analcite. J Inorg Nucl Chem 30 829-837... 

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