Abstract
A compendium of diffusion measurements and their Arrhenius equations for water, carbon dioxide, sulfur, flu- orine, and chlorine in silicate melts similar in composition to natural igneous rocks is presented. Water diffusion in silicic melts is well studied and understood, however little data exists for melts of intermediate to basic com- positions. The data demonstrate that both the water concentration and the anhydrous melt composition affect the diffusion coefficient of water. Carbon dioxide diffusion appears only weakly dependent, at most, on the volatile- free melt composition and no effect of carbon dioxide concentration has been observed, although few experi- ments have been performed. Based upon one study, the addition of water to rhyolitic melts increases carbon dioxide diffusion by orders of magnitude to values similar to that of 6 wt% water. Sulfur diffusion in intermedi- ate to silicic melts depends upon the anhydrous melt composition and the water concentration. In water-bearing silicic melts sulfur diffuses 2 to 3 orders of magnitude slower than water. Chlorine diffusion is affected by both water concentration and anhydrous melt composition; its values are typically between those of water and sulfur. Information on fluorine diffusion is rare, but the volatile-free melt composition exerts a strong control on its dif- fusion. At the present time the diffusion of water, carbon dioxide, sulfur and chlorine can be estimated in silicic melts at magmatic temperatures. The diffusion of water and carbon dioxide in basic to intermediate melts is on- ly known at a limited set of temperatures and compositions. The diffusion data for rhyolitic melts at 800°C to- gether with a standard model for the enrichment of incompatible elements in front of growing crystals demon- strate that rapid crystal growth, greater than 10 −10 ms −1 , can significantly increase the volatile concentrations at the crystal-melt interface and that any of that melt trapped by the formation of melt inclusions may not be rep- resentative of the bulk melt. However, basaltic melt inclusions trapped at 1300°C are more likely to contain bulk melt concentrations of water and carbon dioxide.
Highlights
Volatiles play an enormously important role in igneous processes
The second regime is that of chemical diffusion, which occurs in the presence of a significant chemical potential gradient
Another experimental technique for chemical diffusion allows a mineral with the element of interest to partially dissolve into the melt releasing the element and creating a diffusion profile away from the mineral; this technique often results in multicomponent diffusion, making data analysis difficult
Summary
Volatiles play an enormously important role in igneous processes. The second most abundant volatile in magmas is CO2, whose presence can be found in volcanic gases (Symonds et al, 1994) and as fluid inclusions in minerals (Roedder, 1965, 1979). The halogens, F and Cl, have been measured in volcanic gases and plumes (Symonds et al, 1994), and they are important constituents of hydrous minerals and occasionally are found as halogen minerals in igneous rocks. We apply simple models to explore the effects of volatile diffusion on the compositions of melt inclusions trapped in minerals
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