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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 compositions.
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 volatilefree
melt composition and no effect of carbon dioxide concentration has been observed, although few experiments
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 intermediate
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 diffusion.
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 only
known at a limited set of temperatures and compositions. The diffusion data for rhyolitic melts at 800°C together
with a standard model for the enrichment of incompatible elements in front of growing crystals demonstrate
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 representative
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.
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