Sulfur diffusion in rhyolite melts |
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| Authors: | L L Baker Malcolm J Rutherford |
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| Institution: | (1) Department of Geological Sciences, Brown University, Providence, RI 02912, USA, US |
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| Abstract: | Diffusion rates for sulfur in rhyolite melt have been measured at temperatures of 800–1100° C, water contents of 0–7.3 wt%,
and oxygen fugacities from the quartz-fayalite-magnetite buffer to air. Experiments involved dissolution of anhydrite or pyrrhotite
into rhyolite melt over time scales of hours to days. Electron microprobe analysis was used to measure sulfur concentration
profiles in the quenched glasses. Regression of the diffusion data in dry rhyolite melt gives Dsulfur=0.05·exp{−221±80RT}, which is one to two orders of magnitude slower than diffusion of other common magmatic volatiles such as H2O, CO2 and Cl-. Diffusion of sulfur in melt with 7 wt% dissolved water is 1.5 to 2 orders of magnitude faster than diffusion in the anhydrous
melt, depending on temperature. Sulfur is known to dissolve in silicate melts as at least two different species, S2− and S6+, the proportions of which vary with oxygen fugacity; despite this, oxygen fugacity does not appear to affect sulfur diffusivity
except under extremely oxidizing conditions. This result suggests that diffusion of sulfur is controlled by one species over
a large range in oxygen fugacity. The most likely candidate for the diffusing species is the sulfide ion, S2−. Re-equilibration between S2− and S6+ in oxidized melts must generally be slow compared to S2− diffusion in order to explain the observed results. In a silicic melt undergoing degassing, sulfur will tend to be fractionated
from other volatile species which diffuse more rapidly. This is consistent with analyses of tephra from the 1991 eruption
of Mount Pinatubo, Philippines, and from other high-silica volcanic eruptions.
Received: 26 April 1995 / Accepted: 1 November 1995 |
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