Magmatic sulfur compounds and sulfur diffusion in albite melt at 1 GPa and1300-1500 degrees C

The speciation and diffusion of sulfur in nominally dry albite melt at 1300 -1500 degrees C has been investigated by analyzing glasses formed in piston -cylinder runs using electron microprobe, micro-Raman, infrared-, UV-, and visible-light-spectroscopy, and other techniques. The sulfate ion is very s table in the albite melt even at low f(O2) and is the dominant species in a ll glasses. In the presence of graphite the glass acquired a characteristic strong violet color, as a consequence of conversion of some of the sulfate to S-2(-) and S-3(-) radical anions. The equilibration between S-2(-) and S-3(-) was rapid and both radicals were found to be very stable at elevated temperatures. Both S-2(-), S-3(-), and sulfide diffuse faster then the sul fate anion, so these species will control the diffusion process when presen t. In violet-colored albite glass in which some of the sulfur is S-2(-) and S-3(-), the diffusion coefficient for bulk sulfur diffusion was found to b e: D = 14.7 exp(-458100 J.mol(-1)RT) m(2)/s where T is the temperature in K and R the gas constant. This relation defin es D values significantly lower than those determined for sulfur in dry and esite, dacite, and rhyolite melt, in which the diffusion is controlled by s ulfide. The reason for this difference is the higher degree of melt polymer ization, the stabilization of sulfate over sulfide in the albite melt, and the slow diffusion of S-2(-) and S-3(-) radical anions compared to sulfide.