REDOX CONTROL OF SULFUR DEGASSING IN SILICIC MAGMAS

Explosive eruptions involve mainly silicic magmas in which sulfur solu bility and diffusivity are low. This inhibits sulfur exsolution during magma uprise as compared to more mafic magmas such as basalts. Silici c magmas can nevertheless liberate large quantities of sulfur as shown by the monitoring of SO2 in recent explosive silicic eruptions in are settings, which invariably have displayed an excess of sulfur relativ e to that calculated from melt degassing. If this excess sulfur is sto red in a fluid phase, it implies a strong preference of sulfur for the fluid over the melt under oxidized conditions, with fluid/melt partit ion coefficients varying between 50 and 2612, depending on melt compos ition. Experimentally determined sulfur partition coefficients for a d acite bulk composition confirm this trend and show that in volcanic er uptions displaying excess gaseous sulfur, the magmas were probably flu id-saturated at depth. The experiments show that in more reduced silic ic magmas, those coexisting only with pyrrhotite, the partition coeffi cient decreases dramatically to values around 1, because pyrrhotite lo cks up nearly all the sulfur of the magma. Reevaluation of the sulfur yields of some major historical eruptions in the light of these result s shows that for oxidized magmas, the presence of 1-5 wt % fluid may i ndeed account for the differences observed between the petrologic esti mate of the sulfur yield and that constrained from ice core data. Expl osive eruptions of very large magnitude but involving reduced and cool silicic magmas, such as the Toba or the Bishop events, release only m inor amounts of sulfur and could have consequently negligible long-ter m (years to centuries) atmospherical effects. This redox control on su lfur release diminishes as the melt composition becomes less silicic a nd as temperature increases, because both factors favor more efficient melt sulfur degassing owing to the increased diffusivity of sulfur in silicate melts under such conditions.