BUFFERING OF MELT OXYGEN FUGACITY BY SULFUR REDOX REACTIONS IN CALC-ALKALINE MAGMAS

Calculated oxygen fugacities of magmas from Lascar Volcano, Northern C hile, and the 1991 eruption of Mount Pinatubo, Phillipines, are seen t o increase with decreasing temperature, relative to the fayalite-magne tite-quartz buffer. Isopleths of constant H2S/SO2 at moderate pressure s (2-4 kbar) lie parallel to the data. We conclude that magma mixing p rior to eruption liberated large quantities of SO2 gas by oxidation of dissolved sulphide in the mafic end-member. Oxygen fugacity of the ma gmas was then buffered by reduction of this gas to H2S, leading to inc reasingly oxidized magmas. Such a process might be used to explain the highly oxidized nature of other sulphur-rich evolved calc-alkaline ma gmas, notably the El Chichon trachyandesite.