Empirical calibration of the sulphur valence oxygen barometer from naturaland experimental glasses: method and applications

New data on sulphur valence and magmatic oxidation state for Central Andean volcanic rocks, in combination with published data for experimental and na tural samples, allow derivation of a simple relationship between magma oxid ation state and sulphur speciation. For a number of highly oxidized Central Andean volcanic rocks f(O2) has been calculated using magnetite-ilmenite o r olivine-spinel pairs and the sulphur valence in glasses has been measured using the peak shift of S-K alpha radiation relative to a pyrite standard. Previously published experimental and natural data have been incorporated with a wider range in f(O2) and S valence. The variation in sulphur speciat ion (as S2- or SO42-) as a function of log f(O2) is described by an empiric al polynomial fit which reproduces the data to within +/-0.5 log units and allows use of electron microprobe measurements of the S-K alpha wavelength shift for estimation of magmatic oxygen fugacities. This approach is applic able for f(O2) between FMQ-2 and FMQ+6, encompassing most terrestrial magma s. The method has been used to calculate the f(O2) conditions under which m elt inclusions were trapped in andesitic magmas before magma mixing in two Central Andean volcanoes, and to calculate the oxygen fugacity of a slowly- cooled pyroclastic flow in which the Fe-Ti oxide phases have subsequently r e-equilibrated. In combination with Fe-Ti oxide data, two distinct trends e merge for Lascar volcano. Basaltic andesite-andesitic magma chambers follow T-f(O2) trends which parallel the FMQ buffer curve, indicating ferrous-fer ric iron buffering of oxygen fugacity. Dacitic anhydrite-bearing magmas wit h admired basaltic andesite and andesite follow trends of increasing f(O2) with decreasing temperature, indicative of buffering of f(O2) by SO2-H2S in a co-magmatic phase. This trend continues into the metamorphic aureole of the magma chamber, resulting in highly oxidized (close to magnetite-hematit e) conditions.