Abstract New data on sulphur valence and magmatic oxidation state for Central Andean volcanic rocks, in combination with published data for experimental and natural samples, allow derivation of a simple relationship between magma oxidation state and sulphur speciation. For a number of highly oxidized Central Andean volcanic rocks f O 2 has been calculated using magnetite-ilmenite or olivine-spinel pairs and the sulphur valence in glasses has been measured using the peak shift of S-Kα radiation relative to a pyrite standard. Previously published experimental and natural data have been incorporated with a wider range in f O 2 and S valence. The variation in sulphur speciation (as S 2- or SO 4 2- ) as a function of log f O 2 is described by an empirical polynomial fit which reproduces the data to within ±0.5 log units and allows use of electron microprobe measurements of the S- K α wavelength shift for estimation of magmatic oxygen fugacities. This approach is applicable for f O 2 between FMQ-2 and FMQ+6, encompassing most terrestrial magmas. The method has been used to calculate the in f O 2 conditions under which melt 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 re-equilibrated. In combination with Fe-Ti oxide data, two distinct trends emerge for Lascar volcano. Basaltic andesite-andesitic magma chambers follow T- f O 2 trends which parallel the FMQ buffer curve, indicating ferrous-ferric iron buffering of oxygen fugacity. Dacitic anhydrite-bearing magmas with admixed basaltic andesite and andesite follow trends of increasing f O 2 with decreasing temperature, indicative of buffering of f O 2 by SO 2 -H 2 S in a co-magmatic gas phase. This trend continues into the metamorphic aureole of the magma chamber, resulting in highly oxidized (close to magnetite-hematite) conditions.
This chapter contains sections titled: Introduction Magmas on Earth Eruption of Magmas and Volcanic Plumes Sulfur Behavior in Magmas An Improved Petrologic Method for Estimation of Sulfur Emissions Conclusions Glossary
Hydrothermal experiments have been conducted on a sulfur saturated dacitic melt over a range of pressure, temperature, oxygen fugacity, and melt FeO content in order to examine the effects of these variables on sulfur solubility in fractionated melts. Experiments done under both reducing (GCH, QFM buffers) and oxidizing (MNO, HM buffers) conditions indicate the solubility of sulfur increases with increasing total pressure ( = fluid pressure) over the pressure range 100 to 300 MPa. GCH buffered experiments (1025°C) with 18 to 30 wt % FeO show sulfur solubilities at sulfide saturation ranging from 0.1 to 0.5 wt % S; increasing pressure from 100 to 200 MPa increases sulfur solubility by 500 to 1000 ppm, with the greatest increase observed in more FeO‐rich melts. GCH buffered melts with <5 wt % FeO show no measurable change in sulfur solubility (300±150 ppm S) between 100 and 200 MPa at 1025°C. QFM buffered experiments (1025°C) with 10.0 to 12.5 wt % FeO show sulfur solubility increasing from 600 to 1000 ppm between 100 and 200 MPa. QFM experiments with lower FeO contents (2 to 8 wt %) showed no measurable (±200 ppm S) effect of temperature (912°C to 1025°C) or pressure (100 to 220 MPa) on sulfur solubility. Experiments done under oxidizing conditions of the HM and MNO buffers (1025°C) show sulfur solubilities in melts with 3 to 5 wt % FeO ranging from ∼1400 ppm at 100 MPa to ∼3000 ppm at 300 MPa. More importantly however, the change to more oxidizing conditions is accompanied by a change from sulfide (an FeS‐rich melt) to sulfate (crystalline CaS04) saturated conditions. Stabilization of anhydrite as a magmatic phase (1025°C, 100 to 300 MPa) is also accompanied by a significant increase in sulfur solubility relative to saturation values for more reduced melts with similar FeO contents. The results of this study show that upper crustal oxidation‐reduction and crystal fractionation processes may exert considerable influence on the amount of sulfur contained in magmas erupted at the surface. These results are of basic importance in understanding volatile transport and volcanic degasing processes on planets such as the Earth, Mars, and Venus.
The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non‐uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.
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