Publication Type:Book Chapter
Source:Goldschmidt abstracts 2013, Mineralogical Society, London, United Kingdom, Volume 77, p.1411 (2013)
Keywords:Asia, copper ores, decompression, Far East, fugacity, gold ores, Luzon, magmas, melts, metal ores, models, molybdenum ores, Mount Pinatubo, partitioning, Philippine Islands, PORPHYRY COPPER, porphyry gold, porphyry molybdenum, siliceous composition, sulfur, volatiles
The partitioning of sulfur between silicate melts and coexisting volatile phases is a key parameter to understand igneous processes such as massive S degassing in explosive eruptions (e.g. Mount Pinatubo, June 1991) and the formation of porphyry deposits (which in addition to Cu, Mo, and Au, are S anomalies). It is well-established that the S content at sulfide saturation (SCSS) in a silicate melt increases exponentially with increasing fO (sub 2) regardless of whether the system is anhydrous or the melt is in equilibrium with a hydrous phase. It is also well-established that only S (super 2-) and S (super 6+) species are significant in silicate melts, whereas S (super 4+) species (in addition to S (super 2-) and S (super 6+) ) are significant in volatile phases. Thus, the stability of S (super 4+) species in the volatile phase is an essential parameter controlling S partitioning. Early studies of S behaviour between volatile phases and silicate melts have been used extensively to describe a "sulfur solubility minimum" that has no relevance to SCSS because the experiments in those studies lacked sulfides as run products. However, those experimental studies provide useful reference models when the data are recast in a more appropriate framework (i.e. to express S partitioning between the input gas and the silicate melt). The results show that, at atmospheric pressure, a S partitioning maximum exists at fO (sub 2) conditions that may be attributed to S (super 4+) dominance in volatile phases at low pressure (and that overlap the fO (sub 2) range corresponding to the S (super 2-) to S (super 6+) transition in silicate melts at high pressure). Linking the two types of data (SCSS at high P and D (sub S) (super gas/melt) at atmospheric P) provides a useful model to understand S mass transfer from silicate melts into volatile phases during volatile exsolution and degassing resulting from magma ascent and decompression.
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