Within the first segment of the project focusing on mantle processes, we have so far experimentally identified that the sulfide to sulfate transition occurs at more oxidizing conditions than anticipated, which facilitates relatively high-degree of mantle oxidation by sulfate-bearing slab-derived fluids. This is consistent with the findings in natural samples from the Trans-Mexican Volcanic belt, which show positive correlation between magmatic sulfur concentrations and oxidation state. Indeed, some of these magmas are characterized by extremely high sulfur concentrations (the highest reported to date worldwide) and rather high oxidation state. Relatively primitive mafic magmas at Parinacota also show high oxidation state and relatively high magmatic sulfur abundances. At the same time, there is no significant variation in magma redox as a function of the site and duration of magma storage in the crust. Therefore, these above two field areas indicate that the oxidized nature of arc magmas is primarily obtained at their source in the mantle, answering one of the key questions of the project.
In addition, the study on sulfur speciation at upper crustal pressures found that the fO2 range of the redox transition of sulfur is temperature dependent with significant differences compared to the results of previous ex situ experimental studies and thermodynamic predictions. Furthermore, these experiments have conclusively shown that though radical sulfur ions are present in magmatic fluids, their concentration is below the limits of detection of non-resonant Raman spectrometry over the entire studied range of P-T-fO2 conditions. This is in sharp contrast to recent findings by also in situ studies, which used resonant Raman spectroscopy. These findings have significant implications with regards to ore metal transport in magmatic fluids. In addition, the stability of sulfate species in magmatic fluids was found to be more limited than previously proposed based on ex situ experiments. This has implications for the mechanisms of volcanic sulfate aerosol production, which in turn affect the Earth’s climate and redox balance.
Experiments in progress at mantle pressures quantify sulfur transport in various slab fluids as a function of P-T and fluid composition, whereas field-based investigations will put additional constraints on volatile element – ore metal – redox systematics during magma generation and ascent. The sulfur speciation dataset will be complemented by studying P-T-fO2-melt compositional effects on sulfur speciation in silicate melts. Ultimately, all above data will be used to support model calculations of the geochemical cycle of heterovalent elements in subduction zones and the net long-term effect of subduction zone magmatism on the redox state of the atmosphere – hydrosphere system. In addition, the improved understanding of the interplay between magma redox evolution, sulfur and ore metal budgets on the path from mantle to surface in volcanic arcs will be used to improve models of magmatic-hydrothermal ore genesis.
