Fluid composition in the anhydrous C-O-S system under lower crustal P-T conditions according to thermodynamic modeling

Thermodynamic modeling was used to show that the composition of fluids in the anhydrous subsystem of the C-O-S-H system is dominated by COS, CO, and CS₂ under reducing conditions ( $f_{O_2 } $ < NNO). At an increase in oxygen fugacity up to NNO to NNO + 1 at T = 900–1100°C and lower crustal pressures, the composition of model fluid reaches the maximum content of elemental sulfur. In the presence of water (more than a few mole percent), the compounds of sulfur and carbon are decomposed to form hydrogen sulfide, this is why their content in volcanic gases is negligible. Fluid dominated by carbon dioxide is characteristic of some basic magmas of subduction zones and flood basalt provinces related to superplumes. It is suggested that, under such geodynamic conditions, the COS, CO, and CS₂ components of the fluid dissolve, concentrate, and transport nickel and, possibly, platinum group metals.