Solubility of molybdenite in hydrous granitic melts at 800 °C, 100-200 MPa

The solubility of molybdenite, MoS2, in fluid-saturated, subaluminous to peraluminous granitic melts was determined experimentally using rapid-quench cold-seal pressure vessels at 800 °C and 100-200 MPa, and analysis by laser-ablation ICP-MS. Molybdenite solubility seems to be independent of pressure, but it shows strong variations with oxygen and sulfur fugacity. At constant log fS2 = −1.3 it increases from 0.1-0.7 ppm by weight Mo at the Co-CoO buffer to 29-38 ppm by weight Mo near the MnO-Mn3O4 buffer. The solubility isopleths are nearly parallel to the pyrrhotite-magnetite equilibrium, along which the solubility varies only slightly, from 10 ppmw Mo at the quartz-fayalite-magnetite buffer to 29-38 ppmw Mo at the MnO-Mn3O4 buffer. The observed solubility variations are consistent with the equilibrium MoS2(s) + 3/2O2 = MoO3(l) + S2 and thus confirm that molybdenum(VI) oxide is the predominant species in subaluminous silicate melts at log fO2 = −16 to −11. In addition, the experimental results are well reproduced by a simple thermodynamic model employing the Burnham eight-oxygen formulation for silicate melt species and assuming ideal mixing of dissolved MoO3. The thermodynamic calibration can be used to estimate the molybdenum solubility in subaluminous silicic melts or, for pyrrhotite- and molybdenite-saturated assemblages, the oxygen and sulfur fugacities during magma crystallization.