Development of New Methods to Control Redox Conditions in High-pressure Experiments and their Application to Study Ore Metal Solubilities in Sulfur-bearing Magmatic Fluids
This thesis perfected the method of admixing calculated amounts of H2 gas to the argon pressure medium to impose desired fO2 values in hydrous experiments. Using CoPd alloy redox sensors, the method was shown to be accurate within 0.3 log units for the P-T range of 960 – 2060 bar and 800 – 1100⁰C. The rate of progressive oxidation caused by diffusive H2 loss through the Molybdenum Hafnium Carbide (MHC) pressure vessel walls was determined experimentally and the hydrogen permeability constants of the MHC alloy were calculated. At T = 1000⁰C, fO2 at water saturation increased about 0.36 log units/day, and this value approximately doubled for every 100oC increase in T. A prototype MHC pressure vessel apparatus was developed that can be used with a modified Shaw membrane to impose fO2 precisely and accurately while maintaining rapid quench capability. It was shown that 95% of the imposed hydrogen pressure is attained inside the pressure vessel within 2 hours from the start of the experiment at 800 – 1000⁰C, after which a steady state equilibrium is established. Experiments comparing redox-dependent Cu solubilities in silicate melts at fO2 imposed by redox buffers and identical target fO2 imposed by the hydrogen membrane confirmed consistency between the two methods within 0.3 log units fO2 deviation at T=1000⁰C and P=2000 bar. Experiments using Fe(II)-Mg exchange between olivine and water-saturated melt to constrain Fe3+/Fetotal ratios in the silicate melt yielded results mostly consistent within 1σ error with those predicted by the equation of Kress and Carmichael (1991). The relationship between exchange coefficient of Fe-Ti between magnetite and silicate melt (K_(d,Fe-Ti)^(magnetite/melt)) and fO2 in hydrous mafic magmas was also constrained. Using the new experimental apparatus, the solubilities of Cu, Ag and Au in S-rich Cl-bearing magmatic fluids were determined at T = 900 ⁰C and P = 2000 bar covering a wide range of fO2 from NNO – 0.5 to NNO + 2.5 with 7 steps in-between. Thermodynamic model calculations were performed using the HCh software to determine the dominant species of the metals in the fluids.
magmatic fluids, MHC alloy, porphyry, Pressure vessels, redox, sulfur
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