Aqueous concentration of CO2 in carbon-saturated fluids as a highly sensitive oxybarometer

The CO2 content of aqueous fluids in equilibrium with carbon can be used to retrieve their oxygen fugacity if pressure and temperature are known. Applicable to both natural and experimental systems, we present a new oxybarometer based on the aqueous concentration of CO2 in fluids saturated with either graphite or glass-like carbon, suitable to retrieve their oxygen fugacity. The method was experimentally tested by measuring by mass spectrometry the CO2 content in aqueous fluids coexisting with glass-like carbon buffered externally with Ni-NiO, employing ordered and disordered forms of NiO characterised by small differences in free energy (<5 kJ/mol). Considering analytical uncertainties on CO2 measurements, fO2 values can be resolved with an accuracy of about 0.01 log units, which is one order of magnitude lower than uncertainties affecting conventional solid state redox sensors. The CO2- in-fluid oxybarometer is the first available parameterisation of the fO2 dependency on pressure, temperature and CO2 content of aqueous fluids and can be used for fluids containing >1 mol. % CO2 beneath the graphite-diamond transition.