Aqueous CO2 in carbon-saturated fluids as a highly sensitive oxybarometer: application to the case of the Ni-NiO oxygen buffer.

Abstract:

Oxygen fugacity has a predominant role in the Earth’s interior, controlling processes involving solids, aqueous fluids and melts, affecting phase equilibria and the behaviour of multivalence elements (e.g., Fe, C, S). Being able to quantify its value is fundamental for the interpretation of geological processes and has led to the definition and calibration of several oxybarometers based on solid-solid equilibria or, for instance, melts with a CO₂ content depending on fO₂. Here we present the calibration of a new oxybarometer based on the aqueous concentration of CO₂ in carbon saturated fluids. The CO₂-in-fluid oxybarometer represents the first available parameterization of the fO₂ dependency on pressure, temperature and CO₂ content of aqueous fluids (i.e. XCO₂ = CO2/H₂O + CO₂). It can be used with confidence for natural and synthetic fluids containing > 1 mol% CO₂ and saturated in ordered (graphite) or disordered (glass-like C) carbon. In order to test the methodology, experiments were performed equilibrating an externally buffered and carbon saturated COH fluid, at 1 GPa and 800°C, and measuring with mass spectrometry its XCO₂. As buffers, Ni-NiO assemblages with different NiO precursors were used. As a matter of fact different forms of ordered and disordered NiO are known to be characterized by small variation in the Gibbs free energy (O’Neill and Pownceby 1993) and might consequently affect the fO₂, providing the mean to assess the sensibility of the CO₂-in-fluid oxybarometer. The results of the experiments display how using the XCO₂ as a sensor allows to resolve differences in the fO₂ of 0.001 log units with the uncertainties being in the order of 0.01 log units. Concerning the Ni-NiO buffers, each used buffering assemblage had a different fO₂. We determined the thermodynamic properties of each NiO precursor and finally, using a reference from thermodynamic modeling, we established which buffering assemblage represents the most reliable choice for experiments.

Reference:

O'Neill, H. S. C., & Pownceby, M. I. (1993). Contributions to Mineralogy and Petrology, 114(3), 296-314.