Oxygen Fugacity Buffers and an Interactive Calculator

Abstract

Oxygen fugacity buffers are equilibrium phase assemblages that constrain the chemical potential of oxygen. They may be coexisting minerals or fluids, or their metastable extrapolations. Oxygen fugacity values vary as a function of pressure and temperature, such that raw numbers often have little interpretive value. Instead, oxygen fugacity is generally reported relative to oxygen fugacity buffers that provide convenient and tangible reference curves. The exact calculation of these curves is typically highly complex, involving the solution of thermodynamic relationships using advanced algebra, calculus, and numerical methods. As a result, buffers are often calculated using simple parameterizations which are easily implemented. This simplification of oxygen fugacity buffer curves comes at the cost of accuracy and precision in pressure and temperature space. Occasionally, several parameterizations for a single buffer exist, causing discrepancies in published results. Here, we introduce an online calculator built using the shiny package for the R programming language. The calculator solves the often complex thermodynamics and provides a simple interface for users to manipulate input parameters. All calculations are fully documented and justified. This paper describes some of the common calculation methods for oxygen fugacity buffers, including 1 bar parameterizations, correct usage of tabulated standard pressure data, high-pressure equations of state and phase transitions, gas mixing at low and high pressures, and more. A description of the many challenges and problems in the current literature is provided, and recommendations for future calibrations are suggested.