Calibration of the Fe–Ir redox sensor at high pressure and the oxygen fugacity of laser heated diamond anvil cell experiments

The Fe–Ir binary alloy has been calibrated up to 100 GPa as a part of a thermodynamic model (oxygen fugacity $${(f}_{{\text{O}}_{2}})$$ sensor system) to monitor the redox conditions during high–pressure and high–temperature petrology experiments. The existing Fe–Ir activity–composition relations at 1 bar and ~ 473–2873 K have been updated by including the pressure dependence on the activity–composition relations. We calibrated the volume dependent interaction parameter $${W}^{V}$$ up to 61 GPa and extrapolated the Margules activity model (i.e., $${W}_{\text{Fe}-\text{Ir}}^{G}$$ and $${W}_{\text{Ir}-\text{Fe}}^{G}$$ ) up to 100 GPa by combining the experimentally determined compressibility and compositional data of the alloy, with those of the end member phases in a thermodynamic model. We apply the newly calibrated redox sensor to oxygen fugacity determination during laser heated diamond anvil cell (LHDAC) experiments. In situ LHDAC experiments were performed up to ~ 61 GPa and ~ 2000 K. We tracked the $${f}_{{\text{O}}_{2}}$$ conditions in the DAC by reacting the powdered mixture during laser heating to form a Fe–Ir alloy (sliding redox sensor) and using high–resolution synchrotron Mössbauer Source spectroscopy, powder X–ray diffraction, X–ray absorption near–edge structure spectroscopy, analytical transmission electron microscopy, and chemical analyses down to the nanoscale. The inferred oxygen fugacities measured at the specific P–T conditions of the experiments are ~ 1 log unit below the iron–wüstite buffer, which is lower than estimates obtained with the existing model and compared to multi anvil experiments performed with the same starting mixture at similar P–T conditions.