Towards Redox Control in the Laser-Heated Diamond Anvil Cell

Redox control remains a challenge in laser-heated diamond anvil cell (LHDAC) studies aimed at simulating planetary interiors. While techniques exist to regulate oxygen fugacity (fO₂) in multi-anvil experiments for upper-mantle minerals, they often cannot be extended to pressures of the lower mantle. Here, we present a new approach for creating reducing conditions in the LHDAC using an Ar + 3vol% H_2​ mixture as the pressure medium. We investigate the high-pressure, high-temperature behavior of three iron oxide starting materials. In situ synchrotron X-ray diffraction (XRD) experiments were conducted on α-Fe₂​O_3​ (hematite), ϵ-FeOOH (CaCl2​-type), and Fe₃​O₄​ (magnetite), to assess the effectiveness of this method. Compared to Ar (H₂-free) medium, we observed the formation of more reduced Fe-bearing phases, including Fe₄​O_5​, Fe₅​O_6​,and FeO, depending on the starting material and exact pressure–temperature (P–T) conditions. Our results demonstrate that the Ar + H₂​ pressure medium can drive reduction of ferric to ferrous iron states in the LHDAC environment. These results show a practical method for achieving and maintaining reducing conditions in LHDAC experiments relevant to Earth’s lower mantle, where iron-bearing minerals undergo complex redox reactions under extreme conditions, and they also show how hydrogen can shift phase stability fields in iron oxides, effectively mimicking the reduced environments of planetary interiors.