The oxygen fugacity (fo2) of magmas has a major role in the speciation of dissolved C-O-H molecules and the transport of trace elements eventually released to the atmosphere through time. To date, the experimental determination of the Fe3+ and Fe2+ in solid glasses quenched from magmas in nature (i.e., mid-ocean ridge basalts, MORBs) is needed to calculate their fo2 through previously calibrated equations. No measurements of the Fe3+/∑Fe in basaltic glasses exist as function of pressure and temperature that would allow to model the fo2 of MORBs upon decompression from the Earth’s upper mantle to the surface. This beamtime proposal aims to address the above issue through the determination of,1) the Fe3+/∑Fe ratio of natural glasses from three different mid-ocean ridges whose chemical composition (i.e., major oxides, trace elements and noble gases) is known; 2) the Fe3+/∑Fe of samples in (1) at different P and T using the diamond anvil cell combined with a laser heating system.