Europium in plagioclase-hosted melt inclusions reveals mantle melting modulates oxygen fugacity

To gain insights into the composition and heterogeneity of Earth’s interior, the partial pressure of oxygen (oxygen fugacity, or fO₂) in igneous rocks is characterized. A surprising observation is that relative to reference buffers, fO₂s of mantle melts (mid-ocean ridge basalts, or MORBs) and their presumed mantle sources (abyssal peridotites) differ. Globally, MORBs have near-uniform fO₂s, whereas abyssal peridotites vary by about three orders of magnitude, suggesting these intimately related geologic reservoirs are out of equilibrium. Here, we characterize fO₂s of mantle melting increments represented by plagioclase-hosted melt inclusions, which were entrapped as basaltic melts migrated from their sources toward the seafloor. At temperatures and fO₂s constrained by rare earth element distributions, a range of fO₂s consistent with the abyssal peridotites is recovered. The fO₂s are correlated with geochemical proxies for mantle melting, suggesting partial melting of Earth’s mantle decreases its fO₂, and that the uniformity of MORB fO₂s is a consequence of the melting process and plate tectonic cycling.