Europium in plagioclase-hosted melt inclusions reveals mantle melting modulates oxygen fugacity
Nick Dygert, Gökçe Üstünışık, Roger L. Nielsen
Abstract
Abstract To gain insights into the composition and heterogeneity of Earth’s interior, the partial pressure of oxygen (oxygen fugacity, or f O 2 ) in igneous rocks is characterized. A surprising observation is that relative to reference buffers, f O 2 s of mantle melts (mid-ocean ridge basalts, or MORBs) and their presumed mantle sources (abyssal peridotites) differ. Globally, MORBs have near-uniform f O 2 s, whereas abyssal peridotites vary by about three orders of magnitude, suggesting these intimately related geologic reservoirs are out of equilibrium. Here, we characterize f O 2 s 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 f O 2 s constrained by rare earth element distributions, a range of f O 2 s consistent with the abyssal peridotites is recovered. The f O 2 s are correlated with geochemical proxies for mantle melting, suggesting partial melting of Earth’s mantle decreases its f O 2 , and that the uniformity of MORB f O 2 s is a consequence of the melting process and plate tectonic cycling.