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Abstract To gain insights into the composition and heterogeneity of Earth’s interior, the partial pressure of oxygen (oxygen fugacity, orfO₂) 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-uniformfO₂s, whereas abyssal peridotites vary by about three orders of magnitude, suggesting these intimately related geologic reservoirs are out of equilibrium. Here, we characterizefO₂s 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 andfO₂s constrained by rare earth element distributions, a range offO₂s consistent with the abyssal peridotites is recovered. ThefO₂s are correlated with geochemical proxies for mantle melting, suggesting partial melting of Earth’s mantle decreases itsfO₂, and that the uniformity of MORBfO₂s is a consequence of the melting process and plate tectonic cycling. 

Betts, M.*, Ustunisik, G., Nielsen, R. L., and Daynes, O.*, “Sampling Patterns of Mid-Ocean Ridge (MOR) Magmas as Evidenced by Plagioclase Megacryts and their Melt Inclusions (MI)” Presentation at Goldschmidt Conference, Lyon, France (July 9-14, 2023). 

Daynes, O.*, Ustunisik, G., and Nielsen, R. L., and Betts, M.*, “Depth of Formation of Petrogenetic Processes: Evidence from Plagioclase-Hosted Melt Inclusions” Presentation at Goldschmidt Conference, Lyon, France (July 9-14, 2023). 

Abstract The application of melt inclusions (MI) to infer magmatic processes assumes the MI have remained as constant mass, constant volume systems since the time of trapping. Understanding the effects of both compositional and volumetric re‐equilibration is key for the interpretation of MI data. Although the re‐equilibration behavior MI in quartz and olivine has been studied in some detail, the process is less understood for other MI host phases such as plagioclase, a common phase in igneous rocks. A MI can re‐equilibrate when it experiences pressure and temperature (PT) conditions that differ from formation PT conditions. During laboratory heating, irreversible MI expansion may occur. As a result, the internal pressure within the MI decreases, resulting in chemical and structural changes to the MI and host. We present results of heating experiments on plagioclase‐hosted MI designed to induce volumetric re‐equilibration. The experiments consisted of incrementally heating the MI to temperatures above the homogenization temperatures. At ∼40°C above, the temperature at which the daughter minerals melted, irreversible volume expansion lowered the pressure in the MI, and led to exsolution of CO₂into vapor bubbles. With each additional few degrees of heating, additional episodes of CO₂exsolution, bubble nucleation and expansion of the vapor bubblesoccurred. Re‐equilibration of MI in plagioclase occurred through a combination of ductile and brittle deformation of the host surrounding the MI, whereas previous studies have shown that MI in olivine re‐equilibrate dominantly through ductile deformation associated with movement along dislocations. This behavior is consistent with the differing rheological properties of these phases.