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Publisher's PDFOlivine, orthopyroxene, and spinel compositions within seafloor peridotites yield importantinformation about the nature of Earth’s mantle. Major element compositions of these mineralscan be used to calculate oxygen fugacity, a thermodynamic property critical to understandingphase equilibria in the upper mantle. This study examines how hydrothermal alteration atthe seafloor influences peridotite chemistry. The Tonga Trench (South Pacific Ocean) exposeslithospheric forearc peridotites that range from highly altered to completely unaltered and providesan ideal sample suite for investigating the effect of alteration on spinel peridotite majorelement chemistry and calculated oxygen fugacity. Using the Tonga peridotites, we develop aqualitative alteration scale rooted in traditional point-counting methodology. We show thathigh degrees of serpentinization do not affect mineral parameters such as forsterite numberin olivine, iron site occupancy in orthopyroxene, and Fe3+/SFe ratio in spinel. Additionally,while serpentinization is a redox reaction that leaves behind an oxidized residue, the oxygenfugacity recorded by mantle minerals is unaffected by nearby low-temperature serpentinization.As a result, oxygen fugacity measured by spinel oxybarometry in seafloor peridotitesis representative of mantle processes, rather than an artifact of late-stage seafloor alteration.University of Delaware. Department of Geological Sciences

Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry