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

Abstract

Olivine, 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.