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 aqualitative 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+/ΣFe ratio in spinel. Additionally, while serpentinization is a redox reaction that leaves behind an oxidized residue, the oxygenfugacity recorded by mantle minerals is unaffected by nearby low-temperature serpentinization. As a result, oxygen fugacity measured by spinel oxybarometry in seafloor peridotitesis representative of mantle processes, rather than an artifact of late-stage seafloor alteration.
Bibliographical noteFunding Information:
We thank Trevor Falloon, Sherman Bloomer, and Chris MacLeod for access to the samples, and Robert Stern for his encouragement in working on Tonga. We thank Tim Gooding and Tim Rose for laboratory support at the Smithsonian Institution. We thank Frieder Klein, William Leeman, and an anonymous reviewer for their thorough and insightful suggestions, as well as J. Brendan Murphy for editorial handling. This material is based upon work supported by the National Science Foundation under grant OCE-1433212 to Cottrell and Davis and grant OCE-1434199 to Warren. Birner was supported by a Stanford Graduate Fellowship.