Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry
Date
2016-06-01
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Publisher
Geological Society of America
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.
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Citation
Birner, Suzanne K., et al. "Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry." Geology (2016): G38113-1.