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

Suzanne K. Birner, Jessica M. Warren, Elizabeth Cottrell and Fred A. Davis
Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry
Geology (Boulder) (July 2016) 44 (7): 535-538

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 Fe (super 3+) /Sigma Fe 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.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 44
Serial Issue: 7
Title: Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry
Affiliation: Stanford University, Department of Geological Sciences, Stanford, CA, United States
Pages: 535-538
Published: 201607
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 32
Accession Number: 2016-070841
Categories: Geochemistry of rocks, soils, and sedimentsIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2016178
Illustration Description: illus. incl. sketch map
S25°00'00" - S14°00'00", W175°30'00" - W172°00'00"
Secondary Affiliation: University of Delaware, USA, United StatesNational Museum of Natural History, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201634
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