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The role of C-O-H and oxygen fugacity in subduction-zone garnet peridotites

Nadia Malaspina and Simone Tumiati
The role of C-O-H and oxygen fugacity in subduction-zone garnet peridotites (in Diamonds, the mantle petrologist's best friends, Fabrizio Nestola (editor), Paolo Nimis (editor) and Ross J. Angel (editor))
European Journal of Mineralogy (August 2012) 24 (4): 607-618


C-O-H fluids are released by dehydration, partial melting and/or decarbonation of the slab and transferred to the mantle, where they interact with the surrounding rocks, prompting the growth of carbonates, hydrous minerals and C polymorphs. In the pure C-O-H system, C-saturated fluid speciation is a function of the oxygen chemical potential. Therefore, in natural systems, the fluid speciation can be imposed by the redox state of the rock-forming phases. Alternatively, C-O-H fluids may control the bulk oxidation state of the rock system by redox reactions with the mineral phases. We selected three case studies of garnet-bearing ultramafic rocks (Ulten zone, Italy; Sulu, China; Bardane, Norway), which record metasomatic processes driven by C-O-H fluids at the interface between a subducting slab and the overlying mantle wedge. All these rocks contain carbonates (dolomite-only at P<1.9 GPa at 900 degrees C, magnesite-only at P> 2.4 GPa at 900 degrees C, dolomite+magnesite in between) and hydrous phases (amphibole, phlogopite) equilibrated at some stages in the garnet stability field. The fO (sub 2) values, estimated by analysing the Fe (super 3+) content (skiagite mole fraction) in garnet, indicate that the Ulten and Sulu peridotites record high oxygen fugacities (FMQ to FMQ+2) and a retrograde path with decreasing P and T. The fO (sub 2) values obtained for the Bardane garnet websterites, which record a prograde path with increasing T and P, are up to -2 log units lower than the FMQ. When combined with data for subduction-zone systems (arc lavas and their mantle sources), the studied ultramafic rocks define a trend of decreasing fO (sub 2) with increasing pressure. The Bardane websterites contain C-polymorphs in polyphase inclusions, which precipitated from entrapped metasomatic fluids at ultrahigh pressures. The calculated C-O-H fluid phase in equilibrium with the solid phases consists of mixtures of H (sub 2) O and CO (sub 2) . Semi-quantitative estimates for the Ulten and Sulu peridotites, in which C-polymorphs have not been found, and petrographic constraints for the Ulten peridotites indicate that the C-O-H component of the fluid could consist of H (sub 2) O+CO (sub 2) .

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 24
Serial Issue: 4
Title: The role of C-O-H and oxygen fugacity in subduction-zone garnet peridotites
Title: Diamonds, the mantle petrologist's best friends
Author(s): Malaspina, NadiaTumiati, Simone
Author(s): Nestola, Fabrizioeditor
Author(s): Nimis, Paoloeditor
Author(s): Angel, Ross J.editor
Affiliation: Universita di Milano Bicocca, Dipartimento di Scienze Geologiche e Geotecnologie, Milan, Italy
Affiliation: University of Padua, Department of Geosciences, Padua, Italy
Pages: 607-618
Published: 201208
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Germany
Meeting name: Diamonds, the mantle petrologist's best friends
Meeting location: Bressanone-Brixen, ITA, Italy
Meeting date: 20110221Feb. 21-26, 2011
References: 77
Accession Number: 2012-073343
Categories: Igneous and metamorphic petrologySolid-earth geophysics
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus.
N36°30'00" - N47°30'00", E06°30'00" - E19°00'00"
N58°00'00" - N71°00'00", E04°00'00" - E31°00'00"
Secondary Affiliation: Universita di Milano, ITA, Italy
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Schweizerbart'sche Verlagsbuchhandlung. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201238
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