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Geochemical fingerprints of devolatilization reactions in the high pressure rocks of Ile de Groix, France

English title Geochemical fingerprints of devolatilization reactions in the high pressure rocks of Ile de Groix, France
Applicant Schmidt Susanne
Number 121940
Funding scheme Project funding
Research institution Département des sciences de la Terre Université de Genève
Institution of higher education University of Geneva - GE
Main discipline Mineralogy
Start/End 01.10.2008 - 31.03.2011
Approved amount 127'040.00
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Keywords (10)

High pressure metamorphism; devolatilization reactions; subduction zone; stable isotope analysis; Ile de Groix; Subduction-related HP-metamorphism; dehydration reaction; trace elements; garnet; epidote

Lay Summary (English)

Lead
Lay summary
LeadIn a downgoing slab of a subduction zone devolatilization reactions can generate large quantities of fluids. This project aims to understand the mechanisms and the pathways by which these fluids are released and the resulting changes in the host rock. Background Subduction zones are geologically important sites which produce crust from sediments, oceanic crust, and mantle lithosphere. Physical, petrologic and thermal models for the evolution of subduction zones suggest that devolatilization reactions in the downgoing slab can generate large quantities of fluids during various stages of subduction. The mechanisms and the pathways by which these fluids are released are poorly constrained and have been discussed in various field areas.This study deals with a particular aspect of devolatilization reactions in eclogite, glaucophane, amphibolite and greenschist facies rocks occurring on the Island of Groix (Brittany, France) and will examine the geochemical and petrological compositions of the high-pressure rocks that devolatilization reactions have imprinted on these high-pressure rocks. The strategy used is to thermodynamically and geochemically characterize the high-pressure rocks and determine the geochemical fingerprints of devolatilization reactions left in the rocks. Major, trace and REE-element distribution in the whole rock and in the minerals, as well as oxygen isotope compositions are analyzed and thermodynamic modeling is carried. First ResultsFirst results show that the massive, mafic rocks that have been subducted show limited evidence of trace element and REE mobilization and of fluid-rock interaction. The examined mafic rocks have retained their characteristic element pattern, including a hydrothermal alteration overprint before the subduction process. This suggests that devolatilization reactions do not necessarily imply a high simultaneous trace element release. The trace element contents of OH-bearing phases no longer stable under prevailing conditions are incorporated into the newly formed phases and the high-pressure rocks preserves their primary pre-subduction composition. The systems appear to have been only open for H2O and probably other volatile compositions, but almost all trace and rare-earth elements released during the prograde, peak and retrograde metamorphic path are trapped within the newly formed stable mineral phases, implying that devolatilization is decoupled from trace element migration. SignificanceThis project is concerned with devolatilization reactions in eclogite, blueschist, and greenschist facies rocks of the ancient subduction zone of the Ile de Groix, Brittany (France). It focuses on a particular aspect of high-pressure rocks or the solid state products left behind after devolatilization reactions have occurred. The results may also place constraints on the current discussion concerning mantle melting and metasomatism responsible for arc volcanism.
Direct link to Lay Summary Last update: 21.02.2013

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Associated projects

Number Title Start Funding scheme
113280 Geochemical fingerprints of devolatilization reactions in the high pressure rocks of Ile de Groix, France 01.10.2006 Project funding

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