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Fluid migration above a subducted slab - Thermodynamic and trace element modelling of fluid-rock interaction in partially overprinted eclogite-facies rocks (Sesia Zone, Western Alps)

Journal article
Authors Matthias Konrad-Schmolke
Thomas Zack
P. J. O'Brien
M. Barth
Published in Earth and Planetary Science Letters
Volume 311
Issue 3-4
Pages 287-298
ISSN 0012-821X
Publication year 2011
Published at Department of Earth Sciences
Pages 287-298
Language en
Keywords fluid-rock interaction, subduction zone, fluid migration, slab-mantle interface, trace element transport, high-pressure metamorphism, garnet growth, regional metamorphism, lanzo zone, partitioning experiments, prograde metamorphism, ore-deposits, arc magmas, constraints, flow
Subject categories Earth and Related Environmental Sciences, Geology, Solid earth geology and petrology, Geochemistry

Abstract

The amount and composition of subduction zone fluids and the effect of fluid-rock interaction at a slab-mantle interface have been constrained by thermodynamic and trace element modelling of partially overprinted blueschist-facies rocks from the Sesia Zone (Western Alps). Deformation-induced differences in fluid flux led to a partial preservation of pristine mineral cores in weakly deformed samples that were used to quantify Li, B, Stand Pb distribution during mineral growth, -breakdown and modification induced by fluid-rock interaction. Our results show that Li and 13 budgets are fluid-controlled, thus acting as tracers for fluid-rock interaction processes, whereas Stand Pb budgets are mainly controlled by the fluid-induced formation of epidote. Our calculations show that fluid-rock interaction caused significant Li and B depletion in the affected rocks due to leaching effects, which in turn can lead to a drastic enrichment of these elements in the percolating fluid. Depending on available fluid-mineral trace element distribution coefficients modelled fluid rock ratios were up to 0.06 in weakly deformed samples and at least 0.5 to 4 in shear zone mylonites. These amounts lead to time integrated fluid fluxes of up to 1.4-10(2) m(3) m(-2) in the weakly deformed rocks and 1-8-10(3) m(3) m(-2) in the mylonites. Combined thermodynamic and trace element models can be used to quantify metamorphic fluid fluxes and the associated element transfer in complex, reacting rock systems and help to better understand commonly observed fluid-induced trace element trends in rocks and minerals from different geodynamic environments. (C) 2011 Elsevier B.V. All rights reserved.

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