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Discussion papers
https://doi.org/10.5194/se-2019-140
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/se-2019-140
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 23 Sep 2019

Submitted as: research article | 23 Sep 2019

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This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Solid Earth (SE) and is expected to appear here in due course.

Tracing fluid transfers in subduction zones: an integrated thermodynamic and δ18O fractionation modelling approach

Alice Vho1, Pierre Lanari1, Daniela Rubatto1,2, and Jörg Hermann1 Alice Vho et al.
  • 1Institute of Geological Sciences, University of Bern, CH-3012 Bern, Switzerland
  • 2Institut de Sciences de la Terre, University of Lausanne, CH-1015 Lausanne, Switzerland

Abstract. Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid sources and quantify the conditions of fluid-rock interaction. We present an integrated modelling approach and the computer program PTLOOP that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen fractionation model both based on internally consistent databases. This approach is applied to subduction zone metamorphism to predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure-temperature (P-T) conditions in the subducting slab. The modelled system is composed by a sequence of oceanic crust (mafic) with sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblage and δ18O values of each phase is calculated along a defined P-T path. Fluid-rock interactions may occur as consequence of (1) infiltration of an external fluid into the mafic rocks or (2) transfer of the fluid liberated by dehydration reactions occurring in the mafic rocks into the sedimentary rocks. The effects of interaction with externally-derived fluids on the mineral and bulk δ18O of each rock are quantified for two typical compositions of metabasalts and metasediments with external fluid influx from serpentinite. The dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations in bulk δ18O values, i.e. within 1 ‰. By contrast, the interaction with external fluids may lead to shifts in δ18O up to one order of magnitude larger. Such variations can be detected by analysing in-situ oxygen isotope in key metamorphic minerals such as garnet, white mica and quartz. The simulations show that, when the water released by the slab infiltrates the forearc mantle wedge, it can cause extensive serpentinization within fractions of a Myr and significant oxygen isotope variation at the interface. This technique opens new perspectives to track fluid pathways in subduction zones, to distinguish porous from channelized fluid flows, and to determine the P-T conditions and the extent of fluid/rock interaction.

Alice Vho et al.
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Short summary
This study presents an approach that combines equilibrium thermodynamic modelling with oxygen isotope fractionation modelling for investigating fluid/rock interaction in metamorphic systems. An application to subduction zones shows that chemical and isotopic zoning in minerals can be used to determine feasible fluid sources and the conditions of interaction. Slab-derived fluids can cause oxygen isotope variations in the mantle wedge that may result in anomalous isotopic signatures of arc lavas.
This study presents an approach that combines equilibrium thermodynamic modelling with oxygen...
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