Pervasive fluid-rock interaction in subducted oceanic crust revealed by oxygen isotope zoning in garnet

Thomas Bovay,Pierre Lanari,Daniela Rubatto

doi:10.1007/s00410-021-01806-4

Thomas Bovay, Pierre Lanari + Show 1 more

Open Access

https://doi.org/10.1007/s00410-021-01806-4

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Abstract

Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8‰ in δ18O. Thermodynamic and δ18O models show that the large amount of low δ18O H2O required to change the reactive bulk δ18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 × 105 cm3/cm2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 × 10–20 m2) is comparable to the permeability determined experimentally for blueschist and serpentinites.

Highlights

Increase of pressure and temperature during prograde metamorphism along a subducting slab leads to a series of breakdown reactions involving hydrous phases (e.g. Schmidt and Poli 2003) and is associated with major variation in rock density, rock volume and rock porosity
This study identifies the large amounts of low δ18O fluid generated from antigorite + brucite breakdown reaction in the Zermatt-Saas Zone (ZSZ) serpentinites as the major origin for the external fluid that interacted pervasively with the schists of Theodul Glacier Unit (TGU) during exhumation between 26.5 and 15.5 kbar in the subduction channel
The low bulk δ18O signature of all garnet generations, including those grown during prograde metamorphism, suggests a pre-metamorphic lowering of the bulk rock δ18O, probably through seafloor alteration

Summary

Introduction

Increase of pressure and temperature during prograde metamorphism along a subducting slab leads to a series of breakdown reactions involving hydrous phases (e.g. Schmidt and Poli 2003) and is associated with major variation in rock density, rock volume and rock porosity. Schmidt and Poli 2003) and is associated with major variation in rock density, rock volume and rock porosity. A better understanding of the dynamics of fluid-rock interaction and the associated permeability is crucial to quantify fluid-driven mass transfer processes in subduction zones (Ingebritsen and Manning 1999, 2003, 2010; Manning and Ingebritsen 1999; Konrad-Schmolke et al 2011; Ganzhorn et al 2019)

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