Controls of focused fluid release in subduction zones: insights from experimental dehydration of brucite vein networks in serpentinite

Abstract

Aqueous fluids released by metamorphic dehydration of serpentinites are a key component for seismicity, creep, and geochemical cycling in subduction zones. How these fluids drain and migrate towards the mantle wedge has yet to be fully understood. Here we address the influence of pre-existing structural and mineralogical heterogeneities in serpentinites on dehydration and fluid migration at forearc conditions. We partially dehydrated natural serpentinite containing brucite veins in a piston-cylinder apparatus with a temperature gradient across the conditions of the brucite + antigorite = olivine + fluid reaction (485–520 °C; 1.5 GPa). Micro-tomography, electron microscopy and microstructural analysis of the experimental results, coupled with thermodynamic modelling, show that temperature, mineralogical heterogeneity and variable ingress of external H2 controlled the dehydration extent. Experimentally formed olivine indicates a topotactic relationship between [100]Ol and [0001]Brc, although the resultant fabric is overall random because brucite was randomly oriented. Olivine forms mono-mineralic aggregates along the walls of brucite veins, displaying very high porosity (up to 32%) and permeability (10–13–10–14 m2). Tracing the pre-existing brucite vein network, these aggregates can form a transient network of interconnected, highly permeable fluid channels that allows drainage and may enhance open-system exchange with neighboring lithologies. Infiltration of reduced external fluids can trigger redox dehydration of magnetite + antigorite to Fe-rich olivine, which renews porosity and propagates focused fluid flow. The distribution of brucite and magnetite, especially as vein networks, therefore has a first-order control on how focused fluid drainage and flow paths develop during subduction of serpentinites.