Fluid permeability and speciation in subduction zones

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Subduction-related H2O-rich fluids raise the degree of melting and cause metasomatism and possibly oxidation of the mantle source of island arc magmas. Serpentinites are important carriers of H2O and ferric iron into subduction zones. Although serpentine dehydration has been extensively studied, it is not known whether, and on what timescale the released fluids can migrate into the mantle wedge or whether oxidised species exist in these fluids that could explain arc mantle source oxidation. The mechanism of mantle fluid migration, and whether focused or pervasive flow occurs, is important as it may control the position of island arc volcanos and the extent to which H2O-soluble elements are removed from the slab. If the timescale for fluid transport is slow compared to the rate of subduction, fluids may be subducted into the deep mantle. In preliminary work we have developed a new experimental method for determining the flux of H2O through a mineral assemblage at the conditions of slab dehydration. A fluid source, e.g. serpentinite, is dehydrated to produce a pore fluid overpressure. The fluid migrates through an overlying core of hot-pressed olivine or pyroxene and is captured in an MgO sink on the other side of the core, as it forms brucite. From the volume of brucite formed the fluid flux can be determined. We also place FeS in the serpentinite fluid source and iron metal in the MgO sink to trace the passage of an oxidised fluid phase. The results show that H2O migrates predominantly by diffusion through an olivine assemblage at serpentinite dehydration conditions, rendering olivine rocks effectively impermeable. In this proposal we will examine the conditional limits of this behaviour and determine lattice and grain boundary diffusion coefficients for H2O in mantle assemblages with the new method. The experiments also show that porous flow can occur in orthopyroxene. We will explore the conditions and timescale through which orthopyroxene rich rocks can transport H2O and the effect of grain size and olivine content on this transition. Finally, we will further investigate our preliminary findings that a mobile sulphur bearing fluid is produced by serpentinite dehydration and determine its mobility and redox potential. Thus, we will build on very promising preliminary results to determine the time scale required for H2O-rich fluids to pass through initially dry overlying peridotitic and eclogitic rocks and evaluate whether and to what extent a mobile sulphur-bearing fluid can oxidise the mantle wedge.