Volatiles and Fluids in Subduction Zones

Overview Subproject C1

Recycling of water and volatiles through subduction zones is one of the most fundamental processes affecting the chemical differentiation of the Earth. During the subduction process, metamorphic reactions can prevent deep subduction of volatile components and cause their transport in fluids towards the seafloor, the accretionary prism or the mantle wedge. Hence, the study of subduction zone metamorphism and exhumed fragments of subducted material can provide important constraints on balancing the amount of subducted volatiles and the amounts returned via arc magmatism. Moreover, hydrous fluids/melts represent the catalysts for arc magma generation. Fluids, melts and residual mineral reservoirs are strongly fractionated isotopically and chemically relative to initial compositions during subduction, but it is still debated from which particular source or process the typical arc geochemical signatures are derived.

By studying the mineralogical and geochemical characteristics of subduction-related rocks, associated veins and fluid inclusions, we are planning to obtain critical information on processes operating at depth in subduction zones. Well-established (trace elements, O-H-C isotopes) and relatively novel (halogens, Li-B-N isotopes) geochemical tracers will be used and these results will be integrated in a framework of P-T-X metamorphic conditions to allow us to deduce the origins of subduction zone fluids (metasediments, oceanic crust, serpentinized mantle) and the scale of fluid flow. Moreover, quantitative constraints on element transfer through subduction zones and on the slab contribution to the mantle wedge at subarc depths will be gained. The experimental (piston cylinder, multi-anvil press) determination of solid-fluid and solid-melt-fluid partition coefficients relevant to conditions in the slab beneath the arc and the overlying mantle wedge, respectively, will provide essential constraints to determine chemical characteristics of potential sources for arc magmatism and the geochemical potential of the wedge as source or sink for trace elements and volatiles.

Fig. 1: P-T grid with metamorphic facies showing P-T conditions of high pressure rocks exposed in the working areas and their relation to the different geothermal gradients realized in warm (13 °C/km) to cold (5 °C/km) subduction zones (modified after Peacock and Wang, 1999).