Timing and rates of fluid release during the dehydration of subconducting oceanic crust: reactive fluid flow und high-pressure conditions

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Subduction zones belong to Earth’s most dynamic systems and play an important part in the global volatile cycle and the communication among Earth’s geochemical reservoirs. As subducting plates heat up, their hydrous minerals become progressively unstable, break down and release water. The most prominent dehydration reactions occurring in subduction zones lead to the eclogitisation of the subducting oceanic crust. Field evidence shows that the preferential flow field of released slab fluids is highly channelized and that these fluids tend to react with their wall rocks, thereby serving as agents for the mobilisation and transport of trace elements. It is now evident that the released fluids can reach the mantle wedge and cause melting. The chemical freight of these fluxing fluids is thought to create the distinct chemical signature of the resulting arc magmas. The key towards a better understanding of the subduction zone fluid cycle and its chemical consequences is to obtain in-depth understanding of how fluids flow within and out of the descending slab. The focus will be set here on fluid flow structures found in exhumed high-pressure rocks, from the Chinese Tianshan and New Caledonia, and their evolution in time and space. The project aims to document constraints on the formation of flow structures, the associated fluxes and the duration of distinct intraslab fluid flow events. The goal is 1) to examine how the fluid release of dehydrating slabs occurs, 2) to better understand the evolution of fluid flow and veinforming processes in space and time, and 3) to quantify fluxes and durations of distinct fluid flow events. A better knowledge of these aspects is a pre-requisite to deduce realistic numbers for rates, synmetamorphic porosity and permeability during fluid release.