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Abstract

Slab fluids, which are released by the subducting oceanic lithosphere through compaction and dehydration processes, migrate within the slab upwards to the slab-mantle wedge interface. With that they represent an essential mechanism for the transfer of volatiles from the slab to the mantle wedge. Consequently, hydrophile volatiles can be used as tracers to determine intra-slab fluid flow processes. However, migration processes of slab dehydration fluids, particularly with regards to transport mechanism of redox sensitive elements like sulfur, as well as the evolution of fluid sources during metamorphism of the slab, are not yet understood well.
In this study we investigate an eclogite-facies metabasalt from the HP/LT Akeyazi Metamorphic Complex of the South Tianshan Orogen, NW China. The sample comprises several omphacite-dominated HP veins that crosscut the blueschist-dominated matrix and formed under peak metamorphic conditions. Using mineral chemical analyses combined with in situ δ34S measurements of pyrite, as well as trace element and isotope analyses of C, Sr and Pb in mineral and vein separates, we track fluid sources and the transfer of the redox sensitive elements S and C and reconstruct the metamorphic evolution and the related intra-slab fluid flow of the studied sample.
All isotope systems provide evidence for multiple episodes of fluid-rock interaction with fluids derived from distinct sources. In particular, pyrite grains show patterns in in situ δ34S values which record changes in fluid chemistry that are supported by Sr isotope and trace element compositions. Large pyrite grains recording microbially derived S reveal low temperature seafloor alteration affecting the protolith pillow basalt prior to subduction. This is supported by dolomite δ13C values close to seawater-carbonate compositions. Based on chemical variations, the rock was infiltrated by a two stage intra-slab fluid-flow under peak to prograde metamorphic conditions during ongoing subduction, responsible for the formation of the omphacite-dominated HP-veins. MORB-like pyrite δ34S signatures of around -0.75‰ as well as Sr-mixing lines between AOC and slightly altered basalt indicate that the first HP fluid originated from dehydrating ocean floor basalts. In contrast, negative pyrite δ34S values of around -10.75‰ and Sr signatures correlating with the mixing of altered basalt with seawater454Ma and pelagic clays suggest that the second HP fluid originated from the basalt-seafloor sediment transition. Pathways formed by the first fluid were reused and enlarged by the second fluid, which however also
21st Swiss Geoscience Meeting, Mendrisio 2023
formed new pathways. This sample provides detailed insights into intra-slab fluid flow and fluid-rock interaction processes at HP/LT metamorphic conditions and allows a better understanding of fluid transfer and changes in fluid sources during subduction zone processes.