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Abstract

On Earth today, the recycling of surface water into the mantle occurs in subduction zones. This process is linked to element cycling, arc volcanism, and subduction-zone seismicity. Despite significant progress in understanding the evolution and systematics of subduction, various aspects are poorly understood. This thesis presents new insights into the links between metamorphic reactions, fluid release and transport, mass transfer, and seismicity at blueschist-facies conditions, and the first occurrence of the subduction-driven volatile cycle in deep time. Fluid-rock interaction at blueschist-facies conditions was investigated through geochemical and mineralogical analyses of the Bridge River Blueschist—a unit of subducted oceanic crustal rocks that preserve numerous lawsonite blueschist-facies fluid pathways. The research revealed that the formation of lawsonite blueschist along mm- to cm-scale bands was likely mediated by an external fluid. Mass-balance calculations indicate a metasomatic gain in Na (+24%), Mg (+33%), Si (+21%), and Li (+64%), and a loss of K (-84%), Al (-23%), Rb (-91%), U (-62%), and various HFSE including REE (-22% to -25%). Lithium chronometry modeling using the error-function solution to Fick’s second law indicates that Li isotope diffusion from a metasomatic band into surrounding rock occurred for 1.8 days with an upper limit of 3.7 days, suggesting that fluid fluxes can be extremely rapid and can keep pace with episodic tremor and slip under blueschist-facies conditions. The occurrence of the subduction-driven deep water cycle throughout Earth’s history was investigated using B isotopes, which can trace surface waters in igneous rocks, of a global sample set of Archean granitoids. These rocks show a diversification in B isotope compositions from the Neoarchean onward; c. 40% granitoids <3 Ga have B isotope compositions heavier than that of the mantle, whilst this is c. 15% for granitoids >3 Ga. This trend indicates an increase in surface-derived B in the granitoids’ sources, pointing to increased subduction-driven recycling with time. Altogether, this dissertation shows that lawsonite blueschist-facies metasomatism of oceanic crust can affect water and element cycling and rock structures, fluid flow at lawsonite blueschist-facies can be fast-pulsed, and that the Neoarchean may host the emergence of the subduction-driven water cycle.