Matthijs Smit

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.

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The formation, evolution, and long-term stability of the cratons are central to understanding how the first continents formed and how the Earth operated before the onset of modern plate tectonics. The cratonic mantle keel – the subcontinental lithospheric mantle – plays a critical role in stabilizing the continental lithosphere. However, the timing and nature of processes that shaped the SCLM remain poorly constrained due to the limited availability of geochronological tools applicable to peridotitic mantle rocks. This dissertation addresses this gap by refining and applying high-precision Lu-Hf geochronology to garnet-bearing mantle xenoliths, and by integrating Hf isotope data from tonalite-trondhjemite-granodiorite (TTG) rocks, the earliest products of continental crust formation. This work presents the first large-scale systematic application of low-dispersion internal Lu-Hf isochron dating to mantle xenoliths from the Slave, Kaapvaal, and Siberian cratons. The resulting ages spans over three billion years and demonstrate that preservation of ancient Lu-Hf ages depends on lithology, texture, and metasomatic history. A case study of a recrystallized orthopyroxenite-harzburgite xenolith reveals successive Archean metasomatic events recorded at the grain scale. Complementary TTG zircon data show globally consistent, mildly suprachondritic initial Hf isotope values, suggesting derivation from a long-lived, mildly depleted mantle reservoir. This convergence of mantle and crustal Hf signatures supports the existence of a chemically stable Archean mantle source. Beyond geodynamic implications, this dissertation introduces new methods for handling low-concentration samples and analyzing small-volume garnet separates, improving the feasibility of dating even the most refractory mantle materials. The results support a model of episodic SCLM modification, involving long-lived carbonatitic metasomatism followed by short-lived, localized hydrous overprints prior to kimberlite magmatism. These findings provide new constraints on the geodynamic processes that shaped early Earth and contribute to our broader understanding of mantle reservoirs and crust formation in the Archean.

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The formation of cratonic lithosphere and its participation in continental collision are first-order processes in global tectonics. The Western Gneiss Complex (WGC) of southwestern Norway is a fragment of continental crust that uniquely preserves a complete record of its burial and exhumation during collisional orogeny along with rare fragments of sub-continental lithospheric mantle that were entrained into the terrane during its residence in the mantle. Despite the importance of the WGC for characterising processes operating in the deep crust and mantle during continent-continent collision, its rate and style of burial and exhumation have not been comprehensively studied and the protracted evolution of the included peridotite bodies remains unclear. Lu-Hf garnet and micro-analytical U-Pb rutile geochronology are two powerful tools for lithosphere and tectonics research as they can be used to link ages to conditions of equilibration of rock-forming assemblages. Using these techniques applied to eclogites in the WGC, I constrained the burial rate for continental crust during collisional orogeny to ~5 mm yr-¹, developed a quantitative framework for evaluating geodynamic changes during continental collision, and proposed that deeply buried continental crust is exhumed largely as a flat-slab in the mid-crust, possibly due to erosion of a paleo-plateau in the upper plate. Using Lu-Hf garnet geochronology applied to ultrahigh-pressure (UHP) enstatite-bearing eclogites in the WGC, I provided well-constrained empirical evidence for non-lithostatic eclogitisation, a process that explains the localised occurrence of anomalously-high pressures conditions in deeply buried continental crust. When these research outcomes are compared to the lower plate in the India-Asia collision zone, they demonstrate consistency in the rate and depth of burial and the style of exhumation of continental crust during collisional orogeny. Using Lu-Hf garnet geochronology applied to included peridotite bodies in the WGC, I provided the first well-constrained geochronological evidence for the stabilisation of a buoyant cratonic sub-continental lithospheric mantle in the Archean that melted and recrystallised in concert with major supercontinent break-up intervals. The techniques used herein could be applied to other collisional settings and to other mantle peridotite suites to better constrain the emergence and evolution of global plate tectonics cycles.

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