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Constraining the tectonothermal evolution of subducted oceanic crust using thermodynamic phase equilibrium modeling
Hernández Uribe, David
Hernández Uribe, David
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2020
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2021-06-04
Abstract
Subduction is one of the most important tectonic processes operating on Earth today as it controls major geological processes at convergent plate margins, such as arc magmatism, seismicity, and crustal growth, as well as long-term global geochemical recycling. As such, determining the mineralogical and petrophysical changes that occur during subduction of oceanic crust is key to understand the spatiotemporal evolution and relation between these processes. Previous studies have explored phase changes in mid-ocean ridge basalt (MORB) during subduction; however, most constraints have been derived from experimental petrology, which cannot effectively assess complex natural variables, and the effects of different types of hydrothermal alteration prior subduction on the phase equilibria of subducted MORB have been rarely assessed. Furthermore, recent studies have questioned whether predictions made by numerical simulations of the thermal evolution of active subduction zones are representative of the petrological evolution experienced by subduction-related metamorphic rocks; if not, the previously accepted models of metamorphic phase changes and volatile transport during subduction require reassessment. In this thesis, I address such problems in three separate projects that focus on quantifying the petrological evolution of MORB via thermodynamic phase equilibrium modeling. While experimental petrology provides key insights into the phase equilibria of MORB compositions at different depths, phase equilibrium modeling has risen as a powerful tool to shed light on the evolution of the subducted oceanic crust, as it can put quantitative constraints on the equilibria for any bulk-rock compositions at almost any pressure and temperature (P–T) conditions with relative ease. Particularly, this thesis shows how phase assemblages, the P–T conditions of key devolatilization reactions, and the effect of densification vary for typical MORB along subduction zone geotherms for cold, warm, and average environments. Further, I model and evaluate the effects on the phase equilibria at fore-, sub-, and post-arc depths of different types of hydrothermal alterations affecting the oceanic crust prior to subduction. Finally, I show, using samples of eclogite collected from the Colorado Plateau, how petrological changes that occur on the slab-top during the subduction of oceanic lithosphere can influence geodynamic processes in the overlying plate.
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