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Laboratory and Field Studies of the High P-T Behavior of Zircon with Application to Eclogite-Fluid Interactions and the composition of deep metamorphic fluids in Subduction Zones.
ABSTRACT: Subduction zones, or convergent plate boundaries, mark regions where mature oceanic lithosphere has cooled sufficiently to increase its density above that of the underlying peridotite (asthenospheric mantle), resulting in the oceanic lithosphere sinking deep into the underlying mantle. During the evolution of the oceanic crust, hydrous minerals (i.e. serpentinite) form through sea-water interacting with the basaltic ocean crust. With progressive subduction and increasing pressures and temperatures (metamorphic grade), metamorphic dehydration reactions cause the formation of anhydrous minerals and a separate fluid phase. Fluids generated by hydrous mineral break down play an important role in crust-mantle chemical transfer, the generation of magma at depth, and the geochemical evolution of the Earth. During generation and transport of fluids through the subducted crust, rock-fluid interaction and precipitation of residual minerals progressively change their chemistry. Eventually, these metamorphic fluids reach the oceanic crust-mantle interface and are expelled into the mantle leading to fluid-mantle (peridotite) reactions that (1) reduce the melting temperature for mantle peridotite, allowing magma generation, and (2) allow for transfer of elements from the crust into the mantle. Eclogite is a dense metamorphic rock produced when a mafic protolith experiences pressures exceeding 1.2 GPa (subduction to 45 km) and temperatures >400°C to 1000°C; conditions that fall within the eclogite-facies realm and generally termed High/Ultra-High pressure metamorphism. It is the rock type residual to the extraction of fluids produced during the amphibolite to eclogite metamorphic transition. Because eclogite is the rock type that fluids in subducted oceanic crust will interact with (beneath the volcanic front), surface exposures of exhumed eclogite (Ultra-High pressure metamorphic terrains) are used to study geochemical and metamorphic processes at the crust-mantle interface deep within the subduction zone (up to 120 km). Debate concerning the formation of eclogite within subduction zones centers upon the role of metamorphic fluids initiating/catalyzing eclogitization mineral reactions. While eclogite protoliths are generally anhydrous, the presence of high-grade hydrous-minerals (zoisite, phengite, paragonite, etc) in eclogites argues for an important role for fluids deep in the subduction zone. The importance of zircon in chronological studies is unparalleled by any other mineral found on Earth. Zircons in the Jack Hills, Australia, are the oldest pieces of the Earth known, and zircon grains are ubiquitous in a wide range of lithologies. During growth, zircon incorporates uranium and excludes lead, and its robust structure prevents diffusion of radiogenic Pb. These features make zircon an ideal mineral for U-Pb geochronology in metamorphic environments. Zircons are commonly extracted from eclogites and used to date peak metamorphic conditions; however, such a practice is controversial as we are currently limited in our understanding of zircon growth under high-grade metamorphic conditions. Such a knowledge gap adds great uncertainty to our understanding of deep metamorphic and tectonic processes in subduction zones. A thorough understanding of the chronological relationship between metamorphism and fluid infiltration is critical to deciphering the history of metamorphic terrains and mountain belts around the world. It is the intention of my PhD to identify the influence of metamorphic fluids upon the growth of zircons and thus bridge the gap between geochemical and geochronological interpretations of zircon ages in Ultra-High pressure metamorphic rocks (eclogites and eclogite-facies rocks). I also intend to greatly extend upon our broader understanding of the role of metamorphic fluids in deep subduction zones by identifying geochemical characteristics in zircons that can identify source compositions. My approach will focus on experimentally growing zircons in aqueous fluids at known pressures and temperatures to understand the links between chemistry and conditions of zircon growth. This information will then aid in geochronological interpretations of zircon dates extracted from eclogites collected from the Dabie-Shan metamorphic Ultra-High pressure metamorphic terrane in central China. |