Experimental investigation into the partitioning behavior of hydrothermal Zircon at 1.5 to 3.5 GPa: Tracing metamorphic fluids and crustal evolution processes during High/Ultra-High Pressure Metamorphism

Abstract:

Zircon (ZrSiO4) is the most important mineral available to geoscientists to study the geochemical and geodynamic evolution of the Earth. Its geochemical characterization for silicate melt systems has led to numerous important discoveries concerning the construction and evolution of the continental crust and the environmental conditions of the earliest terrestrial Earth. Recent developments in micron-scale in-situ zircon analysis have seen an exponential growth in its use for understanding geodynamic processes during High and Ultra-high pressure (UHP) metamorphism. Geochemical and textural evidence suggests that zircons in UHP metamorphic terrains grew at sub-solidus conditions in the presence of a hydrous metamorphic fluid. However, to date no experimental studies have been performed to characterize the geochemical behavior of zircon in hydrous metamorphic fluids, limiting its use in metamorphic and hydrothermal environments.

Here I propose to characterize the trace-element partitioning behavior of zircon in hydrothermal fluids at High/Ultra-High Pressure (1.5 to 3.0 GPa) conditions and identify chemical characteristics indicative of growth from a hydrothermal fluid. Trace element concentrations in zircons hosted in high-grade metamorphic lithologies will be combined with measured trace element partition coefficients to reconstruct the source fluid composition from which the zircon grains precipitated. Trace-element compositions of fluids are inherited from the host composition and mineralogy, which are a function of local physical and chemical conditions. This will allow us to link the physical conditions responsible for zircon growth and fluid compositions to zircon U-Pb ages, allowing for refinement of mountain building models and HP/UHP terrane formation and a greater understanding of the production of metamorphic fluids during HP/UHP metamorphism.

Goals:

Resolving the chronological relationship between High/Ultra-High Pressure (HP/UHP) metamorphism and the presence and compositions of hydrous fluids is critical to deciphering the history of metamorphic terrains and mountain belts around the world. It is my intention to (1) experimentally characterize the partitioning of trace-elements between zircon and fluid at HP/UHP conditions, (2) identify chemical characteristics to distinguish zircon grains grown from hydrothermal fluids and those grown from magmatic silicate melts, and (3) identify the influence of metamorphic fluids upon the growth of natural zircon grains from HP/UHP rocks, bridging the gap between geochemical and geochronological interpretations of zircon ages in Ultra-High pressure metamorphic rocks (eclogites and eclogite-facies rocks). The importance of this research will be in extending our understanding of the chronology of high-grade metamorphic terrain formation, the role of metamorphic fluids in deep tectonic environments and as chemical transfer agents in subduction zones, and to increase our understanding of continent scale geodynamic and geochemical processes.