Behavior of Monazite and Zircon During Amphibolite-Facies Fluid-Rock Interaction: Implications for U-Th-Pb Geochronological Interpretations

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EAR-0001084Ague, CarsonHow long does it take to build a mountain range? Geologists can now address this basic question because modern isotopic techniques allow the ages of individual mineral grains in rocks to be determined with unprecedented accuracy. As age determination methods improve, it has become apparent that minerals may preserve a complex record of multiple geologic events separated in time by millions or even billions of years. Fluids, including water-rich solutions and silicate melts, flow through Earth's crust during mountain building, carrying with them heat and chemical species such as greenhouse gases (e.g., CO2) and valuable ore metals. In this regard, a fundamental question actively debated today is: do these migrating fluids impact the U-Th-Pb isotopic systematics of commonly studied minerals like monazite and zircon, providing a record of the actual rates and timing of fluid flow through the Earth during mountain building? To address this question, our proposal seeks to develop new petrological and geochemical criteria to identify the effects of fluid infiltration on the morphology, chemistry, and isotopic systematics of monazite and zircon. The research centers on electron microprobe, rock chemical, and ion-microprobe study of well characterized, Early Paleozoic metamorphic fluid--rock interaction zones cutting the Archaean gneisses of the Napier Complex, East Antarctica. Ultimately, our findings are expected to determine the complete pressure-temperature-time history of the fluid infiltration, and have implications for tectonic and chronological models of the assembly of the Gondwana supercontinent.