The mineral zircon (ZrSiO4) is commonly used to date geological processes, and baddeleyite (ZrO2) is becoming an increasingly important geochronological tool. Although Zr is considered to be immobile and zircon extremely stable and durable, many recent field studies have identified zircon that was hydrothermally altered or precipitated from hydrothermal fluids. Double-capsule cold seal pressure vessel experiments at 2 kbar will be conducted to 1) identify the conditions under which the zirconium-minerals zircon and baddeleyite are soluble in aqueous fluids, and 2) characterize the partitioning of trace elements including Th and U between zircon, aqueous fluids and granitic melts. Zircon-fluid solubility and partitioning experiments will be conducted at 450-750C and variable fluid pH and oxygen fugacity (fO2), while a parallel set of zircon-melt partitioning experiments will be conducted at 750C and variable fO2. Identification of unique trace element signatures of magmatic and hydrothermal zircons will allow for accurate interpretation of zircon ages. For example, trace element concentrations can be used to identify a zircon as hydrothermal, and solubility systematics can be used to constrain the conditions under which it formed, so that the measured age can be interpreted to date the time of fluid infiltration under specific conditions (temperature, fluid pH, fO2).
In addition to aiding interpretation of hydrothermal zircons and their measured ages, the solubility systematics can be used to assess the viability of zircon and baddeleyite for storage of plutonium. This project will fund the thesis research of one current M.S. student, 2-3 future M.S. or Ph.D. students, and 2-3 undergraduate students. Equipment purchased will be used by a large number of investigators in this and other research projects. The grant will also aid in the development of a new interdisciplinary Environmental Science Ph.D. program in collaboration with the Department of Civil and Environmental Engineering at Vanderbilt.
