Collaborative Research: Equilibrium and Kinetic Studies of New Trace Element Thermobarometers

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This project is a collaborative effort between Rensselaer Polytechnic Institute (RPI) and Syracuse University (SU) to develop tools and strategies to "reverse engineer" key minerals of Earth's continental crust - that is, to learn how and when these materials formed and the conditions experienced since their origin. Consider, for example, the benefits of being able to "read" the history of a single grain of sand simply by measuring its content of key chemical elements: Was it formed during an episode of mountain building or during a volcanic eruption? How far did it travel from its source? Was it associated with a potentially valuable ore deposit at the time of formation? This project will involve laboratory synthesis of selected minerals over a range of temperature and pressure conditions, followed by measurement of the amounts of elemental impurities incorporated during growth (the concentrations of aluminum and titanium in quartz, for example, depend strongly on the temperature and pressure of quartz formation). The broader purpose of the project is to develop "chemical tools" for all researchers to use in deciphering events and processes of our planet's past - from mountain building to formation of ore deposits.A decade of experimental research at RPI has focused on evaluating the effects of pressure (P) and temperature (T) on the solubilities of low-abundance elements in key minerals of the continental crust. This enterprise has been called "trace-element thermobarometry" because each application is based on the concentration of a single, marginally compatible element in a common or otherwise strategic mineral phase (e.g., Ti in zircon). These thermobarometers differ in fundamental ways from "conventional" thermobarometers based on major-element phase equilibria, and they have a key advantage: if the system is properly constrained, the concentration of a single impurity in a single mineral can be used as an indicator of its crystallization T and/or P. To date, this effort has produced thermo(baro)meters based on the Ti content of zircon, the Ti content of quartz ("TitaniQ"), the Zr content of rutile, and the Zr content of titanite. The value of these thermobarometers has been enhanced significantly through experimental calibration of the diffusion laws for all relevant impurities in the phases of interest, so users can assess the robustness of the thermobarometers for specific real-world applications. Efforts to date have had significant impact in the geoscience community (as judged by literature citations), but the development of trace-element thermobarometers and the improvement of "old" ones is far from complete. The proposed work is aimed at providing a full toolbox of thermobarometers for crustal systems that includes cross-checks of the various P-T indicators. Specifically, the TitaniQ calibration will be extended to lower P for application to volcanic rocks, and Ti-in-zircon will be more thoroughly assessed for P effects. Entirely new systems and applications will also be pursued, including the development of P-T indicators based on Ti in coesite for ultra high-pressure (UHP) rocks, Al in quartz to complement TitaniQ, Si and Al in rutile, and Ti in both K-spar and in kyanite. Equilibrium studies of all new systems will be complemented by diffusion measurements of the relevant elements. Further, the pressure-volume-temperature properties of fluid inclusions in crystals from experimental run products will be used in conjunction with trace-element thermobarometers to confirm accuracy of our calibrations at relatively low P-T applications. The proposed study involves implementation of techniques specifically in experimental geochemistry, but the applications of our results extend across a substantial expanse of geoscience, including not only igneous and metamorphic petrology but also ore-deposits research, structural geology, tectonics, and sedimentology.