Use and Abuse of Zircon Thermometry - Integrating Modeling, Trace Element Chemistry and Isotopes to Maximize the Use, Limit the Abuse

Overview: Our main objective is to compare magma temperatures obtained from thermodynamic modeling, zircon Ti content, and zircon-quartz inclusion oxygen isotope thermometry to determine optimal conditions for applying zircon thermometry of mid-crustal magmas. Methods to be employed include determining whole-rock chemistry, modeling crystallization history using rhyolite-MELTS, determining zircon Ti concentration, and measuring the oxygen isotope composition of zircon crystals and their quartz inclusions. IM: Determination of accurate temperatures for felsic magmas is a challenging goal in a wide range of studies of crustal petrogenesis. Considerable attention has been paid to using Zr content of melts and Ti content of zircon for constraining magmatic temperatures. However, recent work has shown that in many cases zircon does not begin crystallizing near the liquidus and usually crystallizes over a range of hundreds of degrees. In a pilot study utilizing crystallization modeling and Ti-in-zircon measurements, we showed that crystallization of zircon in a relatively dry, very high Zr (1200 ppm) granitic magma does begin near the liquidus. However, in our simulation of a granite with lower Zr content (320 ppm) zircon crystallization didn't begin until 150 degrees below the liquidus, and modeling by others of a low Zr (150 ppm) granitoid showed zircon reaching saturation closer to 275 degrees below liquidus conditions. These outcomes suggest that neither zircon ages nor trace element compositions in plutonic systems reflect an instant in time and seldom reflect earliest initial thermal conditions of the melt. We propose to significantly build on our initial results where we modeled changes in magmatic temperature and crystallization sequence in four granitoids of varying Zr content. Although extremely well calibrated in the laboratory, the Ti-in-zircon thermometer is limited by geologic factors. For example, most granites do not contain rutile, and although most contain quartz, modeling reveals that quartz saturation (aSiO2 in the melt = 1) may not be achieved until well after zircon begins crystallizing. Our preliminary research revealed several complications in applying zircon thermometry in magmatic systems. These geological limitations result from the complexity in real systems when examining crystallization from liquidus to solidus. The results demonstrate that significant caution must be used in interpreting the meaning of Ti-in-zircon measurements and that thorough field testing of the thermometer is needed. We thus plan to focus on an expanded suite of granites including a suite with moderate Zr content (80-300 ppm) and a suite with high to ultra high Zr (400-1800 ppm). We will use an integrated approach that combines computer modeling (using the rhyolite-MELTS program), and Ti and trace element chemistry of zircon (using ion microprobe methods). Further, we will use a novel approach to oxygen isotope thermometry (using zircon + quartz inclusions as mineral pairs) to compare to results obtained via the modeled and Ti-based approaches. With an integrated approach we will gain considerable insight into the use of zircon in petrogenetic studies, resulting in a much more robust understanding of zircon thermometry. BI: The PIs will continue their practice of having students work at both institutions and be directly trained by the PIs. PI Samson has a new graduate student from Puerto Rico whose thesis will be based on the project, thus we will be training an underrepresented woman scientist. He also has an undergraduate who is already working with the PIs - his research will comprise his senior thesis. PI Moecher has a new female MS student whose work on the project will be the basis of her thesis. Erica Emerson, a PhD student in Science Teaching at SU, will work with the PIs to develop teaching modules designed to aid eighth-grade teachers who teach the fast-paced accelerated Earth science course in New York. Modules will focus on concepts of magmatic evolution, granitic magma characteristics, and chronology. A one day teacher workshop will be held at Syracuse University to discuss the teaching modules, have the PIs and students present key findings, offer lab tours, and provide teachers with hand specimens of the studied granites.