Calibrating thermodynamically valid REE thermobarometers

Rare-earth elements (REE) are widely used to investigate geologic processes. Trace amounts of REE are in minerals like garnet, pyroxene, and hornblende. The primary goal of this project is to develop new calculations that will help scientists determine mineral temperatures and pressures. The new calculations will use data from experiments and theoretical models. Students will use samples and results of this research in course-based undergraduate research experiences. New, publicly available videos will help explain the science used to create the new models. The principal investigator will share lectures with the local community. Rare-earth element (REE) thermobarometry has been applied for more than a decade, but some previous calibrations are not thermodynamically valid and cannot be applied outside the compositional range of the calibrations. Critical assessment of experiments and natural data commonly show discrepancies with equilibration conditions up to hundreds of °C and 2 GPa. This project will develop a thermodynamically-sound theoretical basis for REE thermobarometry, principally involving garnet and clinopyroxene for applications to ultramafic rocks, calc-silicates, and eclogites, but also extending to orthopyroxene and hornblende. This work will critically evaluate competing activity models for trace element mixing in minerals, considering both lattice strain and likely REE substitution mechanisms in each mineral. This work will also require extensive analysis of all minerals in natural, well-equilibrated samples via EPMA and LA-ICP-MS, providing a broad database of mineral compositions of service to other geochemical interests in the Earth Sciences. YouTube videos on thermodynamics will also be developed, both for endmember mineral equilibria and for modeling activities of trace components in minerals in the context of crystal chemically-valid substitution mechanisms. Research samples will be used in Course-based Undergraduate Research Experiences, synergizing undergraduate data collection and interpretation with development of a geochemical database of natural mineral compositions. Educational videos on thermodynamics and outreach lectures on rocks and minerals will be provided to continuing education programs. Overall, this work will establish a new approach to model REE partitioning among minerals, which may inform REE exploration from unconventional sources, such as garnet and other silicates. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.