Reevaluation of metamorphic garnet-based P-T paths and their tectonic implications

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This project investigates the degree to which natural crystals grow in chemical equilibrium. This information is central to the study of metamorphic rocks, especially the pressures and temperatures at which they form. The research examines rocks from several different mountain belts that have been subjected to different pressures and temperatures through geologic time. The project supports the education and training of graduate and undergraduate students, including students from underserved groups. In addition, samples collected during the research are the focus of undergraduate in-class mineralogy and petrology exercises at Boise State University, which serves an unique student demographic (significant percentages of Hispanic, returning, and veteran students). The central focus of this project is to critically evaluate the hypothesis that metamorphic crystals grow substantially out of chemical equilibrium in a rock. A novel approach using inclusion elastic thermobarometry is applied to recover temperature and pressure independent of mineral chemistry. Three main research questions include: (1) Does inclusion elastic barometry and thermometry yield significantly different P-T paths compared to thermodynamically-based methods? (2) Does inclusion elastic barometry and thermometry imply static growth of garnet cores, perhaps even encompassing entire crystals in low-grade rocks? (3) Do differences either in matrix strain or in bulk compositions that shift equilibrium garnet growth to higher temperatures reduce the amount of overstepping, the size of garnet cores, and the bias to calculated P-T paths? Analyses include measurements of pressure and temperature for four different orogenic belts: the northern Appalachians, central Alps, southernmost Chile, and central/eastern Himalaya. Results broadly evaluate the frequency and magnitude of disequilibrium garnet growth, as well as the effects of temperature and strain on garnet nucleation. This research includes a PhD student and undergraduate students to make measurements, interpret data, present results at international meetings, and write up results for publication.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.