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Lexi Thomas Thesis Defense

May 3 @ 9:00 am - 11:00 am MDT

Title: USING RAMAN SPECTROSCOPY TO VALIDATE METAMORPHIC P-T PATHS FROM WESTERN NEW HAMPSHIRE

Abstract: Constructing and interpreting pressure-temperature (P-T) paths in metamorphic rocks is crucial to understanding tectonic processes occurring at mid- to deep-crustal levels. Conventional methods for obtaining P-T paths rely on mineral chemistry, such as chemical zoning in garnets, which assumes growth at chemical equilibrium (Spear et al., 1984). However, studies show that garnet growth can occur with disequilibrium nucleation, which can result in a biased or potentially incorrect P-T path. An alternative, physics-based approach using Raman spectroscopy can also be used to determine P-T paths (Kohn, 2014). This method involves using elastic thermobarometry of quartz inclusions within garnet. In this study, we tested the hypothesis that P-T paths calculated using Raman microspectroscopy yield values for pressure that are comparable to those previously calculated from chemical zoning in garnets. This hypothesis was tested by calculating the entrapment pressure of quartz in garnets from the Hardscrabble Synclinorium, west-central New Hampshire, which can be used to interpret the nucleation pressure of garnet. Two samples were analyzed, K87-21C and D84-1C. We also recalculated these paths using thermodynamic inversion of chemical zoning in garnets. Both Raman results and newly calculated P-T paths we compared to previously calculated P-T paths done by Kohn et al., 1992. Raman spectroscopy isomekes for both samples show a scatter of pressures that do not allow for a robust comparison of Raman methods and thermodynamic modeling, suggesting Raman spectroscopy may not be applicable for calculating P-T paths with low- to moderate-temperature variances.

Advisor: Matt Kohn

Committee Members: Karen Viskupic, CJ Northrup