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

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. However, 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. This method involves using elastic thermobarometry of 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 (QuiG) from the Hardscrabble Synclinorium, west-central New Hampshire. Inclusions in garnet cores yield entrapment pressures of 4-5 kbar at an assumed temperature of ~475 °C; inclusions towards the rims yield higher pressures of 5-6 kbar. Core pressures are higher than anticipated from prior thermodynamic modeling (c. 3 kbar), whereas the change in pressure during garnet growth (~1 kbar) is smaller than previously calculated (~2 kbar). Differences in absolute pressure between the two methods could represent calibration errors. The difference in the change in pressure during garnet growth (1 vs. 2 kbar) could reflect either inability of the QuiG to recover rim pressures, or inaccurate correlation of garnet and plagioclase compositions in thermodynamic models. Regardless, both models suggest garnet grew over a range of pressures (and temperatures), and does not solely represent growth after overstepping, which would produce static garnet growth at constant P-T conditions. Further work is needed to more comprehensively compare core vs. rim QuiG values and model zoning in specific garnets thermodynamically.