Physical (Elastic) vs. Chemical (Thermodynamic) Disparities in Reconstructing Pressure Temperature Paths for Alpine Metamorphic Rocks
College of Arts and Sciences
Department of Geosciences
Dr. Matthew J. Kohn
Chemical zoning in metamorphic garnet is widely used to calculate the pressure-temperature (P-T) history that a rock experiences during formation. An alternative approach is to invert Raman microspectrometric measurements on mineral inclusions to infer P-T conditions during garnet growth. This project compares Raman- based vs. chemically-based P-T paths by (a) making Raman measurements on mineral inclusions in garnets from the Tauern Window, Austria, where a P-T path had been calculated previously using chemistry, (b) inverting Raman measurements to recover inclusion entrapment pressures, and (c) comparing Raman- based and chemically- based P-T paths. The Raman- based P-T path is significantly different from the chemically- based paths. Chemical data suggest Alpine garnets initially formed at high pressure (10 kbar) and grew as pressure decreased to 7 kbar. Raman data instead suggest that garnet grew isobarically at 7-8 kbar. Possibly nucleation kinetics was sufficiently slow that the garnet forming reaction was overstepped and garnet nucleated and grew at nearly constant P-T conditions. Further comparison of Raman- vs. chemically-based approaches may reveal whether these differences are widespread or restricted to certain rock types or metamorphic settings.
Howell, Allie, "Physical (Elastic) vs. Chemical (Thermodynamic) Disparities in Reconstructing Pressure Temperature Paths for Alpine Metamorphic Rocks" (2019). 2019 Undergraduate Research and Scholarship Conference. 71.