Title

Physical (Elastic) vs. Chemical (Thermodynamic) Disparities in Reconstructing Pressure Temperature Paths for Alpine Metamorphic Rocks

Document Type

Student Presentation

Presentation Date

4-15-2019

College

College of Arts and Sciences

Department

Department of Geosciences

Faculty Sponsor

Dr. Matthew J. Kohn

Abstract

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.

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