Pressure-Temperature Record From the Eastern Alps, Austria, Reveals Dynamics of Plate Collision

Metamorphic rocks form and evolve in response to changes in Pressure (P) and Temperature (T). Application of thermodynamics to mineral compositions is commonly used to calculate P-T histories of metamorphic rocks. Geologists use this information to detail and interpret Earth's mountain building events. Here, we test the accuracy of the P-T paths for the eastern Alps constructed 35 years ago (Selverstone et al., 1984, Journal of Petrology, v25, 501-531) using improved thermodynamic calculations.

We first used optical petrography to identify minerals, textures, and metamorphic facies. We then used back-scattered electron imaging on Boise State's Electron Probe Microanalyzer (EPMA) to verify minerals and assess chemical zoning within minerals. Lastly, we used the EPMA to collect individual chemical analyses and applied thermodynamic software to constrain P-T conditions. Whereas Selverstone et al. (1984) report P-T conditions of 7 ± 1 kilobars (25 km depth) and 550 ± 25 degrees °C, our calculations show an indistinguishable pressure of 7 ± 1 kilobars, but a higher temperature of 635 ± 25°C. The higher temperature implies that tectonic plates were warmer than once inferred. Because rocks become less brittle with increasing temperature, brittle phenomena such as earthquakes in the past would have occurred at shallower depths.