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 interpret Earth’s processes. 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 of minerals, textures, and metamorphic facies. We then used Boise State’s Electron Probe Microanalyzer, with back-scattered electron imaging to verify mineral identifications and guided where to collect chemical analyses. Lastly, we used thermodynamic calculation software applied to these chemical data to constrain bounds on 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.