Hexagonal anorthite in pseudotachylyte: a new finding to constrain the thermal history during a seismic event

The determination of the maximum temperature achieved by friction melt (Tmelt) in pseudotachylyte-bearing faults is crucial to estimate earthquake source parameters (e.g., earthquake energy budgets, coseismic fault strength) on a geological basis. Here we investigated the mineralogy of a pseudotachylyte from the Gole Larghe Fault (Italian Alps) by using X-ray powder diffraction, microRaman spectroscopy, and EDS-equipped Field Emission Scanning Electron Microscopy. In particular, we report the presence of the hexagonal polymorph of anorthite CaAl2Si2O8 (dmisteinbergite) in a pseudotachylyte. Experimental work indicated that dmisteinbergite can crystallize at 1200-1400°C by a very rapid quench. Therefore the presence of dmisteinbergite in pseudotachylyte could be a reliable geothermometer for friction melts for which, so far, only approximate estimations Tmelt have been made.