Evidence of dmisteinbergite (hexagonal form of CaAl2Si2O8) in pseudotachylyte: A tool to constrain the thermal history of 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, co-seismic 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, micro-Raman spectro-scopy, and EDS-equipped field emission scanning electron microscopy. In particular, we report the presence of the hexagonal polymorph of CaAl2Si2O8(dmisteinbergite) in a pseudotachylyte. Published experimental work shows dmisteinbergite can crystallize at 1200-1400 °C by rapid quenching. Therefore, the presence of dmisteinbergite in pseudotachylyte could be a reliable geothermometer for friction melts for which Tmelthas only as yet been estimated.