A pseudotachylyte geothermometer

Pseudotachylytes invariably contain a conspicuous concentration of clasts and lithic fragments in their matrices, which reflect brittle wear processes during frictional melting. The ratio of clasts to matrix is interpreted as the ratio of wear to melt (W/M) and measurements of this ratio show a range of 0.1–0.7. Based on models of melting and wear processes it is shown that the ratio W/M is independent of fault displacement, stress, fault area and mineralogy. Thermodynamic considerations indicate that W/M corresponds to the thermodynamic efficiency of the conversion of work to heat and is defined as w/q=(Thigh−Tlow)/Thigh where Thigh refers to the melt temperature and Tlow is the ambient country rock temperature. This provides a simple new technique for estimating the ambient crustal temperature Tcrust, in degrees KelvinTcrust=(1−W/M)Tmelt.Assuming a reasonable value for the temperature of the melt, estimates of country rock temperatures Tcrust for pseudotachylytes from four different localities in the USA indicate a range of 123–387°C. Eclogite-facies pseudotachylytes from western Norway yield a mean country rock temperature of 658°C, in good agreement with independent Fe–Mg exchange geothermometry. This new geothermometer appears to be applicable throughout the entire crust and may provide a better understanding of melting and wear processes during seismic faulting.