Experimental study for noble gas release and exchange under high-speed frictional melting

Isotope ratios of noble gases in certain minerals are believed to be modified easily by their ambient reservoirs when the minerals reach temperatures that are higher than their closure temperatures. The thermal history of a rock or a geologic event, even a fault movement, can therefore be determined quantitatively by application of radiometric dating methods to different minerals. However, neither traditional K–Ar nor Ar–Ar dating methods are readily applicable to faulted rocks because, in the case of faulting, it is difficult to prove that the temperature of an event was greater than the closure temperatures of minerals. onal melting experiments performed on fine grain homogeneous gabbroid with high temperatures induced by frictional heating using a high-velocity friction apparatus were performed to test whether rapid fault movement can reset the noble gas isotope ratio of fault rocks. The temperature on the artificial fault plane is much higher than the closure temperature of the K–Ar system, as inferred from the calculated cooling age. The anticipated rapid equilibration of volatiles during the frictional melting of rocks implies that the noble gas isotope composition/ratio was reset to an atmospheric value during this experiment. We measured noble gas isotopes in a gabbroid sample using laser fusion analysis. The resetting of noble gases is only observed in glass that had melted completely. Rejuvenation and/or apparent increases in the K–Ar ages were only observed in a narrow, 3 mm-wide zone around the fault plane, which appears to be thermally altered and mechanically fractured in thin sections. Because of frictional heating, such glassy materials, called pseudotachylytes, sometimes form along the fault plane in natural faults as a result of complete melting of rock induced by frictional heating. Thus these results suggest that K–Ar ages of fault activity can be quantitatively obtained from the analysis of pseudotachylytes.