Shear heating and clumped isotope reordering in carbonate faults

Natural faults are expected to heat rapidly during seismic slip and to cool quite quickly after the slip event. Here we examine clumped isotope thermometry for its ability to identify such short duration elevated temperature events along frictionally heated carbonate faults. Our approach is based on measured δ₄₇ values that reflect the distribution of oxygen and carbon isotopes in the calcite lattice, measuring the abundance of ¹³C¹⁸O bonds, which is affected by temperature. We examine three types of calcite rock samples: (1) crushed limestone grains that were rapidly heated and then cooled in static laboratory experiments, simulating the temperature cycle experienced by fault rock during an earthquake slip; (2) limestone samples that were experimentally sheared to simulate earthquake slip events; and (3) samples from Fault Mirrors (FMs) collected from principle slip surfaces of three natural carbonate faults. Extensive FM surfaces are believed to form during earthquake slip. Our experimental results show that δ₄₇ values decrease rapidly (in the course of seconds) with increasing temperature and shear velocity. On the other hand, carbonate shear zones from natural faults do not show such δ₄₇ decrease. We suggest that the δ₄₇ response may be controlled by nano-size grains, the high abundance of defects,and highly stressed/strained grain boundaries within the carbonate fault zone that can reduce the activation energy for diffusion, and thus lead to an increased rate of isotopic disordering during shear experiments. In our laboratory experiments the high stress and strain on grain contacts and the presence of nanograins thus allows for rapid disordering so that a change in δ₄₇ occurs in a very short and relatively low intensity heating events. In natural faults it may also lead to isotopic ordering after the cessation of frictional heating thus erasing the high temperature signature of δ₄₇.