ESR-thermochronometry of the MIZ1 borehole, Tono, Japan

Abstract. The electron spin resonance (ESR) of quartz can be used as a low temperature thermochronometric system, however, to date no field validation of the approach has been made. Here we explore the ESR signals of quartz from six samples from the MIZ1 borehole (Tono, Japan). Previous studies have shown that this low-relief region underwent Quaternary exhumation at rates of <0.2 mm yr^-1. We investigate whether quartz ESR signals can resolve such low rates of exhumation, or whether the samples are in thermal equilibrium with ambient borehole temperature. ESR thermochronometry requires that both sample-specific signal saturation and thermal decay are constrained in the laboratory, which makes measurements highly time-consuming. To overcome this, the development of a standardised growth curve (SGC) was explored, which allowed more rapid constraint of the trapped-charge concentrations of each of the samples. Thermal kinetic parameters were determined using an isothermal decay experiment for each individual sample and except for sample MIZ1-08, it was possible to fit all the isothermal decay data together to yield a single set of kinetic parameters that successfully described the dataset. The ESR thermochronometry results show that the MIZ1 samples are in thermal equilibrium for the Al-centre, and that Monte-Carlo inversion of the ESR data yields present-day borehole temperature within 1σ uncertainties for all samples except the lowest temperature sample. In contrast, inversion of the different Ti-centre options (A, B, D) yields temperatures 15–20 °C above contemporary borehole temperature, indicating rock cooling equivalent to a total exhumation of ~1 km over the same period. The cause of this discrepancy is unclear but may relate to sub-linearity of Ti-centre dose response, that led to underestimation of the trapped-charge concentration and hence an overestimation of borehole temperature. Our results validate ESR-thermochronometry of the Al-centre and show that an SGC and common thermal kinetic parameters may be used to expediate sample measurements, however ESR-thermochronometric data from the Ti-centre should be used cautiously until further validation data are available.