ESR-thermochronometry of the MIZ1 borehole, Tono, Japan

東濃地域のMIZ1ボアホールのESR熱年代

King, G. E.*; Bossin, L.*; Kranz-Bartz, M.*; Wen, X.*; Schmidt, C.*; Herman, F.*; 小形 学 ; 末岡 茂   

King, G. E.*; Bossin, L.*; Kranz-Bartz, M.*; Wen, X.*; Schmidt, C.*; Herman, F.*; Ogata, Manabu; Sueoka, Shigeru

Electron spin resonance (ESR) dating of quartz minerals offers a significant advantage over luminescence dating because of its later signal saturation. Whereas the luminescence thermochronometry system is limited to areas experiencing very rapid rock cooling (exhumation) of tens of mm/yr, recent studies have shown that ESR thermochronometry can resolve rates of <1 mm/yr over Quaternary timescales. However, the method has not yet been validated against samples with known thermal histories. To this end, we have investigated six known-thermal history samples from the MIZ1 borehole, Tono, Japan. The natural trapped-charge concentration of the different samples was constrained using a single-aliquot regenerative dose measurement protocol. As the samples had similar properties, we constructed a standardised growth curve to alleviate measurement times. Signal saturation of the Al-centre occurred at ~60 kGy and at ~7 kGy for the Ti-centre. Whereas the Al-centre exhibited single-saturating exponential growth, the Ti-centre exhibited significant sub-linearity in the low dose region, within which the natural trapped-charge concentrations were interpolated. The thermal stability of the different samples was measured using an isothermal holding experiment, whereby samples were dosed in the laboratory before being held at four fixed temperatures for durations ranging from 0 min to 10 h. As the thermal signal loss of the different samples was similar, we were able to fit all samples to derive a single set of thermal kinetic parameters. Finally, the data were inverted for borehole temperature using a Monte-Carlo approach. Whereas the Al-centre of all samples recovered borehole temperature within 1 sigma, the Ti-centre data failed to recover temperature, yielding temperatures ~20-30 deg. C above borehole temperature. The cause is likely related to the observed sub-linearity of the dose response curves which may be indicative of sensitivity change throughout analysis.