Reconstructing the thermal structure of shallow crust in the Tono region using multi-OSL-thermometry of K-feldspar from deep borehole core

Abstract

OSL-thermometry can be used to reconstruct the thermal structure in slowly denuded regions where infrared stimulated luminescence (IRSL) signals of samples obtained from deep boreholes are measured and evaluated with depth (e.g., 1.1–2.3 km depth, corresponding to ∼40–75°C; Guralnik et al., 2015a). Until now, only one study has explored this approach, using a target mineral of Na-feldspar. Therefore, in this study, we applied multi-OSL-thermometry to K-feldspar obtained from deep borehole core samples drilled at the Tono region, central Japan, which is a well-documented thermally stable crustal environment. The IRSL signals at 50, 100, 150, and 225°C (IR50, IR100, IR150, and IR225, respectively) were measured. When a combination of four isothermal holding temperatures (ITL) between 190 and 250°C was used for thermal kinetic parameter derivation, the inverted temperatures for the IR50 signals of the samples at a depth of ∼1 km (∼40°C) were most consistent with the in-situ temperature. However, the inverted temperatures for IR100, IR150, and IR225 signals were inconsistent with the in-situ temperatures even though the apparent geothermal gradients estimated from these IR signals are close to the modern geothermal gradient. These results suggest that the application of OSL-thermometry to K-feldspar in a borehole is useful to reconstruct the paleothermal conditions when adopting appropriate thermal kinetic parameters.