The KTB monazite fission-track profile: initial insights into ultra-low temperature thermochronology from a natural laboratory

Recent advancements in “ultra-low temperature” thermochronology have enabled exploration of the thermal evolution of the upper ∼2 km of the crust, with monazite fission-track (MFT) thermochronology emerging as a promising approach. However, the thermal sensitivity of MFT has mainly been verified through laboratory experiments, without considering geological timescales (10 6 –10 8 years). In this study, we investigated the thermal sensitivity of MFT over geological timescales by analyzing the crustal depth profile of the MFT length from a well-documented thermally stable crustal environment at the German Continental Deep Drilling Program (the KTB). The mean MFT length of the KTB core shows a gradual decrease from the shallow part (9.56 μm at 10 m depth) to the deep part (5.50 μm at 2725 m depth). This result suggests that the MFTs have been annealed depending on the holding temperature of each sample between 25 and 100 Myr. The MFT length profile observed in the KTB suggests that the partial annealing zone of the MFT system is approximately from ≤30 °C up to ∼80 °C over geological timescales. The results of this study support the suggestion of previous research that MFT can be employed as an ultra-low temperature thermochronometric method. On the other hand, the temperature range of the partial annealing zone is likely to be narrower than predicted from extrapolation of short-term annealing experiments using artificial semi-tracks by previous research. The existing annealing models should be reviewed using short-term annealing data using confined tracks and long-term natural annealing data, including the results of this study.