The reconstruction of thermal histories of rocks (thermochronometry) is a fundamental tool both in Earth science and in geological exploration. However, few methods are currently capable of resolving the lowtemperature thermal evolution of the upper 2km of the Earth's crust. Here we introduce a new thermochronometer based on the infrared stimulated luminescence (IRSL) from feldspar, and validate the extrapolation of its response to artificial radiation and heat in the laboratory to natural environmental conditions. Specifically, we present a new detailed Na-feldspar IRSL thermochronology from a well-documented thermallystable crustal environment at the German Continental Deep Drilling Program (KTB). There, the natural luminescence of Na-feldspar extracted from twelve borehole samples (0.1-2.3km depth, corresponding to 10-70 °C) can be either (i) predicted within uncertainties from the current g eothermal gradient, or (ii) inverted into a geothermal palaeogradient of 29 ±2 °Ckm−1, integrating natural thermal conditions over the last 65ka. The demonstrated ability to invert a depth-luminescence dataset into a meaningful geothermal palaeogradient opens new venues for reconstructing recent ambient temperatures of the shallow crust (<0.3Ma, 40-70 °C range), or for studying equally recent and rapid transient cooling in active orogens (<0.3Ma, >200 °CMa−1range). Although Nafeldspar IRSL is prone to field saturation in colder or slower environments, the method's primary relevance appears to be for borehole and tunnel studies, where it may offer remarkably recent (<0.3Ma) information on the thermal structure and history of hydrothermal fields, nuclear waste repositories and hydrocarbon reservoirs.

Optical dating has revolutionized our understanding of Global climate change, Earth surface processes, and human evolution and dispersal over the last ~500 ka. Optical dating is based on an anti-Stokes photon emission generated by electron-hole recombination within quartz or feldspar; it relies, by default, on destructive read-out of the stored chronometric information. We present here a fundamentally new method of optical read-out of the trapped electron population in feldspar. The new signal termed as Infra-Red Photo-Luminescence (IRPL) is a Stokes emission (~1.30 eV) derived from NIR excitation (~1.40 eV) on samples previously exposed to ionizing radiation. Low temperature (7–295 K) spectroscopic and time-resolved investigations suggest that IRPL is generated from excited-to-ground state relaxation within the principal (dosimetry) trap. Since IRPL can be induced even in traps remote from recombination centers, it is likely to contain a stable (non-fading), steady-state component. While IRPL is a powerful tool to understand details of the electron-trapping center, it provides a novel, alternative approach to trapped-charge dating based on direct, non-destructive probing of chronometric information. The possibility of repeated readout of IRPL from individual traps will open opportunities for dating at sub-micron spatial resolution, thus, marking a step change in the optical dating technology.

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Optically stimulated luminescence (OSL) and thermoluminescence (TL) from the metastable states in solids are widely used in luminescent phosphors, dosimetry, geochronology, and thermo- and photo-chronometry. OSL and TL result from a combination of three different processes (charge detrapping, transport, and recombination) and are, therefore, not ideal for characterizing the charge trapping states. Therefore, despite many decades of research, the OSL and TL kinetics and the associated defect systems remain poorly understood in natural minerals. Recently, a radio-photoluminescence (RPL) signal has been discovered in feldspar (K-Na-Ca aluminosilicates occupying > 50% of Earth’s crust) which helps overcome this limitation. This site-selective signal termed infrared photoluminescence (IRPL) arises from radiative relaxation of the excited state of the main electron trapping center (principal trap). In this study, IRPL excitation and emission spectroscopy at cryogenic temperatures reveals two distinct electron-trapping centers (i.e. two principal traps) in feldspar, and helps to determine their trap depths and the excited-state energies. The two trapping centers show the same electron capture cross-sections and the excited-state relaxation lifetimes, but different ground- and excited-state energies. Based on this peculiar combination of trap characteristics, we conclude that that the principal traps consist of the same defect residing at two different crystal sites. The differences in the energy levels of the two principal traps explain their distinct optical and thermal bleaching behavior.