Infra-red stimulated luminescence on K-feldspar: evaluation of a new trapping-detrapping model and perspectives on the late-stage cooling history of the Mont-Blanc massif

Constraining the topographic impact of Quaternary glaciation in the European Alps is important to better assess the control of climate on mountain erosion rates over 10⁴-10⁶ yr timescales. Infra-red stimulated luminescence (IRSL) in feldspar is a dating technique that allows quantification of trapped electrons and the potential reconstruction of rock thermal histories over a timescale of 10⁴-10⁵ years. During the cooling of rocks, ionizing radiation leads to the temporary trapping of electrons in crystal defects. The rate of electron release from these traps depends on the traps’ thermal activation energy as well as their spatial density (controlling their purely athermal loss via quantum mechanical tunnelling). However, interpreting luminescence signals requires that the electron trapping and detrapping models correctly replicate well-constrained thermal histories, both in the laboratory and natural environments. Existing models, such as single saturating exponential (SSE) and general-order kinetics (GOK) for trapping, and band-tail states (BTS) for detrapping, have been previously tested and validated for some benchmark areas. However, these models appear inadequate for our new experimental K-feldspar IRSL dataset from the Mont-Blanc massif (European Alps), e.g. by misfitting laboratory trapping-detrapping behaviour (SSE + BTS) or failing to reproduce dose-dependent isothermal decay curves (GOK). To address these limitations, we introduce a new trapping-detrapping model consisting of a log-normal distribution of trap characteristic doses (D₀ values) and of their thermal lifetimes. This model is internally consistent, mathematically in line with former approaches, verifiable on existing IRSL results from the KTB-borehole, and demonstrates excellent predictive capabilities with respect to our Mont-Blanc dataset. Using this model, we investigated the last 100-kyr cooling history of nine samples from the Mont-Blanc tunnel. Our results suggest that the subsurface cooled by approximately 10–30 °C over the past 20 kyr, implying a potential link to the last deglaciation relating to (1) valley incision and/or (2) cold water infiltration provided by melting glaciers.