Thermoluminescence of feldspar as a multi-thermochronometer to constrain the temporal variation of rock exhumation in the recent past

Abstract

Natural thermoluminescence (TL) in rocks reflects a dynamic equilibrium between radiation-induced TL growth and decay via thermal and athermal pathways. When rocks exhume through Earth's crust and cool from high to low temperature, this equilibrium level increases as the temperature dependent thermal decay decreases. This phenomenon can be exploited to extract thermal histories of rocks. The main advantage of TL is that a single TL glow curve has a wide range of thermal stabilities (lifetime <ka to Ba), and hence can provide multiple constraints on thermal histories. Here we constrain the distribution of kinetic parameters of TL in feldspar using a glow curve deconvolution method and fitting infinitesimal sub-peaks using a general order kinetic model. Each peak corresponds to a different but closely located energy level E. Forward modeling is applied for different time-temperature histories to estimate the sensitivity and limitation of each signal for different cooling rates. The results show that it is possible to constrain thermal histories between ∼30 °C and ∼80 °C. The results also illustrate that shallower traps, i.e. with lower activation energies, can be exploited to constrain lower cooling histories >100 °C/Ma, whereas deeper traps, i.e. with higher activation energies, provide constraints on thermal histories for higher cooling rates (>300 °C/Ma). Finally, we show how the path of rock exhumation (i.e., depth vs. time) can be constrained using an inverse approach. The newly developed methodology is applied to rapidly cooled samples from the Namche Barwa massif, eastern Himalaya to suggest a trend in exhumation rate with time that follows an inverse correlation with global temperature and glaciers equilibrium altitude line (ELA).