Potentials and problems of ESR thermochronometry

Low temperature thermochronometry gives insights into the interaction between surface processes and underlying tectonics. Electron spin resonance (ESR) thermochronometry works by determining the timing and rate at which electrons are trapped and thermally released in quartz in response to in situ ionizing radiation and rock cooling. The technique is sensitive to temperature ranges from 49 to 82 °C (corresponding to cooling rates of 40-1000 °C/Ma). Thus, it has the potential to assess histories of rapid cooling during the Quaternary. ESR thermochronometry was first introduced in the late 1990s, but little work has been done on the technique since. Its application faces many challenges for methodological improvement.

 

This presentation will describe new methods for evaluating realistic trap parameters (i.e., activation energy and frequency factor) of paramagnetic centres and dose rate distribution of ionizing radiation in rocks. Instead of using traditional isothermal annealing experiments, trap parameters were estimated by analysing borehole samples with a well-defined thermal history and record of storage temperature. A first order kinetic model was used to quantify the irradiation-induced trapping and thermally-related detrapping processes. Conventionally, dose rates are calculated by bulk analysis and the assumption of a homogeneous environment, but most rocks are heterogeneous in mineral composition and distribution. Thus, the commonly applied method can result in large uncertainties in calculating beta dose rate, which contributes 60-70% of the total dose rate. A 2D model for solving this problem using direct imaging of mineral distributions within individual rock samples has been developed and tested. ESR thermochronometry has then been applied to estimating the most recent exhumation history of the Namche Barwa massif, at the eastern end of the Himalaya.