Methodological developments in electron spin resonance (ESR) low-temperature thermochronometry

Abstract

Low-temperature thermochronometry provides a means of understanding the interaction between surface processes and underlying tectonics by quantifying the cooling histories of rocks. 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. This technique has great potential in reconstructing thermal histories of the upper ~ 2 km of the Earth's crust during the Quaternary. Its application, however, faces many challenges for methodological improvement since little work has been done on ESR thermochronometry after being first introduced in the late 1990s. Quantitative investigations of thermal histories by ESR thermochronometry rely on the determination of trap parameters of paramagnetic centres in quartz. This study has evaluated the activation energy and frequency factor of Al and Ti centres by analysing borehole samples with well-defined thermal histories and storage temperatures. Fergusons Hill-1 core is located at Otway Basin (Australia) where it is in a thermally steady state, while Eldzhurtinskiy Granite core is located in the Caucasus where it has experienced rapid cooling at a rate of ~ 520 degrees Celsius per million years. The best-fit parameters were determined by a first-order kinetic model which quantifies the irradiation-induced trapping and thermally-related detrapping processes. In heterogeneous rocks, numerical simulation is an ideal solution for accurate estimation of beta dose rate. Identification of mineral distribution is the basis for creating a model, and 2D mapping facilities are more effective than 3D X-ray computed tomography in this aspect. Thus, a 2D model "DosiVox-2D" was established for heterogeneous but isotropic samples, and verified by the comparison with the 3D model and infinite matrix dose method. The practical procedures were then investigated for applying 2D simulation on uniform and layered igneous rocks, including sample selection, mineral mapping, and estimation of radioelement concentrations. Thereafter, ESR thermochronometry was applied to Namche Barwa massif, the eastern Himalayan syntaxis. Samples were collected from a vertical transect to the south of the massif. ESR results, on one hand, have improved our understanding of the localised structural settings. On the other hand, the comparison of cooling history with the region adjacent to the north of the massif, has shed light on the domal development of Namche Barwa massif during Mid-late Pleistocene.