Thermochronometry using red TL of quartz? – Numerical simulation and observations from in-situ drill-hole samples

• We study red TL (RTL) of Variscan granitic bedrock quartz up to 1831 m depth.
• Measurements are compared with numerical simulations of TL glow curves.
• The TL signal around ∼325 °C disappears for temperatures Ta > 53 °C (1831 m depth).
• High temperature RTL signals (>300 °C) are not in field saturation for Ta > 25 °C.
• The RTL signal keeps growing up to doses in the range 6–10 kGy.

As the lifetime of trapped charge in meta-stable states in natural crystals is a function of ambient temperature, the trapped charge concentration – measured for example by ESR or luminescence methods – should contain information on a rock's thermal history. Experiments and modelling, however, confirm that quartz OSL saturates at dose levels too low to be used for thermochronometry in mountain ranges other than tectonically very active Alpine orogens. That is why we propose red TL (RTL) of quartz as a geothermometer that saturates at doses one to two orders of magnitude higher than OSL. We compare numerical simulations of TL glow curves as would be expected at different depths below surface with RTL measurements obtained for samples from a recent drill-hole in the granitic basement of the Variscan Fichtelgebirge (Germany). Natural TL glow curves document gradual thermal erosion of the high temperature (>300 °C) TL with increasing ambient temperature. Moreover, additive RTL dose response of these samples indicates that signals are not in field saturation for ambient temperatures as low as 25 °C, most probably for even lower temperatures. The sample collected near the surface showed an uncommon dose response behaviour.