Geomorphology, Cryosphere and Mountains Faculté des géosciences et de l'environnement Institut des dynamiques de la surface terrestre Low-temperature OSL thermochronology of Namche Barwa & the Mont-Blanc massif [email protected], [email protected], frédé[email protected] & [email protected] Previous thermochronometric data indcate very rapid cooling rates of ≥100 C/Ma (Burg et al., 1998; Seward & Burg, 2008). Provides an ideal test environment for OSL-thermochronology. OSL-thermochron (This study) OSL-thermochron (Guralnik, 2014) 40 30 2 10 0 0 20 40 60 Time (s) 80 100 0 0 1000 2000 3000 Dose (Gy) 4000 5000 AFT age of 0.8 Ma proximal to sample NB140 suggests a cooling rate of ~150 C/Ma. If this cooling rate is input into the model then it is consistent with the n/N ratios, suggesting that cooling rates have not changed over the past 800 ka. 80 60 40 20 Temperature (C) 0 1.0 Temp. 12 ka Temp. 1960 n/N IR50 n/N IR225 0.9 0.8 0.7 50 0.6 40 0.5 30 20 10 4.0 0 2.0 4.0 6.0 8.0 10.0 0 12.0 3.5 3.0 Tectonized zone 2.5 2.0 1.5 1.0 0.5 Mont Blanc shear zone 0 NW 2.0 Variscan metamorphic rocks 0-I 142 5-I 0.8 0.6 4 20 Conclusions 0.4 50 6 1.0 0.0 0.2 60 8 IR50 IR100 IR150 0.0 Measured luminescence signal (Ln) 1.0 n/N 70 0.8 Lx/Tx Signal intensity (counts x 103) (Ln/Tn) 0.4 0.6 Measured n/N 0.8 243 10 0.2 Clear inverse-correlation between n/N values for both signals and temperature at 12 ka reveals that the samples contain a thermal signal. No relationship is recorded with modern tunnel temperature, showing that recent changes in water percolation have no effect. This suggests that periods of water penetration have been short-lived throughout the late Quaternary. after Huntely (2006), Kars et al. (2008), Guralnik (2014) 0.0 0.4 0.6 Measured n/N 0-I D0 0.2 425 Three measurements are made on a Risø automated OSL/TL Reader for each sample. IR50 IR100 IR150 IR225 0.0 0-I OSL samples are prepared under subdued light conditions. Prep involves 1. crushing, 2. sieving, 3. HCl and H2O2 treatment, 4. density separation 5. HF etch (quartz only) Athermal signal depletion can lead to n/N <<1 which does not relate to a thermal signal. Samples screened and only the IR50, IR100 and IR150 signals of the K-feldspar extract of NB140 exhibit a thermal signature. 1.0 Luminescence measurements 1.0 = Samples measured using a MET-protocol (Li and Li, 2012), which involves the measurement of multiple signals (i.e. multi-thermochronometry) for each sample: IR50, IR100, IR150, IR225. after Huntely (2006), Kars et al. (2008), Guralnik (2014) Rock temperature tunnel (C) where E is the trap depth (eV), s is the frequency factor (s), T is temperature (K) and KB is the Boltzman constant. - Environmental dose rate (Gy ka-1) D0 - e-folding dose (Gy) N - max. trapped charge population n - meas. trapped charge population - thermal charge detrapping - athermal charge detrapping 523 ) of an electron in a trap is determined by: Predicted Field Saturation n/N 0.0 0.2 0.4 0.6 0.8 The thernal lifetime ( *Map and exhumation rate control from Seward & Burg (2014) Samples were measured using a post-IR IRSL protocol which involves the measurement of two signals for each sample: IR50 and IR225 (i.e. multi-thermochronometry). 0-F Kinetic parameters *Maréchal et al. (1999) 335 In contrast to other thermochronometers, OSL is a saturating system which limits its application to very rapidly cooling terrain, or elevated temperature settings (e.g. bore holes/tunnels). n/N NB19 NB109 NB131 NB139 NB140 1.0 Sample Exhumation Rate Control AFT 0.8 Ma* AFT 0.8 Ma* Present-day temperatures are depressed by ~20 C relative to those at 12 ka due to percolation of water into the massif following glacial retreat (Maréchal et al., 1999). OSL-thermochronology may provide insights into the longevity of such water incursions. Sample Temp. [C]* 1425−I (24 C) 33 3350-F 2430−I (31 C) 4250−I (39 C) 40 5230-I 5230−I (40 C) 39 4250-I 3350−F (33 C) 31 2430-I 24 1425-I Altitude (km) Optically Stimulated Luminescence (OSL) is a Quaternary dating method. It works through exploiting the time-dependent accumulation of trapped electrons within quartz and feldspar, which occurs when they are protected from light or heat, and are exposed to ionising radiation. Mont-Blanc Massif, France/Italy: exploring hydrothermal histories 0.0 What is OSL? Namche Barwa, Tibet: extracting exhumation rates Predicted Field Saturation n/N 0.2 0.4 0.6 0.8 OSL-thermochronology enables the measurement of recent changes in exhumation rates, beyond the resolution of other systems i.e. over the Quaternary period. Géomorphologie, Cryosphère et Montagnes Mont Blanc granite Dauphinois and Helvetic sediments 4.0 6.0 8.0 10.0 Distance from NW-Entrance (m) 12.0 SE OSL-thermochronology can provide valuable insights into recent cooling and hydrothermal histories where it is applied in rapidly eroding terrains or in elevated temperature settings. Future research will explore different OSL and thermoluminescence signals which may have different kinetic parameters enabling better resolution of cooling/hydrothermal histories and/or application within less rapidly cooling terrains.