OSL-THERMOCHRONOLOGY OF NA- AND K-FELDSPAR FROM NAMCHE BARWA, TIBET

In contrast to other thermochronometric methods, optically stimulated luminescence (OSL)-thermochronology has a relatively low closure temperature (~30-70 ^oC) which offers the potential to constrain near-surface changes in exhumation rates over Quaternary timescales. However, as OSL signals saturate, successful applications of OSL-thermochronology are limited to rapidly exhuming environments or elevated temperature settings (e.g. tunnels or bore holes [1]).

The Namche Barwa massif (eastern Himalayan syntaxis) is thought to have experienced extremely rapid exhumation throughout the late-Cenozoic to Quaternary period [e.g. 2]. This setting is therefore challenging for the application of traditional low-temperature thermochronometers, but provides a useful test-site for the application of OSL-thermochronology in resolving late-stage cooling histories. Six bedrock samples were hand crushed before using conventional methods to extract Na- and K-feldspar fractions. A multiple elevated temperature (MET) protocol was used which comprises infra-red stimulated luminescence (IRSL) measurements at 50, 100, 150 and 225 ^oC to record multiple IRSL signals for each individual sample. The different MET signals may have different thermal stabilities (thus different closure temperatures), and could therefore provide better constraint on cooling and exhumation rates. In addition to thermal trapped-charge depletion, feldspars also exhibit athermal charge loss (fading) which was measured and corrected for.

Preliminary results show that all Na-feldspar IRSL signals are in field saturation, whereas the IRSL₅₀, IRSL₁₀₀ and IRSL₁₅₀ signals of the K-feldspar extracts of some samples exhibit thermal signatures. Incorporating sample specific laboratory-constrained kinetic parameters for these signals into a charge-trapping model, results in a predicted cooling rate of ~150 ^oC Ma^-1for sample NB140, in close agreement with independent cooling rate control from apatite fission-track ages adjacent to the sampling site [2]. These results suggest that OSL-thermochronology has the potential to constrain Quaternary cooling histories and exhumation rates in rapidly-exhuming settings.

[1] Guralnik, B. (Unpubl.) PhD Thesis, ETH-Zurich.

[2] Seward, D. and Burg, J.-P. (2008). Tectonophysics 451, 282-289.