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Rapid denudation of Higher Himalaya during late Pliestocence, evidence from OSL thermochronololgy

De Sarkar S., Mathew G., Pande K., Chauhan N., Singhvi A. K.

Geochronometria

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2013

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Vol. 40, no. 4

304--310

EN

Optically Stimulated Luminescence (OSL) of quartz, with closure temperatures of 30-35°C in conjunction with Apatite Fission Track (AFT; closure temp. ~120°C) and 40Ar-39Ar (biotite closure temperature ~350°C), were used to obtain cooling ages from Higher Himalayan crystalline rocks of Western Arunachal Himalaya (WAH). Cooling age data based on OSL, AFT and Ar-Ar thermochro-nology provide inference on the exhumation – erosion history for three different time intervals over million to thousand year scale. Steady-state exhumation of ~0.5 mm/yr was observed during Miocene (>7.2 Ma) till Early Pleisto-cene (1.8 Ma). Onset of Pleistocene glacial/interglacial conditions from ~1.8 Ma formed glaciated valleys and rapid erosion with rivers incising deep valleys along their course. Erosion enables mid-crustal partial melts to move beneath the weak zone in the valley and causes an erosion-induced tec-tonic uplift. This resulted in a rapid increase in exhumation rate. The OSL thermochronology results suggest increased erosion over ~21 ka period from Late Pleistocene (2.5 mm/yr) to Early Holocene (5.5 mm/yr) and these are to be contrasted with pre 1.8 Ma erosion rate of 0.5 mm/yr. Enhanced ero-sion in the later stage coincides with the periods of de-glaciation during Marine Isotope Stages (MIS) 1 and 2. The results of the present study suggest that in the present setting OSL thermochronology in-formed on the short-term climatic effect on landscape evolution and techniques like the AFT and 40Ar-39Ar provided longer-term exhumation histories.

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Distribution in SAR palaeodoses due to spatial heterogeniety of natural beta dose

Chauhan N., Singhvi A. K.

Geochronometria

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2011

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Vol. 38, no. 3

190-198

EN

In luminescence dating of sediments, Mayya et al. (2006) pointed out that at single grain level, the beta dose for quartz grains is heterogeneous. This heterogeneity arises due the fact that the total potassium in sediment is contributed by few feldspar grains with up to 11-14% stoichiometric potassium (Huntley and Baril, 1997). Beta particles have a range of ~2 mm, which is comparable to grain sizes and inter-grain distances. This fact implies that the spatial fluctuation of beta emitters (K-feldspars) around individual quartz grains results in heterogeneous dose deposition. These fluctua-tions therefore, lead to an inherent spread in palaeodoses received by individual quartz grains. In this study, we compute the spread in single aliquot palaeodoses that arises exclusively due to het-erogeneity in beta radiation dose received by individual grains. We thus postulate that ‘single ali-quots’ (comprising several – typically 100 – heterogeneously irradiated single grains) would have an inherent spread in the palaeodose. In this work, we used Monte Carlo simulations to quantify the ex-tent of spread in palaeodoses arising due to heterogeneity of beta dose and hence put a limit on the precision of age estimation. Simulations results indicated, that, 1) the average of the single aliquot palaeodoses provides the closest approximation to the true palaeodose, 2) the minimum number of al-iquots that are needed to obtain a robust estimate of average palaeodose value depend upon desired precision and the concentration of K, and 3) the ratio of maximum to minimum single aliquot palaeo-dose values for a given K concentration provides a measure of inherent spread arising due to beta dose heterogeneity. Any spread over and above this range, can be ascribed to other sources such as heterogeneous bleaching and sensitivity changes. Radiation dose from other uniformly distributed sources of beta particles (U, Th and Rb) however would reduce this spread.

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Changes in natural OSL sensitivity during single aliquot regeneration procedure and their implications for equivalent dose determination

Singhvi A. K., Stokes S. C., Chauhan N., Nagar Y. C., Jaiswal M. K.

Geochronometria

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2011

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Vol. 38, no. 3

231-241

EN

Measurement of low temperature (90o, C-120o,C) Thermoluminescence (TL) sensitivity of natural quartz samples subjected to pre-heating and optical stimulation indicate that significant sensi-tivity changes can occur during measurement of the natural Optically Stimulated Luminescence (OSL). During the measurement of natural signal, the luminescence sensitivity of samples can change by 40%. The sensitivity changes both during the initial preheat and the measurement of natural OSL. The currently used version of Single Aliquot Regeneration (SAR) protocol measures and corrects for the sensitivity changes after preheat and readout of natural OSL. However, it does not take into ac-count the changes in sensitivity during the readout of the natural signal. We therefore developed a correction procedure so that both the natural and the regenerated OSL in-tensities can be measured and plotted with the same sensitivity and suggest that in the absence of such a correction, a considerable fraction of the SAR based ages could have systematic errors. The correc-tion for the sensitivity is based on the use of sensitivity of 110o, C TL quartz peak, which is correlated to OSL signal (Murray and Roberts, 1998). The use of 110o, C peak provides a reasonable measure of the changes in OSL sensitivity of quartz. A modified Natural Sensitivity Corrected–SAR (NSC-SAR) procedure, that comprises the measurement of, 1) the TL intensity of 110o, C peak for a test dose on sample as received (i.e. natural sample) and, 2) the sensitivity of the 110o,C peak of the same sample after the preheat and read out of the natural OSL, is proposed. This ratio, termed as Natural Correc-tion Factor (NCF), then provides a way to correct for sensitivity changes. Results on samples from diverse depositional environments indicated that the NSC-SAR consistently (without exception) provided improved distribution in paleodoses i.e. a lower scatter compared to the standard SAR protocol. In addition, the use of this protocol also resolved anomalous cases where the intensity of natural OSL was significantly above the saturation intensity of the regenerated OSL. Implicitly, this study implies a caution on the use of palaeodoses obtained from single grains as such a correction is not possible in the currently used automated single grain OSL measurement systems. The only way now on will be to analyze aliquots with only a grain on them.