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  • Author: Ashok Singhvi x
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Anomalous fading and crystalline structure: Studies on individual chondrules from the same parent body

Abstract

Plagioclase feldspar is the major luminescent mineral in meteorites. Thermoluminescence (TL) characteristics, peak temperature (Tm), full width at half maximum (FWHM), ratio of high (HT) to low temperature (LT) peak, and TL sensitivity (TL/dose/mass) to an extent reflect degree of crystallinity of the mineral. The present study explores and establishes a correlation between quantum mechanical anomalous (athermal) fading and structural state by examining TL of individual chondrules. Chondrules were separated using freeze-thaw technique from a single fragment of Dhajala meteorite. The results show large variation in Tm (155−230°C), FWHM (80−210°C) and HT/LT (0.07–0.47) and seem to be positively correlated. TL sensitivity (ranging from 14 to 554 counts/s/Gy/mg) decreases with increasing Tm and FWHM. Large variations in TL parameters (Tm, FWHM, HT/LT, and Sensitivty) suggest that individual chondrules had different degree of crystallization. Thermal annealing experiments suggest that comparatively ordered form of feldspar can be converted to a disordered form by annealing the sample at high temperatures (1000°C) for long time (10 hr) in vacuum (1 mbar pressure) condition and rapidly cooling it. Measured anomalous fading suggest that fading rate increases as the crystal form changes from an ordered state to a disordered state. However, the fading rate becomes nearly negligible for the most disordered feldspars.

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Aspects of Infrared Radioluminescence dosimetry in K-feldspar

Abstract

Infrared radioluminescence (IRRL) of K-feldspar, detected at peak wavelength of 865 nm, is emerging as a potential geochronometric tool. The present study explores and attempts to optimize the IRRL dating protocols and proposes a revised protocol for estimation of palaeodose. UV light (395 nm; 700 mW/cm2) bleach of 800 s was optimum to remove the trapped charges responsible for IRRL and, reduced the interference of radio-phosphorescence due to prior irradiations. Validation of the proposed protocol was carried out by dose recovery tests on mineral and sediment K-feldspar samples of different provenances. An overestimation in dose recovery was observed and was attributed to difference in sensitivity of natural IRRL and regenerated IRRL. The sensitivity changes were significant and systematic and were documented by repeating bleach-IRRL cycles. Corrections for sensitivity changes between natural and regenerated IRRL, gave reliable results and, have now been included in the proposed dating protocol.

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Chronology of red dune aggradations of South India and its Palaeo-environmental significance

Abstract

Red sand dunes occur in the coastal plains of south east and west of Tamil Nadu, India between the coordinates of 8°00′ to 9°30′ N; 77°18′ to 79° 00′ E. OSL dating of these sands indicated aggradations between ∼16-9 ka and ∼9-3 ka in the west and east coasts respectively. Dating results from inland red dunes at the foothills of Western Ghats show a break in deposition at ∼6 ka and aggradation since ∼2 ka. The sand aggradations in the west coast occurred during the transition period when SW monsoon in the area was reestablishing. The dunes attained their stability by 9 ka. In the coastal region, the aggradations were controlled by sea level changes and a local recycling of earlier dunes (in the east coast). In the inland areas, the dune building was controlled by sand supply from fluvial sources.

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

Abstract

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 fluctuations 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 heterogeneity in beta radiation dose received by individual grains. We thus postulate that ‘single aliquots’ (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 extent 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 aliquots 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 palaeodose 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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Editorial
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Changes in natural OSL sensitivity during single aliquot regeneration procedure and their implications for equivalent dose determination

Abstract

Measurement of low temperature (90°C–120°C) Thermoluminescence (TL) sensitivity of natural quartz samples subjected to pre-heating and optical stimulation indicate that significant sensitivity 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 account 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 intensities 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 correction for the sensitivity is based on the use of sensitivity of 110°C TL quartz peak, which is correlated to OSL signal (Murray and Roberts, 1998). The use of 110°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 110°C peak for a test dose on sample as received (i.e. natural sample) and, 2) the sensitivity of the 110°C peak of the same sample after the preheat and read out of the natural OSL, is proposed. This ratio, termed as Natural Correction 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.

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

Abstract

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 thermochronology 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 Pleistocene (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 midcrustal partial melts to move beneath the weak zone in the valley and causes an erosion-induced tectonic 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 erosion in the later stage coincides with the periods of deglaciation during Marine Isotope Stages (MIS) 1 and 2. The results of the present study suggest that in the present setting OSL thermochronology informed 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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An attempt to correct for the fading in million year old basaltic rocks

Abstract

The use of feldspar for luminescence dating has been restricted because of anomalous fading. This has made its application to several important geological problems such as volcanic terrains difficult. Presently, two correction procedures are used to correct for anomalous fading. The present study tests these correction procedures using volcanic samples of known ages spanning the time period of 400 ka to 2.2 Ma. These correction procedures provided grossly underestimated ages (up to 60%). The possible causes for the underestimation are discussed.

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Geological importance of luminescence dates in Oman and the Emirates: An overview

Abstract

In the Wahiba Sands of eastern Oman, luminescence dating of sands enables us to relate wind activity to climatic variations and the monsoon cycle. These changes resulted from Polar glacial/interglacial cyclicity and changes in global sea levels and wind strengths. Luminescence dates show that development of the Sands began over 230 ka ago when the sand-driving winds were the locally arid, northward-blowing SW Monsoon.

During late Quaternary low sea levels, the Tigris-Euphrates river system flowed across the floor of the Persian/Arabian Gulf to the Gulf of Oman SE of the Strait of Hormuz. OSL-dated sands containing calcareous bioclastic fragments deflated from the exposed Gulf floor during glacial low-water periods indicate that during the last glacial cycle, and at least one earlier cycle (∼120–200 ka and possibly as far back as 291 ka), the floor of the Arabian Gulf was exposed. This is deduced from the presence of aeolian dune sands containing bioclastic detritus on the coastal plain of the Emirates and south into Al Liwa (Abu Dhabi), which were built by northern “Shamal” winds. Those calcareous sands now locally overlie sabkhas formed during interglacial high sea levels. Within the present interglacial, marine flooding of the Gulf occurred between about 12 and 6 ka.

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