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Anomalous fading and crystalline structure: studies on individual chondrules from the same parent body

100%

Biswas R. H. , Singhvi A. K.

Geochronometria

2013

tom Vol. 40, no. 4

250--257

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 crys-tallinity 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 chon-drules. Chondrules were separated using freeze-thaw technique from a single fragment of Dhajala me-teorite. 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 an-nealing experiments suggest that comparatively ordered form of feldspar can be converted to a disor-dered 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.

Aspects of Infrared Radioluminescence dosimetry in K-feldspar

80%

Varma V. , Biswas R. H. , Singhvi A. K.

Geochronometria

2013

tom Vol. 40, no. 4

266--273

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 attribut-ed 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.

Optical dating of sediments in Khari River basin and slip rate along Katrol Hill Fault (KHF), Kachchh, India

80%

Kundu H. K. , Thakkar M. G. , Biswas R. H.

Geochronometria

2010

tom Vol. 37

21-28

In the central region of Mainland Kachchh, Western India, the Katrol Hill Fault (KHF) is one of the major E-W trending faults. An understanding of the episodes of reactivation during the past has a bearing on the future seismicity in the region. These reactivations are manifested by offset of elevation of fluvial sediments and scarp-derived colluvium in the Khari River basin, SE of Bharasar (23°11'36.5"N, 69°35'22.6"E). Stratigraphic offsets of the sediments at this site suggest three episodes of reactivation of the KHF during the late Quaternary. Optical dating of samples from sediment strata and top layer of scarp-derived colluvium using Natural Sensitivity Corrected – Single Aliquot Regenerative (NCF-SAR) protocol suggested that these events occurred during the past ~30 ka, with the most recent historic episode around 3.0 ka. Given that a part of the slip recorded in the form of sediments offset, was lost due to erosion after faulting, a lower bound to the time averaged slip rate of the segment of KHF, is inferred to be > 0.23 mm/a during the past 30 ka.