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1

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

Biswas R. H., Singhvi A. K.

Geochronometria

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2013

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Vol. 40, no. 4

250--257

EN

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.

2

Stability of IRSL signals from sedimentary K-feldspar samples

Thomsen K. J., Murray A. S., Mayank J.

Geochronometria

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2011

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Vol. 38, no. 1

1-13

EN

Recent work has identified IR stimulated luminescence signals at elevated temperature from both potassium- and sodium-rich feldspars that have much lower anomalous fading rates than the conventional signal measured using IR stimulation at 50°C. This paper examines the stability of these signals for potassium-rich sedimentary feldspars. We show that the natural post-IR IRSL (pIRIR) signal from a 3.6 Ma old sample is in apparent saturation on a laboratory generated dose response curve, i.e. it does not show detectable fading in nature although a low fading rate is observed on laboratory time scales. We show that the pIRIR signal has a greater thermal stability than the IRSL signal and that the trend in increasing thermal stability is mirrored by a decreasing fading rate. We also investigate the effect of preheat temperature and IR stimulation power on the decay shape and conclude that the data can be explained in terms of either a single- or multiple-trap model. We present evidence that may suggest that at least part of pIRIR signal is derived from a high temperature trap (~550°C thermoluminescence (TL) peak), although again the data can also be explained in terms of a single-trap model. Finally, we present dose response curves and characteristic curvature constants (D0) values for various IRSL signals and conclude that the more stable signals saturate more quickly than the less stable signals and that the initial and final signals saturate at approximately the same level.

3

An attempt to correct for the fading in million year old basaltic rocks

Morthekai P., Jain M., Cunha P. P., Azevedo J. M., Singhvi A. K.

Geochronometria

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2011

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Vol. 38, no. 3

223-230

EN

The use of feldspar for luminescence dating has been restricted because of anomalous fad-ing. 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 peri-od 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.

4

Uncertainties on the luminescence ages and anomalous fading

Zink A.

Geochronometria

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2008

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Vol. 32

47-50

EN

It is well known that some minerals give underestimated luminescence ages due to anomalous fading. The anomalous fading follows a logarithmic decay law characterized by its slope, the socalled fading rate or g-value. Using the fading rate, Huntley and Lamothe (2001) suggested some correction for the fading underestimation of young samples (<40-50 ka). For polymineral fine grains, we observe a fading rate of 0-4%/decade for TL and BL-OSL and 4-6%/decade for IR-OSL. Extending the laboratory observation to archaeological age, the underestimation on the age for 10 ka is estimated to a mean of 5% for TL, 10% for BL-OSL and 45% for IR-OSL. Due to the non-linearity of the Huntley and Lamothe's fading correction, the contribution of the fading to the total uncertainty is estimated by a Monte-Carlo simulation. The inference on dating shows that the uncertainty on the anomalous fading can be a significant term of the combined uncertainty on the age, even for low fading rates.

5

Luminescence dating of neolithic ceramics from Lumea Nouă, Romania

Benea V., Vanderberghe D., Timar A., Van den Haute P., Cosma C., Gligor M., Florescu C.

Geochronometria

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2007

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Vol. 28

9-16

EN

Luminescence dating was applied to four Neolithic pottery fragments excavated at Lumea Nouă (Alba Iulia, Romania), with the purpose of improving the chronology for the archaeological finds, and hence, the occupational history of the site. The single-aliquot regenerative-dose (SAR) protocol was applied to both blue and infrared stimulated luminescence signals from coarse quartz and polymineral fine grains, respectively. A more conventional approach which uses a multiple-aliquot additive-dose protocol and thermoluminescence signals from polymineral fine grains was applied as well. The characteristics of the OSL and IRSL signals were investigated in terms of behaviour in the SAR protocol, dose response and dose recovery. Additionally, anomalous fading measurements of the 410 nm IRSL emission and of the blue TL emission from polymineral fine-grains were performed. Both signals were observed to be affected by anomalous fading. OSL dating of quartz using the SAR protocol is considered to be the technique of choice; it yields an average age of 6.0±0.4 ka (n = 4), which is in agreement with archaeological expectations. The dating results allow refining the typological interpretation of the objects and improve the chronological framework for the site.

6

Anomalous fading of IR-stimulated luminescence from feldspar minerals : some results of the study

Molodkov A., Jaek I., Vasilchenko V.

Geochronometria

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2007

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Vol. 26

11-17

EN

An increasing amount of evidence shows that the use of feldspars in luminescence dating may suffer from significant age underestimates, which are thought to result from anomalous fading. In the hope of finding a solution to the problem we undertook physical investigations of kinetics and temperature dependencies of tunnel afterglow of these minerals. As a result, the method of the direct detection of the tunnel transitions from the dosimetric traps is proposed. Some other relevant results obtained are also presented and discussed. Particularly, the probable reasons of absence of effects of anomalous fading in the feldspar samples investigated in the present work are elucidated.