Luminescence dating - Wikipedia
Luminescence dating is a scientific method which dates certain artifacts Artifacts which can be dated using these methods include ceramics. There are two components involved in evaluating age by luminescence. One is the “equivalent dose” determined from luminescence measurements on mineral. Most luminescence dating methods rely on the assumption that the mineral grains were sufficiently "bleached" at the.
Feldspar Feldspar is another widely used mineral in OSL dating. In terms of chemistry, feldspars are aluminosilicates that form solid solution series with potassium K calcium Ca and sodium Na as end members of a ternary system. Since potassium has an isotope that contributes ionizing radiation in luminescence dating, the potassium in K-feldspars has to be treated as a source of internal dose, in addition to dose contributions from sources external to the grains.
As a result, when dating feldspars, it is necessary to separate K-feldspars from Ca and Na-feldspars and analyze them separately. Compared with quartz, feldspar has a number of attractive luminescence features.
First, feldspar emissions are generally brighter than those from quartz which produces stronger signals.
Luminescence Dating: Applications in Earth Sciences and Archaeology
This means that smaller doses can be measured during analysis. Second, the internal dose from potassium is not susceptible to external influences such as variations in pore water and this allows dose rates to be ascertained more accurately. Third, feldspar can be stimulated using infrared radiation which allows effective separation to be made between the stimulation source and emission wavelengths. The main drawback for feldspar, however, is its susceptibility to anomalous fading [ 64 ].
Anomalous fading occurs when trapped electrons reside in their traps for shorter periods than what would be predicted by physical models such that the luminescence intensity drops over time from the time of irradiation.
Ultimately, the result of anomalous fading is that most feldspar grains yield equivalent doses that are slightly lower than they would in the absence of fading. Correction methods have been developed for dealing with anomalous fading when dating feldspars [ 6566 ].
In terms of emission wavelengths, K-rich feldspars have been reported [ 67 ] to show maximums in the range of — nm violet to blue. Conversely, emissions for some plagioclase feldspars have been reported to appear in the range of — nm blue-green. Other studies, however, have intimated at a more complex emission pattern for feldspars [ 68 ]. Feldspar OSL properties Optical stimulation of luminescence from feldspars has been investigated using visible light.
Early studies employed lasers which included the The emissions were then monitored at shorter wavelengths [ 157 ] and shown to be centered around nm [ 69 ].
The application of OSL stimulation in dating feldspars, however, has been relatively limited because near-infrared stimulation discussed below has been shown to be a more desirable approach. This would indicate that different trap types might be involved [ 50 ]. Apart from green and red stimulation, luminescence in feldspar has been demonstrated using a range of other wavelengths in the region spanning — nm [ 71 ].
Feldspar IRSL properties As mentioned above, wavelengths in the near infrared region peaking around nm can also be used to induce luminescence in feldspars. Since this effect was first noticed [ 72 ], most research in optical dating of feldspars has focused on IRSL stimulation.
The main advantage of using IRSL is that the rest of the visible spectrum can then be used for emission detection. Fine-grained sediments containing mixtures of both plagioclase and K-feldspars have also been demonstrated to display a major stimulation peak around nm as well as a weaker one at nm [ 73 ]. LEDs are much cheaper than lasers and are widely available, making them a desirable alternative.
With plagioclase feldspar, an IRSL emission peak has been identified at nm.
Feldspars stimulated using IRSL following the administration of a laboratory dose also exhibit an emission peak at nm. That peak is not observed in feldspars that have a natural signal.
Applications, Advantages, Age Limits, and Accuracy - DRI Desert Research Institute
When not required during dating, the peak can be removed by preheating the sample to an appropriate temperature. Calcite Thermally stimulated calcite has an emission maximum at nm [ 60 ]. However, efforts to use the mineral in luminescence dating have been encumbered by the limited environmental occurrence of calcite. Calcite also tends to concentrate uranium in its lattice and this complicates dose rate calculations since isotopic disequilibrium of uranium has to be taken into account.
Worth noting is that uranium disequilibrium dating can yield ages from calcite that are more reliable than those obtained using luminescence techniques.
As a result, the incentive to employ luminescence methods in dating calcite has been small. It should be mentioned that some of the earliest, albeit unsuccessful, TL studies that tried to date rocks employed calcite [ 10 ].
Other attempts to use calcite in archaeological dating include a report by Ugumori and Ikeya [ 76 ]. Zircon Zircon is an attractive dosimeter because it usually has a relatively high concentration of uranium.
This yields a dose rate that is relatively constant since it is not susceptible to variations arising from external effects such as changes in water content or burial depth. An associated drawback, however, is that the uranium content of zircon varies between individual grains. Consequently, measurements for dose rate are made on single grains. These slowly decay over time and the ionizing radiation they produce is absorbed by mineral grains in the sediments such as quartz and potassium feldspar.
The radiation causes charge to remain within the grains in structurally unstable "electron traps". The trapped charge accumulates over time at a rate determined by the amount of background radiation at the location where the sample was buried.
Stimulating these mineral grains using either light blue or green for OSL; infrared for IRSL or heat for TL causes a luminescence signal to be emitted as the stored unstable electron energy is released, the intensity of which varies depending on the amount of radiation absorbed during burial and specific properties of the mineral.
Most luminescence dating methods rely on the assumption that the mineral grains were sufficiently "bleached" at the time of the event being dated.The Lumineers - "Stubborn Love" (Official Video)
Quartz OSL ages can be determined typically from toyears BP, and can be reliable when suitable methods are used and proper checks are done.
Boyd, and Donald F. Once the sand grain has been buried and it is no longer exposed to sunlight, the OSL signal starts to accumulate. OSL works because all sediments have some natural radioactivity, caused by the presence of uranium, thorium and potassium isotopes in heavy minerals such as zircons. We analyse the quartz or feldspar minerals in sand deposits.
Optically Stimulated Luminescence
When these quartz or feldspar minerals are exposed to the ionising radiation emitted by the radioactive isotopes in zircons, electrons within the crystals migrate and become trapped in their crystal structure. The number of trapped electrons depends on the total amount of radiation that the mineral has been exposed to. If we assume that the radiation dose rate of the sediment has remained constant over time, then if we measure that dose rate, we can calculate the sample age.
How do we measure the OSL signal? Photograph used with permission of Geoff Duller. The way that we do this is through sampling sand from the landforms in opaque plastic tubes and taking the sample back to a luminescence laboratory where only red light conditions are used.
- Applications, Advantages, Age Limits, and Accuracy
- Optically Stimulated Luminescence
We have to be very careful not to expose the sediments to sunlight when we do this! It is necessary to use red light conditions in the laboratory because the luminescence signal is light sensitive, and red light does not re-set it. We prepare the sample through treating it with acids to remove any calcium carbonate or organic material, and sieve it to get a specific grain size usually between 0.
This instrument stimulates the luminescence signal of the sand through shining the sample with blue or infrared light-emitting-didoes LEDswhich give the electrons enough energy to escape their traps and recombine elsewhere, emitting a photon of light. We measure this emitted light the luminescence and this is the first stage towards measuring the sample age.
Photographs taken by Geoff Duller Aberystwyth University. Blue diodes are most commonly used to stimulate the OSL signal from quartz, which is then detected in the UV emission spectrum. The OSL signal of a potassium K- feldspar sandur sample and its laboratory calibration curve. Figure by Georgina King. We then give our sand sample a range of laboratory radiation doses and measure the luminescence that each dose produces to develop a calibration curve.