Modelling dose rate to single grains of quartz in well-sorted sand samples: The dispersion arising from the presence of potassium feldspars and implications for single grain OSL dating

AbstractIn the OSL dating of sediment, the scatter in equivalent dose (D e) between grains is almost always larger than would be expected due to counting statistics alone. Some scatter may be caused by insufficient (partial) bleaching of some of the grains prior to deposition. In order to date partially bleached sediment, it is essential to estimate the amount of scatter caused by other processes (e.g. grain-to-grain variability in the natural dose rate). Measurements of such scatter are performed at the single-grain level; by contrast, most OSL dating is performed on multi-grain subsamples, for which grain-to-grain scatter is reduced through averaging.Here we provide a model for estimating the expected scatter (i.e. excluding that caused by partial bleaching) for multi-grain aliquots. The model requires as input the single-grain sensitivity distribution, the number of grains in the sub-samples, and the expected scatter at the single-grain level, all of which can be estimated to an adequate degree. The model compares well with measured values of scatter in D e, determined using aliquots of various sizes, and can be used to help produce a minimum-age D e from multi-grain subsamples that is consistent with single-grain data.

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Towards an improvement of optically stimulated luminescence (OSL) age uncertainties: modelling OSL ages with systematic errors, stratigraphic constraints and radiocarbon ages using the R package BayLum

Geochronology ◽

10.5194/gchron-3-229-2021 ◽

2021 ◽

Vol 3 (1) ◽

pp. 229-245

Author(s):

Guillaume Guérin ◽

Christelle Lahaye ◽

Maryam Heydari ◽

Martin Autzen ◽

Jan-Pieter Buylaert ◽

...

Keyword(s):

R Package ◽

Systematic Errors ◽

Covariance Matrices ◽

Optically Stimulated Luminescence ◽

Osl Dating ◽

Dose Rates ◽

Determination Procedure ◽

Single Grain ◽

Measurement Uncertainties ◽

Osl Age

Abstract. Statistical analysis has become increasingly important in optically stimulated luminescence (OSL) dating since it has become possible to measure signals at the single-grain scale. The accuracy of large chronological datasets can benefit from the inclusion, in chronological modelling, of stratigraphic constraints and shared systematic errors. Recently, a number of Bayesian models have been developed for OSL age calculation; the R package “BayLum” presented herein allows different models of this type to be implemented, particularly for samples in stratigraphic order which share systematic errors. We first show how to introduce stratigraphic constraints in BayLum; then, we focus on the construction, based on measurement uncertainties, of dose covariance matrices to account for systematic errors specific to OSL dating. The nature (systematic versus random) of errors affecting OSL ages is discussed, based – as an example – on the dose rate determination procedure at the IRAMAT-CRP2A laboratory (Bordeaux). The effects of the stratigraphic constraints and dose covariance matrices are illustrated on example datasets. In particular, the benefit of combining the modelling of systematic errors with independent ages, unaffected by these errors, is demonstrated. Finally, we discuss other common ways of estimating dose rates and how they may be taken into account in the covariance matrix by other potential users and laboratories. Test datasets are provided as a Supplement to the reader, together with an R markdown tutorial allowing the reproduction of all calculations and figures presented in this study.

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Dating mortar by optically stimulated luminescence: A feasibility study

Geochronometria ◽

10.2478/s13386-011-0002-0 ◽

2011 ◽

Vol 38 (1) ◽

pp. 42-49 ◽

Cited By ~ 28

Author(s):

Christian Goedicke

Keyword(s):

Dose Rate ◽

Short Range ◽

Optically Stimulated Luminescence ◽

Radioactive Equilibrium ◽

Lime Mortar ◽

Modulation Techniques ◽

Single Grain ◽

Optical Bleaching ◽

Number Of Publications

AbstractQuartz is the datable component in mortar. Although its luminescence properties are very well studied, the problem of mortar dating arises from a low level of optical bleaching. In order to reduce the time consuming efforts for dating lime mortar in monuments by optically stimulated luminescence, we investigated the most suitable methods of dose and dose rate determination, and we explored tests which are suitable to predict the datability of a given material. Reverting to the large number of publications on sedimentary partially bleached quartz we found linear modulation techniques especially useful, equally we also recommend the determination of the level of optical depletion before starting analyses. Single grain analysis is regarded as an ultimate but infallible tool to date very poorly bleached quartz from mortar. Dose rate measurements need to take into account the radioactive equilibrium of the uranium decay and the short range inhomogeneity of the environmental gamma radiation field: gamma spectrometry and on-site TLD measurements are best suited.

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Insight into the sediment dynamics of a high-impact low-frequency mass movement event using single-grain feldspar luminescence in the Pokhara valley, Nepal

10.5194/egusphere-egu21-6651 ◽

2021 ◽

Author(s):

Anna-Maartje de Boer ◽

Wolfgang Schwanghart ◽

Jürgen Mey ◽

Jakob Wallinga ◽

Basanta Raj Adhikari ◽

...

Keyword(s):

Sediment Transport ◽

Mass Movement ◽

Low Frequency ◽

High Impact ◽

Osl Dating ◽

Transport Dynamics ◽

Infrared Stimulated Luminescence ◽

Single Grain ◽

Pokhara Valley ◽

Insight Into

Mass movements play an important role in landscape evolution of high mountain areas such as the Himalayas. Yet, establishing numerical age control and reconstructing transport dynamics of past events is challenging. To fill this research gap, we investigated the potential of Optically Stimulated Luminescence (OSL) dating and tracing methods. OSL dating analyses of Himalayan sediments is extremely challenging due to two main reasons: i) the OSL sensitivity of quartz, typically the mineral of choice for dating sediments younger than 100 ka, is poor, and ii) highly turbid conditions during mass movement transport hamper sufficient OSL signal resetting prior to deposition which eventually results in age overestimation. In this study, we aim to bring OSL dating to the test in an extremely challenging environment. First, we assess the applicability of single-grain feldspar dating of mass movement deposits in the Pokhara valley, Nepal. Second, we exploit the poor bleaching mechanisms to get insight into the sediment dynamics of this paleo-mass movement through bleaching proxies. The Pokhara valley is a unique setting for our case-study, considering the availability of an extensive independent radiocarbon dataset (Schwanghart et al., 2016) as a geochronological benchmark.Single-grain infrared stimulated luminescence signals were measured at 50&#176;C (IRSL50) and post-infrared infrared stimulated luminescence signals at 150&#176;C (pIRIR-150). As expected, results show that the IRSL50 signal is better bleached than the pIRIR150 signal. A bootstrapped Minimum Age Model (bMAM) is applied to retrieve the youngest subpopulation to estimate the palaeodose. However, burial ages calculated based on this palaeodose overestimate the radiocarbon ages by an average factor of ~8 (IRSL50) and ~35 (pIRIR150). This shows that dating of the Pokhara Formation with our single-grain approach was not successful. Large inheritances in combination with the scatter in the single-grain dose distributions show that the sediments have been transported prior to deposition under extreme limited light exposure which corresponds well with the highly turbid nature of the sediment laden flood and debris flows that emplaced the Pokhara Formation.To investigate the sediment transport dynamics in more detail we studied three bleaching proxies: the percentage of grains in saturation (2D0 criteria), percentage of well-bleached grains (2&#963; range of bMAM-De) and the overdispersion (OD). Neither of the three bleaching proxies indicate a spatial relationship with run-out distances of the mass movement deposits. We interpret this as virtual absence of bleaching during transport, which reflects the catastrophic nature of the event. While single-grain feldspar dating did not provide reliable burial ages of the Pokhara mass movement deposits, our approach has great potential to provide insight in sediment transport dynamics of high-impact low-frequency mass movement events in mountainous region.ReferencesSchwanghart, W., Bernhardt, A., Stolle, A., Hoelzmann, P., Adhikari, B. R., Andermann, C., ... & Korup, O. (2016). Repeated catastrophic valley infill following medieval earthquakes in the Nepal Himalaya.&#160;Science,&#160;351(6269), 147-150.

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Microscopy studies in support of OSL dating of mortars of historical buildings

Microscopy and Microanalysis ◽

10.1017/s1431927613001190 ◽

2013 ◽

Vol 19 (S4) ◽

pp. 115-116

Author(s):

J. Sanjurjo-Sanchéz ◽

C. Alves ◽

D. Teixeira

Keyword(s):

Optical Microscopy ◽

Dose Rate ◽

Radioactive Decay ◽

Osl Dating ◽

Internal Dose ◽

Historical Buildings ◽

Luminescence Signal ◽

Funding Program ◽

Beta Dose ◽

Quartz Grains

There are several problems regarding the dating of the works in the historical buildings (see a review in Sanjurjo-Sánchez, 2012). Mortars are a potential material for dating of the built environment works, being ubiquitous and not reusable. Optical stimulated luminescence (OSL) dating can be applied to mortars with and without lime and considering the age interval of mortars from thousands of years up to a few decades. OSL dating is based on measuring the accumulated luminescence signal of aliquots (of one or more mineral grains) of constituents from the mortars, resulting from the absorbed dose of surrounding radioactivity (at a constant dose rate), considering contributions both from environment and the radioactive phases of the mortars.In this abstract is considered the relevance of microscopy studies as support for the preparation of samples for dating and the precise age calculations.Microscopy studies can allow identifying samples that cannot be dated due to lack of enough grains of suitable phases as is illustrated in Figure 1a. It is also possible to assess the presence of datable phases. Usually quartz and potassium (K) feldspar are used for dating (phases that can be easily distinguished by scanning electron microscopy and generally by polarizing optical microscopy). For dating, the OSL signal of such datable phases of a mortar must have been bleached before the mortar manufacture and settling, typically by daylight exposure of such minerals. Quartz grains have an easy to bleach signal. K-feldspars grains have a significant internal dose making them less susceptible to environmental variations but they present signal anomalous fading and the luminescence signal is harder to bleach than in quartz grains. However, according to Li B. et al., 2007, the effect of anomalous fading in K-feldspar may be negligible for samples younger than 100 years and there also some proposals for overcoming signal fading and the combination of measurements on K-feldspars and quartz. On the preparation of the sample, microscopy studies can assess the risk of contamination by other phases and, in analogy with what is done in mining studies, define a liberation size that relates to the obtainment of monomineral aliquots. It can also contribute to the identification of the presence of different grainsize fractions of the phases to use in dating (see example in Figure 1b), information that can be used for comparative studies of dose in the different grainsize fractions, such as isochron measurement procedures. One of the main issues for dating is the assessment of the radioactive dose rate that affected the mortar constituents related to the distribution of the contents of U, Th, Rb and K that can occur disperse through the sample or concentrated on some grains. Most OSL studies on sediments relate K content to K-feldspar occurrences. However, in some mortars there could be an important presence of K-micas such as muscovite and the biotite series (see Figure 1b,c) that have a lower K content and a different morphology. The presence of these mica grains has also been considered as potentially disturbing of the signal measurements and procedures for its removal have been proposed by Kortekaas and Murray, 2005. The grainsize distributions of radioactive grains and dated grains can also affect the way the radioactive dose affect the measured dose given that the same amount of K could be distributed in grains with different volume and hence different relation area/mass. Additionally, K-bearing phases can have contents of other radioactive elements affecting the beta dose as is illustrated by Figure 1c showing the presence of halos resulting from radioactive decay of inclusions in biotite crystals. While in general it is admitted that quartz grains had a zero internal dose, the presence of inclusions such as rutile (that can be assessed by optical microscopy) can endanger this assumption. Microscopy studies can also be used to assess precipitation and dissolution process that occur after the preparation of the mortar and that can affect the beta dose for the grains (see Figure 1d). Finally, microscopy studies can be used after dose estimates and can help solve problems that have baffled researchers such as low content of feldspar separations. In the case of mortar samples, for which can be obtained representative sections, the previous considerations show that petrographic models can help in the definition of spatial models for dose rate modelling and the establishment of appropriated age models.Support from Portuguese-Spanish collaboration Project “Ação Integrada E-141/10” (Fundação das Universidades Portuguesas)/“Acción Integrada PT2009-0077” (Ministerio de Ciencia e Innovación) and the Fundação para a Ciência e Tecnologia (pluriannual funding program for research units, PEst-OE/CTE/UI0697/2011, Portuguese funds).

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