Volcans de la Chaîne des Puys (Massif-Central, France) : Les produits du Pariou comme repère chronostratigraphique

La séquence des produits émis par le Pariou est, à l'ouest du volcan, reproductible d'un site à l'autre. Elle comprend depuis la base : des ponces claires ; des lapilli noirs, anguleux, parfois trempés (ponces et lapilli noirs présentant souvent des rubannements) ; des lapilli à faciès de « débourrage », constitués d'un mélange de scories rouges ou noires et de socle, dont la granulométrie diminue vers le haut, de même que la teneur en scories rouges. Enfin, des cendres et lapilli noirs terminent la séquence ou sont parfois interstratifiés dans les niveaux supérieurs des lapilli de débourrage. Il n'y a pas de limite franche entre les différents horizons, ce qui témoigne d'une continuité dans les éruptions. L'épaisseur totale de la formation peut atteindre encore plus de 6m, localement, à 2,3 km du Pariou. Cette séquence caractéristique constitue un excellent repère chronostratigraphique. Elle repose sur des lapilli émoussés à faciès de remaniement sur les volcans Cliersou et Petit Suchet (ou Aumône) et entre le Filhu et le Balmet, ou bien sur des lapilli scoriacés squelettiques à faciès de saupoudrage entre le Côme et le Balmet. Aux points d'observation, l'absence de sols intermédiaires entre les produits du Pariou et les téphras sous-jacentes, d'origine inconnue, suggère que les différentes éruptions se sont succédé rapidement. Les téphras du Pariou reposent sur les scories rouges endogènes du Côme vers le sommet de ce volcan, ce qui indique qu'il a plus de ~9500 ans. Enfin, la présence d'un sol à la surface du Cliersou, sous les produits à faciès de remaniement, suggère une phase de repos assez longue, peut-être supérieure à un millénaire, ce qui conduit à donner à ce volcan un âge égal ou supérieur à 11000 ans environ. Au sud du puy Filhu, une figure d'effondrement dans les téphras du Pariou, de direction nord sud, de 1,4 m de largeur et 40 cm de décrochage vertical est interprétée comme le témoin de l'ouverture d'une faille dans la coulée de lave sous jacente. La fracture, qui se serait donc produite il y a moins de ~9500 ans, est à peu près dans l'axe des failles déjà connues sur le site de Vulcania et la carrière de Lantégy

Residual thermoluminescence for sun-bleached quartz: dependence on pre-exposure radiation dose.

thermoluminescence, à paraître dans Quaternary GeochronologyQuartz grains zeroed by heat and subsequently irradiated - either in the laboratory, or naturally in volcanic deposits - were exposed to sunlight for several months prior to TL measurements. As a result, the residual TL, either Blue or Red, appeared to grow significantly with the radiation-dose experienced by the grains since zeroing. In another experiment, un-heated sedimentary quartz grains, sampled at the very surface of a coastal sand-dune, were irradiated and subsequently exposed to sunlight before TL measurements. The residual Red-TL still exhibited a strong dose-dependence, but at a level which was higher than the natural signal. Finally, it was concluded that the so-called “un-bleachable TL” is not a constant, intrinsic data, for a given quartz sample. Because the residual TL is strongly correlated to the radiation dose experienced since the last zeroing by heat, it can theoretically be quoted in terms of “dose”, as sometimes done. However, this raises the question of its correct evaluation, and, on the other hand, there is a risk of misinterpretation of its physical meaning in the case of un-heated sediments. In the context of sediment dating, the experimentally-estimated residual signal that is routinely subtracted from the total signal for palaeodose estimation is therefore higher than the signal that would have been measured prior to burial. Such effect induces an error which can be significant for Red TL. It can be corrected for, at least roughly, from experimental exploration of the dose-dependence of the residual T

Calibration of a germanium well-detector using 60^{60}Co: the effects of the angular correlation of the two gamma rays emitted in cascade, quantified with Monte Carlo simulations

Radiation sources of 60^Co are commonly measured by means of HPGe gamma spectrometers, either as unknown sources or as calibration sources. However, the two gamma rays that are emitted by this nuclide, at 1.17 MeV and 1.33 MeV respectively, follow each other so rapidly that in the cases where both photons interact with the crystal, the electronics will record a single additive pulse. This is a cascade or coincidence summing effect. Such effect induces in the gamma spectra a "sum-peak", appearing at 2.5 MeV on the energy axis, generated by the pair of photons which have both been entirely absorbed by the detector. Also, the second photon is correlated for direction to the first one, i.e., it will not be emitted at random, with an isotropic probability. Then the question arises of to what extent the two effects, cascade and correlation, might affect the count rates for the three peaks. Various answers have already been published, but without practical solutions. In this context, the present work was devoted to further explore the question by means of Monte Carlo simulation, in the case of a welltype detector. As a result, inside the well the sum peak only is not affected and it allows accurate evaluation of the detector efficiency at 1.25 MeV. Outside the well, near the detector, none of the three peaks can be directly used for efficiency evaluation, unless the relevant corrections can be evaluated. At a distance from the detector, the two single peaks can be used, but with the drawback of a low efficiency

Theoretical and experimental estimation of self-attenuation corrections in determination of 210Pb^{210}Pb by gamma-spectrometry with well Ge detector

article accepté par "Radiation Measurements", sous presseThis paper aims at giving a practical method for routine measurements of the activity of 210Pb in solids by γ-spectrometry with a well Ge crystal. Since the γ-rays emitted by 210Pb have a low energy, 46.5 keV, the proportion of pulses counted in the crystal depends strongly on the sample composition trough self-attenuation within the sample itself. Self-attenuation can be calculated by use of a Monte Carlo method based on a model. In the present work two codes, namely GEANT 4 and a Laboratory code, gave results in agreement. However, both necessitate a good knowledge of the sample composition, of the detector geometry and of the computing programs. In order to circumvent such drawbacks, an experimental technique was developed, where the overall counting efficiency can be derived directly from the transmission of 46.5 keV γ-rays through the sample, by means of a linear relationship. Such correlation was observed experimentally with samples of known activity and the approach was validated by verification that a linear relation preliminary exists between the transmission and the calculated self-absorption

The Clermont radiometric reference rocks: a convenient tool for dosimetric purposes

soumis à Ancient TLBoreholes drilled in 8 different rocks situated in the vicinity of the Clermont TL laboratory have been used for testing and calibrating methods devoted to measure the natural radioactivity. The present paper gives updated data for the nuclide contents and internal dose-rates for those reference media. Two examples of application are presented. The first example shows dose-rate calibration of a scintillator gamma probe using a threshold method. The second example shows a routine technique for TL dosimetry, based on the rocks for dose-rate reference

Volcans de la Chaîne des Puys (Massif Central, France) : point sur la chronologie Vasset-Kilian-Pariou-Chopine

ThermoluminescenceInternational audienceLa compilation des datations radiocarbone de bois carbonisés par leurs déferlantes basales, complétée par des observations téphrochronologiques, permet d'avancer que le puy Chopine, il y a environ 9700 ans, a précédé le Vasset et le Kilian, tous deux péné-contemporains, vers 9400-9300 ans. Les produits du Nouveau Pariou sont recouverts par ceux d'un volcan trachytique, probablement le Kilian. Sous les produits explosifs initiaux du Nouveau Pariou (faciès "Traversin"), les trachytes à amphibole qui avaient été attribués au Kilian, sont vraisemblablement une forme méconnue des trachytes de la phase acide du Pariou lui-même. L'ordre chronologique des éruptions serait donc : Chopine/Pariou/?Vasset?/Kilian, la position du Vasset, hypothétique, restant à confirme

POTENTIAL USE OF THE E' CENTER AS AN INDICATOR OF INITIAL RESETTING IN TL/ESR DATING OF VOLCANIC MATERIALS

International audienceQuartz inclusions embedded in volcanic materials can be used for dattng Quaternary volcanoes by TL or ESR methods. For these techniques the initial zeroing by heat is assumed. However, this assumption can sometimes be questioned either because the temperature was not high, like in phreatomagmatic deposits, or because of contamination, like in thin tephra layers. Preliminary observations suggested that the présence of thé E' center could be an indicator of poor initial zeroing. This possibility was investigated in the present work. Quartz grains from various geological contexts were studied for comparison. Annealing and régénération of the E' center was explored

: La Moutade ash-fall, stratigraphic marker of Alleröd age in northern Limagne (Massif Central, France)

pdf du manuscritNational audienceA basaltic ash layer ("La Moutade tephra") has been identified in several sections in the Northern Limagne : alluviums, swamp and slope deposits. La Nugère volcanic complex is a potential source. Pedo-stratigraphical context, pollen analysis and radiocarbon dating give it an Alleröd age, which is confirmed by a direct thermoluminescence date. Thus, that ash layer can be considered as an important regional stratigraphic marker.Les "tephra de la Moutade", attribués par leur composition chimique à un basalte alcalin, ont été identifiés en Limagne septentrionale dans plusieurs coupes attestant d'environnements sédimentaires variés (dépôts palustres et fluviatiles, formations de versant). Le système éruptif du Puy de la Nugère peut en être l'émetteur. Leur dispersion régionale et leur position chronologique au sein de l'amélioration climatique de l'Alleröd, assurée par des approches croisées dont une datation directe par thermoluminescence, en font un marqueur stratigraphique régional

Sedimentary Dynamics and Tecto-volcanism<br />in the Venosa Basin (Basilicata, Italia)

pdf du manuscritInternational audienceDirect tecto-volcanism is the main process involved in the sedimentary filling of the Venosa lacustrine Basin. Two formations have been distinguished : the Tufarelle Formation and the Piano Regio Formation. In the sedimentary series exposed at the Notarchirico Middle Pleistocene prehistoric site, several volcanic episodes represented by pumice falls, coarse and fine ash-falls and subsequent epiclastic processes have interrupted the lacustrine sedimentation and induced specific environmental changes. Hominids were present immediatly after every volcanic event to exploit the new environment on an opportunistic basis.La Formation de Piano Regio et la Formation de Tufarelle composent le remplissage du Bassin de Venosa. La sédimentation y est contrôlée par l'activité volcanique du Monte Vulture dont il a enregistré les différentes phases d'activité au cours de la fin du Pléistocène ancien et du Pléistocène moyen. A Notarchirico, plusieurs épisodes volcaniques représentés par des ponces, des retombées fines et grossières et leurs différents produits de remaniement ont interrompu la dynamique sédimentaire lacustre et entraîné des modifications environnementales bien particulières. Les occupations successives du site suggèrent une fréquentation opportuniste du milieu volcanique par les Hominidés du Pléistocène moyen