Thierry Pilleyre

Properties of the red TL peak of quartz relevant to thermoluminescence dating

Abstract A well-defined thermoluminescence (TL) peak was observed in several quartz samples from various origins in the range 380–395°C (heating rate 5°C s -1 ) at a wavelength of around 600–620 nm. This red TL peak (RTL), seemingly common in quartz, could be used for dating in the range 10–150 ka, and probably beyond, so a comprehensive study of its main properties seemed worthwhile. Our study included annealing and heat treatment effects, dose rate effects and fading tests. Preliminary results are also given on crushing effects and light exposure sensitivity.

TEPHROSTRATIGRAPHY OF THE LAST 160 ka IN WESTERN LIMAGNE (FRANCE)

The GEOCLER 1 core extracted from the Clermont crater lake sediments and surface studies made at several localities north of Clermont provided numerous stratigraphic, environmental and chronological data (TL and radiocarbon) which allow us to present a chronostratigraphy of the Limagne for the past 160 ka. 118 ash-falls have been recorded in the crater-lake sediments of the Clermont maar. They demonstrate that a basaltic phreatic activity developed along the western fault-scarp of the Limagne between 160 and 70 ka, after which began the Chaine des Puys activity. Moreover, several acid eruptions occured between 160 and 40 ka. From Older Dryas to Atlantic, between 15 and 7 ka, fourteen ash-falls occured and in some cases the source volcanoes have been identified. These include Puy de La Nugere, Puy de Dome, Puy Chopine, Kilian Crater and Puy de Pariou which contribute mainly to the local tephra succession. Prospects for further prehistoric research in volcanic areas are outlined.

A TL model based on deep traps competition

Abstract TL appears as a very complex phenomenon. However, some general features are widely presented by TL materials: low TL yields, nonlinear growth with dose (supralinearity or saturation), lower efficiency for high LET particles etc., which are not explained by the well-known classical one-trap band model. In the past, the competition between TL traps and deep (thermally disconnected) traps was proposed in theoretical models accounting for some specific TL features. Arguments are presented here for a more general role of deep traps, in order to get an overall satisfying (at least qualitatively) description of the TL phenomenon either in the irradiation stage (TL or ESR) or in the heating one. A corresponding theoretical model is proposed. It is outlined that the competition between TL and deep traps leads to spatial correlations between electrons in TL traps and recombination centres (trapped holes). On the other hand, occurrence of recombination is taken into account as well in the irradiation step as during the heating one. As a result, on a very simple basis, the model accounts for supralinear behaviour of TL output with dose, non-exponential shape of TL or ESR growth curves, slight temperature shift of glow peaks with dose and scarceness of observed second-order kinetics.

Sedimentary Dynamics and Tecto-volcanism in the Venosa Basin (Basilicata, Italia)

Direct 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.

TL and ESR of quartz from the astrobleme of Aorounga (Sahara of Chad)

Abstract The present work was intended to evaluate the time that elapsed since the meteorite fall that produced the giant astrobleme of Aorounga (Sahara of Chad). For this purpose the TL and ESR dating techniques using the additive dose method were applied to quartz grains extracted from an impactite and from a sandstone shocked and baked during the impact. The ESR Al centre was measured and resulted in an age of about 800 ka. The red TL and the blue TL showed unusual TL features: the additive dose response curves were marked by an initial saturated part followed by a second rise at around +0.5 kGy; the peaks showed erratic temperature shifts with dose; fading was observed for high temperature peaks. By comparison with previous work using samples baked by lava flows more than 1 Ma ago and presenting some of those features, it was assumed that the minimum age of the astrobleme was of the same order of magnitude. This is in agreement with other observations. A preliminary explanation for those ageing features is proposed. It is suspected that radiation induced traps contribute to the TL of the studied quartz grains. Most probably the ‘malign-behaviour’ of the quartz grains is also connected with shock effects.

Annual dose assessment for luminescence and ESR dating: evaluation of nuclides mobility

Abstract The mobility of nuclides depends on their distribution within different sediment phases. Using a soil science approach, three main phases can be distinguished, which, according to increasing stability, are: — The mobile phase , which includes elements in aqueous solution or elements weakly bonded at the surface of minerals. — The phase that can be mobilised , in oxides, amorphous or weakly crystallised minerals. It can possibly move. — The resistant phase , which cannot move. The proportion of nuclides in the different phases, analysed together with other paedological data, can give information on the local stability of the radioactivity and possibly allow evaluation of error limits and corrections that should be applied to the present dose rate for dating purpose. In the worst cases this can lead to the conclusion that dating would not be reliable. Two case studies show the potential of the approach, which still remains more qualitative than quantitative. In an archaeological site (Ladoux, Puy de Dome, France), streaming surface water brings radioactivity characterised by radium in excess. In volcanic tephra (La Vestide, Bas Vivarrais, France), nuclide mobility is found to be negligible; therefore observed disequilibrium in the U series pre-existed the eruption.