Thermoluminescence dating of sediment baked by Iava flows of the Chaîne des Puys

Abstract

AN earlier attempt¹ to use thermoluminescence (TL) for determining the age of Quaternary lava flows, and in particular of the magnetically reversed² Laschamp and Olby flows of the Chaîne des Puys in the French Massif Central, was frustrated by anomalous fading of the TL. This anomalous fading seems to be exhibited by most of the TL-sensitive minerals in lava and is probably due to wave mechanical leakage of electrons out of traps to nearby defects^3,4. It has not been observed in quartz¹ but only minimal quantities of this mineral are present in the basaltic flows of the Chaine des Puys. Although the TL phototransfer technique is a promising⁵ way of using traps that are immune to anomalous fading, it has not yet proved possible to obtain TL dates directly from volcanic lava (although an empirical technique has achieved relative dating in one particular application⁶). We report here the TL dates for the baking by lava of clay and sediment, thereby avoiding the malign TL characteristics of the lava itself. TL dating of baked clay in the form of archaeological pottery is well established⁷ and the procedures developed in that application were followed here. Some of the dates obtained are of particular interest because of the likely association² of one of the lava flows concerned, that at Royat, with the magnetically reversed flows mentioned above. An average TL age for the latter of 33,000 ± 4,000 yr has been obtained by Valladas et al.^8,9; this is based on quartz extracted from a granitic inclusion found in the Laschamp flow and an underlying quartz pebble heated by the Olby flow.