Alpha-radioactivity-related upper age limit for thermoluminescence dating

A systematic underestimation of the age of mineralisation by the thermoluminescence (TL) technique has been observed in a variety of samples older than Quaternary although their natural TL was not saturated. The samples included calcites, oozes, lime stones, shales, gypsum, basalts and dolerites. It is shown that the TL build-up in nature reaches a dynamic equilibrium level much ahead of the lifetime of the TL trap concerned and is solely determined by the alpha radioactivity of the sample; the validity for TL dating does not exist once such an equilibrium is reached. For the samples considered, the limiting Paleo-alpha dose for TL dating validity works out to be about 150 kilorads; beyond this dose, the TL age and the geological age bears a ratio given by t'/t = a[∫0sup(t)Dsub(α)dt]sup(-b) where a and b are constants and Dsub(α) is the annual alpha irradiation rate in the sample. For a suite of samples with b not = 1, relative dating seems possible by the TL technique. It may be generalised that samples with 1 ppm level of U, Th content cannot be TL dated if they are older than about 500 kiloyears even though their NTL trap lifetimes may be 100 myrs; conversely, a 1000 myr-old sample can be TL dated only if its U, Th content is much less than ppb levels and its TL trap lifetime greater than 1010 years. (author)