Abstract
Thermoluminescence (TL) dating using quartz grain was carried out for 6 samples for old deposits and 26 samples for yonger deposits of the Hakkoda Pyroclastic Flow Deposits. Obtained TL ages are 0.51-0.55 Ma (Average 0.53 Ma) for former ones and 0.19-0.29 Ma (Average 0.25 Ma) for latter ones. These TL ages are 20-40% younger than reported K-Ar ages of 0.65 Ma and 0.40 Ma, respectively. Possible systematic error sources for such age difference are annealing effect in paleodose measurement, radon escape in annual dose evaluation. Adding to other minor effects for systematic errors, TL ages obtained by our usual experiments may have 10-20% younger values than true ages. The difference between TL and K-Ar ages is futher problems but ordinaly K-Ar data of this age range are not so reliable. The errors in measurements are 10-15%, which are calculated from both paleodose measurements and chemical analyses for anuual dose evaluation. In addition, 10% errors are expected in beta-rays contribution in annual dose evaluation. Summarized total error in measurements is about 20%.
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