Optical dating of a Japanese marker tephra using plagioclase

The luminescence characteristics of volcanic plagioclase from an andesitic tephra (Hakone-Tokyo pumice, Hk-TP) from Japan were studied in order to assess if optical dating of plagioclase could be applied to Quaternary tephra. The tephra was shown to contain two kinds of plagioclase grains, labradorite which had a smooth surface and bytownite with a rough surface. The aliquots consisting of smaller grains contained more bytownite; these showed higher luminescence sensitivity and a higher fading rate compared to the larger-grained aliquots containing only labradorite. The fading rates of both OSL and IRSL signals were similar for each grain size. However, the OSL signal showed severe thermal transfer which made it difficult to use for dating. Using the IRSL signal from grains ranging from 150 to 212 μm, an age was obtained in agreement with the independent evidence, once a small fading correction had been applied.