U-series Dating of a Speleothem from Inazumi Cave, Oita Prefecture, Japan

Trace quantities of uranium and thorium were separated from a submerged calcite speleothem (stalactite) sample from Inazumi Cave, Oita Prefecture, using Fe (OH) 3 coprecipitation followed by anion-exchange separation with HNO3 and HCl. The isotope ratios 234U/238U and 230Th/234U were determined by isotope-dilution thermal ionisation mass spectrometry. The speleothem became submerged by accumulation of the Aso welded-tuff, Aso-4, and a resulting rise in the local water table that occurred 85, 000-89, 000 years ago. The age of the analysed portion of the sample was 170, 000 years B.P., suggesting that it was deposited before the earlier Aso-2 and-3 eruptions and has been submerged at least three times.