Chemical composition and radionuclide concentrations were determined for hokutolite, plumbian barite, collected at Peito hot spring, Taiwan. The hokutolite, precipitated on the surface of the base rock, was scraped into 20 layers from the surface. Barium and Pb were determined by inductively coupled plasma-atomic emission spectroscopy, and 228Ra, 226Ra and 210Pb were determined by γ-ray spectrometry. Comparison of the 228Ra/226Ra activity ratio at the surface of the hokutolite with that of the hot spring water suggests that the hokutolite has been isolated from the stream for 60 years. Significantly low fractions of (Ba, Pb) SO4 and changes in Ba/Pb mole ratio in the middle layers of the hokutolite suggest a change of growth condition at that time. Using the smooth and continuous decline in 228Ra/226Ra activity ratio observed in the upper layers of the hokutolite, the growth rate of 0.17 mm year-1 was estimated.
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