Dating of volcanic products by the radioactive disequilibrium system between 238U and 230Th

The radioactive disequilibrium system between 238U and 230Th was observed in some mineral phases of six volcanic products. Four samples of them are the important marker-tephras which provide a certain datum plane in Upper Pleistocene Systems of Japan. As the results of dating by internal isochrons using the (238U/232Th)-(230Th/232Th) diagram, 238U-230Th dates of 35 ± 8 ka, 43 ± 8 ka, 80 ± 2 ka, 82 ± 52 ka, 94 ± 8 ka, and 218 ± 26 ka, were determined for Handa Pyroclastic Flow, Daisen-Kurayoshi Pumice (DKP), Aso-4 Pyroclastic Flow (Aso-4), Ontake I Pumice (Pm-I), Tateyama D Pumice (DPm), and Yamakawa Tuff Breccia, respectively. Mineral of titanomagnetite was confirmed to be the most appropriate phase in this method for dating, as pointed out by CONDOMINES et al. (1982). It can be used also as different subsystems separated in each grain-size fraction (e.g., 100 - 200 mesh, 200 mesh-20 micron and 20 - 8 micron, respectively). The initial 230Th/232Th activity ratio for Pm-I sample were estimated, from the coordinates of an intersection of the internal isochron and the equiline, to be 0.090 ± 0.018. This exceptionally low value must be interpreted by the further study on uranium-series disequilibrium in others derived from the Ontake Volcano. (author)