A Study on concordias in the Uranium-Series Method for Dating

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Two kinds of concordias, [^<230>Th/^<234>U]-[^<234>Th/^<238>U] and [^<230>Th/^<234>U]-[^<231>Pa/^<235>U] diagrams, in the uranium-series dating, are examined in terms of a criterion to evaluate a ^<230>Th/^<234>U date of fossil coral. Because they are most useful to check if a coral sample has been preserved in appropriate condition for dating.More than 70% isotopic data on corals from the Pleistocene Riukiu Limestone (Hanzawa, 1935), were not consistent with the [^<230>U/^<234>U]-[^<234>U/^<238>U] concordia, but were plotted in the higher ^<234>U/^<238>U range. That is, the initial ^<234>U/^<238>U activity ratio estimated from those corals, is not necessarily agreeable with 1.144<plus-minus>0.002 as the value of the present-day sea water, reported by Chen et al. (1986). These facts imply that ^<234>U/^<238>U activity ratio of sea water at the time when fossils grew was somewhat (1 to 2%) higher than the value of Chen et al., or that the uranium isotopes were redistributed in those sam … More ples during their diagenetic alteration, as pointed out by Ku et al. (1990).The reliability of some ^<230>Th/^<234>U dates were critically evaluated by using the [^<230>Th/^<234>U]-[^<231>Pa/^<235>U] concordia. All results on isotopic analyses were plotted on the concordia for wellpreserved corals (of 100 to 95% aragonite) from Kikai Island, whereas those for partly recrystallized or calcite-cemented samples were not on it. Data points from a group of fossil corals seemed, however, to give significantly a straight line. In the case that such line was regarded as a discordia, two ages, namely ages of death and alteration of those samples, can be estimated to 230+50-30 ka and 4<plus-minus>4 ka, respectively. Therefore, the[^<230>Th/^<234>U]-[^<231>Pa/^<235>U] concordia has a possibility to give a reliable uranium-series date for other carbonate-secreting organisms like echinoderms and molluscs, which have been considered not to be appropriate materials for uranium-series dating, due to the open system for uranium isotopes after death of organisms. Less