|Budget Amount *help
¥2,200,000 (Direct Cost : ¥2,200,000)
Fiscal Year 1991 : ¥400,000 (Direct Cost : ¥400,000)
Fiscal Year 1990 : ¥1,800,000 (Direct Cost : ¥1,800,000)
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
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