U-Th radioactive disequilibrium analyses for JCp-1, coral reference distributed by the Geological Survey of Japan

Abstract

The U-Th radioactive disequilibrium measurements with Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICP-MS) were applied to a coral reference; JCp-1 distributed by the Geological Survey of Japan to evaluate homogeneity of the standard material. The two times of standard deviations (2SDs) of [235U/ 238U]isotoperatio, [234U/238U]isotoperatio, U abundance, [230Th/232Th]isotoperatio, and Th abundance results for four repetitive measurements were 0.1%, 1%, 1%, 39%, 15%, respectively. Those of [234U/238U] isotoperatio and U abundance analyses are slightly larger than the analytical reproducibility. Those of Th isotope and abundance analyses are, however, much larger than the analytical reproducibility, suggesting that U is relatively homogeneous in JCp-1, whereas Th is not. Although JCp-1 was prepared from a living coral, the apparent U-Th disequilibrium ages range from 0.82 to 1.14 ka. Th abundances varied from 46 ppb to 56 ppb, which is significantly higher than ordinary coral. It is possible that the observed high Th abundance may have resulted from contamination during preparation steps for the rock powder. Or it may have caused by inclusion of sandy material during the growth of the coral. The presence of residue after nitric acid dissolution indicates the presence of non-carbonate component. The correction of the contribution of 230Th from the contaminant, however, still gives ages older than zero. In addition, [234U/238U]isotoperatio of JCp-1 is higher than the value of present seawater, suggesting the coral may have experienced some diagenesis. Copyright © 2006 by The Geochemical Society of Japan.