ABSTRACT

The radioactive disequilibrium measurements between ²³⁸U and ²³⁰Th with a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) were applied to igneous standard rocks distributed by Geological Survey of Japan and Geological Survey of the United States. We modified sample dissolution and Th separation method, as well as Th isotope analysis by MC-ICPMS described by Nakai et al. (2001). High sensitivity of our ICP-MS enabled to analyze less than 10 ng Th. The abundances of U and Th were determined by an isotope dilution method, using a ²³⁵U depleted uranium reagent and ²³⁰Th prepared by milking of natural U as spikes. We can determine ²³⁸U/²³²Th abundance ratio with a precision of about 2%. This procedure was applied to (²³⁸U/²³⁰Th) radioactive disequilibrium measurements of igneous standard rock samples. Samples older than 350 Ka were found to attain secular equilibrium, indicating the accuracy of our analysis. On the other hand, young igneous standard rocks were found enriched in ²³⁸U relative to ²³⁰Th, (²³⁸U/²³⁰Th)_{activity ratio} > 1, which is often observed for igneous rocks from subduction zones.