238U/230Th disequilibrium measurement for volcanic standard rock samples using a multiple-collector ICPMS.

Abstract

The radioactive disequilibrium measurements between 238 U and 230 Th 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 235 U depleted uranium reagent and 230 Th prepared by milking of natural U as spikes.We can determine 238 U/ 232 Th abundance ratio with a precision of about 2%. This procedure was applied to ( 238 U/ 230 Th) 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 238 U relative to 230 Th, ( 238 U/ 230 Th) activity ratio > 1, which is often observed for igneous rocks from subduction zones.

238U-230Th radioactive disequilibrium in the northern Izu arc: (230Th/232Th) in the sub-arc mantle

“…The analytical scheme for 238 U-230 Th disequilibrium analysis was described in Fukuda and Nakai (2002). Uranium and thorium were purified using anion exchange resin (AGX1-8; Bio-Rad Laboratories Inc.).…”

“…An Aridus micro-concentric nebulizer (Cetac Technologies) was used to introduce the sample solutions. The errors for the ( 230 Th/ 232 Th) and ( 238 U/ 232 Th) ratio were estimated respectively as about 0.5% and 2% (Fukuda and Nakai, 2002). About 200-300 mg of different batches of powdered samples were used for ( 234 U/ 238 U) analyses.…”

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

“…The mixture was centrifuged and the supernatant was discarded. The precipitate was dissolved in 7 M HNO 3 and Th was separated and purified using AG1-X8 resin (see Fukuda and Nakai (2002), for details). The U/TEVA resin was used for U purification following the method of Yokoyama et al (1999).…”

Two types of adakites revealed by 238U-230Th disequilibrium from Daisen Volcano, southwestern Japan

“…The analytical scheme for ( 238 U/ 230 Th) disequilibrium analysis has been described by Fukuda and Nakai (2002). U and Th were purified using anion exchange resin (AGX1-8; Bio-Rad Laboratories Inc.).…”