5:15 PM - 6:45 PM
[SCG51-P04] Development of practical measurement method for 238U-230Th-226Ra disequilibria in volcanic rocks using ICP-MS/MS
Keywords:Volcanic rocks, 238U-230Th-226Ra disequilibria, ICP-MS/MS
The 238U-230Th-226Ra activity ratios in young volcanic rocks (<300,000 years) can provide constraints on physicochemical processes in the subsurface prior to magma eruption. In particular, it has been helpful in the calculation of the chemical contribution of subducting slabs and slab-derived fluids to magma, as well as in quantitative discussions of timescales for magma generation and ascent (Yokoyama et al., 2003). However, the amounts of 230Th and 226Ra in volcanic rocks of island arcs are ultra-trace amounts on the order of femtograms per gram. Therefore, column-based elemental separation, isotope spiking, and the use of large magnetic field mass spectrometers are commonly used, which impose high hurdles in terms of time, money, and technique. This study aims to develop a 238U-230Th-226Ra measurement method that can be practically applied in geochemical laboratories that do not have such specialized techniques and equipment.
In this study, standard rock samples (JB-3 and JR-2) were acid-decomposed and introduced into a mass spectrometer without elemental separation to verify whether interference on the mass spectra of 230Th and 226Ra can be removed. An ICP mass spectrometer (ICP-MS/MS) with He gas flowing in a collision/reaction cell (CRC) between two quadrupole mass separation sections was used for the measurement. As a result of the measurement, no interference effects were observed in the shape of the mass spectrum, and the signal intensity of 226Ra agreed with the calculation based on the sensitivity of the instrument and the concentration of 226Ra in the sample within the error, confirming that ICP-MS/MS can eliminate the effects of mass spectral interference without elemental separation. On the other hand, the concentrations of Th and U deviated significantly from the literature values of the samples, indicating that the recovery rate of Th and U in acid decomposition was problematic. Therefore, in this study, glass beads were prepared from powdered standard rock samples using anhydrous lithium tetraborate as a melting agent. The results of quantitative measurement for the glass beads by LA-ICP-MS showed that the Th and U concentrations agreed with the literature values within the errors. From these results, it is expected that Th and U can be prepared by complete acid decomposition of glass beads, maintaining their original isotope ratios. In this presentation, the analytical method will be evaluated based on the results of ICP-MS/MS measurements of 230Th/238U and 226Ra/230Th ratios in the prepared solution samples.
In this study, standard rock samples (JB-3 and JR-2) were acid-decomposed and introduced into a mass spectrometer without elemental separation to verify whether interference on the mass spectra of 230Th and 226Ra can be removed. An ICP mass spectrometer (ICP-MS/MS) with He gas flowing in a collision/reaction cell (CRC) between two quadrupole mass separation sections was used for the measurement. As a result of the measurement, no interference effects were observed in the shape of the mass spectrum, and the signal intensity of 226Ra agreed with the calculation based on the sensitivity of the instrument and the concentration of 226Ra in the sample within the error, confirming that ICP-MS/MS can eliminate the effects of mass spectral interference without elemental separation. On the other hand, the concentrations of Th and U deviated significantly from the literature values of the samples, indicating that the recovery rate of Th and U in acid decomposition was problematic. Therefore, in this study, glass beads were prepared from powdered standard rock samples using anhydrous lithium tetraborate as a melting agent. The results of quantitative measurement for the glass beads by LA-ICP-MS showed that the Th and U concentrations agreed with the literature values within the errors. From these results, it is expected that Th and U can be prepared by complete acid decomposition of glass beads, maintaining their original isotope ratios. In this presentation, the analytical method will be evaluated based on the results of ICP-MS/MS measurements of 230Th/238U and 226Ra/230Th ratios in the prepared solution samples.