Measurement of²³⁴U/²³⁸U and²³⁰Th/²³²Th in volcanic rocks using the Neptune MC-ICP-MS

Abstract

We present a new multicollector inductively coupled plasma mass spectrometric (MC-ICP-MS) method for measuring isotopic ratios of 234 U/ 238 U and 230 Th/ 232 Th in volcanic rocks using the Thermo Fisher Neptune. Uranium isotope measurements using SRM U010 as bracketing standard produce agreement within several permil for NBL 112A, REMP-18, as well as for equilibrium rock standards. We also demonstrate that uranium standards are not appropriate for correcting thorium isotopic measurements due to differences between U and Th in both mass bias and mass-dependent ion transmission. Use of the thorium synthetic standard, UCSC ThA, as a bracketing standard produces data that agree well with 'accepted values' for other synthetic Th isotope standards and equilibrium rock standards.

Understanding melt generation beneath the slow-spreading Kolbeinsey Ridge using 238U, 230Th, and 231Pa excesses

“…Our analytical methods at the WHOI facility produced uncertainties in keeping with the findings of Richter et al (2006), and as previously reported by Ball et al (2008) and Sims et al (2008b). During analysis of U isotopes using the facility's ThermoFinnigan Neptune, we calibrated SEM yield and simultaneously corrected for mass bias with standard-sample-standard bracketing to monitor SEM efficiency and measurement of the standard U010.…”

“…During analysis of U isotopes using the facility's ThermoFinnigan Neptune, we calibrated SEM yield and simultaneously corrected for mass bias with standard-sample-standard bracketing to monitor SEM efficiency and measurement of the standard U010. Ball et al (2008) observed that the SEM yield correction is error limiting for these analyses. Mass bias drift is corrected and SEM yield is calibrated using U010 corrections (Ball et al, 2008), and mass bias corrections are made using a linear interpolation of U010 238 U/ 236 U = 14,535 ± 149 (Sims et al, 2008b).…”

“…Ball et al (2008) observed that the SEM yield correction is error limiting for these analyses. Mass bias drift is corrected and SEM yield is calibrated using U010 corrections (Ball et al, 2008), and mass bias corrections are made using a linear interpolation of U010 238 U/ 236 U = 14,535 ± 149 (Sims et al, 2008b). The sensitivity of U using this method is 0.1% ion yield (Sims et al, 2008b).…”

“…The sensitivity of U using this method is 0.1% ion yield (Sims et al, 2008b). The accuracy of U isotope analyses by this method is 0.5% for rock standards (Ball et al, 2008), with an internal reproducibility of 0.2-0.5% (2r) (Sims et al, 2008b). All measured U isotope compositions reported in Table 2 were corrected after samplestandard bracketing with the U010 standard as calibrator, using a standard 234 U/ 238 U ratio of 5.4484 Â 10 À5 (Richter et al, 2006), and then corrected again using the U960 standard, which was analyzed several times during most analytical sessions and was assumed to have an accepted 234 U/ 238 U ratio of 5.286 Â 10 À5 (Richter et al, 2006).…”

“…The MC-ICP-MS analytical protocols at WHOI followed those outlined by Hoffmann et al (2007), Ball et al (2008), and Sims et al (2008a). The MC-ICP-MS methods at WHOI include an offline, exponential tailing correction of the 232 amu mass peak on the 230 Th peak, which has been found to be considerably more accurate than linear corrections for Th isotopes, which overestimate the tailing of mass 232 Th on 230 Th (Ball et al, 2008).…”