Measurement of 234U/238U and 230Th/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 ²³⁴U/²³⁸U and ²³⁰Th/²³²Th 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.