Precise and accurate measurement of 226Ra– 230Th– 238U disequilibria in volcanic rocks using plasma ionization multicollector mass spectrometry

Abstract

We describe analytical techniques for the measurement of 226Ra– 230Th– 238U disequilibria in volcanic rocks using plasma ionization multicollector mass spectrometry (MC-ICP-MS). Our methods build upon previous MC-ICP-MS techniques for 232Th/ 230Th and 238U/ 234U measurements with the addition of new chemical and/or mass spectrometric procedures for the determination of Th, U and 226Ra abundances. Using sample sizes that are similar to the requirements of state-of-the-art thermal ionization mass spectrometry (TIMS), we are able to measure Th isotope ratios and 226Ra– 230Th– 238U disequilibria with a precision that is at least ∼2–3× better than TIMS and U isotope ratios with a precision that is comparable to TIMS. Replicate analyses of a 226Ra– 230Th– 234U– 238U radioactive equilibrium rock standard (Table Mountain Latite) demonstrate that our MC-ICP-MS techniques are also highly accurate. An additional bonus is that analyses of Ra, Th and U isotope ratios by MC-ICP-MS are much less time-consuming than by TIMS. These analytical improvements will increase the usefulness of U-series isotopes as high-resolution tracers of the nature and timing of magmatic processes at active volcanoes.