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 ~2-3x 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.