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

We describe analytical techniques for the measurement of Ra-226-Th-230-U-238 disequilibria in volcanic rocks using plasma ionization multicollector mass spectrometry (MC-ICP-MS). Our methods build upon previous MC-ICP-MS techniques for Th-232/Th-230 and U-238/U-234 measurements with the addition of new chemical and/or mass spectrometric procedures for the determination of Th, U and Ra-226 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 Ra-226-Th-230-U-238 disequilibria with a precision that is at least similar to2-3 x better than TIMS and U isotope ratios with a precision that is comparable to TIMS. Replicate analyses of a Ra-226-Th-230-U-234-U-238 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. (C) 2002 Elsevier Science B.V. All rights reserved.