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Abstract: Several analytical techniques are currently used to determine mass-dependent molybdenum isotopic variations in natural materials using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), including different methods for the separation of Mo from the sample and the correction for instrumental mass-dependent isotopic fractionation (instrumental mass bias). Both internal (“double-spiking” using two enriched Mo isotopes) and external (“zirconium doping” with standard-sample bracketing) techniques have been used in previous studies to deal with the effects of instrumental mass bias. The results of these studies have indicated that the precision for Mo isotopic analyses of natural (matrix-bearing) samples is a factor of ∼4–7× better using a double spike. Here we present a detailed study of the ability of MC-ICP-MS to determine, both precisely and accurately, the isotopic composition of Mo extracted from molybdenite using a low blank, high yield two-column procedure for Mo separation and a simple standard-sample bracketing approach to correct for instrumental mass bias. Based on analyses of molybdenites, the precision of this technique is shown to be similar to published double-spike data (within a factor of ∼2). All three of the known types of potential matrix effects in the MC-ICP-MS are also evaluated: automatrix effects, matrix effects due to Zr doping and matrix effects due to elements in the sample other than Mo and Zr. Each of these matrix effects is found to be either insignificant or controllable. Analyses of five molybdenites of hydrothermal origin reveal a range in their Mo isotopic composition that is a factor of ∼4 greater than the previous range reported for such samples. More detailed work is required to elucidate the origin of these mass-dependent Mo isotopic variations in molybdenites. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]