HR: 0800h
AN: V51B-0561    [Abstracts]
TI: The Ins and Outs of U and Th Isotopic Measurements Using a Nu Plasma 1700 MC-ICP- MS
AU: * Pietruszka, A J
EM: apietrus@geology.sdsu.edu
AF: Department of Geological Sciences, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182-1020, United States
AU: Rooney, T O
EM: rooneyt@msu.edu
AF: Department of Geological Sciences, San Diego State University, 5500 Campanile Dr., San Diego, CA 92182-1020, United States
AB: The Nu Plasma 1700 is a relatively new type of large-geometry MC-ICP-MS that offers the potential to improve the accuracy and precision of U and Th isotopic measurements compared to previous instruments (e.g., the Plasma 54-30, or P54-30). The higher resolution of the Nu Plasma 1700 translates to more than a factor of 6 improvement in the abundance sensitivity at high mass (compared to the P54-30), and virtually eliminates the necessity of a tail correction on ²³⁰Th during the measurement of ²³²Th/²³⁰Th ratios (the major source of uncertainty using the P54-30). However, the Nu Plasma 1700 uses a discrete dynode secondary electron multiplier for ion-counting, which has a smaller dynamic range than the Daly detector used by the P54- 30. This prevents the placement of a relatively large ²³⁵U signal on the ion-counter, and thus, requires alternative methods to correct for the effects of instrumental mass bias. Initial efforts in our laboratory have focused on the development of standard-sample bracketing (SSB) techniques to correct for mass bias, with a careful evaluation of possible instrumental artifacts such as matrix effects, drift in the bias between the Faraday collectors and the ion-counting detector (Faraday/IC bias), and non-linearity of both the ion-counting detector and the retardation lens. Experiments with solution standards suggest that the major source of uncertainty on U and Th isotopic measurements using the Nu Plasma 1700 in SSB mode is drift in the Faraday/IC bias. Nevertheless, SSB analyses for U and Th solution standards are reproducible to 0.3% for ²³⁸U/²³⁴U and 0.4% for ²³²Th/²³⁰Th (±2σ), which is similar to the results obtained on the Plasma 54-30 (Luo et al., 1997; Pietruszka et al., 2002). So far, accuracy has been evaluated by repeatedly analyzing the UCSC Th isotopic standard against a previously characterized in-house Th isotopic standard in SSB mode. The result agrees within error of previous determinations for UCSC Th using the Plasma 54-30. Our future development work will focus on a more thorough characterization of the accuracy and precision of the SSB method and a detailed exploration of alternative techniques to correct for mass bias.
DE: 1040 Radiogenic isotope geochemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting