Analytical developments for high-precision measurements of W isotopes in iron meteorites

Abstract

A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than ±0.1 ε on ε¹⁸²W and ε¹⁸⁴W (normalized to ¹⁸⁶W/¹⁸³W). Purification of W was achieved through a two-step, ion-exchange procedure. In most cases, the yield is better than 80%, and purified W solutions are clear of matrix elements and direct isobars of W. The final W solutions were analyzed using a Micromass Isoprobe multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Tests performed on mixtures of terrestrial standards and meteorite samples demonstrate that the method is accurate and that ε¹⁸²W variations as small as ∼0.1 ε can be detected. Analyses of three different aliquote of the Gibeon (IVA) iron meteorite obtained over a period of 6 months show identical ε¹⁸²W values with a weighted mean of 3.38 ± 0.05, consistent with literature data for IVA iron meteorites, and indicating that the metal-silicate differentiation event in its parent body was either contemporaneous with or slightly post-dated (by up to ∼2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure ε¹⁸⁴W accurately and precisely (within ±0.1 ε), which is useful for characterizing cosmogenic and nucleosynthetic effects that may be present in iron meteorites. We also report for the first time measurements of ε¹⁸⁰W, albeit with large error bars (<±4 ε, in most cases). © 2007 American Chemical Society.