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

A procedure was developed to accurately measure the W isotopic compositions of iron meteorites with a precision of better than +/- 0.1 epsilon on epsilon W-182 and epsilon W-184 (normalized to W-186/W-183). 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 epsilon W-182 variations as small as similar to 0.1 epsilon can be detected. Analyses of three different aliquots of the Gibeon (IVA) iron meteorite obtained over a period of 6 months show identical epsilon W-182 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 postdated (by up to similar to 2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure epsilon W-184 accurately and precisely (within +/- 0.1 epsilon), 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 epsilon W-180, albeit with large error bars (<+/- 4 epsilon, in most cases).