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 epsilon on epsilon182W and epsilon184W (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 epsilon182W variations as small as approximately 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 epsilon182W 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 approximately 2.5 My) the formation of refractory inclusions. We demonstrate our ability to measure epsilon184W 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 epsilon180W, albeit with large error bars (<+/-4 epsilon, in most cases).