Tungsten stable isotope compositions of terrestrial samples and meteorites determined by double spike MC-ICPMS

Abstract

Tungsten stable isotopes hold great potential to examine a variety of physical and chemical processes operating during the accretion and differentiation of asteroids and terrestrial planets, such as core formation and mantle–crust differentiation. To assess the magnitude and origin of W isotope fractionations, we determined the W stable isotopic compositions of six USGS geological reference materials, a NIST steel, and 24 iron meteorites and chondrites. The W isotope data were obtained using multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) and using a 180W–183W double spike. Chondrites and iron meteorites exhibit a very narrow range in W stable isotope compositions, resulting in a mean δ184/183W=0.027±0.007‰ (95% conf.) relative to the NIST 3163 W standard. This value represents a good estimate for the W stable isotope composition of bulk planetary bodies from the inner solar system. The δ184/183W of some iron meteorites slightly deviates from this value, most likely due to W isotope fractionations induced during crystallization of the metal cores of iron meteorite parent bodies. The investigated terrestrial silicate rocks exhibit a narrow range in δ184/183W, which for most samples is indistinguishable from the mean value of chondrites and iron meteorites. However, felsic samples tend to be isotopically lighter than mafic samples, indicating that magmatic processes on Earth induced W isotope fractionations. These fractionations are possibly related to the fluid-mobility of W in subduction zones, but more data are needed to test this hypothesis. Given that most terrestrial igneous rocks are isotopically indistinguishable from chondrites and iron meteorites, core formation on Earth does not seem to have induced a measurable isotopic fractionation for W. However, more data are needed to firmly arrive at this conclusion.