No 182W excess in the Ontong Java']Java Plateau source

Small-scale W isotope variations in ancient and modern terrestrial rocks provide insights into Earth's accretion and early differentiation history as well as the long-term evolution of the Earth's mantle. Tungsten isotope studies on such rocks have exploited advances in mass spectrometry, both NTIMS and MC-ICPMS, which now permit the determination of W isotope ratios at unprecedented precision. While W isotope studies performed in different labs by MC-ICPMS and NTIMS generally exhibit excellent agreement, obtaining accurate W isotope data at this level of precision remains analytically challenging. For example, a recent NTIMS study reported a relatively large, +24 ppm excess in W-182/W-184 for a Phanerozoic sample from the Ontong Java Plateau (OJP), but no such W-182/W-184 anomaly was found in another study by MC-ICPMS. The present study aims to resolve the discrepancy between these two previous studies, and more generally to evaluate the agreement between different recent W isotope studies by NTIMS and MC-ICPMS. To this end, we report new Wisotope data for OJP drill core samples obtained by MC-ICPMS. The OJP samples analyzed here exhibit no resolvable W-182/W-184 excess relative to the standards and most terrestrial rocks. Moreover, the OJP samples as well as the terrestrial rock standards analyzed here exhibit small but variable W isotope variations for ratios involving W-183, producing coupled variations in both 'radiogenic' (i.e., W-182/W-184) and 'non-radiogenic' (i.e., W-182/W-184) ratios. These W isotope variations are analytical in origin, induced during sample preparation, and very likely caused by a nuclear field shift isotope fractionation affecting primarily the odd isotope (W-183). The recently reported W-182 excess for an OJP sample may result from this nuclear field shift effect, as the NTIMS analyses had to rely on a double normalization involving the W-183/W-184 ratio. More generally, these results demonstrate that using W-183 data from any MC-ICPMS or NTIMS study requires a careful quantification of any potential analytical W-183 effect. Nevertheless, once such effects are taken into account, then both W-182/W-184 and W-183/W-184 can accurately be determined to a very high level of precision.