Resolution of inter-laboratory discrepancies in Mo isotope data: an intercalibration

Abstract

The molybdenum (Mo) stable isotope system has been applied to a variety of geochemical and environmental problems. In the absence of a universally accepted zero-delta reference material, different groups report their data relative to their adopted in-house standards. Rigorous comparison of results generated in different laboratories using different analytical approaches is only possible if the in-house standards are of identical Mo isotope composition. To determine potential isotopic differences among various standards, the δ⁹⁸Mo (⁹⁸Mo/⁹⁵Mo) values of ten Mo standard solutions were measured as part of this study. For six of these solutions, four laboratories carried out an intercalibration. In contrast to previous results, δ⁹⁸Mo of various in-house standards were found to differ by up to 0.37‰. Renormalisation of our new and published Mo-isotope data available for seawater taken from various sites and the USGS rock reference material SDO-1 relative to NIST-SRM-3134, provides a much better agreement among reported δ⁹⁸Mo values for these samples. Relative to NIST-SRM-3134, the δ⁹⁸Mo of SDO-1 is 0.80 ± 0.14‰ (2s), while oxic, open-ocean seawater is characterised by an average δ⁹⁸Mo of 2.09 ± 0.10‰ (2s). This intercalibration provides a solid platform for comparing and amending existing δ⁹⁸Mo values. In addition, we recommend that future Mo isotope studies adopt NIST-SRM-3134 as a universal zero-delta reference material.