Direct accurate Eu anomaly analysis in very high Ba/Eu silicate samples by triple-quadrupole ICP-MS in MS/MS mass shift mode

Abstract

The concentration of Eu relative to neighbouring rare earth elements (REE) is an important tool in petrology and geobiology. In most sample matrices, precise and accurate middle REE concentration data can be obtained by direct ICP-MS analysis with careful correction of interferences (chiefly, BaO+ on Eu+ and NdO+ on Gd+). However, geologically interesting samples, including sediments from mid-ocean-ridge and other hydrothermal settings, certain limestones, Precambrian cherts, and some alkali-feldspars have such high Ba/Eu ratios that signals on both Eu isotopes are dominated by BaO. This necessitates high resolution mass spectrometry, highly accurate oxide-correction in these matrices, or requires the separation of the REE from the Ba-rich matrix prior to ICP-MS analysis. Here we report experiments with a triple-quadrupole ICP-MS where Eu was analysed as the mass shifted EuO+ after reaction with N2O or O2 for analysis of Palaeoarchaean cherts. Results were compared to conventional REE analysis with a generic oxide correction as well as individual sample oxide correction. It was found that the triple-quadrupole ICP-MS method with N2O reaction and the conventional ICP-MS analysis with individual sample oxide correction yielded indistinguishable middle REE data for samples with Ba/Eu of up to 125,000. The other two methods overestimated the Eu concentration in samples with Ba/Eu > 10,000, leading to spuriously high positive Eu anomalies, which could be misinterpreted as dominant high-temperature hydrothermal venting into the ancient ocean, for which there is currently no convincing evidence. The triple-quadrupole ICP-MS method with N2O reaction offers a fast, low-blank and high-sensitivity solution to REE analysis of high Ba/Eu geological matrices. Based on the insight from these experiments, we propose criteria with which legacy Eu anomaly data can be assessed for accuracy and inclusion into deep time secular trend studies.