High-precision measurement of europium isotopic composition of geological reference materials by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS)

Abstract

To date only few analyses of europium (Eu) isotopic composition of terrestrial and extraterrestrial samples have been reported in the literature. This isotopic systematic remains largely understudied and it is still unknown whether the Eu isotopic composition of the Earth's major reservoirs show measurable variations. Here we present a comprehensive chemical protocol to separate Eu from different sample matrices and a method for measuring high-precision Eu isotopic composition by multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). We show that the chemical separation of Eu by ion chromatography may induce measurable isotopic fractionation and that careful attention must be taken to limit analytical artefacts. As Eu has only two stable isotopes, we investigated different methods to correct as best as possible for instrumental mass bias. We found that doping the samples with samarium (Sm), a close neighbor of Eu, provided the best precision and trueness. Based on complete duplicate analyses, we evaluated the repeatability standard deviation (2SD) of the whole procedure at about 50 ppm on the ¹⁵¹Eu/¹⁵³Eu ratio (i.e. 0.5 ε unit), which is two to four times better than previously published procedures. All igneous and sedimentary reference materials analyzed in this study have Eu isotopic compositions similar to the reference standard NIST 3117a within uncertainties (i.e. ε_Eu = 0 ± 0.5).