Gd Matrix Effects on Eu Isotope Fractionation in Geological Rocks Using MC-ICP-MS: Optimizing Europium Isotope Ratio Measurements in Geological Samples

Abstract

<p></p><p>Eu has only two isotopes (151Eu and 153 Eu). Eu and Gd are one of the rare earth elements that are very difficult to completely separate from each other. Eu isotope ratio can be determined by MC-ICP-MS using internal Sm or Gd spikes to correct for mass discrimination. NIST3117a ultrapure chemical reagent shows almost no Eu isotope fractionation regardless of the kind of normalization isotope pair. However, Eu isotope ratio in the silicate rocks was effected by Gd matrix during MC-ICP-MS measurement if a trace amount of Gd impurity remains in the purified Eu fraction. In this report, we tried to determine optimizing conditions for precise and accurate Europium isotope ratio measurements in geological samples using MC-ICP-MS. The pure Eu fraction with almost no Gd matrix separated from geological samples and NIST3117a ultrapure chemical reagent show almost same degree of Eu isotope fractionation regardless of the kind of normalization isotope pair. However, Eu isotope ratio in the silicate rocks was effected by Gd matrix during MC-ICP-MS measurement using if 154 Gd interference relative to 154 Sm as internal standard is more than ca. 0.1%. Particularly, highly fractionated granite and high silica volcanic rock with extremely low Eu concentration compared to Gd require high – purity Eu separation with a high recovery rate to obtain the true value of the Eu isotope fractionation in the geological rocks. <br></p><br><p></p>

Europium isotope fractionation in highly fractionated igneous rocks with large Eu negative anomaly

“…Recently, Lee and Tanaka (2019) reported a method for determining highly precise and accurate Eu isotope ratio with analytical precision better than ±0.02‰ (2SD) using Sm ( 150 Sm/ 154 Sm) as an internal standard in combination with standard-sample-standard bracketing mass bias correction (C-SSBIN) for ultrapure Eu standard reagents produced by NIST3117a, Aldrich and Perkin Elmer Ltd. However, we noticed a possibility that there may exist artificial Eu isotope fractionation in the geological materials due to incomplete purification between Eu and Gd (Lee and Tanaka, 2021). This incomplete separation is due to 1) the very high content difference between Eu (for example, less than 0.1 ppm in JG2) and Gd (8.10 ppm in JG2) in the highly fractionated granite, and 2) the similar and overlapping distribution coefficient between HIBA and AG50W-X8 resin during column chromatography.…”

“…This indicates that Eu isotope ratio from the geological rocks using MC-ICP-MS had better be determined by Sm normalization method (C-SSBIN) rather than by SSB without Sm normalization under the condition with almost complete Eu purification from Gd. In addition, Lee and Tanaka (2021) proposed that 147 Sm-149 Sm or 147 Sm-154 Sm pair in determination Eu isotope ratio from geological rocks is better than any other as Sm isotope pair for normalization in Eu isotope ratio determination by MC-ICP-MS because of matrix effect due to trace amounts of Gd impurity. JG3 in Fig.…”

“…The magnitude was calculated based on the values of Sm, Eu and Gd from the reference. 4) Eu isotope data are from Lee and Tanaka (2021).…”

“…In addition, those (blue solid symbols) determined by SSB without Sm normalization show more severe fractionation than those by C-SSBIN. Lee and Tanaka (2021) interpreted that these values may indicate pseudo-fractionation of Eu isotope. Generally, Gd concentration in the highly fractionated igneous rocks is 100 times higher than Eu, which cause incomplete Eu purification.…”