Magnesium Isotope Compositions of Natural Reference Materials

This study presents a chemical protocol for the separation of Mg that is particularly adapted to alkali‐rich samples (granite, soil, plants). This protocol was based on a combination of two pre‐existing methods: transition metals were first removed from the sample using an AG‐MP1 anion‐exchange resin, followed by the separation of alkalis (Na, K) and bivalent cations (Ca 2+ , Mn 2+ and Sr 2+ ) using a AG50W‐X12 cation‐exchange resin. This procedure allowed Mg recovery of ∼ 10 0 ± 8%. The [Σcations]/[Mg] molar ratios in all of the final Mg fractions were lower than 0.05. The Mg isotope ratios of eleven reference materials were analysed using two different MC‐ICP‐MS instruments (Isoprobe and Nu Plasma). The long‐term reproducibility, assessed by repeated measurements of Mg standard solutions and natural reference materials, was 0.14‰. The basalt (BE‐N), limestone (Cal‐S) and seawater (BCR‐403) reference materials analysed in this study yielded δ 26 Mg mean values of −0.28 ± 0.08‰, −4.37 ± 0.11‰ and −0.89 ± 0.10‰ respectively, in agreement with published data. The two continental rocks analysed, diorite (DR‐N) and granite (GA), yielded δ 26 Mg mean values of −0.50 ± 0.08‰ and −0.75 ± 0.14‰, respectively. The weathering products, soil (TILL‐1) and river water (NIST SRM 1640), gave δ 26 Mg values of −0.40 ± 0.07‰ and −1.27 ± 0.14‰, respectively. We also present, for the first time, the Mg isotope composition of bulk plant and organic matter. Rye flour (BCR‐381), sea lettuce ( Ulva lactuva ) (BCR‐279), natural hairgrass ( Deschampsia flexuosa ) and lichen (BCR‐482) reference materials gave δ 26 Mg values of −1.10 ± 0.14‰, −0.90 ± 0.19‰, −0.50 ± 0.22‰ and −1.15 ± 0.27‰ respectively. Plant δ 26 Mg values fell within the range defined by published data for chlorophylls.