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 (Ca2+, Mn2+ and Sr2+) using a AG50W-X12 cation-exchange resin. This procedure allowed Mg recovery of similar to 10 0 +/- 8%. The [Sigma cations]/[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 parts per thousand. The basalt (BE-N), limestone (Cal-S) and seawater (BCR-403) reference materials analysed in this study yielded delta Mg-26 mean values of -0.28 +/- 0.08 parts per thousand, -4.37 +/- 0.11 parts per thousand and -0.89 +/- 0.10 parts per thousand respectively, in agreement with published data. The two continental rocks analysed, diorite (DR-N) and granite (GA), yielded delta Mg-26 mean values of -0.50 +/- 0.08 parts per thousand and -0.75 +/- 0.14 parts per thousand, respectively. The weathering products, soil (TILL-1) and river water (NIST SRM 1640), gave delta Mg-26 values of -0.40 +/- 0.07 parts per thousand and -1.27 +/- 0.14 parts per thousand, 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 delta Mg-26 values of -1.10 +/- 0.14 parts per thousand, -0.90 +/- 0.19 parts per thousand, -0.50 +/- 0.22 parts per thousand and -1.15 +/- 0.27 parts per thousand respectively. Plant delta Mg-26 values fell within the range defined by published data for chlorophylls.