Validation of Mg Isotopic Measurements for the Characterisation of Ten Carbonate Reference Materials with Ultra-Low Mg Mass Fractions

Abstract

Magnesium isotope ratios in carbonate rocks and minerals play an important role in tracing geological and biological processes. We report δ²⁶Mg_DSM3 values for the first time in ten carbonate reference materials (GBW07865, GBW07114, GBW(E)070159, GBW07136, GBW07108, GBW03109a, GBW07120, GBW07214a, IAEA-B-7 and IAEA-CO-8) with calcium to magnesium mass ratios around 1300 g g⁻¹ and ultra-low Mg mass fractions of 295 μg g⁻¹. A combination of AG MP-50 (100–200 mesh) and AG 50W-X12 (200–400 mesh) resins for the matrix extraction and multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) were used for the isotopic measurements. This measurement procedure is applicable to materials with high and low Mg mass fractions (e.g., carbonates, lunar highlands anorthosites, ice core). It was validated using various types of reference material. In-house synthetic solutions with 1000 and 1300 g g⁻¹ [Ca]/[Mg] mass ratios yielded δ²⁶Mg_DSM3 values at 2s = ±0.05‰ (n = 62), -4.89‰ and -4.89‰, respectively, indistinguishable from those of the pure Mg solutions, at -4.91‰. δ²⁶Mg_DSM3 values in well characterised carbonatite and carbonate reference materials (such as JDo-1, COQ-1, GBW07133, GBW07217a and GBW07129), with varying MgO and CaO mass fractions, were in agreement with literature values.