Determination of <scp> <i>δ</i> ²⁶ Mg </scp> Values for Geological Reference Materials with Ultra‐Low Magnesium Mass Fractions

The rapidly growing number of studies investigating Mg isotopes in natural systems with ultra‐low Mg mass fractions (e.g., MgO &lt; 0.05% m/m ) highlights the increasing need for precisely calibrated geological reference materials with similarly low Mg levels. However, accurate isotopic characterisation of such materials remains challenging due to limitations in traditional analytical procedures. To address this, we applied the critical mixture double spike (CMDS) technique to perform high‐intermediate precision Mg isotope measurements on nine geological reference materials with low‐Mg mass fractions from China and Japan. Results demonstrate that high‐silica granite JG‐2, alkali feldspar JF‐1, wollastonite GBW03123 and kaolinite GBW03121a exhibit excellent Mg isotopic homogeneity, making them promising candidates for Mg isotope reference materials. In contrast, alkali feldspar JF‐2, GBW03116 and GBW03134 display poorer internal consistency in Mg isotopic compositions among different digestions, likely attributable to a nugget effect arising from heterogeneous Mg distribution within sample powders. Additionally, alkali feldspars exhibit extremely low δ 26 Mg values relative to DSM‐3, ranging from ‐1.05‰ to ‐0.52‰, potentially linked to the high coordination number of Mg in their crystal structures.