Electron probe microanalysis of extremely fine‐grained tephras: new protocols and insights for analyses at the 1 μm scale

ABSTRACT Volcanic ash (tephra) provides an important chronostratigraphic framework for correlating Quaternary records. In ultra‐distal (1500 km) settings such as ice cores and marine sediments, deposits are typically preserved as cryptotephra composed of glass shards m. This poses challenges for accurate geochemical characterization as conventional EPMA–WDS methods require beam diameters of m to limit alkali migration, restricting their application to extremely fine‐grained tephra. Here, we develop and evaluate a quantitative m/1 nA EPMA–WDS method for cryptotephra analysis. Analyses of basaltic to rhyolitic glass standards demonstrate that accurate results are achieved when beam‐sensitive elements are measured early, spectrometer configuration is optimized, and count times are systematically determined by element. Under these conditions, major‐element concentrations deviate by 3% for oxides 1 wt.% and 10% for oxides 1 wt.%, with improved precision relative to existing small‐beam approaches. We validate the m/1 nA method by analyzing cryptotephra from the 1831 CE Zavaritskii eruption (NGRIP1 ice core), producing results indistinguishable from conventional m EPMA–WDS analyses conducted across multiple laboratories. This m EPMA–WDS method extends major‐element analysis to the smallest measurable cryptotephra shards, enabling robust correlation of ultra‐distal archives with their proximal volcanic sources and strengthening the global tephrochronological framework.