Electron Probe Microanalysis of Extremely Fine-Grained Tephras: New Protocols and Insights for Analyses at the 1 µm Scale

Volcanic ash (tephra) from large eruptions is dispersed over broad regions and serves as an important chronostratigraphic marker at regional to hemispheric scales. In ultra-distal (>1500 km) archives such as polar ice cores and marine sediments, the preserved tephra is typically <10 μm in diameter, making accurate major-oxide analysis a persistent challenge for characterizing and correlating these deposits. Although advances in tephra extraction techniques have increased glass shard recovery from these archives, the analytical limitations of characterizing extremely fine-grained glass remain a primary constraint.

This study develops and evaluates a 1 μm/1 nA EPMA–WDS method, implemented without additional external software, that achieves high spatial resolution without inducing significant Na or K migration across low-, intermediate-, and high-silica reference glasses. Method performance strongly depends on the element-to-spectrometer configuration and element-specific count times. Without optimization, both factors induce alkali migration resulting in an up to 6% decrease from standard reference values for either Na or K, particularly in high-silica or alkali-rich compositions. Following parameter optimization, the 1 μm/1 nA configuration yields major-oxide results that deviate by less than 3% from reference values for concentrations exceeding 1 wt.% and 10 wt.% when concentrations are less than 1 wt.%The method improves precision by up to a factor of seven for extremely fine-grained shards compared with existing approaches, including the 3 μm/1 nA configuration, the 3 μm small-beam overlap method, and SEM–EDS at 1 nA.

The optimized configuration exceeds the performance reported in previous assessments of a 1μm/1 nA setup, indicating that the recommended current-density thresholds may be overly conservative. Applied to in-situ, ice-core cryptotephra, the method resolves compositional differences at sufficient granularity to distinguish separate eruptions and discrete eruptive phases. As ultra-distal deposits often contain sparse populations of <10 μm shards, by increasing the quantity of analyzable shards using a reliable 1 μm method, we improve the statistical power of correlation tests between proximal tephra deposits and distal to ultra-distal tephras.

The optimized approach thus facilitates integration of multiple depositional environments within the global tephrochronological framework and provides a reliable analytical method applicable across laboratories with differing instrumental capabilities. The optimized approach facilitates the integration of multiple depositional environments within the global tephrochronological framework while providing EPMA analysts with a reliable, high spatial resolution method.