Phases in fine volcanic ash

Area of Science:

• Geochemistry
• Volcanology
• Atmospheric Science

Background:

• Volcanic ash impacts climate, ecosystems, and health, but its composition and size-dependent properties are poorly understood.
• Existing datasets lack detailed phase information for ash sizes relevant to atmospheric transport and widespread effects.
• Understanding ash composition is crucial for accurate modeling of volcanic impacts.

Purpose of the Study:

• To characterize the phase composition of size-separated volcanic ash fractions.
• To investigate how phase composition varies with grain size, tectonic setting, and bulk chemistry.
• To develop methods for estimating ash phase proportions and density with limited data.

Main Methods:

• X-ray diffraction analysis of 40 ash samples from VEI 2-6 eruptions.
• Size separation of ash into fractions.
• Statistical analysis including regression modeling.

Main Results:

• Fine ash (<45 μm) shows increased crystalline silica and salts, decreased glass and iron oxides.
• Arc and intraplate samples are distinguished by feldspar and clinopyroxene fractions.
• Regressions were developed to estimate glass and major crystal phase proportions.
• Pore-free ash density is well-estimated by clinopyroxene + Fe-oxide fraction, differing between intraplate and arc samples.

Conclusions:

• This study provides a valuable dataset for understanding volcanic ash phase composition and its variability.
• The developed regressions and density estimations improve constraints on atmospheric and environmental ash budgets.
• Accurate ash characterization is essential for refining models of explosive volcanism's impact on the Earth system.