From terrestrial volcanic ashes to planetary surfaces:FTIR spectral constraints on eruption style from surge deposits from Vulcano Island (Italy)

The physical behaviour of silicate magmas and their eruptive style are strongly controlled by melt structure, volatile content, and cooling conditions, reflected in spectral properties. Magma rheology and eruptive style are primarily controlled by volatile-driven modifications of melt structure (especially due to H₂O), which govern fragmentation during magma–water interactions, producing fine, lithic-rich tephra. Spectroscopic techniques provide a powerful means to investigate melt structure and pre-eruptive volatile contents, offering insights into eruption dynamics.
We analysed tephra samples from two phreatomagmatic successions on Vulcano Island (Aeolian Arc, Italy), a natural laboratory to investigate relationships among magma composition, volatile content, and eruption style (Keller, 1980; De Astis et al., 1997). Eleven ash-rich layers were sampled. Field measurements included VNIR reflectance spectra acquired with an ASD FieldSpec spectroradiometer and portable Raman spectroscopy. Diffuse reflectance FTIR spectra were collected using a Bruker Invenio X spectrometer on natural and oven-dried samples (105 °C, 48 h) to evaluate adsorbed water. Quantitative spectral parameters were extracted, including band center, full width at half maximum, and area under the curve in the 300–25000 nm domain. We investigate whether VNIR reflectance spectroscopy and laboratory FTIR measurements can identify spectral criteria diagnostic of eruption style in surge-dominated pyroclastic deposits. Preliminary analyses reveal systematic spectral variations related to volatile content and silicate melt structure. The spectra display absorption features attributed to Fe³⁺, molecular H₂O, OH⁻, Al–OH, and Fe–OH vibrations, enabling extraction of band parameters sensitive to hydration state and polymerization degree. Thermal treatment experiments show reduced band areas and spectral slope associated with H₂O and OH⁻ absorptions after heating, indicating that most water in natural samples is weakly bound or adsorbed. However, water loss varies among stratigraphic levels, reflecting differences in glass content, porosity, and hydration history. Variations in Si–O and Al–O band positions and widths indicate differences in silicate network polymerization, with narrower bands and shifts toward higher wavenumbers consistent with evolved compositions. Overall, the spectral signatures are consistent with highly explosive eruptions involving water-rich, evolved magmas and record internal heterogeneity within the eruptive column, marked by progressive degassing during the eruptive event.
This study contributes to the development of spectral reference datasets of terrestrial volcanic materials, essential for interpreting remote sensing data. By linking spectral features to the physical and chemical characteristics of volcanic deposits and their eruptive context, we constrain the nature of volcanic activity on other planetary bodies.

De Astis, G.F. et al., 1997. Volcanological and petrological evolution of the Vulcano Is land Aeolian arc, southern Tyrrhenian Sea. J. Geophys. Res. 102, 8021–8050.
Keller, J., The island of Vulcano, Rend. Soc. Ital. Mineral. Petrogr., 36, 369–414, 1980