Are the ashes from the latest eruptions (2010–2016) at Turrialba volcano (Costa Rica) related to phreatic or phreatomagmatic events?

The initial eruptive episodes of explosive eruptions are classified as phreatic if the amount of juvenile material (scoria, glass, pumice) is null, and the amount of fresh accidental lithics, and hydrothermally altered lithics, is substantial. Phreatic eruptions have been in some cases recognized as precursory events preceding phreatomagmatic and magmatic eruptive phases. Usually, the lithological features of tephra deposits are investigated and sampled in the field. Investigation of ash samples under binocular microscope or by Back-Scattered Electron (BSE) microscope images of polished sections is usually considered sufficient to typify the fragmentation mechanism of the eruption. The opening eruptive phases at Turrialba volcano, together with the formation of new intracraters (i.e. 2010, 2012, 2014) and the enlargement of the Western Crater (29 October 2014 to present), were classified, in previous papers and internal reports as phreatic. We studied a series of ash samples erupted from 2010 to 2016, with the aim of understanding the fragmentation processes characterizing the vent opening phases. We used SEM + EDS analyses, in addition to field and microscopic investigation. Results showed a composition of accidental lithics of fresh to hydrothermally altered clasts and secondary minerals (82–98%), besides juvenile andesite fragments (2–18%), which leads us to revisit the classification of the initial eruptive phases of Turrialba as phreatomagmatic. Our method allowed the detection of a juvenile component directly involved in an effective magma-water interaction, which was possible only by a scrutinous examination of the glass surface textures by SEM in the range size between 3 and 3.5 phi. We recommend such a type of investigation when the identification of fresh magma in a new eruption is crucial for the preparedness and hazard evaluation at active volcanoes