Characterizing eruption initiation mechanisms and storage conditions at the high-threat Kolumbo volcano, Greece.

Crystals and glass discharged as tephra during explosive volcanic eruptions provide a valuable window into magmatic processes. Kolumbo, a shallow submarine volcano on the Hellenic Arc of the southern Aegean Sea, recently erupted in 1650 CE and was both violently explosive and tsunamigenic, resulting in significant loss of life on neighboring Santorini. Developing a conceptual model of the magma storage and eruption processes at Kolumbo is crucial for improving preparedness in this volcanically active and densely populated region. Using in situ tephra deposits cored during the International Ocean Discovery Program Expedition 398, this study aims to reconstruct the temporal evolution of the magma plumbing system at Kolumbo over the two most recent eruptive units at Kolumbo (1650 CE and an earlier eruption). This research aims to (1) infer pre-eruption magmatic processes and eruption initiation mechanisms, (2) determine magma storage conditions, including pressure, temperature, and H₂O content, and (3) assess temporal variations in the magmatic system at Kolumbo. Methods include the traditional petrologic toolbox of major- and trace-element geochemistry, focusing on minerals and glass to delineate crystallization histories, identify zonation patterns, and infer pressures and temperatures of the crystallizing assemblages. Preliminary results suggest significant differences in eruption initiation mechanisms and magmatic histories between the two eruptive units. The 1650 CE tephra exhibits multiple populations of phenocrysts that preserve textures such as reverse zoning and sieving, consistent with mafic recharge into a silicic reservoir as suggested by previous research. Plagioclase-hosted melt inclusions reveal water content of 4.0–5.25 wt.% for 1650 CE at a storage temperature of 807–847ºC, determined using plagioclase-liquid hygrometry and thermometry (Waters and Lange, 2015; Putirka, 2008). Clinopyroxene-liquid barometry (Petrelli, 2020; Jorgenson, 2022) identifies two clinopyroxene populations, yielding pressures of 4 and 6 kbar, respectively (±0.3 kbar) and temperatures of 914–953ºC (±<3ºC) supportive of mafic injection from a deeper magma source that triggered the 1650 CE eruption. The newly analyzed older eruption was initiated by some other mechanism than mafic injection - either heating from below or depressurization of a highly evolved silicic reservoir. Plagioclase-liquid hygrometry and thermometry indicate water contents of 4.25–5.0 wt.% at a storage temperature of 797–817ºC, though pressures remain unconstrained. These findings offer critical insights into the contrasting magmatic processes driving eruptions at Kolumbo, highlighting the dynamic interplay between mafic recharge and silicic storage.