The origin of fresh glass in phreatic eruptions at Rincón de la Vieja and Turrialba volcanoes, Costa Rica Secondary hydration within hydrothermal systems

Abstract

Despite involving no fresh magma, phreatic eruptions are increasingly found to contain juvenile material. This study seeks to determine whether the glass is the result of an input of new magma or shallow stalled magma passively entrained. Using hydrogen isotope analysis (δD) and water speciation (OH/H₂O_mol), we differentiate between fresh magmatic and rehydrated glasses. At Rincón de la Vieja, glasses exhibit H₂O_total contents up to 0.71 wt.% and δD values between –27‰ and –3‰, indicative of secondary hydration by a single hydrothermal source. In contrast, Turrialba samples show a mixed signal; six samples fall within magmatic δD and water content values, while three exhibit δD values from –99‰ to –104‰ and elevated H₂O contents up to 2.29 wt.%, consistent with secondary hydration by local meteoric waters. Rehydration by hydrothermal fluids supports the conclusion that magma was stored at a shallow level and could be entrained during purely phreatic eruptions.