RESEARCH ARTICLE Isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of Mount St. Helens Angela N. Seligman 1 & Ilya Bindeman 1 & Alexa Van Eaton 2 & Richard Hoblitt 2 Received: 17 March 2017 /Accepted: 27 February 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018Abstract The magmatic degassing history of newly erupted volcanic glass is recorded in its remaining volatile content. However, this history is subsequently overprinted by post-depositional (secondary) hydration, the rates and origins of which are not yet adequately constrained. Here, we present the results of a natural experiment using products of the 1980 eruptions of Mount St. Helens. We measured water concentration, δD glass , and δ 18 O BSG (δ 18 O of the bulk silicate glass) of samples collected during the dry summer months of 1980 and compared them with material resampled in 2015 from the same deposits. Samples collected from the subsurface near gas escape pipes show elevated water concentrations (near 2.0 wt%), and these are associated with lower δD glass (- 110 to - 130‰) and δ 18 O BSG (6.0 to 6.6‰) values than the 1980 glass (- 70 to - 100‰ and 6.8 to 6.9‰, respectively). Samples collected in 2015 from the surface to 10-cm subsurface of the 1980 summer deposits have a small increase in average water contents of 0.1–0.2 wt% but similar δ 18 O BSG (6.8–6.9‰) values compared to the 1980 glass values. These samples, however, show 15‰ higher δD glass values; exchange with meteoric water is expected to yield lower δD glass values. We attribute higher δD glass values in the upper portion of the 1980 deposits collected in 2015 to rehydration by higher δD waters that were degassed for several months to a year from the hot underlying deposits, which hydrated the overlying deposits with relatively high δD gases. Our data also contribute to magmatic degassing of crystal-rich volcanoes. Using the 1980 samples, our recon- structed δD-H 2 O trends for the dacitic Mount St. Helens deposits with rhyolitic groundmass yield a trend that overlaps with the degassing trend for crystal-poor rhyolitic eruptions studied previously elsewhere, suggesting similar behavior of volatiles upon exsolution from magma. Furthermore, our data support previous studies proposing that exsolved volatiles were trapped within a rapidly rising magma and started degassing only at shallow depths during the 1980 eruptions. Keywords Secondary hydration . Volcanic degassing . Mount St. Helens . Hydrogen isotopes . Diffusion . Volcanic glass . Dissolved water concentrations Introduction Processes of degassing of silicic magma in volcanic chimneys and on Earth’ s surface continue to attract active discussion, as it relates to volcanic explosiveness, ignimbrite welding, and fumarolic activity. Effusive, slowly rising silicic eruptions lead to nearly complete degassing, suggesting efficient separation of water from glass at 1 atm pressure, leading to obsidian lavas with as little as 0.1 wt% water (Taylor et al. 1983; Anderson and Fink 1989; Loewen and Bindeman 2015). In contrast, volcanic glass from explosive eruptions usually retains greater amounts of magmatic water (0.1–0.6 wt%), reaching up to 1.6 wt% H 2 O t (total water including molecular and hydroxyl) for rapidly quenched bombs (Newman et al. 1988; Castro et al. 2014; Seligman et al. 2016). Past discussions of water in volcanic glass involved batch versus Rayleigh separation of volatiles, depth of quench, the importance of volatile flux, rehydration, and heterogeneity of the distribution of water and deuterium in eruptive products (Taylor et al. 1983; Newman et al. 1988; Gardner et al. 2017). Upon eruption, transport in the eruption plume may also affect the water con- tent of the glass, due to increased solubility of water in glass with decreasing temperature during cooling (Westrich and Editorial responsibility: M.I. Bursik Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00445-018-1212-6) contains supplementary material, which is available to authorized users. * Angela N. Seligman angie.seligman@gmail.com 1 Department of Earth Sciences, University of Oregon, Eugene, OR 97403, USA 2 David A. Johnston Cascades Volcano Observatory, US Geological Survey, Vancouver, WA 98683, USA Bulletin of Volcanology (2018) 80:37 https://doi.org/10.1007/s00445-018-1212-6