Isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of Mount St. Helens

Angela N Seligman; Ilya N. Bindeman; Alexa R. Van Eaton; Richard P. Hoblitt

doi:10.1007/s00445-018-1212-6

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Isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of Mount St. Helens

Angela N Seligman·Ilya N. Bindeman·Alexa R. Van Eaton·Richard P. Hoblitt

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Abstract

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 reconstructed 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.

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Bulletin of Volcanology Article

Source-reported bounding extent: 46.1322667089571° to 46.31848113932307° latitude; -122.33001708984374° to -122.01690673828124° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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Angela N Seligman, Ilya N. Bindeman, Alexa R. Van Eaton, Richard P. Hoblitt. 2018-03-17. Isotopic insights into the degassing and secondary hydration of volcanic glass from the 1980 eruptions of Mount St. Helens. https://doi.org/10.1007/s00445-018-1212-6

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