Water in volcanic pyroclast: Rehydration or incomplete degassing?

• Positive correlation between water concentration and bulk porosity of volcanic samples.
• Most water concentrations can be explained solely by post-eruptive rehydration of dry glass.
• Most samples underwent high degrees of open-system degassing.
• Few samples from explosive eruptions require primary water concentration of ~1 wt%.
• No water-rich and porosity-poor samples because of a permeability threshold.

The matrix-glass water concentrations in samples from volcanic eruptions of intermediate to highly silicic magmas were measured and compiled. They range from 0.1 wt% to more than 3.5 wt% and show a positive correlation with vesicles surface area over glass volume ratio. Modeling of water diffusion suggests that most of this correlation can be explained by the post-eruptive diffusion of external water at atmospheric temperature and pressure into the matrix-glass, a process referred to as rehydration. Although the precise proportion of primary (magmatic) to secondary (meteoric) water is not determined by our analysis, we find that most samples can be modeled by progressive rehydration of an initially ‘dry’ sample during the time interval between deposition and sample collection at an average rehydration diffusivity of approximately 10−23 m2 s−1. This diffusivity estimate is consistent with values provided in the literature on obsidian hydration dating and with the extrapolation of diffusivity formulations for silicic melts to lower temperatures and pressures.