Hydrogen-isotope evidence for extrusion mechanisms of the Mount St Helens lava dome

Abstract

FROM October 1980 to October 1986 the dacite lava dome at Mount St Helens grew through the accumulation of more than 15 separate lobes during extrusive episodes that lasted from 1 to 24 days, except for an anomalous period of continuous growth throughout most of 1983¹. Over these six years, the growth rate was generally volume predictable; that is, the amount of lava added to the dome in a given eruption was roughly proportional to the time since the preceding extrusion². These well documented eruptions were separated by repose intervals lasting from several months to more than a year². In an attempt to identify the triggering mechanisms for these periodic eruptions, we have used hydrogen isotope analyses to determine water content and deuterium content for 18 samples of the Mount St Helens dome dacite. These isotope data, the first ever collected from an active lava dome, suggest a steady-state process of magma evolution combining crystallization-induced volatile production in the chamber with three different degassing mechanisms: closed-system volatile loss in the magma chamber, open-system volatile release during ascent and kinetically controlled degassing upon eruption at the surface. The data suggest that future dome-building eruptions may require a new influx of volatile-rich magma into the chamber.