Stable Isotopic Studies of Magmatic Degassing

I. Triggering of Mount St. Helens Dome Eruptions

We analyzed water contents and hydrogen isotope analyses of Mount St. Helens dome lavas. The detailed sampling allowed us to compare dD and water content of similarly textured (smooth or vesicular) and positioned (near vent or distal) samples from different dome extrusions. Comparing samples of similar texture and position eliminated isotopic variations that originate during surface flow, and enabled us to look for degassing processes that occurred in the chamber between eruptions. We found that lavas erupted after long repose periods were the most deuterium depleted, consistent with a model where closed system degassing occurs in the magma chamber during repose periods, followed by open system degassing during ascent to the surface, and a final stage of kinetically-controlled gas loss just before cooling. The detection of a kinetic stage of lava degassing had not been detected before this study, but several recent publications have documented this degassing mechanism at other volcanoes, including Mount Unzen (Japan). This work was the cover story in the October 12, 1989 issue of Nature (Anderson and Fink, 1989). Dr. Jon Fink, my former graduate advisor, was the primary collaborator on this project.

Above: Photo of the Mount St. Helens dome in 1985.

II. Comparison of Degassing Processes at the Santiaguito (Guatemala) and Mount St. Helens dome

We used a similar approach to the one described above to study a more complicated series of eruptions at the Santiaguito lava dome in Guatemala. This lava dome has been continually active since 1922, and has gone through several cycles of growth. We found that the lavas are extremely degassed, and no vesicular lava has been erupted since the 1920's. Based on our work (Anderson et al., 1995) we believe that when lava domes reach a critical size, they begin to control the behavior of the volcano as a whole. In other words, a big, strong dome can inhibit the rise of lava to the surface, allowing it to thoroughly degas in transit. As lava domes grow, we expect that subsequent eruptions will require more magmatic pressure to initiate an eruption, but the lavas that reach the surface will lose most of their volatiles during rise from the magma chamber through the conduit. Dr. Jon Fink and Dr. Bill Rose (Michigan Tech) were collaborators on this project.

Small phreatic eruption from the Caliente vent at the Santiaguito lava dome, as viewed from the summit of Santa Maria volcano.