Long-term changes in quiescent degassing at Mount Baker Volcano, Washington, USA; Evidence for a stalled intrusion in 1975 and connection to a deep magma source

• Published in: Journal of volcanology and geothermal research Vol. 186; no. 3; pp. 379 - 386
• Main Authors: Werner, C., Evans, W.C., Poland, M., Tucker, D.S., Doukas, M.P.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier B.V 10.10.2009; Elsevier
• Subjects: chemistry; Crystalline rocks; Earth sciences; Earth, ocean, space; emission; Exact sciences and technology; fumarole; gas; hydrothermal; Igneous and metamorphic rocks petrology, volcanic processes, magmas; intrusion; Mount Baker; Cascade Range; United States; Mount Baker; Washington; chemistry; emission; intrusion; fumarole; Mount Baker; gas; hydrothermal; carbon dioxide; andesites; degassing; isotope ratios; volcanic rocks; volcanoes; igneous rocks; helium; magmas; North America; intrusions; lead; crust; fumaroles; basaltic andesite
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2009.07.006

Long-term changes have occurred in the chemistry, isotopic ratios, and emission rates of gas at Mount Baker volcano following a major thermal perturbation in 1975. In mid-1975 a large pulse in sulfur and carbon dioxide output was observed both in emission rates and in fumarole samples. Emission rates of CO 2 and H 2S were ∼ 950 and 112 t/d, respectively, in 1975; these decreased to ∼ 150 and < 1 t/d by 2007. During the peak of the activity the C/S ratio was the lowest ever observed in the Cascade Range and similar to magmatic signatures observed at other basaltic–andesite volcanoes worldwide. Increases in the C/S ratio and decreases in the CO 2/CH 4 ratio since 1975 suggest a long steady trend back toward a more hydrothermal gas signature. The helium isotope ratio is very high (> 7  R c/ R A), but has declined slightly since the mid-1970s, and δ 13C–CO 2 has decreased by ≥ 1‰ over time. Both trends are expected from a gradually crystallizing magma. While other scenarios are investigated, we conclude that magma intruded the mid- to shallow-crust beneath Mount Baker during the thermal awakening of 1975. Since that time, evidence for fresh magma has waned, but the continued emission of CO 2 and the presence of a long-term hydrothermal system leads us to suspect some continuing connection between the surface and deep convecting magma.