Gas emissions from failed and actual eruptions from Cook Inlet Volcanoes, Alaska, 1989-2006

摘要

Cook Inlet volcanoes that experienced an eruption between 1989 and 2006 had mean gas emission rates that were roughly an order of magnitude higher than at volcanoes where unrest stalled. For the six events studied, mean emission rates for eruptions were ~13,000 t/d CO_2 and 5200 t/d SO_2, but only ~1200 t/d CO_2 and 500 t/d SO_2 for non-eruptive events ('failed eruptions'). Statistical analysis suggests degassing thresholds for eruption on the order of 1500 and 1000 t/d for CO_2 and SO_2, respectively. Emission rates greater than 4000 and 2000 t/d for CO_2 and SO_2, respectively, almost exclusively resulted during eruptive events (the only exception being two measurements at Fourpeaked). While this analysis could suggest that unerupted magmas have lower pre-eruptive volatile contents, we favor the explanations that either the amount of magma feeding actual eruptions is larger than that driving failed eruptions, or that magmas from failed eruptions experience less decompression such that the majority of H_2O remains dissolved and thus insufficient permeability is produced to release the trapped volatile phase (or both). In the majority of unrest and eruption sequences, increases in CO_2 emission relative to SO_2 emission were observed early in the sequence. With time, all events converged to a common molar value of C/S between 0.5 and 2. These geochemical trends argue for roughly similar decompression histories until shallow levels are reached beneath the edifice (i.e., from 20-35 to ~4-6 km) and perhaps roughly similar initial volatile contents in all cases. Early elevated CO_2 levels that we find at these high-latitude, andesitic arc volcanoes have also been observed at mid-latitude, relatively snow-free, basaltic volcanoes such as Stromboli and Etna. Typically such patterns are attributed to injection and decompression of deep (CO_2-rich) magma into a shallower chamber and open system degassing prior to eruption. Here we argue that the C/S trends probably represent tapping of vapor-saturated regions with high C/S, and then gradual degassing of remaining dissolved volatiles as the magma progresses toward the surface. At these volcanoes, however, C/S is often accentuated due to early preferential scrubbing of sulfur gases. The range of equilibrium degassing is consistent with the bulk degassing of a magma with initial CO_2 and S of 0.6 and 0.2 wt.%, respectively, similar to what has been suggested for primitive Redoubt magmas.