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

Abstract

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 2 O 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 highlatitude, 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.

Magma at depth: a retrospective analysis of the 1975 unrest at Mount Baker, Washington, USA

“…The level of gas emissions over time is consistent with changes in gas geochemistry, and the levels reached in 1975 (112t/d H 2 S and 950t/d CO 2 ) are similar to those of other hypothesized 'stalled intrusions' in Alaska (Roman et al 2004;Werner et al 2011), but low compared to eruptive emission rates. Emissions during eruptions at ice-clad andesitic volcanoes similar to Mount Baker typically reach levels in excess of 2000t/d CO 2 , commonly exceeding 10,000t/d (Doukas and Gerlach 1995;Hobbs et al 1991).…”

“…SO 2 emission rates can also exceed 1000t/d. H 2 S emission has not been measured extensively during eruptive activity at such volcanoes, but the measurements that do exist suggest very low H 2 S relative to SO 2 emission (Doukas and McGee 2007;Werner et al 2011). No SO 2 has been detected at Mount Baker, which is expected considering that the maximum sampling and equilibrium temperatures are quite low (150°C and 242°C, respectively;Werner et al 2009).…”

First volatile inventory for Gorely volcano, Kamchatka

“…High-temperature (≥700 C) volcanic gases from Japan clearly converge onto low CO 2 /SO 2 (0.4-3) and SO 2 /HCl (1.8-2.8) ratios, similar to those of Gorely and the KK arc. Measurements at poorly accessible Alaskan volcanoes have been more sporadic, in comparison, (see Werner et al [2011] for a recent review), and since the majority of these volcanoes are glaciated the possibility of gas scrubbing merits attention. However, Werner et al [2011] argued that during recent (1980s to present) volcanic unrests at six Alaskan (Cook Inlet) volcanoes, the gas compositions typically converged toward a relatively narrow CO 2 /SO 2 ratio range of 0.5-2, indicating truly magmatic conditions with little or no sulphur scrubbing.…”

Volcanic SO<sub>2</sub> fluxes derived from satellite data: a survey using OMI, GOME-2, IASI and MODIS

“…Changes in SO 2 flux are often used as an eruption precursor (e.g., Olmos et al, 2007;Inguaggiato et al, 2011;and Werner et al, 2011). The SO 2 flux is a marker of important volcanic processes as, in particular: (1) replenishment of the magmatic system with juvenile magma (e.g., Caltabiano et al, 1994;and Daag et al, 1996), characterized by a gradual increase of the measured SO 2 flux over a relatively long time period, (2) volatile exhaustion of a degassing or erupting magma body.…”