PRECURSORY GAS EMISSIONS TO THE INITIAL MAGMATIC ERUPTION OF REDOUBT VOLCANO (ALASKA) IN 2009

Measurements of volcanic gas emissions, particularly of CO₂, were critical in assessing the level of unrest of Redoubt Volcano, Alaska, in the six months prior to the magmatic eruption beginning 22 March 2009. In October and November 2008, emission rates of CO₂ were on the order of 1800 t/d and SO₂ was below the detection limit (~ 20 t/d). In January 2009, increases in seismicity at the volcano were accompanied by heightened CO₂ emissions, which reached nearly 10,000 t/d, while SO₂ emissions were measured at an unremarkable level of < 300 t/d. During this period of heightened CO₂, glacial ice covering the 1989-90 lava dome began to melt at a high rate. Analysis of glacial melt water collected downstream from the dome showed slightly elevated SO₄, indicating that some of the SO₂ was reacting with these waters, but not enough to account for the expected loss of SO₂ (thousands of tonnes per day). This suggested either that the magna was too deep to degas significant quantities of SO₂ or that deep hydrothermal waters were efficiently removing the SO₂, or perhaps both. The emission rates of CO₂ and SO₂ and estimates of SO₂ scrubbed by meltwater remained relatively constant until a phreatic eruption on 15 March, when SO2 emission rates increased to over 5000 t/d, suggesting that magma had moved to a shallower level. The impressive CO₂ emissions prior to the eruption suggests that intrusion of a deep CO₂-rich magma into the magma chamber at 6-9 km depth was responsible for triggering the unrest. We believe that only once the magma intruded to a shallow level was SO₂ able to degas freely. These data highlight the potential of CO₂ emission rates to provide a key, but often overlooked, parameter for assessing eruption potential. This finding is especially important for ‘wet’ volcanic systems which have the potential to scrub SO₂.