A <scp>CO</scp>₂‐gas precursor to the <scp>M</scp>arch 2015 <scp>V</scp>illarrica volcano eruption

Abstract

Abstract We present here the first volcanic gas compositional time‐series taken prior to a paroxysmal eruption of Villarrica volcano (Chile). Our gas plume observations were obtained using a fully autonomous Multi‐component Gas Analyser System (Multi‐GAS) in the 3 month‐long phase of escalating volcanic activity that culminated into the 3 March 2015 paroxysm, the largest since 1985. Our results demonstrate a temporal evolution of volcanic plume composition, from low CO 2 /SO 2 ratios (0.65‐2.7) during November 2014‐January 2015 to CO 2 /SO 2 ratios up to ≈ 9 then after. The H 2 O/CO 2 ratio simultaneously declined to &lt;38 in the same temporal interval. We use results of volatile saturation models to demonstrate that this evolution toward CO 2 ‐enriched gas was likely caused by unusual supply of deeply sourced gas bubbles. We propose that separate ascent of over‐pressured gas bubbles, originating from at least 20‐35 MPa pressures, was the driver for activity escalation toward the 3 March climax.