Improved understanding of the magmatic system of Soufrière Hills Volcano, Montserrat (SHV) is needed to inform future hazard management strategy, and remaining uncertainties include the depth of magma storage and the source of ongoing gas emissions. Eruptive activity between 1995 and 2010 has been proposed to be sourced from either a dual chamber or transcrustal mush-based magmatic system, with volatile solubility models using H 2 O and CO 2 from melt inclusion (MI) glass estimating depths of 5–6 km. To date, published SHV MI volatile data have neglected the vapour bubbles now known to sequester the bulk of MI magmatic carbon. Total CO 2 concentrations in SHV magma are therefore underestimated, together with volatile-derived entrapment pressures and inferred magma storage depths. Here, we present a new dataset of volatile (H 2 O and total CO 2 ) and major element concentrations in plagioclase- and orthopyroxene-hosted SHV MI, that span almost all of the eruptive activity (Phases 1, 2, 4, and 5), and include the first measurement of bubble-hosted CO 2 for SHV and indeed the Lesser Antilles Arc. Analyses were conducted using Raman spectroscopy, ion microprobe, and electron probe analysis. Dacitic–rhyolitic MI occur within andesitic whole rock compositions. Volatiles in MI glass are similar to published studies (H 2 O 2.47–7.26 wt%; CO 2 13–1243 ppm). However, bubble-hosted CO 2 contributes 9–3,145 ppm, to total inclusion CO 2 with 5%–99% (median 90%) of CO 2 sequestered within bubbles, and total CO 2 concentrations (131–3,230 ppm) are significantly higher than previously published values. Inferred entrapment depths from our dataset range from 5.7 to 17 km – far greater than previous estimates – and support a vertically elongated magmatic system where crystallisation spanned both upper- and mid-crustal depths. Our CO 2 measurements enable new estimation of CO 2 sources and fluxes. As a total of 4.5 Mt of CO 2 was held in SHV magma during the aforementioned phases, the maximum amount of CO 2 that can be emitted from a batch of SHV magma is ∼1500–1750 tonnes/day. Measured CO 2 fluxes are significantly higher, indicating additional input of CO 2 into the system from greater depths. Our study shows that including bubble-hosted CO 2 redefines understanding of the SHV plumbing system.

Melt inclusion bubbles provide new insights into crystallisation depths and CO2 systematics at Soufrière Hills Volcano, Montserrat