Andesitic magma degassing investigated through H 2O vapour–melt partitioning of halogens at Soufrière Hills Volcano, Montserrat (Lesser Antilles)

Abstract

Magma degassing at Soufrière Hills Volcano (SHV) is characterised by an almost permanent SO 2 flux and a HCl production rate which mainly depends on dome growth rate. Degassing processes have been studied through textural, H 2O and halogen analyses of clasts collected between 1995 and 2006 on the dome and in pyroclastic flows and vulcanian eruption deposits. Cl, Br and I are strongly depleted in melts during H 2O degassing with no significant Cl–Br–I fractionation, whereas F is almost unaffected. All magmas erupted at SHV have followed a multi-step degassing path from the magma chamber up to a shallow depth (∼ 1 km, P ∼ 20 MPa). From that depth, however, effusive and explosive paths are distinct; vulcanian eruptions are the result of closed system degassing (CSD), while effusive dome growth is the result of CSD up to a very shallow depth (≤ 200 m, P ∼ 5–2 MPa) followed by open system degassing (OSD). CSD is modelled using the H 2O solubility law, the perfect gas law and partition coefficients of halogens between a rhyolitic melt and H 2O vapour ( d v − l i ). Gas loss characteristic of OSD is modelled using a Rayleigh law. Degassing induced crystallisation is introduced through the ratio of crystallisation and degassing rates, which ranges from 150–500. d v − l Cl for OSD ranges between 50–300, increasing with melt Cl content. For CSD, the lower effective d v − l Cl (∼ 20) is attributed to kinetic effects. Dome forming activity has a greater impact on atmospheric chemistry than vulcanian eruptions because OSD is much more efficient at extracting halogens. The model shows that HCl flux is a good proxy for the dome forming eruption rate. Comparison between model and measured gas compositions suggests a high HBr–BrO conversion rate (BrO/Total Br ∼ 1/3) in the SHV gas plume. The degassing behaviour of Cl, Br and I implies similar Cl/Br (∼ 160) and Br/I (∼ 90) in initial melts, volcanic clasts and high temperature gases. The low Cl/Br at SHV compared to other island arcs (∼ 250–300) is attributed to a shallow, pre-eruptive Br enrichment. The almost permanent dome extrusion at SHV since 1995 has likely had a significant regional atmospheric impact because of the very efficient effusive degassing and the high conversion rate of halogens into reactive species within the gas plume.