Chlorine variations in the magma of Soufriere Hills Volcano, Montserrat: Insights from Cl in hornblende and melt inclusions

The Soufriere Hills Volcano in Montserrat erupts a Cl-rich, porphyritic andesite. HCl degassing accompanies eruption and is dependent on the growth rate of the lava dome. The magma contains hornblende phenocrysts that show repetitive zoning in most elements, including Cl. On the basis of the zoning data, (Cl/OH) ratios in the melt, calculated from partitioning data, increase rimward through each zone, indicating that the phenocrysts formed under conditions of varying (Cl/OH)(m). An empirical relationship between A-site occupancy in the hornblende and temperature implies that crystallisation of each zone is also accompanied by increasing temperature. Each zone ends at a resorption horizon, and crystallisation recommences at lower temperature and (Cl/OH)(m). Melt inclusion H2O and Cl contents for the 8th January 2007 explosive eruption can be explained by closed-system degassing with D-Cl(fl-m) between 5 and 30, or by open-system degassing accompanied by a small amount of crystallisation. However, neither simple closed-system degassing nor convective circulation of magma can explain the positive correlation of (Cl/OH)(m) with temperature. We suggest that the zoning can be caused by accumulation of CO2-rich vapour in the andesite, probably as a result of mafic magma injection into the chamber. Decreasing H2O fugacity and/or increasing Cl-m result in increasing (Cl/OH)(m) while heat transferred with the volatiles causes the rise in temperature. Intermittently, the accumulated fluid is lost to the surface, possibly as a result of renewed eruptive activity. This model requires the CO2-rich fluid to be decoupled from the magma, consistent with previous observations of continuous CO2 emissions at the surface. (C) 2009 Elsevier Ltd. All rights reserved.