Magma chamber dynamics at Soufriere Hills volcano, Montserrat

Implicit in active, steady-state magmatic systems is their persistent activity and unchanging behavior in terms of composition and eruptive styles. The ongoing eruption (July 1995 -- April 2010) of the Soufriere Hills volcano (SHV), Montserrat, provides an ideal setting for understanding this steady-state behavior. Mafic enclaves are ubiquitous in andesitic magma erupted from SHV. The mafic enclaves are basalt to basaltic andesite (49 -- 56 wt. % SiO2) in composition. Based on their chemistry, mineralogy and petrology, they are divided into three types. Prior to intrusion, basaltic magma underwent significant differentiation of amphibole at deep crustal levels. Type 1 (T1) and Type 2 (T2) enclaves represent hybrid magmas which are a mixture of differentiated basaltic magma and the host andesite, while Type 3 (T3) enclaves represent basaltic magma which ponded prior to intrusion and underwent significant additional fractionation of plagioclase. The T1 enclaves sample a vesiculated upper portion of the mixing horizon, while the T2 enclaves sample a less vesiculated, deeper, and slightly more rigid portion of this horizon. The T3 enclaves were near the temperatures of the andesite reservoir at the time of their intrusion; they demonstrate mixing on a physical mixing only, i.e., crystal transfer. The T1 enclaves formed when they reached buoyancy due to vesiculation and detached from the mixing horizon to rise upward in the andesite, whereas T2 enclaves formed during subsequent intrusions, during mafic overturn. The SHV demonstrates periodic and regular explosive activity, for which we can quantify changes in volatile content over time. Volatile analyses from phenocryst-hosted melt inclusions sampled from andesitic pumice cluster at 2.8 -- 5.4 wt. % H2O, with ~ 3000 ppm Cl and negligible CO2. We interpret these volatile contents to mirror conditions in the lower conduit and upper magma reservoir beneath the volcano. Our model of the SHV magmatic system suggests that 1) the mafic magma is providing heat, mass and volatiles to the magmatic system, 2) the magmatic system is unable to evolve to more felsic compositions due to buffering by this mafic magma, 3) CO2 is degassed and lost from the mafic magma as it rises from the deep crust, 4) S exsolves during mixing of the mafic magma and andesitic host and 5) explosive eruptions are sampling magma which is stored in the upper portions of the magma chamber.