Quantification of the Intrusive Magma Fluxes during Magma Chamber Growth at Soufriere Hills Volcano (Montserrat, Lesser Antilles)

Magma fluxes in the crust control the thermal viability and mechanical stability of magma chambers. We estimated the magma fluxes required to generate the negative seismic velocity anomaly observed below Soufrière Hills volcano, Montserrat. Growth of a magma body by accretion of andesitic sills was simulated numerically and the resulting temperatures and melt fractions were used to calculate a synthetic anomaly of seismic wave velocity, which was filtered to be comparable with the velocity anomaly obtained from a tomographic experiment. Petrology indicates that before it was reheated, remobilized and erupted, the temperature of the magma residing in the chamber was about 850°C. We ran simulations where convection is assumed to be low and heat transfer is mostly by conduction and simulations where convection is assumed to be vigorous enough to rapidly cool the magma chamber to 850°C. In both cases, magma chamber growth over the last 350 years results in tomography anomalies that are too strong, unless the magma was emplaced at an unlikely low melt fraction (<0·5). Good fits between the modelled and the observed velocity anomaly were obtained with sills 2–5 km in radius emplaced over 6000–150 000 years, depending on the temperature and melt fraction of the emplaced magma. Because of a trade-off between intrusion dimensions and emplacement durations, the volumetric magma fluxes are restricted to 7 × 10?4 and 5 × 10?3 km3 a?1. The velocity anomaly can be reproduced with a chamber containing high melt-fraction magma or with a mush of crystals and melt. The range of magma ages in the modelled magma chamber is much wider than the crystal residence time of the erupted andesite. This suggests that the eruption taps small pockets of recently assembled magma and that the velocity anomaly is mostly due to a non-eruptible mush

Upper crustal structure of an active volcano from refraction/reflection tomography, Montserrat, Lesser Antilles.

To better understand the volcanic phenomena acting on Montserrat, the SEA-CALIPSO seis-mic experiment (Seismic Experiment with Airgun-source – Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) was conducted in 2007 December with the aim of imaging the upper crust and the magmatic system feeding the active Soufri ?ere Hills Volcano. The 3-D survey covered an area of about 50 × 40 km and involved the deployment of 247 land stations and ocean-bottom seismometers (OBSs). A subset of the data, recorded by four OBSs and four land stations on a southeast to northwest line, has been analysed, and traveltimes have been inverted to obtain a 2-D seismic velocity model through the island. Inverted phases include crustal and sediment P waves and wide-angle reflections. The resulting velocity model reveals the presence of a high velocity body (3.5–5.5 km s?1 ) beneath the island, with highest velocities beneath the Soufri ?ere and Centre Hills, cor responding primarily to the cores of these volcanic edifices, built of a pile of andesite lava domes and subsequent intrusions. In the off-shore region, velocities in the surficial sediment layer vary from 1.5 to 3.0 km s?1 , consistent with a mainly calcareous and volcaniclastic composition. A wide-angle reflector is observed at a depth of ?1200 m below the seabed, and appears to deepen beneath the island. The upper crust beneath this reflector has velocities of 4.0–6.0 km s?1 and is infer red to cor respond to plutonic and hypabyssal rocks and sedimentary material of the old arc. The high velocity region beneath the island, extends into the crust to a depth of at least 5 km, and is believed to be caused by an intrusive complex, possibly of intermediate composition. A low velocity zone, as would be expected in the presence of an active magma chamber, was not observed perhaps due to the limited resolution beneath ?5 km depth. Our results so far provide the first wide-angle seismic constraints on the upper crustal structure of the island to a depth of 10 km, and will help understanding the processes that drive volcanism at Montserrat and other island arc volcanoes