Extensive, water-rich magma reservoir beneath southern Montserrat

•Water in pyroxenes from volcanoes in Montserrat preserve a record of the water content of the melt in which they grew.•The record is used to infer melt water contents up to 9 wt%.•The magma reservoir extends to > 12 km into the crust, where magmas become vapor-saturated.•Water loss in the rims of the crystals suggests fast magma ascent from > 5 km over 5–13 hours prior to a vulcanian explosion.•SSH and SHV magmas are geochemically distinct, despite being only 2 km from eachother and magmas stored at similar depths.•A vertically protracted reservoir exists beneath southern Montserrat, containing heterogeneous bodies of eruptible magma.

South Soufrière Hills and Soufrière Hills volcanoes are 2 km apart at the southern end of the island of Montserrat, West Indies. Their magmas are distinct geochemically, despite these volcanoes having been active contemporaneously at 131–129 ka. We use the water content of pyroxenes and melt inclusion data to reconstruct the bulk water contents of magmas and their depth of storage prior to eruption. Pyroxenes contain up to 281 ppm H2O, with significant variability between crystals and from core to rim in individual crystals. The Al content of the enstatites from Soufrière Hills Volcano (SHV) is used to constrain melt-pyroxene partitioning for H2O. The SHV enstatite cores record melt water contents of 6–9 wt%. Pyroxene and melt inclusion water concentration pairs from South Soufriere Hills basalts independently constrain pyroxene-melt partitioning of water and produces a comparable range in melt water concentrations. Melt inclusions recorded in plagioclase and in pyroxene contain up to 6.3 wt% H2O. When combined with realistic melt CO2 contents, the depth of magma storage for both volcanoes ranges from 5 to 16 km. The data are consistent with a vertically protracted crystal mush in the upper crust beneath the southern part of Montserrat which contains heterogeneous bodies of eruptible magma. The high water contents of the magmas suggest that they contain a high proportion of exsolved fluids, which has implications for the rheology of the mush and timescales for mush reorganisation prior to eruption. A depletion in water in the outer 50–100 μm of a subset of pyroxenes from pumices from a Vulcanian explosion at Soufrière Hills in 2003 is consistent with diffusive loss of hydrogen during magma ascent over 5–13 h. These timescales are similar to the mean time periods between explosions in 1997 and in 2003, raising the possibility that the driving force for this repetitive explosive behaviour lies not in the shallow system, but in the deeper parts of a vertically protracted crustal magma storage system.