Geochemical evolution of the Soufrière Hills volcano, Montserrat, West Indies

The geochemical evolution of Montserrat provides an important background to understanding the current activity of this island arc volcano. Here we present major and trace element, and U-, Th- and O-isotope data for rocks generated in the last 300 kyr that provide constraints on the magmatic processes occurring beneath the volcano. Samples range from low- to medium-K calc-alkaline basalts to dacites. Three suites can be distinguished on the basis of major and trace element compositions: the South Soufrière Hills suite; the Soufrière Hills suite, including the lava from the current eruption; and the mafic inclusions. Magmatic differentiation of the magma that crystallized to form the mafic inclusions appears to have been governed by closed-system processes, modelled by fractional crystallization (F 0·32), whereas the mafic South Soufrière Hills suite evolved in an open system, modelled by continuous magma recharge into a crystallizing reservoir (F 0·7). The Soufrière Hills andesite compositions are attributed to crystal fractionation of the South Soufrière Hills magmas; however, matrix glass compositions fall on a different trend, consistent with partial melting before eruption. Whole-rock 18O values range from 7·0 to 7·4, and are, therefore, slightly enriched compared with primitive arc lavas. This might be due to magmatic fractionation, or the assimilation of up to 20% hydrothermally altered arc crust. Extremely low Nb/Th ratios and low (230Th/232Th) ratios compared with depleted mantle, and relatively high but constant 143Nd/144Nd ratios indicate that the magma source beneath Montserrat is enriched by small (1·2%) amounts of sediment, which was added from the subducting slab probably as a partial melt. High U/Th ratios and large ion lithophile element abundances relative to local sediments suggest that fluid-mobile elements from the dehydrating slab were also added to the wedge, and that the fluid signature in the South Soufrière Hills samples is stronger than in the mafic inclusions. U–Th isotopes are close to secular equilibrium, suggesting that the transfer time of the fluid signature from source to surface is 350 ka. In conjunction with evidence for magma remobilization at Montserrat, much of this time may represent crustal residence, suggesting long time scales of deep-level differentiation relative to the inferred rapid crystallization at shallower levels.