Geochemical evolution of the Soufriere Hills volcano, Montserrat, Lesser Antilles volcanic arc

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 Soufriere Hills suite; the Soufriere 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 greater than or equal to 0.32), whereas the mafic South Soufriere Hills suite evolved in an open system, modelled by continuous magma recharge into a crystallizing reservoir (F greater than or equal to 0.7). The Soufriere Hills andesite compositions are attributed to crystal fractionation of the South Soufriere Hills magmas; however, matrix glass compositions fall on a different trend, consistent with partial melting before eruption. Whole-rock delta(18)O values range from 7.0 to 7.4parts per thousand, 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 (Th-230/Th-232) ratios compared with depleted mantle, and relatively high but constant Nd-143/Nd-144 ratios indicate that the magma source beneath Montserrat is enriched by small (less than or equal to1.2%) 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 Soufriere 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 greater than or equal to350 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.