Volatile behaviour in the 1995-2010 eruption of the Soufrière Hills Volcano, Montserrat recorded by U-series disequilibria in mafic enclaves and andesite host

Abstract

Mafic enclaves brought to the surface in volcanic eruptions offer insight into the deeper sections of a volcanic plumbing system, where the degree of interaction between deeper, mafic magma and the magma reservoir directly feeding a volcanic edifice is not well-constrained. At Soufrière Hills Volcano (SHV) on the island of Montserrat, Lesser Antilles, mafic enclaves have been a ubiquitous feature of the andesitic eruptions during the five phases of the 1995-2010 eruption. We use the short-lived isotopes 210Pb and 226Ra within the Uranium series decay chain, which are sensitive to volatile transfer and loss (due to the loss or gain of the intermediary daughter 222Rn), in both the enclaves and the host andesite to reveal significant time-related information. The sequentially erupted andesites are almost entirely in equilibrium or have deficits of 210Pb with the deficits becoming more pronounced over several eruptive phases. We model that the andesitic reservoirs involved were subject to continuous volatile loss both before and during the eruption. The majority of enclaves, however, have excesses of 210Pb, showing volatile enrichment that lasted over a decade. The highest (210Pb/226Ra)0 ratios are from enclaves in Phase II, indicating that the deeper mafic system was closed to fresh gas influx from Phase III onwards. Enclaves are modelled as being sporadically entrained in the andesite prior to eruption, suggesting that enclave formation is not a triggering mechanism for each eruptive phase.