U-series histories of magmatic volatile phase and enclave development at Soufrière Hills Volcano, Montserrat

Abstract

Injection of volatile-rich mafic magma prior to an eruption may trigger episodes of volcanism and can act to transfer metals from depth. However, petrologic knowledge of the timescales from mafic injection to eruption have thus far been focussed on mineral-scale studies of chemical zoning patterns. The study of mafic enclaves dispersed within eruption products can provide insights into the interaction between deep and shallow reservoirs. We combine ²³⁸U-²³⁰Th-²²⁶Ra-²¹⁰Pb isotope data with trace element concentrations across the interface of two contrasting mafic enclaves in contact with their host andesite from the 2010 eruption at Soufrière Hills Volcano (SHV), Montserrat to investigate the history of mass exchange between the mafic enclave and the andesite host. The application of these time-sensitive isotopes highlights complexities in the transfer of volatiles and metal elements between magmas and the enclaves' potential as eruption triggers. The enclaves exhibit (²¹⁰Pb/²²⁶Ra)₀ ratios >1 consistent with volatile input to the subsurface plumbing system a few decades prior to eruption. Samples of the andesitic host, however, which make up the bulk of the eruptive products, have (²¹⁰Pb/²²⁶Ra)₀ ≤ 1 suggesting no net volatile gain in the decades leading up to eruption, or that melt-volatile interaction is on a timescale unresolvable by ²¹⁰Pb—²²⁶Ra systematics (i.e. <2 years). Variations in trace elements such as Cu, Pb and Ba show loss of a magmatic volatile phase and transport of metals within the deeper part of the plumbing system during differentiation of magmas feeding SHV. Our results do not support that volatile transfer into the andesite via enclaves is a direct trigger of explosive eruptions although the enclaves are likely syn-eruptively formed. ²³⁸U-²³⁰Th-²²⁶Ra-²¹⁰Pb and trace element systematics at SHV support a role for fresh magma influx during periods of unrest, but long-term accumulation of the andesite.