Mafic enclave diversity at the Soufrière Hills Volcano, Montserrat

ABSTRACT The andesitic Soufrière Hills Volcano, Montserrat has been active since 1995, with five phases of dome growth to date. Mafic enclaves are ubiquitous, and supply of mafic magma is interpreted as the main driver for the current eruption. Recent (Phases IV to V) activity has changed from longer periods of dome growth to shorter more intensive episodes. This is coincident with an increase in the abundance and heterogeneity of mafic enclaves, implying that the changes in activity may reflect differences within the magmatic system. The intense Phase V episode lasted ~18 weeks, culminating in the energetic collapse of Feb. 11th 2010. Within these deposits three distinct mafic enclave types have been identified using textural and geochemical variations: (A) Weakly porphyritic basaltic enclaves; with chilled margins and inherited phenocryst proportions of < 8%. (B) Medium to highly porphyritic basaltic-andesite enclaves; with diffuse margins and inherited phenocryst proportions of 16-25%. (C) Composite; a mixture of types A and B indicating two-stage mixing. Textural and petrological variations also includes differences in: vesicularity, modal mineral assemblages, glass abundance, groundmass size and distribution. Inherited phenocrysts show petrological and geochemical characteristics consistent with late stage heating and inheritance from the andesite. Bulk XRF analyses of Phase V enclaves shows a wide compositional range of 48-58wt% SiO2. An observed SiO2 compositional gap between the mafic enclaves and host andesite in the earlier eruptive phases is no longer present, reflecting increasing hybridisation of the mafic enclaves. The majority of enclaves also have some geochemical characteristics distinctive from the earlier phase enclaves, e.g. higher MgO and lower FeO values. Successive replenishments of mafic magma have led to a more complex mixing regime as observed in the composite enclaves. The overall evolution of the geochemistry, coupled with increased textural variability across enclave types implies a correlation with the change in activity in the latter phases.