Remobilization of Andesite Magma by Intrusion of Mafic Magma at the Soufriere Hills Volcano, Montserrat, West Indies

Abstract

The 1995–1999 eruption of the Soufriere Hills volcano, Montserrat, has produced a crystal-rich andesite containing quench-textured mafic inclusions, which show evidence of having been molten when incorporated into the host magma. Individual crystals in the andesite record diverse histories. Amphibole phenocrysts vary from pristine and unaltered to strongly oxidized and pseudomorphed by anhydrous reaction products. Plagioclase phenocrysts are commonly reverse zoned, often with dusty sieve textures. Reverse zoned rims are also common on orthopyroxene phenocrysts. Pyroxene geothermometry gives an average temperature of 858 ± 20°C for orthopyroxene phenocryst cores, whereas reverse zoned rims record temperatures from about 880 to 1050°C. The heterogeneity in mineral rim compositions, zoning patterns and textures is interpreted as reflecting non-uniform reheating and remobilization of the resident magma body by intrusion of hotter mafic magma. Convective remobilization results in mixing together of phenocrysts that have experienced different thermal histories, depending on proximity to the intruding mafic magma. The low temperature and high crystallinity are interpreted as reflecting the presence of a cool, highly crystalline magma body beneath the Soufriere Hills volcano. The petrological observations, in combination with data on seismicity, extrusion rate and SO₂ fluxes, indicate that the current eruption was triggered by recent influx of hot mafic magma.