Injection of vesicular magma into an andesitic dome at the effusive–explosive transition

Abstract

A single, vesicular pyroclast collected from a major (11 × 106 m3) dome collapse eruption of the Soufrière Hills volcano, Montserrat (21st September 1997) was found to contain textural and Raman evidence for multiple decompression events and magma injection into the dome just prior to the onset of dome collapse. Quartz, plagioclase and hornblende phenocrysts contain closed and burst melt inclusions (MIs), the latter in the form of either foam or cylindrical melt extrusions from cavities on fracture surfaces. Fractures within ruptured quartz phenocrysts were infilled with glass, whereas those within plagioclase are entirely free of glass, indicating relatively early fracturing (decompression) of the quartz. This interpretation is supported by slightly higher measured H2OT in burst MIs in quartz (0.44 wt.% ± 0.07, 1 s.d.) compared with plagioclase (0.31 wt.% ± 0.05, 1 s.d.), indicating equilibration on rupturing of the quartz at higher PH2O. H2OT levels in glass from burst MIs in plagioclase are consistent with plagioclase fragmentation at H2O saturation pressures < 5 MPa, at an estimated depth of < 210 m, within the dome. The fragmentation occurred whilst the groundmass glass was still plastic, as evident from the presence of stretched vesicles in glass at fracture terminations. The fractures in plagioclase are empty of vapour phase precipitates (mainly cristobalite that infills matrix vesicles) which constrains the fracturing of plagioclase to the last depressurisation event recorded in the pyroclast, which was the dome collapse which led to the pyroclast being ejected from the dome. We suggest that our study provides the first direct (i.e. non seismic) evidence for the injection of gas-charged magma into the dome at the onset of dome collapse, marking the transition from effusive to explosive eruption, and that the magma injection could have provided the triggering mechanism for dome collapse.