Pre-Eruptive Changes in Physical Conduit Dynamics Recorded in the Final Stage of Phenocryst Growth

Physical changes within a volcanic conduit prior to eruption will affect the processes that immediately follow, including the explosivity and/or duration of the eruptive event. These physical changes, which may include pressurization and heating, will be recorded in crystals that continue to grow as a result of cooling or decompression-induced undercooling in the shallow subsurface. Anorthite (An) values of plagioclase feldspar will vary as a result of pressure or temperature changes, as shown through petrologic experiments on H2O-saturated magmas. Potassium, an incompatible element in plagioclase, may vary in abundance as a result of temperature changes in the magma. Secondary ion mass spectrometry (SIMS) in depth-profiling mode has revealed late-stage changes in the An content of plagioclase phenocrysts that correlate with eruptive style. Plagioclase phenocrysts were extracted from explosive (pumice flows) and effusive (block-and-ash flows) eruptive deposits at Soufrière Hills Volcano (SHV), Montserrat. Crystals from both clast and matrix material were sputtered with an O2+ primary ion beam to a total depth of 18-25 microns. Effusive crystals display very gradual changes in An, with no abrupt variations observed in most of the samples. However, some effusive samples show An increases of 10-20% in the final micron of growth, but do not display a corresponding increase in K. In the effusive crystals, K is ~1000 ppm, but may deviate to slightly higher values when An contents decrease. Explosive crystals show more dramatic An changes 10-20 microns into the crystal surface, but never exhibit the abrupt change in the final micron of growth as observed in some effusive samples. Increased An often (but not always) correlates with increased K concentrations. Optical examination of the depth-profiled crystals reveals that regions of rising An content and stable K values are apparently free of glass and mineral inclusions. Regions with lower An and higher K values appear to represent plagioclase containing quenched melt. Increases in An content, with no corresponding increase in K, appear to represent rising pressure, not temperature, within the environment. We interpret variations in the effusive crystals as oscillatory zoning with some samples displaying a final period of pressurization. We interpret abrupt An and K changes in the explosive samples as a combination of pressurization and heating events. We are currently examining other lines of evidence to constrain pressure and temperature variations within the magma immediately prior to eruption.