HR: 11:35h
AN: V31G-06    [PDF]
TI: Petrological Constraints on the Decompression History of Magma Prior to Vulcanian Explosions at the SoufriŠre Hills Volcano, Montserrat
AU: * Clarke, A
EM: amanda.clarke@asu.edu
AF: Department of Geological Sciences, Arizona State University, Tempe, AZ 871404 United States
AU: Stephens, S
AF: Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AU: Teasdale, R
AF: Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AU: Sparks, R
AF: Department of Earth Sciences, University of Bristol, Bristol, BS8 1RJ United Kingdom
AB: A series of 75 Vulcanian explosions occurred at the SoufriŠre Hills volcano from September 21st to October 22nd 1997. The densities, textures and compositions of plagioclase microlites have been documented for 15 pumice samples. Pre-explosion conduit conditions have been inferred from these data by comparison with experimental decompression experiments of Couch et al. (Journal of Petrology 44, 1477-1502, 2003) on a representative groundmass composition for Montserrat, which established empirically-based relationships between the decompression history and plagioclase composition, plagioclase proportions, crystal size distributions and crystal shape. The natural samples all show very high number densities of microlites (2 x 10$^{3}$ to $>$5 x 10$^{5}$ crystals per mm$^{2}$) and $>$90% of microlites have areas less than 20 square microns. Only the most rapidly decompressed experimental samples even approached reproducing these features of nucleation-dominated crystallization observed in the pumices, although none of the experimental samples yielded number densities that exceeded 10$^{3}$ per mm$^{2}$. Most experiments were confined to pressures of above 30 MPa with a trend from growth-dominated to nucleation-dominated crystallization for fast decompression rates and lower final pressures. Comparison of natural samples to experimental results suggest quench pressures in the range of 20 to 40 MPa. Surface observations indicate a cylindrical conduit with radius of 15 m. Estimated time-averaged flow rates associated with the period of Vulcanian explosions give values of 9 to 13 m$^{3}$/s. If the conduit cross-sectional area were constant with depth and magma flow was steady, then the corresponding ascent rates should produce growth-dominated textures with a much higher proportion of coarse-grained ($>$20 square microns) crystals produced during decompression from 130 to 20 MPa. The following model reconciles pumice sample data with experiments and field observation. We propose that the conduit is not of uniform cross-sectional area. Instead, it is likely that at depths of greater than 1 to 1.5 km the conduit is a narrow dyke which then merges into a wider cylindrical conduit. Each Vulcanian explosion evacuates much of the shallow conduit system. Then, due to the sudden evacuation of the upper conduit and the narrow dyke at depth, magma ascends rapidly (more rapidly than the time-averaged flux rates suggest) to fill up the shallow conduit. The quickly decompressed magma is therefore strongly undercooled and gas supersaturated, conditions which lead to nucleation-dominated microlite crystallization, to subsequent gas pressure increase and ultimately to the next explosive eruption.
DE: 3600 MINERALOGY AND PETROLOGY (replaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting