Diversity of eruptive styles inferred from the microlites of Mt Pelee andesite (Martinique, Lesser Antilles)

We have studied the compositional and textural characteristics of plagioclase microlites from several dome-fonning eruptions (dome, block-and-ash flows, and surges) and plinian fallouts of the recent period of activity of Mt. Pelee in order to infer magma ascent dynamics. Dome-fort-ning and plinian samples display a wide range of plagioclase microlite compositions, from An(35) to An(90) and An(55) to An(90), respectively. Microlites with compositions > An(50-60) crystallized from a mafic melt that intruded the andesitic reservoir in proportion of similar to 10 vol.%. During magma ascent in the conduit, the fraction, number density, and morphology of the decompression-induced microlites (compositions < An50-60) suggest that the plinian magmas experienced a nucleation-dominated crystallization regime with high undercooling. In contrast magmas from domes and block-and-ash flows were dominated by a growth regime with low undercooling, and the surges were influenced by both regimes. According to these crystallization regimes and new experimental data on H2O content of the matrix glasses, we propose magma relative ascent rates and fragmentation pressures for the different eruptive styles observed at Mt. Pelee that may explain why surges are explosive events and why they may or may not be followed by a plinian event. (c) 2007 Elsevier B.V. All rights reserved.