Eruption dynamics inferred from microlite crystallization experiments: Application to Plinian and dome-forming eruptions of Mt Pelée (Martinique, Lesser Antilles).

International audienceDecompression experiments have been conducted to simulate syn-eruptive crystallization in the volcanic conduit, in order to infer magma ascent rates and conditions during dome-forming and Plinian eruptions of silicic arc volcanoes. The experiments were carried out starting from Mt Pelée rhyolitic interstitial melt (76 wt. % SiO2) and consisted in three consecutive steps: hydration, decompression, and annealing. Hydration (saturated and undersaturated) was performed at 850°C and 200 MPa and followed by isothermal decompression, either linearly or stepwise, to a final pressure, Pf, of 30 or 5-10 MPa. Decompression rates range from 0.003 to 25 MPa/min (decompression durations of 15 min to 40 days). Two samples were cooled by 25°C and 50°C during a 3-days step at Pf. Subsequent to decompression, the samples were held up to 15 days at Pf. The experiments generated three types of crystals: pre-, syn-, and post-decompression crystallization. The experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre-decompression crystals on the decompression-induced crystallization. The effects of pre-decompression crystals, decompression rate, undercooling (Pf), and terminal cooling have been determined on plagioclase nucleation, growth, morphology, and composition. The main results i) suggest a positive correlation between decompression rate and the number density of plagioclases nucleated at Pf and ii) highlight the effect of pre-decompression crystals in further decompression-induced crystallization. The relations between the decompression conditions and the plagioclase characteristics have been used to infer Mt Pelée eruption dynamics, suggesting that i) Plinian magmas ascend from the reservoir within less than 1 hour (1-10 m/s), ii) dome and block-and-ash flows magmas ascend within more than 2-5 days, giving time for syn-decompression crystallization around pre-existent microlites, iii) dome magmas evidence long stagnation and cooling at low pressure, and iv) surge magmas ascend without significant crystallization (within less than ~4 days) and massively nucleate plagioclase at very low pressure. The extent and violence of dome destruction may depend on the size/age of the dome, with large/old domes favouring mildly-explosive BAFs, whereas small/young protodomes may generate highly-explosive surges