Eruption Dynamics Inferred from Microlite Crystallization Experiments: Application to Plinian and Dome-forming Eruptions of Mt. Pelee (Martinique, Lesser Antilles)

Decompression experiments have been conducted to simulate syn-eruptive crystallization in volcanic conduits, to infer magma ascent rates and conditions during dome-forming and Plinian eruptions of silicic arc volcanoes. The experiments were carried out on starting material with the composition of Mt. Pelee rhyolitic interstitial melt (76 wt % SiO2) and consisted of three consecutive steps: hydration, decompression, and annealing. Hydration (saturated and undersaturated) was performed at 850 degrees C and 200 MPa and was followed by isothermal decompression, either linearly or stepwise, to a final pressure, P-f, of 30 or 5-10 MPa. Decompression rates range from 0 center dot 003 to 25 MPa min(-1) (decompression durations of 15 min to 40 days). Two samples were cooled by 25 degrees C and 50 degrees C during a 3 day step at P-f. Subsequent to decompression, the samples were held for up to 15 days at P-f. The experiments generated three types of crystals as products of 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 (P-f), and terminal cooling have been determined on plagioclase nucleation, growth, morphology, and composition. The main results suggest a positive correlation between decompression rate and the number density of plagioclase crystals nucleated at P-f and 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. Pelee eruption dynamics, suggesting that (1) Plinian magmas ascend from the reservoir within less than 1 h (1-10 m s(-1)), (2) dome and block-and-ash flow magmas ascend within more than 2-5 days, giving time for syn-decompression crystallization around pre-existent microlites, (3) dome magmas provide evidence for long stagnation and cooling at low pressure, and (4) surge magmas ascend without significant crystallization (within less than similar to 4 days) and massively nucleate plagioclase at very low pressure. The extent and violence of dome destruction may depend on the size or age of the dome, with large or old domes favouring mildly explosive block-and-ash flows, whereas small or young protodomes may generate highly explosive surges.