Mount Pelée andesitic stratovolcano alternates plinian and dome-forming eruptions during its recent period of activity. The causes for either explosive or effusive behaviour of the magma supposedly lie in the degassing and crystallization processes during magma ascent in the volcanic conduit. Therefore, porosity and microlite features have been studied in both plinian and dome-forming products. The plagioclase microlites from the plinian, pyroclastic flow and surge deposits reveal a similar very large compositional range (An47-88) and similar morphological features. The present 1902 and 1929 domes show an additional set of plagioclase microlites larger and more acid in composition (An32-85) than in the other deposits. The results of isobaric crystallization experiments starting from the rhyolitic matrix glass and from a basalt similar to enclaves in the recent andesite suggest that the natural microlites grew from a basaltic liquid composition under magma chamber conditions. A simple re-heating of the andesitic magma body cannot account for the most calcic microlite compositions. However, the An32-44 microlite population in the domes could have been crystallized from the rhyolitic matrix melt at sub-surface conditions. The magma from the pyroclastic flows and surges that lack the microlites grown from the rhyolitic matrix melt, may have risen up within less than 2 days, based on the experimental data. Then the present domes, which emplaced after the pyroclastic flow events, crystallize acid microlites from the matrix melt at low pressures. The experiments provide further information on timescales and water loss during the degassing process in the volcanic conduit. In conclusion, the microlites from Mount Pelée andesite do not seem to play any significant role in the syn-eruptive dynamic processes, while degassing does. However, such inherited microlites may provide useful information on the magma mixing process such as the involved proportions, the pressure-temperature conditions, and mixing timescales.
Petrological and experimental constraints on microlite crystallization and degassing at Mount Pelée (Antilles).