Textures, water content and degassing of silicic andesites from recent plinian and dome-forming eruptions at Mount Pelee volcano (Martinique, Lesser Antilles arc)

Previous petrological and phase-equilibrium experimental studies on recent silicic andesites from Mount Pelee volcano have evidenced comparable pre-er uptive conditions for plinian and dome-forming (pelean herein) eruptions, i mplying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the Mount Pelee magmas during eruptio n. To further investigate the degassing conditions of plinian and pelean ma gmas of Mount Pelee, we study here the most recent Mount Pelee's products ( P1 at 650 years B.P., 1902, and 1929 eruptions, which cover a range of plin ian and pelean lithologies) for bulk-rock vesicularities, glass water conte nts (glass inclusions in phenocrysts and matrix glasses) and microtextures. Water contents of glass inclusions are scattered in the plinian pumices bu t on average compare with the experimentally-deduced pre-eruptive melt wate r content (i.e., 5.3-6.3 wt.%), whereas they are much lower in the dominant pelean Lithologies (crystalline, poorly vesicular lithics and dome samples ). This indicates that the glass inclusions of the pelean products have und ergone strong leakage and do not represent pre-eruptive water contents. The water content of the pyroclast matrix glasses are thought to closely repre sent the residual water content in the melt at the time of fragmentation. D etermination of the water contents of both the pre-eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn-eru ptive degassing. We find the amount of water exsolved during the eruptive p rocess to be higher in the pelean products than in the plinian ones, typica lly 90-100 and 65-70% of the initial water content, respectively. The vesic ularities calculated from the amount of exsolved water compare with the mea sured vesicularities for the plinian pumices, consistent with a closed-syst em, near-equilibrium degassing up to fragmentation. By contrast, the low re sidual water contents, low groundmass vesicularities and extensive groundma ss crystallization of the pelean products are direct evidence of open-syste m degassing. Microtextural features, including silica-bearing and silica-fr ee voids in the pelean lithologies may represent a two-stage vesiculation. (C) 2000 Elsevier Science B.V. All rights reserved.