Petrological and experimental constraints on magma storage for large pumiceous eruptions in Dominica island (Lesser Antilles)

Abstract

The general question of the generation of large-volume silicic eruptions is here addressed through the experimental determination of the storage conditions of the primary magmas that generated ignimbritic eruptions at Dominica Island (Lesser Antilles) during the 24–51 ka period of time. The basal plinian fallouts and pumice pyroclastic flows from the large-volume (~ 5 km3 DRE) events of Layou, Roseau and Grand Fond were investigated, together with the smaller ignimbritic eruptions of Grand Bay and Grande Savane. All samples are dacitic (63–66 wt% SiO2) and contain ~ 30 vol% phenocrysts of plagioclase (~ 21 vol%), orthopyroxene (~ 5 vol%) and Fe-Ti oxides (< 1 vol%), in a rhyolitic matrix glass. The most differentiated samples contain additional amphibole (up to 5 vol%) and quartz. Crystallization experiments were performed starting from Layou and Roseau pumice samples at 800 to 900 °C, 200 to 400 MPa, ~ ΔNNO + 1 and for H2O-saturated and H2O-undersaturated conditions. The main phase contents, assemblages and compositions of both natural samples were reproduced experimentally at ~ 850 °C, ΔNNO + 0.6, 7–8 wt% melt H2O and ~ 400 MPa (~ 16 km depth) consistent with magma ponding at the mid-crustal discontinuity. There is also evidence of more differentiated magma batches that may reflect a plumbing system with a significant vertical extension. The relationships between the chamber depth, width and volume argue for eruptions that do not form collapse calderas, in agreement with field evidence. The erupted magma volumes in Dominica are more than five times larger than those emitted in the neighbouring islands (Martinique, Guadeloupe, Montserrat; < 1 km3), which may be explained by a locally extensional tectonic context that favoured assembly of large magma bodies, but also by the rarity of frequently draining upper crustal reservoirs (as evidenced on the neighbouring volcanic systems) that favoured deep accumulation of large volumes of magma during this period and time for differentiation to dacitic compositions.