Magma storage conditions and control of eruption regime in silicic volcanoes: experimental evidence from Mt. Pelée

Differences of eruption regimes in silicic volcanoes, e.g. effusive versus explosive, have commonly been ascribed either to stratification of volatiles in the magma storage region or to gas loss through permeable conduit walls. Recent Plinian and Pelean eruptions of silicic andesite magmas from Mt. Pelée (P1: 650 yr B.P., 1902, 1929) show no systematic variations in bulk rock and phenocryst and glass compositions. Rare coexisting FeTi oxide pairs in Pelean products yieldT between 840 and 902°C, and ΔNNO between +0.4 and +0.8. Pre-eruptive melt H2O contents, calculated from plagioclase-melt equilibria, span values from 1.9 to 5.5 wt%. Glass inclusions from the P1 Plinian fallout have H2O contents between 4.2 and 7.1 wt%. In contrast, the Pelean inclusions have H2O contents commonly <3 wt%, due to post-entrapment modifications upon eruption. Phase equilibrium studies allow pre-eruptive conditions to be precisely determined and demonstrate that recent eruptions, either Plinian or Pelean, tapped magmas with melt H2O contents of 5.3–6.3 wt%, stored at 2 ± 0.5 kbar, 875–900°C and ΔNNO = +0.4-0.8. Differences in eruptive style at Mt. Pelée are unrelated to systematic variations in pre-eruptive magmatic H2O concentrations, but may be caused by contrasting modes of degassing in the conduit.