Dynamics Of Ascent, Crystallization And Degassing Of Water-Rich Basaltic Magmas: A Melt Inclusion Study

Volatile species (H2O, CO2, S, Cl, F) play a key role in the dynamics of ascent, crystallization and eruption of basaltic magmas. Here I present a synthesis of data on dissolved volatiles probed in olivine-hosted melt inclusions that are representative of water-rich basaltic magmas. Samples were emplaced during explosive eruptions that led to lava fountains and tephra deposits at Mt Etna (2001 and 2002 lateral eruptions) and Vesuvius (over the last three centuries) and to gas jets and production of pumice at Stromboli (historical and present-day activities). Most of the data were recently published: Etna [1,2], Stromboli [3] and Vesuvius [4]. The morphologies of the melt inclusions and their compositions - that either match that of the carrier magma (Etna) or are more primitive (Stromboli, Vesuvius) - reflect differing dynamics of magma crystallization and ascent. As a whole these magmas have moderately to high initial water concentrations (2.7 to 4.5 wt%), with the highest values in K-tephritic magmas of Vesuvius. A large part of the dissolved water and other volatiles is lost at low pressure that induces extensive crystallization (including plagioclase) and significant changes of the physical properties of magma stored in the upper parts of the volcanic conduits. Independently of the initial water concentrations, the evolutions of H2O and CO2 dissolved in melt inclusions of Mt Etna and Vesuvius strongly suggest, in both cases, the re-equilibration of the melt with a CO2-rich gas phase and its dehydration at depth. Such a process could be ubiquitous at open conduit basaltic volcanoes typically having a high CO2 flux. New data on melt inclusions confirm that sulfur is dominantly dissolved as sulfate in these water-rich basaltic melts. They also indicate that sulfite species may be generated during X-ray analysis of water-bearing glasses. This species occurs in experimental glasses only under specific oxidizing conditions [5]. As a whole the total bulk loss of sulfur from magma is >95% whereas the extent of Cl degassing depends on the dynamics of magma ascent and extrusion. The later feature is well illustrated at Mt Etna and Stromboli where magmas show a comparable Cl/H2O ratio. Melt inclusions allowed us (i) to discuss the mechanisms at the origin of lava fountains and (ii) to model the evolution of the dissolved and exsolved S, Cl and F concentrations and the S/Cl and S/F ratios of the gas phase in function of the eruptive style at Mt Etna [2]. These data provide a reference background to interpret the compositional variations of the surface gas emissions. [1] Spilliaert et al., 2006, JGR 111; [2] Spilliaert et al., 2006, EPSL 248; [3] Métrich et al., 2005, GRL 32; [4] Marianelli et al., 2006, GRL 32; [5] Métrich et al., 2005, GCA 69, A51 Suppl.