The Gas Factory: segregated exsolved volatiles derived from unerupted magmas fuel arc eruptions and persistent degassing

Abstract:

Open vent, persistently degassing volcanoes in subduction zones, such as Stromboli (Italy), Villarica (Chile), Bagana and Manam (Papua New Guinea), Soufriere Hills (West Indies), Fuego and Pacaya volcanoes (Guatemala), among others, produce high gas fluxes and infrequent lava dome, violent strombolian or ‘paroxysmal’ eruptions. Gas fluxes produced at these volcanoes are far higher than can be supplied by erupting magma and are integral to the dynamics of such systems. The excess gases are generated during prolonged vapour-saturated fractionation of water-rich basalt in crustal reservoirs, which produces a segregated exsolved hydrous volatile phase which may flux through overlying shallow magma reservoirs. Sulfur and chlorine and other minor species partition into the exsolved volatile phase. The surface manifestation of such activity is quiescent and persistent open vent degassing (irrespective of eruption of magma) and gas-rich explosive and dome-building activity. The excessive gas fluxes at the surface are a sign of extensive intrusive magmatic activity in the crust beneath, aided in many cases by extensional tectonics in the overriding plate. Here we illustrate the conceptual picture of gas-dominated arc volcanoes driven by extensive magmatic intrusion and degassing at depth, based on our recent work characterising the gas and petrological systematics of the volcanic examples above.