Petrological insights into pre- and syn-eruptive degassing at Ambrym volcano, Vanuatu
Abstract This study is a petrological investigation of magmatic conditions and degassing processes at Ambrym volcano, a basaltic island arc volcano recently identified as a prodigious emitter of volcanic gases. Typical activity at this volcano entails heavy degassing from quasipermanent lava lakes, with Strombolian activity of fluctuating intensity. This is punctuated on an approximately decadal timescale by moderate sub-Plinian paroxysms, often accompanied by overflow of lava from the intracaldera cones and via flank fissures. This study focuses primarily on basaltic scoria recently erupted from a central lava lake during a period of heavy degassing, in order to interpret volatile pathways through the system and how these relate to gas emissions at the surface. Lavas from basaltic intracaldera and flank lava flows have also been briefly examined to give a broader insight into magma storage associated with variations in volcanic activity, and detailed study has been made of a trachyandesitic lava flow erupted in 1986, a geochemically unusual event compared to the majority of larger events in historical time. Petrography, major element modelling, mineral chemistry and dissolved volatile contents indicate that Ambrym is a water-poor, CO2-rich system, characterised chiefly by quiescent convection-driven degassing through two central intracaldera cones. Examination of the 1986 trachyandesite suggests that it derives from interaction of basaltic magma with a body of trachydacitic melt residing in the shallow crust.
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