Geochemical evidence for relict degassing pathways preserved in andesite

Andesitic arc volcanoes degas large quantities of volatiles; evidence for vapour transport in the erupted lavas is rarely preserved and poorly understood, but is crucial for understanding eruption style. We present geochemical evidence for the transport of metal-bearing vapour in shear zones preserved in lavas erupted from Soufrière Hills Volcano, Montserrat. Textural evidence suggests that shear-induced brittle failure occurred in a narrow zone (at the conduit wall or in the lava dome). Elevated metal concentrations (Cu, Au, Ag, Pb, Zn) within the zones indicate that the fractures acted as a transient pathway for metal-bearing magmatic gases. During slip, frictional heating to temperatures of >1000 °C caused partial melting at the slip surface. Resorption of volatiles and metals into the partial melt preserved the geochemical signature of magmatic vapour in the shear zone. Cordierite, which is highly unusual in volcanic rocks, crystallised from the peraluminous partial melt, with metal-bearing sulphides and oxides. The shear zones provide the first geochemical evidence for vapour segregation and transport through viscous andesite magmas and provide an insight into controls on eruption style.