The good, the bad, and the bubbly: Textural controls on secondary vesiculation of silicic magma through thermal heating
Volcanic eruptions are driven by magma buoyancy caused by volatiles exsolving to form a separate gas phase. Gas overpressurization within the melt and subsequent fragmentation determines whether an eruption will be explosive (fragmentation) or effusive (no fragmentation). Therefore, a thorough understanding of factors which affect the ability of a melt to exsolve and retain or lose these volatiles effectively is required. Bubble nucleation and subsequent growth are the processes by which volatiles exsolve from the melt into an exsolved gas phase. As continued vesiculation occurs, bubbles may begin to interact and coalesce at a point called the percolation threshold, permeable pathways through the melt will then form until the permeability threshold is reached allowing volatiles to outgas, reducing overpressure. Currently, research has focused on the effects that pressure, temperature, and composition have on volatile solubility and eventual vesiculation. However, erupted bombs consisting of assemblages of heterogenous pyroclasts that were subsequently sintered and welded into a conduit-forming plug during the Cordón Caulle 2011-2012 eruption display vesiculation trends within naturally occurring obsidian pyroclasts formed by sintering of particles or quenched melt that cannot be resolved by volatile solubility effects alone. The erupted products have consistently low volatile contents with little variability present across erupted samples (0.07-0.32 wt. % H2O). Within these pyroclasts, the heterogeneity of internal textures is visible when viewed using backscattered electron (BSE) imaging or X-ray computed tomography (XCT) as clear borders exist between regions that are of a clastogenic (sintered) origin, of a quenched melt origin, or formed by variable forms of vesiculation. The heterogeneity of internal textures present within even a obsidian pyroclastic domain led to the hypothesis that foaming discrepancies observed within individual clasts were due to pre-existing textures promoting or inhibiting secondary vesiculation in the shallow conduit. This secondary vesiculation occurs through near isobaric temperature increases in the shallow conduit, following primary vesiculation and volatile exsolution associated with isothermal decompression from storage at depth. To test this hypothesis, obsidian samples of 1cm x 1-2cm x 1-3cm were heated above their glass transition temperature (Tg), between 850-910°C, to allow obsidians to vesiculate as the melt would have in the conduit. The textures of the samples were characterised before and after heating using x-ray computed tomography. The results show that within the slightly volatile supersaturated samples variable foaming was observed for each independent texture within obsidian pyroclasts, with foaming preferentially occurring within regions that contained pre-existing, isolated bubbles. These experiments show that limited thermally driven in situ foaming of relatively dense clasts containing small isolated bubbles, can increase overpressure if the domain doesn’t expand as bubbles form without increasing permeability leading to gas overpressure within this smaller region and localised explosions in order to clear this blockage, explaining the hybrid effusive-explosive eruptions observed at Cordón Caulle.