Abstract
The mechanisms of hazardous silicic eruptions are controlled by complex, poorly-understood conduit processes. Observations of recent Chilean rhyolite eruptions have revealed the importance of hybrid activity, involving simultaneous explosive and effusive emissions from a common vent. Such behaviour hinges upon the ability of gas to decouple from magma in the shallow conduit. Tuffisite veins are increasingly suspected to be a key facilitator of outgassing, as they repeatedly provide a transient permeable escape route for volcanic gases. Intersection of foam domains by tuffisite veins appears critical to efficient outgassing. However, knowledge is currently lacking into textural heterogeneities within shallow conduits, their relationship with tuffisite vein propagation, and the implications for fragmentation and degassing processes. Similarly, the magmatic vesiculation response to upper conduit pressure perturbations, such as those related to the slip of dense magma plugs, remains largely undefined. Here we provide a detailed characterization of an exceptionally large tuffisite vein within a rhyolitic obsidian bomb ejected during transitional explosive-effusive activity at Chaiten, Chile in May 2008. Vein textures and chemistry provide a time-integrated record of the invasion of a dense upper conduit plug by deeper fragmented magma. Quantitative textural analysis reveals diverse vesiculation histories of various juvenile clast types. Using vesicle size distributions, bubble number densities, zones of diffusive water depletion, and glass H2O concentrations, we propose a multi-step degassing/fragmentation history, spanning deep degassing to explosive bomb ejection. Rapid decompression events of ~3-4 MPa are associated with fragmentation of foam and dense magma at ~200-350 metres depth in the conduit, permitting vertical gas and pyroclast mobility over hundreds of metres. Permeable pathway occlusion in the dense conduit plug by pyroclast accumulation and sintering preceded ultimate bomb ejection, which then triggered a final bubble nucleation event. Our results highlight how the vesiculation response of magma to decompression events is highly sensitive to the local melt volatile concentration, which is strongly spatially heterogeneous. Repeated opening of pervasive tuffisite vein networks promotes this heterogeneity, allowing juxtaposition of variably volatile-rich magma fragments that are derived from a wide range of depths in the conduit. This process enables efficient but explosive removal of gas from rhyolitic
Highlights
Explosive-effusive behavioral transitions during silicic volcanic eruptions remain poorly understood, yet are typical of rhyolitic activity (e.g., Eichelberger et al, 1986; Gonnermann and Manga, 2003)
The density of the host obsidian slightly exceeds that of the bulk tuffisite vein (2281 vs. 2070 kgm−3), because of its lower porosity (0.04 vs. 0.15), the pore-free tuffisite vein density is marginally higher (Table 1)
The data presented in this paper provide new constraints on fluid and pyroclast transport through tuffisite veins, which controls the episodic, explosive gas-magma separation that characterizes transitional rhyolite eruptions (Schipper et al, 2013; Castro et al, 2014)
Summary
Explosive-effusive behavioral transitions during silicic volcanic eruptions remain poorly understood, yet are typical of rhyolitic activity (e.g., Eichelberger et al, 1986; Gonnermann and Manga, 2003). Chaitén was closely followed by the 2011–2012 Cordón Caulle eruption, in Chile, which is providing additional insights into vent and plume processes (e.g., Schipper et al, 2013; Bonadonna et al, 2015) Both recent eruptions were dominated by hybrid explosive-effusive behavior, which requires a new paradigm to explain the nature of eruptive controls (Schipper et al, 2013; Castro et al, 2014). The latter, which are generated via shear stresses or gas overpressure, are sometimes preserved as particle-clogged tuffisite veins hosted by ballistic bombs, conduitfilling lava or country rock (Heiken et al, 1988; Stasiuk et al, 1996; Gonnermann and Manga, 2003; Tuffen et al, 2003; Cabrera et al, 2011; Holland et al, 2011; Castro et al, 2012, 2014; Berlo et al, 2013)
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