Conduit Dynamics in Transitional Rhyolitic Activity Recorded by Tuffisite Vein Textures from the 2008–2009 Chaitén Eruption

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 behavior 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 Chaitén, 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 H 2 O 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 ∼200-360 m depth in the conduit, permitting vertical gas and pyroclast mobility over hundreds of meters. 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 Saubin et al.Chaitén Tuffisites Record Conduit Processes 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 magma and creates a complex textural collage within dense rhyolitic lava, in which neighboring fused clasts may have experienced vastly different degassing histories.

The Permeability Evolution of Tuffisites and Implications for Outgassing Through Dense Rhyolitic Magma

“…Peak heights at 3,550 cm −1 (H 2 O t ) and 1,630 cm −1 (H 2 O m ) were determined using 18‐point linear baseline corrections. Using the Beer‐Lambert law, a glass density of 2,281 kg/m 3 (Saubin et al, ), and absorption coefficients of 80 l mol −1 /cm (3,550 cm −1 ; Ihinger et al, ) and 55 l mol −1 cm (1,630 cm −1 ; Newman et al, ; Okumura et al, ), we converted these data to species concentrations. The combined uncertainty of this method, which depends on the wafer thickness and density and the choice of molar absorption coefficient, is typically <10% (von Aulock et al, ).…”

“…Some juvenile clasts in the BTB (Figure ), CH5G (Figure d), and B1 (Figures e and f) samples have vesiculated centers leading to a frothed appearance. Quantification of vesicle size distributions and H 2 O concentrations in sample BTB (Saubin et al, ) has facilitated a detailed reconstruction of the relative timing of clast vesiculation and fracture opening, together with the evolution of gas pressure in the system. Results show that, for sample BTB, the strongly vesiculated clasts had vesiculated prior to their incorporation into the fracture by pressurized gas from deeper in the conduit.…”

“…For example, the photograph taken during the 10 January 2012 activity at Cordón Caulle ( t = +4 s) shows that localized outgassing occurred via pathways with fracture (i.e., plane) geometries (Figure a), rather than outgassing through permeable foam (e.g., Eichelberger et al, ). Indeed, recent experimental work by Ryan et al () has shown that it is difficult to create permeability in initially impermeable, high‐porosity foams, describing them as “persistently impermeable.” Dense obsidian bombs emitted during hybrid activity characteristically hosted tuffisites, which are centimetric fractures infilled with pyroclastic material (Castro et al, ; Heiken et al, ; Saubin et al, ; Stasiuk et al, ; Tuffen et al, ; Figure a). Tuffisites have been interpreted to record the transient opening and occlusion of the permeable pathways that provide fleeting escape routes for pressurized gas prior to eventual blockage and violent ejection (Saubin et al, ).…”

“…Indeed, recent experimental work by Ryan et al () has shown that it is difficult to create permeability in initially impermeable, high‐porosity foams, describing them as “persistently impermeable.” Dense obsidian bombs emitted during hybrid activity characteristically hosted tuffisites, which are centimetric fractures infilled with pyroclastic material (Castro et al, ; Heiken et al, ; Saubin et al, ; Stasiuk et al, ; Tuffen et al, ; Figure a). Tuffisites have been interpreted to record the transient opening and occlusion of the permeable pathways that provide fleeting escape routes for pressurized gas prior to eventual blockage and violent ejection (Saubin et al, ). H 2 O concentration gradients at the tuffisite‐host rock interface (Berlo et al, ; Castro et al, ) and within fracture‐filling clasts (Saubin et al, ) provide constraints on tuffisite depths (hundreds of meters), timescales of fracture opening (tens of minutes to several hours), and gas pressure changes associated with fracture opening (reductions of up to several MPa).…”

“…The BTB tuffisite is 30 mm wide with remarkably planar walls. It is connected to a network of sub‐millimeter subsidiary tuffisites in the dense obsidian host material (Saubin et al, ). (b) Photograph of the decametric breadcrust bomb from the June 2011 hybrid activity at Cordón Caulle.…”