Unravelling textural heterogeneity in obsidian: Shear-induced outgassing in the Rocche Rosse flow

Abstract

Obsidian flow emplacement is a complex and understudied aspect of silicic volcanism. Of particular importance is the question of how highly viscous magma can lose sufficient gas in order to erupt effusively as a lava flow. Using an array of methods we study the extreme textural heterogeneity of the Rocche Rosse obsidian flow in Lipari, a 2 km long, 100 m thick,~800 year old lava flow, with respect to outgassing and emplacement mechanisms. 2D and 3D vesicle analyses and density measurements are used to classify the lava into four textural types: 'glassy' obsidian (b15% vesicles), 'pumiceous' lava (N 40% vesicles), high aspect ratio, 'shear banded' lava (20-40% vesicles) and low aspect ratio, 'frothy' obsidian with 30-60% vesicles. Textural heterogeneity is observed on all scales (m to μm) and occurs as the result of strongly localised strain. Magnetic fabric, described by oblate and prolate susceptibility ellipsoids, records high and variable degrees of shearing throughout the flow. Total water contents are derived using both thermogravimetry and infrared spectroscopy to quantify primary (magmatic) and secondary (meteoric) water. Glass water contents are between 0.08-0.25 wt.%. Water analysis also reveals an increase in water content from glassy obsidian bands towards 'frothy' bands of 0.06-0.08 wt.%, reflecting preferential vesiculation of higher water bands and an extreme sensitivity of obsidian degassing to water content. We present an outgassing model that reconciles textural, volatile and magnetic data to indicate that obsidian is generated from multiple shear-induced outgassing cycles, whereby vesicular magma outgasses and densifies through bubble collapse and fracture healing to form obsidian, which then re-vesiculates to produce 'dry' vesicular magma. Repetition of this cycle throughout magma ascent results in the low water contents of the Rocche Rosse lavas and the final stage in the degassing cycle determines final lava porosity. Heterogeneities in lava rheology (vesicularity, water content, microlite content, viscosity) play a vital role in the structural evolution of an obsidian flow and overprint flow-scale morphology. Post-emplacement hydration also depends heavily on local strain, whereby connectivity of vesicles as a result of shear deformation governs sample rehydration by meteoric water, a process previously correlated to lava vesicularity alone.

Emplacement of the Rocche Rosse rhyolite lava flow (Lipari, Aeolian Islands)

“…Rhyolitic lava flows show a complex internal sequence with variably vesicular, devitrified and deformed textures, and complex surface deformation patterns (Fink 1978(Fink , 1983Manley and Fink 1987;Shields et al 2016). The traditional view of rhyolitic flow interiors includes (from the surface downwards) an upper breccia, thick obsidian, foliated rhyolite, thinner obsidian and a basal breccia (Fink 1978(Fink , 1983Manley and Fink 1987).…”

“…Shields et al (2016) identified four textural units in the Rocche Rosse: type 1-glassy obsidian, which is spherulitic and has a low vesicularity (< 15%); type 2-'frothy' obsidian, which is vesicular (30-60%) and glassy, with large elongated vesicles perpendicular to banding; type 3-pumiceous lava, which is highly vesicular (40-60%), with variable vesicle sizes; type 4-shear banded lava, characterised by a low-medium vesicularity (20-40%), and highly deformed and coalesced vesicles. Glassy obsidian and shear banded lavas occur as massive and/or platy outcrops, pumiceous lava is observed throughout the flow and 'frothy' obsidian makes up fold hinges, tension gashes, micro folding and extensional fractures (Shields et al 2016).…”

“…It has been previously reported that the Rocche Rosse behaved as a Bingham fluid during emplacement (Clay et al 2013), which can promote strain localisation and folding. Strain localisation affects the rheological behaviour of lavas, and rheological contrasts between layers, such as differences in viscosity, capillary number and yield strength (Castro and Cashman 1999;Gonnerman and Manga 2005;Shields et al 2016), can promote shear zonation. Due to the high viscosity of flowing rhyolitic lavas, the last increments of flow-related strain are recorded in the structures that are evident in the final solidified flow (Clay et al 2013).…”

“…However, knowledge of these influences on the apparent viscosity of a bubbly lava is limited (Stein and Spera 1992;Quane and Russell 2005;Pistone et al 2012;Chevrel et al 2013), and they are therefore not included in viscosity calculations here. The estimated flow surface viscosity was 10 8 to 10 10 Pa s, using rhyolite glass chemistry from Bullock (2015); eruptive temperature range estimates (750 to 900°C) of Cas and Wright (1987), Gottsmann and Dingwell (2001) and Davì et al (2010); and the water content (0.08-0.25 wt%) of Shields et al (2016). Deformation is heterogeneous and multi scale, varying greatly and non-systematically across flow (Shields et al 2016).…”

“…Previous work on the Rocche Rosse has focused on fragmentation and transport mechanisms of the associated pyroclastic sequence (Dellino and La Volpe 1995), the thermal history of flow frontal ramps, i.e. ridges of fold structures extending downward into the flow by tens of metres (Gottsmann and Dingwell 2001), magmatic history (Gioncada et al 2003(Gioncada et al , 2005Davì et al 2009Davì et al , 2010 and textural heterogeneity (Gonnermann and Manga 2003;Clay et al 2013;Shields et al 2016). Ramp structures of the Rocche Rosse were also documented by Hall (1978) and Cas and Wright (1987), inferring a compressive regime and build-up of lava behind a brecciated flow front, hindering flow advance.…”