Evidence for the development of permeability anisotropy in lava domes and volcanic conduits

International audienceThe ease at which exsolving volatiles can migrate though magma and outgas influences the explosivity of a volcanic eruption. Volcanic rocks often contain discrete discontinuities, providing snapshots of strain localisation processes that occur during magma ascent and extrusion. Whether these features comprise pathways for or barriers to fluid flow is thus of relevance for volcanic eruption and gas emission modelling. We report here on nine discontinuity-bearing andesite blocks collected from Volcán de Colima, Mexico. We present a systematic porosity and permeability study of fifty cores obtained from the blocks collected, and interpret the genetic processes of the discontinuities through detailed microstructural examination. Bands in pumiceous blocks were inferred to be relicts of inhomogeneous bubble expansion which, despite significantly increasing porosity, do not markedly affect permeability. Other discontinuities in our blocks are interpreted to be shear strain-induced flow banding, cavitation porosity, and/or variably healed fractures. In each of these cases, an increase in permeability (up to around three orders of magnitude) was measured relative to the host material. A final sample contained a band of lower porosity than the host rock, characterised by variably infilled pores. In this case, the band was an order of magnitude less permeable than the host rock, highlighting the complex interplay between dilatant and densifying processes in magma. We therefore present evidence for significant permeability anisotropy within the conduit and/or dome of a volcanic system. We suggest that the abundance and distribution of strain localisation features will influence the escape or entrapment of volatiles and therefore the evolution of pore pressure within active volcanic systems. Using a simple upscaling model, we illustrate the relative importance of permeable structures over different lengthscales. Strain localisation processes resulting in permeability anisotropy are likely to play an important role in the style, magnitude, and recurrence interval of volcanic eruptions

Petrological architecture of a magmatic shear zone: A multidisciplinary investigation of strain localisation during magma ascent at Unzen volcano, Japan

Shearing of magma during ascent can promote strain localisation near the conduit margins. Anymechanical and thermal discontinuities associated with such events may alter the chemical, physicaland rheological stability of the magma and thus its propensity to erupt. Lava spines can record suchprocesses, preserving a range of macroscopic and microscopic deformation textures, attributed toshearing and friction, as magma ascends through the viscous-brittle transition. Here, we use a multi-disciplinary approach combining petrology, microstructures, crystallography, magnetics and experi-mentation to assess the evidence, role and extent of shearing across a marginal shear zone of the1994–1995 lava spine at Unzen volcano, Japan. Our results show that crystals can effectively moni-tor stress conditions during magma ascent, with viscous remobilisation, crystal plasticity and com-minution all systematically increasing towards the spine margin. Accompanying this, we find anincrease in mineral destabilisation in the form of pargasitic amphibole breakdown displaying tex-tural variations across the shear zone, from symplectitic to granular rims towards the spine margin.In addition, the compaction of pores, chemical and textural alteration of interstitial glass and mag-netic variations all change systematically with shear intensity. The strong correlation between thedegree of shearing, crystal deformation and disequilibrium features, together with distinct magneticproperties, implies a localised thermal input due to shear and frictional processes near the conduitmargin during magma ascent. This was accompanied by late-stage or post-emplacement fluid- andgas-induced alteration of the gouge, as well as oxidation and glass devitrification. Understandingand recognising evidence for strain localisation during magma ascent may, therefore, be vital whenassessing factors that regulate the style of volcanic eruptions, which may provide insights into thecryptic shifts from effusive to explosive activity as observed at many active lava dome

Frictional melt homogenisation during fault slip: Geochemical, textural and rheological fingerprints

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Thermal resilience of microcracked andesitic dome rocks

International audienceThe strength of the rocks forming a lava dome informs on its structural stability, important for volcanic hazard assessments. Dome-forming rocks are persistently challenged by thermal stresses from recurring eruptive events that may reduce their strength and jeopardise the structural stability of the dome. Here, we present a series of experiments to better understand the impact of thermal stresses on the strength of an andesitic dome rock from Volcán de Colima (Mexico), a volcano that has witnessed some substantial dome collapses in recent years. Uniaxial compressive strength (UCS) was first tested at room temperature on as-collected samples and samples that had undergone either slow (heated and cooled at 1 °C/min) or shock (heated at 1 °C/min and shock-cooled in cold water) thermal stressing to target temperatures of 400–700 °C. Slow- and shock-cooling thermal stressing did not measurably alter sample strength, connected porosity, or permeability. UCS tests performed at high in-situ temperatures (400–700 °C), however, showed an increase in sample strength and stiffness. We interpret that the resistance of this rock to thermal stresses results from both the presence of abundant pre-existing microcracks and the thermal stability of its mineral assemblage. Unchanged physical properties for the thermally stressed samples deformed at room temperature suggests that the pre-existing microcracks close and reopen, respectively, as the rock expands and contracts during heating and cooling to accommodate the volumetric changes without further microcracking. The increase in strength and stiffness at high in-situ temperatures can be explained by the closure of microcracks due to thermal expansion. These observations suggest that the strength of microcracked dome rocks (1) may be slightly higher when hot (below the glass transition of the groundmass glass), although “upscaled” strength estimates highlight that dome strength will be largely unchanged by an increase in temperature, (2) may only be reduced following the first thermal stressing event, and (3) may not be further reduced by repeated thermal stressing events. Therefore, thermal perturbations, often observed at active domes, may not, as perhaps expected, repeatedly degrade the strength of individual blocks forming the lava dome and therefore may not jeopardise dome stability

The influence of water on the strength of Neapolitan Yellow Tuff, the most widely used building stone in Naples (Italy)

International audienc

Facile production of nanocomposites of carbon nanotubes and polycaprolactone with high aspect ratios with potential applications in drug delivery

Facile route to polymer carbon nanotube nanocomposites.</p