Lava flow internal structure found from AMS and textural data: An example in methodology from the Chaîne des Puys, France

Loock, Sébastien; Diot, Hervé; Vries, Benjamín van Wyk de; Launeau, Patrick; Merle, Olivier; Vadeboin, F.; Petronis, Michael

doi:10.1016/j.jvolgeores.2008.08.017

Cited by 26 publications

(14 citation statements)

References 39 publications

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“…In contrast to x-ray tomography AMS analysis can be performed in minutes, with very high precision, and does not require high computational capabilities. By combining AMS results with a thorough analysis of rock textures one can obtain information on lava flow dynamics, and specifically quantify shear strain (Tarling & Hrouda, 1993;Cañón Tapia et al, 1997;Loock et al, 2008;Ferré et al, 2014). Many studies identify correlations between the shape of the magnetic susceptibility ellipsoid and the shape of the strain ellipsoid (Borradaile, 1988;Borradaile, 1991;Rochette et al, 1992;Cañón Tapia et al, 1997;Cañón Tapia, 1994;Cañón Tapia & Castro, 2004;Almqvist et al, 2012), however they are not necessarily equal (Borradaile, 1988;Rochette et al, 1992).…”

Section: Magnetic Propertiesmentioning

confidence: 99%

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

Shields

Mader

Caricchi

et al. 2016

Journal of Volcanology and Geothermal Research

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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.

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Section: Magnetic Propertiesmentioning

confidence: 99%

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

Shields

Mader

Caricchi

et al. 2016

Journal of Volcanology and Geothermal Research

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“…Two-dimensional image analysis on rock-sections gives direct quantification of the SPO fabric ellipse (Launeau et al, 1990), while tensorial reconstruction applied to any three perpendicular sections yields the SPO ellipsoid (Owens, 1984;Launeau and Cruden, 1998;Launeau and Robin, 2005). Many studies have applied, sometimes combining, these two approaches to determine the flow dynamics during magma emplacement in dikes (Philpotts and Asher, 1994;Tauxe et al, 1998;Callot et al, 2001Geoffroy et al, 2002;Féménias et al, 2004;Poland et al, 2004;Bascou et al, 2005, Philpotts andPhilpotts, 2007;Aubourg et al, 2008, Hastie et al, 2011, lava flows (Cañón-Tapia et al, 1995;Cañón-Tapia, 2005;Loock et al, 2008) and domes (Arbaret et al, 1993;Cañón-Tapia and Castro, 2004;Závada et al, 2009). When identical, AMS and SPO fabric properties are linked to magma flow dynamics by applying the theory of the motion and rotation of the crystals in the sheared medium (Dragoni et al, 1997).…”

Section: Introductionmentioning

confidence: 99%

Magnetic and shape fabrics of magnetite in simple shear flows

Arbaret

Launeau

Diot

et al. 2013

Journal of Volcanology and Geothermal Research

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International audienceThe magnetite fabrics measured by anisotropy of magnetic susceptibility (AMS) and by shape preferred orientation (SPO) optical methods are classically used as flow kinematics indicators in lava flows. The development of magnetite fabrics during simple shear strains γ ≤ 20 was performed using a suspension of 1% volume fraction of multidomain magnetite randomly contained in a mixture of silicone and wax. We measured AMS fabric and SPO ellipsoids by calculating a quadratic shape tensor from oriented thin-sections. For γ < 8, fabrics obey to the theoretical model of rotation of Jeffery (1922). Fabrics are usable to determine the flow kinematics, including the amount of applied finite strain. For γ > 8, fabric elements, foliation and lineation, are stabilised closely parallel to the flow plane and the shear direction, respectively. Two- and three-dimensional numerical simulations using measured aspect ratios of magnetite point out that the large scattering of aspect ratios and the initial orientation distribution of particles are together responsible for a wide-ranging loss of periodicity. The stable AMS and SPO fabrics observed at large strains in experiments are the result of these primary fabric properties combined to collisions between particles and, possibly, their complex three-dimensional shapes. In addition, the constant angular relationship observed at large strains between fabrics and flow components is related to the transient collisions. Consequently, the determination of the lava flow kinematics by using fabric properties measured either by AMS or by SPO analyses should be indubitably associated to a detailed study of the three-dimensional shape of the solid carriers. Regularly shaped populations of low elongated particles will be capable to produce cyclic to oscillating fabrics, while the fabric of elongated particles will be more sensitive to the shape parameters and collisions, ultimately favouring stable fabrics at large strains

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“…Attempts to obtain a simple relationship between deformation and some aspect of AMS results (orientation of principal susceptibilites, degree of anisotropy or shape of the associated ellipsoid; e.g., Hrouda 1993;Hrouda et al 2005;Loock et al 2008;Petronis et al 2013) have shown that it is not possible to obtain a universally valid relationship that can be applied to rocks of different composition. This is the case because of the dependence of the AMS on the habit of the minerals present on each sample, and on their degree of alignment, which might be different for each mineral species even in the same sample (Cañón-Tapia and Castro 2004).…”

Section: Anisotropy Of Magnetic Susceptibilitymentioning

confidence: 99%

Mechanism of formation of volcanic bombs: insights from a pilot study of anisotropy of magnetic susceptibility and preliminary assessment of analytical models

Cañón‐Tapia

2017

Bull Volcanol

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Volcanic bombs and achneliths are a special type of pyroclastic fragments formed by mildly explosive volcanic eruptions. Models explaining the general shapes of those particles can be divided in two broad categories. The most popular envisages the acquisition of shapes of volcanic bombs as the result of the rush of air acting on a fluid clot during flight, and it includes many variants. The less commonly quoted model envisages their shapes as the result of forces acting at the moment of ejection of liquid from the magma pool in the conduit, experiencing an almost negligible modification through its travel through air. Quantitative evidence supporting either of those two models is limited. In this work, I explore the extent to which the anisotropy of magnetic susceptibility (AMS) might be useful in the study of mechanisms of formation of volcanic bombs by comparing measurements made on two spindle and two bread-crusted bombs. The results of this pilot study reveal that the degree of anisotropy of spindle bombs is larger, and their principal susceptibility axes are better clustered than on bread-crusted bombs. Also, the orientation of the principal susceptibility axes is consistent with two specific models (one of the in-flight variants and the general ejection model). Consequently, the reported AMS measurements, albeit limited in number, indicate that it is reasonable to focus attention on only two specific models to explain the acquisition of the shapes of volcanic bombs. Based on a parallel theoretical assessment of analytical models, a third alternative is outlined, envisaging volcanic bomb formation as a two-stage process that involves the bursting of large (~m) gas bubbles on the surface of a magma pond. The new model advanced here is also consistent with the reported AMS results, and constitutes a working hypothesis that should be tested by future studies richer in data. Fortunately, since this work also establishes that AMS can be used to determine magnetic fabric in small size samples, the possibility to expand this study to a larger collection of bombs is granted.

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Lava flow internal structure found from AMS and textural data: An example in methodology from the Chaîne des Puys, France

Cited by 26 publications

References 39 publications

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

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

Magnetic and shape fabrics of magnetite in simple shear flows

Mechanism of formation of volcanic bombs: insights from a pilot study of anisotropy of magnetic susceptibility and preliminary assessment of analytical models

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