Deformation experiments of bubble‐ and crystal‐bearing magmas: Rheological and microstructural analysis

Abstract

[1] Simple shear deformation experiments on three-phase, hydrous, haplogranitic magmas, composed of quartz crystals (24-65 vol.%), CO 2 -rich gas bubbles (9-12 vol.%) and melt in different proportions, were performed with a Paterson-type rock deformation apparatus. Strain rates from 5 Á 10 À6 s À1 to 4 Á 10 À3 s À1 were applied at temperatures between 723 and 1023 K and at pressure of 200 MPa. The results show that the three-phase suspension rheology is strongly strain rate dependent (non-Newtonian behavior). Two non-Newtonian regimes were observed: shear thinning (viscosity decreases with increasing strain rate) and shear thickening (viscosity increases with increasing strain rate). Shear thinning occurs in crystal-rich magmas (55-65 vol.% crystals; 9-10 vol.% bubbles) as a result of crystal size reduction and shear zoning. Shear thickening prevails in dilute suspensions (24 vol.% crystals; 12 vol.% bubbles), where bubble coalescence and outgassing dominate. At intermediate crystallinity (44 vol.% crystals; 12 vol.% bubbles) both shear thickening and thinning occur. Based on the microstructural observations using synchrotron radiation X-ray tomographic microscopy, bubbles can develop two different shapes: oblate at low temperature (<873 K) and prolate at high temperature (>873 K). These differences in shape are caused by different conditions of flow: unsteady flow, where the relaxation time of the bubbles is much longer than the timescale of deformation (oblate shapes); steady flow, where bubbles are in their equilibrium deformation state (prolate shapes). Three-phase magmas are characterized by a rheological behavior that is substantially different with respect to suspensions containing only crystals or only gas bubbles.Citation: Pistone, M., L. Caricchi, P. Ulmer, L. Burlini, P. Ardia, E. Reusser, F. Marone, and L. Arbaret (2012), Deformation experiments of bubble-and crystal-bearing magmas: Rheological and microstructural analysis,

Syn-Emplacement Fracturing in the Sandfell Laccolith, Eastern Iceland—Implications for Rhyolite Intrusion Growth and Volcanic Hazards

“…Such a rheological transition of the magma from viscous flow to jamming may be related to a viscosity increase induced by crystallization and/or porosity reduction, degassing and cooling (Cashman and Blundy, 2000;Sparks et al, 2000;Giordano et al, 2008;Costa et al, 2009;Cordonnier et al, 2012;Pistone et al, 2012Pistone et al, , 2013Pistone et al, , 2015Pistone et al, , 2016Heap et al, 2015;Kennedy et al, 2016;Kushnir et al, 2017). Anderson and Fink (1992) described vesicular bands ("striations") in an obsidian lava flow, which they attributed to cooling, because they formed perpendicular to cooling joints.…”

“…In a magma (multiphase fluid) with a crystal fraction <65%, the strain rate, at which the rheological transition to brittle deformation occurs, strongly depends on the viscosity of the magma, which, in turn, is controlled by temperature, crystal content and shape, bubble fraction and shape, and melt composition, including the volatile content (Dingwell, 1997;Caricchi et al, 2007;Giordano et al, 2008;Cordonnier et al, 2012;Pistone et al, 2012Pistone et al, , 2013Pistone et al, , 2016Gonnermann and Manga, 2013). A non-Newtonian rheology in magma has been shown to be manifested by strain localization in the melt-phase and the onset of a strain-dependent rheology (e.g., Caricchi et al, 2007;Cordonnier et al, 2012;Pistone et al, 2016).…”

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

“…Pinkerton and Sparks 1978;Lipman et al 1985;Manga et al 1998;Rust and Manga 2002;Llewellin and Manga 2005). 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).…”

Rheological controls on the eruption potential and style of an andesite volcano: A case study from Mt. Ruapehu, New Zealand

“…These zones have been shown to focus permeability resulting in significant outgassing (e.g., Caricchi et al, 2011;Pistone et al, 2012).…”

“…Few studies exist of the more complex -and realistic -situation of three-phase suspen-3 sions (e.g., Shields et al, 2014). These include the rheological study of synthetic high-viscosity magmas containing both crystals and bubbles (e.g., Pistone et al, 2012Pistone et al, , 2013. More recently, Truby et al (2014) present a constitutive model which uses the bubble suspension as an 'effective medium' in which the crystals reside, i.e.…”