DEVELOPMENT OF SHEAR ZONES IN CRYSTAL-BEARING MAGMAS: EVIDENCE FROM HIGH-RESOLUTION NEUTRON COMPUTED TOMOGRAPHY

During ascent and eruption, magma/lava is subject to high stresses, which promote the localization of strain along shear zones. The complex flow dynamics of non-Newtonian crystal-bearing lavas is, to date, poorly understood and deserves attention as it is the primary control on magma transport and ultimately, eruption style. Here, we combine strain-step deformation experiments on crystal-bearing magma to simultaneous acoustic emission monitoring and neutron computed tomography image analysis (between each strain step) to analyse the behaviour of the crystal phases as well as the development of fractures in shear zones.

Acoustic emission and post-deformation optical analyses reveal that cracking is very important across the ductile-brittle transition. The deformation of magma in compression induce the development of shear zones at an angle ranging between 30 and 45° to the maximum stress field, depending on the applied stress and the ratio of ductile to brittle response. The crystals, in particular, appear to influence crack propagation and in some cases strong, intact crystals may deviate crack propagation. Moreover, there was a tendency for the plagioclase and amphibole crystals (in contrast to oxides or pyroxenes) to fragment during the deformation. At high stresses, macroscopic cracks propagated across crystals and cataclastic shear zones developed in which crystals were completely pulverized.

Examination of the sequence of neutron computed tomograms allowed us to track in-situ the evolution of crystals and cracks during lava deformation. The shear zones are found to thin with an increase in applied stress. Physically, the state of the crystals vary profoundly across the shear zones, which host variable amounts of cracks

The evolution of shear zones within the samples can be directly correlated with lava rheology across the ductile-brittle transition. This work aims to elaborate a deterministic model of failure, which may refine current volcanic eruption models.