Microstructural Analysis of Mont Gerbier de Jonc volcanic dome, France

Abstract

Magnetic susceptibility (AMS) has previously been employed to determine the magnetic properties and magnetic fabrics of the Mont Gerbier de Jonc (MGDJ) volcanic dome, located in the Massif Central, France, allowing an interpretation of the preferred orientation of minerals (von Siebenthal, 2010). However, this research has not been tested against the physical orientation of minerals. This study investigates the physical orientation of minerals comprising MGDJ in order to understand the mechanical transitions in a cooling magma, and the ability of X-ray Computed Tomography (XRCT) as a method of fabric analysis. Microstructures from orientated samples were analyzed using optical microscopy and XRCT to understand the relationship between fabric geometry and crystallographic preferred orientation. Relative orientation with respect to north and shape characteristics of sanidine phenocrysts in one sample were described using XRCT to segment and isolate these grains, and they proved to be good indicators for fabric characteristics. Understanding mechanical behaviors in magma will help with the prediction of pace and violence of eruptions.