Unravelling magmatic deformation histories: unravelling complex kinematic indicators in tube pumices using X-ray tomography

Abstract:

The bubble geometries preserved in highly vesiculated rhyolitic pumices provide kinematic indicators of the state of stress and deformation immediately prior to fragmentation. Here we present a suite of high resolution (1-15µm/voxel) 3D x-ray tomography data from tube pumices that enable us to piece together this complex deformation history. Qualitative visualization and quantitative geometric analysis of each individual bubble (10’s of thousands per 3D volume) in the images reveals complex internal morphologies and bubble interactions; many features of which are not preserved during traditional sample preparation. Even in this end member example of tube pumices where simple shear is dominant, we can identify complex and spatially variable deformation during ascent, and multiple stages of spatially variable brittle and ductile deformation. From relationships between bubble size, geometry, and elongation we define variably orientated but strong deformation perpendicular to the tube axis. From this we define the evolving stress field and identify implying temporal and spatial variability in bubble deformation. The data indicate complex responses to changing stress fields, with significant strain localization, including significant “kink band” late stage shortening along the major elongation axis, and we interpret this information to better understand the physico-chemical conditions and mechanisms operating during explosive volcanism.