2D and 3D fabric quantification of conduit textures to understand eruption dynamics and mechanisms: unique in situ example of Mt Unzen

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The Unzen Scientific Drilling Project (USDP) at Mt Unzen volcano penetrated the magmatic conduit, a key part of the volcanic system; but one that is little studied as it is rarely sampled. In this study we have access to these unique in situ samples, which allows us to investigate what is thought to be the conduit of the last eruption (1991-1995). The system is known to be cyclic and the textural information recorded in these conduit rocks holds key information about magmatic movement, periodic and cyclic dike emplacement, and eruption dynamics that is important for understanding Mt Unzen; and many other dome-building volcanoes at subduction zones. The project focusses on making high resolution 2D and 3D observations in order to quantify the textural information (porosity, mineral alignments etc.) preserved in the conduit samples; textures that can be used to reveal the different stages of magma supply and changes in flow-rate and/or deformation (strain-rates). The project benefits significantly from the breadth of expertise in the project group, and a full multi-disciplinary feasibility study has been performed using all the intended methods. Key of the work is the ability for non-destructive 3D imaging to accurately define the structures in these extremely limited sample volumes, allowing careful placement and minimal destructive analysis to be required. Our analysis will use the latest 2D SEM and 3D X-ray tomographic methods to define the nature of these fabrics in relation to conduit formation, laminar and/or turbulent flow, and the transition from brittle to ductile deformation which all effect eruption mechanisms. By integrating the core textural information we will obtain that from additional magnetic fabric, rock mechanical data and limited geochemical analysis we will gain a better understanding of all conduit processes: from initial intrusion to the later stage hydrothermal alteration. Furthermore, by linking the observed conduit textures to those seen in erupted surface samples, the project will also enable better interpretation of field and geochemical data used to assess eruption mechanisms and volcano stability. The intended outcomes include: a better understanding of the Mt Unzen system, a better framework for interpreting textural information preserved in erupted material (to constrain behavior at depth), and improved conceptual and mechanical-rheological models of the behaviour of magma in conduits. This work focusses on the exceptional and unique samples available from conduit of Mt Unzen, but will have results that are immediately transferable to other dome-building volcanoes at subduction zones.