3D textural characterization of rocks representing S. Miguel Island (Azores, Portugal)

The role of the physical and microstructural properties of lavas impacts the dynamics of magma ascent and of the volcanic edifice itself. In the context of a doctoral project, ten samples representative of the central volcanoes of S. Miguel Island (Azores, Portugal), priorily collected in available outcrops in the island, were microstructurally assessed for the first time in the INGV-OV (Naples). Imaging was processed by the ZEISS Xradia Versa 410 X-ray computed microtomography. This tool enables accurate and complete textural characterization of rocks by providing 3D images of the samples. Posteriorly, the rocks were analysed with a dedicated image analysis software to resolve the internal microstructure of the samples. determining several key properties (porosity, permeability, fracturation, and crystal content) that are of major relevance for a posterior physical and mechanical assessment. This publication results from work conducted under the transnational access/national open access action at magLab, INGV-OV (Naples, Italy) supported by WP3 ILGE - MEET project, PNRR - EU Next Generation Europe program, MUR grant number D53C22001400005.

X-ray computed microtomography (X-μCT) : ZEISS Xradia Versa 410 (INGV-OV) This micro-CT scan is equipped with a microfocus X-ray source capable of energies from 40 to 150 kV and detectors with magnifications ranging from 0.4X to 20X (resolution down to 0.9 μm). Subsequent image analysis using the PerGeos Software for mineral and void space segmentation, as well as greyscale image optimization. Pore Network Models and permeability simulations were also performed on this software using dedicated modules.

X-ray imaging was performed in absorption mode acquiring 1601 2D radiographs (projections) over a 360° rotation with 100 to 120 kV voltage, 10 W power, variable exposure time (0.7 to 1.2 s). Appropriate source filters were used to minimise beam hardening. An optical magnification of 0.4× was used, resulting in a nominal voxel size between 14.68 μ mand15.37 μm. The acquired projections were then reconstructed through a filtered back-projection algorithm using XRMReconstructor software by ZEISS, providing 3D greyscale images of the internal microstructure of the samples, proportional to the X-ray attenuation coefficient of the rock phases, which is a function of their density and composition. Unconfined compressive strength (UCS) experiments combined with μCT imaging were performed on four representative cylinder specimens (i.e., BAf14, BF3, BF13, and T31 samples) with a ~ 7.5 mm diameter and a length-diameter ratio of approximately 1.9. The test was conducted using a loading rate of 0.03 mm/min and a Deben in-situ load cell stage (CT5000H250), with a maximum load capacity of 5210 N, incorporated into the Zeiss equipment of the INGV-OV. Pre and post-test 3D scans were obtained, and Young’s modulus (or elastic modulus; E) was calculated based on stress-strain curves derived from the plate displacement. X-ray imaging of the cylinders was per formed collecting 801 projections over a total angular scan of 180◦ (plus fan angle) with a scan time between 2.75 and 3.5 s per projection at 120 kV and 10 W and using an optical magnification of 0.4× to obtain 3D images with a voxel size of 15μm