Quantitative textural analysis of Vulcanian pyroclasts (Montserrat) using multi-scale X-ray computed microtomography: comparison with results from 2D image analysis
X-ray computed microtomography (μCT) was carried out on four pyroclasts from the 1997 Vulcanian explosions of Soufrière Hills Volcano, Montserrat. Three-dimensional data from multiple image stacks with different spatial resolutions (0.37, 4–8, and 17.4 μm px−1) were combined to generate size distributions of vesicles, inter-vesicle throats, crystals, and Fe–Ti oxides over a 3.4–860-μm size range, and to compare the results with those obtained by 2D image analysis on the same samples. Qualitative textural observations are in good agreement with those made in 2D, but μCT provides better resolution of textural features and spatial relationships. Calculation of size distributions requires automated decoalescence of the connected vesicle network. Problems related to this process, in part due to the high porosity of pumice, result in potential artefacts in the calculated size distributions, which are discussed in detail. The main modes of the 3D vesicle volume distributions are systematically shifted to larger sizes compared with those of the 2D distributions. Sample total vesicularities obtained in 3D are within 13 vol.% of those found in 2D, and within 10 vol.% of those measured by He-pycnometry. Total number densities of vesicles and Fe–Ti oxides from the two methods are consistent only to the first order, 3D values ranging from 37% to 309% of those in 2D. Vesicle coalescence, investigated by examining inter-vesicle throat size distributions, occurred in all pyroclasts between neighbouring vesicles of many sizes. The larger the vesicle, the more connected it is.
KeywordsX-ray computed microtomography Vesicle size distribution Vesicle number density Coalescence Vulcanian explosions Soufrière Hills Volcano Montserrat
The project was partly funded by the French Agence Nationale de la Recherche (ANR-EXPLANT, Contract No ANR-05-CATT-003 to C. Martel), and partly by ERC grant 202844 under the European FP7. The study benefited from discussions with Jonathan Castro and Wim Degruyter. Marco Stampanoni, Emmanuel Le Trong and David Picard helped us at the Swiss Light Source, and Armelle Basillais acquired the IPROS stacks at the Institut de Prévention et de Recherche sur l’Ostéoporose (IPROS), Orléans. We thank Thomas Shea and an anonymous reviewer for their thoughtful and constructive reviews of the manuscript.
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