Quantitative textural analysis of Vulcanian pyroclasts (Montserrat) using multi-scale X-ray computed microtomography: comparison with results from 2D image analysis |
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Authors: | Thomas Giachetti L Arbaret T H Druitt K Kelfoun |
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Institution: | 1.Université Blaise Pascal, Laboratoire Magmas et Volcans,Clermont Université,Clermont-Ferrand,France;2.CNRS, UMR 6524, LMV,Clermont-Ferrand,France;3.IRD, R 163, LMV,Clermont-Ferrand,France;4.Institut des Sciences de la Terre d’Orléans,Université d’Orléans,Orléans Cedex 2,France;5.GEOLAB,Clermont-Ferrand Cedex 1,France |
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Abstract: | 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. |
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