High-resolution 3D analyses of the shape and internal constituents of small volcanic ash particles: The contribution of SEM micro-computed tomography (SEM micro-CT)

Abstract

The morphology of small volcanic ash particles is fundamental to our understanding of magma fragmentation, and in transport modeling of volcanic plumes and clouds. Until recently, the analysis of 3D features in small objects (b250 μm) was either restricted to extrapolations from 2D approaches, partial stereo-imaging, or CT methods having limited spatial resolution and/or accessibility. In this study, an X-ray computed-tomography technique known as SEM micro-CT, also called 3D X-ray ultramicroscopy (3D XuM), was used to investigate the 3D morphology of small volcanic ash particles (125-250 μm sieve fraction), as well as their vesicle and microcrystal distribution. The samples were selected from four stratigraphically well-established tephra layers of the Meerfelder Maar (West Eifel Volcanic Field, Germany). Resolution tests performed on a Beametr v1 pattern sample along with Monte Carlo simulations of X-ray emission volumes indicated that a spatial resolution of 0.65 μm was obtained for X-ray shadow projections using a standard thermionic SEM and a bulk brass target as X-ray source. Analysis of a smaller volcanic ash particle (64-125 μm sieve fraction) showed that features with volumes N 20 μm 3 (~3.5 μm in diameter) can be successfully reconstructed and quantified. In addition, new functionalities of the Blob3D software were developed to allow the particle shape factors frequently used as input parameters in ash transport and dispersion models to be calculated. This study indicates that SEM micro-CT is very well suited to quantify the various aspects of shape in fine volcanic ash, and potentially also to investigate the 3D morphology and internal structure of any object b 0.1 mm 3 .

Eifel maars: Quantitative shape characterization of juvenile ash particles (Eifel Volcanic Field, Germany)

“…A detailed description of Mertes and Schmincke (1985) and Schmincke (2007 Büchel and Mertes (1982) and Schmincke (2007). the SEM micro-CT technique and its applicability to volcanic ash is presented in Vonlanthen et al (2014).…”

“…Smoothing reduces the resolution of the contour. Therefore, a relatively low Gaussian smoothing value (S = 2 in the Bruker's NRecon software, see Vonlanthen et al, 2014 for details) was applied. This value turned out to have little effect on D1 (D1 = 1.150 for S = 1 and D1 = 1.113 for S = 2) and minimal on D2 (D2 = 1.146 for S = 1 and D2 = 1.135 for S = 2), but it is nevertheless sufficient to eliminate the artifacts (Fig.…”

“…3. Test of the influence of the degree of smoothing (S = 1 − S = 6) on fractal dimensions calculated for an SEM micro-CT slice of sample MFM45-4 (for details see Vonlanthen et al, 2014). and highly silica-undersaturated (i.e.…”

“…6. Consistency tests of the fractal technique using several cross sections obtained from the 3D SEM micro-CT reconstruction of the particle MFM45-4 (see Vonlanthen et al, 2014). A) Influence of the cross section orientation on the calculated fractal dimensions; B) Influence of the location of parallel cross sections on the calculated fractal dimensions.…”

Mechanism of formation of volcanic bombs: insights from a pilot study of anisotropy of magnetic susceptibility and preliminary assessment of analytical models

“…From a microscopic perspective, the relation between particle morphology and eruptive and transport mechanisms has been abundantly explored (e.g., Dellino and Liotino 2002;Ersoy et al 2006Ersoy et al , 2008Heiken and Wohletz 1985;Leibrandt and Le Pennec 2015;Liu et al 2015;Rausch et al 2015;Riley et al 2003;Sheridan and Kortemeier 1987;Sheridan and Marshall 1983;Vonlanthen et al 2015). Heiken (1972) considered that the shape of particles in the ash size range is mostly dependent on the shape of magmatic gas bubbles at the moment of fragmentation: flat and platy shards arise from broken vesicle walls, and droplet shapes are produced from low viscosity magma where the effect of surface tension is likely to become more important.…”