Imaging 3D pumice textures using Synchrotron X-ray tomography: understanding andesitic Subplinian eruptions at Mt. Ruapehu, New Zealand

The role of bubbles in the complex rheology of andesitic magmatic suspensions was examined in pumice clasts from 22,500 to 10,000 yr BP subplinian eruptions (0.3 to 0.5 km3) at Mt. Ruapehu. Three contrasting pumice textural end-members were imaged with high resolution X-ray tomography (4.45 and 1.8 μm pixel) to investigate variations between vesiculation, degassing and fragmentation processes. These textures include: Type 1) Coarsely vesicular, foliated pumice clasts, with average total porosities of 80% (76.5% + 7 connected) and average bulk densities of 0.64 g/cm3 + 0.25 (volumetric number density, Nv=2.8-5.4 x 105cm-3). Vesicle size distribution is exponential, and vesicles are sub-spherical to ellipsoidal, strongly oriented and aligned, are 24 to 1777 μm in size (equivalent diameters), with smooth vesicle walls and variable degrees of coalescence. Bubble walls vary from flat films to slightly curved. Type 2) Finely vesicular, fibrous pumice clasts, with average total porosities of 73% (69% +5 connected) and bulk densities of 0.87 g/cm3 + 0.14 (Nv=5.3 x 106 cm-3). Vesicle size distribution is exponential, with ellipsoidal to highly deformed and contorted vesicles; 10 to 2370 μm in size, with thin bubble walls varying from flat to highly wrinkled films. Vesicles are abundant along pyroxene walls and line thin (30-80 μm wide) cracks within phenocrysts. Type 3) Very finely vesicular pumice clasts, with average total porosities of 54% (50% + 12 connected) and bulk densities of 1.48 g/cm3 + 0.35 (Nv=4.2 x 105 cm-3). Vesicle size distribution is exponential, with irregular vesicles between 24 and 8400 μm. Vesicles are crudely oriented, but locally aligned, and have thick bubble walls. Based on these textures, and the facies architecture of associated deposits, we infer conditions of vesiculation and bubble growth prior to eruption. Textures within type 1 clasts suggest uninhibited bubble growth, where the dominant foliation of these clasts suggests formation in regions of high strain rate gradients, shortly before fragmentation. Type 1 clasts are dominant in deposits from initially sustained eruptive columns that progressively wane, as indicated by normally graded fallout deposits. Textures within type 2 indicate multiple stages of deformation before fragmentation, where convolute bubble shapes and concave bubble walls indicate bubble collapse. Type 2 clasts are dominant in deposits from partially collapsing ash columns that formed interfingering pyroclastic density current and fallout deposits. Textures within type 3 suggest a more viscous regime, with relatively lower porosities and narrower bubble aperture sizes, likely formed from more volatile-poor magma. Type 3 clasts are dominant in deposits from sustained, steady eruptive columns, represented by massive, lithic-rich fallout beds. The coexistence of two or three pumice textures in the same stratigraphic level indicates heterogeneous strain rate and viscosity regimes in a single erupting magma body, where the rheology of fragmenting magma varies across the conduit. However, all pumice texture types point towards dominantly homogeneous nucleation and decompression-induced fragmentation.