5:15 PM - 7:15 PM
[SVC33-P05] Vesicles and crystals are often next to each other: A CT observation of the lava erupted at Unzen Volcano
Keywords:CT, lava, Unzen, vesicle, crystal, segmentation
Outgassing, the separation of volcanic gases from the melt phase during the ascent of magma and is sufficiently released into the atmosphere, can suppress the explosion, resulting in effusive eruptions. Permeable gas flow through the interconnected bubble networks may provide outgassing pathways, but the longevity of such a structure is still under debate. The importance of the crystal framework has been suggested (Lindoo et al., 2017). However, it is not obvious whether it really exists or whether it also exists in silicic magma. Therefore, we observed the positional relationship between vesicles and crystals in dacitic lava.
We observed lava samples from the Unzen Fugen-dake Volcano 1990-1995 eruption: bread crust bomb (BCB), lava from pyroclastic flow deposits (PFD), and two lava spine samples. X-ray computed tomography (CT) is useful for observing the internal structure of lava, but discerning plagioclase crystals from silicate melt is challenging in CT. Here, in addition to the CT of the general X-ray examination system, we used Synchrotron radiation X-ray and performed three kinds of X-ray scanning with different image pixel sizes: micro-CT, computed laminography (CL), and nano-CT. The CL and the nano-CT were performed at SPring-8. Then, we segmented reconstructed 3D images into vesicles, crystals, and a melt phase (glass at room temperature).
The segmented reconstructed 3D images show that large visible vesicles appeared to be in contact with crystals. Vesicles are widely dispersed in BCB and most localized in the shear-deformed lava spine (spine-shear). In spine-shear, we could visualize vesicles extending in narrow gaps between crystals. By combining the observation of the segmented reconstructed 3D images with SEM analysis, we found that there were sheet-shaped vesicles between crystals of various kinds of minerals in spine-shear. On the other hand, in BCB, large vesicles are connected through numerous vesicles sufficiently smaller than 1 mm, which are not resolved by micro-CT. Still, high-resolution imaging made it possible to visualize them. It was also observed that crystals found in BCB were often next to vesicles.
The vesicularity of BCB corresponds to a depth of ∼0.5 km. The melt phase at this depth is deformable in terms of viscosity. Therefore, the bubble films separating growing bubbles eventually rupture, gas escapes, and then bubbly magma shrinks. The low vesicularity (∼0.1) magma, as is observed in the spine and PFD that have erupted to the surface and formed lava domes, would lose the permeable pathways. However, if crystals covered with bubbles in shrinking magma approach each other, the crystals can contact each other with bubbles in between. In this case, crystals can make a framework to maintain the permeable network. Near the conduit wall, crystals would have been lined up by shear deformation due to the magma ascent, efficiently creating outgassing pathways.
We observed lava samples from the Unzen Fugen-dake Volcano 1990-1995 eruption: bread crust bomb (BCB), lava from pyroclastic flow deposits (PFD), and two lava spine samples. X-ray computed tomography (CT) is useful for observing the internal structure of lava, but discerning plagioclase crystals from silicate melt is challenging in CT. Here, in addition to the CT of the general X-ray examination system, we used Synchrotron radiation X-ray and performed three kinds of X-ray scanning with different image pixel sizes: micro-CT, computed laminography (CL), and nano-CT. The CL and the nano-CT were performed at SPring-8. Then, we segmented reconstructed 3D images into vesicles, crystals, and a melt phase (glass at room temperature).
The segmented reconstructed 3D images show that large visible vesicles appeared to be in contact with crystals. Vesicles are widely dispersed in BCB and most localized in the shear-deformed lava spine (spine-shear). In spine-shear, we could visualize vesicles extending in narrow gaps between crystals. By combining the observation of the segmented reconstructed 3D images with SEM analysis, we found that there were sheet-shaped vesicles between crystals of various kinds of minerals in spine-shear. On the other hand, in BCB, large vesicles are connected through numerous vesicles sufficiently smaller than 1 mm, which are not resolved by micro-CT. Still, high-resolution imaging made it possible to visualize them. It was also observed that crystals found in BCB were often next to vesicles.
The vesicularity of BCB corresponds to a depth of ∼0.5 km. The melt phase at this depth is deformable in terms of viscosity. Therefore, the bubble films separating growing bubbles eventually rupture, gas escapes, and then bubbly magma shrinks. The low vesicularity (∼0.1) magma, as is observed in the spine and PFD that have erupted to the surface and formed lava domes, would lose the permeable pathways. However, if crystals covered with bubbles in shrinking magma approach each other, the crystals can contact each other with bubbles in between. In this case, crystals can make a framework to maintain the permeable network. Near the conduit wall, crystals would have been lined up by shear deformation due to the magma ascent, efficiently creating outgassing pathways.