2D and 3D fabric quantification of conduit textures to understand eruption dynamics and mechanisms: unique in situ example of Mt Unzen
Mineralogy, Petrology and Geochemistry
Final Report Abstract
The nature of sub-volcanic alteration is usually only observable after erosion and exhumation at old inactive volcanoes, via geochemical changes in hydrothermal fluids sampled at the surface, via relatively low-resolution geophysical methods or can be inferred from erupted products. These methods are spatially or temporally removed from the real subsurface and thus provide only indirect information. In contrast, the ICDP deep drilling of the Mt Unzen volcano subsurface affords a snapshot into the in situ interaction between the dacitic dykes that fed dome-forming eruptions and the sub-volcanic hydrothermal system, where the most recent lava dome eruption occurred between 1990 and 1995. Here, we analyze drill core samples from hole USDP-4, constraining their degree and type of alteration. We identify and characterize two clay alteration stages. These observations imply that the early clay- forming fluid was acidic and probably had a magmatic component. The porosity in the dyke samples is dominantly fracture-hosted, and fracture-filling mineralization is common, suggesting that the dykes were fractured during magma transport, emplacement and cooling, and that subsequent permeable circulation of hydrothermal fluids led to pore clogging and potential partial sealing of the pore network on a timescale of ~ 9 years from cessation of the last eruption. These observations, in concert with evidence that intermediate, crystal-bearing magmas are susceptible to fracturing during ascent and emplacement, lead us to suggest that arc volcanoes enclosed in highly fractured country rock are susceptible to rapid hydrothermal circulation and alteration, with implications for the development of fluid flow, mineralization, stress regime and volcanic edifice structural stability. The additional detailed study of the Mount Unzen spine reveals the competing occurrence of compactional and dilational shear regimes during magma ascent in volcanic conduits. At depth, in areas subjected to high effective pressure, shearing may induce pore compaction, thereby lowering the permeability of the system and inhibiting lateral outgassing to the country rock. At shallower depth, where the effective pressure may be low, shearing may favor localized dilation that enhances permeability. Both shear regimes result in the development of permeability anisotropy, with permeability generally being highest parallel or sub-parallel to the direction of extrusion and lowest perpendicular to the shear plane. The observation of shearing mode overprints suggests that fluctuations in effective pressure and strain rates, during stick-slip cycles, may result in magma switching between compactant and dilational shearing regimes, thus dynamically reshaping fluid circulation at a range of scales and in turn controlling outgassing efficiency during magma ascent and eruption.
Publications
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Rapid alteration of fractured volcanic conduits beneath Mt Unzen. Bulletin of Volcanology, 83(5).
Yilmaz, Tim I.; Wadsworth, Fabian B.; Gilg, H. Albert; Hess, Kai-Uwe; Kendrick, Jackie E.; Wallace, Paul A.; Lavallée, Yan; Utley, James; Vasseur, Jérémie; Nakada, Setsuya & Dingwell, Donald B.
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Transient conduit permeability controlled by a shift between compactant shear and dilatant rupture at Unzen volcano (Japan). Solid Earth, 13(5), 875-900.
Lavallée, Yan; Miwa, Takahiro; Ashworth, James D.; Wallace, Paul A.; Kendrick, Jackie E.; Coats, Rebecca; Lamur, Anthony; Hornby, Adrian; Hess, Kai-Uwe; Matsushima, Takeshi; Nakada, Setsuya; Shimizu, Hiroshi; Ruthensteiner, Bernhard & Tuffen, Hugh
