Mechanism of Volcanic Eruptions; Microstructure of fluid-bearing rocks

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1 Michihiko Nakamura Title/Affiliation Specialized Field Research Subject Associate Professor/ Department of Earth Sciences, Graduate School of Sciences, Tohoku University Petrology, Volcanology Mechanism of Volcanic Eruptions; Microstructure of fluid-bearing rocks Objectives: (1) Since bubble growth and expansion in magmas are the driving forces of violent volcanic eruptions, the mechanism of degassing (outgassing) has been a central topic in volcanology. In the last two decades, the permeable gas-flow hypothesis has been widely accepted as an explanation of the degassing of viscous silicic magmas, in which bubbles scarcely rise in the time scale of eruptions. As magma ascends, the solubility of volatiles decreases and the bubbles expand due to decompression, resulting in an increase in magma vesicularity and the formation of foam. This foam should be highly permeable for effective degassing. However, mechanisms of the permeable flow degassing have been poorly understood. To address this issue, Nakamura has been carried out integrated study of experimental volcanology, petrography and permeability measurement. (2) Knowledge of the fundamental mechanism of fluid-rock interaction is indispensable to understand geochemical dynamics. The mechanisms generally assumed to be rate-limiting during chemical exchange are lattice diffusion and dissolution-reprecipitation. This assumption is, however, often difficult to test in natural rock samples because grain-scale chemical traces of fluids can be easily blurred by annealing and late stage geologic events. Previous studies have pointed out that the two conventional processes fail to explain some geochemical observations such as enhanced chemical exchange in rock recrystallization through coarsening, exsolution and stress deformation, but the fundamental processes leading to enhanced exchange remain uncertain. In order to investigate the mechanism of fluid-rock interaction, we have carried out hydrothermal experiments at 1200ºC and 1.2 GPa, where Ni infiltrated into synthesized dunites. Results: (1) Mechanisms of permeable flow degassing of magmas In this period, four international papers were published and one paper has been submitted. * We presented a relation between permeability and vesicularity of pumices in a pyroclastic flow, which revealed the processes of fragmentation, degassing and compaction of the magmatic foam. The newly obtained relation is different from the conventional power law curve, suggesting the necessity to review the conventional interpretation of permeability development in magmas (Nakamura et al., 2008, J. Volcanol. Geotherm. Res.) * To investigate the behavior of bubbles in a hypothetical open-system condition, we performed a series of vesiculation experiments on natural rhyolitic obsidian using a newly designed semipermeable cell. We found that diffusive dehydration occurred from the sample surface, and the bubbles were resorbed into the melt. The numerical calculation shows that the bubble-free obsidian layers with a width of a few millimeters, which are often observed in natural lava flows, requires the degassing paths open for at least a

2 few hundred hours. (Yoshimura and Nakamura, 2008, J. Volcanol. Geotherm. Res.; S. Yoshimura is a Ph.D. student awarded SDC by GCOE in 2008, and DC by JSPS from 2009.) * To investigate the shear-induced evolution of bubble microstructures, we performed for the first time a series of deformation experiments on vesiculated rhyolitic melts by twisting columnar obsidians with an originally-developed torsional deformation high-temperature apparatus. The experimental results demonstrate that shear strain and strain rate control the degree of bubble deformation and coalescence. The bubble connectivity starts to increase for a vesicularity of vol%. A connectivity of >80% was achieved for a vesicularity of approximately 40 vol% and 10 rotations at 0.5 rpm, corresponding to a maximum strain of 30 at a strain rate of s -1. We infer that the bubbles form large interconnecting channelized networks near the conduit wall before the magma reaches to this depth (Okumura, Nakamura et al., 2008, J, Geophys. Res.; S. Okumura is a GCOE-PD). * We experimentally demonstrated that shear deformation dramatically increases the magma permeability parallel to the shear direction via the enhancement of bubble coalescence and the networking of tube-like bubbles. When shear strain is large, the permeability sharply increases at a vesicularity of 30 vol.%. The gas velocity along the direction of magma flow is inferred to be large enough for gas escape to be significant during magma ascent of effusive eruption. A simple model of the magma flow along volcanic conduits indicates that magma deformation results in degassing at greater depths than in the case where the magma is isotropically vesiculated. The ratio of the radius of the volcanic conduit to its length may control the degree of magma deformation and consequently the eruption explosivity (Okumura, Nakamura et al., 2009, Earth Planet. Sci. Lett.). * Another companion paper entitled "Shear deformation experiments of vesicular rhyolite: brittle fracturing, degassing, and compaction of magmas" by Okumura, Nakamura et al. has been submitted to J. Volcanol. Geotherm. Res. (2) Role of grain growth on chemical transport in fluid-bearing rocks * We evaluated experimentally the effect of grain growth on the cation exchange between synthesized forsterite aggregates (i.e., dunite) and Ni-rich aqueous fluid at 1.2 GPa and 1200 C. The grain boundary (GB) migration caused Ni-enrichment in the area swept by the GBs, in a fashion similar to that reported for stable isotope exchange in the quartz aggregates. The progress of the grain growth resulted in an increase in the average nickel concentration in the dunites of up to 8.15 times that calculated for a system having stationary GBs. The overall diffusivity of the nickel along the wet GBs and interconnected fluid networks was found to be m 3 /s, which is 4 5 orders of magnitude higher than the grain boundary diffusivity in the dry dunite. These results show that the grain growth rate is a fundamental factor in the evaluation of the time-scale of chemical homogenization in the upper mantle (Ohuchi, Nakamura et al., submitted to Contrib. Mineral. Petrol.; this experimental study was carried out when Ohuchi was a 21COE-PD). Publications: Journals: 1. Nakamura, M., Ohtaki, K. and Takeuchi, S., Permeability and pore-connectivity variation of pumices from a single pyroclastic flow eruption: Implications for partial fragmentation, J. Volcanol. Geotherm. Res., 176, , doi: /j.jvolgeores , 2008.

3 2. Okumura, S., Nakamura, M., Tsuchiyama, A., Nakano, T. and Uesugi, K., Evolution of bubble microstructure in sheared rhyolite: Formation of a channel-like bubble network, J. Geophys. Res., 113, B07208, doi: /2007jb005362, Okumura, S., Nakamura, M., Takeuchi, S., Tsuchiyama, A., Nakano, T., Uesugi, K., Magma deformation may induce non-explosive volcanism via degassing through bubble networks, Earth Planet. Sci. Lett., 281, , doi: /j.epsl , Ohuchi, T., Nakamura, M., Michibayashi, K., Effect of grain growth on cation exchange between dunite and fluid: implications for chemical homogenization in the upper mantle, submitted to Contrib. Mineral., Petrol. 5. Okumura, S., Nakamura, M., Nakano, T., Uesugi, K., Tsuchiyama, A., Shear deformation experiments on vesicular rhyolite: Implications for brittle fracturing, degassing, and compaction of magmas in volcanic conduits, submitted to J. Geophys. Res. 6. Yoshimura, S. and Nakamura, M. Diffusive dehydration and bubble resorption during open-system degassing of rhyolitic melts, Journal of Volcanology and Geothermal Research, 178, 72 80, doi: /j.jvolgeores , Nakamura, M., Analyses of pyroclastic deposits to investigate dynamics of volcanic eruptions, submitted to J. Geol. Soc. Japan (in Japanese with English abstract). 8. Nakamura, M., Kasai, Y., Sato, N. and Yoshimura, S. Application of Hydrogen Isotope Geochemistry to Volcanology: Recent Perspective on Eruption Dynamics. Proceedings of the 5th Internatinal Workshop on Water Dynamics, Sendai, Japan, September 2007, American Institute of Physics Conference Proceedings, 987, 93 99, Books: Contribution to books: 1. Nakamura, M., An Approach to Volcanic Explosions: Towards a better understanding of eruption mechanisms with application to volcanic hazard mtigation, (Eds., Y. Ida and H. Taniguchi), University of Tokyo Press, 53-62, 77-87, 2009 (in Japanese). Symposium Participations: 1. Okumura S., Nakamura, M., Takeuchi, S., Tsuchiyama, A., Nakano, T. and Uesugi, K., Magma deformation may induce non-explosive volcanism via degassing through bubble networks.international Association of Volcanology and Chemistry of the Earth s Interior, Reykjavík, Iceland, August 17 22, Nakamura, M., Ohuchi, T., Michibayashi, K., Chemical transport between minerals and fluids enhanced by grian boundary migration: a case of compatible elements, The 2nd International Symposium on Interface Mineralogy, March 9-12, 2009, Sendai, Japan. 3. Yoshimura, S., and Nakamura, M., Growth and resorption of bubbles by chemical exchange between disequilibrium fluids, Americal Geophysical Union Fall Meeting, San Francisco, Decmber 15 19, Nakamura, M., Takeuchi, S., Otaki, K., Magma Permeability Revisited: Development with vesiculation and fragmentation in a single eruption, JPGU Meeting 2008, May 25-30, 2008, Makuhari, Japan. 5. Okumura, S., Nakamura, M., Tsuchiyama, A., Nakano, T. and Uesugi, Experimental deformation of

4 viscous fluid with gas bubbles: Microstructure and permeability, and implication for volcanic eruption, JPGU Meeting 2008, May 25-30, 2008, Makuhari, Japan. 6. Okumura, S., Nakamura, M., Uesugi, K., Tsuchiyama, A., Nakano, T., Experimental formation of brittle fracture in magma: Shear fracturing and fragmentation of magma induce open-system degassing? JPGU Meeting 2008, May 25-30, 2008, Makuhari, Japan. 7. Okumura, S., Nakamura, M., Effect of relaxation of bubble shape by surface tension and magma compaction on permeable degassing of magma, 2008 Fall meeting of the Volcanological Society of Japan, Oct , 2008, Morioka, Japan. 8. Okumura, S., Nakamura, M., Uesugi, K., Nakano, T., Tsuchiyama, A., Open-system degassing of flowing and stagnant magmas in a volcanic conduit, JPGU Meeting 2009, May 16-21, 2009, Makuhari, Japan. 9. Yoshimura, S., and Nakamura, M., Diffusive fractionation of H2O and CO2 by vesiculation of rhyolitic melts,jpgu Meeting 2008, May 25-30, 2008, Makuhari, Japan. 10. Nakamura, M., Sato, N., Volcanic eruption dynamics inferred from subunit analysis of pyroclastic deposits, 2008 Annual Meeting of the Geological Society of Japan, Sep , 2008, Akita, Japan (Invited). 11. Matsui, R., Nakamura, M., Yoshida, T., Nagahashi, Y., A petrological study on the magma plumbing systems in the Niijima and Shikinejima volcanoes, Izu Islands, 2008 Annual Meeting of Japan Association of Mineralogical Sciences, Sep , 2008, Akita, Japan. 12. Nakamura, M., Tamura, S., Iguchi, M., Miki, D., Triggering Mechanism of the Three Historical Eruptions of the Sakurajima Volcano: Constraint from the Mineral Chemistry of Magnetite, 2008 annual meeting of the Disaster Prevention Research Institute (DPRI) of the Kyoto University, Feb.24-25, 2009, Kyoto, Japan. 13. Kichise, T., Nakamura, M., Yasui, M., Yoshida, T., Nagahashi, Y., Origin of juvenile lithic fragments in the Asama A,B' and B pumice-fall eruptions, 2008 Fall meeting of the Volcanological Society of Japan, Oct , 2008, Morioka, Japan. 14. Nakamura, M., Sato, N., Shallow-level degassing in an intermittent explosive eruption: A case study of Asama Volcano Tenmei eruption, 2008 Fall meeting of the Volcanological Society of Japan, Oct , 2008, Morioka, Japan. 15. Yoshimura, S., Nakamura, M., Chemical interaction between H2O-rich bubbles and CO2-rich fluid, 2008 Fall meeting of the Volcanological Society of Japan, Oct , 2008, Morioka, Japan. 16. Abe, M., Nakamura, M., Experimental study of aqueous fluid distribution in amphibolitic lower crust, JPGU Meeting 2009, May 16-21, 2009, Makuhari, Japan. 17. Okumura, H., Nakamura, M., Crystallization experiments in the system Ab-Qtz-H2O: Role of supercritical fluid and flux elements, JPGU Meeting 2009, May 16-21, 2009, Makuhari, Japan. 18. Tamura, S., Nakamura, M., Iguchi, M., Miki, D., Comparative petrology of the historical large eruptions of the Sakurajima Volcano: constraints from magnetite chemistry, JPGU Meeting 2009, May 16-21, 2009, Makuhari, Japan. 19. Kichise, T., Nakamura, M., Yasui, M., Yoshida, T., Nagahashi, Y., Petrogenetic study of dense fragments included in pumice-fall eruption; case study of Asama volcano at 1108 and 1128 eruptions, JPGU Meeting 2009, May 16-21, 2009, Makuhari, Japan.

5 20. Matsui, R., Nakamura, M., Yoshiki, K., Kuritani, T., Yoshida, T., Suzuki, T., Nagahashi, Y., A petrological study on the magma plumbing systems in the Niijima and Shikinejima volcanoes, Izu Islands, JPGU Meeting 2009, May 16-21, 2009, Makuhari, Japan. 21. Yoshimura, S., Nakamura, M., Healing experiments of fractures in a rhyolitic melt: Estimation of lifetime of degassing pathways, JPGU Meeting 2009, May 16-21, 2009, Makuhari, Japan.