[S-VC33] Mechanism of volcanic eruptions

[SVC33-03] Petrological constraints on shallow conduit processes during fountain-collapse pyroclastic flows in the Asama 1783 eruption.

*Takuma Muraki¹, Michihiko Nakamura¹, Mayumi Mujin¹, Maya Yasui² (1.Department of Earth Science, Graduate school of science, Tohoku University, 2.Department of Earth and Environmental Sciences, College of Humanities and Sciences, Nihon University)

Keywords:Asama volcano, welding, pyroclastic flow, fountain-collapse, explosive–effusive transition, nanolite

Following fragmentation in volcanic conduits, some intermediate to silicic magmas may not reach the surface directly but weld or fallback, producing eruption-style diversity, as indicated by the common occurrence of clastogenic lava, obsidian pyroclasts, and syn-eruptive juvenile dense fragments. In the 1783 (Ten-mei era) eruption of the Asama volcano, these processes might have occurred synchronously with fountain-collapse pyroclastic flows (the Agatsuma pyroclastic flow) in the transition period from the explosive to the effusive eruptions highlighting their role in controlling eruption explosivity. We collected the samples from the upper part of the Agatsuma pyroclastic flow deposits and observed them with an optical microscope, field emission type scanning electron microscopy, and X-ray computed microtomography. The weakly vesiculated juvenile blocks (WVJB) were coated with welded volcanic ashes. They sometimes formed agglomerate-like clusters (AC) in size up to ~ 1 m across composed of pebble to boulder-sized WVJBs, which characterize the pyroclastic flow. The ash particles and WVJBs have diverse welding characteristics and groundmass crystallization microstructures. We compared them with those of our previous heating and crystallization experimental products to untangle the pressure (P)-oxygen fugacity (fO₂)-temperature (T)-time (t) paths in the syn- and post-fragmentation processes. Because the surface tension-driven relaxation and volcanic glass welding were generally faster than the groundmass crystallization observed in the samples by one to two orders of magnitudes, the ash coating and AC formation processes can be divided into two stages: the final welding and preceded crystallization. The shape and crystallinity of volcanic ashes in the coating ranged from angular glass shards (almost nanolite- and ultrananolite-free) to spherical ones with diverse proportions of nanolite and ultrananolite. On the other hand, the internal WVJBs are sometimes composed of a few smaller welded blocks and generally show higher groundmass crystallinity than volcanic ashes. The relative modal abundances of plagioclase and magnetite suggest that the crystallization of the ashes and WVJBs occurred in an oxidized condition. This microstructural diversity showed that the ashes and WVJBs took various P-T-fO₂-t paths before final welding, from quenching in the air to annealing near the fragmentation depth, leading to the interpretation of conduit processes including fall back into the conduit, welding, and re-fragmentation and their repetition. Comparison with the experimental studies allows us to constrain the time course of eruptive phenomena, i.e., the maximum crystallization duration that corresponds to the residence time in the shallow conduit is comparable to the time interval from the pre-climactic sub-Plinian phase to the end of the climactic Plinian eruption, in which the Agatsuma pyroclastic flows occurred, while the timescale of welding corresponds to the intervals of sub-Plinian eruptions. Based on these constraints, we propose that the fountain collapse-type pyroclastic flows were repeatedly generated through the shallow conduit being filled with welded ashes and blocks during the pre-climax phase of the eruption.