Slow ascent of unusually hot intermediate magmas triggering Strombolian to sub-Plinian eruptions

Monday, 14 December 2020: 16:15
Virtual
Charline Lormand1, Georg F Zellmer2, Geoff Kilgour3, Karoly Nemeth4, Alan S Palmer4, Naoya Sakamoto5, Hisayoshi Yurimoto6, Takeshi Kuritani6, Yoshiyuki Iizuka7 and Anja Moebis4, (1)Massey University, Volcanic Risk Solutions, Palmerston North, New Zealand, (2)Massey University, Volcanic Risk Solutions, Palmeston North, New Zealand, (3)GNS Science, Wairakei Research Center, Taupo, New Zealand, (4)Massey University, Palmerston North, New Zealand, (5)Hokkaido University, Isotope Imaging Laboratory, Sapporo, Japan, (6)Hokkaido University, Sapporo, Japan, (7)Academia Sinica, Institute of Earth Sciences, Taipei, Taiwan
Recorded Presentation
Abstract:
To assess whether magma ascent rates control the style of volcanic eruption, we have studied the petrography, geochemistry and size distribution of microlites of plagioclase and pyroxene from historical eruptions from Tongariro, Ruapehu and Ngauruhoe volcanoes located in the Tongariro Volcanic Centre (TgVC), New Zealand. The studied deposits represent glassy andesitic and dacitic tephra shards from several eruptive formations. Covering a range in eruption styles and sizes from Strombolian to sub-Plinian, these samples provide an excellent opportunity to explore fundamental volcanic processes such as pre-eruptive magma ascent processes. Our quantitative petrographic analysis shows that larger microlites display complex growth zoning, and only the smallest crystals have formed during magma ascent in the conduit. By combining orthopyroxene geothermometry, plagioclase hygrometry, and MELTS modelling, we show that these microlites nucleated at maximum pressures of 550 MPa from hot andesitic magmas (> 1130 ˚C) with low H2O content (0-1.5 wt%). Size distributions of more than 60,000 microlites yield concave-up curves, and the slopes of the pyroxene microlite size distributions, in combination with well-constrained orthopyroxene crystal growth rates from one studied tephra, indicate microlite population growth times of 3 ± 1 days, irrespective of eruption style. These data imply that microlites form in response to cooling of melts ascending at velocities of < 5 cm s-1 prior to H2O exsolution, which only occurs at < 33 MPa. The maximum magma ascent rate in the upper conduit, calculated using the exsolution of water during final decompression, is of 12 m s-1, i.e. at least an order of magnitude lower than the hypersonic vent velocities typical of sub-Plinian eruptions. This implies that magma ascent from depths of an average of 4 km occurs in dykes, and that vent velocities at the surface are controlled by conduit restriction towards the surface and shallow fragmentation.
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