INV10d-0001
Using crystal textures and compositional zoning to investigate magma storage and transport timescales across tectonic settings

Tuesday, 15 December 2020: 18:00
Fiona Couperthwaite1, Adam Kent2, Paul J Wallace3, Dan J Morgan4, Oliver Nebel5, Thor Thordarson6 and Jason Harvey4, (1)Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, (2)Oregon State University, Corvallis, United States, (3)University of Oregon, Department of Earth Sciences, Eugene, OR, United States, (4)University of Leeds, School of Earth and Environment, Leeds, United Kingdom, (5)Monash University, School of Earth, Atmosphere and Environment, Melbourne, VIC, Australia, (6)University of Iceland, Nordic Volcanological Center, Institute of Earth Sciences, Reykjavik, Iceland
Abstract:
Crystals found in volcanic deposits often record the ambient magma conditions and its storage and transport processes prior to an eruption. Considering their textures and utilising petrological tools (thermometry and diffusion modelling), we reconstruct the conditions and processes that these crystals, and the magma in which they are carried, experienced in the subsurface. We use olivine crystals in tephras and/or lavas from Mauna Loa, HI (hotspot), Red Rock Volcanic Complex, Australia (intraplate) and Four In One, Cascades (continental subduction) to deduce the magma storage and transport network below. The Moinui flow at Mauna Loa contains two distinctive olivine populations - equant cumulates (forsterite (Fo) core compositions Fo90-88) and platy olivines (Fo83-82 cores). Diffusion timescales are also bi-modal; weeks-years (equant) and days up to a few weeks (platy). Combining textural and compositional evidence with the diffusion timescales, we follow the journey of the equant crystals from depth to a long period of shallower storage. Nucleation of the bladed olivines occurred later upon magma ascent to the surface, maturing during emplacement of the flow. At Red Rock Volcanic Complex we again found at least two populations of olivine –normally zoned olivines (Fo91-88 cores) with thick overgrowths, and a smaller, reversely zoned population (Fo67-62 cores) together with crystals exhibiting reaction textures. We capture the characteristic timescale between disaggregation of olivine in the subsurface (mostly < 1 month) and the eventual eruption. We use a tephra sequence at Four in One, to investigate the magma plumbing structure and dynamics at high-resolution, by analysing the zoning or lack of zoning patterns with crystal textures. The timescales captured in the olivine reveals prolongued times of subsurface melt storage and lithospheric interaction. Tectonic setting defines melt volume, focused melt supply and with this the existence of incubating magma chambers.
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