Megacrystals track magma convection between reservoir and surface

• Published in: Earth and Planetary Science Letters
• Main Authors: Moussallam, Yves, Oppenheimer, Clive, Scaillet, Bruno, Buisman, Iris, Kimball, Christine, Dunbar, Nelia, Burgisser, Alain, Schipper, C. Ian, Andújar, Joan, Kyle, Philip
• Other Authors: Institut des Sciences de la Terre d'Orléans - UMR7327 (ISTO), Centre National de la Recherche Scientifique (CNRS)-Université d'Orléans (UO)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire des Sciences de l'Univers en région Centre (OSUC), Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire de Paris, Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS)-Bureau de Recherches Géologiques et Minières (BRGM) (BRGM), Department of Geography Cambridge, UK, University of Cambridge UK (CAM), Department of Earth Sciences Cambridge, UK, Department of Earth and Environmental Science Socorro, New Mexico Institute of Mining and Technology New Mexico Tech (NMT), Institut des Sciences de la Terre (ISTerre), Université Joseph Fourier - Grenoble 1 (UJF)-Institut Français des Sciences et Technologies des Transports, de l'Aménagement et des Réseaux (IFSTTAR)-Institut national des sciences de l'Univers (INSU - CNRS)-Institut de recherche pour le développement IRD : UR219-PRES Université de Grenoble-Université Savoie Mont Blanc (USMB Université de Savoie Université de Chambéry )-Centre National de la Recherche Scientifique (CNRS), School of Geography, Victoria University Melbourne, National Science Foundation (Division of Polar Programs) under grant ANT1142083, European Project: 279790,EC:FP7:ERC,ERC-2011-StG_20101014,ELECTROLITH(2011)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2015
• Subjects: melt inclusion; megacrystal; anorthoclase; convection; bi-directional flow; crystal zoning; [SDU]Sciences of the Universe [physics]; [SDU.STU]Sciences of the Universe [physics]/Earth Sciences; [SDU.STU.VO]Sciences of the Universe [physics]/Earth Sciences/Volcanology; Lava Lake; Antarc*; Antarctica
• Online Access: https://hal-insu.archives-ouvertes.fr/insu-01124396; https://hal-insu.archives-ouvertes.fr/insu-01124396/document; https://hal-insu.archives-ouvertes.fr/insu-01124396/file/Moussallam-EPSL-2015.pdf; https://doi.org/10.1016/j.epsl.2014.12.022

International audience Active volcanoes are typically fed by magmatic reservoirs situated within the upper crust. The development of thermal and/or compositional gradients in such magma chambers may lead to vigorous convection as inferred from theoretical models and evidence for magma mixing recorded in volcanic rocks. Bi-directional flow is also inferred to prevail in the conduits of numerous persistently-active volcanoes based on observed gas and thermal emissions at the surface, as well as experiments with analogue models. However, more direct evidence for such exchange flows has hitherto been lacking. Here, we analyse the remarkable oscillatory zoning of anorthoclase feldspar megacrystals erupted from the lava lake of Erebus volcano, Antarctica. A comprehensive approach, combining phase equilibria, solubility experiments and melt inclusion and textural analyses shows that the chemical profiles are best explained as a result of multiple episodes of magma transport between a deeper reservoir and the lava lake at the surface. Individual crystals have repeatedly travelled up-and-down the plumbing system, over distances of up to several kilometers, presumably as a consequence of entrainment in the bulk magma flow. Our findings thus corroborate the model of bi-directional flow in magmatic conduits. They also imply contrasting flow regimes in reservoir and conduit, with vigorousconvection in the former (regular convective cycles of ∼150 days at a speed of ∼0.5mms−1) and more complex cycles of exchange flow and re-entrainment in the latter. We estimate that typical, 1-cm-wide crystals should be at least 14 years old, and can record several (from 1to 3) complete cycles between the reservoir and the lava lake via the conduit. This persistent recycling of phonolitic magma is likely sustained by CO2fluxing, suggesting that accumulation of mafic magma in the lower crust is volumetrically more significant than that of evolved magma within the edifice.