Experimental Phase-equilibrium Constraints on the Phonolite Magmatic System of Erebus Volcano, Antarctica

• Published in: Journal of Petrology
• Main Authors: Moussallam, Yves, Oppenheimer, Clive, Scaillet, Bruno, Kyle, Philip R.
• Other Authors: Department of Geography Cambridge, UK, University of Cambridge UK (CAM), Institut des Sciences de la Terre d'Orléans - UMR7327 (ISTO), Bureau de Recherches Géologiques et Minières (BRGM) (BRGM)-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)-Institut national des sciences de l'Univers (INSU - CNRS)-Université d'Orléans (UO)-Centre National de la Recherche Scientifique (CNRS), New Mexico Institute of Mining and Technology New Mexico Tech (NMT)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2013
• Subjects: lava lake; phase equilibrium; CO2; degassing; Erebus; phonolite; [SDU.STU.VO]Sciences of the Universe [physics]/Earth Sciences/Volcanology; [SDE.MCG]Environmental Sciences/Global Changes; Lava Lake; Antarc*; Antarctica
• Online Access: https://insu.hal.science/insu-00813668; https://insu.hal.science/insu-00813668/document; https://insu.hal.science/insu-00813668/file/JPet-Moussallam.pdf; https://doi.org/10.1093/petrology/egt012

International audience Field observations and petrological studies have recently advanced understanding of the magmatic system of Erebus volcano, renowned for its sustained CO2-rich degassing, and long-lived phonolitic lava lake. However, this body of work has highlighted uncertainty in several key parameters, including the magma temperature, redox state and the depth of the reservoir presumed to maintain the lava lake. Here, we use experimentally determined phase equilibria to constrain these unknowns. The experiments ranged in temperature from 900 to 1025°C, in pressure from atmospheric to 300 MPa, in water content from 0 to 8 wt %, and in oxygen fugacity from NNO + 4 (where NNO is nickel-nickel oxide) to QFM - 2 (where QFM is quartz-fayalite-magnetite). The natural system was experimentally reproduced at 950 ± 25°C, a pressure below 200 MPa, redox conditions between QFM and QFM - 1, and remarkably low water contents of less than 0*5 wt %. These findings help in understanding petrological observations, including melt inclusion data, as well as the measured composition of gas emissions from the lava lake. Biotite and amphibole appear in the crystallization sequence at around 925°C, even under very dry conditions (biotite). Both biotite and amphibole are absent in the phonolites erupted over the last 20 kyr at Erebus. The constant abundance of anorthoclase observed in the erupted lavas and bombs indicates that the shallow magmatic system feeding the Erebus lava lake (below pressures of 200 MPa) has been thermally buffered at 950 ± 25°C over this time period, possibly reflecting steady-state connection with the deep feeding system rooted in the mantle. Combined with recent seismological data, our results suggest that if a large phonolitic reservoir exists, then it should lie in the depth range 4-7*5 km. The tight constraints on temperature and redox conditions will be valuable for future thermodynamical and rheological modelling.