2020 | Halogen transport from magma reservoirs to the atmosphere: constraints from experiments and nature
Volcanic arcs enable mass transport between the Earth’s interior and Atmosphere. An important part of this geochemical cycle is the emission of volatile compounds as volcanic plumes. Analysis of plume chemistry can help to monitor volcanic unrest, forecast eruptions, and provide quantitative constraints on environmental and climate impacts of volcanic activity. Determining how certain volcanic gas exhalations link to different deep magma reservoir processes like crystal and bubble growth and magma ascent is a fertile area of research (e.g., Schipper et al., 2019). Although significant progress has been made in the study of some volatiles (e.g. H2O, CO2, S), our understanding of the highly-reactive halogens (e.g., Cl, F) that are ubiquitous in volcanic plumes is lacking (Aiuppa et al. 2009). Here we aim to address this prob- lem, by making some of the first experimental determinations of halogen diffusivity at magmatic condi- tions, and linking these to observations of halogen (Cl and F) release from active volcanoes in the Vanua- tu arc. In this multi-disciplinary effort, we will first experimentally constrain halogen (Cl, F) diffu- sivities in trachyandesitic melts—representative of many evolved basalt matrix melts—via the diffusion couple technique. We will then link these unique data to field measurements of halogen flux from a known halogen- rich volcano, Ambrym (Vanuatu) (Bani et al. 2009). There are currently no halogen diffusivities available for trachyandesitic melts, hindering our ability to estimate rates of halogen exsolution into bub- bles, and to ultimately forward-model outgassing of these hazardous elements into the atmosphere. In order to put the experimental data in a natural context, we will partner with the ambitious Volca- no Waka Lab project (www.wakalab.org), which was awarded to Dr. C. Ian Schipper of Victoria University of Wellington. This project aims to determine the flux of volatile compounds from the volcanic arcs of Melanesia (including Vanuatu), which host the most prodigiously-degassing and understudied volcanoes on Earth (Carn et al. 2017). Furthermore, the project will balance gas emissions against what is known about petrological reservoirs and processes in the region. Ambrym volcano is the ideal location for the study of halogen fluxes for several reasons. Firstly, it is the world’s leading emitter of volcanic gases, meaning it is guaranteed to provide rich plumes where high-quality data can be collected (Allard et al. 2016). Secondly, its emissions are a product of relatively quiescent, passive degassing, meaning it is not a hazardous place to work (Allard et al. 2016). Thirdly, it has already been shown to be a major emitter of halogens, to the point that its exhalations have had demonstrable health impacts on local populations (Allibone et al. 2012). The proposed work will thus address a significant shortcoming in our understanding of halogen mobility in volcanic systems, while addressing the specific impacts of the world’s most intense emitter of volcanic gases. It will furthermore contribute to an ambitious international effort to constrain volatile release from subduction zones, and the balance of elements across the lithosphere-atmosphere divide.
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