Autoren:	Feisel, Yves
Hauptberichter:	Castro, Jonathan M.
Titel:	Halogen diffusion in silicic melt and implications for magmatic processes
Online-Publikationsdatum:	22-Mär-2023
Erscheinungsdatum:	2023
Sprache des Dokuments:	Englisch Englisch
Zusammenfassung/Abstract:	Dissolved magmatic volatiles control many substantial physico-chemical properties of silicate melt which is why they are of profound importance to the nature and style of volcanic eruptions. Even though the halogens account only for a small fraction of the typical volcanic volatile budget, they exhibit characteristics that stand out among the volatiles and may affect fundamental melt properties and — once released from the volcano — impact the environment and atmosphere. Moreover, their properties in silicate melts bear the potential to utilize halogen degassing behavior to characterize magmatic processes at shallow crustal levels. To support the development of a detailed understanding of halogen behavior in silicate melts, this thesis presents experimental results of halogen (F, Cl, Br, I) diffusion experiments in silica-rich natural melts under both dry and hydrous conditions. The diffusion couple technique was applied with a range of different experimental setups at temperatures characteristic of natural magmatic systems (750–1200 °C) and pressures of 0.1 and 160 MPa. Experimental samples were evaluated using a variety of analytical methods. The experiments reveal that halogen diffusion in silicic melt follows Arrhenian behavior and is strongly dependent on the melt structure. In anhydrous silicic melt, halogen diffusivity is inversely correlated with the ionic radius of the diffusing species, spanning a range of diffusivities of 3–4 orders of magnitude among F, Cl, Br, and I. Upon the addition of 1.5 wt.% of H2O to the melt, the diffusivity of all halogens increases significantly due to the depolymerizing effect of water. The increase is most pronounced for the largest halogen iodine and is subsequently less for the smaller halogens, leading to a narrower diffusive range of only 1–2 orders of magnitude and, consequently, merely a weak correlation of diffusivity and ionic radius compared to the anhydrous case. This behavior is interpreted to represent a gradual decrease of the influence of ionic porosity on the diffusion-rate, making the process of bond-breaking more relevant as a diffusion-rate-limiting mechanism with increasing water content. In a related experimental study, the effects of F and Cl on the phase equilibria of a near-liquidus hydrous trachyte melt was investigated by employing a novel disequilibrium approach which involved diffusion couple experiments at temperatures of 925–990 °C and 100 MPa. The results demonstrate a pronounced decrease of the melt’s liquidus temperature by ~50 °C, which is induced by the diffusion of F and Cl into a formerly halogen-depleted clinopyroxene-bearing melt. These observations suggest that the liquidus decrease is a consequence of clinopyroxene dissolution by increased solubility of its main cationic components (Fe, Mg, Ca) due to their complexation with F and Cl. These findings illustrate that even low fluctuations in halogen concentration can strongly influence the physical properties of the melt, e.g., by shifting a melt’s liquidus temperature due to pre- or syn-eruptive volcanic degassing. The findings of this thesis provide a significant addition to the understanding of magmatic halogen behavior and include the first ever diffusion data of iodine in silicate melt. Moreover, the results may be applied in future studies to characterize magmatic processes at depth, such as bubble growth or magma ascent, and may ultimately help to identify volcanic unrest of silicic volcanoes.
DDC-Sachgruppe:	550 Geowissenschaften 550 Earth sciences
Veröffentlichende Institution:	Johannes Gutenberg-Universität Mainz
Organisationseinheit:	FB 09 Chemie, Pharmazie u. Geowissensch.
Veröffentlichungsort:	Mainz
ROR:	https://ror.org/023b0x485
DOI:	http://doi.org/10.25358/openscience-8924
URN:	urn:nbn:de:hebis:77-openscience-170f7e40-59cf-4e49-8a07-9a843d0d08ad4
Version:	Original work Original work
Publikationstyp:	Dissertation Dissertation
Nutzungsrechte:	Urheberrechtsschutz
Informationen zu den Nutzungsrechten:	http://rightsstatements.org/vocab/InC/1.0/
Umfang:	xvi, 175 Seiten, Illustrationen, Diagramme
Enthalten in den Sammlungen:	JGU-Publikationen