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Partitioning of elements between high-temperature, low-density aqueous fluid and silicate melt as derived from volcanic gas geochemistry

Zelenski, M, Simakin, A, Taran, Y, Kamenetsky, VS ORCID: 0000-0002-2734-8790 and Malik, N 2020 , 'Partitioning of elements between high-temperature, low-density aqueous fluid and silicate melt as derived from volcanic gas geochemistry' , Geochimica et Cosmochimica Acta, vol. 295 , pp. 112-134 , doi: 10.1016/j.gca.2020.12.011.

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Abstract

By comparing high-quality volcanic gas and whole rock compositions, we calculated the apparent (observed) mass partition coefficients Kd* for 58 elements on six basaltic volcanoes located in arc and rift/hotspot settings. The inferred Kd* vary from ∼1100 for sulfur to 0.0001 for zirconium, i.e., within seven orders of magnitude. Only 14 elements have Kd* > 1, including highly volatile S, Se, Te and halogens, as well as Tl, Re, Os, Bi, Cd, Au, In and As. Alkali metals have Kd* in the range from 0.1 for Cs to 0.01 for Na. Partition coefficients of other rock-forming elements are Kd* in sulfur-rich, HCl-poor and reduced rift/hotspot gases. Degassing directly from the free melt surface is negligible; deep gas passing through the erupting vent is quickly overwhelmed by the signal of low-pressure degassing. Equilibration of rising bubbles with the surrounding melt almost eliminates the difference between Kd* calculated for degassing lava flows (no connection with deep magma) and for lava lakes and open-vent volcanoes (convective mass exchange with deep magma takes place). Diffusion does not strongly affect the apparent partitioning of magmas degassing at surface. Gas bubbles growing in near-surface silicate melts at atmospheric pressure have a large density difference compared to the surrounding melt of 12–15 thousand times. This leads to the rapid expansion of such bubbles and a decrease in the thickness of the diffusion boundary layer in the melt due to its stretching around the growing bubble, which sharply decreases diffusion fractionation. As a result, the apparent partition coefficients (Kd*) for degassing basaltic volcanoes are close to the equilibrium ones (Kd) for most of the elements. The partition coefficients of volatile elements (S and Cl) calculated from the comparison of volcanic gas and rock compositions are in agreement with the values determined previously via experiments or theoretical modeling.

Item Type: Article
Authors/Creators:Zelenski, M and Simakin, A and Taran, Y and Kamenetsky, VS and Malik, N
Keywords: element partitioning, volcanic gas, silicate melt, bubble growth, diffusion, equilibrium
Journal or Publication Title: Geochimica et Cosmochimica Acta
Publisher: Pergamon-Elsevier Science Ltd
ISSN: 0016-7037
DOI / ID Number: 10.1016/j.gca.2020.12.011
Copyright Information:

Copyright 2020 Elsevier Ltd.

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