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Across-arc variations in K-isotope ratios in lavas of the Izu arc: Evidence for progressive depletion of the slab in K and similarly mobile elements

Across-arc variations in K-isotope ratios in lavas of the Izu arc: Evidence for progressive depletion of the slab in K and similarly mobile elements
Across-arc variations in K-isotope ratios in lavas of the Izu arc: Evidence for progressive depletion of the slab in K and similarly mobile elements

In subduction zones, fluids rise from the slab to the mantle, causing metasomatism and flux melting of the mantle to produce arc magmas. The transfer of material from slab to mantle and, in turn, to arc crust is an important control on the long-term chemical evolution of the mantle and continental crust. In this study, we investigate the transport of K in subduction zones by exploring the systematics of K stable-isotope variations in lavas of the Izu arc. We find that the Izu lavas have isotopically heavy K relative to estimates for midocean ridge basalt (MORB)-source upper mantle. Moreover, the δ 41K values of the lavas are clearly heavier than those of subducting sediments and are probably heavier than subducting altered ocean crust. An across-arc decrease in δ 41K values is apparent. Arc-front lavas are heavier than the mantle by about 0.22‰ (median), whereas rear-arc lavas are heavier by only about 0.08‰ (median). The heavy K-isotope compositions of the arc lavas may arise from isotopic fractionation during slab dehydration, where light K is preferentially retained in phases such as phengite in the slab. The across-arc decrease in δ 41K values may be due to progressive breakdown of these phases, and to associated depletion of the slab in heavy K. Variations in the relative contributions of different source materials—igneous ocean crust, sediment, and mantle peridotite—may also play a role. In particular, we explore a possibility, motivated by radiogenic-isotope studies, that the slab signal in K isotopes may be attenuated in the rear arc as a result of extensive fluid-mantle interaction. If K isotopes do track slab dehydration, then K isotopes provide insight into the transfer of K and similarly mobile elements out of the slab and into the upper mantle and arc crust. Lastly, we observe extreme isotopic variations in some of the lavas, which we interpret to result from crustal-level or Earth-surface processes that affect only a subset of the lavas.

Izu arc, mantle wedge, phengite, potassium isotopes, slab, subduction
0012-821X
Parendo, Christopher A.
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Jacobsen, Stein B.
3db2f47d-604a-4ce5-8ece-0c1421e1f7cb
Kimura, Jun-ichi
78b1b00d-3857-45e3-8a70-2e4f8d977b86
Taylor, Rex N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Parendo, Christopher A.
55d4b0da-cfcd-4dc5-9421-92e981d6ae9b
Jacobsen, Stein B.
3db2f47d-604a-4ce5-8ece-0c1421e1f7cb
Kimura, Jun-ichi
78b1b00d-3857-45e3-8a70-2e4f8d977b86
Taylor, Rex N.
094be7fd-ef61-4acd-a795-7daba2bc6183

Parendo, Christopher A., Jacobsen, Stein B., Kimura, Jun-ichi and Taylor, Rex N. (2022) Across-arc variations in K-isotope ratios in lavas of the Izu arc: Evidence for progressive depletion of the slab in K and similarly mobile elements. Earth and Planetary Science Letters, 578, [117291]. (doi:10.1016/j.epsl.2021.117291).

Record type: Article

Abstract

In subduction zones, fluids rise from the slab to the mantle, causing metasomatism and flux melting of the mantle to produce arc magmas. The transfer of material from slab to mantle and, in turn, to arc crust is an important control on the long-term chemical evolution of the mantle and continental crust. In this study, we investigate the transport of K in subduction zones by exploring the systematics of K stable-isotope variations in lavas of the Izu arc. We find that the Izu lavas have isotopically heavy K relative to estimates for midocean ridge basalt (MORB)-source upper mantle. Moreover, the δ 41K values of the lavas are clearly heavier than those of subducting sediments and are probably heavier than subducting altered ocean crust. An across-arc decrease in δ 41K values is apparent. Arc-front lavas are heavier than the mantle by about 0.22‰ (median), whereas rear-arc lavas are heavier by only about 0.08‰ (median). The heavy K-isotope compositions of the arc lavas may arise from isotopic fractionation during slab dehydration, where light K is preferentially retained in phases such as phengite in the slab. The across-arc decrease in δ 41K values may be due to progressive breakdown of these phases, and to associated depletion of the slab in heavy K. Variations in the relative contributions of different source materials—igneous ocean crust, sediment, and mantle peridotite—may also play a role. In particular, we explore a possibility, motivated by radiogenic-isotope studies, that the slab signal in K isotopes may be attenuated in the rear arc as a result of extensive fluid-mantle interaction. If K isotopes do track slab dehydration, then K isotopes provide insight into the transfer of K and similarly mobile elements out of the slab and into the upper mantle and arc crust. Lastly, we observe extreme isotopic variations in some of the lavas, which we interpret to result from crustal-level or Earth-surface processes that affect only a subset of the lavas.

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Across-arc variations in K-isotope ratios in lavas of the Izu arc - Evidence for progressive depletion of the slab in K and similarly mobile elements - Accepted Manuscript
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Accepted/In Press date: 7 November 2021
e-pub ahead of print date: 22 November 2021
Published date: 15 January 2022
Keywords: Izu arc, mantle wedge, phengite, potassium isotopes, slab, subduction

Identifiers

Local EPrints ID: 453016
URI: http://eprints.soton.ac.uk/id/eprint/453016
ISSN: 0012-821X
PURE UUID: 59e9449d-355d-4722-b273-8b57e8db3570
ORCID for Rex N. Taylor: ORCID iD orcid.org/0000-0002-9367-0294

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Date deposited: 07 Jan 2022 17:02
Last modified: 17 Mar 2024 07:02

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Contributors

Author: Christopher A. Parendo
Author: Stein B. Jacobsen
Author: Jun-ichi Kimura
Author: Rex N. Taylor ORCID iD

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