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SW Pacific arc and backarc lavas and the role of slab-bend serpentinites in the global halogen cycle
journal contribution
posted on 2023-05-20, 08:57 authored by Kendrick, MA, Leonid Danyushevsky, Trevor FalloonTrevor Falloon, Woodhead, JD, Arculus, RJ, Ireland, TIn order to better understand the global halogen cycle and the behaviour of halogens during subduction, we investigated the concentrations of F, Cl, Br and I in suites of rare submarine glasses formed during subduction initiation on the Hunter Ridge (N. Fiji Basin) and from the northernmost part of the Tonga Arc. Additional submarine glasses from mature arcs and backarcs including the Tonga arc (Volcano A), North Fiji, Lau, Manus and Woodlark Basins were also investigated. The aim was to constrain the relative abundances of all four halogens (F/Cl, Br/Cl and I/Cl) in subducted components in proto-arc, arc and backarc lavas for the first time. This enables an assessment of slab-fluid evolution from incipient arcs to mature backarcs, and a comparison of halogens in slab-fluids with possible subduction inputs and other mantle outputs. The various arc and backarc lavas investigated are strongly enriched in Cl, Br and I relative to Nb, and weakly enriched in F compared to Pr, demonstrating metasomatism of incipient sub-arc mantle and sub-arc/backarc mantle by slab-components with strikingly similar ranges of F/Cl (0.11 ± 0.09), Br/Cl (0.0028 ± 0.0008) and I/Cl (0.000013-0.001) (2 standard deviations). The F/Cl ratio, which is much higher than seawater and other surface fluids, is attributed to enhanced mobility of F in saline fluids in sub-arc environments, with slab-melts ascribed an additional minor role. The slab-fluid Br/Cl and I/Cl ratios are strikingly similar to altered oceanic crust and seafloor serpentinites, and distinctly different from sediment pore waters, sediments and most forearc serpentinites. The median Br/Cl and I/Cl ratios of slab fluids in sub-arc and sub-backarcs are also indistinguishable from median mid-ocean ridge and ocean island basalt values, which have I/Cl ratios lower than estimated for the primitive mantle. The data are interpreted to indicate that sedimentary volatile components present in sediments and forearc serpentinites are efficiently returned to the surface through forearc regions. In contrast, halogens in altered ocean crust and lithospheric serpentinites are recycled through magmatic arcs and into the deeper mantle with the result that the mantle's I/Cl ratio has decreased over time. Mass balance considerations suggest that serpentinisation of the lithospheric mantle at the slab-bend immediately prior to subduction is more significant than estimated previously and is driven by seawater-derived fluids without sedimentary components. Lithospheric serpentinites are likely to represent a dominant source of halogens and other volatiles, including noble gases, in the Earth's mantle.
Funding
Australian Research Council
AMIRA International Ltd
ARC C of E Industry Partner $ to be allocated
Anglo American Exploration Philippines Inc
AngloGold Ashanti Australia Limited
Australian National University
BHP Billiton Ltd
Barrick (Australia Pacific) PTY Limited
CSIRO Earth Science & Resource Engineering
Mineral Resources Tasmania
Minerals Council of Australia
Newcrest Mining Limited
Newmont Australia Ltd
Oz Minerals Australia Limited
Rio Tinto Exploration
St Barbara Limited
Teck Cominco Limited
University of Melbourne
University of Queensland
Zinifex Australia Ltd
History
Publication title
Earth and Planetary Science LettersVolume
530Article number
115921Number
115921Pagination
1-12ISSN
0012-821XDepartment/School
School of Natural SciencesPublisher
Elsevier Science BvPlace of publication
Po Box 211, Amsterdam, Netherlands, 1000 AeRights statement
©2019 Elsevier B.V. All rights reserved.Repository Status
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