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Noble gases recycled into the mantle through cold subduction zones

Journal article
Authors Andrew Smye
Colin Jackson
Matthias Konrad-Schmolke
Marc Hesse
Steve Parman
David Shuster
Chris Ballentine
Published in Earth and Planetary Science Letters
Volume 471
Pages 65-73
ISSN 0012-821X
Publication year 2017
Published at Department of Earth Sciences
Pages 65-73
Language en
Links https://doi.org/10.1016/j.epsl.2017...
Subject categories Geochemistry, Geosciences, Multidisciplinary

Abstract

Subduction of hydrous and carbonated oceanic lithosphere replenishes the mantle volatile inventory. Substantial uncertainties exist on the magnitudes of the recycled volatile fluxes and it is unclear whether Earth surface reservoirs are undergoing net-loss or net-gain of H2O and CO2. Here, we use noble gases as tracers for deep volatile cycling. Specifically, we construct and apply a kinetic model to estimate the effect of subduction zone metamorphism on the elemental composition of noble gases in amphibole a common constituent of altered oceanic crust. We show that progressive dehydration of the slab leads to the extraction of noble gases, linking noble gas recycling to H2O. Noble gases are strongly fractionated within hot subduction zones, whereas minimal fractionation occurs along colder subduction geotherms. In the context of our modelling, this implies that the mantle heavy noble gas inventory is dominated by the injection of noble gases through cold subduction zones. For cold subduction zones, we estimate a present-day bulk recycling efficiency, past the depth of amphibole breakdown, of 5-35% and 60-80% for 36Ar and H2O bound within oceanic crust, respectively. Given that hotter subduction dominates over geologic history, this result highlights the importance of cooler subduction zones in regassing the mantle and in affecting the modern volatile budget of Earth's interior. (C) 2017 Elsevier B.V. All rights reserved.

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