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Article Dans Une Revue Scientific Reports Année : 2017

Highly oxidising fluids generated during serpentinite breakdown in subduction zones

Baptiste Debret
D A Sverjensky
  • Fonction : Auteur

Résumé

Subduction zones facilitate chemical exchanges between Earth's deep interior and volcanism that affects habitability of the surface environment. Lavas erupted at subduction zones are oxidized and release volatile species. These features may reflect a modification of the oxidation state of the sub-arc mantle by hydrous, oxidizing sulfate and/or carbonate-bearing fluids derived from subducting slabs. But the reason that the fluids are oxidizing has been unclear. Here we use theoretical chemical mass transfer calculations to predict the redox state of fluids generated during serpentinite dehydration. Specifically, the breakdown of antigorite to olivine, enstatite, and chlorite generates fluids with high oxygen fugacities, close to the hematite-magnetite buffer, that can contain significant amounts of sulfate. The migration of these fluids from the slab to the mantle wedge could therefore provide the oxidized source for the genesis of primary arc magmas that release gases to the atmosphere during volcanism. Our results also show that the evolution of oxygen fugacity in serpentinite during subduction is sensitive to the amount of sulfides and potentially metal alloys in bulk rock, possibly producing redox heterogeneities in subducting slabs. During subduction, the increase of pressure and temperature conditions in the subducting plate results in hydrous mineral breakdown and the release of volatile-rich fluids. The oxidized or reduced nature of the released fluids is controlled by mineral local equilibrium changes and can be monitored through the oxygen fugacity (fO 2). Although large variabilities of fO 2 were shown to occur in subduction zones, especially in melange zones constituting the plate/mantle interface 1 , previous petrological and geochemical studies have shown that the redox state of the subducted mafic crust does not change during prograde metamorphism and that its redox budget remains relatively constant 2. Therefore, other sources of fluids must be considered to explain the oxidized nature of arc magmas 3-5. Serpentinized mantle peridotites can constitute a major source of water at depth in subduction zones 6. Those rocks comprise a significant part of subducting oceanic lithosphere hydrated near the seafloor at slow or ultra-slow spreading ridges 7, 8 or slab-bending related faults 9 , and are also present as part of the slab-wedge interface or mantle wedge that is percolated by aqueous fluids emanating from the slab during subduction 10, 11. Serpentinites are thus ubiquitous in subduction zones. Furthermore, several studies have suggested that the breakdown of these rocks is the most likely source of fluid-mobile elements (e.g. B), volatiles and halogens enriched in arc magmas (e.g. 12-17). The dehydration of serpentinized mantle results in a net decrease in bulk rock volatile concentrations 6, 16, 17 and Fe 3+ /∑e 18, 19. In addition, observations of hematite-magnetite assemblages in dehydrated serpentinites 20, 21 suggest a high fO 2 , from one to five log units above the Quartz-Fayalite-Magnetite (QFM) oxygen buffer, during antigorite breakdown in subduction zones which could be compatible with the release of oxidized S and C in slab-derived fluids. If correct, this model would have major implications for the cycling of volatiles between the Earth's surface and interior, volcanic degassing (e.g. SO X , CO X species emitted from volcanoes), and the occurrence of metal sulfide ore deposits 22, 23. Although several studies have indicated some evidence of variations of fO 2 in antigorite-bearing serpentinites that can be related to the extent of initial serpentinization 24, 25 , there is at this time no consensus on the evolution of fO 2 or the redox state of serpentinite-derived fluids during antigorite breakdown in subduction zones.
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Dates et versions

hal-03976630 , version 1 (07-02-2023)

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Baptiste Debret, D A Sverjensky. Highly oxidising fluids generated during serpentinite breakdown in subduction zones. Scientific Reports, 2017, 7, ⟨10.1038/s41598-017-09626-y⟩. ⟨hal-03976630⟩
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