Abstract
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.
Highlights
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
Observations of hematite-magnetite assemblages in dehydrated serpentinites[20, 21] suggest a high fO2, 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
Several studies have indicated some evidence of variations of fO2 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 fO2 or the redox state of serpentinite-derived fluids during antigorite breakdown in subduction zones
Summary
During the first part of the reaction path (logξ < −4), the crystallization of olivine and chlorite is accompanied by a progressive increases of fO2 up to four log units above QFM oxygen buffer (Fig. 1a). At these conditions, hematite becomes in equilibrium with magnetite and buffers the fO2. At such P-T-fO2-pH conditions, volatile and redox-sensitive elements, such as sulfur or carbon, are expected to be mobilized under in their oxidized form (e.g. CO20aq) or SO42−(aq)) rather than reduced species (e.g. CH4(aq) or HS−(aq); Fig. 1a)
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