Highly oxidising fluids generated during serpentinite breakdown in subduction zones

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.

High‐P metamorphism of rodingites during serpentinite dehydration (Cerro del Almirez, Southern Spain): Implications for the redox state in subduction zones

“…Beyond the dehydration front, that is, within the Chl‐harzburgite domain, all the metarodingite bodies are transformed into Ep‐metarodingite, and Grand‐metarodingite only occurs as scarce relicts in the core of some boudins. This supports the hypothesis that flux of high amounts (~9 wt% H 2 O; Padrón‐Navarta, Tommasi, et al., ) of oxidizing fluids (Debret & Sverjensky, ; Debret et al., ; Merkulova et al., ) released during Atg‐breakdown in serpentinite favored the transformation of Grand‐metarodingite into Ep‐metarodingite (Figure , stage 6).…”

“…The oxidizing capacity of these fluids relative to a reference state (the redox budget; Evans, ) is of great importance for the composition of arc magmatism and arc‐related ore deposits (Evans & Powell, and references therein). The role of dehydration reactions in serpentinite for the redox state of subduction zones has been well studied (Debret & Sverjensky, ; Debret et al., , ; Merkulova et al., ). These studies emphasized the importance of fluids released from serpentinites as oxidizing agents of adjacent slab lithologies (mostly anhydrous mantle peridotites) at the metre scale.…”

“…The breakdown of antigorite in serpentinite is the main dehydration reaction occurring at high pressure in subduction zones (Padrón‐Navarta, Tommasi, et al., ; Ulmer & Trommsdorff, ). Owing to serpentinite dehydration, oxidizing fluids are released (Debret & Sverjensky, ; Debret et al., , ; Merkulova et al., ) and may interact with subducted lithologies, including metarodingites, inducing mineralogical and/or compositional changes in rocks and fluids. Therefore, metarodingites from Cerro del Almirez are ideal for unravelling how fluids from dehydration reactions may trigger metamorphic/metasomatic reactions in subducted mafic rocks, and how these reactions affect the compositions of the slab recycled deep in the mantle and of fluids that migrate to inner regions of the mantle wedge where arc magmas are generated.…”

Subduction metamorphism of serpentinite‐hosted carbonates beyond antigorite‐serpentinite dehydration (Nevado‐Filábride Complex, Spain)

“…Because it releases high amounts of H 2 O‐rich fluids—9 wt% H 2 O at ~660°C—dehydration of Atg‐serpentinite is potentially the most relevant devolatilization reaction to mobilize carbon at sub‐arc depths into fluids from slab and mantle wedge lithologies (Bromiley & Pawley, ; Padrón‐Navarta, Hermann, Garrido, López Sánchez‐Vizcaíno, & Gómez‐Pugnaire, ; Rüpke, Morgan, Hort, & Connolly, ; Ulmer & Trommsdorff, ). There is mounting evidence that fluids generated during high‐pressure (HP) deserpentinization are oxidizing—close to the hematite‐magnetite oxygen buffer—and are mildly alkaline (Alt et al., ; Debret et al., ; Debret & Sverjensky, ; Evans, Reddy, Tomkins, Crossley, & Frost, ; Galvez et al., ). If so, deserpentinization fluids could mobilize both reduced and inorganic carbon as CO 2(aq) and HCO 3 – (Facq, Daniel, Montagnac, Cardon, & Sverjensky, ).…”

Isotopic Compositions of Sulfides in Exhumed High‐Pressure Terranes: Implications for Sulfur Cycling in Subduction Zones

“…Increasing pressure and temperature conditions during subduction drive metamorphic reactions in the slab to produce hydrous fluids or silicate melts, which transport material into the overlying system that eventually produces the magmatic arc (e.g., Hacker, ; Manning, ; Marschall & Schumacher, ). Despite the important role sulfur may play in the redox of the slab‐arc system and trace metal cycling, a limited number of studies have focused on sulfur mobilization from the slab during subduction metamorphism (Alt, Shanks, et al, , Alt, Garrido, et al, ; Canil & Fellows, ; Crossley et al, ; Debret & Sverjensky, ; Evans & Powell, ; Evans et al, , ; Jego & Dasgupta, ; LaFlamme et al, ; Lee et al, ; Tomkins & Evans, ).…”