Redox species and oxygen fugacity of slab-derived fluids: Implications for mantle oxidation and deep carbon-sulfur cycling

Abstract

The generation and migration of slab-derived fluids modulate subduction zone seismicity, arc magmatism, and deep volatile cycling. However, the redox species and oxygen fugacity (fO2) (hereafter expressed as log units relative to the fayalite–magnetite–quartz buffer, △FMQ) of slab-derived fluids are highly debated. Here we conducted phase equilibria modeling on altered oceanic crust (AOC) and serpentinites along typical subduction geotherms in the C-S-bearing system over a pressure range of 0.5–6 GPa. With the averaged compositions of AOC and serpentinite, our calculated results show that oxidized carbon-sulfur species dominate slab-derived fluids during slab subduction. As a result, slab-derived fluids are highly oxidized and at or above the typical △FMQ values of arc magmas at forearc to subarc depths. The predicted oxidized carbon and sulfur species are compatible with natural observations in fluid inclusions from many oceanic HP metamorphic rocks. More importantly, it is revealed that, the redox state of slab-derived fluids is primarily controlled by the redox budget (RB) of the slab prior to subduction. Subduction-zone thermal structure, however, only exerts a minor influence on the slab-derived fluid fO2, which is supported by the similar fO2 ranges in arc lavas from cold and hot subduction zones. Our models further show that, if an open system is assumed, most of carbon (>70%) and sulfur (>50%) in cold subducted AOC and serpentinite would be lost at subarc depths. Small amounts of carbon and sulfur could be transported into the deeper mantle via closed-system subduction and open-system cold subduction, supplying the source materials for volatile-rich intraplate magmas and superdeep diamonds.

Coupled Cycling of Carbon and Water in the Form of Hydrous Carbonatitic Liquids in the Subarc Region

“…Organic species were excluded from the computation because HKF data for ions of amino acids do not extrapolate plausibly to high pressures (Connolly & Galvez, 2018). The MgSiC + species was also excluded due to its unrealistically high concentration (H. Y.-B. Li et al, 2022).…”

“…The MgSiC + species was also excluded due to its unrealistically high concentration (H. Li et al., 2022; Y.‐B. Li et al., 2022).…”

“…More recently, the fluid-saturated melting of subducted sediment or basalt was proposed to occur at relatively low temperatures via flux melting, that is, by the addition of water that sterns from the dehydration of underlying serpentinized peridotite (H. Y.-B. Li et al, 2022;Spandler et al, 2007;Turner & Langmuir, 2022).…”

“…More recently, the fluid–saturated melting of subducted sediment or basalt was proposed to occur at relatively low temperatures via flux melting, that is, by the addition of water that sterns from the dehydration of underlying serpentinized peridotite (H. Li et al., 2022; Y.‐B. Li et al., 2022; Spandler et al., 2007; Turner & Langmuir, 2022). Analogously, meta–ophicarbonate situated at the top of the slab or at the base of the mantle wedge likely undergoes flux melting, too.…”

Sulfur disproportionation in deep COHS slab fluids drives mantle wedge oxidation

“…In subducting lithosphere, sulfur mineral stability depends on initial sulfur content, rock composition (e.g., serpentinites, eclogites, and metasediments), pressure-temperature (P-T) path, and redox conditions. In serpentinites and eclogites, thermodynamic modeling predicts pyrrhotite at forearc conditions (14)(15)(16)(17). In eclogites and serpentinites, pyrite is thermodynamically predicted to be the most common sulfur phase at prevailing fo 2 conditions during subarc metamorphism along cold subduction geotherms [<ΔFMQ + 1; fo 2 expressed as the logarithmic unit difference relative to the FMQ (fayalite-magnetite-quartz buffer)] ( [14][15][16][17].…”