We calculated pressure-temperature (P–T) pseudosections and electrolytic fluid speciation for different slab components of AOC (Staudigel et al., 1989) (in the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–CO₂–S₂–O₂ system) and serpentinite (Deschamps et al., 2013) (CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–CO₂–S₂–O₂–Cr₂O₃), using Perple_X 6.9.1 (Connolly, 2005; Galvez et al., 2015, 2016; Connolly and Galvez, 2018) and the HP62/HP622 and DEW19 thermodynamic databases (Holland and Powell, 2011; Huang and Sverjensky, 2019). The DEW model enables us to calculate equilibrium between minerals, aqueous solute, and solvent species up to 6 GPa and 1200°C (Sverjensky et al., 2014; Huang and Sverjensky, 2019), which covers our modeling P–T range (0.5–6.0 GPa and 400–1000°C). This model is not applicable to melt, although slab melting seems likely to occur in some hot subduction zones (Syracuse et al., 2010; Hernández-Uribe et al., 2020). Here we only investigated the nature of AOC- and serpentinite-derived fluids because the oceanic crust is the volumetrically largest fluid source, and the hydrated lithospheric mantle is the major source of redox budget (Evans and Tomkins, 2021). Na₂O and K₂O were neglected in the abyssal serpentinites due to their low contents (Deschamps et al., 2013). Cl is a common and important component in slab-derived fluids (e.g., Jarrard, 2003; Bekaert et al., 2021). However, recent experiments suggest that the role of Cl in enhancing the solubility and mobility of carbonates and Fe³⁺ under subduction zone conditions is limited (Sanchez-Valle et al., 2017; Li and Wang., 2022), so we excluded Cl from the present model. The bulk compositions and solid solution models used are listed in Supplementary Table S1 and S2, respectively. The initial oxidation state of the redox-sensitive elements (Fe, C, and S) in a system is specified by the amount of excess O₂ [ 0.25 n ( F e 3 + ) + n ( C 4 + ) − 0.5 n ( S 2 − ) − 0.25 n ( S − ) + 1.5 n ( S 6 + ) ] , thus requiring the oxidation state of iron, carbon, and sulfur in AOC and serpentinite (Supplementary Table S1) as prior knowledge. The lagged speciation algorithm (Connolly and Galvez, 2018), which allows the mass balance between solids and fluids, was used to derive the electrolytic fluid speciation and concentration. For conditions where all C- or S-bearing minerals are completely dissolved, we set aq_bad_results to ignore bad results in the calculation. Moreover, we resampled those results by meemum. exe and “interim_results” set to true to examine if they are equal to those calculated by werami. exe (Supplementary Figure S1). We considered the neutrally charged COHS (H₂O, CO₂, CH_4, and H₂S)-solvent model with a non-linear subdivision scheme. The equation of state (EoS) for H₂O and CO₂ is Pitzer-Sterner (PS) EoS (Pitzer and Sterner, 1995), whereas for other solvents is the Modified Redlich Kwong (MRK) (Connolly and Cesare, 1993). None of the neutral C species (e.g., CO_2,aq, CH_4,aq, H₂CO_3,aq) and S species (e.g., H₂S_,aq, SO_2,aq) were considered solutes for consistency with the use of a COHS solvent. The MgSiC⁺ and HFeO₂ ^- species were also excluded because of their unrealistically high concentrations at the P–T conditions of interest (Connolly, personal communications; Peng et al., 2020; Spranitz et al., 2022).

In this study, we calculated electrolytic fluid speciation by assuming both a closed and open system along the Honshu (cold subduction) and Cascadia (hot subduction) geotherms. Fluid fractionation in the open system follows the Rayleigh fractionation model (Connolly and Galvez, 2018; Walters et al., 2020a) at about 2°C intervals from 400°C (where major dehydration reactions get initiated) until H₂O is fully extracted from the system. The closed-system modeling does not allow fluid escape during dehydration but still provides a convenient reference for comparison (Evans and Powell, 2015; Connolly and Galvez, 2018). The two types of subduction geotherms used in this study represent a rapid convergence (8 cm/y) of ∼129 Myr old crust with a slab dip of 29^o and a slow convergence (3 cm/y) of ∼7 Myr young crust with a slab dip of 20^o (Syracuse et al., 2010), respectively. The broad P–T conditions considered here cover a suite of subduction zone environments.

Although fO₂ is an important parameter that suggests whether the slab-derived fluids have the potential to oxidize the mantle or not, it is independent of the quantity of the redox-sensitive elements (e.g., Giggenbach, 1992; Evans, 2012). Therefore, slab fluid-related redox budget fluxes rather than fO₂ alone are more suitable for discussing mantle oxidation. Here, we followed the equations of Evans et al. (2012) to evaluate the oxidation capacity of the slab-derived fluids on the upper mantle.

According to the relationship between the mantle fO₂ and redox budget (Evans et al. (2012), pp. 27), we have ∆ F M Q = 4.77 + 1.61 ⁡ ln R B M ¯ (1) where R B M ¯ is the mantle redox budget in mol kg⁻¹ R B M ¯ evolves as a function of time as slab fluid-related redox budget is added to the mantle during slab subduction: R B M ¯ = R B M i ¯ + R B ˙ ∆ t M w (2) R B ˙ = ∑ ( S ∙ ρ ∙ h ∙ C l ) (3) where R B M i ¯ is the initial mantle redox budget (0.042 mol/kg, Li and Lee, 2004). R B ˙ is the slab fluid-related redox budget flux relative to the mantle reference state (Evans, 2006). ∆ t is time. M_W is the mass of a subduction-affected mantle wedge [5.46 × 10²⁰ kg, Evans et al. (2012)]. S is the subducted area (km²/year), ρ and h are the density (kg/m³) and thickness of subducted materials, respectively. C_l is carbon and sulfur concentrations in slab-derived fluids.

Mineral assemblages (expressed as common rock types) of AOC and serpentinite at 0.5–6 GPa and 400–1000°C are shown in Figure 1. Detailed labeled phase diagrams are provided in Supplementary Figure S2. Along the cold subduction (e.g., Honshu geotherm) (Figure 1A), lawsonite, talc, and glaucophane are the major hydrous minerals up to 670°C (2.7 GPa) in AOC. Carbon-bearing minerals include graphite and magnesite, stable at T<420°C and T<630°C, respectively. Pyrite is the only sulfur-bearing phase below 670°C. Above 670°C, the subducted AOC only contains limited H₂O contents (2–5 vol% muscovite), and all the carbon and sulfur would be incorporated into the aqueous fluid. Along the hot subducted geotherm, AOC has hydrous minerals (epidote, glaucophane, and chlorite) at temperatures lower than 780°C (2.4 GPa), with dolomite and pyrite as the only carbon- and sulfur-bearing phases below 680°C. Deep subduction of AOC along both geotherms would release ∼20 vol% fluids (Supplementary Figure S3).

In the serpentinite system, mineral assemblages and proportions along different geotherms are similar (Figure 1B and Supplementary Figure S3), consistent with previous predictions (e.g., Evans and Powell, 2015). The hydrous minerals of brucite, antigorite, and chlorite would dehydrate gradually with increasing temperature. Brucite converts to olivine at 480–520°C, where antigorite starts to dehydrate and finally transforms to olivine, orthopyroxene, and chlorite at ∼630°C. Aragonite converts to dolomite between 1.5 GPa (hot subduction) and 2.5 GPa (cold subduction) at ∼530°C. Dolomite would be replaced by magnesite at ∼600°C, which is close to the temperature of pyrite disappearance. All magnesite would dissolve into the aqueous fluid during antigorite breakdown, accompanied by hematite precipitation. Pyrite and anhydrite are the major sulfur-bearing phases in the serpentinite system. Pyrite is only stable at <600°C, whereas anhydrite can appear up to 750°C and 4.0 GPa in Honshu and 920°C and 3.0 GPa in Cascadia.

The composition and redox-sensitive speciation of carbon- and sulfur-bearing fluids equilibrated with subducted slab in a closed system are calculated using lagged speciation algorithm (Connolly and Galvez, 2018) (Figure 2). The open-system model (Rayleigh fractionation, Supplementary Figure S4) gives almost identical results as does the closed-system model, such similarity was also revealed by modeling for subduction zone sediment-derived fluids (Connolly and Galvez, 2018). Figure 2 only shows the redox-sensitive C- and S-bearing species in the fluid, other bulk composition-sensitive metal-complex species involving elements (Na, K, Mg, etc.) are available in Supplementary Figure S5. The major C-bearing aqueous species in the AOC-derived fluids are CO₂, H C O 3 − , Fe(HCOO)⁺, and C O 3 2 − , independent of subduction zone thermal structure. CH₄ only occurs at T<∼450 °C (cold) or T<∼550°C (hot); its concentration in the Cascadia model is 10 mmol/kg, approximately 2 orders of magnitude greater than those in the Honshu model at 450°C (Figures 2A,B). The S-bearing aqueous species are HS^–-dominant at <500 °C in the Honshu model or HS^–and H₂S-dominant at <580°C in the Cascadia model. The concentrations of H S O 4 − , H S O 3 − and S O 4 2 − along both geotherms drastically increase with temperature and reach a concentration level of 500–1000, 100–200, and 10–100 mmol/kg, respectively, which is 1–2 orders of magnitude greater than those of HS^–and H₂S. S 3 − becomes a considerable S-bearing specie only in the fluids released by hot subducted AOC; however, its concentration decreases rapidly from 30 to 0.1 mmol/kg beyond the subarc depths.

The main C-bearing species in serpentinite-derived fluids are CO₂, H C O 3 − , and Fe(HCOO)⁺. Their concentrations in fluids increase significantly from 0.1 to 0.3 to 200–300, 0.3–4 to 1–20, and <0.1 to 10–20 mmol/kg at T<630°C, respectively. Above 630°C, these aqueous carbon species along both geotherms would not display significant concentration variations, though H C O 3 − and Fe(HCOO)⁺ can decrease their concentrations from 10 to 1 mmol/kg and ∼1 to 0.5 mmol/kg in the hot subduction model, respectively. C O 3 2 − only occurs in the cold subduction model and has a low concentration of <0.3 mmol/kg at forearc to subarc depths (Figure 2C). The S-bearing aqueous species in serpentinite-derived fluids are complex and include CaSO_4,aq, S O 4 2 − , HS^–, H₂S, H S O 4 − , and H S O 3 − . HS^–and H₂S only appear at forearc depths, whereas the H S O 4 − and H S O 3 − concentrations increase towards subarc depths. In the Honshu model, CaSO_4,aq concentrations decrease significantly from 1000 to 7 mmol/kg in the forearc region but increase beyond subarc depths (Figure 2C). In the Cascadia model, CaSO_4,aq concentrations are relatively low (∼0.4 mmol/kg) and this specie is restricted to forearc depths (Figure 2D). S O 4 2 − has an overall constant concentration of ∼20 mmol/kg in the Honshu model but variable concentrations of 0.4–8 mmol/kg in the Cascadia model.

Although reduced species such as CH₄, HS^–, H₂S and S 3 − could reach significant concentrations of >10 mmol/kg in fluids at forearc depths, the oxidized species such as CO₂, H C O 3 − , C O 3 2 − , H S O 4 − , H S O 3 − , and S O 4 2 − become dominated in subarc fluids.

Oxygen fugacity (fO₂), the most common variable used to quantify the redox state of slab-derived fluids, is reflected by the above-mentioned redox-sensitive species. Notably, the fO₂ values of slab-derived fluids in open and closed systems do not show significant differences (Figure 3). Overall, the AOC- and serpentinite-derived fluids along different geotherms exhibit similar fO₂ patterns from forearc to subarc depths. Compared to the fayalite–magnetite–quartz (FMQ) buffer, AOC-derived fluids have positive ∼△FMQ increasing towards subarc depths (Figures 3A,B). The fO₂ values of the serpentinite-derived fluids are almost constant (∼△FMQ+2) at T<∼600–610°C and sharply increase by 1.5–2 log units at ∼630 °C accompanied by solid C-S-bearing phase transition and hydrous mineral dehydration (Figures 3C,D).

Redox budget (RB), a parameter featuring the initial redox state of subducted materials before subduction, refers to the total number of transferred electrons among the multivalent elements such as iron, carbon, and sulfur (Evans, 2012) relative to a reference state. It can affect fluid speciation (Evans and Powell, 2015; Walters et al., 2020a) and the fO₂ of slab-derived fluids. In this study, we used the equation of Evans (2006) RB [ n ( F e 3 + ) + 4 n ( C 4 + ) + 0 n ( S 2 − ) + n ( S − ) + 8 n ( S 6 + ) ] , for a reference state of Fe as Fe²⁺, C as C⁰, and S as S^2-) to adjust the RB values of AOC and serpentinite. To better understand the effects of bulk RB on the fO₂ of slab-derived fluids, we calculated a set of T/P-RB pseudosections (See Supplementary Figure S6 for details) along the Honshu and Cascadia subduction geotherms, as well as △FMQ isopleths, in the AOC and serpentinite systems (Figure 4). Our results show that slab-derived fluids have a large fO₂ range, spanning from △FMQ+0 to △FMQ+6 in AOC and from △FMQ–6.5 to △FMQ+4.5 in serpentinite. The △FMQ isopleths exhibit vertical S-shaped patterns across the diagram, implying that the fluid fO₂ is mainly controlled by the RB of subducted reservoirs. Given a fixed RB for AOC (11.63) and MORB (9.37), the subduction of these rocks can increase the fluid fO₂ by 1.5–2 log units (Figures 4A,B). However, the fO₂ of serpentinite-derived fluid can increase significantly during subduction at low RB and temperature conditions (Figures 4C,D). The fluid fO₂ differences induced by subduction zone thermal structure are generally less than 1 log unit for the same reservoir (Figures 4A–D).

Our new models show that, with increasing PT conditions, the deep subduction-zone fluids are gradually dominated by oxidized carbon (CO₂, H C O 3 − ; C O 3 2 − ) and sulfur species ( H S O 4 − ; H S O 3 − ; S O 4 2 − ) (Figure 2), regardless of the subduction-zone thermal structure and open/closed system behavior (Figure 2, Supplementary Figures S7, S8, S4). This prediction is broadly consistent with the oxidized C-S species commonly observed in fluid inclusions from HP metamorphic rocks (Scambelluri and Philippot, 2001; Frezzotti and Ferrando, 2015). In addition, our results also suggest that Fe(HCOO)⁺ can reach a concentration of more than 1–10 mmol/kg, which is consistent with the abiotic species observed in the solubility experiments of eclogite and peridotites (Huang and Sverjensky, 2019).

Our calculated results can also explain different fluid inclusions observed in different UHP oceanic eclogites from cold subduction zones, such as those from the western Alps and southwestern Tianshan. The former contains oxidized C and S species ( H C O 3 − ; C O 3 2 − ; S O 4 2 − ) (Frezzotti et al., 2011), the latter consists of reduced CH₄ and H₂ (Tao et al., 2018) or H₂S and HS^– (Li et al., 2020). The oxidized C-S species in the Alps samples could be attributed to the elevated oxidation (high RB) of subducted AOC before subduction (Supplementary Figure S7). However, abundant sulfide minerals in the Tianshan eclogites can lead to a low RB (Li et al., 2016), which would result in AOC-derived fluids dominated by reduced species of H₂S and HS^– (Supplementary Figure S7).

Low-P experiments indicate that S 3 − is an important specie enhancing the enrichment of soft metals (Au, Cu, and Mo) in arc magmas (Pokrovski and Dubrovinsky, 2011; Pokrovski and Dubessy, 2015). However, the behavior of this species under HP conditions was not constrained in previous modeling works (Walters et al., 2020a; Evans and Frost, 2021). We predict that S 3 − in AOC-derived fluids becomes significantly abundant in hot subduction zones (Figure 2B), and can be stable under moderate fO₂ conditions (△FMQ+1 to △FMQ+2) at subarc depths (Figures 4B,Supplementary Figure S7). These predictions are consistent with the formation of large porphyry Cu-Au ore deposits commonly triggered by hot subduction processes (Cooke et al., 2005; Sun et al., 2010; Zhang et al., 2017).

Methane (CH₄) is a critical carbon specie in subduction zone fluids. CH₄-bearing fluid inclusions were discovered in oceanic eclogites and HP ophicarbonates from the southwestern Tianshan (Tao et al., 2018; Peng et al., 2020), HP-LT ophicarbonates from the western Alps (Brovarone et al., 2017, 2020) and the Alpine Corsica (Galvez et al., 2013; Brovarone et al., 2020), and metamorphosed harzburgite from the North Qilian HP-LT metamorphic belt (Song et al., 2009). All these findings indicate that abiotic CH₄ may be common in the forearc region of oceanic subduction zones. Our models show that the AOC-derived fluids would have abiotic CH₄ species at forearc depths (Figures 2A,B), and low RB values of AOC favor the CH₄ production (Supplementary Figure S7). In addition, subducted serpentinite with low RB values can also release considerable CH₄ (Supplementary Figure S8). Previous studies only considered cold subduction zones as the key reservoir of abiotic methane (e.g., Zhang et al., 2022), according to the natural HP-LT sample record in cold oceanic subduction zones. The CH₄ production in hot subduction zones is still enigmatic due to the scarcity of exhumed HP rocks in these environments. Our models predict that, at forearc depths, hot subduction zones with low RB can produce more CH₄ than cold subduction zones (Figures 2A,B). Therefore, the forearc regions of hot subduction zones could also be a potentially key production factory of methane, which needs to be considered further during evaluating methane flux in global subduction zones.

It has been long proposed that AOC-derived fluids have high fO₂ because of high Fe³⁺ content relative to sediments and serpentinites (Evans et al., 2012). However, this is challenged by the low solubility of Fe³⁺ in aqueous fluids, as revealed by several experiments (Simon et al., 2004; Sanchez-Valle et al., 2017). Our results suggest that the fO₂ of slab fluids is largely controlled by the RB values of the protoliths of subducted materials (Figure 4). Subducted MORB or sulfide-rich AOC (RB ≤9.37) could release reduced fluids (fO₂ below or near △FMQ) equilibrated with pyrite at forearc to subarc depths (Figures 4A,Supplementary Figure S7A). This prediction matches petrological records in the Tianshan pyrite-rich eclogites and HP veins (Li et al., 2016, 2020; Tao et al., 2018). With higher RB (>11.63), AOC-derived fluids are highly oxidized (△FMQ+2 to △FMQ+6), broadly consistent with the high fO₂ records in oceanic eclogites from North Qilian (△FMQ+0 to +4, Cao et al., 2011), Songduo in southern Tibet (△FMQ+2, Liu et al., 2016), and Syros in Greece (△FMQ+2 to +4, Walters et al., 2020b). Moreover, the fO₂ of AOC-derived fluids from forearc to subarc depths can increase by 2–2.5 log units during dehydration (Figures 3A,B), further implying that deep subduction AOC-derived fluids are oxidized.

The fO₂ of serpentinite-derived fluids is indicated to have a wide range of △FMQ–4 to △FMQ+5 (e.g., Peretti et al., 1992; Evans and Powell, 2015; Debret and Sverjensky, 2017), which is broadly consistent with our modeled results (Figures 4C,D). Such large variations most likely result from heterogeneous pre-subduction redox states in serpentinites (RB=2.14–12.82) (Evans and Powell, 2015). For example, at low RB (<4.81) conditions, serpentinite-derived fluids are in equilibrium with iron and pyrrhotite (Supplementary Figure S6) and have fO₂ ranging from △FMQ–6.5 to △FMQ–2.5 at <500°C, which is supported by the presence of reduced phases (awaruite, native copper, pyrrhotite, etc.) in some serpentinites (Frost, 1985; Peretti et al., 1992; Galvez et al., 2013). Our results also show that the dehydration of antigorite, accompanied with the breakdown of pyrite, would release oxidized fluids equilibrated with hematite and anhydrite with a fO₂ increase by 2 log units (from △FMQ+2 to △FMQ+4) (Figures 3C,D). This is supported by the presence of hematite in subducted serpentinites (e.g., Frost, 1985; Bach et al., 2004; Debret et al., 2015) and experimental products (Maurice et al., 2020).

Our modeling results indicate that the fO₂ of slab-derived fluids is controlled by not only Fe³⁺ but also redox-sensitive C-S species in the slab. Therefore, the RB of the subducted slab is the first-order factor affecting the oxygen fugacity of slab fluids. Subduction zone thermal structure has little influence on the fO₂ of slab-derived fluids, with variation generally below 1 log unit at fixed RB (Figure 4). This variation is much smaller than the petrological observations in oceanic eclogites and serpentinites.

To evaluate whether the oxidized fluids from dehydrating AOC and serpentinite can oxidize the subarc mantle or not, Eqs 1. and 2, and 3 were used to estimate C- and S-related oxidation fluxes released by subducted slab and timescales for mantle oxidation. We assumed a subducted area of 2.45 km²/year (Jarrard, 2003) and densities of 3000 kg/m³ for AOC (Bekaert et al., 2021) and 2800 kg/m³ for serpentinite (Duan et al., 2022) with different thicknesses (Figure 5) in the calculation. Our results show that AOC- and serpentinite-related oxidation fluxes to subarc mantle are 18.6–43.3 × 10¹² mol/year and 0.67–7.64 × 10¹² mol/year for cold subduction and 21.3–49.8 × 10¹² mol/year and 0.45–5.13 × 10¹² mol/year for hot subduction. With such oxidation fluxes, the mantle fO₂ can be elevated by one to two log units on a timescale of less than 1–10 Ma. Therefore, the oxidized C-S species in the fluids are expected to oxidize Fe²⁺ in the mantle wedge (Kelley and Cottrell, 2009), which is responsible for the △FMQ to △FMQ+3 range in fO₂ reported from subarc peridotite xenoliths (Ballhaus, 1993; Parkinson and Arculus, 1999; Peslier et al., 2002; Evans et al., 2012). The occurrence of anhydrite and S O 4 2 − -bearing inclusions in arc peridotite xenoliths (Bénard et al., 2018) also reflects the oxidized fluids penetrating the subarc mantle. Recent investigations on the Mariana arc lavas revealed that the subarc mantle gradually becomes oxidized and increases its fO₂ from typical mantle values (△FMQ–1 to 0) during subduction initiation to △FMQ≥1 after around 2–4 Ma subduction (Brounce et al., 2015). This implies that the sub-arc mantle is oxidized over the most lifetime of subduction.

Our results show that subduction zone thermal structure has limited influence on the fO₂ of slab-derived fluids. This prediction can be further tested by arc lavas from global cold and hot subduction zones (Cottrell et al., 2021), both of which show similar fO₂ average values (1.02±0.45 and 1.23±0.63) and ranges mostly spanning from △FMQ to △FMQ+3 (Supplementary Figure S9). The input of heterogeneous RB materials into subduction zones might also contribute to the large fO₂ variations (up to 3 log units) in the same arc lavas, such as Marianas and Altiplano.

For a long-term geological timescale, the atmospheric O₂ concentration was significantly increased from less than 1% to the present atmospheric level during the Ediacaran (a so-called Neoproterozoic Great Oxidation Event) (Lyons et al., 2014), which would have increased the oxidation state and redox budget of the pre-subduction slab. This is further supported by the elevated seawater sulfate during this period (Farquhar et al., 2010). After the Neoproterozoic Great Oxidation Event, the input of high RB materials into subduction zones likely induced slab fluid oxidation (Figure 4). These oxidized agents were progressively introduced into and interacted with the subarc mantle, producing high-fO₂ arc magmas and potentially generating porphyry Cu deposits (PCDs) (Richards, 2015; Sun et al., 2015). This may be one of the key mechanisms to explain why most PCDs were formed at <550 Ma (Liu et al., 2020).

Based on averaged carbon and sulfur concentrations in AOC and serpentinites (Staudigel et al., 1989; Alt et al., 2012; Evans, 2012; Bekaert et al., 2021), our models can provide important constraints on the deep C-S cycle in subduction zones (Figure 6). Assuming a closed system, all carbon- and sulfur-rich phases in AOC and serpentinites would be dissolved into aqueous fluids during deep subduction beyond ∼3 GPa. The closed system in subduction zones may be occasional and temporary but exists, as supported by some in situ HP vugs in oceanic eclogites (Angiboust and Raimondo, 2022). Subduction of the fully C-S-dissolved fluids enclosed within the slab could deliver carbon and sulfur into the deep mantle beyond subarc depths. However, for most cases, releasing fluids proceeded as an open system, which is indicated by the common occurrence of HP-veins penetrating eclogites and mantle wedge rocks (e.g., Guo et al., 2012; Plümper et al., 2017; Bloch et al., 2018). For an open system, most of carbon (>70%) and sulfur (>50%) in cold subducted AOC and serpentinites would be incorporated into aqueous fluids, which could deliver such components into the overlying mantle wedge and arc lavas (Figures 6A,C). In addition, subducted sediments may lose >40–65% of carbon and sulfur to the subarc mantle during subduction (Li et al., 2020; Stewart and Ague, 2020; Chen et al., 2021). Therefore, most subducted carbon and sulfur would complete their deep journey at subarc depths, confirming the arc system as a key region for C-S recycling (Connolly, 2005; Kelemen and Manning, 2015; Foley and Fischer, 2017; Li et al., 2020). However, the hot subducted AOC and serpentinites will lose all carbon and sulfur (except for serpentinite sulfur) at depths shallower than 70 km in the case of behaving as an open system for the fluids. (Figures 6B,D). Therefore, the closed-system subduction and open-system cold subduction can transport carbon and sulfur into the deep mantle beyond subarc depths. Besides, the heterogeneous composition (Farsang et al., 2021) and redox state (Supplementary Figures S6A, C) of carbon and sulfur in subducted slab may be also helpful for deep C-S cycling. These deep subducted carbon and sulfur would contribute to the volatile-rich mantle source of deep-seated magmas (Chalapathi Rao and Lehmann, 2011; Sakuyama et al., 2013) and the formation of superdeep diamonds with sulfide inclusions (Kaminsky, 2012; Smith et al., 2016).