Mantle wedge oxidation from deserpentinization modulated by sediment-derived fluids

Abstract

The Earth's mantle is oxygen-breathing through the sink of oxidised tectonic plates at convergent margins 1,2 . Ocean floor serpentinisation increases the bulk oxidation state of iron relative to dry oceanic mantle and results in a variable intake of other redox-sensitive elements such as sulphur 2,3 . The reversibility of seafloor oxidation in subduction zones during high-pressure dehydration of serpentinite ("deserpentinisation") at subarc depths and the capacity of the resulting fluids to oxidise the mantle source of arc basalts are highly contested [4][5][6][7][8][9][10][11][12][13][14][15] .Thermodynamic modelling 8,10,15 , experiments 6,12,14 , and metaperidotite study in exhumed highpressure terrains 4,15,16 result in differing estimates of the redox state of deserpentinisation fluids, ranging from low to highly oxidant. Here we show that although intrinsic deserpentinisation fluids are highly oxidant, the infiltration of small fractions of external fluids equilibrated with metasedimentary rocks strongly modulates their redox state and oxidation-reduction capacity explaining the observed discrepancies in their redox state. Infiltration of fluids equilibrated with graphite-bearing sediments reduces the oxidant, intrinsic deserpentinisation fluids to oxygen fugacities similar to those observed in most graphite-furnace experiments and natural metaperidotites. However, infiltration of CO2-bearing fluids equilibrated with modern GLOSS generates sulphate-rich, highly oxidising deserpentinisation fluids. We show that such GLOSSinfiltrated deserpentinisation fluids can effectively oxidise the mantle wedge of cold to hot subduction zones potentially accounting for the presumed oxidised nature of the source of arc basalts.

Sulfur disproportionation in deep COHS slab fluids drives mantle wedge oxidation

“…Consequently, further increase in mantle wedge redox state to FMQ and ΔFMQ + 1 values requires the reduction of S +6 and S 4+ in fluids.The overall mantle wedge oxidation process at arc magma genesis depth depends on the H 2 O flux and, thus, the P - T conditions at which the dehydration reactions initiate. Cold subduction zones are more likely to be characterized by high fluid fluxes ( 54 ), thus reaching the condition of a fully oxidized mantle wedge (i.e., by increasing f o 2 of ~3 log units) faster than hot ones (i.e., <5 Myr) ( 59 ). Whatever the case, the modeled oxidation rate suggests that the subarc mantle is fully oxidized over the entire duration of the arc magmatic activity.…”

Chromium Isotope Behavior During Serpentinite Dehydration in Oceanic Subduction Zones

“…As mentioned above, both rock types show overlapping Al 2 O 3 , V, and Sc, indicating a similar magmatic history (Figure 4), which is also supported by the lack of systematic differences in Fe isotopes among both rock types (Debret et al., 2021). In addition to textural variations pointing to metamorphic growth of Chl‐harzburgite after a serpentinite protolith (Dilissen et al., 2018, 2021; Padrón‐Navarta et al., 2015), thermodynamic modeling shows that open‐system serpentinite dehydration can account for the oxidation differences between the two types of rocks (Padrón‐Navarta et al., 2023). Therefore, the observed higher average δ 53 Cr of the Chl‐harzburgites than that of Atg‐serpentinites by ∼0.08‰ cannot be dominated by the heterogeneity of the protolith.…”

“…Serpentinites are widely considered key agents in geodynamic and geochemical processes in subduction zones (Deschamps et al., 2013; Scambelluri et al., 2004, 2019; Spandler & Pirard, 2013). Serpentinite dehydration in subduction zones not only modifies the geochemical composition of the subducting slab, but also transfers unique geochemical signatures to the overlying mantle wedge (e.g., Alt et al., 2012; Harvey et al., 2014; Scambelluri & Tonarini, 2012; Shen et al., 2021; Tonarini et al., 2011) and plays a key role in the oxidation of arc magmas (e.g., Debret et al., 2020; Padrón‐Navarta et al., 2023; Y. X. Zhang et al., 2021). As serpentinite dehydration can generate Cl‐rich fluids (Kendrick et al., 2011, 2018; Scambelluri et al., 1997, 2004, 2015), such a process is expected to not only release volatiles and fluid‐mobile elements (FMEs) but also potentially mobilize Cr.…”

Fluid Redox Fingerprint of the CaCO3+Antigorite Dehydration Reaction in Subducted Metacarbonate Sediments

“…Antigorite is the dominant constituent of subducted serpentinites and ophicarbonates, and it is also common in the impure marbles derived from metamorphism and the hydration of dolostones (e.g., [5]; Figure 1a). Antigorite dehydration is under scrutiny from the petrologic community due to its ambiguous role in influencing the geochemistry and, most importantly, the redox of fluids that interact with the mantle wedge [6][7][8][9][10][11][12][13][14][15][16][17]. More specifically, different views are reported in the literature, stemming from the different strategies adopted for investigating this petrologic process, i.e., direct observations from the study of natural samples or indirect observations from experiments or thermodynamic modelling.…”