The Subducting Slab as a Chromatographic Column: Regimes of Sub‐Solidus Mass Transport as a Function of Lithospheric Hydration State, With Special Reference to the Fate of Carbonate

Abstract

When oceanic lithospheres sink into the mantle in subduction zones, a fraction of their mass dissolves into mobile fluids that are released at increasingly higher pressure and temperature (P-T) conditions (Frezzotti

Sulfur disproportionation in deep COHS slab fluids drives mantle wedge oxidation

“…Given the widespread occurrence of subducted sediment signatures in arc magmatism (52), believed to result from the influence of sediment-buffered slab fluids, it is reasonable to assume that fluid transport in the uppermost section of a subducting slab at subarc depths occurs by porous flow. Such a process, as also shown here, is fully capable of resetting serpentinitederived and AOC-derived fluid chemistry to that of the sediment layer within a few meters of infiltration (34,36).…”

“…From such a low value, it is apparent that most sulfur entering the subarc mantle has to be delivered by S-bearing fluids rising from the entire subducting slab lithologies. These fluids can quickly flow through fracture networks or slowly permeate and reequilibrate with subducted sediments before entering the subarc mantle ( 34 , 36 , 49 ).…”

“…2, E and F) Such a composite fluid composition has been further mass-balanced with the global H 2 O flux produced by global subductions between 100 and 150 km [1.8 × 10 11 to 8.0 × 10 11 kg/year; estimated by (54)]. Assuming that all subduction zone fluids are fully chemically reset by interacting with the subducted sediments before entering the subarc mantle (34,36), the H 2 O flux has been assumed to be entirely equilibrated with a sediment column composed solely of metacarbonate sediments. Overall, these calculations allow for the drawing of a relation between the time it takes for a fluid with a specific redox capacity to oxidize the subarc mantle from a starting fo 2 value to a desired one.…”

“…In this study, we explore, using electrolytic fluid thermodynamic modeling, the dissolution mechanisms and the redox-dependent speciation of S and C in aqueous fluids during subduction at subarc depths within carbonate metasediments in the f o 2 range from ΔFMQ − 1 to ΔFMQ + 1. We performed deep fluid thermodynamic modeling in metasediments since these rocks, despite their small volume compared to the rest of the slab, potentially behave as powerful “oxidative fluid filters” ( 34 ) at the top of a downgoing slab, imprinting their geochemical fingerprint to fluids leaving the slab (see Supplementary Text for further information) ( 34 – 36 ). We find that, as observed in further modeling in other slab lithologies, at high P and T (3 to 4.3 GPa and ~560° to 730°C), pyrite dissolution in aqueous fluids occurs by S 1− disproportionation into sulfate, bisulfite, and sulfide species.…”

Carbon Storage in the Forearc Produced by Buoyant Diapirs of Subducted Sediment

“…At subarc depths, aqueous dissolution of carbonates in the sediments and crust is thought to carry the majority of the carbon into the source of arc magma in the mantle wedge (Ague & Nicolescu, 2014; W. Chen et al., 2023; Farsang et al., 2021), but this is limited to conditions with high aqueous fluid production and high Ca + content carbonates. During the infiltration of a water‐bearing fluid at forearc depths, metamorphic decarbonation reactions have been suggested to be a significant mechanism for releasing carbon from sediments (e.g., carbonated siliciclastic) and altered ocean crust (Gorman et al., 2006; Stewart & Ague, 2020; Tian et al., 2019; Zhong & Galvez, 2022). Diapirs have been proposed to be more efficient than metamorphism and melting during recycling of subducted carbon (Behn et al., 2011; C. Chen et al., 2021; Marschall & Schumacher, 2012).…”