Extended Data Fig. 8: Mantle wedge oxidation capacity of deserpentinization fluids modulated by graphite-bearing metasediments derived fluids.

(a) Modification at the slab surface of the Δlog₁₀fO₂[FMQ] and the concentration of HSO₄⁻ —relative to the intrinsic deserpentinization fluid (ID)— during infiltration of fluids equilibrated with metasedimentary rocks with a high reducing capacity (graphite-bearing metapelite) for a worldwide compilation of subduction zones^83,84 (colour-coded for the pressure at which the serpentinite dehydrates at the slab surface, Source Data). (b) The capacity of these modified, serpentinite-derived fluids (empty dots in a) to oxidize the mantle wedge on top of the slab at near wet-solidus conditions is computed for the hottest (Central Cascadia) and coldest (Tonga) subduction zones. A minimum value range of Δlog₁₀fO₂[FMQ] inferred for oxidized IAB source and recorded by high-pressure metasomatized mantle atop of the slab^47,90 is given as a horizontal blue-shaded range. Sediment (graphite-bearing)-serpentinite derived fluids have a variable capacity to oxidize the mantle wedge for hot and cold subduction zones, a variable potential that is directly related to the contrasting solubility of HSO₄⁻ for the two extreme thermal cases. The metasomatized mantle wedge has an initially depleted composition⁸⁵. Squares and stars on the red and blue lines indicate the condition range limits at which pyrrhotite (Po), or anhydrite (anh) are the stable minerals hosting S in the rocks. For an ultradepleted MORB mantle, see Extended Data Fig. 7. For interaction with sediments with low reducing capacity (GLOSS), see Fig. 4 in the main text.