Extended Data Fig. 2: Redox-sensitive elements in the solid phases for a high-pressure serpentinite.

The absolute concentration of redox-sensitive components in the solids (expressed as mol/kg of rock Al98-05a) for the pseudosection shown in Supplementary Figure 1 (see also Fig. 3a in the main text for the contouring of oxygen fugacity relative to the buffer FMQ). All panels were computed from the absolute amounts of mineral phases containing oxygen-sensitive components and their concentration in pure and solid solution endmembers from WERAMI outputs. Computations used the back-calculated method for fluid speciation in PerpleX, except for panel S⁶⁺ which was computed using the lagged speciation method that allows mass balance constraints in the region below the complete serpentinite dehydration. The last panel shows the redox budget of the rock referred to the whole mantle reference redox state (Methods).