Thermodynamic constraints on devolatilization of variably altered oceanic ultramafic rocks during subduction: Implications for subarc mantle oxidation

Fluids released by devolatilization of subducted serpentinites at subarc depths trigger partial melting of the overlying mantle wedge and contribute to arc magmatism. The subarc mantle is more oxidized relative to the oceanic mantle, but the potential role of fluids derived from serpentinites during subduction in this oxidation remains contentious. Here, we compile bulk compositions of variably altered oceanic ultramafic rocks at slow- to ultraslow-spreading mid-ocean ridges worldwide, including partially and completely serpentinized ultramafic rocks, carbonate-bearing serpentinites (referred to as ophicarbonates), and talc-altered serpentinites. Using thermodynamic modeling, we quantify the oxygen fugacity (fO₂) of fluids produced during breakdown of antigorite, chlorite, and talc, which are the major water carriers in these lithologies under subarc conditions, along typical subduction geotherms. Results show that the redox states of the rocks prior to subduction play an important role in the fO₂ of the deep-released fluids and that the subduction geotherms play a minor role. Partially and completely serpentinized ultramafic rocks and ophicarbonates with initial Fe³⁺/Fe_total ratios of 0.45, 0.84, and 0.78 generate fluids with increasing fO₂ to 2.4–2.8, 3.7–4.0, and 3.0–3.4 log units above the fayalite–magnetite–quartz (FMQ) buffer, respectively, during antigorite dehydration, which remains almost constant during chlorite dehydration. These calculations, combined with previous experimental and modeling results, suggest that oxidized fluids are liberated through antigorite and chlorite breakdown in subducted serpentinites, and the fluid fO₂ may be positively linked to the initial bulk Fe³⁺/Fe_total ratios. Inconsistency occurs between the modeling results and the sample-based study, given that the transformation of S-bearing phases in natural rocks is likely more complicated. In contrast, talc-altered serpentinites have relatively low Fe³⁺/Fe_total ratios and total Fe contents, and fluids characterized by decreasing fO₂ to ∼0.5 log units below the FMQ buffer can be produced during primary devolatilization. Quantification of fluid-mediated mass transfer indicates that dehydration of antigorite and chlorite in partially serpentinized ultramafic rocks in subducted oceanic slabs can oxidize the subarc mantle on typical subduction timescales, particularly along the cold to intermediate geotherms.