Redox evolution of metamorphic COHS fluids in subduction zones: Insights from automated thermodynamic modeling with ThermoPathX

The flux of carbon, sulfur, and other volatiles between Earth’s mantle and surface plays a fundamental role in shaping the planet’s long-term geochemical cycles. However, modeling the fate of these volatiles at elevated pressures and tracking their oxidation state remains challenging. These difficulties are reflected in the large uncertainties that persist in global carbon and sulfur budget estimates.

Recent advances in thermodynamic modeling have incorporated electrolytic aqueous fluid speciation and open-system frameworks, significantly improving our understanding of slab devolatilization. These developments have clarified the roles of carbon and sulfur dissolution, mass transfer, and associated redox conditions during subduction. At the same time, the volume and complexity of model outputs have increased substantially, creating a need for modern tools for efficient data handling, processing, and visualization.

To investigate the redox budget and COHS fluid speciation across a global suite of subduction thermal models, we developed ThermoPathX, a software framework for automated thermodynamic modeling. ThermoPathX uses PerpleX [1] as its computational engine and enables streamlined preparation and execution of one- and two-dimensional (X–Y) models, followed by the construction of semi-3D (X–Y–Z) models through an iterative workflow based on extracted and processed results. This multi-dimensional approach allows simultaneous analysis of fluid-release pulses, fluid composition and speciation, and oxygen fugacity during prograde metamorphism along a wide range of subduction-zone P–T paths.

We tested ThermoPathX's capacity to explore the potential effects of thermal regimes and initial redox budget on fluid redox capacity by examining the evolution of metapelites during subduction metamorphism. Continental metapelites show a systematic decrease in Fe³⁺/ΣFe with increasing metamorphic grade during regional metamorphism [2]. Here, we examine whether a similar trend occurs in subduction-zone metapelites and whether such behavior can be explained by intrinsic (closed-system) devolatilization, or instead requires open-system interaction with externally derived reduced fluids. Our modeling indicates that intrinsic devolatilization alone is sufficient to reduce the bulk Fe³⁺/ ΣFe ratio and the overall redox budget, driven by the loss of oxidized volatile components in aqueous fluids due to the oxidation of graphite and the reduction of ferric iron in silicates in the rock. This reduction is more pronounced along warm subduction geotherms.

[1] Connolly, 2009 (doi: 10.1029/2009GC002540); [2] Forshaw & Pattison, 2023 (doi: 10.1130/G50542.1)

This research work was funded by the European Commission – NextGenerationEU, through Momentum CSIC Programme: Develop Your Digital Talent (MMT24-IACT-01; M.Bukała) and the ERC CdG, OZ: Deep Earth’s Oxygen recycling at subduction Zones Grant Agreement 101088573.