New perspectives on the oxygen and deep water cycle

There is sustained interest in the potential coupling between geodynamic processes and the deep volatile cycle, including oxygen and other redox-sensitive elements. Most current global mass-balance estimates are based on the redox budget [1] or redox capacity [2] of subducted inputs with highly variable compositions and hydration states (sediments, mafic and ultramafic rocks), and on corresponding global outputs. Past efforts have largely focused on the deep-water cycle, particularly in subduction zone settings. However, the evolution of the redox state and redox budget of these diverse inputs during subduction has only recently begun to be addressed, and many fundamental questions remain unresolved. A key issue is the extent to which oxidised species are transferred from the slab to the mantle wedge. This problem is difficult to evaluate when subducting lithologies are assumed to behave independently and as closed systems during dehydration reactions. Increasing evidence instead points to significant exchange of aqueous fluids among contrasting lithologies, with critical and non-linear effects on the redox capacity of fluids ultimately transferred to the mantle wedge [3]. In addition, high-pressure hydrodynamics, driven by dynamic permeability changes in compacting rheologies, remain poorly constrained. The role of the mantle wedge as a potential oxygen reservoir is therefore an emerging topic of interest.

In this contribution, we present a series of natural, theoretical, and experimental case studies based on analyses of COHS components in different lithologies. These observations are complemented by bulk and in situ stable-isotope data, which further support mixing of aqueous fluids from different sources under variable pressure–temperature conditions. Most mafic and ultramafic input lithologies show prograde evolution under highly oxidising conditions and possess a high redox budget. However, interaction with lithologies containing minor amounts of reduced phases, such as graphite-bearing metapelites, produces distinctive petrological and geochemical signatures and substantially reduces the oxidising capacity of the interacting lithologies. In particular, sulphur and carbon efficiently track these interactions and represent the most effective vectors for redox-budget transfer from the slab to the mantle wedge. New data on the role of the cold mantle wedge as an oxygen reservoir are also presented.

Overall, these observations highlight the need to integrate lithological interactions and fluid exchange into models of subduction-zone processes, accounting for secular and global variations in input lithologies. Future constraints on ferric iron in key high-pressure hydrous phases and on the stability of sulphur-bearing phases will enable the development of improved thermodynamic models, leading to more robust predictions of the redox capacity of high-pressure aqueous fluids.

[1] Evans (2012) Earth-Science Reviews, 113, [2] Galvez, M. E., Müntener, O., & Jaccard, S. L. (2025). Geophysical Research Letters, 52 [3] Padrón-Navarta et al. (2023) Nature Geosciences, 16,

This project has been funded through the ERC project OZ (DOI: 10.3030/101088573).