Redox Conditions and Melting Processes in Magmatic Systems

Redox conditions within magmatic systems—defined by the balance between oxidising and reducing species—critically control the speciation of multivalent elements (notably iron and sulfur), the stability of mineral phases and the onset of partial melting. Variations in oxygen fugacity influence melt viscosity, crystallisation trajectories and the partitioning of trace elements, thereby shaping magma chemistry and eruption dynamics. In subduction settings, fluids released from dehydrating slabs can modify the redox state of the overlying mantle wedge, imparting an oxidised signature to arc magmas and affecting volatile release to the atmosphere. At mid‐ocean ridges and within mantle plumes, inherited redox heterogeneities and processes such as degassing and crustal assimilation further diversify melt compositions. A thorough understanding of these redox–melting interactions is essential for reconstructing Earth's deep volatile cycles, volcanic emissions and the formation of ore deposits.

Research from Nature Portfolio

Recent studies have revealed that aqueous fluids ascending from dehydrating slabs undergo significant redox modification by thin layers of subducted metasedimentary rock, which can oxidise the slab‐derived flux via the removal of H₂, CH₄ and H₂S and thereby elevate oxygen fugacity in the mantle wedge to levels characteristic of arc magma generation. Global compilations of Archean to Proterozoic basalts, interpreted through V–Ti redox proxies, demonstrate a ~1 log‐unit rise in upper‐mantle oxygen fugacity between 3.8 and 2.5 Ga, coincident with shifts in Th/Nb and Nd isotopic ratios that implicate crustal recycling in driving mantle oxidation. Foundational mass‐transfer models of serpentinite breakdown further show that the transformation of antigorite to olivine, enstatite and chlorite at subduction‐zone pressures generates fluids at or above the hematite–magnetite buffer, enriched in sulfate, providing a proximate oxidising agent for sub‐arc magmatism.

Redox Conditions and Melting Processes in Magmatic Systems publication trend

The graph below shows the total number of articles in redox conditions and melting processes in magmatic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Oxygen fugacity (fO₂): A measure of the effective chemical potential of oxygen, governing the redox state of a mineral or melt.

Mantle wedge: The region of the upper mantle above a subducting tectonic plate where slab‐derived fluids induce partial melting.

Melt inclusion: A small pocket of melt trapped within a growing mineral, preserving the composition and redox state of the host magma at the time of entrapment.

Oxybarometer: A proxy or calibration, often based on element partitioning, that quantifies the redox state of a melt or mineral assemblage.

References

1. Slab-derived devolatilization fluids oxidized by subducted metasedimentary rocks. Nature Geoscience (2022).
2. Oxidation of Archean upper mantle caused by crustal recycling. Nature Communications (2022).
3. Highly oxidising fluids generated during serpentinite breakdown in subduction zones. Scientific Reports (2017).
4. Oldhamite: a new link in upper mantle for C–O–S–Ca cycles and an indicator for planetary habitability. National Science Review (2023).
5. Effect of redox on Fe–Mg–Mn exchange between olivine and melt and an oxybarometer for basalts. Contributions to Mineralogy and Petrology (2020).