Sulfur degassed at volcanic arcs calls for dissolved sulfate ions (S ⁶⁺ ) released by subduction-zone fluids, oxidizing (in association with carbon) the subarc mantle, but sulfur speciation in subduction fluids at subarc depths remains unclear. We apply electrolytic fluid thermodynamics to model the dissolution behavior of pyrite in metacarbonate sediments as a function of P , T and rock redox state up to 4.3 gigapascals and 730°C. At subarc depth and the redox conditions of the fayalite-magnetite-quartz oxygen buffer, pyrite dissolution releases oxidized sulfur in fluids by disproportionation into sulfate, bisulfite, and sulfide species. These findings indicate that oxidized, sulfur-rich carbon-oxygen-hydrogen-sulfur (COHS) fluids form within subducting slabs at depths greater than 100 kilometers independent from slab redox state and that sulfur can be more effective than the concomitantly dissolved carbon at oxidizing the mantle wedge, especially when carbonates are stable.

Metamorphic reactions are commonly driven to completion within shear zones thanks to fluid circulation, making the re‐equilibration of the mineral assemblage one of the dominant processes. Despite the important role of H₂O in such processes, forward thermodynamic modelling calculations commonly assume either H₂O‐saturated conditions or only fluid loss during prograde evolution to peak conditions. These assumptions influence the understanding of shear zones during the retrograde evolution. Here, we investigate the P–T–MH₂O retrograde evolution of the Mt. Bracco Shear Zone (MBSZ), an Alpine ductile tectonic contact which marks the boundary between two HP units in the Dora‐Maira Massif (Western Alps, Italy). After the eclogite‐facies peak (at 500–520°C and 1.8–2.2 GPa), the subsequent mylonitic event is constrained at amphibolite‐facies conditions, continuing its evolution at decreasing pressure and temperature during rock exhumation, from ~590°C, 1.0 GPa down to ~520°C, 0.7 GPa. The P/T–MH₂O forward modelling highlights different behaviour for the two analysed samples. After reaching a minimum H₂O content at the transition from eclogite‐ to amphibolite‐facies conditions, a significant fluid gain is modelled for only one of the two analysed samples just before the mylonitic event. The MBSZ then evolves towards H₂O‐undersaturated conditions. This work thus underlines the necessity of investigating the H₂O evolution within shear zones, as the H₂O content is susceptible to change through the P–T path, due to dehydration reactions or fluid infiltration events. Furthermore, lithological heterogeneities influence possible different fluid circulation regimes in shear zones, resulting in externally or internally derived fluid gain.

Despite the fact that rock textures depend on the 3D spatial distribution of minerals, our tectono-metamorphic reconstructions are mostly based on a 2D visualisation (i.e. thin sections). This work compares 2D and 3D investigations of petrography and microstructures, modal abundances, and local bulk rock composition and their implication for P–T estimates, showing the pros and cons and reliability of 2D analysis. For this purpose, a chloritoid–garnet-bearing mica schist from the Dora-Maira Massif in the Western Alps has been chosen. In particular, for 2D a thin section scan has been combined with chemical X-ray maps, whereas for 3D the X-ray computerised axial microtomography (µCT) has been applied. Two-dimensional investigations are readily accessible and straightforward but do not consider the entire rock volume features. Conversely, the rise of 3D techniques offers a more comprehensive and realistic representation of metamorphic features in the 3D space. However, they are computationally intensive, requiring specialised tools and expertise. The choice between these approaches should be based on the research aims, available resources, and the level of detail needed to address specific scientific questions. Nevertheless, despite differences in the modal distribution, the estimated bulk rock compositions and relative thermodynamic modelled phase fields show similarities when comparing the 2D and 3D results. Also, since different thin section cut orientations may influence the results and consequent interpretations, three different cuts from the 3D model have been extrapolated and discussed (i.e. XZ, YZ, and XY planes of the finite-strain ellipsoid). This study quantitatively corroborates the reliability of the thin section approach for tectono-metamorphic reconstructions, still emphasising that 3D visualisation can help understand rock textures.

Antigorite dehydration is a process able to release, in comparison with other minerals, the highest amount of H₂O from a subducting slab. The released fluid delivers critical elements (e.g., S, Cu, and REE) to the overlying subarc mantle, modifying the mantle source of arc magmas and related ore deposits. Whether antigorite breakdown produces oxidising or reducing fluids is debated. Whereas previous studies have investigated antigorite dehydration in serpentinites (i.e., in a (C)AMFS-H₂O system), this contribution is devoted to the CMFS-COHS carbonate system, which is representative of the metacarbonate sediments (or carbonate-dominated ophicarbonate rocks) that sit atop the slab. Thermodynamic modelling is used to investigate the redox effect of the carbonate-buffered antigorite dehydration reactions (i.e., brucite breakdown and antigorite breakdown) on electrolytic fluid geochemistry as a function of P-T-fO₂. The influence of P-T-fO₂ conditions on the solubility of C and S, solute-bound H₂ and O₂, fluid pH, the average valence states of dissolved C and S, and the fluid redox budget indicates that, in metacarbonate sediments, the CaCO₃+antigorite reaction tends to produce reducing fluids. However, the redox state of such fluids is buffered not only by the redox state of the system but also, most importantly, by concomitantly dissolving redox-sensitive minerals (i.e., carbonates, graphite, pyrite, and anhydrite). A qualitative correlation between the redox state of the system and the possible depth of fluid release into the mantle wedge is also derived.