The observation that primitive arc magmas are more oxidized than mid-ocean-ridge basalts has led to the paradigm that slab-derived fluids carry SO2 and CO2 that metasomatize and oxidize the sub-arc mantle wedge. We combine petrography and thermodynamic modelling to quantify the oxygen fugacity (fO2) and speciation of the fluids generated by serpentinite dehydration during subduction. Silicate-magnetite assemblages maintain fO2 conditions similar to the quartz-fayalite-magnetite (QFM) buffer at fore-arc conditions. Sulphides are stable under such conditions and aqueous fluids contain minor S. At sub-arc depth, dehydration occurs under more reducing conditions producing aqueous fluids carrying H2S. This finding brings into question current models in which serpentinite-derived fluids are the cause of oxidized arc magmatism and has major implications for the global volatile cycle, as well as for redox processes controlling subduction zone geodynamics.

Magnetite stability in ultramafic systems undergoing subduction plays a major role in controlling redox states of the fluids liberated upon dehydration reactions, as well as of residual rocks. Despite their relevance for the evaluation of the redox conditions, the systematics and geochemistry of oxide minerals have remained poorly constrained in subducted ultramafic rocks. We present a detailed petrological and geochemical study of magnetite in hydrous ultramafic rocks from Cerro del Almirez (Spain). Our results indicate that prograde to peak magnetite, ilmenite-hematite solid solution minerals, and sulfides coexist in both antigorite-serpentinite and chlorite-harzburgite at ca. 670 °C / 1.6 GPa, displaying successive crystallization stages. Each stage is characterized by specific mineral compositions. In antigorite-serpentinite, magnetite inherited from seafloor hydration and recrystallized during subduction has moderate Cr (Cr2O3 < 10 wt.%) and low Al and V concentrations. In chlorite-harzburgite, polygonal magnetite is in textural equilibrium with olivine, orthopyroxene, chlorite, pentlandite, and ilmenite-hematite solid solution minerals. The Cr2O3 contents of this magnetite are up to 19 wt.%, higher than any magnetite data obtained for antigorite-serpentinite, along with higher Al, and V, derived from antigorite breakdown, and lower Mn concentrations. This polygonal magnetite displays conspicuous core to rim zoning as recognized on elemental maps. Cr-V-Al-Fe3+ mass balance calculations, assuming conservative behavior of total Fe3+ and Al, were employed to model magnetite compositions and modes in the partially dehydrated product chlorite-harzburgite starting from antigorite-serpentinite, as well as in the serpentinite protolith starting from the chlorite-harzburgite. The model results disagree with measured Cr and V compositions in magnetite from antigorite-serpentinites and chlorite-harzburgites. This indicates that these two rock types had different initial bulk compositions and thus cannot be directly compared. Our mass balance analysis also reveals that in order to account for the mass conservation of fluid-immobile elements such as Cr-V-Al-Fe3+, new magnetite formation is required across the antigorite-breakdown reaction. Complete recrystallization and formation of new magnetite in equilibrium with peak olivine (Mg# 89-91), chlorite (Mg# ∼95), orthopyroxene (Mg# 90-91), and pentlandite buffer the released fluid to redox conditions of ∼1 log unit above the quartz-fayalite-magnetite (QFM) buffer. This is consistent with the observation that the Fe-Ti solid solution minerals (hemo-ilmenite and ilmeno-hematite) crystallized as homogeneous phases and exsolved upon exhumation and cooling. We conclude that antigorite-dehydration reaction fluids carry only a moderate redox budget and therefore may not be the only reason why arc magmas are comparatively oxidized.

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Hydration of the oceanic mantle is a fundamental process of the global water cycle promoting chemical and volumetric changes and facilitating mantle exhumation along detachment faults. At which depth these processes occur and how fluids circulate along ductile mantle shear zones are, however, less well constrained. Here we present field, chemical, and microstructural evidence of hydration processes of peridotite mylonites within an upper mantle shear zone from an Alpine ophiolite (Lanzo massif, Italy). Mylonitic and ultramylonitic areas of the anastomosing shear zone are enriched in Cl-bearing amphibole. Electron backscatter diffraction (EBSD) data indicate the activation of the (100)[001] amphibole slip system arguing for synkinematic growth and deformation at temperatures consistent with Mg-hornblende stability between 800°C and 850°C. High Cl contents in amphibole (0.15-0.61 wt%) as well as oxygen isotope data (δ 18 O whole-rock between 4.4‰ and 4.7‰) indicate accumulation and focusing of seawater-derived fluid in mylonitic and ultramylonitic domains. Such hydration processes are consistent with strain partitioning between water-poor (less deformed) and water-rich (intensely deformed) layers, consistent with changes in olivine and pyroxene crystallographic preferred orientations (CPOs). Our results support recent geophysical data from ultraslow spreading mid-ocean ridge systems that fluids might penetrate beyond the stability of serpentine to depth between 6 and 15 km. Such peridotite shear zones act as fluid pathways for long-lived detachment faults or oceanic transform faults, along which upper mantle rocks are exhumed to the seafloor. Fracturing and fluid flow along such peridotite shear zones might be recorded by deep microseismicity along ultraslow spreading ridges.