Hydration of a Mantle Shear Zone Beyond Serpentine Stability: A Possible Link to Microseismicity Along Ultraslow Spreading Ridges?

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.