Dissolution-Precipitation dominated deformation in (ultra)high-pressure serpentinites from the Zermatt-Saas Meta-Ophiolite

Serpentinites are key components in subduction zones, acting as primary carriers of water into the deep Earth and critically influencing seismic behavior. Several studies suggest that fluid-saturated deformation in serpentinized subduction channels may control a variety of processes associated with Intermediate-depth seismicity (~50 to 300 km depth) . A key problem in their rheology is the discrepancy between experimentally deformed serpentinites, which exhibit predominantly brittle behavior, and their naturally deformed counterparts, which show ductile fabrics. While the dominant deformation mechanism in subduction settings—whether crystal plasticity, dissolution-precipitation, or a combination— also remains poorly documented. Moreover, there is a lack of constraints on how serpentinites deform during and after partial dehydration at (ultra)high-pressure conditions and transformation to olivine and pyroxene-dominated assemblages. To address these issues, we present an integrated microstructural and geochemical study of serpentinites across a hectometer-scale strain gradient within the Zermatt-Saas meta-ophiolite, documenting the evolution of deformation and major element mobility during subduction and exhumation. In low-strain samples, dehydration forms coarse-grained olivine-diopside-clinohumite-magnetite veins. Host antigorite shows weak crystallographic preferred orientations (CPOs) and twinning. With increasing strain, deformation localizes around these veins, where olivine develops a weak B-type CPO, but with grains showing no evidence of intracrystalline deformation. Progressively, antigorite develops a strong, penetrative foliation with a (001) maximum normal to foliation and grain size reduction, while olivine veins are folded and boudinaged. Low angle grain boundaries are related to fracturing of olivine. In high-strain serpentinite mylonites, transposed olivine veins form isoclinal folds, and S-C' fabrics develop. Antigorite CPO strength increases considerably, something that is not observed for olivine. Whole thin section XRF mapping reveals an increase of Ni and S in the more deformed serpentinites, where pentlandite defines the C' fabric and wraps around olivine porphyroclasts. Antigorite mm thick bands show Cr depletion accompanied by grain size reduction, while Fe-Mn occur normally associated with each other. In the transposed olivine veins there is an increase of Fe content in comparison to the original olivine vein composition.  When present, Al-rich phases such as chlorite are mostly undeformed but can breakdown locally and transform into tremolite + magnetite in late shear bands. Our data document a fluid-assisted progression from localized brittle-ductile to distributed ductile deformation. Microstructural and chemical evidence indicate that deformation was primarily controlled by dissolution-precipitation processes, with limited crystal plasticity in antigorite and predominantly brittle olivine deformation. This study provides a rare dataset on metamorphic olivine deformation in subduction zones and highlights the fundamental coupling between element mobility, metamorphic reactions, and strain localization in the subduction interface and mantle wedge.