Microstructural Evolution of High- and Low-Strain Serpentinites from the Zermatt-Saas Meta-Ophiolite: Insights into Antigorite and Olivine Deformation at Intermediate-Depth Seismicity Depths

Serpentinites play a critical role in subduction zones due to their unique mechanical properties, which influence tectonic and seismic processes and facilitate deformation along the subduction interface. A long-standing question is the discrepancy between experimentally deformed serpentinites, which exhibit brittle/brittle-ductile microstructures, and naturally deformed serpentinites, which predominantly show ductile features. Additionally, there is a strong debate on whether deformation in antigorite bearing rocks is driven by crystal plasticity, dissolution-precipitation, or a combination of both. Moreover, studies on deformation in partially dehydrated or hydrated serpentinites (containing metamorphic olivine and clinopyroxene), subducted down to (ultra)high pressure conditions, remain scarce. To address these issues, we conducted a detailed microstructural study of serpentinites from a hectometer-scale strain gradient zone within the Zermatt-Saas meta-ophiolite, examining deformation mechanisms in antigorite and olivine at depths relevant to intermediate-depth earthquakes and subsequent exhumation across mantle wedge conditions.

In low-strain serpentinites, dehydration of brucite-antigorite produces coarse-grained olivine-diopside-clinohumite-magnetite veins (“olivine veins”), while the host antigorite displays mesh textures, weak crystallographic preferred orientations (CPOs), and evidence of twinning. Deformation begins to localize around olivine veins, where olivine exhibits a B-type CPO with [010] parallel to the pole of foliation and [001] parallel to the lineation but no internal deformation. With increasing strain, antigorite foliation becomes continuous and penetrative, accompanied by CPO strengthening, grain size reduction, and localized folding and boudinage of olivine, where the CPO strength also increases. High-strain domains exhibit mylonitic fabrics, intense antigorite foliation with (001) maxima aligned to the pole of foliation and (010) parallel to lineation, and transposed olivine vein folds reduced to isoclinal rootless folds. Additionally S-C’ foliations form locally, with fine-grained olivine fibers coating C’ planes, and pressure shadows around olivine porphyroclasts containing olivine-diopside mixtures forming mm-scale bands within antigorite foliations. The olivine grains in the pressure shadows also present a strong B-type olivine CPO.

Our findings highlight a progressive transition from brittle-ductile to ductile deformation in serpentinites in a fluid-rich environment. This deformation seems to be controlled by dissolution-precipitation processes and dislocation creep. Furthermore, this study provides one of the few datasets of deformation of metamorphic olivine in subduction zones. The conditions documented are not only relevant for the oceanic lithosphere but also for the mantle wedge near the subduction channel, offering critical insights into the interplay of deformation, metamorphism, and fluid-rock interactions in these tectonic settings.