Localized incipient dehydration in antigorite serpentinite during deformation experiments at subduction zone conditions

Abstract. Intermediate-depth earthquakes are commonly linked to slab dehydration, with dehydration embrittlement of antigorite proposed as a dominant mechanism of brittle failure. However, experimental results increasingly suggest that classical dehydration embrittlement alone may not fully explain intermediate-depth seismicity. Recent work shows that localized incipient dehydration can occur in antigorite due to chemical heterogeneities and stress, forming olivine-rich veins that are also observed in natural serpentinites. These sites may represent regions of strain localization where failure can develop as deformation proceeds. To determine whether deformation promotes localized incipient dehydration relative to hydrostatic conditions, and whether this process contributes to antigorite brittle failure, we investigated the incipient dehydration of antigorite using a six-ram multi-anvil apparatus under both hydrostatic and deviatoric stress conditions. Experiments were conducted across the antigorite stability field at pressures of 3 GPa, temperatures of 530–712 °C, and strain rates of 10-4- 10-5 s-1. Localized incipient dehydration occurs within the antigorite stability field during both static and deformation experiments. Nanocrystalline clusters, veins, and networks containing olivine and pyroxene are observed in all experiments. Localized dehydration is enhanced during deformation experiments, leading to the development of distinct microstructures relative to static conditions. However, this process promotes failure only at fast strain rates (~10⁻⁴ s⁻¹), suggesting that incipient or even complete dehydration alone is insufficient to cause embrittlement at laboratory conditions.