The Legacy of plastic deformation and pre-existing microstructures during olivine serpentinization

High-temperature crystal-plastic deformation of olivine is an inevitable consequence of asthenospheric mantle flow. In this environment olivine commonly yields by dislocation creep to form dislocation walls. Solid-state diffusion along these imperfections is faster than through the bulk crystal. At low temperatures, olivine serpentinization modifies the petrophysical and geochemical properties of the oceanic lithosphere. Under these conditions olivine generally fails to compositionally readjust via diffusion and re-equilibrates via dissolution-reprecipitation. Although plastic deformation and serpentinization are decoupled, we identified compositional readjustments expressed as striped zonings in partially serpentinized and deformed olivine grains from the Leka Ophiolite Complex (LOC), Norway. Combining focused ion beam sample preparation and (scanning) transmission electron microscopy reveals that every zone is immediately related to a subgrain boundary composed of edge dislocations. The zonings are a result of the Fe-Mg exchange potential between olivine and antigorite, where Fe enrichment or depletion is controlled by the silica activity imposed on the system by orthopyroxene and magnetite stability. Nanometer-sized serpentine precipitates along olivine dislocation walls and crystallographic relationships gained by electron backscatter diffraction (EBSD) suggest that hydration was also initiated along these intracrystalline imperfections. The legacy of pre-existing microstructures in the LOC is exhibited by localized occurances of parallel but highly misorientated olivine grains seperated by plastically deformed diopside lamellae. Microtomography combined with EBSD suggest that these areas are a consequence of a hydration-dehydration sequence to result in a complex pseudomorphism of primary mantle orthopyroxene to olivine and the cryptic survival of the primary microstructure. Our observations provide new insights into the role of primary microstructures and crystal-plastic deformation on subsequent fluid-mediated reactions.