Deformation and reaction in the microstructural record of oceanic serpentinites

Ultramafic rocks exposed adjacent to mid-ocean ridges in the footwall to large slip oceanic detachment faults provide unique insight into deformation and reaction when transforming from peridotite to serpentinite. In contrast to orogenic serpentinites, oceanic serpentinites have not subjected to superposed metamorphic and/or tectonic overprinting. A suite of samples from mostly fresh peridotites (~20% alteration), with preserved olivine and pyroxene, to completely serpentinized rocks (100% alteration), dominated by serpentine (lizardite) and magnetite, were collected from a ~1.2 km long drill core from IODP Expedition 399 at the Atlantis Massif oceanic core complex.

A combined approach of synchrotron diffraction and electron backscatter diffraction in order to analyze the crystallographic preferred orientation (CPO), and micro X-ray fluorescence mapping and optical microscopy in order to image and analyze the microstructure, is used to explore the variable microstructures.

Magnetite forms polycrystalline aggregates defining a foliation, which ranges from anastomosing to highly parallel. In partially serpentinized, mylonitic peridotites showing olivine grain size reduction and CPO development; magnetite aggregates trace the preexisting mylonitic fabric. Lizardite and magnetite both have a variable CPO strength and different CPO types, suggesting that different processes and parameters influence the formation of these microstructures. Further, late stage deformation, is evident from microfaulting, sheared serpentine veins and dissolution features. The individual contributions of deformation and serpentinization reaction to the final microstructure will be evaluated and discussed.