Deformation microstructures in serpentinites from the Atlantis Massif Oceanic Core Complex

Oceanic core complexes are a common feature along slow-spreading mid-ocean ridges. Serpentinized mantle rocks are exposed at the seafloor in the footwall to large-scale detachment faults. While it is likely that the exposed and rotated footwall has experienced deformation, it is unclear how internal footwall deformation is accommodated by the ultramafic rocks. One example of such an oceanic core complex is the Atlantis Massif at the Mid-Atlantic ridge (30° N) drilled by International Ocean Discovery Program (IODP) Expedition 399. Site U1601 provides the unique opportunity to understand any deformation recorded in serpentinized mantle rocks over >1.2 km depth.

To better understand the depth distribution of deformation and the associated deformation mechanisms, we combine microstructure and crystallographic preferred orientation (CPO) analysis by means of Scanning Electron Microscopy techniques, Micro-X-ray Fluorescence Mapping and synchrotron high energy X-ray diffraction. Results show variable microstructures ranging from zoned mesh cells with no CPO, to foliated samples with a strong CPO of both serpentine and magnetite, to serpentinite samples exhibiting deformation microstructures like kinking, faulting and dissolution-precipitation. The origin of characteristic microstructures and CPOs, whether formed due to serpentinization, deformation, or mutual interaction, will be discussed.