The seismic signature of serpentinite in subduction zones: A rock physics approach

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Serpentinites are rocks that contain a significant proportion of serpentines, which form by hydrothermal alteration of basic silicates (e.g., olivine). These rocks form primarily in the upper oceanic crust, due to hydrothermal circulation of oceanic water along the mid-oceanic ridges where new oceanic crust is generated. As a consequence, the oceanic crust that enters subduction zones is thought to be serpentinised extensively, at least in its upper part. The presence of serpentinite near the subduction interface is expected to have a key influence on subduction zone dynamics, because serpentine minerals have peculiar mechanical and physical properties: they are very weak compared to other crustal and mantle rocks, and they dehydrate (i.e., undergo chemical transformations and release free water) upon heating. The latter effect has dramatic consequences on the effective stress state in the subducting slab, and is thought to play a fundamental role in the generation of slow slip events, intermediate-depth earthquakes, arc volcanism, and water recycling in the mantle.The exact role of serpentinites in subdcution processes is however difficult to quantify precisely since the exact location and amount of serpentine minerals in subduction zones remains poorly known. In order to test whether serpentinites are indeed responsible for the aforementioned features of subduction zones, it is of primary importance to be able to demonstrate their presence or absence at depth. Seismic imaging is the most robust observational constraint available, but the precise identification of serpentinites using seismic methods is difficult. Significant progress has been achieved in the determination of the elastic properties and seismic speeds of serpentine (antigorite, lizardite) single crystals. However, the deformation and dehydration of serpentinites has been shown to systematically induce significant cracking. The microcracks generated by deformation and dehydration may well remain open at depth in subdcution zones, at least temporarily, due to the elevated fluid pressures arising from dehydration itself and buoyancy-driven fluid migration. Microcracking can potentially have strong, first order effects on seismic properties and anisotropy, but remains poorly quantified in serpentinites. In this project we propose to dramatically improve our ability to link seismic observables to the presence of serpentinite by (1) experimentally measure the seismic properties of serpentinites during deformation and dehydration, (2) quantify the microstructural evolution and the relationships between microcrack orientation and crystallographic preferred orientation, and (3) model the effects of microcracks on seismic wave speeds using effective medium approaches. Our study is expected to provide a robust chracterisation of the seismic signature of deformed and dehydrating serpentinites, and thus have a direct impact on the intrepretation of seismic images. In addition, our data will contribute to a better understanding of the deformation and dehydration mechanisms that are key aspects of subduction zone dynamics.