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

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.

The goal of this project is to advance our understanding of the role of serpentinites in subduction zones, by establishing datasets and developing models for the prediction of their seismic properties, and hence dramatically improving our ability to detect them using seismic imaging. This project is fundamental in nature and we identified the following non-academic beneficiaries:

(1) in the short term: Students in higher education (undergraduate and postgraduate) engaged in geology and geophysics degrees, but also more generally in STEM subjects. Our project combines laboratory experiments, mathematical modelling and state-of-the-art nanostructural characterisation methods; the dissemination of our research to a wide audience through the UCL and Oxford communication channels is likely to spark the interest of A-level and undergraduate students in quantitative disciplines (chemistry, mathematics, physics) who then might join Earth Sciences. This will contribute to the achievement of NERC's training objectives, which include the supply of a skilled workforce in adequation with industry needs.

(2) in the long term: Industries specialised in exploration geophysics and oil and gas recovery. Complex materials such as serpentinites, which contain a significant proportion of hydrous phyllosilicates, have a structure similar to shales, which are of tremendous importance for the oil industry (primarily as a caprock, but also potentially as unconventional sources of hydrocarbons). Hence, the improvement of our understanding of serpentinites constitutes an important step towards better predictions of shale's mechanical behaviour and seismic signature.

We will engage with potential beneficiaries through the dissemination of our results by liasing with the comminucation teams at UCL and Oxford, which both have a strong online and media presence.

We make the choice to focus our impact activities on improving teaching and training in higher education; we plan to organise a 3 days short course on mineral and rock physics, aimed at 4th year undergraduate and postgraduate students. Based on the observation that mineral and rock physics is at the crossroads between disciplines such as geology, chemistry and physics (material sciences), we recognise the need to build bridges between each separate disciplines and to train students who will have a wider and stronger cross-disciplinary background. The planned workshop are in line with the need to attract STEM students into environment and geological sciences.