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

Lead Research Organisation: University College London
Department Name: Earth Sciences

Abstract

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 subduction 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 subduction 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 characterisation of the seismic signature of deformed and dehydrating serpentinites, and thus have a direct impact on the interpretation 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.

Planned Impact

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.

Publications

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Description By using a range of experimental rock deformation techniques, we have discovered that antigorite, a common rock type in subduction zones, is a much stronger material than previously thought. Under low pressure and temperature conditions, antigorite is brittle and has a strength commensurate to that of other low porosity silicate rocks. However, in contrast with other materials, antigorite is essentially nondilatant, which is a unique behaviour for a rock. Experiments conducted under elevated pressure and temperature indicate that antigorite is also relatively strong, and does not behave purely by crystal plastic processes even at high temperatures. This raises the question of the underlying microscale deformation mechanisms activated during antigorite deformation. Our preliminary modelling results suggest that this mineral is likely to deform by shear microcracking and delamination of its crystal structure, a process previously overlooked. Our results have implications for the rheology of subduction interfaces and the stresses that drive plate tectonics.
Exploitation Route I do not know yet.
Sectors Environment

 
Description Brown 
Organisation Brown University
Country United States 
Sector Academic/University 
PI Contribution Performed a series of high pressure deformation experiments in a Griggs-type apparatus.
Collaborator Contribution Provided technical support and making apparatus and laboratory available to us. Provided scientific input in the discussion of experimental data.
Impact We are now preparing 2 publications related to the experiments performed there (not yet submitted, in preparation).
Start Year 2016
 
Description Oxford 
Organisation University of Oxford
Country United Kingdom 
Sector Academic/University 
PI Contribution Performed nanoindentation experiments in the Materials department and Scanning Electron Microscope observations in the Earth Sciences Department (wth co-PI Lars Hansen).
Collaborator Contribution Makng equipment available to us and in-depth scientific discussions about the results of the experiments and observations.
Impact We have published already 2 papers related to this collaboration, and are currently writing 3 additional papers to be submitted this year.
Start Year 2016