A new method for mapping stresses in mantle rocks: Dislocation density from electron-backscatter diffraction

Lead Research Organisation: University of Oxford
Department Name: Earth Sciences

Abstract

The viscous flow of solid rock in Earth's deep interior fundamentally controls a variety of large-scale processes ranging from the formation of tectonic plate boundaries to the timescale of sea-level rise associated with receding ice sheets to the dynamics of the deepest portions of earthquake-generating faults. The most common approach to learning about the flow of rocks at extreme conditions is to conduct experiments in a laboratory on small samples at relatively short timescales. The relevant processes in the Earth, however, occur over many kilometers and millions of years, scales much larger than those accessible in the laboratory. For scientists to know how best to extrapolate from small scales to large scales when predicting the behavior of real Earth processes, they need a robust understanding of the microscopic physical mechanisms controlling the flow of rocks at extreme conditions.

Several sophisticated models exist describing the viscous behavior of rocks. However, considerable uncertainty persists regarding their details, and there is a corresponding lack of suitable analytical techniques to test those details. Therefore, we propose to adapt a new technique for quantifying the intricacies of the viscous flow of rocks. This technique, recently developed for analyzing small-scale structures in metals, allows defects in the constituent crystals-known as dislocations-to be mapped quickly and precisely at high resolution in an electron microscope. We will conduct laboratory deformation experiments on single crystals of olivine, the dominant mineral in the upper mantle, to establish a relationship between the density of dislocations and the deformation conditions. Determining this relationship will enable us to map deformation conditions at very fine scales in synthetic and natural olivine-rich rocks deformed in variety of settings. We will then compare the results of this mapping to predictions from current models for olivine deformation, using the results to rule out models that do not capture the essential physical processes and to refine and update the ones that do. The project will thus yield a new method in the Earth sciences applicable to a wide range of minerals, and it will address several outstanding problems in the deformation of upper-mantle rocks.

Planned Impact

The proposed research forms an interdisciplinary blue skies project, focussed on providing fundamental understanding that can underpin the application of laboratory-derived models of rock deformation to large-scale geological processes. The progress of such processes - over timescales ranging from very short to geologic in length - is hidden from direct observation and measurement and yet is crucial to addressing key outstanding problems in geodynamics with major societal and economic impact on communities worldwide. Such issues include the identification, assessment and mitigation of (intra-plate) earthquake and volcanic hazards and even the extraction of oil, gas and other mineral wealth using modern techniques such as induced hydraulic fracturing or 'fracking'.

The immediate end users engaged in modelling rock deformation, who would directly benefit from the project results, are primarily engaged within the academic community. However, the results of the project are likely to have down the line benefits for non-academic users - both industrial and governmental - involved in the application of rock deformation modelling for fossil fuel and mineral extraction, large scale civil engineering, construction, earthquake hazard mitigation and volcanic threat monitoring.

There is clearly, therefore, potential for the proposed work to have very significant impact and I can outline a number of clear mechanisms in order to deliver impact to a range of beneficiaries. Academic peers and end-users will be addressed via standard direct and indirect routes. The indirect path features dissemination of results and engagement with the academic modelling community via international conference presentations, collaborative travel, and publishing of data on University websites and NERC data repositories in order to drive the model development from which end users will benefit. Potential end users will also be engaged directly and informed of the progress of the project via delivery of presentations at a variety of general Earth Sciences and focused rock deformation academic conferences, which have historically featured presentation and discussion amongst the academic community and wider stakeholders in industries ranging from mineral/fossil fuel extraction to civil engineering.

Locally, presence of the Oxford-Shell Geoscience Lbaoratory will facilitate immediate and on-goining interaction with the fossil fuel extraction industry, which has a current research focus on teh deformation of mudrocks (rather than crystalline rocks) in sedimentary basins. Oxford Earth Sciences also has a strong mineral studies group, led by Professor Laurence Robb, with numerous direct links with mineral and mining companies whose interests will rock deformation are closely aligned with the research focus of this project.

More broadly, Oxford has a highly effective press office, with one officer dedicated to the work of the division that includes Earth Sciences. The Oxford science press officer also runs the Oxford Science Blog that has a wide reach (including social media such as Twitter and Facebook) and aims to communicate to the public, via the Internet, current research outputs by Oxford researchers. Oxford is also part of i-Tunes U, with considerable potential for the dissemination of material to the global public in the form of downloads and podcasts. Through the University and NERC, there are also substantial opportunities to deliver impact via outreach science, and we expect to allow the project PDRA to take a lead in these activities.

Publications

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Britton T (2016) Tutorial: Crystal orientations and EBSD - Or which way is up? in Materials Characterization

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Hansen L (2016) Viscous anisotropy of textured olivine aggregates: 2. Micromechanical model in Journal of Geophysical Research: Solid Earth

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Hansen LN (2016) Olivine anisotropy suggests Gutenberg discontinuity is not the base of the lithosphere. in Proceedings of the National Academy of Sciences of the United States of America

 
Description We have developed a new technique for measuring crystal defects in olivine, the dominant mineral in Earth's upper mantle. This technique allows unparalleled resolution, and opens up many possibilities for quantifying the microphysics of rock deformation. A paper on this technique has been published and advertised in order to disseminate this methodology to the Earth Science community.
We've also published a benchmark paper establishing the common defect structures in deformed olivine that will act as a key reference for future studies interested in interpreting olivine microstructures. In two separate papers, we've used this technique to decipher the underlying physics behind grain growth in deformed olivine and behind grain rotation in partially molten mantle rocks. Another paper was published in Science Advances which uses this technique to establish that the strength of olivine depends on the volume of material being deformed and resolves a long standing controversy about the strength of the lithosphere.
In addition, recent results indicate that distinct differences between different deformation regimes can be ascertained through this new methodology. Thus, rocks deformed in past events can now be analyzed and the style of deformation in ancient tectonic processes determined. Furthermore, the observed differences in dislocation structures produced by different mechanisms of deformation are inspiring the development of new microphysical models for the flow of Earth's upper mantle. These new models will significantly impact our understanding of plate tectonics and its link to mantle convection.

We've also published the first paper apply this technique to quartz, establishing criteria for interpretation of common microstructural features, and to calcite, establishing the microphysics that controls the onset of brittle faulting in the upper crust. It is safe to say that we've exceeded our predicted output for the duration of this grant.
Exploitation Route The microphysical models we're developing can be incorporated into large-scale simulations by geodynamicists.

Furthermore, the methodology we've developed can now be applied to a wide range of other geological materials with relevance to other parts of the deforming solid earth.

Our work has also shed light on several aspects of dislocation measurement that will improve future work on metals investigated with Materials Science applications.
Sectors Aerospace, Defence and Marine,Education,Energy,Environment,Manufacturing, including Industrial Biotechology

URL http://lnhansen.wordpress.com
 
Title High-angular resolution electron backscatter diffraction data (HR-EBSD) from olivine and quartz 
Description This dataset is supplemental to the paper Wallis et al. (2019) and contains data derived from distortion of crystal lattices measured using conventional electron backscatter diffraction (EBSD) and high-angular resolution electron backscatter diffraction (HR-EBSD). The datasets include lattice misorientation, elastic-strain heterogeneity, residual-stress heterogeneity, and densities of geometrically necessary dislocations in olivine and quartz. We intend the data and associated paper to demonstrate key aspects of the HR-EBSD technique and to draw comparisons with conventional EBSD. As the paper by Wallis et al. (2019) is a review paper, several of the datasets have also been present in, or are otherwise related to, additional previous publications listed below . Data are provided as 55 tab delimited .txt files organised by the figure in which they appear within Wallis et al. (2019). Data types are indicated in the file names. Please consult the data description file for detailed explanations. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
 
Description Collaboration with colleage at University of Liverpool 
Organisation University of Liverpool
Department Department of Electrical Engineering and Electronics
Country United Kingdom 
Sector Academic/University 
PI Contribution Applying developed characterization method to a new mineral, plagioclase, which is an important test of the methods general applicability to earth materials.
Collaborator Contribution Provided samples for analysis and insight into data interpretation.
Impact No outputs yet. Data collection is currently ongoing.
Start Year 2017
 
Description Collaboration with colleagues at University of Pennsylvania 
Organisation Showcase Cinema
Country United Kingdom 
Sector Private 
PI Contribution Applied characterization technique developed as a part of the funded research to Dr. Qi's research, helping with a major breakthrough in our understanding of the behavior of partially molten rocks. Have also worked with Prof. Goldsby on interpreting results of indentation experiments. Results have led to a major new understanding in the mechanical properties earth materials, resolving disagreement among 40 years worth of laboratory-based studies.
Collaborator Contribution Access given to a variety of experimental samples. Collaboration on data interpretation. Input on manuscript preparation
Impact Manuscript based on work with Chao Qi in preparation for submission to Earth and Planetary Science Letters. Manuscript based on work with David Goldsby in review at Science Advances.
Start Year 2015
 
Description Collaboration with colleagues at University of Washington St. Louis 
Organisation Washington University in St Louis
Country United States 
Sector Academic/University 
PI Contribution We have applied newly developmed method to samples of olivine that were initially deformed and subsequently annealed at high temperature. Our maps of crystal defect densities reveal how the microstructure evolves during long static periods at high temperature, providing new insight into the seismic characteristics of Earth's upper mantle.
Collaborator Contribution Conducted experiments, provided samples for analysis, spearheaded manuscript writing.
Impact A manuscript is in publication for submission to Geophysical Research Letters
Start Year 2016