RheoMan: a five-year, ERC-funded (Advanced Grant), project to model the rheology of the Earth's mantle

Feb 27, 2014 Publication in Nature Results

A new model for polycrystalline, olivine-rich rock deformation

A previously neglected type of crystal defect called disclinations can explain how the most abundant mineral in the Earth’s upper mantle is able to deform under stress. This solution to the ‘olivine deformation paradox’, presented this week in Nature, should lead to a better understanding of upper-mantle dynamics — from the atomic scale to global mantle flow.

P. Cordier, S. Demouchy, B. Beausir, V. Taupin, F. Barou & C. Fressengeas (2014) Disclinations provide the missing mechanism for deforming olivine-rich rocks in the mantle. 507, 51-56,  DOI : 10.1038/nature13043.

see also the News & Views from Greg Hirth associated with this publication

On geological timescales, the solid minerals of the Earth’s upper mantle deform in a viscous fashion, a process generally understood to be facilitated by the motion of linear crystal defects known as dislocations. In detail, though, this explanation has been challenged by the fact that the mineral olivine, which comprises 60–70% of the upper mantle, has a crystal symmetry that prevents the observed deformation from occurring solely by this mechanism.



We propose that another class of linear crystal defects, known as disclinations, can account for the ‘missing’ deformation. Although disclinations, like dislocations, were first characterized more than 100 years ago, they have only recently been recognized to be pervasive in polycrystalline metals. Using electron diffraction mapping (EBSD), we show that disclinations are also abundant in experimentally and naturally deformed olivine samples.



Combining these observations with a model for disclination-based deformation, we demonstrate that disclinations can provide the missing mechanism for plastic flow in olivine, and hence for upper-mantle deformation.