Spanning the scales from atomic to rock microstructure to planetary seismology

Abstract:

There is strong connection from atomic structure (sub-nm scale) to microstructure (mm scale); for example the elastic properties of minerals are directly linked to their atomic arrangement. This link between atomic arrangement and elastic properties has been extensively exploited in recent years with ab initio methods providing the elastic tensors of many minerals in extreme conditions. The use of ab initio modeling to predict the behavior of dislocations pioneered by Patrick Cordier’s group in Lille provides another link between atomic structure and deformation at the crystal scale. These data may be further up-scaled via the prediction of the evolution of crystal preferred orientations (CPO) using polycrystalline modeling codes (e.g.,VPSC) to complete the link to anisotropic physical properties at the scale of the Earth deformation patterns.

The link between upper mantle seismology and the microstructure of peridotite rocks recovered at the Earth’s surface was one the first cases where a quantitative estimate of anisotropy measured by seismology could be directly compared with rock samples to provide results in good agreement, even though the length scales measurement differed by several orders of magnitude. The agreement between seismology with propagation path lengths of hundreds to thousands of km and physical properties of rock samples of a few cm³implies that in the upper mantle the flow field must be coherent over large distances. Large-scale convection currents at the planetary scale are clearly a key factor in creating this coherence, but analysis of seismic data also points to coherence in the deformation patterns at the scale of hundreds to thousands of km in the lithospheric mantle.

Yet the interpretation of seismic anisotropy data in terms of flow patterns in the mantle relies on our knowledge of the crystal scale plasticity and on the CPO evolution. Ab-initio modelling of crystal plasticity is an essential tool here. In the upper mantle, this knowledge may be constrained by the direct analysis of deformed peridotites. To contribute to this effort we have been compiling a database of mantle rocks microstructures and CPO.