Appraisal of D-Rex parameterization in simulating olivine crystallographic preferred orientation (CPO) evolution using microstructural properties

Olivine is a major phase in the upper mantle and its crystallographic preferred orientation (CPO) carries strong implications for the interpretation of seismic anisotropy and geodynamic models of the upper mantle. The computational model D-Rex (Kaminski et al., 2004) is often used to depict the evolution of olivine CPO under various flow patterns. In its tracing of the crystallographic orientation of olivine and orthopyroxene aggregate D-Rex includes the process of dynamic recrystallization, a fundamental process associated with deformation under dislocation creep. Dynamic recrystallization and deformation mechanism are incorporated in D-Rex via different parameters - the efficiency of nucleation of new grains, grain boundary mobility, and the threshold value below which the grains deform by grain boundary sliding (GBS) (i.e., deformation does not result in rotation or recrystallization). These parameters were benchmarked with experiments to fit the overall CPO evolution. While D-Rex is set to predict the CPO, an appraisal of other pivotal microstructural properties like grain size, dislocation density, and recrystallization fraction has been neglected. Here, we use the implementation of D-Rex within ASPECT to model the shearing of grain to first trace the microstructural properties and further test how they are affected by the dynamic recrystallization parameters. We synthesize the results of tens of runs under a range of parameter space and strains. We observe that for grain size distribution, as the nucleation and threshold value for GBS increase the spread of grain sizes is relatively low while increasing mobility causes a large spread of grain size associated with a small group of grains that dominate the overall CPO. At low strains, the intermediate-sized grains are the major contributor to the CPO while with increasing strain, a smaller fraction of large grains dominate the CPO. Further, we find that the biggest grains keep growing bigger, while the smallest grains oscillate around the GBS threshold. Our analysis highlights the gap between the natural evolution of olivine microstructure and the microstructural properties evolution in D-Rex. The use and implications of different suggested parameters will be discussed. 

  Kaminski, É., Ribe, N. M., & Browaeys, J. T. (2004). D-Rex, a program for calculation of seismic anisotropy due to crystal lattice preferred orientation in the convective upper mantle. Geophysical Journal International, 158(2), 744–752. https://doi.org/10.1111/j.1365-246X.2004.02308.x