Some Issues on the Lattice-Preferred Orientation in Olivine-Rich Rocks

Abstract:

During the last several years, various new results were reported on the LPO (lattice-preferred orientation) of olivine that would require major modifications to the paradigms established by the previous studies (for a review see (Karato et al., 2008)). They include (i) the development of strong LPO in the diffusion creep (Miyazaki et al., 2013), (ii) the development of the B-type fabric under dry, low stress torsion deformation at the presence of a small amount of melt (Kohlstedt and Holtzman, 2009) (also Holtzman et al., 2003), and (iii) the development of the A-type fabric under water-rich conditions (Ohuchi and Irifune, 2013).

I will discuss these results based partly on the review of existing results and new results from my lab on olivine morphology. (Miyazaki et al., 2013) reported that substantial LPO develops in diffusion creep in forsterite-diopside aggregates and also the intensity of LPO increases with temperature. They attributed this to the anisotropic crystal morphology of forsterite that appears to depend on temperature. I first note that these results are similar to often observed non-negligible LPO in the diffusion creep (or superplasticity) regime when conditions are close to the boundary to dislocation creep regime (calcite: (Pieri et al., 2001; Walker et al., 1991), perovskite: (Karato et al., 1995)). And in these cases, non-negligible LPO in diffusion creep regime is attributed to the contribution from dislocation glide near the conditions close to the transition to dislocation creep regime. The observed temperature dependence can be interpreted by this model. I will report the results of experimental studies on olivine morphology as a function of temperature to test this model.

The causes for the B-type LPO in partially molten olivine, and the A-type LPO under water-rich conditions are enigmatic. Possible causes for these observations will be discussed based partly on the current knowledge of deformation of olivine.