Relationships Between Olivine CPO and Deformation Parameters in Naturally Deformed Rocks and Implications for Mantle Seismic Anisotropy

Abstract

We analyze peridotites from a wide range of tectonic settings to investigate relationships between olivine crystallographic preferred orientation (CPO) and deformation conditions in naturally deformed rocks. These samples preserve the five olivine CPO types (A‐ through E‐type) that rock deformation experiments have suggested are controlled by water content, temperature, stress magnitude, and pressure. The naturally deformed specimens newly investigated here (65 samples) and compiled from an extensive literature review (445 samples) reveal that these factors may matter less than deformation history and/or geometry. Some trends support those predicted by experimentally determined parametric dependence, but several observations disagree—namely, that all CPO types are able to form at very low water contents and stresses and that there is no clear relationship between water content and CPO type. This implies that at the low stresses typical of deformation in the mantle, CPO type more commonly varies as a function of strain geometry. Because olivine CPO is primarily responsible for seismic anisotropy in the upper mantle, the results of this study have several implications. These include (1) the many olivine CPO types recorded in samples from individual localities may explain some of the complex seismic anisotropy patterns observed in the continental mantle, and (2) B‐type CPO—where olivine's “fast axes” align perpendicular to flow direction—occurs under many more conditions than traditionally thought. This study highlights the need for more experiments and the difficulty in using olivine CPO in naturally deformed peridotites to infer deformation conditions.

Evolution of the Josephine Peridotite Shear Zones: 2. Influences on Olivine CPO Evolution

“…This is counter to our natural samples, which do not show strong correlations between texture strength and shear strain ( Figure 6; Skemer et al, 2010;Warren et al, 2008). Olivine CPOs from other peridotite localities (e.g., Ben Ismaïl & Mainprice, 1998;Bernard et al, 2019) also exhibit similar texture strengths to our samples, though strain is unquantified in these examples. The overprediction of texture strength by the Hansen, Conrad, et al (2016) model is observed in other texture models (e.g., VPSC in Signorelli & Tommasi, 2015).…”

Dynamics of the lithosphere and upper mantle in light of seismic anisotropy

“…The most common, A-type LPO regime [Karato et al, 2008;Mainprice, 2007] appears most prevalent among xenolith and mantle massif samples [Ben Ismail and Mainprice, 1998;Bernard et al, 2019]. The corresponding modeled LPO predictions of best-fit hexagonal symmetry axis alignment in flow are broadly consistent with the orientation of the longest FSE axis.…”

“…If we consider the range of natural xenolith and ophiolite samples, all of the LPO types documented in the lab are indeed found in global compilations [Mainprice, 2007;Bernard et al, 2019]. However, samples from individual sites show a range of different LPOs, at presumably similar deformation conditions in terms of volatile content and deviatoric stress.…”

“…Among the modern studies, Zhang and Karato [1995] and the largestrain experiments of Bystricky et al [2000] showed how olivine a-axes cluster within the shear plane for the common, A type fabric, or the high-stress D type. A-type LPO is found most commonly in natural samples [Ben Ismail and Mainprice, 1998;Bernard et al, 2019], and provides the most straightforward link between anisotropy and flow as was explored in section 3.3.…”

“…However, samples from individual sites show a range of different LPOs, at presumably similar deformation conditions in terms of volatile content and deviatoric stress. Moreover, when such conditions are estimated from the samples, no clear systematics akin to the laboratory-derived phase diagrams arise [Bernard et al, 2019]. This implies that the style and history of deformation may be more important in controlling natural sample LPO, and perhaps, by inference, seismic anisotropy, particularly in the lithosphere where deformation is commonly more localized and where preservation potential is high.…”