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

We analyze peridotites from a wide range of tectonic settings to investigate relationshipsbetween olivine crystallographic preferred orientation (CPO) and deformation conditions in naturallydeformed rocks. These samples preserve the five olivine CPO types (A- through E-type) that rockdeformation experiments have suggested are controlled by water content, temperature, stress magnitude,and pressure. The naturally deformed specimens newly investigated here (65 samples) and compiled froman extensive literature review (445 samples) reveal that these factors may matter less than deformationhistory and/or geometry. Some trends support those predicted by experimentally determined parametricdependence, but several observations disagree—namely, thatallCPO types are able to form at very lowwater 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 variesas a function of strain geometry. Because olivine CPO is primarily responsible for seismic anisotropy in theupper mantle, the results of this study have several implications. These include (1) the many olivine CPOtypes recorded in samples from individual localities may explain some of the complex seismic anisotropypatterns observed in the continental mantle, and (2) B-type CPO—where olivine's “fast axes” alignperpendicular to flow direction—occurs under many more conditions than traditionally thought. Thisstudy highlights the need for more experiments and the difficulty in using olivine CPO in naturallydeformed peridotites to infer deformation conditions.
Geochemistry, Geophysics, Geosystems, 20 (7)
ISSN:1525-2027