Seismic anisotropy in the upper mantle is produced primarily by lattice-preferred orientations (LPO) in olivine formed during ductile deformation. Because seismic anisotropy is one of the principal means of characterizing upper mantle flow directions, it is critical to understand how LPO is affected by deformation conditions, including temperature, pressure, water content and strain magnitude. The factors controlling LPO development in olivine have been investigated experimentally for several decades. Most recently, high temperature and pressure laboratory experiments demonstrated that water content and stress magnitude each play key roles in the development of LPO in olivine under experimental conditions. This seminal experimental work not only reproduced the fabric most commonly observed in olivine in natural rocks (known as A-type), but also identified several new types of LPO that form under different water and stress conditions (B-type through E-type). The existence and occurrence of these less common fabric types in natural rocks have important implications for seismic anisotropy interpretation, which has traditionally assumed only the existence of A-type fabric. Because experiments are conducted at ~7 orders of magnitude faster strain rates and under much narrower PT conditions than typically present in Earth’s mantle, it is unclear whether their results can be directly extrapolated to natural peridotites.
This presentation will discuss the results of a study of 65 peridotites from a wide range of tectonic settings, investigating whether naturally deformed samples agree with these experiments. The samples, which include xenoliths, and ophiolitic and continental peridotites, preserve all six documented olivine LPO types, as determined through electron backscattered diffraction (EBSD). Water contents in major mineral phases were obtained using Secondary Ion Mass Spectrometry (SIMS), stress magnitudes using paleopiezometry, and strain geometry using X-Ray CT data. The samples investigated here were also combined with an extensive literature review of over 500 published samples from 52 localities. In all, this compilation of natural samples reveals that rather than water and stress, olivine LPO types seem to vary as a function of temperature, strain geometry, and possibly strain magnitude. Additionally, we find that B-type LPO, which significantly affects how flow direction is inferred from azimuthal anisotropy, occurs under many more conditions than traditionally thought. This study highlights the difficulty in using olivine LPO in natural peridotites to infer deformation conditions and the need for more rock deformation experiments targeting olivine LPO development.