Iron‐rich olivine is mechanically weaker than olivine of mantle composition, ca. Fo90, and thus is more amenable to study under a wide range of laboratory conditions. To investigate the effects of iron content on deformation‐produced crystallographic preferred orientation (CPO) and grain size, we analyzed the microstructures of olivine samples with compositions of Fo70, Fo50, and Fo0that were deformed in torsion under either anhydrous or hydrous conditions at 300 MPa. Electron backscatter diffraction (EBSD) observations reveal a transition in CPO from D‐type fabric, induced by dislocation glide on both the (010)[100] and the (001)[100] slip systems, at low strains, to A‐type fabric, caused by dislocation glide on the (010)[100] slip system, at high strains for all of our samples, independent of iron content and hydrous/anhydrous conditions. A similar evolution of fabric with increasing strain is also reported to occur for Fo90. Radial seismic anisotropy increases with increasing strain, reaching a maximum value of ∼1.15 at a shear strain of ∼3.5 for each sample, demonstrating that the seismic anisotropy of naturally deformed olivine‐rich rocks can be well approximated by that of iron‐rich olivine. Based on EBSD observations, we derived a piezometer for which recrystallized grain size decreases inversely with stress to the ∼1.2 power. Also, recrystallized grain size increases with increasing iron content. Our experimental results contribute to understanding the microstructural evolution in the mantle of not only Earth but also Mars, where the iron content in olivine is higher.
Seismic anisotropy arises in the upper mantle due to the alignment of olivine crystal lattices and is often used to interpret mantle flow direction. Experiments on the evolution of olivine crystal‐preferred orientation (CPO) have found that the texture that develops is dependent on many factors, including water content, differential stress, preexisting CPO, and deformation kinematics. To evaluate the role of these factors in naturally deformed samples, we present microstructural transects across three shear zones in the Josephine Peridotite. Samples from these shear zones exhibit a mixture of A‐type textures, which have been associated with dry conditions and primary activation of the olivine [100](010) slip system, and of E‐type textures, which have been associated with wetter conditions and primary activation of the [100](001) slip system. CPOs with characteristics of both A‐type and E‐type textures are also present. CPO type does not evolve systematically as a function of either strain or water content. We used a micromechanical model to evaluate the roles of preexisting texture and kinematics on olivine CPO evolution. We find that the preexisting texture controls CPO evolution at strains up to 5 during simple shear. Kinematics involving a combination of simple shear and pure shear can explain the olivine CPOs at higher strain. Hence, preexisting CPOs and deformation kinematics should be considered in the interpretation of CPOs measured in naturally deformed rocks and of large‐scale patterns in upper‐mantle seismic anisotropy.
more » « less- NSF-PAR ID:
- 10443482
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 124
- Issue:
- 12
- ISSN:
- 2169-9313
- Page Range / eLocation ID:
- p. 12763-12781
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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. -
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