Evolution of olivine lattice preferred orientation during simple shear in the mantle

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Abstract

Understanding the variation of olivine lattice preferred orientation (LPO) as a function of shear strain is important for models that relate seismic anisotropy to the kinematics of deformation. We present results on the evolution of olivine orientation as a function of shear strain in samples from a shear zone in the Josephine Peridotite (southwest Oregon). We find that the LPO in harzburgites re-orients from a pre-existing LPO outside the shear zone to a new LPO with the olivine [100] maximum aligned sub-parallel to the shear direction between 168% and 258% shear strain. The strain at which [100] aligns with the shear plane is slightly higher than that observed in experimental samples, which do not have an initial LPO. While our observations broadly agree with the experimental observations, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, olivine re-alignment appears to be dominated by slip on both (010)[100] and (001)[100], due to the orientation of the pre-existing LPO. Fabric strengths, quantified using both the J- and M-indices, do not increase with increasing shear strain. Unlike experimental observations, our natural samples do not have a secondary LPO peak. The lack of a secondary peak suggests that subgrain rotation recrystallization dominates over grain boundary migration during fabric re-alignment. Harzburgites exhibit girdle patterns among [010] and [001] axes, while a dunite has point maxima. Combined with the observation that harzburgites are finer grained than dunites, we speculate that additional phases (i.e., pyroxenes) limit olivine grain growth and promote grain boundary sliding. Grain boundary sliding may relax the requirement for slip on the hardest olivine system, enhancing activation of the two easiest olivine slip systems, resulting in the [010] and [001] girdle patterns. Overall, our results provide an improved framework for calibration of LPO evolution models.

Introduction

Understanding olivine orientation as a function of shear strain is critical for quantifying relationships between the kinematics of deformation and the direction and magnitude of seismic anisotropy. For example, constraining the variation of olivine lattice preferred orientation (LPO) produced during simple shear is key to interpreting seismic anisotropy in terms of upper mantle convection (Hess, 1964, Nicolas and Christensen, 1987, Ribe, 1992, Mainprice and Silver, 1993, Blackman and Kendall, 2002, Wenk, 2002). The relationships among olivine deformation, LPO development and seismic anisotropy have been examined experimentally (Nicolas et al., 1973, Zhang and Karato, 1995, Bystricky et al., 2000). Observations from these experiments have been used to place constraints on models (e.g., Ribe and Yu, 1991, Wenk and Tomé, 1999, Tommasi et al., 2000, Kaminski and Ribe, 2001, Blackman et al., 2002, Conrad et al., 2007) that predict LPO development and thus upper mantle seismic anisotropy. Application of these models to deformation in the earth is improved by comparison of experimental results to rocks deformed under natural conditions, i.e., at lower stress and strain rate than can be achieved in laboratory experiments. To this end, we analyzed the evolution of olivine LPO as a function of shear strain in naturally deformed peridotites from a shear zone in the Josephine Peridotite in southwest Oregon.

Mantle anisotropy results from ductile flow in the asthenosphere by dislocation creep, which produces alignment of elastically anisotropic minerals. Olivine and orthopyroxene, the dominant mineral phases in the upper mantle, have orthorhombic symmetry and are anisotropic (Vp anisotropies of 22% and 16%, respectively; Nicolas and Christensen, 1987). At upper mantle pressure and temperature conditions, they deform by dislocation creep, resulting in an LPO. Deformation is principally accommodated by slip on (010)[100] and (001)[100] in olivine and on (100)[001] in orthopyroxene. At depths greater than 250 km, anisotropy rapidly decreases and this has been interpreted as either a transition to diffusion creep (Karato, 1992) or to dislocation creep with a different slip system (Mainprice et al., 2005).

Zhang and Karato (1995) carried out simple shear experiments on olivine aggregates at 1200 °C and 1300 °C over a range of shear strains to investigate olivine fabric evolution. They found that the originally random fabric of their aggregates developed an LPO with a [100] maximum parallel to the flow direction by a shear strain of ~ 150%, as had previously been suggested experimentally by Nicolas et al. (1973). The Nicolas et al. (1973) experiments were performed in an axial geometry, but bubbles in olivine grains aligned with the flow direction at high strain and were interpreted to have deformed by simple shear. Bystricky et al. (2000) demonstrated that the [100] alignment persists to high shear strains (~ 500%).

Initial theoretical treatments of olivine LPO assumed that olivine grain orientations are controlled by finite strain (e.g., McKenzie, 1979). As (010)[100] has the lowest critical resolved shear stress (Durham and Goetze, 1977, Bai et al., 1991), the olivine [100] axis was predicted to align with the finite strain ellipsoid (McKenzie, 1979, Ribe, 1992). However, experimental results (Nicolas et al., 1973, Zhang and Karato, 1995, Bystricky et al., 2000) indicate that the olivine [100] maximum only coincides with the finite strain ellipsoid at strains < 100%. This alignment may be more a coincidence than an indication of control on the fabric by the strain geometry. In viscoplastic self-consistent (VPSC) models (Wenk et al., 1991, Lebensohn and Tomé, 1993, Tommasi et al., 2000) the olivine [100] maximum approaches the flow direction at a rate intermediate between the finite strain model and experimental observations. In models that include dynamic recrystallization (e.g., Wenk and Tomé, 1999, Kaminski and Ribe, 2001), crystal nucleation and growth rates are varied so as to fit LPO evolution to the experimental observations. For example, the DRex model (Kaminski and Ribe, 2001, Kaminski and Ribe, 2002) achieves a good fit to the experimental data and includes a parameterization to predict the time-scale for LPO evolution. These model predictions, however, are dependent on the validity of the extrapolation of the experimental data to the low strain rates that prevail in the mantle.

We present data from peridotite samples to test the extrapolation of experimental relationships for LPO development (Nicolas et al., 1973, Zhang and Karato, 1995, Bystricky et al., 2000) to natural conditions. Studies of deformation in naturally deformed peridotites are often hindered by the lack of a well-defined finite strain marker. However, the Josephine Peridotite is ideal for the analysis of fabric evolution with shear strain as it has a pre-existing foliation, defined by variations in pyroxene content, which provides a passive strain marker, as shown in Fig. 1. In addition, variations in pyroxene content permit assessment of the effects of second phases on olivine LPO development.

Section snippets

Field observations

The Josephine Peridotite in southwestern Oregon is the mantle section of a ~ 150 Ma ophiolite from a fore-arc or back-arc setting (Dick, 1976, Harper, 1984, Kelemen and Dick, 1995). The peridotite is predominantly composed of harzburgite, with pyroxene-rich layers in some localities (Dick and Sinton, 1979). A series of shear zones, described by Loney and Himmelberg (1976) and Kelemen and Dick (1995), outcrop over a distance of 300 m in the Fresno Bench area of the Josephine Peridotite. The shear

Methods

We analyzed olivine fabrics in harzburgites from the widest of the Josephine shear zones, shown in Fig. 1. The shear plane is approximately vertical, based on observations of how it cuts across topography along strike and the similar orientation of nearby shear zones with higher strains (Kelemen and Dick, 1995). Based on our field observations and those of Kelemen and Dick (1995), the shear plane is oriented at 035°/90°. The lineation plunge of 50°NE was determined from outcrop-scale

Results

From analyses of nine samples across the Josephine shear zone, we find that the olivine [100] maximum, initially oriented at 62° counterclockwise to the shear plane, is aligned parallel to the shear direction at the center of the shear zone. To visually demonstrate the change in olivine orientation with strain, EBSD orientation maps and inverse pole figures of a low strain and a high strain sample are shown in Fig. 3. Olivine is colored as a function of the angle between the [100] axis and the

Discussion

Our results on olivine LPO evolution during simple shear extend observations of LPO variations to lower stresses and strain rates than are available from experimental datasets (Zhang and Karato, 1995, Bystricky et al., 2000). While our observations broadly agree with the experimental data, our results suggest that a pre-existing LPO influences the strain necessary for LPO alignment with the shear direction. In addition, the pre-existing LPO and presence of additional phases affect the behavior

Conclusions

Our results on olivine LPO evolution during shear are consistent with the conclusion from experimental data (Nicolas et al., 1973, Zhang and Karato, 1995, Bystricky et al., 2000) that olivine LPO aligns with the shear direction during deformation. However, alignment of naturally deformed samples requires higher strain, which we suggest is due to the orientation of the pre-existing LPO. Our results extend the observations of how olivine LPO evolves within simple deformation kinematics to lower

Acknowledgments

This work benefited from discussions with M. Behn, L. Montési, H.J.B. Dick, A. Tommasi, É. Kaminski, L. Mehl and J. Tullis. H.J.B. Dick provided insight in the field. K. Hanghøj, M. Billen, B. deMartin, L. Montési and M. Sundberg helped with fieldwork. Early work by S. Singletary provided initial results on LPO variation across the shear zone. We thank L. Kerr at the Marine Biological Laboratory for keeping the SEM in operating condition. A. Tommasi and É. Kaminski kindly shared results from

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  • Cited by (0)

    1

    Now at: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road, NW, Washington DC 20015, United States.

    2

    Now at: Department of Geological Sciences, Brown University, Providence, RI 02912, United States.

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