Evolution of olivine lattice preferred orientation during simple shear in the mantle
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
References (52)
- et al.
A 3-D kinematic model of fabric development in polycrystalline aggregates: comparisons with experimental and natural examples
J. Struct. Geol.
(1987) The distribution of disorientation angles if all relative orientations of neighbouring grains are equally probable
Scr. Metall.
(1979)- et al.
A kinematic model for recrystallization and texture development in olivine polycrystals
Earth Planet. Sci. Lett.
(2001) - et al.
A self-consistent anisotropic approach for the simulation of plastic deformation and texture development of polycrystals: application to zirconium alloys
Acta Metall. Mater.
(1993) - et al.
A scanning electron microscope study of the effects of dynamic recrystallization on lattice preferred orientation in olivine
Tectonophysics
(2002) - et al.
Interpretation of SKS-waves using samples from the subcontinental lithosphere
Phys. Earth Planet. Inter.
(1993) Shear zone geometry: a review
J. Struct. Geol.
(1980)- et al.
The misorientation index: development of a new method for calculating the strength of lattice-preferred orientation
Tectonophysics
(2005) - et al.
Simple shear deformation of olivine aggregates
Tectonophysics
(2000) - et al.
High-temperature creep of olivine single crystals 1. Mechanical results for buffered samples
J. Geophys. Res.
(1991)
An olivine fabric database: an overview of upper mantle fabrics and seismic anisotropy
Tectonophysics
Seismic anisotropy in the upper mantle: 2. Predictions for current plate boundary flow models
Geochem. Geophys. Geosyst.
Seismic anisotropy of the upper mantle: 1. Factors that affect mineral texture and effective elastic properties
Geochem. Geophys. Geosyst.
Texture Analysis in Materials Science: Mathematical Models
High shear strain of olivine aggregates: rheological and seismic consequences
Science
Global mantle flow and the development of seismic anisotropy: differences between the oceanic and continental upper mantle
J. Geophys. Res.
Compositional layering in alpine peridotites: evidence for pressure solution creep in the mantle
J. Geol.
Dynamic recrystallization and strain softening of olivine aggregates in the laboratory and the lithosphere
Plastic flow of oriented single crystals of olivine 1. mechanical data
J. Geophys. Res.
The Josephine ophiolite, northwestern California
Geol. Soc. Am. Bull.
Seismic anisotropy of the uppermost mantle under oceans
Nature
Rheology of the upper mantle and the mantle wedge: a view from the experimentalists
Timescales for the evolution of seismic anisotropy in mantle flow
Geochem. Geophys. Geosyst.
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.