Characterization of the glide planes of the [001] screw dislocations in olivine using electron tomography (original) (raw)
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Activities of olivine slip systems in the upper mantle
Physics of the Earth and Planetary Interiors, 2012
We investigated the effect of pressure (P) on olivine [1 0 0](0 0 1) and [0 0 1](1 0 0) dislocation slip systems by carrying out deformation experiments in the Deformation-DIA apparatus (D-DIA) on single crystals of Mg 2 SiO 4 forsterite (Fo100) and San Carlos (SC) olivine (Fo89), at P ranging from 5.7 to 9.7 GPa, temperature T = 1473 and 1673 K, differential stress r in the range 140-1500 MPa, and in water-poor conditions. Specimens were deformed in axisymmetry compression along the so-called [1 0 1] c crystallographic direction, which promotes the dual slip of [1 0 0] dislocations in (0 0 1) plane and [0 0 1] dislocations in (1 0 0) plane. Constant r and specimen strain rates (_ e) were monitored in situ by synchrotron X-ray diffraction and radiography, respectively. Comparison of the obtained high-P rheological data with room-P data, previously reported by Darot and Gueguen (1981) for Fo100 and Bai et al. (1991) for SC olivine, allowed quantifying the activation volume V Ã in classical creep power laws. We obtain V Ã = 9.1 ± 1.6 cm 3 /mol for Fo100. For SC olivine, we obtain V Ã = 10.7 ± 5.0 cm 3 /mol taking into account the oxygenfugacity uncertainty during the high-P runs. These results, combined with previous reports, provide complete sets of parameters for quantifying the activities of olivine dislocation slip systems. Extrapolation of the rheological laws obtained for SC olivine crystals to conditions representative of natural deformations show that [1 0 0](0 1 0) slip largely dominates deformation in the shallow upper mantle. At depths greater than 65kmalonga20−Maoceanicgeothermor65 km along a 20-Ma oceanic geotherm or 65kmalonga20−Maoceanicgeothermor155 km along a continental geotherm, the dual activity of [1 0 0](0 0 1) and [0 0 1](1 0 0) slips becomes comparable to that of [1 0 0](0 1 0) slip. At depths greater than $240 km, [0 0 1](0 1 0) slip becomes dominant over all other investigated slip systems. Such changes in olivine dislocation-slips relative activity provide a straightforward explanation for the seismic anisotropy contrast and attenuation with depth observed in the Earth's upper mantle.
Hardening mechanisms in olivine single crystal deformed at 1090 °C: an electron tomography study
Philosophical Magazine, 2017
The dislocation microstructures in a single crystal of olivine deformed experimentally in uniaxial compression at 1090 °C and under a confining pressure of 300 MPa, have been investigated by transmission electron tomography in order to better understand deformation mechanisms at the microscale relevant for lithospheric mantle deformations. Investigation by electron tomography reveals microstructures, which are more complex than previously described, composed of [0 0 1] and [1 0 0] dislocations commonly exhibiting 3D configurations. Numerous mechanisms such as climb, crossslip, double cross-slip as well as interactions like junction formations and collinear annihilations are the source of this complexity. The diversity observed advocates for microscale deformation of olivine significantly less simple than classic dislocation creep reported in metals or ice close to melting temperature. Deciphering mechanism of hardening in olivine at temperatures where ionic diffusion is slow and is then expected to play very little role is crucial to better understand and thus model deformation at larger scale and at temperatures (900-1100 °C) highly relevant for the lithospheric mantle.
Experimental deformation of olivine single crystals at mantle pressures and temperatures
2009
Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented San CarlosB B B olivine single crystals, at pressure (P) ranging from 3.5 to 8.5 GPa, temperature (T) from 1373 to 1673 K, and in poor water condition. Oxygen fugacity (fOB 2 B) was maintained within the olivine stability field and contact with enstatite powder ensured an orthopyroxene activity aB opx B = 1. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane, here called respectively a-slip and c-slip. Constant applied stress (σ) and specimen strain rates (ε) were monitored in situ using timeresolved x-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of run products revealed that dislocation creep was responsible for sample deformation. Comparison of the obtained high-P deformation data with the data obtained at room-P by Bai et al. (1991)-on identical materials deformed at comparable T-σ-fOB 2 B-aB opx B conditions-allowed quantifying the P effect on a-slip and c-slip rheological laws. A slip transition with increasing pressure, from dominant a-slip to dominant c-slip, is documented. a-slip appears sensitive to pressure, which translates into the high activation volume VB a B * = 12 ± 4 cmP 3 P/mol in the corresponding rheological law, while pressure has little effect on c-slip with VB c B * = 3 ± 4 cmP 3 P/mol. These results may explain the discrepancy between olivine low-P and high-P deformation data which has been debated in the literature for more than a decade.
Deformation of olivine at mantle pressure using the D-DIA
European Journal of Mineralogy, 2006
Knowledge of the rheological properties of mantle materials is critical in modeling the dynamics of the Earth. The hightemperature flow law of olivine defined at mantle conditions is especially important since the pressure dependence of rheology may affect our estimation of the strength of olivine in the Earth's interior. In this study, steady-state high-temperature (up to 1473 K) deformation experiments of polycrystalline olivine (average grain size e 10 µm) at pressure up to 9.6 GPa, were conducted using a Deformation-DIA (D-DIA) high-pressure apparatus and synchrotron X-ray radiation. The oxygen fugacity (f O2) during the runs was in-between the iron-wustite and the Ni/NiO buffers' f O2. The water content of the polycrystalline samples was generally about 150 to 200 wt. ppm but was as low as 35 wt ppm. Typically, 30 % strain was generated during the uniaxial compression. Sample lengths during the deformation process as well as the differential stresses were monitored in situ by X-ray radiography and diffraction, respectively. The strain rate was derived with an accuracy of 10-6 s-1. Differential stress was measured at constant strain rate (~10-5 s-1) using a multi-element solid-state detector combined with a conical slit. Recovered specimens were investigated by optical and transmission electron microscopy (TEM). TEM shows that dislocation glide was the dominant deformation mechanism throughout the experiment. Evidence of dislocation climb and cross-slip as active mechanisms are also reported. Deformation data show little or no dependence of the dislocation creep flow with pressure, yielding to an activation volume V * of 0 " 5 cm 3 /mol. These new data are consistent with the high-temperature rheological laws at lower pressures, as reported previously.
High-temperature creep of olivine crystals from four localities
Physics of the Earth and Planetary Interiors, 1994
To compare the rheologies of olivine samples from various tectonic settings, an experimental investigation of the high-temperature creep behavior of crystals from China (Fo 89_91), Norway (Fo93) and the Red Sea (Fog) was carried out. The results were examined in the context of the well-documented flow data for olivine from the USA (Fo89_93). Both the major element and the minor element compositions differed depending on locality. Creep experiments were performed in an 1 atm deformation rig at temperatures from 1200 to 1500°Cand compressive stresses from 15 to 175 MPa. Oxygen fugacity was varied over the full stability field of olivine. During deformation along three orientations, the samples remained in contact with orthopyroxene. The mechanical results for samples from all three localities can be summarized as follows: For the [110J~orientation, three rate-controlling creep mechanisms were observed; each can be described by a power-law relationship. Activation energies of 160 U mol-1, 1100 U mo1 1 and 380 U mol 1 and oxygen fugacity exponents of 0.37, 0.04 and 0.13 were obtained for the low-, intermediate-and high-temperature creep regimes, respectively. For the [101]õ rientation, activation energies of 270 U mol 1 and 660 U mol 1 and oxygen fugacity exponents of 0.30 and 0.05 were measured for the low-and high-oxygen fugacity creep regimes, respectively. For the [011]~orientation, an activation energy of 510 U mo11 was obtained over the full range of oxygen fugacity; however, oxygen fugacity exponents of 0.02 and 0.31 were determined for the low-and high-oxygen fugacity creep regimes, respectively. The stress exponent is approximately 3.5 for all creep regimes. Within experimental error, the creep results for olivine from all three localities are the same as those reported for San Carlos olivine single crystals. Therefore, moderate differences in forsterite content (from Fo 89 to Fo93) and in the concentrations of minor elements (such as Ni, Cr and Mn) do not significantly affect the creep behavior of olivine. Consequently, rheological data obtained for olivine samples from specific localities can be reasonably applied to model plastic deformation processes in the upper mantle. *. (a) (Bai et al. 1991 Bai and Kohlstedt 1992).
Rheology of olivine and the strength of the lithosphere
Geophysical Research Letters, 1990
In order to throw some light on the rheological behavior of the upper mantle of the Earth, a detailed series of high-temperature deformation experiments has been performed on olivine single crystals at 1 atto pressure under highly controlled thermodynamic conditions. The effects of stress, temperature, oxygen fugacity and orthopyroxene activity on the deformation rate of olivine have been carefully measured for the major hightemperature slip systems. These experimental results have been extrapolated to the pressures and strain rates of the upper mantle to provide new insight into the mechanical behavior of the mantle and a better constrained approach to extrapolation of laboratory deformation data to the conditions present in the upper mantle. We suggest that extrapolations in stress provide the most reliable means for extending laboratory creep data to upper mantle strain rates. We also predict that, in the dislocation creep field, the bulk of the strain in olivine in the upper mantle is accommodated by the (010)[100] slip system. In addition, true mantle strengths are probably most accurately modelled using the experimental data for samples oriented favoring the (010)[001] slip system. However, the reliable prediction of mantle strengths requires highly accurate temperature versus depth relationships and good indications of the local oxygen fugacity in the mantle.
IOP Conference Series: Materials Science and Engineering, 2015
The paper presents the results of investigations of deformed natural polycrystalline оlivine. The relationship of the structure of polycrystalline olivine grains to three modal size distributions has been revealed. Grains of different size were observed to be strained at threshold temperatures of 950, 775, and 650°C. It has been demonstrated that the microstructure develops as the dislocation mechanism changes from diffusion creep to grain boundary sliding. The changes in deformation mechanisms promote the change in the preferred crystallographic orientations of olivine from type A to type D and then to type B. The relation of the transitions between different types of orientations to the conditions of deformation in the lower layers of the lithosphere at the plate boundaries is discussed.