MR34A-01 INVITED
Rheology of hcp-iron up to 19 GPa and 600 K in the D-DIA
Stress-strain curves, i.e., relations between the differential stress and macroscopic sample strain, of polycrystalline hcp-iron have been obtained at pressures up to 19 GPa, three different temperatures (600, 400, and 300 K), and various strain rates using the deformation-DIA coupled with monochromatic X-rays. The experiment was carried out at the GSECARS bending magnet beamline 13-BM-D at the Advanced Photon Source (Argonne, IL, USA). We used two sintered diamond anvils on the down-stream side in the DDIA, to serve as windows for diffracted X-rays. The starting material was a bcc-iron rod (0.5 mm in diameter and 0.6 mm in length). The generated temperature was inferred from the input power using a power-temperature relation which had been determined in a separate run. The cell assembly was pressurized isotropically up to a load of 50 tons. At this load, the sample was still bcc-phase and the generated pressure was about 15 GPa. At this fixed load, the sample was heated up to 700 K, and the phase transition from bcc to hcp was observed. After the synthesis of hcp-phase, five independent stress-strain curves were obtained on axial shortening and the sample exhibited ductile behavior in all. Above 4 percent axial strain, sample stresses reach saturation and the sample exhibited steady-state deformation. Stress exponents at temperatures of 400 and 600 K were determined to be 31 and 7, respectively. These results indicate that hcp-iron deforms in plasticity regime below 400 K and that the dominant deformation mechanism at 600 K may be low temperature power-law creep. The overall deformation behavior for hcp-iron is consistent with that of zinc, suggesting that the deformation mechanism map of hcp-iron resembles those of other hexagonal metals.
MR34A-02
High-pressure creep of serpentine, interseismic deformation and initiation of subduction
Serpentines, phyllosilicates resulting from mantle hydration, have a low viscosity compared with other mantle and slab materials within subduction zones. They have a global geodynamic importance on the timescale of mantle convection because a serpentinite layer may decouple the mantle wedge from the subducting slab, therefore influencing plate tectonics regime on the Earth. The seismic implications are far reaching as serpentinite rheology may govern stress build-up and downdip relaxation over the slab surface, which are critical parameters for earthquake triggering and for the downdip extent of major ruptures. However, limitation of apparatus has restricted previous high temperature deformation experiments on serpentinites to pressures below 0.7 GPa, and the lack of data at relevant P-T impeded quantification of serpentine rheology influence on subduction zones dynamics. We present in situ deformation experiments on the high-pressure variety antigorite, at low strain rates and pressure-temperature (P-T) of 1 and 4 GPa and 200 to 500C, respectively, i.e. over most of the antigorite stability field, using the Deformation-DIA (D-DIA) apparatus coupled with synchrotron X-ray at 13-BM-D at GSE-CARS (Advanced Photo Source). Strain rates and stresses were obtained respectively from in-situ monitoring the sample length with X-ray radiographs, and azimuthal dependence of d-spacings on diffraction patterns. The determined stress-strain curves were fitted to a power-law equation including both temperature and pressure dependence. Regardless of the temperature, serpentinized mantle at the slab surface has a low viscosity that allows localizing the deformation and impeding stress build-up. The consequences of such a rheology for subduction zones dynamics at short and long term include limitation of the downdip propagation of large earthquakes and viscous relaxation as an origin of post-seismic deformations and slow earthquakes. The low viscosity of serpentinized faults in the oceanic lithosphere makes them possible sites for subduction initiation.
MR34A-03 INVITED
Effect of phase transition on the P wave velocity in regions of coexisting phases
The bulk modulus, defined as the ratio of pressure change to resulting volume strain, generally reflects the elastic restoring force. In regions that are undergoing phase transitions, additional contribution to the volume strain are possible. At equilibrium, a small pressure increase will increase the amount of the high pressure (small volume) phase while a pressure decrease will increase the amount of the large volume phase. Thus, the bulk modulus of a material at conditions inside a two phase loop will exhibit a very soft bulk modulus if the perturbing pressure is slowly applied relative to the equilibrium time constant. Typically the effective bulk modulus will be an order of magnitude lower than the elastic bulk modulus. If the time scales are similar, then energy will be absorbed, accompanied with a reduction in the bulk quality factor, QK, and some softening of the bulk modulus. If the equilibrium time scale is shorter or equal to the P wave period, then we should expect two phase regions (including olivine to wadsleyite, wadsleyite to ringwoodite, ringwoodite to perovskite, pyroxene to garnet, and garnet to perovskite) to have anomalously slow P waves and, possibly, bulk attenuation. Here we report laboratory data in which bulk modulus is softened by a stress induced variation of the proportion of coexisting phases. We use Fa70Fo30 olivine as our sample. Experiments are performed in a multi-anvil high pressure apparatus (Deformation DIA) using synchrotron (NSLS) X-ray radiation as the probing tool. Pressure is up to 12 GPa and temperature is up to 1400 oC. Measurements were carried out within the binary loop where alpha-gamma olivine phases coexist. We apply uniaxil oscillation stress onto the sample and Young's modulus and Q-1 are measured at frequency of 0.1-0.01Hz. Our results indicate that the sinusoidal force applied to the sample in olivine-ringwoodite region has much lower bulk modulus and higher Q-1 than at in the single phase regions. Our data are consistent with a diffusion controlled model of Jackson (2007), where the characteristic time decreases with decreasing strain. If we extrapolate the model to Earth conditions with probing P waves, we conclude a relaxation time of the reaction of less than one second. This suggests that the two phase zones in the Earth should have slow P waves.
MR34A-04
In-situ Laser Heating and Pressure Change With Radial Diffraction to Investigate Deformation of Deep Earth Relevant Minerals
Abstract: Many deep Earth mineral phases have stability fields that are accessible only with diamond anvil cell (DAC), and currently this remains the only method for studying these mineral phases at pressures relevant to the deep Earth. So far radial diffraction DAC experiments have had two serious limitations, pressure and stress could only be applied incrementally ex-situ and deformation was limited to ambient temperature. These limitations bring into question the applicability of these experiments to deformation behavior in the deep earth where minerals are deforming at high-temperature and pressure. To address this issue we developed a novel combination of remotely controlled radial DAC with in-situ laser heating. This enables us to change pressure and thus stress on the sample while at high temperature. For remotely controlling the pressure we constructed a holding frame which can be used for different radial cell designs. The DAC is placed within the holding frame together with a gas-driven membrane. Inflating the membrane pushes the piston into the cylinder which is retained by the frame. While the membrane applies force from the bottom, the top of the assembly provides optical access for one- sided laser heating. The laser is directed from the top, vertically along the symmetry axis of the DAC onto the sample. The gas-pressure can be controlled remotely from outside the hutch, thus allowing for pressure change during heating and X-ray exposure. Using in-situ laser heating we induce recrystallization in a sample of Mg0.75Fe0.25O that had been deformed at room temperature. We observe grain growth and texture strengthening upon recrystallization. The remote pressure control is used to deform bcc Fe into the hcp Fe stability field and then back into the bcc phase on decompression. We observe development of strong textures in both the bcc phase and the hcp phase of Fe as well as texture change during decompression of the hcp phase. By combining the techniques, we convert in-situ a sample of natural San Carlos olivine (Fo90.7Fa9.3) into an assemblage of perovskite and periclase in the DAC. This sample was deformed at pressures from 30 to 50 GPa and at a temperature of 1100 ± 100 K. Both perovskite and periclase develop texture during deformation with periclase developing the stronger texture of the two. Texture in periclase is different from that obtained in room temperature compression experiments and this could be due to deformation in a two phase aggregate or deformation at high temperature. Elastic lattice strains are significantly lower in the perovskite and periclase assemblage when deformed at high temperature as compared to a room temperature experiment.
MR34A-05 INVITED
First-Principles Study of Mg-Perovskite Deformation Mechanisms Under Lower Mantle Conditions
It is widely accepted that MgSiO3 Perovskite is one of the most abundant minerals of the Earth's mantle. Therefore, the convective flow and the seismic properties of the Earth's deep mantle should depend strongly on the mechanical and physical properties of this mineral. Global studies of seismic anisotropy of the lower mantle suggest that the lower mantle is essentially isotropic. These results are commonly interpreted as indicating that diffusion creep dominates in the lower mantle. However, recent studies on olivine at high-pressure have shown that isotropic seismic properties do not necessarily imply the absence of Crystal Preferred Orientation (CPO) and should not be considered as a strong evidence for diffusion creep (Mainprice et al. Nature 2005). To evaluate the role of plastic deformation and lattice preferred orientation in the development of seismic anisotropy, it is important to have a good understanding of the deformation behavior of minerals at conditions of the Earth's deep mantle. Whereas, this information is generally difficult to obtain through experiments, atomic scale modeling of dislocation properties in minerals have recently shown that relative activities of slip systems in minerals can be predicted (Carrez et al. Nature 2007, Ferre et al. PEPI 2007). In this study, we propose a theoretical modeling of dislocation glide systems in MgSiO3 Perovskite based on ab initio calculation and the Peierls-Nabarro model. These results are used as an input for a polycrystalline Visco-Plastic Self-Consistent (VPSC) models of CPO development. Our results show that clear CPO can develop. The plastic anisotropy of perovskite and the texture development remain fairly similar in the pressure range 30- 100 GPa with a dominant [010](100) slip system. Our results are in good agreement with recent experimental results of Wenk et al. (EPSL 2004). The seismic anisotropy of MgSiO3 Perovskite aggregates is calculated using the CPO predicted from the ab initio/VPSC model for a given shear strain and single crystal elastic constants at lower mantle pressures and temperatures (ranging from P=30 to 100 GPa and T=1500 and 3500 K). At the top of the lower mantle, we predict a relatively weak anisotropy for P and S waves, with higher values for P waves (2.9% for P waves and 1.6% for S waves at 1500K ). The weak nature of the predicted seismic anisotropy is in general agreement with global seismology observations, without the need to involve mechanisms that do not produce CPO.
MR34A-06 INVITED
Deformation of dry Olivine up to 11 GPa and 2100 K Using a Rotational Drickamer Apparatus
Characterizing the pressure effects on plastic deformation of olivine is important for understanding mantle dynamics and evolution of the Earth. Although many experimental studies have been performed on olivine deformation, the magnitude of the pressure effects (activation volume, V*) remain controversial. We use the rotational Drickamer apparatus (RDA) to determine the pressure dependence of creep strength of olivine in the dislocation creep regime. Deformation experiments on dry hot-pressed polycrystalline San Carlos olivine have been conducted at 6-11 GPa, 1500-2100 K and strain rates of 0.9-7.4 × 10-5 s-1 using the RDA at X17B2, Brookhaven National Laboratory. Shear deformation experiments up to the strain of 0.45 were performed. Near steady-state deformation was observed after the shear strain of 0.04-0.24. The stress was measured from the dependence of lattice spacing on the orientation for (130), (131), (112), (122), (140), (211) and (241) planes, as well as dislocation densities. We found that (i) the strength of olivine at steady-state deformation at these pressures is much larger than those at lower pressures, (ii) the variation of stress values inferred from different lattice planes decreases with temperature. We conclude that the creep strength of olivine for dislocation creep increases significantly with pressure and that the plastic anisotropy in olivine decreases with temperature.
MR34A-07
Measurement of Activation Volume of Dry Olivine at High Pressure
Despite considerable effort to measure the activation volume V* of creep of olivine using a new generation of high-pressure deformation machines, namely the Rotational Drickamer Apparatus (RDA) and the Deformation- DIA (D-DIA) in conjunction with synchrotron x-ray sources, progress has been marred by a combination of an apparently weak signal (i.e., low V*) and measurement noise. The latter has a broad spectrum of causes, from inherent limitations of measurement systems (of stress, primarily) to limited time available for creep experiments at synchrotron sources to investigator-induced variations of sample state during measurement. We report here measurements following a significant advance in regulation of sample state: deforming polycrystalline olivine in a dry state under more uniform mechanical conditions gives us our first clear signal of a positive activation volume. The advance has been made possible by a choice of assembly materials that assures anhydrous conditions around the sample, and a technique for testing in the absence of a thermocouple within the sample assembly. The key to the anhydrous assembly is a self-gasketing D-DIA cube of hybrid composition: a sphere of mullite embedded in a cube of unfired pyrophyllite, where the diameter of the sphere matches the edge length of the cube (6-mm in our case). Unfired pyrophyllite is an excellent gasketing material, being soft and (in notable contrast with mullite) non-friable. Additionally, as a cube-filling "web" around the mullite sphere, the pyrophyllite is ideally configured for D-DIA self gasketing: maximum volume at the cube corners, minimum (zero) volume at cube faces. Thus the configuration under pressure is a bone-dry mullite pressure medium gasketed by pyrophyllite. Removal of the thermocouple from the deformation piston results in a demonstrably more symmetric deformation column within the assembly for the entire duration of the experiment, and the absence of the thermocouple is reliably compensated by off-line calibration of furnace power vs temperature. Results of creep experiments in the new cell indicate a value of V*/n (where n is the stress exponent) of 3 ± 1 × 10-6 m3/mol between 3 and 6 GPa. The value of n will be better resolved in future experiments, but if the deformation is in the dislocation creep regime (n ~ 3.5), then V* ~ 10 × 10-6 m3/mol.
MR34A-08
Experimental Deformation of San Carlos Olivine Single Crystal at Mantle P and T: Evidences for a Slip-System Transition with increasing P.
Seismic velocity anisotropies observed in the shallow upper mantle are interpreted from lattice preferred
orientations (LPO) produced experimentally in olivine, which depends on the dominant dislocation slip systems.
At low pressure P<3 GPa, mantle temperature (T), and in dry conditions, olivine [100] dislocation slip
dominates the less active [001] slip. This tends to align crystal fast velocity [100] axis with the principal shear
direction. Yet recent deformation experiments carried out at P>3 GPa (Couvy et al., 2004, EJM, 16, 877;
Raterron et al., 2007, Am. Min., 92, 1436), as well as a theoretical study based on first-principle calculations
(Durinck et al., 2005, PCM, 32, 646), show that, in forsterite, [001] slip may be dominant in the (P,T) range of
the deep upper mantle. This would promote a shear-parallel slow-velocity [001] axis and may explain the
seismic-velocity attenuation with depth observed at depth >200 km (Mainprice et al., 2005, Nature, 433, 731).
Although such [100]-slip/[001]-slip transition has previously been attributed to the presence of water (e.g., Jung et
al., 2006, Tectonophysics, 421, 1), evidences for shear parallel [001] axis are observed in nature at high P>4
GPa in dry subduction context (Xu et al., Tectonophysics, 421, 111). Whether such pressure-induced slip
transition is an important process in the deep upper mantle is still debated in the literature (see, Karato, 2007,
Tectonophysics, 429, 287, and Ji et al., 2007, Tectonophysics , 429, 291), and this debate has strong
implications for our understanding of upper mantle convective flows.
In order to determine the P effect on Fe-bearing olivine [100] and [001] slip activities, deformation experiments
were carried out on San Carlos olivine oriented crystals at P>3 GPa and 1100°