MR43A-0971
Diffusion creep, grain rotation and mantle anisotropy
The mantle deforms by some combination of diffusion creep and dislocation creep. It is well established that dislocation creep gives rise to crystallographic preferred orientations (CPO) which in turn lead to seismic anisotropy. Consequently seismic anisotropy may be interpreted as indicating the action of dislocation creep, and an absence of anisotropy as indicating diffusion creep. One assumption involved is that diffusion creep and the accompanying grain boundary sliding cause random grain rotations which destroy CPO. So, if a mantle rock has deformed in dislocation creep, developing a CPO, and then moves into a strain rate/temperature/grain size regime promoting diffusion creep as the dominant mechanism, the CPO will be destroyed. Here I test this assumption via a numerical model (named "DiffForm") for diffusion creep. The model is grain- scale and hence predicts the finite rotations of individual grains through time as the polycrystal deforms in grain boundary diffusion creep accompanied by sliding. The rotation evolution depends on the details of the starting microstructure, but for a variety of deformation types and initial microstructures simulations show that the rotation rates decrease through time. At large strains the microstructures reach quasi-steady states in which little further rotation occurs. This implies that diffusion creep can weaken a pre-existing CPO but not destroy it, so regions of anisotropic mantle may, in fact, be deforming by diffusion creep, with the CPO a weakened but still potent inherited feature.
MR43A-0972
Micromechanical Modelling of the Viscoplastic Behavior of Olivine
Efforts to couple mantle flow models with predictions of mineral deformation typically ignore rheological impacts that texture development may have on the flow evolution. Olivine crystals have only three easy slip systems for dislocation glide, leading to strong mechanical interactions between grains as deformation proceeds. These intergranular interactions are also responsible for very large viscoplastic anisotropy when polycrystals exhibit pronounced Lattice Preferred Orientations (LPO). Using a full field polycrystal plasticity model for creep in dry polycrystalline olivine under thermomechanical conditions prevailing in the upper mantle, it is shown that very large stress and strain rate heterogeneities build up at the grain scale upon deformation. Field heterogeneities increase with the strength of the hard slip system incorporated for the sake of enabling general deformation. Compared with several nonlinear mean field approaches for polycrystal plasticity, all based on the Self- Consistent scheme, only the recent Second Order procedure, which is based on a variational method, really captures the effect of intraphase stress heterogeneities on the effective viscoplastic behavior and on the local stress and strain rate intragranular fluctuations. Results compare very well with those of the full field method, at a significantly reduced computation effort. We anticipate that this model is the best model to date for accurate, from the physical point of view, micromechanical modeling of upper mantle peridotite. The "tangent" model, most commonly used in geophysical studies of the mantle, departs significantly from the full-field reference solutions. Olivine polycrystals are found to be able to deform with only four independent slip systems, the hard system perhaps being provided by dislocation climb (often observed in experimental work) and/or grain boundary mechanisms (sliding, migration). The resistance of this accommodation process may essentially control the flow stress of olivine polycrystals. First attempt to assess the effect of the rheological anisotropy (associated with LPO development) on typical in situ convective flow will also been shown.
MR43A-0973
Interpreting In-situ Synchrotron X-ray Diffraction Measurements from Deforming Quartz Using An Elastic Plastic Self Consistent (EPSC) Model
The use of synchrotron x-ray diffraction combined with the recently developed deformation DIA apparatus (D-DIA) has facilitated a new class of deformation experiments in which the state of stress at the grain scale can be observed during controlled deformation experiments. The grain scale nature of the data allows us to examine the distribution of stress across various subpopulations of grains in a polycrystal during deformation, which will ultimately lead to a more sophisticated understanding of polycrystalline deformation in general. A challenge that has come along with these new technological advances is that the data reveal a richer more complex picture of polycrystalline deformation than was anticipated. The grain scale data appear to mirror the heterogeneity of strain commonly observed in naturally and experimentally deformed monomineralic polycrystals, in which heavily deformed grains can be found adjacent to grains that exhibit relatively little deformation. A number of workers have now observed substantial variations in stress levels between grains in deforming monomineralic polycrystalline earth materials. Determining the method to properly interpret the stress data and cast it in terms of macroscopic stresses is important because it is the aggregate properties which are useful for geodynamic calculations. We are exploring the use of elastic plastic self consistent (EPSC) models, developed by metallurgists for describing in-situ neutron diffraction observations of deforming metals, for use in interpreting x- ray diffraction data from deforming silicates. We are conducting deformation experiments on Arkansas novaculite using the D-DIA apparatus on beamline X17b2 at the NSLS. During deformation experiments we are able to observe the behavior of the (100), (101) and (112) lattice reflections of quartz. Strain in our experiments is measured by periodically taking a radiograph of the sample (which is bounded by metal foils) and comparing the length of the sample to a radiograph taken immediately before the start of the deformation experiment. We are able to model the behavior of the (100), (101) and (112) reflections using an EPSC model in which basal and prism <a> slips are activated. An interesting outcome of the EPSC model is the prediction that the macroscopic stress experienced by the sample should be greater than the stress calculated from any of the reflections that we observed. This observation serves as a caution against using reflection stresses as a proxy for the macroscopic stress in in-situ deformation experiments.
MR43A-0974
Evaluation of stress in high pressure radial diffraction: application to hcp Co
Understanding the coupling between elastic and plastic behaviour in hcp Co plastically deformed is important as it can serve as a starting model for improving our understanding of hcp-Fe, the main constituent of the Earth's inner core. For many years, the radial diffraction technique has been used to study mechanical properties under pressure. In those experiments, a polycrystalline sample is plastically deformed between two diamond anvils and lattice spacings are measured using diffraction, with the incoming x-ray beam perpendicular to the compression direction. From the variations of the d-spacings with the diffraction angle, we deduce information on the hydrostatic and deviatoric stress in the sample, while the variations of diffraction intensities provide information on the lattice preferred orientations within the polycrystal. Theories have been developed to relate the observed lattice strains to elastic moduli and stress within the sample (1). However, those models do not account for the effect of plastic deformation and, as a consequence, stress determinations can be inconsistent between lattice planes. In particular, experiments on cobalt have shown that plasticity effects on lattice strains were particularly large in hcp metals (2). This implies that the elastic moduli previously measured for hcp-iron using this technique are not directly related to single-crystal elastic moduli(3). Addressing this problem requires us to consider plastic relaxation, in addition to elastic effects. This can be done using polycrystal elasto-plastic models, which account for slip activity and the threshold stresses associated with their activation. Here, we present new results on modeling radial diffraction experiments using an elasto-plastic self-consistent (EPSC) model and show how the model can be used to interpret radial diffraction data on hcp-Co. More important, we also show how this can be used to derive information about the active slip systems and their critical stress of activation. (1) A.K. Singh, C. Balasingh, Mao, R.J. Hemley & J. Shu, Analysis of lattice strains measured under non- hydrostatic pressure, J. Appl. Phys., 1998, 83, 7567-7575 (2) S. Merkel, N. Miyajima, D. Antonangeli, G. Fiquet & T. Yagi, Lattice preferred orientation and stress in polycrystalline hcp-Co plastically deformed under high pressure, J. Appl. Phys., 2006, 100, 023510 (3) D. Antonangeli, S. Merkel & D. L. Farber, Elastic anisotropy in hcp metals at high pressure and the sound wave anisotropy of the Earth's inner core, Geophys. Res. Lett., 2006, 33, L24303
MR43A-0975
New Experimental Method for In Situ Determination of Material Textures at Simultaneous High-Pressure and –Temperature by Means of Radial Diffraction in the Diamond Anvil Cell.
Radial diffraction in the diamond anvil cell (DAC) has long been used to determine the stress state of materials under non-hydrostatic compression. This technique is also a major tool to investigate textures and infer deformation mechanisms in high pressure minerals. However, most of these experiments have been conducted at ambient temperatures and therefore the results of these measurements may be difficult to extrapolate to the deep Earth. Here, we present a new experimental design that was tested at HPCAT sector 16 BMD of the Advanced Photon Source. This method allows in situ measurement of stresses and textures in the DAC at simultaneously high- pressures and –temperatures. Details of this new technique that uses radial diffraction geometry are discussed, including the uses of amorphous boron gaskets, external heating using graphite heater, and membrane pressure control. Current coverage in pressure and temperature (~30 GPa and 1100 oC). The use of the method will be demonstrated with in situ texture measurements on the high-pressure phases of iron. In the experiment, we were able to observe strong textures in bcc-Fe, track the evolution of the texture with increasing temperature and during the bcc to fcc phase transition. Finally, we observed plastic deformation in the fcc phase between 5 and 15 GPa at 850 oC till the nucleation of hcp-Fe.
MR43A-0976
Material Yield Strength and Anisotropy in the Nonhydrostatic Diamond Anvil Cell: Implications for Mantle and Core Minerals
Hydrostaticity in the diamond anvil cell can be considered as a special case of the generalized three dimensional stress environment in the diamond anvil cell. Recognizing and measuring the extent of non-hydrostaticity can help elucidate measurements of physical properties of mantle and core materials. The amount of differential stress that is supported by a mineral's lattice can be interpreted as a lower-bound on its yield strength, while strength differences among different lattice planes indicate the presence of elastic and/or plastic anisotropy in the sample. Corresponding lattice textural evolution may also help indicate modes of deformation, and may help connect the mineral physics rheology measurements to seismological observations. In addition, measuring the stress environment in the diamond anvil cell is required to make accurate measurements of pressures to interpret phase stability and equation of state data. Here, we present new measurements of yield strength and lattice anisotropy for a materials relevant to the Earth's mantle and core: grossular garnet and Os metal. The differential stress supported by grossular garnet was inferred from synchrotron X-ray measurements of lattice strains in the radial diffraction geometry, and was found to increase from 1.3(±0.6) GPa (at P = 5.8(±1.1) GPa) to 4.1(±0.4) GPa (at P=19(±1.0) GPa ). These results are consistent with inferred strength values for majorite garnet from measurements in the diamond cell normal geometry[1], bolstering the idea that garnet-structured materials may all have similar strengths. However, similar radial diffraction measurements on ringwoodite[2] suggest that this spinel-structured phase is stronger. In fact, the grossular garnet has a similar yield strength as hydrous ringwoodite[3]. This result suggests that the presence of water in the transition zone may not be required to explain a weak rheology, and therefore, models of transition zone behavior built assuming garnet is the high strength phase may need to be revised. The transition metal osmium can be considered as a material analogue for the high pressure behavior of the core mineral, iron. However, the elastic behavior of hexagonal materials has been notoriously difficult to determine using radial diffraction methods. Our measurements of elastic and plastic deformation of Os metal high pressures using in-situ high pressure X-ray diffraction in the radial geometry show that Os has the highest yield strength observed for any pure metal, supporting up to 10 GPa at a pressure of 26 GPa. Furthermore, our data indicate changes in the non-hydrostatic c/a ratio and clear lattice preferred orientation effects at pressures above 15 GPa. We present these results, and examine their implications in light of seismological measurements of inner core anisotropy. References: [1] Kavner, A., S. V. Sinogeikin, J. D. Bass, and R. Jeanloz, "Strength and equation of state of majorite", J. Geophys. Res., 105 pp. 5963-5971, 2000. [2] Kavner, A. and Duffy, T. S., "Strength and elasticity of ringwoodite at upper mantle pressures", Geophys. Res. Lett. 28 p. 2691, 2001 [3] Kavner, A., "Elasticity and strength of hydrous ringwoodite ", Earth Planet. Sci. Lett., 214 pp 645-654. 2003.
MR43A-0977
Attenuation and modulus softening at high pressure and high temperature
We report an experimental method to study the anelastic properties of materials at high pressure and high temperature. The multi-anvil high pressure deformation device, used to apply a cyclic loading force onto the sample, can reach 15 GPa and 2000K. A synchrotron X-ray radiation source provides time resolved images of the sample and reference material. The images yield stress and strain as a function of time, stresses are derived from the reference material, strains from the sample. Strain as small as 10-5 can be resolved. We have obtained experimental results which exhibit resolvable attenuation factor (Q-1) and Young's modulus (E) at deep Earth conditions. Samples of San Carlos olivine, MgO, and a two phase mixture of olivine and ringwoodite for Fo30 have been studied. For San Carlos, these results are in quantitative agreement with previously reported lower pressure data and suggest that temperature and grain size have dominating effect on these properties with a very small pressure dependence of attenuation. The two phase region of olivine – ringwoodite suggest that bulk modulus softening results from driving the phase transformation by the oscillating load. This latter observation is consistent with a diffusion based model of Jackson. This model further predicts a softening of seismic P waves in regions of coexisting phases in the Earth.
MR43A-0978
Are CaIrO3 and MgGeO3 isomechanical to MgSiO3-post-perovskite ?
The recent discovery of MgSiO3 post-perovskite (pPv) and prediction of its elastic properties using atomistic modelling has major implications for the interpretation of seismic anisotropy of the D" layer. Because they don't take into account lattice preferred orientations induced by convective flow, the elastic properties are not sufficient to interpret seismic anisotropy and it is necessary to investigate the plasticity of this mineral. However, it is well known that pressure and temperature near the core-mantle boundary make experimental studies extremely difficult. To circumvent this difficulty, experimental studies are often carried out on analogous phases (stable at lower pressures) which are supposed to exhibit the same mechanical properties as the high-pressure phase. In this work, we calculate the dislocation properties of MgGeO3 pPv at 120GPa and CaIrO3 pPv at ambient pressure using the Peierls-Nabarro (PN) model. The so-called PN model is a fundamental concept of the dislocation theory which describes the resistance of the lattice to dislocation motion, a very important factor for the plasticity of silicates. The PN model also provides an analytical description of the dislocation core and of its potential spreading in the glide plane. Known for several decades, the PN model has triggered a renewed interest when Christian and Vitek (1970) showed that realistic models of dislocations could be built by incorporating generalized stacking faults (GSF) into the PN model. Here, we use the ab initio total-energy package VASP to calculate GSF, which are incorporated in the PN model. In that way, we obtain a model of the dislocation core profile and the value of the stress required to move a dislocation (the so-called Peierls stress) for ten slip systems of each compound. These results are compared to those recently published on MgSiO3 post- perovskite to assess the potential relevance of the analogue approach in studying the rheology of the D" layer. We show that, besides the crystal structure, atomic bonding is an important feature in constraining plastic strain anisotropy. The greatest contrast between Ca-O and Ir-O bond strengths compared to Mg-O and Ge-O or Si-O makes CaIrO3 behave very distinctly from MgSiO3. Although still present, differences are smaller between MgGeO3 and MgSiO3.
MR43A-0979
High-pressure deformation experiments with Kawai-type apparatus for triaxial deformation (KATD)
Knowledge of rheological property of mantle constituent minerals is important for understanding of material behavior in the Earthfs deep interior. Most of previous studies on rheology of mantle minerals with controlled deformation have been limited to low-pressure (<3 GPa) due to experimental difficulty. Recently, deformation- DIA apparatus (D-DIA) and rotational Drickamer apparatus (RDA) have been developed for deformation experiments at higher pressures (e.g. Wang et al., 2003; Yamazaki and Karato, 2001). However, reported experimental pressure condition using D-DIA is still limited to <10 GPa, and experiments at low differential stress condition (< 1 GPa) with RDA is still difficult. We have installed new high-pressure deformation apparatus "Kawai-type Apparatus for Triaxial Deformation (KATD)" at Magma Factory, Tokyo Institute of Technology. The KATD is a modification of cubic-type Kawai-type multi-anvil apparatus with top and bottom differential rams. Since achievable maximum pressure using Kawai- type apparatus (with WC anvils) is about 30 GPa, deformation experiments up to 30 GPa is expected to be possible using KATD. Deformation with very low stress level is expected to be possible using KATD because compression can be done quasi-hydrostatically. In this presentation, basic performance of KATD and preliminary results of deformation experiments at high- pressure and high-temperature will be shown. In the most recent experiments, sintered (Mg0.85Fe0.15)O sample was successfully deformed up to shear strain of >1 at ~15 GPa and 1473 K using hard Al2O3 piston .
MR43A-0980
Olivine Instability: An Experimental View of Mechanism of Deep Earthquakes
Olivine (¦Á-(Mg,Fe)2SiO4) is the major constituent of the upper mantle and the ocean lithosphere. In subduction zone, where the earthquakes happen, the rheology of slab is mainly controlled by that of olivine. Several different mechanisms for deep focus earthquakes have been suggested, which include olivine instability (Bridgman, 1936; Orowan, 1960; Post, 1977; Ogawa, 1987; Hobbs and Ord, 1988; Kao and Chen, 1995), shear-induced melting (Griggs, 1954, 1972; Griggs and Handin, 1960; Griggs and Baker, 1969), phase transformation (Bridgman, 1945; Benioff, 1963; Meade and Jeanloz, 1989), dehydration of hydrous specimens (Meade and Jeanloz, 1991), and olivine metastability-induced anticrack (Green and Houston, 1995). Since the low temperature of the ¡°cold" slab, which can be as low as 600°C in transition zone, olivine may still exist there and thus its shear instability may still be the possible mechanism for the deep-focus earthquakes. In our experimental study on deformation of San Carlos olivine at subduction zone conditions carried out on a D-DIA apparatus, Sam85 at X17B2, NSLS, we observed that the transitional temperature between regimes of insensitive to temperature and sensitive to temperature can be as high as 900°C or even higher for the annealed polycrystal olivine sample, while that for unannealed sample can be as low as 450°C. Our results for the unannealed sample are consistent to the result of Raterron et al (2004), which is concluded from the relaxation experiments. The annealed and unannealed olivine can be present the natural olivine in non-fault systems and that in pre-existing fault systems in subduction zone, respectively. We therefore propose a new olivine instability model with a ¡°sandwich" formation for the deep focus earthquakes: In this model the pre-existing fault system is surrounded by the no-fault systems. When the slab dives down, the olivine in both systems undergoes a stress- build-up process and can hold very high stress in both cases. However, as it keeps diving to the transition zone, the slab is heated and its temperature arrives at the boundary temperature from the insensitive temperature regime to the sensitive to temperature regime for the olivine in pre-existing fault system. As a result, while the olivine in non-fault system is still in regime of insensitive to temperature and can still hold the built high stress, the olivine in pre-existing fault system can not hold the built stress any more and gives a stress release. The pre- existing fault is re-active and heat from the re-active fault accelerates the ongoing process. Earthquake happens.
MR43A-0981
Microstructural evolution of garnet in a greenschist facies transpression zone
Natural observations, laboratory experiments, and theoretical modeling support the interpretation of Grt plasticity in the lower crust and upper mantle; however, these processes are thought to be of little importance in shallow to middle crustal levels. Multiple textural varieties of Grt from the western boundary (Mt. Dumplin high strain zone) of an upper greenschist facies dextral transpression zone in southern New England, USA, display mesoscopic and microscopic evidence of syn-tectonic deformation and recrystallization. These microstructures were examined further by optical microscopy, electron probe microanalysis, orientation contrast imaging (OCI), and automated electron backscatter diffraction (EBSD) in order to understand possible low-grade deformation mechanisms and their significance. The N-S-striking shear zone dips steeply W, the mylonitic foliation is defined by aligned Ms- Chl-Rt, layers of Qtz and fine-grained Grt; Qtz-Chl-Ms and fine-grained Grt aggregates define lineations that plunge moderately SW. S-C-C' fabrics, asymmetric folds and porphyroclasts (delta- and sigma-type) are well developed on foliation-normal/lineation-parallel planes, and display sinistral kinematics; surfaces normal to foliation and normal to lineation exhibit strong asymmetries that indicate normal motion. Pre-tectonic mineral parageneses consist of St pseudomorphed by Chl-Ms-Ctd, Als pseudomorphed by Ms, and coarse-grained Grt and Ab porphyroclasts with associated asymmetric tails. Grt is manifest as three types: 1) equant Grt porphyroclasts; 2) elongate Grt aggregates consisting of 50-100 ƒİm equant Grt porphyroblasts; 3) type 1-type 2 transitional Grt morphology. Elemental x-ray mapping of Ca and Mn reveals at least two periods of growth in Grt types 1 and 3, and one period of growth in type 2 that correlates with type 1 and 3 rims; Mg is completely homogenized. Detailed mapping of type 3 Grt cores reveals ¡¥fractured' Ca-enriched cores ¡¥healed' with Ca- depleted composition. OCI of type 1 Grt shows no internal substructure. OCI of type 2 Grt also shows rare internal substructure (finer-grained equant inclusions with low angle boundaries), and EBSD shows aggregates have CPO symmetrical to tectonic fabric (parallel to lineation), high angle grain boundaries, and neighbor- neighbor grain pairs correlate with random grain pair distributions. Type 2 Grt also displays ¡¥stacking' structures where individual porphyroblasts are stacked vertically and grain boundaries are at low angle. OCI of type 3 Grt shows considerable internal substructure of three varieties: A) substructure boundaries that coincide with Ca-depleted compositions in cores; B) 50-100 ƒİm equant substructures included in rims; C) substructure boundaries in rims that ¡¥nucleate' from substructure A boundaries in cores. Detailed EBSD traverses across all substructure boundaries indicate rotation around rational crystallographic axes. Observations suggest that early amphibolite facies Grt (type 1 and 3 cores) was deformed non-penetratively by plastic deformation or sub-critical fracture (type 3 Grt, substructure A). Type 2 Grt nucleated pre- to syn-tectonically, at least partially through the consumption of type 3 Grt porphyroclasts, and was included in type 1 and 3 rims by rigid body rotations (substructure B). Substructures C in type 3 Grt rims are inherited from pre-existing crystallographic anisotropies in cores (substructure A). Additionally, type 2 Grt was deformed syn-tectonically to produce CPOs, likely as a result of flattening associated with transpression.
MR43A-0982
Rheology of Partially Molten Spinel Lherzolite at High Temperature
Uniaxial creep experiments were performed on synthetic spinel lherzolite samples with grain sizes of 25- 50μm in a gas medium dead-load creep rig to investigate the deformation behavior of partially molten upper mantle. The room pressure experiments were conducted at conditions of elevated temperatures of 1140-1200 °C and well-controlled oxygen fugacities of 10-6-10-10atm. Differential stresses of 4-74MPa were applied to yield strain rates between 10-8 and 10-5 s-1. By fitting the steady-state creep strength with empirical power law, we obtain an oxygen fugacity exponent of 0.05 ± 0.02, a stress exponent of 1.5 ± 0.4, and an activation energy of 1428 ± 48 KJ/mol for the high-temperature deformation of a partially molten spinel lherzolite. The oxygen fugacity exponent, m, is comparable to those obtained for deformation of olivine single crystal, which suggests that deformation of pyroxene doesn't depend significantly on oxygen fugacity as olivine. The creep activation energy obtained in this study is about twice of those from previous studies. SEM observations show about 2-4% melt existing as melt pockets in triple junctions and melt films between grain boundaries in deformed spinel lherzolite. The stress exponent of 1.5 suggests that both dislocation creep and melt-enhanced diffusion creep contribute significantly to the deformation of lherzolite. Our results provide constrains on the rheology and deformation microstructures of partially molten upper mantle.