MR33B-1850
High-Pressure Strength and Elasticity of a Composite MgO and NaCl Sample in the Diamond Anvil Cell
Earth consists of multiphase materials and therefore knowledge of the plastic and elastic properties of the multiphase aggregates under high pressures is of central importance for understanding dynamic processes within the Earth. Interpretation of static high-pressure experiments also requires knowledge of mixtures and composite materials. We have initiated an effort to systematically study the behavior of mixtures in the diamond anvil cell (DAC). The strength and elasticity study of an MgO and NaCl mixture (volume ratio 1:4) has been investigated to 55.5 GPa using a DAC. In-situ radial synchrotron X-ray diffraction measurements were carried out at beamline X17C of the National Synchrotron Light Source, Brookhaven National Laboratory. Our results showed that NaCl B1-B2 phase transformation initiated at about 27.5 GPa and completed at around 30.2 GPa, reflecting the transition pressure discrepancy observed in previous reports. We also found that the NaCl B1-B2 transition affects remarkably rheological properties of accompanying MgO by releasing some of the built-up stress in MgO. Our results show that plastic behavior of the MgO and NaCl mixture is different from that in a single phase. The weaker NaCl phase accommodates most of the strain within the mixture sample. The strength weakening of NaCl appears across the B1-B2 transition, decreasing the differential stress from 0.55(17) GPa to 0.19(4) GPa. The differential stress of NaCl B1 phase can be described by t=0.0244P-0.1367, where t and P are the pressures in GPa. The differential stress supported by the NaCl B2 phase over the pressure range of 30.2-55.5 GPa can be expressed by t=0.0833P- 2.3850, reaching 2.25(25) GPa at the maximum applied pressure. Our elasticity data suggest that MgO remains elastic but NaCl deforms plastically. The importance of plastic deformation to the application of lattice strain theory was examined.
MR33B-1851
An Electron Backscatter Diffraction Investigation of Plastic Deformation in Pyrite: Microstructural Changes, Slip Systems and a Revised Deformation Mechanism map
Experimentally deformed single-crystal and polycrystalline pyrite samples have been investigated using electron backscatter diffraction (EBSD). The single-crystal samples were loaded parallel to <100> or <110> and deformed at a strain rate of 10-5 s-1 and temperatures of 600°C and 700°C. Schmid factor results advocate that <110> loading activates the well established {001}<100> slip system while <100> loading activates the {110}<1-10> slip system. However, while boundary trace analysis and lattice rotation axes indicate {110}<1-10> is possible, it requires concurrent activation of a complex arrangement of slip planes and vectors. In contrast determination of 75MPa as the required critical resolved shear stress (CRSS) for {001}<100> activation, suggests crystal misalignment of ~5-15° to the loading direction in <100> loaded crystals would be sufficient to activate the {001}<100> slip system. Thus, {001}<100> is considered the most likely active slip system in deformed pyrite. This is supported by {001}<1-10> slip being uncommon in polycrystalline pyrite samples. Polycrystalline pyrite samples deformed experimentally at strain rates of 10-4 s-1 and 10-5 s-1 and temperatures between 450-700°C preserve evidence for plastic deformation, specifically dislocation creep. Similar results are recorded in naturally deformed samples (~320-610°C) suggesting dislocation creep is widespread in deformed pyrite and operates at temperatures far lower than previously anticipated (~ 425°C). Combining the EBSD results with the stress-strain curve data from experimental deformation allows construction of a revised pyrite deformation mechanism map. This map corresponds to polycrystalline pyrite with a grain size of ~35μm and suggests the brittle-plastic transition at geological strain-rates occurs at ~320°C. Pyrite trace-element geochemistry is widely used in mineral deposit characterisation, and increasingly evidence is presented documenting trace element zonation in pyrite. While such zonation may relate to growth processes, this study raises the possibility that it may rather be related to diffusion pathways facilitated by plastic deformation processes that are much more widespread in pyrite than is generally accepted.
MR33B-1852
Creep of Fine-grained Gabbro in dry Condition
Natural fine-grained gabbro were deformed at 300MPa confining pressure in a paterson-type deformation apparatus in GFZ. Creep tests were performed at temperatures ranging from 950-1150'C, stresses from 25-500 MPa, and strain rates between2.3x10-4 to 6.7x10-8s-1. The fine-grained gabbro is composed of 60 vol percent plagioclase, 30 vol percent pyroxene, 10 vol percent magnetite and ilmenite. The samples were dried at 1000¡®C for 167 hours before experiments. FTIR measurements show a water content of 0.008 wt percent H2O for starting samples, and 0.03 wt percent H2O for deformed samples. We performed three kinds of tests: stress step creep tests, temperature step creep test and constant stress creep with a long creep time. The data of stress-stepping creep tests and the constant stress creep test with long creep time show that the strain rates under the same stress level were increasing with cumulated creep time beyond a threshold time, which is 24 hours for temperature up to 1050 ¡®C and 5 hours for temperature of 1100 ¡®C, and a linear relation with slope of 1.0 was found between logarithm of strain rate and logarithm of accumulated time, suggesting time-proportional strain-rate enhancement, or equivalently, time-weakening effect of flow strength. Microstructural observations of deformed samples show that melt films occurred between grain boundaries of samples, and the melt contents increase with the creep time, indicating the mechanism of the weakening behavior. The strain rate enhancement related to melt fraction agrees to the data of Dimanov et al. [2000], and is fitted well with the model of Paterson [2000]. In order to determine a steady-state flow law with the effect of melt film excluded, the original steady-state strain rates are converted to the case with t=24 hours for experiments with temperatures up to 1050 ¡®C, and data for temperature of 1100 ¡®C are converted to the case with t=5 hours. The time-corrected creep data were fitted to the most commonly used power flow law, deriving a stress exponent of n=4, activation energy of Q= 644 kJmol-1, and pre-exponential factor in log scale of log A=10.3 MPans-1. The microstructures observed indicate that the deformation of the samples is in the dislocation creep regime. Free dislocations were observed in both pyroxene and plagioclase grains and dislocation walls and subgrain rotation were observed in pyroxene grains. Very fine-grained pyroxene and olivine altered from pyroxene grains suggest that dynamic recrystallization or reaction-accompanied dynamic recrystallization occurred in the samples during deformation with temperatures over 1000 ¡®C. The value of stress exponent and activation energy determined in this study lie between the two end-member values of anorthite [Rybacki and Dresen, 2000] and diopside [Dimanov et al., 2005]. References Dimanov, A., R. Wirth, G. Dresen, 2000. Tectonophysics, 328. Dimanov, A., and G. Dresen, 2005. J. Geophys. Res., 110(B7). Paterson, M.S., 2000. Tectonophysics. Rybacki, E., and G. Dresen, 2000. J. Geophys. Res., 105.
MR33B-1853
Deciphering P-T-t Paths from Reaction Microstructures in Metamorphic Rocks: a New Approach by Means of Three-Dimensional Finite Element Modelling
One of the main goals of metamorphic petrology is to obtain information on the variations of metamorphic P-T conditions during orogenesis (P-T-t paths). For this purpose petrologists are aware of the potentiality of studying reaction microstructures, although results are not always satisfactory as in most cases qualitative approaches, failing on the real meaning of specific microstructral relationships, are often adopted. Thus, the present research aimed to study the petrogenetic meaning of reaction microstructure in metamorphic rocks through the formulation of a new true three-dimensional finite-element model. For this purpose, different petrologically well studied metamorphic microstructural situations have been selected, in order to identify information, variables and constraints fundamental for the development of the model. A generalised finite-elements model (FEM) has been developed, applicable to any microstructural situation, independently on grain-size and distribution of minerals in the matrix, and able to also consider growth anisotropies, intracrystalline diffusion, pressure solution, and possibly anisotropy of the strain field. This model is based on a combination of the usual diffusion linear equations used in current irreversible thermodynamic models, providing constraints on absolute values of diffusion coefficients of chemical components, chemical potential gradients and time of reactions during metamorphism, starting from information on textural anisotropies observed in metamorphic rocks. In the model, parameterization is given by diffusion, convection and reaction coefficients of each chemical species within each finite element, which dimension is equal to the spatial resolution of the experimentally measured input data (i.e. SEM elemental maps). Thus, parameterization is able to describe locally heterogeneous reaction phenomena although based on a basically linear partial derivative differential model. Such a discretization of the continuum model is by far simpler, although slightly less efficient, than usual approaches based on adaptive discretization (adaptive FEM) and/or mechanisms of grain boundary description and localization. In a generic microstructural situation, number and tortuosity of mineral phase interfaces are so large that an explicit geometric description would be virtually unfeasible. At present, the model is under tuning using real microstructural situations observed in garnet + staurolite +/- kyanite micaschists of the Australpine Basement (Eastern Alps, Northern Italy).
MR33B-1854
Deformation of MgSiO3 perovskite at high pressure using diamond anvil cells and in- situ radial diffraction
Magnesium silicate perovskite is thought to be the major constituent of the lower mantle. Consequently, knowledge of the deformation behavior of MgSiO3 perovskite is important for understanding the geodynamic behavior in the deep Earth as well as interpreting observed seismic anisotropy. One mechanism that can generate anisotropy is dislocation glide and associated development of texture or lattice preferred orientation (LPO). Radial x-ray diffraction provides a method for studying, in-situ, the development of elastic lattice strains as well as LPO in polycrystalline samples. Previous measurements using diamond anvil cells (DAC) and radial diffraction documented three different texture types in MgSiO3 perovskite depending on starting material (enstatite, olivine, and ringwoodite). In order to understand these differences we have performed new experiments using a DAC with a membrane pressure-control system and the possibility of simultaneous laser heating. We have also employed improved gasket technology (amorphous boron and kapton gaskets as well as cubic boron nitride and kapton gaskets) to minimizes extraneous diffraction from the gasket. These new experiments have confirmed previous results and allowed a more detailed investigation of the MgSiO3 system. After transformation from olivine, perovskite has [100] axes aligned with the compression direction. This is an elastically weak direction, and it is possible that this texture results from mechanical 110 twinning. Perovskite transformed from ringwoodite develops a 012 texture, and for the case of perovskite synthesized from enstatite a 001 texture is observed. These two texture types can be explained by activity of (010)[100], (100)[010], and (001)<110> slip systems that are known to occur in perovskite structures. A first attempt at applying simultaneous laser heating and deformation did not appreciably change orientation patterns for perovskite synthesized from enstatite.
MR33B-1855
Strain heterogeneity during dislocation creep in Carrara Marble using a micro-scale strain-mapping technique
We mapped the heterogeneity of strain produced during creep experiments done at strain rates of 1e-5 to 1e-4 (inverse seconds) and at 700-1000 K, on split cylinders of Carrara marble. The micro-scale mapping technique used pre- and post-deformation observations of a fine grid of markers deposited on one surface of the split cylinders. The local strain, which could be measured at a scale of one micron or less, could be as large as three times that of the total bulk aggregate strain, which was typically between 0.10 and 0.20. Localized deformation could be observed along twins and grain boundaries, but could also be observed as patchy strain variation in intergranular regions, probably indicating the development of a subgrain structure. A Schmid factor analysis suggests that multiple slip systems were activated during the compression tests, as expected for these conditions. The von Mises equivalent strain, averaged over the entire grid area, is the same as the imposed strain, but the strain averaged over a single grain can vary by a factor of two times the bulk strain. Simple VPSC models are not capable of explaining the grain to grain variations, or the localized intergranular heterogeneity.
MR33B-1856
Shear Zone Development and Rheology in the Deep Orogenic Crust
Within the Central Gneiss Belt (CGB) of the southwestern Grenville Province, Ontario, Canada, a number of allocthonous lithotectonic domains are juxtaposed along crustal-scale shear zones. Extensive exposure of variably reworked granulites of the interior Parry Sound domain (iPSD) has enabled investigation of the structural and petrologic character of domain-bounding shear zones within the deep orogenic crust. Recent detailed mapping and structural data collected along the southwestern margin of the iPSD is consistent with the suggestion of Culshaw et al. (in prep) that spaced outcrop-scale shear zones have coalesced and progressively reworked layered granulites into a transposed amphibolite-facies tectonite. The tectonites comprise the Twelve Mile Bay Shear Zone (TMBSZ), which separates the iPSD from para-autocthonous rocks to the south. This study investigates the grain- and outcrop-scale mechanisms involved in shear zone development and attempts to quantify the associated changes in rock rheology. Northwest of TMBSZ, samples collected across individual outcrop-scale shear zones (i.e., across large strain gradients) have distinct differences in mineralogy and microstructure. In mafic layers the original granulite texture and cpx + opx + pl + hbl +/- grt assemblage is commonly retained away from the shear zones within unsheared "panels". With proximity to the shear zones pyroxenes and garnet are progressively consumed in hydration reactions producing hornblende and biotite, which define a new planar foliation within the highly attenuated and deflected layering. Felsic layers generally have only minor mineralogical changes across the zones, but develop an increasingly intense and recrystallized structural fabric into the sheared margin. The shear zones are commonly cored by variably deformed pegmatite dikes that were emplaced prior to, or during the early stages of shearing. Evidence for incipient shear zone formation along mineralized fracture sets that cut unsheared granulites, often with clear centimeter-decimeter wide alteration halos, is preserved in adjacent rocks closer to the domain interior. Approaching the TMBSZ, the proportion of undeformed panel is decreased considerably and a finer-grained tectonite fabric becomes dominant. Panels in this area are more podiform, and relict layering is often at a much lower angle to the transposed fabric that wraps it. Large feldspar porphyroclasts, often with extensive tails parallel to layering, are commonly observed in the tectonite suggesting that these rocks represent widened and strongly attenuated pegmatite-cored shear zones. Thus, a six-stage conceptual model is proposed in which iPSD granulites are reworked into amphibolite facies tectonites (of the TMBSZ) through growth of spaced shear zones that initiated on fluid-filled fractures, and progressive consumption and rotation of relict granulite panels. Previous experimental studies have shown that changes in rock mineralogy and microstructure can have a substantial effect on bulk rheology and the partitioning of strain. For reactions where relatively rigid phases such as garnet and pyroxene are replaced by well-aligned biotite and amphibole, orders of magnitude strength drops may be possible. Structural data and observations are used here to constrain grain- and outcrop-scale numerical modeling exercises that evaluate the 1) strength contrasts between the shear zones and the granulite protolith, 2) bulk strength of crustal blocks with variable proportions of shear zones, and 3) sensitivity to various physical parameters.
MR33B-1857
Mechanisms of Subcritical Cracking in Calcite
Brittle materials are characterized by a critical stress intensity factor above which they will fail catastrophically by dynamic cracking. However, it has been observed that materials can also fail at much lower stresses, through slow crack growth, often referred to as subcritical cracking. This phenomenon can take place even in vacuum, but is greatly enhanced by water and other reactive species in the environment. For a given material and environmental condition there is a systematic relationship between the crack tip velocity and the stress intensity factor. The presence of a lower stress limit to subcritical cracking has been predicted from thermodynamics but has not been firmly demonstrated experimentally. This parameter would control the long- term strength of geological materials. Subcritical cracking must necessarily be important in controlling the rock strength in near-surface processes where water and other active species are present and the displacements and stresses are low. Weathering is one example of such a process. Modelling has shown that fracture networks generated by a high degree of subcritical cracking will percolate at much lower fracture densities than purely stochastical fracture networks. This has important implications for how water can move through the crust. Understanding the mechanisms for subcritical crack growth in geological materials is also important in assessing the stability and long term performance of sequestration reservoirs for CO2 or nuclear waste. The mechanism for stress corrosion is well known for glasses and quartz. For carbonate minerals, the mechanism for subcritical crack growth has not been identified, and the only experimental studies on calcitic materials have been on polycrystalline rocks such as marble. Suggested mechanisms include stress corrosion (weakening reactions at the crack tip), preferential dissolution at the crack tip with rapid removal of dissolved species, and environmentally controlled microplasticity (pile-up of dislocations in the process zone around the crack tip). In our experiment, we study the subcritical growth of a cleavage crack through a single calcite crystal. We use the well documented Double Torsion method which allows for easy study of a tensile crack growing in the specimen. We find that the environmental conditions, and in particular the availability of water, has a significant influence on both the crack tip velocity at a given load, but also the behaviour of the crack movement. Based on our experimental data, we propose what mechanisms are dominant for subcritical crack growth in calcite at various load levels.
MR33B-1858
Experimental Deformation of Olivine Single Crystal at Mantle P and T: Pressure Effect on Olivine Dislocation Slip-System Activities
Seismic velocity anisotropies observed in the 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 high-pressure deformation experiments (Couvy et al., 2004, EJM, 16, 877; Raterron et al., 2007, Am. Min., 92, 1436; Raterron et al., 2008, Phys. Earth Planet. Int., doi:10.1016/j.pepi.2008.07.026) show that [001](010) slip system dominates [100](010) system in the (P,T) range of the deep upper mantle. This may promote a shear-parallel slow-velocity [001] axis and may explain the seismic-velocity attenuation observed at depth >200 km (Mainprice et al., 2005, Nature, 433, 731). In order to further constrain the effect of P on olivine slip system activities, which is classically quantified by the activation volume V* in power creep laws, deformation experiments were carried out in poor water condition, at P>5 GPa and T=1400°C, on pure forsterite (Fo100) and San Carlos olivine crystals, using the Deformation-DIA apparatus at the X17B2 beamline of the NSLS (Upton, NY). Ten crystals were oriented in order to active either [100] slip alone or [001] slip alone in (010) plane, or both [100](001) and [001](100) systems together. Constant applied stress σ <300 MPa and specimen strain rates were monitored in situ using time-resolved x-ray diffraction and radiography, respectively, for a total of 27 investigated steady state conditions. The obtained rheological data were compared with data previously obtained in comparable T and σ conditions, but at room P, by Darot and Gueguen (1981, JGR, 86, 6219) for Fo100 and by Bai et al. (1991, JGR, 96, 2441) for San Carlos olivine. This new set of data confirms previous deformation data obtained at high pressure, i.e., it translates into a high activation volume V*>14 cm3/mol for [100](010) and a low V*<9 cm3/mol for [001](010)in both Fo100 and Fe-bearing olivine. We also show that the combined activity of [100](001) and [001](100) systems translates into V*=12± 3 cm3/mol, suggesting that pressure also reduces both system activities. All together, our data show that olivine [001](010) system dominates deformation at the mantle P and T.
MR33B-1859
Polycrystalline Garnet Porphyroblasts, an EBSD Study
Polycrystalline garnet porphyroblasts (PGP's) are significant because their formation provides information about metamorphic crystalline mechanisms, in particular during early stages of crystal growth, which may differ from those governing later stages; and because their existence may affect the chemical and structural evolution of metamorphic rocks. For example, the extent of element exchange between the garnet interior and the matrix may be affected by the presence of grain boundaries within PGP's. There have been several previous studies of PGP's but important questions about them remain; e.g. whether early coalescence is a common method by which garnets crystallize, whether grains rotate during growth to attain an energetically favorable grain-grain contact, and whether deformation and/or precursor minerals or other chemical or mechanical heterogeneities influence the formation of PGP's. PGP's have been detected by us in several different localities including; micaschist from SE Vermont (USA), including locality S35j of Rosenfeld (1968); the Solitude Range (British Columbia, Canada); the Southern Menderes Massif (Turkey); and three zones (garnet, staurolite, kyanite) from the Dutchess County Barrovian sequence in NY (USA). We have identified two types of PGP: cryptic and morphologically distinct. Cryptic PGP have no obvious morphological expression of the high angle boundaries within them and appear to be a single crystal. Morphologically distinct PGP have an obvious depression in the outer grain boundary where it is intersected by the internal grain boundary. Most PGP's contain inclusion trails and the high angle grain boundaries crosscut the trend of these as well as the inclusions themselves. PGP also show major element growth zoning that is not influenced by the internal grain boundaries except in rare cases. PGP's comprise ~ 5-35% of the garnet populations analyzed. More than 95% of the PGP's we have analyzed are comprised of 2-3 domains; the rest contain 4-16 domains. The prevalence of low- energy boundaries within the PGP's analyzed thus far is 20 ± 2%, slightly greater than the 15% predicted for a random disrtibution in cubic minerals.
MR33B-1860
The Effect of Cement Distribution, Abundance, and Morphology on the Mechanical Behavior of Granular Porous Media.
The material properties of clastic sedimentary rocks are a function of multiple parameters, including intrinsic characteristics such as porosity and cement content. The latter are typically characterized in bulk; cement type and percentage and percent porosity are documented, but cement distribution and resulting pore geometry are rarely described. However, natural sandstones have grain-cement-pore geometries that are largely dictated by grain and cement mineralogy. Quartz cement, for example, is typically precipitated as overgrowths on quartz grains with a geometry that is crystallographically controlled. Therefore, we have explored the elastic mechanical effects of natural cement geometry, distribution, and abundance in quartz arenites with a series of 2-D, finite element simulations of multi-grain assemblies. In the model domains, circular quartz grains are bonded together with variable amounts of quartz cement. Cement geometries are either simple grain-bridging bands or idealized quartz overgrowths. The simulations demonstrate that as cement bridges lengthen, stress is more distributed, point loads dissipate, and contact stiffness increases, regardless of cement geometry. Importantly, this is true even though the overgrowth model has more than three times the cement and 25% less pore space than the grain-bridging model. The locations of stress concentrations, however, do vary with cement geometry. To extend our understanding of the importance of cement distribution in granular systems from 2-D to 3-D, 2-grain assemblies constructed of spherical glass beads connected by varying amounts of ice cement were tested in shear and tension. Results provide a bridge between nature and theory, which we frame in terms of a conceptual model of the effect of progressive cementation on the elastic behavior of porous media.
MR33B-1861
Carbon-bearing silicate grains of texture formation on phlogopite crystals from Shimonoseki-shi, Yamaguchi-ken, Japan
Introduction: Carbon contribution to crustal rocks is significant to produce circulated materials with carbon
among atmosphere, sea-water and land rocks (Miura, 2007,2008), though detections of carbon contents are
used to be very difficult by normal inorganic analyzers. The present purpose of the paper is to elucidate
carbon effects to textures of mineral crystals from dynamic reaction processes.Sample and Observation: The
present samples used in this study are phlogopite crystals from druses of basaltic lavas, which are found
from Kifune-cho and Mutsure-jima, Shimonoseki-shi, Yamaguchi-ken, Japan. Texture of phlogopite is
observed by the FE-SEM with EDX analyzer, Yamaguchi, Japan. Result of Observation: Phlogopite crystal
shows precipitated growth patterns with various feature and spirals (including five-sided or three-sided) on
Fe and F -rich phlogopite crystal plates. The detailed FE-SEM observation shows the following two textures.
1) Fe and Ti-rich phlogopite crystals are intruded and surrounded irregularly by carbon-rich fluid as the vein-
textures without regular texture of growth pattern. 2) Basement of Fe and Ti-rich phlogopite crystals (without
carbon) are precipitated by Fe-Mg-bearing unknown X1 phase (carbon-bearing grains mixed with phlogopite-
like grains) under growth (spirals) patterns connected by many grains, where many grains show unknown X2
phase (carbon-rich phases with K-Mg-Fe-Al-Si-O grains in composition). Step patterns reported as
polysynthetic twin-like feature show two phases of carbon-rich and carbon-poor grains. All zonal textures and
growth spirals with five or three-sided pattern consist of many small micro-grains with spherule-like grains X2.
Formation: The present data indicate that zonal and growth spirals reported previously are not phlogopite
crystal growth but micro-grain assemblages on phlogopite crystals during formation of final stage of druses in
basaltic rocks. Many micro-grains are considered to be formed irregularly by rapid reaction as vapor-gas
states of carbon and fluorine (without chlorine)on phlogopite plates during quenching processes after
basaltic magma events. Summary: Carbon effects on circulation process on the Earth can be found in the
crystal rocks as carbon-bearing micro-grains (unknown X1 and X2) on phlogopite crystals found in druses
from Shimonoseki-shi, Yamaguchi-ken, Japan, where the micro-grains with spherule texture are formed as
gas explosive and quenched processes on phlogopite plates during high-temperature formation (without sea-
water in composition). References: Miura Y.(2008): AOGS Meeting (Busan) , Abstract IWG04-D412. Miura
Y.(2007): Frontiers in Mineral Sciences 2007 (Univ. of Cambridge, UK), 223.
http://www.asiaoceania.org
MR33B-1862
Structures and growth textures of Japanese twin boundaries in quartz
Growth textures and atomic configurations of Japanese twin boundaries in quartz were studied by the observation of natural samples and by computational simulations. Samples used in this study are collected from Narushima, Nagasaki Prefecture, Japan. The samples were first polished, and then etched by hydrofluoric acid for several minutes. The etched figures were observed by phase-contrast reflection microscopy and scanning electron microscopy. From these observations, high concentration of Brazil twin lamellae is found near the composition plane of Japanese twin. Observations of cathode luminescence images reveal that the development of Brazil twin lamellae at {112̅2} composition plane of Japanese twin is directly related to the preferential growth of Japanese twin along the composition plane. Atomic configurations at {112̅2} composition planes of Japanese twin were simulated by using molecular dynamics simulations and the energy minimization method. The simulated structures proved that {112̅2} or {1̅1̅22} composition planes are the only composition planes that do not introduce unsatisfied bonding between Si and O atoms. When the composition plane is different from these planes, some kind of defect structures, like dislocations, are inevitably introduced. In the case of Brazil twin, screw dislocations are also known to be incorporated where orientation of Brazil twin boundary changes from one orientation to another. On the other hand, in the case where Brazil twin boundaries intersect with {112̅2} composition planes of Japanese twin, we found that structures are kept coherently without any unsatisfied bonding. This result means that {112̅2} composition planes of Japanese twin are the crystallographic sites having more than one possible stacking structures. Observations in this study indicate that {112̅2} composition plane of Japanese twin serves as a source of Brazil twin during the course of crystal growth.
MR33B-1863
Measurements of Low-Temperature Plasticity in dry Olivine Using the D-DIA
Important unresolved issues in geodynamics demand a better understanding of the strength of the lithosphere. To date, most deformation experiments on mantle rocks have concentrated on the high- temperature creep regime. In the present study, we have carried out deformation experiments in a D-DIA apparatus at beam line X17B2 at the National Synchrotron Light Source using a dry mullite-pyrophyllite hybrid cell assembly. We deformed polycrystalline samples of San Carlos olivine + 5% enstatite in triaxial compression at relatively low temperatures of 673 to 1273 K, high pressures (mean stress) from 4 to 10 GPa and constant strain rate of about 3.0×10-5s-1. The state of stress was calculated from changes in lattice spacing measured by x-ray diffraction in energy dispersive mode; plastic strain was recorded directly from x-radiographs of the sample assembly during deformation. We found that strength was weakly dependent on temperature at cooler temperatures corresponding to the shallower portion of the lithosphere, decreasing from 3.5 to 3.0 (±0.5) GPa over the range 700 – 1100 K, and turned more strongly temperature-dependent above 1100 K. The temperature dependence eventually reached a value corresponding to that of high-temperature creep around 1400 K. Our strength values at lower temperatures are in good agreement with those determined by Evans and Goetze (1979) based on microindendation experiments, but diverge as temperature increases (our measured strengths are higher). There is poor agreement across the entire temperature range with values reported by Raterron et al. (2004) based on stress relaxation tests, ours being higher by a factor of 2 or more above 800 K. Can anybody say crème brûlèe?
MR33B-1864
Elastic Plastic Self Consistent (EPSC) Modeling of Plastic Deformation in Olivine
We report on in-situ synchrotron x-ray diffraction from high pressure deformation experiments conducted using fayalite and San Carlos olivine. The experiments were conducted using the D-DIA apparatus at beamline X17b2 at the NSLS. During deformation experiments we observe the diffraction behavior of a number of lattice reflections for lattice planes oriented perpendicular to compression as well as those that contain the compression direction. We measure sample strain 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 used elastic plastic self consistent (EPSC) modeling to analyze diffraction from the sample during deformation. Our EPSC models of olivine indicate that the operation of various slip systems impacts in-situ diffraction in different ways. For example, we find that most slip systems with a [100] burger's vector primarily affect the (120) and (131) reflections in contrast to [001] slip on (100) and {110} as well as [100] slip (001) which primarily affect the (101), (334) and (112) reflections. We also observe that the effect of slip on diffraction for planes oriented perpendicular to the compression direction and those containing the compression direction are not related in a simple or consistent way for all slip systems. In our previous work, we showed that stresses calculated by taking differentials between lattice strains from these two orientations did not produce a robust estimate of the macroscopic stress. Here we show that in addition, the practice of using the differential lattice strain discards valuable information.
MR33B-1865
Strength of Single Crystal of Orthopyroxene Under Lithospheric Conditions
The strength of the lithosphere is an important parameter that controls the tectonic style of a planet. In most of the previous studies, the power-law creep laws of constituent minerals were used to estimate the strength of the lithosphere that predict exceedingly high strength of the lithosphere which would not allow the operation of plate tectonics. In this report, we present new results on the strength of orthopyroxene that indicate that orthopyroxene may have much smaller strength than olivine at low temperatures. Previous studies on deformation of orthopyroxene showed that the strength of orthopyroxene is controlled by the competition of slip (+ recrystallization) and a phase transformation to clinopyroxene (Raleigh et al., 1971; Coe et al., 1973). The latter is less thermally activated and hence the strength of orthopyroxene at low temperatures is mostly controlled by the phase transformation, and is smaller than those determined by slip (+ recrystallization). In fact, in many naturally deformed peridotites at high stresses (low temperatures), evidence of ortho to clino phase transformation is reported. However, the previous studies have major limitations: (1) the stress magnitude was determined by an external load cell in a solid-medium apparatus and has large uncertainties, (2) the mechanical data are fitted using a power-law equation for both regimes, but the validity of the power-law formula is questionable for deformation by a phase transformation. Consequently, we have initiated a series of experimental studies in which we will quantify the strength of orthopyroxene at relatively low temperature conditions. We use a Griggs apparatus at P=1-2 GPa and T to 1500 K for a range of strain-rate. In order to reduce the friction, we use CsCl as a confining medium. In most of experiments, we deform orthopyroxene together with olivine to determine the relative strength of these two minerals. The strength of a sample is estimated from the load-cell reading as well as the dislocation density of olivine (Karato and Jung, 2003). Our preliminary observations showed that orthopyroxene is weaker than olivine at low temperatures. We will report further experimental observations on the strength of orthopyroxene and discuss some possible implications for the strength of the lithosphere.
MR33B-1866
Deformation induced slip-system transition in olivine at high pressures
Deformation-induced olivine fabrics provide vital information for understanding seismic anisotropy in the upper mantle since olivine is the primary mineral in this region. Our recent analysis of the fabric of deformed olivine aggregates has documented the influence of deformation conditions (e.g., temperature and stress) on the fabric evolution of olivine at high pressures. Experiments were carried out using a deformation-DIA on an X-ray beamline at the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory. Samples of polycrystalline olivine were deformed at a constant displacement rate of ~ 3.0 × 10- 5 s-1 over true axial strains of ~ 30% at temperatures of 673 to 1573 K and pressures of 5 to 9 GPa. Electron backscattered diffraction (EBSD) analyses of the fabrics of the deformed samples reveal a transition in slip system. For low temperatures (T < 1173 K) and high stresses (σ > 2 GPa), the normals to the (100) lattice planes are parallel to the maximum principal stress; while at high temperatures (T > 1473 K) and lower stresses (σ < 1 GPa), the normals to the (010) lattice planes switch to be parallel to the maximum principal stress. This transition in fabric occurs even if pressure is fixed, indicating that pressure is not a primarily factor causing the transition in dominant slip system. Our observations indicate that deformation of olivine is dominated by the (100) [001] slip system at low temperatures and high stresses and by the (010) [100] slip system at high temperatures and low stresses.
MR33B-1867
Rheological and Textural Memory of Marbles: Examples from Complicated Torsion Experiments
We performed a new set of torsion experiments on Carrara marble with the aim of better understanding the evolution of rheology, fabric and microstructures under complicated strain path, and evaluate the effect of pre-existing, strain-induced anisotropy on the mechanical and microstructural evolution of deformed rocks. Different types of deformation experiments were conducted at a temperature of 1000 K, 300 MPa confining pressure, and 3x10-4 s-1 shear strain rate. Microstructural examinations were carried out using light and electron microscopy. Fabric analysis was performed through electron backscattered diffraction (EBSD) measurements. The stress-strain relationships from the experiments indicate that the mechanical response of Carrara marble depends on the deformation history. After a first ductile deformation event the marble becomes permanently softened and is able to undergo plastic strain at relatively lower stresses compared to previously undeformed marble. After shear strain reversal of γ = 2 or less the original microstructure is restored; in highly re-strained samples (γ = ± 3, 4 and 5) dynamic recrystallization mechanisms create an evident foliation whose angle in respect to the shear plane defines a shear sense criterion which is in agreement with the reversed sense of shearing. After a minimum shear strain reversal of γ = 3 the pre-existing foliation is totally overprinted. The fabric produced during strain reversal experiments shows features, which are comparable with those of single-stage experiments; but the strength of the crystallographic preferred orientation (CPO) is significantly lower, indicating complex microstructural interactions during plastic reactivation. Our tests indicate that the strain weakening in Carrara marble is predominantly caused by grain size refinement through dynamic recrystallization. The development of a crystallographic preferred orientation contributes towards only about one third of the total weakening observed.