deformation induced crystallographic preferred orientation of hcp iron an experimental study using a deformation dia apparatus abstract shear and uniaxial deformation experiments on hexagonal close packed iron hcp fe was conducted using a deformation dia apparatus at a pressure of 13 17 gpa and a temperature of 723 k to determine its deformation induced crystallographic preferred orientation cpo development of the cpo in the deforming sample is determined in situ based on two dimensional x ray diffraction using monochromatic synchrotron x rays in the shear deformation geometry the 0001 and 11 2 0 axes gradually align to be sub parallel to the shear plane normal and shear direction respectively from the initial random texture in the uniaxial compression and tensile geometry the 0001 and 11 2 0 axes respectively gradually align along the direction of the uniaxial deformation axis these results suggest that basal slip 0001 11 2 0 is the dominant slip system in hcp fe under the studied deformation conditions the p wave anisotropy for a shear deformed sample was calculated using elastic constants at the inner core condition by recent ab initio calculations strength of the calculated anisotropy was comparable to or higher than axisymmetric anisotropy in earth s inner core

global upper mantle structure from finite frequency surface wave tomography 1 we report global shear wave velocity structure and radial anisotropy in the upper mantle obtained using finite frequency surface wave tomography based upon complete three dimensional born sensitivity kernels because wavefront healing effects are properly taken into account finite frequency surface wave tomography improves the resolution of small scale mantle heterogeneities especially for deep anomalies that are constrained by the longest period surface waves in our finite frequency model ffsw1 the globally averaged radial anisotropy shows a transition from positive sh sv to negative anisotropy sv sh at about 220 km consistent with a change in the dominant mantle circulation pattern from predominantly horizontal flow at shallow depths to vertical flow at greater depths the radial anisotropy beneath cratons and the old pacific plate agrees well with previous studies however our model exhibits a strong negative radial anisotropy at depths greater than 120 km beneath mid ocean ridges a feature that is not present in previous upper mantle models more interestingly the depth extent of the ridge anomalies is distinctly different beneath fast and slow spreading centers anomalies beneath fast spreading centers are stronger but the strength decreases rapidly below 250 km in contrast beneath slow spreading centers such as the northern mid atlantic ridge and the red sea anomalies extend down at least to the top of the transition zone the different depth extent of the ridge anomalies suggests that the primary driving force of slow spreading seafloor may be different from that of fast spreading seafloor and that active upwelling beneath slow spreading ridges may play a major role in the opening of the seafloor

evidence for two distinct modes of large scale ocean circulation changes over the last century through its nonlinear dynamics and involvement in past abrupt climate shifts the thermohaline circulation thc represents a key element for the understanding of rapid climate changes the expected thc weakening under global warming is characterized by large uncertainties and it is therefore of significant importance to identify ocean circulation changes over the last century by applying various statistical techniques on two global sea surface temperature datasets two thc related modes are separated the first one involves relatively slow adjustment of the whole conveyor belt circulation and has an interhemispherically symmetric pattern the second mode is associated with the relatively fast adjustment of the north atlantic overturning cell and has the seesaw structure based on the separation of these two patterns the authors show that the global conveyor has been weakeningsince the late 1930sand that the north atlanticoverturningcell suffered an abrupt shift around 1970 the distinction between the two modes provides also a new frame for interpreting past abrupt climate changes

creep mechanisms in the lithospheric mantle inferred from deformation of iron free forsterite aggregates at 900 1200 c abstract to further constrain the plasticity of rocks in the uppermost lithospheric mantle deformation experiments were carried out on forsterite aggregates using a gas medium apparatus paterson press at 300 mpa 900 1200 c and nearly constant strain rates of 10 5 s 1 the starting material was a synthetic iron free forsterite aggregate with an average grain size of 2 8 m and 2 3 of iron free enstatite eight deformation experiments were performed as well as an additional static annealing to characterize grain growth the maximum stresses obtained range from 480 to 1870 mpa below 1000 c where stress significantly exceeds confining pressure and based on microstructural observations grain boundary mediated creep is observed with evidences of sliding and cavitation gaping at grain boundaries at 1050 1200 c where pseudo steady state could be achieved the microstructures are very different and show evidences of dislocation activity resulting from the activation of several dislocation slip systems with increasing temperature when compared to rheology laws previously obtained from similar experiments the temperature dependence of iron free olivine creep is similar to the one of its iron bearing counterpart at high temperature 1200 c at temperatures 1000 c however the strength of iron free olivine is higher than for iron bearing olivine the deformation induced textures obtained show that grain boundary sliding gbs lead to cavitation which was activated in response to large differential stresses i e beyond the goetze criterion given these high stress conditions our results cannot be directly applied to deformation of the earth s mantle at large scale nevertheless they highlight the key role played by grain boundaries in producing strain at lithospheric temperatures when crystal plastic mechanisms remain inefficient

plastic deformation of wadsleyite and olivine at high pressure and high temperature using a rotational drickamer apparatus rda abstract large strain plastic deformation experiments of wadsleyite and olivine were conducted using a rotational drickamer apparatus rda up to pressure and temperature conditions corresponding to the earth s mantle transition zone sintered ring shaped mg fe 2sio4 wadsleyite and olivine samples were deformed at p 16 gpa and t 1600 and 1800 k and p 11 gpa and t 1800 k respectively with equivalent strain rate of e e 6 10 5 s 1 in situ observations of deforming samples were carried out using the synchrotron radiation facility at brookhaven national laboratory nsls x17b2 stress was measured by x ray diffraction at six different angles with respect to the compression axis the stress estimated by x ray diffraction was in good agreement with the stress estimated from dislocation density for olivine strain was determined using x ray radiographs of a strain marker re or mo foil deformation of samples with a rda involves both uniaxial compression and simple shear a new formulation is developed to analyze both components to determine the rheological properties of a sample stress strain curves show strain hardening up to the equivalent strain of e 0 2 followed by the quasi steady state deformation wadsleyite is found to be stronger than olivine compared at similar conditions and the creep strength of olivine at p 11 gpa is much higher than those at lower pressures

high temperature elasticity of iron bearing olivines the first high temperature data on the nine adiabatic elastic moduli for iron bearing olivine are reported these measurements are on two single crystal specimens of natural olivine at ambient pressure and from room temperature to a maximum of 1500 k the two specimens contain 8 and 9 modal percent fayalite which required the oxygen fugacity be controlled at high temperature to preserve their chemical stability the rectangular parallelepiped resonance apparatus was adapted to buffer the specimens from the atmosphere with a mixture of co and co2 gas a small increase 1 2 gpa in the adiabatic bulk modulus of each specimen over that of end member forsterite was found the data are high quality to extreme temperatures with good agreement found when comparing the temperature derivatives of the elastic moduli of the two specimens neither specimen exhibits measurable nonlinear temperature dependence in the computed isotropic bulk and shear moduli which is in contrast to published forsterite data the temperature derivatives of the isotropic bulk modulus ks are 1 69 1 80 10 2 gpa k 1 for the two olivine specimens and the shear modulus g derivatives are 1 38 1 36 10 2 gpa k 1 these derivatives are only slightly larger in magnitude than ks t p 1 56 10 2 and g t p 1 30 10 2 gpa k 1 found previously for iron bearing olivine over a very small temperature range there are also no significant differences between the temperature derivatives found here and the average derivatives of end member forsterite from data retrieved over a slightly larger temperature range several dimensionless parameters have been calculated from these results and are discussed in view of systematics which bear on high pressure phases in earth s transition zone one result from these systematics related to the seismic velocities in the earth and especially the shear wave velocities is that an olivine content of less than 50 is implied at the 400 km discontinuity if earth s upper mantle is isochemical furthermore the substitution of almost 10 iron for magnesium at the forsterite end of the olivine solid solution series has little effect on the dimensionless parameters or on the temperature derivatives of the elastic moduli at high temperature

shear deformation of dry polycrystalline olivine under deep upper mantle conditions using a rotational drickamer apparatus rda abstract shear deformation experiments on dry hot pressed polycrystalline san carlos olivine have been conducted at 4 9 9 6 gpa 1300 1870 k and strain rates of 0 6 7 4 10 5 s 1 using a rotational drickamer apparatus rda at a synchrotron facility the stress was measured from the orientational dependence of lattice spacing for the 1 3 0 1 3 1 1 1 2 1 2 2 1 4 0 and 2 4 1 planes as well as from the dislocation densities based on the mechanical and microstructural observations we infer that deformation occurs by power law creep involving dislocation glide and climb under high temperature conditions whereas deformation is due to exponential creep through the peierls mechanism at relatively low temperatures in both regimes the strength of olivine at steady state deformation at these pressures is much larger than those at lower pressures the activation volumes v is estimated to be 15 20 10 6 m 3 mol for the power law creep combined with the previous results under wet conditions we present the recommended values of flow law parameters on olivine for power law creep constrained by high pressure deformation experiments

growth kinetics of forsterite reaction rims at high pressure abstract growth kinetics of forsterite fo reaction rims between periclase per and enstatite en were studied experimentally at pressure p and temperature t conditions of 3 0 11 1 gpa and 1473 1873 k respectively pt markers originally placed at the per en interface were always observed at the per fo interface which indicates that mg and o are the diffusing species in fo rim growth mg o coupled diffusion the presence of some en inclusions in fo grains and the growth rate of the fo rim suggests that grain boundary diffusion is dominant rather than lattice diffusion considering the very fast grain boundary diffusion of o in olivine the mg o coupled grain boundary diffusion in fo is deduced to be rate limited by the diffusivity of mg based on an analysis of data collected under dry conditions the product of the mg grain boundary diffusion coefficient d gb and the effective grain boundary width was determined to be d gb d gb 0 exp e pv rt with d gb 0 10 9 68 1 51 m 3 s e 379 44 kj mol and v 1 9 1 4 cm 3 mol our results combined with previously reported data on mg lattice diffusion in fo suggest that for mg the significance of grain boundary diffusion increases with depth in the earth s upper mantle although lattice diffusion is still dominant for typical mantle grain sizes of 1 10 mm

rheological contrast between garnet and clinopyroxene in the mantle wedge an example from higashi akaishi peridotite mass sw japan abstract garnet clinopyroxenites occur within foliated dunite in the higashi akaishi peridotite mass located within the subduction type high pressure low temperature sanbagawa metamorphic belt the garnet clinopyroxenites contain 3 80 garnet and garnet and clinopyroxene are homogeneously distributed garnet crystals contain extensive regular dislocation arrays and dislocation networks suggesting that dislocation creep was the dominant deformation mechanism analyses of crystallographic orientation maps indicate similar grain sizes and aspect ratios for garnet and clinopyroxene regardless of modal composition indicating that these minerals deformed with similar degree of plasticity moreover indexes of crystallographic fabric intensity i e j index and m index for both garnet and clinopyroxene tend to increase with increasing modal composition of garnet fourier transform infrared spectroscopy analysis revealed that water content in garnet is 60 ppm whereas that in clinopyroxene is 70 ppm olivine crystal preferred orientations in the higashi akaishi peridotite mass characterized by 0 0 1 0 1 0 are thought to have developed during deformation under wet conditions consequently we argue that the presence of water could act to enhance garnet plasticity during deformation the results reveal contrasting influences of water on the deformation of garnet and diopside under wet conditions compared with dry the strain rate increases by two orders of magnitude for garnet but by an order of magnitude for diopside given the influence of water on the creep strength of garnet garnet within the higashi akaishi mass may have become significantly as weak as clinopyroxene during deformation

modeling the effect of freshwater pulses on the early holocene climate the influence of high frequency climate variability freshwater 4 67 10 14 m 3 into the labrador sea at three different constant rates 1 5 sv 1 sv 1 10 6 m 3 s 1 i n 10 years 0 75 sv in 20 years and 0 3 sv in 50 years for each rate five ensemble experiments have been performed varying in initial conditions the freshwater pulses produce a weakening of the thermohaline circulation the perturbed state is in agreement with proxy evidence for the 8 2 ka event two types of recovery of the thermohaline circulation occurred differing in time scale 1 200 years and 2 200 years in the experiments with 10 year and 20 year pulses both types of recovery were observed this suggests that the model response is unpredictable in the range of parameters studied here it is hypothesized that the unpredictability is associated with annual to decadal climate variability our results demonstrate that several types of recovery may exist with the same kind of perturbation the interpretation of events observed in proxy data may be thus more complex than realized until now since the magnitude and duration of climatic events caused by freshwater pulses is likely to depend strongly on nonlinear dynamics inside the coupled atmosphere sea ice ocean system index terms 3344 meteorology and atmospheric dynamics paleoclimatology 4255 oceanography general numerical modeling 4532 oceanography physical general circulation 4215 oceanography general climate and interannual variability 3309 9325 information related to geographic region atlantic ocean keywords holocene thermohaline circulation predictability meltwater coupled climate model

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

strain localization in olivine aggregates at high temperature a laboratory comparison of constant strain rate and constant stress boundary conditions we performed high strain torsion experiments on aggregates of fo50 olivine to test the influence of imposed boundary conditions on localizing deformation we deformed both solid and thin walled cylinders of fo50 either at constant strain rate or at constant stress samples deformed in constant strain rate experiments reached a peak stress followed by weakening at a continually decreasing weakening rate in contrast samples deformed in constant stress experiments weakened at an accelerating weakening rate localization is manifested in samples deformed at constant stress as irregularities along strain markers s c foliations and torsional buckling of thin walled cylinders in contrast samples deformed at constant strain rate deformed homogeneously grain boundary maps created with electron backscatter diffraction data indicate that high strain regions in constant stress samples correlate with finer grain sizes and stronger crystallographic fabrics since the dominant deformation mechanism is grain size sensitive heterogeneous recrystallization leads to strain localization in finer grained regions however variations in strength are not large enough to initiate localization in constant strain rate experiments the magnitude of grain size heterogeneity remains relatively constant with increasing strain implying that shear zones are maintained throughout the experiments even as non localizing regions recrystallize based on our results we propose that deformation driven at constant stress in earth s lithosphere will easily localize even if structural heterogeneities are not initially present

interpretation of sks waves using samples from the subcontinental lithosphere abstract the seismic properties of five south african kimberlite nodules have been calculated from petrofabric measurements these garnet lherzolite and harzburgite composition nodules 56 80 olivine are known from previous studies to have originated at depths of 120 170 km in the subcontinental lithosphere their in situ seismic properties have been calculated by extrapolating the elastic constants to the appropriate temperature 900 1050 c and pressure 3 0 3 5 gpa conditions the average of the five samples has a maximum s wave anisotropy v s of 3 7 which is sufficiently high to explain previously reported teleseismic sks delay times t in continental shield areas of up to 1 7 s the biggest delay time is observed when the foliation is vertical the lineation horizontal and the fastest s wave polarized parallel to the lineation i e an orientation for transcurrent motion delay times of over 1 s can only be generated by this orientation a strong mantle crust mechanical coupling is suggested for such situations the potential use of seismic anisotropy as an indicator of strain in the lithosphere has been investigated using experimental and simulated fabric data for olivine the most abundant phase 70 in the upper mantle the v p and v s seismic anisotropy increases with fabric strength and finite strain recrystallization tends to reduce fabric strength and seismic anisotropy resulting in saturation values for experimental and simulated fabrics which correspond to approximately 50 60 strain or 8 9 v s for pure olivine aggregates a survey of 25 naturally deformed peridotites of oceanic origin suggests an average maximum s wave anisotropy for the olivine component of 9 or about 8 for a lherzolite or harzburgite rock composition when the orthopyroxene component is taken into account the ophiolite samples are twice as anisotropic as the kimberlite nodules if an average anisotropy value is representative of the lithosphere then the sks delay times represent variations in anisotropic layer thickness for delay times over 1 s as the orientation is constrained to be foliation vertical and lineation horizontal the magnitude of s wave anisotropy is less sensitive than v p to variations in olivine volume fraction in range 50 100 and to deformation or fabric intensity further suggesting that s wave anisotropy is particularly apt for the determination of the anisotropic lithosphere thickness for smaller delay times there is some trade off between structural orientation and thickness

effect of water and stress on the lattice preferred orientation of olivine the influence of water and stress on the lattice preferred orientation lpo of olivine aggregates was investigated through large strain shear deformation experiments at high pressures and temperatures p 0 5 2 1 gpa t 1470 1570 k under both waterpoor and water rich conditions the specimens are hot pressed synthetic olivine aggregates or single crystals of olivine water was supplied to the sample by decomposition of a mixture of talc and brucite deformation experiments were conducted up to shear strain 6 using the griggs apparatus where water fugacity was up to 13 gpa at the pressure of 2 gpa the water content in olivine saturated with water increases with increasing pressure and the solubility of water in olivine at p 0 5 2 gpa was 400 1200 ppm h si several new types of lpo in olivine are found depending on water content and stress samples deformed in waterpoor conditions show a conventional lpo of olivine where the olivine 100 axis is subparallel to the shear direction the 010 plane subparallel to the shear plane type a however we identified three new types type b c and e of lpo of olivine depending on the water content and stress the type b lpo of olivine which was found at relatively high stress and or under moderate to high water content conditions is characterized by the olivine 001 axis subparallel to the shear direction the 010 plane subparallel to the shear plane the type c lpo which was found at low stress and under water rich conditions is characterized by the olivine 001 axis subparallel to the shear direction the 100 plane subparallel to the shear plane the type e lpo which was found under low stress and moderate water content is characterized by the olivine 100 axis subparallel to the shear direction the 001 plane subparallel to the shear plane observations by transmission electron microscopy tem and scanning electron microscopy sem show that the dislocations in water poor samples type a are curved and both b 100 and b 001 dislocations have a similar population numerous subgrains are seen in water poor samples in backscattered electron images in contrast water rich samples both type b and type c contain mostly b 001 dislocations and dislocations are straight and sub grain boundaries are rare compared to those in water poor samples these observations suggest that 1 dominant slip systems in olivine change with water fugacity and stress and 2 grain boundary migration is enhanced in the presence of water seismic anisotropy corresponding to the fabrics under water rich condition is significantly different from that under water poor condition consequently the relationship between seismic anisotropy and flow geometry in water rich regions is expected to be

deformation of olivine orthopyroxene aggregates at high pressure and temperature implications for the seismic properties of the asthenosphere abstract the effect of pressure temperature and composition on the development of crystal preferred orientations cpo and seismic properties of olivine orthopyroxene aggregates were investigated using samples containing olivine and 12 5 25 and 50 vol of orthopyroxene the samples were deformed in simple shear at a constant strain rate of 10 4 s 1 with total shear strains between 0 5 and 1 3 at pressures of 3 5 and 8 gpa and temperatures of 1300 1400 and 1500 c respectively olivine s cpo vary as a function of the orthopyroxene content all samples have their olivine 010 axes normal to the foliation samples with 12 5 and 25 orthopyroxene have their 001 axes parallel to the lineation b type whereas the samples with 50 orthopyroxene have their 100 axes oriented parallel to the lineation a type at 3 gpa we propose that olivine cpo may result from a variation between two types of diffusion accommodated grain boundary sliding difgbs mechanisms at higher pressure the relative contribution of difgbs and dislocation related mechanisms depends on the volume of secondary phases for low orthopyroxene contents dislocation related mechanisms prevail and induce the development of b type cpo whereas for higher amount of orthopyroxene difgbs controls the deformation and leads to a type cpo orthopyroxene s cpo strength increases with increasing pressure and temperature and is characterized by the concentration of 100 and 010 axes normal to the foliation and 001 close to the lineation the seismic properties show that deformation in pyroxene poor and rich peridotites are consistent with the seismic anisotropy observed in intraplate regions where the mantle flow is horizontal conversely only pyroxene rich peridotites deformed through difgbs could explain the vs h vs v

change of olivine a axis alignment induced by water origin of seismic anisotropy in subduction zones abstract the effects of water on the crystallographic preferred orientation cpo of olivine aggregates were investigated through simple shear deformation experiments under asthenospheric upper mantle conditions pressure 2 1 5 2 gpa temperature 1490 1830 k using a deformation dia apparatus formation of the a type olivine fabric developed by the 010 100 slip system was observed under water depleted conditions coh 1000 ppm h si in comparison with fabrics of anhydrous olivine 111 ppm h si those of olivine having higher water contents 625 ppm h si in olivine showed stronger anisotropic properties e g p wave anisotropy s wave polarization anisotropy and the ratio of horizontally and vertically polarized shear waves the water induced olivine cpo transition from a type to b type like fabric accompanies a change in the alignment of the seismic fast a axes resulting in flow parallel and flow perpendicular shear wave splitting under water depleted and water rich conditions respectively the rotation of the fast direction of shear wave splitting across an arc which is observed in many subduction zones is well explained by the likely bimodal water distribution along the mantle wedge i e water rich in fore arc and water depleted in back arc regions and the developments of two different types of olivine fabrics i e b type like fabric in fore arc and a type fabric in back arc regions

surface and deep ocean interactions during the cold climate event 8200 years ago evidence from a north atlantic deep sea sediment core reveals that the largest climatic perturbation in our present interglacial the 8200 year event is marked by two distinct cooling events in the subpolar north atlantic at 8490 and 8290 years ago an associated reduction in deep flow speed provides evidence of a significant change to a major downwelling limb of the atlantic meridional overturning circulation the existence of a distinct surface freshening signal during these events strongly suggests that the sequenced surface and deep ocean changes were forced by pulsed meltwater outbursts from a multistep final drainage of the proglacial lakes associated with the decaying laurentide ice sheet margin

d rex a program for calculation of seismic anisotropy due to crystal lattice preferred orientation in the convective upper mantle summary models of development of lattice preferred orientation lpo of crystals aggregates in convec tive flow are necessary to interpret the anisotropic seismic signature of the earth s upper mantle for that purpose we previously developed a model of lpo evolution in olivine aggregates by plastic deformation and dynamic recrystallization by subgrain rotation and grain boundary migration this paper presents a refined version of that model called d rex for dynamic recrystallization induced lpo a public version of which is made available on our web site the code displays two new features 1 enstatite is incorporated in the aggregates and 2 grain boundary sliding gbs of small grains is taken into account enstatite is incorporated on the assumption of no direct interaction with olivine the fast a axis of enstatite grains tend to be parallel to the slow c axis of olivine which dilutes the total anisotropy grain boundary sliding is included using a threshold dimensionless volume fraction defined as the ratio of the initial size of the grains over the size for which gbs is the dominant mechanism of deformation grains with a dimensionless volume smaller than do not rotate by plastic deformation and their strain energy is set to zero comparison with torsion experiments at very large strain constrains the threshold dimensionless volume to 0 3 0 1 the incorporation of grain boundary sliding prevents the lpo from becoming singular at large strains and yields more realistic predictions our kinematic formalism and the model s semi analytical character insures that it is fast robust and stable it can be applied efficiently to arbitrary 3 d convective flows

in situ observation of crystallographic preferred orientation of deforming olivine at high pressure and high temperature abstract simple shear deformation experiments on polycrystalline olivine and olivine single crystal were conducted at pressures of 1 3 3 8 gpa and temperatures of 1223 1573 k to understand the achievement of steady state fabric strength and the process of dynamic recrystallization development of crystallographic preferred orientation cpo of olivine was evaluated from two dimensional x ray diffraction patterns and shear strain was measured from x ray radiographs the steady state fabric strength of the a type fabric was achieved within total shear strain of 2 at strains higher than 1 an increase in concentration of the 0 1 0 axes mainly contributes to an increase in fabric strength at strains higher than 2 the magnitude of v sh v sv i e ratio of horizontally and vertically polarized shear wave velocities scarcely increased in most of the runs the v sh v sv of peridotite 70 vol olivine 30 vol minor phases having the steady state a type olivine fabric coincides with that of recent global one dimensional models under the assumption of horizontal flow suggesting that the seismic anisotropy observed in the shallow upper mantle is mostly explained by the development of a type olivine fabric experimental results on the deformation of single crystal olivine showed that the cpo of olivine is influenced by the initial orientation of the starting single crystal because strain is concentrated in the recrystallized areas and the relic of the starting single crystal remains in the upper mantle the old cpo of olivine developed in the past may affect the olivine cpo developed in the present

the effect of dynamic recrystallization on olivine fabric and seismic anisotropy insight from a ductile shear zone oman ophiolite abstract subhorizontal mantle structures subparallel to the moho are rotated into nw se subvertical orientations across a shear zone in a sinistral sense of shear within the northern fizh mantle section of the oman ophiolite dynamic recrystallization resulted in grain size reduction of olivine and the development of porphyroclastic texture mean olivine grain size stabilized at 0 7 mm within the shear zone center this may reflect the steady state grain size of dynamically recrystallized olivine as determined by the deviatoric stress which in this case was as low as 10 mpa crystal preferred orientation cpo patterns of olivine are consistently 100 fiber or partial fiber texture indicating that olivine slip systems did not change during shearing dynamic recrystallization causes a weakening of olivine fabric intensity toward the shear zone center but this weakening is counterbalanced by cpo strengthening due to dislocation glide this process resulted in an abrupt decrease in seismic anisotropy at the center of the shear zone in contrast to a gradual decrease in olivine fabric intensity and mean grain size the measured seismic anisotropy patterns did not change in ways that would be significantly measurable by seismological observations despite the development of the shear zone dispersion of both p and s waves in the shear zone may be of little effect with respect to the overall seismic anisotropy this is not only because the shear zone occurs substantially in a narrow region but also because the seismic anisotropy is weaker in the shear zone than the high t structure region it suggests that a record of simple systematic seismic anisotropy observed in the upper mantle may indicate a simplified mantle flow structure as localized structures may be obscured in the region of the observation

probability of radial anisotropy in the deep mantle article i nfo it is well established that the earth s uppermost mantle is anisotropic but observations of anisotropy in the deeper mantle have been more ambiguous radial anisotropy the discrepancy between love and rayleigh waves was included in the top 220 km of prem but there is no consensus whether anisotropy is present below that depth fundamental mode surface waves for commonly used periods up to 200 s are sensitive to structure in the first few hundred kilometers and therefore do not provide information on anisotropy below higher mode surface waves however have sensitivities that extend to and below the transition zone and should thus give insight about anisotropy at greater depths but they are very difficult to measure we previously developed a new technique to measure higher mode surface wave phase velocities with consistent uncertainties these data are used here to construct probability density functions of a radially anisotropic earth model down to approximately 1500 km in the uppermost mantle we obtain a high probability of faster horizontally polarized shear wave speed likely to be related to plate motion in the asthenosphere and transition zone however we find a high probability of faster vertically polarized shear wave speed to a depth of 1500 km in the lower mantle we see no significant shear wave anisotropy this is consistent with results from laboratory measurements which show that lower mantle minerals are anisotropic but lpo is unlikely to develop in the pressure temperature conditions present in the mid mantle 2008 elsevier b v all rights reserved

mantle dynamics inferred from the crystallographic preferred orientation of bridgmanite seismic shear wave anisotropy is observed in earth s uppermost lower mantle around several subducted slabs the anisotropy caused by the deformation induced crystallographic preferred orientation cpo of bridgmanite perovskite structured mg fe sio3 is the most plausible explanation for these seismic observations however the rheological properties of bridgmanite are largely unknown uniaxial deformation experiments have been carried out to determine the deformation texture of bridgmanite but the dominant slip system the slip direction and plane has not been determined here we report the cpo pattern and dominant slip system of bridgmanite under conditions that correspond to the uppermost lower mantle 25 gigapascals and 1 873 kelvin obtained through simple shear deformation experiments using the kawai type deformation dia apparatus the fabrics obtained are characterized by 100 perpendicular to the shear plane and 001 parallel to the shear direction implying that the dominant slip system of bridgmanite is 001 100 the observed seismic shear wave anisotropies near several subducted slabs tonga kermadec kurile peru and java can be explained in terms of the cpo of bridgmanite as induced by mantle flow parallel to the direction of subduction

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

the effect of deformation history on the evolution of olivine cpo abstract olivine crystallographic preferred orientation cpo is the primary cause of seismic anisotropy in the upper mantle in tectonic environments with complex flow patterns for example corner flow near mid ocean ridges or subducting slabs the interpretation of seismic anisotropy may be complicated by evolving thermochemical deformation conditions and the integrated deformation history to understand how deformation history influences cpo evolution deformation experiments were conducted on samples of aheim dunite which has a strong pre existing texture experiments were performed in a triaxial geometry using a griggs apparatus at p 1 gpa t 1473 k up to a maximum strain of 0 7 to simulate different deformation histories samples were deformed in three different configurations with the pre existing foliation perpendicular oblique and parallel to the shortening axis of deformation distinct patterns of cpo development are observed for each experimental configuration likewise texture strength symmetry and orientation evolved differently in each set of experiments these data are interpreted as evidence that cpo did not reach steady state and that achieving steady state texture requires larger strains than previously thought it is concluded that the integrated deformation history plays a significant role in cpo evolution and the consequent interpretation of seismic anisotropy in earth s mantle

simple tidal prism models revisited simple tidal prism models for well mixed estuaries have been in use for some time and are discussed in most text books on estuaries the appeal of this model is its simplicity however there are several flaws in the logic behind the model these flaws are pointed out and a more theoretically correct simple tidal prism model is derived in doing so it is made clear which effects can in theory be neglected and which can not

a minimum thermodynamic model for the bipolar seesaw 1 the simplest possible model is proposed to explain a large fraction of the millennial climate variability measured in the isotopic composition of antarctic ice cores the model results from the classic bipolar seesaw by coupling it to a heat reservoir in this thermal bipolar seesaw the heat reservoir convolves northern time signals with a characteristic timescale applying the model to the data of grip and byrd we demonstrate that maximum correlation can be obtained using a timescale of about 1000 1500 years higher correlations are obtained by first filtering out the long term variability which is due to astronomical and greenhouse gas forcing and not part of the thermal bipolar seesaw the model resolves the apparent confusion whether northern and southern climate records are in or out of phase synchronous or time lagged

new type of olivine fabric from deformation experiments at modest water content and low stress a new type of olivine fabric was found by high strain simple shear deformation experiments in the presence of trace amounts of water at 0 5 2 2 gpa and 1470 1570 k in this fabric called e type fabric the olivine 100 axis is subparallel to the shear direction and the 001 plane is parallel to the shear plane this geometry suggests that the 100 001 slip system makes the dominant contribution to total strain this fabric is dominant at a modest water content 200 coh 1000 h 10 6 si at low stresses and high temperatures some mylonites from peridotite massifs show this type of olivine fabric which suggests the presence of water during the shear localization the seismic anisotropy caused by this fabric is qualitatively similar to that by dry fabric a type but the magnitudes of anisotropy are different between the two types for horizontal flow the amplitude of vsh vsv anisotropy is weaker but the amplitude of shear wave splitting is stronger for the e type fabric than for the a type dry fabric seismic anisotropy in the oceanic upper mantle may be due to the olivine e type fabric

a scanning electron microscope study of the effects of dynamic recrystallization on lattice preferred orientation in olivine effects of dynamic recrystallization on lattice preferred orientation lpo in olivine were investigated through the combination of two sem based techniques electron backscattered diffraction ebsd technique for crystallographic orientation measurement and backscattered electron imaging bei for dislocation observation samples are experimentally deformed olivine aggregates in simple shear geometry in the sample deformed at t 1473 k and high stresses 480 mpa only incipient dynamic recrystallization is observed along grain boundaries orientations of these small recrystallized grains are more random than that of relict grains suggesting an important role of grain boundary sliding at this stage of recrystallization in the sample deformed at t 1573 k and low stress 160 mpa dynamic recrystallization is nearly complete and the lpo is characterized by two 100 peaks one peak is located at the orientation subparallel to the shear direction and is dominated by grains with high schmid factor the other occurs at high angles to the shear direction and is due to the contribution from grains with low schmid factor grains with high schmid factor tend to have higher dislocation densities than those with low schmid factor based on these observations we identify two mechanisms by which dynamic recrystallization affects lpo 1 enhancement of grain boundary sliding due to grain size reduction leading to the modification of lpo caused by the relaxation of constraint for deformation 2 grain boundary migration by which grains with lower dislocation densities grow at the expense of grains with higher dislocation densities based on the deformation mechanism maps and stress versus recrystallized grain size relation we suggest that the first mechanism always plays an important role whereas the second mechanism has an important effect only under limited conditions

global mantle flow and the development of seismic anisotropy differences between the oceanic and continental upper mantle 1 viscous shear in the asthenosphere accommodates relative motion between earth s surface plates and underlying mantle generating lattice preferred orientation lpo in olivine aggregates and a seismically anisotropic fabric because this fabric develops with the evolving mantle flow field observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small we use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and lpo and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements for asthenosphere 500 km from plate boundaries simple shear rotates the lpo toward the infinite strain axis isa the lpo after infinite deformation faster than the isa changes along flow lines thus we expect the isa to approximate lpo throughout most of the asthenosphere greatly simplifying lpo predictions because strain integration along flow lines is unnecessary approximating lpo with the isa and assuming a type fabric olivine a axis parallel to isa we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy average misfit 13 continental anisotropy is less well fit average misfit 41 but lateral variations in lithospheric thickness improve the fit in some continental areas this suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents the contribution of the oceanic lithosphere is likely smaller because it is thinner younger and less deformed than its continental counterpart

on the lehmann discontinuity the lehmann discontinuity 200km in the upper mantle is proposed to result from a change in the preferred orientation of olivine as a result of a change in deformation mechanism in the shallow upper mantle deformation is due to dislocation creep and results in an anisotropic structure whereas in the deeper upper mantle diffusion creep becomes dominant due to a smaller activation volume resulting in an isotropic structure for anisotropy due to dislocation glide corresponding to a horizontal flow the resultant anisotropy will yield vsh vsv and vph vpv thus for waves propagating nearly vertically e g scs waves there will be discontinuous jumps in the s and p wave velocities at this discontinuity in this model the depth of the lehmann discontinuity depends on the deformation conditions such as stress level geotherm water fugacity and grain size giving a natural explanation for its observed variation with tectonic setting

in situ stress strain measurements in a deformation dia apparatus at p t conditions of the upper part of the mantle transition zone we report on technical improvements in experiments with a deformation dia d dia apparatus which enable the study of the rheology of solid materials at p t conditions of the earth s mantle transition zone dimensions of the anvil truncation pressure medium and gasket were optimized to achieve deformation experiments above 13 gpa with a relatively low press load 0 7 mn to minimize the damage of the x ray transparent second stage anvils the adoption of low x ray absorbing material e g cubic bn anvils graphite window in a lacro3 heater along the x ray path enabled quantitative determination of stress and strain of a sample by means of simultaneous in situ x ray radial diffraction and radiography using synchrotron radiation at spring 8 based on the new technique a uniaxial deformation experiment with a strain rate of 3 88 10 5 s 1 and strains up to 25 5 was carried out on wadsleyite at a pressure of 14 5 gpa and a temperature of 1700 k

protracted fabric evolution in olivine implications for the relationship among strain crystallographic fabric and seismic anisotropy abstract crystallographic fabrics in olivine rich rocks provide critical information on conditions and mechanisms of deformation as well as seismic properties of earth s upper mantle previous interpretations of fabrics produced in laboratory experiments were complicated by uncertainty as to whether the steady state fabric was attained to examine the systematics of the evolution of olivine crystallographic fabrics at high strain we conducted torsion experiments on olivine aggregates to shear strains of up to 20 our results demonstrate that a steady state fabric is not reached until a shear strain 10 a much higher value than previously thought necessary fabrics characterized by girdles of 010 and 001 axes or by clusters of 010 and 001 axes are both observed until now these fabrics were associated with either two different deformation mechanisms or two different sets of deformation conditions here we establish that both fabrics are in fact part of the same evolutionary process an eigenvalue analysis allows the fabric shape to be quantitatively correlated with the magnitude of shear strain misorientation analysis suggests that the observed fabric evolution results from the competition of the two easiest slip systems in olivine 010 100 and 001 100 our results open up the possibility of using olivine crystallographic fabrics or seismic anisotropy to quantitatively evaluate strain histories in both field studies and geophysical investigations of upper mantle rocks

the effect of dynamic recrystallization on olivine crystal preferred orientations in mantle xenoliths deformed under varied stress conditions abstract spinel peridotite xenoliths from persani mountains southeastern carpathians romania were submitted to deformation by dislocation creep accompanied by dynamic recrystallization under variable stress and temperature conditions predominance of low angle boundaries parallel to 100 with well defined 0vw rotation axes in olivine indicates that subgrain rotation is the main recrystallization mechanism and that dislocation glide occurs mainly in 100 0kl systems analysis of olivine crystal preferred orientations highlights that recrystallization results in dispersion of the orientations of recrystallized grains relatively to the parent grains this dispersion may be quantified by a dimensionless dispersion factor defined as the ratio of the random component of the 100 axis distributions between the recrystallized grains and porphyroclasts this factor is largely independent from the overall fabric strength the number of grains analyzed and the recrystallized grain size the olivine cpo strength and anisotropy at the rock scale quantified by j index are largely controlled by the volume of recrystallized grains which depends on finite strain comparison of olivine cpo in coarse and fine grained porphyroclastic peridotites shows that deviatoric stress and temperature conditions play nevertheless a role on the cpo evolution during recrystallization selective grain growth which is favored at high temperature and low stresses does counteract the dispersion produced by the nucleation processes

pressure induced slip system transition in forsterite single crystal rheological properties at mantle pressure and temperature deformation experiments were carried out in a deformation dia high pressure apparatus d dia on oriented mg2sio4 olivine fo100 single crystals at pressure p ranging from 2 1 to 7 5 gpa in the temperature t range 1373 1677 k and in dry conditions these experiments were designed to investigate the effect of pressure on olivine dislocation slip system activities responsible for the lattice preferred orientations observed in the upper mantle two compression directions were tested promoting either 100 slip alone or 001 slip alone in 010 crystallographic plane constant applied stress sigma and specimen strain rates formula were monitored in situ using time resolved x ray synchrotron diffraction and radiography respectively transmission electron microscopy tem investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation a slip transition with increasing pressure from a dominant 100 slip to a dominant 001 slip is documented extrapolation of the obtained rheological laws to upper mantle p t and sigma conditions suggests that 001 slip activity becomes comparable to 100 slip activity in the deep upper mantle while 001 slip is mostly dominant in subduction zones these results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle and for the puzzling seismic anisotropy anomaliesmore commonly observed in subduction zones less

high pressure and temperature fabric transitions in olivine and variations in upper mantle seismic anisotropy the effects of pressure on crystallographic preferred orientation cpo of olivine aggregates were investigated through simple shear deformation experiments at pressures between 2 1 and 7 6 gpa and temperatures of 1493 1673 k under dry conditions using a deformation dia apparatus and the variations in seismic anisotropy were evaluated under the earth s upper mantle conditions we found that the monotonic decrease in seismic anisotropy with depth is caused by the pressure dependency of the seismic properties of a type developed by the 010 100 100 slip system olivine fabric while the rapid decrease is caused by the fabric transition from a type to b c type by the hk0 001 001 slip systems at 7 6 gpa and 1673 k moreover an alternative transition from a type fabric to b type like fabric by the 010 001 001 slip system occurs at 7 6 gpa and lower temperature these two temperature dependent fabric transitions occurring at 7 6 gpa result in low seismic anisotropy with vsh vsv the ratio of horizontally and vertically polarized shear waves 1 at low temperatures i e old continental mantle conditions and vsh vsv 1 at high temperatures i e oceanic mantle conditions at greater depths consistent with seismological observations thus the variations of cpo with pressure and temperature in olivine under dry conditions can explain the seismic anisotropy signatures observed in the upper mantle without invoking other mechanisms

the misorientation index development of a new method for calculating the strength of lattice preferred orientation abstract using orientation data from experimentally deformed olivine we explore some practical problems with the j index a commonly applied measure of fabric strength we show that the j index is highly dependent on several factors including the number of discrete data in the orientation distribution function odf and arbitrary numerical parameters specified for its calculation because of this non uniqueness we conclude that the j index is difficult to interpret and should only be applied with caution as an alternative to the j index we propose a new measure of fabric strength that is based on the distribution of uncorrelated misorientation angles this m index is shown to be insensitive to the parameters specified for its calculation for typical deformed olivine samples we show that 150 discrete data are adequate to quantify fabric strength using the m index technique the m index correlates well with seismic anisotropy particularly for materials of the same fabric type therefore we conclude that the m index technique is well suited for the quantification of fabric strength and the comparison of like materials

development of a type olivine fabric in water rich deep upper mantle abstract water controls the activity of slip systems in olivine resulting in various types of olivine crystallographic preferred orientation i e fabric in mantle rocks the a type olivine fabric is the most commonly observed olivine fabric in natural peridotites development of a type olivine fabric developed by the 010 100 slip system is known to be limited to the water poor conditions of the shallow upper mantle 2130 ppm h si in olivine while b type fabric by the 010 001 slip system was observed under moderately wet conditions 750 2130 ppm h si developments of c type by the 100 001 slip system fabric was limited to water poor conditions v sh v sv the ratio of horizontally and vertically polarized shear waves with depth in the global one dimensional models is well explained by the olivine fabrics developed in the horizontal flow of a water poor mantle only a type olivine fabric can explain the vertical mantle flow which associates the seismic anisotropy of v sh v sv 200 km depth a strong anomaly of v sh v sv

in situ observation of texture development in olivine ringwoodite magnesiowustite and silicate perovskite at high pressure magnesium silicates are the dominant minerals in the earth s mantle their preferred orientation is important for understanding the rheology and seismic anisotropy in the deep earth here we report results of radial synchrotron diffraction diamond anvil cell dac experiments on san carlos olivine axially compressed to 50 gpa experiments were performed at room temperature except for brief laser heating to induce phase transformations high stresses and development of preferred orientation were observed in diffraction images quantitative texture information was obtained by analyzing the images with the rietveld method with increasing pressure between 9 and 43 gpa olivine develops a texture with 001 axes perpendicular to the compression direction that is compatible with hk0 001 pencil glide ringwoodite between 20 and 50 gpa develops weak preferred orientation with 011 lattice planes perpendicular to the compression direction after the phase transformation perovskite and magnesiowqstite display transformation textures that are then modified by continuing deformation magnesiowqstite has a weak h111i maximum parallel to the compression direction that changes towards h001i with increasing deformation perovskite transforming from olivine has a pronounced 100 transformation texture and with increasing deformation a 012 maximum develops the transformation texture is probably produced by mechanical 110 twinning and nucleation in orientations that minimize elastic strain energy the deformation texture of perovskite is due to slip d 2004 elsevier b v all rights reserved

characterisation of deformation and flow mechanics around porphyroclasts in a calcite marble ultramylonite by means of ebsd analysis abstract this paper studies the flow heterogeneity around porphyroclasts associated with greenschist facies deformation of a calcite marble shear zone microstructural data from electron backscatter diffraction analyses ebsd are used to constrain the flow mechanics of this dominantly non coaxial type of deformation the microstructure of the undisturbed ultramylonite grain size range 5 100 m mean 40 m is interpreted to represent steady state time independent flow conditions with flow planes parallel to the shear zone boundary single calcite porphyroclasts grain size 1 3 mm caused flow perturbation in the fine grained marble ultramylonite it is the shape in particular of these rigid porphyroclasts that controls their rotational behaviour during deformation and therefore the development of specific flow fabrics the flow planes around elongated rhomboidal stable porphyroclasts change the orientation to become roughly parallel to the porphyroclast margin whereas the geometry of flow planes around nearly equant rotating porphyroclasts describes a type flow pattern we infer that to some extent decoupling at the clast matrix interface has occurred to guarantee a stable orientation of elongated porphyroclasts but was not sufficient to reduce the rotation rate of equant clasts to zero according to the flow deflection the general crystallographic preferred orientation cpo with its single c axis maximum perpendicular to the flow plane is rotated about an axis which is sub parallel to the kinematic rotation axis of the shear zone ultramylonite microstructures cpos and misorientation data are best explained by the dual operation of grain size insensitive dislocation creep with recovery and recrystallization and grain size sensitive diffusion creep mechanisms the limited grain size reduction around porphyroclasts suggests that the grain size insensitive mechanisms controlled rheology