mechanism of the olivine to spinel phase transformation in ni2sio4 an electron microscope study of the olivine to spinel transformation in ni2sio4 has revealed the nature of the defect structures in the transforming olivine immediately ahead of the migrating olivine spinel interphase boundary the residual olivine has a high density of 100 and 001 burger s vector dislocations 1 to 5 109 cm 2 no stacking faults or dissociated dislocations are observed in the residual olivine although a significant number 108 cm 2 of 010 dislocations are activated by the extreme synthesis conditions 52 kbar 1100 c this result may have important applications in the study of paleo deformation of olivine the mechanism of the transformation in the fine ground sample appears to be a nucleation and growth process with no evidence to support a martensitic mechanism

a new self organizing mechanism for deep focus earthquakes the mechanism of deep focus earthquakes has been a puzzle since their discovery almost 70 years ago1 2 because brittle fracture and frictional sliding at depths in excess of 100 200 km would require unrealistic rock strengths3 4 rock strength does increase with pressure but a few hundred mpa equivalent to 10 20 km depth suffices to inhibit most fracture and elevated temperature activates ductile mechanisms that operate at stresses less than the fracture stength a range of mechanisms has been proposed for deep earthquakes including plastic instabilities5 7 shear induced melting8 11 and instabilities accompanying recrystallization12 13 or polymorphic phase transformation14 23 each of these proposed mechanisms has exhibited certain inherent weaknesses see kirby22 for review and discussion here we report experimental observations of high pressure faulting of metastable mg2geo4 olivine as it undergoes incipient transformation to a spinel 24 25 structure we present a model in which this faulting arises from the generation propagation and linking up of spinel filled anticracks26 when applied to the olivine spinel transformation in the earth s mantle the anticrack model27 satisfactorily accounts for the similarities and differences between shallow and deep earthquakes and removes the problems associated with frictional sliding

the effect of hot spots on the oceanic age depth relation hot spots located both midplate and on midocean ridges can bias the empirical age depth relation ocean floor bathymetry for seafloor greater than 80 m y b p in age has been previously used to determine if the oceanic lithosphere behaves as a cooling half space or as a cooling plate however large areas of the ocean basins do not fit a simple age depth relation anomalously shallow swells approximately 1000 km wide and 1 km high surround hot spots such as iceland the azores and the cape verde islands by sorting seafloor depth data by distance to the nearest hot spot track we show that the along isochron variation in depth in the north atlantic correlates with distance from a hot spot track due to the absence in the north atlantic of seafloor both old and far from hot spots we compare depth versus age at constant distance and do not observe flattening of the age depth relation in any distance range the age depth relation depth meters 2700 295 t m y 1 2 derived for seafloor 1200 to 1800 km from a hot spot track represents our best estimate of the age depth relation appropriate to seafloor unaffected by a hot spot track in the north atlantic the previously inferred flattening of the age depth relation appears to be caused by the inclusion of hot spot swells this implies that hot spots may be the primary means of heat transfer from the deeper mantle to the base of the lithosphere

depth and age in the north pacific the concept of the oceanic lithosphere as a cooling thermal boundary layer on top of a viscous upper mantle accounts for the increase in depth with age of the ocean floor analyses of the exact nature of this relation provide constraints on the rheology and structure of this layer in past studies problems with uncertainties of age and in sediment thickness have limited the value of this relation on older ocean floor in this study we evaluate different tectonic histories for the quiet zones and construct a revised isochron chart for the north pacific we check and modify the current sediment thickness charts using new deep sea drilling data and revised ideas concerning reflectors in the southwestern north pacific averaging the synbaps digital data base into half by half degree elements and correcting for sediment load we plot basement depth versus age for the entire north pacific because of the skewness in the data associated with seamounts we present the data as contours about the mode and find that two thirds of the observations lie within 300 m of this value on crust between 0 and 80 ma the ocean floor subsides with the square root of age on ocean floor older than 100 ma the depths in general flatten with age with areally correlateable swells and valleys the hawaiian and line island swells correlate positively and the mariana basin valley negatively with geoid height the flattening in the depths indicates that the thermal boundary layer has reached equilibrium conditions under older ocean floor further the correlation of the swells and valleys with geoid height presents evidence for organized convection in the upper mantle beneath older lithosphere in the north pacific

geoid anomalies over cooling lithosphere source for a third kernel of upper mantle thermal parameters and thus an inversion summary the geoid anomaly over an isostatic mass redistribution on an infinite flat earth is simply 27 c g times the differential mass moment on a spheroidal earth the limiting value can never quite be reached because the external effect of a spherical shell of mass dipole is identically zero so distant portions of the spherical surface subtract from the effect of the region under the point of observation the width of the integrating kernel varies with the depth of the mass distribution but the observable fraction of the limiting geoid slope can be estimated at least for ocean basins of moderate extent where it should reach about 90 per cent for very small 3 cm yr 1 ocean basins the observable geoid slope is smaller a consideration of published geoid data together with gravity anomalies in the same area compared to the theoretical free geoid gravity anomalies gravity observed on the geoid leads with great caution to a crude estimate of the thermally produced slope of 0 14 0 02 m myr 1 given a value of the thermal geoid signal it is possible to conduct an inversion for the upper mantle thermal parameters if two the density and specific heat are assumed to be known the parameter kernels for the heat flow topographic decay and geoid give three equations to solve for expansion coefficient diffusivity and initial temperature ti the results are different from common modelling assumptions 3 1 10 5 c 1 17 per cent 0 0065 cm2 s l 45 per cent and 1660 c 29 per cent manipulation of the equations in the case of general temperature variable parameters leads to partial explanations appears multiplied by k0 7 k0 conductivity at 0 7 of t1 at surface temperature while k is weighted toward the lower values in the low to medium temperature range an estimate of t1 using the shape of the temperature variations of k and k in the most reliable available data for olivine remains at about 1660 c 30 per cent in spite of the large error band the higher initial temperature is plausible because decompression of upwelling material produces enough partial melting to generate the oceanic crust the latent heat of melting must be supplied from somewhere and the only other possible source is a large amount of mechanical work applied to the system in the divergent flow

provenance of tin in the late bronze age balkans based on probabilistic and spatial analysis of sn isotopes abstract we report the largest published dataset to date of sn isotopic compositions of bronze age artifacts 338 along with 150 cassiterite samples 75 new from six potential tin ore sources from which the tin in these artifacts were thought to have likely originated the artifacts are from a broad area central europe through the central balkans and the six tin sources are cornwall three sites in the erzgebirge and two sites in serbia a clustering analysis on mean site level isotopic values of 124sn identifies regional variation that can be attributed to the use of different tin ore sources in different regions therefore geographically meaningful regions were identified to group the bronze age artifact assemblages and a probabilistic bayesian analysis was performed to determine the proportional contribution of each tin source to each regional assemblage artifacts enriched in heavy isotopes 124sn 0 7 that cluster in west central serbia are likely associated with the ores from mt cer in west serbia mixed artifact assemblages high and low 124sn in this region are attributed to the use of cassiterite from the two serbian sites mt cer and mt bukulja moderate composition artifacts that occur north of the middle danube in vojvodina transylvania and central europe are likely associated primarily with ores from the west pluton of the erzgebirge compositionally light bronzes 124sn

a novel approach to lead isotope provenance studies of tin and bronze applications to south african botswanan and romanian artifacts lead isotopic ratios of cassiterite the dominant ore of tin evolve after crystallization through decay of uranium u and thorium th to lead pb due to the relatively elevated u pb ratios of this mineral we show that the pb isotopic ratios of smelted tin at rooiberg south africa form an isochron with a model age that matches the known geological age 2 ga of the host granite for the rooiberg cassiterite deposits since the pb isotopic ratios of many prehistoric tin and bronze artifacts throughout southern africa also fall on this isochron we deduce that they were made with tin from either the rooiberg deposits or similar age deposits that exist nearby in addition we show that bronze artifacts from romania define an isochron corresponding to a variscan age 0 3 ga suggesting a central or western european tin deposit as its source since no variscan tin is known from the neighboring carpathian mountains implications of this approach for provenance studies of tin and bronzes around the world are examined given various major tin deposits and their age distribution

mechanisms of transformation and deformation in mg1 8fe0 2sio4 olivine and wadsleyite under non hydrostatic stress abstract we have studied the effect of non hydrostatic stress on the mechanisms of the olivine wadsleyite ringwoodite phase transformations and deformation mechanisms of olivine and wadsleyite at high pressure experiments were performed at 900 c in the stability field 15 gpa for 0 5 h and in the stability field 16 gpa for 11 h using a multianvil apparatus with san carlos olivine as the starting material a sample assembly designed to produce non hydrostatic stress was used the deformed samples have been characterised using optical and transmission electron microscopy remnant olivine contains high densities of mostly c dislocations and deformation occurs by dislocation glide involving the slip systems 010 001 and 100 001 in wadsleyite dislocations are in a climb configuration which suggests that self diffusion of si and or o is much faster in wadsleyite than in olivine at 900 c wadsleyite also contains 010 stacking faults which are interpreted to be growth defects that anneal out with time during the olivine wadsleyite transformation non hydrostatic stress results in anisotropic reaction textures wadsleyite nucleates preferentially on olivine grain boundaries that are oriented at a high angle to the direction of principal compressive stress and or the growth of wadsleyite occurs preferentially in this direction wadsleyite transforms to ringwoodite by two competing mechanisms i coherent intracrystalline nucleation on a dislocations probably where they intersect 010 stacking faults and ii partially coherent nucleation at wadsleyite grain boundaries non hydrostatic stress enhances transformation rates by increasing the density of dislocations which act as nucleation sites for ringwoodite although the samples were partially reacted under non hydrostatic stress there is no evidence for transformational faulting

tin isotope characterization of bronze artifacts of the central balkans abstract isotopic analysis has proved to be an effective approach to determine the provenance of copper ore sources for the production of bronze artifacts more recently methods for sn isotopic analysis of bronze have been developed however the viability of tin isotopes as a means to define groupings that may be attributed to varying ore sources production methods or recycling is still in question in part this is due to the numerically and or geographically limited nature of published datasets this study reports on the sn isotopic composition of 52 artifacts from the later bronze age 1500 1100 bce from serbia and western romania the majority of samples cluster between 0 4 and 0 8 per mil for 124 sn and 0 2 and 0 4 per mil for 120 sn relative to nist srm 3161a and this isotopic grouping of bronze artifacts occurs across serbia however groupings of isotopically heavier and lighter artifacts are evident and each corresponds to a more limited geographic range artifacts associated with higher sn values are limited to the vojvodina region of northern serbia whereas a cluster of bronzes with lower sn isotopic signatures are constrained to the banat along the serbia romania border and transylvania one low value outlier corresponds to an uncontextualized find near krusevac at the southern extent of the study area geographic correlation of the low value cluster with known tin mineralization in transylvania and the moderate value cluster with placer tin deposits of western serbia suggests that these distinct bronze sn isotopic signatures might reflect exploitation of different tin ores the small cluster of high sn isotopic values from bronzes from the vojvodina region might reflect bronze recycling in this area that lies furthest from both known tin ore sources

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

transformation mechanisms of san carlos olivine to mgfe 2sio4 β phase under subduction zone conditions abstract to understand the mechanisms of the high pressure transformations of mg0 9fe0 1 2sio4 under transition zone conditions we have experimentally reacted san carlos olivine fo90 at 1000 c and 13 gpa in a multianvil split sphere apparatus the reaction products and microstructures have been studied in detail by transmission electron microscopy under these disequilibrium conditions san carlos olivine transforms to phase and spinel with mg fe ratios which are not consistent with equilibrium compositions the transformation to both phases occurs by a nucleation and growth mechanism involving nucleation at olivine grain boundaries both phases contain characteristic stacking faults on 010 phase and 110 spinel and some spinel grains show extreme degrees of disorder although there is uncertainty in the exact stability field of the experiment the results are consistent with formation of metastable spinel probably within either the or stability fields metastable spinel progressively transforms back to phase by a shear like mechanism involving a highly disordered spinelloid phase which may be a transitional phase in the transformation based on these results and available kinetic data metastable formation of spinel in a subducting oceanic slab within the or stability fields is possible at temperatures below 1000 c

elasticity of natural majorite and ringwoodite from the catherwood meteorite sound velocities and elastic moduli of natural polycrystalline majorite mg0 78fe0 21ca0 01 sio3 and ringwoodite mg0 75fe0 25 2sio4 from the catherwood meteorite were measured by brillouin spectroscopy these are the first acoustic measurements for such natural high pressure phases with fe contents comparable to that of the earth s mantle the adiabatic bulk modulus of majorite is ks 164 4 gpa and the shear modulus is 87 2 gpa which are typical values for garnet structured silicates both the elastic moduli and the sound velocities of natural cubic majorite are slightly lower than values for aluminosilicate garnets with comparable fe contents moreover the bulk and shear moduli of natural majorite are indistinguishable from those of pure mg majorite and the dependence of these moduli on fe content is small in contrast ringwoodite ks 193 3 113 2 exhibits a pronounced change in the aggregate elastic moduli and sound velocities with increasing fe content the addition of 10 fe to phase changes the velocity by an amount comparable to the velocity change across the transition in mg2sio4 it is therefore necessary to account for the effect of fe in constructing mineralogical models of the transition zone

an olivine to beta phase transformation mechanism mg2sio4 fine grained powder samples of mg2sio4 forsterite phase have been partially transformed to the beta phase in a 2000 ton uniaxial split sphere apparatus ussa 2000 at p 15 gpa and t 1000 c in runs of 5 minutes duration this technique allowed us to catch the first steps of the transformation in mg2sio4 a tem study of the run products showed that a structurally disordered phase is an intermediate of the reaction the oxygen sublattice of this intermediate phase is well ordered but a very large density of planar faults on 010 planes destroys the long range order in the cation sublattices a two steps mechanism for the transformation is therefore probable involving first the nucleation of the structurally disordered phase then the ordering of this phase these two steps may be associated with low energy activation barriers and could thus explain the fast kinetics of the transformation

metastability of enstatite in deep subducting lithosphere olivine and mg fe sio3 pyroxene the most abundant minerals in the earth s upper mantle are believed to transform to high pressure phases at 400 km depth1 5 the possible metastable persistence of olivine to greater depths in some subduction zones with consequences for the origin of deep focus earthquakes and the dynamics of subduction has been discussed extensively6 14 but the role of other mantle minerals has not been considered we report here an experimental study of the kinetic behaviour of the magnesian pyroxene enstatite mgsio3 at upper mantle conditions we find that whereas forsterite mg olivine mg2sio4 transforms rapidly to phase at 1 200 c and 16 gpa the transformation of enstatite to phase plus stishovite on the same timescale requires much higher temperatures at lower temperatures enstatite transforms directly to the ilmenite structure but only at pressures greater than 20 gpa enstatite should therefore persist metastably to greater depths than olivine in subduction zones transforming directly to the ilmenite structure the enstatite ilmenite transformation is accompanied by a large decrease in volume which should increase the stresses in subducting slabs change the buoyancy forces that drive subduction and might also contribute to the origin of deep focus earthquakes

elasticity of olivine α beta β and spinel γ polymorphs of germanates and silicates summary ultrasonic data for the velocities of germanate and silicate compounds in the olivine a beta 8 and spinel y crystal structures have been determined as a function of pressure to 7 5 kbar at room temperature for polycrystalline specimens hot pressed at pressures up to 65 kbar the a y phase transformations are characterized by the following velocity u density p relationships a the velocity jumps are twice the percentage magnitude of the density jumps b the ratio vp v of the compressional to shear velocity is either approximately constant or increases slightly across the transitions and c the slopes linear or logarithmic on v p diagrams for the a y transition are comparable to those produced by isothermal compression or isobaric expansion of the low pressure olivine phase the behaviour of up u is a feature common to many phase transformations however the observed relationships a and c while similar to those for the quartz coesite transition in si02 are in marked contrast with those for the coesite rutile pyroxene garnet and pyroxeneilmenite transformations these latter transitions involve increases in cation anion co ordination and nearest neighbour distances whereas the olivine spinel and olivine beta phase transformations do not such crystallographic details may be diagnostic for interpretations of the composition and mineralogy of the earth s transition zone systematic trends in the elastic properties for isostructural sequences support the concept of germanates as models for the elasticity of their silicate analogues this scheme is applied to estimate the bulk modulus of the spinel polymorph of mg si04 k 2 06 0 05 mbar comparison of the new elasticity data with recent earth models in the vicinity of the 400 km discontinuity reveals that only model 1066b of gilbert dziewonski is compatible with the experimental data for the u y and a 8 transformations

sound velocities and elastic properties of fe bearing wadsleyite and ringwoodite the sound velocities and single crystal elastic moduli of phase wadsleyite and phase ringwoodite of mg fe 2sio4 with fe fe mg ratios of 0 075 and 0 09 respectively have been determined at ambient conditions by brillouin spectroscopy both compressional and shear wave aggregate velocities decrease with increasing fe content in both phases but the magnitude of this decrease is different for the two phases the adiabatic bulk modulus ks of fe bearing mg2sio4 ks 170 2 gpa is indistinguishable from that of the mg end member within experimental uncertainty whereas ks of mg fe 2sio4 increases rapidly with increasing iron content the shear moduli of both phases decrease with increasing fe content our measurements indicate that the velocity and impedance contrasts between olivine and mg fe 2sio4 are independent of fe content for mg rich compositions but the contrast for the mg fe 2sio4 transition increases significantly with increasing fe content the new data support a previous estimate of 40 10 for the olivine content of the upper mantle and suggest that less than 50 mg fe 2sio4 is sufficient to account for the observed impedance contrasts at depths of both 410 km and 520 km unless the effect of fe on elastic properties is accounted for it is difficult to account for both the 410 and 520 km discontinuities with a single olivine content

dislocation enhanced selective dissolution an examination of mechanical aspects using deformation mechanism maps abstract analysis of dislocation configuration energetics reveals that free dislocations are responsible for the greatest strain energy per unit volume further interpretation suggests that pile ups of free dislocations called the dislocation enhanced selective dissolution desd microstructure have the greatest potential for selective dissolution these mechanical arguments are identical for phase transformations activated by dislocation strain the desd microstructure is characteristic of low dislocation mobility and need not produce macroscopic evidence of strain therefore a desd field is proposed for relatively low homologous temperature and shear stress on a deformation mechanism map an exploration of the effects of solution transfer deformation in a polymineralic assemblage via dislocation mechanism maps suggests that desd has the potential to produce more anisotropic textures than pressure solution

interfacial friction induced pressure and implications for the formation and preservation of intergranular coesite in metamorphic rocks abstract the finding of intergranular coesite and coesite pseudomorphs has been taken as critical evidence to support that the formation of coesite was related to regional ultrahigh pressure uhp metamorphism rather than the pressure vessel effect previous laboratory deformation experiments found that coesite occurs in practically undeformed strain forbidden zones immediately adjacent to the piston sample interfaces while the mean stress applied to the specimen is remarkably lower than the quartz coesite equilibrium boundary the formation and preservation of intergranular coesite in eclogites from uhp metamorphic terranes and the occurrence of coesite in strain forbidden zones within experimentally deformed rocks can be more satisfactorily explained by the theory of interfacial friction induced pressure which is a well known in metal forging during certain episodes of fast tectonic deformation under high transient differential stresses the interfacial friction can induce very significant deformation pressures in thin layers of weak materials e g sio2 between large garnet crystals that are refractory and mechanically strong thus act as excellent anvils the local deformation pressure which deviates from the lithostatic value is likely an intrinsic property of highly constrained flows of weak materials between strong walls at least on a microstructural scale

effects of isostasy on large scale geoid signal i geoid anomaly over an earth like planet summary the geoid anomaly due to lithospheric changes such as cooling is a second order quantity which is highly sensitive to the definition of isostasy many different terms must be summed over a range of depths so a simplified test planet is required to make sure the algorithm is working correctly our model planet consists of an ocean underlain by lithosphere and asthenosphere there are two ocean ridges along 0 and 180 longitudes and two trenches along 90 and 270 longitudes the oceanic plates are moving away from the ridge with a velocity of 5 cm yr l at the equator pressure is assumed constant at the compensation depth which is itself an equipotential surface we use the method of rings in which a set of 18 rings about an arbitrary pole observation point cover the whole earth this method allows the anomaly source depth to be taken into account changes in the equipotential surface height both above and below any ring induce mass changes due to infilling with sea water and mantle material respectively these mass changes cause further changes in the geoidal height over and below all the other rings and the problem renders itself into a set of linear simultaneous equations the final geoid is a product of seven second order effects the direct dipole effect due to the density differences in the cooling lithosphere lister 1982 produces a geoidal elevation of sll m and 3 11 m over the ridge and the trench respectively the direct mass effect vening meinesz 1946 due to the fact that a column on a sphere is pie shaped and not straight sided as on a flat earth model produces a geoidal elevation of s12m and 3 11 m over the ridge and the trench respectively the final geoidal elevation due to the direct and indirect effects is s26 m and 3 26 m over the ridge and the trench respectively

rheology of mg2geo4 olivine and spinel harzburgite implications for earth s mantle transition zone geophysical modeling requires an upper mantle low viscosity zone lvz recent results suggest that the lvz could be within the mantle transition zone mtz or above it depending on boundary conditions the rheology of the olivine dominated uppermost mantle is well constrained experimentally but not the mtz here we report first time measurement of the rheology of mg2geo4 spinel analogue of ringwoodite the dominant mineral of the lower mtz in the dislocation creep regime and compare it with that of mg2geo4 olivine in both cases we incorporate 15 20 pyroxene the second most abundant mineral in the upper mantle to provide a realistic comparison of rock rheologies under similar conditions spinel dominated material is about twice as strong as olivine dominated material our results suggest that unless h2o weakening of ringwoodite or phase transformation induced grain size reduction occurs in the lower mtz it will be more viscous than above the 410 km discontinuity

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

composition of the upper mantle geophysical tests of two petrological models the elastic properties of candidate mantle phases are used to test the viability of olivine rich pyrolitic and cao al 2o 3 rich eclogitic assemblages for the mantle high temperature adiabats for each phase of interest are constructed and compared to mantle seismic properties both pyrolitic and eclogitic assemblages satisfy the seismic properties between 200 and 400 km between 400 and 670 km depth an eclogitic assemblage yields a superior match to velocities and velocity gradients the 400 km seismic discontinuity may represent a chemical boundary between pyrolite and picritic eclogite piclogite or phase transformations in the olivine orthopyroxene components of a piclogitic assemblage containing about 16 olivine high velocity gradients in the transition zone may be explained by the transformation of ca rich cpx to majorite garnet seismic properties at the top of the lower mantle are consistent with pyrolite piclogite or perovskite implying that the 670 km discontinuity may be a chemical boundary

kinking in mg2geo4 olivine an ebsd study we have used electron backscatter diffraction ebsd to characterize the internal deformation and orientation of olivine grains within an experimentally deformed mg2geo4 olivine aggregate we observe that a significant fraction of the grains contain kink bands our observations of kink band geometry combined with the work of earlier authors indicate that 001 slip is the dominant slip system operating in mg2geo4 olivine at 1000 1473 k at 0 6 1 3 gpa since mg2geo4 is a high pressure analog for natural compositions of olivine this is consistent with the observations made by others that the dominant slip system for olivine in the mantle switches from 100 to 001 as pressure increases we also observed that kink bands formed in all measured olivine grains with 001 axes oriented within 35 of the compression direction we thus conclude that in a randomly oriented polycrystal undergoing uniaxial deformation at low to moderate temperatures 18 of all grains will deform by kinking and that kink band formation is an important accommodation mechanism for olivine at these conditions

anisotropic growth in the olivine spinel transformation of mg2geo4 under nonhydrostatic stress abstract the olivine spinel phase transformation in mg 2 geo 4 does not occur by a martensitic mechanism the evidence from samples transformed in a griggs type solid medium deformation apparatus are 1 1 lack of microstructural features in the olivine phase which can be specifically associated with a martensitic mechanism 2 2 the orientation relationship between the two phases that is predicted by the martensitic mechanism does not occur nor is there any apparent consistency of relative orientations 3 3 application of a differential stress to the transforming sample resulted in an anisotropic growth rate for the spinel phase indicating that growth was externally controlled rather than crystallographically controlled anisotropic growth of the spinel phase results in elongation of the residual olivine phase grains in the plane normal to the direction of maximum principal compressive stress a velocity ratio of 1 7 0 7 5 4 has been determined for the growth rate of the spinel from measurements on residual olivine grains the interphase grain boundary in samples transformed under stress has cusp shaped fingers of spinel with a blunt end separated by thin spikes of olivine samples transformed isostatically do not exhibit this feature providing further confirmation of anisotropic growth of the spinel the preferred growth of the spinel is consistent with a theory of phase transformation under nonhydrostatic stress the predicted spinel finger shape based on this theory is generally consistent with observed shapes except for the blunt end the discrepancy may be due to surface energy which has not been considered here or to local deviations of the applied macroscopic stress

global isostatic geoid anomalies for plate and boundary layer models of the lithosphere abstract isostatic geoid anomalies are usually interpreted using a flat earth one dimensional idealization isostatic anomalies on the spherical self gravitating earth differ from this idealization because 1 degree one terms in the spherical harmonic expansion vanish 2 each term in the spherical harmonic expansion is multiplied by l 2 l 0 5 relative to the flat earth case 3 mass in cones rather than straight sided columns is constant and 4 further deformation of the earth is induced by the gravitational attraction of the deformation caused by the isostatic potential anomaly when the effect of each of these is quantified the second third and fourth nearly cancel leaving the degree one over the horizon effect providing the most important difference calculations of model isostatic geoid anomalies for the spherical analogues developed here of the plate and boundary layer thermal models show that this effect can bias estimates of geoid slopes by over 20 although the effect is usually less than 5 the geoid anomalies for these two models are quite different over old ocean basins but they are unlikely to be distinguishable on the basis of global geoid observations owing to the presence of other larger perturbations in the geoid stripping the effects of plate aging and a hypothetical uniform 35 km thick isostatically compensated continental crust from the observed geoid emphasizes that the largest amplitude geoid anomaly is the geoid low of almost 120 m over west antarctica this anomaly is a factor of two greater in amplitude than the low of 60 m over sri lanka

lateral heterogeneity in the convecting mantle thermally induced lateral heterogeneity in the earth s mantle has been studied in terms of idealized two dimensional constant viscosity numerical models of mantle convection steady model solutions of the temperature and velocity fields were analyzed with respect to both the spatial and spectral signatures of their inherent lateral heterogeneity by examining the changes in spectral signature produced by varying the rayleigh number the degree of internal heating and the depth in the convection cell it has been possible to identify those features of the spectra of lateral heterogeneity which are most indicative of these variations particular attention has been paid to the spectral characteristics of thermal boundary layers within the convecting layer the spectral analysis of known temperature fields represents the forward problem corresponding to the inverse problem of inferring the thermal structure of the mantle from the spectral components of its lateral heterogeneity as determined by seismic tomography the forward problem presented here illustrates a new approach in convection modeling which may ultimately provide important diagnostic criteria for the interpretation of recent tomographic data

creep mechanism in mg2geo4 effects of a phase transition we deformed polycrystalline mg2geo4 olivine and spinel in a griggs type solid medium apparatus flow of mg2geo4 olivine can be represented by 6 5 l07 3 5 exp 105 kcal mol rt where kbar is the differential stress and s 1 is the natural strain rate for mg2geo4 spinel the flow law is 2 6 104 2exp 73 kcal mol rt the low stress exponent and activation enthalpy coupled with fine grain size 3 m suggest that mg2geo4 spinel deformed by a superplastic mechanism flow parameters for the olivine phase suggest a dislocation creep mechanism comparison of theoretical superplastic flow laws for mg2geo4 olivine with the spinel phase data suggests that strain rates in mg2geo4 spinel are only about a factor of 3 lower than for mg2geo4 olivine of the same grain size a similar estimate holds for dislocation creep of the two phases if it is controlled by diffusion transformation from mg2geo4 olivine to spinel reduced grain size to approximately 3 m thus we might expect a similar reduction in grain size in the earth s transition zone which could result in superplastic deformation of the transformed phase and cause a weak decoupling zone at the transition boundary in the mantle superplasticity brought about by transformation induced reduction in grain size may also provide a mechanism for deep focus earthquakes and an explanation for the correlation observed between their distribution with depth and the depths of phase transitions within the mantle

reflection properties of phase transition and compositional change models of the 670 km discontinuity seismic observations of the amplitude ratio amplitude p 670p amplitude p p suggest a reflection coefficient r 7 13 for reflections from the underside of the 670 km discontinuity this can be compared with calculated reflection coefficients to constrain the nature of the mantle transition region velocity and density profiles through the transition region were calculated for olivine pyroxene garnet and olivine garnet model mineral assemblages for models with identical upper and lower mantle compositions the computed reflection coefficients from the phase transitions pertinent to the 670 km discontinuity are less than 0 5 these phase transitions produce their maximum r if they are effectively discontinuous which gives r 2 3 except for the garnet to perovskite transformation which gives r 8 in general given realistic impedance contrasts across a phase transition the change in material properties must occur over a region less than 3 km thick for r to be of the order of 5 a discontinuous change in chemical composition at 670 km depth coupled with a change in phase can also produce a reflection coefficient of the observed magnitude r 2 3 for a first order change in phase at 670 km with different iron concentrations on each side of the discontinuity and r 4 6 for cases with a discontinuous increase in silica content based on our analysis the thermal and chemical boundary layers which would occur across such a chemical discontinuity would probably have negligible effects on the reflection coefficient thus although limited by considerable uncertainties the mineralogical and seismic data suggest a two layered model of the mantle with a discontinuous change in both phase and composition at 670 km

the geoid effect of compensated topography and uncompensated oceanic trenches the geoid is becoming increasingly important in interpretation of global tectonics most of the topography of the earth is isostatically compensated so removal of its effect from the geoid is appropriate before tectonic modeling the oceanic trenches however are dynamically depressed features and cannot be isostatically compensated in the classical way continental topography compensated at 35 km gives intracontinental geoidal undulations of up to 15 m over mountain ranges in a spherical harmonic expansion to order and degree 22 oceanic topography compensated at 40 km reasonable for the thermally supported long wavelengths matches the 10 m difference between old continents and old oceans in a detailed nasa gsfc geoid removing the assumed compensation for the oceanic trenches leaves negative anomalies of up to 9 m amplitude caused by their uncompensated mass deficit this mass deficit acts as a partial regional compensation for the excess mass of the subducting slabs and partly explains why geoidal and gravity anomalies over the cold slabs are less than thermal models predict

textural evolution during high pressure dehydration of serpentinite to peridotite and its relation to stress orientations and kinematics of subducting slabs insights from the almirez ultramafic massif abstract the almirez ultramafic massif se spain preserves the transformation of high p antigorite atg serpentinite to chlorite chl harzburgite 1 6 1 9 gpa 680 710 c a metamorphic reaction that is the primary source of water at the intermediate depth of subducting slabs we present a detailed ct and ebsd study of oriented samples across the atg serpentinite dehydration isograd to investigate the textural evolution during serpentinite dehydration to peridotite and its relation to stress orientations and the kinematics of subducting slabs above the atg out isograd atg serpentinite shows a prograde mylonitic foliation and a weak shape preferred orientation spo of oxide aggregates defining a n s stretching lineation the antigorite crystal preferred orientation cpo is characterized by 001 atg perpendicular to the foliation and the poles to 100 atg and 010 atg distributed in a girdle like symmetry with 100 atg nearly parallel to the stretching lineation the antigorite microstructure and cpo are consistent with deformation by dislocation creep twinning and dissolution precipitation creep these textures record the long term shear deformation near the slab interface where the main compressive stress 1 was at an acute angle to the foliation below the atg out isograd atg serpentinite dehydrated to unfoliated coarse grainedchl harzburgite with granofels or spinifex textures distributed in alternating decameter sized lenses crystallization of granofels and spinifex chl harzburgite records respectively a sequence of slow and fast fluid draining events during serpentinite dehydration under the same orientation of the principal stresses that resulted in the shear deformation of the atg serpentinite both textural types exhibit coarse grained textures with systematic mineral cpos and spos and microstructures without evidence of major ductile deformation the texture of the granofels chl harzburgite formed by a topotactic dehydration reaction after atg serpentinite coupled to compaction leading to an olivine layering subparallel to the atg serpentinite foliation the olivines of granofels chl harzburgite are rounded and display a weak cpo that can be accounted for by the topotactic reaction 100 atg 100 ol and 001 atg 010 ol after atg serpentinite similarly orthopyroxene shows a marked cpo consistent with the topotactic reaction 100 opx 001 atg and 001 opx 100 atg spinifex chl harzburgite displays systematic mineral spo and cpo spinifex olivines are tabular on 100 ol and elongated along 001 ol c b a and define a ese wnw platelet lineation the average texture is characterized by 001 ol opx subparallel to a strong ese wnw oxide aggregate lineation and 100 ol opx and 001 ol opx within a plane of similar orientation to the atg serpentinite foliation the spos and cpos of spinifex chl harzburgites are composed of up to four orientation populations of tabular olivines where one population is volumetrically dominant in all samples relative to the main orientation population the cpo of olivine shows clustered distribution of 100 ol and 010 ol maxima rotated at systematic angles around 001 ol these clustered cpos and spos show a remarkable correlation with the orientation of the principal paleostresses the preponderant orientation population of tabular olivines lies on the plane of maximum compression 2 3 plane and has 010 ol and its 100 ol tabular faces nearly perpendicular to the atg serpentinite foliation this population can be accounted for by oriented growth of platy crystals perpendicular to 1 the other populations lie in the plane perpendicular to the least compressive stress 3 and the planes of maximum shear the driving force and causes for the oriented crystallization of tabular olivine in these planes is uncertain but their correlation with paleostresses suggests a cause effect relationship spinifex and granofels chl harzburgites show a marked ese wnw oxide aggregate lineation that differs in orientation from that of atg serpentinite and is approximately parallel to the intermediate compressive stress 2 these lineations and the platelet lineation of spinifex chl harzburgite may be due to along strike fluid flow below the permeability barrier that constituted the atg out dehydration isograd our study shows that the kinematics of the slab paleostresses and fluid flow exert a dynamic control on the textures of atg serpentinite dehydrating to peridotite in subducting slabs

interface kinetics grain scale deformation and polymorphism deviatoric stress generated within subducting slabs by the olivine spinel transformation has been modeled by assuming the phases to be simply connected rather than comprising a mixture in which one phase is embedded within another here we use a simplified model to explain how transformation strain is incorporated into a continuum model and we then use the simplified model to explain quantitatively the origin of the unreasonably large deviatoric stresses predicted by existing slab models we review experiments on the transformation of single crystal samples and argue that they are consistent with the occurrence at the grain scale of deviatoric stresses comparable with those predicted erroneously to exist at the slab scale by those slab models using a simple example we show that although large deviatoric stresses can exist at the grain scale their average over a sample containing many grains can be hydrostatic this leads us to the problem of modeling the microscale structure we outline the therm

effects of isostasy on large scale geoid signal ii geoid anomaly over the earth summary the geoid anomaly due to the oceanic and the continental lithosphere is a second order quantity which is sensitive to the definition of isostasy as the geoid and the compensation level 2200 km depth are both equipotential surfaces further potential changes are induced due to infilling of sea water above and mantle material below until equilibrium is maintained the final geoid is composed of seven second order effects in this work the surface of the earth is divided into 36 x 18 spherical trapezoids 10 x 10 each and age is assigned to each trapezoid containing oceanic lithosphere using the map of larson et al the geoid anomaly is calculated using the method of rings in which a set of 18 rings about an arbitrary point observation point covers the whole earth over most of the earth the direct dipole effect is dominant relative to the direct mass effect the final geoid anomaly after the self deformation effect can be up to twice as much as the initial mass and dipole signals the calculated geoid follows some major observed anomalies over the earth such as the geoid high in the west pacific and the low in the indian ocean but is poorly correlated with the observed geoid due to large and wide anomalies presumably caused by flow in the mantle such as the low over antarctica these results suggest although not statistically significant that the residual geoid variance reaches a minimum if the reference density under the oceanic lithosphere of 3 28 g is slightly higher than the 3 268 cm under the continental lithosphere

on poisson s ratio and composition of the earth s lower mantle abstract poisson s ratio of the lower mantle calculated from recently published values of seismic wave velocities and extrapolated to atmospheric pressure and room temperature is found to be in the range 0 23 0 25 these values are compared with the values of poisson s ratio calculated for binary mixtures of mgsio3 perovskite and magnesiowustite with various iron contents current values of the experimental error on measured elastic moduli give little hope to be able to discriminate between pyrolite and chondritic lower mantles both are acceptable if the shear modulus of perovskite is in the upper range of liebermann et al estimates a similar calculation using the seismic parameter confirms the results obtained by considering poisson s ratio and further constrains the value of the shear modulus of perovskite to lie between 1600 and 1700 kilobars for current mantle models to remain plausible chemical stratification of the mantle is therefore possible but not required by seismological data

metastable mantle phase transformations and deep earthquakes in subducting oceanic lithosphere earth s deepest earthquakes occur as a population in subducting or previously subducted lithosphere at depths ranging from about 325 to 690 km this depth interval closely brackets the mantle transition zone characterized by rapid seismic velocity increases resulting from the transformation of upper mantle minerals to higher pressure phases deep earthquakes thus provide the primary direct evidence for subduction of the lithosphere to these depths and allow us to investigate the deep thermal thermodynamic and mechanical ferment inside slabs numerical simulations of reaction rates show that the olivine spinel transformation should be kinetically hindered in old cold slabs descending into the transition zone thus wedge shaped zones of metastable peridotite probably persist to depths of more than 600 km laboratory deformation experiments on some metastable minerals display a shear instability called transformational faulting this instability involves sudden failure by localized superplasticity in thin shear zones where the metastable host mineral transforms to a denser finer grained phase hence in cold slabs such faulting is expected for the polymorphic reactions in which olivine transforms to the spinel structure and clinoenstatite transforms to ilmenite it is thus natural to hypothesize that deep earthquakes result from transformational faulting in metastable peridotite wedges within cold slabs this consideration of the mineralogical states of slabs augments the traditional largely thermal view of slab processes and explains some previously enigmatic slab features it explains why deep seismicity occurs only in the approximate depth range of the mantle transition zone where minerals in downgoing slabs should transform to spinel and ilmenite structures the onset of deep shocks at about 325 km is consistent with the onset of metastability near the equilibrium phase boundary in the slab even if a slab penetrates into the lower mantle earthquakes should cease at depths near 700 km because the seismogenic phase transformations in the slab are completed or can no longer occur substantial metastability is expected only in old cold slabs consistent with the observed restriction of deep earthquakes to those settings earthquakes should be restricted to the cold cores of slabs as in any model in which the seismicity is temperature controlled via the distribution of metastability however the geometries of recent large deep earthquakes pose a challenge for any such models transformational faulting may give insight into why deep shocks lack appreciable aftershocks and why their source characteristics including focal mechanisms indicating localized shear failure rather than implosive deformation are so similar to those of shallow earthquakes finally metastable phase changes in slabs would produce an internal source of stress in addition to those due to the weight of the sinking slab such internal stresses may explain the occurrence of earthquakes in portions of lithosphere which have foundered to the bottom of the transition zone and or are detached from subducting slabs metastability in downgoing slabs could have considerable geodynamic significance metastable wedges would reduce the negative buoyancy of slabs decrease the driving force for subduction and influence the state of stress in slabs heat released by metastable phase changes would raise temperatures within slabs and facilitate the transformation of spinel to the lower mantle mineral assemblage causing slabs to equilibrate more rapidly with the ambient mantle and thus contribute to the cessation of deep seismicity because wedge formation should occur only for fast subducting slabs it may act as a parachute and contribute to regulating plate speeds wedge formation would also have consequences for mantle evolution because the density of a slab stagnated near the bottom of the transition zone would increase as it heats up and the wedge transforms to denser spinel favoring the subsequent sinking of the slab into the lower mantle

isostatic geoid anomalies on a sphere long wavelength geoid anomalies due to lateral variations in the density or thickness of a thin layer in local isostatic equilibrium on the surface of a sphere depend sensitively on the assumed state of stress within the layer a number of common intuitive definitions of local isostasy generally believed to be essentially identical are associated with quite different states of isostatic stress and the corresponding theoretical geoid anomalies can vary by more than a factor of 2 this sensitivity of the theoretical anomaly to the exact definition of local isostasy constitutes an obstacle to any proposed program of interpreting or exploiting observed global isostatic geoid anomalies such as those associated with the aging of the oceanic lithosphere

evolution of grain sizes and orientations during phase transitions in hydrous mg2sio4 transformation microstructures in mantle minerals such as mg fe 2sio4 are critical for predicting the rheological properties of rocks and the interpretation of seismic observations we present in situ multi grain x ray diffraction experiments on hydrous mg2sio4 at the p t conditions relevant for deep cold subducting slabs up to 40 gpa and 850 c at a low experimental strain rate 4 10 6s 1 we monitor the orientations of hundreds of grains and grain size variations during the series of forsterite wadsleyite ringwoodite phase transformations microtextural results indicate that the and an intermediate phase grow incoherently relatively to the host phase consistent with a nucleation and growth model the and phase exhibit orientation relationships which are in agreement with previous ex situ observations the and intermediate show texturing due to moderate differential stress in the sample both the and transformation induce significant reductions of the mean sample grain size of up to 90 that starts prior to the appearance of the daughter phase apart from the in the newly formed and phases the nucleation rate is faster than the growth rate inhibiting the formation of large grains these results on grain orientations and grain size reductions in relation to transformation kinetics should allow refining existing slab strength models

flow in the mantle inferred from the low degree harmonics of the geopotential summary flow in the earth s mantle is inferred from continental drift its distribution may be obtained from the pattern of zones of compression and tension in the earth s crust and from the low harmonics of the geopotential the origin of the flow is held to be convection the depth to which the flow extends will be discussed the relation of the flow to the earthquake mechanism will be considered