gas hydrates clathrates causing pore water freshening and oxygen isotope fractionation in deep water sedimentary sections of terrigenous continental margins the occurrence of gas hydrates in deep water sections of the continental margins predicted from anomalous acoustic reflectors on seismic profiles has been confirmed by recent deep sea drilling results on the pacific continental slope off guatemala gas hydrates were brought up for the first time from two holes 497 498a drilled during leg 67 of the dsdp in water depths of 2360 and 5500 m respectively the hydrates occur in organic matter rich pleistocene to miocene terrigenous sediments in the hydrate bearing zone a marked decrease in interstitial water chlorinities was observed starting at about 10 20 m subbottom depth pore waters at the bottom of the holes near 400 m subbottom have as little as half the chlorinity of seawater i e 9 similar but less pronounced trends were observed during previous legs of the dsdp in other hydrate prone segments of the continental margins where recharge of fresh water from the continent can be excluded e g leg 11 the crystallization of hydrates like ice excludes salt ions from the crystal structure during burial the dissolved salts are separated from the solids subsidence results in a downward motion of the solids including hydrates relative to the pore fluids thawing of hydrates during recovery releases fresh water which is remixed with the pore fluid not involved in hydrate formation the volume of the latter decreases downhole thus causing downward decreasing salinity chlorinity hydrate formation is responsible for oxygen isotope fractionation with18o enrichment in the hydrate explaining increasingly more positive 18o values in the pore fluids recovered after hydrate dissociation with depth

a kinematic model for recrystallization and texture development in olivine polycrystals the interpretation of seismic anisotropy in the mantle requires a knowledge of the relationship between the lattice preferred orientation lpo of crystals and the convective flow field in order better to understand this link we present a model for the evolution of lpo in olivine aggregates that deform by both intracrystalline slip and dynamic recrystallization dynamic recrystallization depends on the dislocation density of the grains which is a function of the applied local stress grains with a large density of dislocations lower their bulk strain energy by nucleating strain free sub grains at a rate proportional to a dimensionless nucleation parameter grains with high energy are then invaded by grains with low energy by grain boundary migration at a rate proportional to a dimensionless grain boundary mobility m the value of is constrained by observed lpo patterns in experimentally deformed olivine aggregates and m is constrained by the temporal evolution of the strength of the lpo for m 125 75 and 3 the model predictions agree well with the experimental results numerical calculations of lpo using our model are significantly faster than those based on viscoplastic self consistent or equilibrium based theories making the model especially suitable for applications for complex convective flows

a thermodynamically self consistent damage equation for grain size evolution during dynamic recrystallization summary we employ basic non equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium under the assumption that the whole grain size distribution remains self similar we show that the grain size reduction is controlled by the rate of mechanical dissipation in agreement with recent findings our formalism is self consistent with mass and energy conservation laws and allows a mixed rheology as an example we consider the case where the grain size distribution is lognormal as is often experimentally observed this distribution can be used to compute both the kinetics of diffusion between grains and of dynamic recrystallization the experimentally deduced kinetics of grain size coarsening indicates that large grains grow faster than what is assumed in classical normal grain growth theory we discuss the implications of this model for a mineral that can be deformed under both dislocation creep and grain size sensitive diffusion creep using experimental data of olivine our predictions of the piezometric equilibrium in the dislocation creep regime are in very good agreement with the observations for this major mantle forming mineral we show that grain size reduction occurs even when the average grain size is in diffusion creep because the largest grains of the grain size distribution can still undergo recrystallization the resulting rheology that we predict for olivine is time dependent and more non linear than in dislocation creep as the deformation rate remains an increasing function of the deviatoric stress this rheology is not localizing

abrupt tectonics and rapid slab detachment with grain damage a simple model for necking and detachment of subducting slabs is developed to include the coupling between grain sensitive rheology and grain size evolution with damage necking is triggered by thickened buoyant crust entrained into a subduction zone in which case grain damage accelerates necking and allows for relatively rapid slab detachment i e within 1 my depending on the size of the crustal plug thick continental crustal plugs can cause rapid necking while smaller plugs characteristic of ocean plateaux cause slower necking oceanic lithosphere with normal or slightly thickened crust subducts without necking the model potentially explains how large plateaux or continental crust drawn into subduction zones can cause slab loss and rapid changes in plate motion and or induce abrupt continental rebound

structural reactivation in plate tectonics controlled by olivine crystal anisotropy structures formed during ancient tectonic events are commonly reactivated during subsequent tectonism numerical models point to mechanical anisotropy arising from the inherited orientation of crystals of the mineral olivine in the lithospheric mantle as the cause of this behaviour

tectonic plate generation and two phase damage void growth versus grain size reduction 1 the generation of plate tectonics from mantle convection requires shear localization in order to yield narrow weak plate boundaries separating broad strong plates a plausible shear localizing mechanism involves damage e g distributed microcracking one theoretical model of which involves two phase mechanics two phase damage theory employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a description of void generation and microcracking and hence weakening failure and shear localization here we examine the application of this theory to the problem of generating plate like behavior from convective type divergent convergent poloidal motion through a source sink formulation we extend the previous damage theory to consider two possible damage effects 1 growth and nucleation of voids associated with dilation of the host matrix and 2 increasing fineness i e reducing coarseness of the mixture by for example grain size reduction void generating damage is found to be poor at plate generation because of the predominance of dilational motion that is adverse to the development of plate like flow fineness generating damage is found to be very efficient at generating plate like behavior if we assume that the matrix viscosity is a simple function of grain void size as is typical for diffusion creep the implied grain size reduction mechanism is different from that of dynamic recrystallization and appears to be more capable of generating the requisite shear localization for forming tectonic plates from mantle flow

halide systematics in comparison with nutrient distributions in sites 1033b and 1034b saanich inlet odp leg 169s abstract the entire suite of halogens was measured in the pore fluids of hole 1033b and 1034b from saanich inlet odp leg 169s the fast sedimentation rates and large amount of organic carbon burial coupled with anoxia of the overlying waters promotes an advanced stage of diagenesis within the sediment column chloride interstitial water profiles suggest salinity variations within the waters of saanich inlet concentration profiles for iodide and bromide support the argument that they are produced through the degradation of organic matter although the concentration increases in i and br indicate that these halides are not regenerated in similar proportions to marine organic matter it appears that iodide and bromide are regenerated to similar degrees within the sediment column and in similar proportions to the sediment halide concentrations fluoride porewater values show a complicated pattern most likely caused by secondary reactions involving complexation with mg2 carbonate fluorapatite precipitation carbonate mineral diagenesis and or uptake into alumino silicate minerals

50 ma initiation of hawaiian emperor bend records major change in pacific plate motion the hawaiian emperor bend has played a prominent yet controversial role in deciphering past pacific plate motions and the tempo of plate motion change new ages for volcanoes of the central and southern emperor chain define large changes in volcanic migration rate with little associated change in the chain s trend which suggests that the bend did not form by slowing of the hawaiian hot spot initiation of the bend near kimmei seamount about 50 million years ago ma was coincident with realignment of pacific spreading centers and early magmatism in western pacific arcs consistent with formation of the bend by changed pacific plate motion

oriented grain growth and modification of frozen anisotropy in the lithospheric mantle abstract seismic anisotropy throughout the oceanic lithosphere is often assumed to be generated by fossilized texture formed during deformation at asthenospheric temperatures close to the ridge here we investigate the effect of high temperature and high pressure static annealing on the texture of previously deformed olivine aggregates to simulate residence of deformed peridotite in the lithosphere our experiments indicate that the orientation and magnitude of crystallographic preferred orientation cpo will evolve due to the preferential growth of grains with low dislocation densities these observations suggest that texture and stored elastic strain energy promote a style of grain growth that modifies the cpo of a deformed aggregate we demonstrate that these microstructural changes alter the orientation distributions and magnitudes of seismic wave velocities and anisotropy therefore static annealing may complicate the inference of past deformation kinematics from seismic anisotropy in the lithosphere

rheology of two phase systems a microphysical and observational approach abstract ductile shear zones commonly contain distinctive bands of high strain rock characterized by intimately mixed fine grained two phase or polyphase material these ultramylonite bands are weaker than the surrounding material and may play a critical role in strain localization how such zones develop how the phases become evenly dispersed the bulk rheology and the controls on grain size are all unclear the following generic scenario may resolve some of these questions 1 dislocation creep and dynamic recrystallization cause grain size reduction commonly the recrystallized grain sizes of the two phases differ 2 grain size reduction causes a switch to grain boundary diffusion creep which requires grain boundary sliding diffusion allows one phase to fill spaces that open between grains of the other this will happen most rapidly in the finer grained phase the grain size of the resulting mixture is therefore controlled by that of the finer grained phase this leads to mixing and dispersion of the two phases producing a fine grained evenly dispersed two phase aggregate 3 the bulk rheology will be controlled by grain boundary diffusion creep of the two phases with the grain size controlled by the finer grained phase bulk flow laws can be developed for quartz feldspar and olivine orthopyroxene ultramylonites based on these concepts using appropriate mixing laws

initiation of plate tectonics from post magma ocean thermochemical convection leading theories for the presence of plate tectonics on earth typically appeal to the role of present day conditions in promoting rheological weakening of the lithosphere however it is unknown whether the conditions of the early earth were favorable for plate tectonics or any form of subduction and thus how subduction begins is unclear using physical models based on grain damage a grainsize feedback mechanism capable of producing plate like mantle convection we demonstrate that subduction was possible on the hadean earth hereafter referred to as proto subduction or proto plate tectonics that proto subduction differed from modern day plate tectonics and that it could initiate rapidly scaling laws for convection with grain damage show that though either higher mantle temperatures or higher surface temperatures lead to slower plates proto subduction with plate speeds of 1 75 cm yr can still be maintained in the hadean even with a co2 rich primordial atmosphere furthermore when the mantle potential temperature is high e g above 2000 k the mode of subduction switches to a sluggish subduction style where downwellings are drip like and plate boundaries are diffuse finally numerical models of post magma ocean mantle convection demonstrate that proto plate tectonics likely initiates within 100 myr of magma ocean solidification consistent with evidence from hadean zircons after the initiation of proto subduction non plate tectonic sluggish subduction prevails giving way to modern style plate tectonics as both the mantle interior and climate cool hadean proto subduction may hasten the onset of modern plate tectonics by drawing excess co2 out of the atmosphere and cooling the climate

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

a simple model of plate generation from mantle flow summary a simple model of non newtoniancreeping ow is used to evaluate classes of rheologieswhichallow viscous mantle ow to becomeplate like the model describes shallow layerlithosphericmotion driven by sources and sinks the sources represent spreading ridges while the sinks rep resent subduction zones the sources and sinks thus also prescribe the poloidal component ofthe surface ow eld the toroidal strike slip component of the ow eld is found via thesolution of the stokes equation with non newtonian rheology as a rst basic investigation ofthe model the horizontal divergence from the two dimensional rectangular velocity eld ofolson bercovici 1991 is used for the source sink eld the degree to which the induced uid ow reproducesthe rectangularplate is used to measure the success of differentrheologiesin generating plate like ows results indicate that power law rheologies even in the limit ofvery high power law index can only produce modest plate like ow for example the ratioof toroidal to poloidal kinetic energy for a source sink eld derived from a square plate is atbest 0 65 whereas a perfect square plate has a ratio of 1 0 moreover the power law rheologyappears to reach an asymptotic limit in its ability to produce plate like behavior this impliesthat plate tectonics is unlikely to arise from a power law rheology even in the limit of very high a class of rheologies that yield signi cantly more promisi ng results arise from the carreaupseudo plastic rheology with the power law index taken to be 0 one rheology in thisclass is the continuum model for stick slip earthquake behavior of whitehead gans 1974 which is essentially the carreau equation with 1 this class of rheologies referred to asthe stick slip rheologies induces a toroidal to poloidal kinetic energy ratio for a square plates ssource sink function which can be as high as0 9 the viscosity or strength distribution forthis class of rheologies also appears more plate like showing fairly uniform high viscosity re gions pseudo plates and sharply de ned low viscosity zones pseudo margins in contrast even the most nonlinear power law rheology produces spatially varying high viscosity regionsand relatively smooth low viscosity margins the greater success of the stick slip rheologiesin producing plates is attributed to a self lubricating mechanism in which the transfer of mo mentum from regions of high shear to low shear is inhibited in contrast even in the limit ofin nite power law index a power law rheology can retard but never prohibit momentum trans fer this feature is essential to the sharpening of velocity pro les into plate like pro les whichis illustrated with a simple boundary layertheory key words plate tectonics mantle convection non newtonian ow toroidal poloidal cou pling

toroidal poloidal energy partitioning and global lithospheric rotation during cenozoic time the motion of surficial plates can be split into two terms the toroidal field related to shear between plates or to spin around their centres and the poloidal field associated with horizontal divergence at ridges and subductions recent papers have suggested that the present day plate motion minimizes the ratio of toroidal to poloidal energies 1 and that the existence of a global lithospheric rotation results from viscosity variations between oceans and continents 2 this paper tests and confirms these two hypotheses using plate reconstructions during cenozoic time the partition of energy between the two modes always favours the poloidal velocities the global rotation computed using a viscosity one order of magnitude lower below oceans than below continents agrees with observations the existence of a non linear rheology in a uniform asthenosphere leads to a satisfactory location of the global lithospheric rotation but with too small an amplitude

mechanisms of shear localization in the continental lithosphere inference from the deformation microstructures of peridotites from the ivrea zone northwestern italy the ultramafic massif of balmuccia northwestern italy shows a variety of deformation fabrics including some localized shear zones that resulted in nearly complete melting pseudotachylyte a series of peridotite specimens were collected near one of the pseudotachylyte fault veins to investigate the mechanisms of shear localization the microstructural analyses show at least three deformation stages the first stage i is nearly homogeneous deformation at low stress 3 mpa and high temperature 1300 1500 k followed by localized deformation stage ii at a higher stress 60 mpa and moderate temperature 1000 1150 k and finally semi brittle deformation stage iii at a very high stress 400 mpa that resulted in the formation of pseudotachylyte the stage ii deformation resulted in relatively small strains in most areas but significant localized deformation leading to dynamic recrystallization occurred the degree of which increases systematically toward the fault zone based on this observation together with the observation that the brittle deformation post dated dynamic recrystallization we conclude that the shear localization in this locality occurred as a result of ductile deformation rather than brittle deformation various mechanisms of shear localization in the ductile regime are examined and we conclude that the grain size reduction due to dynamic recrystallization at relatively high stresses and moderate temperatures is a probable mechanism of shear localization the intermediate regime between dislocation and diffusion creep where rheology is grain size sensitive yet continuous recrystallization occurs due to dislocation creep is suggested to play an important role in shear localization in the upper mantle the conditions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization it is shown that shear localization occurs at relatively low temperatures and or high stresses localized deformation in the continental lithosphere will occur well below the brittle ductile transition 10 km depth down to 40 80 km depth depending upon the geothermal gradient and water fugacity

on the initiation of subduction zones analysis of the relation between intraplate stress fields and lithospheric rheology leads to greater insight into the role that initiation of subduction plays in the tectonic evolution of the lithosphere numerical model studies show that if after a short evolution of a passive margin time span a few tens of million years subduction has not yet started continued aging of the passive margin alone does not result in conditions more favorable for transformation into an active margin

subduction initiation at a strike slip plate boundary the cenozoic pacific australian plate boundary south of new zealand 1 we first present a synthesis of the macquarie ridge complex mrc tectonic structures as well as paleo reconstruction models of the kinematic evolution of the pacific australia plate boundary south of new zealand since the eocene we then ascertain the geodynamical conditions that preceded subduction initiation and identify the nature and structures of the crust that first subducted at the puysegur subduction zone this synthesis is used to produce a subduction initiation model for the puysegur region concomitant to inception of the alpine fault ca 23 ma a 150 km wide transpressive relay zone developed along puysegur bank inherited structures enabling localization of compressive deformation right lateral motion at the relay zone has juxtaposed oceanic and continental crusts facilitating inception of subduction and controlling the subduction vergence subsequently the puysegur subduction zone initiated at the transpressive relay zone ca 20 ma upper and lower plate inherited structures guided and facilitated the lengthening of the subduction zone during the neogene the four individual segments of the mrc represent different stages of incipient subduction whose development depends on local geodynamical conditions and lithospheric heterogeneities the example of the mrc demonstrates that subduction can initiate from an oceanic spreading center through progressive changes in plate kinematics within a 10 15 myr time frame index terms 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158 8155 tectonophysics plate motions general 9604 information related to geologic time cenozoic keywords subduction initiation pacific australia plate motion cenezoic macquarie ridge

the other isotopes research avenues based on 36ar 37ar and 38ar the earliest publications on 40 ar 39 ar dating recognized the usefulness of 39 ar 37 ar in providing chemical clues to the species releasing argon during stepped heating while 37 ar measurements are required for interference corrections aside from these essential roles 36 ar 37 ar and 38 ar have other useful applications the dominance of ca as the target nucleus for the production of 38 ar and 36 ar from spallation by cosmic rays has led to the determination of cosmic ray exposure ages from 38 ar 37 ar 38 ar production from cl has an important role in the study of ore minerals containing saline fluid inclusions both in dating and in understanding theirgenesis combining 38 ar 36 arratioswith micro thermometricdeterminations ofsalinity pro vides a way to determine absolute concentrations of noble gases in ore fluids absorption of cosmic ray produced secondary neutrons by 35 cl and 37 cl provides another means to study cosmic ray exposure of meteorites 36 s excesses in meteoritic sodalite provide evidence of now extinct 36 cl in the early solar system surprisingly there is little or no evidence of excess 36 ar beyond what can be accounted for by secondary neutron exposure we have recently devised a method to identify monoisotopic 36 ar from 36 cl decay

grain damage hysteresis and plate tectonic states abstract shear localization in the lithosphere is an essential ingredient for understanding how and why plate tectonics is generated from mantle convection on terrestrial planets the theoretical model for grain damage and pinning in two phase polycrystalline rocks provides a frame work for understanding lithospheric shear weakening and plate generation and is consistent with laboratory and field observations of mylonites grain size evolves through the competition between coarsening which drives grain growth and damage which drives grain reduction the interface between crystalline phases controls zener pinning which impedes grain growth damage to the interface enhances the zener pinning effect which then reduces grain size forcing the rheology into the grain size dependent diffusion creep regime this process thus allows damage and rheological weakening to co exist providing a necessary positive self weakening feedback moreover because pinning inhibits grain growth it promotes shear zone longevity and plate boundary inheritance however the suppression of interface damage at low interface curvature wherein inter grain mixing is inefficient and other energy sinks of deformational work are potentially more facile causes a hysteresis effect in which three possible equilibrium grain sizes for a given stress coexist 1 a stable large grain weakly deforming state 2 a stable small grain rapidly deforming state analogous to ultramylonites and 3 an unstable intermediate grain size state perhaps comparable to protomylonites a comparison of the model to field data suggests that shear localized zones of small grain mylonites and ultra mylonites exist at a lower stress than the co existing large grain porphyroclasts rather than as predicted by paleopiezometers or paleowattmeters at a much higher stress this interpretation of field data thus allows localization to relieve instead of accumulate stress the model also predicts that a lithosphere that deforms at a given stress can acquire two stable deformation regimes indicative of plate like flows i e it permits the coexistence of both slowly deforming plate interiors and rapidly deforming plate boundaries earth seems to exist squarely inside the hysteresis loop and thus can have coexisting deformation states while venus appears to straddle the end of the loop where only the weakly deforming branch exists

halide systematics in interstitial waters of ocean drilling sediment cores abstract the distribution of dissolved halides cl br i in interstitial waters of deep drill holes in the ocean is discussed for a number of environments hemi pelagic sediments hydrate containing sediments sediments affected by halide brines and also in hydrothermal environments the importance of the diagenesis of organic c in the production of i and br is discussed with reference to the observations of dissolved so 4 2 and nh 4 in the pore fluids more importantly in the zone of methano genesis below the so 4 2 reduction zone and usually associated with high organic c accumulation rates increases in i and br are of importance and usually are much higher than the increases observed in the so 4 2 reduction zone in pore fluids affected by dissolution of evaporites the br cl is often a strong indicator of the degree of evaporation of seawater demonstrated from results obtained in the mediterranean sea dissolution of halites on the other hand leads to br cl much lower than those of sea water in hydrothermal sediments there is no doubt that i and br contents of the fluids are influenced by hydrothermal alteration of the sediments and their organic c but signals are not clear the present study provides an overview of previous dsdp and odp studies of halide distributions in sedimentary pore waters in addition to other contributions on halide distributions in sedimentary pore waters in this symposium volume

collapse of passive margins by lithospheric damage and plunging grain size abstract the collapse of passive margins has been proposed as a possible mechanism for the spontaneous initiation of subduction in order for a new trench to form at the junction between oceanic and continental plates the cold and stiff oceanic lithosphere must be weakened sufficiently to deform at tectonic rates such rates are especially hard to attain in the cold ductile portion of the lithosphere at which the mantle lithosphere reaches peak strength the amount of weakening required for the lithosphere to deform in this tectonic setting is dictated by the available stress stress in a cooling passive margin increases with time e g due to ridge push and is augmented by stresses present in the lithosphere at the onset of rifting e g due to drag from underlying mantle flow increasing stress has the potential to weaken the ductile portion of the lithosphere by dislocation creep or by decreasing grain size in conjunction with a grain size sensitive rheology like diffusion creep while the increasing stress acts to weaken the lithosphere the decreasing temperature acts to stiffen it and the dominance of one effect or the other determines whether the margin might weaken and collapse here we present a model of the thermal and mechanical evolution of a passive margin wherein we predict formation of a weak shear zone that spans a significant depth range of the ductile portion of the lithosphere stiffening due to cooling is offset by weakening due to grain size reduction driven by the combination of imposed stresses and grain damage weakening via grain damage is modest when ridge push is the only source of stress in the lithosphere making the collapse of a passive margin unlikely in this scenario however adding even a small stress contribution from mantle drag results in damage and weakening of a significantly larger portion of the lithosphere we posit that rapid grain size reduction in the ductile portion of the lithosphere can enable or at least significantly facilitate the collapse of a passive margin and initiate a new subduction zone we use this model to estimate the conditions for passive margin collapse for modern and ancient earth as well as for venus

esr dating of the type cromerian freshwater bed at west runton u k abstract we performed esr dating on tooth enamel from three in situ locations within the west runton freshwater bed fw bed which is stratigraphically below fluvial sands of anglian origin the teeth contained little uranium which made esr ages essentially independent of the choice of uranium uptake model however this advantage was offset by a critical dependence of the esr ages upon the moisture content of the sediments care was taken to account for self shielding effects arising from the large size of the elephant tooth using the value of 10 moisture content observed during 1992 in sediments exposed on the cliff face gives ages of 346 55 eu and 350 52 lu ka corresponding to the oxygen isotope stage 11 10 boundary additional moisture content measurements of different parts of the fw bed in 1995 yielded a mean value of 25 using values of 20 and 30 moisture content we obtain ages from 420 90 to 460 80 ka eu 1 errors because the faunal and floral data indicate that the fw bed was an interglacial deposit the esr results suggest that the fw bed could have been deposited in either isotope stages 11 or 13 however independent faunal and stratigraphic evidence favours a stage 13 age or earlier this is discussed in relation to the esr data these results are consistent with prevailing views e g bridgland 1994 that the overlying anglian deposits correlate with oxygen isotope stage 12

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

energetics of a two phase model of lithospheric damage shear localization and plate boundary formation summary the two phase theory for compaction and damage proposed by bercovici et al 2001a j geophys res 106 8887 8906 employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a model for damage void generation and microcracking and a continuum description of weakening failure and shear localization here we examine further variations of the model which consider 1 how interfacial surface energy when averaged over the mixture appears to be partitioned between phases 2 how variability in deformational work partitioning greatly facilitates localization and 3 how damage and localization are manifested in heat output and bulk energy exchange microphysical considerations of molecular bonding and activation energy suggest that the apparent partitioning of surface energy between phases goes as the viscosity of the phases when such partitioning is used in the two phase theory it captures the melt compaction theory of mckenzie 1984 j petrol 25 713 765 exactly as well as the void damage theory proposed in a companion paper ricard bercovici submitted calculations of 1 d shear localization with this variation of the theory still show at least three possible regimes of damage and localization at low stress is weak localization with diffuse slowly evolving shear bands at higher stress strong localization with narrow rapidly growing bands exists and at yet higher shear stress it is possible for the system to undergo broadly distributed damage and no localization however the intensity of localization is strongly controlled by the variability of the deformational work partitioning with dilation rate represented by the parameter f or 1 extreme localization is allowed with sharp profiles in porosity weak zones nearly discontinuous separation velocities and effectively singular dilation rates finally the bulk heat output is examined for the 1 d system to discern how much deformational work is effectively stored as surface energy in the high stress distributed damage cases heat output is reduced as more interfacial surface energy is created yet in either the weak or strong localizing cases the system always releases surface energy regardless of the presence of damage or not and thus slightly more heat is in fact released than energy is input through external work moreover increased levels of damage represented by the maximum work partitioning f make the localizing system release surface energy faster as damage enhances phase separation and focusing of the porosity field thus yielding more rapid loss of net interfacial surface area however when cases with different levels of damage are compared at similar stages of development say the peak porosity of the localization it is apparent that increased damage causes smaller relative heat release and retards loss of net interfacial surface energy the energetics and energy partitioning of this damage and shear localization model are applied to estimating the energy costs of forming plate boundaries and generating plates from mantle convection

fluorine mobility during early diagenesis of carbonate sediment an indicator of mineral transformations abstract the abundant occurrence of calcium carbonate minerals in marine sediment their well documented dissolution or precipitation during diagenesis and their high f content suggest that carbonate mineral diagenesis may be an important influence on f behavior in marine sediment to test this hypothesis we have examined the geochemistry of f in shallow carbonate mud bank sediment of florida bay usa the f content of biogenic calcium carbonate in florida bay varies with mineralogy positively correlates with the mg content of calcite and occurs in similar abundance to sr high mg calcite f sr 2 4 mol mol aragonite 0 53 0 79 mol mol low mg calcite 0 22 mol mol models of porewater distributions direct solute flux measurements and reaction rate estimates over the upper 0 16 cm of sediment from bob allen key bank predict net fluxes out of the sediment of 170 mol f m2 d 230 mol sr m2 d and 5 6 mmol ca m2 d the net solute flux ratio of f sr 0 71 mol mol is consistent with dissolution of aragonite or high mg calcite but f ca and sr ca ratios 28 mmol mol and 40 mmol mol respectively are 3 10 that of biogenic carbonate sources selective dissolution of a high f and sr content phase or more likely based on dissolution experiments concurrent dissolution and reprecipitation of phases with different f and sr contents account for this discrepancy the loss of f sr and ca to fluorapatite precipitation as predicted from a stoichiometric model of phosphate release can be added to the transport reaction model predicted net fluxes a resulting more complete mass balance model incorporating both carbonate mineral reactions and fluorapatite formation yields total release estimates of f sr and ca of 770 mol 332 mol and 53 mmol m2 d respectively calcium carbonate minerals apparently undergo transformation whereby 34 mmol high mg calcite m2 d and 19 mmol aragonite m2 d dissolve and 46 mmol low mg calcite m2 d precipitates ignoring other cations besides ca if no fluorapatite formation occurs the required fluxes are that 23 mmol aragonite m2 d dissolves and 3 4 mmol high mg calcite m2 d and 14 mmol low mg calcite m2 d precipitate net loss of caco3 from the deposit by dissolution is 6 7 mmol m2 d 3 of the accumulation flux the magnitude of these fluxes could cause significant mineralogical and chemical changes on rapid time scales in nearshore carbonate sediment fluorine is probably mobile in other carbonate deposits undergoing diagenetic alteration on short and longer time scales and is a powerful additional constraint on the rates and nature of carbonate mineral diagenesis

ice sheet variability around the north atlantic ocean during the last deglaciation millennial scale variability in the flux of ice rafted detritus to north atlantic sediments during the last glacial period has been interpreted to reflect a climate forced increase in the discharge of icebergs from ice sheet margins surrounding the northern north atlantic ocean 1 but the relationship between ice sheet variability and climate change is not clear as both the sources of ice rafted detritus and the ice marginal processes are varied and complex 2 4 terrestrial records are helpful in unravelling this complexity because they can demonstrate the scale of ice sheet oscillations and whether the ice sheet or sector was advancing or retreating with respect to climate change here we constrain the age and anatomy of a prominent readvance of the british ice sheet in the northern irish sea region at similar to 14 14 c kyr bp similar to 16 4 calendar kyr bp the analysis indicates that the british ice sheet participated in an iceberg discharge episode known as heinrich event 1 comparison with other terrestrial and marine ice sheet records suggests that the dynamic collapse of the laurentide ice sheet beginning at 14 6 15 0 c 14 kyr bp1 4 similar to 17 2 17 6 calendar kyr bp 5 initiated varied responses from other ice sheet margins around the northern north atlantic region these observations support the argument that the release of icebergs and meltwater during heinrich event 1 disrupted the north atlantic thermohaline circulation 6 8 leading to a delay or reversal of deglaciation of the northern hemisphere and at least as far south as 40 degrees s for two to three thousand years 5 9 10 suggesting a climate forcing and response similar to that of the ensuing younger dryas cold snap 11 12

generation of plate tectonics with two phase grain damage and pinning source sink model and toroidal flow abstract the grain damage and pinning mechanism of bercovici and ricard 2012 for lithospheric shear localization is employed in two dimensional flow calculations to test its ability to generate toroidal strike slip motion and influence plate evolution this mechanism posits that damage to the interface between phases in a polycrystalline material like peridotite composed primarily of olivine and pyroxene increases the number of small zener pinning surfaces which then constrain mineral grains to ever smaller sizes regardless of creep mechanism this effect allows a self softening feedback in which damage and grain reduction can co exist with a grain size dependent diffusion creep rheology moreover grain growth and weak zone healing are greatly impeded by zener pinning thereby leading to long lived relic weak zones the fluid dynamical calculations employ source sink driven flow as a proxy for convective poloidal flow upwelling downwelling and divergent convergent motion and the coupling of this flow with non linear rheological mechanisms excites toroidal or strike slip motion the numerical experiments show that pure dislocation creep rheology and grain damage without zener pinning as occurs in a single phase assemblages permit only weak localization and toroidal flow however the full grain damage with pinning readily allows focussed localization and intense plate like toroidal motion and strike slip deformation rapid plate motion changes are also tested with abrupt rotations of the source sink field after a plate like configuration is developed the post rotation flow and material property fields retain memory of the original configuration for extensive periods leading to suboptimally aligned plate boundaries e g strike slip margins non parallel to plate motion oblique subduction and highly localized weak and long lived acute plate boundary junctions such as at what is observed at the aleutian kurile intersection the grain damage and pinning theory therefore readily satisfies key plate tectonic metrics of localized toroidal motion and plate boundary inheritance and thus provides a predictive theory for the generation of plate tectonics on earth and other planets

catastrophic resurfacing and episodic subduction on venus abstract there is clear evidence that a catastrophic resurfacing event occurred on venus in the relatively recent past a primary data source for this resurfacing event is the spatial distribution of impact craters in this paper we apply the pair correlation technique to the observed crater distribution and find that the result is identical to that for a random distribution in order to test the sensitivity of the technique we also apply it to the spatial distribution of coronae on venus for the coronae we find substantial deviations from a random distribution one explanation for the catastrophic resurfacing is the episodic subduction hypothesis we model episodic subduction using a thermal boundary layer stability analysis we find that episodic subduction events with intervals of 500 to 700 myr can transport only 15 25 of the radiogenic heat produced within the planet we suggest that the remainder of the heat must be lost to the surface during a period of vigorous tectonic activity following the subduction event but prior to the subsequent stabilization of a global lithosphere

creep and strain dependent microstructures of synthetic anorthite diopside aggregates abstract we investigated plastic deformation of fine grained synthetic diopside and two phase anorthite diopside aggregates in triaxial compression and torsion up to 5 temperature confining pressure and stress ranged between 950 1180 c 200 400 mpa and 5 500 mpa respectively water content of samples ranged between 0 005 0 002 and 0 075 0 025 wt h 2 o all samples deformed in linear viscous creep with a stress exponent of 1 0 0 2 the activation energy ranged between 571 53 and 290 28 kj mol depending on mineralogy and water content sample strength depended on water fugacity with an exponent of 1 55 0 25 samples deformed in torsion and coaxial compression gave similar flow laws in spite of significant differences in the corresponding microstructures scanning and transmission electron microscopy of two phase samples deformed in torsion showed phase mixing cavitation and dislocation processes we suggest that linear viscous creep of fine grained two phase aggregates involved grain boundary sliding accommodated by grain boundary diffusion and significant dislocation accommodation at high strains we also observed that cavity coalescence and microcracking led to sample failure hence dynamic instabilities may exist in high strain shear zones accommodating viscous deformation in the lower continental crust

mechanisms for the generation of plate tectonics by two phase grain damage and pinning abstract weakening and shear localization in the lithosphere are essential ingredients for understanding how and whether plate tectonics is generated from mantle convection on terrestrial planets we present a new theoretical model for the mechanism of lithospheric weakening and shear localization and hence plate generation through damage grain evolution and zener pinning in two phase polycrystalline lithospheric rocks grain size evolves through the competition of coarsening which drives grain growth with damage which drives grain reduction however in a two phase medium the interface between phases induces zener pinning which impedes grain growth and facilitates damage the size of the pinning surfaces is given by the roughness of the interface and damage to the interface causes smaller pinning surfaces which in turn drive down the grain size forcing the rheology into the grain size dependent diffusion creep regime this process allows damage and rheological weakening to co exist which is normally considered impossible in single phase assemblages moreover pinning greatly inhibits grain growth and shear zone healing which is much faster in single phase materials hence the resulting shear localization is rapid less than 1 myr but the healing time for a dormant weak zone is very slow greater than 100 myrs these effects therefore permit rapidly forming and long lived plate boundaries the model therefore provides a key ingredient and predictive theory for the generation of plate tectonics on earth and other planets

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

integrating satellite observations and modern climate measurements with the recent sedimentary record an example from southeast alaska 1 assessments of climate change over time scales that exceed the last 100 years require robust integration of high quality instrument records with high resolution paleoclimate proxy data in this study we show that the recent biogenic sediments accumulating in two temperate ice free fjords in southeast alaska preserve evidence of north pacific ocean climate variability as recorded by both instrument networks and satellite observations multicore samples ew0408 32mc and ew0408 43mc were investigated with 137cs and excess 210pb geochronometry three dimensional computed tomography high resolution scanning xrf geochemistry and organic stable isotope analyses ew0408 32mc 57 162 n 135 357 w 146 m depth is a moderately bioturbated continuous record that spans ad 1930 2004 ew0408 43mc 56 965 n 135 268 w 91 m depth is composed of laminated diatom oozes a turbidite and a hypopycnal plume river flood deposit a discontinuous event based varve chronology indicates 43mc spans ad 1940 1981 decadal scale fluctuations in sedimentary br cl ratios accurately reflect changes in marine organic matter accumulation that display the same temporal pattern as that of the pacific decadal oscillation an estimated sitka summer productivity parameter calibrated using seawifs satellite observations support these relationships the correlation of north pacific climate regime states primary productivity and sediment geochemistry indicate the accumulation of biogenic sediment in southeast alaska temperate fjords can be used as a sensitive recorder of past productivity variability and by inference past climate conditions in the high latitude gulf of alaska

volatile composition of microinclusions in diamonds from the panda kimberlite canada implications for chemical and isotopic heterogeneity in the mantle abstract in order to better investigate the compositions and the origins of fluids associated with diamond growth we have carried out combined noble gas he and ar c and n isotope k ca and halogen cl br i determinations on fragments of individual microinclusion bearing diamonds from the panda kimberlite north west territories canada the fluid concentrations of halogens and noble gases in panda diamonds are enriched by several orders of magnitude over typical upper mantle abundances however noble gas c and n isotopic ratios 3he 4he 4 6 ra 40ar 36ar 20 000 30 000 13c 4 5 to 6 9 and 15n 1 2 to 8 8 are within the worldwide range determined for fibrous diamonds and similar to the mid ocean ridge basalt morb source value the high 36ar content of the diamonds 1 10 9 cm3 g is at least an order of magnitude higher than any previously reported mantle sample and enables the 36ar content of the subcontinental lithospheric mantle to be estimated at 0 6 10 12 cm3 g again similar to estimates for the morb source three fluid types distinguished on the basis of ca k cl compositions are consistent with carbonatitic silicic and saline end members identified in previous studies of diamonds from worldwide sources these fluid end members also have distinct halogen ratios br cl and i cl the role of subducted seawater derived halogens originally invoked to explain some of the halogen ratio variations in diamonds is not considered an essential component in the formation of the fluids in contrast it is considered that large halogen fractionation of a primitive mantle ratio occurs during fluid melt partitioning in forming silicic fluids and during separation of an immiscible saline fluid

land sea correlations in the pleistocene based on isoleucine epimerization in non marine molluscs the correlation between the oxygen isotope stratigraphy of the oceans and continental stratigraphic records is one of the main challenges for quaternary research most continental classifications however are based on techniques and hypotheses that predate recent advances nevertheless the deep sea oxygen isotope global stratigraphic framework has been extended to the continents by means of some long sequences1 but these are exceptional and regional geology is unsatisfactorily classified for correlation with the oxygen isotope signal because successive geological events produced similar evidence homotaxis geochronometric dating is essential for correlation but available methods are highly site regional or sample specific fortunately non marine molluscs are ubiquitous in time and space and here we use the time dependent epimerization of l isoleucine in these fossils to subdivide the pleistocene of the british isles and to identify more events than recognized by the existing classification2 by calibrating the relative aminostratigraphic scale with independent dating methods we have set up a geochronology which is the basis for land sea correlations back to oxygen isotope stage 15

formation and structure of lithospheric shear zones with damage shear localization is an integral part of tectonics on earth we examine the role of two different forms of microstructural weakening voids and grain size reduction in the formation of shear zones as a function of depth the evolution of grain size and voids employed in this study is determined within the frame work of two phase damage theory the shear zone model is characterized by two dimensional simple shear which allows for lithostatic pressure to influence the evolution of void generating damage we consider cases with pure void generating damage pure grain size reducing damage and combined void generating and grain size reducing damage the introduction of lithostatic pressure alters void evolution and specifically leads to a suppression of void generation at depth grain size reducing damage produces the most significant localization differences in the time scale and efficacy of the two different damage mechanisms result from the mechanical constraints of viscous compaction and dilation imposed on void generating damage cases with combined void generating and grain size reducing damage lead to a more complicated interaction with grain size reduction driving porosity to the flanks of the central shear zone this result is proposed to be relevant for interpreting the void and grain size microstructures observed in faults on earth

hydrothermal geochemistry of sediments and pore waters in escanaba trough odp leg 169 geochemical studies of pore fluids and solid phases in two ocean drilling program odp drill sites sites 1037 and 1038 in the escanaba trough off northern california have provided further data on the hydrothermal processes associated with the spreading of the gorda ridge previous work in the area of odp site 1038 includes the discovery of a hydrothermal system and associated sulfide deposits centered around an uplifted sediment hill in this sedimented extensional environment this earlier work provided some insights into the present nature of venting however only deep drilling investigations can provide the means to fully understand the genesis and evolution of this system and associated hydrothermal deposits odp leg 169 is the third deep drilling operation to explore the magnitude genesis and evolution of hydrothermal systems on sedimented ridges previous studies centered on the guaymas basin in the gulf of california and the middle valley in the ne pacific ocean pore water studies in the reference odp site 1037 and in the hydrothermally active area of odp site 1038 have revealed the presence of a complex system of hydrothermally originated fluids whereas the data in the reference site indicate recent hydrothermal activity in the basal part of the drill site the evidence in site 1038 suggests that fluids of hydrothermal origin spread out at shallow depths around the central hill causing substantial sediment alteration as well as deposition of hydrothermal sulfides in the near surface zone of the sediments a second major discovery at site 1038 was the evidence for fluid phase separation at depth at temperatures possibly in excess of 400 c this conclusion is based on the presence of both low cl and high cl fluids the latter appear to be advected rapidly towards the surface presumably along cracks and faults the low cl fluids however appear to be transported laterally along sandy horizons in the sediments thus signifying two very different migration pathways for high cl and low cl hydrothermally phase separated fluids studies of the organic geochemistry of dissolved gases and matured organic matter corroborate these findings of extensive hydrothermal alteration of the sediments

a source sink model of the generation of plate tectonics from non newtonian mantle flow a model of mantle convection which generates plate tectonics requires strain rate or stress dependent rheology in order to produce strong platelike flows with weak margins as well as strike slip deformation and plate spin i e toroidal motion here we employ a simple model of source sink driven surface flow to determine the form of such a rheology that is appropriate for earth s present day plate motions in this model lithospheric motion is treated as shallow layer flow driven by sources and sinks which correspond to spreading centers and subduction zones respectively the source sink field is derived from the horizontal divergence of plate velocities and thus directly prescribes poloidal motion the toroidal flow field is solved through the non newtonian stokes equation for shallow layer tangential flow on the surface of a sphere two plate motion models are used to derive the source sink field the first is an idealized square plate which is used to explore the basic aspects of the model the second is an analytically continuous model of earth s present day plates bercovici and wessel 1994 as originally implied in the simpler cartesian version of this model bercovici 1993 the classical power law rheologies do not generate platelike flows as well as the hypothetical whitehead gans stick slip rheology which incorporates a simple self lubrication mechanism for the idealized plate geometry the power law rheologies yield much more diffuse strike slip shear i e radial vorticity than the stick slip rheology for the present day plate geometry the power law rheologies fail to reproduce the original proportion of left and right lateral strike slip shear whereas the stick slip rheology gives almost exactly the right proportion none of the fluid rheologies examined however produce more than approximately 60 of the original maximum shear for either plate model the viscosity fields produced by the power law rheologies are diffuse and the viscosity lows over strike slip shear zones or pseudo margins are not as small as over the prescribed convergent divergent margins in contrast the stick slip rheology generates very platelike viscosity fields with sharp gradients at the plate boundaries and margins with almost uniformly low viscosity quantitative comparisons with the toroidal poloidal kinetic energy partitioning and vorticity fields of the original plate model are also examined and the stick slip rheology is generally found to yield the most favorable comparisons power law rheologies with high viscosity contrasts however lead to almost equally favorable comparisons though these also yield the least platelike viscosity fields this implies that the magnitude of toroidal flow and platelike strength distributions are not necessarily related arid thus may present independent constraints on the determination of a self consistent plate mantle rheology the results of this study however predict that if such a rheology can indeed be uniquely determined it is likely to be in the class of stick slip self lubricating rheologies

determination of δ18o of seawater in the deep ocean during the last glacial maximum we present a method for determining the 18o of seawater in the deep ocean during the last glacial maximum from the measured 18o values of deep sea pore fluids using data from deep sea drilling project dsdp site 576 in the western pacific this method yields a glacial to interglacial change in 18osw of 1 0 0 25 this value for 18osw is the first direct measurement of deep ocean 18o for the last glacial maximum and avoids the problems of spatial and temporal variability of the 18o of surface water implicit in previous determinations more precise higher resolution pore fluid measurements are required to improve this determination

formation of lithospheric shear zones effect of temperature on two phase grain damage abstract shear localization in the lithosphere is a characteristic feature of plate tectonic boundaries and is evident in the presence of small grain mylonites localization and mylonitization in the ductile portion of the lithosphere can arise when its polymineralic material deforms by a grain size sensitive rheology in combination with zener pinning which can impede or possibly even reverse grain growth and thus promotes a self softening feedback mechanism however the efficacy of this mechanism is not ubiquitous and depends on lithospheric conditions such as temperature and stress therefore we explore the conditions under which self weakening takes place and in particular the effect of temperature and deformation state stress or strain rate on these conditions in our model the lithosphere like polymineralic material is deformed in a two dimensional simple shear driven by constant stress or strain rate the mineral grains evolve to a stable size which is obtained when the rate of coarsening by normal grain growth and the rate of grain size reduction by damage are in balance damage involves processes by which some of the deformational energy gets transferred into surface energy this can happen by i dynamic recrystallization grain damage and ii stretching deforming and stirring the material interface interface damage the influence of temperature enters through rheological laws which govern the rate of work and damage grain growth kinetics and the damage partitioning fraction which is the fraction of deformational work that goes into creating new surface energy we demonstrate that a two phase damage model in which the partitioning fraction depends on both temperature and roughness of the interface between the phases can successfully match the field data including the reported correlation of grain size and temperature the increasing dominance of dislocation creep at higher temperatures and a large range of grain sizes observed across the depth of a single shear zone

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

the interaction of viscous heating with grain size dependent rheology in the formation of localized slip zones the formation of localized shear zones is important for understanding many local and global processes in geodynamics we have developed a self consistent thermal mechanical model together with a rheology which depends on temperature strain rate and grain size distribution the grain size distribution has contributions from both dynamic recrystallization and grain growth processes and is governed locally by a nonlinear ordinary differential equation a one dimensional model with 104 points is employed to resolve all of the scales involving grain size and temperature we found that grain growth inhibits the development of shear zones and that there is a delicate interplay between viscous heating and grain growth process in determining whether narrow fault zones are developed quickly for realistic parameters of rheology and grain boundary processes for wet olivine the magnitude of the rate of grain growth is crucial to determine whether shear zones are stable or unstable at temperature t 1000 k or shear stress 100 mpa

dynamic recrystallization and phase mixing in experimentally deformed peridotite article history ductile shear zones evolve through complex feedbacks between microstructure and rheology in the mantle shear zones often display mylonitic or ultramylonitic microstructures characterized by extensive grain size reduction and well mixed polymineralic domains the feedback between the formation of these microstructures and grain size sensitive deformation is often considered an important weakening mechanism in high temperature mantle shear zones to understand better the processes that transform coarse grained tectonites into fine grained and well mixed mylonites we have performed triaxial deformation experiments on synthetic peridotite samples comprised of mm scale olivine and orthopyroxene clasts experiments were conducted in a griggs apparatus at a confining pressure of 1 gpa temperatures of 1400 to 1550 k and strain rates of 10 5 10 6 s 1 under nominally dry conditions experiments yield deformed samples with macroscopic natural strains ranging from 0 31 to 0 74 samples are partially recrystallized with neoblasts ranging in grain size from 2 25 m at these deformation conditions unrecrystallized mm sized clasts deform by grain size insensitive dislocation creep while recrystallized grains deform by grain size sensitive dislocation accommodated grain boundary sliding disgbs using electron backscatter diffraction ebsd we investigate the interfaces between recrystallized olivine and orthopyroxene domains at the majority of these interfaces no mixing is observed however many boundaries are serrated which is interpreted to be the result of surface tension driven phase boundary migration in a few cases we observe small degrees of mixing with enclaves of one phase completely surrounded by the other phase this mixing occurs only within a few recrystallized grain length scales of the interface these results demonstrate that the serial processes of dynamic recrystallization phase boundary migration and disgbs deformation are a plausible mechanism for solid state phase mixing however more extensive deformation may be needed to reproduce the near steady state microstructure observed in highly deformed mantle shear zones

subduction zone infancy examples from the eocene izu bonin mariana and jurassic california arcs a new model for the earliest stages in the evolution of subduction zones is developed from recent geologic studies of the izu bonin mariana ibm arc system and then applied to late jurassic ophiolites of cailfornia the model accounts for several key observations about the earliest stages in the evolution of the ibm system 1 subduction nucleated along an active transform boundary which separated younger less dense lithosphere in the west from older more dense lithosphere to the east 2 initial arc magmatic activity occupied a much broader zone than existed later 3 initial magmatism extended up to the modern trench over a region now characterized by subnormal heat flow 4 early are magmatism was characterized by depleted tholeiitic and ultra depleted boninitic magmas indicating that melting was more extensive and involved more depleted mantle than is found anywhere else on earth 5 early arc magmatism was strongly extensional with crust forming in a manner similar to slow spreading ridges and 6 crust production rates were 120 to 180 km 3 km ma several times greater than for mature arc systems these observations require that the earliest stages of subduction involve rapid retreat of the trench we infer that this resulted from continuous subsidence of denser lithosphere along the transform fault this resulted in strong extension and thinning of younger more buoyant lithosphere to the west this extension was accompanied by the flow of water from the sinking oceanic lithosphere to the base of the extending lithosphere and the underlying asthenosphere addition of water and asthenospheric upwelling led to catastrophic melting which continued until lithosphere subsidence was replaced by lithospheric subduction application of the subduction zone infancy model to the late jurassic ophiolites of california provides a framework in which to understand the rapid formation of oceanic crust with strong arc affinities between the younger sierran magmatic arc and the franciscan subduction complex provides a mechanism for the formation and subsidence of the great valley forearc basin and explains the limited duration of high t high p metamorphism experienced by franciscan melanges

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

halogen cycling and precious metal enrichment in sub volcanic magmatic systems insights from the rum layered intrusion scotland abstract the cycling of the halogens in sub volcanic systems may play an important role in magmatic ore deposit formation and the emission of climate altering gases via volcanic outgassing however the mobile and reactive nature of the halogens as well as their typically low abundances in most intrusive igneous rocks means that their behaviour in shallow crustal level magmatic systems is difficult to constrain we present data from the 60 ma rum layered suite nw scotland utilising the neutron irradiation noble gas mass spectrometry ni ngms technique to provide insight into the provenance and behaviour of cl br and i in magma chamber and precious metal enrichment processes as a well characterised and superbly exposed example of an open system sub volcanic magmatic body the rum intrusion is an ideal locality to investigate halogen behaviour in this context our data reveal concentrations of

pore water fluoride in peru continental margin sediments uptake from seawater abstract fluoride analyses display downward decreasing pore water gradients in peru shelf phosphatic muds that require diffusion from the overlying seawater into the sediment column and removal by reaction within the upper few tens of centimeters presumably by incorporation into carbonate fluorapatite the profiles can be modeled as first order f removal with rate constants of 3 yr 1 and asymptotic f concentrations deep in the cores of 35 45 m almost one half the seawater value the integrated flux of fluoride from seawater into organic rich shelf sediments in coastal upwelling zones phosphatic muds yields a contemporaneous global f burial of 0 54 10 10 mol f yr 1 about one fifth the burial in other sinks mostly carbonates and opal the associated burial flux of phosphorus in shelf phosphorites is about 1 6 10 10 mol p yr 1 comparable to p burial in the deep sea with organic matter 1 4 10 10 mol yr 1 and biogenic carbonates 1 4 10 10 mol yr 1 thus phosphorite formation on the peru shelf is a significant contemporaneous process

bromine and iodine in peru margin sediments and pore fluids implications for fluid origins at the peruvian convergent margin two distinct pore fluid regimes are recognized from differences in their cl concentrations the slope pore fluids are characterized by low cl concentrations but elevated br and i concentrations due to biogenic production the shelf pore fluids exhibit elevated cl and br concentrations due to diffusive mixing with an evaporitic brine in the slope pore fluids the br i and nh4 concentrations are elevated following bacterial decomposition of organic matter but the i concentrations are in excess of those expected based on mass balance calculations using nh4 and br concentrations the slope sediment organic matter which is enriched in iodine from oxidationreduction processes at the oxygenated sediment water interface is responsible for this enrichment the increases in dissolved i and the i enrichments relative to nh4 and br correlate well with sedimentation rates because of differential trapping following regeneration the pore fluid i br ratios suggest that membrane ion fiitration is not a major cause of the decreases in cl concentrations other possible sources for low cl water including meteoric water clathrate dissociation and or mineral dehydration reactions imply that the diluting component of the slope low cl fluids has flowed at least 1 km through the sediment the low bottom water oxygenation in the shelf is responsible for the low if any enrichment of iodine in the shelf sediments fluctuations in bottom water oxygen concentrations in the past however may be responsible for the observed variations in the sediment ibr ratios comparison of na cl and br cl molar ratios in the pore fluids shows that the shelf high cl fluid formed from mixing with a brine that formed from seawater concentrated by twelve to nineteen times and probably was modified by halite dissolution this dense brine located below the sediment sections drilled appears to have flowed a distance 500 km through the sediment

dynamics of subduction initiation at preexisting fault zones we show that a new subduction zone can initiate at a preexisting dipping fault zone with reasonable plate forces consistent with what is observed in the western pacific the dynamics of subduction initiation within a viscoelastic medium has been systematically explored with the finite element method we investigate the compression of oceanic lithosphere with both force and velocity boundary conditions and track the thermal structure in which heat is transported by both advection and diffusion the viscosity of the medium is non newtonian and temperature dependent we also examine the influence of a perfectly plastic yield stress a new method to model the preexisting weak zone as a graded fault with remeshing is introduced the first 400 km of plate convergence is examined during which time there are profound changes in plate boundary dynamics initially topography across the plate boundary is characterized by over 1000 m of uplift close to the trench subsequently a bathymetric depression develops on the overriding plate with an initial subsidence rate of 125 m myr for a slab subducting at 2 cm yr tectonic subsidence of the overriding plate is primarily dependent upon the depth of slab penetration for a plate force of 4 10 12 n m subduction initiates even with a fault shear stress of 3 mpa the resistance most consistent with seismological observations however subduction will not initiate when this fault shear stress is 5 mpa our results are consistent with plate reconstructions which predict the initiation of subduction across preexisting weak zones within oceanic basins

generation of plate tectonics from lithosphere mantle flow and void volatile self lubrication abstract the formation of plate tectonics from mantle convection necessarily requires nonlinear rheological behavior recent studies suggest that self lubricating rheological mechanisms are most capable of generating plate like motion out of fluid flows the basic paradigm of self lubrication is nominally derived from the feedback between viscous heating and temperature dependent viscosity here we propose a new idealized self lubrication mechanism based on void e g pore and or microcrack generation and volatile e g water ingestion we test this void volatile self lubrication mechanism in a source sink flow model this leads to a basic nonlinear system which permits the excitation of strike slip toroidal motion a necessary ingredient of plate like motion out of purely divergent i e poloidal or characteristically convective flow with relatively inviscid void filling volatiles the void volatile mechanism yields a state of highly plate like motion i e with uniformly strong plate interiors weak margins and extremely focussed strike slip shear zones moreover the void volatile model obeys a chemical diffusion time scale that is typically much longer than the thermal convection time scale the model thus complies with the observation that plate boundaries are long lived and survive even while inactive the void volatile model of self lubrication therefore predicts self focussing shear zones plate generation and plate boundary longevity through what has long been suspected to be a key ingredient for the existence of plate tectonics i e water

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

age spreading rates and spreading asymmetry of the world s ocean crust we present four companion digital models of the age age uncertainty spreading rates and spreading asymmetries of the world s ocean basins as geographic and mercator grids with 2 arc min resolution the grids include data from all the major ocean basins as well as detailed reconstructions of back arc basins the age spreading rate and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments the age uncertainties for grid cells coinciding with marine magnetic anomaly identifications observed or rotated to their conjugate ridge flanks are based on the difference between gridded age and observed age the uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges resulting in ridge jumps toward hot spots we also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age depth relationships the new set of grids helps to investigate prominent negative depth anomalies which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow a combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems

why is terrestrial subduction one sided subduction of the lithosphere at convergent plate boundaries takes place asymmetrically the subducted slab sinks downward while the overriding plate moves horizontally one sided subduction in contrast global mantle convection models generally predict downwelling of both plates at convergent margins two sided subduction we carried out two dimensional 2 d numerical experiments with a mineralogical thermomechanical viscoelastic plastic model to elucidate the cause of one sided subduction our experiments show that the stability intensity and mode of subduction depend mainly on slab strength and the amount of weak hydrated rocks present above the slab two sided subduction occurs at low slab strength sin 0 15 the weak interface is maintained by the release of fluids from the subducted oceanic crust as a consequence of metamorphism the resulting weak interplate zone localizes deformation at the interface and decouples the strong plates facilitating asymmetric plate movement our work suggests that high plate strength and the presence of water are major factors controlling the style of plate tectonics driven by self sustaining one sided subduction processes

origin of the middle pleistocene transition by ice sheet erosion of regolith the transition in the middle pleistocene 0 9 ma seen in 18o deep sea core records from relatively low amplitude high frequency 41 kyr to high amplitude low frequency 100 kyr ice volume variations under essentially the same orbital forcing can be attributed to a change from an all soft bedded to a mixed hard soft bedded laurentide ice sheet through glacial erosion of a thick regolith and resulting exposure of unweathered crystalline bedrock a one dimensional ice sheet and bedrock model which includes transport of sediment and ice by subglacial sediment deformation demonstrates that a widespread deforming sediment layer maintains thin ice sheets before the transition which respond linearly to the dominant 23 and 41 kyr orbital forcing progressive removal of the sediment layer eventually causes a transition to thicker ice sheets whose dominant timescale of change 100 kyr reflects nonlinear deglaciation processes in model simulations over the last 3 ma initialized with no ice and a uniform 50 m sediment layer the time series of ice volume and extent agree in several important aspects with the observed records

rheological weakening of olivine orthopyroxene aggregates due to phase mixing part 2 microstructural development to understand the processes involved in phase mixing during deformation and the resulting changes in rheological behavior we conducted torsion experiments on samples of iron rich olivine plus orthopyroxene the experiments were conducted at a temperature t of 1200 c and a confining pressure p of 300 mpa using a gas medium deformation apparatus samples composed of olivine plus 26 orthopyroxene were deformed to outer radius shear strains up to 26 in samples deformed to lower strains of 4 elongated olivine and pyroxene grains form a compositional layering already by this strain mixtures of small equant grains of olivine and pyroxene begin to develop and continue to evolve with increasing strain the ratios of olivine to pyroxene grain size in deformed samples follow the zener relationship indicating that pyroxene grains effectively pin the grain boundaries of olivine and inhibit grain growth due to the reduction in grain size the dominant deformation mechanism changes as a function of strain the microstructural development forming more thoroughly mixed fine grained olivine pyroxene aggregates can be explained by the difference in diffusivity among me fe or mg o and si with transport of meo significantly faster than that of sio2 these mechanical and associated microstructural properties provide important constraints for understanding rheological weakening and strain localization in upper mantle rocks

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

on the extent of mantle hydration caused by plate bending when bent at subduction zones oceanic plates are damaged by normal faulting and this bending related faulting is widely believed to cause deep mantle hydration down to 20 30 km deep the buoyancy of water or equivalently confining pressure however makes it difficult to bring water down even if faulting is deep extension associated with plate bending generates negative dynamic pressure but the magnitude of such dynamic pressure is shown to be insufficient to overcome confining pressure seismic velocity anomalies that have been used to infer the extent of mantle hydration are reviewed and it is suggested that small crack like porosities which can be produced by thermal cracking and further enhanced by bending related faulting is sufficient to explain such velocity anomalies the presence of such porosities however does not necessarily lead to the substantial hydration of oceanic plates because of confining pressure whereas the depth extent of bending generated porosities is uncertain the theory of thermal cracking can be used to place a lower bound on the amount of water contained in the slab mantle 0 03 0 07 wt h2o and this lower bound is suggested to be more than sufficient to explain the lower plane earthquakes of the double seismic zone by dehydration embrittlement

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

grain size sensitive rheology of orthopyroxene the grain size sensitive rheology of orthopyroxene is investigated using data from rheological and microstructural studies a deformation mechanism map is constructed assuming that orthopyroxene deforms by two independent mechanisms dislocation creep and diffusion creep the field boundary between these mechanisms is defined using two approaches first experimental data from lawlis 1998 which show a deviation from non linear power law behavior at low stresses are used to prescribe the location of the field boundary second a new orthopyroxene grain size piezometer is used as a microstructural constraint to the field boundary at constant temperature both approaches yield sub parallel field boundaries separated in grain size by a factor of only 2 5 extrapolating to lithospheric conditions the deformation mechanism transition occurs at a grain size of 150 500 m consistent with observations from nature as the transition from dislocation to diffusion creep may promote shear localization grain size reduction of orthopyroxene may play a prominent role in plate boundary deformation

geochemical constraints on the distribution and rates of debromination in the deep subseafloor biosphere abstract organic matter in marine sediments is degraded through a range of diverse metabolic pathways which are dependent on substrate availability environmental conditions and microbial ecology the rates and systematics of these metabolic pathways affect long term global geochemical cycles and the degradation of organic matter in the subsurface marine environment organohalide respiration is one of these pathways that has been hypothesized to be widely active in the deep biosphere with the carbon halogen bonds being broken through microbially mediated redox reactions besides directly providing energy to microbes in marine sediments and allowing bromine to cycle back into the overlying ocean organobromine respiration may also be closely linked to nitrogen and carbon cycling in anoxic marine sediments here we investigate the distribution and rates of debromination by tracking the production of dissolved bromide br with depth in pore water sampled at several continental margins pore water profiles of br and ammonium nh 4 concentrations from the krishna godavari k g basin on the southeastern margin of india indicate a common distribution of rates of debromination and nh 4 production in continental margin sediments and suggest that the pools of bioavailable nitrogen and organobromine compounds are likely geochemically associated at these sites dissolved br and total solid phase bromine concentration profiles from the k g basin and costa rica margin indicate the most rapid debromination occurs in the upper 10 20 m of the sediment column the rates of debromination in the sediment column from the costa rica cascadia and nankai convergent margins are estimated using numerical reaction transport modeling of pore water br concentration profiles to constrain the maximum amount of metabolic energy that could be provided to the microbial communities through organobromine respiration the modeled rates of debromination provide an upper limit to organobromine respiration activity because other debromination processes may also be responsible for an unknown fraction of these geochemically derived rates modeled rates of debromination on the order of 10 1 10 3 mol m 2 y 1 indicate that the maximum amount of energy that is potentially provided through organobromine respiration is low relative to other metabolic pathways such as sulfate reduction and methanogenesis however organobromine respiration may still serve an important niche in the microbial community and debromination is an important part of the oceanic bromine cycle

grain coarsening maps a new tool to predict microfabric evolution of polymineralic rocks polymineralic rocks undergo grain coarsening with increasing temperature in both static and deformational environments as long as no mineral reactions occur the grain coarsening in such rocks is complex because the different phases influence each other and it is this interaction that controls the rate of grain coarsening of the entire aggregate we present a mathematical approach to investigate coupled grain coarsening using a set of microstructural parameters including grain size and volume fraction of both second phases and matrix mineral in combination with temperature information based on samples from polymineralic carbonate mylonites that were deformed at different temperatures we demonstrate how the mathematical relation can be calibrated for this natural system using such data sets for other lithologies grain coarsening maps can be generated which allow the prediction of microstructural evolution in polymineralic rocks such predictions are crucial for all subdisciplines in the earth sciences that require fundamental knowledge about microstructural changes and rheology of an orogen at different depths such as structural geology geophysics geodynamics and metamorphic petrology

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

authigenic apatite formation and burial in sediments from non upwelling continental margin environments abstract evidence for precipitation of authigenic carbonate fluorapatite cfa in long island sound and mississippi delta sediments suggests that formation of cfa is not restricted to environments of active coastal upwelling we present porewater data suggestive of cfa formation in both these areas application of a sequential leaching procedure designed specifically to separate authigenic carbonate fluorapatite from other phosphorus containing phases including detrital apatite of igneous or metamorphic origin provides strong supporting evidence for authigenic apatite formation in these sediments the size of the authigenic apatite reservoir increases with depth indicating continued formation of cfa during early diagenesis this depth increase is mirrored by a decrease in solid phase organic p at both sites suggesting that cfa is forming at the expense of organic p mass balance considerations application of diagenetic models to interstitial water nutrient data and the saturation state of the interstitial water are consistent with this interpretation diagenetic redistribution of phosphorus among the different solid phase reservoirs is observed at both sites and results in near perfect retention of p by these sediments over the depth intervals sampled formation of cfa in continental margins which do not conform to the classically defined regions of phosphorite formation renders cfa a quantitatively more important sink than has previously been recognized including this reservoir as a newly identified sink for reactive p in the ocean the residence time of p in the modern ocean must be revised downward the implication for ancient oceans of cfa formation in continental margin sediments other than phosphorites is that phosphorite formation may be less a representation of episodicity in removal of reactive p from the oceans than of localized concentration of cfa in phosphatic sediments by secondary physical processes

amino acid geochronology resolution and precision in carbonate fossils changes in indigenous proteins preserved in carbonate skeletons can be used to estimate the time elapsed since death of the organism racemization the most widely used reaction in amino acid geochronology refers to the inversion of l amino acids to their d configuration for the protein amino acid l isoleucine racemization occurs about only one of the two chiral carbon atoms this reaction epimerization proceeds from an initial dl ratio near zero to an equilibrium ratio of about 1 30 the reaction rate is controlled primarily by temperature and to a lesser degree by the nature of the proteins themselves a variable related to taxonomy and the extent of protein degradation analyzing monospecific samples minimizes the taxonomic variable the resolving power of dl ratios as relative age indices is a function of the reaction rate degree of natural variability and analytical uncertainty natural variability is defined as the spread in ratios in five well preserved shells of the same age from a single stratum and is primarily related to taxonomy it can range from 20 or more in taxa of low reliability to 1 in taxa of greatest integrity 5 to 10 is to be expected analytical uncertainty within a single laboratory is relatively insignificant amounting to no more than 2 but interlaboratory comparisons suggest caution in comparing ratios determined at different laboratories or by different techniques sampling appropriate portions of a shell is essential to minimize variability in dl ratios site temperature exerts the greatest control on reaction rate hence resolving power for tropical sites 25 c age differences 1 ka can be resolved within the holocene and ca 5 to 10 ka for older samples as the rate decreases equilibrium is attained after 150 to 300 ka the reaction rate is substantially lower at mid latitude sites ca 10 c where equilibrium requires about 2 ma and resolution within the last glacial cycle is no better than 10 to 20 ka arctic sites 10 c show almost no measurable racemization within the holocene and in extreme cases even within the last 100 ka equilibrium is not attained in 10 ma the extent of amino acid racemization epimerization in carbonate fossils provides chronological information for the critical time period beyond radiocarbon dating dl ratios can be used directly as relative age indices and with suitable calibration can provide reasonable estimates of absolute age and or effective diagenetic temperature improving the accuracy of racemization based age estimates requires a better understanding of racemization kinetics in carbonate systems

evidence of phase nucleation during olivine diffusion creep a new perspective for mantle strain localisation for the past decades grain size reduction leading to diffusion creep in olivine is believed to be a very important process for strain localisation in the lithospheric mantle however the mechanisms of grain size reduction in this regime are still poorly understood e g platt 2015 here we show new experimental results that document grain size reduction and material weakening during wet olivine diffusion creep while occurring for both mono phase and two phase aggregates grain size reduction is coeval with strain localisation and local phase mixing in olivine pyroxene aggregates based on evidence of fluid inclusions and cracks filled with a fine grained phase mixture we conclude that grain size reduces as a result of fluid assisted nucleation that takes place in the presence of an aqueous fluid during diffusion creep cavitation induced by grain boundary sliding creep cavitation can be inferred and may play a critical role for olivine grain size reduction amongst their implications for rock rheology in general our findings highlight a key process for strain localisation in the ductile uppermost mantle

effects of water on dynamically recrystallized grain size of olivine abstract recrystallized grain size versus stress relationship for olivine has been determined under water poor and water rich conditions we found that when the water content in olivine exceeds 800 ppm h si the size of recrystallized grains is about two to three times larger than under water poor conditions compared at the same stress level microstructural observations of deformed samples show that the ratio of recrystallized grain size to that of subgrains increases significantly with water content and that the volume fraction of grains containing subgrain boundaries is smaller under water rich conditions than under water poor conditions these observations suggest that the enhancement of grain boundary mobility by water is more significant than that of dislocation recovery leading to a larger size of recrystallized grains under water rich conditions

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

microfabrics in carbonate mylonites along a large scale shear zone helvetic alps abstract carbonate mylonites with varying proportions of second phase minerals were collected at positions of increasing metamorphic grade along the basal thrust of the morcles nappe helvetic nappes switzerland variations of temperature stress and strain rate changes in chemistry of solid and fluid phases and differing degrees of strain localization and annealing were tracked by measuring the shapes mean sizes and size distributions of both matrix and second phase grains as well as crystal preferred orientation cpo of the matrix field structures suggest that strain rate was constant along the fault the mean and distribution of the calcite grain sizes were affected most profoundly by temperature increased temperature presumably accompanied by decreased stress correlated with larger mean sizes and wider size distributions at a given location the matrix grains in mylonites with more second phase particles are on average smaller have narrower size distributions and have more elongate shapes for example mylonites with 50 vol of second phases have matrix grain sizes half that of pure mylonites changes in calcite chemistry and the presence of synkinematic fluids seemed to influence microfabric only weakly temporal variations in conditions such as exhumation induced cooling apparently provoke changes in temperature stress and strain rate along the nappe these changes result in further strain localization during retrograde conditions and cause the grain size to be reduced by an additional 50 the matrix cpo strengthens with increasing temperature or strain but weakens and rotates with increasing second phase content these fabric changes suggest differing rates of grain growth grain size reduction and development of cpo owing to variations in the deformation conditions and perhaps mechanisms to interpret natural mylonite structures or to extrapolate mechanical data to natural situations requires careful characterization of the microfabric and in particular second phase minerals

only four major 100 ka glaciations during the brunhes chron early attempts at land sea correlations between continental glaciations and the marine 18o record were premature without a high resolution continental stratigraphy existing schemes of continental subdivision and correlation based facies floras and facies faunas do not provide the necessary resolution this paper reports on a new high resolution subdivision and correlation of the pleistocene of the british isles subdivision and correlation is accomplished primarily by aminostratigraphy based on aminozones which is calibrated by other geochronological means u series tl osl esr 14c palaeomagnetic and 36cl rock exposure dating for correlation with oxygen isotope stratigraphy these data are then used to test the hypotheses of shackleton 1987 and raymo 1997 that only four major glaciations occurred in the brunhes chron during oxygen isotope stages 16 12 6 and 2 their hypotheses are confirmed except for stage 2 when at least part of the british ice sheet was at its maximum somewhat earlier during stages 4 and 3

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

evolution of a polymineralic mantle shear zone and the role of second phases in the localization of deformation 1 the influence of second phases e g pyroxenes on olivine grain size was studied by quantitative microfabric analyses of samples of the hilti massif mantle shear zone semail ophiolite oman the microstructures range from porphyroclastic tectonites to ultramylonites from outside to the center of the shear zone starting at conditions of ridge related flow they formed under continuous cooling leading to progressive strain localization the dependence of the average olivine grain size on the second phase content can be split into a second phase controlled and a dynamic recrystallization controlled field in the former the olivine grain size is related to the ratio between the second phase grain size and volume fraction zener parameter in the latter dynamic recrystallization manifested by a balance between grain growth and grain size reduction processes yields a stable olivine grain size in both fields the average olivine and second phase grain size decreases with decreasing temperature combining the microstructural information with deformation mechanism maps suggests that the porphyroclastic tectonites 1100 c and mylonites 800 c formed under the predominance of dislocation creep since olivine rich layers are intercalated with layer parallel polymineralic bands in the mylonites nearly equiviscous conditions can be assumed in the ultramylonites diffusion creep represents the major deformation mechanism in the polymineralic layers it is this switch in deformation mechanism from dislocation creep to diffusion creep that forces strain to localize in the fine grained polymineralic domains at low temperatures 700 c underlining the role of the second phases on strain localization in cooling mantle rocks

rheological weakening of olivine orthopyroxene aggregates due to phase mixing 1 mechanical behavior to understand the processes involved in rheological weakening due to phase mixing in olivine orthopyroxene aggregates we conducted high strain torsion experiments on two phase samples at a temperature of 1200 c and a confining pressure of 300 mpa samples composed of iron rich olivine plus 26 orthopyroxene were deformed to outer radius shear strains of up to 26 values for the stress exponent n and grain size exponent p were determined based on least squares fits of the strain rate stress and grain size data to a power law creep equation both at smaller strains 3 and at larger strains 24 microstructural observations demonstrate that with increasing shear strain grain size decreased and mixtures of small equant grains of olivine and pyroxene developed the values of n and p combined with associated changes in microstructure demonstrated that our samples deformed i by dislocation accommodated grain boundary sliding with subgrains present at lower strains and ii by dislocation accommodated grain boundary sliding with subgrains absent at higher strains the evolution of both the mechanical and the microstructural properties observed in this study provide insights into the dynamic processes associated with rheological weakening and strain localization in the plastically deforming portion of the lithosphere