plate generation in a simple model of lithosphere mantle flow with dynamic self lubrication one of the more enigmatic features of the earth s style of mantle convection is plate tectonics itself in particular the existence of strike slip or toroidal motion toroidal motion is uncharacteristic of basic thermal convection but necessarily forms through the interaction of convective flow and nonlinear rheological mechanisms recent studies have implied that the empirically determined power law rheologies of mantle silicates are not sufficient to generate the requisite toroidal motion a simple source sink model of mantle or lithospheric flow shows that dynamic self lubrication which arises through the coupling of viscous heating and temperature dependent viscosity is highly successful at generating strike slip motion in particular as the viscosity of the fluid system becomes more temperature dependent the toroidal flow field makes an abrupt transition from a state of weak unplate like motion to a state with intense and extremely focused structure in essence the fluid dynamical model develops strike slip faults

viscous heating in silicate melts an experimental and numerical comparison 1 the transition from newtonian to non newtonian flow of silicate melts is commonly manifested as shear thinning at conditions of high stress and strain rate shear thinning may strongly influence the dynamics of magmatic flows but the details of its microscopic origins are not fully understood here we consider viscous heating and thermomechanical coupling as a potential cause of shear thinning we compare the results of laboratory uniaxial compression experiments of a silicate melt with the results of thermomechanical numerical simulations corresponding to the experimental setup both the experimental and numerical results concord and indicate that the reduction of the temperature dependent viscosity in flowing silicate melts is a result of viscous heating viscous heating was quantified for glasses with viscosities ranging from 108 to 1011 pa s and strain rates from 10 5 to 100 s 1 the results of 48 compression experiments indicate that the transition from newtonian to non newtonian flow in the silicate melt occurs at a brinkmann number i e ratio of heat gained to heat lost around 1 whereas brittle behavior dominates the melt deformation when the deborah number i e ratio of viscoelastic relaxation time to characteristic deformation time is larger than around 0 01 the observed viscous heating significantly contributes to the viscosity decrease observed in high stress strain rate experiments and questions our current understanding of the non newtonian deformation behavior of silicate melts

numerical modelling of spontaneous slab breakoff dynamics during continental collision slab detachment or breakoff is directly associated with phenomena like morphological orogenesis occurrenceofearthquakesandmagmatism atdepththedetachmentprocessisslowand characterized by viscous rheolgy whereas closer to the surface the process is relatively fast and plastic using a 2d mantle model 1500 km deep and 4000 km wide we investigated with finite difference and marker in cell numerical techniques the impact of slab age convergence rate and phase transitions on the viscous mode of slab detachment in contrast to previous studies exploring simplifiedbreakoffmodelsinwhichtheblockageresponsibleforinducingbreakoffiskinematically prescribed we constructed a fully dynamic coupled petrological thermomechanical model of viscousslabbreakoff inthismodel forcedsubductionofa700 km longoceanicplatewasfollowed bycollisionoftwocontinentalplatesandspontaneousslabblockingresultingfromthebuoyancyof thecontinentalcrustonceithadbeensubductedtoadepthof100 124 km typically fivephasesof model development can be distinguished a oceanic slab subduction and bending b continental collision initiation followed by the spontaneous slab blocking thermal relaxation and unbending in experiments with old oceanic plates in this phase slab roll back occurs c slab stretching and necking d slab breakoff and accelerated sinking and e post breakoff relaxation our experiments confirm a correlation between slab age and the time of spontaneous viscous breakoff as previously identified in simplified breakoff models the results also demonstrate a non linear dependence of the duration of the breakoff event on slab age a positive correlation being characteristic of young 50 ma slabs while for older slabs the correlation is negative the increasing duration of the breakoff with slab age in young slabs is attributed to the slab thermal thickness which increases both the slab thermal relaxation time and duration of the neckingprocess inolderslabsthistendencyiscounteractedbynegativeslabbuoyancy whichgen eratehigherstressesthatfacilitateslabneckingandbreakoff apredictionfromourbreakoffmodels isthattheolivine wadsleyitetransitionplaysanimportantroleinlocalizingviscousslabbreakoffat depths of 410 510 km due to the buoyancy effects of the transition

internal heating and thermal constraints on the mantle we have carried out numerical simulations to study the average temperature and thermal structure of an internally heated mantle it is found that in an incompressible mantle the hottest parts are localized in regions of slow flow in the middle and upper mantle whereas in a compressible mantle they occur over laterally extensive regions near the core mantle boundary cmb for chondritic concentrations of heat producing elements hpe s the average thermal conductivity in the mantle must be high to avoid heating the core and the temperature of the cmb tcmb is low to avoid large scale melting the mantle may have been extensively molten in the archaean

constraints on minimum velocity variance for seismic traveltime tomography summary traveltime data together with known spacing between sources and receivers for seismic transmission tomography can be used to determine rigorous constraints on the minimum and maximum wave speeds in the propagating medium these constraints lead to a new minimum variance criterion on inversion algorithms these results do not improve the velocity reconstructions directly but provide rigorous and easily computed figures of merit to help evaluate the difficulty of the reconstruction problem and t he performance of tomographic inversion codes specifically these criteria may be used during preprocessing to decide whether linear or non linear traveltime tomography methods are required for analysing a given data set or during post processing to determine whether regularization methods used to constrain the maximum model variance were overly restrictive

the shallowater aubrite evidence for origin by planetesimal impacts shallowater is the only known unbrecciated igneous aubrite enstatite achondrite it is a coarse grained orthopyroxenite consisting of in vol poikilitic orthoenstatite crystals 80 up to 4 5 cm in size which contain as inclusions and in the interstices xenoliths of an assemblage of twinned low ca clinoenstatite 1 forsterite 2 9 plagioclase 2 5 metallic fe ni 3 3 troilite 2 9 schreibersite 0 4 weathered opaques 8 and traces of niningerite and oldhamite conclusions 1 shallowater is of igneous origin 2 shallowater could not have formed by internal igneous nor by impact melting processes either on the h or l enstatite chondrite or aubrite parent bodies instead shallowater appears to be a sample of yet a fourth enstatite type asteroid 3 shallowater experienced an extraordinarily complex and unusual three stage cooling history as determined by mineral stability considerations and modelling of micro structural evolution as a function of cooling rate stage 1 very fast cooling from 1 580 c to somewhere above 712 c at a rate of 100 c hour through 1000 c above 1000 c the rate was probably much faster stage 2 very slow cooling annealing from 712 to 680 c at a rate of 7 5 c 106 y stage 3 fast cooling from 680 to 600 c at a rate of 0 5 c day and from 600 to 300 c at a rate of 0 4 c day 4 this complex cooling history suggests an equally unusual origin for shallowater as follows a completely or partly molten asteroid of nearly pure enstatite composition was broken up by low velocity impact with a solid e like object the melt cooled very fast stage 1 due to incorporation of cold 20 projectile debris the xenoliths fragments reassembled into a rubble pile body while t was 712 c and deeply buried materials like the object that is now shallowater cooled very slowly from 712 to 680 c stage 2 the anneal of stage 2 requires burial at a depth of 40 km on a 100 km diameter asteroid excavation from this depth by impact to account for the fast cooling of stage 3 can be accounted for in two scenarios scenario 1 during catastrophic impact the shallowater object could have been by chance near the surface within 10 m of a very large fragment thus accounting for the fast cooling and for surviving for 4 5 109 y scenario 2 the shallowater object may have been excavated by break up and subsequently buried near the surface of a gravitationally reassembled body this would require yet a second break up and reassembly episode 5 the xenoliths represent an enstatite like meteorite type unknown as an individual fall not unexpected for a rock type from an asteroid that appears to have been largely destroyed early in the history of the solar system

manganese chromium systematics in sulfides of unequilibrated enstatite chondrites we measured with a secondary ion mass spectrometer mn cr ratios and cr isotopes in individual grains of mn bearing sulfides i e sphalerites zns alabandites mns and niningerites mgs in nine unequilibrated enstatite chondrites uecs the goals were to determine whether live 53mn half life 3 7 ma was incorporated in these objects at the time of their isotopic closure and to establish whether mn cr systematics in sulfides in uecs can be used as a high resolution chronometer to constrain formation time differences between these meteorites sulfide grains analysed in four of these uecs mac 88136 el3 mac 88184 el3 mac 88180 el3 and indarch eh4 have clear 53cr excesses these 53cr excesses can be very large 53cr 52cr ranges up to 18 400 the largest 53cr excess measured so far and in some grains are well correlated with the mn cr ratios thus they were most likely produced by the in situ decay of 53mn in the meteorite samples in the remaining five meteorites no detectable excesses of 53cr were found and only upper limits on the initial 53mn 55mn ratios could be established the four meteorites with 53cr excesses show variations in the inferred 53mn 55mn ratios in various sulfide grains of the same meteorite the mn cr systematics in these sulfides were disturbed during and or after the decay of 53mn by varying degrees of reequilibration provided 53mn was homogeneously distributed in the region of the early solar system where these objects formed the data suggest that the time of the last isotopic equilibration of sulfides in el chondrites occurred at least 3 ma after a similar episode in eh chondrites

lithospheric thinning during the evolution of continental large igneous provinces a case study from the north atlantic tertiary province lithospheric thickness can act as an important physical control on the composition of asthenosphere derived melts by restricting the minimum depth at which decompression melting can occur a case study of plume related tertiary lavas from the isle of mull scotland has revealed the presence of three successive magma types which become progressively more depleted in the incompatible trace elements geochemical modeling suggests that melting initially took place within the garnet iherzolite stability field but subsequently involved more extensive melting 10 of spinel iherzolite at shallower mantle depths these results can be explained in terms of a progressively thinning lithosphere beneath the region during the evolution of the volcanic center the observed degree of lithospheric thinning cannot be explained by extension alone and it appears that some relatively rapid lower lithospheric erosion by the plume head is also required theoretical modeling suggests that lithospheric erosion can be a comparatively rapid process and this has important implications for the various starting plume models

abee and related eh chondrite impact melt breccias abstract the abee eh chondrite is an impact melt breccia about 80 90 of the chondrules were melted many of those that remain have been partly resorbed euhedral enstatite phenocrysts are abundant many have nucleated on the surfaces of relict chondrules and now protrude into kamacite globules euhedral laths of graphite essentially identical to those produced magmatically on earth occur throughout the rock abee experienced two episodes of impact melting with an intervening period of brecciation the first impact melting event produced a relatively homogeneous rock with abundant melt upon cooling the rock was shattered by bombarding meteoroids igneous oldhamite rich dark inclusions were introduced at this time a second impact event produced melt which engulfed the abee fragments and dark inclusions primarily digesting the small ones heat lost to the surrounding clastic debris caused the groundmass melt to quench previously described diamonds in abee containing typical solar system isotopic compositions of n and xe were probably produced from graphite by shock several other eh chondrites may also be impact melt breccias the most extensively melted of these is adhi kot which contains shocked but largely unmelted chondrule rich clasts as well as chondrule free sulfide and silica rich clasts that also appear to be impact products

ar ar ages and thermal histories of enstatite meteorites abstract compared with ordinary chondrites there is a relative paucity of chronological and other data to define the early thermal histories of enstatite parent bodies in this study we report 39ar 40ar dating results for five el chondrites khairpur pillistfer hvittis blithfield and forrest five eh chondrites parsa saint marks indarch bethune and reckling peak 80259 three igneous textured enstatite meteorites that represent impact melts on enstatite chondrite parent bodies zaklodzie queen alexandra range 97348 and queen alexandra range 97289 and three aubrites norton county bishopville and cumberland falls several ar ar age spectra show unusual 39ar recoil effects possibly the result of some of the k residing in unusual sulfide minerals such as djerfisherite and rodderite and other age spectra show 40ar diffusion loss few additional ar ar ages for enstatite meteorites are available in the literature when all available ar ar data on enstatite meteorites are considered preferred ages of nine chondrites and one aubrite show a range of 4 50 4 54 ga whereas five other meteorites show only lower age limits over 4 35 4 46 ga ar ar ages of several enstatite chondrites are as old or older as the oldest ar ar ages of ordinary chondrites which suggests that enstatite chondrites may have derived from somewhat smaller parent bodies or were metamorphosed to lower temperatures compared to other chondrite types many enstatite meteorites are brecciated and or shocked and some of the younger ar ar ages may record these impact events although impact heating of ordinary chondrites within the last 1 ga is relatively common for ordinary chondrites only bethune gives any significant evidence for such a young event

temperature dependent newtonian and non newtonian convection implications for lithospheric processes convection studies of temperature dependent newtonian and non newtonian rheology in aspect ratio seven boxes have been carried out for volume averaged rayleigh numbers between o 105 and o 107 large lateral viscosity variations o 104 are found for non newtonian cases with total temperature dependent viscosity variations up to 250 flow structures for both rheologies are distinguished by large scale circulations with relatively stable descending limbs much larger hot thermal anomalies are found near the surface for non newtonian rheology lithospheric thinning is facilitated by non newtonian rheology because of stress softening and the lubrication of the descending limbs by hot diapirs

an early i xe age for cb chondrite chondrule formation and a re evaluation of the closure age of shallowater enstatite the iodine xenon system has been analyzed in samples of 7 chondrules from the cb chondrites gujba and hammadah al hamra hah 237 one sample from gujba defined a high temperature iodine xenon isochron corresponding to closure 1 87 0 4 ma before closure of shallowater enstatite motivated by this result we employ outlier rejection to re evaluate the shallowater age leading to a modified value of 4562 3 0 4 ma 1 in this process the datum obtained by combining our i xe age for gujba with the literature pb pb age is rejected as an outlier indicating that in this sample the i xe system closed earlier than the accepted pb pb age of chondrules from cb chondrites the need for a formation environment distinct from that of chondrules from other meteorites is thus reduced

dynamic topography of passive continental margins and their hinterlands since the cretaceous abstract even though it is well accepted that the earth s surface topography has been affected by mantle convection induced dynamic topography its magnitude and time dependence remain controversial the dynamic influence to topographic change along continental margins is particularly difficult to unravel because their stratigraphic record is dominated by tectonic subsidence caused by rifting we follow a three fold approach to estimate dynamic topographic change along passive margins based on a set of seven global mantle convection models we first demonstrate that a geodynamic forward model that includes adiabatic and viscous heating in addition to internal heating from radiogenic sources and a mantle viscosity profile with a gradual increase in viscosity below the mantle transition zone provides a greatly improved match to the spectral range of residual topography end members as compared with previous models at very long wavelengths spherical degrees 2 3 we then combine global sea level estimates with predicted surface dynamic topography to evaluate the match between predicted continental flooding patterns and published paleo coastlines by comparing predicted versus geologically reconstructed land fractions and spatial overlaps of flooded regions for individual continents since 140 ma modelled versus geologically reconstructed land fractions match within 10 for most models and the spatial overlaps of inundated regions are mostly between 85 and 100 for the cenozoic dropping to about 75 100 in the cretaceous regions that have been strongly affected by mantle plumes are generally not captured well in our models as plumes are suppressed in most of them and our models with dynamically evolving plumes do not replicate the location and timing of observed plume products we categorise the evolution of modelled dynamic topography in both continental interiors and along passive margins using cluster analysis to investigate how clusters of similar dynamic topography time series are distributed spatially a subdivision of four clusters is found to best reveal end members of dynamic topography evolution along passive margins and their hinterlands differentiating topographic stability long term pronounced subsidence initial stability over a dynamic high followed by moderate subsidence and regions that are relatively proximal to subduction zones with varied dynamic topography histories along passive continental margins the most commonly observed process is a gradual motion from dynamic highs towards lows during the fragmentation of pangea reflecting the location of many passive margins now over slabs sinking in the lower mantle our best fit model results in up to 500 150 m of total dynamic subsidence of continental interiors while along passive margins the maximum predicted dynamic topographic change over 140 million years is about 350 150 m of subsidence models with plumes exhibit clusters of transient passive margin uplift of about 200 200 m but are mainly characterised by long term subsidence of up to 400 m the good overall match between predicted dynamic topography to geologically mapped paleo coastlines makes a convincing case that mantle driven topographic change is a critical component of relative sea level change and indeed the main driving force for generating the observed geometries and timings of large scale continental inundation through time

the diversity of tectonics from fluid dynamical modeling of the lithosphere mantle system abstract numerical modeling of the lithosphere has been carried out from many points of view most popular and prevalent is the use of the kinematic boundary conditions to move the lithosphere about in the dynamics we have approached the numerical modeling of the lithosphere mantle system by treating this as a fluid dynamical system in which crustal and mantle rheologies and thermal and compositional buoyant forces play dominant roles in delivering the dynamics in a self consistent manner we have investigated the development of extensional and compressional tectonic regimes from variations of the crustal density and compare their temporal evolutions in comparing with kinematically driven thickening of the lithosphere we find that the timescales are more self consistently determined in a full fluid dynamical treatment than in kinematic models we emphasize the delicate nature of balancing the chemical buoyancy of the crust with the thermal negative buoyancy of the mantle lithosphere as the modes and associated timescales of the delamination process are controlled critically by their relative importance the role of an actively participating buoyant crust cannot by ignored in the overall lithosphere mantle dynamics

manganese chromium isotope systematics of enstatite meteorites abstract we present results of a study of the 53 mn 53 cr isotope systematics in the enstatite chondrites and achondrites aubrites the goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short lived radionuclide 53 mn our earlier work lugmair and shukolyukov 1998 shukolyukov and lugmair 2000a has shown that the asteroid belt bodies are characterized by essentially the same initial 53 mn abundance however we have found the presence of a gradient in the abundance of the radiogenic 53 cr between the earth moon system mars and the asteroid vesta if this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated this can be explained either by an early volatility controlled mn cr fractionation in the nebula or by an original radially heterogeneous distribution of 53 mn the enstatite chondrites are suggested to form in the inner zones of the solar nebula much closer to the sun than the ordinary chondrites therefore their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial 53 mn we have studied the bulk samples of the eh4 chondrites indarch and abee and the el6 chondrite khairpur although these meteorites have essentially the same mn cr ratio as the ordinary chondrites the relative abundance of the radiogenic 53 cr is three times smaller than in the ordinary chondrites because these meteorites are primitive undifferentiated and no mn cr fractionation had occurred within their parent bodies this difference is a strong argument in favor of an initially heterogeneous distribution of 53 mn in the early inner solar system this finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula using the characteristic 53 cr excess of the enstatite chondrites and the observed gradient their place of origin falls at about 1 4 au or somewhat closer to the sun i e 1 0 1 4 au we also present chronological results for the enstatite chondrites and achondrites the absolute 53 mn 53 cr ages of the eh4 chondrites are old 4565 ma the el6 chondrite khairpur is 4 5 ma younger which is in good agreement with the 129 i 129 xe data from the literature the age of the aubrite pena blanca spring appears to be similar to those of the enstatite chondrites while that of the aubrite bishopville is at least 10 ma younger which is also in agreement with the 129 i 129 xe data the results from bulk samples of aubrites indicate that the last mn cr fractionation in their parent body occurred 4563 ma ago and imply an evolution of the mn cr isotope system in an environment with an higher than chondritic mn cr ratio for several millions of years

dynamical consequences on fast subducting slabs from a self regulating mechanism due to viscous heating in variable viscosity convection the authors have studied 2 d time dependent convection for a rheology which is both non newtonian and temperature dependent strong effects associated with viscous heating are found in the downwelling sheets which are heated on both sides with an intensity around o 10 sup 2 times the chondritic value the magnitude of viscous heating increases strongly with the subduction speed the slab interior is weakened by viscous heating and slab breakoff then takes place this process provides a self regulating mechanism for governing the speed of intact slabs able to reach the deep mantle timescales associated with viscous heating are quite short a few million years internal heating by radioactivity decreases the amount of shear heating 13 refs 5 figs

layered convection induced by phase transitions we report a systematic study on the conditions under which an endothermic phase transition can enforce layered convection two dimensional numerical calculations of convection in a domain containing a divariant phase change were performed in the framework of the extended boussinesq approximation i e considering the effects of adiabatic gradient latent heat and frictional heating in the energy equation we find that the critical value of the negative clapeyron slope which must be surpassed in order to induce layered convection decreases in magnitude with increasing rayleigh number ra in the range 104 ra 2 106 near the critical clapeyron slope vacillations between double and single layer convection or strongly leaking double layer convection are possible the breakdown into layers is influenced very little by the latent heat release but depends solely on the phase boundary deflection caused by lateral temperature differences the value of the critical clapeyron slope also seems little affected by the width of the transition zone or by its depth a possible superplastic rheology within the transition zone would tend to favor layered convection scaling the model results to the 670 km discontinuity in the earth s mantle as a possible endothermic phase boundary we estimate the critical clapeyron slope to be in the range of 4 to 8 mpa k 40 to 80 bar k the possibility that the spinel perovskite periclase transition is within this range appears to be remote but certainly cannot be neglected

thermomechanical modeling of pulsation tectonics and consequences on lithospheric dynamics pulsating phenomena can control continental breakup and the formation and closing of oceanic basins using a two dimensional time dependent convection model with a non linear temperature and depth dependent rheology we have obtained self consistently such types of pulsating behavior very hot mantle plumes ascend to the surface where they can quickly thin the lithosphere down to a few kilometers an episode of intense shear heating then ensues which is comparable to the length of the main episode for flood volcanism lasting a couple of myrs the highest plate velocities occur during this period when both the lithosphere and slab are profusely lubricated by the impingement of the hot plume

thermomechanical modelling of slab detachment abstract slab detachment or breakoff is appreciated as an important geological process as shown by recent tomographic imaging using a 2 d upper mantle model 660 km deep and 2000 km wide we have investigated with a 2 d finite difference and marker in cell numerical technique the multi resolutional character of the thermomechanical phenomena related to this complex geological process our experiments show that this process can be initiated in form of slab necking by a prolonged 8 30 my period of slab weakening due to thermal diffusion 60 c my due to strong positive thermal feedback from shear heating detached slab fragments sink rapidly with a tendency for coherent rotation the influence of temperature and pressure dependent thermal conductivity on the process of thermal weakening of the slab is quite significant this supports the idea that the breakoff process is triggered by thermal diffusion on a time scale linearly dependent on heat conductivity rapid topographic changes and increasing volcanic activities due to the melting of subducted oceanic crust are possible scenarios of this vigorously driven geodynamic process

various influences on plumes and dynamics in time dependent compressible mantle convection in 3 d spherical shell the effects on the dynamics of compressible mantle convection from increasing rayleigh number viscosity stratification and internal heating are investigated in a time dependent 3 d spherical shell model resolution of up to spherical harmonics of degree 192 and 150 higher order finite difference grid points unevenly distributed in the radial direction have been employed surface rayleigh number varies from 5 104 to o 107 the viscosity profiles used range from constant to a depth dependent viscosity profile the adams williamson adiabatic assumption is adopted for describing the equation of state both depth dependences of thermal expansivity and thermal conductivity are included we have focused on the viscosity stratification in the lower mantle the mean temperature profiles are very sensitive to the input parameters such as the amount of internal heating and the dissipation number in the isoviscous model the number of hot plumes decreases for low rayleigh number 5 104 and increases dramatically with higher rayleigh number 106 in the model with a high viscosity zone in the lower mantle the number of plumes is small and does not change too much when the rayleigh number varies from 3 106 to 8 106 in the purely base heated viscously stratified models well organized triple junctions of the downwelling sheets are developed near the surface and interconnected cold walls of downwellings are produced around upwelling plumes the presence of internal heating increases the number of upwelling plumes and breaks up the interconnection of downwelling sheets strong viscous heating in the compressible solutions is found near the top and bottom at the higher values of rayleigh number the laterally averaged value of viscous dissipation is comparable with the chondritic strength of internal heating whether the maximum viscous heating is found in upwelling plumes or downwelling flows depends on the presence of internal heating and the mechanical boundary conditions the effects of compressibility on local structures and plume morphologies can be significant however these compressible effects are moderate on the global patterns

great earthquakes and slab pull interaction between seismic coupling and plate slab coupling great earthquakes the few largest earthquakes that account for most of the earth s seismic energy release have occurred at only a few subduction zones around the world strong locking or seismic coupling of the interface between plates at certain subduction zones is often invoked to explain these great earthquakes although past studies have correlated strong seismic coupling with a compressional stress environment that is characterized by back arc compression and caused by trenchward motion of the overriding plate the consequences of this compressional environment for the tectonic forces that drive global plate motions are not yet clear to examine these consequences we compared subduction zone earthquake magnitudes to tectonically constrained estimates of the degree to which each slab transmits its excess weight as a direct pull force on a subducting plate at seismically uncoupled subduction zones that generate only moderate sized earthquakes we find that slabs must transmit nearly their entire upper mantle weight as a pull force on the subducting plate at seismically coupled subduction zones that produce great earthquakes however we find that slabs must be nearly completely detached from their subducting plates this suggests that slabs subducting in a compressional environment experience stress induced weakening that prevents the effective transmission of the slab pull force convergent mantle flow above a descending slab that becomes decoupled from its surface plate may induce additional surface compression that further locks the subduction zone and leads to additional slab decoupling and detachment the resulting redistribution of plate driving forces may be responsible for rapid changes in plate motion c 2003 elsevier b v all rights reserved

comparison of mixing properties in convection with the particle line method spatial resolution in mixing processes is an acute problem we propose a line method akin to the contour dynamics technique which is an extension of the particle method but with the particles redistributed on the line with time we have used up to 10 5 particles per line and ten lines to investigate the dynamical and structural properties of mixing for both newtonian and non newtonian temper ature dependent viscosity convection in 2d geometry the spatial structures and the time history of the lines formed in newtonian convection are dierent from those produced in non newtonian convection which has the tendency for producing long living horizontal structures ecient mixing in the upper mantle would be inhibited by non newtonian rheology

the i xe chronometer and the early solar system we review the development of the i xe technique and how its data are interpreted and specify the best current practices individual mineral phases or components can yield interpretable trends in initial 129i 127i ratio whereas whole rock i xe ages are often hard to interpret because of the diversity of host phases many of which are secondary varying standardizations in early work require caution only samples calibrated against shallowater enstatite or bjurble can contribute reliably to the emerging i xe chronology of the early solar system although sparse data for which i xe and mn cr can be compared suggest that the two systems are concordant among ordinary chondrite samples we derive a new age for the closure of the shallowater enstatite standard of 4563 3 0 4 myr from the relationship between the i xe and pb pb systems this yields absolute i xe ages and allows data from this and other systems to be tested by attempting to construct a common chronology of events in the early solar system absolute i xe dates for aqueous and igneous processes are consistent with other systems consideration of the i xe host phases in cais and dark inclusions demonstrates that here the chronometer records aqueous alteration of pre existing material the ranges of chondrule ages deduced from the al mg and i xe systems in semarkona ll3 0 and chainpur ll3 4 are consistent chainpur i xe data exhibit a greater range of ages than semarkona possibly reflecting a greater degree of parent body processing however individual chondrules show little or no evidence of such processing determining the host phase s responsible for high temperature correlations may resolve the issue

the evolution of material surfaces in convection with variable viscosity as monitored by a characteristics based method we have studied the evolution of material surfaces in strongly time dependent convection for both newtonian and non newtonian temperature and depth dependent rheologies a spline characteristic method has been employed the method of characteristics is second order in time and fourth order in space our strategy is to employ a very dense grid for the material surface field with about 50 to 100 times more points than for the temperature field grid we have detected sharp entrainment of the surrounding material into both the ascending and descending flow structures we have observed the breakoff process involving the descending plume mixing takes place very differently for the two rheologies with time small vortical features are developed inside the newtonian plumes while unmixed islands still persist in the non newtonian flow

micro x ray diffraction assessment of shock stage in enstatite chondrites abstract a new method for assessing the shock stage of enstatite chondrites has been developed using in situ micro x ray diffraction xrd to measure the full width at half maximum fwhm of peak intensity distributed along the direction of the debye rings or chi angle corresponding to individual lattice reflections in two dimensional xrd patterns this xrd technique differs from previous xrd shock characterization methods it does not require single crystals or powders in situ xrd has been applied to polished thin sections and whole rock meteorite samples three frequently observed orthoenstatite reflections were measured 020 610 and 131 these were selected as they did not overlap with diffraction lines from other phases enstatite chondrites are commonly fine grained stained or darkened by weathering shock induced oxidation and metal sulfide inclusions furthermore most e chondrites have little olivine or plagioclase these characteristics inhibit transmitted light petrography nevertheless shock stages have been assigned macalpine hills mac 02837 el3 s3 pecora escarpment pca 91020 el3 s5 mac 02747 el4 s4 thiel mountains til 91714 el5 s2 allan hills alha 81021 el6 s2 elephant moraine eet 87746 eh3 s3 meteorite hills met 00783 eh4 s4 eet 96135 eh4 5 s2 lewis cliff lew 88180 eh5 s2 queen alexandra range que 94204 eh7 s2 lapaz icefield lap 02225 eh impact melt s1 for the six with published shock stages there is agreement with the published classification fwhm plotted against petrographic shock stage demonstrates positive linear correlation fwhm ranges corresponding to shock stages were assigned as follows s1 3 5 s6 not measured slabs of abee eh impact melt breccia and northwest africa nwa 2212 el6 were examined using xrd alone fwhm values place both in the s2 range consistent with literature values micro xrd analysis may be applicable to other shocked orthopyroxene bearing rocks

three dimensional flow in the subslab mantle three dimensional models of mantle flow at subduction zones make it possible to explain the common occurrence of trench parallel subslab seismic anisotropy subslab flow becomes inherently three dimensional when slab driven flow interacts with a wide variety of ambient background mantle flow conditions this interaction depends on slab geometries mechanical coupling parameters and lower mantle viscosities deflection of subslab mantle flow is a robust feature for all model parameters and geometries as the slab acts as an obstruction to the ambient background mantle flow background mantle flow can become trench perpendicular or trench parallel subslab flow depending on whether the ambient background mantle flow is deflected beneath the bottom of the slab or toward the edge of the slab the first case is especially prominent in models with short slabs that do not penetrate into the lower mantle the second case is especially prominent in models with long steep slabs the results are also highly sensitive to the amount of mechanical coupling between the subducting plate and the mantle beneath it high levels of coupling create a boundary layer of trench perpendicular entrained flow pushing the deflection due to the obstructing slab away from the slab we compare our subslab flow model predictions with a global set of seismic anisotropy fast directions in the subslab mantle and find generally good agreement between the anisotropy observations dominantly trench parallel or trench perpendicular and the mantle flow directions predicted for decoupled systems

first known el5 chondrite evidence for dual genetic sequence for enstatite chondrites the enstatite chondrites form two compositionally distinct groups the eh and el chondrites and four petrologic types types 3 6 where each type represents increasing degrees of metamorphic alteration1 2 prior to the recovery of the antarctic meteorite reckling peak a80259 the subject of the present report all known el chondrites were petrologic type 6 whereas all known eh chondrites were types 3 5 it has long been appreciated that the variations in bulk composition preclude a simple conversion of eh4 material into el6 material3 7 however complex models involving simultaneous variations in bulk composition and petrologic type have been discussed1 5 8 9 and may be implied by other classification schemes in common use that of anders keil type i intermediate type ii and that of van schmus and wood e3 e4 e5 e6 we report here the discovery of the first el5 chondrite an el5 breaks the eh3 4 eh5 el6 sequence and indicates that the enstatite chondrites constitute two discrete isochemical metamorphic sequences eh3 5 and el5 6

i xe ages of enstatite chondrites abstract in order to elucidate the early thermal history of enstatite chondrite parent bodies we determined 129i 129xe whole rock ages of enstatite chondrites 5 eh 2 el one eh impact melt relative to the shallowater reference meteorite 4562 3 0 4 ma all errors are 1 i xe ages of both el6 chondrites lon 94100 4 38 0 60 ma and neuschwanstein 3 87 0 73 ma negative sign indicates ages younger than shallowater agree well with data of other el6 chondrites lon 94100 displayed a second isochron at lower temperatures equivalent to a younger age of 5 25 1 17 ma perhaps reflecting different retention temperatures of respective carrier phases during sequential cooling the enstatite chondrites abee eh4 indarch eh4 eet 96135 eh4 5 and st marks eh5 encompass a i xe age range of 0 57 1 05 ma eet 96135 1 to 0 45 0 72 ma abee again in agreement with previously reported ages of eh chondrites only the age of st marks differs strongly from previously reported younger ages now being more in accordance with other members of the eh clan the eh3 chondrite sahara 97096 showed the youngest i xe age of 7 87 0 46 ma distinctly younger than other i xe ages of eh chondrites including other eh3s due to the apparent high retention temperature of the i xe system in enstatite estimated 800 c this young age implies a later resetting of the i xe system by a severe thermal likely impact induced event the eh impact melt lap 02225 records a similarly young thermal event though no isochron relationship could be established the data fall within an apparent i xe age range of 5 to 15 ma similar to sahara 97096 overall eh chondrite parent body experienced a thermal history determined by a complex interplay between impact disturbances and parent body metamorphism

129i 127i variations among enstatite chondrites abstract this paper presents the 129i 127i composition for 11 different enstatite chondrites there is a well defined hiatus in iodine isotopic compositions separating the enstatite chondrites the hiatus correlates with a similar hiatus defined by variations in chemical composition both major and minor elements eh 4 5 chondrites have 129i 127i ratios 20 higher 4 m y older than el 6 chondrites the enstatite chondrites are the third meteorite group joining silicates in iab irons and c3v carbonaceous chondrites in which 129i 127i correlates with chemical composition within each enstatite chondrite group the variations in initial iodine isotopic composition is less than 4 comparable to the experimental resolution the 20 difference between eh and el may indicate a maximum iodine isotopic heterogeneity between the eh and el nebular reservoirs alternatively if eh and el chondrites evolved from a homogeneous iodine isotopic reservoir then the el group must have evolved 4 0 m y longer than the eh group and either in an environment with a much lower i xe ratio than presently observed or one which was open to the loss of radiogenic 129xe the latter case may imply a nebular environment and would therefore be consistent with the open system evolution required to produce the chemical hiatus the duration of subsequent evolution in the high i xe environment must have been 1 m y and probably represents thermal metamorphism in two different parent bodies

testing an integrated chronology i xe analysis of enstatite meteorites and a eucrite we have determined initial 129i 127i ratios for mineral concentrates of four enstatite meteorites and a eucrite in the case of the enstatite meteorites the inferred ages are associated with the pyroxene rich separates giving pyroxene closure ages relative to the shallowater standard of indarch eh4 0 04 0 67 ma khairpur el6 4 22 0 67 ma khor temiki aubrite 0 06 ma and itqiy enstatite achondrite 2 6 2 6 ma negative ages indicate closure after shallowater no separate from the cumulate eucrite asuka a 881394 yielded a consistent ratio though excess 129xe was observed in a feldspar separate suggesting disturbance by thermal metamorphism within 25 ma of closure in shallowater iodine 129 ages are mapped to the absolute pb pb time scale using the calibration proposed by gilmour et al 2006 who place the closure age of shallowater at 4563 3 0 4 ma comparison of the combined 129i pb data with associated 53mn ages for objects that have been dated by both systems indicates that all three chronometers evolved concordantly in the early solar system the enstatite chondrites are offset from the linear array described by asteroid belt objects when 53mn ages are plotted against combined 129i pb data supporting the suggestion that 53mn was radially heterogeneous in the early solar system

deflection of mantle flow beneath subducting slabs and the origin of subslab anisotropy global compilations of subslab shear wave splitting parameters show a mix of trench parallel and trench perpendicular fast directions that often directly contradict predictions from two dimensional models of slab entrained flow here we show that subslab anisotropy is consistent with three dimensional geodynamic models that feature the interaction between subducting slabs and regional mantle flow each model represents a specific region for which high quality source side shear wave splitting data are available we compare the distribution of finite strain in the models with shear wave splitting observations showing that both trench parallel and trench perpendicular fast directions can be explained by deflection of regional mantle flow around or beneath subducted slabs subslab maximum elongation directions calculated from our models depend on a combination of geometry factors such as slab dip angle and maximum depth mechanical parameters such as decoupling between the slab and the subjacent mantle and the orientation and magnitude of the regional mantle flow

the role of shear heating in lubricating mantle flow abstract the role of viscous heating in mantle flows has been studied within the framework of a steady state two dimensional cartesian model with a composite non newtonian and newtonian rheology there is temperature dependence in the upper mantle rheology and temperature and pressure dependent rheology in the lower mantle we have considered adiabatic and viscous heating using the extended boussinesq approximation we have selected as the only control variable the temperature at the core mantle boundary cmb we have compared systematically both the boussinesq and extended boussinesq solutions for the same rheology and same amount of internal heating by varying the temperature at the cmb from 2000 k to around 10 000 k viscosity contrasts in the system increase with the vigor of convection and reach up to five orders in magnitude both the vigor of convection and the global viscous heating increases systematically with the temperature at the cmb the rms velocity shows a power law type dependence on the temperature at the cmb with a power law exponent of 1 5 for high cmb temperatures greater then 3500 k a focused concentration of shear heating promotes the development of high strain rate and low viscosity zones next to descending flows which is primarily non newtonian in character in the high cmb temperature regime much more pronounced shear zones are developed along the downwelling for models with viscous heating present than for the boussinesq flows illustrating the important role played by mechanical heating on lubricating limbs in mantle circulation

the i xe record of alteration in the allende cv chondrite abstract complex i xe and mineralogical studies have been performed on four heavily altered allende fine grained spinel rich ca al rich inclusions cais and four allende dark inclusions dis showing various degrees of iron alkali metasomatic alteration the cais are largely composed of fe rich spinel al diopside and secondary nepheline and sodalite the dis consist of chondrules and allende like matrix composed of lath shaped fayalitic olivine nepheline sodalite and ca fe rich pyroxene andradite feni sulfide nodules chondrule phenocrysts are extensively or completely replaced by fayalitic olivine nepheline and sodalite metal nodules are replaced by feni sulfides andradite and ca fe rich pyroxenes the chondrules and matrices are crosscut by ca fe rich pyroxene feni sulfide fayalitic olivine veins dis are surrounded by continuous ca rich rims composed of andradite wollastonite kirschsteinite and ca fe rich pyroxenes whereas the outer portions of the inclusions are depleted in ca three cais yield well defined i xe isochrons with ages 3 1 0 2 3 0 0 2 and 3 7 0 2 ma younger than the shallowater internal standard 4566 2 ma similar release profiles suggest the same iodine carrier most probably sodalite for all four cais the allende dis yield i xe ages from 0 8 0 3 to 1 9 0 2 ma older than shallowater based on the petrographic observations we infer that the dis experienced at least two stage alteration during an early stage of the alteration which took place in an asteroidal setting but not in the current location of the dis chondrule silicates were replaced by secondary fayalitic olivine nepheline and sodalite calcium lost from the chondrules was redeposited as ca fe rich pyroxene veins and ca fe rich pyroxene andradite nodules in the matrix the second stage of alteration resulted in mobilization of ca from the dis and its re deposition as ca rich rims composed of ca fe rich pyroxenes andradite and wollastonite around the dis we interpret i xe ages of the dis as time of their alteration prior incorporation into allende the younger i xe ages of the fine grained spinel rich cais may reflect hydrothermal alteration of the allende host which could have occurred contemporaneously with the second stage of alteration of the allende dis the lack of evidence for the disturbance of i xe system in the allende dis may suggest that fluid responsible for the alteration of the allende cais was in equilibrium with the i and xe bearing phases of the dis