comparison of dynamic flow models for the central aleutian and tonga kermadec subduction zones using three dimensional 3 d instantaneous dynamic models of the central aleutian subduction zone we show that a low viscosity low density deltarho 10 kg m3 region in the wedge is needed to match observations of topography the geoid and the stress state in the slab and overriding plate previous models of the tonga kermadec subduction zone also require a low viscosity low density deltarho 20 kg m3 region in the wedge a low viscosity region in the wedge decouples the overriding plate from the downward flow in the wedge caused by the sinking slab reducing the magnitude of negative dynamic topography on the overriding plate and changing the state of stress in the slab from down dip tension to down dip compression if the low viscosity region is large as in the tonga kermadec models then the decoupling may also cause the overriding plate to go into extension these are the first dynamic models to simultaneously match the state of stress in the slab and overriding plate and suggest that the combination of stress state within the slab and overriding plate may provide an additional constraint on the viscosity structure within subduction zones

mn cr isotope systematics of the d orbigny angrite we have conducted a detailed study of the mn cr systematics of the angrite d orbigny here we report cr isotopic abundances and mn cr ratios in olivine pyroxene glass chromite and bulk rock samples from d orbigny 53cr excesses in these samples correlate well with their respective mn cr ratios and define an isochron with a slope that corresponds to an initial 53mn 55mn ratio 3 24 0 04 x 10 6 and initial 53cr 52cr ratio of 53 0 30 0 03 at the time of isotopic closure the 53mn 55mn ratio of the d orbigny bulk rock is more than two fold the 53mn 55mn ratio of the angrites lewis cliff 86010 lew and angra dos reis ador and implies an older mn cr age of 4562 9 0 6 ma for d orbigny relative to a pb pb age of 4557 8 0 5 ma for lew and ador one of the most unusual aspects of d orbigny is the presence of glass a phase that has not been identified in any of the other angrites the mn cr data for glass and a pyroxene fraction found in druses indicate that they formed contemporaneously with the main phases of the meteorite since the mn cr age of d orbigny is 5 ma years older than the angrites lew and ador d orbigny likely represents an earlier stage in the evolution of the angrite parent body

re os systematics in chondrites and the fractionation of the platinum group elements in the early solar system we have investigated the re os system for samples of whole rock metal and sulfide from ordinary chondrites using closed system analytical techniques we found complete exchange between sample and tracer isotopes for silicate containing samples and obtained precise and reliable re os concentration measurements results on two group iva iron meteorites and on a silicate rich iron steinbach iva an are consistent with the iva ivb isochron and support the previous observation that iva ivb irons may be slightly older than iiab irons data on whole rock fragments and metal rich separates from the st severin chondrite ll6 show a large range in 1870re 188 os and in 187 os 188 os which makes possible in principle the determination of a re os internal isochron on a chondrite for the first time this re os fractionation may be due to partial melting of fenis and macroscopic redistribution of metal and sulfide the st severin data show a good correlation line on a 187 re 187 os evolution diagram if this is considered to represent an internal isochron it gives an age t 4 68 0 15 ae 187 re 1 64 10 11 a 1 and an initial 187 os 188 os 0 0 0953 0 0013 this age is in agreement with but slightly older than the more precise 187 re 187 os age for the iiab irons as well as for irons from other groups t 4 61 0 01 ae a st severin sulfide nodule has very low re and os concentrations and shows a young re os model age 2 3 ae indicating recent element remobilization whole rock and metal rich separates of h group chondrites h3 to h6 yield restricted ranges in 187 re 188 os 0 42 0 47 and 187 os 188 os 0 128 0 133 there is a systematic difference between re os in the metal extracted from a chondrite and the bulk chondrite this shows that there is a small but significant re os fractionation within subsystems contained in the chondrites from whole rock samples of h group chondrites we calculate a mean 187 re 188 os 0 423 0 007 and 187 os 188 os 0 12863 0 00046 which may characterize the evolution of an average chondritic reservoir for re os the ordinary chondrite data plot close to the iiab isochron although the deviations found are larger than found for the irons the re os chronometer in iron meteorites is apparently controlled by the re os fractionation due to fractional crystallization of liquid metal re os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies in contrast the behavior in ordinary chondrites while also dominated by the metal phases must reflect fractionation and transport on a local macroscopic scale within the chondrites between the metal phases after aggregation due to partial melting of fenis or represent variable re os fractionation of the metal phases prior to the accretion of the chondrites however for st severin we attribute the major re os fractionation to early heating of the meteorite above the fe fes eutectic we do not consider that the re os fractionation observed in other chondrites is due to the redistribution of re and os during chondrite metamorphism including shock but it may plausibly represent earlier stages of re os fractionation for the different feni metal constituents prior to accretion

isotopic abundances inferences on solar system and planetary evolution members of the royal swedish academy ladies and gentlemen in thinking over why i am here today i recalled the story of tom sawyer a character of mark twain s on a beautiful bright saturday morning when all of his friends were playing he appeared by the wall in front of his aunt s house with a bucket of whitewash paint and a long handled brush before him stood about thirty meters of continuous wooden fence three meters tall he dipped his brush and passed it over a plank and then repeated it then he stopped and compared the insignificant whitewashed streak with the far reaching continent of unpainted fence and became melancholy then he took up the brush again and went to work a friend came by and teased him for working on saturday when he could be swimming or playing tom said it wasn t so bad because a boy didn t get the chance to whitewash a fence every day tom took a new brush stroke at the fence and stepped back to note the beauty of the effect his friend got interested and then asked permission to make a stroke or two he then offered tom the core of an apple he had just started to nibble on tom told him that it was doubtful that his friend could help because this was the front fence and that a very good and sophisticated job had to be done possibly only but one child in a thousand possibly two thousand could do it well after this denial his friend just had to try and offered tom the whole apple for a chance to paint then more young friends came around to attempt this challenging task by the end of the afternoon tom was the wealthiest boy in town he had twelve marbles a key that didn t unlock anything a piece of blue glass to look through a tin soldier a dog collar but no dog and lots of other valuable things the fence had been coated three times and if he had not run out of whitewash tom would have bankrupted every kid in the village my presence here is due to an ability to attract young talent to help whitewash the fence the only difference is that we have just started on the job

age of the eucrite caldera from convergence of long lived and short lived chronometers abstract the long lived 147 sm 143nd chronometer has been used in conjunction with the short lived 146 sm 142 nd and 53 mn 53 cr chronometers to constrain the time of formation for the eucrite caldera the combination of these three chronometers has produced results that are consistent with one another and converge on a precise high resolution age of 4537 12 ma within 95 confidence limits so far this is the youngest crystallization age that has been confirmed for a noncumulate eucrite thus this basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated asteroidal parent body most likely between 17 and 41 million years after the formation of the first condensates in the early solar system from these age constraints it is evident that caldera either formed within a magma body on a large 500 km asteroid or in an extensive impact melt on the crust of its parent body

the depth of trenches as a function of the subduction rate and age of the lithosphere abstract the trench is one of the most characteristic features of the subduction zones for a given arc or for all the subduction zones put together relationships between their depth variations the variations of the subduction rate and the age of the sinking lithosphere can be established certain other parameters have been examined with those mentioned above but without success it will be demonstrated that the observed relationships can be interpreted in terms of an elastic model and of a viscous model

crystallization recrystallization and impact metamorphic ages of eucrites y792510 and y791186 abstract monomict eucrites y792510 and y791186 of pyroxene metamorphic type 6 contain areas of recrystallized mesostasis and granoblastic pyroxene textural comparisons suggest that the mesostasis areas in these eucrites originally resembled analogous areas in the pristine basaltic eucrite clast y75011 84 39ar 40ar ages were determined for microbreccias y791186 87 and y792510 62 and crystalline clast y792510 65 the 39ar 40ar ages for y791186 87 and y792510 62 are identical within error limits at 3 46 0 18 ga and 3 40 0 16 ga 2 respectively both these samples were involved in an impact event in which ar was quantitatively degassed 3 43 0 10 ga ago y792510 65 has an apparently older 39ar 40ar age of 3 60 0 04 ga 2 suggesting that it was degassed in a separate event or to a lesser degree rb sr and sm nd ages were determined for both y792510 clasts the rb sr age of 3 26 0 24 ga 2 for y792510 65 was totally reset by the impact event s whereas the rb sr age of 4 26 0 62 ga 2 for y792510 62 either was partially reset by the same event s or by earlier recrystallization of mesostasis the 147sm 143nd age calculated for y792510 65 including all data is 4 67 0 22 ga 2 but is likely to have been affected by weathering in the antarctic it becomes 4 57 0 09 ga if only data for the major minerals plagioclase and pyroxene are included in the isochron the 147sm 143nd age of y792510 62 is 4 34 0 06 ga 2 for the same mineral phases but may be biased by the presence of weathered ca phosphates in the pyroxene separate the 146sm 142nd age of y792510 65 is 4 42 0 06 0 04 ga corresponding to initial 146sm 144sm 0 0030 0 0010 relative to 146sm 144sm lew 0 0076 0 0009 for the lew86010 angrite at 4 558 ga ago the y792510 65 ages from the two sm nd chronometers are concordant at 4 48 ga which may be the crystallization age of this eucritic basalt late crystallization alternatively these sm nd ages may have been affected by the event in which ca phosphate and augite in mesostasis were recrystallized and augite and chromite in granoblastic areas were derived from cloudy pyroxenes late recrystallization the subsolidus recrystallization may be a consequence of a thermal spike in the midst of an otherwise monotonically cooling environment resulting from impact cratering of the hot crust of the newly formed eucrite parent body pyroxene homogenization probably occurred prior to formation of granoblastic pyroxene and close to the time of initial crystallization mn cr data provide only a hint of live 53mn t1 2 3 7ma and imply either chromium isotopic equilibration after decay of 53mn or late crystallization impact brecciation and lithification probably accompanied thermal annealing 3 4 3 6 ga ago as given by the 39ar 40ar ages the rb sr age of y792510 65 is consistent with this interpretation the 39ar 40ar and rb sr systematics of minute k rich inclusions in plagioclase and pyroxene formed by previous impact and thermal annealing events probably were reset then these young impact metamorphic ages demonstrate that impact resetting of radiometric ages on the hed parent body asteroid 4 vesta occurred over an interval of at least 0 7 ga

live iron 60 in the early solar system isotopic analyses of nickel in samples from the differentiated meteorite chervony kut revealed the presence of relative excesses of 60ni ranging from 2 4 up to 50 parts per 104 these isotopic excesses are from the decay of the now extinct short lived nuclide 60fe and provide clear evidence for the existence of 60fe over large scales in the early solar system not only was 60fe present at the time of melting and differentiation that is fe ni fractionation of the parent body of chervony kut but also later at the time when basaltic magma solidified at or near the surface of the planetesimal the inferred abundance of 60fe suggests that its decay alone could have provided sufficient heat to melt small diameters of several hundred kilometers planetary bodies shortly after their accretion

chromium isotope anomaly in an impactite sample from the el gygytgyn structure russia evidence for a ureilite projectile the 3 6 ma 18 km diameter el gygytgyn impact structure arctic russia is unique among the currently known terrestrial impact craters in that it is the only one that was formed in acid volcanic rocks previous analyses of impactites from el gygytgyn showed minor enrichments of the siderophile elements including ir which together with distinct cr enrichments gave rise to speculation that an achondritic projectile was involved we studied the major and trace element composition in samples from the new icdp drill core obtained near the center of the structure as well as the chromium isotopic composition of an impact glass sample collected on the surface several suevitic breccias from the upper part of the suevite sequence in the drill core show higher cr and ni contents compared with felsic volcanic rocks in the lower part of the core and from surface samples however it is difficult to unambiguously establish a meteoritic component from trace element data as input from rare mafic target rocks is a possibility in contrast the cr isotopic composition of the impact glass sample yielded a nonterrestrial e 54 cr value of 0 72 0 31 2 std err this negative e 54 cr is different from known carbonaceous chondrite values e 54 cr of 0 95 to 1 65 but is nearly identical to reported values for ureilites approximately 0 77 the value is however also within analytical error of eucrites approximately 0 38 and ordinary chondrites approximately 0 42 given the chemical signatures found in previous analyses of el gytgytgyn impactites and the similarity of our cr isotopic data to ureilites we suggest that the impacting asteroid could have been an f type asteroid of mixed composition similar to the recent almahata sitta fall in sudan

early solar system events and timescale some recent information on the mn cr and al mg systems is reviewed this information is used to derive constraints on the timing of processes and events which took place in the early solar system using reasonable assumptions a timeline is constructed where the estimated age of the solar system is 4571 ma this age is taken to mark the time when most calcium aluminum rich inclusions cais were starting to form a process that may have lasted for several 105 years almost contemporaneously small planetesimals have accreted that served to store these cais for later dispersal among larger planetesimals by the time large numbers of planetesimals of several tens of kilometers in size had formed the interior of these objects started to melt through the decay of 26al collisional disruption of these planetesimals allowed gases dust and melt to escape into the surrounding space the fine droplets of melt reacted with gas and dust to form chondrules which after rapid cooling were partially re accreted onto the residual rubble pile this process of primary chondrule formation in most cases involving several generations of planetesimals most plausibly lasted only for 2 ma towards the end of this period and during the following 3 to 4 ma planetary objects of several hundred kilometers in size were formed they still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers with basaltic volcanism occurring within a few million years

the dynamical control of subduction parameters on surface topography the long wavelength surface deflection of earth s outermost rocky shell is mainly controlled by large scale dynamic processes like isostasy or mantle flow the largest topographic amplitudes are therefore observed at plate boundaries due to the presence of large thermal heterogeneities and strong tectonic forces distinct vertical surface deflections are particularly apparent at convergent plate boundaries mostly due to the convergence and asymmetric sinking of the plates having a mantle convection model with a free surface that is able to reproduce both realistic single sided subduction and long wavelength surface topography self consistently we are now able to better investigate this interaction we separate the topographic signal into distinct features and quantify the individual topographic contribution of several controlling subduction parameters results are diagnosed by splitting the topographic signal into isostatic and residual components and by considering various physical aspects like viscous dissipation during plate bending performing several systematic suites of experiments we are then able to quantify the topographic impact of the buoyancy rheology and geometry of the subduction zone system to each and every topographic feature at a subduction zone and to provide corresponding scaling laws we identify slab dip and slightly less importantly slab buoyancy as the major agents controlling surface topography at subduction zones on earth only the island arc high and the back arc depression extent are mainly controlled by plate strength overall his modeling study sets the basis to better constrain deep seated mantle structures and their physical properties via the observed surface topography on present day earth and back through time

dynamics of retreating slabs 2 insights from three dimensional laboratory experiments 1 a laboratory analogue of a three layer linear viscous slab upper mantle lower mantle system is established in a silicone putty honey and crystallized honey tank experiment the same setup as in the numerical investigation part 1 is used we focus on the interaction of the slab with the induced passive mantle flow by widely varying the mantle volume flux boundary conditions in our numerical experiments the lateral volume flux was set to zero in interpreting the results relative to the real earth the base of the box is taken as the bottom of the mantle convection system while the lateral boundaries may be associated with the presence of other nearby slabs dynamic force equilibrium assessed on the basis of an analytical review of forces is described for four different phases 1 the subduction initiation instability 2 the accelerating dynamic free fall phase of the slab 3 the dynamic interaction with the 660 km discontinuity and 4 a final phase of steady state trench retreat phase 3 is an important feature not observed in the numerical experiments this highly dynamic phase of interrupted trench retreat can therefore be attributed to boundary conditions on mantle volume flux on the basis of integration constants of force equilibrium in phases 2 and 4 we identify two different classes of volume flux one in which the lateral boundary can be considered open and the other class where it is closed closed boundary condition cases are obtained if any of the lateral box boundaries are 600 km away from the slab assuming a one to one relation between trench retreat and back arc spreading enigmatic observations of episodic opening of back arc basins can be explained by our experimental observations

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

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

constraints on the venusian crustal thickness variations in the isostatic stagnant lid approximation 1 the isostatic stagnant lid isl approximation is applied to the long wavelength venusian topography and geoid to place constraints on the thickness of the crust and lithosphere previous studies showed that a purely thermal isostasy model of the lithosphere the stagnant lid accounts for a significant portion of the topography and geoid anomalies here we assume that the remaining discrepancy between the observed and model geoid is due to crustal thickness variations and that the crust the lithosphere and the mantle are in a state of double isostatic equilibrium the lithosphere is isostatically floating on top of the mantle and the crust is isostatically floating on top of the lithosphere the addition of another variable the crustal thickness allows an exact fit to the observed topography and geoid this also makes the solution non unique an exact fit to topography and geoid can be achieved in a broad range of average lithospheric and crustal thicknesses additional constraints such as the gabbro eclogite phase transition and the existence of relatively young volcanism limit this range to some extent the inferred variations in the crustal thickness are less sensitive to model assumptions and generally agree with the results obtained in previous studies

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

evidence for the presence of 53mn in the early solar system chromium isotopic anomalies appear to be widespread in the allende inclusions clear anomalies are shown in the 53cr 52cr isotopic ratio deviations from the terrestrial value are correlated with the mn cr ratios indicating that 53mn half life t 3 7 106y existed at the time of formation of the meteorite inclusions the initial 53cr 52cr in the inclusions is lower than the terrestrial value recent results have shown that chromium isotopic anomalies are frequent on the 54cr isotope birck and allegre 1984 at the same time our results suggested the possibility of some anomalies on the 53cr isotope which can be produced by the decay of 53mn the present article presents improved precision measurements on the same inclusions of the allende meteorite and an extension of the measurements to mineral separates these results are the first evidence for live 53mn in the early solar system noted 53mn in the following

slab dip vs lithosphere age no direct function one paradigm of subduction relates the dip of the slab to the buoyancy of the downgoing lithosphere along subduction zones with the negative buoyancy proportional to the age of the oceanic lithosphere we measured the dip of the slab down to depths of 250 km along 164 sections crossing 13 subduction zones and compared it with the age of the subducting oceanic lithosphere both at the trench and at depth we show here that this relationship is far more irregular than previously suggested and that it is not possible to simply correlate the increase of the slab dip to the increasing age of the downgoing cooler lithosphere younger oceanic lithosphere may show steeper dip than older segments of slabs e g central america vs south america in contrast with predictions of models considering only slab pull the combination of slab age and subduction rate better accounts for slab dip however the correlation is not satisfactory correlation coefficient equal to 0 450 these results suggest that supplemental forces or constraints have to be accounted for such as thickness and shape of the hangingwall plate absolute plate velocity presence of lateral density variations in the hosting upper mantle effects of accretion erosion subduction of oceanic plateaus and slab deformation due to the motion of the mantle relative to the subducting plate d 2005 elsevier b v all rights reserved

buildup of a dynamically supported orogenic plateau numerical modeling of the zagros central iran case study the iranian plateau is a vast inland region with a smooth average elevation of c 1 5 km formed at the rear of the zagros orogen as a result of the arabia eurasia collision i e over the last 30 35 myr this collision zone is of particular interest due to its disputed resemblance to the faster himalayan collision which gave birth to the tibetan plateau around 50 myr ago recent studies have suggested that a recent 10 5 ma slab break off event below central iran caused the formation of the iranian plateau here we test several hypotheses through large scale 3082 3 590 km numerical models of continental subduction mod els that incorporate a free upper surface erosion rheological stratification brittle elastic ductile rheologies and metamorphic phase changes density and physical properties and account for the specific crustal and thermal structure of the arabian and iranian continental lithospheres we test the impact of the transition from oceanic to continental subduction and the topographic consequences of the progressive slowdown of the convergence rate during continental subduction our results demonstrate the role of mantle flow beneath the overriding plate initiated as an indirect consequence of slab break off this flow creates a dynamic topography support during continental subduction and results in delamination of the overriding plate lithospheric mantle followed by isostatic readjustment hence of further uplift and maintenance of a plateau like topography without significant crustal thickening the slowdown of the convergence rate dur ing the development of the continental subduction collision phase largely contributes to this process by controlling the timing and depth of slab break off

horizontal subduction and truncation of the cocos plate beneath central mexico 1 based on analysis of data from a trans mexico temporary broadband seismic network centered on mexico city we report that the subducting cocos plate beneath central mexico is horizontal and tectonically underplates the base of the crust for a distance of 250 km from the trench it is decoupled from the crust by a very thin low viscosity zone the plate plunges into the mantle near mexico city but is truncated at a depth of 500 km probably due to an e w propagating tear in the cocos slab unlike the shallow slab subduction in peru and chile there is active volcanism along the trans mexican volcanic belt tmvb that lies much further inland than regions to either side where subduction dip is not horizontal geodynamical modeling indicates that a thin weak layer such as imaged by the seismic experiment can explain the flat subduction

mn cr relative sensitivity factor in ferromagnesian olivines defined for sims measurements with a cameca ims 1280 ion microprobe implications for dating secondary fayalite abstract the short lived radionuclide 53mn which decays to 53cr with a half life of 3 7 myr is useful for sequencing objects that formed within the first 20 myr of solar system evolution 53mn 53cr relative chronology enables aqueously formed secondary minerals such as fayalite and various carbonates in ordinary and carbonaceous chondrites to be dated thereby providing chronological constraints on aqueous alteration processes in situ measurements of mn cr isotope systematics in fayalite by secondary ion mass spectrometry sims require consideration of the relative sensitivities of the 55mn and 52cr ions for which a relative sensitivity factor rsf 55mn 52cr sims 55mn 52cr true is defined using appropriate standards in the past san carlos olivine fa 10 was commonly used for this purpose but a growing body of evidence suggests that it is an unsuitable standard for meteoritic fayalite fa 90 natural fayalite also cannot be used as a standard because it contains only trace amounts of chromium which makes determining a true 55mn 52cr ratio and its degree of heterogeneity very difficult to investigate the dependence of the mn cr rsf on ferromagnesian olivine compositions we synthesized a suite of compositionally homogeneous mn cr bearing liquidus phase ferromagnesian olivines fa31 99 manganese chromium isotopic measurements of san carlos olivine and synthesized ferromagnesian olivines using the university of hawai i cameca ims 1280 sims show that the rsf for fa10 is 0 9 it increases rapidly between fa10 and fa31 and reaches a plateau value of 1 5 0 1 for fa 34 the rsf is time dependent it increases during the measurements of olivines with fayalite content 50 the rsf measured on ferroan olivine fa 90 is influenced by pit shape whereas the rsf measured on magnesian olivine fa10 is less sensitive to changes in pit shape for these reasons 53mn 53cr systematics of chondritic fayalite fa 90 should be determined using standards of similar composition that are measured under the same analytical conditions as the unknown the 53mn 53cr ages of secondary fayalites fa90 100 in the elephant moraine eet 90161 l3 05 vicencia ll3 2 asuka 881317 cv3 and macalpine hills mac 88107 c3 chondrites 2 4 1 3 1 8 4 0 1 1 1 4 4 2 0 7 0 8 and 5 1 0 4 0 5 myrs after cv cais respectively are 3 myr older when using an rsf measured on a matrix matched fa99 standard rather than on a san carlos olivine the inferred 53mn 53cr ages of fayalite formation are consistent with the ages reported for calcites in cm chondrites measured with similarly matrix matched standards suggesting an early onset of aqueous alteration on the ordinary and carbonaceous chondrite parent bodies heated by decay of 26al

contributors to chromium isotope variation of meteorites we report the results of a comprehensive high precision survey of the cr isotopic compositions of primitive chondrites along with some differentiated meteorites to ensure complete dissolution of our samples they were first fused with lithium borate tetraborate at 1050 1000 c relative to the nist cr standard srm 3112a carbonaceous chondrites exhibit excesses in 54cr 52cr from 0 4 to 1 6e 1e 1 part in 10 000 and ordinary chondrites display a common 54cr 52cr deficit of 0 4e analyses of acid digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54cr enriched carrier which is characterized by a large excess in 54cr 52cr up to 200e associated with a very small deficit in 53cr 52cr 2e we did not find 54cr anomalies in either bulk enstatite chondrites or in leachates of their acid digestion residues this either requires that the enstatite chondrite parent bodies did not incorporate the 54cr anomaly carrier phase during their accretion or the phase was destroyed by parent body metamorphism chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the nist srm 3112a cr standard the eucrite and martian meteorites studied exhibit small deficits in 54cr 52cr the 54cr 52cr variations among different meteorite classes suggest that there was a spatial and or temporal heterogeneity in the distribution of a 54cr rich component in the inner solar system we confirm the correlated excesses in 54cr 52cr and 53cr 52cr for bulk carbonaceous chondrites but the new data yield a steeper slope 6 6 than that reported in shukolyukov and lugmair 2006 the correlated excesses may affect the use of the mn cr chronometer in carbonaceous chondrites we could not confirm the bulk carbonaceous chondrite mn cr isochron reported by shukolyukov and lugmair 2006 and moynier et al 2007 mostly because we find much smaller total variations in e53cr 0 2 all bulk chondrites have small e53cr excesses up to 0 3 relative to the earth most likely reflecting the sub chondritic mn cr ratio of the earth the e53cr variations in chondrites do seem to grossly correlate with mn cr and yield an initial solar system 53mn 55mn value of 5 4 2 4 10 6 corresponding to an absolute age of 4566 4 2 2 ma nuclear interactions with cosmic rays result in coupled excesses in e54cr and e53cr with a 4 1 ratio in phases with high fe cr these are most dramatically demonstrated in the iron meteorite carbo showing excesses in e54cr of up to 140e these new results show that the mn cr chronometer should be used with caution in samples minerals with high fe cr and long cosmic ray exposure ages

chromium isotopic homogeneity between the moon the earth and enstatite chondrites abstract among the elements exhibiting non mass dependent isotopic variations in meteorites chromium cr has been central in arguing for an isotopic homogeneity between the earth and the moon thus questioning physical models of moon formation however the cr isotopic composition of the moon relies on two samples only which define an average value that is slightly different from the terrestrial standard here by determining the cr isotopic composition of 17 lunar 9 terrestrial and 5 enstatite chondrite samples we re assess the isotopic similarity between these different planetary bodies and provide the first robust estimate for the moon in average terrestrial and enstatite samples show similar e 54 cr on the other hand lunar samples show variables excesses of 53cr and 54cr compared to terrestrial and enstatite chondrites samples with correlated e 53 cr and e 54 cr per 10 000 deviation of the 53cr 52cr and 54cr 52cr ratios normalized to the 50cr 52cr ratio from the nist srm 3112a cr standard unlike previous suggestions we show for the first time that cosmic irradiation can affect significantly the cr isotopic composition of lunar materials moreover we also suggest that rather than spallation reactions neutron capture effects are the dominant process controlling the cr isotope composition of lunar igneous rocks this is supported by the correlation between e 53 cr and e 54 cr and 150sm 152sm ratios after correction of these effects the average e 54 cr of the moon is indistinguishable from the terrestrial and enstatite chondrite materials reinforcing the idea of an earth moon enstatite chondrite system homogeneity this is compatible with the most recent scenarios of moon formation suggesting an efficient physical homogenization after a high energy impact on a fast spinning earth and or with an impactor originating from the same reservoir in the inner proto planetary disk as the earth and enstatite chondrites and having similar composition

origin and chronology of chondritic components a review abstract mineralogical observations chemical and oxygen isotope compositions absolute 207pb 206pb ages and short lived isotope systematics 7be 7li 10be 10b 26al 26mg 36cl 36s 41ca 41k 53mn 53cr 60fe 60ni 182hf 182w of refractory inclusions ca al rich inclusions cais and amoeboid olivine aggregates aoas chondrules and matrices from primitive unmetamorphosed chondrites are reviewed in an attempt to test i the x wind model vs the shock wave model of the origin of chondritic components and ii irradiation vs stellar origin of short lived radionuclides the data reviewed are consistent with an external stellar origin for most short lived radionuclides 7be 10be and 36cl are important exceptions and a shock wave model for chondrule formation and provide a sound basis for early solar system chronology they are inconsistent with the x wind model for the origin of chondritic components and a local irradiation origin of 26al 41ca and 53mn 10be is heterogeneously distributed among cais indicating its formation by local irradiation and precluding its use for the early solar system chronology 41ca 41k and 60fe 60ni systematics are important for understanding the astrophysical setting of solar system formation and origin of short lived radionuclides but so far have limited implications for the chronology of chondritic components the chronological significance of oxygen isotope compositions of chondritic components is limited the following general picture of formation of chondritic components is inferred cais and aoas were the first solids formed in the solar nebula 4567 4568 myr ago possibly within a period of 10 and have 26al 27al 0 similar to those in chondrules 5 nebular gas the majority of chondrules within a chondrite group may have formed over a much shorter period of time

the nature of the kt impactor a 54cr reappraisal according to the variations observed in the various meteorite classes 54cr represents an isotopic tool for planetary body discrimination in the search for the nature of the cretaceous tertiary kt impactor cr isotopic measurements were already performed at kt boundary the current work is aimed to modernize cr isotopic data with high precision measurements to confirm a carbonaceous chondrite type infall the contribution of which is also estimated isotopic signatures of two marine clays caravaca stevens klint exhibit an isotopic ratio which would represent a mixing of a carbonaceous chondrite of cm2 type with terrestrial material in a ratio 6 to 19 a single impactor may account for both marine and continental cr isotopic signatures

the role of the overriding plate thermal state on slab dip variability and on the occurrence of flat subduction 1 slab dip varies significantly both between different and along single subduction zones provided that old subducting plates are colder and denser than young plates variations in the slab dip should correlate with slab age however recent statistical analyses do not show this expected correlation we present the results of non newtonian numerical dynamic models where subduction is driven by means of a kinematic boundary condition we systematically vary the age of both the overriding and subducting plates in order to test these effects on the slab dip at different depth ranges we find that colder overriding plates result in shallower slab dips and episodes of flat slab subduction as a result of the increased suction force in the mantle wedge the influence of the thermal state of the overriding plate on slab dip is shown here to be more important than that of the age of subducting lithosphere modeling results are qualitatively compared to the large dip variability of the cocos slab including a flat slab segment we suggest that this variability is likely related to the change of the thermal state of the overriding plates with flat subduction occurring under cold lithosphere in southwestern mexico and steep subduction under the warmer lithosphere of the northwestern caribbean plate

plate force due to bending at subduction zones 1 bending of the lithosphere at subduction zones generates stresses that contribute to the force balance on plates we determine the net horizontal force on subducting plates by imposing a simple kinematic description of strain in the equations that govern the mechanical equilibrium of a thin viscous sheet the magnitude of the force depends on the velocity thickness and curvature of the subducting plate using representative values for old oceanic lithosphere we obtain an effective horizontal force that is nearly 40 of the buoyancy force due to slabs in the upper mantle

mn cr systematics in primitive meteorites insights from mineral separation and partial dissolution abstract cr isotopic compositions have been measured on carbonaceous chondrites cc tafassasset paris niger i nwa 5958 nwa 8157 and jbilet winselwan in bulk samples the 54 cr 52 cr ratios expressed as e 54 cr range from 0 93 to 1 58 e units these values are in agreement with values characteristic for distinct petrologic types despite this 54 cr heterogeneity the variability in the 53 cr 52 cr ratios expressed as e 53 cr of 0 2 e units and the mn cr ratios is consistent with the previous finding of an isochron in the mn cr evolution diagram the mn cr ratio in cc corresponds to variable abundances of high t condensate formed and separated at the beginning of the solar system thus the canonical 53 mn 55 mn ratio can be defined based on a consistent chronology for u pb and mn cr between the earliest objects formed in the solar nebula and the d orbigny angrite we define a canonical 53 mn 55 mn ratio and e 53 cr i of 6 8 10 6 and 0 177 respectively the internal mn cr systematics in tafassasset and paris were studied by two approaches leaching technique and mineral separation despite variable e 54 cr values up to 30 e linear co variations were found between e 53 cr and mn cr ratio the mineral separates as well as the leachates of tafassasset fall on a common isochron indicating that 1 cooling of the tafassasset s parent body occurred at 4563 5 0 25 ma and that 2 54 cr is decoupled from the other isotopes even though temperatures 900 c have been reached during metamorphism in the case of paris the leachates form an alignment with a 53 mn 55 mn ratio higher than the canonical value this alignment is not an isochron but rather a mixing line based on leachates from various cm and ci we propose the occurrence of three distinct cr reservoirs in meteoritic material pure54 high53 and low53 characterized by a e 53 cr and e 54 cr of 0 and 25 000 2 17 and 8 and 0 5 and 151 respectively pure54 has already been described and is carried by highly refractory nano spinel high53 is mn rich and most probably carried by sulfides in the matrix whereas low53 is characterized by low mn cr ratios and it is sensitive to metamorphism this component could correspond to mineral phases such as refractory oxides and carbide variable mixing proportions of high53 and low53 would explain the larger than expected uncertainty mswd of 5 5 on the cc bulk regression line a monte carlo simulation allows us to evaluate the impact of the dispersion of the initial cr isotopic ratios as a function of variable high53 the co variation of the mn cr ratio and the e 53 cr defined by the mineral separates from paris corresponds to an age of 4566 44 0 66 0 75 ma while their e 54 cr still differ by at least 0 42 e this age is likely to date the segregation of forsteritic olivines most probably from type i chondrules from fayalitic olivines from type ii chondrules and given the sampling procedure by handpicking of hundreds of grains corresponds to the average age of chondrule formation

on topography and geoid from 2 d stagnant lid convection calculations 1 temperature dependent convection in the stagnant lid regime with a frank kamenetskii and arrhenius viscosity formulation are compared when properly scaled the nusselt number and root mean square velocity of the fluid for the two viscosity formulations are similar but the surface stresses and hence predicted dynamic topography and geoid differ significantly the arrhenius viscosity formulation results are insensitive to a viscosity cutoff at the high viscosity limit as long as the cutoff is 104 times the basal viscosity the condition number for stagnant lid convection matrices using a penalty formulation is smaller than the penalty number times viscosity contrast value a reasonable estimate of the condition number explaining why large viscosity contrast convection problems with the penalty method agree with other formulations and analytic solutions the difference in magnitude between the maximum and minimum dynamic topography could be used to constrain the magnitude of the viscosity contrast across the stagnant lid in planetary bodies

geometry of subduction and depth of the seismogenic zone in the guerrero gap mexico the western coast of guerrero in southern mexico has been identified as a seismic gap on the middle american trench in which no large earthquakes have occurred at least since 1908 it has been suggested that the seismic energy accumulated since 1908 will eventually be released by a large earthquake a permanent seismic network was installed to monitor the seismicity of this mature seismic gap and to understand the geometry of the subducted slab beneath this region the seismicity defines an unusual distribution along two bands of seismic activity parallel to the coast the resulting geometry of the subduction zone shows that the cocos plate dips at a shallow angle beneath the north american plate to a depth of 40 km from there the subducted slab is bent upward following a subhorizontal trajectory extending inland at a depth of 50 km this plate geometry is reminiscent of that found in peru and central argentina two other regions where a young oceanic plate is being subducted in mexico however the slab underplates an overriding plate which is only half as thick as that observed in south america a possible oceanic origin of the allochtonous terranes comprising southern mexico may explain the presence of the anomalously thin lithosphere in this region

isotopic evidence for the cretaceous tertiary impactor and its type high precision mass spectrometric analysis of chromium in sediment samples from the cretaceous tertiary k t boundary coincident with the extinction of numerous organisms on earth confirms the cosmic origin of the k t phenomenon the isotopic composition of chromium in k t boundary samples from stevns klint denmark and caravaca spain is different from that of earth and indicates its extraterrestrial source the chromium isotopic signature is consistent with a carbonaceous chondrite type impactor the observed differences in the chromium isotopic composition among various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with earth during its history

physicochemical isotope anomalies isotopic composition of refractory elements can be modified by physical processes such as distillation and sputtering in unexpected patterns distillation enriches the heavy isotopes in the residue and the light isotopes in the vapor however current models appear to be inadequate to describe the detailed mass dependence in particular for large fractionations coarse and fine grained inclusions from the allende meteorite exhibit correlated isotope effects in mg both as mass dependent fractionation and residual anomalies this isotope pattern can be duplicated by high temperature distillation in the laboratory a ubiquitous property of meteoritic inclusions for mg as well as for most of the other elements where measurements exist is mass dependent fractionation in contrast terrestrial materials such as microtektites tektite buttons as well as lunar orange and green glass spheres have normal mg isotopic composition a subset of interplanetary dust particles labelled as chondritic aggregates exhibit excesses in sup 26 mg and deuterium anomalies sputtering is expected to be a dominant mechanism in the destruction of grains within interstellar dust clouds an active proto sun as well as the present solar wind and solar flare flux are of sufficient intensity to sputter significant amounts of material laboratory experiments in mg show widespread isotope effects including residual supmore 26 mg excesses and mass dependent fractionation it is possible that the sup 26 mg excesses in interplanetary dust is related to sputtering by energetic solar wind particles the implication if the laboratory distillation and sputtering effects are discussed and contrasted with the anomalies in meteoritic inclusions the other extraterrestrial materials the authors have access to less

manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites whole chondrule mn cr isochrons are presented for chondrules separated from the chainpur ll3 4 and bishunpur ll3 1 meteorites the chondrules were initially surveyed by instrumental neutron activation analysis ll chondrite normalized mn cr mn fe and sc fe served to identify chondrules with unusually high or low mn cr ratios and to correlate the abundances of other elements to sc the most refractory element measured a subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive x ray spectrometer prior to high precision cr isotopic analyses 53cr 52cr correlates with 55mn 52cr to give initial 53mn 55mn i 9 4 1 7 10 6 for chainpur chondrules and 53mn 55mn i 9 5 3 1 10 6 for bishunpur chondrules the corresponding chondrule formation intervals are respectively tlew 10 1 ma for chainpur and 10 2 ma for bishunpur relative to the time of igneous crystallization of the lewis cliff lew 86010 angrite because mn sc correlates positively with mn cr for both the chainpur and bishunpur chondrules indicating dependence of the mn cr ratio on the relative volatility of the elements we identify the event dated by the isochrons as volatility driven elemental fractionation for chondrule precursors in the solar nebula thus our data suggest that the precursors to ll chondrules condensed from the nebula 5 8 2 7 ma after the time when initial 53mn 55mn i 2 8 0 3 10 5 for calcium aluminum rich inclusions cais our preferred value determined from data for a mineral separates of type b allende cai br1 b spinels from efremovka cai e38 and c bulk chondrites mn cr formation intervals for meteorites are presented relative to average i mn 53mn 55mn ch 9 46 10 6 for chondrules mn cr ratios for radiogenic growth of 53cr in the solar nebula and later reservoirs are calculated relative to average i mn 53cr i 9 46 0 08 10 6 0 23 0 08 for chondrules inferred values of mn cr lie within expected ranges thus it appears that evolution of the cr isotopic composition can be traced from condensation of cais via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids measured values of 53cr for individual chondrules exhibit the entire range of values that has been observed as initial 53cr values for samples from various planetary objects and which has been attributed to radial heterogeneity in initial 53mn 55mn in the early solar system estimated 55mn 52cr 0 42 0 05 for the bulk earth combined with 53cr 0 for the earth plots very close to the chondrule isochrons so that the earth appears to have the mn cr systematics of a refractory chondrule thus the earth apparently formed from material that had been depleted in mn relative to cr contemporaneously with condensation of chondrule precursors if as seems likely the earth s core formed after complete decay of 53mn there must have been little differential partitioning of mn and cr at that time

paleogene calcareous nannoplankton biostratigraphy of northwestern europe abstract the classic paleogene stages thanetian cuisian lutetian bartonian ludian and stampian defined in the epicontinental deposits of the anglo paris basin s are placed within a standard biostratigraphic framework based on an analysis of their calcareous nannoplankton floras relatively precise biostratigraphic correlations can be made with some chronostratigraphic units e g lutetian while biostratigraphic resolution of others e g ludian is limited important hiatuses are recognized between certain chronostratigraphic units only the base of the lutetian late np14 and bartonian mid np16 stages can be precisely determined in terms of calcareous nannoplankton zones with the recent study of the magnetic polarity history of the paleogene of southern england and recent advances in magnetobiostratigraphic correlations in deep sea cores it is possible to interpret the paleogene history of the anglo paris basin s in a synoptic global framework in which paleomagnetic stratigraphy provides the chronologic and eustatic variations the graphic units of northwestern europe within rather than at the boundaries of i e at dynamic element redefinition of the boundaries of the standard paleogene chronostratigraphic unconformities eustatically controlled depositional cycles would result in more precise correlations in terms of standard calcareous nannoplankton biostratigraphy

dynamic surface topography a new interpretation based upon mantle flow models derived from seismic tomography the very long wavelength i e in the degree range l 1 8 dynamic topography at the earth s surface estimated by isostatic reduction of the observed topography is shown to be in good agreement with the topography predicted by a model of viscous flow in the mantle this flow model was previously derived by independent consideration of the observed nonhydrostatic geoid and is based upon a recent model of large scale seismic shear velocity heterogeneity in the mantle our estimate and prediction of the long wavelength dynamic surface topography reveals significant depressions 2 3 km of the continental shields relative to their hydrostatic reference positions

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

manganese chromium isotope systematics of carbonaceous chondrites abstract in this article we present the results of cr isotope investigations of different types of carbonaceous chondrites and of the pallasite eagle station the 53 cr 52 cr ratios in the bulk samples of carbonaceous chondrites are correlated with 55 mn 52 cr ratios the slope of the correlation line yields a 53 mn 55 mn ratio of 8 5 1 5 10 6 at the time of mn cr fractionation mapping this ratio onto an absolute time scale yields a time for this event of 4568 1 0 8 1 1 ma ago this time is very similar to the formation age of efremovka cais of 4567 2 0 6 ma y amelin a n krot i d hutcheon a a ulyanov lead isotopic ages of chondrules and calcium aluminum rich inclusions science 297 2002 1678 1683 to a time of the chondrule formation of 4568 1 ma ago l e nyquist d lindstrom d mittlefehldt c y shih h wiesmann s wentworth r martinez manganese chromium formation intervals for chondrules from the bishunpur and chainpur meteorites meteorit planet sci 36 2001 911 938 which most likely constrains early global high temperature mn cr fractionation in a nebular setting the bulk samples of carbonaceous chondrites exhibit clear 54 cr excesses 54 cr that are correlated with the 53 cr excesses 53 cr and also with mn cr ratios one possible explanation of this correlation is that 54 cr is also radiogenic like 53 cr and was formed by the decay of the short lived parent radionuclide 54 mn the very short half life of 54 mn of 312 days would require that both short lived radionuclides 53 mn and 54 mn were generated locally in spallation reactions during the early period of an active sun the alternative and possibly more plausible explanation is the heterogeneous addition of presolar material the presolar component enriched in 54 cr is mostly contained in the matrix of carbonaceous chondrites the relative amount of matrix decreases in the sequence ci cm co cv a large proportion of mn is associated with the matrix while cr preferentially resides in the chondrules thus the mn cr ratio also follows the sequence ci cm co cv and is correlated with 54 cr the acid resistant residues of carbonaceous chondrites are characterized by relatively large excesses of 54 cr and moderate deficits of 53 cr the magnitude of these excesses and deficits decreases in the sequence ci cm cv and may imply that cr in the cm and cv residues is increasingly more equilibrated with cr from the rest of the meteorites the 53 cr 52 cr and 54 cr 52 cr ratios in the residues are anti correlated indicating that there are at least two cr components of possibly presolar origin all residues have large excesses of the most neutron rich ti isotope 50 ti the pattern of 50 ti does not exactly follow that observed for 54 cr but it is generally similar the cr isotope systematic of the pallasite eagle station indicates that the precursor of this meteorite was a cv type material the 53 mn 53 cr system indicates that the cr isotopes equilibrated in this meteorite 4557 5 0 6 ma ago

53mn 53cr systematics of carbonates in cm chondrites implications for the timing and duration of aqueous alteration abstract cm chondrites contain carbonates and other secondary minerals such as phyllosilicates sulfides sulfates oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid we report in situ mn cr isotope measurements in the highly aqueously altered cm2 1 chondrites que 93005 and alh 83100 using secondary ion mass spectrometry cameca ims 1270 ion microprobe the 53 cr excesses are correlated with the 53 mn 55 mn ratio and result from the in situ decay of 53 mn a short lived radioisotope with a half life of 3 7 ma if we assume that carbonate grains in samples que 93005 and alh 83100 are cogenetic then the excesses define initial 53 mn 55 mn ratios 53 mn 55 mn 0 of 4 1 1 2 10 6 and 5 1 1 7 10 6 respectively these values are comparable to those in carbonates from other cm chondrites as reported in the literature initial 53 mn 55 mn ratios for calculated model isochrones for individual carbonate grains range from 3 8 1 4 10 6 to 4 8 2 1 10 6 for que 93005 and from 3 1 1 6 10 6 to 1 3 0 5 10 5 for alh 83100 a possible interpretation for the ranges in 53 mn 55 mn 0 could be that alteration in individual cm chondrites was episodic and occurred over an extended period of time however isochrones based on the entire set of carbonate grains in each of the cm chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in cm chondrites of different subtypes and that alteration on the cm parent asteroid started contemporaneously with or shortly after cai formation and lasted at least 4 ma

the distribution of short lived radioisotopes in the early solar system and the chronology of asteroid accretion differentiation and secondary mineralization abstract we evaluate initial 26 al 27 al i 53 mn 55 mn i and 182 hf 180 hf i ratios together with 207 pb 206 pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common closed isotopic system i e the early solar nebula the relative initial isotopic abundances of 26 al 53 mn and 182 hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values here denoted by i al ss i mn ss and i hf ss respectively i mn ss and i hf ss 9 1 1 7 10 6 and 1 07 0 08 10 4 respectively correspond to canonical i al ss 5 1 10 5 i hf ss so determined is consistent with i hf ss 9 72 0 44 10 5 directly determined from an internal hf w isochron for cai minerals i mn ss is within error of the lowest value directly measured for cais we suggest that erratically higher values measured for cais in carbonaceous chondrites may reflect proton irradiation of unaccreted cais by the early sun after other asteroids destined for melting by 26 al decay had already accreted the 53 mn incorporated within such asteroids would have been shielded from further local spallogenic contributions from within the solar system the relative initial isotopic abundances of the short lived nuclides are less consistent with the 207 pb 206 pb ages of the corresponding materials than with one another the best consistency of short and long lived chronometers is obtained for 182 hf 180 hf i and the 207 pb 206 pb ages of angrites 182 hf 180 hf i decreases with decreasing 207 pb 206 pb ages at the rate expected from the 8 90 0 09 ma half life of 182 hf the model solar system age thus determined is t ss hf w 4568 3 0 7 ma 26 al 27 al i and 53 mn 55 mn i are less consistent with 207 pb 206 pb ages of the corresponding meteorites but yield t ss mn cr 4568 2 0 5 ma relative to i al ss 5 1 10 5 and a 207 pb 206 pb age of 4558 55 0 15 ma for the lew86010 angrite the mn cr method with i mn ss 9 1 1 7 10 6 is useful for dating accretion if identified with chondrule formation primary igneous events and secondary mineralization on asteroid parent bodies all of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies for i mn ss 9 1 1 7 10 6 parent body differentiation is found to extend at least to 5 ma post t ss i e until differentiation of the angrite parent body 4563 5 ma ago or 4564 5 ma ago using the directly measured 207 pb 206 pb ages of the d orbigny clan angrites the 1 ma difference is characteristic of a remaining inconsistency for the d orbigny clan between the al mg and mn cr chronometers on one hand and the 207 pb 206 pb chronometer on the other differentiation of the iiiab iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body and at nearly the same time as one another as shown by the mn cr ages of iiiab irons and ureilites respectively the latest recorded episodes of secondary mineralization are for carbonates on the ci carbonaceous chondrite parent body and fayalites on the cv carbonaceous chondrite parent body both extending to 10 ma post t ss

controls on trench topography from dynamic models of subducted slabs a finite element method with constrained elements and lagrange multipliers is used to study tectonic faults in a viscous medium a fault representing the interface between overriding and subducting plates has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included the fault causes stress to concentrate in its vicinity giving rise to a weak plate margin and a mobile plate if a power law rheology is used surface dynamic topography with either a newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin this suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow our models show that trench depth increases with fault dip angle slab dip angle slab length and age of oceanic lithosphere just prior to subduction the influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length these relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths for those subduction zones with oceanic lithosphere on both sides of the trench observed trench depths have been corrected for fault and slab dip angles based on the relationships from the dynamic models after correction to a common set of parameters trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict comparison between the trench depths corrected for fault and slab dip angles and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 mpa to 30 mpa depending on model details

reassessing the origin and chronology of the unique achondrite asuka 881394 implications for distribution of 26al in the early solar system abstract the achondrite asuka 881394 has mineralogy broadly similar to that of eucrites but is isotopically chemically and texturally distinct from them previous u pb chronology shows that it is very old forming within the first 0 8 ma of the formation of calcium aluminum rich inclusions cais however the age difference between asuka 881394 and other very old solar system materials cais quenched angrites measured with the 26al 26mg and 53mn 53cr extinct radionuclide chronometers and 207pb 206pb chronometer is not the same this could be interpreted to reflect heterogeneity in the distribution of 26al and 53mn in the early solar system given the significant implications for the early solar system chronology if 26al and 53mn are indeed heterogeneously distributed in this study we further investigate the origin of asuka 881394 and the apparent age discordance between short lived and absolute chronometers by focusing on measurement of its e54cr composition renewed measurements of the absolute pb pb age and new high precision measurements of its 26al 26mg systematics new cr isotope data places additional constraints on the origin of asuka 881394 its e 54cr value of 0 37 0 10e is resolvable outside of uncertainty from hed meteorites 0 72 0 10e reinforcing evidence from oxygen isotopic analyses that suggest it originated from a distinct parent body unlikely to be 4 vesta new pb pb analyses combined with using a directly measured 238u 235u ratio for age calculation result in a recalculated pb pb age of 4564 95 0 53 ma 1 5 ma younger than the previously reported age with this age asuka 881394 remains one of the oldest known achondrites in our solar system new high precision 26al 26mg data produce an initial 26al 27al ratio of 1 48 0 12 10 6 within error of previous data this ratio corresponds to an al mg age of 4563 69 0 36 ma or 4564 83 0 21 ma relative to cais and the d orbigny angrite respectively thus depending on which age anchor is used the 26al 26mg age is either 1 26 ma or 0 12 ma younger than the new pb pb age the latter being unresolved within analytical uncertainty though a potential age discrepancy between 26al 26mg and pb pb could be a result of heterogeneous distribution of 26al we demonstrate with our new high precision mg isotope data in conjunction with petrographic evidence that the mg isotope system has been disturbed in asuka 881394 we suggest that the 26al 26mg system closed to diffusion after the u pb system either due to slow cooling on the parent body or low grade metamorphic re equilibration of mg thus we can satisfactorily explain the observed age discrepancy between 26al 26mg and u pb systems in asuka 881394 without invoking heterogeneous distribution of 26al 27al ratio in the early solar system comparison of the asuka 881394 data with that of other anomalous achondrites from distinct parent bodies suggests that these could also have evolved from a source region with a canonical 26al 27al ratio because these achondrites have significant differences in their e54cr 17o systematics which could be indicative of location within the early protoplanetary disk it is consistent with homogeneous distribution of 26al in the early solar system angrites remain an outlier either because they evolved from a source with a lower 26al 27al ratio or because their 26al 26mg or pb pb data are problematic in either case this suggests that basaltic angrites are questionable as the age anchor for the entire solar system as a whole and other very old well preserved achondrites should be considered for that role

aluminum 26 in h4 chondrites implications for its production and its usefulness as a fine scale chronometer for early solar system events in order to investigate whether or not 26al can be used as a fine scale chronometer for early solar system events we measured with an ion microprobe mg isotopes and al mg ratios in separated plagioclase olivine and pyroxene crystals from the h4 chondrites ste marguerite forest vale beaver creek and quenggouk and compared the results with the canonical 26al 27al ratio for ca al rich inclusions cais for ste marguerite sm and forest vale fv pb pb and mn cr ages have previously been determined gpel et al 1994 polnau et al 2000 polnau and lugmair 2001 plagioclase grains from these two meteorites show clear excesses of 26mg the 26al 27al ratios inferred from these excesses and from isotopically normal mg in pyroxene and olivine are 2 87 0 64 x 10 7 for sm and 1 52 0 52 x 10 7 for fv the differences between these ratios and the ratio of 5 x 10 5 in cais indicate time differences of 5 4 0 1 ma and 6 1 0 2 ma for sm and fv respectively these differences are in agreement with the absolute pb pb ages for cais and sm and fv phosphates but there are large discrepancies between the u pb and mn cr system for the relative ages for cais sm and fv for example mn cr ages of carbonates from kaidun are older than the pb pb age of cais however even if we require that cais are older than these carbonates the time difference between this adjusted cai age and the mn cr ages of sm and fv require that 26al was widely distributed in the early solar system at the time of cai formation and was not mostly present in cais a feature of the x wind model proposed by shu and collaborators gounelle et al 2001 shu et al 2001 from this we conclude that there was enough 26al to melt small planetary bodies as long as they formed within 2 ma of cais and that 26al can serve as a fine scale chronometer for early solar system events

on the relationships between slab dip back arc stress upper plate absolute motion and crustal nature in subduction zones 1 statistical analysis of modern oceanic subduction zone parameters such as the age of a downgoing plate or the absolute plate motions is performed in order to investigate which parameter controls the dip of a slab and conversely what the influence of slab geometry is on upper plate behavior for that purpose parameters have been determined from global databases along 159 transects from all subduction zones that are not perturbed by nearby collision or ridge plateau seamount subduction on the basis of tomographic images slabs that penetrate through or lie on the 670 km discontinuity are also identified the results of the statistical analysis are as follows 1 back arc stress correlates with slab dip i e back arc spreading is observed for deep dips deeper than 125 km larger than 50 whereas back arc shortening occurs only for deep dips less than 30 2 slab dip correlates with absolute motion of the overriding plate the correlation is even better when the slab lies on or even more penetrates through the 670 km discontinuity 3 slabs dip more steeply by about 20 on average beneath oceanic overriding plates than beneath continental ones 4 slabs dip more steeply on average by about 10 near edges 5 slab dip does not correlate with the magnitude of slab pull the age of subducting lithosphere at the trench the thermal regime of the subducting lithosphere the convergence rate or the subduction polarity east versus west the present study provides evidence that the upper plate absolute motion plays an important role on slab dip as well as on upper plate strain retreating overriding plates are often oceanic ones and thus may partially explain the steeper slab dips beneath oceanic upper plates one can infer that low slab dips correlate well with compression in continental advancing upper plates whereas steep dips are often associated with extension in oceanic retreating upper plates excess weight of old slabs is often counterbalanced by other forces probably asthenospheric in origin such as lateral mantle flow near slab edges or anchor forces to determine slab dip

on early solar system chronology implications of an heterogeneous spatial distribution of 26al and 53mn abstract early solar system chronology is usually built with the assumption that the distribution of short lived radionuclides was homogeneous through the solar accretion disk at present there is no unambiguous evidence for a homogeneous distribution of short lived radionuclides in the solar accretion disk while some data point to a heterogeneous distribution of short lived radionuclides in this paper we explore a possible chronology based on a heterogeneous distribution of 26 al and 53 mn in the accretion disk our basic assumption is that the different abundances of extinct short lived radionuclides in calcium aluminium rich inclusions cais and chondrules are due to spatial rather than temporal differences we develop a simple model where cais and chondrules form contemporaneously in different spatial locations and are characterised by distinct initial 26 al and 53 mn abundances in this model all evolved bodies are supposed to be originally chondritic i e to be made of a mixture of cais chondrules and matrix this mixture determines the initial content in 26 al and 53 mn of a chondritic parent body as a function of its cai and chondrule abundance fraction this approach enables us to calculate coherent 26 al and 53 mn ages from the agglomeration of the parent body precursors cais and chondrules until the isotopic closure of 26 al and 53 mn thereafter called 26 al 53 mn age we calculate such 26 al 53 mn ages for a diversity of evolved objects with the constraint that they should be found for realistic chondritic parent body precursors i e objects having similar or identical petrograpy to the existing chondrite groups the so defined age of the d orbigny angrite is 4 3 1 1 myr for the asuka 881394 eucrite 2 8 1 0 myr for the h4 chondrite sainte marguerite 3 myr and for h4 forest vale 5 myr the calculated 26 al 53 mn ages give timescales for the evolution of the respective parent bodies meteorites that can be investigated in the light of further petrographic studies we anchor the calculated relative chronology to an absolute chronology using absolute pb pb ages and relative hf w ages of the objects under scrutiny the precursors of sainte marguerite and forest vale agglomerated at the same time 4565 8 1 2 ma ago the precursors of eucrites asuka 881394 agglomerated 4564 8 1 2 ma ago the precursors of angrites agglomerated late 4561 5 1 8 ma ago our model provides a fully compatible al mg mn cr pb pb chronology and is shown to be robust to reasonable changes in the input parameters the calculated initial 26 al 27 al ratios are high enough to have 26 al as a possible heat source for differentiation

influence of geometry and eclogitization on oceanic plateau subduction abstract subduction is driven by the negative buoyancy of the slab which depends on both the temperature and composition of the lithospheric plate in the case of subduction of an oceanic plateau larger thickness of the lower density crust and harzburgite layers can locally decrease the negative buoyancy available to drive subduction however the ability of a plateau to have an impact on subduction depends on the total buoyancy of the slab and therefore also depends on size of the plateau and the basalt to eclogite transition which substantially increases the density of the crust using 2 d numerical finite element models of subduction we investigate the role of eclogitization and oceanic plateau size thickness and length in the process of oceanic plateau subduction model results show that eclogitization of the crust substantially increases the chances that a plateau will be subducted because the extra buoyancy of larger plateaus is lost through the phase transition for kinematically driven models all plateaus are subducted regardless of the thickness or width extrapolation of these results to 3 d geometry shows that sufficient slab buoyancy is available to sustain subduction for narrow plateaus with slab widths 6 7 times the plateau width depending on plateau thickness and length in dynamically driven models subduction of oceanic plateaus of various sizes can substantially slow subduction and large plateaus 200 km long and 25 km thick can lead to break off of the slab and underplating of the plateau

53mn 53cr dating of fayalite formation in the cv3 chondrite mokoia evidence for asteroidal alteration fayalite grains in chondrules in the oxidized aqueously altered cv3 chondrite mokoia have large excesses of radiogenic chromium 53 these excesses indicate the in situ decay of short lived manganese 53 half life 3 7 million years and define an initial53mn 55mn ratio of 2 32 0 18 10 6 this ratio is comparable to values for carbonates in ci and cm chondrites and for several classes of differentiated meteorites mokoia fayalites formed 7 to 16 million years after allende calcium aluminum rich inclusions during hydrothermal activity on a geologically active asteroid after chondritic components had ceased forming in the solar nebula

nickel and chromium isotopes in allende inclusions high precision nickel and chromium isotopic measurements have been carried out on nine allende inclusions 62ni 64ni excesses are present in at least three of the samples the magnitudes of the excesses are about 3 e units 1 e unit 1 part in 10 000 for64ni and about 1 e unit for62ni chromium isotopes measured for some of the same samples show general excesses on54cr of about 7 e units while small deficits are observed for53cr considering these isotope anomalies except those for53cr together with the excesses present in the samples of other neutron rich isotopes of the iron group elements the most likely mechanism responsible for these anomalies is a neutron rich statistical equilibrium process in addition there is an indication of elevated60ni in almost every inclusion measured however for alk 111 this excess was clearly resolved by our experimental technique this effect may be related to the decay of now extinct60fe but the present data are not sufficient to clearly demonstrate its existence in the early solar system an upper limit of 1 6 10 6 can be calculated for60fe 56fe at the time these allende inclusions crystallized

dynamic uplift during slab flattening subduction exerts a strong control on surface topography and is the main cause of large vertical motions in continents including past events of large scale marine flooding and tilting the mechanism is dynamic deflection the sinking of dense subducted lithosphere gives rise to stresses that directly pull down the surface here we show that subduction does not always lead to downward deflections of the earth s surface subduction of young lithosphere at shallow angles flat subduction leaves it neutrally or even positively buoyant with respect to underlying mantle because the lithosphere is relatively warm compared with older lithosphere and because the thickened and hence drier oceanic crust does not undergo the transformation of basalt to denser eclogite accounting for neutrally buoyant flat segments along with large variations in slab morphology in the south american subduction zone explains along strike and temporal changes in dynamic topography observed in the geologic record since the beginning of the cenozoic our results show that the transition from normal subduction to slab flattening generates dynamic uplift preventing back arc marine flooding