thermal histories of the core and mantle recognition that the cooling of the core is accomplished by conduction of heat into a thermal boundary layer d at the base of the mantle partly decouples calculations of the thermal histories of the core and mantle both are controlled by the temperature dependent rheology of the mantle but in different ways thermal parameters of the earth are more tightly constrained than hitherto by demanding that they satisfy both core and mantle histories we require evolution from an early state in which the temperatures of the top of the core and the base of the mantle were both very close to the mantle solidus to the present state in which a temperature increment estimated to be 800 k has developed across d the thermal history is not very dependent upon the assumption of newtonian or non newtonian mantle rheology the thermal boundary layer at the base of the mantle i e d developed within the first few hundred million years and the temperature increment across it is still increasing slowly in our preferred model the present temperature at the top of the core is 3800 k and the mantle temperature extrapolated to the core boundary without the thermal boundary layer is 3000 k the mantle solidus is 3860 k these temperatures could be varied within quite wide limits without seriously affecting our conclusions core gravitational energy release is found to have been remarkably constant at 3 1011 w nearly 20 of the core heat flux for the past 3 109 y although the total terrestrial heat flux has decreased by a factor of 2 or 3 in that time this gravitational energy can power the chemical dynamo in spite of a core heat flux that is less than that required by conduction down an adiabatic gradient in the outer core part of the gravitational energy is used to redistribute the excess heat back into the core leaving 1 8 1011 w to drive the dynamo at no time was the dynamo thermally driven and the present radioactive heating in the core is negligibly small the dynamo can persist indefinitely into the future available power 1010 y from now is estimated to be 0 3 1011 w if linear mantle rheology is assumed or more if mantle rheology is non linear the assumption that the gravitational constant decreases with time imposes an implausible rate of decrease in dynamo energy with conventional thermodynamics it also requires radiogenic heating of the mantle considerably in excess of the likely content of radioactive elements

gravity field of the moon from the gravity recovery and interior laboratory grail mission spacecraft to spacecraft tracking observations from the gravity recovery and interior laboratory grail have been used to construct a gravitational field of the moon to spherical harmonic degree and order 420 the grail field reveals features not previously resolved including tectonic structures volcanic landforms basin rings crater central peaks and numerous simple craters from degrees 80 through 300 over 98 of the gravitational signature is associated with topography a result that reflects the preservation of crater relief in highly fractured crust the remaining 2 represents fine details of subsurface structure not previously resolved grail elucidates the role of impact bombardment in homogenizing the distribution of shallow density anomalies on terrestrial planetary bodies

provenance and weathering control on river bed sediments of the eastern tibetan plateau and the russian far east abstract we investigated the petrography and major trace and rare earth element compositions of the bed sediments from the large rivers draining the eastern tibetan plateau etp the huang he yellow chang jiang yangtze hong he red mekong and salween the combined drainage spans tropical temperate and arid climate zones in addition samples from the lena river draining the verkhoyansk and sette daban ranges in an arctic to sub arctic climate zone in siberia were also studied for their major and trace element composition the sub angular texture the quartz feldspar rock fragment classification in the lithic arenite field and the relatively low chemical index of alteration cia 42 79 indicate that these sediments are texturally petrographically and chemically immature and immobile trace elements can be used to obtain some information about the lithologic provenance the etp river sediments are felsic and similar to the upper continental crust with the salween most felsic and the lena most mafic the sediments show a gradation in maturity that is not clearly related to present climatic or physiographic regime this suggests that maturity itself is a relict feature inherited from previous sedimentary cycles the high physical erosion in such high energy and relief systems seems to inhibit intense weathering and manifests a provenance control from local rocks on the sediment compositions

holocene transgression recorded by sand composition in the mesotidal galician coastline nw spain this study confirms several inferences regarding holocene coastal dynamics and climate through a petrographic modal analysis of 60 holocene sand samples recovered in seven sites along the nw coast of the iberian peninsula fluvial sand can be discriminated from more mature intertidal and aeolian sand according to texture and composition fluvial sand contains soil products and coastal sand has significant bioclasts quartzofeldspathic sand appears in the western area produced by the erosion of granite and granitoid and quartzolithic sand occurs in the eastern area produced by the erosion of metasediment changes in sand composition during holocene deposition are manifested by an increase in modern carbonate clasts mc correlated with the holocene transgression episodes of faster sea level rise and subsequent erosion of surrounding cliffs are indicated by the preservation of high proportions of feldspar in intertidal sand in contrast fluvial sand is characterized by greater quartz enrichment these inferences were confirmed by petrographic indices carbonate clasts total clasts mc t total feldspars monocrystalline quartz f qm and plagioclase total feldspars p f the different maturity of intertidal and aeolian sands is revealed by their variable quartz contents despite similar proportions of plagioclase and k feldspar this suggests mechanical abrasion as the main factor controlling maturity in contrast fluvial sand shows depleted plagioclase contents as the result of inland weathering processes intertidal beach and aeolian sands are essentially the products of the erosion of coastal cliffs and head deposits with only the scarce contribution of fluvial drainages the long distance transport of galician coastal sands is discarded based on the close relationship between their composition and that of local sand sources our findings indicate that short distance transport of sediments from the west closed off coastal wetlands and occluded estuarine mouths during the holocene transgression by deposition on sediment trap zones along the irregularly shaped galician coast

the coupled effects of mantle mixing and a water dependent viscosity on the surface ocean abstract water content plays a vital role in determining mantle rheology and thus mantle convection and plate tectonics most parameterised convection models predict that earth initially underwent a period of rapid degassing and heating followed by a slow and sustained period of regassing and cooling however these models assume water is instantaneously mixed and homogeneously distributed into the mantle this is a limitation because the mixing time for water entering and leaving the mantle is a function of the rayleigh number which varies dramatically with water content temperature and through time here we present an adapted parametrised model crowley et al 2011 to include the coupled effects of the time scale of mixing with a water dependent viscosity we consider two mixing types first where the mixing time is constant throughout the model and second where mixing time varies as a response to an evolving rayleigh number we find that facilitated by the effects of water content in the melt region at mid ocean ridges a constant mixing time can induce long periods of degassing the inclusion of a variable mixing time dependent on the rayleigh number acts to limit the period of degassing and also results in more water being stored in the mantle and less at the surface than in both the constant and instantaneous mixing cases mixing time cannot be more than 2 billion years as large mixing times trap water in the mantle leaving a dry surface even small changes in the surface ocean induced by mixing times on the order of 0 1 gyrs can cause changes in the global mean sea level on the order of 10 s of metres these changes in sea level could easily uncover topographic highs in the bathymetry potentially aiding sub aerial erosion a process thought to be important in early earth evolution even in this relatively simple model the inclusion of a mixing time between water entering and leaving the mantle creates a more dynamic water cycle

a note on continents and the earth s urey ratio abstract the urey ratio mantle heat production divided by heat loss is a key constraint for thermal history models recent urey ratio estimates are in the range of 0 21 0 49 it has been well noted in the literature that classic thermal history models predict a larger value the extent of the discrepancy is considered significant enough to be deemed a paradox the urey ratio paradox has motivated a number of studies and some considerable debates classic thermal history models and the majority of those that have followed do not explicitly consider the effects of continents for consistency the present day observations they seek to match should be adjusted to account for continents we use mantle convection simulations with continents to argue that the adjusted ratio shifts to 0 33 0 76 classic thermal history models predict a urey ratio that although at the high end is within this range this suggests that the paradox has been exaggerated it also points toward a first order role for continents in the earth s thermal evolution

stirring geochemistry in mantle convection models with stiff plates and slabs abstract numerical models of mantle convection are presented that readily yield midocean ridge basalt morb and oceanic island basalt oib ages equaling or exceeding the apparent 1 8 ga lead isotopic ages of trace element heterogeneities in the mantle these models feature high viscosity surface plates and subducting lithosphere and higher viscosities in the lower mantle the formation and subduction of oceanic crust are simulated by means of tracers that represent a basaltic component the models are run at the full mantle rayleigh number and take account of faster mantle overturning and deeper melting in the past more than 97 of the mantle is processed in these models including the expected excess density of former oceanic crust readily accounts for the depletion of morb source relative to oib sources a novel finding is of gravitational settling of dense tracers within the low viscosity upper mantle as well as at the base of the mantle the models suggest as well that the seismological observation of a change in tomographic character in the deep mantle might be explained without the need to postulate a separate layer in the deep mantle these results expand the range of models with the potential to reconcile geochemical and geophysical observations of the mantle

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

south america and a few grains of sand part 1 beach sands to study correlations between petrography and provenance of modern sands and thus contribute to improved interpretation of ancient sandstones this study reports on the petrography of 218 modern south american beach sands obtained from a shoreline of 25 300 km judged by quartz feldspar rock fragment ratios there are five major associations pacific 24 16 60 argentina 28 14 58 brazil 91 4 5 caribbean east 59 8 23 and caribbean west 54 17 29 study of potash feldspar volcanic rock fragments and the sio 2 al 2 o 3 ratio supplement q f rf ratios tectonics are the dominant control on the mineralogy both the pacific and brazilian associations which form 75 of south america s coastline identify correctly tectonic setting that is active margin composition for the pacific and passive margin for the brazilian however the argentine association would yield a serious error because sand with active margin composition overprints a passive margin variation of total quartz and feld

paleoclimate and extensional tectonics of short lived lacustrine environments lower cretaceous of the panormide southern tethyan carbonate platform nw sicily abstract subaerial erosion and continental sedimentation interbedded with shallow water carbonates are unequivocal stratigraphic records to evaluate paleoenvironmental and paleoclimate evolution of emerged landmass stratigraphic analysis of the cretaceous monte gallo section of the mesozoic panormide carbonate platform in the northern side of the palermo mountains nw sicily records a peculiar continental derived clays that interrupted the shallow water carbonate sedimentation these clays rest with lenticular geometries above the tectonically enhanced subaerial erosional unconformity of the barremian lower aptian requienid limestones and are covered by the upper cretaceous rudistid limestone sedimentological investigation combined with mineralogical and petrographic results reveal the occurrence of alkaline to saline lake clays deposition in pond filling depositional environment recording stressed conditions evaporation especially in its final living phase they were formed when a half graben tilted block tectonics produced footwall uplift of the gallo faulted blocks carbonate platform paleoclimate evaluations of the continental derived clays highlighted that a period of warm humid conditions which favoured their formation interrupted the uniform warm climate conditions highlighting a greenhouse climate phase

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

the evolution of hadean eoarchaean geodynamics abstract geodynamic modelling of hadean eoarchaean tectonics typically requires higher rates of internal heat production and higher mantle temperatures in models that possess temperature dependent viscosity and a yield criterion in such models under hadean conditions for a wide range of geodynamic configurations and modelling approaches subduction has a propensity to fail this has led to the suggestion that the predominant tectonic regime in the hadean was stagnant lid convection with intermittent recycling events various lines of evidence support this suggestion from i the long mixing time of mantle isotopic anomalies or compositional heterogeneities such as 142nd 182w and platinum group elements to ii the long residence time of the hadean protolith to the jack hills zircons and iii thermal evolution models which typically require lower heat flux in the past to avoid the archaean thermal catastrophe the framework provided by stagnant lid or episodic overturn convection iv provides an explanation for the formation of early archaean ttgs and greenstones and v explains the interleaving arc plume sequence observed in many archaean terranes suggesting subduction initiation events may have been common increasing their preservation potential implications include a low magnetic field strength in the hadean which is consistent with emerging paleointensity data from these times

an early lunar core dynamo driven by thermochemical mantle convection although the moon currently has no internally generated magnetic field palaeomagnetic data combined with radiometric ages of apollo samples provide evidence for such a magnetic field from 3 9 to 3 6 billion years gyr ago1 possibly owing to an ancient lunar dynamo1 2 but the presence of a lunar dynamo during this time period is difficult to explain1 2 3 4 because thermal evolution models for the moon5 yield insufficient core heat flux to power a dynamo after 4 2 gyr ago here we show that a transient increase in core heat flux after an overturn of an initially stratified lunar mantle might explain the existence and timing of an early lunar dynamo using a three dimensional spherical convection model6 we show that a dense layer enriched in radioactive elements a thermal blanket at the base of the lunar mantle can initially prevent core cooling thereby inhibiting core convection and magnetic field generation subsequent radioactive heating progressively increases the buoyancy of the thermal blanket ultimately causing it to rise back into the mantle the removal of the thermal blanket proposed to explain the eruption of thorium and titanium rich lunar mare basalts7 plausibly results in a core heat flux sufficient to power a short lived lunar dynamo

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

mixing by time dependent convection abstract two dimensional numerical flow models are employed to study the mixing of passive heterogeneities in the earth s mantle both simple kinematic models of time dependent flow and dynamic convection models are studied initially small blocks of tracers are inserted into the flow and their progressive dispersal is monitored through time some of the tracers also serve as infinitesimal strain markers three different regimes of mixing are observed a all heterogeneities are rapidly strained and stirred according to a turbulent mixing law where the size of a blob decreases exponentially with time b most heterogeneities are only slowly strained and possibly follow a laminar mixing law with a linear decrease with time of the size of a heterogeneity c the size reduction proceeds rapidly in part of the fluid but some poorly stirred islands persist for a long time all three regimes are found in various kinematic flow models while in the dynamic convection models only regime a is observed the effective strain rate in the exponential turbulent mixing law is more than an order of magnitude smaller than the average flow strain rate when the results from the dynamic convection models with variable viscosity are scaled to the upper mantle mixing over small length scales would be achieved after a few hundred million years for whole mantle convection with a significant increase of viscosity with depth mixing times could be in the order of 1 2 ga in the latter case it appears marginally possible that part of the isotopic heterogeneity observed in oceanic basalts has evolved within the convecting mantle rather than in physically separated reservoirs

extensive crustal extraction in earth s early history inferred from molybdenum isotopes estimates of the volume of the earliest crust based on zircon ages and radiogenic isotopes remain equivocal stable isotope systems such as molybdenum have the potential to provide further constraints but remain underused due to the lack of complementarity between mantle and crustal reservoirs here we present molybdenum isotope data for archaean komatiites and phanerozoic komatiites and picrites and demonstrate that their mantle sources all possess subchondritic signatures complementary to the superchondritic continental crust these results confirm that the present day degree of mantle depletion was achieved by 3 5 billion years ago and that earth has been in a steady state with respect to molybdenum recycling mass balance modelling shows that this early mantle depletion requires the extraction of a far greater volume of mafic dominated protocrust than previously thought more than twice the volume of the continental crust today implying rapid crustal growth and destruction in the first billion years of earth s history

progressive mixing of meteoritic veneer into the early earth s deep mantle komatiites are ancient volcanic rocks mostly over 2 7 billion years old from the archaean era that formed through high degrees of partial melting of the mantle and therefore provide reliable information on bulk mantle compositions1 in particular the platinum group element pge contents of komatiites provide a unique source of information on core formation mantle differentiation and possibly core mantle interaction2 3 4 5 6 7 8 most of the available pge data on komatiites are from late archaean 2 7 2 9 gyr old or early proterozoic 2 0 2 5 gyr old samples here we show that most early archaean 3 5 3 2 gyr old komatiites from the barberton greenstone belt of south africa and the pilbara craton of western australia are depleted in pge relative to late archaean and younger komatiites early archaean komatiites record a signal of pge depletion in the lower mantle resulting from core formation this signal diminishes with time owing to progressive mixing in to the deep mantle of pge enriched cosmic material that the earth accreted as the late veneer during the early archaean 4 5 3 8 gyr ago meteorite bombardment

the tungsten isotopic composition of the earth s mantle before the terminal bombardment it has long been speculated that a late heavy bombardment of earth by meteoritic material replenished the mantle s budget of siderophile iron loving elements such as tungsten that were largely lost to the core during its segregation however evidence for this late veneer remains indirect and its influence has been much debated matthias willbold and colleagues present high precision tungsten isotope analyses of ancient greenland rocks and show that they have significantly higher 182w 184w ratios than modern terrestrial samples this finding is in good agreement with the expected influence of a meteoritic late heavy bombardment they speculate that both the tungsten isotope data and the observed decrease in 142nd 144nd neodymium ratios can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection many precious iron loving metals such as gold are surprisingly abundant in the accessible parts of the earth given the efficiency with which core formation should have removed them to the planet s deep interior1 one explanation of their over abundance is a late veneer a flux of meteorites added to the earth after core formation as a terminal bombardment that culminated in the cratering of the moon2 some 3 8 billion year old rocks from isua greenland are derived from sources that retain an isotopic memory of events pre dating this cataclysmic meteorite shower3 4 these isua samples thus provide a window on the composition of the earth before such a late veneer and allow a direct test of its importance in modifying the composition of the planet using high precision less than 6 parts per million 2 standard deviations tungsten isotope analyses of these rocks here we show that they have a isotopic tungsten ratio 182w 184w that is significantly higher about 13 parts per million than modern terrestrial samples this finding is in good agreement with the expected influence of a late veneer we also show that alternative interpretations such as partial remixing of a deep mantle reservoir formed in the hadean eon5 6 more than four billion years ago or core mantle interaction7 do not explain the w isotope data well the decrease in mantle 182w 184w occurs during the archean eon about four to three billion years ago potentially on the same timescale as a notable decrease in 142nd 144nd refs 3 and 6 we speculate that both observations can be explained if late meteorite bombardment triggered the onset of the current style of mantle convection

paleo to mesoarchean polymetamorphism in the barberton granite greenstone belt south africa constraints from u pb monazite and lu hf garnet geochronology on the tectonic processes that shaped the belt discussion cutts et al 2014 1 combined thermodynamic modeling of compositional change in metamorphic garnet with metamorphic peak and retrograde pressure temperature p t data derived from quantitative p t pseudosections to determine clockwise p t paths for several samples from the tjakastad

onset of solid state mantle convection and mixing during magma ocean solidification energy sources involved in the early stages of planetary formation can cause partial or even complete melting of the mantle of terrestrial bodies leading to the formation of magma oceans upon planetary cooling solidification is expected to take place from the bottom upwards because of the steeper slope of the liquid adiabat with respect to the liquidus elkins tanton 2012 solomatov 2015 fractional solidification in particular can lead to the formation of a compositional layering that can play a fundamental role for the subsequent long term dynamics and evolution of the interior tosi et al 2013 plesa et al 2014 in order to assess to what extent primordial compositional heterogeneities generated upon magma ocean solidification can be preserved we investigate the cooling and solidification of a whole mantle magma ocean along with the conditions that allow solid state convection to start mixing the mantle before solidification has completed to this end we run 2 d numerical simulations in cylindrical geometry using the finite volume code gaia huttig et al 2013 we treat the liquid magma ocean in a parametrized fashion while we self consistently solve the conservation equations of thermochemical convection in the growing solid mantle accounting for pressure temperature and melt dependent rheology we consider two end member cases fractional crystallization where melt is instantaneously extracted into the overlying liquid leaving beneath a differentiated mantle and batch crystallization where melt remains in contact with the silicate matrix throughout solidification causing no differentiation by testing the effects of different cooling rates and rayleigh numbers we show that for a lifetime of the liquid magma ocean between 1 and 10 myr lebrun et al 2013 the onset of solid state convection prior to complete mantle crystallization is possible and that part or even all of the compositional heterogeneities generated upon fractionation can be erased by efficient mantle stirring figure 1 we discuss the consequences of our findings in relation to the early and long term evolution of compositional heterogeneities generated via fractional crystallization of magma oceans in terrestrial bodies with emphasis on mars thermochemical history

natural weathering mechanisms of almandine garnet the weathering of almandine garnet in the oxidized vadose zone of saprolite near otto north carolina begins at grain boundaries and along fractures traversing the garnet grains radially oriented fibrous intergrowths of gibbsite and goethite form layers of uniform thickness which grow by centripetal replacement as weathering proceeds the contact between the garnet surface and the layer of weathering products is sharp and smooth and garnet corners are rounded large well defined etch pits are absent on the underlying garnet surface these observations suggest that diffusion transport of reactants and or products through the gibbsite goethite layer is the rate limiting step in the weathering of almandine garnet in the oxidizing environment of the saprolite in soils overlying the saprolite garnet surfaces are directly exposed to weathering solutions without the intervening surface layer such unprotected grains in soils and stream sediments exhibit numerous large well developed etch pits surface reaction control rather than transport control prevails during weathering of almandine garnet in the absence of protective surface layers of weathering products biochemical or biological processes in the soil apparently prevent the gibbsite goethite layer from forming or persisting soil solutions can therefore react directly with the garnet surface rather than requiring reactants or products to diffuse through the gibbsite goethite coating as in the saprolite the mechanism and rate limiting step of almandine garnet weathering is apparently strongly dependent on the chemical environment in which weathering occurs

provenance of the carboniferous hochwipfel formation karawanken mountains austria slovenia geochemistry versus petrography this study focuses on the carboniferous sediments of the karawanken mountains austria slovenia to identify possible source areas and their geotectonic setting provenance analyses have been applied using petrographical and geochemical approaches what also lead to an evaluation of the used methods within the hochwipfel formation five stratigraphic related petrofacies groups can be defined using the sandstone component inventory homogenous provenance results based on major element and trace element data suggest an active continental margin continental island arc with probably an intraplate signature provenance analyses of light minerals point to source areas with basement rocks mainly metamorphic arc material and sedimentary rocks in combination the rapid and highly variable change of provenance signatures within the stratigraphic succession of the petrofacies types excludes a single provenance in the western part of the karawanken mountains additionally changes in composition of the individual petrofacies groups correlate with the stratigraphy in the eastern part varying contents of garnet chromite hornblende and epidote within the sedimentary succession of the hochwipfel sandstones can partly be attributed to geodynamical changes in the hinterland the chemical compositions of garnets tourmalines and chromites confirm the results of both geochemistry and sediment petrography in a quantitative approach based on petrography we can see an equal mixture of all source areas whereas quantification with a geochemical approach results in c 11 of material that can be attributed to a passive continental margin source 28 to 37 to an active continental margin c 44 to a magmatic arc setting and 16 to 19 to an oceanic within plate source area the resulting model for the geodynamical setting of the hochwipfel basin during the late palaeozoic can be best explained by a forearc basin in the eastern part and various pull apart basins in the western part connected through bypasses at the continental margin of gondwana and the palaeotethys as an additional outcome this study clarifies that geochemically derived provenance results based on bulk rock and mineral composition recognize best complex geodynamical settings and a mixture of sources since they incorporate best the provenance significant lithic components on the other hand it points out that an application of additional petrological approaches does not necessarily lead to further information and an elimination of errors but could be essential to define general information of the investigated stratigraphy like the correlation of deposits with stratigraphy petrofacies groups

pre 3750 ma supracrustal rocks from the nuvvuagittuq supracrustal belt northern quebec geochemistry and u pb ion microprobe zircon geochronology guided by high resolution mapping 1 50 scale was used to define a minimum age of ca 3750 ma for supracrustal rocks from the nuvvuagittuq supracrustal belt nsb in the northern superior province quebec canada mineralogy and geochemistry of critical field relationships preserved at the porpoise cove locality describe a supracrustal succession of mafic amphibolites and ultramafic rocks finely banded quartz magnetite units with intermixed coarse grained ferruginous quartz pyroxene rocks and quartz biotite schists that superficially resemble polymict metaconglomerates with large up to 10 cm deformed polycrystalline quartz and mafic clasts all units in the mapped outcrop have sharp lithological contacts narrow dominantly trondhjemitic orthogneiss sheets locally preserve intrusive contact relationships to the supracrustals the total extent of the supracrustal enclave is 8k m 2 it is strongly deformed and the full deformation history appears to be shared by all units with later modifications from leucogranitoid intrusions the quartz biotite schists record complex zircon growth at 3500 and 2800 ma interpreted to reflect metamorphic events zircons separated from orthogneisses in the enclave including one sheet that transects a banded quartz pyroxene magnetite unit yield magmatic 207 pb 206 pb ages of ca 3750 ma these ages are slightly younger than earlier provisional reports for an nsb orthogneiss from porpoise cove the nuvvuagittuq supracrustals are the oldest rocks thus far reported for the minto block they overlap in age with the ca 3 70 3 83 ga isua supracrustal belt and akilia association rocks in west greenland and they represent an important new area for exploration of the early earth

degree 1 mantle convection and the crustal dichotomy on mars the surface of mars consists of an old heavily cratered and elevated southern hemisphere and younger resurfaced and depressed northern hemisphere a feature often termed the crustal dichotomy the global crustal structure zuber et al 2000 revealed by topography and gravity data from the mars global surveyor spacecraft and the possible late formation of the boundary zone between the hemispheres mcgill and dimitriou 1990 have been proposed to indicate an endogenic origin for the dichotomy however degree 1 mantle convection that is required for any endogenic process to be viable cannot be produced with conventional mantle convection models schubert et al 1990 we have studied the role of radially stratified viscosity on mantle deformation by using rayleigh taylor instability analyses in a spherical shell geometry our analyses reveal that when mantle viscosity is stratified with a weak asthenosphere deformation at long wavelengths is more efficient than that at short wavelengths the weaker the asthenosphere the longer the wavelength at which the deformation is the most efficient a thicker asthenosphere also favors the deformation at long wavelengths both the rayleigh taylor instability analyses and numerical modeling of mantle convection show that degree 1 convection can be produced within the martian mantle provided that the mantle had a weak asthenosphere v500 km thick and v10 2 times weaker than the underlying mantle early in planetary history the degree 1 convection causes preferential heating of one hemisphere that may explain the primary features associated with the dichotomy in crustal structure fl 2001 elsevier science b v all rights reserved

effect of plate bending on the urey ratio and the thermal evolution of the mantle abstract the bending of tectonic plates as they subduct causes resistance to plate motions and mantle convection it has been proposed that this effect could keep plate velocities relatively constant with time and it would imply relatively high mantle temperatures through much of earth history and relatively rapid cooling at present it also implies a low urey ratio compatible with that inferred from cosmochemistry here it is confirmed that bending resistance only plays a significant role if plate thickness is determined mainly by dehydration stiffening accompanying melting rather than by conductive cooling even then the bending resistance is quite sensitive to the radius of curvature of the subducting plate observed radii are generally larger than the 200 km assumed in some studies ranging up to 600 km or more furthermore radii of curvature tend to adjust so as to prevent bending resistance from becoming large when these factors are accounted for calculations show that bending resistance is unlikely to have been a large factor through earth history and the thermal evolution of the mantle is unlikely to have been affected very much the resolution of the urey ratio problem should then be sought elsewhere

late accretion as a natural consequence of planetary growth the mantles of the terrestrial planets contain elemental abundances that suggest accretion continued at a late stage after core formation geochemical data of meteorites from differentiated asteroids are consistent with such a late accretion event suggesting that the phenomenon occurred throughout the solar system and was related to planet formation

deep earth recycling in the hadean and constraints on surface tectonics the hadean mantle was efficiently heated from high internal heat production high rates of impact bombardment and primordial heat from accretion as a result a strong case is made for extremely high internal temperatures low internal viscosities and extremely vigorous mantle convection previous studies of mixing in such high rayleigh number convective environments indicate that chemically heterogeneous mantle anomalies should have efficiently remixed into the mantle on timescales of less than 100 myr however 142nd and 182w isotope studies indicate that heterogeneous mantle domains survived without mixing for over 2 gyr at odds with expected mixing rates similarly concentrations of platinum group elements in archean komatiites purportedly due to the later veneer of meteoritic addition on the earth only achieve current levels at 2 7 ga indicating a time lag of almost 1 to 2 gyr in mixing this material thoroughly into the mantle previous studies have sought to explain slow archean mantle mixing via mantle layering due to endothermic phase changes or anomalously viscous blobs of material with limited efficacy here we pursue another explanation for inefficient mantle mixing in the hadean tectonic regime a number of lines of evidence suggest that resurfacing in the archean was episodic and extending these models to hadean times implies the hadean was characterized by long periods of tectonic quiescence we explore mixing times in 3d spherical cap models of mantle convection which incorporate vertically stratified and temperature dependent viscosities we show that mixing in stagnant lid regimes is at the extreme over an order of magnitude less efficient than mobile lid mixing and for plausible rayleigh numbers and internal heat production the lag in hadean convective recycling can be explained the attractiveness of this model is that it not only explains the long lived 142nd and 182w anomalies but also posits an explanation for the delay between accretion of the late veneer between 4 5 to 3 8 ga on a stagnant surface and its fully mixed signature apparent in elevated pges in 2 7 ga komatiites it also provides an explanation for the 400 myrs of immobility of the mafic protolith from which the jack hill zircons were sourced and retards early heat loss from the mantle providing a solution to the archean thermal catastrophe of parameterized earth evolution models

geoframework coupling multiple models of mantle convection within a computational framework solver coupling can extend the capability of existing modeling software and provide a new venue to address previously intractable problems a software package has been developed to couple geophysical solvers demonstrating a method to accurately and efficiently solve multiscale geophysical problems with reengineered software using a computational framework pyre pyre is a modeling framework capable of handling all aspects of the specification and launching of numerical investigations we restructured and ported citcoms a finite element code for mantle convection into the pyre framework two citcoms solvers are coupled to investigate the interaction of a plume at high resolution with global mantle flow at low resolution a comparison of the coupled models with parameterized models demonstrates the accuracy and efficiency of the coupled models and illustrates the limitations and utility of parameterized models

an archaean heavy bombardment from a destabilized extension of the asteroid belt the late heavy bombardment lasted much longer than previously thought up to 1 7 billion years ago on earth with impacts on the moon and earth coming mostly from the e belt survivor hungaria asteroids the late heavy bombardment was a period of time generally put at about 4 1 billion to 3 8 billion years ago when the inner planets of the solar system were subjected to a high frequency barrage of asteroids this left its mark on the moon but on earth the craters quickly disappeared owing to tectonic processes and erosion in the first of two papers on the bombardment brandon johnson and jay melosh determine the properties of the asteroids by looking at spherule beds layers of debris ejected during the impacts the thickness of spherule layers is expected to vary according to the size of the impactor and the speed at which it hit earth this historical record of impacts indicates that the number of projectiles colliding with earth was substantially higher 3 5 billion years ago than it is today with a gradual decline in the number of strikes after the late heavy bombardment bottke et al modelled the evolution of an asteroid belt that extended farther towards mars than the present one they find that most of the impactors traced by the spherule beds probably originated in this e belt which was disrupted during migrations of some of the giant planets the barrage of comets and asteroids that produced many young lunar basins craters over 300 kilometres in diameter has frequently been called the late heavy bombardment1 lhb many assume the lhb ended about 3 7 to 3 8 billion years gyr ago with the formation of orientale basin2 3 evidence for lhb sized blasts on earth however extend into the archaean and early proterozoic eons in the form of impact spherule beds globally distributed ejecta layers created by chicxulub sized or larger cratering events4 at least seven spherule beds have been found that formed between 3 23 and 3 47 gyr ago four between 2 49 and 2 63 gyr ago and one between 1 7 and 2 1 gyr ago5 6 7 8 9 here we report that the lhb lasted much longer than previously thought with most late impactors coming from the e belt an extended and now largely extinct portion of the asteroid belt between 1 7 and 2 1 astronomical units from earth this region was destabilized by late giant planet migration10 11 12 13 e belt survivors now make up the high inclination hungaria asteroids14 15 scaling from the observed hungaria asteroids we find that e belt projectiles made about ten lunar basins between 3 7 and 4 1 gyr ago they also produced about 15 terrestrial basins between 2 5 and 3 7 gyr ago as well as around 70 and four chicxulub sized or larger craters on the earth and moon respectively between 1 7 and 3 7 gyr ago these rates reproduce impact spherule bed and lunar crater constraints

short term episodicity of archaean plate tectonics by combining geochemical data and geodynamical models evidence is provided to address the existence and style of archaean plate tectonics a topic of vigorous debate for decades using careful analyses of lithostratigraphic archaean assemblages and numerical model results we illustrate that a short term episodic style of subduction was a viable style of tectonics in the early earth modeling results show how due to the low strength of slabs in a hotter earth frequent slab break off events prevented a modern style long lived subduction system and resulted in frequent cessation and re initiation of the subduction process on a typical time scale of a few million years results fit with geochemical observations that suggest frequent alternation of arc style and non arc style volcanism on a similarly short time scale such tectonics could provide the link between early pre plate tectonic style of tectonics or stagnant lid convection and modern style plate tectonics in which short term episodes of proto subduction evolved over time into a longer term more successful style of plate tectonics as mantle temperature decayed

petrography and chemistry of sandstones from the swiss molasse basin an archive of the oligocene to miocene evolution of the central alps oligocene to miocene fluvial sandstones from the swiss molasse basin were analysed for sandstone framework composition heavy minerals whole rock geochemistry and detrital chrome spinel chemistry samples were taken from the proximal part of the basin close to the alpine main thrust and are chronostratigraphically calibrated between 31 and 13 ma sandstone composition allows the identification of different source rocks and their variation in time and space place constraints on the oligocene to miocene evolution of the central alps in the eastern part of the basin sandstones document a normal unroofing sequence with the downcutting from austroalpine sedimentary cover into austroalpine crystalline rocks and slightly later at 21 ma into penninic ophiolites in the central part downcutting into crystalline basement rocks occurred at 25 ma and the removal of the sedimentary cover was much more advanced than in the east this may be interpreted as a first signal from the doming of the lepontine area at 20 ma extensional tectonics in the hinterland led to the first exposure of low grade metamorphic rocks from the footwall of the simplon fault in the central alps erosion of these rocks persisted up to the youngest sediments at 13 ma in the western part of the basin a contribution from granitoid and ultra mafic rocks is documented as early as 28 ma the source for the ultra mafic detritus is penninic ophiolites from the piemonte zone of the western alps which were already exposed at the surface at that time

the influence of climate and topography on rock fragment abundance in modern fluival sands of the southern blue ridge mountains north carolina chemical weathering influences the detrital composition of sand size sediment derived from source areas subject to different amounts of precipitation in the coweeta basin north carolina of the grain types studied rock fragments are most sensitive to chemical degradation therefore their abundance is the best indicator of cumulative weathering effects destruction of sand size rock fragments by chemical weathering is a function of both the intensity and duration of chemical weathering experienced by grains in regoliths of the source area in the coweeta basin the intensity of chemical weathering is directly related to the climate via effective precipitation in individual subbasins whereas the duration of chemical weathering is inversely related to the relief ratio of the watershed therefore soils in watersheds with low relief ratios and high discharge per unit area experience the most extensive chemical weathering and sediments derived from these watersheds contain the lowest percentage of rock fragments the effects of climate alone cannot explain the systematic variation of rock fragment abundance in sediments from the coweeta basin the compositional imprint left on these sediments by chemical weathering is a function of both climate and topographic slope in the sediment source area

guidelines for assessing the provenance of mesozoic and cenozoic clastic successions sourced by pre jurassic basement complexes in southernmost north america

detrital modes in sedimenticlastic sands from low order streams in the iberian range spain the potential for sand generation by different sedimentary rocks the composition of modern stream sands derived from sedimentary source rocks in the iberian range has been analyzed in order to evaluate the contributions of the different bedrock types mainly sandstones limestones and dolostones temperate to subhumid climate and short transport conditions promote a weathering limited denudation regime as expected sand composition proved to be essentially quartzolithic with variable amounts of penecontemporaneous carbonates sand compositional data were compared with the exposure areas of the different bedrocks in the drainage sub basins considered for semi quantitative assessment of the sand generation potential of each bedrock type siliciclastic formations sandstones appear to be by far the most significant sand producers with sand generation indices sgis in the medium sand fraction ranging from 4 to 20 i e 4 to 20 times greater than the sgi of carbonate rocks composition and texture are the main factors controlling carbonate sand generation sparitic limestones yield higher sgis 2 8 to 20 when source terrains are constituted exclusively by carbonate rocks high sparite grain content in the sands is enhanced by supplies from additional sources such as calcitized dolostones dolomicrite sources are strongly under represented in the sands analyzed very low sgi whereas the proportion of micritic limestone grains tends to be an accurate reflection of that bedrock at the source even though the results presented here refer to the first stage of sand generation with negligible transport effects we think they may be helpful in the analysis and reconstruction of source terrains in ancient sedimenticlastic deposits

thermochemical structures within a spherical mantle superplumes or piles 1 heterogeneous compositional mantle models are frequently invoked to explain some observations obtained from geochemistry and seismology in particular two regions in the earth s lower mantle under the pacific and africa are often interpreted as being either piles or superplumes of dense material we perform numerical modeling of thermochemical convection in a three dimensional spherical geometry to investigate whether the geometry can focus a dense chemical component into a small number of isolated rounded piles or superplumes of material we study the effect of temperature and compositionally dependent viscosity and we find two different modes of dense layer deformation temperature dependent rheology leads to a low viscosity dense layer which is passively swept aside by downwellings into linear piles that are spread throughout the entire lower mantle the addition of compositionally dependent rheology in which the dense layer is more viscous results in active deformation of the dense material forming large isolated superplumes our results indicate that piles and superplumes are separate features which in general do not occur together and in order for isolated rounded superplumes to form an intrinsic compositional viscosity increase is required for the dense material

exceptions to the relationship between plate tectonics and sandstone composition abstract routine use of qfl qplsmlvm and lmlslv plate tectonic provenance diagrams has led to the recognition of error populations for these sandstones the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams the anomalous sandstones can be separated into at least four categories i sandstones deposited during the transition between tectonic regimes may be derived in part from relict source rocks ii the relative enrichment of sandstones in detrital quartz by weathering and or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data iii sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields iv the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams and is an unresolved problem the first two categories represent permanent problems in the use of sandstone composition as an indicator of plate tectonic setting whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate tectonic setting

compositional variations in deep sea gravity flow deposits a case study from the voirons flysch voirons wagital complex chablais prealps france abstract grain size control is frequently invoked to explain the compositional variation of detrital sediments in fluvial sediments a similar possibility is here investigated in the fossil deep sea gravity flow deposits from the voirons flysch voirons wagital complex chablais prealps about 270 samples of diverse depositional settings were analysed using the framework composition including cement and porosity mutti facies and grain size measurements six lithofacies have been identified from cluster analysis they correspond to channel l1 to l3 and lobe l4 and l5 depositional settings and to reworked turbidites l6 our results emphasize the close relationship between petrography and grain size which alters the detrital signal from extrabasinal sources the abundant polycrystalline component of proximal deposits is progressively removed while skeletal grains and additional intrabasinal grains become more abundant in distal deposits alongside porosity is reduced by the simultaneous increase of cement which constrains subsequent diagenetic modifications hence proximal depositional settings keep most of the inherited signal from extrabasinal sources whereas distal deposits will be homogenised when plotted in provenance analysis diagrams the successive depositional settings show distinct fields possibly corresponding to different tectonic settings of the dickinson model this compositional variation thus creates a bias in provenance interpretation resulting from both grain size control and flow mechanisms finally our results emphasize that petrographic data must be handled with care in provenance analyses especially when comparing various deep sea gravity flow systems with different grain size ranges

highly siderophile element abundances in eoarchean komatiite and basalt protoliths plume derived mg rich volcanic rocks komatiites high mg basalts and their metamorphic equivalents can record secular changes in the highly siderophile element hse abundances of mantle sources an apparent secular time dependent enrichment trend in hse abundances from paleoarchean to paleoproterozoic mantle derived rocks could represent the protracted homogenization of a late veneer chondritic contaminant into the pre late veneer komatiite source to search for a possible time dependence of a late accretion signature in the eoarchean mantle we report new data from rare 3700 myr old mafic and ultramafic schists locked in supracrustal belts from the inukjuak domain quebec canada and the akilia association west greenland our analysis shows that some of these experienced hse mobility and or include a cumulate component touboul et al in chem geol 383 63 75 2014 whereas several of the oldest samples show some of the most depleted hse abundances measured for rocks of this composition we consider these new data for the oldest documented rocks of komatiite protolith in light of the late veneer hypothesis

sand composition changes across key boundaries of siliciclastic and hybrid depositional sequences abstract sand composition of arenite successions is sensitive to a suite of factors operating between initial grain production and final diagenesis on a variety of spatial and temporal scales seven allogenic factors the relative importance of which relies upon the complex interaction between tectonics eustasy and climate play a decisive role in dictating petrofacies distribution within siliciclastic to hybrid depositional sequences these factors include i tectonic exhumation ii physical and chemical rock breakdown iii change in sediment flux iv change in source basin physiography v shelf colonization by organisms vi generation of chemical grains and vii volcanism autogenic processes may locally have a considerable influence on the type and amount of sediment supplied to the basin thus interfering with the external control based upon literature data a conceptual framework of expected compositional changes across the key surfaces for sequence stratigraphic interpretation is outlined two case histories from the miocene shelf to turbidite deposits of the northern apennines and the quaternary alluvial to nearshore succession of the adriatic coast respectively are used as references to illustrate how arenite petrofacies changes can be framed into a sequence stratigraphic scheme on multiple timescales

comment on neodymium 142 evidence for hadean mafic crust o neil et al reports 26 september 2008 p 1828 presented neodymium 142 data for rocks from northern quebec canada and suggested that these rocks may represent the oldest preserved crustal section on earth we argue that the age of the rocks is based on a spurious correlation between rocks that are probably not co genetic and negative 142 nd anomalies that may be the result of an analytical artifact

early earth a new recipe for old crust the processes responsible for the growth of earth s first continents are enigmatic the geochemical signature of 4 billion year old rocks discovered in canada points to a key role for shallow magmatic processes above upwelling mantle rocks

exploitation of faunal resources by marine hunter gatherer groups during the middle holocene at the copaca 1 site atacama desert coast abstract the article presents the results of a study conducted on an assemblage of archeofaunal remains from the copaca 1 archeological site located on the arid coast of northern chile the site corresponds to an extensive shell midden that was used generally as an occupational site and specifically as a funerary one by specialized marine hunter gatherers exclusively during the archaic period the analysis of the faunal remains enabled a general description of the use of local fauna throughout the site s cultural sequence which range from 7866 to 5040 cal bp according to the results obtained marine and terrestrial fauna including marine and terrestrial mammals sea birds pelagic and oceanic fish mollusks crustaceans and equinoderms were used as a source of both food and technological implements during the middle holocene this implies that the human groups that inhabited copaca 1 accessed most of the ecoanthropic spheres of the southern cone of the southwestern pacific coast from early times onward an adaptation dated since 12 000 bp in the south western coast from southern peru and northern chile

early martian mantle overturn inferred from isotopic composition of nakhlite meteorites following the crystallization of a magma ocean the martian mantle probably underwent an overturning event but its initiation timing and geochemical consequences are poorly constrained isotopic data for martian meteorites and numerical simulations provide strong evidence for early overturning in the martian mantle