the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

on the sensitivity of parameterized convection to the rate of decay of internal heat sources thermal histories for the earth based on parameterized mantle convection models have previously indicated that the earth departs significantly from a steady thermal state the non steady state is manifest by a present day excess of heat loss over heat production we investigate the dependence of the heat production heat loss ratio and mantle viscosity on heat source decay rate by varying the relative and absolute concentrations of the heat producing isotopes of k u and th in the mantle we examine three models with progressively increasing effective half lives respectively characterized by k u ratios of 6 104 chondritic 1 104 terrestrial and 0 25 104 low ka u each of these models can be constrained to yield a common present day heat flow mantle temperature and mantle viscosity thus demonstrating the inability of those present day constraints to differentiate between widely varying blends of the earth s nuclear fuel the present day heat production heat loss ratio ranges from about 73 for the chondritic model to about 86 for the low k u model other possible criteria diagnostic of the earth s radioisotope mix are the present day isotopic abundances the low k u model requires u and th greater by a factor of 2 5 over the chondritic model and k less by a factor of 10 however measurements of concentrations in candidate mantle rocks reveal a range that far exceeds the relatively small variations that distinguish the different compositional models paleothermal conditions also may be indicative of the earth s radioisotope blend the thermal state in the archean calculated for the low k u earth model is characterized by a mean mantle temperature about 100 c hotter and heat flow about 2 times greater than the present day whereas a chondritic earth model yields a mantle temperature 300 c hotter and heat flow 4 times greater however current estimates of paleo heat flow and mantle melting temperatures are burdened with uncertainties sufficiently large to preclude their use as criteria diagnostic of the earth s radioisotope endowment

volcanic degassing of argon and helium and the history of crustal production on venus we develop a new methodology linking 40ar and 4he degassing to crustal production on venus in order to examine two different scenarios for the history of magmatism on that planet both consistent with recent analyses of the cratering record of the venus surface the first scenario includes episodic global resurfacing events and modest levels of magmatism between such events while the second scenario invokes different rates of steady magmatism before and after a given transition time our degassing models include distinct mantle crustal and atmospheric reservoirs diffusive transfer of 40ar and 4he from the crust to the atmosphere is also taken into account the 40ar abundance in the present atmosphere reflects the integrated degassing and magmatism over most of planetary history in contrast helium escapes from the planetary atmosphere within a characteristic residence time of 200 myr to 1 8 gyr so the present atmospheric 4he abundance is relevant to the volume of magma produced during the last global resurfacing event and the rate of the magmatism subsequent to that event under the first scenario and to the two steady rates of magmatism and the transition time under the second unfortunately large uncertainties in mineral partition coefficients and in the ar and he mixing ratios in the lower atmosphere of venus presently prevent the use of our degassing models to distinguish among crustal formation history models we therefore explore the influence of these uncertain parameters on the degassing history in order to identify those new laboratory and in situ measurements that will most strongly constrain crustal production history as an important step in the development of the ar degassing model the k budget in the bulk silicate fraction of venus is re examined on the basis of venera and vega ray measurements of k u and th concentrations in surface materials for u and th concentrations in the bulk silicate planet of 18 29 ppb and 64 94 ppb respectively the degree of mantle melting that formed the surface materials at the venera 9 and 10 and vega 1 and 2 landing sites is calculated to lie between 0 02 and 0 16 the degree of melting calculated for materials at each landing site is combined with measurements of surface k concentration to estimate the k concentration in the bulk silicate portion of the planet at between 100 and 300 ppm these results suggest that venus and earth have similar heat production

experimental estimation of velocities and anisotropy of a series of swedish crystalline rocks and ores to provide a guide for future deep 1 5 km seismic mineral exploration and to better understand the nature of reflections imaged by surface reflection seismic data in two mining camps and a carbonatite complex of sweden more than 50 rock and ore samples were collected and measured for their seismic velocities the samples are geographically from the northern and central parts of sweden ranging from metallic ore deposits meta volcanic and meta intrusive rocks to deformed and metamorphosed rocks first ultrasonic measurements of p and s wave velocities at both atmospheric and elevated pressures using 0 5 mhz p and s wave transducers were conducted the ultrasonic measurements suggest that most of the measured velocities show positive correlation with the density of the samples with an exception of a massive sulphide ore sample that shows significant low p and s wave velocities the low p and s wave velocities are attributed to the mineral texture of the sample and partly lower pyrite content in comparison with a similar type sample obtained from norway which shows significantly higher p and s wave velocities later an iron ore sample from the central part of sweden was measured using a low frequency 0 1 50 hz apparatus to provide comparison with the ultrasonic velocity measurements the low frequency measurements indicate that the iron ore sample has minimal dispersion and attenuation the iron ore sample shows the highest acoustic impedance among our samples suggesting that these deposits are favourable targets for seismic methods this is further demonstrated by a real seismic section acquired over an iron ore mine in the central part of sweden finally a laser interferometer device was used to analyse elastic anisotropy of five rock samples taken from a major deformation zone in order to provide insights into the nature of reflections observed from the deformation zone up to 10 velocity anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone using the laser interferometery measurements however the origin of the reflections from the major deformation zone is attributed to a combination of anisotropy and amphibolite lenses within the deformation zone

mantle devolatilization and convection implications for the thermal history of the earth the depletion of volatiles from the earth s mantle over its history may have had significant effects on thermal convection due to the effects of volatiles on silicate rheology experimental data suggest that the activation energy for solid state creep in a volatile free environment is as much as 25 greater than in a volatile rich environment and thus devolatilization can lead to a substantial increase in viscosity in a devolatilizing system gradually increasing temperatures relative to those in a system with volatile independent rheology may be required to maintain moderate viscosities and convection we investigate this postulated effect by incorporating a time dependent increase in activation temperature simulating progressive mantle devolatilization in a parameterized convection model volatiles significantly affect the earth s thermal evolution when the dependence of mantle rheology upon volatile content is sufficiently strong the total volatile loss is sufficiently large and devolatilization is relatively slow and continuous for models with an increase of 20 in activation energy over earth history the present day ratio of heat production to heat loss urey ratio approaches or exceeds 1 0 when volatile loss is relatively continuous with rapid early volatile loss the upper bound on the present day urey ratio is about 0 8 to 0 9 comparable to the limit with volatile independent rheology present uncertainties in these factors permit an earth that may be close to a steady thermal state or perhaps even warming up

cooling of the earth in the archaean consequences of pressure release melting in a hotter mantle a model is presented to describe the cooling of the earth in the archaean at the higher archaean mantle temperatures pressure release melting starts deeper and generates a thicker basaltic or komatiitic crust and depleted harzburgite layer compared with the present day situation intrinsic compositional stability and lack of mechanical coherency renders the mechanism of plate tectonics ineffective it is proposed that the archaean continents stabilised early on top of a compositionally stratified root in the archaean oceanic lithosphere hydrated upper crust can founder and recycle through its high pressure phase eclogite eclogite remelting and new pressure release melting generates new crustal material migration of magma and latent heat release by solidification at the surface provides an efficient mechanism to cool the mantle by several hundreds of degrees during the archaean this can satisfactorily explain the occurrence of high extrusion temperature komatiites and lower extrusion temperature basalts in greenstone belts as being derived from the same source by different mechanisms

parameterized convection and the thermal evolution of the earth abstract thermal convection is an efficient means of extracting heat from the interior of the earth heat lost from the earth s interior comes from the decay of radioactive isotopes and the cooling of the earth to determine the magnitude of each contribution detailed studies of the convective mechanism must be carried out one approach to this problem is to parameterize a nondimensional mantle temperature in terms of the mantle rayleigh number the time variation of the radioactive heat production and the transient cooling of the entire earth are included also included is the differentiation of radioactive isotopes into the continents important conclusions are 1 1 that between 25 and 30 of the surface heat flow can be attributed to the cooling of the earth this gives a mean concentration of uranium in the mantle close to 30 ppb 2 2 that two scales of mantle convection are occurring the first associated with convection of the surface plates and the second a turbulent convection beneath the surface plates 3 3 that the mean temperature of the mantle in archean times was 100 150 k hotter than at present but that the mean thickness of the lithosphere was about one half the present value 4 4 that 60 to 80 of the area of the continents was formed prior to 3 b y b p

effects of water dependent creep rate on the volatile exchange between mantle and surface reservoirs abstract a parameterized model of mantle convection including the volatile exchange between mantle and surface reservoirs is used to study the thermal history of the earth the influence of dissolved volatiles on mantle rheology is reformulated the weakening of mantle materials is described by a functional relationship between creep rate and water fugacity we use flow law parameters of diffusion creep in olivine under dry and wet conditions the mantle degassing rate is considered as directly proportional to the seafloor spreading rate which in turn is dependent on the mantle heat flow to calculate the spreading rate we assume that the heat flow under the mid ocean ridges is double the average mantle heat flow the rate of regassing also depends on the seafloor spreading rate as well as on other factors such as the efficiency of volatile recycling through island arc volcanism both mechanisms degassing and regassing are coupled self consistently with the help of a parameterized convection model under implementation of a different formulation of mantle viscosity the model is run for 4 6 gyr time series of average mantle temperature volatile loss mantle viscosity mantle heat flow rayleigh number and urey ratio are calculated the effects of changing initial parameters have been tested and found negligible for the final results mantle water is outgassed rapidly within a time scale of less than 200 myr for all numerical simulations the present day values of calculated parameters are in the generally accepted range

present day thermal field and mesozoic cenozoic thermal evolution of the western bredasdorp basin south africa an integrated 3d numerical forward modelling approach abstract we analyse a crust scale 3d structural model of the western bredasdorp basin wbb located offshore southern south africa to assess both the spatial and temporal development of geothermal gradients in the southern african passive transform continental margin the results of simulations of the present day conductive thermal field were used to constrain the heat flow evolution during 3d numerical modelling of the paleo thermal regime besides conforming to the regional geotectonic framework the ensuing models are largely consistent with measured temperatures the largest control on present day temperatures is exerted by the basin s present day geometry and distribution of thermal properties which in turn trace back to the earliest tectonic event involving crustal thinning and significant syn rift sedimentation implying from the modelled shallow and deep geotherms we find that the variations of present day surface heat fluxes in the wbb are much more dictated by the long term crustal radiogenic heat production in concert with the interaction of low thermally conductive sediments and high thermally conductive crustal rocks rather than by the present day mantle heat flow in addition model calibration with measured vitrinite reflectance trends suggests that three major phases of thermal disequilibrium resulting in high heat flow during syn rift and post rift times characterise the distribution of maximum paleo temperatures and paleo geotherms in terms of subsidence the syn rift tectonic event plays the most significant role in basin evolution the height and width of the syn rift heat flow anomalies and their lateral variations reflect a direct consequence of instantaneous crustal stretching leading to rapid burial and lateral differences in crustal thicknesses and amounts of radiogenic heat produced the post rift heat flow peaks are interpreted to be related to a late cretaceous mantle related hotspot event and a late miocene episode of margin uplift as indicated by validated vitrinite trends and the modelled distribution of eroded sediments the hotspot event represents a more localised thermal event while the late cenozoic erosion and inferred uplift depict a regional character the modelled evolution of deep heat flow further suggests that ancient thermal perturbations have little influence on the present day thermal field

determination of groundwater recharge regime and flowpath in the lower heihe river basin in an arid area of northwest china by using environmental tracers implications for vegetation degradation in the ejina oasis abstract environmental tracers cfcs stable isotopes 18o 2h and 3h and major ions were employed to study river infiltration and groundwater recharge in the aquifer system in the basin of the lower heihe river northwest china three groups of waters have been recognized 1 young groundwater connected to the river with large variation of cfc apparent ages ranging from 40 a and being depleted in 18o and 2h compared with the river water and 3 groundwater in gurinai a grassland located about 100 km from the river in which the predominant discharge is from the badain jaran desert with cfc apparent ages ranging from 25 to 50 a and being enriched in 18o and 2h compared to the river water the groundwater along the river contains cfcs and 3h down to depths of about 120 m and the shallow groundwater exhibits cfc apparent ages in a wide range which are not dependent on the well depth groundwaters along the river show a similar trend of enrichment in 18o and 2h as the river water whereas groundwaters in depression cones are depleted in heavier isotopes and have low cfc and 3h concentrations the cfc apparent age of the groundwater increases with increasing distance downstream indicating that the dominant part of the groundwater is from infiltration of river water in the upper reaches modifications of groundwater recharge are reflected in variations of stable isotope compositions as well as cfc and 3h concentrations in the groundwater that was recharged from the river over the last decades despite recharging from river water groundwater abstraction has induced a water balance deficit the riparian ecosystem in the ejina oasis is constrained by both decreased river flow and increased groundwater abstraction the vegetation degradation in the ejina oasis is controlled not only by natural aridification but also worsened by heavy groundwater abstraction and decreased river flow

glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity summary a gravitationally self consistent theory for relative sea level variations forced by pleistocene deglaciation events is employed to explore the extent to which rsl and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth a trade off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle taking full account of such uncertainty plausible values of the viscosity beneath the transition region are bounded above by 1023 poise cgs units the preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over hudson bay the calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of wurm wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them

seismic imaging and potential field modelling to delineate structures hosting vhms deposits in the skellefte ore district northern sweden the skellefte district in northern sweden is a roughly 150 by 50 km2 large early proterozoic massive sulphide belt based on high resolution reflection seismic data along two parallel seismic profi

modelling the global carbon cycle for the past and future evolution of the earth system the earth may be described as a global system consisting of the components solid earth hydrosphere atmosphere and biosphere this system evolves under the external influence of increasing solar luminosity in spite of this changing external forcing the earth s climate has been stabilized by negative feedbacks against global freezing in the past faint young sun paradox the future long term trend of further increasing solar luminosity will cause a further atmospheric co2 decrease atmospheric co2 will fall below the critical level for photosynthesis and the plant based biosphere will die out in the present paper we propose a modelling study of the evolution of the carbon cycle from the archaean to the planetary future our model is based on a paper published previously by caldeira and kasting caldeira k kasting j f 1992 the life span of the biosphere revisited nature 360 721 723 the difference of the current study with respect to this work resides in the forcing function used for the silicate weathering rate while caldeira and kasting used a constant weathering rate over time we calculate the time evolution of this rate by assuming a balance between the weathering flux and the co2 release flux by volcanism and metamorphism we use the geodynamics theory to couple the two internal forcing functions continental area for weathering and spreading for co2 release flux which were generally considered as independent in previous models this coupling introduces an additional feedback in the system we find a warmer climate in the past and a shortening of the life span of the biosphere up to some hundred million years

delayed cooling of the earth s mantle due to variable thermal conductivity and the formation of a low conductivity zone abstract the thermal conductivity of mantle materials decreases with higher temperatures and increases with greater pressure due to phonon mechanism but increases with temperature for radiative transfer this trade off attribute allows for the formation of a low conductivity zone lcz within the top thermal boundary layer in mantle convection we have studied with a two dimensional 2 d cartesian model the potential of this low conductivity zone in retarding secular cooling of the mantle for a longer period of time than models with constant thermal conductivity using a recently proposed model for thermal conductivity and an adiabatic boundary condition for the bottom of the mantle we have carried out a set of numerical experiments within the framework of the extended boussinesq approximation for constant viscosity depth dependent thermal expansivity and variable thermal conductivity with surface rayleigh numbers between around 106 and around 107 we have employed internal heating between two and four times the chondritic level and half life time values of 2 5 ga and 5 ga the cooling rate of the mantle can be decreased by the feedback interaction between mantle conductivity and internal heating which gives rise to slower sinking cold currents the retardation time increases with the strength of radiogenic heating and can be as long as a couple of billion years for high initial heating rates four times the chondritic value however increasing the radiative contribution of the conductivity speeds up the cooling process the less the radiative contribution is the more mantle cooling would be retarded in the course of adiabatic cooling we find the paradoxical situation in which the effective rayleigh number of the mantle can actually increase with time we suggest then that because of the possibilities for the formation of low conductivity zones adjacent to the thermal boundary layers which bring forth unexpected consequences mantle thermal conductivity should be considered as a fundamentally important factor in constraining the thermal evolution of both the core and mantle

internally heated mantle convection and the thermal and degassing history of the earth a parameterized internally heated convection model for the earth has been developed the mantle viscosity is temperature and volatile content dependent a heat flow rayleigh number relationship appropriate for an internally heated mantle is assumed for each model an initial homologous temperature and bulk water content are assumed and the initial radiogenic heat production is constrained to give a present day heat flow of 0 07 w m 2 the model is run for 4 6 gyr and temperature heat flow degassing and regassing rates stress and viscosity are calculated a nominal case is established which shows good agreement with accepted mantle values the effects of changing various parameters have also been tested all cases show rapid cooling early in the planet s history and strong self regulation of viscosity due to the temperature and volatile content dependence the values chosen for initial temperature and volatile content have negligible effects on the results while assumptions about present day heat flow viscosity and mantle volatile content can have important consequences the effects of weakly stress dependent viscosity are examined within the bounds of this model and are found to be small mantle water is typically outgassed rapidly to reach an equilibrium concentration on a time scale of less than 200 m y for almost all models the main exception being for models which start out with temperatures well below the melting temperature for some cases the degassing efficiency is allowed to decrease with time as the mantle cools which combined with the rapid early loss of volatiles leads to a net regassing of the mantle to the present day and loss of water from the surface

focussed time dependent martian volcanism from chemical differentiation coupled with variable thermal conductivity recently the orbiter camera on the mars global surveyor imaged on some lava flows the sparse cratering and thence young age of these flows indicates that volcanic activity on mars is long lived and perhaps pulsating these pulses beg for an explanation not yet provided by one dimensional convection models simulating the thermal and compositional evolution of the martian interior we have for the first time simulated the thermochemical evolution of mars with a two dimensional model for the entire planetary history our results show that variable thermal conductivity k t p can maintain sublithospheric hot spots over time scales of several hundred ma and is essential in the focussing of hot mantle upwellings at almost the same location this scenario can explain the persistent pulsating and focussed volcanism in the tharsis region

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

structural evolution of the warrawoona greenstone belt and adjoining granitoid complexes pilbara craton australia implications for archaean tectonic processes abstract the pilbara granite greenstone terrain 3 6 2 8 ga australia is quoted as the key area formed by passive gravity driven solid state diapirism but contrasting views have been reported recently which led us to re examine the key area the warrawoona greenstone belt and the adjoining mt edgar and corunna downs granitoid complexes we found i an asymmetric non synclinal distribution of metamorphic grade and deformation intensity across the warrawoona greenstone belt ii a major shear zone on the margin of the mt edgar granitoid complex with a unidirectional stretching lineation pattern iii a ne trending transcurrent shear zone within the central gneiss complex of the mt edgar granitoid complex iv coeval magmatic activity and doming of the granitoid complexes and v the loss of about 9 km of stratigraphic units in the warrawoona greenstone belt the area has had a prolonged history with at least four deformation phases between ca 3 47 and 2 8 ga the main domal geometry of the mt edgar granitoid complex was obtained during active ne sw extension at 3 31 ga d x 2 which appears to been in response to e w compression possibly the e w compression resulted in crustal thickening and partial melting probably of a 3 46 3 43 ga granitic basement causing crustal weakening collapse and extrusion of the 3 31 ga granitoids extension at 3 31 ga was accommodated by the mt edgar shear zone a mid crustal detachment and by the associated beaton well zone which acted as a transfer zone within the gneissic basement doming of the mt edgar granitoid complex is interpreted to have occurred in an active extensional core complex type mode an indication for deformation prior to 3 32 ga comes from syntectonically intruded gabbro diorite and dolerite sills within the mt edgar shear zone which intruded possibly as early as 3 46 3 45 ga d x 1 ultramafic schistose xenoliths within the gabbro diorite intrusions are the oldest observed deformed rocks formed during d x post doming deformation involved ne sw compression d x 3 followed by dextral transcurrent deformation d x 4 during nw se compression the latter may be related to the regional phase of deformation at 2 93 ga during which the eastern and western pilbara were amalgamated comparison of this deformation sequence to others within the marble bar domain suggests a common tectonic history from 3 46 ga onwards our structural and kinematic data especially the 3 31 ga unidirectional stretching lineation pattern lead us to reject solid state diapirism due to convectional overturn as the main cause for doming of the mt edgar granitoid complex we suggest that the cause lies in a plate tectonic process

biological fe oxidation controlled deposition of banded iron formation in the ca 3770 ma isua supracrustal belt west greenland the redox balance of the archean atmosphere ocean system is among the most significant uncertainties in our understanding of the earliest history of earth s surface zone most workers agree that oxygen did not constitute a significant proportion of the atmosphere until after ca 2 45 ga after the great oxidation event but there is less agreement on when o2 production began and how this may have been consumed by reduced species such as fe ii in the oceans the fe redox cycle through time has been traced using banded iron formations bifs and fe isotopes are increasingly used to constrain the conditions of earth s paleoenvironments including the pathways of formation of bifs iron isotope analyses of bifs from the 3 7 to 3 8 ga isua supracrustal belt isb obtained by micro sampling of magnetite rich layers and conventional analysis as well as by in situ femtosecond laser ablation fs la icp ms indicate a consistently narrow range of non zero 56fe values analysis of magnetite by fs la icp ms allows for precise and accurate micron scale analyses without the problems of orientation effects that are associated with secondary ion mass spectrometry sims analyses magnetite 56fe values range from 0 4 to 1 1 among different bands but within individual layers magnetite grains are mostly homogeneous although these bifs have been metamorphosed to amphibolite facies the metamorphism can neither explain the range in fe isotope compositions across bands nor that between hand samples the isotopic compositions therefore reflect primary low temperature sedimentary values the positive 56fe values measured from the isb magnetites are best explained by deposition of fe iii oxides produced by partial oxidation of fe ii rich ocean water a dispersion reaction model which accounts for rates of hydrothermal fe ii aq input rates of oxidation and rates of fe oh 3 settling suggests exceptionally low o2 contents 0 001 of modern o2 contents in the photic zone such low levels suggest an anoxygenic pathway is more likely and the data can be well modeled by anoxygenic photosynthetic fe ii oxidation comparison of the fe isotope data from the isua bifs with those from the 2 5 ga bifs from the hamersley and transvaal basins australia and south africa respectively suggests a striking difference in fe sources and pathways the 2 5 ga magnetite facies bifs of australia and south africa have 56fe values that range from 1 2 to 1 2 over small scales and are on average close to 0 which is significantly lower than those reported here from the isua bifs the wide range in fe isotope compositions for the hamersley and transvaal bifs in concert with c and o isotope data have been interpreted to reflect bacterial dissimilatory fe iii reduction dir the absence of low 56fe values in the isua bifs as well as the lack of fine scale isotopic heterogeneity may indicate formation prior to widespread dir

parameterized thermal convection in a layered region and the thermal history of the earth parameterized convective schemes for constant viscosity fluids are used to investigate the influence of layering on the relationship between the rate of heat loss from and heat supply to a number of convecting layers of different viscosity and thicknesses simple analytic expressions are obtained which accurately predict the behavior to a step decrease in the heat supply and show that layering considerably increases the response time the same arguments are applied to a planet consisting of spherical shells they show that the response of the earth s heat loss to the decay of radioactive elements depends strongly on whether or not mantle convection extends from the core to the surface if it does so the surface heat flux is only about 50 greater than the heat generation rate however if upper and lower mantle convection occurs in two separate shells as seismological and geochemical arguments suggest the response time is increased the present surface heat flux is then principally controlled by the thermal structure of the earth at the time it was formed the best estimate of the present ratio of the heat loss to heat generation is about two and this value is consistent with the behavior of an earth model consisting of a two layered mantle overlying a core