geodynamo solar wind and magnetopause 3 4 to 3 45 billion years ago stellar wind standoff by a planetary magnetic field prevents atmospheric erosion and water loss although the early earth retained its water and atmosphere and thus evolved as a habitable planet little is known about earth s magnetic field strength during that time we report paleointensity results from single silicate crystals bearing magnetic inclusions that record a geodynamo 3 4 to 3 45 billion years ago the measured field strength is 50 to 70 that of the present day field when combined with a greater paleoarchean solar wind pressure the paleofield strength data suggest steady state magnetopause standoff distances of 5 earth radii similar to values observed during recent coronal mass ejection events the data also suggest lower latitude aurora and increases in polar cap area as well as heating expansion and volatile loss from the exosphere that would have affected long term atmospheric composition

palaeomagnetism of archaean rocks of the onverwacht group barberton greenstone belt southern africa evidence for a stable and potentially reversing geomagnetic field at ca 3 5 ga abstract palaeomagnetic data from the palaeoarchaean era 3 2 3 6 ga have the potential to provide us with a great deal of information about early conditions within and processes affecting the earth s core mantle and surface environment here we present new data obtained from some of the oldest palaeomagnetic recorders in the world igneous and sedimentary rocks from the onverwacht group of the barberton greenstone belt kaapvaal craton southern africa our palaeomagnetic measurements strengthen a recently published positive conglomerate test y usui j a tarduno m watkeys a hofmann and r d cottrell 2009 and our new u pb date constrains the conglomerate to older than 3455 8 ma the new palaeomagnetic data from other units are nontrivial to interpret and are of uncertain reliability when taken individually similar we argue to all other published palaeomagnetic data of this age nonetheless four poles two new two derived from published data produced from high temperature components of magnetisation recorded in the komati noisy and hooggenoeg formations exhibit considerably improved clustering when their directions are corrected for differences in attitude resulting from a large fold structure dated at 3 23 ga on the basis of this enhanced consistency in stratigraphic coordinates the positive conglomerate test and the absence of any clear indications of their remagnetisation from comparison with younger poles we argue that these are the most trustworthy palaeomagnetic results yet produced from any rocks of palaeoarchaean age when taken in conjunction with published data the new results present the most compelling evidence to date that the earth had a stable geomagnetic field at ca 3 5 ga in addition to presenting tentative evidence that it was undergoing polarity reversals the data do not appear to support a claim made previously from palaeoarchean palaeomagnetic data from the pilbara craton y suganuma y hamano s niitsuma m hoashi t hisamitsu n niitsuma k kodama and m nedachi 2006 of extremely rapid latitudinal plate motion during this period finally when compared with similarly aged data from the pilbara craton western australia the new data do not rule out the hypothesis that the two cratons were conjoined at this point in their history in the supercraton vaalbara

a hadean to paleoarchean geodynamo recorded by single zircon crystals knowing when the geodynamo started is important for understanding the evolution of the core the atmosphere and life on earth we report full vector paleointensity measurements of archean to hadean zircons bearing magnetic inclusions from the jack hills conglomerate western australia to reconstruct the early geodynamo history data from zircons between 3 3 billion and 4 2 billion years old record magnetic fields varying between 1 0 and 0 12 times recent equatorial field strengths a hadean geomagnetic field requires a core mantle heat flow exceeding the adiabatic value and is suggestive of plate tectonics and or advective magmatic heat transport the existence of a terrestrial magnetic field before the late heavy bombardment is supported by terrestrial nitrogen isotopic evidence and implies that early atmospheric evolution on both earth and mars was regulated by dynamo behavior

zircon radiation damage ages abstract the suggestion that the accumulation of radiation damage in zircons could be used as a dating method was first proposed in the 1950s in the original technique radiation damage was determined by x ray diffraction however this is not suitable for the small sample analysis needed in modern geochronology it is now possible to measure radiation damage and u and th contents on micron sized areas on the polished surface of zircon grains using raman spectroscopy and sims mass spectrometry this opens the way for a reassessment of the potential of radiation damage ages and the purpose of this contribution is to re examine the technique through its application to zircons from two granitoids from the archean yilgarn craton and ancient detrital zircons from the jack hills in western australia the three examples show internally consistent radiation damage ages that are also in accord with independent geochronological evidence the 420 110 ma radiation damage age for the darling range granite is coincident with 500 400 ma biotite rb sr ages in the same region of the yilgarn craton likewise a tonalite from the youanmi terrane in the craton with a 1650 150 ma zircon radiation damage age lies within the domain of a 1600 ma event recorded by biotite rb sr ages the jack hills zircons have a 1120 130 ma radiation damage age that is explained by a complex damage accumulation and annealing history culminating in a mild heating event indicated by biotite ar ar ages of about 1140 ma the positive results for the three case histories suggest that radiation damage ages could play a useful role in dating low temperature thermal events

thermal evolution of earth with magnesium precipitation in the core vigorous convection in earth s core powers our global magnetic field which has survived for over three billion years in this study we calculate the rate of entropy production available to drive the dynamo throughout geologic time using one dimensional parameterizations of the evolution of earth s core and mantle to prevent a thermal catastrophe in models with realistic urey ratios we avoid the conventional scaling for plate tectonics in favor of one featuring reduced convective vigor for hotter mantle we present multiple simulations that capture the effects of uncertainties in key parameters like the rheology of the lower mantle and the overall thermal budget simple scaling laws imply that the heat flow across the core mantle boundary was elevated by less than a factor of two in the past relative to the present another process like the precipitation of magnesium bearing minerals is therefore required to sustain convection prior to the nucleation of the inner core roughly one billion years ago especially given the recent upward revision to the thermal conductivity of the core simulations that include precipitation lack a dramatic increase in entropy production associated with the formation of the inner core complicating attempts to determine its age using paleomagnetic measurements of field intensity because mantle dynamics impose strict limits on the amount of heat extracted from the core we find that the addition of radioactive isotopes like potassium 40 implies less entropy production today and in the past on terrestrial planets like venus with more sluggish mantle convection even precipitation of elements like magnesium may not sustain a dynamo if cooling rates are too slow

inclusion localised crystal plasticity dynamic porosity and fast diffusion pathway generation in zircon abstract a population of oscillatory zoned igneous zircon grains in a javanese andesite contains fluid and mineral inclusions up to 10 m across trapped during zircon growth orientation contrast imaging and orientation mapping by electron backscatter diffraction reveal that crystal plastic deformation overprints growth zoning and has localized around 1 10 m pores and inclusions cumulative crystallographic misorientation of up to 25 around pores and inclusions in zircon is predominantly accommodated by low angle 010 slip a distinctly different population of sub micron pores is present along subgrain boundaries and their triple junctions these are interpreted to have formed as a geometric consequence of dislocation interaction during crystal plasticity dislocation creep microstructures are spatially related to differences in cathodoluminescence spectra that indicate variations in the abundance of cl active rare earth elements the extent of the modification suggests deformation related fast pathway diffusion distances that are over five orders of magnitude greater than expected for volume diffusion this enhanced diffusion is interpreted to represent a combination of fast diffusion pathways associated with creep cavitation dislocations and along low angle boundaries these new data indicate that ductile deformation localised around inclusions can provide fast pathways for geochemical exchange these pathways may provide links to the zircon grain boundary thus negating the widely held assumption that inclusions in fracture free zircon are geochemically armoured once they are physically enclosed

mechanisms and consequences of intra crystalline enrichment of ancient radiogenic pb in detrital hadean zircons from the jack hills western australia abstract the recent discovery of pb radiogenic lead enriched domains peds in zircon using high resolution scanning ion imaging atom probe tomography and transmission electron microscopy indicates that the u pb isotopic system in zircon can be affected not only by pb loss but also by intra crystalline pb enrichment however the formation mechanism of peds and the consequences for in situ u pb dating remain elusive to further understand these issues scanning ion imaging and u pb dating were carried out on 51 hadean detrital zircon grains from the jack hills western australia of these 8 grains were found to contain micrometer scale peds with elevated 207pb and 206pb in the ion images isotope profiles across these peds confirm that they represent unsupported pb resulting from intra grain mobilization and local enrichment of pb during ancient overprinting events the peds are rare in most grains with only 1 3 pb hotspots present in any 70 m 70 m imaged area and are located either in dark cl high u growth zones or associated with bright cl low th recrystallized domains suggesting that they were likely formed by local scale processes the peds located in dark cl high u growth zones likely resulted from pb trapped in local interconnected pathways produced by radiation damage during thermal overprinting events those associated with bright cl low th domains are interpreted as pb concentrated in nanoscale pores formed during fluid present recrystallization of non metamict zircon our interpretations therefore imply that intra crystalline enrichment of pb in zircon can occur at various scales and through diverse mechanisms age calculations based on the ion images demonstrate that incorporation of peds in conventional in situ sims spots produces spuriously old ages that are either reversely discordant or concordant depending on the time of pb mobilization and the analytical precision the implication is that careful investigation of intra grain distribution of u th pb isotopes is required in order to better understand the u pb isotopic system and to obtain reliable crystallization ages for ancient zircons affected by complex pb mobilization and redistribution

an early geodynamo driven by exsolution of mantle components from earth s core recent palaeomagnetic observations report the existence of a magnetic field on earth that is at least 3 45 billion years old compositional buoyancy caused by inner core growth is the primary driver of earth s present day geodynamo but the inner core is too young to explain the existence of a magnetic field before about one billion years ago theoretical models propose that the exsolution of magnesium oxide the major constituent of earth s mantle from the core provided a major source of the energy required to drive an early dynamo but experimental evidence for the incorporation of mantle components into the core has been lacking indeed terrestrial core formation occurred in the early molten earth by gravitational segregation of immiscible metal and silicate melts transporting iron loving siderophile elements from the silicate mantle to the metallic core and leaving rock loving lithophile mantle components behind here we present experiments showing that magnesium oxide dissolves in core forming iron melt at very high temperatures using core formation models we show that extreme events during earth s accretion such as the moon forming giant impact could have contributed large amounts of magnesium to the early core as the core subsequently cooled exsolution of buoyant magnesium oxide would have taken place at the core mantle boundary generating a substantial amount of gravitational energy as a result of compositional buoyancy this amount of energy is comparable to if not more than that produced by inner core growth resolving the conundrum posed by the existence of an ancient magnetic field prior to the formation of the inner core

constraints from material properties on the dynamics and evolution of earth s core the material properties of the earth s core have been better constrained by recent technical and computational advances the properties imply that the core was once hot but is cooling quickly and the inner core is young

numerical tsunami simulation including elastic loading and seawater density stratification systemic discrepancies between observed and modeled tsunami wave speeds were previously identified for two recent major tsunamis the 2010 maule and 2011 tohoku events to account for these discrepancies we developed a numerical tsunami propagation code solving the shallow water equation and including the effects of elastic loading of the seafloor by the tsunami as well as a linear density profile in the seawater column we show here that both effects are important to explain the commonly observed difference between observations and simulations we conclude that the density variation in the seawater column affects the wave speed without changing the waveform whereas the loading effect has an effect on the wave speed and the waveform showing a negative phase before the main arrival due to the depression of the seafloor surrounding the tsunami wave the combination of both effects is needed to achieve a better match between observations and simulations

experimental hydrothermal alteration of partially metamict zircon we present the results of a series of hydrothermal experiments on grains from two partially metamict zircon samples from sri lanka in the temperature range 350 to 650 c and with different solutions 2 m alcl 3 2 m cacl 2 pure h 2 o and a multi cation solution under these conditions sharply bounded reaction fronts penetrated into the zircon grains and developed complex lobate and rim structures that resemble structures found in natural zircon systems the reaction zones are characterized by a marked increase in the cathodoluminescence intensity a decrease of the back scattered electron emission and an increased degree of structural order as revealed by micro raman and infrared spectroscopy sensitive high resolution ion microprobe and electron microprobe measurements revealed that the altered areas gained solvent cations e g ca 2 ba 2 mg 2 al 3 from the solution and lost variable amounts of zr si hf the ree u th as well as radiogenic pb a comparison between dry and hydrothermal annealing trends shows that the kinetics of structural recovery including recrystallization of the amorphous phase in metamict zircon is strongly enhanced under hydrothermal conditions this finding suggests that water catalyzes structural recovery processes in metamict zircon we found that the structure of the reacted areas does not resemble that of crystalline zircon i e is still characterized by a temperature dependent degree of disorder which would not be expected if the reaction is controlled by a coupled dissolution and re precipitation process instead the alteration process can be described best by a diffusion reaction recrystallization model in this model it is postulated that the diffusion of water into the metamict structure is the driving force for moving recrystallization fronts we found that the rate and the extent of solid state recrystallization of the amorphous phase is an important factor in determining the mobility of trace elements this interpretation is indicated by the observation that trace elements including u and th were preferentially lost during the reaction with a fluid at low temperatures where recrystallization of the amorphous material was slow or not activated at all the observed chemical alteration patterns are believed to reflect a competition between the kinetics of long range diffusion and ion exchange and the kinetics of the short range diffusion necessary for the recrystallization process

pervasive remagnetization of detrital zircon host rocks in the jack hills western australia and implications for records of the early geodynamo it currently is unknown when earth s dynamo magnetic field originated paleomagnetic studies indicate that a field with an intensity similar to that of the present day existed 3 5 billion years ago ga detrital zircon crystals found in the jack hills of western australia are some of the very few samples known to substantially predate this time with crystallization ages ranging from 3 0 4 38 ga these zircons might preserve a record of the missing first billion years of earth s magnetic field history however a key unknown is the age and origin of magnetization in the jack hills zircons the identification of 3 9 ga i e hadean field records requires first establishing that the zircons have avoided remagnetization since being deposited in quartz rich conglomerates at 2 65 3 05 ga to address this issue we have conducted paleomagnetic conglomerate baked contact and fold tests in combination with u pb geochronology to establish the timing of the metamorphic and alteration events and the peak temperatures experienced by the zircon host rocks these tests include the first conglomerate test directly on the hadean zircon bearing conglomerate at erawandoo hill although we observed little evidence for remagnetization by recent lightning strikes we found that the hadean zircon bearing rocks and surrounding region have been pervasively remagnetized with the final major overprinting likely due to thermal and or aqueous effects from the emplacement of the warakurna large igneous province at 1070 million years ago ma although localized regions of the jack hills might have escaped complete remagnetization there currently is no robust evidence for pre depositional 3 0 ga magnetization in the jack hills detrital zircons

secondary magnetite in ancient zircon precludes analysis of a hadean geodynamo zircon crystals from the jack hills western australia are one of the few surviving mineralogical records of earth s first 500 million years and have been proposed to contain a paleomagnetic record of the hadean geodynamo a prerequisite for the preservation of hadean magnetization is the presence of primary magnetic inclusions within pristine igneous zircon to date no images of the magnetic recorders within ancient zircon have been presented here we use high resolution transmission electron microscopy to demonstrate that all observed inclusions are secondary features formed via two distinct mechanisms magnetite is produced via a pipe diffusion mechanism whereby iron diffuses into radiation damaged zircon along the cores of dislocations and is precipitated inside nanopores and also during low temperature recrystallization of radiation damaged zircon in the presence of an aqueous fluid although these magnetites can be recognized as secondary using transmission electron microscopy they otherwise occur in regions that are indistinguishable from pristine igneous zircon and carry remanent magnetization that postdates the crystallization age by at least several hundred million years without microscopic evidence ruling out secondary magnetite the paleomagnetic case for a hadean eoarchean geodynamo cannot yet been made

secondary magnetic inclusions in detrital zircons from the jack hills western australia and implications for the origin of the geodynamo the time of origin of earth s dynamo is unknown detrital zircon crystals containing ferromagnetic inclusions from the jack hills of western australia have the potential to contain the oldest records of the geodynamo it has recently been argued that magnetization in these zircons indicates that an active dynamo existed as far back as 4 2 ga however the ages of ferromagnetic inclusions in the zircons are unknown here we present the first detailed characterization of the mineralogy and spatial distribution of ferromagnetic minerals in jack hills detrital zircons we demonstrate that ferromagnetic minerals in most jack hills zircons are commonly located in cracks and on the zircons exteriors hematite is observed to dominate the magnetization of many zircons while other zircons also contain significant quantities of magnetite and goethite this indicates that the magnetization of most zircons is likely to be dominantly carried by secondary minerals that could be hundreds of millions to billions of years younger than the zircons crystallization ages we conclude that the existence of the geodynamo prior to 3 5 ga has yet to be established

the high conductivity of iron and thermal evolution of the earth s core abstract we measured the electrical resistivity of iron and iron silicon alloy to 100 gpa the resistivity of iron was also calculated to core pressures combined with the first geophysical model accounting for saturation resistivity of core metal the present results show that the thermal conductivity of the outermost core is greater than 90 w m k these values are significantly higher than conventional estimates implying rapid secular core cooling an inner core younger than 1 ga and ubiquitous melting of the lowermost mantle during the early earth an enhanced conductivity with depth suppresses convection in the deep core such that its center may have been stably stratified prior to the onset of inner core crystallization a present heat flow in excess of 10 tw is likely required to explain the observed dynamo characteristics

signals from the ancient geodynamo a paleomagnetic field test on the jack hills metaconglomerate abstract the oldest history of earth s magnetic field cannot be directly read from extant bulk rocks because of subsequent metamorphism at temperatures close to or exceeding the curie temperature of common magnetic minerals the jack hills metasediments of western australia which have seen lower peak metamorphic temperatures contain zircons as old as 4 4 billion yr old to assess whether these sediments can retain an ancient signal of the geodynamo we present a paleomagnetic conglomerate test on a cobble bearing jack hills unit thermal demagnetization reveals a distinct magnetic component with high unblocking temperatures between 550 and 580 c that passes the conglomerate test indicating magnetization prior to deposition of the conglomerate this result together with rock magnetic data indicates that the high unblocking temperature component is carried by magnetite which records magnetization in an ambient field and the simplest explanation is that a dynamo was present existing geochronological data imply that the clasts could contain mixtures of minerals extending to ages only slightly older than the maximum depositional age at 3 05 billion yr ago however the positive conglomerate test reported here indicates that the jack hills metasediments have the potential to record paleorchean to hadean magnetic fields on a clast or sub clast mineral scale

multi scale three dimensional characterization of iron particles in dusty olivine implications for paleomagnetism of chondritic meteorites dusty olivine olivine containing multiple sub micrometer inclusions of metallic iron in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence capable of maintaining a faithful record of pre accretionary magnetization acquired during chondrule formation here we show how the magnetic architecture of a single dusty olivine grain from the semarkona ll3 0 ordinary chondrite meteorite can be fully characterized in three dimensions using a combination of focused ion beam nanotomography fib nt electron tomography and finite element micromagnetic modeling we present a three dimensional 3d volume reconstruction of a dusty olivine grain obtained by selective milling through a region of interest in a series of sequential 20 nm slices which are then imaged using scanning electron microscopy the data provide a quantitative description of the iron particle ensemble including the distribution of particle sizes shapes interparticle spacings and orientations iron particles are predominantly oblate ellipsoids with average radii 242 94 199 80 123 58 nm using analytical tem we observe that the particles nucleate on sub grain boundaries and are loosely arranged in a series of sheets parallel to 001 of the olivine host this is in agreement with the orientation data collected using the fib nt and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host the shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets individual particle geometries are converted to a finite element mesh and used to perform micromagnetic simulations the majority of particles adopt a single vortex state with bulk spins that rotate around a central vortex core we observed no particles that are in a true single domain state the results of the micromagnetic simulations challenge some preconceived ideas about the remanence carrying properties of vortex states there is often not a simple predictive relationship between the major intermediate and minor axes of the particles and the remanence vector imparted in different fields although the orientation of the vortex core is determined largely by the ellipsoidal geometry i e parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids the core and remanence vectors can sometimes lie at very large tens of degrees angles to the principal axes the subtle details of the morphology can control the overall remanence state leading in some cases to a dominant contribution from the bulk spins to the net remanence with profound implications for predicting the anisotropy of the sample the particles have very high switching fields several hundred millitesla demonstrating their high stability and suitability for paleointensity studies

hadean age for a post magma ocean zircon confirmed by atom probe tomography the only physical evidence from the earliest phases of earth s evolution comes from zircons ancient mineral grains that can be dated using the u th pb geochronometer 1 oxygen isotope ratios from such zircons have been used to infer when the hydrosphere and conditions habitable to life were established 2 3 chemical homogenization of earth s crust and the existence of a magma ocean have not been dated directly but must have occurred earlier 4 however the accuracy of the u pbzirconagescanplausiblybebiasedbypoorlyunderstood processes of intracrystalline pb mobility 5 7 here we use atomprobe tomography 8 to identify and map individual atoms in the oldest concordant grain from earth a 4 4 gyr old hadean zircon with a high temperature overgrowth that formed about 1gyrafterthemineral score isolatednanoclusters measuring about 10nm and spaced 10 50nm apart are enriched in incompatible elements including radiogenic pb with unusually high 207 pb 206 pbratios wedemonstratethatthelengthscales of these clusters make u pb age biasing impossible and that they formed during the later reheating event our tomography data thereby confirm that any mixing event of the silicate earth must have occurred before 4 4gyr ago consistent with

paleomagnetism indicates that primary magnetite in zircon records a strong hadean geodynamo determining the age of the geomagnetic field is of paramount importance for understanding the evolution of the planet because the field shields the atmosphere from erosion by the solar wind the absence or presence of the geomagnetic field also provides a unique gauge of early core conditions evidence for a geomagnetic field 4 2 billion year gy old just a few hundred million years after the lunar forming giant impact has come from paleomagnetic analyses of zircons of the jack hills western australia herein we provide new paleomagnetic and electron microscope analyses that attest to the presence of a primary magnetic remanence carried by magnetite in these zircons and new geochemical data indicating that select hadean zircons have escaped magnetic resetting since their formation new paleointensity and pb pb radiometric age data from additional zircons meeting robust selection criteria provide further evidence for the fidelity of the magnetic record and suggest a period of high geomagnetic field strength at 4 1 to 4 0 billion years ago ga that may represent efficient convection related to chemical precipitation in earth s hadean liquid iron core

venus a thick basal magma ocean may exist today basal magma oceans develop in earth and venus after accretion as their mantles solidify from the middle outwards fractional crystallization of the basal mantle is buffered by the core and radiogen