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

geomagnetic paleointensity derived from single plagioclase crystals we suggest a new approach for the determination of absolute geomagnetic paleointensity that uses single plagioclase crystals to avoid problems caused by alteration during thellier thellier analyses transmission electron microscope imaging and rock magnetic data indicate that plagioclase crystals from a recent basalt flow in hawaii 1955 kilauea eruption contain pseudo single to single domain titanomagnetite inclusions these feldspars yield paleointensity values that agree within error with magnetic observatory data and paleointensity values reported from whole rock samples these results suggest that single plagioclase crystals from older igneous rocks may provide a viable source of paleointensity data

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

prospects for an ancient dynamo and modern crustal remanent magnetism on venus abstract venus lacks an internally generated magnetic field today whether one existed in the past is unknown but critical to atmospheric evolution and potential habitability canonical models assume the core of venus is cooling too slowly for convection and thus a magnetic dynamo to occur today core mantle heat flow is suppressed in these models after a putative transition in mantle dynamics associated with widespread volcanic resurfacing however recent studies of impact craters and other surface features support more steady heat loss over geologic time precipitation of mgo and or sio2 from the core can also drive compositional convection even with slow cooling here we reevaluate the likelihood that venus has an earth like at least partially liquid and chemically homogeneous core using numerical simulations of the coupled atmosphere surface mantle core evolution an earth like core is only compatible with the modern lack of a dynamo if the thermal conductivity of core material is towards the higher end of modern estimates i e 100 w m 1 k 1 if lower estimates like 40 50 w m 1 k 1 are actually correct then we favor recent proposals that the core has completely solidified or preserved primordial stratification any simulation initialized with a homogeneous liquid core predicts a global magnetic field with earth like surface strength for 2 3 billion years after accretion consistent with all available observations and also sporadic activity within the surface age while temperatures remain below the curie point of magnetite therefore future spacecraft missions should prioritize the first ever magnetometer measurements below the ionosphere to search for crustal remanent magnetism

zircon shrimp dating confirms a palaeoarchaean supracrustal terrain in the southeastern kaapvaal craton southern africa abstract we report shrimp zircon ages for felsic volcanic rocks of the early archaean nondweni greenstone belt ngb located in northern kwazulu natal province south africa the ngb is part of a chain of greenstone belt remnants that occurs south of the well known barberton greenstone belt bgb two samples of felsic volcanic rocks of the basal toggekry formation yielded zircon ages of 3 53 ga whereas significantly younger ages of 3 41 ga were obtained for two samples of felsic rocks from near the top of the witkop formation the latter date now constrains the age of stromatolite like structures in cherts associated with the felsic rocks which probably represent some of the oldest preserved evidence of life on earth the zircon ages also indicate that the ngb correlates well chronologically with the bgb and is one of the oldest volcano sedimentary successions in the southeastern kaapvaal craton geochemical data for the felsic ngb rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust possibly in a back arc environment initial e nd values for ngb rocks range from 1 2 to 0 3 and suggest involvement of pre greenstone continental crust that was possibly extensive in the southeastern part of the kaapvaal craton during palaeoarchaean times

thermoremanence anhysteretic remanence and susceptibility of submicron magnetites nonlinear field dependence and variation with grain size we have measured initial susceptibility 0 and the dependence of anhysteretic remanent magnetization arm and thermoremanent magnetization trm on applied field for seven samples of magnetite with mean grain sizes from 40 to 540 nm trm acquisition is nonlinear in geomagnetically relevant weak fields contrary to the usual assumption made in paleointensity determination over the 40 540 nm range 0 varies with particle shape but only weakly with particle size d and can be used to correct for varying magnetite concentrations in sediment cores arm is strongly size dependent over the same range and is the best parameter for monitoring grain size variations in natural samples arm and trm have similar variations as d 1 for d 1 m suggesting a common source for this pseudo single domain psd dependence on grain size however trm is 10 20 times more intense than arm in grains around 0 2 m in size the trm microstate seems to be a two domain structure whereas the arm microstate may be a vortex structure which has never before been convincingly demonstrated by magnetic measurements independent evidence comes from theoretical fits to trm and arm field dependence data in moderate and strong fields trm in 215 540 nm magnetites is explained by two domain theory but arm is not in smaller grains d 0 1 m a psd theory predicts that trm is carried by single domain sd moments of entire grains but arm resides in moments 1 per grain 20 40 times weaker these remanence levels match those of metastable sd and vortex states respectively we therefore propose that magnetite grains in the 0 1 0 5 m size range can remain in metastable sd or two domain states following acquisition of trm but revert to a vortex ground state when field cycled for example in arm acquisition or alternating field demagnetization

the early archaean nondweni greenstone belt southern kaapvaal craton south africa part i stratigraphy sedimentology mineralization and depositional environment abstract the nondweni greenstone belt is an exceptionally well preserved 3 4 ga succession of predominantly volcanic rocks near the southern margin of the kaapvaal craton nondweni group rocks show both similarities and differences to the barberton sequence of similar age 300 km to the north in contrast with the latter none of the nondweni lavas exceed 23 mgo and spinifex textures are exclusively of pyroxene type the characteristics of the lavas and magma genesis are discussed in part ii of this series of papers three formations are recognized in the nondweni group the lowermost mogongolozi formation comprises basaltic andesites komatiitic andesites and basalts with intercalated coarser grained pyroxenites and gabbros the toggekry formation is a felsic unit of quartz sericite schists with small cu zn massive sulphide ore bodies of volcanogenic origin the uppermost witkop formation comprises mainly basalts komatiites and komatiitic basalts intercalated with chert layers and capped by a clastic sedimentary succession the total exposed thickness of the nondweni group is 16 500 m is the result of duplication of stratigraphy and tectonic thickening during early d1 thrusting in the bedding plane of much thinner lithological units tuffaceous sediments comprise a relatively minor component of the stratigraphy but are indicative of depositional environment banded chert layers are largely derived from accumulations of accretionary lapilli tuffs although a diverse origin is indicated for some varieties at the base of the witkop formation a number of undeformed banded coarse grained graphitic quartzite bodies entirely devoid of sedimentary structures are considered to have originated as hydrothermal deposits resulting from fumarolic activity selective brecciation of layers resulted from explosive release of fluids and gases the textures of these bodies are in marked contrast to the highly rounded pseudo conglomerates from the clastic sedimentary succession at the top of the witkop formation this 500 m thick succession passes upwards through reworked tuffs and alluvial fan deposits into a zone of evaporites and stromatolites swallow tail twin crystals of barite and an evaluation of crystal interfacial angles are indicative f replacement of primary evaporative gypsum most primary features of the nondweni greenstone belt suggest a shallow water or subaerial depositional environment and together with characteristics of the magma composition indicate a tectonic setting in a subduction related continental margin setting

the effect of cooling rate on the intensity of thermoremanent magnetization trm acquired by assemblages of pseudo single domain multidomain and interacting single domain grains summary experiments designed to measure the absolute palaeointensity of the geomagnetic field generally do so by comparing the ancient thermoremanent magnetization trm retained by an igneous rock with a new trm imparted in the laboratory one problem with this procedure is that the relative magnitudes of the ancient and laboratory trms may be influenced not only by the external field intensities at the time the two coolings took place but also by the rate at which the coolings themselves occurred here we present new measurements of this cooling rate effect obtained from treatments in the laboratory differing in cooling rate by a factor of 200 synthetic samples containing sized ferrimagnetic grains were used in the experiments theoretical considerations and previous experiments have indicated the cooling rate effect to be dependent on domain state increases in trm magnitude of more than 7percent per order of magnitude decrease in cooling rate have been reported for assemblages of non interacting single domain sd grains here we focus on magnetite grains in the less well studied pseudo single domain psd and multidomain md states using a range of applied field intensities to impart the trms for the first time we also measure the cooling rate effect in grains of titanomagnetite that have been oxyexsolved so that they contain strongly interacting sd lamellae in all cases the cooling rate effect measured was in the same sense as alreadyobservedinidealmagneticallynon interactingsdgrainsbutwasconsiderablyweaker on average the effect did not exceed 3percent increase in trm per order of magnitude decrease in cooling rate and did not show any systematic dependence on appliedfield intensity insomesamples containing coarsergrains thecoolingrateeffectwasnotdistinguishablefrom zero the sense and magnitude of the cooling rate effect remain uncertain in truly md grains as different studies have produced discrepant results for the more practically relevant case of psd and interacting sd grains which commonly dominate the trm in igneous rocks however it appears that we can be more confident in our assertions the cooling rate effect in such materials is in the same sense as in non interacting sd grains but smaller a consequence of long range ordering in lavas and small intrusions containing these it is unlikely to exceed 10percent although a correction should always be attempted the results of palaeointensity studies based upon such samples will generally not be severely biased

a whole rock absolute paleointensity determination of dacites from the duffer formation ca 3 467 ga of the pilbara craton australia an impossible task abstract we have conducted a whole rock type magnetic and absolute paleointensity determination of the red dacite of the duffer formation from the pilbara craton australia the age of the dated rock unit is 3467 5 ma 95 confidence vector analyses results of the step wise alternating field demagnetization nrm up to 100 mt and thermal demagnetization from nrm up to 650 c yield three components of magnetization curie point determinations indicate three characteristic temperatures one at 150 200 c a second one at 450 c and a third one at 580 c magnetic grain size experiments were performed on small specimens with a variable field translation balance vftb the coercivity of remanence hcr suggests that the nrm is carried by low coercivity grains that are associated with a magnetite fraction as is shown by the high temperature component with blocking temperatures above 450 c and up to at least 580 c the ratios of the hysteresis parameters plotted as a modified day diagram show that most grain sizes are scattered within the single domain sd and the superparamagnetic and single domain sp sd domain ranges in addition to the rock magnetic experiments we have performed absolute paleointensity experiments on the samples using the modified thellier coe double heating method to determine the paleointensities partial trm p trm checks were performed systematically to document magnetomineralogical changes during heating the temperature was incremented by steps of 50 c between room temperature and 590 c the paleointensity determinations were obtained from the slope of arai diagrams our paleointensity results indicate that the paleofield obtained was 6 4 0 68 n 11 micro teslas with a virtual dipole moment vdm of 1 51 0 81 1022 am2 from a medium to high temperature component ranging from 300 to 590 c that has been interpreted to be the oldest magnetization yet recorded in paleomagnetic studies of the duffer formation the absolute paleointensity is relatively low and we interpret this low paleofield bias a result of a thermochemical remanent magnetization tcrm process that indicates a possible underestimate of the paleofield by a factor of four for the red dacite of the duffer fm

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

assessing the inner core nucleation paradox with atomic scale simulations abstract we investigate the conditions required to freeze liquid iron and iron alloys near the centre of earth s core it is usually assumed that inner core growth begins once the ambient core temperature falls below the melting temperature of the iron alloy at earth s centre however additional under cooling is required to overcome the energy barrier associated with creating a solid liquid interface predictions based on classical nucleation theory cnt have estimated a required undercooling of 1000 k which cannot be reconciled with predicted core cooling rates of 100 k gyr 1 this apparent contradiction has been called the inner core nucleation paradox here we address three major uncertainties in the application of cnt to inner core nucleation using atomic scale simulations first we simulate freezing in fe and fe o liquids at core conditions to self consistently constrain all parameters required by the cnt equations second we test the basic validity of cnt by directly calculating the waiting time to observe freezing events in fe and fe o liquids third we investigate the influence of wave like forcings applied to the atomic simulations which have been suggested as a means to significantly reduce the energy barrier our results are consistent with cnt in the computationally accessible parameter regime though error estimates on the waiting time can reach 50 of the measurement at the largest undercooling temperatures using cnt to extrapolate to inner core conditions yields estimated undercooling of 730 20 k for the pure iron system and 675 35 k for the fe o system forcings corresponding to large pressure variations of o 10 gpa reduce these values by 100 k while our undercooling estimates are significantly lower than previous estimates they are not low enough to resolve the inner core nucleation paradox

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

evidence for a 3 45 billion year old magnetic remanence hints of an ancient geodynamo from conglomerates of south africa 1 paleomagnetic and rock magnetic analyses of 3445 million year old dacite conglomerate clasts and parent body rocks from the barberton greenstone belt south africa define two dominant components of magnetization one component unblocked at low temperature is an overprint acquired 180 million years ago the other component is unblocked at high temperatures and passes a conglomerate test indicating that this component is older than the depositional age of the conglomerate 3416 ma the high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes archean overprinting and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations alternatively this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo including magnetization by an external field related to solar wind interaction with the atmosphere such exotic mechanisms can be tested with the acquisition of paleointensity data while more scattered than paleomagnetic data recording the more recent geomagnetic field the high unblocking temperature component in the dacite parent body shows some consistency and the simplest explanation of the data is that they reflect a geodynamo 3445 million years ago

geomagnetic paleointensity at 2 41 ga as recorded by the widgiemooltha dike swarm western australia abstract absolute geomagnetic paleointensity measurements were conducted on samples from six mafic dikes of the 2 41 ga widgiemooltha swarm western australia rock magnetic analyses indicate that the paleointensity signal is carried by nearly stoichiometric pseudosingle domain magnetite and or low ti titanomagnetite paleointensity values were determined using the thellier double heating method supplemented by low temperature demagnetizations the ltd thellier method in order to reduce the effect of magnetic remanence carried by large pseudosingle domain and multidomain magnetite grains thirty one samples from five dikes yielded successful paleointensity determinations with the mean value of 41 2 3 8 t which corresponds to a virtual dipole moment of 6 65 0 98 a m 2 the mean and range of paleofield strength values are similar to those of the recent earth s magnetic field and are consistent with a compositionally driven geodynamo established by the earliest paleoproterozoic era the existence of a stable dipolar geomagnetic field during the proterozoic indicated by paleointensity and paleodirectional data from this and prior studies seems incompatible with a young age of the inner core suggested by recent models of the earth s thermal evolution acquisition of reliable paleointensity estimates using the novel approaches such as the ltd thellier method is crucial in constraining the development of more realistic earth like models of long term geodynamo behavior

paleointensity determination on a 1 786 ga old gabbro from hoting central sweden abstract paleointensities from precambrian rocks are rare and might be biased by remagnetization processes here we present new analyses of samples from a 1 786 ga gabbro near hoting central sweden rock magnetic and mineralogical analyses indicate that one of the sites site 5 may be pristine whereas the others exhibit evidence of alteration characteristic remanent magnetization was determined using principal component analysis for each sample and was compared with results obtained in a previous study of elming et al 2009 intensity measurements from site 5 show higher values compared to those of the other sites suggesting that alteration processes may lead to underestimation of the field intensity after cooling rate and anisotropy correction the field moment at 1 786 ga was estimated to be 25 6 3 3 zam 2 and 15 2 6 1 zam 2 from site 5 only and from all sites respectively we consider the result from site 5 to be more accurate owing to the lack of evidence for alteration our estimates agree well with the proterozoic vdm values suggested by biggin et al 2009

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

paleointensity during periods of rapid reversal a case study from the middle jurassic shamrock batholith western nevada paleointensity is one of the least deter mined parameters in geomagnetism yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the earth s core and lower mantle we test two important hy potheses that affect our understanding of how paleointensity has changed in earth s past 1 the mesozoic dipole low mdl hypoth esis and 2 the inverse relationship between geomagnetic polarity reversal rate and paleo intensity we report paleointensity results determined using both the coe modified thellier method and the izzi protocol on the medium grained hornblende biotite granite of the middle jurassic shamrock batholith in the southern singatse range western nevada usa previous zircon u pb dating gives an age of batholith emplacement at ca 165 8 0 4 ma this age estimate is coincident with a period of unusually high reversal frequency 10 reversals ma in the middle jurassic remanence in the shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase the labora tory un blocking temperature range for the well de fined thermal remanent magnetization is nar row between 540 and 575 c and median destructive fields are between 40 and 50 mt thus we infer that the age of rema nence ac quisition is approximated by the u pb age estimate for pluton emplacement two sets of quality criteria were used to assess the paleo intensity results the first set yielded an 87 success rate while the second set using more stringent criteria resulted in only a 20 suc cess rate mean paleointensity values for the shamrock batholith using the loose and strict quality criteria are 34 0 6 4 t n 13 and 33 6 9 6 t n 3 respectively with errors reported at one standard deviation after correction for both remanence anisot ropy and cooling rate the mean values that pass loose and strict criteria are respec tively 17 9 2 7 t and 17 5 2 6 t the uncorrected and corrected paleointensity values for the strict estimates yield respec tive mean virtual dipole moments vdms of 62 7 18 zam 2 z 10 21 and 32 7 4 9 zam 2 the uncorrected value is within error of the vdms reported for plagioclase grains isolated from ca 160 ma basalts from ocean drilling program site 801 however these values have an absolute difference ranging from 6 to 20 zam 2 while the corrected vdm value is more consistent with the esti mated average jurassic field strength of 29 zam 2 using a recent estimate for the long term stable field strength of 42 zam 2 our corrected values add further credence to both the mdl and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses

cooling history of earth s core with high thermal conductivity abstract thermal evolution models of earth s core constrain the power available to the geodynamo process that generates the geomagnetic field the evolution of the solid inner core and the thermal history of the overlying mantle recent upward revision of the thermal conductivity of liquid iron mixtures by a factor of 2 3 has drastically reduced the estimated power available to generate the present day geomagnetic field moreover this high conductivity increases the amount of heat that is conducted out of the core down the adiabatic gradient bringing it into line with the highest estimates of present day core mantle boundary heat flow these issues raise problems with the standard scenario of core cooling in which the core has remained completely well mixed and relatively cool for the past 3 5 ga this paper presents cooling histories for earth s core spanning the last 3 5 ga to constrain the thermodynamic conditions corresponding to marginal dynamo evolution i e where the ohmic dissipation remains just positive over time the radial variation of core properties is represented by polynomials which gives good agreement with radial profiles derived from seismological and mineralogical data and allows the governing energy and entropy equations to be solved analytically time dependent evolution of liquid and solid light element concentrations the melting curve and gravitational energy are calculated for an fe o s si model of core chemistry a suite of cooling histories are presented by varying the inner core boundary density jump thermal conductivity and amount of radiogenic heat production in the core all models where the core remains superadiabatic predict an inner core age of 600 myr about two times younger than estimates based on old lower thermal conductivity estimates and core temperatures that exceed present estimates of the lower mantle solidus prior to the last 0 5 1 5 ga allowing the top of the core to become strongly subadiabatic in recent times pushes the onset of inner core nucleation back to 1 5 gyr but the ancient core temperature still implies a partially molten mantle prior to 2 ga based on these results the scenario of a long lived basal magma ocean and subadiabatic present day core seems hard to avoid

growth fault structure and stratigraphic architecture of the buck ridge volcano sedimentary complex upper hooggenoeg formation barberton greenstone belt south africa abstract the 3 45 3 42 ga buck ridge volcano sedimentary complex br vsc forms the uppermost part of the hooggenoeg formation in the early archaean onverwacht group of the barberton greenstone belt south africa the complex consists mainly of massive and pillowed basalts and felsic quartz plagioclase porphyritic rocks which are capped by pervasively silicified sedimentary rocks deposition of the felsic extrusive and volcani clastic sedimentary rocks occurred in shallow water to emersion during an extensional tectonic regime the extensional regime is expressed by a number of listric normal faults that transect the entire 2 km thick complex the syndepositional i e growth fault character of the faults is indicated by variations in thickness and facies of rock units across the faults and by systematic rotation of the fault blocks broadly contemporaneous with the faulting felsic rocks intruded the complex into the highest levels the geometry of these intrusive bodies was controlled by the major growth faults extension also created space for the intrusion of approximately bedding perpendicular felsic and mafic dykes and black chert veins the new field observations and u pb zircon shrimp data have pinned the timing of events during and after deposition of the br vsc the growth fault model accounts well for previously unexplained structural observations that include both extensional and compressional features recognition of an early extensional phase of deformation demonstrates further similarities between the tectonic histories of the barberton greenstone belt and the greenstone belts of the east pilbara in australia

implications of a long lived basal magma ocean in generating earth s ancient magnetic field 1 observations of earth s magnetic field extending back to 3 45 billion years ago indicate that generation by a core dynamo must be sustained over most of earth s history however recent estimates of thermal and electrical conductivity of liquid iron at core conditions from mineral physics experiments indicate that adiabatic heat flux is approximately 15 tw nearly three times larger than previously thought exacerbating difficulties for driving a core dynamo throughout earth history by convective core cooling alone here we explore the geomagnetic consequences of a basal magma ocean layer in the lowermost mantle hypothesized to exist in the early earth and perhaps surviving until well after the archean while the modern solid lower mantle is an electromagnetic insulator electrical conductivities of silicate melts are known to be higher though as yet they are unconstrained for lowermost mantle conditions we consider a range of possible electrical conductivities and find that for the highest electrical conductivities considered a long lived basal magma ocean could be a primary dynamo source region this would suggest the proposed three magnetic eras observed in paleomagnetic data originate from distinct sources for dynamo generation from 4 5 to 2 45 ga within a basal magma ocean from 2 25 to 0 4 ga within a superadiabatically cooled liquid core and from 0 4 ga to present within a quasi adiabatic core that includes a solidifying inner core

effect of grain size and domain state on thermal demagnetization tails thermal demagnetization of viscous remanence vrm and partial thermoremanence ptrm of 20 and 135 m natural magnetites reveals a broad spectrum f tub of unblocking temperatures tub both and tb the blocking temperature in contrast 0 04 m single domain sd grains demagnetize sharply at tub tb high and low tub tails of f tub were wider for 135 m multidomain md grains than for 20 m psd grains 10 mt alternating field demagnetization rendered the vrm of the 20 m magnetite more sd like in subsequent thermal cleaning selectively erasing both low and high tub tails of f tub primary and secondary remanences are then cleanly separated and non linear thellier paleointensity determination becomes linear our results agree well with tb tub data for 105 year thermoviscous overprints in the milton monzonite over the range of log time common to the two studies because tub av tb for psd and md as well as sd grains the pullaiah et al paleothermometry method will work if tub av values are used instead of tub max values

palaeointensity of the 1 3 billion yr old gardar basalts southern greenland revisited no evidence for onset of inner core growth

detrital magnetite and chromite in jack hills quartzite cobbles further evidence for the preservation of primary magnetizations and new insights into sediment provenance abstract the magnetization of zircons from sedimentary rocks of the jack hills yilgarn craton western australia provide evidence for a hadean to paleoarchean geodynamo 4 0 to 4 2 billion years old these magnetizations pass a microconglomerate test attesting to the fidelity of jack hills zircons as recorders of these most ancient magnetic signals the lack of pervasive remagnetization of the jack hills is also documented through a positive conglomerate test conducted on cobble sized clasts a key element of the latter test is the preservation of a high unblocking temperature magnetization that can survive peak metamorphic temperatures rock magnetic studies suggest the mineral carrier is magnetite herein we investigate the magnetic mineral carriers in cobble samples through scanning electron microscope and microprobe analyses conduct an inter laboratory paleomagnetic study to evaluate sensitivities required to evaluate the weak magnetizations carried by the jack hills sediments and assess provenance information constrained by the opaque minerals these data confirm magnetite as a detrital phase and the presence of high unblocking temperature magnetizations further supporting the posit that the jack hills sediments can preserve primary magnetic signatures we note that some of these magnetizations are near the measurement resolution of standard cryogenic magnetometers and thus exacting laboratory procedures are required to uncover these signals in addition to magnetite the cobbles contain an assemblage of mg poor cr fe chromites ni sulfides and pyrrhotite that suggest a source in a layered intrusion different from the granitoid source of the zircons any hadean rock fragment in these sediments if present remains elusive

geomagnetic field strength 3 2 billion years ago recorded by single silicate crystals analysis of silicate rocks from the archaean kaapvaal craton is used to obtain palaeodirections and intensities of the earth s magnetic field some 3 2 billion years ago and finds that the field strength was within 50 per cent of the present day value indicating that a viable magnetosphere sheltered the early earth s atmosphere from solar wind erosion

exsolved magnetite inclusions in silicates features determining their remanence behavior submicroscopic needle shaped titanomagnetite inclusions exsolved in silicate minerals commonly occur in mafic intrusive rocks and are protected from alteration by their silicate hosts making them excellent candidates for paleomagnetic studies a suite of samples containing clinopyroxene and plagioclase hosted magnetite inclusions from five geologically diverse sites was examined using magnetic force microscopy to image the inclusions magnetic domain state alternating field demagnetization experiments indicate that some inclusions are more stable recorders than others the two factors controlling the remanence behavior of the inclusions are internal microstructures and inclusion dimensions magnetite ulvospinel unmixing within an inclusion subdivides the original titanomagnetite solid solution into a boxwork structure composed of 103 105 magnetite prisms separated by thin ulvospinel lamellae the conversion of multidomain sized needles into assemblages of interacting single domains increases the coercivity and hence relaxation time of the inclusions and results in a thermochemical magnetic remanence in samples without this exsolution microstructure the inclusions diameters determine coercivity and their magnetization is thermoremanent both styles of high coercivity inclusions successfully record paleomagnetic directions in mesozoic rocks and their ubiquity within silicate minerals clinopyroxene and plagioclase of mafic intrusive rocks indicates their value as chemically and magnetically stable tools for elucidating the ancient magnetic field marine magnetic anomalies and crustal kinematics

some global features of palaeointensity in geological time summary a global palaeointensity data base was constructed from all published data from volcanic rocks in geological time older than 0 03 ma the data base contains a total of 1123 flow mean data retrieved from 83 original papers various features of the earth s dipole moment were examined from the data which are based on thellier and shaw methods long term variation of the earth s dipole moment seems to have existed in the past 300 ma with a broad minimum at 120 180 ma as suggested by prevot et al 1990 however due to limited site distribution we cannot regard this mesozoic dipole low as being completely established precambrian palaeointensity data are still insufficient to conclude any long term variation in this time range although the geodynamo processes of moderate magnitude definitely existed in the early time of the earth s history the 2 test was applied to the distribution of the virtual dipole moment for the past 5 ma in which the transitional data were excluded the results indicate that the distribution of virtual dipole moment is better represented by a log normal distribution rather than a normal distribution and this tendency seems to be true for the past 20 ma the relation of mean palaeointensity versus palaeomagnetic colatitude was examined for the past 10 ma for the data excluding transitions the relation is concordant with a theoretical curve from a geocentric axial dipole this reconfirms that the dipole field was dominant in the past geomagnetic field which is the dipole hypothesis in palaeomagnetism on the other hand the virtual dipole moment is much smaller for transitional palaeomagnetic fields and a virtual geomagnetic pole lower than 45 seems to be a reasonable criterion to be categorized as a transition the mean dipole moment for the pole latitude of 30 n 50 n band is larger than that for 30 s 50 s indicating that there might have been a persistent asymmetry of palaeointensity between normal and reversed states or some kind of geometrical asymmetry between the northern and southern hemispheres

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

earth science another energy source for the geodynamo magnesium is not usually considered to be a constituent of earth s core but its presence there has now been proposed to explain an ongoing enigma the identity of the energy sources that drive our planet s magnetic field see letter p 387 existing models of the earth s core dynamo which generates earth s global magnetic field are unable to explain its apparent longevity it has existed for more than 3 4 billion years most of the earth s history in particular the thermal conductivity of iron is now estimated to be higher than previously thought implying that thermal convection and radiogenic heating would not have been sufficient to sustain the geodynamo here joseph o rourke and david stevenson show that the precipitation of magnesium bearing minerals from the earth s core can serve as an alternative power source for the geodynamo they suggest that equilibration at high temperatures in the aftermath of giant impacts on the early earth allowed a small amount of magnesium to partition into the core and show that the transport of magnesium as oxide or silicate from the cooling core to underplate the mantle is a more efficient source of buoyancy than inner core growth such a dynamo could survive throughout geologic time even if core radiogenic heating is minimal and core cooling is slow

cluster analysis on a sphere application to magnetizations from metasediments of the jack hills western australia abstract metasediments of the jack hills contain the oldest known terrestrial minerals in the form of zircons nearly 4 4 billion years old paleointensity data from these zircons provide evidence for a hadean geodynamo as old as 4 2 billion years old given the importance of these zircons for constraining the earliest history of the core it is vital to understand the fidelity of the zircon record a fundamental aspect providing context for the preservation of primary magnetic signals is the nature of overprints predicted to have been imparted on rocks of the jack hills due to archean to proterozoic metamorphic events to be viable magnetic records of a hadean geodynamo zircon magnetization directions should differ from these secondary magnetizations to evaluate these secondary magnetizations we report paleomagnetic analyses of a comprehensive sampling of 68 quartzite cobble sized clasts from the jack hills metasediments 0 5 to 1 0 km from the discovery site which has yielded the oldest zircons and paleofield estimates while application of standard paleomagnetic tests suggests that the ensemble of cobble directions cannot be distinguished from those drawn from a random distribution a new cluster analysis of directions on a sphere and non parametric resampling approaches reveal significant directions amongst subsets of the data one isolated at the lowest temperature analyzed 200 to 300 c declination dec 316 8 inclination inc 51 1 appears to be dominated by the present day field another isolated at higher but still relatively low unblocking temperatures that we call intermediate of 350 500 c dec 243 8 inc 9 5 agrees with a magnetic overprint isolated from the secondary cr fe mica fuchsite isolated from the jack hills discovery site passing a field test at the 80 confidence level no evidence is found in our data or in the data of others collected on similar jack hills lithologies for a widespread 1 ga remagnetization event instead we interpret the most prevalent secondary magnetization of the quartzite i e intermediate unblocking and the fuchsite characteristic remanent magnetization to be 2 65 ga in age coincident with peak metamorphism as high as ca 475 c of the jack hills the presence of this distinct secondary magnetization its difference from that recorded by jack hills zircons at high unblocking temperatures and the lack of a dominant remagnetization direction at high unblocking temperatures in the cobble data the expected result for a primary magnetization lends further support to the fidelity of the hadean geomagnetic record the presence of the secondary magnetization also lends support to the conclusion that most of the jack hills metasediments were deposited in the archean with only minor reworking and potential tectonic interleaving of proterozoic components overall the application of the new directional cluster analysis presented here has the potential to reveal magnetic directions in highly scattered data sets typical of weakly magnetized coarse grained sedimentary rocks

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

the effects of secondary mineral formation on coe type paleointensity determinations theory and simulation thellier type experiments are the most widely applied approaches for determining the absolute paleointensities of earth s magnetic field one major problem however is that specimens are prone to thermal alteration due to the intensive thermal treatment during experiments linear arai plots with acceptable partial thermal remanent magnetization ptrm checks have been considered as evidence for the absence of or negligible effects of thermal alteration and as reliable indicators of high quality paleointensity estimates however by simulating the coe variant of the thellier method on assemblages of single domain sd magnetite particles it is demonstrated that new magnetic minerals which form during thermal treatments can result in linear concave up or concave down arai plots depending on the magnetic properties of both the primary and secondary magnetic phases among this range of behavior pseudoideal arai plots which are linear with acceptable ptrm check statistics would lead to paleointensity underestimates it is further demonstrated that ptrm checks are proportional to the degree of underestimate with a magnetic granulometry dependency for sd particles due to the complexity of this dependency ptrm check statistics are only comparable when specimens have similar magnetic properties this suggests that a universal threshold for ptrm check statistics is not likely to be effective since the criteria of linearity and low ptrm check statistics are insufficient to guarantee the fidelity of the estimates auxiliary rock magnetic methods such as temperature dependent hysteresis parameters and anhysteretic remanent magnetization are highly recommended to identify the presence of alteration

detecting the oldest geodynamo and attendant shielding from the solar wind implications for habitability abstract the onset and nature of the earliest geomagnetic field is important for understanding the evolution of the core atmosphere and life on earth a record of the early geodynamo is preserved in ancient silicate crystals containing minute magnetic inclusions these data indicate the presence of a geodynamo during the paleoarchean between 3 4 and 3 45 billion years ago while the magnetic field sheltered earth s atmosphere from erosion at this time standoff of the solar wind was greatly reduced and similar to that during modern extreme solar storms these conditions suggest that intense radiation from the young sun may have modified the atmosphere of the young earth by promoting loss of volatiles including water such effects would have been more pronounced if the field were absent or very weak prior to 3 45 billion years ago as suggested by some models of lower mantle evolution the frontier is thus trying to obtain geomagnetic field records that are 3 45 billion years old as well as constraining solar wind pressure for these times in this review we suggest pathways for constraining these parameters and the attendant history of earth s deep interior hydrosphere and atmosphere in particular we discuss new estimates for solar wind pressure for the first 700 million years of earth history the competing effects of magnetic shielding versus solar ion collection and bounds on the detection level of a geodynamo imposed by the presence of external fields we also discuss the prospects for constraining hadean paleoarchean magnetic field strength using paleointensity analyses of zircons

magnesium partitioning between silicate melt and liquid iron using first principles molecular dynamics implications for the early thermal history of the earth s core abstract the high conductivity of the earth s core discovered through first principles and experimental studies requires that the core must start very hot and cool down slowly to generate the earth s magnetic field by thermal buoyancy the requirement is difficult to satisfy due to the fast cooling of the overlying magma ocean and consequently of the underlying core this is in direct conflict with the early appearance of the earth s paleomagnetic field recently it was proposed that significant amount of magnesium mg can be partitioned into the core through the high temperature created by the moon forming giant impact due to its intrinsic low solubility subsequent cooling would cause mg precipitation to generate compositional buoyancy to power the geodynamo in the early history of the earth here we show using first principles molecular dynamics simulations that the equilibrium constant of magnesium dissolution in molten iron depends on temperature entirely consistent with recent experimental data we further show that mg partitioned into the core during giant impacts and reaching a concentration of about 2 wt can precipitate out at around 3 5 ga much earlier than the onset of inner core nucleation during the subsequent evolution of the earth silicon si concentration of the earth s core will remain constant while mg and oxygen o concentrations decrease significantly consequently the current si concentration in the core reflects the accretion processes of the earth while o and mg concentrations in the core is the combined result of both accretion and the subsequent evolution of the earth core forward modeling shows that for mgo precipitation to provide enough power to generate the magnetic field in the early history of the earth initially high silicon content of the core is preferred which is accommodated readily in the grand tack accretion scenario the geodynamo driven by mgo precipitation explains the secular decline of palaeomagnetic field intensity in the early history of the earth

primary pseudo single and single domain magnetite inclusions in quartzite cobbles of the jack hills western australia implications for the hadean geodynamo

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

paleointensity estimates from historic and modern hawaiian lava flows using glassy basalt as a primary source material abstract published paleointensity estimates derived from lavas extruded in known fields are highly variable and rarely recover the expected field strength within an accuracy of better than 10 inconsistent results on modern volcanic rocks lend even greater uncertainty to intensity experiments performed on lava flows emplaced during periods of unknown geomagnetic field strength the majority of published paleointensity data are collected from the slowly cooled massive centers of lava flows where the magnetic grains are more likely to be multi domain and produce non ideal experimental results glassy volcanic material found on subaerial lava flow tops and in sub aqueous and subglacial environments however is rapidly cooled and therefore most likely of all volcanic materials to behave as single domain particles demanded by neel theory we present a new paleointensity study of historic and modern hawaiian lavas and test the viability of subaerially emplaced glassy basaltic material as an accurate recorder of magnetic field intensity six of eight lava flows sampled on the big island of hawaii 1843 1859 1935 1950 1960 1990 c e produce well behaved arai plots and recover an average intensity to within 2 7 t of the expected field strength or better than 8 accuracy we apply very strict selection criteria including a minimum of three specimens per site to prevent extraneous field estimates from affecting the final results individual volcanic glass results from the 1960 c e lava flow have a much lower variance than published data from the same volcanic unit glassy materials should therefore be collected wherever possible as they allow recovery of geomagnetic field strength with unprecedented accuracy

constraints on asteroid magnetic field evolution and the radii of meteorite parent bodies from thermal modelling abstract paleomagnetic measurements of ancient terrestrial and extraterrestrial samples indicate that numerous planetary bodies generated magnetic fields through core dynamo activity during the early solar system the existence timing intensity and stability of these fields are governed by the internal transfer of heat throughout their parent bodies thus paleomagnetic records preserved in natural samples can contain key information regarding the accretion and thermochemical history of the rocky bodies in our solar system however models capable of predicting these field properties across the entire active lifetime of a planetary core that could relate the processes occurring within these bodies to features in these records and provide such information are limited here we perform asteroid thermal evolution models across suites of radii accretion times and thermal diffusivities with the aim of predicting when fully and partially differentiated asteroids generated magnetic fields we find that dynamo activity in both types of asteroid is delayed until 4 5 5 5 myr after calcium aluminium rich inclusion formation due to the partitioning of 26al into the silicate portion of the body during differentiation and large early surface heat fluxes followed by a brief period

evolving core conditions ca 2 billion years ago detected by paleosecular variation abstract paleomagnetic data provide one of the few probes available to interrogate early evolution of the core here we apply this probe by examining the latitudinal dependence of paleosecular variation psv data derived from high quality paleomagnetic data collected from proterozoic and neoarchean rocks these data define a neoarchean geomagnetic field that was more dipolar than that during proterozoic times indicating a change in core conditions the signals observed may reflect a change in forcing of the dynamo and an early onset of inner core growth we propose a model that links evolution of the core mantle and crust in three principal phases i before approximately 3 5 ga an entirely liquid core may not have hosted a geodynamo if heat transport was sufficient across the core mantle boundary however a geodynamo could have been generated if so sources in the shallow outer core could have been more important for generating the dynamo relative to deeper convection resulting in a field that was less dipolar than that generated in later times ii cooling of the lower mantle between ca 2 and 3 5 billion years ago was promoted by deep subduction and possibly coincided with inner core growth the geodynamo during this episode was deeply seated producing a highly dipolar surface magnetic field iii after ca 2 billion years ago continued subduction led to large scale core mantle boundary compositional and heat flux heterogeneity with these changes shallow core contributions to the geomagnetic field grew in importance resulting in a less dipolar field

mantle control of the geodynamo consequences of top down regulation the mantle global circulation including deep subduction and lower mantle superplumes exerts first order controls on the evolution of the core the history of the geodynamo and the structure of the geomagnetic field mantle global circulation models that include realistic plate motions deep subduction and compositional heterogeneity similar to the observed large low seismic velocity provinces in the lower mantle predict that the present day global average heat flux at the core mantle boundary cmb exceeds 85 mw m 2 this is sufficient to drive the present day geodynamo by thermochemical convection and implies a very young inner core with inner core nucleation between 400 and 1100 ma the mantle global circulation also generates spatially heterogeneous heat flux at the cmb with peak to peak lateral variations exceeding 100 mw m 2 such extreme lateral variability in cmb heat flux in conjunction with the high thermal conductivity of the core implies that the liquid outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces numerical dynamo simulations show how this pattern of heterogeneous boundary heat flux affects flow in the outer core producing localized circulation patterns beneath the cmb tied to the mantle heterogeneity and long lived deviations from axial symmetry in the geomagnetic field

widespread occurrence of silicate hosted magnetic mineral inclusions in marine sediments and their contribution to paleomagnetic recording magnetic mineral inclusions occur commonly within other larger mineral phases in igneous rocks and have been demonstrated to preserve important paleomagnetic signals while the usefulness of magnetic inclusions in igneous rocks have been explored extensively their presence in sediments has only been speculated upon the contribution of magnetic inclusions to the magnetization of sediments therefore has been elusive in this study we use transmission electron microscope tem and magnetic methods to demonstrate the widespread preservation of silicate hosted magnetic inclusions in marine sedimentary settings tem analysis reveals detailed information about the microstructure chemical composition grain size and spatial arrangement of nanoscale magnetic mineral inclusions within larger silicate particles our results confirm the expectation that silicate minerals can protect magnetic mineral inclusions from sulfate reducing diagenesis and increase significantly the preservation potential of iron oxides in inclusions magnetic inclusions should therefore be considered as a potentially important source of fine grained magnetic mineral assemblages and represent a missing link in a wide range of sedimentary paleomagnetic and environmental magnetic studies in addition we present depositional remanent magnetization drm modeling results to assess the paleomagnetic recording capability of magnetic inclusions our simulation demonstrates that deposition of larger silicate particles with magnetic inclusions will be controlled by gravitational and hydrodynamic forces rather than by geomagnetic torques thus even though these large silicates may contain ideal single domain particles they can not contribute meaningfully to paleomagnetic recording however smaller silicate grains e g silt and clay sized silicates with unidirectionally magnetized magnetic inclusions can potentially record a reliable drm

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

anomalous rare earth element yttrium and zirconium mobility associated with uranium mineralization the mobility of ree y and zr during the variscan uranium mineralization event in high grade metasediments of the bohemian massif czech republic has been investigated to understand their behaviour in alkaline hydrothermal alterations the ree patterns together with high mobility of y and zr point to movement of ree y zr and u by highly mineralising oxidising basinal fluids in the permian their high mobility during the origin of an economically important uranium mineralization is supported by the occurrence of y and zr rich coffinite

the kiaman reversed polarity superchron at kiama toward a field strength estimate based on single silicate crystals abstract we present the first geomagnetic paleointensity measurements on single plagioclase crystals from lavas formed during the 262 318 ma permo carboniferous reverse polarity superchron commonly known as the kiaman rock magnetic experiments suggest that feldspar crystals separated from two lavas of the kiaman superchron type area near kiama australia carry pseudo single domain low ti magnetic inclusions capable of faithfully retaining a primary paleointensity signal thellier paleointensity experiments yield virtual dipole moments of 8 5 0 2 1022 am2 and 9 3 0 3 1022 am2 for these crystals furthermore these results lack the alteration problems that prohibit paleointensity and sometimes paleodirection determination using whole rock lava samples from the type area the derived field strengths are 5 6 times higher than some other virtual dipole moment vdm values of kiaman age derived from whole rock igneous samples this suggests either high paleointensity variability during the kiaman superchron or that unrecognized non ideal magnetic behavior has compromised some prior paleointensity results reported from whole rocks elsewhere

multidecadally resolved polarity oscillations during a geomagnetic excursion polarity reversals of the geomagnetic field have occurred through billions of years of earth history and were first revealed in the early 20th century almost a century later details of transitional field behavior during geomagnetic reversals and excursions remain poorly known here we present a multidecadally resolved geomagnetic excursion record from a radioisotopically dated chinese stalagmite at 107 91 thousand years before present with age precision of several decades the duration of geomagnetic directional oscillations ranged from several centuries at 106 103 thousand years before present to millennia at 98 92 thousand years before present with one abrupt reversal transition occurring in one to two centuries when the field was weakest these features indicate prolonged geodynamo instability repeated asymmetrical interhemispheric polarity drifts associated with weak dipole fields likely originated in earth s deep interior if such rapid polarity changes occurred in future they could severely affect satellites and human society

in search of high fidelity geomagnetic paleointensities a comparison of single plagioclase crystal and whole rock thellier thellier analyses the strength of earth s magnetic field on timescales greater than a million years is known primarily from thellier thellier analyses of igneous rocks in the thellier thellier method the natural remanent magnetization nrm is compared with a thermoremanent magnetization trm imparted during heating in a known laboratory field unfortunately the multiple heating steps required by the method often result in alteration of magnetic minerals in clay bearing groundmass of whole rock samples this alteration may result in the creation of new magnetic minerals that could affect trm acquisition analyses of single plagioclase crystals which contain magnetic inclusions that retain paleofield values may offer a way to retrieve paleointensity data that is less affected by alteration we test this hypothesis through a study of whole rocks and single plagioclase crystals from a mid cretaceous 113 115 ma basalt flow of the rajmahal traps northeastern india magnetic hysteresis and thermal demagnetization data from plagioclase crystals and whole rock samples are similar suggesting a common magnetic mineralogy however hysteresis data from heated whole rock samples indicate that a fine grained magnetic phase has formed this effect was either absent or much less pronounced in the single plagioclase crystals tested paleointensity data from these crystals suggest a paleofield value of 65 1 5 3 t n 15 data from whole rock samples overlap these values but are on average lower we attribute the difference in this study to the preferential growth of new magnetic minerals in the whole rock samples resulting in an enhanced acquisition of trm and shallowing of the slope of nrm lost versus trm gained curves these data suggest that subtle alteration effects may bias thellier thellier results from some whole rock samples toward low paleofield values in these cases the single plagioclase crystal approach may yield more reliable estimates of geomagnetic paleointensity

dipole strength and variation of the time averaged reversing and nonreversing geodynamo based on thellier analyses of single plagioclase crystals 1 single plagioclase crystals separated from lavas can contain minute magnetic inclusions that faithfully record the geomagnetic field while the silicate host limits natural and experimentally induced alteration here we use paleointensity analyses of plagioclase crystals to examine three characteristic geodynamo regimes spanning the last 200 myr an interval of moderate reversal frequency during the paleogene the nonreversing field of the cretaceous normal polarity superchron and a period of very high reversal occurrence during the late jurassic an inverse relationship between reversal rate and intensity is supported furthermore the strength of the time averaged reversing field is more variable than that of the nonreversing field these observations are consistent with an active lower mantle shaping core mantle boundary heat flux and efficiency of the geodynamo on timescales of tens of millions of years

young inner core inferred from ediacaran ultra low geomagnetic field intensity an enduring mystery about earth has been the age of its solid inner core plausible yet contrasting core thermal conductivity values lead to inner core growth initiation ages that span 2 billion years from 0 5 to 2 5 billion years ago palaeomagnetic data provide a direct probe of past core conditions but heretofore field strength data were lacking for the youngest predicted inner core onset ages here we present palaeointensity data from the ediacaran 565 million years old sept iles intrusive suite measured on single plagioclase and clinopyroxene crystals that hosted single domain magnetic inclusions these data indicate a time averaged dipole moment of 0 7 1022 a m2 the lowest value yet reported for the geodynamo from extant rocks and more than ten times smaller than the strength of the present day field palaeomagnetic directional studies of these crystals define two polarities with an unusually high angular dispersion s 26 at a low latitude together with 14 other directional data sets that suggest a hyper reversal frequency these extraordinary low field strengths suggest an anomalous field behaviour consistent with predictions of geodynamo simulations high thermal conductivities and an ediacaran onset age of inner core growth a late onset of inner core growth is inferred from ultra low palaeomagnetic field strengths about 565 million years ago as measured in magnetic inclusions in ediacaran crystals

hazard analysis of li shan landslide in taiwan abstract in this study a method coupling limit equilibrium analysis monte carlo analysis and geography information system gis was implemented to evaluate landslide hazard with consideration of spatial uncertainties with the gis the three dimensional surface topography underground geomaterial distribution and groundwater level were processed for slope stability analysis in addition mechanical properties of the geomaterials were considered as random variables instead of single values slope sections and groundwater levels were also randomly adopted through a monte carlo sampling process a distribution of safety factor and probability of failure were determined this approach was applied to hazard assessment of the li shan landslide in central taiwan through a series of analyses the hazards of sliding bodies of the li shan landslide were estimated and ranked it shows the influence of ground water level is significant which reveals the importance of ground water control and monitoring the results show the safety of the northeast and southeast areas of the landslide have been significantly improved since the ground water levels have been lowered by the drainage system spatially the hazard moderately dissipates from center area in the post remediation phase

the signature of inner core nucleation on the geodynamo abstract energy considerations indicate that the power delivered to the present day geodynamo comes mainly from the growth of the solid inner core through light element and latent heat releases the nucleation of the inner core was therefore a major transition for the geodynamo here we use numerical dynamo simulations linked by thermochemical evolution of the core to investigate the effects of inner core nucleation icn on the geodynamo and identify possible icn footprints in the palaeomagnetic field our results suggest that a strong field dipole dominated dynamo prior and after icn the uniformitarian scenario is most likely we predict little footprint of icn on the surface field intensity consistent with the observed lack of a long term trend in paleointensity data instead we find that the best proxy for inner core growth is an increase in axial octupole strength with age from present day to icn which might be resolvable in the palaeomagnetic record we confirm the existence of weak field multipolar dynamos prior to icn suggesting the dynamo might have transiently reached a weak field state the catastrophic scenario the narrow stability domain of these weak field dynamos implies that the catastrophic scenario is less probable than the uniformitarian scenario

thermal history of the 3 5 3 2 ga onverwacht and fig tree groups barberton greenstone belt south africa inferred by raman microspectroscopy of carbonaceous material raman spectra of carbonaceous material were collected in situ from samples of cherts of the onverwacht and fig tree groups in the central barberton greenstone belt the spectra feature two dominant peaks characteristic of disordered carbon the d peak at 1310 cm 21 and the o peak at 1580 1600 cm 21 d peak positions and relative peak intensities and areas indicate that all samples have been altered to lower greenschist facies or above no correlation was observed between maximum temperature and stratigraphic position or degree of hydrothermal alteration implying that metamorphism in the central barberton greenstone belt was regional and unaccompanied by the flow of large quantities of hydrothermal fluids samples from the marble bar chert of the pilbara block western australia have been heated to the same extent as samples from barberton this study demonstrates the use of raman spectra of carbonaceous material as a sensitive geother mometer for low temperature metamorphic facies this application could also be used to establish the antiquity of putative microfossils from metamorphic terranes

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

an assessment of long duration geodynamo simulations using new paleomagnetic modeling criteria qpm abstract long term temporal variations of the magnetic field timescales 10 myr characterized from paleomagnetic data have been hypothesized to reflect the evolution of earth s deep interior and couplings between the core and mantle by tying observed changes in the paleomagnetic record to mechanisms predicted from numerical geodynamo simulations we have a unique tool for assessing changes in the deep interior back in time however numerical simulations are not run in an earth like parameter regime and assessing how well they reproduce the geomagnetic field is difficult criteria have been proposed to determine the level of spatial and temporal agreement between simulations and observations spanning historical and holocene timescales but no such criteria exist for longer timescales here we present a new set of five criteria quality of paleomagnetic modeling criteria q pm that assess the degree of semblance between a simulated dynamo and the temporal and spatial variations of the long term 10 myr paleomagnetic field these criteria measure inclination anomaly virtual geomagnetic pole dispersion at the equator latitudinal variation in virtual geomagnetic pole dispersion normalized width of virtual dipole moment distribution and dipole field reversals we have assessed 46 geodynamo simulations using the q pm criteria the simulations have each been run for the equivalent of at least 300 kyr span reversing and non reversing regimes and include either homogeneous or heterogeneous heat flux boundary conditions we find that none of our simulations reproduce all salient aspects of the long term paleomagnetic field behavior for the past 10 myr nevertheless our simulations bracket earth values suggesting that an earth like simulation is feasible within the available computationally accessible parameter space this new set of criteria can inform future simulations that aim to reproduce all aspects of earth s long term magnetic field behavior

core evolution driven by mantle global circulation abstract reconstructions of the phanerozoic history of mantle global circulation that include past plate motions are used to constrain the thermochemical evolution of the core according to our mantle global circulation models the present day global average heat flux at the core mantle boundary lies in the range 80 90 mw m 2 with peak to peak long wavelength lateral variations up to 100 mw m 2 associated with compositional and thermal heterogeneity in the d layer for core thermal conductivity in the range k 100 130 w m 1 k 1 we infer that the present day outer core is thermally unstable beneath the high seismic velocity regions in the lower mantle but thermally stable beneath the large low seismic velocity provinces a numerical dynamo shows how this boundary heat flux heterogeneity generates departures from axial symmetry in the time average geomagnetic field and the pattern of flow in the outer core standard thermochemical evolution models of the core driven by mantle global circulation heat flow predict inner core nucleation between 400 and 1100 ma with thermal conductivity k 100 w m 1 k 1 the core heat flow derived from our mantle global circulation models is adequate for maintaining the geodynamo since inner core nucleation supercritical for dynamo action by thermal convection just prior to inner core nucleation and marginal for inner core convection

new constraints on the variation of the geomagnetic field during the late neolithic period archaeointensity results from sichuan southwestern china we have carried out an archaeomagnetic study on a late neolithic locality liujiazhai in sichuan southwestern china we pull together various dating techniques including radiocarbon analysis optically stimulated luminescence dating stratigraphic information as well as archaeological and archaeomagnetic estimations to constrain the age of the studied samples rock magnetic results indicate thermally stable fine grained magnetite or titanomagnetite as the dominant magnetic carriers more than half of the specimens 141 246 in the paleointensity experiment pass the selection criteria and are considered to record robust intensity values the virtual axial dipole moments range from approximately 2 8 to 7 8 1022 am2 with an average of 5 9 1022 am2 indicating that the geomagnetic intensity around 3000 before the common era b c e is overall lower than the present field intensity 9 8 1022 am2 of this area the new results from liujiazhai are generally consistent with the published data of similar age but deviate from the only available model of cals10k 1b at certain time periods making them important for future improvements of the model those data are significant for constraining the variation of geomagnetic field intensity between 3100 and 2600 b c e and improving the regional model of eastern asia

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

paleointensity results from the late archaean modipe gabbro of botswana 1 rock magnetic and thellier thellier coe paleointensity results are reported for a new collection of samples from the 2 78 ga modipe gabbro of botswana the magnetic properties are very favorable leading to an unusually high success rate and a well constrained result of 36 40 t 95 confidence interval we discuss the long standing problem of allowing for the enormous differences between the natural and laboratory cooling rates and apply a temperature dependent correction derived from first order reversal curve forc data whereas pure single domain sd corrections often lead to quite large decreases in paleointensity estimates sometimes exceeding 50 we find a modest increase of about 10 the earth s magnetic dipole moment in the late archaean is thereby estimated to have been 6 1022 am2

rock magnetism of tiny exsolved magnetite in plagioclase from a paleoarchean granitoid in the pilbara craton granitoids are widespread in precambrian terranes as well as the phanerozoic orogenic belts but they have garnered little attention in paleomagnetic studies because granitoids often contain abundant coarse grained magnetically unstable oxides in this study the first example of tiny needle shaped exsolved oxides in plagioclase in a paleoarchean granitoid is reported the magnetic properties of single plagioclase crystals with the exsolved oxide inclusions have been studied to determine their paleomagnetic recording fidelity demagnetization experiments and hysteresis parameters indicate that the oxide inclusions are near stoichiometric magnetite and magnetically very stable first order reversal curve forc diagrams reveal negligible magnetostatic interactions minimal interactions are also reflected by very efficient acquisition of anhysteretic remanent magnetization single plagioclase crystals exhibit strong magnetic remanence anisotropies which require corrections to their paleodirectional and paleointensity data nonetheless quantitative consideration of anisotropy tensors of the single plagioclase crystals indicates that the bias can be mitigated by properly averaging data from a few tens of single crystals from the nonlinear thermoremanence acquisition of the plagioclase crystals we estimate that the plagioclase crystals can reconstruct paleointensity up to 50 t local metamorphic condition suggests that those magnetite may carry remanence of 3 2 to 3 3 ga we suggest that exsolved magnetite in granitoids is potentially a suitable target for the study of the early history of the geomagnetic field and prompt detailed microscopic investigations as well as paleomagnetic tests to constrain the age of remanence

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

the stratigraphy of the 3 5 3 2 ga barberton greenstone belt revisited a single zircon ion microprobe study abstract recent field and geochemical studies indicate a need to test the stratigraphy of the ca 3 5 ga barberton greenstone belt as it is presently adopted 1 2 this work uses the ion microprobe shrimp to attempt such a test results show that 1 volcaniclastic sediments of the theespruit formation