lunar surface magnetic fields and their interaction with the solar wind results from lunar prospector the magnetometer and electron reflectometer experiment on the lunar prospector spacecraft has obtained maps of lunar crustal magnetic fields and observed the interaction between the solar wind and regions of strong crustal magnetic fields at high selenographic latitude 30 s to 80 s and low 100 kilometers altitude electron reflection maps of the regions antipodal to the imbrium and serenitatis impact basins extending to 80 s latitude show that crustal magnetic fields fill most of the antipodal zones of those basins this finding provides further evidence for the hypothesis that basin forming impacts result in magnetization of the lunar crust at their antipodes the crustal magnetic fields of the imbrium antipode region are strong enough to deflect the solar wind and form a miniature 100 to several hundred kilometers across magnetosphere magnetosheath and bow shock system

a long lived lunar dynamo driven by continuous mechanical stirring lunar rocks contain a record of an ancient magnetic field that seems to have persisted for more than 400 million years and which has been attributed to a lunar dynamo models of conventional dynamos driven by thermal or compositional convection have had difficulty reproducing the existence and apparently long duration of the lunar dynamo here we investigate an alternative mechanism of dynamo generation continuous mechanical stirring arising from the differential motion due to earth driven precession of the lunar spin axis between the solid silicate mantle and the liquid core beneath we show that the fluid motions and the power required to drive a dynamo operating continuously for more than one billion years and generating a magnetic field that had an intensity of more than one microtesla 4 2 billion years ago3 are readily obtained by mechanical stirring the magnetic field is predicted to decrease with time and to shut off naturally when the moon recedes far enough from earth that the dissipated power is insufficient to drive a dynamo in our nominal model this occurred at about 48 earth radii 2 7 billion years ago thus lunar palaeomagnetic measurements may be able to constrain the poorly known early orbital evolution of the moon this mechanism may also be applicable to dynamos in other bodies such as large asteroids

magnetism of a very young lunar glass recent paleomagnetic studies of apollo samples have established that a core dynamo existed on the moon from at least 4 2 to 3 56 billion years ga because there is no lunar dynamo today a longstanding mystery has been the origin of magnetization in very young lunar samples 10 t core dynamo field nor impact generated fields

lunar paleointensity measurements implications for lunar magnetic evolution abstract we analyze published and new paleointensity data from apollo samples to reexamine the hypothesis of an early 3 9 3 6 ga lunar dynamo our new paleointensity experiments on four samples use modern absolute and relative measurement techniques with ages ranging from 3 3 to 4 3 ga bracketing the putative period of an ancient lunar field samples 60015 anorthosite and 76535 troctolite failed during absolute paleointensity experiments samples 72215 and 62235 impact breccias recorded a complicated multi component magnetic history that includes a low temperature 500 c component associated with a low intensity 2 t similar multi component behavior has been observed in several published absolute intensity experiments on lunar samples additional material from 72215 and 62235 was subjected to a relative paleointensity experiment a saturation isothermal remanent magnetization or sirm experiment neither sample provided unambiguous evidence for a thermal origin of the recorded remanent magnetization we test several magnetization scenarios in an attempt to explain the complex magnetization recorded in lunar samples specifically an overprint from exposure to a small magnetic field an isothermal remanent magnetization results in multi component behavior similar to absolute paleointensity results from which we could not recover the correct magnitude of the original thermal remanent magnetization in light of these new experiments and a thorough re evaluation of existing paleointensity measurements we conclude that although some samples with ages of 3 6 to 3 9 ga are strongly magnetized and sometimes exhibit stable directional behavior it has not been demonstrated that these observations indicate a primary thermal remanence particularly problematic in the interpretation of lunar sample magnetizations are the effects of shock as relative paleointensity measurements for lunar samples are calibrated using absolute paleointensities the lack of acceptable absolute paleointensity measurements renders the interpretation of relative paleointensity measurements unreliable consequently current paleointensity measurements do not support the existence of a 3 9 3 6 ga lunar dynamo with 100 t surface fields a result that is in better agreement with satellite measurements of crustal magnetism and that presents fewer challenges for thermal evolution and dynamo models

a low oxygen tolerant east pacific flat clam posidonotis semiplicata from the lower jurassic of the canadian cordillera flat clams represent a loosely defined group of thin shelled flat valved bivalves their distribution and high concentration in some especially mesozoic rocks have attracted interest and several contrasting paleoecologic interpretations have been proposed to explain their occurrence we present a comprehensive study of posidonotis semiplicata hyatt an early jurassic flat clam common in western north and south america posidonotis symmetrica hyatt posidonotis balteata crickmay and posidonotis cancellata leanza are interpreted as subjective junior synonyms in north america p semiplicata occurs in the upper sinemurian to lower toarcian of several cordilleran allochthonous terranes the south american record is restricted to upper pliensbachian to lower toarcian occurrences suggesting that the species originated in the northeast pacific in late sinemurian time and spread to the southeast pacific by the late pliensbachian the closely related posidonotis dainellii losacco sporadically kno

magnetic fidelity of lunar samples and implications for an ancient core dynamo abstract some lunar rocks have stable magnetizations that provide compelling evidence for an ancient lunar core dynamo however a longstanding problem has been interpreting the unstable alternating field af demagnetization behavior observed in the remaining majority of the apollo lunar sample suite similar unstable behavior has also been observed for many meteorites it is unclear whether the behavior of these samples indicates that they formed in the absence of an ancient magnetizing field or whether they simply have poor magnetic recording properties it is necessary to distinguish between these two possibilities in order to determine whether paleomagnetic fields were present on a sample s parent body to address this issue we analyzed five samples whose rock magnetic properties span the full suite of observed demagnetization behaviors mare basalts 15556 15016 12017 10020 and troctolite 76535 we demonstrate that the effects of spurious anhysteretic remanent magnetization arm during af demagnetization in combination with multidomain magnetic carriers and magnetic remanence anisotropy are likely responsible for the apparently poor magnetic behavior of many lunar samples therefore the unstable af demagnetization behavior observed for many lunar samples is not alone sufficient for ruling out the presence of an ancient lunar core dynamo nevertheless spurious arm may explain the observation of surprisingly high 1 t apollo era paleointensity measurements for samples thought to have formed after the cessation of a lunar core dynamo

south pole aitken basin magnetic anomalies evidence for the true polar wander of moon and a lunar dynamo reversal the analysis of the lunar prospector magnetic data over south pole aitken basin reveals two categories of magnetic anomalies with distinctly different polarities modeling the anomalies in terms of vertical prism source bodies shows two separate true polar wander paths of the moon one is driven by the giant surface mass deficiency resulted from the formation of the south pole aitken basin and the other by the giant surface mass concentrations mascons formed inside the large impact basins such as serenitatis and imbrium and a vast volcanic deposit on procellarum terrane a total of 100o true polar wander path suggests that the source bodies are formed and magnetized during a long period moreover the distinctly different polarities of the two categories of the anomalies supports the idea that the core dynamo underwent a reversal or shutdown and started up with a different polarity caused by the latter impacts the impact induced shock pressures of 10 20 gpa traveling inside the lunar core the appreciable angular momentum transfer to the lunar mantle by the impacting bodies and the impact induced stratification of the core probably reversed the polarity of the core dynamo we also model the north crisium magnetic anomaly extracted from the lunar prospector magnetic data the paleomagnetic pole position obtained for the source body reveals that the north crisium anomaly belongs to the first category of the anomalies

lunar surface magnetic field concentrations antipodal to young large impact basins abstract we find that the major regions of strong surface magnetic fields detected by planetary electron reflection magnetometry perm measurements of the apollo 15 and 16 subsatellites are located near the antipodes of four young large ringed impact basins orientale imbrium serenitatis and crisium statistical analysis of the 26 impact basins whose antipodes fall within the 35 s to 35 n latitude band covered by perm measurements shows that the antipodal regions of older basins have median surface magnetic fields of 0 2 nt compared with 0 5 1 5 nt for the four youngest basins although the mechanisms for producing the surface magnetic field concentrations antipodal to impact basins are not well understood these results indicate a period of strong lunar magnetic fields between 3 85 and 3 6 aeons consistent with lunar sample paleomagnetic data the strong surface magnetic fields near the basin antipodes may have resulted from concentration of a preexisting ambient magnetic field at the antipode by the partially ionized vapor cloud produced in hypervelocity basin forming impacts and acquisition of shock remanent magnetization from the impact of secondary ejecta

initial mapping and interpretation of lunar crustal magnetic anomalies using lunar prospector magnetometer data maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on lunar prospector are presented on the lunar nearside relatively strong anomalies are mapped over the reiner gamma formation on western oceanus procellarum and over the rima sirsalis rille on the southwestern border of oceanus procellarum the main rima sirsalis anomaly does not correlate well with the rille itself but is centered over an imbrian aged smooth plains unit interpreted as primary or secondary basin ejecta the stronger reiner gamma anomalies correlate with the locations of both the main reiner gamma albedo marking and its northeastward extension both the rima sirsalis and the reiner gamma anomalies are extended in directions approximately radial to the center of the imbrium basin this alignment suggests that imbrium basin ejecta materials lying in many cases beneath the visible mare surface are the sources of the nearside anomalies if so then the albedo markings associated with the stronger refiner gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment two regions of extensive magnetic anomalies are mapped in regions centered on the ingenii basin on the south central farside and near the crater gerasimovic on the southeastern farside these regions are approximately antipodal to the imbrium and crisium basins respectively the imbrium antipode anomaly group is the most areally extensive on the moon while the largest anomaly in the crisium antipode group is the strongest detected by the lunar prospector magnetometer a consideration of the expected antipodal effects of basin forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts in both regions the strongest individual anomalies correlate with swirl like albedo markings of the reiner gamma class visible on available orbital photography

a review of lunar paleointensity data and implications for the origin of lunar magnetism the saturation remanence normalization method of paleointensity estimation has been applied to more than 50 lunar samples and has been calibrated in terms of standard intensity determinations done on 14 samples these results suggest that a lunar surface field comparable in intensity to the earth s magnetic field existed from about 3 6 to 3 8 ae absolute paleointensity estimates and lunar surface field strength determinations are consistent with this observation the data presented here differ from previous descriptions of the variation in the lunar field with time in that a the oldest samples 3 9 ae are among the most weakly magnetized and b an order of magnitude drop in paleointensity is suggested between the time of extrusion of the apollo 11 low and high potassium basalts the limited range in age of the strongly magnetized lunar samples and the absence of shocked samples in this set argues against a solar source for the lunar paleofield or for fields arising from transient phenomena such as impacts the coincidence of this high field era with the termination of basin forming impacts and the beginning of mare basalt extrusion suggests a common cause a close approach of the moon to the earth at about 3 8 ae might account for all three phenomena the confined duration of the strong lunar field may allow for the assignment of relative ages for certain lunar samples and surface features

paleointensity determination using anhysteretic remanence and saturation isothermal remanence 1 determining the strength of ancient planetary magnetic field is pivotal to understanding the evolution of planets and asteroids in the solar system while the thellier type double heating technique provides the most faithful field strength information for rocks carrying a thermoremanent magnetization trm many extraterrestrial rock samples respond unfavorably to heat treatment the present study systematically examined the two well known normalization techniques that avoid any heating by comparing the ratios of trm anhysteretic remanent magnetization arm and trm saturation isothermal remanent magnetization sirm both the ratios of trm arm and trm sirm are dependent on the grain size as well as the volume concentration of magnetite the remanence ratios also were found to increase as the alternating field increased during demagnetization for fine grained magnetite a new calibration relation of 2 60 1 32 for the trm arm and of 3 62 1 28 10 2 for the trm sirm was defined for an external field of 50 t the trm sirm is superior to trm arm because the former showed less dispersion in the grain size dependence the volume concentration dependence and the stability against af demagnetization in addition the standard error of the mean for the trm sirm ratio was smaller than that for the trm arm ratio thus whenever heating is inapplicable the sirm method seems to be a better choice than the arm method however it should be emphasized that the uncertainty of trm arm and trm sirm is still nearly an order of magnitude larger than that of the high fidelity thellier estimation and thus must be limited in use only when samples are irreversibly altered during heating in practice the best approach is to carry out both arm and sirm methods and check whether the two estimations agree within the uncertainties

the origin of the moon and geophysical consequences summary the classical theory of tidal friction developed by sir george darwin has been used by a number of investigators to gain information about the origin of the moon by tracing its orbital evolution backward in time the present paper extends tidal theory further by making the dissipation function frequency dependent where frequency is defined as the relative angular rate of the satellite and planet in order to test this modified theory empirically it is first applied to a study of the origin of phobos and deimos the two satellites of mars contrary to earlier results which maintained their present low eccentricity orbits it is found that their initial orbits are highly elongated ellipses suggesting an origin in terms of captured cometary nuclei when the frequency dependent theory even in a two dimensional version is applied to the more complicated earth moon system it leads to a set of dynamical constraints on the origin of the moon which can be used together with geophysical evidence and evidence of the lunar surface structure the most likely origin of the moon is found to be capture as a separate body rather than its formation in the vicinity of the earth from smaller planetesimals earlier developments of tidal theory had required an initial orbit of very high inclination 90 or even greater and initial obliquity of the earth s axis of 90 all of which would have made capture extremely unlikely and further required the dissipation of unreasonable amounts of rotational kinetic energy urey had favoured capture of the moon for geochemical reasons we now support this hypothesis because of dynamical arguments evidence is presented to support the occurrence of intensive heating and volcanism both in the earth and in the moon approximately four billion years ago thus capture of the moon may explain the apparently anomalously early development of the earth s atmosphere and oceans

can the lunar crust be magnetized by shock experimental groundtruth abstract since the first evidence of magnetized lunar crust two mechanisms of magnetization have been suggested to account for lunar magnetism thermoremanent magnetization trm or shock remanent magnetization srm we present here the first experimental acquisition of shock remanence by lunar rocks in the 0 1 2 gpa range and discuss their implications for the interpretation of the paleomagnetic record of these rocks as well as for the distribution of magnetic anomalies revealed by orbital data laser shock experiments in controlled magnetic fields performed on lunar mare basalts demonstrated that in the presence of an ambient field these rocks can be magnetized significantly starting at low pressure 0 1 gpa hydrostatic loading experiments up to 1 8 gpa in controlled magnetic fields were used to impart piezo remanent magnetization an analogue for shock remanent magnetization to mare basalts and highland regolith breccias these experiments allow quantifying the shock remanence as a function of pressure and ambient field regarding the lunar antipodal magnetic anomaly model our results show that lunar soils regolith breccia and about 40 of lunar highland rocks comprising regolith and impact melt breccia in the upper crust can be magnetized by low pressure shocks for typical lunar rocks dominated by multidomain feni with low ni content the maximum remanent magnetization that can be acquired during a low pressure shock

multiple causes of interannual sea surface temperature variability in the equatorial atlantic ocean the tropical atlantic ocean shows sea surface temperature variability on interannual timescales observational and model data suggest that some of this variability can be attributed to the advection of anomalously warm northern subtropical waters toward the equator

the early magnetic field and primeval satellite system of the moon clues to planetary formation from the stable remanent magnetization of the apollo igneous rocks and high grade breccias the existence of a primeval lunar magnetic field was inferred the palaeointensities of the samples rise rapidly to a maximum at 3 9 ga then decrease exponentially to 3 2 ga strongly suggesting that the moon had a field generated in a core the existence of which was inferred from its non hydrostatic figure modelling of the apollo 15 and 16 subsatellite magnetic anomalies by p j coleman l l hood and c t russell gave palaeomagnetic directions of crustal strata this enabled n pole positions to be calculated which were empirically found to form three bipolar groups the mean poles of which define on the core dynamo hypothesis three axes of rotation different from the present these were dated as pre nectarian lower nectarian and upper nectarian imbrian multi ring basins of these ages were found to lie close to the corresponding palaeo equators the impacting bodies were therefore satellites not asteroids or comets their velocities before collision can be shown from basin asymmetries to be nearly equatorial the consequent changes in the moment of inertia tensor by basin formation caused these successive reorientations of the moon relative to its axis of rotation in space the three mean poles form a 90 spherical triangle the explanation is that the moon had three satellites the orbits of each decayed they broke up at the roche limit into smaller bodies which produced impact basins near the equator the moon then reorientated according to euler s principle before the next group of impacts lunar palaeomagnetism and especially the inferences that the moon has an iron core that segregated late and had a primeval satellite system may provide important constraints on theories of lunar and planetary formation

nonlinear thermoremanence acquisition and implications for paleointensity data abstract in paleointensity studies thermoremanence is generally regarded as a linear function of ambient magnetic field at low fields comparable to that of the present day earth we find pronounced nonlinearity at low fields for a class of materials with silicate hosted magnetite that otherwise perform well in paleointensity experiments we model this nonlinearity with narrow size ranges of large acicular single domain grains which are most likely in a vortex state i e nonuniformly magnetized sometimes labeled pseudosingle domain simple trm theory predicts that even certain single domain particles will also exhibit a nonlinear response saturating in fields as low as the earth s such behavior although likely to be rare may bias some paleointensity estimates the bias is especially pronounced when the laboratory field is higher than the ancient field fortunately the fundamental assumption that thermoremanence is proportional to applied field can and should be routinely checked at the end of successful paleointensity experiments by adding two extra heating steps

formation and evolution of a lunar core from ilmenite rich magma ocean cumulates abstract in the absence of comprehensive seismic data coverage the size composition and physical state of the lunar core are still debated it has been suggested that a dense ilmenite rich layer which originally crystallised near the top of the lunar magma ocean may have sunk to the centre of the moon to form either an outer core surrounding a small metallic inner core or a complete core if the lunar metallic iron content is insignificant here we study the formation gravitational stability and thermal evolution of both an ilmenite rich outer core and a full ilmenite rich core using a two dimensional cylindrical thermo chemical convection model gravity acceleration in the lunar mantle decreases quickly with depth since the gravity acceleration directly influences the buoyancy of materials the low gravity acceleration near the centre was explicitly taken into account core formation and evolution were investigated by assessing the effects of varying two parameters the mg density and the internal heat production of the ilmenite rich layer varying these parameters changes the compositional and thermal buoyancy of the dense layer models show that a stable ilmenite rich outer core may indeed have formed in the lunar interior and that its density depends on the internal heating in and the mg of the ilmenite rich layer furthermore the sharpness of the core mantle boundary is shown to depend on the internal heat production in the ilmenite rich material surprisingly a higher internal heat production results in a sharper core mantle boundary and a higher ilmenite content in the core this is caused by lower viscosities as a result of higher temperatures although maximum core temperatures vary between different models by 700 1000 k around 2 gyr after the start of the models present day estimates vary by only about 350 500 k further narrowing of the range of internal heating values is essential for a better determination of both the present day lunar core temperature and its physical state

detailed study of the mare crisium northern magnetic anomaly low altitude lunar prospector magnetometer lp mag data for mare crisium show two magnetic anomalies near its inner northern and southern edges because these features are located inside a basin they were likely formed by slow cooling of the basin s melt or the partially melted mantle instead of by any impact related shock magnetization process therefore they are important for assessing the nature of the ancient dynamo field that produced them in this study we confine our attention to the simpler northern anomaly cna and use low altitude 22 km lp data to model its source body as a dipole and magnetized disks of different radii we infer that the source is likely located 20 30 km from the surface and horizontally localized within a small region 1 or 30 km radius the surface field intensity calculated from the best fit dipole is in good agreement with that obtained from lp electron reflectometer lp er data our magnetization directions are substantially different from two previous studies largely due to using lower altitude data to perform our inversions we also find a surprising sensitivity to small changes in source body latitude 1 the magnetic paleopoles implied by our best fit models are distant from previous estimates by up to 50 of great circle arc and are substantially distant from the moon s present rotation axis our results demonstrate how multiple altitude datasets must be used when estimating paleopoles and other properties of even the simplest of the moon s magnetic anomalies

global mapping of lunar crustal magnetic fields by lunar prospector abstract the lunar prospector electron reflectometer has obtained the first global map of lunar crustal magnetic fields revealing that the effects of basin forming impacts dominate the large scale distribution of remanent magnetic fields on the moon the weakest surface magnetic fields 40 nt are diametrically opposite to these same basins this pattern is present though less pronounced for several other post nectarian impact basins larger than 500 km in diameter the reduced strength and clarity of the pattern for older basins may be attributed to 1 demagnetization from many smaller impacts which erases antipodal magnetic signatures over time 2 superposition effects from other large impacts and 3 variation in the strength of the ambient magnetizing field the absence of fringing fields stronger than 1 nt around the perimeter of the imbrium basin or associated with craters within the basin implies that any uniform magnetization of the impact melt must be weaker than 10 6 g cm3 g 1 this limits the strength of any steady ambient magnetic field to no more than 0 1 oe at the lunar surface while the basin cooled for tens of millions of years following the imbrium impact 3 8 billion years ago

antipodal effects of lunar basin forming impacts initial 3d simulations and comparisons with observations abstract 3d simulations of basin scale lunar impacts are carried out to investigate a the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins and b the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the south pole aitken spa basin the simulations demonstrate that a basin forming impact produces a massive hot partially ionized cloud of vapor and melt that expands thermally around the moon converging near the basin antipode approximately 1 h after the impact for typical impact parameters in agreement with previous work analytic calculations of the interaction of this vapor melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region the time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode the latter produce antipodal shock stresses within the range of 5 25 gpa where stable shock remanent magnetization srm of lunar soils has been found experimentally to occur calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally this suggests that pre existing ejecta materials which would also contain abundant metallic iron remanence carriers may be important anomaly sources a possibility that is consistent with enhanced magnetic anomalies observed peripheral to spa the latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick spa ejecta mantle occurring near the antipodes of the imbrium and serenitatis impacts together with converging seismic compressional waves these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of spa near the imbrium antipode allowing the ascent of magma with enhanced kreep concentrations

how accurately can nrm sirm determine the ancient planetary magnetic field intensity abstract the present study tested grain size dependence of thermoremanent magnetization trm saturation isothermal remanent magnetization sirm as well as its stability against demagnetization the trm sirm ratio is dependent on the grain size of magnetite suggesting that a strong constraint on grain size characterization is necessary to be used as a paleointensity proxy in planetary magnetism in addition the trm sirm ratio increases as the alternating field increases for fine grained magnetite accuracy of trm sirm was investigated using historic showa lava erupted in 1946 it was observed that the natural remanent magnetization nrm sirm ratio of 0 032 is comparable to the field intensity of 46 80 t however the uncertainty of nrm sirm was an order of magnitude larger than that of the companion thellier estimation therefore the nrm sirm ratio can only provide crude estimations on the absolute planetary magnetic field intensity in practice anisotropy correction is advisable to reduce the scatter of nrm sirm

tides and the evolution of the earth moon system summary a model of the tides in a hemispherical ocean is used to investigate the effect of changes in the earth s rotation rate on the power dissipated by the ocean tides the results obtained are then used in an idealized astronomical model to investigate how they affect the history of the earth moon system using the tidal model it is found that at rotation rates higher than that of the present earth the power dissipated by the semi diurnal tides in the ocean drops off rapidly as a result of the increased tidal frequency thus if the earth s rotation rate is doubled from its present value then the rate of energy dissipation in the ocean is reduced to approximately one third of its present value and the tidal torque is reduced by a factor of about 6 the present value for secular acceleration of the moon calculated from the results of the tidal model is 30 5 arcsec century 2 using this value in the astronomical model which has the moon and sun in circular orbits above the equator and assuming that the tidal torque is independent of the tidal frequency the gerstenkorn event is predicted to have occurred 1 3 109 yr ago when the astronomical model is run with a torque determined at all times from the tidal model the reduction in the energy dissipated early in the history of the system leads to a gerstenkorn date of 5 3 109 yr ago however dissipation within the solid earth is found to be important early in the history of the system and when this effect is included it gives a date for the gerstenkorn event of 3 9 109 yr ago

evidence for an early archean geomagnetic field a paleomagnetic study of the komati formation barberton greenstone belt south africa 3 5 ga old peridotitic komatiites of the komati formation barberton greenstone belt south africa have a characteristic nrm natural remanent magnetization that is stable during alternating field and thermal demagnetization the thermal nature of this remanence is demonstrated by successful thellier type paleo intensity determinations on some samples in the temperature range 350 to 575 c the characteristic nrm is an almost total thermal overprint acquired during unroofing and cooling following pervasive greenschist metamorphism at 3470 20 ma the mean direction of magnetization after cleaning is d 326 i 81 k 60 95 8 n 7 sites with a paleopole at 11 s 21 e p m 15 the paleopole falls in southern africa ancestral southern africa was therefore in polar latitudes around 3 5 ga the same position it occupied in the late archean and early proterozoic paleofield intensities are about one half present day intensities for polar latitudes a vigorous earth s magnetic field was already in existence in the early archean implying that the core had a suitable size composition and convective flow to support dynamo action