effet de la vitesse de refroidissement sur l intensite de l aimantation thermoremanente etude experimentale consequences theoriques the experiments reported here compare intensity and thermal resistance of two thermal remanent magnetizations trm successively acquired by the same sample under a magnetic field but at two different cooling speeds the samples one with a base of cro2 powder and the others derived from oceanic basalts were representative of the different types of structure domains characteristic of magnetic grains the experimental procedure established aimed at limiting the various factors of uncertainty mineralogical and structural modifications related to heating inhomogeneity of the sample temperature at crucial stages of each experiment effects of undesirable magnetizations acquired concurrently with the trm the main conclusion of this study is that the trm intensity increases as a function of time of cooling independently of the types of structure domains carried by the magnetic grains the greatest effect is with the cro2 monodomain the trm obtained between 130 and 100 c during a cooling period of 120

magnetism in rocks rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks sediments soils and organisms its applications include environmental magnetism magnetic anisotropy sources of continental and oceanic magnetic anomalies records of geomagnetic field variations and polarity reversals and the paleomagnetic record of plate motions and the wilson cycle this paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas pseudo single domain behavior magnetic domains and micromagnetic structures diagnostic tests of the type and stability of remanent magnetization magnetic microanalysis thermoviscous remagnetization and chemical remanent magnetization other areas including sediment and soil magnetism are covered in a companion paper by verosub and roberts on environmental magnetism

separating multidomain and single domain like remanences in pseudo single domain magnetites 215 540 nm by low temperature demagnetization it has long been hypothesized that the pseudo single domain properties of submicron magnetites are a result of coexisting multidomain and single domain like remanences we have separated these two types of remanence by low temperature demagnetization ltd in which samples are cooled through the low temperature magnetite isotropic point and rewarmed to room temperature all in zero field at the isotropic point the first crystalline anisotropy constant vanishes and multidomain remanence due to displaced domain walls will be unpinned we measured stepwise alternating field af and thermal demagnetization curves of weak field thermoremanent magnetization trm and anhysteretic remanent magnetization and of saturation isothermal remanent magnetization irms both with and without prior ltd for magnetites with mean particle sizes of 215 390 and 540 nm the memory fraction of remanence recovered after ltd which represents the single domain like component decreased from 44 for 215 nm grains to 23 for 540 nm grains in the case of irms memory of trm was approximately twice as large af decay curves were soft and multidomain like before ltd but became harder and single domain like in shape after ltd ltd removed most low unblocking temperature trm and irms distributed low unblocking temperatures are therefore a multidomain phenomenon probably resulting from successive adjustments of domain walls to changing internal fields and pinning strengths during heating

high paleointensities of the geomagnetic field from thermomagnetic studies on rift valley pillow basalts from the mid atlantic ridge nineteen pillow basalts dredged within the rift valley of the mid atlantic ridge at 36 8on were studied by the thellier stepwise heating method in order to determine the paleointensity of the geomagnetic field when they erupted on to the sea floor previously reported fission track ages are 2 000 to 6 000 years for the youngest rocks mainly olivine basalts and 10 000 to 100 000 years for the others mainly plagioclase basalts and pyroxene basalts all but three pillow basalts meet the conditions commonly considered as indicative of quite reliable paleointensity estimates stability of the direction of nrm during its thermal demagnetization constant ratio of nrm trm natural remanent magnetization to thermoremanent magnetization over 50 or more of the original nrm intensity 80 to 94 for 11 specimens and reproducibility of low temperature partial trm ptrm however strong field thermomagnetic measurements indicate that 11 of these 16 samples display a significant increase in curie temperature 15 to 80oc during the paleointensity experiments below 250oc notwithstanding the linearity of the nrm trm plot in this temperature interval this alteration probably due to low temperature oxidation of the specimens seems typical of young pillow basalts and may result in paleointensity estimates which are too high this result shows that excellence of remanence tests nrmtrm linearity and ptrm stability does not ensure the absence of chemical changes during the thellier experiments and therefore the validity of the paleointensity obtained the assumption that reliable paleointensities are obtained when the curie point increase is 10oc or less led to the selection of five of our specimens all from the youngest group their mean paleointensity is 64 2 20 5 xt standard deviation and the corresponding virtual axial dipole moment vadm is 11 5 3 7 x 1022 a m 2 given the variations of the vadm of the earth s magnetic field over the last 6 000 years as established from archeomagnetic studies our paleointensity results suggest that the latest eruptions on the inner floor of the rift valley at 36 8on occurred 1 500 1 000 years ago

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

determination of geomagnetic palaeointensities in vacuum summary specimens with low and intermediate curie points from six hawaiian historic basalt flows were used for the determination of the intensity of the historic geomagnetic field by the thelliers method the specimens were heated either in air or in a vacuum of lo torr the regional intensity of the earth s magnetic field at the time of extrusion of these rocks is known from direct observations the palaeointensities determined in vacuum from four low and intermediate curie point flows are correct within the uncertainty caused by the local field anomalies whereas those determined in air from the same four flows are of lesser quality and accuracy unaltered submarine basalts have similarly low curie points and thus may also be amenable to palaeointensity determination in vacuum the behaviour of the remaining two flows which had higher curie points was more erratic and they yielded less accurate palaeointensities regardless of whether they were determined in air or vacuum the thelliers technique thellier thellier 1959 is one of the most widely used methods for the determination of palaeointensities of the earth s magnetic field from rocks it consists of comparing the thermal spectrum of the natural remanent magnetization nrm of an igneous rock with that of an artifically induced thermoremanent magnetization trm in the same specimen it is assumed that the nrm is a trm acquired during the initial cooling of the rock and that its original thermal spectrum remained unchanged throughout the history of the rock in practice these conditions are not easily met because of two reasons first the nrm acquired by the rock at the time of its formation may be different from that measured in the laboratory because of viscous decay or the superimposition on the nrm of secondary components of magnetization in some cases however this problem may be overcome by thermal cleaning which is an integral part of this technique second when rocks are heated they undergo alterations which affect the chemical composition size and shape of their magnetic mineral grains the effect of these changes may be such that the nrm and trm are produced in two magnetically different materials thus rendering the determination of palaeointensity unreliable because oxidation is the change most likely to occur when heating is carried out in air many authors have recommended the use of high curie temperature rocks which

paleointensity determination magnetic domain structure and selection criteria this special section is focused on the impact of kittel s theory of magnetic domains on rock magnetism and this paper is focused on the implications for paleointensity determinations absolute paleointensity determination is based on specific properties of thermoremanent magnetizations trms mainly the linear dependence of a trm s intensity on the inducing field and the additivity and independence of partial trms these properties are exhibited by populations of noninteracting single domain sd grains but not by populations of multidomain grains the situation is more complex in the pseudo single domain population the most common one in natural rocks usable for paleointensity studies which sometimes exhibit sd like behavior but not always if the experiments were restricted to sd rocks it would not be possible to find enough outcrops to have the spatial and temporal resolution needed for interpretation of the data in terms of geodynamo however as paleointensity determination is very time consuming a preselection of suitable samples is necessary a new procedure will be proposed using a direct test of partial trm s properties with a fully automated experiment fast to implement

deformation mechanisms and stability analysis of undermined sedimentary rocks in the shallow subsurface abstract the deformation mechanisms and the stability of an undermined sedimentary rock layers were studied using the numerical model flac itasca 2006 specifically validated for such configurations the lowermost stratum in a sequence of successive layers is modeled as multi fractured voussoir beam boundary conditions material properties and geometrical constraints for the numerical modeling were chosen to reflect rock mass properties that are characteristic to sedimentary rock masses a series of ordered models spanning over a representative range is presented the indeterminacy parameters of the voussoir beam model are determined using non linear regression of the numerical results over a dimensionless space the results show that the thickness coefficient of the compressive arch n formed within the deflecting beam is non uniform at the abutment na ranges from 0 3 to 0 4 inversely proportional to beam stiffness at the mid span nm ranges from 0 4 to 0 5 snap trough failure of the beam occurs for beams of normalized composite stiffness en

a new quality check for absolute palaeointensities of the earth magnetic field absolute palaeointensity determinations of recent lavas from mt etna have been carried out using the koenigsberger thellier thellier ktt method but with two different data presentations the first which is the standard method compares the natural remanent magnetization nrm demagnetized and the thermoremanent magnetization trm acquired in successive temperature intervals cumulatively the second compares the nrm and trm in the successive intervals in an incremental manner the latter method appears to distinguish aberrant behaviour better than the classical method such aberrant behaviour can be related to changes in hysteresis ratio parameters saturation remanent magnetization to saturation magnetization and remanent coercive force to coercive force which are reflected by displacements in the day et al 1977 plot these changes in hysteresis parameters are caused by irreversible changes in domain state or transformations of magnetic phases brought about by the thermal cycling the changes in hysteresis parameters serve as a useful quality factor for absolute palaeointensity determinations

an improved methodology for thellier type paleointensity determination in igneous rocks and its usefulness for verifying primary thermoremanence we propose a new methodology for thellier type paleointensity experiments in standard paleointensity experiments the blocking temperatures of most stable remanences are often not reached before thermal alteration of magnetic mineralogy begins and subsequent data have to be discarded we emphasize that when alteration begins at low temperature and continues throughout the experiment which then fails in standard analyses the resulting unblocking temperature tub spectrum of the alteration product does not necessarily overlap with the whole tub spectrum of natural remanent magnetization nrm because tub depends on physical characteristics of the new grains not the temperatures at which they were formed nrm may survive in an uncontaminated higher tub window if alteration remanence only has low tub then partial thermoremanent magnetization ptrm checks at temperature step tj 1 can be used to correct for alteration that has occurred at tj and furthermore thermal demagnetization of a full trm acquired in the laboratory at the end of the progressive thellier thellier experiment should reveal the true uncontaminated tub spectrum of the nrm in higher temperature intervals we propose a new experimental sequence which allows monitoring and correction of alteration and provides two semi independent estimates of palaeointensity in the circumstances described above we illustrate the issues involved through new experimental work on igneous rocks from a paleozoic charnockitic syenite where we unravel trm from multidomain isothermal remanence despite massive alteration during laboratory heating finally we point out the value of these experiments for verifying primary trm in igneous rocks indicating properties that distinguish trm from other remanence

determination of geomagnetic palaeointensities from the quaternary west eifel volcanic field germany summary determination of geomagnetic palaeointensities has been attempted for 418 specimens from 51 volcanic units of the quaternary west eifel volcanic field this paper describes the measuring procedure using a modification of the thellier method the analysis of the derived data points the determination of linear segments and the quality of the individual palaeointensity values calculated from the corresponding slopes according to a series of acceptance and rejection criteria finally 312 reliable individual palaeointensity results have been obtained about 73 per cent of the accepted regression lines have correlation coefficients better than 0 990 and less than 10 per cent of the accepted thellier experiments have quality factors less than 1 0 the carriers of magnetization of the west eifel volcanic rocks are titanomagnetites with a broad range of titanium contents and oxidation states in a more or less unweathered mostly foiditic rock matrix accompanying rock magnetic experiments allowed a rough classification of the specimens corresponding to the oxidation state of the titanomagnetites specimens with highly oxidized titanomagnetites have been the most successful candidates for the thellier experiments however a prediction of the suitability of the specimens for the thellier experiments by sample selection according to this classification was not always appropriate instead for most volcanic units pilot specimens were first measured to determine the suitability of a specific sample type the temperature interval from which the evaluation of palaeointensity was possible and the best laboratory field intensity to keep the slope errors of the thellier experiments to a minimum palaeointensity determination was not possible for 14 volcanic units because only unsuitable specimens were available for each of the 37 remaining volcanic units at least five successful thellier experiments have been carried out leading to a final mean palaeointensity result these 37 values have relative standard errors of between 4 and 23 per cent mostly around 10 per cent specimens with differing rock magnetic properties from the same volcanic unit yielded consistent palaeointensity values therefore all west eifel palaeointensity results have to be regarded as reliable estimates of the earth s magnetic palaeofield strength mean palaeointensities range from 7 to 62 pt with an unexpectedly large number of palaeointensity values less than 30 pt about half of the low palaeointensity values are associated with intermediate virtual geomagnetic pole latitudes which may be due to a geomagnetic excursion

absolute paleointensity and magnetomineralogical changes determinations of absolute paleointensity are often hampered by magnetomineralogical changes produced within samples of lava flows during successive heatings such changes which directly affect the capacity for a sample to acquire a partial thermoremanent magnetization ptrm are reflected by negative ptrm checks a deviation between the original ptrm and a repeated measurement after heating at a higher temperature because the deviations indicated by the ptrm checks can provide a direct estimate of the effects to the ptrms we suggest that they can be used to correct ptrms the correction method we present requires additional measurements in particular ptrm checks performed after each ptrm and specific diagrams to detect acquisition of chemical remanence performing multiple successive ptrm checks can also provide information on the blocking temperatures of the alteration product these experiments conducted on lava flows from different localities show that the magnetomineralogical changes mainly involved grains with blocking temperatures lower than the last heating step the corrections have been tested on recent and historic lava flows from several localities we have also compared results on samples from the same lava flows that were heated in air and in vacuum the mean paleointensity values were found to lie within less than 10 from the expected field intensity at each site the corrections resulted in the recovery of reliable paleointensity determinations for about twice as many specimens heated in vacuum and 65 additional data for specimens heated in air successive heatings at the same temperature in air are often accompanied by a time dependent oxidation of the natural remanent magnetization which is responsible for paleointensity values lower than the expected field a problem that is not related to the corrections we conclude that corrections can be used with fairly high confidence to the ptrms performed in vacuum and in air

experiments on trm intensity dependence on cooling rate palaeointensity studies have an important role to play in understanding the origin and time variation of the geomagnetic field however theoretical considerations indicate that the intensity of magnetization recorded by a rock or archaeological sample will be dependent on the rate at which it cooled as well as field magnitude experiments on synthetic magnetite and titanomagnetites of varying grain sizes indicate that for acicular magnetite at low concentration slower cooled trm is stronger than faster cooled trm but for all other samples slower cooled trm is weaker this is the predicted behaviour however the magnitude of the effect is much greater than the predicted kinetic effects and indicates that the magnetization of sub microscopic magnetite rich inclusions in the titanomagnetites strongly controls the trm intensity of the matrix

intensity of the earth s magnetic field since precambrian from thellier type palaeointensity data and inferences on the thermal history of the core summary we present a compilation of palaeointensity data obtained by the thellier method from magmatic rocks up to 3 5 gyr old no apparent very long term variation occurs from present to early precambrian the overall data are compatible with a gravitationally powered dynamo model in which the inner core is some 4 0 gyr old however palaeointensity data are still too incomplete to dismiss the possibility of a more complex field history with a geodynamo first thermally driven then gravitationally powered despite the limited number of data currently available large differences in the dipole field moment are clearly documented with a time scale of the order of 50 100myr the changes in intensity can be due to repetitive instabilities of the thermal boundary layer at the base of the mantle

paleointensity of the earth s magnetic field in the jurassic new results from a thellier study of the lesotho basalt southern africa thellier paleointensity experiments carried out on the jurassic 180 m y old karoo basalt lesotho southern africa provided paleointensity estimates for 40 samples from 10 distinct lava flows with reversed magnetization mean paleointensity was found to be equal to 24 2 11 3 t standard deviation with a minimum of 12 t and a maximum of 41 3 t this corresponds to a mean virtual dipole moment vdm of 4 6 2 0 1022 am2 the between flow dispersion is ascribed to paleosecular variation which thus would approach up to 40 of the mean paleointensity as is observed from the published paleointensity data set covering last 20 m y this result yields further evidence for the period of low geomagnetic field in jurassic to early cretaceous time mesozoic dipole low a symmetrical intensity versus time pattern is suggested for this time interval with a minimum vdm of 3 1022 am2 in late jurassic and substantially higher 4 5 1022 am2 vdms in early to middle jurassic and early cretaceous respectively

changes in trm and arm in a basalt due to laboratory heating abstract laboratory heating in air often results in irreversible changes of remanence properties in natural rocks such an effect was traced for a recent basalt sample containing titanomagnetite x 0 4 as the main remanence carrier for heating times between 3 and 300 min it is shown that the change can be explained by the scheme of high temperature oxidation titanomagnetite psd md iron rich phase sd ilmenite remanent magnetizations in both the original and heated samples are the mixtures of two components high blocking temperature high coercivity fraction and low blocking temperature low coercivity fraction the trm and arm show distinctly different behaviors in this change showing the limitation of using the arm as an analog of the trm

paleomagnetism of the lesotho basalt southern africa paleomagnetic study of four sections from middle jurassic karoo lava located in lesotho and south africa yielded a new paleomagnetic pole for africa with coordinates 71 6 n 273 5 e a95 3 7 k 33 n 47 paleosecular variation estimated as the angular standard deviation of vgps distribution is 14 1 2 4 1 8 which is considerably lower than expected for this time interval and latitudinal band preliminary paleointensity results indicate a low field intensity during the acquisition of the magnetization this new result tends to support a possible correlation between secular variation and dipole intensity when the lower to middle jurassic paleomagnetic data set for africa is considered a difference of 13 9 8 7 of arc is found to exist between the mean northwest and south african poles furthermore while the northwest african pole is in very good agreement with the lower to middle jurassic north american pole whichever set of rotation parameters is used to close the north atlantic ocean the south african pole falls some 12 15 of arc apart as intraplate deformation models developed to achieve tighter fit of west gondwana prior to the opening of the south atlantic fail to account fully for the difference between northwest and south african poles the pre cretaceous apparent polar wander path for africa has to be considered with caution

slope movement crisis on the east flank of mt etna volcano models for eruption triggering and forecasting abstract benchmarks installed on the upper eastern flank of mt etna in 1982 have subsided continually since then with the rate of subsidence twice accelerating prior to eruptions the first of these eruptions was in december 1985 and the second in september 1989 this pattern of accelerating downslope movement has also been observed prior to landslides and recent work applying knowledge of the failure of materials has shown that analysis of the inverse rate of these movements can be used to predict the time of failure post eruption analyses of geodetic or seismic data from volcanoes has shown that in several cases this approach could have been used to forecast eruptions weeks in advance applying the same principles to the accelerating subsidence on mt etna s eastern flank prior to the eruptions of 1985 and 1989 shows that rough estimates of eruption dates could have been obtained several months in advance these observations also suggest a speculative eruption triggering mechanism involving an interplay between slope creep deformation and extensional weakening over the zone of intrusion

thermal demagnetization of partial thermoremanent magnetization three well characterized glass ceramic samples containing magnetite crystals of different domain states have been given partial thermoremanent magnetizations ptrms in the temperature interval 400 350 c for varying lengths of time 10 min to 5 days and in magnetic fields from 0 5 to 5 oe 0 05 0 5 mt the thermal demagnetization behavior of the ptrms changes with domain state in the sense that with decreasing coercivity increasing grain size demagnetization curves become less steep and may not completely demagnetize until heated to the curie temperature numerical calculations based on neel s single domain sd theory invoking field dependence and experimentally determined as well as theoretical volume distributions based on power laws have been performed to predict viscous magnetization acquisition and demagnetization temperatures we observe that experimental results do not match any of the theoretical predictions there is no discernible field dependence of demagnetization curves for the sd and pseudo single domain samples and even for the soft multidomain md sample in the field range investigated an important consequence for thermally derived paleointensity data is that they may not be entirely reliable especially if the sample remanence is due to md grains

possible mechanisms causing failure of thellier palaeointensity experiments in some basalts summary the normally magnetized zone of the jurassic lesotho basalts although providing apparently quite reliable palaeofield directions kosterov perrin 1996 shows anomalous behaviour when studied in vacuum using the thellier palaeointensity method typically the slope of the natural remanent magnetization thermoremanent magnetization nrm trm curves is very steep at intermediate temperatures 200 to 400 460 c in order to elucidate the reasons for such an anomalous behaviour six representative samples from a total of 74 studied using this method were subjected to a variety of analyses these experiments indicate that the magnetic properties are dominated by pseudo single domain psd magnetite grains some 1 mm in size resulting from high temperature oxidation of titanomagnetite laboratory heatings in vacuum up to the curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k similarly the k t curves in vacuum are with a single exception rather reproducible since the laboratory trms yield almost ideal nrm trm plots the anomalous nrm trm plot is presumably due to some peculiarity of the natural trm the partial trm ptrm acquisition capacity in the moderate temperature range cooling from 200 to 20 c is generally very strongly reduced after heating to 270 c which indicates that some magnetic alteration has already occurred at these temperatures hysteresis measurements between room temperature and the curie temperature t c show that some small less than 10 per cent but significant irreversible changes in hysteresis characteristics also occur during heating in particular the coercive force h c0 at room temperature is typically reduced after heating at a moderate temperature 175 c but increases after treatments at 475 c and more pronouncedly at 580 c the saturation magnetization j s0 remains unchanged except for a very small decrease less than 5 per cent occurring in some samples after the two latter treatments these changes are most clearly seen on h c t j s t bilogarithmic plots which show that the moderate temperature change in coercivity can extend up to 200 250 c thus hysteresis measurements as a function of temperature over a promising tool for sample pre selection for thellier experiments alternating field demagnetization and cycling of ptrms at liquid nitrogen temperature suggest that the blocking mechanism is largely multidomain like near room temperature but becomes less so as the curie point is approached the main reason for the failure of the thellier experiments is the loss of a fraction of the nrm natural trm at temperatures apparently lower than the blocking temperatures in nature it is suggested that this anomalous behaviour results from the reorganization of the domain structure of the psd grains during heating this transformation which seems to be triggered by the coercivity decrease observed at very moderate temperatures can reduce the nrm intensity without requiring any correlated ptrm acquisition