archeomagnetic study of seven contemporaneous kilns from murcia spain new archeointensity values have been determined from seven spanish kilns sampled at the same archeological site and dated by archeological constraints as between 1100 and 1200 a d the directions of the characteristic remanent magnetization and paleointensities have been obtained from classical thellier experiments conducted on 69 samples collected from the walls of the kilns thermoremanent magnetization trm anisotropy and the cooling rate effect upon trm intensity acquisition have been investigated in all the samples differences between mean archeointensities per site before and after corrections for cooling rate are between 3 and 17 the study of these seven contemporaneous kilns coming from the same archeological site and treated in the same way in the laboratory allows us to discuss the different factors that can cause the dispersion between sites generally observed in archeointensity studies the results confirm on one hand the need for cooling rate corrections in this kind of study and on the other hand the need to obtain several archeointensity determinations per century in order to construct a reliable archeointensity curve finally the new data provide a robust mean intensity 60 3 3 3 t at the latitude of paris for the middle of the xii century in western europe and constrain better the evolution of the geomagnetic field intensity during medieval times in this area

first archaeomagnetic secular variation curve for the iberian peninsula comparison with other data from western europe and with global geomagnetic field models 1 a first secular variation sv curve for the iberian peninsula was computed by hierarchical bayesian method using a total of 134 archaeomagnetic directions with ages ranging from 775 to 1959 a d a general agreement is observed between the iberian curve and the french and german sv curves although some interesting differences were found such as the occurrence of lower inclinations between the 11th and 14th centuries in the iberian curve the analysis of these three reference curves indicates that sv in western europe is characterized by three major directional changes at 125 200 and 1350 a d it is suggested that these cusps are regional features of the geomagnetic field the iberian curve has been compared with the predictions of the jackson calsk7k 2 and hongre global models despite large differences recognized between these models even for the dipolar terms they predict reasonably well the iberian archaeomagnetic sv

paleomagnetism from deception island south shetlands archipelago antarctica new insights into the interpretation of the volcanic evolution using a geomagnetic model deception island shows the most recent exposed active volcanism in the northern boundary of the bransfield trough the succession of the volcanic sequence in the island is broadly divided into pre and post caldera collapse units although a well constrained chronological identification of the well defined successive volcanic episodes is still needed a new paleomagnetic investigation was carried out on 157 samples grouped in 20 sites from the volcanic deposits of deception island south shetlands archipelago antarctic peninsula region distributed in 1 volcanic breccia 3 sites and lavas 2 sites prior to the caldera collapse 2 lavas emplaced after the caldera collapse 10 sites and 3 dikes cutting pre and the lowermost post caldera collapse units 5 sites the information revealed by paleomagnetism provides new data about the evolution of the multi episodic volcanic edifice of this quaternary volcano suggesting that the present day position of the volcanic materials is close to their original emplacement position the new data have been combined with previous paleomagnetic results in order to tentatively propose an age when comparing the paleomagnetic data with a global geomagnetic model despite the uncertainties in the use of averaged paleomagnetic data per volcanic units the new data in combination with tephra occurrences noted elsewhere in the region suggest that the pre caldera units f1 and f2 erupted before 12 000 year bc the caldera collapse took place at about 8300 year bc and post caldera units s1 and s2 are younger than 2000 year bc

secular variation of the earth s magnetic field in the balkan region during the last eight millennia based on archaeomagnetic data summary the first archaeomagnetic secular variation sv curves for the whole southern balkan peninsula are presented these are based on all data within a 700 km circle centred at thessaloniki 40 60on 23 00oe this data set consists of 325 directional and 625 intensity data mainly from greece bulgaria serbia and southern hungary some data from southern italy are also included the sliding moving window technique was used to calculate a continuous sv curve for intensity while the directional sv curves were calculated using the bivariate extension of the fisher statistics these curves are well constrained and clearly show the main features of the geomagnetic field variation in this region during the last eight millennia comparisons with the predictions of the scha dif 3k and scha dif 8k regional and the cals7k 2 and arch3k 1 global geomagnetic field models show a good agreement for the last 3000 years but differences for older times the balkan sv curves identify several rapid changes of the geomagnetic field in eastern europe and can be used as reference curves for archaeomagnetic dating in the balkan peninsula

a german catalogue of archaeomagnetic data summary a catalogue has been compiled of existing published and unpublished archaeomagnetic directional data from sites in germany the data comprise 125 results dated mainly in the past two millennia the stability of the natural remanent magnetization was proven for most structures with at least a thellier viscosity test although for the majority of the data the direction is based on the characteristic remanent magnetization obtained from demagnetization experiments rock magnetic experiments carried out on the samples from many of the sites reveal that the dominant magnetic carrier is magnetite often oxidized or with impurities for many sites the archaeological age estimate is supported by physical dating methods while the roman epoch 0 400 ad and the period from medieval to modern times 800 1700 ad are reasonably well covered with data the time interval in between and the first millennium bc are only poorly covered the geographical distribution of data throughout germany shows a concentration along the rhine valley during roman times with in general a better coverage to the north nevertheless this data set clearly shows the secular variation during the past three millennia and it extends the european archaeomagnetic data set considerably

the igneous petrology and magmatic evolution of the midcontinent rift system klewin k w and shirey s b 1992 the igneous petrology and magmatic evolution of the midcontinent rift system in p a ziegler editor geodynamics of rifting volume ii case history studies on rifts north and south america and africa tectonophysics 213 33 40 the 1100 ma midcontinent rift system of north america is one of the world s great igneous provinces containing over one million km3 of mafic volcanic and plutonic rocks volcanic rocks range from picrite to rhyolite although tholeiitic basalt predominates plutonic rocks range from peridotite to granite with plagioclase rich gabbros and troctolites being most abundant the igneous history of the rift can be divided into early and late stages flows of the early stage form a trend having low alumina high magnesia picrite as the primitive endmember whereas flows from the late stage form a trend having high alumina olivine tholeiite as the primitive endmember the early basalts have enriched nd isotopic compositions and were derived largely from old heterogeneous subcontinental lithosphere the late basalts have chondritic to depleted nd isotopic compositions and were derived from large degree partial melts of a mantle plume that had mixed with isotopically depleted asthenospheric mantle apparently melting began initially in the subcontinental lithosphere as it was stretched subsequently a lower mantle plume which had entrained asthenospheric mantle during its ascent rose beneath the rift and melted at large degrees to yield the voluminous later basalts

geochemistry and paleomagnetism of keweenawan basalt in the subsurface of nebraska abstract basalts of keweenawan age mesoproterozoic extend from the lake superior region southwestward into the subsurface of nebraska and kansas one of the best examples of these subsurface basalts occurs in the williams 1 radenslaben well in saunders county nebraska the well is located on the flank of a linear magnetic high within the confines of the midcontinent rift system a core from the well preserves 186 continuous feet 56 7 m of keweenawan basalt five lava flows are present in the core the flows have a coarse ophitic texture and consist of labradorite augite fe ti oxides olivine pseudomorphs chlorite the primary mineralogy is well preserved in the top three flows a through c which represent almost 75 of the total core interval sporadic small patches of low grade secondary minerals are concentrated in the upper portions flow tops of flows a through c but decrease to minimal amounts in the massive flow interiors in contrast the bottom two flows d and e and an alteration zone on the top of flow b show the effects of incipient burial metamorphism and contain chlorite albitized plagioclase prehnite epidote and k feldspar megacrysts a paleomagnetic reversal is recorded in the middle of flow c the rock below the reversal has reversed polarity and the rock above normal polarity there is no evidence that any part of the core has been faulted the original stratigraphic sequence is preserved the age of the reverse normal polarity in these rocks is not constrained but is similar in sequence to the main keweenawan reversal recorded at 1097 ma the geochemical data suggest however that the polarity reversal may be later and record an event that is not presently recognized in the exposed flows of the lake superior region the overall tholeiitic character of these basalts is demonstrated by their primary mineralogy cipw normative chemistry and immobile trace element concentrations trace element abundances in flows a b and c are primary and have not been modified by the burial metamorphism when normalized to chondrites the trace elements form in a tight coherent array however flows d and e clearly show the chemical effects of the burial metamorphism and some of the trace elements for example ba rb sr na and to a lesser extent k have been mobilized the radenslaben basalts have geochemical similarities to e morb but with an additional lree and lile enrichment component they are similar in trace element and nd isotopic characteristics to the main later sequence of keweenawan flood basalts exposed at lake superior in containing relatively depleted lree and lile enrichment vectors and initial positive nd values

recent achievements in archaeomagnetic dating in the iberian peninsula application to roman and mediaeval spanish structures archaeomagnetic studies in spain have undergone a significant progress during the last few years and a reference curve of the directional variation of the geomagnetic field over the past two millennia is now available for the iberian peninsula these recent developments have made archaeomagnetism a straightforward dating tool for spain and portugal the aim of this work is to illustrate how this secular variation curve can be used to date the last use of several burnt structures from spain four combustion structures from three archaeological sites with ages ranging from roman to mediaeval times have been studied and archaeomagnetically dated the directions of the characteristic remanent magnetization of each structure have been obtained from classical thermal and alternating field af demagnetization procedures and a mean direction for each combustion structure has been obtained these directional results have been compared with the new reference curve for iberia providing archaeomagnetic dates for the last use of the kilns the inferred ages obtained from this study are consistent with the archaeological evidence available

estimate of the magnetic anisotropy effect on the archaeomagnetic inclination of ancient bricks abstract the magnetic fabric of 59 bricks coming from 5 ancient kilns has been studied by measuring the anisotropy of magnetic susceptibility ams and the anisotropy of isothermal airm anhysteretic aarm and thermal atrm remanent magnetization the bricks are characterized by a well developed magnetic fabric that matches their flat shape the shape of the anisotropy ellipsoids is in almost all cases oblate with the maximum and intermediate axes lying parallel to the large face of the brick and the minimum axis perpendicular to it the directions of the principal axes are almost the same irrespectively of the type of anisotropy measured whereas the degree of anisotropy of the airm aarm and atrm is much higher than the ams as the bricks lie horizontally within the kiln the planar magnetic fabric results in an inclination shallowing of the archaeomagnetic direction with respect to that of the earth s magnetic field at the time of their last cooling estimation of this effect on the grounds of atrm measurements yields a shallowing that varies from 4 to 10 for individual samples such inclination difference may significantly bias archeomagnetic dating for the case of the canosa late roman kiln it leads to a dating error of more than two centuries

petrology of the keweenawan mamainse point lavas ontario petrogenesis and continental rift evolution the mamainse point formation forms a 5250 m thick sequence of flood basalts ranging from high mgo picrites and basalts to basaltic andesites that formed in the keweenawan 1100 ma midcontinent rift of north america this section is the most continuous and complete sequence of keweenawan lavas and records almost the entire igneous history of the rift the section consists of numerous stratigraphic groups of lavas that have distinct chemical characteristics a variety of petrogenetic processes including partial melting of different sources at decreasing depth periodically replenished tapped and fractionated magma chambers crustal contamination and polybaric fractionation were responsible for the evolution of the magmas the dominant petrogenetic process changes with stratigraphy which suggests that the physical evolution of the rift controlled which process operated at any time the initial fracturing of the crust presumably allowed high mgo liquids generated at depth to be erupted the early stage of rifting was apparently slow and allowed the formation of long lived magma chambers in which complex processes such replenishment mixing and assimilation contamination could occur the late stage of rifting however was apparently vigorous and allowed voluminous magmas from throughout the crust to be erupted these conclusions from the chemostratigraphy agree with the physical stratigraphy which indicates a change in tectonic style from an early stage of broad subsidence to a late stage of active graben formation

paleointensity of the geomagnetic field recovered on archaeomagnetic sites from france abstract twenty new paleointensity determinations have been obtained from archaeomagnetic sites from france time intervals investigated range from 65 to 380 ad and from 1370 to 1700 ad two hundred forty two samples of tiles and bricks were treated with the classical thellier method for paleointensity determination rock magnetic studies identify magnetite substituted maghemite and hematite as the main magnetic minerals as is usual with archaeomagnetic samples especially tiles magnetic anisotropy was observed in all samples the effect of thermoremanent magnetization trm anisotropy upon the paleointensity values was corrected after the determination of the tensor of trm anisotropy in each sample these determinations allow corrections of the field strength obtained which are usually lower than 20 the effect of the cooling rate on the trm intensity acquired was also determined and seems relatively low on our samples most frequently eight to 10 samples per site were used to calculate the mean intensity standard deviations are typically lower than 7 of the average value during the roman period small variations of the field intensity in paris are observed between 62 and 70 t while during the more recent period the intensity of the field decreases from 62 to 52 t an analysis of the paleointensity data available for the last 2000 years obtained from sites in western europe was performed using a weighing factor which takes into account the number and type of the samples studied as well as the technique used for the paleointensity determination this weighing factor clearly shows that most of the existing data might not be reliable the analysis performed indicates that more data is needed especially from the time interval 500 to 1400 ad before it would be possible to determine a reference curve of the time variations of the archaeomagnetic intensity of the field in western europe

chemostratigraphy of the neoproterozoic alona bay lavas ontario a basal sequence of flood basalt lavas associated with the neoproterozoic midcontinent rift system crops out in alona bay along the southeastern shore of lake superior in ontario the alona bay lava succession is about 1200 m thick and lies just north of the well studied contemporaneous mamainse point formation detailed chemostratigraphy of the alona bay lavas suggests they are grossly correlative with the basal portion of the mamainse point formation for instance like the basal part of mamainse point formation the alona bay section contains numerous high mgo lavas and can be subdivided into 4 5 groups with distinct chemical characteristics chemical variations within the alona bay groups are largely the result of fractional crystallization likely at moderate pressures one small group of alona bay lavas also carries the compositional imprint of crustal contamination the remaining inter group chemical distinctions at alona bay are the consequence of temporal changes in partial melting and source ch

ensembles of low degree archeomagnetic field models for the past three millennia abstract we introduce ensembles of time varying archeomagnetic field models consisting of a reference model a mean model and a thousand individual models we present a set of three such ensembles built from archeomagnetic volcanic and sedimentary data sets that cover the past three millennia these ensembles can be used to describe the field at any location from the core surface to the magnetosphere and assess the way uncertainties due to the limited distribution and quality of the data affect any of its component or parameter such as individual gauss coefficients they provide alternative and we argue more complete descriptions of the archeomagnetic field to those provided by previously published archeomagnetic field models being better suited to existing and emerging needs such as those of geomagnetic data assimilation we present the data sets we rely on essentially the same as those used by other recent archeomagnetic field models and describe how errors affecting the data and errors due to non modelled small spatial scales of the field are taken into account we next explain our modeling strategy and motivation for building low degree spherical harmonic degree ensembles of models we carry on a number of end to end simulations to both illustrate the usefulness of such ensembles and point at the type of errors one should expect practical illustrations of what can be done with these three ensembles of models with examples of geomagnetic inferences are also described northern high latitude flux patches for instance appear to be the most robust features of all these patches tend to fluctuate but clearly have some favored locations resulting in the same clear signature with three tongues over northern america europe and asia in the time averaged field at the core mantle boundary similar to what had been found in earlier models inferences about the field behavior in the southern hemisphere are more difficult to draw still some suggestions that the well known present south atlantic reversed patch could have arisen as early as in 1500 a d are found in some of the ensembles we otherwise confirm that most of the current archeomagnetic field model limitations are related to a number of sediment cores identified as producing frequent outliers in the modeling process we provide evidence that such cores are likely affected by timing errors and timing delays between magnetization lock in and sediment deposition that future more advanced treatment should be able to handle all source files for the three ensembles of models together with appropriate matlab applications can be downloaded from http geomag ipgp fr download archeo fm zip

rock magnetic analyses as a tool to investigate archaeological fired sediments a case study of mirador cave sierra de atapuerca spain summary here we report a detailed mineral magnetic study of neolithic burnt levels in the mirador cave sierra de atapuerca burgos spain to reconstruct the burning history and to investigate their suitability for archaeomagnetic purposes as a consequence of the ancient burning a characteristic facies sequence was developed along the holocene stratigraphy from top to bottom it includes i 2 10 cm ash layers ii 2 cm underlying rubefied layers and iii clay mainly unburnt and of variable thickness in some cases a thermally altered facies 2 6 cm with a heterogeneous texture was identified facies tf usually between rubefactions and the unburnt levels 126 oriented samples from 4 units mir12 15 18 and 21 and a 2 m section all comprised between units mir21 6380 40 14c bp and mir9 5090 40 14c bp were analysed with rock magnetic methods in addition bulk sediment from each facies that comprise the neolithic sequence was investigated measurements included stepwise alternating field and thermal demagnetization of natural remanent magnetization nrm viscosity experiments determination of the anisotropy of the magnetic susceptibility ams the susceptibility frequency dependence at room temperature and determination of the temperature dependence of the susceptibility additional experiments consisted in the determination of the behaviour of anhysteretic and isothermal irm remanences magnetic hysteresis loops first order reversal curve diagrams and thermal demagnetization of three axial irm it appeared that the facies all show a fairly similar magnetic mineralogy and grain size dominated by low titanium magnetite that is often partially maghaemitized main differences constitute the amount of superparamagnetic particles that is higher in unburnt strata concurring with a less well defined nrm behaviour the magnetic mineral concentration is notably higher in ashes this homogeneity strongly suggests that similar sources and burial conditions prevailed during neolithic times agreeing with archaeological observations and favoured by rapid burial conditions very limited alterations have been deduced ams data revealed the absence of fluid flow in the ash lenses sampled demagnetization revealed a stable single nrm component in ashes a single or two component nrm in rubefactions and less stable multicomponent behaviour in clays in ashes magnetic minerals are likely secondary in origin formed by low temperature oxidization soon after burning although this thermochemical nature of the nrm invalidates the use of these sediments for palaeointensity studies archaeomagnetic directional data can be successfully obtained because the burning and oxidation are closely confined in time

testing the reliability of detrital cave sediments as recorders of paleomagnetic secular variations seso cave system central pyrenees spain abstract a paleomagnetic study has been carried out on a waterlaid detrital sedimentary sequence of 240 cm thick within the seso cave system west central pyrenees in these sediments seven charcoal samples were dated using 14c ams ranging from 2080 to 650 cal yr bp 130 bc 1300 ad two levels of human occupation of the cave have been recognized by ceramics associated to the iberian period and to the roman period respectively the detrital sedimentary sequence is made of autochthonous piping detached material from the eocene marls host rock inside of the cavity and allochthonous stream transported sediments from the outside sediments the autochthonous material first 100 cm made of fine grain laminated sediments lutites and marls corresponds to pond facies the allochthonous material 190 240 cm is made of lutites and sands and corresponds to stream facies and both facies are mixed from 100 to 190 cm the increase in sedimentation rate towards the end of the sequence stream facies points to an intensification of the alluvial activity as a possible consequence of a more arid climate during the medieval climate anomaly for the paleomagnetic study 44 discrete cylindrical samples were taken along the detrital sequence the values of the natural remanent magnetization and magnetic susceptibility are significantly lower in the pond sediments than in the stream sediments the declination and inclination of the paleomagnetic characteristic component sister samples analyzed by both alternating field and temperature demagnetizing procedure of each depth point is compared to the spanish archeomagnetic catalog and available geomagnetic models arch3k 1 cals3k 4 cals10k 1b and scha dif 3k in order to determine the accuracy of these sediments recording the earth s magnetic field results suggest that these sediments poorly record the earth s magnetic field however paleomagnetic inclination shows similar results between both demagnetizing methods and the inclination is well recorded especially in the younger stream facies the lack of archeological remains with absolute dates from 925 to 1545 cal yr bp in the iberian paleomagnetic secular variation reference curve has prevented up to now the study of that time period therefore the inclination data from the seso cave deposit is the first record of the iberian paleomagnetic secular variation during most part of the medieval time and they are closer to the inclination values of one geomagnetic model cals10k 1b

stratigraphy and geochemistry in the proterozoic mafic volcanic rocks of the nagu korpo area sw finland the volcanic rocks of the nagu korpo area in sw finland form a thin 1000 m formation that is folded into narrow synforms separated by migmatite filled antiforms in nagu the volcanic formation comprises three units a lower unit of subvolcanic banded gabbro sills a middle unit of amphibolitic volcanic rocks and an upper unit consisting of a thin ultramafic volcanic rock layer the volcanic formation is interlayered within a thicker sequence of metasedimentary gneisses geochemically the metavolcanic rocks are tholeiitic and plots based on ti y zr and other trace elements show that they resemble recent within plate lavas morb and mantle normalised trace element plots show patterns strongly differing from those of recent arc lavas indicating a tectonic environment of plate boundary related initial rifting the upper ultramafic lavas are enriched in lree relative to the lavas of the much thicker amphibolitic middle unit below a model for the initial formation of the volcanic rocks of the svecofennian fold belt must acknowledge both the episode of initial rifting and the later evolution of an island arc like environment

updated iberian archeomagnetic catalogue new full vector paleosecular variation curve for the last three millennia authors acknowledge the spanish ministry of economy cgl2014 54112 r and cgl2017 87015 p projects bes 2015 074575 and bes 2016 077257 grants the spanish ministry of education fpu14 02422 grant and the autonomous community of madrid through the european youth employment initiative pej15 amb ai 0203 m g p and m r acknowledge the ramon y cajal program and the cgl2015 63888 r project of the spanish ministry of economy and competitiveness financial support was also given by the pics international program for scientific cooperation cnrs france and csic spain

a catalogue of spanish archaeomagnetic data a total of 58 new archaeomagnetic directions has been determined from archaeological structures in spain together with five previous results they allow the compilation of the first archaeomagnetic catalogue for spain which includes 63 directions with ages ranging between the 2nd century bc and the 20th century ad characteristic remanence directions have been obtained from stepwise thermal and alternating field demagnetization the hierarchical structure has been respected in the calculation of the mean site directions rock magnetic experiments reveal that the main magnetic carrier is magnetite or titanomagnetite with different titanium contents the age estimate of the studied structures is generally well justified by archaeological constraints for six structures the proposed date is also supported by physical methods the data are in close agreement with the french secular variation sv curve this catalogue represents the first step in the construction of a sv curve for the iberian peninsula which will be of much use in archaeomagnetic dating and in modelling of the earth s magnetic field in western europe

continuous geomagnetic field models for the past 7 millennia 2 cals7k 1 we present two continuous global geomagnetic field models for recent millennia cals3k 2 covering the past 3000 years and cals7k 2 covering 7000 years from 5000 bc to 1950 ad the models were determined by regularized least squares inversion of archeomagnetic and paleomagnetic data using spherical harmonics in space and cubic b splines in time they are derived from a greatly increased number of paleomagnetic directional data compared to previous efforts and for the first time a significant amount of archeointensity data is used in this kind of global model allowing the determination of evolution of geomagnetic dipole strength while data accuracy and dating uncertainties remain a limitation reliable low resolution global models can be obtained the results agree well with previous results from virtual axial dipole moment vadm studies from archeomagnetic intensity data apart from a systematic offset in strength a comparison of model predictions with the previous 3000 year model cals3k 1 gives general agreement but also some significant differences particularly for the early epochs the new models suggest that the prominent two northern hemisphere flux lobes are more stationary than cals3k 1 implied extending considerably the time span of stationary flux lobes observed in historical models between 5000 bc and 2000 bc there are time intervals of weak dipole moment where dipole power is exceeded by low degree nondipole power at the core mantle boundary model coefficients and evaluation code can be obtained from the earthref digital archive erda together with animations and snapshots plots for every 100 years at http www earthref org detailed urls for the different material are listed in appendix a

crustal contamination identified in keweenawan osler group tholeiites ontario a trace element perspective in the lake superior area voluminous middle proterozoic tholeiitic basalt with minor rhyolite and interflow sediments occupy the midcontinent rift and overlie archean and older proterozoic basement the 3000 m thick section of gently dipping 10 sse basalts on the black bay peninsula is divided into three suites based on the relationship between stratigraphy petrography and geochemical variations e g mg number a1 2 o 3 la sm and gd yb the lower suite consists of augite olivine porphyritic lavas with high mg number sio 2 tio 2 and gd yb but low la sm the overlying central suite flows are either aphyric or plagioclase porphyritic and have lower mg number sio 2 la sm and gd yb decline upward through the central suite the upper suite lavas are either aphyric or plagioclase porphyritic and element ratios plot on slightly different trends compared to the central suite the lower suite has a narrow compositional range the average of the four most primitive lavas is used

paleosecular variation of the earth magnetic field at the canary islands over the last 15 ka abstract we report on new paleomagnetic directions obtained from 38 lava flows located at tenerife and gran canaria canary islands spain one flow is a historical one 1706 ad and 28 other flows are dated by radiocarbon between 1550 ad and about 13 200 bc nine other flows are not dated but they have stratigraphic links with the other flows thermomagnetic curves unblocking temperatures and coercivities suggest that the main carrier of the remanent magnetization is titanomagnetite with various titanium contents in the pseudo single domain range paleodirections were obtained by thermal and alternating field demagnetization on more than 400 specimens the two youngest flows yield directions well consistent with the data previously published from the canary islands but only covering the last 500 yr comparison with model predictions indicates that the models account on the long term for most of the data however on short term scale a better agreement is observed with the archeomagnetic based model predictions arch3k and sha dif 14k two time intervals between 25 bc and 85 ad and around 600 700 bc however are characterized by more variable paleomagnetic directions suggesting that the variability of the earth magnetic field was faster than predicted by the models on a wider geographical scale a rather good consistency is observed between the canarian dataset and those from northern africa spain and azores field information is well consistent with paleomagnetic information for the undated sites and they both confirm that the eruption rate of the gran canaria volcanic system was high around 600 bc 1000 bc and 4650 bc refined ages could be obtained for two of the undated sites using archeomagnetic dating combined with the data previously published for the last 500 yr this new dataset is the first long psv record available for the canary islands significantly contributing to the archeomagnetic paleomagnetic database at latitudes lower than 30 n improvement in the time resolution of model predictions is now needed to allow more detailed data model comparisons on short time scale

intensity of the earth s magnetic field in greece during the last five millennia new data from greek pottery abstract new archaeointensity results have been obtained from the study of four ceramic collections coming from four different archaeological sites in greece the ages of the ceramic fragments based on archaeological constrains and radiocarbon analysis range from 2200 bc to 565 ad low field magnetic susceptibility versus temperature reveals a good thermal stability for most of the samples however for some samples the thermomagnetic curves are not reversible indicating mineralogical changes during heating isothermal remanent magnetisation irm acquisition curves and thermal demagnetisation of three orthogonal irm components have also been performed the rock magnetic results identify magnetite and or ti magnetite as the main magnetic carriers in the studied samples classical thellier experiments with regular partial thermoremanent magnetisation ptrm checks have been conducted on 125 specimens belonging to 34 independent ceramic fragments only 61 archaeointensity determinations at specimen level that correspond to linear nrm trm plots were used for the calculation of the site mean archaeointensities the effect of the anisotropy of the thermoremanent magnetisation trm and cooling rate upon trm intensity acquisition have been investigated in all specimens the maximum difference between the trm anisotropy corrected and uncorrected intensities is around 30 at specimen level confirming that the trm effect can be very important in ceramic samples cooling rate correction factors determined per specimen are up to 10 with only one exception that reaches 35 despite the moderate success rate of archaeointensity determination around 50 reliable mean site intensities have been obtained with in situ intensities ranging from 53 6 4 1 to 69 3 3 9 t corresponding to virtual axial dipole moments from 9 2 0 7 to 11 9 0 7 10 22 am 2 the new data are reasonably consistent with other available data for the studied region as well as with the sv reference curves for greece and the south balkan peninsula and the regional and global geomagnetic field models combined with previously published data from the area they confirm that important changes of the earth s magnetic field intensity occurred in greece during the last five millennia for some periods the available archaeointensity data for the balkan area show a large dispersion even for data corresponding to high quality intensity standards whereas for other periods their limited number prevents an accurate description of geomagnetic field intensity changes this evidences the need of new reliable and well dated archaeointensity data in order to obtain a robust description of geomagnetic field intensity changes during the last five millennia in this area

archaeomagnetic secular variation in germany during the past 2500 years summary the german archaeomagnetic data set was supplemented with 35 new directions from ger man sites mainly dating from the past 3000 yr together with archaeomagnetic data from the neighbouring countries of germany a database of 166 reliable archaeomagnetic directions has been compiled for the reference area which is defined by a 500 km radius circle around gottingen the retrieved directions come from well dated archaeological structures and about 40 per cent of the dating relay on natural science methods such as radiocarbon thermolumines cence dendrochronology dating or historical documents from this data set a secular variation sv reference curve has been calculated using a bivariate algorithm which fits a natural cubic spline based on roughness penalty to declination inclination and time simultaneously the error tube surrounding this curve was obtained from bayesian modelling of the experimental errors which can also take stratigraphic information into account the obtained sv reference curve for the past 2500 yr is similar to that from france but also significant differences are seen comparison of the curves does not show a simple westward drift of the sv pattern the german reference curve allows archaeomagnetic dating in the reference area and extends this dating technique to sites situated in middle europe

archaeomagnetic evidence for the age of a roman pottery kiln from calahorra spain summary the first report of a palaeomagnetic study of a roman pottery kiln is presented individual bricks from the walls of the heating chamber have been sampled and measured in order to determine their palaeomagnetic directions the measurements indicate that the innermost part of the structure was heated well above the curie point of haematite enabling the use of the palaeomagnetic declination and inclination after the axial dipole correction and fitting to the paris i t curve to infer the age of the last heating event

early ordovician continental break up in variscan europe ndsr isotope and trace element evidence from bimodal igneous associations of the southern massif central france abstract the high grade marvejols group and the low grade albigeois cevennes sedimentary sequence contain bimodal igneous rocks of early ordovician age which are representative of a widespread thermal event in the european variscides comparison of their nd sr isotope and trace element characteristics provides additional evidence for their origin in an ensialic extensional setting as an alternative to the back arc model proposed by previous authors we propose that these associations record a continental break up episode unrelated to contemporaneous subduction in this model the widespread early paleozoic bimodal magmatism marks the birth of a mid european oceanic arm we propose that the break up was controlled by both transtensional processes and mantle plume activity high grade associations such as the marvejols group and low grade bimodal associations such as the albigeois cevennes might be regarded as representing opposite rifted passive margins the northern marvejols type margin was involved in a late ordovician silurian subduction like process with hp ht metamorphism in contrast the southern albigeois cevennes margin remained largely unaffected possibly as part of the overriding plate of the subduction zone pseudo calc alkaline signatures unrelated to subduction processes may occur in magmatic rocks associated with continental break up episodes in this case negative nb anomalies are produced by the addition of crustal components enriched in th and lree relative to nb this inferred alternative origin of nb anomalies has important bearing on the paleogeodynamic settings based on geochemical data

new archaeointensity data from italy and geomagnetic field intensity variation in the italian peninsula we present new archaeointensity results from three italian kilns situated at ascoli satriano vagnari and fontanetto po obtained with the thellier modified by coe double heating method these data complement the directional results previously published all sites are dated on the basis of archaeological information and or thermoluminescence dating the results are corrected for the anisotropy of the thermoremanent magnetization and the cooling rate effects the new data are compared with previously published archaeointensity data from italy and nearby countries within 900 km radius from viterbo an initial data set including archaeointesity data mainly coming from italy france switzerland czech republic slovakia hungary greece and bulgaria has been compiled after the application of strict selection criteria the most reliable data have been used for the calculation of a preliminary italian intensity secular variation sv curve for the last 3000 yr the new curve covers the 300 bc 400 ad and 1200 1900 ad periods it is established by means of sliding windows of 200 yr shifted by 100 yr the lack of reliable data for the 1000 200 bc and 400 1200 ad time intervals does not permit the calculation of a continuous curve clearly more high quality archaeointensity data from italy and europe are still needed to draw a robust intensity sv curve for the italian peninsula that could be used for archaeomagnetic dating in combination with the directional data

a new contribution to the archaeomagnetic study of a roman pottery kiln from calahorra spain summary results from archaeomagnetic research on a roman pottery kiln are presented in a previous paper pares et al 1992 a collection of samples from the kiln s heating chamber was studied and palaeodirection results were obtained in the same paper dating was performed on the basis of the secular variation reference curve of the inclination at paris in the present study the palaeointensity was evaluated on the basis of the same collection of samples the mean value obtained is 60 9pt a comparison of this value with other available archaeointensity values obtained from neighbouring territories confirms the date estimated by pares et al 1992kthe middle of the 1st century ad

regional and local patterns of low grade metamorphism in the north shore volcanic group minnesota usa the north shore volcanic group in northern minnesota is part of the middle proterozoic keweenawan sequence one of the largest plateau lava provinces in the world the primary geochemistry of the basalts suggests that volcanism occurred in an intracontinental rift environment the subaerial lava flows mainly amygdaloidal olivine tholeiites and tholeiites have undergone low grade metamorphism from zeolite to lower greenschist facies on the basis of alteration phases replacing the primary magmatic minerals infilling amygdales and veins and replacing secondary minerals the following zones have been distinguished 1 thomsonite scolecite smectite 2 heulandite stilbite smectite 3 laumontitechlorite albite 4 laumontite chlorite albite prehnite pumpellyite and 5 epidote chlorite albite actinolite zone in addition to the overall zonation based on mineral parageneses zonations in the composition of the ab content of the newly formed albite replacing primary ca rich plagioclase and of the newly formed mafic phyllosilicates are observed within the sequence and within single flows mafic phyllosilicates in the upper part of the sequence mainly smectites and mixed layer smectite chlorites display high si and ca na k contents whereas in the lower part of the sequence the amounts of si and ca na k are markedly lower mainly chlorites and mixed layer chlorite smectites similar zonations are observed within the individual flows the albite content of the newly formed plagioclase is highest and the si and ca na k content of the phyllosilicates lowest in the amygdaloidal flow top while the opposite is true for the massive flow interior the above features suggest that the overall pattern is one of burial type metamorphism associated with extension in the rift setting in detail the mineral assemblages are controlled not only by the stratigraphic position but also by the flow morphology controlling permeability whose effect on the assemblages is most pronounced in the stratigraphically upper parts this suggests that at the first stages of alteration lowest grade the patterns of fluid flow were important effects in controlling the assemblages at greater burial depth assemblages are more homogeneous perhaps representative of a more even and pervasive flow pattern using the observed assemblages at face value to define grade and or facies different conditions would be assigned within the different morphological flow portions thus at low grade metamorphic conditions it is essential to integrate assemblages from different morphological flow portions in order to define satisfactorily the overall metamorphic conditions

coastal upwelling onshore offshore circulation equatorward coastal jet and poleward undercurrent over a continental shelf slope abstract the onshore offshore circulation equatorward coastal jet and poleward undercurrent associated with coastal upwelling are studied with numerical models the model ocean has a continental shelf slope uniform in the longshort direction and is forced by the wind stress with a limited longshore extent the thermocline intersects the shelf slope and the internal radius of deformation is smaller than the width of the shelf slope this may be a typical situation for coastal upwelling regions such as those off oregon and northwest africa as the initial response to the onset of the winds the ekman offshore flow and the compensating onshore flow are induced and the equatorward flow develops over the shelf slope when the first mode coastal trapped wave from the equatorward edge of the forcing region arrives the onshore compensating flow offshore of the coastal area begins to decrease in strength and eventually offshore flow accompanied by downwelling dominates thus the upwelling tends to be confined t

high quality microwave archaeointensity determinations from an early 18th century ad english brick kiln summary samples of bricks from the dogmersfield park post medieval brick kiln hampshire uk have been used to determine the intensity of the geomagnetic field during the early 18th century ad extensive magnetic mineralogical analyses have been carried out revealing weathered zones and multidomain dominated areas within the bricks likewise statistical data treatment has been used to identify mineralogical heterogeneities once the most suitable parts for the study were selected archaeointensity determinations were carried out applying coe s double step variant of the thellier method the 14 ghz microwave system used in this study was developed at liverpool university and the thermal demagnetizer is a commercial instrument the former uses microwaves instead of heat to demagnetize the sample the latter uses heat and was used for comparison data from the microwave system were of very high quality and uncertainties assigned to single measurements are smaller than the intrinsic variability of the archaeointensity stored within the bricks both ptrm and ptrm tail checks rarely fail and even pass when the sample is almost fully demagnetized microwave intensities group around a central value 53 t slightly higher than the present day field intensity and agree well with previous studies and geomagnetic field models thermal results exhibit higher and more scattered values some samples show evidence of laboratory thermal alteration the different behaviour of samples when subjected to microwave and thermal experiments appears to be linked with the magnetic mineralogy high ti titanomagnetite easy microwave demagnetization without alteration is achieved whereas sample alteration is almost unavoidable during thermal demagnetization high ti titanomagnetite seems to be the ideal mineralogy for microwave experiments instead of pure magnetite preferred in conventional thellier experiments the last part of this paper discusses the usefulness of conventional alteration tests applied to microwave experiments it appears that for this technique negative alteration checks can originate from poor reproducibility of the absorbed microwave power

three millennia of directional variation of the earth s magnetic field in western europe as revealed by archeological artefacts directional secular variation of the geomagnetic field over the last 2000 years has been defined in western europe from numerous archeomagnetic studies however the number of archeomagnetic results for older periods is much more limited for this reason we present new data obtained from fired archeological structures found in two french sites loupiac and aspiran dated within the first millennium b c latest bronze earliest iron age transition 850 700 b c and iron age 525 475 b c respectively from a compilation of archeomagnetic results from western europe great britain italy and france and northern africa tunisia we propose a directional secular variation curve for western europe that covers the entire first millennium b c this curve exhibits a large clockwise motion with rapid changes during the first half of the millennium while the last four centuries b c are characterized by weak variations

an assessment of the age and tectonic setting of volcanics near the base of the windermere supergroup in northeastern washington implications for latest proterozoic earliest cambrian continental separation mafic metavolcanics of the huckleberry formation in northeastern washington occur near the base of the windermere supergroup a sequence of immature clastic rocks thought to have been deposited dur

magma genesis in a late proterozoic proto oceanic rift ree and other trace element data from the keweenawan mamainse point formation ontario canada abstract the keweenawan mamainse point formation of ontario consists of a sequence of tholeiitic lavas and transitional basalt dykes with trace element characteristics similar to other proto oceanic rift sequences stratigraphic variation of geochemistry allows the subdivision of the sequence into five series each of which has suffered shallow level fractional crystallization differences in incompatible element ratios and ree patterns however can only be explained by variations in the melting process ree modelling suggests that parental magmas may have been derived from a heterogeneous mantle with flat chondritic and lree depleted sources however the systematic evolution of ree patterns and other trace elements are better explained by dynamic partial melting within two subsequent rising mantle diapirs

widespread occurrence of a novel high coercivity thermally stable low unblocking temperature magnetic phase in heated archeological material 1 the widespread occurrence of a novel high coercivity magnetic phase in well heated archeological material is reported its properties are defined when it represents the dominant magnetic phase although it is nearly always found as part of a mixture of magnetic phases they are as follows very high coercivity remanence coercivity 600 mt low unblocking temperatures 200 c and high degree of thermal stability this last property distinguishing it from goethite the phase shows striking similarities to magnetic phases produced by thermal decomposition of nontronite an fe rich clay where decomposition occurs after prolonged heating in air to high temperatures conditions suffered by well heated archeomagnetic material preliminary results of mossbauer and x ray diffraction spectroscopy suggest that the phase is more likely to be a substituted hematite rather than fe cristobalite or a variant of fe2o3

mafic rocks in a deep crustal segment of the variscides the gory sowie sw poland evidence for crustal contamination in an extensional setting coronitic metagabbronorites so called hyperites and metabasites occur within gneisses migmatites and minor granulites in the high grade metamorphic gory sowie block in the sudetes sw poland incompatible trace element and end data combined with field and petrographic evidence suggest that three groups may be distinguished 1 enriched amphibolites zr nb ca 8 end500 1 2 5 2 depleted amphibolites zr nb 30 end500 5 6 and 3 metagabbronorites zr nb 13 19 end500 scattered between 5 3 and 1 these characteristics favour an extensional within plate setting consistent with their field occurrence as small bodies scattered within metasedimentary and felsic metaigneous gneisses in that setting enriched mantle sources were partially melted to produce a range of basic magmas showing affinities with continental tholeiites within plate type basalts presumably in a more advanced stage of rifting more depleted varieties transitional to n morb were emplaced these mafic magmas were variably contaminated during their ascent in the crust the metagabbronorites with their well preserved igneous textures were previously interpreted as having been intruded after the main deformation and after the peak of amphibolite facies metamorphism some authors suggesting a relation to the circum gory sowie ophiolites however the geochemical characteristics and p t estimates suggest that the metagabbronorites are unrelated to the ophiolitic gabbros found in the neighbourhood of the gory sowie block and in contrast to them have experienced a complex polybaric p t path

continuous geomagnetic field models for the past 7 millennia 1 a new global data compilation 1 a global data set of archeomagnetic and paleomagnetic data covering the past 7000 years has been compiled it consists of 16 085 results of inclination 13 080 of declination and 3188 of intensity for the time span 5000 bc to 1950 ad declination and inclination data come partly from existing databases and partly from original literature a new global compilation of intensity data for the millennial scale is included data and dating uncertainties are discussed as we attempted to obtain an internally coherent data set the global distribution of the data is very inhomogeneous in both time and space all the data are compared to predictions from the previous 3000 year global model cals3k 1 this collection of data will be useful for global secular variation studies and geomagnetic field modeling although southern hemisphere data are still underrepresented in particular we will use it in a further study to update and extend the existing global model cals3k 1 the huge increase in data compared to the previous compilation will result in significant changes from current models as we might have missed some suitable data we encourage the reader to notify us about any data that have not been included yet and might fit in as improving our global millennial scale models remains our aim for the future the data files described in this paper are available from the earthref digital archive erda at http earthref org cgi bin erda cgi n 331

assessing post depositional processes in archaeological cave fires through the analysis of archaeomagnetic vectors abstract this paper presents a methodological application of archaeomagnetism and rock magnetism as a tool to evaluate post depositional mechanical alterations in archaeological cave fires most taphonomic and post depositional studies on anthropogenic sediments have mainly focused on the diagenetic alterations that these contexts undergo from a geochemical point of view however physical alterations are still largely assessed from mere macroscopic observations of the burnt facies comprising these fires this paper compares the archaeomagnetic directions recorded in the ashes of two holocene fires from the mirador cave sierra de atapuerca burgos spain one apparently well preserved and the other clearly bioturbated vector analyses of archaeomagnetic directions together with the study of magnetic properties combined with field macroscopic observations can provide a powerful tool to assess when a fire is actually in situ the anisotropy of magnetic susceptibility ams in both fires exhibits a dominant sedimentary fabric produced by compaction and also exhibits differences between the areas mechanically disturbed within these fires from those which are not the following set of magnetic features was identified when an archaeological fire preserves its primary position i univectoral and high intensity nrm orthogonal demagnetisation diagrams in ashes ii koenigsberger q n ratio values higher than unity indicative of an undisturbed thermo remanence trm or a partial thermoremanence p trm and iii a good clustering of characteristic directions defining an statistically representative mean archaeomagnetic direction the concurrence of these observations can be used as criterion to determine when an archaeological cave fire is physically in situ thus allowing a correct interpretation of the archaeological record

high coercivity remanence in baked clay materials used in archeomagnetism a study of the high coercivity remanence in archeological baked clays has been carried out more than 150 specimens from 46 sites across europe have been analyzed selected on the basis of the presence of a fraction of their natural remanence that was resistant to alternating field demagnetization to 100 mt the study was based on the stability of isothermal remanence to alternating field and thermal demagnetization and its variation on cooling to liquid nitrogen temperature results indicate that the high coercivity remanence may be carried by magnetite hematite and in isolated cases partially oxidized magnetite and goethite in addition a high coercivity thermally stable low unblocking temperature phase has been identified the unblocking temperatures of both the isothermal remanence and the alternating field resistant natural remanence exhibit similar unblocking temperatures suggesting that the same phases carry both signals the high coercivity low unblocking temperature phase contributes to the natural remanence sometimes carrying a stable direction and behaving ideally during palaeointensity experiments and sometimes not an unambiguous mineralogical identification of this phase is lacking although likely candidates include hemoilmentite related to clay source lithology and substituted hematite or magnetic ferri cristabolite both possible products of thermal transformation of iron bearing clays

rift wide correlation of 1 1 ga midcontinent rift system basalts implications for multiple mantle sources during rift development magmatism that accompanied the 1 1 ga midcontinent rift system mrs is attributed to the upwelling and decompression melting of a mantle plume beneath north america five distinctive flood basalt compositions are recognized in the rift related basalt succession along the south shore of western lake superior based on stratigraphically correlated major element trace element and nd isotopic analyses these distinctive compositions can be correlated with equivalent basalt types in comparable stratigraphic positions in other mrs localities around western lake superior four of these compositions are also recognized at mamainse point more than 200 km away in eastern lake superior these regionally correlative basalt compositions provide the basis for determining the sequential contribution of various mantle sources to flood basalt magmatism during rift development extending a model originally developed for eastern lake superior in this refined model the earliest basalts were derived from small degrees of partial melting at great depth of an enriched ocean island type plume mantle source e value of about 0 followed by magmas representing melts from this plume source and interaction with another mantle source most likely continental lithospheric mantle e 0 the

a basaltic ferrobasaltic granulite association oonagalabi gneiss complex central australia magmatic variation in an early proterozoic rift extremely fractionated basaltic to ferrobasaltic amphibolites and granulites comprise two spatially associated mafic tholeiitic suites deformed sills within the early proterozoic oonagalabi basement gneiss complex harts range central australia the metatholeiites are characterised by high to very high feo tio2 and p2o5 contents and variable depletion in cao and al2o3 despite similar zr nb ratios the rocks from the two suites show different degrees of enrichment in lree and other immobile incompatible elements the basaltic melts which were parental to the two mafic suites were not comagmatic and the rocks cannot be related simply by fractionation of realistic assemblages of low pressure fractionating phases

self reversed magnetization held by martite in basalt flows from the 1 1 billion year old keweenawan rift canada in some basalt flows of the 1 1 billion year old keweenawan rift exposed at mamainse point ontario there is a magnetic phase that holds a remanence antiparallel to the populations of magnetite and hematite that are typical of flows in the succession the paleomagnetic and geological context of this component demonstrates that it is not a chemicaloverprint whoseremanence is attributableto a subsequently reversed geomagnetic field but that the component is a self reversal of the primary magnetization here we use rock magnetic experiments and mossbauer spectroscopy to show that this phase occurs in the most oxidized flowsandisheldbya fine grainedpopulationofhematitethatacquiredits self reversedremanencethrough interactions with a phase of lower blocking temperature we propose that this self reversed hematite is a manifestation of the self reversal phenomena that has been observed to occur experimentally during the transformation of maghemite to hematite and thathas been attributed to negative exchange coupling across the crystal lattices we suggest that hematite formed in association with iron silicates in the basalt flows carries a remanence reflective of the field in which it formed but that martite hematite pseudomorphed after magnetite which formed through the progressive oxidation of magnetite to maghemite with subsequent inversion to hematite carries the self reversed remanence this study marks the second time that a naturally occurring self reversed magnetization has been attributed to this mechanism and marks the oldestreportedinstanceof anytypeof self reversedremanence themartitethatformsthrough thisprocess has the ability to retain a record of the negative exchange coupling from the time of its formation for hundreds of millions of years

new archaeomagnetic data recovered from the study of roman and visigothic remains from central spain 3rd 7th centuries summary new archaeomagnetic results from four heated combustion structures recovered from two archaeological sites in central spain are reported they have been dated by archaeological evidence and in two cases by radiocarbon dating rock magnetic experiments indicate low coercivity magnetic phases such as magnetite and thermally stable maghaemite as the main carriers of the remanent magnetization haematite has been observed in poorly heated baked clays archaeomagnetic directions have been obtained from either alternating field or thermal demagnetization experiments performed on 57 specimens coming from 46 independently oriented samples the four well defined archaeomagnetic directions obtained are in good agreement with previous archaeomagnetic data and with recent regional and global field models they define the beginning of easterly declination drift that was initiated around 350 400 ad and culminated around 800 850 ad and delineate the maximum in inclination that took place around 600 650 ad in addition classical thellier thellier experiments including thermal remanent magnetization anisotropy and cooling rate corrections were conducted on 23 specimens only 13 specimens corresponding to well defined single component behaviour gave reliable results new mean archaeointensities have been obtained for two of the four studied structures vbk1 64 2 5 0 t and vbt1 62 4 2 6 t the new data suggest that two relative intensity maxima occurred in western europe around 320 and 630 ad being of lower magnitude that observed in eastern europe

new archeointensity data from spain and the geomagnetic dipole moment in western europe over the past 2000 years archeomagnetic studies on 14 kilns a group of jar fragments and a collection of baked bricks dated between 1000 and 1959 ad plus one roman pottery kiln have been conducted in order to obtain high quality archeointensity data to enhance the western european database the thellier method with corrections for anisotropy of thermoremanent magnetization trm and for cooling rate dependence upon trm acquisition was used the effect of trm anisotropy is only important for the bricks where the corrected and uncorrected mean intensities differ by more than 5 cooling rate correction factors determined per sample are up to 15 and per site up to 7 7 our 17 new data together with 62 previously published results were used to obtain by bayesian modeling the geomagnetic field intensity over the past two millennia for western europe our results indicate that geomagnetic intensity remained more or less constant between the 1st and 4th centuries and between the 14th and 16th centuries mean values at paris around 65 and 57 t respectively whereas an important decrease occurs between 1600 and 1800 ad the detailed evolution of geomagnetic field intensity during the high middle ages is not yet well established despite their differences geomagnetic global models predict our results reasonably well this work indicates the need to obtain a number of archeointensity data for each time interval in order to reliably record variations of the geomagnetic field and to test whether any relationship exists between field intensity and climate

evidence for rapid geomagnetic field intensity variations in western europe over the past 800 years from new french archeointensity data abstract the number of reliable archeointensity determinations obtained from western europe for the past millennium remains limited moreover the large scatter between different datasets available is puzzling the present study analyzed 31 new groups of baked clay ceramic or brick fragments sampled in france 29 groups and in belgium 2 groups these groups contain several fragments collected from different artefacts and are precisely dated principally from historical constraints between the xiiith and the xixth centuries additionally we re evaluated 14 intensity values that we previously obtained from the same time period the fragments were analyzed using two different thermal methods 1 the in field zero field iz or the izzi version of the classical thellier and thellier method and 2 the triaxe protocol that involves high temperature magnetization measurements data were corrected for the anisotropy of thermoremanent magnetization trm and the dependence of trm acquisition on the cooling rate was taken into account in the different protocols archeointensity data obtained on twin specimens sampled from the same fragment and using both experimental techniques generally show a good agreement i e within 5 at the fragment and at the site level all retained site level averaged intensity results 43 of 45 groups have standard deviations of less than 5 t furthermore groups of approximately the same age have very consistent archeointensity altogether the data presented herein recover a detailed and smoothed geomagnetic field intensity variation curve characterized by two peaks in intensity the first during the second half of the xivth century and the second around ad 1600 followed by a significant decreasing trend in intensity during most the xviith and xviiith centuries this evolution does not satisfactorily fit with the expected intensity values for france derived from geomagnetic field models relying on a different evolution of the axial dipole moment our results lead us to propose that the axial dipole moment decreased from ad 1600 to the end of the xviiith century then slightly increased up to ad 1850 before decreasing again to present day

metallogeny of the midcontinent rift system of north america abstract the 1 1 ga midcontinent rift system of north america is one of the world s major continental rifts and hosts a variety of mineral deposits the rocks and mineral deposits of this 2000 km long rift are exposed only in the lake superior region in the lake superior region the rift cuts across precambrian basement terranes ranging in age from 1850 ma to more than 3500 ma where exposed the rift consists of widespread tholeiitic basalt flows with local interlayered rhyolite and clastic sedimentary rocks beneath the center of lake superior the volcanic and sedimentary rocks are more than 30 km deep as shown by recent seismic reflection profiles this region hosts two major classes of mineral deposits magmatic and hydrothermal all important mineral production in this region has come from hydrothermal deposits rift related hydrothermal deposits include four main types 1 native copper deposits in basalts and interflow sediments 2 sediment hosted copper sulfide and native copper 3 copper sulfide veins and lodes hosted by rift related volcanic and sedimentary rocks and 4 polymetallic five element veins in the surrounding archean country rocks the scarcity of sulfur within the rift rocks resulted in the formation of very large deposits of native metals where hydrothermal sulfides occur i e shale hosted copper sulfides the source of sulfur was local sedimentary rocks magmatic deposits have locally supported exploration and minor production but most are subeconomic presently these deposits occur in intrusions exposed near the margins of the rift and include cu ni pge and ti fe v in the duluth complex u ree nb in small carbonatites and breccia pipes resulting from local hydrothermal activity around small felsic intrusions mineralization associated with some magmatic bodies resulted from the concentration of incompatible elements during fractional crystallization most of the sulfide deposits in intrusions however contain sulfur derived from country rocks the interaction between magma and country rocks was important in generation of the magmatic cu ni sulfide deposits a mantle plume origin has been proposed for the formation of the midcontinent rift more than 1 million km3 of mafic magma was erupted in the rift and a comparable volume of mafic intrusions are inferred beneath the rift providing a ready and structurally confined supply of mafic source rocks that were available for leaching of metals by basinal brines these brines were heated by a steep geothermal gradient that resulted from the melting and underplating of magma derived from the plume hydrothermal deposits were emplaced for at least 30 40 m y after rift magmatism and extension ceased this time lag may reflect either the time required to heat deeply buried rocks and fluids within the rift or may be due to the timing of post rift compression that may have provided the driving mechanism for expulsion of hydrothermal fluids from deep portions of the rift