seismic and geoelectric evidence for collisional and extensional events in the fennoscandian shield implications for precambrian crustal evolution extensive deep seismic and electromagnetic sounding programs have resulted in tens of deep refraction seismic profiles normal incidence reflection profiles several hundred magnetotelluric soundings and several magnetovariational array observations results are compiled into four maps 1 moho depth 2 thickness of the lower crustal high velocity layer vp 7 km s 3 depth to the upper interface of the high velocity layer and 4 distribution of upper and middle crustal electrical conductors the moho depth is approximately 40 km in the archaean domain and varies between 40 and 65 km in the svecofennian domain most of the thickness variation 0 24 km is concentrated in the lower crustal high velocity layer electrically the crust is formed of several rather homogeneous either resistive or conductive blocks that are bounded by upper and middle crustal conductive zones the cratonized archaean crust experienced an extensional phase during which a thin mafic lower crustal high velocity layer was produced in association with mafic dykes 2 2 2 1 ga and well conducting volcanogenic and metasedimentary belts 2 5 2 0 ga the moho of the archaean domain was consequently reformed in the early proterozoic major thickening of the crust 50 65 km occurred when a svecofennian island arc and the archaean craton collided the svecofennian orogeny 1 76 1 9 ga resulted in a collage of metasedimentary rocks squeezed between crustal blocks of the island arc the internal terrane boundaries are seen as good reflectors and or inclined or vertical conductors that extend at least to the middle crust the thick high velocity 7 km s lower crust is a combination of mafic lower crust of the island arc crustal blocks and mafic additions from either delamination of the lithosphere after the collision or from a period of late svecofennian subduction the collisional boundary is also preserved as a discontinuous conductive zone between the archaean and svecofennian domains the thickened crust was subsequently thinned to 42 km along e w striking zones during an extensional period characterized by the intrusion of anorogenic rapakivi granitoid batholiths coeval mafic dykes and gabbro anorthosite bodies 1 6 1 5 ga the crust of the central part of the shield has since remained virtually intact

unraveling the evolution of chondrite parent asteroids by precise u pb dating and thermal modeling u th pb isotopic data are reported for mineral fractions individual chondrules and fractions of chondrule fragments from the equilibrated ordinary chondrite richardton h5 chondrules and milligram sized fractions of pyroxene rich chondrule fragments contain highly radiogenic pb and concordant or nearly concordant u th pb isotopic systems and are suitable for precise pb pb age determinations olivine and sulfide have low u concentrations and contain less radiogenic pb the ages of individual chondrules pyroxene rich and phosphate fractions are determined using u pb and pb pb isochron and model date calculations the pb pb isochron date of 4562 7 1 7 ma of the richardton chondrules and chondrule fragments is resolved from the pb pb isochron date of 4550 7 2 6 ma obtained from multiple phosphate fractions possible biases of the isochron dates due to single stage approximation of multi stage evolution contamination with modern common pb and disturbance to the system by reheating are examined and are found to be insignificant the chondrule and phosphate dates are interpreted as the timing of cessation of pb diffusion during cooling following metamorphism in chondrite parent bodies the difference in estimated closure temperatures 950 1150 k for pyroxenes and 700 800 k for phosphates temperature estimates are based on published diffusion rates for pb in pyroxenes and apatite allows evaluation of the average cooling rate at 26 13 k million years for the richardton parent body over the period of 4563 4551 my thermal modeling of the h chondrite parent body which is assumed to be asteroid 6 hebe heated by decay of 26al suggests a scenario in which accretion initiated at 1 7 m y after formation of calcium aluminum rich inclusions and continued for 3 5 m y

implications of the carbonaceous chondrite mn cr isochron for the formation of early refractory planetesimals and chondrules abstract an excellent 53 mn 53 cr isochron for bulk ci cm co cv cb and ungrouped c3 chondrites seems to suggest that each carbonaceous chondrite group acquired its mn cr ratio 4568 1 myr ago this age is indistinguishable from the age of ca al rich inclusions cais which is considered to be the start of the solar system t 0 however carbonaceous chondrites were not assembled until at least 1 5 5 myr after t 0 to judge by the 207 pb 206 pb and 26 al 26 mg ages of the chondrules within them and by the fact that they were not melted by heat from the decay of 26 al presumably therefore these meteorites inherited their bulk mn cr isochron from precursor materials which experienced mn cr fractionation at t 0 as a possible physical mechanism for how the isochron was established initially and later inherited by the carbonaceous chondrites we propose the rapid formation at t 0 of planetesimals that were variably depleted in moderately volatile elements and hence had variably low mn cr the planetesimals and the undepleted high mn cr primitive dust from which they were made shared the same initial e 53 cr and therefore evolved on an isochron we suggest that later impact disruption of the planetesimals produced dusty debris which became mixed in various proportions with unprocessed high mn cr dust before accreting to the carbonaceous chondrite parent bodies with mixing in a closed system the isochron was unchanged we infer that some debris rich material was converted to chondrules prior to accretion the chondrules could have been formed by flash melting of the mixed dust or could instead have been made directly by the impact splashing of molten planetesimals or by condensation from impact generated vapor plumes

re os systematics in chondrites and the fractionation of the platinum group elements in the early solar system we have investigated the re os system for samples of whole rock metal and sulfide from ordinary chondrites using closed system analytical techniques we found complete exchange between sample and tracer isotopes for silicate containing samples and obtained precise and reliable re os concentration measurements results on two group iva iron meteorites and on a silicate rich iron steinbach iva an are consistent with the iva ivb isochron and support the previous observation that iva ivb irons may be slightly older than iiab irons data on whole rock fragments and metal rich separates from the st severin chondrite ll6 show a large range in 1870re 188 os and in 187 os 188 os which makes possible in principle the determination of a re os internal isochron on a chondrite for the first time this re os fractionation may be due to partial melting of fenis and macroscopic redistribution of metal and sulfide the st severin data show a good correlation line on a 187 re 187 os evolution diagram if this is considered to represent an internal isochron it gives an age t 4 68 0 15 ae 187 re 1 64 10 11 a 1 and an initial 187 os 188 os 0 0 0953 0 0013 this age is in agreement with but slightly older than the more precise 187 re 187 os age for the iiab irons as well as for irons from other groups t 4 61 0 01 ae a st severin sulfide nodule has very low re and os concentrations and shows a young re os model age 2 3 ae indicating recent element remobilization whole rock and metal rich separates of h group chondrites h3 to h6 yield restricted ranges in 187 re 188 os 0 42 0 47 and 187 os 188 os 0 128 0 133 there is a systematic difference between re os in the metal extracted from a chondrite and the bulk chondrite this shows that there is a small but significant re os fractionation within subsystems contained in the chondrites from whole rock samples of h group chondrites we calculate a mean 187 re 188 os 0 423 0 007 and 187 os 188 os 0 12863 0 00046 which may characterize the evolution of an average chondritic reservoir for re os the ordinary chondrite data plot close to the iiab isochron although the deviations found are larger than found for the irons the re os chronometer in iron meteorites is apparently controlled by the re os fractionation due to fractional crystallization of liquid metal re os ages of iron meteorites give the time of crystallization of metal segregations and cores of early planetary bodies in contrast the behavior in ordinary chondrites while also dominated by the metal phases must reflect fractionation and transport on a local macroscopic scale within the chondrites between the metal phases after aggregation due to partial melting of fenis or represent variable re os fractionation of the metal phases prior to the accretion of the chondrites however for st severin we attribute the major re os fractionation to early heating of the meteorite above the fe fes eutectic we do not consider that the re os fractionation observed in other chondrites is due to the redistribution of re and os during chondrite metamorphism including shock but it may plausibly represent earlier stages of re os fractionation for the different feni metal constituents prior to accretion

low field variation of magnetic susceptibility as a tool for magnetic mineralogy of rocks abstract low field variation of magnetic susceptibility was investigated on a collection of several hundreds specimens of various minerals and rocks using the kly 4s kappabridge the measurement is fully automated being executed in 21 distinct fields ranging from 2 to 450 a m in one frequency of 875 hz the measurement is rapid 7 min per specimen so that large collections of specimens can be investigated the results can be processed both graphically and mathematically for the latter processing parameters of two kinds were introduced one characterizes the susceptibility change with field the other one characterizes the field in which the susceptibility no longer obeys the rayleigh law and starts becoming more complex the results were evaluated statistically remarkable differences were revealed between individual minerals and between some rock types for example the field variation of susceptibility of pyrrhotite is in general an order of magnitude larger than that of titanomagnetite the susceptibility increase in pyrrhotite starts at the field an order of magnitude lower than that of titanomagnetite low field variation of susceptibility then appears as an interesting phenomeon that helps in the identification of magnetic minerals and in some cases also in assessing the compositional variation of them

magnetic susceptibility of the rock matrix related to magnetic fabric studies non ferromagnetic minerals constitute what is called the rock matrix whose susceptibility k1 is directly accessible using only high magnetic fields measurements on minerals and a wide range of rock types show that k1 is mainly due to paramagnetism and hardly exceeds 10 3 si with an anisotropy degree p less than 1 35 different methods to estimate the role of the matrix component in low field susceptibility k and its anisotropy include petrological and chemical analysis evolution of p vs k low temperature studies and comparison of k with remanent magnetizations

transpressional tectonics during the emplacement of pasupugallu gabbro pluton western margin of eastern ghats mobile belt india evidence from ams fabrics abstract the pasupugallu gabbro anorthosite pluton intruded the transition zone between the mesoproterozoic eastern ghats granulites and the meso to paleoproterozoic amphibolites and migmatitic gneisses of east dharwar craton southern india the pluton is bound by shear zones at its entire eastern northern and southwestern margins the objective of this work is to study the internal fabric of the pluton to understand the tectonic setting during its emplacement essentially by means of anisotropy of magnetic susceptibility ams a total number of 107 samples have been subjected to ams and irm studies the magnetic fabric is principally ferromagnetic in origin as the magnetic susceptibility values are 10 3 si which is further supported by irm studies the magnetic fabrics correspond well with the field measurements and petrographic studies they show two lobes in both the northern and southern parts which display concentric inward dipping helicoidal foliation trajectories with interlayered steep and gently dipping domains the magnetic lineation pattern suggests that the magma might have ascended through the central lobes and extended towards outer margins particularly to the west the constrictional strains occur at the eastern margin while flattening strains mark the western margin ams studies together with our field structural observations suggest that the magma has been emplaced into a chamber dilational jog induced by crustal scale dextral transpressional tectonic regime probably during the onset of grenvilian event

a model for planetesimal meltdown by 26al and its implications for meteorite parent bodies the melting of planetesimals heated by 26 al has been modelled using a new finite difference method that incorporates convection as an example we consider a planetesimal with a radius of 64 km which accretes instantaneously at t 0 75 myr after the formation of calcium aluminum rich inclusions from cold 250 k nebular dust with 50 porosity at t 0 9 myr t 700 k the planetesimal shrinks to a radius of 50 km due to sintering at t 1 2 myr t 1425 k the fully insulated interior deeper than a few kilometers starts to melt and at t 1 5 myr t 1725 k with 50 melting convection starts by t 2 myr the planetesimal is a globe of molten convecting slurry inside a thin residual crust from about t 2 5 myr the crust thickens rapidly as the power of 26 al fades planetesimals probably melt in this manner when they accrete before t 1 3 myr and are large enough to insulate themselves r 20 km for accretion at t 0 rising to 80 km at t 1 3 myr melting behavior will also be affected by the level of 60 fe in nebular dust by the extent of devolatilization reactions and basalt segregation during heating and by gradual accretion the model suggests that a the parent bodies of differentiated meteorites had accreted before about t 1 5 to 2 myr and before most chondritic parent bodies had formed and b that molten planetesimals may be a source for chondrule melt droplets

high spatial resolution ion microprobe measurements refine chronology of carbonate formation in orgueil aqueous activity on meteorite parent bodies is indicated by the presence of carbonates high spatial resolution ion microprobe analyses of nine individual carbonate grains four dolomites five breunnerites from the orgueil meteorite reveal linear correlations between 53cr excesses and mn cr ratios in all grains indicative of in situ decay of radioactive 53mn half life 3 7 million years the well defined isochrons appear to have chronological significance if this is the case then 53mn 55mn ratios between 2 1 and 4 7 x 10 6 are inferred for the time of carbonate formation and absolute ages of between 4561 and 4565 ma are calculated systematic uncertainty of 2 ma dolomites tend to have formed slightly earlier than the breunnerites our data imply extensive aqueous activity on the orgueil parent body over a period of several million years starting 3 4 myr after formation of the solar system that most likely was the result of impact heating and latent heat from the decay of radioactive 26al and 60fe

age of the eucrite caldera from convergence of long lived and short lived chronometers abstract the long lived 147 sm 143nd chronometer has been used in conjunction with the short lived 146 sm 142 nd and 53 mn 53 cr chronometers to constrain the time of formation for the eucrite caldera the combination of these three chronometers has produced results that are consistent with one another and converge on a precise high resolution age of 4537 12 ma within 95 confidence limits so far this is the youngest crystallization age that has been confirmed for a noncumulate eucrite thus this basaltic meteorite formed relatively late as compared to other noncumulate eucrites on a differentiated asteroidal parent body most likely between 17 and 41 million years after the formation of the first condensates in the early solar system from these age constraints it is evident that caldera either formed within a magma body on a large 500 km asteroid or in an extensive impact melt on the crust of its parent body

early solar system events and timescale some recent information on the mn cr and al mg systems is reviewed this information is used to derive constraints on the timing of processes and events which took place in the early solar system using reasonable assumptions a timeline is constructed where the estimated age of the solar system is 4571 ma this age is taken to mark the time when most calcium aluminum rich inclusions cais were starting to form a process that may have lasted for several 105 years almost contemporaneously small planetesimals have accreted that served to store these cais for later dispersal among larger planetesimals by the time large numbers of planetesimals of several tens of kilometers in size had formed the interior of these objects started to melt through the decay of 26al collisional disruption of these planetesimals allowed gases dust and melt to escape into the surrounding space the fine droplets of melt reacted with gas and dust to form chondrules which after rapid cooling were partially re accreted onto the residual rubble pile this process of primary chondrule formation in most cases involving several generations of planetesimals most plausibly lasted only for 2 ma towards the end of this period and during the following 3 to 4 ma planetary objects of several hundred kilometers in size were formed they still stored enough energy to continue melting from the inside to finally differentiate into chemically stratified layers with basaltic volcanism occurring within a few million years

live iron 60 in the early solar system isotopic analyses of nickel in samples from the differentiated meteorite chervony kut revealed the presence of relative excesses of 60ni ranging from 2 4 up to 50 parts per 104 these isotopic excesses are from the decay of the now extinct short lived nuclide 60fe and provide clear evidence for the existence of 60fe over large scales in the early solar system not only was 60fe present at the time of melting and differentiation that is fe ni fractionation of the parent body of chervony kut but also later at the time when basaltic magma solidified at or near the surface of the planetesimal the inferred abundance of 60fe suggests that its decay alone could have provided sufficient heat to melt small diameters of several hundred kilometers planetary bodies shortly after their accretion

the duration of ordinary chondrite metamorphism inferred from tungsten microdistribution in metal abstract precise ratios of w and re relative to ir determined by laser ablation icp ms reveal that matrix metal in equilibrated ordinary chondrites eoc exhibits a correlated variation of these ratios in unequilibrated ordinary chondrites uoc the w ir ratio varies by over two orders of magnitude w ir 0 003 0 6 and shows no correlation with any other siderophile element most matrix metal in uoc is depleted in w relative to metal in eoc thus w must enter metal during metamorphism the correlation between w ir and re ir is evident in type 4 chondrites this implies that the abundance and isotopic composition of w is set in ordinary chondrite metal by petrologic type 4 and that the reductant must be exhausted to limit further isotopic exchange as indicated by the 182 hf 182 w ages of ordinary chondrite metal lee and halliday science 274 1996 1876 1879 this offers a means of dating the onset of metamorphic heating of oc parent bodies a discrepancy in ages between 182 hf 182 w lee and halliday science 274 1996 1876 1879 and 207 pb 206 pb gopel et al earth planet sci lett 121 1994 153 171 or 129 i 129 xe brazzle et al geochim cosmochim acta 63 1999 739 760 dating is real and measures the interval of metamorphism the time difference between reduction of w during the onset of heating and isotopic closure of pb or xe by cooling e g t m 2 12 myr for h chondrites the 187 re 187 os and 182 hf 182 w ages are set by the same process metamorphic equilibration establishing an important link between an absolute chronometer t 1 2 187 re 41 6 billion years and a short lived radioisotope system t 1 2 182 hf 9 myr

validity of archaeomagnetic field recording an experimental pottery kiln at coppengrave germany palaeomagnetic data obtained from archaeological materials are used for reconstructions of the earth s magnetic field of the past millennia while many studies tested the reliability of this recorder for palaeointensity only a few studies did this for direction the study presents an archaeomagnetic and rock magnetic investigation applied to an experimental pottery kiln which was operated in 2003 to produce stone ware this kind of high quality pottery needs a temperature of at least 1160 c shortly before heating of the kiln direct absolute measurements of the absolute geomagnetic field vector have been carried out close to it after cooling of the kiln 24 oriented palaeomagnetic samples have been taken although curie temperatures are about 580 c that is the typical temperature for magnetite thermal as well as alternating field demagnetisations reveal also a considerable amount of hematite as magnetic carrier this mixture of magnetite and hematite is dominated by pseudo single domain grains demagnetisation removed in some cases weak secondary components but in most cases the specimens carried a single component thermoremanent magnetisation the mean characteristic remanent magnetisation direction agrees on 95 per cent confidence level with the directly measured field direction archaeointensity was obtained from five specimens with the thellier coe method and with the multiple specimen palaeointensity domain state corrected method six of these specimens also provided a result of the dekkers bohnel method which overestimated the archaeointensity by about 9 per cent compared to the direct value while after correction for fraction the value agrees very well for the multiple specimen palaeointensity domain state corrected method only fractions between 25 and 75 per cent have been used and specimens showing alteration have been excluded above 450 c many specimens showed alteration of the magnetic grains because median destructive temperatures were often above this value in most cases the fraction was less than 50 per cent nevertheless the obtained intensity 48 48 0 24 is on 95 per cent confidence level in agreement with the direct observation behaviour of the specimens during the thellier experiments was not ideal because of narrow unblocking temperature spectra and alteration nevertheless the obtained mean archaeointensity is also in agreement with the direct field observation here the relative palaeointensity error is about 6 per cent and very high compared the multiple specimen palaeointensity domain state corrected method the investigation demonstrates that a pottery kiln can provide a very precise estimate of the ancient geomagnetic field vector

evidence for the presence of 53mn in the early solar system chromium isotopic anomalies appear to be widespread in the allende inclusions clear anomalies are shown in the 53cr 52cr isotopic ratio deviations from the terrestrial value are correlated with the mn cr ratios indicating that 53mn half life t 3 7 106y existed at the time of formation of the meteorite inclusions the initial 53cr 52cr in the inclusions is lower than the terrestrial value recent results have shown that chromium isotopic anomalies are frequent on the 54cr isotope birck and allegre 1984 at the same time our results suggested the possibility of some anomalies on the 53cr isotope which can be produced by the decay of 53mn the present article presents improved precision measurements on the same inclusions of the allende meteorite and an extension of the measurements to mineral separates these results are the first evidence for live 53mn in the early solar system noted 53mn in the following

source field problems in northern parts of the baltic shield abstract theoretical calculations using a stationary ionospheric electrojet model are presented for a resistivity model of interest in studies of the baltic shield the earth model includes the undistorted east european platform resistivity versus depth model and crustal and asthenospheric conducting layers the results show that close to the source region for periods 15 min the magnetic field depends only weakly on the earth s conductivity accordingly the resolution of apparent resistivity curves obtained by the horizontal spatial gradient hsg technique becomes poor at longer periods close to the source region a comparison with experimental hsg apparent resistivity from kiruna and kevo in northern scandinavia supports the existence of high conductivity in the lower crust the sharp decrease of apparent resistivity at periods 1 h may within the limits of experimental error be a result of the non uniform source rather than an indication of a good conductor in the upper mantle

an electrical resistivity crustal section from the alps to the baltic sea central segment of the egt abstract electrical resistivity models derived from magnetotelluric and magnetovariational measurements in the past 10 years have been compiled along the central segment of the european geotraverse along the profile low resistivity structures exist particularly in the middle and upper crust and probably mask deeper low resistivity zones these structures can be related to different tectonic units along the profile the prominent features are the two dimensional low resistivity structures at the transition from the moldanubian to the saxothuringian and from the saxothuringian to the rhenohercynian unit of the variscan belt the depth correlation of the low resistivity horizon in the rhenish massif with a seismic low reflectivity band the very high conductance of an upper to middle crustal layer in the north german lowlands and its cessation at the river elbe towards the baltic sea the reduction in the resistivity of these zones is very likely due to graphite at mid crustal depths it is obvious that there exists a general increase of electrical conductance in the upper half of the crust from south to north of the central segment which may be due to increasing accumulation of graphite and successive tectonically controlled ordering processes this phenomenon may by relating it to the growing crustal age from south to north be the indicator of a maturing continental crust

anomalous magnetic fabrics of dikes in the stable single domain superparamagnetic threshold this research has been partly funded by project cgl2008 04264 bte and by grant prx14 00303 to carles soriano

magnetic properties opaque mineralogy and magnetic anisotropies of serpentinized peridotites from odp hole 670a near the mid atlantic ridge abstract magnetic mineralogy and magnetic properties of 16 samples of serpentinized peridotites harzburgites from odp hole 670a are studied the degree of serpentinization was determined by a new method which combines saturation magnetization and whole rock iron content a unique curie temperature t c of 580 c corresponding to pure magnetite was observed microprobe analyses whole rock metal content and curie temperatures indicate the absence of awaruite ni 3 fe or native metals hysteresis parameters reveal the contribution of pseudo single domain psd grains in samples with a low degree of serpentinization and large multi domain md grains in highly serpentinized samples with j rs j s 0 08 natural remanent magnetization nrm is mainly unstable and possibly composite particularly in the highly serpentinized samples the intensity j a of anhysteretic remanent magnetization arm in the geomagnetic field is much higher than j n intensity of nrm with j a j n averaging 2 4 the koenigsberger ratio q is low and susceptibility k depends on degree of serpentinization ranging from 1 2 to 51 5 10 6 m 3 kg 1 nrm inclinations are scattered with some tendency to lie near the horizontal plane which is close to the foliation plane anisotropies of susceptibility of arm and of saturation remanent magnetization srm were studied and compared with spinel foliation determined for three less serpentinized samples in these samples magnetic anisotropies seem to be mimetic of high temperature deformation before serpentinization when the degree of serpentinization is high later magnetic fabrics probably related to magnetite veins dominate previous anisotropies the temperature dependence of susceptibility and of viscous remanent magnetization vrm were studied the thermal variation of susceptibility shows a shallow hopkinson effect at about 300 400 c time dependence of induced and remanent magnetization increases with temperature reaching maximum values at about 300 400 c based on these observations induced magnetization and vrm of lower crustal and upper mantle rocks and their contribution to marine magnetic anomalies are discussed

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

magnetotelluric array in the central finnish lapland i extreme data characteristics abstract we present magnetotelluric mt data from the northern fennoscandian shield which show extreme behavior due to the complex geoelectric setting left behind by the tectonic reworking of the archaean bedrock during the palaeoproterozoic the core of the mt data set from the central finnish lapland were acquired in 2014 in the context of the masca project extreme data are defined by the condition where the determinant of the real and or imaginary part of the impedance tensor becomes negative this is related to but is a more anomalous condition than out of quadrant off diagonal impedance phases physically these are associated to reversal of e g the electric field as compared to its direction for one dimensional resistivity structure so far only few examples of extreme data have been explicitly reported in geophysical literature data previously classified as erroneous might belong to the class of extreme data instead and data quality analysis should take this into account a resistivity contrast of at least ca 1 1000 is required to observe extreme data as indicated by synthetic modelling studies extreme data are challenging from a modelling point of view and thus also drive the need for new developments in forward and inverse codes for mt in this study behavior of the phase tensor parameterization in case of extreme data is clarified the quadrants of the principal value phases a piece of information inherently missing from the phase tensor can be recovered if the parameters are followed as a function of period the behavior of the determinant average of the impedance tensor is investigated since out of quadrant determinant phases are sometimes observed in conjunction with extreme data a special condition is demonstrated which links out of quadrant determinant phases and extreme data to negative determinant of the galvanic distortion tensor

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

age and isotopic relationships among the angrites lewis cliff 86010 and angra dos reis results of a wide ranging isotopic investigation of the unique antarctican angrite lew 86010 lew are presented together with a reassessment of the type angrite angra dos reis ador the principal objectives of this study are to obtain precise radiometric ages initial sr isotopic compositions and to search for the erstwhile presence of the short lived nuclei 146sm and 26al via their daughter products the isotopic compositions of sm u ca and ti were also measured this allows a detailed appraisal to be made of the relations between and the geneology of these two angrites lew proves to be severely contaminated with modern terrestrial pb which is shown to result from terrestrial weathering nevertheless concordant pb pb model ages of pyroxene separates were obtained 2 4 55784 52 ga for lew and 4 55780 42 ga for ador uranium isotopic compositions are normal within error the inferred initial pb isotopic composition is within error of primordial pb as defined by canon diablo troilite because of the extreme upb ratios of both angrites this places an upper limit of 2 ma on the time between volatile element loss of the angrite parent body or its precursor planetesimals and the final crystallization of the angrites as differentiates initial 87sr86sr ratios were found to be indistinguishable in lew ador and the cumulate eucrite moore county the derived initial 87sr86sr for all three meteorites is 0 698970 15 2 a 147sm 143nd isochron for lew was obtained yielding an age of 4 553 34 ga with an initial 143nd144nd of 0 506682 49 2 146sm142nd isotope systematics were also measured anomalies on 142nd arising from extinct 146sm were clearly resolved resulting in an initial 146sm144sm of 0 0071 17 with nd142 2 57 0 62 at the time of isotopic closure both of these sm nd methods imply derivation of lew from a reservoir with chondritic smnd ratio they are also quite consistent with the previously reported systematics for ador overall the age and isotopic similarities between lew and ador are striking it suggests almost simultaneous production on the same asteroid even though recent experimental studies imply that the two are not comagmatic calcium and titanium do not exhibit enrichments in the n rich isotopes in contrast to cais making it unlikely that the angrite parent body is inherently rich in early condensates from the solar nebula no evidence was found for live 26al in lew when combined with the pb pb age and initial 87sr86sr data this firmly excludes 26al being an important heat source in the early solar system heat supplied for for example 1 the differentiation of the angrite and eucrite parent bodies and 2 metamorphism of the chondrite parent bodies must come from some other source the debate over live vs extinct 26al in the earliest solar system remains unresolved by this new data from lew however both the angrite and eucrite parent bodies have extremely low rbsr and high upb ratios compared to the solar nebula therefore the common initial 87sr86sr of 0 698970 can be firmly associated with the u pb angrite age above and the formation of both bodies this enables an absolute chronology of the early solar system to be established based upon 87sr86sr differences the oldest cais are a minimum of 11 4 ma older than the angrites i e 4 569 5 ga the age and isotopic constraints are discussed with respect to current collapse condensation and accretion timescales calculated for the solar nebula

significance of iron isotope mineral fractionation in pallasites and iron meteorites for the core mantle differentiation of terrestrial planets abstract seven bulk chondrites with 57 fe 54 fe values between 0 1 and 0 relative to irmm 14 tend to be slightly lighter than 11 bulk iron meteorites which have 57 fe 54 fe values ranging from 0 04 to 0 2 at the mineral scale taenite from two iron meteorites cranbourne and toluca shows 57 fe 54 fe values heavier by up to 0 3 than their kamacite counterpart thus calling into question the significance of bulk iron meteorite data on three pallasites esquel marjalahti and springwater we measured a heavier iron isotope composition for the metal fractions compared to the coexisting olivines as previously observed on two other pallasites eagle station and imilac but the range of 57 fe 54 fe differences from 0 32 to 0 07 is larger than that originally found troilite from two pallasites appears to be even heavier than the metal fraction whereas schreibersite is lighter than its olivine counterpart there is thus a general tendency for minerals within a given rock to show a heavier fe isotope composition as the coordination number of fe increases although troilite is an exception to this rule iron meteorites are classically considered as remnants of asteroid cores and pallasites as core mantle interfaces the simultaneous finding that the metal fractions of pallasites have a higher 57 fe 54 fe signature than the coexisting olivines and that the iron meteorites are slightly heavier than chondrites could be taken as an indication that planetary core mantle differentiation is accompanied by sizeable iron isotope fractionation in this hypothesis mass balance constraints imply that resultant planetary mantles should be isotopically lighter than the chondritic starting material that is not observed however since all planetary mantles analyzed so far have 57 fe 54 fe values equivalent to or heavier than those of chondrites it thus appears that the moderate temperature and pressure metal silicate fractionation that occurred in pallasite and iron parent bodies is not readily transposable to planets as far as fe isotopes are concerned instead these mantle signatures could reflect equilibrium fractionation at a higher temperature or the lack of a global core mantle equilibration at the planetary scale overall these new results show that the mass dependent isotopic scatter observed among inner solar system bodies from the bulk rock to the planetary scale 0 3 57 fe 54 fe is more restricted than previously thought this likely confirms a homogenization process that occurred in the protoplanetary accretion disk between refractory inclusion condensation and chondrule formation

imaging the continental upper mantle using electromagnetic methods abstract the internal structure of the continental lithosphere holds the key to its creation and development and this internal structure can be determined using appropriate seismic and electromagnetic methods these two are complementary in that the seismic parameters usually represent bulk properties of the rock whereas electrical conductivity is primarily a function of the connectivity of a minor constituent of the rock matrix such as the presence of a conducting mineral phase e g carbon in graphite form or of a fluid phase e g partial melt or volatiles in particular conductivity is especially sensitive to the top of the asthenosphere generally considered to be a region of interconnected partial melt knowledge of the geometry of the lithosphere asthenosphere boundary is important as this boundary partially controls the geodynamic processes that create modify and destroy the lithosphere accordingly collocated seismic and electromagnetic experiments result in superior knowledge than would be obtained from using each on its own this paper describes the state of knowledge of the continental upper mantle obtained primarily from the natural source magnetotelluric technique and outlines how hypotheses and models regarding the development of cratonic lithosphere can be tested using deep probing electromagnetic surveying the resolution properties of the method show the difficulties that can be encountered if there is conducting material in the crust examples of data and interpretations from various regions around the globe are discussed to demonstrate the correlation of electromagnetic and seismic observations of the lithosphere asthenosphere boundary also the observations from laboratory measurements on candidate mineralogies representative of the mantle such as olivine are presented

heat flow and thermal structure of the lithosphere across the baltic shield and northern tornquist zone abstract terrestrial heat flow data from archean and proterozoic provinces of the baltic shield its southwestern margin and the tectonic transition to the younger tectonic units of central europe northern segment of the european geotraverse are compiled and examined for local and regional variations a 2 d numerical lithospheric temperature and heat flux model is presented covering 1800 km of the deep seismic fennolora profile across the baltic shield and 500 km of eugeno s northern eugemi seismic lines across the northern tornquist zone and the danish and north german basins thermal lithosphere defined as the outer layer with relatively low temperatures t c in which heat transfer is dominantly by conduction is found to vary markedly in thickness from 85 110 km beneath basins and tornquist zone to about 150 km in central shield areas and 200 250 km in areas of low heat flow along the northern shield profile these results are in very good agreement with seismological information from surface wave dispersion analysis and the refraction wide angle seismic interpretation of fennolora data the combined thermal and seismologic evidence indicates that the uppermost mantle region stratified in seismic velocity is restricted to the upper relatively cold thermal layer which is interpreted to constitute shield lithosphere local and to some extent regional variations in surface heat flow are found to be closely related to variations in near surface radiogenic heat production for the southern baltic shield a heat flow heat production relationship of q 0 32 8 7 6 h 0 is derived regional increase in heat flow from northern and central shield provinces to the shield margin and the tornquist zone is interpreted to be related partly to differences in upper crustal heat generation partly and in southernmost areas mainly to an increased heat flux from the upper mantle upper mantle heat flux is estimated at 20 25 mw m 2 along the northern and central shield transect increasing to 30 40 mw m 2 beneath the shield margin basins and tornquist zone lithospheric thickness variations in this region of northern europe are found to be closely associated with variations in heat flux from the uppermost mantle and perhaps to a large extent controlled by geodynamically determined lateral differences in upper mantle heat flux earthquake focal depth data and information on crustal strength show baltic shield seismicity to be limited generally to the upper 20 25 km brittle part of the crust at temperatures less than 300 400 c maximum source depth of magnetic anomalies at temperatures near 600 c is in the deepest part of the crust near the crust mantle boundary in the southern region about 25 km depth along the tornquist zone and about 35 40 km in southern shield areas and in the uppermost mantle 60 70 km depth in the northern shield areas some petrologic implications are discussed the present deep crust is supposed to be at lower temperature pressure conditions than at early stages of formation however also the present deepest parts of the shield crust 35 40 km are within the stability field of eclogites the general absence of significant gravity anomalies in areas of marked crustal thickness variations indicate high density eclogites to be present in significant amounts in the deep shield crust

process of magnetite fabric development during granite deformation this study evaluates the fabric defined by magnetite grains in a syntectonically deformed granite and deciphers the processes that led to magnetite fabric development anisotropy of anhysteretic re

verification and interpretation of the i xe chronometer abstract comparisons of i xe and pb pb relative ages are made for phosphate and feldspar separates from 12 different meteorites in all cases where i xe ages can be measured in phosphate i xe and pb pb chronometers agree within experimental uncertainty no discordant phosphate samples are observed with the exception of allegan i xe ages for feldspar separates generally agree with pb pb ages for the corresponding phosphate the general concordancy observed between i xe and pb pb chronometers suggests that the i xe system is a reliable and interpretable chronometer when applied to single minerals systems allegan feldspar differs from the other feldspar separates predating its phosphate by 17 ma with an i xe age indistinguishable from those of orgueil and murchison magnetites among the oldest observed

origin and chronology of chondritic components a review abstract mineralogical observations chemical and oxygen isotope compositions absolute 207pb 206pb ages and short lived isotope systematics 7be 7li 10be 10b 26al 26mg 36cl 36s 41ca 41k 53mn 53cr 60fe 60ni 182hf 182w of refractory inclusions ca al rich inclusions cais and amoeboid olivine aggregates aoas chondrules and matrices from primitive unmetamorphosed chondrites are reviewed in an attempt to test i the x wind model vs the shock wave model of the origin of chondritic components and ii irradiation vs stellar origin of short lived radionuclides the data reviewed are consistent with an external stellar origin for most short lived radionuclides 7be 10be and 36cl are important exceptions and a shock wave model for chondrule formation and provide a sound basis for early solar system chronology they are inconsistent with the x wind model for the origin of chondritic components and a local irradiation origin of 26al 41ca and 53mn 10be is heterogeneously distributed among cais indicating its formation by local irradiation and precluding its use for the early solar system chronology 41ca 41k and 60fe 60ni systematics are important for understanding the astrophysical setting of solar system formation and origin of short lived radionuclides but so far have limited implications for the chronology of chondritic components the chronological significance of oxygen isotope compositions of chondritic components is limited the following general picture of formation of chondritic components is inferred cais and aoas were the first solids formed in the solar nebula 4567 4568 myr ago possibly within a period of 10 and have 26al 27al 0 similar to those in chondrules 5 nebular gas the majority of chondrules within a chondrite group may have formed over a much shorter period of time

anisotropy of magnetic remanence a brief review of mineralogical sources physical origins and geological applications and comparison with susceptibility anisotropy the magnetic fabric of rocks and sediments is most commonly characterized in terms of the anisotropy of low field magnetic susceptibility ams however alternative methods based on remanent magnetization measured in the absence of a magnetic field rather than induced magnetization measured in the applied field have distinct advantages for certain geological applications this is particularly true for 1 adjunct studies in paleomagnetism in order to assess the fidelity with which a natural remanence records the paleofield orientation 2 studies of weakly magnetic or weakly deformed rocks for which susceptibility anisotropy is very difficult to measure precisely and 3 quantitative applications such as strain estimation the fundamental differences between susceptibility and remanence and their respective anisotropies are due to several factors 1 susceptibility arises from all of the minerals present in a sample whereas remanence is carried exclusively by a relatively small number of ferromagnetic minerals 2 ferromagnetic minerals are generally more anisotropic than para and diamagnetic minerals 3 for ferromagnetic minerals remanence is inevitably more anisotropic than susceptibility and 4 a number of common minerals including single domain magnetites possess an inverse anisotropy of susceptibility i e they tend to have minimum susceptibility parallel to the long axis of an individual particle remanence is immune to this phenomenon as a consequence of all these factors remanence anisotropy may generally provide a better quantitative estimate of the actual distribution of particle orientations in a rock sample

on induction effects at eiscat and image magnetometer stations summary when ground based magnetometer data are used for ionospheric magnetospheric studies it is important to know the external ionospheric magnetospheric and internal telluric contributions to the observed total variation to study them we calculate geomagnetic induction vectors and apply the siebert kertz separation method using data from the eiscat and image magnetometer stations in fennoscandia induction vectors in the period range 20 2560s show the expected and dominating ocean effect near the arctic ocean at masi a small scale inland anomaly is detected in addition to the ocean effect at hankasalmi a man made anomaly due to the nearby stare radar is present the source effect on induction vectors at high latitudes is briefly investigated by separating the magnetic variations of several bay events at image stations into external and internal parts we study the importance of the large scale induction in the recordings depending very much on the particular event the internal horizontal part is 10 30 per cent of the total field in the auroral electrojet region and its relative contribution increases in more distant regions the internal contribution of the vertical field at inland stations is 10 40 per cent having an opposite sign to the total field or 10 30 per cent having the same sign as the total field depending on the location with respect to the ionospheric currents and to the ocean results are also discussed using a thin sheet model

the origin of tectonic lineation in extensional basins combined neutron texture and magnetic analyses on undeformed clays abstract in extensional sedimentary basins fine grained sediments that appear undeformed at the outcrop scale can carry a magnetic fabric consistent with the regional deformation pattern the origin of the magnetic lineation which is often found in extensional basins is not yet well understood in clays from extensional basins in southern italy the magnetic lineation is tectonically controlled and oriented perpendicular to the main normal faults a combined analysis of magnetic and mineral fabrics was made to gain insight into the processes that lead to a lineation in extensional settings low field high field and low temperature susceptibility measurements were used to distinguish the ferrimagnetic and paramagnetic contributions to the magnetic susceptibility and its anisotropy the magnetic anisotropy of the sediments is predominantly carried by paramagnetic phyllosilicates neutron texture analysis was used to evaluate the spatial distribution of chlorite basal planes results demonstrate that the orientation of the magnetic lineation is related to the spatial distribution of chlorite lying parallel to the common axis of differently oriented basal planes a quantitative correlation between the magnetic and rock fabric was made comparing the low and high field magnetic anisotropy ams hfa to the theoretical anisotropy calculated from the chlorite preferred orientation a good linear correlation is found between the degree of theoretical anisotropy and the ams and hfa results show that the integrated approach of magnetic and mineral fabric investigations represents a valid alternative tool for detecting grain scale and regional deformation patterns in weakly deformed extensional basins where macroscopic evidence of deformation is often not visible

inverse magnetic fabric in carbonate bearing rocks abstract the observation of a magnetic susceptibility ellipsoid whose maximum axis corresponds to the minimum axis of petrofabric pole of bedding or schistosity is referred to as an inverse magnetic fabric the investigation of the magnetic properties of some ferroan carbonate monocrystals and paramagnetic limestones demonstrates that c axis preferred orientation of paramagnetic carbonates results in a maximum susceptibility parallel to the flattening direction inverse magnetic fabrics due to magnetite are also encountered in weakly deformed limestones a mineralogical model based on the property of single domain grains to have a zero susceptibility parallel to their long axis is proposed however more complex cases are also encountered

chondrule textures and precursor grain size an experimental study chondrule formation appears to have been a major event in the early solar system but chondrule properties do not allow us to distinguish between several possible formation mechanisms the physical nature of the precursors especially grain size must affect the textures of the chondrules they yield when heated we melted precursors of different grain sizes including extremely fine grained crystalline aggregates analogous to nebular condensates to see whether objects resembling most natural chondrules can be crystallized with one minute heating and moderate cooling rates the grain size of the charges depends directly on the grain size of the starting material for temperatures up to very close to the liquidus temperature a single rapid heating of condensate like material thus produces very fine grained chondrules like dark zoned chondrules for a very wide range of peak temperatures it is incapable of generating the observed textural distribution of chondrules which are predominantly porphyritic the simplest model for chondrules a single heating of unmodified condensate material therefore appears unrealistic coarse grained chondrules might be formed from fine grained precursors by extended heating with evaporation leading to coarsening or by multiple reheating events with higher temperatures in subsequent events otherwise an origin from annealed condensates planetary rocks or by condensation of liquid and crystals is required

observations of the electrical asthenosphere beneath scandinavia the recent recognition that long period i e of the order of hours electromagnetic induction studies could play a major role in the detection of the asthenosphere has led to much interest amongst the geophysical and geological communities of the geomagnetic response functions derived for differing tectonic environments experiments carried out on the ocean bottom have met with considerable success in delineating the electrical asthenosphere i e a local maximum in electrical conductivity minimum in electrical resistivity in the upper mantle in this paper observations of the time varying magnetic field recorded in three regions of scandinavia northern sweden kiruna kir northern finland northeastern norway kevo kev and southern finland sauvamaki sau are analysed in order to obtain estimates of the inductive response function c for each region the estimated response functions are compared with one from the centre of the east european platform eep and it is shown that the induced eddy currents at periods of the order of 103 104 s in the three regions flow much closer to the surface than under the platform centre specifically at a period of 3000 s these currents are flowing at depths of the order of kev 120 km kir 180 km sau 210 km eep 280 km implying that the transition to a conducting zone of 0 2 s m occurs at around these depths one dimensional inversion of ckev and ckir shows that there must exist a good conducting zone of 0 1 1 0 s m under each of the two regions of 40 km minimum thickness at depths of kev 105 115 km kir 160 185 km this is to be contrasted with eep where the d profile displays a monotonically decreasing resistivity with depth reaching 0 1 s m at 300 km finally a possible temperature range for the asthenosphere consistent with the deduced conducvitity is discussed it is shown that at present there is insufficient knowledge of the conditions water content melt fraction etc likely to prevail in the asthenosphere to narrow down the probable range of 900 1500 c

source effects and the interpretation of geomagnetic sounding data at sub auroral latitudes summary conventional techniques of geomagnetic deep sounding use functions of vertical to horizontal field variations to estimate either horizontal layering or lateral discontinuities in conductivity within the earth when the conductivity varies with depth alone gradient methods are also used to estimate the ambient conductivity at depth of penetration in sub auroral regions and in the period range 15 60 min the assumption of source field uniformity basic to both methods does not always hold and the techniques of interpretation become questionable using three dimensional source models electrojet field aligned currents we show how the closeness of the source can distort transfer function estimates and thus z h ratios estimates of source field gradients appear to be less error prone at high latitudes than standard interpretations of internal external field ratios

a comparative study of magnetic anisotropy measurement techniques in relation to rock magnetic properties abstract magnetic anisotropy measurements are becoming increasingly common to many studies within the different disciplines of geology involving sedimentary igneous and metamorphic rocks a plethora of techniques exists for measuring magnetic anisotropy of rocks some are rapid and non destructive while others are more labor intensive or may result in alteration of the magnetic minerals all however have the potential of revealing a wealth of information when measured and interpreted correctly in broad terms anisotropy techniques subdivide into measurements of susceptibility remanence and torque here we consider the first two of these anisotropy of magnetic susceptibility ams is by far the most utilized and measures composite fabrics magnetic susceptibilities in high fields and low temperatures however are being increasingly used to isolate the paramagnetic contribution to the fabrics when distinguishing between fabrics carried by different ferromagnetic phases or to separate these from the diamagnetic and paramagnetic contributions to the fabric then remanence anisotropy techniques become necessary anisotropies of thermal remanence atrms of anhysteretic remanence aarm and of isothermal remanence airm are the most common examples remanence anisotropy may be measured over the full spectrum of magnetic coercivities or over a targeted range e g partial or aparm moreover anisotropies may be calculated using only the resolved field parallel component of the vector in which case a minimum of six different orientations is necessary to obtain a complete symmetric tensor or using the three components full vector of the measured magnetic vectors e g avarm in which case three orthogonal applied magnetizations are the minimum requirement in this study we utilize a variety of magnetic remanence room temperature techniques to measure remanence anisotropy of selected coarse and finer grained gneiss granulitic specimens with well pronounced fabrics results are compared to room temperature ams and are interpreted in terms of the applicability of instrumentation technique to specific rock magnetic properties

mechanical anisotropies and mechanisms of mafic magma ascent in the middle continental crust the sondalo magmatic system n italy

the use of the anisotropy of magnetic remanence in the resolution of the anisotropy of magnetic susceptibility into its ferromagnetic and paramagnetic components abstract the anisotropy of magnetic susceptibility ams is often controlled by both ferromagnetic sensu lato and paramagnetic minerals the anisotropy of magnetic remanence amr is solely controlled by ferromagnetic minerals jelinek trav geophys 37 1993 introduced a tensor derived from the isothermal amr whose normalized form equals the normalized susceptibility tensor provided that the ferromagnetic fraction is represented by multi domain magnetite the present paper shows the close correlation between these tensors for a collection of strongly magnetic specimens containing multi domain magnetite in addition acceptable correlation between the tensors was also found for a collection of specimens containing single domain magnetite a new method is developed for the ams resolution into ferromagnetic and paramagnetic components using the amr some examples are presented of this resolution in mafic microgranular enclaves in granodiorite and in gneisses of the ktb borehole

the magnetic anisotropy of mantle peridotites example from the twin sisters dunite washington abstract the twin sisters dunite massif washington state provides unweathered peridotites for magnetic analysis these rocks have been chosen to test a new magnetic fabric based on measuring the anisotropy of magnetic susceptibility ams in high field with a vibrating sample magnetometer although the dunites are fresh they host small quantities of secondary magnetite formed along cracks under low field conditions magnetite masks the magnetic fabric of olivine and thus the magnetic fabric does not reflect mantle flow deformation above the magnetic saturation at a field of 1 t the ferrimagnetic component is removed and the high field slope represents the paramagnetic ams only this high field ams originates from the magnetocrystalline anisotropy of olivine the high field ams are compared with lattice preferred orientation lpo fabric data obtained by electron backscatter diffraction lpo point distributions are formed by high temperature plastic flow and match the high field measurements in contrast girdle lpos present notable departures from the high field ams axes

modelling electromagnetic responses of 2 d structures due to spatially non uniform inducing fields analysis of magnetotelluric source effects at coastlines summary in previous works we presented 2 d and 3 d magnetotelluric modelling methods based on rayleigh fourier expansions these methods are an alternative to finite element and finitedifference techniques and are especially suitable for modelling multilayered structures with smooth irregular boundaries here we generalize the 2 d method for the calculation of the electromagnetic response of 2 d structures to arbitrary spatially non uniform 2 d and 3 d inducing magnetic fields these fields are characteristic of low and high latitude regions we calculate the response to different 2 d and 3 d sources of a 2 d structure representative of the conductivity distribution which could be found at a coastline which includes deep conductive anomalies in the lower crust and upper mantle then we investigate source effects comparing these responses to that obtained for a uniform source these effects become noticeable for periods greater than approximately 6 h and increase with the period of the source they are highly dependent on the morphology of the source and also on the orientation of the external field relative to the strike direction of the structure in various cases they totally mask the uniform source response

a gelation analogy of crustal formation derived from fractal conductive structures 1 the statistical evaluation of numerical random media models and of electromagnetic array data provides evidence of fractal conductive structures in the middle and lower crust during the search for an electrical conduction mechanism that is compatible with the geophysical anomalies in the middle and lower crust random resistor networks were developed these networks contain two types of resistors representing the rock matrix and the conductive phase here we demonstrate that random resistor network models can also explain the statistical properties of the electric field distortion measured in large scale electromagnetic array experiments correspondence between measured and modeled distortion statistics is obtained if the structures formed by the two resistor types in the networks have a fractal geometry this can indicate that the natural conductive networks also have a fractal geometry and stay close to a percolation threshold a simple scale model of crust formation mechanics is considered in order to find an analog medium and an analog process that creates such a geometry we suggest gelation as a rheological analogy that allows for a coexistence of ductile and brittle behavior of material in the lower crust this process forms fractal structures in the host matrix and fractal conductive networks provided that conductive material is available at that time of crustal evolution the statistical evaluation of the field data provides evidence for a fractal structure with an upper bound of the size of fennoscandia

differentiation history of the mesosiderite parent body constraints from trace elements and manganese chromium isotope systematics in vaca muerta silicate clasts abstract we report here the results of a study of trace element microdistributions and 53mn 53cr systematics in several basaltic and orthopyroxenitic clasts from the vaca muerta mesosiderite ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that following igneous crystallization these clasts underwent extensive metamorphic equilibration that resulted in intra and inter grain redistribution of elements there is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents such as phosphorus derived from the mesosiderite metal at the time of metal silicate mixing furthermore our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal silicate mixing event after taking into account the effects of these postmagmatic secondary processes comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites respectively suggests that the primary igneous petrogenesis including parent magma and source compositions of vaca muerta silicates were similar to those of achondritic meteorites of the howardite eucrite diogenite hed association internal 53mn 53cr isochrons obtained for two basaltic pebble 16 and 4679 and two orthopyroxenitic 4659 and 4670 clasts show that chromium isotopes are equilibrated within each clast nevertheless just as for noncumulate eucrites and diogenites 53cr excesses in whole rock samples of the basaltic clasts 1 01 e in pebble 16 1 07 e in 4679 are significantly higher than in the orthopyroxene rich clasts 0 62 e in 4659 0 53 e in 4670 as in the case of the hed parent body this suggests that mn cr fractionation in the parent body of the vaca muerta silicate clasts occurred very early in the history of the solar system when 53mn was still extant however the slope of the 53mn 53cr isochron defined by the whole rock samples of vaca muerta clasts corresponding to a 53mn 55mn ratio of 3 3 0 6 10 6 is distinctly lower than that defined by the hed whole rock samples corresponding to a 53mn 55mn ratio of 4 7 0 5 10 6 indicating that the global mn cr fractionation event that established mantle source reservoirs on the parent body of the vaca muerta silicate clasts occurred 2 ma after a similar event on the hed parent body

interpretation quantitative de l anisotropie de susceptibilite magnetique abstract we propose a model in which the magnetic susceptibility anisotropy is considered as resulting from the distinct properties of two rock components this model suggests that differences in the susceptibility ellipsoid can be principally related to mineralogical variations thus some difficulties in the quantitative interpretation of anisotropy data may be explained this model is consistent with a detailed analysis carried on dalradian rocks from scotland a new methodology of anisotropy analysis is derived it is based on the fact that in specimens from a same sample the susceptibility ellipsoid often varies but relations nevertheless exist between values of the principal and mean susceptibilities for each sample measurements are made on 6 12 specimens the study of the relations between principal and mean susceptibilities permits separation of the anisotropies of the rock components the anisotropy determination appearing the best for the high susceptibility component

deep array electromagnetic sounding on the baltic shield external excitation model and implications for upper mantle conductivity studies abstract the bear array of simultaneous electromagnetic em observations probes the deep crustal and upper mantle conductivity structure of the baltic shield searching for the lithosphere asthenosphere boundary beneath the adequate interpretation of the results of this unique high latitude natural field em sounding requires proper understanding of the actual external excitation conditions because conventionally used plane wave model assumptions may be substantially violated in the vicinity of inhomogeneous polar sources the paper presents an overview of the morphology and statistics of source distortions in the bear em field transfer functions tf and the ways of their suppression the stability of the final tf estimates obtained with the exclusion of intensive non stationary auroral effects is further justified the external excitation model effective for the whole bear observation period is inferred from the array distribution of the inter station geomagnetic transfer functions the model is supported by the results of polar ionosphere magnetosphere current system studies based on the simultaneous ground and satellite geomagnetic observations and sets bounds for the plane wave approach in the bear data interpretation to avoid unfounded inferences on the upper mantle electrical properties the signatures of the lithosphere asthenospere boundary under fennoscandia derived from the bear data are summarized and its resolution within the traditional plane wave interpretational paradigm is analysed assuming the presented external source pattern and estimated tf uncertainties caused by the source inhomogeneity

the anisotropy of magnetic susceptibility ams in low grade cleaved pelitic rocks influence of cleavage bedding angle and type and relative orientation of magnetic carriers

u pb and 207pb 206pb ages of zircons from basaltic eucrites implications for early basaltic volcanism on the eucrite parent body we have undertaken petrologic and shrimp u th pb isotopic studies on zircons from basaltic eucrites yamato y 75011 y 792510 asuka a 881388 a 881467 and padvarninkai with different thermal and shock histories eucritic zircons are associated with ilmenite in most cases and have subhedral shapes in unmetamorphosed and metamorphosed eucrites some zircons in highly metamorphosed eucrites with granulitic texture occur alone in pyroxene and typically have rounded to subrounded shapes due to recrystallization superchondritic zr hf ratios of eucritic zircons indicate that they crystallized from incompatible element rich melts after crystallization of ilmenite concentrations of uranium and thorium in zircons in the unmetamorphosed eucrite y 75011 are higher than those in metamorphosed eucrites the u pb systems of eucritic zircons are almost concordant but some zircon grains show reverse discordance radiogenic lead loss up to 48 from zircons is observed in the shock melted eucrite padvarninkai the 207pb 206pb ages of zircon in y 75011 4550 9 ma n 5 are nearly identical within analytical uncertainty to the ages of zircons from the metamorphosed eucrite y 792510 4545 15 ma n 13 the highly metamorphosed eucrites a 881388 4555 54 ma n 5 and a 881467 4558 13 ma n 8 and the shock melted eucrite padvarninkai 4555 13 ma n 18 the averaged 207pb 206pb age of zircon from five eucrites analyzed in this study is 4554 7 ma 95 confidence limits n 49 indistinguishable from the averaged u pb age 4552 9 ma of the same samples because of the high closure temperature of lead in zircon tclosure 1050 c with a cooling rate of 0 2 c yr the 207pb 206pb ages of eucritic zircon do not represent metamorphic ages but crystallization ages of extrusive lavas this fact strongly suggests that volcanism of the eucrite parent body occurred at a very early stage of the solar system history 7 20 ma after cai formation 4567 2 0 6 ma thus basaltic eucrites crystallized from parental magmas within a short interval following the differentiation of their parent body the u pb ages of eucritic zircons are older than the u pb sm nd and rb sr ages of some basaltic eucrites which is consistent with differences in closure temperatures of each isotopic system and suggests that thermal and shock metamorphism affected the isotopic systems of pyroxene plagioclase and phosphates

geoelectric studies and conductivity structures of the eastern and northern parts of the baltic shield abstract large resistive regions ideally having a horizontal size of about 200 km are needed to study reliably the conductivity distribution of the lower crust of the baltic scandinavian shield by deep electromagnetic em methods the overburden of the shield consists of quaternary sediments and is thin with a low conductance this makes the shield favourable for em studies the numerous conductive zones e g schist belts in the uppermost crust however require careful two dimensional modelling in order to obtain reliable electrical cross sections of the earth s crust electromagnetic methods magnetometer array studies and magnetotelluric profiling have proven to be a useful tool in studies of the lithospheric geophysics of the shield the crust of the finnish and soviet baltic shield can be divided into geoelectrically different blocks in central finland the blocks are surrounded by narrow highly conductive band like formations they are located in the middle crust and their upper surface often occurs in the 6 10 km region the resisitivities are typically 5 10 m the greatest anomalous crustal regions have a conductance of more than 20 000 s the lower crust is very conductive in most parts of the shield two dimensional models of long magnetotelluric profiles indicate the great variety and complexity of the geoelectric structure of the crust

abundance major element composition and size of components and matrix in cv co and acfer 094 chondrites abstract the relative abundances and chemical compositions of the macroscopic components or inclusions chondrules and refractory inclusions and fine grained mineral matrix in chondritic meteorites provide constraints on astrophysical theories of inclusion formation and chondrite accretion we present new techniques for analysis of low count pixel si mg ca al ti and fe x ray intensity maps of rock sections and apply them to large areas of co and cv chondrites and the ungrouped acfer 094 chondrite for many thousands of manually segmented and type identified inclusions we are able to assess pixel by pixel the major element content of each inclusion we quantify the total fraction of refractory elements accounted for by various types of inclusion and matrix among co chondrites both matrix and inclusion mg si ratios approach the solar and bulk co ratio with increasing petrologic grade but si remains enriched in inclusions relative to matrix the oxidized cv chondrites with higher matrix inclusion ratios exhibit more severe aqueous alteration oxidation and their excess matrix accounts for their higher porosity relative to reduced cv chondrites porosity could accommodate an original ice component of matrix as the direct cause of local alteration of oxidized cv chondrites we confirm that major element abundances among inclusions differ greatly across a wide range of co and cv chondrites these abundances in all cases add up to near chondritic solar bulk abundance ratios in these chondrites despite wide variations in matrix inclusion ratios and inclusion sizes chondrite components are complementary this complementarity provides a robust meteoritic constraint for astrophysical disk models

very long period magnetotellurics at tucson observatory implications for mantle conductivity in a companion paper egbert et al this issue we describe the estimation of very long period 0 16 t 91 days magnetotelluric mt impedances from 11 years of data collected at the tucson geomagnetic observatory here we discuss the implications of these data for mantle conductivity using minimum norm flattest and smoothest inversions we find simple one dimensional models of electrical conductivity in the depth range 0 1500 km we use forward modeling a linearized resolution analysis and constrained one dimensional inverions to delineate the range of models which are consistent with the estimated impedances although the mt data have limited resolution large scale vertical averages of mantle conductivity are well constrained we reach the following conclusions concerning mantle conductivity beneath tucson 1 the upper 200 km has a conductance of order 104 siemens s this anomalously high conductance may be concentrated in an aesthenospheric high conductivity layer but the geometry of the conductive zone is not constrained 2 typical conductivities in the transition zone 400 700 km are 0 1 0 3 s m 1 a step increase to reach this value at or near the 400 km olivine spinel phase transition is consistent with but not required by the data an upper mantle which is resistive throughout 0 05 s m 1 or less is not allowed by the data 3 resolvable large scale averages of conductivity increase from 0 2 s m 1 to 1 0 s m 1 between 600 and 900 km depth a range of models including those with step increases and step decreases at the 670 km seismic discontinuity are consistent with the data 4 between 900 1500 km conductivity increases slowly average conductivities in this region are of the order of 1 s m 1 to within a factor of 2 or 3 while limited zones of highly resistive mantle are consistent with the data a lower mantle which is resistive throughout is not models in which conductivity is always above 5 s m 1 below 1000 km can also be ruled out in conjunction with improved laboratory estimates of electrical conductivities of mantle minerals at high temperatures and pressures these constraints can provide important clues to the composition and physical state of the mantle

are high temperature fractionations in the solar nebula preserved in highly siderophile element systematics of the earth s mantle the relative abundances of the highly siderophile elements hse os ir ru pt rh and pd in relatively pristine lherzolites differ from solar abundance ratios and are several orders of magnitude higher than predicted for equilibrium distribution between metal silicate core mantle the samples are characterized by a mean ca al ratio of 1 18 0 09 m and a mean ca si ratio of 0 10 0 01 m overlapping with a mean ca al of 1 069 0 044 m and a mean ca si of 0 081 0 023 m found in chondrites wasson and kallemeyn 1988 interestingly the ci normalized abundance pattern shows decreasing solar system normalized abundances with increasing condensation temperatures the abundance of the moderately volatile element pd is about 2x higher than those in the most refractory siderophiles ir and os thus the hse systematics of upper mantle samples suggest that the late bombardment which added these elements to the accreting earth more closely resembles materials of highly reduced eh or el chondrites than carbonaceous chondrites in fact the hse in the earth mantle are even more fractionated than the enstatite chondritesan indication that some inner solar system materials were more highly fractionated than the latter

on the interpretation of normal and inverse magnetic fabric in dikes examples from the eger graben nw bohemian massif abstract recent studies of igneous rocks indicate that the predominant occurrence of normal inverse fabric in dikes may either reflect the presence of multi domain md single domain sd grains or it may result from different orientation mechanisms of magnetic minerals in magmas of different viscosities the ambiguity in physical vs geological cause of normal inverse magnetic fabric must be answered before any successful geological interpretation of magnetic fabric can be made in order to address this problem we studied magnetic fabric of selected dikes associated with the sw ne trending eger graben nw bohemian massif the studied area offered very extensive collection of rock types basanite bostonite camptonite tinguaite and trachybasalt magnetic susceptibility varies according to rock type and reflects the relative contents of magnetic minerals in most cases titanomagnetite with variable ti content was identified as main magnetic carrier the degree of anisotropy is relatively low in most cases less than 10 the shape of anisotropy ellipsoid ranges from slightly prolate to neutral and oblate several different types of magnetic fabric using anisotropy of low field magnetic susceptibility ams were observed in studied dikes so called normal and inverse magnetic fabrics and anomalous magnetic fabric comparing all studied sites it seems that the type of magnetic fabric is lithology dependent normal magnetic fabric with magnetic foliations and subhorizontal magnetic lineations both parallel to the dike margins was found in bostonite and trachybasalt dikes inverse magnetic fabric with magnetic lineations and magnetic foliations perpendicular to the dike margins was found in camptonite dikes anisotropy of anhysteretic remanent magnetization amr indicate that the observed inverse magnetic fabric may be caused by the presence of sd magnetic grains amr fabric being normal with respect to dike margins in contrast to that no single domain particles were revealed using frequency dependence and anhysteretic susceptibility measurements the ams measured in variable weak magnetic fields is field dependent for camptonite dike and field independent for other rock types i e bostonite basanite tinguaite and trachybasalt for further flow direction and tectonic interpretations of magnetic fabric in dikes it is suggested to use preferably the amr fabric at least for dikes which demonstrate significant field dependence of ams as it reflects the true rock fabric more accurately than ams fabric

the problem of the lower boundary of the pleistocene in eastern lithuania abstract a sequence of lacustrine and alluvial sediments represented by stratified sand silt and rarely clay over 20 m thick is widespread in the anyksciai and vilnius districts in eastern and south eastern lithuania respectively these sediments are exposed only in eastern lithuania in the sventoji river valley and its tributaries here they overlie devonian bedrock and are overlain by the pleistocene glacial deposits the results of different geological investigations palaeobotanical mineralogical geochemical etc indicate that these sediments probably formed in the basins and rivers that date from the neogene to the early pleistocene stratigraphical subdivision and correlation of sediments from the transition zone between the neogene and quaternary are problematic for several reasons this article presents new results of palaeomagnetic investigations of these sediments from five outcrops accompanied by lithological studies the sediments were laid down during a relatively long and variable period of palaeomagnetic reversals comparable to the matuyama and gauss polarity chrons the boundary between the brunhes and matuyama polarity chrons has been traced in the upper part of the sections the stratigraphical subdivision and correlation of the entire sequence as well as the identification of the neogene quaternary boundary in these sediments remain undetermined

resolution of ferrimagnetic and paramagnetic anisotropies in rocks using combined low field and high field measurements summary a method of separation of ferrimagnetic and paramagnetic components of magnetic anisotropy from the total anisotropy is developed from the anisotropy measurement of a specimen in a low field susceptibility anisotropy and from measurement in two high fields stronger than the saturation field of the ferrimagnetic fraction present the following anisotropy components can be calculated susceptibility anisotropy field independent of the paramagnetic fraction low field susceptibility anisotropy of the ferrimagnetic fraction and high field anisotropy of the ferrimagnetic fraction errors possibly arising from imperfect saturation of the ferrimagnetic fraction in the field practically available are analysed

the geoelectric model of the polar profile northern finland abstract electromagnetic soundings have been made in order to construct a geoelectrical conductivity model of the crust along the european geotraverse egt polar profile forty magnetotelluric mt soundings eighteen audiomagnetotelluric amt soundings and ten magnetohydrodynamic mhd soundings were made on the main polar profile polar i and ten more mt soundings on a parallel profile polar ii 40 km to the southeast of the main profile analysis of simultaneous recordings by the eiscat magnetometer chain and thin sheet modelling of the effect of the barents sea indicate that neither the source field effects nor the presence of the ocean are significant at periods below 200 s in the measurement area the magnetotelluric data have been modelled with two dimensional models representing the regional structure along the profiles in addition to the regional structure a thin inhomogeneous surface layer is included in the models in order to explain some local features of the measured response functions although details of the surface electrical structures are poorly resolved the gross features of the geoelectrical cross section are considered to be reliable the results divide the polar profile into three different blocks the better conducting karasjok kittila greenstone belt in the south has an average resistivity of less than 10 m the more resistant lapland granulite belt with a resistivity between 100 and 200 m is underlain by conductive the geoelectric cross section agrees in gross detail with the corresponding gravity refraction seismic and reflection seismic cross sections of the polar profile all methods indicated a similar shape for the southwestern part of the lapland granulite belt i e granulites have a gently ne dipping boundary against the underlying karelian province in the northeastern part of the granulite belt the geoelectric model and the gravimetric model show a rather steep s dipping boundary against the inari terrain northeast of the granulite belt

a theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals summary susceptibility thermo remanent magnetization trm and isothermal remanent magnetization irm anisotropy ellipsoids have been determined for several rock samples the results indicate that the ellipsoid of initial susceptibility is less anisotropic than the trm and low field irm ellipsoids which are found experimentally to be of identical shape this suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non destructive low field irm measurements such irm measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured the results for a series of artificial anisotropic samples containing magnetite particles of different sizes in the range 0 2 90 m were very similar to those for the rocks in contrast a comparison of the susceptibility and irm ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory such results imply that susceptibility and irm ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single domain particles magnetization confined to the easy axis or whether the particles are essentially multidomain

upb systematics of phosphates from equilibrated ordinary chondrites u pb systematics were determined from fifteen phosphate separates from equilibrated ordinary chondrites and from small bulk fragments of the same meteorites the high 238u204pb ratios of thirteen of these phosphate separates lead to extremely radiogenic pb whose 206pb204pb ratios range from 250 up to 3500 the pb pb model ages for these phosphates range from 4 563 to 4 502 ga with an analytical precision of 106 y and the u pb system is apparently concordant the time interval observed 60 106 y reflects the thermal processing of the equilibrated chondrites and is consistent with that previously derived from the rb sr k ar and pu chronologies the pb pb ages of the phosphates from the seven h chondrites show a negative correlation versus their metamorphic grade this is the first clear relationship ever observed between a long lived chronometer and the intensity of metamorphism as reflected by metamorphic grade assuming that the pb pb age indicates the accurate u pb closure time in phosphates the pb pb chronology is compatible with the model of a layered h chondrite parent body however this interpretation of the u pb systematics is not unique it postulates a slow cooling of the equilibrated materials at high temperature in apparent conflict with petrological observations except for the h chondrites which agree rather well with pu systematics comparison of the pb pb chronology with published radiochronometric data does not reveal simple correlations in the present debate concerning the thermal history of chondrites the chronometric information derived from each isotope system is interpreted as the time of its thermal closure however this basic assumption may not be correct for all the radiochronologies and must be evaluated before the radiochronometric data can be applied as compelling time constraints for the period of 4 56 4 4 ga of proto planetary history

discerning magmatic flow patterns in shallow level basaltic dykes from the ne rift zone of tenerife spain using the anisotropy of magnetic susceptibility ams technique discerning magmatic flow patterns in shallow level basaltic dykes from the north east rift zone of tenerife spain using the anisotropy of magnetic susceptibility ams technique

frequency dependent ams of rocks a tool for the investigation of the fabric of ultrafine magnetic particles abstract in some geological processes new very fine grained magnetic minerals may originate the variation in content of these minerals is routinely investigated by frequency dependent magnetic susceptibility which is traditionally interpreted in terms of the presence of viscous superparamagnetic sp particles in addition to stable single domain ssd and multidomain md magnetic particles in addition the fabric of these grains can be investigated through the frequency dependent ams through standard ams measurement at different frequencies one can evaluate the contribution of sp particles to the whole rock ams appropriate methods were developed various rocks soils and ceramics showing frequency dependent magnetic susceptibility were investigated measurable changes of ams with operating frequency were revealed and attempts are made of their fabric interpretation

decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield decomposition analysis of the bear magnetotelluric data implications for the upper mantle conductivity in the fennoscandian shield

oxygen silicon and mn cr isotopes of fayalite in the kaba oxidized cv3 chondrite constraints for its formation history the iron rich olivine end member fayalite occurs in the matrix chondrules ca al rich inclusions cais silicate aggregates and dark inclusions in the kaba and mokoia oxidized cv3 chondrites in most occurrences fayalite is associated with magnetite and troilite to help constrain the origin of the fayalite fa98 100 we measured oxygen and silicon isotopic compositions and mn cr systematics in fayalite from two petrographic settings of the kaba meteorite one setting consists of big fayalite laths embedded in the matrix and radiating from a core of fine grained magnetite and sulfide while the other setting consists of small fayalite magnetite sulfide assemblages within or at the surface of type i barred or porphyritic olivine chondrules oxygen in the big fayalite laths and small chondrule fayalites falls on the terrestrial fractionation line and is distinct from that in chondrule forsterites which are enriched in 16o 17o 4 oxygen in the big fayalite laths may be isotopically heavier than that in chondrule fayalites silicon isotopes suggest that forsterite is 1 amu heavier than adjacent fayalite within kaba chondrules however we were unable to confirm large silicon isotopic differences among fayalites reported previously the mn cr data for big kaba fayalites give an initial 53mn 55mn ratio of 2 07 0 17 10 6 consistent with literature results on mokoia chondrule fayalites the combined data suggest that fayalites in both petrographic settings formed at about the same time 9 7 ma after the formation of cais our data indicate that those fayalite magnetite troilite assemblages replacing metal inside and around chondrules formed by aqueous alteration on the meteorite parent body the formation site and mechanism for the big fayalite laths is less clear but the petrographic setting indicates that they did not form in situ none of the models that have been suggested for formation of these fayalites is entirely satisfactory

study on finite strain and its relationship with magnetic fabric along ailao shan red river shear zone 1 the finite strain along ailao shan red river shear zone has been studied the axial ratios of strain markers and shear strains vary greatly arising principally from deformation intensity and from contrasting competencies ellipticity of chloritic amygdales in basalt ranges from 6 79 to 10 47 corresponding shear strain gamma xz 2 22 2 93 and ellipticity of garnets from 1 56 to 2 95 gamma xz 0 45 1 14 in core gneiss the s c angle of the sheared rocks varies from 2degrees to 28degrees gamma sc 1 35 28 60 mostly in a range 2degrees to 7degrees optical observation thermal demagnetization and hysteresis curves were conducted to determine the nature of minerals and the contribution of paramagnetic and ferromagnetic minerals to magnetic susceptibility the result shows that ferromagnetic minerals and hornblende dominate the magnetic fabric of samples with high to very high susceptibility and play an important role in the magnetic fabric of samples with low to intermediate susceptibility the ferromagnetic minerals are mainly pseudo single domain and multidomain ti poor magnetite most samples have their magnetic lineation lying between the c and s surfaces in the remaining samples deviations from this common alignment principally resulted from the magnetic minerals being deflected by porphyroclasts mineral cleavage and cracks the axial ratio correlation is very poor between the magnetic and actual or imaginary strain ellipsoid calculated from the s c fabric this poor correlation suggests that the shape of the finite strain ellipsoid cannot be evaluated using the magnetic fabric when the latter is a combined result from multiple minerals and especially when the shape effect of multidomain titanomagnetite is important

magnetic fabric and rheology of co mingled magmas in the nasavrky plutonic complex e bohemia implications for intrusive strain regime and emplacement mechanism abstract the fabrics of mafic microgranular enclaves mme and of the host granodiorite of the old granodiorite intrusion of the nasavrky plutonic complex e bohemia were investigated by means of magnetic anisotropy the magnetic fabric in mme is oriented coaxially with the magnetic fabric of the host granodiorite which is undoubtedly magmatic intrusive in origin consequently the magnetic fabric in mme originated during the same process as the magnetic fabric in granodiorite i e during the granodiorite emplacement in addition also the degree of anisotropy and the shapes of susceptibility ellipsoids are very similar in mme and in granodiorite which probably indicates that the mme had probably a similar viscosity to that of granodiorite and behaved as passive markers whose shapes resembled those of the intrusive strain ellipsoids petrological and geochemical approaches coupled with rheological modelling allow the prediction of the physical states of magmas and of the development of their internal fabrics observed fabrics were compared to the results of numerical modelling of multiparticle systems slowly moving in viscous fluid the steep magnetic foliations and almost vertical magnetic lineations suggest that today s erosion level represents a relatively deep intrusive level where magma flowed vertically and its intrusion was controlled by the shape of the feeder zone used for magma ascension

from qualitative to quantitative magnetic anisotropy analysis the prospect of finite strain calibration abstract a method of separating low field susceptibility anisotropies due to ferrimagnetics and the matrix proposed hitherto in cases where the principal axis of these two components are merged is generalized in cases where the axes are distinct the method consists of analysing in specimens close in the same sample anisotropies the differences of which are due to the variation of respective percentages of the two components although the strain rate remains constant one thus uses the cause which itself renders global anisotropy unusable a first application is presented where a control using the anisotropy of isothermal remanent magnetization allows the validity of the method to be tested a discussion of the choice of the best ellipsoid relative either to the matrix or to the ferrimagnetics to give the best indication of relative finite strain rate is opened

on the electrical crust mantle structure in fennoscandia no moho and the asthenosphere revealed summary an interpretation of the geomagnetic inductive response function c 0 observed at kiruna in northern sweden is herein undertaken the bounds of acceptable solutions are initially discovered by a monte carlo random search procedure and the best fitting solutions are examined by the application of linear theory to the problem the data are shown to have a higher degree of internal consistency than that described by the estimated variances of each datum a further monte carlo inversion of the variance reduced data set gives solutions with well defined model parameters the two major features of the models are 1 a small or non existent electrical conductivity variation across the seismic moho boundary and 2 the unequivocal existence of an electrical asthenosphere under the fenno scandian shield beginning at a depth of between 155 185 km and of 60km minimum thickness both of these observations have seismic counterparts finally possible mantle temperature profiles are deduced which depend on the assumptions and laboratory data employed

differences between magnetic and mineral fabrics in low grade cleaved siliciclastic pelites a case study from the anglo brabant deformation belt belgium abstract theoretically within a given pelitic rock in which the main paramagnetic carriers are white mica and chlorite and in which judging from the ams parameters ams is controlled by the paramagnetic carriers the results of x ray pole figure goniometry should show a qualitative relationship with ams this idea is tested on single phase deformed low grade pelites of the lower palaeozoic brabant massif belgium representing the southeastern part of the anglo brabant deformation belt for the vast majority of the samples investigated the different lf ams parameters suggest a paramagnetic control on lf ams different parameters of lf ams measured at 77 k low t lf ams and a comparison of these with the lf ams parameters indicate that lf ams is controlled virtually entirely by paramagnetic carriers high field torque magnetometry hf ams indicates further that ams is controlled entirely by paramagnetic carriers for the vast majority of the samples two apparent discrepancies are observed however these are 1 a frequent mismatch between the lf ams fabric on the one hand and the paramagnetic fabric indicated by low t lf ams and hf ams on the other hand and 2 a rather common mismatch between the preferred orientation of chlorite and white mica suggested by x ray pole figure goniometry on the one hand and the lf ams fabric low t lf ams fabric and hf ams paramagnetic fabric on the other hand these apparent discrepancies may partly be attributed to the composite nature of both the lf ams fabric and the paramagnetic susceptibility fabric the presence of different populations of paramagnetic carriers possibly in combination with minor traces of ferromagnetic s l carriers which are not picked up by the methods applied may explain some of the mismatches observed and are compatible with the basin evolution history of the brabant massif in addition it is possible that the low temperatures low t lf ams and high fields hf ams are capable of enhancing one of the paramagnetic phases thus leading to discrepancies between methods that define the mineral and the magnetic fabrics the results of this study show that even when the lf ams data suggest that the lf ams of a given pelitic rock in which phyllosilicates form the main paramagnetic carriers is controlled entirely by paramagnetic carriers alternative methods such as phyllosilicate x ray pole figure goniometry will not necessarily reflect the paramagnetic carrier orientation although a number of cases have successfully correlated lf ams with mineral fabric and occasionally even strain this study serves as an example for how methods to define mineral and magnetic fabrics can be sensitive in detecting different paramagnetic phases

a method for the separation of paramagnetic ferrimagnetic and haematite magnetic subfabrics using high field torque magnetometry summary in this study the contribution of the paramagnetic ferrimagnetic and haematite components to the magnetic anisotropy is separated by means of high field torque magnetometry torque measurements at different fields which are high enough to saturate the ferrimagnetic minerals however still low enough that the torque resulting from the haematite is linear with field allow for the separation of the three magnetic anisotropy components the method has been applied to haematite single crystals in which no paramagnetic or ferrimagnetic components have been found contribute to the torque signal the mean direction of the poles to the crystallographic basal plane in the haematite single crystals is subparallel to the minimum susceptibility direction measured in low field the separation analysis has also been applied to highly deformed red beds from the lower glarus nappe complex switzerland no ferrimagnetic phases are present in the rocks and therefore they cannot contribute to the anisotropy of magnetic susceptibility the magnetic fabric arises from a paramagnetic subfabric carried by the phyllosilicate minerals and haematite in which the basal planes of both phases are in the cleavage plane the measured magnetic lineation seen in low field anisotropy of magnetic susceptibility appears to be an apparent lineation that arises from a weak girdling of haematite and the paramagnetic minerals conforming the rock

new archeomagnetic secular variation data from central europe ii intensities abstract variations of the geomagnetic field during the past millennia can be obtained by investigating archeological material the spatial distribution of the available data across europe is rather uneven and only about 15 of the sites provide a full vector record of the geomagnetic field here we report 39 new reliable paleointensity data for sites from austria germany and switzerland with ages ranging between 1500 bce and 1900 ce for 31 of them directional information is available too paleointensities were determined with thellier type techniques with the multiple specimen domain state corrected paleointensity protocol or with both techniques corrections for anisotropy of thermoremanent magnetization were carried out for the structures showing small variations of only a few per cent in most cases cooling rate dependence was tested for 16 structures often strong alteration during the experiments was observed seven successful cooling rate corrections were applied which lowered the paleointensity values by about 10 values obtained from the multiple specimen technique are generally somewhat lower than those from the thellier experiments rock magnetic measurements revealed magnetite type minerals mostly in pseudo single domain range as main magnetic carrier but also high coercive mineral components like hematite or epsilon iron oxide are present the new data are mostly in good agreement with published paleointensities they support the presence of strong intensity variations around 800 ce and in the millennium bce the new full vector data reveal that the deep paleointensity minimum observed from german data around 850 bce is accompanied by large variations in direction with declinations up to 50

plane confined magnetic lineations in mingled mafic and felsic magmas the sazava pluton bohemian massif abstract multiple magnetic lineations were revealed using the anisotropy of magnetic susceptibility ams and anisotropy of magnetic remanence amr methods in a domain of mafic felsic magma mingling at the northwestern margin of the sazava pluton central bohemian plutonic complex bohemian massif gabbrodioritic enclave swarms and sheets are steeply oriented and exhibit magnetic lineations plunging at steep to moderate angles mostly 40 70 whereas in their tonalitic host lineations plunge from vertical to horizontal angles but mostly less than 40 the magnetic lineations in both rock types spread along a nne ssw steep plane that could be simplified as representing an average margin parallel magmatic foliation in the pluton concordant with an average regional cleavage in the wall rock the plane confined lineations are interpreted as having recorded the heterogeneous superposition of two processes 1 vertical stretching during emplacement and magma mingling which left behind the steep lineations and 2 regional tectonic stretching which progressively rotated the mineral grains in rheologically weaker domains chiefly in the host tonalite to form the sub horizontal lineation the average foliation bearing the multiple lineations is interpreted as a composite foliation that recorded both margin perpendicular shortening during emplacement overprinted by coaxial regional tectonic shortening this example reaffirms that 1 magmatic fabrics in crystallizing magmas can record accumulated strain resulting from both emplacement and regional tectonic deformation and that 2 separating magmatic and also magnetic fabrics related exclusively to the internal chamber processes from fabrics caused by regional tectonic deformation is problematic or even impossible in cases where composite fabrics are recognized

nickel and chromium isotopes in allende inclusions high precision nickel and chromium isotopic measurements have been carried out on nine allende inclusions 62ni 64ni excesses are present in at least three of the samples the magnitudes of the excesses are about 3 e units 1 e unit 1 part in 10 000 for64ni and about 1 e unit for62ni chromium isotopes measured for some of the same samples show general excesses on54cr of about 7 e units while small deficits are observed for53cr considering these isotope anomalies except those for53cr together with the excesses present in the samples of other neutron rich isotopes of the iron group elements the most likely mechanism responsible for these anomalies is a neutron rich statistical equilibrium process in addition there is an indication of elevated60ni in almost every inclusion measured however for alk 111 this excess was clearly resolved by our experimental technique this effect may be related to the decay of now extinct60fe but the present data are not sufficient to clearly demonstrate its existence in the early solar system an upper limit of 1 6 10 6 can be calculated for60fe 56fe at the time these allende inclusions crystallized

identification of multicomponent anisotropies in rocks using various field and temperature values in a cryogenic magnetometer abstract a new technique involving magnetization measurements on a rotating sample in a cryogenic magnetometer with variable field from 0 to 4 t and variable temperature is presented it can be used to separate and identify multi component anisotropy in rocks the results obtained for anisotropy due to magnetite haematite pyrrhotite or phyllosilicates are interpreted according to different magnetization models

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

pleistocene magnetochronology of early hominin sites at ceprano and fontana ranuccio italy article i nfo paleomagnetic analyses were conducted on two cores drilled at ceprano in central italy where an incomplete hominin cranium was discovered in 1994 as well as on two additional cores from the nearby site of fontana ranuccio that yielded hominin remains associated with an acheulean industry no evidence for the 0 78 ma brunhes matuyama boundary was found at ceprano down to 45 m below the level that yielded the hominin cranium the ceprano lithostratigraphy and the paleomagnetic age constraints are broadly consistent with the stratigraphy of the liri lacustrine sequence of the latina valley constrained by published k ar ages between 0 6 and 0 35 ma and according to an age model with magnetic susceptibility supported by pollen facies data suggest that the level that yielded the hominin cranium has an age of 0 45 0 05 0 10 ma evidence for the brunhes matuyama boundary was found at fontana ranuccio about 40 m below the hominin level consistent with a k ar age of 0 46 ma reported for this level hence the ceprano and fontana ranuccio hominin occurrences may be of very similar mid brunhes age

electromagnetic studies in the fennoscandian shield electrical conductivity of precambrian crust abstract electromagnetic em investigations of the 1980s in the fennoscandian baltic shield produced an unique and unified em data set studies include regional investigations by the magnetovariational mv method with large lateral sampling distance investigations of anomalous conductivity structures by magnetotelluric mt soundings and other em and electrical methods audio mt soundings d c dipole dipole and vlf resistivity profilings with shorter sampling distance and studies of the near surface conductivity by airborne em surveys the variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures the precambrian of the fennoscandian shield is characterized by roughly nw se directed elongated belts of conductors which separate more resistive crustal blocks the latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including 1 the kittila vetrenny poyas conductor 2 the lapland granulite belt and inari pechenga imandra varzuga conductors 3 the archaean proterozoic boundary conductor and 4 the southern finland conductor the conductive belts orogenic conductors indicate places where crustal masses collided and were finally sealed together enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite and sulphide bearing metasedimentary rocks estimations of the lower crustal conductivity indicate a laterally heterogeneous lower crust in the fennoscandian shield archaean lower crust seems to be in general more resistive than the early proterozoic lower crust of the karelian and svecofennian domains the lower crust in the southwestern part of the svecofennian domain and in the sveconorwegian domain seems to be more resistive than in the central part of the svecofennian domain