is the asthenosphere electrically anisotropic abstract two techniques for resolving the conductance of an electrically conductive asthenosphere are presented the first technique combines observatory electromagnetic data in the period range of the daily variation and longer which provides penetration depths of 400 km and deeper with laboratory data of the conductivity of upper mantle materials the conductance can be estimated by this technique but the geometry e g the exact depth is not resolved the second technique extends the classical magnetotelluric mt tensor estimation and decomposition to the period range 1000 30 000 s if an electrical asthenosphere exists then in this period range the electromagnetic field penetrates into it 1000 s but also through it 30 000 s the concept of regional strike directions which has been successfully used to obtain crustal anisotropy directions is again employed at these long periods the surprising result is that in the depth range of the asthenosphere a strong directional dependence of the conductance again occurs although large arrays will be necessary in order to prove that this is really conductivity anisotropy in the asthenosphere first field results from arrays 100 200 km in extent are encouraging the concept of a superposition of crustal and upper mantle anisotropy explains to some extent why earlier magnetotelluric determinations of the asthenosphere had little success only in the fortunate case where a mt experiment was performed not parallel to the direction of the crustal conductor but parallel to the asthenospheric one is the second structure resolved electrical anisotropy in the uppermost mantle supports the concept of intracrystalline water which allows for a contribution of hydrogen diffusity to the conductivity of olivine this diffusity is highly anisotropic with respect to the crystal axis and if the crystals are partly aligned the conductivity measured with mt is also direction dependent

trace element abundances in hibonites from the murchison carbonaceous chondrite constraints on high temperature processes in the solar nebula abundances of trace elements including 14 rare earth elements ree and 8 other refractory trace elements rte have been measured in 33 hibonite and 3 perovskite bearing grains from the murchison carbonaceous chondrite the normalized abundance patterns show a good correlation with three morphological and chemical groups of hibonites sixteen platy hibonite crystals placs typically have patterns which are depleted in the relatively volatile trace elements v nb ba eu and yb group iii three placs also show large depletions in ce one plac is enriched in the ultrarefractory ree while two other placs have fractionated patterns enriched in the light ree with positive tm anomalies the placs commonly have a roll off in the abundances of the hree which is consistent with solid liquid partitioning three blue aggregates bags have patterns with low overall abundances and depletions in the ultrarefractory ree except tm group ii spinel hibonite grains shibs show a variety of patterns mainly of group ii but 4 shibs have relatively smooth patterns with anomalies in the volatile ree ce eu and yb the perovskites have patterns similar to group ii but with excesses in eu and yb the hree roll off in the placs and the igneous morphologies of the shibs indicate that these grains formed by the crystallization of melts distillation may be important for the bags and two of the placs which have large isotopic mass fractionation effects placs have the simplest trace element patterns and the largest ti isotopic anomalies and probably were the first hibonites to form in the early solar system the shibs have trace element signatures that require multiple processing episodes they are somewhat complementary to the placs in that they are enriched in the volatile elements which are depleted in the placs on average the shibs contain isotopically less anomalous ti than the placs however shibs contain 26mg excesses that are consistent with the in situ decay of 26al whereas placs generally have only small or no excesses of 26mg despite high 27al24mg while an overall relationship exists between the morphology chemistry and ti and mg isotopic systematics of meteoritic hibonites they do not as yet fit into a simple scenario for the formation of the solar system

introduction to electrical properties of the earth s mantle the mantle is the largest of the concentric regions that constitute the earth s interior lying beneath an oceanic or continental crust which may be 6 to 75 or more km thick the mantle region extends to the boundary with the earth s liquid core the most detailed characterization of the mantle is provided by the velocity depth profiles obtained from the seismic waves dziewonski and anderson 1981 a generally abrupt rise in velocity the mohorovicic discontinuity bounds the crust and the upper mantle between about 100 and 220 km deep a low velocity zone is encountered beyond which the velocity increases continuously and in steps near 400 670 and perhaps 770 km the name lower mantle is assigned to the region from about 1000 km to the start of the core at 2890 km deep

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

al correlated26mg excess in a large ca al rich inclusion of the leoville meteorite abstract minerals from a large 1 5 cm diameter type b inclusion of the c3v carbonaceous chondrite leoville have been analyzed mass spectrometrically for their mg isotopic composition 25 mg 24 mg ratios were found to be indistinguishable in all cases from those of terrestrial mg while in anorthites 26 mg is overabundant by up to 15 the excess 26 mg 26 mg is linearly correlated with the 27 al 24 mg ratio with the constant of proportionality 26 mg 27 al 5 9 0 3 10 5 this is in agreement with values reported for allende and another type b inclusion from leoville it is at variance with the twenty times higher ratio found in leoville hibonites

on the vertical gradient and associated heterogeneity in mantle electrical conductivity abstract equivalent scalar magnetotelluric response functions obtained from the robust analysis of geomagnetic field daily mean values at 16 magnetic observatories have been systematically inverted data from these sites were found to contain sufficient one dimensional 1 d data consistent with required source field assumptions for meaningful interpretation and were also found to have acceptably well bounded response function confidence limits previous work is reviewed which indicates that significant lateral heterogeneity in mantle conductivity structure is required by the data and that most of these data are adequately modeled by a series of simple two layered earth models with a conductivity jump at depths of 500 1000 km regional variations in upper mantle conductivity have previously been suggested by systematic inversion using the h algorithm these features are examined in the restricted model space of conductance delta functions the statistical significance of these variations is also investigated new evidence is found here for the coast effect at very long periods as well as related evidence for the presence of electrically conductive zones in the upper mantle beneath the pacific ocean semi norm minimization inversion techniques are applied to directly place strong constraints on the sharpness of the vertical conductivity gradient in the mid mantle the mid mantle conductivity jump is required to be abrupt with values beneath this mid mantle transition zone of o 1 s m 1 at all sites

titanium isotopic anomalies in hibonites from the murchison carbonaceous chondrite abstract the isotopic compositions of titanium in eight grains of hibonite caai 12 o 19 from the carbonaceous chondrite murchison have been determined by high precision secondary ion mass spectrometry using an ion microprobe the titanium in the hibonites varies greatly in 50 ti from about 42 to 8 permil relative to terrestrial with smaller up to 4 permil but clearly resolvable effects in 46 ti and 48 ti these results complement ion probe measurements by fahey et al 1985 of a 100 permil excess of 50 ti in a hibonite grain from the carbonaceous chondrite murray and confirm the presence of widespread negative anomalies suggested by the results of hutcheon et al 1983 on hibonites from murchison the magnitude of these variations seems explicable only in terms of nucleogenic processes which produced extremely variable titanium isotopic abundances in the hibonite source materials the hibonites evidently did not participate to the same extent as most material in the mixing and homogenisation processes that accompanied the formation and later evolution of the solar system thus significant source materials of the hibonites may be the supernova condensates of clayton 1978 and may support the concept of chemical memory clayton 1978 niemeyer and lugmair 1984

electromagnetic induction fields in the deep ocean off california oceanic and ionospheric sources summary a 56 day long measurement of seafloor electromagnetic field fluctuations is separated into parts coherent and incoherent with the horizontal magnetic field at a terrestrial observatory 1700 km distant but at the same geomagnetic latitude least squares methods show that the m2 and n2 ocean tides are significant in the seafloor data but not on land spectra of the remaining data show high seafloor electric to seafloor magnetic field coherences but markedly lower seafloor to land values especially for the horizontal electric field model studies show that the correlated parts of the seafloor spectra are consistent with induction by broad scale ionospheric sources while the uncorrelated parts are explained by a combination of a barotropic long wave continuum and an internal wave component the oceanic electromagnetic field is discussed in terms of its modal structure and wavelength dependence and implications for seafloor magnetotellurics may be profound

presolar isotopic and chemical signatures in hibonite bearing refractory inclusions from the murchison carbonaceous chondrite magnesium calcium and titanium isotopic compositions as well as trace element abundances have been determined by ion microprobe mass spectrometry for 37 hibonite bearing inclusions from the murchison carbonaceous chondrite these and previously analyzed hibonite bearing inclusions can be divided into three main groups based on morphological and mineralogical criteria platy crystal fragments placs generally have depletions in the relatively volatile refractory trace elements eu yb mb v ba little or no evidence of 26al i e little or no radiogenic 26mg 26mg and widely variable ca and ti isotopic compositions one plac 13 13 has excess 48ca of 104 and 50ti of 273 the largest effects yet measured in these elements four blue aggregates bags are distinctive in having low overall trace element abundances with large relative depletions in the ultrarefractory elements gd er lu zr hf y sc and mass fractionated mg isotopic compositions enriched in the heavy isotopes large 50ti deficits have been measured in three of the four bags the only bag analyzed for its ca isotopic composition also shows a deficit in 48ca spinel hibonite inclusions shibs show a wide variety of morphologies ranging from bladed hibonite crystals within spinel to spherules of hibonite and spinel to large uniform hibonite crystals within spinel shibs consisting of bladed hibonite in spinel or hibonite spinel spherules typically have ultrarefractory depleted trace element patterns although uncommon ultrarefractory enriched patterns were also found 26mg at a level consistent with 5 10 5 27al and generally small 10 ca and ti isotopic anomalies one of the shib spherules 7 170 has highly unusual systematics with a large 50ti deficit only a small 48ca deficit and no 26mg the large hibonite crystals within spinel have variable trace element patterns commonly with anomalies in the relatively volatile elements ce eu and yb 26mg is present in some of these inclusions but none of this type of shib has large ca or ti isotopic anomalies while at least four distinct components are required to model the ti isotopic compositions three components are sufficient to describe the variations in ca isotopic compositions measured thus far the systematics of plac 13 13 with large excesses of 48ca and 50ti match the predictions of the multiple zone mixing model for nucleosynthesis in supernova ejecta to a maximum neutron excess of 0 159 compositions depleted in 48ca and 50ti are unlikely to represent discrete nucleosynthetic components but represent material with a shortfall of neutron rich material relative to the solar system abundances additional components must be present but are not easily resolved within the morphological groups of hibonite bearing inclusions a few have exceptional chemical and isotopic characteristics such as shib spherule 7 170 however independent of the morphological groups there are correlations in hibonite isotopic systematics and chemistry that hold for all inclusions studied 1 no inclusion has both a large 50ti anomaly and 26mg and 2 inclusions depleted in the ultrarefractory elements have 26mg with the exception of those which have a large 50ti anomaly these features clearly establish a relationship between chemical and isotopic characteristics of the inclusions which must predate their formation several components were combined probably as mixtures of dust grains or crystals and were melted to form refractory inclusions the nucleosynthetic anomalies must be inherited from stellar environments it is possible that the depletions in ultrarefractory elements are also a memory of stellar condensation and are not necessarily related to a condensation event in the solar nebula as has generally been assumed

lithospheric and sublithospheric anisotropy beneath central southeastern brazil constrained by long period magnetotelluric data abstract electric anisotropy calculated from geoelectric strikes of magnetotelluric mt data and seismic anisotropy derived from shear wave splitting parameters are jointly analyzed to estimate the degree and orientation of strain in the subcontinental mantle of central southeastern brazil high quality long period mt soundings are available at 90 sites concentrated along four profiles at the southwestern and southern borders of the paleoproterozoic archean sao francisco craton this data set is complemented by a previous study of sks and skks splitting measurements available at more than 40 sites covering a slightly wider region for this study the mt data were processed with modern techniques including recovery of the undistorted em field polarizations through correction of static shift and phase mixing due to galvanic distortions three dimensional forward modelling of mt and gds geomagnetic depth soundings transfer functions supports interpretation of deep electrical anisotropy in the region magnetotelluric phase responses of orthogonal propagation modes present slight splitting at long periods for most of the sites indicative of overall electrically low mantle anisotropy to depths greater than 250 km the electrical strike azimuths are parallel to the fast nw directions of shear wave splitting along the southern borders of the sao francisco craton suggesting that the seismic anisotropy also resides within the same depth range since this direction is very different from that of present day south american westward absolute plate motion it is inferred that no significant lateral mantle flow or deformation related to the plate motion is observed under the study area either because it is absent or the rigid lithosphere is much thicker than expected to explain the prevailing electric and seismic common direction it is suggested that a mechanical coupling between lithospheric and sublithospheric mantle exists a relic strain possibly resulting from ancient continental collision processes is interpreted to have induced a general alignment of olivine down to mantle depths beneath the continental rigid plate the observed slightly enhanced conductive texture could be associated with hydrogen diffusion along the aligned olivine a axis especially within mantle patches subjected to metasomatic processes the coincidence of mantle strikes with the trend of surface deformation pattern also suggests that crust lithospheric and sublithospheric upper mantle have deformed and translated coherently preserving the regional nw direction since the tectono thermal event responsible for the deformation distinct electric azimuths from the general nw trend are observed in regions probably perturbed by cretaceous rift related intraplate magmatism and in zones of complex deep structures produced by superposed nearly simultaneous oblique neoproterozoic collisions in the southeast of the sao francisco craton

joint inversion of satellite detected tidal and magnetospheric signals constrains electrical conductivity and water content of the upper mantle and transition zone we present a new global electrical conductivity model of earth s mantle the model was derived by using a novel methodology which is based on inverting satellite magnetic field measurements from different sources simultaneously specifically we estimated responses of magnetospheric origin and ocean tidal magnetic signals from the most recent swarm and champ data the challenging task of properly accounting for the ocean effect in the data was addressed through full three dimensional solution of maxwell s equations we show that simultaneous inversion of magnetospheric and tidal magnetic signals results in a model with much improved resolution comparison with laboratory based conductivity profiles shows that obtained models are compatible with a pyrolytic composition and a water content of 0 01 wt and 0 1 wt in the upper mantle and transition zone respectively

correlated morphological chemical and isotopic characteristics of hibonites from the murchison carbonaceous chondrite twenty four hibonite dominated refractory grains from the murchison carbonaceous chondrite have been identified from polished grain mounts of density separates and have been morphologically and chemically divided into three groups 1 14 colourless platy crystal fragments placs with less than 2 7 tio2 2 3 pleochroic blue aggregates bags which are composed of crystal plates and fragments and have tio2 concentrations between 5 1 and 6 5 3 six spinel hibonite spherules and crystal aggregates shibs often with spinel fe silicate and clinopyroxene rims the maximum tio2 concentrations of hibonite in the shibs ranges from 3 5 to 7 6 but the tio2 concentration in one grain was heterogeneous and ranged from 0 5 to 5 0 hibonite in all types is stoichiometric with ti and mg predominantly in a coupled substitution of mg2 and ti4 for 2al3 one grain with a rounded hibonite core does not clearly fit into the three designated groups the hibonites have been analysed by ion microprobe mass spectrometry for their mg and ti isotopic compositions twelve of the placs have 26al27al 0 ratios less than 1 10 5 but the other two placs have 26al27al 0 of 5 5 10 5 and 7 7 10 5 the excess 26mg may reflect initial mg isotopic heterogeneities in the source region of the placs or be the result of in situ26al decay in which case the distribution of 26al was heterogeneous three bags show substantial positive mass fractionation of mg one grain has excess 26mg while the other two have 26mg depletions the 26mg depletions indicate the preservation of mg isotopic anomalies in the bags but the excess 26mg in one grain could be due to in situ26al decay the six shibs have excess 26mg that is correlated with 27al24mg the mg isotopic systematics of the shibs are consistent with the in situ decay of 26al with 26al27al 0 of ca 5 10 5 the unclassified hibonite has a clearly resolved deficit in 26mg titanium isotopic anomalies are common in the hibonites measured here the largest anomalies are in 50ti with variations in 50ti48ti ranging from 50 to 16 relative to terrestrial smaller anomalies are present in 49ti and 47ti the ti isotopic compositions are not directly correlated with morphological type but the placs show the largest variations in isotopic composition whereas the shibs are generally close to terrestrial the presence of hibonite populations with distinct morphological chemical and mg isotopic systematics suggests that there were several hibonite formation episodes within the early solar system the hibonites probably formed by the melting of refractory dust aggregates during local transient thermal events

the telluric field in northern germany induced by tidal motion in the north sea summary by faraday s law of induction tidal currents of the north sea induce electric currents which to a certain extent spread out far inland their influence on the telluric field in northern germany was investigated time variations of the telluric field at six field sites in northern germany were recorded continuously for at least 2 months during 1984 together with variations of the magnetic field at gottingen transfer functions between the telluric and magnetic fields were calculated from the dst and d continuum from 0 5 to 8cpd cycles per day they are presented as continuous functions of frequency by orthogonal polynomials this serves a to estimate the local telluric dc distortion matrix by comparing the transfer function values of the sq harmonics at 1 2 3 4cpd with undistorted impedances obtained by schmucker s sq analysis b to eliminate that part of the induced telluric field which is correlated with magnetic field variations in the uncorrelated part of the spectra a significant peak appears at the lunar frequency m2 24 12 4206 cpd the chapman miller method yields amplitude and phase of the mzvariation in the uncorrelated telluric fields local influences are eliminated using the inverse of the distortion matrices mentioned above the amplitudes then vary between 0 3 and 0 6 mv km the direction and phase of the uncorrelated telluric field at five field sites reflect well the pattern of the tidal current in the north sea along the coast and coincide with the assumed motion induced electric currents

fun with panurge high mass resolution ion microprobe measurements of mg in allende inclusions the performance characteristics of panurge a modified cameca ims3f ion microprobe have been studied at a mass resolving power of 5000 for the purpose of determining isotopic ratios at a precision level approaching that of counting statistics using beam switching the techniques used for this type of measurement are described using this approach the isotopic composition of mg and si and the atomic ratio of al mg in minerals from the allende inclusion wa and the allende fun inclusion cl have been measured with the ion microprobe at high mass resolving power enrichments in 26 mg of up to 260 have been found mg and al mg measurements on cogenetic spinel inclusions and host plagioclase crystals yield mg al isochrons in excellent agreement with precise mineral isochrons determined by thermal emission mass spectrometry the measurements confirm the presence of substantial excess 26 mg in wa 26 mg 27 al 5 x 10 5 and its near absence in cl 26 mg 27 al 4 x 10 6 in wa plagioclase data for which 27 al 24 mg 300 to 1000 define a linear array with 26 mg 27 al 3 x 10 5 and with initial 26 mg 24 mg composition 30 greater than in high mg phases this suggests a metamorphic reequilibration of mg in allende plagioclase at least 0 6 my after wa formation there were no variations in detected 26 mg 27 al in wa plagioclase associated with concentration of 26 mg into isolated clusters we have confirmed by ion probe measurements that the mg composition in allende cl is highly fractionated and is uniform among pyroxene melilite plagioclase spinel crystals and spinel included in melilite and plagioclase crystals likewise the si composition is mass fractionated and is the same in pyroxene melilite and plagioclase

ion microprobe studies of efremovka cais i magnesium isotope composition five coarse grained ca al rich refractory inclusions cais from the efremovka cv3 chondrite are analysed for their magnesium isotopic composition using an ion microprobe the analysed inclusions represent the three common cai types a b1 and b2 data for magnesium isotopic composition of the major mineral phases melilite spinel fassaite and anorthite in these inclusions are used to determine intrinsic magnesium isotopic fractionations and to identify nonlinear effect in 26mg due to the decay of extinct 26al the magnesium isotopic mass fractionation f mg generally favours the heavier isotopes as is expected in the case of coarse grained cais the f mg data for two of the inclusions however show contrasting trends sympathetic behaviour found in both isotopic and petrographic data in one type b1 inclusion suggests that the parent melt of this inclusion experienced a short duration volatilization event prior to crystallization the isotopic data also suggest possible presence of relict spinel in this inclusion that constrains the cooling rate of the parent melt to 70 c h during crystallization hot and dense nebular settings that allow for such fast cooling can only be localized in nature the pristine nature of the efremovka cais as evident from the lack of secondary alteration products in them is well supported by the isotopic data three of the five inclusions yield well behaved mg al isochrons with 26al27al 0 close to the canonical value of 5 10 5 the anorthite data for another inclusion suggest minor disturbance in the isotopic systematics and yield a nearly well behaved isochron with a slightly lower value of 26al27al 0 data for melilite and anorthite in the fifth inclusion define apparent isochrons that suggest partial re equilibration and exchange of magnesium isotopes between these two phases the efremovka data are suggestive of a relatively homogeneous distribution of 26al in the solar nebula

divalent cation diffusion in mg2sio4 spinel ringwoodite β phase wadsleyite and olivine implications for the electrical conductivity of the mantle mg fe and mg ni interdiffusion coefficients have been measured in single crystal olivine alpha mg ni fe sub 2 sio sub 4 polycrystalline beta phase beta mg ni fe sub 2 sio sub 4 and silicate spinel gamma mg ni fe sub 2 sio sub 4 at 1473 k between 1 and 14 gpa under controlled thermodynamic conditions in olivine d sub mg ni sup alpha ranges from 10 sup 17 m sup 2 s sup 1 at 1 gpa to 4x10 sup 18 m sup 2 s sup 1 at 9 gpa and d sub mg fe sup alpha ranges from 3x10 sup 15 m sup 2 s sup 1 at 1 gpa to 10 sup 15 m sup 2 s sup 1 at 4 gpa at 9 gpa the cation diffusion rates in gamma spinel are approx 3 orders of magnitude higher than those in olivine between 9 and 14 gpa d sub mg ni sup gamma decreases from 8x10 sup 15 to 9x10 sup 16 m sup 2 s sup 1 and d sub mg fe sup gamma decreases from 2x10 sup 13 to 5x10 sup 14 m sup 2 s sup 1 between 10 and 14 gpa the diffusion coefficients for both fe and ni are similar in magnitude and pressure dependence between beta phase and gamma spinel the activation volumes for mg ni diffusion are 3 2 and 6 7 cm3 mol 1 in the alpha and gamma beta phases respectively and for mg fe diffusion are 5 4more and 6 1 cm3 mol 1 in the alpha and gamma beta phases respectively our results demonstrate that extreme differences in transport properties are expected to occur across the 400 km discontinuity a model for electrical conductivity in the upper mantle involving two mechanisms for electrical conduction with one mechanism being associated with a hydrogen or defect related conduction process and the other involving a straightforward application of the nernst einstein equation to our cationic diffusion data reproduces geomagnetic sounding results for the upper mantle and shallow transition zone this modeling yields an order of magnitude increase in electrical conductivity across the 400 km discontinuity with little or no change in conductivity occurring near 520 km depth our modeling suggests that divalent cation diffusion is the dominant mechanism of charge transport between 400 and 670 km depth and that the high pressure olivine polymorphs dominate the electrical behavior of the deep upper mantle comparison between field based deep mantle conductivity profiles derived from magnetotelluric studies and extrapolation of our model suggests that a discontinuity in electrical transport properties exists at the base of the transition zone as well similarly given the dramatic differences in chemical diffusion between different polymorphs of olivine it is possible that other transport property mediated processes such as viscous flow could dramatically change across the 400 and 670 km discontinuities such changes are less likely to occur at 520 km depth c 2000 american geophysical union less

physicochemical isotope anomalies isotopic composition of refractory elements can be modified by physical processes such as distillation and sputtering in unexpected patterns distillation enriches the heavy isotopes in the residue and the light isotopes in the vapor however current models appear to be inadequate to describe the detailed mass dependence in particular for large fractionations coarse and fine grained inclusions from the allende meteorite exhibit correlated isotope effects in mg both as mass dependent fractionation and residual anomalies this isotope pattern can be duplicated by high temperature distillation in the laboratory a ubiquitous property of meteoritic inclusions for mg as well as for most of the other elements where measurements exist is mass dependent fractionation in contrast terrestrial materials such as microtektites tektite buttons as well as lunar orange and green glass spheres have normal mg isotopic composition a subset of interplanetary dust particles labelled as chondritic aggregates exhibit excesses in sup 26 mg and deuterium anomalies sputtering is expected to be a dominant mechanism in the destruction of grains within interstellar dust clouds an active proto sun as well as the present solar wind and solar flare flux are of sufficient intensity to sputter significant amounts of material laboratory experiments in mg show widespread isotope effects including residual supmore 26 mg excesses and mass dependent fractionation it is possible that the sup 26 mg excesses in interplanetary dust is related to sputtering by energetic solar wind particles the implication if the laboratory distillation and sputtering effects are discussed and contrasted with the anomalies in meteoritic inclusions the other extraterrestrial materials the authors have access to less

hibonite bearing microspherules a new type of refractory inclusions with large isotopic anomalies reported are petrographic descriptions major and trace element chemistry and mg ca and ti isotopic compositions of a new class of refractory inclusions that consist of spherules composed of hibonite and a silicate glass the distinctive features of these inclusions are excesses in 48ca and 50ti in both glass and hibonite and 26mg depletions relative to terrestrial isotopic compositions three spherules have been examined and analyzed one from the lance co3 meteorite and two from the murchison cm2 meteorite lance 3413 1 31 la3413 1 31 and murchison 7 228 mur7 228 have euhedral to subhedral hibonite crystals enclosed within glass murchison 7 753 mur7 753 has a rounded hibonite core with several small inclusions of perovskite a small fragment of glass is attached to the hibonite and an fe silicate rim is imperfectly preserved around the grain la3413 1 31 has a group ii ree pattern mur7 228 a refractory pattern with depletions in the relatively volatile elements sr ba nb v and eu and mur7 753 a pattern characterized by the prior removal of an ultrarefractory component and overall fractionation of all rees the partitioning of the lrees between hibonite and glass in mur7 228 is consistent with equilibrium hibonite liquid partition coefficients previously determined la3413 1 31 shows much less partitioning while mur7 753 shows no evidence for partitioning and preserves an unequilibrated refractory component highly enriched in gd all spherules have initial magnesium depleted in 26mg by around 3 relative to terrestrial mg but only mur7 228 shows evidence for in situ decay of 26al with an initial 26al27al of 1 7 0 7 10 5 both hibonite and glass in all three spherules show excesses of 48ca and 50ti ranging up to 40 and 20 respectively relative to terrestrial ca and ti spherules such as these are rare and the only other occurrence is in the unique chondrite alh85085 the hibonite in the spherules shows similarities to isotopically anomalous hibonite crystal fragments placs but it is unlikely that the spherules formed by remelting of placs the precursors include isotopically anomalous ca ti carriers but also isotopically normal refractory components that probably formed as condensates the spherules formed by melting of these precursors under disequilibrium conditions and rapid cooling after hibonite crystallization these inclusions must have formed early prior to the dilution of isotopic anomalies by mixing processes and in an area characterized by excesses of 48ca and 50ti depletions of 26mg and lack of 26al

compatibility of induction methods for mantle soundings 1 formulations that form the basis of experimental impedances in induction soundings result from the impedance boundary conditions or from the simplified theoretical models the formulations are essentially different for the magnetotelluric and magnetovariation sounding methods in order to increase reliability of mantle investigations studies of the mantle s electrical properties are often carried out by the joint inversion of impedances obtained by both sounding methods a forward modeling approach is used to verify the accuracy of merging the long period impedances obtained by the magnetotelluric and magnetovariation methods the spherical modeling of the responses above 2 d and 3 d mantle inhomogeneities has shown that the different induction methods can give mutually inconsistent results and the combination of their responses can be problematic in practice for this reason much attention is given to the generalized horizontal spatial gradient sounding method which results in impedance functions that in space and frequency domains closely resemble the magnetotelluric impedances in this study some interesting properties of the induction arrows above a spherical inhomogeneity excited by an inhomogeneous external field are estimated for long periods a final comprehensive model assuming a shell of realistic conductance at the earth s surface is evidence that the generalized horizontal spatial gradient method is promising for the study of mantle inhomogeneities and can be reliably used in combination with the magnetotelluric method in a specific way

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

magnesium isotopic compositions of olivine spinel and hibonite from the murchison carbonaceous chondrite the mg isotopic compositions of olivine spinel and hibonite from the murchison carbonaceous chondrite have been determined by secondary ion mass spectrometry the olivines exhibit only marginal effects they have no detectable intrinsic mass fractionation however after correcting for mass fractionation the mean residual 26mg24mg for the five grains is slightly in excess of the terrestrial value blue spinel which often contains inclusions of hibonite and perovskite is characterised by positive fractionation enriched in heavier isotopes ranging from 1 to 46 permil amu and with a roughly constant residual of 1 04 0 25 2 m permil these characteristics can be derived from an initial terrestrial mg isotopic composition through varying degrees of mass fractionation and the addition of 26mg from 26al decay the hibonites show both positive and negative fractionation and residuals three deep blue granular hibonites show large fractionation from 10 to 17 permil amu with residuals ranging from 1 8 to 9 7 permil the remaining five colourless hibonites are closer to the terrestrial composition the fractionation for these grains varies from 5 to 3 permil amu and the residual from 7 0 to 2 5 permil there is a hint of a correlation between fractionation and residual effects

pressure effect on electrical conductivity of mantle olivine abstract we report new measurements of electrical conductivity of olivine that were made in a multi anvil press using solid buffers mo moo2 to stabilize oxygen partial pressure the temperature range was 1000 c 1400 c and the pressure range was 4 10 gpa when pressure effects are interpreted as activation volumes in the arrhenius equation they yield values of order 0 6 0 6 cm3 mole values of the pre exponential factor 0 are of the order of hundreds of s m and the zero pressure activation energy is about 1 5 ev in agreement with laboratory measurements at lower pressure conditions in addition we analyzed older literature data which produced a similar result but with larger variation the common practice of assuming that the pressure effect on electrical conductivity of olivine in the earth s upper mantle depends weakly on pressure has rested on uncertain grounds these new observations suggest that for upper mantle conditions depths of 80 200 km neglecting pressure effects on olivine conductivity is justified a weak pressure effect also supports the small polaron fe3 model as the dominant conduction mechanism although the mainly positive activation volumes argue for a component of lattice deformation when applied to the depth range 200 400 km in the earth the laboratory data yield conductivities of order 10 2 s m slightly lower than the range of geophysical conductivity profiles but well within this range after allowing for probable effects of oxygen activity and temperature uncertainty

chromium isotopic anomalies in the murchison meteorite abstract the abundances of chromium isotopes in refractory inclusions from the allende meteorite show wide spread anomalies the chromium isotope anomalies are similar in pattern to the anomalies discovered in calcium and titanium the largest effects occur at the neutron rich isotopes 48 ca 50 ti and 54 cr individual cr rich pink spinels from the murchison meteorite exhibit large and variable excesses in 53 cr and 54 cr including the largest 53 cr anomaly so far reported 53 cr 52 cr 3 2 0 6 magnesium isotopes in murchison cr poor blue spinels also show variable anomalies in 26 mg including mass dependent fractionation favoring the lighter isotopes the 53 54 cr and 26 mg anomalies in murchison spinels are indicative of a heterogeneous distribution of magnesium and chromium isotopes in the early solar nebula and require a contribution from several nucleosynthetic components in addition to physicochemical processing

the 3d electromagnetic response of the earth to ring current and auroral oval excitation summary we report on a new compilation of functions representing the geoelectromagnetic response of the earth s mantle the c response functions and newly defined d response functions the latter of which constitutes a sensitive indicator of lateral heterogeneities in conductivity were estimated for periods from 5 to 106 7 days the spatial coverage and statistical quality of these response function estimates were improved over previous compilations in part by making use of a newly assembled global catalogue of magnetic observatory hourly mean values and by applying a statistically robust form of empirical orthogonal function eof analysis the eof results suggest that for periods longer than 5 days the geomagnetic field variation can be represented predominantly by a single eigenmode we have identified that the primarysource fields of the dominant mode consist of two current systems in the same direction an equatorial ring current in the magnetosphere and two conjugate auroral oval ring currents in the ionosphere the influence of the auroral current system is seen at surprisingly low latitudes 40 and 50 for the vertical and northern components respectively which results in non negligible bias on the conventional c response at high latitudes a preliminary correction of the auroral current effect has been made on the c and d responses obtained for data from 55 and 52 observatories respectively at latitudes from 60 to 60 we also present a set of approximate equivalent internal mantle electrical current distributions that prove a useful indicator of the presence of deep earth heterogeneities these new sets of electromagnetic information will increase the resolving power of 3d distribution of electrical conductivity in the earth s mantle

on the combination of the magnetotelluric and the geomagnetic depthsounding method for resolving an electrical conductivity increase at 400 km depth the simultaneous use of two different passive electromagnetic em sounding techniques for resolving the upper mantle electrical conductivity structure is emphasized the geomagnetic gm method uses magnetic field variations at periods between 3 hours and 600 hours yielding penetration depths between 300 km and 1000 km the magnetotelluric mt method is applied to field variations at periods between a few seconds and 16 hours and yields penetration depths between a few km and 600 km this is an extension of both methods using additional d sub st data for mt and additional s sub q data for gm in application to field data from western europe the resulting overlap of the target depths includes an electrical conductivity increase from 10 sup minus 2 s m at 300 km to 1 s m at 700 km depth while a single conductivity jump at either 400 500 or 700 km cannot explain the broadband data although the olivine to spinel transition is a possible explanation of the conductivity enhancement at 400 km depth another mineralogy could also occur this conductivity increase has not been resolved in other sub continental profiles and lateral heterogeneity is probable in this depth range 27 refs 3 figs 2 tabs

new evidence for extraordinarily rapid change of the geomagnetic field during a reversal palaeomagnetic results from lava flows recording a geomagnetic polarity reversal at steens mountain oregon suggest the occurrence of brief episodes of astonishingly rapid field change of six degrees per day the evidence is large systematic variations in the direction of remanent magnetization as a function of the temperature of thermal demagnetization and of vertical position within a single flow which are most simply explained by the hypothesis that the field was changing direction as the flow cooled