magnetic remanence in the murchison meteorite the murchison meteorite is a carbonaceous chondrite containing a small amount of chondrules various inclusions and matrix with occasional porphyroblasts of olivine and or pyroxene it also contains amino acids that may have served as the necessary components for the origin of life magnetic analyses of murchison identify an ultrasoft magnetic component due to superparamagnetism as a significant part of the magnetic remanence the rest of the remanence may be due to electric discharge in the form of lightning bolts that may have formed the amino acids the level of magnetic remanence does not support this possibility and points to a minimum ambient field of the remanence acquisition we support our observation by showing that normalized mineral magnetic acquisition properties establish a calibration curve suitable for rough paleofield determination when using this approach 1 2 of the natural remanence left in terrestrial rocks with trm and or crm determines the geomagnetic field intensity irrespective of grain size or type of magnetic mineral with the exception of hematite the same method is applied to the murchison meteorite where the measured meteorite remanence determines the paleofield minimum intensity of 200 2000 nt during and or after the formation of the parent body

an empirical failure criterion for rocks and jointed rock masses abstract a new triaxial strength equation is proposed as fitting the available laboratory triaxial test data it is shown how the five triaxial parameters unconfined compressive tensile and shear strengths coefficient of friction and exponent in the triaxial equation can be inter related by three equations a relation between intact rock strength rock mass strength ratio and barton s rock quality 1974 is next developed based on geomechanical data for four longwall panels and in situ coal strength data this relation and the coefficient of internal friction from barton s classification are used in the three inter relations to evaluate the rest of the triaxial parameters for rock masses performance of the failure criterion is studied using practical cases of rock structures in coal and coal measure strata viz five longwall caving panels and four coal pillars

pronounced changes in paleo wind direction and dust sources during mis3b recorded in the tacheng loess northwest china abstract central asia loess deposits can yield detailed records of aridification across the asian interior as well as the atmospheric circulation in this study we presented grain size heavy mineral assemblage rock magnetism and anisotropy of magnetic susceptibility of a loess section in the tacheng basin in xinjiang region nw china the results of anisotropy of magnetic susceptibility indicated that a near surface se wind prevailed in the tacheng area since marine isotope stage mis 3 while a near surface sw prevailing wind developed during mis3b we inferred that the change of wind direction was due to glacial advance during mis3b combining the result of anisotropy of magnetic susceptibility with grain size heavy mineral assemblage data and previously published low field mass susceptibility revealed that dust transported by the near surface sw wind had a greater ability to enhance magnetic susceptibility and higher amphibole epidote ratio additionally a strong siberian high during heinrich event h 5 at ca 48 ka blocked the advection of moisture into the region via mid latitude westerlies causing glacial recession and the weakening of the prevailing sw wind our study has implications for identifying changes in paleo wind direction and dust source which can aid understanding of the influence of westerlies and the siberian high on dust accumulation as well as their interactions

probabilistic performance based approaches to the static and seismic assessment of rock cavities abstract this paper demonstrates the applicability of a probabilistic performance based approach to assess the stability of rock cavities under gravity and seismic loads with reference to the case study of a neapolitan room and pillar cave located inside a rocky hill an advanced 3d model was generated importing in the numerical code the data acquired during a laser scanning based geometrical survey the careful reproduction of the on site geometry was found to improve the reliability of the results of static and dynamic analyses because i the load eccentricity induced by the irregular shape of the hill reduces the stability of pillars ii the localization of the roof crack depends on the shape of the ceiling and furthermore iii the global dynamic behaviour of the hill is affected by topographic effects the safety factors of rooms and pillars computed after the seismic analyses were compared to their values before earthquake to highlight earthquake induced modifications of the static equilibrium under gravity loads some considerations on the resilience of the cavity are reported based on the latter comparison

linear and nonlinear thellier paleointensity behavior of natural minerals 1 we tested single domain sd and multidomain md theories of the acquisition and thermal demagnetization of thermoremanent magnetization trm and partial trm ptrm in thellier paleointensity determination using as samples plagioclase and mafic minerals extracted from matachewan diabase dikes susceptibility hysteresis and coercivity data indicate that the plagioclase contains mainly sd grains of magnetite while the mafic minerals clinopyroxene biotite and actinolite contain mainly md magnetite the extracted minerals were used in simulated thellier experiments with laboratory trm as a proxy for natural remanent magnetization nrm the plagioclase behaved ideally as predicted by neel sd theory giving a linear nrm ptrm arai plot with positive ptrm checks the mafic minerals behaved nonideally with strongly curved arai plots sagging below the ideal sd line in accord with observations for md magnetites and with predictions that in heating to moderate temperatures t more trm is lost than can be regained as ptrm causing convex down arai plots in experiments with ptrm parallel to nrm all ptrm tail checks were zero but ptrm checks increasingly deviated from the arai curve toward the sd line as t increased however in repeat thellier experiments ptrms at all t agreed with the original ptrms the negative ptrm checks were therefore not due to physicochemical alteration but rather to departure from initial states with different magnetic histories than those of the original ptrms the inability of ptrm checks to reliably distinguish between md behavior and irreversible alteration could lead to overestimates of paleointensity from straight line fits to the low t unaltered part of the arai curve or underestimates from fits to high t data alternating field cleaning to 15 mt before each thellier step made the arai plot only marginally less curved low temperature demagnetization ltd zero field cycling through the 120 k verwey transition hardly changed the curvature of the arai plot for ptrm parallel to nrm for ptrm perpendicular to nrm ltd led to an inflected arai curve straddling the ideal sd line data of either form would not give reliable paleointensity values no presently available pretreatment renders the paleointensity data of md grains usable sd and nearly sd grains which behave ideally in the thellier experiment remain the only useful recorders of absolute paleointensity

magnetic properties of the olivenza meteorite possible implications for its evolution and an early solar system magnetic field abstract the magnetic properties of samples of the olivenza chondrite ll5 obtained from four collections have been investigated the natural remanent magnetization nrm consists of a very stable primary component which is randomly scattered in direction on a scale of 1 mm 3 or less within the samples and a secondary magnetization widely varying in intensity and probably also in direction the origin of the secondary nrm is not clear and may be of terrestrial origin it is concluded that the nrm is carried by the ordered nickel iron mineral tetrataenite the origin of the primary nrm could be a magnetic field associated with the solar nebula out of which the metal grains condensed and acquired a thermo remanent magnetization trm or olivenza could be a fine grained breccia the constituent fragments possessing randomly directed magnetization the implications for the origin and evolution of olivenza and its parent body if the former magnetizing process has occurred are discussed

high frequency barotropic ocean variability observed by grace and satellite altimetry 1 previous theoretical and model based studies of the relationship between ocean bottom pressure pb and sea level suggest primarily barotropic variability at mid to high latitudes for scales greater than a few hundred kilometers and periods less than a few months we use 7 day grace solutions and equivalent satellite altimetry maps spatially smoothed over 750 km to investigate the relation between pb and the observed fields are significantly coherent at high latitudes for periods of 14 180 days and mid latitudes for periods of 30 100 days the admittance amplitude between observed pb and is close to 1 for higher frequencies and drops off for lower frequencies the results provide for the first time global observational evidence for the barotropic nature of large scale ocean variability at mid and high latitudes we also demonstrate that grace data contain significant information over the oceans at periods 60 days i e shorter than the nominal monthly time resolution

near 13 day barotropic ocean response to the atmospheric forcing in the north pacific 1 in the kuroshio extension system study kess east of japan bottom pressure observations over the 2 year study period exhibit strong high frequency variability near 13 days the first cyclostationary empirical orthogonal function mode for the band pass filtered kess bottom pressure explains about 57 of the near 13 day variance and exhibits almost in phase variability in space with a hint of westward propagation the 13 day variability is strong during the winter and is driven by the large scale wind stress curl over a broad region of the north pacific modeling results over the north pacific closely follow the observations and indicate that topography confines the barotropic response to the west of emperor seamount chain and slows the westward propagation of the near 13 day bottom pressure variability

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

paleomagnetism of impact spherules from lonar crater india and a test for impact generated fields article planetary surfaces have been ubiquitously melted by meteoroid impacts throughout solar system history the resulting impact melts form some of the youngest igneous samples from rocky bodies like the moon mars and asteroids upon cooling these melts may record any ambient planetary magnetic fields as well as postulated transient fields generated by impact plasmas impact generated fields have been proposed as a key alternative to the core dynamo hypothesis for the paleomagnetism of extraterrestrial bodies here we describe a paleomagnetic study of basaltic impact glasses from the lonar impact crater situated in the deccan traps in maharashtra india previous theoretical work predicts extremely strong magnetic fields possibly n1 000 times the earth s surface field may have been transiently generated during the lonar impact we find that the glasses contain a natural remanent magnetization nrm whose properties depend strikingly on sample mass small b0 5 g splash form samples demagnetize erratically and are inefficiently magnetized while larger irregularly shaped samples contain a stable component that is efficiently magnetized similar to lonar basalts however the rock magnetic recording properties of these samples are uncorrelated with mass therefore we conclude that the size dependence of the nrm reflects a difference in how the samples acquired thermoremanence the splash forms of the smaller samples indicate they cooled during flight and therefore that they were magnetized while in motion explaining their weak and unstable nrm this motional nrm is a new manifestation of thermoremanent magnetization not observed before in geologic samples no glasses contain evidence for any strong n 100 t impact generated fields

metal phases in ordinary chondrites magnetic hysteresis properties and implications for thermal history magnetic properties are sensitive proxies to characterize feni metal phases in meteorites we present a data set of magnetic hysteresis properties of 91 ordinary chondrite falls we show that hysteresis properties are distinctive of individual meteorites while homogeneous among meteorite subsamples except for the most primitive chondrites these properties can be explained by a mixture of multidomain kamacite that dominates the induced magnetism and tetrataenite both in the cloudy zone as single domain grains and as larger multidomain grains in plessite and in the rim of zoned taenite dominates the remanent magnetism in agreement with previous microscopic magnetic observations the bulk metal contents derived from magnetic measurements are in agreement with those estimated previously from chemical analyses we evidence a decreasing metal content with increasing petrologic type in ordinary chondrites compatible with oxidation of metal during thermal metamorphism types 5 and 6 ordinary chondrites have higher tetrataenite content than type 4 chondrites this is compatible with lower cooling rates in the 650 450 c interval for higher petrographic types consistent with an onion shell model but is more likely the result of the oxidation of ordinary chondrites with increasing metamorphism in equilibrated chondrites shock related transient heating events above approximately 500 c result in the disordering of tetrataenite and associated drastic change in magnetic properties as a good indicator of the amount of tetrataenite hysteresis properties are a very sensitive proxy of the thermal history of ordinary chondrites revealing low cooling rates during thermal metamorphism and high cooling rates e g following shock reheating or excavation after thermal metamorphism our data strengthen the view that the poor magnetic recording properties of multidomain kamacite and the secondary origin of tetrataenite make equilibrated ordinary chondrites challenging targets for paleomagnetic study

mechanical properties and damage constitutive model of coal in coal rock combined body abstract coal and rock mass bear loads together in underground engineering such as coal mining and roadway support resulting in different mechanical properties from those of pure coal and rock mass accurately obtaining the mechanical properties of coal in coal rock combined body can help to properly understand the deformation and failure characteristics of the underground coal mass first an experimental method to obtain the stress and strain of coal in coal rock combined samples using strain gauges was proposed and validated then tests on thirteen types of coal rock combined samples were conducted the results show that the rock experienced a strain recovery and played a loading effect on the failure of coal when coal rock samples failed the compressive strength of coal in the sample increased with the rock strength and decreased with the coal rock height ratio furthermore two damage constitutive models of coal were established by serially connecting one damage body with one or two newton body s to reveal the influences of rock s on the mechanical behavior of coal in coal rock samples finally test results were fitted using the two models it was showed that the two models can accurately describe the stress strain curves of coal in coal rock samples and reflect the influences of rock s coal rock combination form and coal rock height ratio the fitting constants of the two models have clear physical meanings and can be easily obtained which are expected to be widely used in underground engineering

a non isostatic global sea level response to barometric pressure near 5 days changes in surface atmospheric pressure usually induce an isostatic response in the ocean characterized by an adjustment in sea level at the rate of approximately 1 cm hpa nonisostatic signals are however observed at many tropical tide gauges at periods near 5 days our analysis of satellite observations reveals the global nature of these signals involving nonisostatic sea level fluctuations with nearly uniform phase within separate basins and an out of phase oscillation between the atlantic and pacific oceans comparisons with a barotropic model further link the observed signals to forcing by a large scale pressure wave in the atmosphere and show that constrictions between basins prevent an isostatic response from being established the apparent forced nonresonant nature of the nonisostatic response contrasts with the predominantly resonant ocean dynamics at higher frequencies and may be relevant to explain the behavior of the long period tides

time variations in geomagnetic intensity 1 after many years spent by paleomagnetists studying the directional behavior of the earth s magnetic field at all possible timescales detailed measurements of field intensity are now needed to document the variations of the entire vector and to analyze the time evolution of the field components a significant step has been achieved by combining intensity records derived from archeological materials and from lava flows in order to extract the global field changes over the past 12 kyr a second significant step was due to the emergence of coherent records of relative paleointensity using the remanent magnetization of sediments to retrace the evolution of the dipole field a third step was the juxtaposition of these signals with those derived from cosmogenic isotopes contemporaneous with the acquisition of records new techniques have been developed to constrain the geomagnetic origin of the signals much activity has also been devoted to improving the quality of determinations of absolute paleointensity from volcanic rocks with new materials proper selection of samples and investigations of complex changes in magnetization during laboratory experiments altogether these developments brought us from a situation where the field changes were restricted to the past 40 kyr to the emergence of a coherent picture of the changes in the geomagnetic dipole moment for at least the past 1 myr on longer timescales the field variability and its average behavior is relatively well documented for the past 400 myr section 3 gives a summary of most methods and techniques that are presently used to track the field intensity changes in the past in each case current limits and potential promises are discussed the section 4 describes the field variations measured so far over various timescales covered by the archeomagnetic and the paleomagnetic records preference has always been given to composite records and databases in order to extract and discuss major and global geomagnetic features special attention has been devoted to discussing the degree of confidence to be put in the data by considering the integration of multiple data sets involving different techniques and or materials

meteorites found on misfits flat dry lake nevada meteorites have been found on the small misfits flat dry lakebed near stagecoach nevada 119 382w 39 348n since the first find on sept 22 2013 a total of 58 stones of weathering stage w2 3 with a combined mass of 339 g have been collected in 19 visits to the area this small 3 3 3 6 km lakebed is now a newly designated dense collection area dca most meteorites were found in a small 350 180 m area along the north shore and most are fragments of several broken individual stones three of these fragments were classified as an ll4 5 of shock stage s2 now named misfits flat 001 one of which stone mf33 fell 8 1 1 3 ka ago based on the 14c terrestrial age assuming it came from a 20 80 cm diameter meteoroid in addition a small darkly crusted meteorite mf34 now named misfits flat 002 was found 820 m wsw from the main mass this meteorite is classified as an ll5 ordinary chondrite with shock stage s4 5 the meteorite is saturated in 14c at 63 dpm kg 1 suggesting it originated from the center of a 0 5 m diameter meteoroid or deep inside a 1 0 m meteoroid less than 300 yr ago accounts exist of a fireball seen at 13 15 ut on march 2 1895 that are consistent with the find location of misfits flat 002

deviation of long period tides from equilibrium kinematics and geostrophy new empirical estimates of the long period fortnightly mf tide obtained from topex poseidon t p altimeter data confirm significant basin scale deviations from equilibrium elevations in the low latitude pacific have reduced amplitude and lag those in the atlantic by 308 or more these interbasin amplitude and phase variations are robust features that are reproduced by numerical solutions of the shallow water equations even for a constant depth ocean with schematic interconnected rectangular basins a simplified analytical model for cooscillating connected basins also reproduces the principal features observed in the empirical solutions this simple model is largely kinematic zonally averaged elevations within a simple closed basin would be nearly in equilibrium with the gravitational potential except for a constant offset required to conserve mass with connected basins these offsets are mostly eliminated by interbasin mass flux because of rotation this flux occurs mostly in a narrow boundary layer across the mouth and at the western edge of each basin and geostrophic balance in this zone supports small residual offsets and phase shifts between basins the simple model predicts that this effect should decrease roughly linearly with frequency a result that is confirmed by numerical modeling and empirical t p estimates of the monthly mm tidal constituent this model also explains some aspects of the anomalous nonisostatic response of the ocean to atmospheric pressure forcing at periods of around 5 days

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

toward a robust normalized magnetic paleointensity method applied to meteorites abstract we propose a new paleointensity method based on normalization by isothermal remanent magnetization derivative vs alternating field rem method it provides an estimate of the absolute paleointensity with an uncertainty of about a factor two contrary to methods using normalization by total isothermal remanent magnetization or anhysteretic magnetization it is applicable to multicomponent magnetizations artificial or natural isothermal remanent magnetizations can be recognized and relaxation of natural remanent magnetization can be taken into account it is applicable for magnetizations carried by magnetite feni alloys and pyrrhotite the rem method is of particular interest to estimate paleointensities in meteorites as these materials are often characterized by complex multicomponent magnetization of various origins impact magnetization thermoremanence isothermal remanent magnetization and metastable magnetic minerals not suitable for thellier experiments however like other paleointensity methods the rem method underestimates the true paleointensity for meteorite samples with magnetizations that are heterogeneous on spatial scales below the sampling scale for l ordinary chondrites an upper limit of 1 t is proposed for the paleofield tentative paleofield estimates in the 0 05 0 5 t range are proposed for ll ordinary chondrites which is much lower than previous results the first paleofield estimates for rumuruti chondrites indicate the presence of magnetic fields around 6 t during the last major impact on their parent body aubrites and hed achondrites paleofields of at least 10 t may suggest the existence of a dynamo field during the cooling of their parent body results from martian meteorites are scattered between 1 and 24 t and may represent the crustal magnetic field of the planet after dynamo shutdown carbonaceous chondrites provide contrasting results with possible evidence of strong field mt processes that clearly require additional studies

did the carbonaceous chondrites evolve in the crustal regions of partially differentiated asteroids 1 carbonaceous chondrites are considered to be the most primitive meteorites that contain some of the earliest formed solar system phases e g ca al rich inclusions and chondrules the parent bodies of the carbonaceous chondrites are traditionally known to have experienced aqueous alteration however the recent paleomagnetic records of the cv chondrite allende indicate remnant magnetism that suggests that these chondrites possibly evolved on the crustal region of a partially differentiated asteroid having a convective molten iron core we present results based on our comprehensive numerical simulations of the scenario involving melting and planetary differentiation of asteroids the possibility of a sizable carbonaceous chondritic crust on these differentiated bodies is explored the simulations indicate that it could be possible to explain the paleomagnetic records of the chondrites by the dynamo generated magnetic field from the convective molten iron core apart from the slow linear accretion scenario we envisage a scenario involving two episodes of accretion of the asteroids an early rapid accretion of the asteroids in the initial stage perhaps within the initial 2 myr would produce a sizable molten iron core subsequent to differentiation to produce the required magnetic fields this would be followed by a slow accretion of consolidated chondritic crust perhaps over several million years there is also a possibility of the disrupted expulsion of the chondritic crust by the internal pressures generated by gases released during hydration dehydration reactions

the fall recovery classification and initial characterization of the hamburg michigan h4 chondrite the hamburg meteorite fell on january 16 2018 near hamburg michigan after a fireball event widely observed in the u s midwest and in ontario canada several fragments fell onto frozen surfaces of lakes and thanks to weather radar data were recovered days after the fall the studied rock fragments show no or little signs of terrestrial weathering here we present the initial results from an international consortium study to describe the fall characterize the meteorite and probe the collision history of hamburg about 1 kg of recovered meteorites was initially reported petrology mineral chemistry trace element and organic chemistry and o and cr isotopic compositions are characteristic of h4 chondrites cosmic ray exposure ages based on cosmogenic 3he 21ne and 38ar are 12 ma and roughly agree with each other noble gas data as well as the cosmogenic 10be concentration point to a small 40 60 cm diameter meteoroid an 40ar 39ar age of 4532 24 ma indicates no major impact event occurring later in its evolutionary history consistent with data of other h4 chondrites microanalyses of phosphates with la icpms give an average pb pb age of 4549 36 ma this is in good agreement with the average sims pb pb phosphate age of 4535 3 9 5 ma and u pb concordia age of 4535 10 ma the weighted average age of 4541 6 9 5 ma reflects the metamorphic phosphate crystallization age after parent body formation in the early solar system

the barotropic electromagnetic and pressure experiment 1 barotropic current response to atmospheric forcing during 1986 1987 an array of horizontal electrometers which measure the subinertial barotropic velocity was deployed for 11 months in the central north pacific to study atmospherically forced motions in a low eddy kinetic energy region significant coherence is found to be ubiquitous between the barotropic velocity and one or more surface atmospheric variables over the north pacific the maximum squared coherence between currents and wind stress curl at all periods greater than 3 days is typically 0 6 rarely below 0 4 and occasionally above 0 8 and is dominantly nonlocal although local coherence is seen at some sites especially at the shorter periods both the coherence and the intersite pattern similarity are strongest at the periods of relative maxima in the barotropic current autospectra e g at 6 8 10 14 and 20 50 days the locations at which the wind stress curl forces the motions at each site and for each period have been determined while there is substantial intersite variability some of the observed patterns of squared coherence over the north pacific are very similar to those predicted by simple linear smooth bottomed quasi geostrophic models of atmospherically forced barotropic currents the forcing location is occasionally found to be the same spot for all oceanic instruments especially at the autospectral maxima in barotropic current more commonly distinct forcing locations are found for each instrument even when closely spaced suggesting topographic influence in other cases multiple forcing locations are implicated for the motions at a single station all of this site and period dependent variability of the forcing location is incompatible with the simple models and is due either to the known spatial inhomogeneity of the curl forcing field or to topographic effects the importance of the former is emphasized using simple arguments based on analytic transfer functions between the atmospheric and oceanic variables and on observed wavenumber spectra for wind stress curl quantitative assessment of topographic effects must await suitable numerical models in addition maximum likelihood wavenumber spectra of the barotropic currents have been estimated in those bands with an adequate number of coherent sensors the wavenumbers at the spectral peaks are compatible with the dispersion relation for rossby waves modified by the observed large scale linearly sloping topography which shoals to the northeast the observed group velocities are in general quite consistent with the inferred forcing locations based on the ocean atmosphere coherences

magnetic characteristics of cv chondrules with paleointensity implications 1 we have conducted a detailed magnetic study on 45 chondrules from two carbonaceous chondrites of the cv type 1 mokoia and 2 allende allende has been previously extensively studied and is thought to have a high potential of retaining an extra terrestrial paleofield few paleomagnetic studies of mokoia have previously been undertaken we report a range of magnetic measurements including hysteresis first order reversal curve analysis forcs demagnetization characteristics and isothermal remanent irm acquisition behavior on both mokoia and allende chondrules the mokoia chondrules displayed more single domain like behavior than the allende chondrules suggesting smaller grain sizes and higher magnetic stability the mokoia chondrules also had higher average concentrations of magnetic minerals and a larger range of magnetic characteristics than the allende chondrules irm acquisition analysis found that both sets of chondrules have the same dominant magnetic mineral likely to be a feni phase taenite kamacite and or awaruite contributing to 48 of the mokoia chondrules and 42 of the allende chondrule characteristics forc analysis revealed that generally the allende chondrules displayed low field coercivity distributions with little interactions and the mokoia chondrules show clear single domain like distributions paleointensity estimates for the two meteorites using the remc and preisach methods yielded estimates between 13 and 60 t and 3 56 t respectively for allende and 3 140 t and 1 110 t respectively for mokoia from the data we suggest that mokoia chondrules carry a non primary remagnetization and while allende is more likely than mokoia to retain its primary magnetization it also displays signs of post accretionary magnetization

magnetic study of magnetite in the tagish lake meteorite the saturation magnetization saturation remanent magnetization the coercive and remanent coercive force were determined at room and liquid nitrogen temperatures for three pieces of the tagish lake meteorite the results are compared to similar data for four other chondrites allende murray orgueil and murchison the data suggests that the tagish lake meteorite is magnetically homogeneous and is not as magnetically hard as the comparison chondrites the magnetization measurements indicate that it contains about 10 11 multi domain magnetite magnetic susceptibility measurements on all the samples from 77 k to room temperature showed a verwey transition for all the samples which contain a significant amount of multi domain magnetite the coercive force data further indicate that the magnetite in tagish lake is multi domain and that the grain size is small and approximately 4 9 m

strength reduction on saturation of coal and coal measures rocks with implications for coal pillar strength coal and associated coal measures rocks including shales mudstones and sandstones may lose significant strength on water saturation due to the absorption of water stress corrosion or mineralogical changes to the clays within the rock matrix as most coal seams are natural aquifers water ingress into underground mine workings is common and is typically controlled during mining by dewatering pumps after a mine has closed unless ongoing pumping is maintained workings will flood and coal pillars and the pillar roof and floor foundations will revert to fully saturated conditions this paper reports on a study of how flooding effects pillar strength in an abandoned coal mine in queensland australia the reduction in strength of coal and coal measures rocks between unsaturated and saturated water moisture contents has been quantified through physical testing these strength data along with the distribution of lithologies in a coal pillar from the mine were used as inputs for numerical modelling of the strength reduction of a coal pillar on water saturation it was found that the modelled pillar strength reduction could be estimated by the average strength reduction of all the lithological components of the pillar weighted by the thickness of each lithology

aspects of the validation of magnetic remanence in meteorites meteorite magnetic records constitute physical evidence of processes acting during early solar system evolution consequently the validation of these records is important in meteorite research the first step in the validation process should be the rem value the rem value is the ratio of natural remanence nrm to saturation remanent magnetization imparted by a 1 t magnetic field sirm the rem values range over 3 to 4 orders of magnitude for stony meteorites and for chondrules from allende c3v s1 bjurbole l4 s1 and chainpur ll3 s1 meteorites the rem values computed from published nrm and sirm data identify many orders of magnitude range in the rem values including rem values 100 10 3 these data suggest a dependence for the nrm intensity on the curatorial location from which the sample was obtained any earth rock acquiring thermoremanent magnetization trm in the geomagnetic field has a restricted range in rem mostly between 5 and 50 10 3 the exception being the mineral hematite in the multidomain size range the only terrestrial samples with rem much greater than 100 10 3 are those struck by lightning the rem value provides a physical basis for recognition between valid records and those that might be contaminated the isothermal remanence acquisition ra curve is presented as a contamination curve that allows an indication of the level of magnetic field contamination required to give the computed rem rm sirm value in the case of the bjurbole and chainpur chondrules with rem values 100 10 3 the ra curve indicates that unrealistically large contamination magnetic fields would be required to give rem values greater than 100 10 3 this would suggest contamination other than by a hand magnet that is normally available to an experimenter this would require an explanation that would involve large magnetic fields during chondrule formation or some extraordinary remanence acquisition mechanism that remains to be described magnetic contamination experiments using 80 and 40 mt magnets demonstrate that the rem values and extent of modification of the magnetic vector record are mineralogy dependent and this is mostly related to the amount and characteristics of the mineral tetrataenite the complexity of the meteorite records suggest validation of the record as a first step the rem value is the first physical statement that can be made in this validation

modified mohr coulomb criterion for non linear triaxial and polyaxial strength of jointed rocks abstract rocks encountered in civil and mining engineering fields are invariably jointed and act under triaxial or polyaxial stress conditions the mohr coulomb shear strength criterion is the most widely used criterion for jointed rocks in its present form there are two major limitations of this criterion firstly it considers the strength response to be linear and secondly the effect of the intermediate principal stress on the strength behaviour is ignored a modified non linear form of mohr coulomb strength criterion has been suggested in this study to overcome these limitations barton s concept of critical state for rocks has been imbibed in the linear mohr coulomb criterion to deduce a semi empirical expression for non linear criterion however the shear strength parameters of the conventional mohr coulomb criterion are used in the proposed criterion the proposed criterion is a simple and rational nonlinear polyaxial strength criterion for anisotropic jointed rocks in an earlier publication 1 the applicability of the criterion was evaluated for intact rocks in present paper the criterion is extended to jointed rocks which are anisotropic in nature the applicability of the proposed criterion has been verified by applying it to extensive experimental data on triaxial and polyaxial test results on jointed rocks available from literature applicability of the criterion to explain rock burst conditions for some indian rocks is also demonstrated

experimental constraints on magnetic stability of chondrules and the paleomagnetic significance of dusty olivines dynamic crystallization experiments are conducted under a magnetic field to determine both magnetic and mineralogical properties of chondrules the experiment reproduced synthetic dusty olivine samples that were formed by a high temperature reduction of an initially fayalitic olivine backscattered electron microscopy observations confirmed that synthetic dusty olivine contains abundant fine submicron sized ni poor fe inclusions in the cores of mgo rich olivine grains similar to that in natural chondrules alternating field demagnetization experiments of dusty olivine samples indicate mean destructive fields of up to 80 mt suggesting the submicron sized fe inclusions are a carrier of stable remanence in natural chondrules fine fe inclusions in the dusty olivine may have been armored against chemical alteration by surrounding host olivine crystals since the fine fe inclusions were probably heated above the curie temperature during the last chondrule forming events the fine fe inclusions in dusty olivine can acquire thermal remanent magnetization during the chondrule formation event theoretical time temperature relation of such fine grained fe kamacite grains suggested that a paleomagnetic data observed above 490 c in thermal demagnetization experiments of dusty olivines is reliable despite the low grade metamorphism of unequilibrated ordinary chondrites e g ll3 0 therefore the presence of fine fe inclusions in dusty olivine in unequilibrated ordinary chondrites constrains that such dusty olivine in chondrules is a good candidate as an un altered and stable magnetic recorder of the early solar magnetic field

polycrystalline taenite and metallographic cooling rates of chondrites reply to comments of a w r bevan and h j axon 1981

433 eros problems with the meteorite magnetism record in attempting an asteroid match the magnetometer experiment mag onboard the near shoemaker spacecraft detected no global scale magnetization and established a maximum magnetization of 2 1x10 6 am 2 kg 1 for asteroid 433 eros this is in sharp contrast with the estimated magnetization of other s class asteroids gaspra 2 4 x 10 2 am 2 kg 1 braille 2 8 x 10 2 am 2 kg 1 and is below published values for all types of ordinary chondrites this includes the l ll types considered to most closely match 433 eros based on preliminary interpretations of near remote geochemical experiments the ordinary chondrite meteorite magnetization intensity data was reviewed in order to assess the reasonableness of an asteroid meteorite match based on magnetic property measurements nrm natural remanent magnetization intensities for the ordinary chondrite meteorites show at least a 2 order of magnitude range within each of the h l ll groups all well above the 2 1 x10 6 am 2 kg 1 level for 433 eros the rem values ratio of the nrm to the sirm saturation remanent magnetization range over 3 orders of magnitude for all chondrite groups indicating no clear relationship between nrm and the amount of magnetic material levels of magnetic noise in chondrite meteorites can be as much as 70 or more of the nrm consequently published values of the nrm should be considered suspect unless careful evaluation of the noise sources is done goddard studies of per unit mass intensities in large 10 000gm and small down to 1 gm samples from the same meteorite demonstrate magnetic intensity decreases as size increases this would appear to be explained by demagnetization due to magnetic vector randomness at unknown scale sizes in the larger samples this would then argue for some level of demagnetization of large objects such as an asteroid the possibility that 433 eros is an ll chondrite cannot be discounted

evidence from the rb sr system for 4 4 ga alteration of chondrules in the allende cv3 parent body the timing and processes of alteration in the cv parent body are investigated by the analysis of sr isotopes major and trace elements and petrographic type and distribution of the secondary minerals nepheline and sodalite in 22 chondrules from the allende cv3 chondrite the sr isotopic compositions of the chondrules are scattered around the 4 0 ga reference line on the 87sr 86sr evolution diagram indicating that the chondrules have been affected by late thermal alteration event s in the parent body the degree of alteration determined for individual chondrules based on the distribution of nepheline and sodalite is unrelated to the disturbance of the rb sr system suggesting that the alteration process that produced nepheline and sodalite is different from the thermal process that disturbed the rb sr system of the chondrules considering the geochemical behavior of rb and sr the main host phase of sr in chondrules is likely to be mesostasis which could be most susceptible to late thermal alteration as there is a poor connection between the alteration degree determined from abundances of nepheline and sodalite and the disturbance of rb sr isotopic system we consider the mesostasis to provide a constraint on the late parent body alteration process from this point of view 23 mesostasis rich chondrules including those from literature data were selected the selected chondrules are closely correlated on the 87sr 86sr evolution diagram with an inferred age of 4 36 0 08 ga this correlation would represent an age of the final major sr isotopic redistribution of the chondrules in the parent body

the anomalous demagnetization behaviour of chondritic meteorites abstract alternating field af demagnetization of chondritic samples often shows anomalous results such as large directional and intensity changes saw tooth intensity vs demagnetizing field curves are also prevalent an attempt to explain this behaviour is presented using a computer model in which individual mineral grains can be magnetized in a variety of different ways a simulated demagnetization can then be carried out to examine the results it was found that the experimental behaviour of chondrites can be successfully mimicked by loading the computer model with a series of randomly orientated and sized vectors the parameters of the model can be changed to reflect different trends seen in experimental data many published results can be modelled using this method a known magnetic mineralogy can be modelled and an unknown mineralogy deduced from af demagnetization curves only by comparing data from mutually orientated samples can true stable regions for palaeointensity measurements be identified calling into question some previous estimates of field strength from meteorites

nonheating methods for absolute paleointensity determination comparison and calibration using synthetic and natural magnetite bearing samples nonheating paleointensity methods utilize an anhysteretic remanent magnetization arm or a saturation isothermal remanent magnetization to model the natural thermal remanent magnetization trm to avoid heating induced alteration we report the results of paleointensity experiments using the arm pseudo thellier and ratio of equivalent magnetization rem methods conducted to investigate their relative efficiency in recovering the true paleofield strength and to provide additional estimates of their calibration factors the experiments on synthetic magnetite bearing samples representing single domain sd and pseudo single domain psd magnetic states indicated that the correction factors for the arm based methods depend on the magnetic grain size domain state changing from 6 3 for sd grains to 4 1 for 1 5 m psd grains the pseudo thellier method yielded correct absolute paleointensity values when normalization by the trm arm demagnetization slope was used when applied to samples of lava flows and dikes from the 32 kyr lemptegy volcano france both the arm and pseudo thellier methods produced similar paleointensity estimates 28 0 5 1 t and 26 9 4 7 t respectively consistent with the available thellier data for the 31 33 kyr time interval the correction factors estimated from our synthetic samples for the rem 3000 and for remc 1500 and rem 1500 variants are consistent with the previously published estimates however all rem variants yielded unrealistically high estimates 110 t of the paleofield strength from our natural samples our experimental results support arm as a better proxy of trm and suggest that the arm based methods currently represent the best alternative to heating based absolute paleointensity method

a preisach method for estimating absolute paleofield intensity under the constraint of using only isothermal measurements 2 experimental testing 1 this paper reports an empirical test of a new nonheating preisach based protocol for determining the absolute ancient magnetic field intensity paleointensity using a selection of synthetic samples and a large suite of modern lavas and pyroclastic lithic samples generally the preisach paleointensity estimates compare favorably with the expected field intensities 1 for the synthetic samples displaying single domain like behavior the method returned the correct result for the known field while multidomain samples yielded an underestimate and 2 averaging 168 post 1850 a d lavas yielded a value 6 within the expected field value these preisach paleointensity estimates are also compared with paleointensity determinations made on the same suite of samples using standard thellier type heating protocols and the nonheating remanence rem method the preisach paleointensity estimates compare favorably for samples that yielded correct thellier type determinations including the synthetic single domain like samples in addition the preisach method produced estimates for cases where the thellier type estimates failed a possible selection criterion was identified median destructive field which was found to improve the paleointensity estimates in some sample suites for the investigated sample set the preisach method was found to be much more accurate than the rem method the importance of cooling rate on the preisach paleointensity estimate is also examined

magnetic microstructures of metal grains in equilibrated ordinary chondrites and implications for paleomagnetism of meteorites meteorites are a primary source of information about past magnetic field in the solar system yet the small scale magnetic properties of feni metals which are the magnetic carriers of most meteorites are poorly known we study here the magnetic microstructures of feni metals in two equilibrated chondrites two types of tetrataenite bearing microstructures are revealed 1 zoned taenite particles that consist of a cloudy zone 20 250 nm large tetrataenite precipitates embedded in ni poor matrix and a 1 10 m thick tetrataenite rim 2 zoneless plessite particles that consist of large tetrataenite grains 1 m embedded in a kamacite matrix magneto optical imaging of saturation remanence shows that the submicron sized tetrataenite islands in cloudy zone carry a much stronger remanence than the m sized tetrataenite crystals in the tetrataenite rims and plessite micron scale mapping of coercivity of remanence bcr shows that the center part of the cloudy zone has finer tetrataenite grains 20 nm and higher bcr values 1 t than the outer part 250 nm and 400 mt respectively these results suggest that the micron sized tetrataenite is in a multi domain state whereas the submicron sized tetrataenite in the cloudy zone are in a single domain like state and may be regarded as a potentially good paleomagnetic recorder in meteorites the stability of the remanent magnetization in ordinary chondrites is a function of the amount of the cloudy zones of the zoned taenite grains rather than the bulk amount of tetrataenite the fe ni phase diagram indicates that precursor of tetrataenite is paramagnetic when the metamorphic temperature was above 350 400 c therefore the remanent magnetization of tetrataenite cannot be an evidence of early magnetic activity on the parent body e g dynamo activity during the first 10 to 50 myr after the peak of metamorphism assuming 900 c peak temperature and 50 100 c myr cooling rate our tem observations show that tetrataenite has a homogeneous crystallographic orientation in an individual zoned taenite grain in low or null field tetrataenite may acquire a spontaneous magnetization whose direction is controlled by this crystallographic orientation which varies from grain to grain the small scale heterogeneity of remanence observed in equilibrated chondrites may imply that no significant magnetic field e g dynamo field was present during cooling below 350 400 c

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

magnetic classification of stony meteorites 2 non ordinary chondrites a database of magnetic susceptibility measurements on different non ordinary chondrites c e r and ungrouped populations is presented and compared to our previous similar work on ordinary chondrites it provides an exhaustive study of the amount of iron nickel magnetic phases essentially metal and magnetite in these meteorites in contrast with all the other classes cm and cv show a wide range of magnetic mineral content with a two orders of magnitude variation of whether this is due to primary parent body differences metamorphism or alteration remains unclear c34 and c2 yield similar values to the ones shown by ck and cm respectively by order of increasing the classes with well grouped are r co ck ci kak cr e ch cb based on magnetism eh and el classes have indistinguishable metal content outliers that we suggest may need to have their classifications reconsidered are acfer 202 co elephant moraine eet 96026 c45 meteorite hills met 01149 and northwest africa nwa 521 ck asuka a 88198 lapaz icefield lap 031156 and sahara 98248 r values can also be used to define affinities of ungrouped chondrites and propose pairing particularly in the case of cm and cv meteorites

near 5 day nonisostatic response of the atlantic ocean to atmospheric surface pressure deduced from sub surface and bottom pressure measurements 1 nonisostatic ocean responses to atmospheric pressure have been observed in tropical regions of the pacific and atlantic oceans at periods near 5 days barotropic ocean model simulations coupled to atmospheric forcing predicted that this nonisostatic ocean response is driven not regionally but globally by the rossby haurwitz wave to date little observational evidence has been provided to support the model simulations especially at extratropical latitudes here we present the basin scale nature of the nonisostatic response in the atlantic ocean near 5 day periods using four historical long term 1 5 year sub surface and bottom pressure measurements spanning from 16 s to 37 n joint analysis of them together with global gridded atmospheric pressure reveals a basin scale nonisostatic sea level fluctuation in the north and tropical atlantic oceans near 5 day periods with almost uniform phase it also confirms that the driving force for this near 5 day fluctuation is the westward propagating rossby haurwitz wave

evidence of a 4 6 day barotropic planetary oscillation of the pacific ocean abstract spectral analysis of scattered island and coastal tide gage records from the pacific ocean reveals the presence of a coherent sea level fluctuation at 4 6 days period the oscillation is distinct from baroclinic inertia gravity wave fluctuations of sea level at the same periods that are trapped to the central pacific equatorial zone concomitant spectral analysis of island surface weather data demonstrates that sea level is forced by surface atmospheric pressure but does not respond statically like an inverted barometer the basinwide character and uniform westward propagation of the oscillation suggest the presence of a barotropic planetary wave s however the oscillation is strongly attenuated with an estimated energy e folding time of less than three days

evidence for an impact induced magnetic fabric in allende and exogenous alternatives to the core dynamo theory for allende magnetization we conducted a paleomagnetic study of the matrix of allende cv3 chondritic meteorite isolating the matrix s primary remanent magnetization measuring its magnetic fabric and estimating the ancient magnetic field intensity a strong planar magnetic fabric was identified the remanent magnetization of the matrix was aligned within this plane suggesting a mechanism relating the magnetic fabric and remanence the intensity of the matrix s remanent magnetization was found to be consistent and low 6 t the primary magnetic mineral was found to be pyrrhotite given the thermal history of allende we conclude that the remanent magnetization was formed during or after an impact event recent mesoscale impact modeling where chondrules and matrix are resolved has shown that low velocity collisions can generate significant matrix temperatures as pore space compaction attenuates shock energy and dramatically increases the amount of heating nonporous chondrules are unaffected and act as heat sinks so matrix temperature excursions are brief we extend this work to model allende and show that a 1 km s planar impact generates bulk porosity matrix porosity and fabric in our target that match the observed values bimodal mixtures of a highly porous matrix and nominally zero porosity chondrules make chondrites uniquely capable of recording transient or unstable fields targets that have uniform porosity e g terrestrial impact craters will not record transient or unstable fields rather than a core dynamo it is therefore possible that the origin of the magnetic field in allende was the impact itself or a nebula field recorded during transient impact heating

magnetic characterization of the new magnetic mineral tetrataenite and its contrast with isochemical taenite abstract taenite with 48 57 ni is magnetically soft but when transformed via atomic ordering below 320 c it becomes the new magnetic mineral tetrataenite with considerable magnetic hardness and other distinctive magnetic properties tetrataenite is ubiquitous in chondrite meteorites occurring as discrete grains as a clear taenite rim and in the etched cloudy zone measured remanent coercivities range from 30 mt in appley bridge ll4 to 600 mt in some bjurbole l4 chondrules this range suggests a considerable variation in tetrataenite properties that might be related to degree of order size and properties of the ordered regions possible shock induced effects and other causes including the initial grain size particularly in the cloudy zone tetrataenite is characterized during heating to the apparent curie point near 550 c and taenite is characterized during subsequent cooling to room temperature this record is possible because of the large magnetic anisotropy contrasts between tetrataenite and taenite and the fact that heating to the apparent curie point destroys the magnetic anisotropy of tetrataenite thereby allowing a taenite cooling record failure to recognize the presence of tetrataenite can result in considerable misinterpretation of thermomagnetic analysis since the curve shapes for tetrataenite and taenite can be different magnetic hysteresis loops thermomagnetic curves coercivity temperature curves and saturation remanence thermal demagnetization curves provide unquestionable records of two vastly different magnetic materials classical magnetic techniques can be used to monitor the progress of disordering of the tetrataenite during isothermal annealing magnetic hysteresis loop measurements of a bjurbole chondrule at 488 c clearly record the magnetic parameter changes associated with disordering because of the large coercivity and other distinctive properties tetrataenite exerts significant control on the bulk magnetic properties of meteorites regardless of its volumetric significance paleofield estimates to date have not considered the role of tetrataenite and should therefore be accepted with caution

magnetic properties of the ll5 ordinary chondrite chelyabinsk fall of february 15 2013 here we characterize the magnetic properties of the chelyabinsk chondrite ll5 s4 w0 and constrain the composition concentration grain size distribution and mineral fabric of the meteorite s magnetic mineral assemblage data were collected from 10 to 1073 k and include measurements of low field magnetic susceptibility v0 the anisotropy of v0 hysteresis loops first order reversal curves m spectroscopy and x ray microtomography the rem and rem 0 paleointensity protocols suggest that the only magnetizations recorded by the chondrite are components of the earth s magnetic field acquired during entry into our planet s atmosphere the chelyabinsk chondrite consists of light and dark lithologies fragments of the light lithology show logv0 4 57 0 09 s d n 135 while the dark lithology shows 4 65 0 09 n 39 where v0 is in 10 9 m 3 kg 1 thus chelyabinsk is three times more magnetic than the average ll5 fall but is similar to a subgroup of metal rich ll5 chondrites paragould aldsworth bawku richmond and l ll5 chondrites glanerbrug knyahinya the meteorite s room temperature magnetization is dominated by multidomain feni alloys taenite and kamacite no tetrataenite is present however below approximately 75 k remanence is dominated by chromite the metal contents of the light and dark lithologies are 3 7 and 4 1 wt respectively and are based on values of saturation magnetization

micromagnetic coercivity distributions and interactions in chondrules with implications for paleointensities of the early solar system 1 chondrules in chondritic meteorites record the earliest stages of formation of the solar system potentially providing information about the magnitude of early magnetic fields and early physical and chemical conditions using first order reversal curves forcs we map the coercivity distributions and interactions of 32 chondrules from the allende karoonda and bjurbole meteorites distinctly different distributions and interactions exist for the three meteorites the coercivity distributions are lognormal shaped with bjurbole distributions being bimodal or trimodal the highest coercivity mode in the bjurbole chondrules is derived from tetrataenite which interacts strongly with the lower coercivity grains in a manner unlike that seen in terrestrial rocks such strong interactions have the potential to bias paleointensity estimates moreover because a significant portion of the coercivity distributions for most of the chondrules is 10 mt low coercivity magnetic overprints are common therefore paleointensities based on the rem method which rely on ratios of the natural remanent magnetization nrm to the saturation isothermal remanent magnetization irm without magnetic cleaning will probably be biased the paleointensity bias is found to be about an order of magnitude for most chondrules with low coercivity overprints paleointensity estimates based on a method we call remc which uses nrm irm ratios after magnetic cleaning avoid this overprinting bias allende chondrules which are the most pristine and possibly record the paleofield of the early solar system have a mean remc paleointensity of 10 4 mt karoonda and bjurbole chondrules which have experienced some thermal alteration have remc paleointensities of 4 6 and 3 2 mt respectively

study on the structural characteristics of the overburden under thick loose layer and thin bed rock for safety of mining coal seam the geological conditions of the first working face at the baodian coal mine of the yanzhou mining district in china are examined to determine the feasibility of modifying coal pillars that prevent water seepage to those that prevent sand flow the coal pillars are under a thick loose layer of sediment and thin layer of bedrock after analyzing the structure of the rock and soil mass above the coal seam the thickness of the bedrock bottom clay layer and bottom aquifer of two detecting roadways are discussed at the center of the xingxing anticline the thinnest of the bedrock is only about 30 m however the thickness of the bottom clay layer is at least 10 m and more and the jurassic rock sandstone is the thinnest with low levels of water and a layer of weathered material the bottom aquifer has low water yield with no pressure the similarity theory is used to construct a prototype with 30 m high bedrock the conclusion is that the height of the caved zone height is about 21 3 m after the complete extraction of no 3 coal seam and the results are in agreement with the experimental data the results show that it is feasible for the first working face to use a coal pillar to prevent sand flow moreover the findings in this study can act as an important reference source for predicting the height of the overburden that would cause damage and reasonable advancement of mining that is considered to be safe under similar geomining conditions

the implications of the magnetism of ordinary chondrite meteorites abstract the magnetic properties of eleven ordinary chondrites eight ll chondrites and three l chondrites have been analysed the samples were repeatedly fragmented and oriented to a common reference direction natural remanent magnetisation nrm susceptibility and anisotropy of susceptibility measured at each stage the response of some fragments to alternating field a f demagnetisation thermal demagnetisation isothermal remanent magnetisation irm and magnetic hysteresis has been measured it was found that the orientation of the stable nrm was random down to a scale of 1 mm 3 the dominant magnetic carrier in types 4 6 was the ordered iron nickel mineral tetrataenite the type 3 s measured contained no tetrataenite and the nrm was carried by fine grained taenite a magnetic fabric analogous to a physical fabric was found in all samples in all but two the fabric was foliated tuxtuac and wold cottage showed lineation thermal demagnetisations of nrm showed a trend towards higher blocking temperatures for type 6 chondrites over types 3 5 because the fabric was continuous in the majority of samples and the nrm randomly orientated it was concluded that the magnetic carriers were magnetised before emplacement in the meteorite and that the meteorites are not finescale breccias the preservation of the random nrm leads to the preference of hot accretion as the mechanism for producing chondritic textures as opposed to metamorphic reheating as this would tend to erase the random magnetisation

trm in low magnetic fields a minimum field that can be recorded by large multidomain grains abstract thermally acquired remanent magnetization is important for the estimation of the past magnetic field present at the time of cooling rocks that cool slowly commonly contain magnetic grains of millimeter scale this study investigated 1 mm sized magnetic minerals of iron iron nickel magnetite and hematite and concluded that the thermoremanent magnetization trm acquired by these grains did not accurately record the ambient magnetic fields less than 1 t instead the trm of these grains fluctuated around a constant value consequently the magnetic grain ability to record the ambient field accurately is reduced above the critical field trm acquisition is governed by an empirical law and is proportional to saturation magnetization m s the efficiency of trm is inversely proportional to the mineral s saturation magnetization m s and is related to the number of domains in the magnetic grains the absolute field for which we have an onset of trm sensitivity is inversely proportional to the size of the magnetic grain these results have implications for previous reports of random directions in meteorites during alternating field demagnetization or thermal demagnetization of trm extraterrestrial magnetic fields in our solar system are weaker than the geomagnetic field by several orders of magnitude extraterrestrial rocks commonly contain large iron based magnetic minerals as a common part of their composition and therefore ignoring this behavior of multidomain grains can result in erroneous paleofield estimates

first order reversal curve forc diagrams for pseudo single domain magnetites at high temperature abstract the recently developed first order reversal curve forc technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations however there are still some gaps in the basic understanding of forc diagrams in particular the behavior of pseudo single domain psd grains and the contribution of magnetostatic interactions in this paper we address some of these problems we report the first forc diagrams measurements on narrowly sized and well characterized synthetic psd through multidomain md magnetite samples the forc diagrams evolve with grain size from single domain sd like to md like through the psd grain size range since each sample contains grains of essentially a single size individual psd grains evidently contain contributions from both sd like and md like magnetic moments in proportions that vary with grain size the evolving forc diagrams cannot be due to physical mixtures of sd and md grains of widely different sizes the forc diagrams were all asymmetric small psd samples have forc diagrams with a distinctive closed contour structure the distributions of the larger md grains display no peak and lie closer to the interaction field axis to assess the effect of magnetostatic interactions we measured forc diagrams between room temperature and the curie temperature on heating the forc distributions contract without changing shape until 500 c above this temperature the diagrams become more md like and in addition become more symmetric the temperature dependence of the interaction field parameter is proportional to that of the saturation magnetization in accordance with neel s interpretation of the preisach diagram the decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions the magnetic hysteresis parameters as a function of temperature were determined from the forc curves as the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature

long lived magnetism on chondrite parent bodies abstract we present evidence for both early and late stage magnetic activity on the cv and l ll parent bodies respectively from chondrules in vigarano and bjurbole using micro ct scans to re orientate chondrules to their in situ positions we present a new micron scale protocol for the paleomagnetic conglomerate test the paleomagnetic conglomerate test determines at 95 confidence whether clasts within a conglomerate were magnetized before or after agglomeration i e for a chondritic meteorite whether the chondrules carry a pre or post accretionary remanent magnetization we found both meteorites passed the conglomerate test i e the chondrules had randomly orientated magnetizations vigarano s heterogeneous magnetization is likely of shock origin due to the 10 to 20 gpa impacts that brecciated its precursor material on the parent body and transported it to re accrete as the vigarano breccia the magnetization was likely acquired during the break up of the original body indicating a cv parent body dynamo was active 9 ma after solar system formation bjurbole s magnetization is due to tetrataenite which transformed from taenite as the parent body cooled to below 320 c when an ambient magnetic field imparted a remanence we argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution and a l ll parent body dynamo must have been active at least 80 to 140 ma after peak metamorphism primitive chondrites did not originate from entirely primitive never molten and or differentiated parent bodies primitive chondrite parent bodies consisted of a differentiated interior sustaining a long lived magnetic dynamo encrusted by a layer of incrementally accreted primitive meteoritic material the different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies and or formation location in the protoplanetary disk