sources of magnetic susceptibility in a slate abstract magnetic susceptibility and its anisotropy in the borrowdale volcanic slates at kentmere in the english lake district are attributed largely to preferred orientation of a paramagnetic chlorite of diabantite ripidolite composition in units of 10 6 cgs g the principal susceptibilities for the slates are 9 61 9 42 8 69 and for the chlorite grains the minimum anisotropy is represented by principal susceptibilities of 11 57 11 22 and 9 15 because the magnetic susceptibility is carried by a tightly packed matrix forming mineral that has recrystallised during the deformation it is not possible to imagine simple grain rotation as being responsible for the anisotropy of susceptibility

magnetic susceptibility fabrics in slates structural mineralogical and lithological influences abstract a single penetrative tectonic deformation of archean greywackes accompanied by low grade metamorphism generated low field magnetic susceptibility fabrics that are coaxial with the rock s schistosity although there are multiple sources of magnetic susceptibility pre metamorphic magnetite and the metamorphic minerals pyrrhotite biotite and chlorite variety thuringite daphnite aphrosiderite are responsible for the anisotropy of susceptibility these minerals grew with preferred crystallographic orientations influenced by the prevailing stress regime on its own rotation cannot be invoked to explain the alignment of the metamorphic minerals there are varying proportions of at least two of these minerals in most specimens for 153 specimens the degree of anisotropy increases as the mean susceptibility increases over the range from 100 to 700 in units of 10 6 s i theory shows that this is to be expected either due to increasing the traces of strongly oriented ferrimagnetics or due to increasing the proportion of chlorite in the latter case this simultaneously increased its preferred orientation by closer packing stress influenced crystallization of chlorite rapidly achieves saturation alignment and a limiting magnetic anisotropy although strain may continue to increase samples taken through individual graded beds show that the susceptibility ellipsoid varies from oblate in slaty rocks to a neutral shape in sandier rocks however its orientation is always structurally controlled with the minimum susceptibility axis perpendicular to schistosity

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

field impressed anisotropies of magnetic susceptibility and remanence in minerals the application of alternating af or direct df magnetic fields to samples containing dispersed particles of magnetite titanomagnetite or maghemite has been found to alter significantly the measured low field susceptibility anisotropy even though the particle orientation is unchanged the acquisition of isothermal remanent magnetization irm is also strongly dependent on the previous af treatment given to the samples a method of quantifying the changes brought about by application of the fields is described the samples studied were intrinsically isotropic or only weakly anisotropic and the computed impressed susceptibility or irm ellipsoids produced by the applied fields were ellipsoids of revolution with the unique axis aligned with the applied field axis the size and shape of the field impressed ellipsoids were strongly dependent upon particle size these effects might thus form the basis of a rapid nondestructive means of estimating particle size in rocks the results also have important implications for magnetic fabric analysis and the static af demagnetization of rocks in particular to ensure that the intrinsic magnetic fabric is measured without any field impressed component of anisotropy samples should not be subjected to static af demagnetization prior to the determination of either low field susceptibility anisotropy or irm anisotropy samples should also not carry a significant remanence such as an irm acquired in a field greater than about 5 mt prior to low field susceptibility anisotropy analysis

a two stage exhumation of the variscan crust u pb la icp ms and rb sr ages from greater kabylia maghrebides the significance and role of major shear zones are paramount to understanding continental deformation and the exhumation of deep crustal levels la icpms u pb dating of monazites combined with rb sr analyses of biotites from an anatectic metapelite from greater kabylia algeria highlights the history of shear zone development and the subsequent exhumation of deep crustal levels in the internal zones of the maghrebides monazites give an age of 275 4 4 1 ma 2r dating the thermal peak coeval with anatexis this age is identical to those recorded in other crystalline terranes from south easternmost europe i e south alpine and austro alpine domains that suffered crustal thinning during the continental rifting predating the tethys opening rb sr analyses of biotites yield a cooling age of 23 7 1 1 ma related to the exhumation of the buried variscan crust during the miocene as an extrusive slice roughly coeval with the emplacement of nappes and shortly followed by lithospheric extension leading to the opening of the alboran sea

subtle stretching lineation revealed by magnetic fabric of callovian oxfordian black shales french alps abstract the anisotropy of magnetic susceptibility ams of callovian oxfordian black shales from the subalpine massifs of the french alps has been measured the magnetic susceptibility of these formations originates from ferromagnetic titanomagnetite and paramagnetic phyllosilicates illite and chlorite although the magnetic fabric is complicated by the interaction of planar and linear structural elements and the presence of variable amounts of minerals two types of lineation are preserved a stretching lineation which exhibits counterclockwise rotation as a result of deformation from south to north and an intersection lineation parallel to the fold axis this set of new structural data together with semiquantitative analysis of ams is in agreement with the geodynamic evolution of the western alps

a comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials some results from lake sediments abstract a new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility arm i e specific arm obtained in a 1 oe steady field which is particularly sensitive to the single domain sd and small pseudo single domain psd grains of the finer magnetite fraction a second parameter low field susceptibility is relatively more sensitive to the coarser magnetite fraction larger psd and smaller multidomain md grains we can then obtain a measure of the ratio of coarse to fine grain magnetite for large numbers of samples by plotting arversus a simple idealized model based on sized magnetite samples is proposed to explain the use of the armversus plot for detecting relative grain size changes in the magnetic content of natural materials the sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of arm and are used to test the model the model is used to interpret the magnetic variations observed and the interpretations are supported by high field hysteresis measurements of the same sediments the combination of the high field hysteresis method of day et al 1 and the arm vs method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite the armvs method should be used as an intial means of identifying distinct groups of samples the high field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation

the influence of deformation mechanisms on magnetic fabrics in weakly deformed rocks abstract magnetic fabrics of weakly deformed rocks are not simply correlated with the principal finite strain directions depositional magnetic fabrics persist at 30 shortening where the rocks are deformed by particulate flow grain boundary sliding rocks with a spaced stylolitic cleavage believed to be formed by pressure solution and shortened by 20 50 have maximum susceptibility parallel to the intersection of cleavage and bedding this type of magnetic fabric is attributed to the interference of a depositional magnetic fabric between cleavage surfaces with a tectonic magnetic fabric in the cleavage surfaces

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

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

magnetic fabric arising from paramagnetic phyllosilicate minerals in mudrocks the magnetic fabric of mudrock samples from the westbury member of the penarth group rhaetian the blue lias formation hettangian to sinemurian and the oxford clay callovian to oxfordian has been measured using a high field torque magnetometer the magnetic susceptibility anisotropy of these mudrocks originates from paramagnetic phyllosilicate minerals of which the most important is considered to be detrital chlorite the magnetic fabric preserves both a lineation and a foliation which are of a primary style this magnetic fabric is sensitive to the effects of recent weathering due to the ease with which chlorite is oxidized during weathering a model is proposed utilizing compaction to explain the acquisition of this magnetic fabric this model infers that the magnetic fabric represents in both degree and direction a measure of the preferred crystallographic orientation of the paramagnetic phyllosilicate minerals the magnetic fabric data suggest that the kmax direction corresponds with the quartz grain long axis preferred orientation and therefore may be used as a palaeocurrent indicator as in titanomagnetite bearing sediments evidence is presented to suggest that paramagnetic susceptibility anisotropy may be common in other mudrocks

anisotropies of partial anhysteretic remanence and susceptibility in compacted black shales grainsize and composition dependent magnetic fabric we have quantified the magnetic fabric of the highly compacted heebner shale pennsylvanian of kansas in terms of the anisotropies of partial anhysteretic remanent magnetization parm and low field susceptibility hysteresis loops exhibit almost purely paramagnetic behavior and the ratio of high field linear susceptibility to low field susceptibility is very close to one the susceptibility anisotropy therefore reflects the fabric of the paramagnetic matrix while the remanent anisotropies are due to the preferred orientations of ferrimagnetic grains both the paramagnetic and ferrimagnetic fabrics are dominated by a horizontal foliation with a weak northeasterly lineation but the parm anisotropies are generally much larger than the susceptibility anisotropy the degree of foliation measured with parm s decreases markedly with increasing coercivities indicating stronger foliation in the coarser grainsize fraction in contrast the parm mean lineations are grainsize independent suggesting a post compactional origin related to tectonic compression the susceptibility lineation appears to have a slightly different azimuth and may therefore be due to paleocurrents

synthetic magnetic fabrics in a plasticene medium mixtures of ferrimagnetic grains with a supporting medium of paramagnetic plasticene permit hypotheses concerning the interpretation of assemblages of different minerals to be confirmed the deformation of such plasticene mixtures in pure shear transpression and simple shear emphasize the dominance of the initial anisotropy over the end result for low strains 25 shortening in pure shear for minor amounts of multidomainal ferrimagnetic grains the deformation experiments indicate that the magnetic lineation rotates rapidly into the maximum extension direction for the three strain histories tested although the magnetic fabric directions are reliable kinematic indicators after modest strain in simple shear the symmetry of the fabric is not a plane strain fabric great discrepancies occur in the case of assemblages single domain femmagnetic grains in plasticene in pure shear these produce inverse fabrics in which the magnetic lineation spins towards parallelism with the maximum shortening direction

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

use of anisotropy to determine the origin of characteristic remanence in the siwalik red beds of northern pakistan it is often difficult or impossible to determine the origin of the characteristic remanent magnetization of red beds from the bulk remanence alone however anisotropy of remanence or susceptibility is strongly controlled by the statistical alignment of hematite grains this in turn may reflect the development of the magnetic fabric of the sediment over time so the shape of the anisotropy ellipsoid may provide clues to the origin of remanence in this work we make a study of the anisotropy of magnetic susceptibility ams in siwalik red beds of miocene age from northern pakistan comparison of the results with detailed petrographic studies and other information suggests that advanced soil development leads to the destruction of primary fabrics and often with it a coherent magnetization furthermore it should be possible to use ams fabric information to quantify the degree of pedogenesis in these miocene soils we attempted to determine the anisotropy of isothermal remanence air but found ams to be the technique of choice because of apparent changes in coercivity during air experiments we interpret the air data as resulting from metastable domains in hematite grains which change domain state during the air experiment

composite magnetic fabric in weakly deformed black shales abstract the anisotropy of magnetic susceptibility ams of weakly deformed sediments of the subalpine terres noires french alps is due to phyllosilicates matrix contribution and detrital titanomagnetites ferromagnetic contribution of variable grain size various rock magnetic and anisotropy techniques have been carried out to understand the origin of the scattering of maximum and minimum susceptibility axes we demonstrate the major contribution of the matrix both to susceptibility and ams anisotropy even when mean matrix susceptibility is weak this suggests that phyllosilicates are better oriented within the foliation plane and that intrinsic anisotropy of the detrital titanomagnetite grains is probably weaker than expected also the separation of the two magnetic subfabrics of three selected sites by coupling together low field ams hfa high field ams and aas anisotropy of anhysteretic remanence demonstrates the existence of a competition between two magnetic lineations and an interchange of the intermediate and minimum ams axes as a result of fine titanomagnetite this is the first evidence of the so called intermediate fabric as predicted by theory

magnetic anisotropy in the trenton limestone results of a new technique anisotropy of anhysteretic susceptibility a new method for determining magnetic anisotropy using anhysteric remanence susceptibility is described the magnetic fabric of a collection of trenton limestone specimens has been determined using this method as well as by conventional anisotropy of magnetic susceptibility the results demonstrate the usefulness of the new method for finding the magnetic fabric of rock units such as the trenton in which the bulk magnetic susceptibilities are low a model is proposed to explain the observed foliated and lineated fabric as a consequence of overburden compaction and regional horizontal stresses the original fabric is inferred to have been isotropic the anisotropy resides in secondary magnetite of late paleozoic age it is argued that the observed magnetic fabric must therefore be alleghenian or younger in age our method has the potential to determine paleostress directions in carbonates elsewhere provided our assumptions are correct

entrainment in compositionally buoyant plumes abstract the forms and growth histories of compositionally buoyant plumes were investigated experimentally by emitting a lower density fluid into a higher density fluid from a point source and also by allowing multiple plumes to rise from an interface between layers of fluid with different densities of special interest is the observation that some lower density plumes entrain higher density host material as they rise this process which occurs even at very low reynolds numbers may be analogous to the mixing of geological materials in igneous plumes and salt diapirs entrainment of host fluid up through the base of the carapace occurs in plumes that experience simultaneous cap expansion and vertical ascent at a rate that is dependent on the internal vortical velocity such plumes have a viscosity contrast between the host and source fluids near unity and a cap stem diameter ratio of about 2 3 plumes that entrain host material change shape from spherical to a form with a flat base and bluntly pointed tip and decelerate due to an increase in effective density relative to the density of the surrounding host material the ascent of all plumes with a host source viscosity ratio within the range 0 1 21 and a density contrast of about 1 5 can be modeled as stokesian provided that entrainment is accounted for by use of an effective density effective density is simply the weighted average density of the plume and entrained host material

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

cosmic ray exposure and pre atmospheric size of the gebel kamil iron meteorite cosmogenic he ne and ar as well as the radionuclides 10be 26al 36cl 41ca 53mn and 60fe have been determined on samples from the gebel kamil ungrouped ni rich iron meteorite by noble gas mass spectrometry and accelerator mass spectrometry ams respectively the meteorite is associated with the kamil crater in southern egypt which is about 45 m in diameter samples originate from an individual large fragment individual as well as from shrapnel concentrations of all cosmogenic nuclides stable and radioactive are lower by a factor 3 4 in the shrapnel samples than in the individual assuming negligible 36cl decay during terrestrial residence indicated by the young crater age 85 cm i e a pre atmospheric mass 20 tons with a preferred radius of 115 120 cm 50 60 tons the analyzed samples came from a depth of approximately 20 cm individual and approximately 50 80 cm shrapnel the size of the gebel kamil meteoroid determined in this work is close to estimates based on impact cratering models combined with expectations for ablation during passage through the atmosphere folco et al 2010 2011

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