neoformed magnetic minerals as an indicator of moderate burial the key example of middle paleozoic sedimentary rocks west virginia in order to help unravel the thermal history of middle paleozoic sedimentary rocks in west virginia a rock magnetic study was conducted with a focus on the marcellus shale vitrinite reflectance fluid inclusions microthermometry and conodont alteration index data yield contradictory burial temperature within the range 150 250 c 302 482 f the characterization of magnetite and pyrrhotite may be used as an index to track burial temperature around 200 c 392 f low temperature and room temperature magnetic measurements were performed in order to determine the magnetic assemblage three magnetic assemblages were identified that were stratigraphically distributed the goethite and nanosized magnetite a1 assemblage is mainly found in the clinton group oriskany sandstone stratigraphic interval silurian lower devonian nanosized fraction of magnetite and probably pyrrhotite a3 assemblage essentially constitutes the marcellus shale chemung formation sequence devonian microsized pyrrhotite is the typical mineral for a2 that is only identified near the alleghenian structural front overall the rare occurrence of micron pyrrhotite in our samples suggests that the study area has not experienced burial temperatures higher than 200 c 392 f

discrimination of biogenic and detrital magnetite through a double verwey transition temperature magnetite occurs widely in natural environments in both inorganic and biogenic forms discrimination of the origin of magnetite has important implications from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings in this study we present rock magnetic and electron microscopic analyses of marine sediments from the continental margin of oman low temperature magnetic data reveal two distinct verwey transition tv temperatures that are associated with the presence of biogenic and inorganic magnetite this interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses our study justifies the use of two distinct tv temperatures as a diagnostic signature for discriminating inorganic and biogenic magnetite simple low temperature magnetic measurements therefore provide a tool to recognize rapidly the origin of magnetite within natural samples in addition our analyses reveal progressive down core dissolution of detrital and biogenic magnetite but with preservation of significant amounts of fine grained magnetite within sediments that have been subjected to severe diagenetic alteration we demonstrate that preservation of magnetite in such environments is due to protection of fine grained magnetite inclusions within silicate hosts our results therefore also provide new insights into diagenetic processes in marine sediments

the low temperature magnetic signature of fe rich serpentine in cm2 chondrites comparison with terrestrial cronstedtite and evolution with the degree of alteration 1 we have studied the low temperature magnetic properties of one ci and five cm2 chondrites well characterized in the literature and of cronstedtite the fe end member of the fe mg serpentine solid solution cronstedtite is highly anisotropic and its relaxation properties below 10 k suggest a glassy magnetic behavior frustration of the magnetic interactions in octahedral layers would account for the observed magnetic freezing with a possible role of the random distribution of ferric iron in tetrahedral layers based on the comparison between powders and crystals characterized by scxrd and empa we show that grain size and substitution of octahedral fe by mg induce changes in the position of the low tmagnetic susceptibility peak a low tpeak is also found in each of the cm2 chondrites but not in the ci chondrite it is interpreted as the signature of fe serpentines which are considered by most authors to have mostly formed during asteroidal aqueous alteration events the low tmagnetic properties of the less altered meteorites compare well with those of cronstedtite and the position of the susceptibility peak varies with the published degree of alteration by comparing magnetic data on reference cronstedtite magnetic and published mineralogical data on cm2 chondrites we suggest that the evolution of the low tmagnetic signature of the latter reflects bulk changes in fe serpentines chemical composition rather than only mixing between fe rich and mg rich serpentines this is analogous to previous observations on the changes in fe incorporation to mg serpentines during progressive serpentinization of abyssal peridotites

magnetic properties of variably serpentinized peridotites and their implication for the evolution of oceanic core complexes serpentinization of ultramafic rocks during hydrothermal alteration at mid ocean ridges profoundly changes the physical chemical rheological and magnetic properties of the oceanic lithosphere there is renewed interest in this process following the discovery of widespread exposures of serpentinized mantle on the seafloor in slow spreading oceans unroofing of mantle rocks in these settings is achieved by displacement along oceanic detachment faults which eventually results in structures known as oceanic core complexes occs however we have limited understanding of the mechanisms of serpentinization at the seafloor and in particular their relationship with the evolution of occs since magnetite is a direct product of serpentinization the magnetic properties of variably serpentinized peridotites can provide unique insights into these mechanisms and their evolution in the oceanic lithosphere here we present new results from an integrated rock magnetic paleomagnetic and petrological study of variably serpentinized peridotites from the first fossil occ recognized in an ophiolite integration with existing data from mid ocean ridge related abyssal peridotites recovered from several scientific ocean drilling sites yields the first magnetic database from peridotites extending across the complete range 0 100 of degrees of serpentinization variations in a range of magnetic parameters with serpentinization and associated paleomagnetic data provide i key constraints on the mechanism s of serpentinization at mid ocean ridges ii insights on the potential for serpentinized peridotites to contribute to marine magnetic anomalies and iii evidence that leads to a new conceptual model for the evolution of serpentinization and related remanence acquisition at occs

new late permian paleomagnetic data from argentina refinement of the apparent polar wander path of gondwana the late paleozoic early mesozoic apparent polar wander path of gondwana is largely constructed from relatively old paleomagnetic results many of which are considered unreliable by modern standards paleomagnetic results derived from sedimentary sequences which are generally poorly dated and prone to inclination shallowing are especially common here we report the results of a joint paleomagnetic geochronologic study of a volcanic complex in central argentina u pb dating of zircons has yielded a robust age estimate of 263 0 1 6 2 0 ma for the complex paleomagnetic analysis has revealed a pretilting primary permian magnetization with dual polarities rock magnetic experiments have identified pseudo single domain titano magnetite and hematite as the mineralogic carriers of the magnetization lightning induced isothermal remagnetizations are widespread in the low coercivity magnetic carriers the resulting paleomagnetic pole is 80 1 s 349 0 e a95 3 3 n 35 and it improves a late permian mean pole calculated from a filtered south american paleomagnetic data set more broadly this new high quality igneous based paleomagnetic pole falls between the previously distinct late permian segments of the laurussian and gondwanan apparent polar wander paths suggesting that the long recognized disparity between these large paleomagnetic data sets may be primarily due to the inclusion of low quality or systemically biased data

rock magnetic criteria for the detection of biogenic magnetite abstract we report results on the magnetic properties of magnetites produced by magnetotactic and dissimilatory iron reducing bacteria magnetotactic bacterial mtb strains ms1 mv1 and mv2 and dissimilatory iron reducing bacterium strain gs 15 grown in pure cultures were used in this study our results suggest that a combination of room temperature coercivity analysis and low temperature remanence measurements provides a characteristic magnetic signature for intact chains of single domain sd particles of magnetite from mtbs the most useful magnetic property measurements include 1 acquisition and demagnetization of isothermal remanent magnetization irm using static pulse and alternating fields 2 acquisition of anhysteretic remanent magnetization arm and 3 thermal dependence of low temperature 20 k saturation irm after cooling in zero field zfc or in a 2 5 t field fc from 300 k however potentially the most diagnostic magnetic parameter for magnetosome chain identification in bulk sediment samples is related to the difference between low temperature zero field and field cooled sirms on warming through the verwey transition t 100 k intact chains of unoxidized magnetite magnetosomes have ratios of fc zfc greater than 2 where the parameter is a measure of the amount of remanence lost by warming through the verwey transition disruption of the chain structure or conversion of the magnetosomes to maghemite reduces the fc zfc ratio to around 1 similar to values observed for some inorganic magnetite maghemite greigite and gs 15 particles numerical simulations of fc zfc ratios for simple binary mixtures of magnetosome chains and inorganic magnetic fractions suggest that the fc zfc parameter can be a sensitive indicator of biogenic magnetite in the form of intact chains of magnetite magnetosomes and can be a useful magnetic technique for identifying them in whole sediment samples the strength of our approach lies in the comparative ease and rapidity with which magnetic measurements can be made compared to techniques such as electron microscopy

observations of pkp df and pkp bc across the united kingdom implications for studies of attenuation in the earth s core summary we demonstrate that pkp df and pkp bc recorded by a network of broad band stations in the united kingdom from earthquakes in the southwest pacific show considerable spatial variation in amplitude and frequency content we suggest that strong heterogeneity in apparent attenuation at the base of the mantle related to ultra low velocity zones ulvz beneath the southwest pacific and or possibly to the east of iceland may explain the anomalous observations of df and bc across the uk the length scale of attenuating ulvzs required to explain the observations is similar to that estimated recently for a southwest pacific ulvz by wen helmberger 1998 that is low wave speed low apparent q regions of 80 km height separated by steep sided valleys of width 40 km with typical lowermost mantle wave speeds high apparent q if the apparent attenuation is due to anelasticity alone disregarding the possible contribution from scattering the minimum ulvz q values required to explain the amplitude of 2 hz bc are about 15 30 consistent with the interpretation of the ulvzs as zones of partial melt at the base of the mantle this suggests that estimates of inner core anelastic attenuation inferred from df bc spectral ratios will be biased if df and or bc cross ulvzs at the base of the mantle under the source and or receiver further the presence of attenuating ulvzs may explain the large scatter in estimates of inner core q from global studies

reconstruction of low temperature 100 c burial in sedimentary basins a comparison of geothermometer in the intracontinental paris basin abstract the thermal evolution of sediments in the shallowest part of sedimentary basins is challenging to investigate as most of the geothermometers are at their application limits this is typically the case of the callovian oxfordian claystones at the eastern border of the paris basin a recent 2000 m deep well penetrated the entire jurassic and triassic sedimentary series and permitted coring of sediments never reached by previous drilling in this part of the basin thanks to the use of several independent geothermometers apatite fission track thermochronology fluid inclusion microthermometry clay mineralogy and sedimentary organic matter evolution the burial history of the mesozoic series has been reconstructed modeled and calibrated using kinetic algorithms relevant to the various paleothermometers used in addition to these conventional methods an innovative approach was developed using the temperature dependence of the magnetic mineral assemblage within clay rich sediments mageval thermal modeling indicates that the callovian oxfordian and the lower triassic experienced maximum burial temperatures of 50 5 c and 90 10 c respectively i e about 25 c higher than present day this temperature offset implies the erosion of several hundred meters of sediments which depending on the thermal flux and conductivity considered most likely constituted upper cretaceous chalk deposits falling sea level resulted in the exposure and subsequent dismantling of the chalk cover at the beginning of cenozoic times the eastern border of the basin was then subjected to slow erosion and discrete tectonic deformation

thermal alteration of pyrite to pyrrhotite during earthquakes new evidence of seismic slip in the rock record seismic slip zones convey important information on earthquake energy dissipation and rupture processes however geological records of earthquakes along exhumed faults remain scarce they can be traced with a variety of methods that establish the frictional heating of seismic slip although each has certain assets and disadvantages here we describe a mineral magnetic method to identify seismic slip along with its peak temperature through examination of magnetic mineral assemblages within a fault zone in deep sea sediments cored from the japan trench one of the seismically most active regions around japan during the integrated ocean drilling program expedition 343 the japan trench fast drilling project fault zone sediments and adjacent host sediments were analyzed mineral magnetically supplemented by scanning electron microscope observations with associated energy dispersive x ray spectroscopy analyses the presence of the magnetic mineral pyrrhotite appears to be restricted to three fault zones occurring at 697 720 and 801 m below sea floor in the frontal prism sediments while it is absent in the adjacent host sediments elevated temperatures and coseismic hot fluids as a consequence of frictional heating during earthquake rupture induced partial reaction of preexisting pyrite to pyrrhotite the presence of pyrrhotite in combination with pyrite to pyrrhotite reaction kinetics constrains the peak temperature to between 640 and 800 c the integrated mineral magnetic microscopic and kinetic approach adopted here is a useful tool to identify seismic slip along faults without frictional melt and establish the associated maximum temperature

ferromagnetic resonance and magnetic characteristics of intact magnetosome chains in magnetospirillum gryphiswaldense abstract the magnetic characteristics of intact magnetosome chains in magnetospirillum gryphiswaldense bacteria were investigated by means of static and dynamic magnetic analyses and ferromagnetic resonance spectroscopy the nano sized magnetosomes are generally in a stable single domain state but magnetosomes smaller than 30 nm characteristic of superparamagnetic magnetite particles were also found alternating current ac susceptibility indicates that all magnetosomes are blocked below 150 k at room temperature the anisotropy of m gryphiswaldense is dominated by the shape of the magnetosome chains low temperature ferromagnetic resonance fmr spectroscopy indicates that this dominant shape anisotropy can affect the detection of the verwey transition at 100 k the static and dynamic magnetic analyses show that the verwey transition is smeared and that our magnetotactic bacteria fail the moskowitz test this failure is explained by the biomineralization of non stoichiometric magnetosomes this interpretation is based on the increase in high field susceptibility and the distinct peak in the out of phase component of the ac susceptibility below 50 k these results are attributed to freezing of spins associated with defect structures in the core and at the surface of nano sized magnetosomes the results obtained from m gryphiswaldense demonstrate that intrinsic properties of nano sized magnetosomes are significantly influenced by non stoichiometry and by the anisotropy excited from their arrangement in the bacteria

progressive and punctuated magnetic mineral diagenesis the rock magnetic record of multiple fluid inputs and progressive pyritization in a volcano bounded basin iodp site u1437 izu rear arc

biomineralization crystallography and magnetic properties of bullet shaped magnetite magnetosomes in giant rod magnetotactic bacteria article i nfo magnetosomes produced by magnetotactic bacteria are of great interest for understanding bacterial biomineralization along with sedimentary magnetism and environmental magnetism one of the most intriguing species magnetobacterium bavaricum can synthesize hundreds of bullet shaped magnetite magnetosomes per cell which contribute significantly to magnetic properties of sediments however the biomineralization mechanism and magnetic properties of such magnetosomes remain unknown in this paper we have conducted a comprehensive study of the crystallography and magnetic properties of bullet shaped magnetosomes formed by uncultivated giant rod magnetotactic bacteria referred to as myr 1 recently discovered in lake miyun beijing china transmission electron microscopy observations reveal that each myr 1 cell contains hundreds of bullet shaped magnetite magnetosomes which are arranged into 3 5 braid like bundles of chains the formation of the bullet shaped magnetosomes can be divided into two stages initial isotropic growth up to 20 nm followed by elongation along the 100 direction which is unusual compared with the expected 111 magnetic easy axis although the 100 orientation is the hard axis of the face centered cubic magnetite the shape anisotropy of bullet shaped magnetosomes and intra bundle magnetostatic interactions confine the magnetization direction of the chain along the long axis of the cell bundle due to each bundle of magnetosome chains effectively behaving as an elongated single domain particle the myr 1 cells show high coercivity weak intra bundle magnetostatic interaction and high ratio these results provide new insights into the biomineralization process and magnetic properties of bullet shaped magnetite magnetosomes

ferromagnetic resonance and low temperature magnetic tests for biogenic magnetite magnetite is both a common inorganic rock forming mineral and a biogenic product formed by a diversity of organisms magnetotactic bacteria produce intracellular magnetites of high purity and crystallinity magnetosomes arranged in linear chains of crystals magnetosomes and their fossils magnetofossils have been identified using transmission electron microscopy tem in sediments dating back to 510 570 ma and possibly in 4 ga carbonates in martian meteorite alh84001 we present the results from two rock magnetic analyses the low temperature moskowitz test and ferromagnetic resonance fmr applied to dozens of samples of magnetite and other materials the magnetites in these samples are of diverse composition size shape and origin biologically induced extracellular biologically controlled magnetosomes and chiton teeth magnetofossil synthetic and natural inorganic we confirm that the moskowitz test is a distinctive indicator for magnetotactic bacteria and provide the first direct experimental evidence that this is accomplished via sensitivity to the magnetosome chain structure we also demonstrate that the fmr spectra of four different strains of magnetotactic bacteria and a magnetofossil bearing carbonate have a form distinct from all other samples measured in this study we suggest that this signature also results from the magnetosomes unique arrangement in chains because fmr can rapidly identify samples with large fractions of intact isolated magnetosome chains it could be a powerful tool for identifying magnetofossils in sediments

a strong angular dependence of magnetic properties of magnetosome chains implications for rock magnetism and paleomagnetism 1 single domain magnetite particles produced by magnetotactic bacteria magnetosomes and aligned in chains are of great interest in the biosciences and geosciences here we investigated angular variation of magnetic properties of aligned magnetospirillum magneticum amb 1 cells each of which contains one single fragmental chain of magnetosomes with measurements at increasing angles from the chain direction we observed that i the hysteresis loop gradually changes from nearly rectangular to a ramp like shape e g bc and remanence decrease ii the acquisition and demagnetization curves of irm shift toward higher fields e g bcr increases and iii the forc diagram shifts toward higher coercivity fields e g bc forc increases for low temperature results compared to unoriented samples the samples containing aligned chains have a much lower remanence loss of field cooled fc and zero field cooled zfc remanence upon warming through the verwey transition higher ratio fc zfc for the measurement parallel to the chain direction and lower ratio larger fc and zfc values for the perpendicular measurement micromagnetic simulations confirm the experimental observations and reveal that the magnetization reversal of magnetosome chain appears to be noncoherent at low angles and coherent at high angles the simulations also demonstrate that the angular dependence of magnetic properties is related to the dispersion degree of individual chains indicating that effects of anisotropy need to be accounted for when using rock magnetism to identify magnetosomes or magnetofossils once they have been preserved in aligned chains additionally this study experimentally demonstrates an empirical correspondence of the parameter bc forc to bcr rather than bc at least for magnetite chains with strong shape anisotropy this suggests forc analysis is a good discriminant of magnetofossils in sediments and rocks

low temperature magnetic behaviour near 35 k in unmetamorphosed claystones summary there is growing evidence that the magnetic assemblage of claystones illustrated by low temperature magnetic transitions at 120 k and 35 k may be representative of the peak burial temperature in the so called oil window 60 150 c in previous studies it was proposed that this magnetic assemblage is characterized by fine grained pyrrhotite fe7s8 and magnetite fe3o4 however evidence of pyrrhotite was not so obvious and the magnetic transition near 35 k of fine grained pyrrhotite has similarities with those of siderite feco3 or rhodochrosite mnco3 here we propose some diagnostic tests to distinct 50 k behaviours by studying claystones from netherlands borehole and borneo prism that experienced different peak burial temperatures we perform magnetic susceptibility temperature dependency of sirm zfc rt sirm and field cooled hysteresis loops on cooling of rt sirm 300 k to 10 k we applied a magnetic field of 5 t to enhance neel type magnetic transition it is found that our samples can be classified in two categories based on the shape of the rt sirm curve one displaying an abrupt break in slope in the remanence at 30 k called n behaviour and the other one characterized by a progressive increase of the remanent magnetization by 80 k named p behaviour the first category contains essentially magnetite and fe mn carbonates the second one magnetite and probably iron sulphides

formation of tabular single domain magnetite induced by geobacter metallireducens gs 15 distinct morphological characteristics of magnetite formed intracellularly by magnetic bacteria magnetosome are invoked as compelling evidence for biological activity on earth and possibly on mars crystals of magnetite produced extracellularly by a variety of bacteria including geobacter metallireducens gs 15 thermophilic bacteria and psychrotolerant bacteria are however traditionally not thought to have nearly as distinct morphologies the size and shape of extracellular magnetite depend on the culture conditions and type of bacteria under typical co2 rich culture conditions gs 15 is known to produce superparamagnetic magnetite crystal diameters of approximately 30 nm in the current study we were able to produce a unique form of tabular single domain magnetite under nontraditional low co2 culture conditions this magnetite has a distinct crystal habit and magnetic properties this magnetite could be used as a biosignature to recognize ancient biological activities in terrestrial and extraterrestrial environments and also may be a major carrier of the magnetization in natural sediments

the thermodynamic effect of nonhydrostatic stress on the verwey transition abstract experimental results on the change in the verwey transition temperature in magnetite under compression exhibit puzzling variability with slopes ranging from 6 to 16 k gpa our thermodynamic analysis of the verwey transition in magnetite explains much of this variability in terms of the transformation strain as magnetite changes in crystal structure from cubic to monoclinic because this strain involves a much larger change in shape than in volume the change in temperature of the phase boundary t v can be much more sensitive to nonhydrostatic stress than to hydrostatic pressure uniaxial compression and tension both are predicted to increase t v an effect that is opposite in sign and for favorable orientations up to six times greater per gigapascal than for pressure moreover because the monoclinic twin orientation with highest t v is thermodynamically favored our treatment also shows how any desired twin may be selected uniquely by applying stress of appropriate orientation thereby removing obstacles to understanding many kinds of low temperature phenomena in magnetite caused by the presence of multiple twins a similar analysis for the thermodynamic effects of strong magnetic fields is outlined predicting that t v will generally be lowered again the twin for which t v is highest i e is lowered the least will be the most stable twin selectivity per gigapascal of stress is up to two to three times stronger than that per tesla of magnetic field

paleomagnetic recording fidelity of nonideal magnetic systems a suite of near identical magnetite nanodot samples produced by electron beam lithography have been used to test the thermomagnetic recording fidelity of particles in the 74 333 nm size range the grain size range most commonly found in rocks in addition to controlled grain size the samples had identical particle spacings meaning that intergrain magnetostatic interactions could be controlled their magnetic hysteresis parameters were indicative of particles thought not to be ideal magnetic recorders however the samples were found to be excellent thermomagnetic recorders of the magnetic field direction they were also found to be relatively good recorders of the field intensity in a standard paleointensity experiment the samples intensities were all within 15 of the expected answer and the mean of the samples within 3 of the actual field these nonideal magnetic systems have been shown to be reliable records of the geomagnetic field in terms of both direction and intensity even though their magnetic hysteresis characteristics indicate less than ideal magnetic grains

tectonic climatic and diagenetic control of magnetic properties of sediments from kumano basin nankai margin southwestern japan abstract integrated ocean drilling program iodp site c0002 was cored in the kumano basin which is a forearc basin in the nankai margin of southwest japan the uppermost 1052 5 m of sediment in the accretionary prism slope and overlying forearc basin was recovered discontinuously during expedition 315 magnetic minerals from site c0002 were studied to understand sedimentary tectonic and diagenetic processes in this forearc environment four rock magnetic units a b c and d are identified in the forearc basin sediments of unit a high concentrations of detrital magnetic iron oxides in surface sediments give way to progressive diagenetic dissolution with depth nevertheless large titanomagnetite particles survived diagenetic dissolution along with magnetic nanoinclusions within host silicates that preserve a relict magnetic signal that records climatic modulation of sediment delivery to the kumano basin the authigenic iron sulphides ferrimagnetic greigite and paramagnetic pyrite formed in association with gas hydrates and anaerobic oxidation of methane in the forearc basin sediments of unit b methane venting from deeper within the slope sediments of unit c and accretionary prism sediments of unit d has given rise to formation of pyrite aggregates along sediment fractures and has contributed to further diagenetic depletion of magnetic signals in these units structural evolution of the kumano basin has caused migration of the gas hydrate stability zone ghsz through time so that the site c0002 sediments record both the present and fossil ghsz positions magnetic properties of kumano basin sediments provide insights into the interplay between diagenetic climatic and tectonic processes that controlled the sedimentary evolution of this forearc basin

mapping hydrocarbon charge points in the wessex basin using seismic geochemistry and mineral magnetics abstract this study reports a multidisciplinary approach to determining hydrocarbon charge points and migration in the wessex basin southern england geochemical analysis of reservoir core material bridport sandstone and inferior oolite using gas chromatography mass spectrometry gc ms suggests that the oil in the wessex basin is from a single source and that small variations in environmentally sensitive biomarkers are likely due to small differences in maturity or depositional conditions during the formation of the oil over millions of years using seismic data basin modelling revealed two potential hydrocarbon migration pathways from the hanging wall of the purbeck fault into the sherwood sandstone reservoir at wytch farm one of these potential pathways is represented by cores termed creech and the other bushey farm to try to distinguish between the two potential pathways cores were studied using mineral magnetic techniques the magnetic signature was characterised using low temperature

magnetic hysteresis of pseudo single domain and multidomain magnetite below the verwey transition abstract hysteresis loops have been measured as a function of temperature between 10 k and room temperature for two size fractions of crushed magnetite 1 5 m pseudo single domain psd and 100 150 m multidomain md the material under study appears to be fairly close to stoichiometric magnetite with a verwey transition temperature of 115 k for the md sample and 118 k for the psd sample for both samples hysteresis properties below the verwey transition depend critically on whether or not the samples were cooled through the transition in the magnetic field for the psd sample an even more complex behavior has been observed temperature dependences of coercive force obtained after zero field cooling differ significantly for the demagnetized versus magnetized starting magnetic state

the effects of annealing and concentration on the hysteresis properties of magnetite around the psd md transition abstract hysteresis parameters h cr h c j rs j s and their ratios h cr h c j rs j s have been measured for a large number of accurately prepared grain size fractions of magnetite in the range between 5 and 150 m for several grain size fractions three different concentrations of magnetite are used 100 0 1 and 0 002 vol most of the measurements were repeated after annealing the specimens to 600 c for some specimens in the pseudo single psd and multidomain md range h c and h cr have been measured as functions of temperature plots of the results from h c h cr h c and j rs j s versus the grain size reveal curves with a convex and a concave part concentration and annealing affects the values of the hysteresis parameters especially for grains coarser than 25 m but the shape of the curves remains the same the inflection point from convex to concave for all curves occurs at 25 m and it appears to be independent of concentration and annealing it is therefore proposed to define the transition from psd to md as the inflection point of these curves

authigenesis of magnetic minerals in gas hydrate bearing sediments in the nankai trough offshore japan gas hydrate occurrence is one of the possible mechanisms invoked for iron sulfide formation a high resolution rock magnetic study was conducted in iodp expedition 316 hole c0008c located in the megasplay fault zone of the nankai trough offshore japan in this particular zone no bottom simulating reflectors bsr indicating the base of the gas hydrate stability field have been identified two hundred and eighteen pleistocene samples were collected from 70 to 110 m csf in order to document the changes in the concentration grain size and rock magnetic parameters of magnetic minerals through the gas hydrate bearing horizons two different populations of magnetic grains are recognized in the pseudosingle domain range three types of magnetic mineral assemblages are identified iron oxides magnetite ferrimagnetic iron sulfides greigite and pyrrhotite and their mixture greigite and pyrrhotite are authigenic and constitute six layers called is1 is6 is1 is3 is4 and is6 are associated with pore water anomalies suggesting the occurrence of gas hydrates and anoxic conditions is2 and is5 are probable gas hydrates horizons although there is no independent data to confirm it the remaining intervals are mainly composed of detrital iron oxides and paramagnetic iron sulfides two scenarios based on different diagenetic stages are proposed to explain the variations in the magnetic properties and mineralogy over the studied interval the results suggest that rock magnetism appears useful to better constrain the gas hydrate distribution in hole c0008c and counterbalances the low resolution of pore water analyses and the absence of a bsr

magnetic fabrics in the bjerkreim sokndal layered intrusion rogaland southern norway mineral sources and geological significance abstract magnetic anisotropy can provide important information about mineral fabrics and thus magmatic processes particularly when it is known how multiple mineral species contribute to the anisotropy it may also affect the direction of induced or remanent magnetization with important consequences for paleomagnetic studies or the interpretation of magnetic anomalies here we aim at describing the magnetic fabrics in the bjerkreim sokndal layered intrusion and identifying their carriers anisotropies of magnetic susceptibility and remanence were measured on samples covering different geographic locations and stratigraphic units within the bjerkreim sokndal layered intrusion the intrusion is characterized by magmatic layering and has a synform structure with strong foliation on the limbs detailed comparison between magnetic and mineral fabric shows that they are not necessarily coaxial but the minimum susceptibility and minimum anhysteretic remanence are generally normal to the foliation or the magmatic layering the minimum susceptibility and anhysteretic remanence are associated with pyroxene 100 axes and the maximum susceptibility and anhysteretic remanence are sub parallel to the pyroxene 001 axes in layers mcu ivc and mcu ive for which electron backscatter data are available however the paramagnetic anisotropy of pyroxene is too weak to explain the observed anisotropy we propose that the magnetic anisotropy of magnetite free specimens is carried by hemo ilmenite exsolutions within pyroxene in addition to pyroxene itself when present multi domain magnetite dominates both the anisotropy of magnetic susceptibility and anhysteretic remanence due to shape preferred orientation and distribution anisotropy the orientation of the magnetic fabric appears independent of carrier due to their common deformation history but the degree of anisotropy is stronger for magnetite bearing specimens the results of this study will facilitate future structural interpretations and may be used to correct for magnetization deflection

petrogenesis of late early cretaceous high silica granites from thebangong nujiang suture zone central tibet abstract high silica granites are common in collisional orogenic belts however the petrogenesis of such granites and the geodynamic processes associated with their emplacement are debated in this study we present zircon u pb ages and hf isotopes and whole rock geochemical and sr nd isotopic compositions for biotite granites from the jiang co area in the bangong nujiang suture zone central tibet zircon u pb dating reveals that the jiang co granites were emplaced during the late early cretaceous ca 114 ma these granites are characterized by high and variable sio2 69 7 78 2 wt and k2o 4 7 8 4 wt and low mgo 0 09 1 25 wt and p2o5

the stardalur magnetic anomaly revisited new insights into a complex cooling and alteration history abstract this study provides new rock magnetic and magneto mineralogical data including mossbauer spectroscopy of basaltic drill cores from the stardalur volcanic complex iceland in order to better understand the strong magnetic anomaly which is caused by an extraordinary high natural remanent magnetization nrm nrm and magnetic susceptibility display a positive linear correlation r 2 0 81 and reach very high values up to 121 a m and 148 10 3 si although a curie temperature of 580 c and a verwey transition at about 160 c is indicative of magnetite t heating experiments in argon and air atmosphere and thermal demagnetization measurements of nrm revealed a slight cation deficiency according to induced remanent magnetization experiments the remanence is carried solely by this low coercive phase minor titanomaghemite with a t c at about 340 c only occurs in samples with larger oxide grains 20 80 m high vesicle abundances and the exsolution texture of fe ti oxides suggest subaerial extrusion of the lava a high oxygen fugacity probably above the nno buffer and a low ti ti fe ratio of the basaltic melt are suggested as a precondition for high concentration of magnetic minerals and therefore high primary trm during high temperature oxidation ilmenite exsolution lamellae developed in titanomagnetite and symplectic magnetite pyroxene formed by the breakdown of olivine this secondary magnetite grown at temperatures above the curie temperature increases the primary trm early stage hydrothermal alteration below about 375 c led to maghemitization of titano magnetite clearly indicated by shrinkage cracks and irreversible t curves during later stage hydrothermal alteration nrm intensity increased slightly due to the growth of secondary magnetite at lower temperatures about 250 300 c this hydrothermally formed magnetite acquired only a low crm but increased magnetic susceptibility significantly according to our results it is suggested that hydrothermal alteration does not necessarily lower remanent magnetization but contributes to an increase in magnetization the interplay of the three factors melt composition small grain sizes of secondary magnetite due to decomposition of silicates and new formation under hydrothermal conditions caused the strong magnetic anomaly at the surface

a test of ice self collection kernels using aircraft data abstract aircraft observations from the cirrus regional study of tropical anvils and cirrus layers crystal florida area cirrus experiment face campaign obtained in the anvil of a large convective storm from 26 july 2002 are presented during this flight a lagrangian spiral descent was made allowing the evolution of the ice particle size distribution to be followed relative humidities during 1 km from 11 to 3 c of the descent were within 4 of ice saturation it was assumed that the ice particle size distribution was evolving through the process of aggregation alone three idealized ice ice collection kernels were used in a model of ice aggregation and compared to the observed ice particle size distribution evolution a geometric sweep out kernel a golovin sum of particle masses kernel and a modified golovin kernel sum of particle masses raised to a power the golovin kernel performed worst the sweep out kernel produced good agreement with the observations when a constant aggregation eff

low temperature magnetic behavior of titanomagnetites titanium substitution has a significant effect on the temperature dependence 5 300 k of low temperature saturation remanence and of initial susceptibility in synthetic and natural titanomagnetite fe3 xtixo4 0 x 0 4 and electronic and lattice relaxation phenomena over the entire titanomagnetite compositional range these features produce diagnostic magnetic behavior useful for titanomagnetite identification in natural samples moreover the apparent similarities between the thermal decay of low temperature remanence of titanomagnetite with pyrrhotite pure magnetite and superparamagnetic phases can lead to complications in interpreting low temperature remanence data when titanomagnetite is present

identification of nanocrystalline goethite in reduced clay formations application to the callovian oxfordian formation of bure france the callovian oxfordian cox clay formation in the paris basin france has been the target of many studies investigating the feasibility of deep nuclear waste disposal in a reduced clay formation to determine the mobility of radionuclides in the host rock formation modeling of the porewater chemistry particularly iron solute concentrations is necessary notably this study aims to understand the supersaturation of fe iii oxyhydroxides given by models fe iii oxyhydroxides have been identified magnetically in unpreserved callovian oxfordian samples in this study a set of magnetic measurements are used to detect the fe bearing magnetic minerals present in the cox clay formation a core sample from the borehole for1118 preserved from air since its collection is the target of this study the magnetic measurements performed show that magnetite and goethite are the main magnetic minerals 0 2 together with probable greigite and occur in low concentrations goethite occurs as nanoparticles dispersed in the clayey matrix and not enclosed in other minerals or in organic matter it is unlikely that the goethite is an alteration by product as particular care was undertaken this finding resolves the discrepancies between observations and previous modeling results

origin and implications of two verwey transitions in the basement rocks of the vredefort meteorite crater south africa abstract two populations of magnetite exist in the shocked basement rocks of the vredefort meteorite impact crater one associated with original crustal genesis and metamorphism around 3 0 ga and the other related to the impact itself at 2 02 ga pre impact magnetite is mostly micron to millimeter in size lying within the multidomain to pseudo single domain range the second population of magnetite is less than 10 m in size and formed within the interstices of planar deformation features or within the reaction rims of biotite both of which were created during impact our study shows that each of these populations possesses specific verwey transition temperatures one around 124 k associated with pre impact magnetite and the other around 102 k associated with impact related magnetite the high temperature verwey transition is attributed to stoichiometric magnetite while the low temperature verwey transition to non stoichiometric magnetite pre impact rocks containing both verwey transitions are ubiquitous throughout the crater pseudotachylites formed during impact have a single verwey transition spanning temperatures from 94 to 111 k heating the basement rocks above 550 600 c for 3 min or above 500 c for 1 h irreversibly modifies the 124 k verwey transition by shifting it to lower temperatures based on these findings it is possible that no wholesale heating of the crater occurred above 550 600 c for 3 min or above 500 c for 1 h during or since the time of impact although some places of more localized heating are identified an unresolved problem remains to reconcile these data with temperatures thought to exist in the crust during and after impact

toward a new 250 c pyrrhotite magnetite geothermometer for claystones abstract we investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation immature burial temperature 40 c of bure callovo oxfordian claystones in the basin of paris france early mature burial temperature 85 c of opalinus lower dogger claystones from the mont terri anticline in front of the jura fold belt switzerland and mature to overmature burial temperature 7 s 8 the p transition is also detected in early mature to mature opalinus and chartreuse claystones we conduct additional experimental heating of natural opalinus claystones one set of experiments is referred to as short term heating 1 h from 100 c to 200 c it is dedicated to an investigation of the effect of short lived heating processes in geology a second set of heating experiments is designed to approach burial conditions using a gold capsule in burial like experiments we heated opalinus claystones from 150 c to 250 c for several weeks under a pressure of 100 mpa in both experiments we observe a correlative diminution of the pyrrhotite signature at 35 k with increasing temperature we interpret this trend as the appearance of magnetite we derive a parameter pm from the warming curve of a saturated isothermal remanent magnetization acquired at 10 k zfc we report on a consistent evolution of pm with temperature in the range of 40 c to 250 c including natural samples heated samples at 95 c and burial like heated samples pm first increases between 40 c up to 85 c implying that pyrrhotite gradually dominates the magnetic assemblage at low temperature for temperatures above 85 c pm decreases up to 250 c implying that the formation of magnetite gradually overshadows the magnetic input of pyrrhotite pm values obtained from mature to overmature claystones from the chartreuse are lower than the pm values obtained from the burial like heated opalinus claystones suggesting that the formation of magnetite is driven by kinetics the continuous trend of the pm parameter suggests that the magnetic properties of pyrrhotite magnetite claystones can be used to infer paleo temperatures and we propose to name this geothermometer mageval

fabrics of migmatites and the relationships between partial melting and deformation in high grade transpressional shear zones the espinho branco anatexite borborema province ne brazil abstract the espinho branco anatexite located within a transcurrent high temperature shear zone in ne brazil was the subject of a comprehensive petrostructural study anisotropy of magnetic susceptibility ams anisotropy of anhysteretic remanence aar electron backscatter diffraction ebsd to evaluate the compatibility of different fabrics with the kinematics of melt deformation magnetite dominates susceptibilities larger than 1 msi and biotite displays 001 lattice directions consistent with ams k 3 axes in contrast migmatites with a susceptibility lower than 0 5 msi and no visible mesoscopic foliation provide crystallographic fabrics distinct from ams and aar however aar remains consistent with the regional strain field these results suggest that the correlation of field ams and crystallographic fabrics is not always straightforward despite the relatively simple organisation of the magnetic fabric in the anatexite we conclude that ams recorded the final stages of the strain field in the migmatite irrespective of its complex mesoscale structures and contrasting crystallographic fabrics

effects of internal stress on remanence intensity jumps across the verwey transition for multi domain magnetite abstract the magnetic properties of magnetite fe3o4 are strongly dependent on the internal stress related to stress controlled regions and to closure domains associated with defects the contribution of internal stress to the low temperature magnetic properties of magnetite was tested using annealed and unannealed multi domain md magnetites during low temperature cooling a room temperature induced saturation isothermal remanent magnetization sirm increased abruptly at the verwey transition tv 122 k in particular the absolute intensity jump vj defined as the jump in sirm at tv upon cooling resulted from the high coercivity fraction of md grains we observe that annealing significantly reduces internal stress and thus decreases the average microcoercivity comparison of the alternating field af demagnetization spectra of vj both for annealed and unannealed magnetites directly links vj to the internal stress it is likely that removal of the closure domain associated with stress controlled regions was dominant when the peak af was less than the average micromagnetic coercivity hc resulting in a net increase of vj with increasing af however when the af exceeded the hc threshold vj decreased because the stress controlled regions were demagnetized such observations could therefore be useful for estimating the hc of md magnetite

continuous production of nanosized magnetite through low grade burial geological processes such as burial can lead to remagnetization in rocks due to neoformed magnetic minerals that have passed a critical volume called blocking volume in this study we designed a heating experiment for claystones obtained from the paris basin france in the 50 130 c temperature range in order to simulate 20 nm and 20 nm minerals respectively this result led us to propose a conceptual model of nucleation and growth process of magnetite during low grade burial from 2 to 4 km depth ultrafine magnetite 20 nm was predominant over single domain magnetite 20 nm for 4 km depth transposed to natural conditions our heating steps experiment suggested that claystone type rocks are remagnetized during burial for temperatures higher than 200 c the extrapolation of our results indicated that burial remagnetization due to the chemical remanent magnetization might be larger than the natural remanent magnetization 2012 american geophysical union all rights reserved

paleomagnetism and rock magnetism of the ca 1 87 ga pearson formation northwest territories canada a test of vertical axis rotation within the great slave basin abstract a geometrically quantitative plate kinematic model based on paleomagnetism for the initial assembly of laurentia has taken form in the past few decades within this framework there remains but one problematic interval of data predominantly from the slave craton which is the 1 96 1 87 ga coronation apparent polar wander path apwp the coronation apwp shows large 110 back and forth oscillations that are difficult to explain in terms of plate motion nonetheless poles from the coronation apwp have been incorporated in various paleogeographic reconstructions of laurentia and the supercontinent nuna pointing to the importance of testing its veracity in this study we conducted a detailed paleomagnetic and rock magnetic study of the ca 1 87 ga pearson formation east arm of great slave lake northwest territories canada our results show that pearson formation yields a characteristic remanent magnetization carried by single domain or small pseudo single domain magnetite the age of the magnetization is constrained to be older than paleoproterozoic deformation and is interpreted as primary paleomagnetic declinations reveal a one to one correlation with local structural attitudes indicating that some small blocks in the fold belt likely experienced significant 60 vertical axis rotations presumably related to large dextral displacements along the mcdonald fault system alternative explanations such as true polar wander or a non dipole magnetic field are considered less parsimonious for the data presented here it is suspected that some existing christie bay group poles the stark and tochatwi formations which were sampled in areas with anomalous structural attitudes and differ from time equivalent poles obtained from areas of the slave craton far from major transcurrent faults may similarly suffer from vertical axis rotation we suggest further study before using possibly rotated christie bay group poles for paleogeographic reconstructions

volcano tectonic evolution of a linear volcanic ridge pico faial ridge azores triple junction assessed by paleomagnetic studies abstract the morphology of volcanic oceanic islands results from the interplay between constructive and destructive processes and tectonics in this study the analysis of the paleomagnetic directions obtained on well dated volcanic rocks is used as a tool to assess tilting related to tectonics and large scale volcano instability along the pico faial linear volcanic ridge azores triple junction central north atlantic for this purpose 530 specimens from 46 lava flows and one dyke from pico and faial islands were submitted to thermal and alternating magnetic fields demagnetizations detailed rock magnetic analyses including thermomagnetic analyses and classical high magnetic field experiments revealed titanomagnetites with different ti content as the primary magnetic carrier capable of recording stable remanent magnetizations in both islands the paleomagnetic analysis yields a characteristic remanent magnetization which presents island mean direction with normal and reversed polarities in agreement with the islands location and the age of the studied lava flows indicating a primary thermo remanent magnetization field observations and paleomagnetic data show that lava flows were emplaced on pre existing slopes and were later affected by significant tilting in faial island magmatic inflation and normal faults making up an island scale graben can be responsible for the tilting in pico island inflation related to magma intrusion during flow emplacement can be at the origin of the inferred tilting whereas gradual downward movement of the se flank by slumping processes appears mostly translational

domain wall dynamics in 4c pyrrhotite at low temperature summary monoclinic 4c pyrrhotite fe7s8 is ferrimagnetic due to an ordered defect structure with alternating vacancy and vacancy free sublattices its low temperature magnetic transition near 35k is characterized by the distinct increase in coercivity and remanent magnetization the increase of these parameters has been attributed to changes in the domain wall structure we present static and dynamic magnetization data of a powder sample to study the domain wall dynamics across the low temperature transition the amplitude dependent ac susceptibility and the ferromagnetic resonance spectroscopy indicate that the hardening of the domain wall pinning at the transition occurs simultaneously with the decrease in initial saturation remanent magnetization these two effects are explained by the enhanced inhomogeneity of the bulk material caused by the persistency of the ordered vacancies and by newly formed defects due to localized distortion of fe ii sites in the vacancy free sublattice the generated localized defects are the link between the domain wall dynamics and the low temperature transition in 4c pyrrhotite

support for an a type pangea reconstruction from high fidelity late permian and early to middle triassic paleomagnetic data from argentina 1 a major disparity is observed between the late paleozoic early mesozoic apparent polar wander paths apwps of laurussia and gondwana when the landmasses are re assembled in a conventional a type pangea this discrepancy has endured from the earliest paleomagnetic reconstructions of the supercontinent and has prompted discussions of non dipole paleomagnetic fields and alternative paleogeographic models here we report on a joint paleomagnetic geochronologic study of late permian and early to middle triassic volcanic and volcaniclastic rocks from argentina which demonstrates support for an a type model without requiring modification to the geocentric axial dipole hypothesis new shrimp u pb and 40ar 39ar isotopic dating has reinforced the inferred age of the sequences which we estimate at 264 ma upper choiyoi group and 245 ma puesto viejo group field stability tests demonstrate that the volcanic rocks are carrying early primary magnetizations which yield paleopoles 73 7 s 315 6 e a95 4 1 n 40 upper choiyoi and 76 7 s 312 4 e a95 7 3 n 14 puesto viejo a comprehensive magnetic fabric analysis is used to evaluate structural restorations and to correct for magnetization anisotropy the paleomagnetic results derived from volcaniclastic rocks are interpreted to be affected by inclination shallowing and corrections are discussed a comparison of these new results with the existing permian triassic paleomagnetic data from gondwana suggests the presence of widespread bias in the latter we contend that such bias can explain the observed apwp disparity at least for late permian middle triassic time and that alternative paleogeographic reconstructions or non dipole paleomagnetic fields do not need to be invoked to resolve the discrepancy

low temperature remanence in stable single domain magnetite 1 a numerical model is developed in order to simulate the loss of low temperature remanence while warming through the verwey transition we concentrate on the amount of remanence lost fc and zfc during field cooled and zero field cooled low temperature demagnetization curves for stable single domain ssd particles of 1 inorganic magnetite and 2 biogenic magnetite in the form of magnetosome chains the model predicts that delta ratios fc zfc for inorganic ssd magnetite increase from 1 to 1 6 with increasing aspect ratio and that magnetosome chains have elevated delta ratios of at least 2 0 disaggregated magnetosomes are predicted to behave similarly to inorganic acicular ssd magnetite the numerical results agree with experimental results

magnetic hysteresis of 0 6 110 μm magnetites across the verwey transition we report saturation magnetization ms saturation remanence mrs coercive force hc and remanence coercivity hcr as a function of grain size d and temperature t for 0 6 135 m magnetites

grain size dependent magnetic discrimination of iceland and south greenland terrestrial sediments in the northern north atlantic sediment record abstract we use isothermal and temperature dependent in field and magnetic remanence methods together with electron microscopy to characterize different sieved size fractions from terrestrial sediments collected in iceland and southern greenland the magnetic fraction of greenland silts 3 63 m and sands 63 m is primarily composed of near stoichiometric magnetite that may be oxidized in the finer clay

metastability of domain state in multidomain magnetite consequences for remanence acquisition experimental studies on isothermal remanent magnetization irm acquisition of synthetic specimens containing crushed natural magnetite in seven grain sizes from 5 10 m to 100 150 m show that the initial demagnetization state before irm acquisition strongly affects the samples ability to acquire irm it is easier to acquire irm from an initially thermally demagnetized state than from an alternating field af demagnetized state and low temperature demagnetization of the thermally demagnetized state increases resistance to irm acquisition these data are interpreted as confirmation of the existence of a number of possible domain states metastable states which have variable capacity for remanence acquisition our results also demonstrate that activation of domain structure is much more effective by af than by direct dc fields we suggest that the continually changing external field experienced during af demagnetization allows the magnetic structure to reorganize incrementally towards the demagnetized state requiring a considerably lower peak field about 3 times lower to achieve the same demagnetization effect thermoremanent magnetization trm in larger grain sizes is partially self reversed a difference in grain size dependence is observed for a number of magnetic parameters all parameters vary with grain size below about 50 m above 50 m parameters associated with coercivity af demagnetization or low temperature treatment hcr saturation irm sirm memory trm memory median acquisition field maf of irm after af median destructive fields mdf of sirm are independent of grain size parameters associated with trm or thermal demagnetization mdf oftrm maf of irm after thermal demagnetization maf of irm after thermal demagnetization followed by low temperature demagnetization self reversal as percentage of trm show grain size dependency throughout the grain size range our interpretation of the data invokes the existence of subtle material inhomogeneities in both natural and synthetic magnetites giving rise to a limited range of curie temperatures below 580 c perhaps down to 560 c we suggest that their control over the local energy minima states available to the grain is more significant in the trm or thermally demagnetized state while shielding by soft domain walls is dominant in the af demagnetized state where walls are in the simplest domain configuration leading to differing grain size dependencies of these different states

anomalous unblocking temperatures viscosity and frequency dependent susceptibility in the chemically remagnetized trenton limestone abstract chemical remagnetization of many carbonate rock units occurred by diagenetic alteration of iron sulfide precursors and by authigenesis of fine grained magnetite which can commonly be recognized by a distinctive set of rock magnetic attributes these carbonates also typically exhibit anomalously high unblocking temperatures for recent thermoviscous components that have overprinted the characteristic chemical remanence recent work has shown that departures from neel theory as applied in the model of pullaiah et al can generally be attributed to vrm carried by multidomain particles to which the theory does not apply in the remagnetized carbonates however rock magnetic data all point to an overwhelming dominance of fine superparamagnetic and stable single domain particles therefore we explore possible alternative causes for the anomalous unblocking temperatures including maghemitization and magnetocrystalline rather than shape anisotropy new blocking unblocking models for these cases however fail to match the observed behavior leading to the conclusion that the anomalous unblocking temperatures are indeed probably attributable to multidomain carriers a possible alternative explanation is an increase in coercivity due to maghemitization of single domain particles after acquisition of the viscous overprint causing a significant increase in their unblocking temperatures new frequency dependent susceptibility measurements at low temperatures allow us to focus on the time temperature behavior of the particles at the superparamagnetic stable single domain threshold and we find that these do closely follow the predictions of neel theory as formulated in the models of pullaiah et al and walton middleton schmidt

low temperature cycling of isothermal and anhysteretic remanence microcoercivity and magnetic memory abstract this paper reports low temperature cycling ltc through the verwey transition of anhysteretic remanence arm partial arms and partially demagnetised saturation isothermal remanence sirm induced at room temperature in pseudo single domain and multidomain md magnetite the remanences were cooled in zero field to 50 k and then heated back to room temperature by inducing partial arms over different field ranges and by partially alternating field demagnetising sirms it was possible to isolate both low coercive force and high coercive force fractions of remanence on cooling through the verwey transition a sharp increase in the remanence was observed the relative size of the jump increased as the high coercive force fraction was increasingly isolated this behaviour is interpreted as being due to both an increase in the single domain multidomain threshold size on cooling through the verwey transition and to the reduction or elimination of closure domains in the low temperature phase in addition the memory ratio i e the fraction of remanence remaining after ltc divided by the initial remanence was found to be higher for the high coercive force fraction than the low coercive force fraction in our interpretation the high coercivity fraction behaviour is associated with reversible domain re organisation effects whilst the low coercive force fraction s behaviour is associated with irreversible domain re organisation and de nucleation processes due to the decrease in magnetocrystalline anisotropy on cooling to the verwey transition the high coercive force fraction is likely to be magnetoelastically controlled thus a rock displaying high coercive force behaviour is likely to carry a palaeomagnetically meaningful remanence with high unblocking temperatures in addition ltc analysis can be used to identify the domain state dominating the natural remanence in magnetite bearing rocks

determination of magnetic properties of rocks by analysis of demagnetization curves hematite ilmenite bearing rocks from sw sweden granulites from the high grade sveconorwegian corresponding to grenvillian gneiss terrane of sw sweden were investigated in order to relate magnetic properties to the granulite facies mineralogy and the amphibolite facies retrograde mineralogy we used standard paleomagnetic techniques susceptibility measurements electron and optical microscopy and a new way to analyze demagnetization curves based on vector difference sums the new technique allowed quantitation of the relative contributions of partial natural remanent magnetization nrm components as well as numerical extraction of thermal and coercive force alternating field af unblocking data two major components were identified one stable which is carried by hematite ilmenite and one less stable carried by multidomain md magnetite the age of the hematite ilmenite component is circa 930 ma and it has steep negative inclination coercivities in excess of 150 mt and unblocking temperatures between 540 degrees c and 640 degrees c two corresponding poles are reported 1 paleolatitude paleolongitude plat plong 218 degrees 24 3 with dp dm 9 4 degrees 9 9 degrees paleopole and 2 plat plong 203 degrees 51 degrees with d p d m 4 4 degrees 5 6 degrees vgp the md magnetite component is a recent or cretaceous paleogene steeply positive inclined nrm component associated with strong induced magnetization plat plong 2 degrees 74 degrees and d p d m 11 degrees 14 degrees vgp the md magnetite was unblocked at conditions of 6 40 mt and 60 400 degrees c our new analysis method was successful in showing that retrograde amphibole formation strongly decreases the importance of the hematite ilmenite component on behalf of the md magnetite it has a diminishing effect on a negative aeromagnetic anomaly in the area the 930 ma components constitute 80 0 of the vector difference sums depending on rock composition however as this fraction goes down to 60 the induced magnetization seems to outweigh the hematite ilmenite influence on aeromagnetic anomalies due to its opposite direction these rocks still have a strong and stable hematite ilmenite dominated nrm less

paleomagnetic study on the permian rocks of the indochina block and its implications for paleogeographic configuration and northward drifting of cathaysialand in the paleo tethys

review of the low temperature magnetic properties of magnetite from a rock magnetic perspective the low temperature magnetic properties of magnetite are reviewed and implications for rock magnetism considered the behaviour of fundamental properties of magnetite at low temperatures near the verwey transition tv are documented and attention is given to various verwey transition theories the low temperature behaviour of the magnetic energies that control domain structure is reviewed in detail for the first time in rock magnetic literature the low temperature anomaly in spontaneous magnetization ms is documented and the differences between the saturation magnetization and ms near the verwey transition are discussed it is argued that the low temperature behaviour of the magnetocrystalline anisotropy and in particular the anomaly at tv is most likely to affect multidomain remanence during low temperature cycling for multidomain crystals it is calculated that the large increase in magnetocrystalline anisotropy intensity and reduction in symmetry on cooling through tv is likely to reduce the stability of closure domains

low temperature magnetic properties of pelagic carbonates oxidation of biogenic magnetite and identification of magnetosome chains 1 pelagic marine carbonates provide important records of past environmental change we carried out detailed low temperature magnetic measurements on biogenic magnetite bearing sediments from the southern ocean ocean drilling program odp holes 738b 738c 689d and 690c and on samples containing whole magnetotactic bacteria cells we document a range of low temperature magnetic properties including reversible humped low temperature cycling ltc curves different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of ltc curves a dipole spring mechanism in magnetosome chains is introduced to explain reversible ltc curves this dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite similar to published results for uniaxial stable single domain sd particles the dipole spring mechanism reversibly restores the remanence during warming in ltc measurements this supports a previous idea that remanence of magnetosome chains is completely reversible during ltc experiments we suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains although the presence of isolated uniaxial stable sd particles and magnetically interacting particles can complicate this test magnetic measurements through the eocene section of odp hole 738b reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic hard component co occurrence of these two magnetic fingerprints during the late eocene in the southern ocean indicates less oxic conditions probably due to increased oceanic primary productivity and organic carbon burial

thermal magnetic behaviour of al substituted haematite mixed with clay minerals and its geological significance s u m m a r y clay minerals and al substituted haematite al hm usually coexist in soils and sediments however effects of clay minerals on al hm during thermal magnetic measurements in argon environment have not been well studied in order to quantify such effects a series of al hm samples were synthesized and were then mixed with clay minerals illite chlorite kaolinite and ca montmorillonite the temperature dependence of magnetic susceptibility curves in an argon environment showed that al substituted magnetite was produced during the thermal treatment via the reduction of al hm by the clay mineral which leads to a significant magnetic enhancement of the thermal products in addition the reductive capacity varies among different types of clay minerals that is illite chlorite kaolinite ca montmorillonite furthermore the iron content in the clay minerals and al content of al hm are two predominant factors controlling the reduced haematite content the iron is released from the clay minerals and provides the reducing agent while al decreases the crystallinity of haematite and thus facilitates the chemical reaction therefore the thermal magnetic measurements can be used to quantify the al content of al hm in natural samples our study provides significant information for palaeomagnetism and environmental magnetism studies such as thermal magnetic analysis and palaeomagnetic intensity reconstruction using ancient pottery and kilns

impact of climate change on the magnetic mineral assemblage in marine sediments from izu rear arc nw pacific ocean over the last 1 myr abstract a rock magnetic study was conducted on upper pleistocene marine sediments from international ocean discovery program expedition 350 site u1437 in order to highlight the paleoenvironmental changes in the nw pacific ocean influenced by the east asian monsoon regime remanent magnetization analyses hysteresis properties first order reversal curves and low temperature magnetic measurements were carried out along with electron microscope observations the results indicate that coarse grained titano magnetite is the dominant magnetic phase in the magnetic mineral assemblage time series analysis supports that this assemblage is modulated by global climate changes over the last 1 myr during the interglacial stages magnetic minerals are more abundant and are dominated by coarse grained titano magnetite of both terrigenous likely from mainland china and volcanic izu arc front japan origin during the glacial stages the magnetic mineral content is lower probably reflecting partial dissolution of ti magnetite and the magnetic assemblage is composed of terrigenous coarse grained titano magnetite and of higher coercivity presumably finer eolian particles likely hematite as a result of the enhancement of the winter monsoon in continental asia the magnetic mineral assemblage reflects a superimposition of volcanic and global climate signals bulk organic geochemical analyses for total and organic carbon as well as nitrogen and sulfur contents confirm a climatic signature in the composition of the sediments with more oxygenated water masses being present during glacial periods additional x ray fluorescence measurements on bulk samples indicate various origins of the sediment particles with both proximal and distal sources

controls on emplacement of the caledonian ross of mull granite nw scotland anisotropy of magnetic susceptibility and magmatic and regional structures abstract anisotropy of magnetic susceptibility ams was measured from 139 sites across the exposed portion of the caledonian ross of mull granite argyllshire nw scotland to investigate the internal architecture of the pluton field and petrographic observations support the results of the ams study and a complementary suite of rock magnetic experiments was used to quantify the magnetic mineralogy of the granite ams data reveal a fabric that records a partial tectonic overprint of an emplacement related magma inflow fabric the partially preserved inflow pattern indicates a south to north emplacement of subhorizontal sheets that coalesced to form a tabular pluton the ams data are remarkably consistent across the intrusion with clear north south strikes and trends dominating the magnetic foliation and lineation patterns respectively these ams fabrics are discordant to internal zonation in the granite in the southernmost part of the granite ams data are parallel to bedding cleavage orientations preserved in many large 100 m proterozoic moine blocks the scarcity of moine blocks in the northern part of the intrusion and the prevalence of randomly oriented stoped blocks suggest that this part of the intrusion is nearer to the ceiling of the pluton the partial tectonic overprint consistent with east west compression occurred during the latter stages of emplacement and mainly affected the younger granite facies rm2 while the oldest granite facies rm1 and certain magmatic structures diorite enclaves preserve the original emplacement fabric an alternative explanation interprets the magnetic lineations as an intersection fabric between a shallowly dipping emplacement fabric and a steep tectonic fabric we argue that emplacement was not associated with eastward orogenic collapse of the scandian moine nappes as proposed by previous workers but occurred either before orogenic collapse or during a period of compressional reactivation

chains clumps and strings magnetofossil taphonomy with ferromagnetic resonance spectroscopy magnetotactic bacteria produce intracellular crystals of magnetite or greigite the properties of which have been shaped by evolution to maximize the magnetic moment per atom of iron intracellular bacterial magnetite therefore possesses traits amenable to detection by physical techniques typically narrow size and shape distributions single domain size and arrangement in linear chains and often crystal elongation past strategies for searching for bacterial magnetofossils using physical techniques have focused on identifying samples containing significant amounts of single domain magnetite or with narrow coercivity distributions searching for additional of traits would however increase the likelihood that candidate magnetofossils are truly of biological origin ferromagnetic resonance spectroscopy fmr is in theory capable of detecting the distinctive magnetic anisotropy produced by chain arrangement and crystal elongation here we present analyses of intact and lysed magnetotactic bacteria dilutions of synthetic magnetite and sedimentary samples of modern carbonates from the great bahama bank oligocene miocene deep sea muds from the south atlantic and pleistocene lacustrine deposits from mono basin california we demonstrate that fmr can distinguish between intact bacterial magnetite chains collapsed chains and linear strings of magnetite formed by physical processes we also show that sediments in which the magnetization is likely carried by bacterial magnetite have fmr spectra resembling those of intact or altered bacterial magnetite chains