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

simulating migrated and inverted seismic data by filtering a geologic model the simulation of migrated and inverted data is hampered by the high computational cost of generating 3d synthetic data followed by processes of migration and inversion for example simulating the migrated seismic signature of subtle stratigraphic traps demands the expensive exercise of 3d forward modeling followed by 3d migration of the synthetic seismograms this computational cost can be overcome using a strategy for simulating migrated and inverted data by filtering a geologic model with 3d spatial resolution and angle filters respectively a key property of the approach is this the geologic model that describes a target zone is decoupled from the macrovelocity model used to compute the filters the process enables a target orientedapproach by which a geologically detailed earth model describing a reservoir is adjusted without having to recalculate the filters because a spatial resolution filter combines the results of the modeling and migration operators the simulated images can be compared directly to a real migration image we decompose the spatial resolution filter into two parts and show that applying one of those parts produces output directly comparable to 1d inverted real data two dimensional synthetic examples that include seismic uncertainties demonstrate the usefulness of the approach results from a real data example show that horizontal smearing which is not simulated by the 1d convolution model result is essential to understand the seismic expression of the deformation related to sulfate dissolution and karst collapse

associations between burial diagenesis of smectite chemical remagnetization and magnetite authigenesis in the vocontian trough se france results of a paleomagnetic rock magnetic geochemical and petrographic study on jurassic and cretaceous carbonates in the vocontian trough support a hypothesized connection between burial diagenetic alteration of smectite and the widespread occurrence of a chemical remanent magnetization crm carried by magnetite where smectite has altered to other clay minerals limestones are characterized by a prefolding secondary normal polarity magnetization throughout the basin the magnetization is interpreted to be a crm based on low burial depths which cannot cause thermoviscous resetting where significant smectite is still present the crm is absent weakly developed and where the clays show no evidence for burial alteration the units are characterized by a primary magnetization crm intensity also varies with the amount of smectite and burial isothermal anhysteretic and natural remanent magnetization intensities increase where smectite has altered both stratigraphically and geographically this is interpreted to indicate magnetite authigenesis associated with clay diagenesis superparamagnetic magnetite is more dominant in highly altered units based on the results of low temperature experiments all sections away from the alps have 87sr 86sr values that are similar to coeval seawater and stable isotopes of carbon and oxygen show no sign of alteration orogenic type fluids therefore are not a likely agent of remagnetization near the alps the rocks are characterized by an additional reversed polarity component which is interpreted to reflect acquisition of the crm through a reversal a postfolding magnetization is also present there and strontium isotopic ratios are higher than elsewhere in the basin and might indicate some alteration by orogenic type fluids we conclude that burial diagenesis of smectite is the likely cause for the development of the widespread crm in the vocontian trough and that this mechanism might explain widespread chemical remagnetization elsewhere

magnetite authigenesis and diagenetic paleotemperatures across the northern appalachian basin the concentration of magnetite in the lower and middle devonian helderberg and onondaga carbonate formations varies in a distinct pattern along an east west profile across new york state magnetite concentrations are uniformly low in the western half of the profile increase sharply eastward to a peak value near syracuse and decrease farther east the pattern strongly resembles previously reported variations in the extent of clay mineral diagenesis that reflect differences in paleotemperatures along the profile previous paleomagnetic studies have documented that the magnetite carries a well defined magnetization of pennsylvanian permian age and magnetite authigenesis is therefore no younger than late paleozoic we attribute the correlation between high magnetite concentration and high illite content to temperature dependent diagenesis triggered by orogenic fluids the large proportion of secondary magnetite indicates that over most of the area the mechanism of late paleozoic remagnetization was principally chemical thermal effects played a less direct role in the remagnetization of the strata by controlling the extent of the chemical processes that resulted in overprinting of the original remanence

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

regional patterns of magnetite authigenesis in the appalachian basin implications for the mechanism of late paleozoic remagnetization we report paleomagnetic and rock magnetic data from sedimentary carbonates of the appalachian basin that have implications for the mechanism of widespread late paleozoic remagnetization in this rock type sampling was accomplished mostly along two across basin transects one in devonian carbonates of new york state and the other in ordovician and mississippian carbonates of tennessee and alabama paleomagnetic investigations of the new york devonian carbonates along a transect from albany to buffalo indicate that these rocks were completely remagnetized in alleghenian time rock magnetic studies show that magnetite concentration varies in a regular fashion along the transect with a local maximum near syracuse and decreasing gradually to the east and west this pattern shows a striking correlation with the degree of diagenetic alteration of clay minerals in a devonian bentonite horizon which took place during alleghenian time we conclude that most of the magnetite present in these carbonates is of authigenic origin and that clay mineral alteration and magnetite authigenesis were coeval late paleozoic events that were controlled by the same diagenetic factors thermoviscous remagnetization processes cannot be ruled out in this setting in light of the observed degree of thermal maturity however our results suggest that chemical factors played a critical role in remagnetization since they allow us to infer that magnetite authigenesis and remagnetization are about the same age we propose a geochemical model for magnetite authigenesis wherein the iron is derived from detrital smectites during diagenetic illitization which is triggered by the introduction of potassium rich brines results from the tennessee transect from nashville to chattanooga show a very different pattern paleomagnetic studies indicate that late paleozoic remagnetization has affected ordovician carbonates of the nashville dome paleozoic carbonates in the overthrust belt near chattanooga also carry the late paleozoic remagnetization however mississippian carbonates between the dome and the overthrust belt are very weakly magnetized and show no evidence for late paleozoic remagnetization samples show only a present field magnetization or at one locality in northwestern alabama a dual polarity magnetization of probable mississippian age rock magnetic studies indicate that higher concentrations of magnetite are present in the remagnetized rocks of the nashville dome and the overthrust belt than in the nonremagnetized mississippian carbonates we therefore conclude that magnetite authigenesis and remagnetization are related events in this setting and that the remagnetization must be due to chemical processes the presence of a remagnetization shadow in the younger rocks now exposed between the fold thrust belt and the flexural arch is attributed to westward flow of magnetite forming aqueous fluids through deep sub carboniferous aquifers during alleghenian time upward migration of the fluids through the mississippian carbonates may have been prevented by the devono mississippian chattanooga shale an impermeable stratum that is present throughout the region

evolution of the raman spectrum of carbonaceous material in low grade metasediments of the glarus alps switzerland terra nova 22 354 360 2010 abstract the raman spectrum of carbonaceous material cm from advanced diagenesis 200 c to low grade metamorphism 320 c is documented in the helvetic flysch of the glarus alps switzerland the spectrum is complex with several defect bands at 1200 d4 1350 d1 1500 d3 and 1620 d2 cm 1 we document the evolution of these bands relative to the graphite g band with increasing metamorphic grade and we show that this qualitative evolution may be used as a proxy for temperature in the glarus alps we develop a robust peak fitting method and propose quantitative parameters that may be used as proxies for thermal metamorphism in this region further work in other geological contexts is needed to assess whether the spectral evolution observed in the glarus alps may be generalized and to critically assess the potential for calibrating a general empirical and quantitative thermometer based on the raman spectrum of cm in low grade rocks

pyrite alteration and neoformed magnetic minerals in the fault zone of the chi chi earthquake mw7 6 1999 evidence for frictional heating and co seismic fluids 1 during an earthquake physical and chemical transformations lead to alteration and formation of minerals in the gouge layer altered and neoformed minerals can be used as tracers of some earthquake processes in this study we investigate pyrite and magnetic minerals within the host chinshui siltstone and the 16 cm thick gouge this gouge hosts the principal slip zone of chi chi earthquake mw7 6 1999 in the chinshui siltstone pyrite framboids of various sizes and euhedral pyrite are observed the magnetic mineral assemblage comprises stoichiometric magnetite greigite and fine grained pyrrhotite the pyrite content is generally reduced in the gouge compared to the wall rock the magnetic mineral assemblage in the gouge consists of goethite pyrrhotite and partially oxidized magnetite the pyrrhotite goethite and some magnetite are neoformed pyrrhotite likely formed from high temperature decomposition of pyrite 500 c generated during co seismic slip of repeated earthquakes goethite is inferred to have formed from hot aqueous co seismic fluid 350 c in association with the 1999 chi chi event elevated fluid temperatures can also explain the partial alteration of magnetite and the retrograde alteration of some pyrrhotite to pyrite we suggest that characterization of neoformed magnetic minerals can provide important information for studying earthquake slip zones in sediment derived fault gouge

the magnetization of clay rich rocks in sedimentary basins low temperature experimental formation of magnetic carriers in natural samples abstract rocks are often magnetically altered during burial thus providing a useful method to study their thermo chemical history in order to improve the understanding of burial diagenetic magnetization we experimentally synthesized magnetite in natural samples lower jurassic argillites from the paris basin were heated to 150 c under a confined atmosphere of argon when a magnetic field was applied during the heating a chemical remanent magnetization was acquired rock magnetism studies reveal that most of the newly formed magnetic carriers were magnetite using anhysteretic remanent magnetization as a measure of the quantity of magnetite an increase of up to 150 after heating was observed ferrimagnetic iron sulphide and hematite are observed to have formed in samples composed of less than 3 calcite and new magnetite is sometimes completely absent in samples containing less than 0 5 calcite hysteresis measurements suggest that the new magnetite is in the pseudo single domain state scanning electron microscope studies indicate that the new magnetic carriers are associated with pre existing pyrite the magnetic transformations during the experiments involved a fluid which reacted with iron bearing and sulfur bearing minerals in ways depending on local redox conditions this study shows that magnetite can be produced in argillaceous sediments at the normal temperatures and pressures found in sedimentary basins without any external supply from long range fluid flow

replacement of pyrite framboids by magnetite in limestone and implications for palaeomagnetism limestones are an important source of palaeomagnetic data with magnetite the dominant carrier of the magnetization because the magnetite is present in low concentrations however it has been directly observed only rarely and in trace amounts in acid treated separates such residues are composed primarily of spheres the ultimate origin of which has been enigmatic7 yet interpretation of the palaeomagnetic data depends on a knowledge of the origin of the magnetite which has been inferred to carry either a chemical or a viscous remanent magnetization here we present electron microscope data obtained from a sequence of samples of onondaga limestone from an east west traverse across northern new york which unambiguously show that magnetite spheres are derived by alteration and replacement of framboidal pyrite remagnetization is thus caused by chemical rather than viscous processes this chemical remanent magnetization is compatible with a fluid mediated event occurring on a regional scale induced by tectonic stress of alleghenian late palaeozoic age

analysis of the resolution function in seismic prestack depth imaging we consider the problem of estimating subsurface quantities such as velocity or reflectivity from seismic measurements because of a limited aperture and band limited signals the output from a seismic prestack reconstruction method is a distorted or blurred image this distortion can be computed using the concept of resolution function which is a quantity readily accessible in the fourier space of the model the key parameter is the scattering wavenumber which at a particular image point is defined by the incident and scattered ray directions in a given background model any location in any background model can be considered in general the resolution function will depend on the following four quantities the background velocity model the frequency bandwidth the wavefield type and the acquisition geometry we first establish the resolution function for a general scattering model assuming local reaction we then adapt this result for two well known scattering models born and kirchhoff for each of these approximations the corresponding resolution function is derived and discussed finally by employing a simple synthetic data example we demonstrate the ability of the resolution function to predict the image distortions

sem stem observations of magnetite in carbonates of eastern north america evidence for chemical remagnettzation during the alleghenian orogeny early to middle paleozoic carbonates of eastern north america have been pervasively remagnetized in order to determine the process of remagnetization scanning and scanning transmission electron microscopy have been used to characterize magnetite in thin sections and in concentrated separates samples included ordovician knox carbonates from east tennessee ordovician trenton limestone and devonian onondaga and helderberg limestones from new york and ordovician trenton carbonates from michigan inclusions of authigenic minerals within magnetite grains lack of cations other than iron and a variety of textural relations all imply that the magnetite is authigenic these data are consistent with estimates that paleotemperatures never exceeded values that would reset magnetic directions the remagnetization is thus a chemical remanent magnetization crm rather than viscous remanent magnetization vrm as the timing of remagnetization corresponds to the alleghenian orogeny the observed relations imply stress induced crystallization of magnetite that was mediated by fluids consistent with but not requiring fluid flow on a regional basis

grain size dependence of the magnetic behavior of pyrrhotite during its low temperature transition at 34 k the grain size dependent behavior of pyrrhotite during a magnetic lt transition at 34 k is suitable for granulometric purposes cycling down to liquid helium temperature suppresses soft md jrs more than hard sd jrs so that it may be effective in removing soft secondary nrm components the domain structure of large pyrrhotite grains changes from md at room temperature to sd or psd below the lt transition

raman spectra of carbonaceous material in metasediments a new geothermometer metasedimentary rocks generally contain carbonaceous material cm deriving from the evolution of organic matter originally present in the host sedimentary rock during metamorphic processes this organic matter is progressively transformed into graphite s s and the degree of organisation of cm is known as a reliable indicator of metamorphic grade in this study the degree of organisation of cm was systematically characterised by raman microspectroscopy across several mesozoic and cenozoic reference metamorphic belts this degree of organisation including within sample heterogeneity was quantified by the relative area of the defect band r2 ratio the results from the schistes lustres western alps and sanbagawa japan cross sections show that 1 even through simple visual inspection changes in the cm raman spectrum appear sensitive to variations of metamorphic grade 2 there is an excellent agreement between the r2 values calculated for the two sections when considering samples with an equivalent metamorphic grade and 3 the evolution of the r2 ratio with metamorphic grade is controlled by temperature t along the tinos cross section greece which is characterised by a strong gradient of greenschist facies overprint on eclogite facies rocks the r2 ratio is nearly constant consequently the degree of organisation of cm is not affected by the retrogression and records peak metamorphic conditions more generally analysis of 54 samples representative of high temperature low pressure to high pressure low temperature metamorphic gradients shows that there is a linear correlation between the r2 ratio and the peak temperature t c 445 r2 641 whatever the metamorphic gradient and probably the organic precursor the raman spectrum of cm can therefore be used as a geothermometer of the maximum temperature conditions reached during regional metamorphism temperature can be estimated to 50 c in the range 330 650 c a few technical indications are given for optimal application

on the relation between seismic interferometry and the migration resolution function seismic interferometry refers to the process of retrieving new seismic responses by crosscorrelating seismic observations at different receiver locations seismic migration is the process of forming an image of the subsurface by wavefield extrapolation comparing the expressions for backward propagation known from migration literature with the green s function representations for seismic interferometry reveals that these seemingly distinct concepts are mathematically equivalent the frequency domain representation for the resolution function of migration is identical to that for the green s function retrieved by seismic interferometry or its square in the case of double focusing in practice they differ because the involved green s functions in seismic interferometry are all defined in the actual medium whereas in migration one of the green s functions is defined in a background medium

diagenetic sources of stable remanence in remagnetized paleozoic cratonic carbonates a rock magnetic study hysteresis measurements lowrie fuller tests cisowski tests and low temperature demagnetization experiments on samples of the knox dolomite trenton limestone and onondaga limestone indicate that the remanence in these remagnetized carbonate units resides principally in single domain magnetite in contrast to previous interpretations volumetrically however a significant fraction of the magnetite is coarser grained as previous scanning electron microscopic work on magnetic extracts has shown these conclusions are based mainly on the characteristic wasp waisted shape of the hysteresis loops and on the anomalously high ratios of remanent coercivity to bulk coercivity both of which clearly indicate a bimodal distribution of coercivities grain sizes the magnetization is controlled mainly by cubic magnetocrystalline anisotropy consistent with the spheroidal particles commonly observed in extracts hysteresis trends show that especially the finest particles lack shape anisotropy in view of these conclusions the two components of natural remanence in these rocks are interpreted as 1 a late paleozoic kiaman chemical remanence carried by single domain diagenetic magnetite and 2 a cenozoic thermoviscous overprint residing principally in coarser pseudosingle domain and multidomain magnetite also of late paleozoic diagenetic origin

directional oriented wavefield imaging a new wave based subsurface illumination imaging condition for reverse time migration the key objective of an imaging algorithm is to produce accurate and high resolution images of the subsurface geology however significant wavefield distortions occur due to wave propagation through complex structures and irregular acquisition geometries causing uneven wavefield illumination at the target therefore conventional imaging conditions are unable to correctly compensate for variable illumination effects we propose a generalised wave based imaging condition which incorporates a weighting function based on energy illumination at each subsurface reflection and azimuth angles our proposed imaging kernel named as the directional oriented wavefield imaging compensates for illumination effects produced by possible surface obstructions during acquisition sparse geometries employed in the field and complex velocity models an integral part of the directional oriented wavefield imaging condition is a methodology for applying down going up going wavefield decomposition to both source and receiver extrapolated wavefields this type of wavefield decomposition eliminates low frequency artefacts and scattering noise caused by the two way wave equation and can facilitate the robust estimation for energy fluxes of wavefields required for the seismic illumination analysis then based on the estimation of the respective wavefield propagation vectors and associated directions we evaluate the illumination energy for each subsurface location as a function of image depth point and subsurface azimuth and reflection angles thus the final directionaloriented wavefield imaging kernel is a cross correlation of the decomposed source and receiver wavefields weighted by the illuminated energy estimated at each depth location the application of the directional oriented wavefield imaging condition can be employed during the generation of both depth stacked images and azimuth reflection angle domain common image gathers numerical examples using synthetic and real data demonstrate that the new imaging condition can properly image complex wave paths and produce high fidelity depth sections

finite difference calculation of travel times the travel times of the first arriving seismic waves through any velocity structure can be rapidly computed on a two or three dimensional numerical grid by finite difference extrapolation from point to point wavefronts rather than the traditional rays are tracked head waves are properly treated and shadow zones are filled by the appropriate diffractions differences of less than 0 03 per cent are found between the results of this technique and raytracing for a complex two dimensional model this scheme is useful for the windowing of finite difference calculations to increase computing speed and promises to aid in earthquake location tomographic inversion and kirchhoff migration in structures that have strong lateral velocity gradients

evidence for a relationship between hydrocarbons and authigenic magnetite establishing a relationship between hydrocarbon migration and the precipitation of authigenic magnetite in sedimentary rocks is of significant interest with respect to 1 elucidating mechanisms for remagnetization and establishing the origin of secondary magnetizations residing in magnetite 2 developing a method to date hydrocarbon migration events by determining the time of remanence acquisition by palaeomagnetic methods and 3 evaluating whether searching for anomalous concentrations of diagenetic magnetic minerals and or aeromagnetic anomalies is justified as a relatively inexpensive exploration tool the direct association of hydrocarbon migration and the precipitation of authigenic magnetite has however not been established in this paper we report palaeomagnetic rock magnetic petrographic and geochemical results of a study of samples from permian spelebthems and gilsonite found in the ordovician arbuckle group in southern oklahoma the results indicate that there is a genetic relationship between hydrocarbon migration and the precipitation of authigenic magnetite

fluid control of localized mineral domains in limestone pressure solution structures grain to grain and stylolitic solution structures in two central appalachian siluro devonian limestone macroscale folds contain one of four distinct mineral assemblages that are characterized by the dominant iron phase mineral present 1 chloritegillitegpyritegcalciteg quartzgtio2ggoethite 2 chloritegillitegpyrite altered to iron oxide hydroxidegcalcite gquartzgtio2 3 chloritegilliteg magnetitegcalcitegquartz and 4 chloritegilliteggoethitegcalcitegquartzgtio2 optical reflectance microscopy and sem eds was used to characterize the mineralogy and mineral morphology of these structures geochemical modeling was used to constrain the conditions of formation and preservation the primary control on solution structure mineral assemblage was the redox conditions present in the solution structures during burial and deformation the redox conditions on the microscale may have been controlled by the local fluid chemistry and the presence absence of hydrocarbons and organic acids within the formation fluids and the influx of externally derived fluids by fracture formation during the folding process the wide variation in mineralogy of the solution structures shows that they were chemical factories where a variety of chemical reactions took place during rock dissolution in particular the formation of authigenic magnetite in solution structures has significant implications for paleomagnetic applications and use of anisotropy of anhysteretic remanent magnetization and anisotropy of magnetic susceptibility fabrics q 2005 elsevier ltd all rights reserved

review and update of the applications of organic petrology part 1 geological applications abstract organic petrology developed as coal petrology at the beginning of the 20th century dedicated mainly to the study of coals because of their utilization in industry coal petrology was then considered a branch of coal science later with the development of specialized nomenclature classification of coal components and the standardization and improvement of analytical microscopical methods this discipline expanded in interests and name becoming organic petrology organic petrology carries a broader context being as well a tool applied in the study of dispersed organic matter in sedimentary rocks due to its importance in exploration for fossil fuel resources at present organic petrology is a discipline widely recognized for its role in fundamental and applied research with respect to both coal utilization and in geosciences throughout the 20th century several important monographs have been published on the discipline of organic petrology including stach s textbook of coal petrology 1st edition 1935 2nd 1975 3rd 1982 updated as the more general organic petrology by taylor et al 1998 more recently the text applied coal petrology the role of petrology in coal utilization was published by suarez ruiz and crelling 2008 this review is the first in a two part review series that describes and updates the role of organic petrology in geosciences a second part complementing this one and focused on the applications of organic petrology to other scientific fields will follow

modeling of marine seismic profiles in the t x and τ p domains in many cases comparison of real data with synthetic seismograms provides additional constraints on the velocity depth profiles obtained with simple inversion techniques obtaining a satisfactory match between the real and computed data usually requires several trials with different models but can be performed rapidly if the theoretical seismograms are themselves easily interpretable i e if the major contributions which make up the synthetic traces can be identified and separated in horizontally stratified media modeling is further simplified and is faster if the simulation techniques are implemented in the domain of the intercept time and the ray parameter p the generalized reflection and transmission matrix method is well suited for these purposes and may be used to generate synthetic seismograms in both the t x and p planes computation of plane wave seismograms is straightforward and merely corresponds to an inverse fourier transform of the overall reflectivity matrix for each ray parameter

towards pyrrhotite magnetite geothermometry in low grade metamorphic carbonates of the tethyan himalayas shiar khola central nepal abstract in mesozoic metacarbonates of the tethyan himalayas shiar khola area central nepal two characteristic remanent magnetisations chrm1 and chrm2 were identified by their unblocking temperature spectra the chrm1 is carried by pyrrhotite unblocking temperature 270 360 c and the chrm2 by magnetite unblocking temperature spectra 430 580 c the temperature related formation of pyrrhotite at the expense of primary magnetite during low grade metamorphism in marly carbonates allows the determination of thermal gradients by the pyrrhotite magnetite ratio this new method can be used as a geothermometer for t 300 c in low grade metamorphic carbonates where other methods are not available this method is applied for the first time in the tethyan himalayas of central nepal in the shiar khola valley systematic variations in the ferrimagnetic content of the metacarbonates along an e w profile were detected by the ratio of remanence intensity of pyrrhotite to magnetite derived from natural remanent magnetisation rpyr mag and saturation magnetisation spyr mag over a stretch of 10 km the rpyr mag and spyr mag increase from w to e from 0 42 to 0 91 and 0 48 to 1 0 respectively based on temperature estimates the eastern part experienced upper anchizone epizone 250 300 c conditions while the western part underwent only diagenesis 200 c the temperature gradient and the temperature ranges suggested are consistent with the findings of the calcite twin lamellae geothermometry which is a non magnetic method

the late diagenetic conversion of pyrite to magnetite by organically complexed ferric iron abstract the geochemistry of late stage diagenetic replacement of pyrite by magnetite was investigated by laboratory experiments of pyrite dissolution and magnetite precipitation involving organically complexed ferric iron the design of the experiments is based on the hypothesized geochemical conditions for in situ diagenesis in the pennsylvanian age belden formation an organic rich carbonate in northwestern colorado with pyrite grains rimmed with magnetite that contains a cretaceous paleopole position the geochemistry is applicable to both deep and near surface reactions involving pyrite magnetite and organic matter and bears upon the geologic interpretation of secondary chemical remanent magnetizations crm aqueous solutions of ferric ligand complexes and pyrite were heated 60 c for 60 days in the absence of light and at low partial pressures of oxygen additional experiments with each ligand were performed both in the presence of bentonite and without ferric iron ligand only raising the ph to 8 5 produced a ferrous hydroxide precipitate and subsequent heating 90 c resulted in the formation of magnetite ferric complexes of oxalate salicylate and acetylacetonate resulted in pyrite dissolution and magnetite formation the activity of strongly reducing ligands e g catechol or unsaturated fe iii exchange capacity particularly in experiments with bentonite or with solutions of uncomplexed ligands can result in an effective reduction of ferric iron concentration and prevent pyrite dissolution and or interfere with the crystallization of magnetite the bentonite acted to adsorb dissolved species and also underwent direct dissolution especially by the oxalate ligands although these reactions could occur at any depth a continuing supply of molecular oxygen more available in near surface settings would result in the formation of hematite or the production of acidic sulfate conditions whereas at greater depths or restricted porosity the redox gradation is halted at magnetite the results of this study thus provide a likely geochemical mechanism for the replacement of pyrite by magnetite during the latter stages of diagenesis

high precision three dimensional paleothermometry derived from paleomagnetic data in an alpine metamorphic unit in the dauphinoise zone of the french western alps a posttectonic thermoremanent magnetization carried by pyrrhotite has been isolated the paleomagnetic signature is interpreted as the record of successive partial thermoremanent magnetizations of opposite directions acquired during the early miocene slow cooling of the epimetamorphic marly limestones of the studied area a local vertical thermal gradient of 22 2 c km was derived from the unblocking temperature of a reversal recorded at different altitudes using this vertical gradient we have produced a paleotemperature map at the time of the reversal about 20 ma the map shows a horizontal regional thermal gradient of 7 c km the higher temperatures were toward the crystalline basement paleomagnetic directions demonstrate that this horizontal gradient was present during cooling its origin can be explained by large volume fluid circulation along the major fault that forms the boundary between the crystalline basement and its liassic cover

magnetic transition at 30 34 kelvin in pyrrhotite insight into a widespread occurrence of this mineral in rocks abstract a characteristic magnetic transition at 30 34 k is shown to provide a powerful tool for the identification of pyrrhotite with concentration down to 10 ppm through the same low temperature techniques as applied to magnetite and hematite extended down to liquid helium temperature a review of rock magnetic and petrological data on pyrrhotite suggests that this mineral should be considered as a major carrier of paleomatnetic signals unblocking temperature up to 350 c and extreme resistance against af may be encountered in fine grained pyrrhotite

features and evolution of slip structures in badlands areas se spain abstract slips are little known mud coated structures several centimeters thick produced by gravity driven flows on free vertical surfaces we have differentiated a sequence formed by three main morphologies mud drips slips and pseudo stalactites this study located in the badlands of the guadix and baza basins se spain suggests that formation of muddy morphologies on subvertical walls is determined by the physico chemical characteristics of colloidal suspensions slurries related to rainy periods and does not indicate any substrate decay multiple braided microchannels internally structured like irregular book pages are commonly found each formed by mudflows during rainy episodes these morphologies constitute a temporary sink of fine erosive materials they consist of scarcely sorted mixtures of heterogeneous matter of a detrital cohesive nature with low clay contents which inherit the features of the parent materials the current semiarid environmental conditions appear to favor conservation rather than development the differences observed in the slips of the guadix and baza basins can be determined by the different geomorphological evolution of the two basins and to a lesser extent by the properties of the slurries that fed the slips

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

the occurrence and origin of late paleozoic remagnetization in the sedimentary rocks of north america although it has been known for 25 years that some paleozoic sedimentary rock units in europe and north america were remagnetized during pennsylvanian or permian time it is only very recently that the complex and widespread nature of the late paleozoic remagnetization phenomenon has been generally acknowledged it is now recognized that many paleozoic paleomagnetic poles for north america that were once considered reliable are in fact the result of remagnetization and as a consequence the paleomagnetic data base for the paleozoic is undergoing rapid and drastic revision the causes of late paleozoic remagnetization in north america are currently the focus of much interest and active research the remagnetization can reside in either hematite or magnetite and different remagnetization mechanisms have been important in different settings chemical remagnetization processes some related to specific diagenetic events are dominant in hematite bearing sandstones and carbonates in magnetite bearing carbonates both chemical and thermoviscous remagnetization processes appear to have been important but it is difficult to determine which process is the dominant one in some settings some of the observed remagnetizations can be linked to the migration of chemically active and perhaps hot fluids during the mountain building events that affected much of the continent during the late paleozoic paleomagnetic studies promise to be important in assessing the role of orogeny in driving fluid migrations within sedimentary basins and in constraining the age of the migrations and the nature of the fluids

3 d migration of cross spread data resolution and amplitude aspects ideally full prestack migration involves a 4 d integration along the source and receiver coordinates obviously for cross spread data typical for land acquisition this 4 d integration cannot be effectuated an analysis is presented of the resolution and amplitude behavior of full prestack migration applied to cross spread data moreover a proposal is made to modify operators that partly compensate for the diminishing effects of the cross spread aquisition geometry on amplitudes and resolution

paleomagnetism of the notch peak contact metamorphic aureole revisited pyrrhotite from magnetite pyrite under submetamorphic conditions 1 on the basis of new demagnetization and rock magnetic experiments a scattered two polarity remagnetization extending throughout the contact metamorphic aureole surrounding the jurassic notch peak granitic stock in western utah is now thought to reside in metamorphic pyrrhotite rather than in fine grained hematite in part the pyrrhotite formed by in situ reduction of magnetite by pyrite and organic compounds it could not have been formed by the large scale introduction of reducing fluids as isotopic evidence indicates that these metalimestones remained impermeable at higher grades some pyrrhotite probably also formed by the reaction of pyrite and fe bearing silicates as magnetite destruction and pyrrhotite formation extends into rocks that are not obviously metamorphosed the reactions responsible could proceed at very low less than 200 c temperatures and the feasibility of such low temperature reactions in a closed system is shown by detailed thermodynamic calculations this study demonstrates that under reducing conditions in the presence of sufficient pyrite pyrrhotite can form at the expense of magnetite under very mild conditions indeed magnetite may be destroyed merely by deep burial such a mechanism may help explain the paucity of primary magnetite in ancient sedimentary rocks particularly since the single domain and pseudo single domain grains capable of retaining a stable paleomagnetic signature over geologic time are extremely fine grained and hence particularly vulnerable to destruction

time temperature relations for the remagnetization of pyrrhotite fe7s8 and their use in estimating paleotemperatures abstract paleotemperature controls the maturation of coal and hydrocarbons in sedimentary basins and is also important in determining paleogeothermal gradient and hence tectonic style in exhumed metamorphic terrains one method of estimating paleotemperature analyses the partial remagnetization of a rock due to heating in thick volcanic or sedimentary sequences over subcrustal heat sources such as plumes or at convergent plate margins the overprinted natural remanent magnetization nrm of a rock records both the age and the paleotemperature of remagnetization but a temperature correction from laboratory to geological time scales is required using theoretical time temperature relations time temperature relations are well known for magnetite fe 3 o 4 but are reported here for the first time for pyrrhotite fe 7 s 8 another common nrm carrier data for each mineral separately yield independent estimates of paleotemperature if geologically reasonable estimates of heating time can be made paleotemperature can be estimated without geological input if data for both minerals are combined together with the age of remagnetization determined from the paleomagnetic pole of the nrm overprint these paleotemperature estimates can be used to infer the history of heating and uplift following burial as a test case we examine thermally acquired nrm overprints carried by pyrrhotite fe 7 s 8 and magnetite fe 3 o 4 in the milton monzonite of southeastern australia these overprints record a heating event about 100 ma ago probably thermal doming prior to rifting of the tasman sea that upgraded coal rank in the sydney basin extrapolating from laboratory to geological times using the new time temperature contours for pyrrhotite we estimate that the presently exposed sydney basin in the vicinity of the milton monzonite was remagnetized by heating to 165 30 c for 100 ka assuming a paleogeothermal gradient of 70 c km appropriate for young or incipient rifts the depth of burial at the time of remagnetization is estimated to have been 2 3 0 4 km this figure is in excellent agreement with independent estimates based on reflectance data for the coal accessory mineral vitrinite

metamorphic control of the magnetic mineralogy of black shales in the swiss alps toward the use of magnetic isogrades abstract the magnetic susceptibility of the helvetic jurassic black shales sampled around the aar and gothard massifs in the swiss alps is due to iron bearing silicates magnetite or pyrrhotite it exhibits a clear correlation with the alpine metamorphism from zeolite to amphibolite facies it firstly decreases due to the breakdown of original magnetite then sharply increases when pyrite is transformed into magnetic pyrrhotite using numerous field measurements of the susceptibility it was possible to carry out a fast and precise mapping of the pyrrhotite in isograde which appears to correspond closely to the stilpnomelane out isograde the matrix susceptibility obtained by means of high field measurements with a cryogenic magnetometer is due to iron bearing silicates and shows only smooth variations with metamorphism both monoclinic and hexagonal pyrrhotites are observed in the high grade area

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

imprinting chemical remanent magnetization in claystones at 95 c abstract thermal effects related to burial and hydrothermal alteration leads to chemical remanent magnetization crm we present an experimental study of crm production by heating claystones at 95 c a vertical magnetic field of 2 mt was applied to the claystones during heating and the evolution of the remanence during heating in air is monitored intermittently for up to four months solid fragments 9 to 26 g of claystones are included in a teflon holder that is placed in the oven under a controlled atmosphere newly formed grains acquire a crm and a thermoviscous magnetization tvrm both being parallel to the applied magnetic field crm is related to the amount of newly formed grains that pass the critical volume during the reaction to measure the acquired remanence the claystones are first cooled in a zero magnetic field and then measured using a 2g squid magnetometer in the frame of the research programme on the feasibility of radioactive waste disposal in a deep geological formation we investigate the magnetic transformation of mont terri lower dogger claystones switzerland due to thermal imprinting at 95 c we simulate the dehydration that occurs in the walls of galleries after excavation when interstitial water evaporates and rehydration when the galleries are refilled allowing water to move towards dehydrated zones during dehydration the remanence gains one order of magnitude at the beginning of the experiment and then it follows a linear rate of 0 23 0 07 ma m 1 day between 3 and 14 days the magnetic susceptibility increases by a few percent the increase of the remanence and of the magnetic susceptibility stops after 15 days mass monitoring indicates that interstitial water evaporates when remanence and magnetic susceptibility stabilizes during rehydration the remanence increases again whilst magnetic susceptibility drops by a few percent after 20 days the remanence during rehydration follows a rate of 0 42 0 15 ma m 1 day by contrast when rehydration takes place later after 66 days the rate is much lower 0 09 0 04 ma m 1 day low temperature investigation of magnetic properties indicates an initial magnetic assemblage of magnetite and pyrrhotite newly formed magnetite and hematite carry the remanence we propose that magnetite is formed at the expense of pyrite hematite results from the progressive oxidation of newly formed magnetite our results suggest the possibility that any claystones that pass the oil window can be remagnetized due to the unique action of temperature