acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians lombard a d van der pluijm b a and van der voo r 1991 acadian and alleghenian remagnetization of the jim pond formation central western maine northern appalachians tectonophysics 186 279 291 samples were collected from ten sites of the late cambrian early ordovician jim pond formation for paleomagnetic study stepwise thermal demagnetization reveals three separable components of magnetization component i is typically removed by 350ac it is subparallel to the present day field 354o 76 vs 342 72o at the site location 45 3on 289 4 e and is considered to be a recent partial overprint component ii without tilt correction is a south southeasterly and shallow direction mean 165 o o k 31 4 a s5 8 6 o that is removed over an intermediate temperature range 350 600 c component iii without tilt correction is a northeasterly and shallow upward direction mean lo 24o k 21 5 ass 7 3 and is removed over the highest temperature range 480 to 69ooc though not statistically significant for components ii and iii the precision parameter k decreases and the as5 increases when tilt correction is applied suggesting that both are post folding magnetizations component ii without tilt correction has a corresponding paleomagnetic pole located at 43on 130 e dp dm 4 3 8 6 o which falls near the late carboniferous segment of the laurentian apparent polar wander path apwp component iii without tilt correction has a corresponding pole located at 32 n 98 e dp dm 4 7o 7 8 which falls near the lower middle devonian segment of the apwp we conclude that the jim pond formation has undergone two paleozoic remagnetization events one in the early to middle devonian and a second one in the late paleozoic the ages of these remagnetizations coincide with the timing of major erogenic activity in the area i e the acadian and alleghenian respectively the remagnetization event associated with the acadian pulse can be recognized in other paleomagnetic investigations in the northern appalachians

preservation of a magnetization carried by magnetite inclusions during amphibolite grade metamorphic conditions an example from the sarek dykeswarm northern swedish caledonides an outcrop consisting of a 608 myr old sheeted dyke complex and dyke intruded metasediments has been investigated palaeomagnetically it is located in the interior of a nappe the basal and upper parts of which suffered penetrative deformation and amphibolite grade metamorphism during the caledonian orogeny the only trace from the orogeny at the outcrop is a widely spaced network of brittle and ductile shear zones crosscutting dyke contacts foliated amphibolite consisting of hornblende quartz plagioclase garnet and sphene occur locally within them the opaque mineralogy of the dykes differs from that commonly found in continental dykes with ilmenite and pyrrhotite being the major visible opaque phases and there are indications that magnetite inclusions exsolved from silicates carry a primary remanence even though ar 39 40 ar mica ages were reset at ca 430 myr during the orogeny a secondary component carried by pyrrhotite is less prominent but falls on the baltican apw path at 430 myr the inferred primary component has a large directional uncertainty but indicates low palaeolatitudes for the baltscandian margin during rifting less

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

a genetic link between remagnetization and potassic metasomatism in the devonian onondaga formation northern appalachian basin whole rock chemical analyses indicate that the onondaga formation along a rochester syracuse new york transect has a marked eastward increasing trend in k al ratio which correlates with an increase in authigenic magnetite content and degree of diagenetic illitization an eastward increase in the content of some authigenic minerals including k feldspar quartz and fe rich chlorite in the rocks is indicated by petrographic and x ray diffraction studies these observations suggest a link between remagnetization and potassic alteration of fe bearing silicates in the onondaga two hypothetical reactions are proposed in which magnetite authigenesis is related to the breakdown of fe bearing smectite in the presence of potassium products of these reactions include authigenic k feldspar illite quartz and chlorite as well as magnetite our results suggest that remagnetization was triggered by a basin wide migration of k rich fluids during the alleghenian orogeny

thermoviscous remagnetization in some appalachian limestones experimental evidence shows that blocking temperature relaxation time theory for single domain magnetite can grossly underestimate laboratory thermal demagnetization temperatures of a present day viscous remanence in devonian limestones of new york state thermoviscous remagnetization at moderately elevated burial temperatures during the late paleozoic cannot therefore be readily excluded for the origin of the stable secondary component in these rocks more generally these results emphasize that single domain thermal activation theory cannot be assumed a priori to correctly predict temperature of thermoviscous remanence acquisition in all magnetite bearing rocks the prob leto

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

methods in rock magnetism and paleomagnetism techniques and instrumentation david w collinson chapman and hall london 1983 xiv 503 pp stg 35 00 hardback

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

experimental deformation of synthetic magnetite bearing calcite sandstones effects on remanence bulk magnetic properties and magnetic anisotropy we have quantified effects of experimental deformation on the magnetic properties of a set of synthetic calcite sandstone samples containing magnetite the deformation was carried out in a microcomputer controlled apparatus that adjusted the applied differential stress as needed to maintain a constant strain rate of 10 5 s 1 most samples were deformed under dry conditions but a few were deformed with a pore fluid present the samples deformed under dry conditions required substantially higher differential stresses macroscopically ductile shortening strains of up to 25 produced the following irreversible changes in magnetic properties 1 increased bulk coercivity remanence coercivity and mean anhysteretic remanence susceptibility 2 decreased mean low field susceptibility 3 decreases in the component of remanence parallel to shortening 4 smaller decreases for most samples in the component normal to shortening resulting in a net rotation of the remanence away from the shortening axis 5 larger decreases in the normal component in a few samples resulting in a net rotation of the remanence towards the shortening axis 6 increased magnetic anisotropy and 7 increased deformation of initial magnetic ellipsoids a comparison of data for samples deformed under dry and wet conditions higher and lower differential stresses respectively indicates that remanence reorientation and susceptibility anisotropy are controlled primarily by bulk strain i e rotation and displacement of particles whereas coercivity and anhysteretic anisotropy are controlled dominantly by microstrain or intragranular stress

remanence stability and magnetic fabric development in synthetic shear zones deformed at 500 c shear experiments were performed on magnetite bearing calcite aggregates to examine magnetic fabric development and remanence stability in a deforming system using elevated temperature and pressure to encourage deformation by crystal plastic processes samples composed of 1 wt pseudo single domain magnetite 1 2 m in a calcite matrix were created with either strong or weak initial fabrics and deformed in coaxial simple shear to strains up to 1 5 at constant strain rates between 6 10 5 and 1 10 4 s 1 at 500 c and confining pressure of 300 mpa samples were given weak field thermal remanent magnetizations prior to deformation demagnetization of postdeformation remanence reveals that a primary remanent magnetization can withstand deformation at pressures and temperatures approximately equivalent to greenschist facies metamorphic conditions on laboratory time scales but this stability is found to depend on the character of the predeformation fabric the origin of secondary remanence components acquired during deformation is uncertain but is likely to partially result from thermal viscous remagnetization complete postdeformation remagnetization in initially anisotropic samples appears to involve a stress softening or piezoremanent magnetization mechanism postdeformation anisotropy measurements show progressive changes in magnetic fabric strength with strain in the absence of a strong initial magnetic anisotropy magnetic fabric intensity increases linearly as a function of strain however deformation that overprinted an existing fabric results in an apparent decrease of the initial anisotropy at low strains followed by rapid increases in magnetic fabric strength with increasing strain our results underscore the important role that initial fabric can play in determining the character of deformation fabrics

remagnetization by basinal fluids testing the hypothesis in the viola limestone southern oklahoma migration of orogenic or basinal fluids is a recently invoked mechanism to explain the widespread presence of late paleozoic secondary magnetizations in the rocks of north america paleomagnetic and geochemical results from the ordovician viola limestone in southern oklahoma are evaluated to assess the role of basinal fluids in leading to secondary magnetizations in the unit the viola limestone contains what we interpret to be a pervasive pennsylvanian synfolding magnetization residing in magnetite and a localized permian magnetization which resides in hematite and occurs in alteration zones around mineralized veins both secondary magnetizations are interpreted as chemical remanent magnetizations crm based on low burial temperatures and the presence of authigenic magnetic phases the relative proportion of the permian crm in hematite gradually decreases whereas the magnetite crm increases with distance from the veins fluid inclusion and sr isotope studies indicate that the vein mineralization calcite with mississippi valley type oxides and sulfides precipitated from basinal fluids which were warm saline and radiogenic the radiogenic 87sr 86sr values of the limestones in the alteration zones and the fact that there is more significant alteration closer to the veins suggests that the basinal fluids were also responsible for alteration in the limestones the coincidence of the geochemical and remagnetization trends suggests that the permian crm dates the migration of basinal fluids through the veins geochemical results from the viola limestone containing the pervasive crm indicate that it is relatively unaltered with no evidence for basinal fluids the lack of evidence for basinal fluids suggests that other mechanisms for the origin of the pervasive crm need to be tested the results of this study indicate that flow of basinal fluids was focused in veins and only locally altered the host limestone

pervasive remagnetization of paleozoic rocks acquired at the time of mesozoic folding in the south china block a total of 152 paleomagnetic samples was collected from 23 sites distributed over six paleozoic formations with ages ranging between middle ordovician and early permian in the tangshan type section in the nanjing area of south china stepwise thermal demagnetization has successfully resolved two magnetization components from these samples component a with northerly shallow tilt corrected directions resides in approximately 95 of the total number of samples analyzed this component passes fold tests at the 99 significance level in three formations the overall mean direction of component a is 350 3 tilt corrected a95 7 2 component b isolated from 19 samples in two formations cl and d3 yields negative fold tests at the 95 significance level and is therefore younger than the mesozoic folding the overall site mean direction of component b is 354 4 in situ a95 8 4 the tilt corrected direction of component a clearly resembles the in situ direction of component b indicating that both components are likely to be of approximately the same age we infer that they are both remagnetizations associated with the deformation a review of the paleozoic paleomagnetic data reported by other authors from adjacent areas shows that similar remagnetization situations may prevail it is concluded that the northeast part of south china has experienced pervasive remagnetizations at the time of folding the direction of this folding induced remagnetization yields a new well defined paleopole position 56 n 314 e for south china although this paleopole is significantly different from mesozoic poles reported to date we infer that a triassic to jurassic age is most likely for this remagnetization

impact induced phase transformations at 50 60 gpa in continental crust an epma and atem study strongly shocked gneiss fragments from the allochthonous polymict breccia of the 23 ma old haughton impact crater were studied by optical microscopy electron probe microanalysis epma and analytical transmission electron microscopy atem the study is focused on the substantiation of shock induced phase transformations in rock forming minerals and their implications for post shock thermal regimes and impact age dating in the gneiss fragments the original layering is largerly preserved although all the minerals have experienced dramatic phase transformations chemically the various shock generated decomposition regions still reflect the pre shock mineralogy however significant heterogeneities exist on the 10 100 m scale four different regions of decomposition products can be distinguished epma 1 shocked biotite rich regions with strong variations in si al k fe and mg on the 10 m scale 2 shocked quartz with about 3 al and k and traces of fe and mg 3 a region with al si 1 which does not correspond to any pre shock mineral and 4 shocked k feldspar showing a significant deficit in k and traces of fe and mg on the nanometer scale a uniform bimodal reaction pattern emerges all minerals are transformed to new crystals embedded in a silica rich glass the shock generated crystals are 1 spinel in shocked biotite 2 quartz cristobalite and coesite in shocked quartz 3 an unknown polymorph of corundum al2o3 in feldspar and in the al si 1 region and 4 mullite in association with sio2 glass in shocked sillimanite grains all crystals range in size between 0 05 and 1 0 m taking into account estimated durations and pressure temperature conditions for shock wave passage and post shock temperatures the reactions observed can be ascribed to high temperatures during and after pressure release the different mineral glass parageneses suggest temperatures in excess of 1400 c moreover given the small size of the newly formed minerals very rapid cooling down to 1200 c is required

remagnetisation of a rock analogue during experimental triaxial deformation abstract synthetic specimens containing pseudo single domain magnetite were given a univectorial anhysteretic remanent magnetisation they were deformed in a ductile manner at room temperature under a confining pressure of 150 mpa 1 5 kbar the strain rate was 10 5 s 1 and the maximum strain achieved was 23 shortening a weak 0 035 mt magnetic field was present in the pressure vessel during deformation hydrostatic compaction alone produced pressure demagnetisation of low coercivity components of remanence but did not change remanence directions macroscopic differential stress was needed to deform the specimens and this also permitted syndeformational remagnetisation for values of more than 20 mpa 200 bars this added new components of piezo remanent magnetisation prm approximately parallel to the direction of the magnetic field in the pressure vessel they were acquired in all coercivity intervals but the largest components were added below 35 mt the final remanence orientation and its spatial distribution of vector components were dictated by the relative orientations of the remagnetising field and the initial remanence prm contributions almost completely masked the changes related to strain induced grain rotation at these low strains the directions of the externally imposed stress strain axes may not significantly influence the direction of new remanence components prm acquisition occurs when differential stress exceeds a threshold value that probably depends on the previous stress history of the magnetite grains in part this may explain the origin of syntectonic remagnetisation in some naturally deformed rocks where the remanence is not reset by recrystallisation or thermal or chemical processes

further studies of carbonate remagnetization in the northern appalachian basin paleomagnetic and rock magnetic studies of sedimentary carbonates in the northern appalachian basin demonstrate that the late paleozoic remagnetization in this area is probably related to authigenesis of single domain magnetite fifteen sites within middle devonian carbonates were sampled along a transect extending from albany new york to the algonquin arch in southeastern ontario paleomagnetic studies show that all sites except the two westernmost sites on the algonquin arch were remagnetized during alleghenian time the very weakly magnetic unremagnetized sites retain a dual polarity magnetization of probable devonian age in the remagnetized sites both the size of the kiaman components and magnetite concentration estimated from low field susceptibility x are highest in the central part of the transect and lower on either side x ray powder diffraction analyses demonstrate that k feldspar is also most abundant in the middle of the transect with lower concentrations to the west and east we suggest that formation of authigenic magnetite and k feldspar may be related in this setting sites with the largest kiaman components have the highest unblocking temperatures and the largest ratios of anhysteretic remanent magnetization arm to bulk susceptibility arm x indicating that these sites have the greatest amounts of single domain magnetite several remagnetized sites exhibit northerly shallow magnetic components which may be kiaman normal directions we conclude that remagnetization along the new york ontario transect is a chemical remanent magnetization resulting primarily from authigenesis of single domain magnetite we present a model in which pulses of heated fluids triggered magnetite formation scanning electron microscope observation of magnetic extracts from 14 sites show that most of the resolvable magnetite grains are multidomain grains of irregular shape many sites also contain a small percentage of magnetic spheres and or euhedral magnetite particles a majority of the magnetite grains contain fe as the only cation but many also contain minor amounts of other cations including various combinations of cr ni ti zn mn si al ca and mg single domain magnetite is not resolvable with our observational techniques but its presence is inferred from rock magnetic tests much of this magnetite may be of authigenic origin

synfolding magnetization in the jurassic preuss sandstone wyoming idaho utah thrust belt the jurassic preuss sandstone exposed in five thrust plates of the wyoming idaho utah thrust belt carries directions of remanent magnetization that group most tightly after only partial unfolding field petrographic and rock magnetic evidence indicates that the carrier of this magnetization is detrital low ti titanomagnetite the detrital titanomagnetite was remagnetized at low temperatures 75 150 c probably completely during folding anisotropy of magnetic susceptibility and petrographic observations indicate that the detrital titanomagnetite has been affected by tectonic strain we suggest that low temperature remagnetization of the detrital titanomagnetite was either a viscous partial thermoremanent magnetization the acquisition of which was enhanced by stress or a piezoremanent magnetization that involved stress induced movement of domain walls during intracrystalline strain or was a combination of the two mechanisms stress may promote remagnetization at temperatures much lower than predicted by current theoretical models other mechanisms such as acquisition of chemical remanent magnetization during folding deflection of a prefolding magnetization by internal strain or combination of components of magnetization with different direction cannot account for the geometry of magnetization in the preuss the locus of acquisition of synfolding magnetization in the preuss migrated in conjunction with deformation in the thrust belt a model is presented in which synfolding magnetization was acquired during cooling and folding as strata moved up thrust ramps a lack of reverse polarity directions remains a puzzling feature of the remanence the remanent direction is tentatively interpreted to reflect the predominant polarity state during its acquisition over an extended rather than a discrete time period during folding in late cretaceous and early tertiary periods of predominantly normal polarity

remagnetizations and thrusting in the idaho wyoming overthrust belt a study on the relationship between deformation and paleomagnetism was carried out in the frontal segment of the idaho wyoming overthrust belt samples were taken from the jurassic twin creek unit of the most easterly thrust the prospect thrust for paleomagnetic study detailed analysis of the paleomagnetic data showed a synfolding remagnetization with a paleopole latitude longitude 83 n 286 e located on the tertiary segment of the north american apparent polar wander path apwp the age of the paleopole roughly coincides with the age of thrusting on the prospect thrust as determined by synorogenic deposits the magnetization was found to reside in magnetite spherules previous data from the absaroka and darby thrust sheets that lie to the west of the prospect thrust were reevaluated and they also indicate a synfolding age for the remagnetization the corresponding paleopole 64 n 162 e lies on the late mesozoic segment of the north american apwp for these more interior thrust sheets the apparent age of the remagnetization is older than that for the exterior prospect thrust sheet but again roughly corresponds with the inferred age of thrusting for the sampled thrusts we conclude that remagnetization in the idaho wyoming overthrust belt is temporally related to deformation and we suggest that authigenic magnetite growth occurred during discrete fluid pulses that were concurrent with thrusting and folding events

experimental stress remagnetization of magnetite abstract pseudo single domain psd and multidomain md magnetite grains remagnetize in weak magnetic field 30 t during experimental triaxial deformation the magnetite is supported in a calcite cement matrix minor remagnetization occurs with hydrostatic stress of 100 mpa significant remagnetization requires hydrostatic pressure of 150 mpa with differential stress of 5 mpa superposed on the sample intergranular differential stresses must be much higher due to amplification at grain asperities stress remagnetization does not need chemical or thermal energy new components of magnetic remanence are added parallel to the remagnetizing field however this only happens in grains or parts of grains with coercivities of remanence 60 mt grains with coercivities of 20 55 mt remember the primary magnetization and are not stress magnetized these coercivity limits do not depend on the differential stress or strain rate of the experiment the spatial distribution of vector components of remanence was isolated by af demagnetization after deforming a magnetized sample the components of remanence spread along a partial great circle between the initial remanence and the direction of the remagnetizing field the directions of the original magnetization and the remagnetizing field are the only factors controlling the course of the remagnetization path triaxial deformation shortened these samples by if the experimental results are transferable to nature it is possible that a pulse of excess crustal stress 25 mpa could partially remagnetize low dislocation density magnetite the experiments show that the directions of the remagnetizing field and the primary magnetization are the only variables that affect the demagnetization vector path we do not need to know the strain history coaxial or noncoaxial finite strains or principal strain directions to detect the paleofield directions unlike paleomagnetic vectors affected by large strains permanent changes in magnetic structure occur as a result of experimental deformation regardless of the differential stress the proportion of grains with remanence coercivities

alternating field demagnetization of rocks and the problem of gyromagnetic remanence abstract alternating field a f demagnetization has proved to be a very reliable technique for separating the magnetization components of rock samples the method is subject to errors caused by either imperfection of the technique or by intrinsic properties of a rock recently stephenson 1 2 introduced the term gyroremanent magnetization grm for a disturbing remanent magnetization that can be acquired by magnetic material during tumbling or stationary a f demagnetization the implications for the routine a f demagnetization of anisotropic rock samples seemed to be very serious here however a method is presented on how to avoid the effect of grm on results obtained from stationary a f demagnetization

remanent magnetism and ductile deformation in an experimentally deformed magnetite bearing limestone abstract a limestone with pseudo single domain psd pure magnetite is given an isothermal remanent magnetization irm samples are deformed triaxially at 200 mpa 2 kbar confining pressure at room temperature and at a strain rate of 10 5 s 1 the limestone deforms in a ductile manner by twinning to produce sufficiently homogeneously strained specimens up to about 20 shortening the uniform saturation irm rotates away from the shortening axis approximately by an amount expected for the homogeneous strain of a non material line marker hydrostatic compaction alone shows that this is time dependent and compatible with progressive damage of the ferrimagnetic grains strain produces an increase in coercivity of the magnetite an increase in its saturation remanence and an increase in its anisotropy of magnetization with respect to irm these observations are compatible with reduction in effective grain size and change of shape of magnetite experimental deformation reduces the intensity of magnetisation chiefly by the removal of low coercivity components of remanence when there is a pure irm vector

dissolution behaviour of fine grained magnetite and maghemite in the citrate bicarbonate dithionite extraction method mineral magnetic properties of soils and sediments are increasingly used as proxy parameters for environmental and palaeoclimate analysis to investigate which magnetic minerals contribute to the environmental signal in the samples chemical techniques such as the citrate bicarbonate dithionite cbd extraction method have been introduced in environmental magnetism studies this technique is assumed to distinguish lithogenic magnetite from pedogenic maghemite mineral content in soils and sediments unfortunately interpretation of the cbd extractions is not straightforward because the procedure is sometimes more suitable for distinction between grain size than for distinction between minerals the procedure of the cbd extraction technique was investigated to determine the influence of extraction temperature and iron oxide concentration on the dissolution behaviour of the samples synthetic samples were extracted at three different temperatures 60o 70o and 80oc at similar iron oxide concentration 5 wt and for three different concentrations 0 1 wt 1 wt and 5 wt at the same temperature 60oc our results show that a lower extraction temperature reduces the dissolution rate for all samples while decrease in iron oxide concentration increases the dissolution rate thus the parameters in the cbd procedure have a major influence on the dissolution behaviour of the samples in practice this means that when natural samples of differing iron oxide concentration are extracted with this technique the results of the extractions cannot be compared therefore the outcome of this type of extraction experiment can only be accurately interpreted when the effect of the procedure on the dissolution behaviour is taken into account 1999 elsevier science b v 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

experimental strain of isothermal remanent magnetization in ductile sandstone abstract a macroscopically ductile sandstone to which a homogeneous isothermal remanent magnetization irm was applied has been deformed at 150 mpa confining pressure and constant strain rate of 10 5 s 1 hydrostatic compaction does not produce a deflection of the irm vector although it is reduced in intensity pure shear producing shortening in the range 2 to 35 steadily reduces the intensity of magnetization but also homogeneously rotates the remanence vector toward the plane of flattening the amount of rotation is slightly less than that expected for a non material line undergoing homogeneous strain deformation selectively removes weakly coercive components of remanence as revealed by alternating field af demagnetization during deformation a weak deformational viscous remanent magnetization dvrm is acquired from the pressure vessel this is different from a conventional viscous remanent magnetization vrm in that it is not acquired when the specimen is subject to hydrostatic confining pressure alone even for periods three times longer than the longest deformation test

magnetite dissolution and neocrystallization during cleavage formation paleomagnetic study of the martinsburg formation lehigh gap pennsylvania paleomagnetic study of the shale to slate transition in the ordovician martinsburg formation at lehigh gap pennsylvania reveals that the remanence of the relatively undeformed shales predates cleavage formation and that the slates were remagnetized during cleavage development the shales display two components of remanence component 1 c1 was the only magnetization direction in the majority of the shales and the first removed component in 10 shale specimens component 2 c2 is observed as a single component remanence with a low 300 400 c unblocking temperature range in samples from one shale station and as a second removed component in 10 shale specimens the mean directions are declination inclination 347 04 346 47 in situ tilt corrected for c1 and 80 63 135 40 for c2 the pencil slates and well cleaved slates both display single component remanences the mean directions are 198 12 201 26 for the pencil slates and 199 24 225 54 for the well cleaved slates using the relationship between cleavage development and remagnetization in the slates the timing of cleavage development and folding and the regional geology we can constrain the relative ages of the remanences as follows 1 the shale c1 direction is most likely prefolding and represents a primary upper ordovician direction 2 the slate directions are younger in age than the shale directions and are probably of permo carboniferous alleghenian age these results show that remagnetization occurred during development of slaty cleavage in these rocks we interpret the mechanism for this remagnetization to have been strain induced dissolution and neocrystallization of magnetite the additional difference in the characteristic directions of the pencil slates and well cleaved slates may be attributed to strain reorientation of the pencil slate remanence after new growth of magnetite the unknown temporal relationship between dissolution and neocrystallization and strain precludes any quantitative attempt to correct remanence for strain in these samples our study shows that paleomagnetic studies of deformed sediments must consider the possibility of dissolution and neocrystallization of remanence carriers in addition to strain induced rotation

remagnetization of the tonoloway formation and the helderberg group in the central appalachians testing the origin of syntilting magnetizations summary palaeomagnetic rock magnetic geochemical and strain studies of the helderberg group and the tonoloway formation in the valley and ridge province of west virginia were performed to test the origin of syntilting magnetizations and to test the mechanisms of chemical remanent magnetization crm acquisition the test for a connection between strain and remagnetization was performed by comparing the types and levels of strain with the magnetic properties between the generally coarse grained thickly bedded helderberg and the thinly bedded and finer grained tonoloway standard tilt tests as well as optimal differential untilting from two transects on the wills mountain anticline indicate that the helderberg contains a pervasive late syntilting 30 per cent untilting characteristic remanent magnetization chrm whereas the tonoloway contains a well defined early syntilting chrm 1 76 per cent untilting as well as a chrm 2 which is similar to the chrm in the helderberg the chrms reside in magnetite the rock magnetic results from the two units are similar although the helderberg samples have a finer apparent grain size perhaps as a result of magnetic hardening strain data reveal an apparent correlation between the syntilting magnetizations and types degrees of pressure solution strain pressure solution strain is higher in the helderberg and lower in the tonoloway strain may have caused rotation of magnetic minerals but it not clear that the differences in strain between the units were high enough to cause the rotations that are needed to explain the tilt test results several other viable hypotheses were also investigated one hypothesis is a strain enhanced chemical process that caused dissolution and precipitation of new magnetite in solution structures during folding a preliminary test of this hypothesis produced inconclusive results another hypothesis consistent with some of the characteristics of the magnetizations in the helderberg and tonoloway units is that the rocks were partially to completely remagnetized by a piezoremanent remagnetization process geochemical results show that origin of the remanence carrying grains cannot be attributed to orogenic type fluids but were probably caused by a burial diagenetic process

on conducting the modified slug test in tight formations the method introduced by bredehoeft and papadopulos 1980 for conducting a modified slug test in tight formations does not assure the condition of approximate equilibrium necessary at the start of the test in addition compressibility in the shut in well can be significantly larger than the compressibility of water which bredehoeft and papadopulos indicate should be used in the calculations errors that may result from using the original test procedure are discussed and a modified procedure and testing arrangement are presented

thermal alteration of the magnetic mineralogy in ferruginous rocks the northampton ironstone contains the paramagnetic minerals siderite and berthierine and a trace of the ferrimagnetic mineral magnetite the magnetic fabric of this sedimentary rock as defined by the anisotropy of magnetic susceptibility is controlled by bedding compaction but is inverse i e the maximum susceptibility axes are normal to the bedding plane this inverse fabric is attributed to the dominating presence of siderite the rocks were incrementally heated to 600 c and the magnetic fabric was measured at room temperature and at near liquid nitrogen temperature after each heating step cooling the samples in liquid nitrogen enhances the paramagnetic contribution to the magnetic fabric after heating the samples above 250 c the room temperature magnetic fabric became normal with the minimum susceptibility axes perpendicular to the bedding plane the low temperature magnetic fabric remains inverse until the samples are heated to about 500 c after heating to higher temperatures a normal fabric was observed at both room temperature and low temperature rock magnetic evidence shows that magnetite is created during the heating the start of mineralogical changes in the rock was detected by electron spin resonance spectroscopy powder x ray diffraction and acquisition of isothermal remanence berthierine starts to break down at approximately 250 c and is totally oxidized by 500 c the creation of magnetite in the heating experiments corresponds with this mineralogical change the change in the room temperature magnetic fabric is associated with the decomposition of berthierine part of which alters to magnetite the new magnetite phase has a normal magnetic fabric which overprints the inverse magnetic fabric of siderite

strain and magnetic remanence abstract experimental data may be compatible with the hypothesis that a single direction of magnetic remanence rotates as a rigid marker with strains up to 40 shortening in coaxial perfect flattening x y z detailed agreement with the passive line model is relatively poor for the specimens in which remanance is carried by magnetite however for this range of strains the differences with the passive line model wettstein s equation are so slight that the latter model may be more easily employed to de strain or restore deformed remanance to its original attitude in the case of hematite bearing remanences the differences between the passive line and rigid marker model are even smaller because of the higher aspect ratios of grains of hematite therefore it is suggested that wettstein s equation may be safely used to restore remanence after even higher strains where the remanence is carried by hematite