cratered cobbles in triassic buntsandstein conglomerates in northeastern spain an indicator of shock deformation in the vicinity of large impacts quartzite cobbles in lower triassic buntsandstein conglomerates from northeastern spain display unusual millimeter to centimeter sized circular craters commonly having central mounds and surrounded by radial fractures the conglomerates are also marked by intense fracturing down to microscopic scale these features have traditionally been attributed to tectonic compression and pressure dissolution at cobble contacts sections through the cratered cobbles reveal pervasive internal fracturing segments detached along concave spall fractures and zones marked by quartz grains with planar deformation features comparison with results of impact experiments on artificial conglomerates suggests that these features were produced by internal accelerations grain collisions and spallation related to shock wave propagation through inhomogeneous deposits the proximity of the outcrops to the azuara and proposed rubielos de la cerida impact structures suggests that shock deformation of conglomerates can provide an easily recognizable regional impact signature

lower silurian aeronian megafaunal and conodont biofacies of the northwestern michigan basin lower silurian llandovery aeronian carbonates of the burnt bluff group northwestern michigan basin represent a transect along a southward dipping ramp that extends from tidal flat to basin environments benthic megafaunas include an ostracod biofacies tidal flat stromatoporoid coral biofacies very shallow subtidal pentamerid crinozoan and crinozoan stromatoporoid biofacies deeper subtidal and a crinozoan sponge biofacies distal ramp and basin the crinozoan sponge biofacies which includes diverse small crinozoan ossicles 19 types of siliceous sponge spicules and at least 65 other taxa has a biota that is similar to those of silurian continental margins megafaunal biofacies indicate an early silurian gradient going from a shoreline in the north to water depths of perhaps 60 m in the south the burnt bluff group contains conodonts of the icriodella discreta icriodella deflecta zone as an onshore biofacies with panderodus kockelella ozarkodina icriodella and oulodus and an offshore biofacies with panderodus walliserodus and aspelundia rcsumc les carbonates du silurien infkrieur llandovery aeronien du groupe de burnt bluff dans la rcgion nord ouest du bassin de michigan rephentent un transect le long d une rampe inclince vers le sud s ktalant d un environnement d estran tidal a celui d un bassin les mkgafaunes benthiques incluent un biofaciks a ostracodes estran tidal un biofaciks a stromatoporoi des coraux infratidal trks peu profond des biofaciks a pentamkrides crinozoaires et crinozoaires stromatoporoi des infratidal profond et un biofaciks a crinozoaires cponges rampe distale et bassin le biofaciks a crinozoaires cponges qui inclut diverses petites plaques de crinozoaires 19 types de spicules d cponges siliceux et au moins 65 autres taxons posskde un biote qui est similaire a ceux des marges continentales siluriennes les biofaciks mkgafauniques indiquent un gradient au silurien prccoce allant d une ligne de rivage dans le nord jusqu a des profondeurs d eau atteignant peut etre 60 m dans le sud le groupe de burnt bluff contient des conodontes de la zone a icriodella discreta icriodella deflecta comme biofaciks intracdtier accompagncs de panderodus kockelella ozarkodina icriodella et oulodus et un biofaciks extrac6tier avec panderodus walliserodus et aspelundia traduit par la rcdaction

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

inferring domain state from magnetic hysteresis in high coercivity dolerites bearing magnetite with ilmenite lamellae abstract hysteresis loops were measured for 16 dolerite samples from separate precambrian dykes whose remanence is probably primary remanence is carried by grains of magnetite subdivided into fine particles usually by ilmenite lamellae as seems common in mafic igneous rocks of high coercivity coercive force h c ranges from 9 to 40 10 3 a m 110 500 oe the ratio j rs j s of saturation remanence to saturation magnetization ranges from 0 10 to 0 45 j rs j s varies in approximate proportion to h c with a constant of proportionality appropriate for pseudo single domain magnetite with one or two domain walls per particle a cooling cycle to 77k causes large 40 on average demagnetization of j rs in most samples supporting the dominance of pseudo single domain magnetite most samples also show a large 29 on average decrease in h c on cooling to 140k suggesting magnetostrictive control of h c perhaps through internal stresses opposing domain wall motion only the three dolerites with h c greater than 30 10 3 a m 380 oe show little change in h c on cooling suggesting dominance by single domain particles with shape anisotropy in magnetite grains subdivided into single domain particles by ilmenite lamellae magnetic interaction may lower j rs j s from the expected 0 5 some theories 1 2 suggests that this is due to a self demagnetizing field from magnetic poles on the surface of each grain of magnetite subdivided by ilmenite and should be corrected for another theory 3 suggests that this correction is not necessary because the self demagnetizing field is already approximately cancelled due to magnetic poles inside each subdivided grain at the ends of magnetite particles the latter suggestion is favoured by our three highest h c dolerites whose j rs j s averages 0 42 0 04 before and 0 66 0 06 after correction for the self demagnetizing field from poles on the surface of magnetite ilmenite grains applying this correction to our other dolerites usually raises j rs j s significantly above 0 5 which is unreasonable for pseudo single domain magnetite hence correcting for self demagnetizing fields from poles on the surface of magnetite ilmenite grains in rocks is not recommended whatever the domain state

diagenetic origin for quartz pebble conglomerates the occurrence of quartz pebble conglomerates qpc in the rock record increases backward through time from the tertiary through the precambrian the positive correlation between qpc abundance and age is valid both for numbers of reported qpc and for qpc as a percentage of all conglomerate and at both the era and the period level qpc are usually interpreted as being due to intense chemical weathering protracted transport or sediment recycling but none of these can account for the age distribution of qpc which is the opposite of the global mass age distribution for sedimentary rocks precambrian and tertiary conglomerates with similar sources and sedimentology have vastly different clast populations nonquartzose clasts being much more abundant in the younger rocks comparison of the petrology of qpc and polymict conglomerates shows that qpc have consistently higher proportions of diagenetic secondary matrix and pressure solved grain contacts we conclude that diagenetic factors play an important role in qpc formation by preferentially destroying less durable clasts

recognition of an infracambrian source rock based on biomarkers in the baghewala 1 oil india heavy sulfur rich oil produced from the infracambrian 540 640 ma jodhpur formation in the baghewala 1 well represents a new exploration play in the bikaner nagaur basin in india and the punjab basin in pakistan the baghewala 1 oil is nonbiodegraded and thermal maturation dependent biomarker ratios indicate generation from the source rock within the early oil window age diagnostic and source dependent biomarkers indicate that the oil originated from algal and bacterial organic matter with no higher plant input in an infracambrian carbonate rich source rock deposited under anoxic marine conditions these characteristics support a local origin of the baghewala 1 oil from organic rich laminated dolomites in the infracambrian bilara formation significant amounts of petroleum could originate from equivalents of the proposed bilara source rock in the punjab basin pakistan where the precambrian to lower paleozoic section is thicker and more deeply buried than in india deeper burial of the source rock in the punjab basin than in the bikaner nagaur basin could generate more mature equivalents of the baghewala 1 oil the baghewala 1 oil is geochemically similar to another heavy oil from the infracambrian salt range series in the nearby karampur 1 well in pakistan and to oils derived from carbonate evaporite facies of the infracambrian huqf group about 2000 km 1243 mi to the southwest in the eastern flank province of southern oman these findings are consistent with published evidence that subsiding rift basins in northwest india pakistan and southern oman were in close proximity during the infracambrian along the middle eastern edge of gondwanaland

early cambrian microbial reefs archaeocyathan inter reef communities and associated facies of the yangtze platform abstract a sedimentological and paleoecological study of part of the northern yangtze platform of china was conducted three sections of the early cambrian atdabanian botomian stage xiannudong formation where studied and reveal that stromatolite and thrombolite reefs flourished in close association with an inter reef community of archaeocyaths and other organisms adjacent to the reef environment were migrating ooid shoals oncoids peloids and silty sandy lime mud no archaeocyathan reefs were observed in any of the measured sections but irregular archaeocyaths were found in some microbial reefs and archaeocyaths form the bulk of the debris in float rudstone that was deposited between microbial reefs we interpret the archaeocyath rich float rudstone to represent a storm churned inter reef deposit typically regular type archaeocyaths dominated the float rudstone while irregular type archaeocyaths were found in abundance dwelling within the microbial reefs this trend changed upsection as irregular type archaeocyaths dominated both the float rudstone and in the microbial reefs the oncolite and peloidal mudstone are interpreted as representing a quiet water lagoonal environment created by the presence of the microbial reefs and ooid shoals during large storms material from the ooid shoals or microbial reefs would wash into the lagoon the silty sandy micrite represents a quiet water setting which could either be a deep subtidal environment or a restricted lagoonal environment the facies associations of these three sections along with other published data indicate that the yangtze platform at least during the early and middle cambrian was a carbonate platform and equatorially located

a new rock magnetic approach to selecting sediments for geomagnetic paleointensity studies application to paleointensity for the last 4000 years the hypothesis that the ratio of detrital remanent magnetization to anhysteretic remanent magnetization drm arm for sediment samples is a measure of relative geomagnetic paleointensity is critically evaluated by two distinct approaches one approach is a detailed rock magnetic examination of the implicit assumptions of the drm arm method and the construction of a selection process by which to identify sediments that conform to requirements satisfying these assumptions sediments are uniform with respect to drm arm ratio if they contain magnetite in the 1 15 ym particle size range as the predominant magnetic mineral and have variations in magnetite content of less than 20 30 times the minimum concentration the drm arm ratios of these sediments should provide estimates of relative geomagnetic paleointensity relative particle size variations in magnetite are detected with a plot of anhysteretic susceptibility xarm versus low field susceptibility x and the size range 1 15 m is approximately identified by high field hysteresis parameters a rock magnetic evaluation of leboeuf lake sediments with these techniques indicates that these sediments are suitable for a relative plaeointensity study the second approach to evaluating the drm arm ratio as a measure of relative paleointensity is direct comparison with absolute paleointensity data a comparison between the leboeuf lake estimates and thellier thellier results from the western united states supports the conclusion that suitable sediments can record geomagnetic paleointensity fluctuations

saturation magnetostriction and its low temperature variation inferred for natural titanomaghemites implications for internal stress control of coercivity in oceanic basalts summary highly oxidized titanomaghemite in oceanic basalts often carries remanent magnetization of high coercivity stability helping preserve the oceanic magnetic anomaly pattern we study the source of this high coercivity in four oceanic basalts from odp sites 238 572d 470a and 556 containing highly oxidized titanomaghemite titanium content parameter x 0 55 and oxidation parameter z 0 9 on average most of the titanomaghemite is likely in singledomain grains with uniaxial anisotropy because the ratio of saturation remanence j rs to saturation magnetization j s approaches 0 50 j rs j s 0 46 on average we show that the uniaxial anisotropy is very likely magnetostrictively controlled through internal stresses i in the titanomaghemite grains this allows us to use a novel indirect method to estimate the saturation magnetostriction s of the titanomaghemite a saturation remanence j rs is given along the axis of a cylindrical sample of each basalt then a small compression is applied

paleomagnetic and rock magnetic results from lower crustal rocks of iodp site u1309 implication for thermal and accretion history of the atlantis massif abstract integrated ocean drilling program iodp expeditions 304 305 recovered a total of 1 4 km sequence of lower crustal gabbroic and minor ultramafic rocks from the atlantis massif oceanic core complex on the western flank of the mid atlantic ridge mar at 30 n we conducted an integrated paleomagnetic and rock magnetic study on this sequence to help address the interplay between magmatism and detachment faulting detailed thermal and alternating field demagnetization results demonstrate that stable components of magnetization of mainly reversed polarity with unblocking temperatures below the curie temperature of magnetite are retained in gabbroic rocks at iodp site u1309 several samples also contain multicomponent remanences of both normal and reversed polarities that were acquired over sharply defined blocking temperature intervals providing evidence for localized reheating of some intervals during both normal and reversed polarity periods results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier rocks recovered at site u1309d the overall magnetic inclination of hole u1309d is 35 implying significant up to 50 counterclockwise viewed to the north rotation of the footwall around a horizontal axis parallel to the rift axis 010 may have occurred the tectonic rotations inferred by the paleomagnetic data suggest that the original fault orientation dipped relative steeply toward the spreading axis and subsequently rotated to a shallower angle coupled with the newly published u pb zircon ages for hole u1309d rocks grimes c b john b e wooden j l 2008 protracted construction of gabbroic crust at a slow spreading ridge constraints from 206pb 238u zircon ages from atlantis massif and iodp hole 1309d 30 n mar geochem geophys geosyst 9 q08012 doi 1029 2008gc002063 the new paleomagnetic data provide temporal and thermal constraints on the accretion history of the atlantis massif

domain size closure domains and the importance of magnetostriction in magnetite abstract previous micromagnetic calculations invoking magnetostatic exchange and crystalline anisotropy energies predict about an order of magnitude more domains in magnetite than have experimentally been observed domains observed on magnetite by the magneto optic kerr effect are of similar size as previously observed ones by the bitter technique simple but quite accurate domain calculations show that for rectangular parallelepipeds of magnetite it is the closure four domain configuration that should be of lowest energy for all multidomain grain sizes unless magnetostrictive energies are considered for the observed domain pattern it is shown that the domains themselves must carry significant magnetoelastic energies

magnetostratigraphy and mid palaeolatitude vgp dispersion during the permo carboniferous superchron results from parana basin southern brazil rhythmites summary a detailed magnetostratigraphic and rock magnetism study of two late palaeozoic rhythmite exposures itu and rio do sul from the itarare group parana basin brazil is presented in this paper after stepwise alterning field procedures and thermal cleaning were performed samples from both collections show reversed characteristic magnetization components which is expected for late palaeozoic rocks however the itu rocks presented an odd flat inclination pattern that could not be corrected with mathematical methods based on the virtual geomagnetic pole vgp distributions correlation tests between the maximum anisotropy of the magnetic susceptibility axis k1 and the magnetic declination indicated a possible mechanical influence on the remanence acquisition the rio do sul sequence displayed medium to high inclinations and provided a high quality palaeomagnetic pole after shallowing corrections of f 0 8 of 347 5 e 63 2 s n 119 a95 3 3 k 31 which is in accordance with the palaeozoic apparent wander pole path of south america the angular dispersion sb for the distribution of the vgps calculated on the basis of both the 45 cut off angle and vandamme method was compared to the best fit model g for mid latitudes both of the sb results are in reasonable agreement with the predicted palaeo latitudinal s relationship during the cretaceous normal superchron cns although the sb value after the vandamme cut off has been applied is a little lower than expected this result in addition to those for low palaeolatitudes during the permo carboniferous reversed superchron pcrs previously reported indicates that the low secular variation regime for the geodynamo that has already been discovered in the cns might have also been predominant during the pcrs

magnetism in rocks rock magnetism is the study of induced and remanent magnetization of ferrimagnetic mineral grains in rocks sediments soils and organisms its applications include environmental magnetism magnetic anisotropy sources of continental and oceanic magnetic anomalies records of geomagnetic field variations and polarity reversals and the paleomagnetic record of plate motions and the wilson cycle this paper reviews the beginnings of rock magnetism and then traces the development of six particularly interesting areas pseudo single domain behavior magnetic domains and micromagnetic structures diagnostic tests of the type and stability of remanent magnetization magnetic microanalysis thermoviscous remagnetization and chemical remanent magnetization other areas including sediment and soil magnetism are covered in a companion paper by verosub and roberts on environmental magnetism

kinematic history of the narragansett basin massachusetts and rhode island constraints on late paleozoic plate reconstructions the formation of the narragansett basin of rhode island and massachusetts and the polyphase deformation geometry of the pennsylvanian age basin sediments can be explained by collision and strike slip interactions between an avalonian microcontinent the european north american plate and the african plate during the late paleozoic sinistral movement between an avalonian microcontinent and the european north american plate resulted in formation of pull apart basins subsequently africa moved toward north america along an east west trending dextral megashear zone and collided with the eastern margin of southern north america the narragansett basin is located on the avalonian microcontinent near the suture with the north american continent and in the transition zone between the megashear and the collision tectonic regimes the basin shape stratigraphy and sedimentary relationships can be explained by the coalescence of two composite grabens which formed at different times the thicker northern graben is only affected by one deformation whereas the southern graben shows evidence of four distinct phases of postrifting deformation the first two coaxial folding deformations of the basin sediments are attributed to the closure of the basin complex during collision of africa and north america the third and fourth deformations are interpreted as sinistral r riedal and dextral shears respectively associated with the east west trending megashear system a prograde barrovian metamorphic front started in the western portion of the southern graben slightly after the first deformation and migrated northward and eastward where it is synchronous with the second deformation

magma flow inferred from preferred orientations of plagioclase of the rio ceara mirim dyke swarm ne brazil and its ams significance

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

magnetic properties of a soil from southeastern brazil in the presence of magnetic biomineralization by social insects abstract it is known that the migratory ant pachycondyla marginata and its prey the termite neocapritermes opacus synthesize magnetic minerals which may contribute to the magnetic characteristics of soil a comparative study of the magnetic properties of the minerals found in those species as well as their nests and surrounding soil aimed at investigating the contribution of the biogenic minerals to the magnetization of the soil partially oxidized magnetite t v 120 k was found in the pachycondyla marginata as shown by the verwey transition in the zfc fc and rtsirm curves low temperature remanence measurements of the neocapritermes opacus indicated the presence of maghemite and or titanomaghemite while partially oxidized pseudo single domain magnetites and or titanomaghemite were recognized in the soil and insect nests based on the similarity in the magnetic characteristic of the samples it is concluded that there is significant influence of biogenic mineralization upon the pedogenic contribution

unraveling the magnetic carriers of igneous cores from the atlantic pacific and the southern indian oceans with rock magnetic characterization previous rock magnetic investigations of oceanic basement samples have been mainly restricted by single drill holes of dredge collections in this study we present new and published data on the rock magnetic properties of igneous rock samples recovered during seven ocean drilling program odp legs in the atlantic pacific and the southern indian oceans the recovered igneous rocks from these legs are mainly basaltic flows both submarine and subaerial diabase sills and serpentinized peridotites with approximate ages of 140 000 years 10 12 34 69 100 108 and 110 121 ma which offers an opportunity to investigate magnetic properties and mineral changes attending alteration of basement rocks over a wide range of ages from variable tectonic settings titanomagnetite and titanomaghemite are commonly present in igneous rock samples of the ontong java plateau the kerguelen plateau broken ridge the eastern equatorial pacific the middle atlantic ridge trans atlantic geotraverse area and the newfoundland iberia rifted margins for basaltic rock samples three general groups a b and c respectively can be divided in terms of rock magnetic properties samples from group a have a single phase of ti poor titanomagnetite with curie temperatures ranging between 480 and 580 degrees c and exhibit a strong verwey transition in the vicinity of 110 k basalts from this group are most likely good paleomagnetic recorders and probably have preserved original and stable magnetic remanences group b is mainly observed in pillow lavas and is characterized by a curie temperature of 260 280 degrees c which is typical of low temperature oxidized titanomaghemite or titanium rich titanomagnetite the low temperature curves for group b do not show the verwey transition group c has more than one curie temperature which suggests the presence of multiple magnetic phases the thermomagnetic signature indicates the inversion of titanomaghemite to a strongly magnetized magnetite as shown by the irreversible cooling curves for the serpentinized peridotites recovered from both sides of the newfoundland iberia rifted margins results from low temperature measurements show that titano magnetites are present in the dark colored peridotites with a strong verwey transition in the vicinity of 110 k and with field and frequency dependent susceptibility curves that resemble those of synthetic tmo in contrast to the magnetic properties observed from the dark colored peridotites the low temperature curves for the yellow brown colored peridotites did not show any verwey transition thermomagnetic analysis also failed to show evidence for titanomagnetites the remanent magnetization is carried by a thermally unstable mineral that breaks down at about 420 degrees c probably maghemite the magnetic signatures of the serpentinized peridotites recovered from both sides of the newfoundland iberia rift appear not in conflict with the notion that conjugate margins will have generally similar crustal structure and evolution history our rock magnetic data indicate that the differences in the rock magnetic properties of basaltic rocks are mainly a function of mineralogy and alteration there is no apparent coincidence between the age of the rocks and the degree of low temperature alteration as suggested by the presence of nearly unoxidized titanomagnetite in the cretaceous aged basalts and by an examination of curie temperature versus sample age that does not unambiguously show a positive relationship between the degree of low temperature oxidation and crust ages c 206 elsevier b v all rights reserved less

magnetic hysteresis in young mid ocean ridge basalts dominant cubic anisotropy magnetic hysteresis data from young mid ocean ridge basalts include samples with saturation remanence to saturation magnetization mrs ms ratios greater than 0 5 the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy under the usual assumption of dominant uniaxial anisotropy the narrow single domain grain size distribution implied by these high mrs ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains dominant cubic anisotropy provides a plausible explanation for the high mrs ms ratios and if generally valid requires reinterpretation of granulometric and domain state inferences made from hysteresis data

further evidence of widespread permian remagnetization in the north patagonian massif argentina abstract a paleomagnetic study on accurately dated 472 476 ma early ordovician undeformed granitoids exposed in the northeastern margin of the north patagonian massif 41 5 s 65 0 w was carried out with the main goal of comparing the early ordovician paleogeographic position of patagonia relative to gondwana about one hundred specimens were processed from ten sites on the granitoids plus two sites on the quartzites and sandstones of the unconformably overlying early devonian sierra grande formation seven out of the twelve sites all in the granitoids provided consistent directions of a characteristic remanent magnetization structural correction of paleomagnetic data from plutons could only be applied at a few sites from bedding attitudes of overlying devonian or tertiary sedimentary rocks the paleomagnetic study failed to recover the primary remanence thermal demagnetization isolated a reversed magnetic component with high unblocking temperatures magnetic properties and petrographic analyses suggest that remagnetization is carried by secondary magnetic minerals that grew from circulating high temperature fluids inferred from the pervasive to localized hydrothermal alteration this led to the formation of magnetite and or hematite after biotite a paleomagnetic pole pp was computed from the mean site directions the position of this pp 011 e 65 0 s a95 12 k 24 5 on the apparent polar wander path of south america suggests that this magnetization was probably acquired during the early permian syntectonic remagnetization during permian times has been reported for sandstones of the lower member of the devonian sierra grande formation at nearby outcrops our data suggest that the remagnetization was pervasive and affected the ordovician granitoids as well many south american permian paleomagnetic poles have been derived from syn to postectonic magnetizations along the southwestern gondwana margin from the argentine precordillera to the sierras australes belt and related to the ca 290 270 ma san rafael tectonic phase slightly younger remagnetizations have been found affecting sedimentary rocks exposed in the foreland both in argentina and uruguay suggesting a migration of the remagnetization front towards the north northeast the similarity of the position of our new remagnetized pole from the north patagonian massif to those from units in southern gondwana confirms that patagonia was already part of gondwana by the early permian whether the remagnetization of the ordovician granitoids is associated to the widespread and well dated in 290 280 ma magmatism that affected the northern patagonia and a major part of southwestern gondwana during the permian or can be attributed to tectonic processes associated to the collision of patagonia or post collision adjustment of minor plates is discussed

direct observation of ferrimagnetic ferroelastic domain interactions in magnetite below the verwey transition abstract the magnetic behaviour of magnetite at low temperatures is dominated by its transformation to a monoclinic crystal structure that is simultaneously ferrimagnetic ferroelastic and ferroelectric below 125 k the verwey transition here we use electron microscopy to reveal the relationship between ferrimagnetic and ferroelastic domain structures in monoclinic magnetite we present dynamic observations of magnetite during heating and cooling across the verwey transition revealing a diversity of unexpected interaction phenomena between crystallographic twins and magnetic domain walls this study provides a new understanding of the low temperature magnetic properties of magnetite that will affect a broad range of rock magnetic studies from the interpretation of magnetic remanence in terrestrial rocks and sediments to the search for biogenic magnetite in extraterrestrial materials

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

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

the steens mountain oregon geomagnetic polarity transition 2 field intensity variations and discussion of reversal models we carried out an extensive paleointensity study of the 15 5 0 3 m y miocene reversed to normal polarity transition recorded in lava flows from steens mountain south central oregon one hundred eighty five samples from the collection whose paleodirectional study is reported by mankinen et al this issue were chosen for paleointensity investigations because of their low viscosity index high curie point and reversibility or near reversibility of the strong field magnetization curve versus temperature application of the thellier stepwise double heating method was very successful yielding 157 usable paleointensity estimates corresponding to 73 distinct lava flows after grouping successive lava flows that did not differ significantly in direction and intensity we obtained 51 distinguishable complete field vectors of which 10 are reversed 28 are transitional and 13 are normal the record is complex quite unlike that predicted by simple flooding or standing nondipole field models it begins with an estimated several thousand years of reversed polarity with an average intensity of 31 5 8 5 t about one third lower than the expected miocene intensity this difference is interpreted as a long term reduction of the dipole moment prior to the reversal when site directions and intensities are considered truly transitional directions and intensities appear almost at the same time at the beginning of the transition and they disappear simultaneously at the end of the reversal large deviations in declination occur during this approximately 4500 1000 year transition period that are compatible with roughly similar average magnitudes of zonal and nonzonal field components at the site the transitional intensity is generally low with an average of 10 9 4 9 t for directions more than 45 away from the dipole field and a minimum of about 5 t the root mean square of the three field components x y and z are of the same order of magnitude for the transitional field and the historical nondipole field at the site latitude however a field intensity increase to pretransitional values occurs when the field temporarily reaches normal directions which suggests that dipolar structure could have been briefly regenerated during the transition in an aborted attempt to reestablish a stationary field changes in the field vector are progressive but jerky with at least two and possibly three large swings at astonishingly high rates each of those transitional geomagnetic impulses occurs when the field intensity is low less than 10 t and is followed by an interval of directional stasis during which the magnitude of the field increases greatly for the best documented geomagnetic impulse the rapid directional change corresponds to a vectorial intensity change of 6700 2700 nt yr 1 which is about 15 50 times larger than the maximum rate of change of the nondipole field observed during the last centuries the occurrence of geomagnetic impulses seems to support reversal models assuming an increase in the level of turbulence within the liquid core during transitions the record closes with an estimated several thousand years of normal polarity with an average intensity of 46 7 20 1 t agreeing with the expected miocene value however the occurrence of rather large and apparently rapid intensity fluctuations accompanied by little change in direction suggests that the newly reestablished dipole was still somewhat unstable

sponge spicules from the lower cambrian in the yanjiahe formation south china the earliest biomineralizing sponge record abstract sponges are the earliest metazoans and according to molecular clocks have a deep history back into the cryogenian however the record of their biomineralized elements below the ediacaran cambrian boundary has been controversial here we present the first and earliest indisputable record of hexactinellid spicules in the lowest cambrian terreneuvian below the small shelly fossil anabarites trisulcatus protohertzina anabarica assemblage zone zone i spicules recovered from cherts of the lowest yanjiahe formation in the yichang hubei province south china include monaxons diaxons triaxons and that are all siliceous in composition this earliest record of silica biomineralization by filter feeding metazoan suggests sponges have contributed to the cycling of silica in the oceanic system as early as the beginning of the phanerozoic these results along with a review of previously reported sponge occurrences suggest a two step evolution on sponges of the early cambrian

low temperature first order reversal curve forc diagrams for synthetic and natural samples first order reversal curve forc diagrams were measured on a variety of synthetic and natural submarine basalts and potsherds samples as well as for a mixture between room temperature and 10 k measuring forc diagrams allowed us to examine the behavior of the coercivity field and interaction field distributions with decreasing temperature generally as the temperature is decreased forc contours were observed to expand in all directions e g the maximum coercivity peak migrates toward a higher coercivity for synthetic magnetite samples there were abrupt changes in the forc distribution at the verwey transition 120 k while in the more complex natural magnetite samples the forc distribution gradually progressed with temperature as the temperature decreased forc diagrams were found to display different domain state characteristics for example samples with dominant superparamagnetic signals became single domain sd like a sd like sample stayed sd like and a psd like sample became more sd like in addition to these general features we also observed some more specialized features first in both synthetic and natural magnetite samples a secondary higher coercivity peak is occasionally present below the verwey transition which we suggest is a twinning contribution second the coercivity increases by a factor 15 between 300 k and 20 k in some of the seamount samples third the effect of field cooling zero field cooling on forc diagrams is negligible with the hysteresis parameters hc and hcr displaying a greater dependency finally it is shown that in mixtures of magnetite and hematite the hematite contribution disappears on cooling below the morin transition leaving a strong well defined magnetite signal

absolute paleointensity at 1 27 ga from the mackenzie dyke swarm canada 1 paleointensity studies have been conducted on 6 mafic dykes from the 1270 ma mackenzie swarm in the slave province canada the mean direction of the characteristic magnetization coincides with results of an earlier study in which the primary origin of the magnetization was established on the basis of a contact test high unblocking temperatures magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains of magnetite paleointensity experiments were conducted with a specially designed oven using a revised version of the thellier coe method thirteen successful determinations of paleointensity were obtained for 4 dykes the paleofield estimates vary between 4 3 and 22 1 t yielding virtual dipole moments vdms between 1 3 0 2 and 4 5 0 9 1022 am2 these new results increase the number of low field determinations during the precambrian which largely dominate the database with an averaged field of 3 1 2 5 1022 am2 they also emphasize the importance of additional studies to understand the differences with the strong paleointensities obtained using new techniques

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

the upper kaena sedimentary geomagnetic reversal record from southern sicily a detailed paleomagnetic and rock magnetic record of the upper kaena polarity reversal was determined from late pliocene marine marls in southern sicily two separate characteristic magnetization components were distinguished a low temperature lt component between 200 and 50 c and a high temperature ht component between 500 and 580 610 c the two components record the reversal in a slightly different way but both were acquired in an early stage following deposition most probably by authigenic biogenic formation of magnetic minerals the actual r n reversal via e and horizontal inclinations seems to take 3 3 kyr and is followed by a stable normal interval 3 3 kyr and subsequently by a significant excursion 50 60 in declination 3 6 kyr relative intensities were lower during a time span of 5 6 kyr and show a minimum of 10 of the nontransitional values but during the excursion no intensity changes are apparent the vgps of the ht component are confined to a longitude between india and australia those of the lt component oscillate between this same longitude and one passing over north and south america we conclude that most if riot all observed features are largely determined by the remanence acquisition process which is complicated by authigenic formation of magnetic minerals and cyclically fluctuating paleoredox conditions this has not only resulted in considerable smoothing but also in delayed nrm acquisition in specific sedimentary intervals we believe that the observed excursion is a sedimentary artifact and not related to geomagnetic field behavior during transitions we emphasize that many reported geomagnetic features may in fact be due to the sedimentary remanence acquisition process

differences between magnetic domain imaging observations and theory a quasi two dimensional model is developed to calculate domain structures in magnetite grains containing two to 20 domains the calculation shows that domains do not consist of uniformly magnetized regions consistent with the conclusions previously carried out on sub micron grains but that they become more uniformly magnetized for grains containing several domains consistent with domain imaging results in spite of improvement over previous calculations there remains a significant difference between the number of domains observed and calculated

the magnetism of a glacial aeolianite sequence from lanzarote canary islands coupling between luvic calcisol formation and saharan dust trapping processes during wet deposition events off northwestern sahara summary in order to better document the climatic origin of pedogenized loess deposits west of sahara rock magnetic measurements were performed on a last glacial coastal sand sequence from la mala lm quarry lanzarote canary islands containing six interbedded loess palaeosol units hysteresis and susceptibility data point to a coarse grained magnetic enhancement in the coastal sand which contrasts with the superparamagnetic sp to pseudo single domain psd behaviour of the saharan loess and palaeosol high and low temperature experiments show that oxidized titanomagnetite contributions dominate in the sand while fine grained sp to psd iron oxidation products such as maghemite and goethite are evidenced in the saharan loess palaeosol units at room temperature the detrital psd multidomain titanomagnetite contribution of local origin is tentatively estimated from the ac field dependence of magnetic susceptibility surface oxidation of detrital spinel grains and authigenesis of fine grained iron oxides including sp goethite are proposed to explain the magnetic properties of the primarily fine saharan dust bearing material the dry local climate of the present day and late holocene means that loess deposits are not preserved the six pedogenized loess units however point to dust trapping under semi arid wetter conditions probably illustrating periods of reduced latitudinal temperature gradients and climate variability of the north atlantic climate respectively these findings suggest that both the canary palaeosol and its content of ultra fine iron oxide might be constrained by wet deposition and trapping of fine saharan dust

the physical isolation and identification of carriers of geologically stable remanent magnetization paleomagnetic and rock magnetic microanalysis and electron microscopy paleomagnetic and rock magnetic microanalysis of fully oriented microspecimens provides a more direct understanding of the source and origin of geologically stable remanent magnetizations scanning transmission electron microscopy allows direct observation and characterization of magnetic phases we provide a brief history of micromagnetic research describe a functional microspecimen preparation method evaluate instrumentation and present results of three combined micromagnetic stem studies

microfossils from the neoproterozoic gangolihat formation kumaun lesser himalaya their stratigraphic and evolutionary significance abstract a well preserved microbial assemblage from the gangolihat dolomite kumaun lesser himalaya is described in the present paper the present assemblage contains both filamentous septate and non septate and coccoidal microfossils besides acritarchs and sponge spicules the assemblage includes 23 species belonging to 12 genera siphonophycus typicum is the most common species in the entire assemblage the assemblage shows an abundance of filaments over coccoids and acritarchs the assemblage includes siphonophycus septatum siphonophyscus typicum siphonophycus robustum siphonophycus capitaneum siphonophycus kestron and siphonophycus punctatum gunflintia minuta gunflintia grandis gunflintia barghoornii oscillatoriopsis media oscillatoriopsis obtusa oscillatoriopsis cuboides oscillatoriopsis sp nostocomorpha chlorogloeaopsis contexta obruchevella parvissima sphaerophycus parvum myxococcoides minor leiosphaeridia crassa trachyhystrichosphaera vidalii trachyhystrichosphaera sp micrhystridium pallidum cymatiosphaera minuta and sponge spicules of monaxons and hexactinellid affinity the assemblage suggests a neoproterozoic age for gangolihat dolomite

middle late cambrian rankenella girvanella reefs of the mila formation northern iran following the collapse of the early cambrian archaeocyathan calcimicrobial reef consortium the middle late cambrian furongian was an interval dominated by purely microbial dendrolite and stromatolite reefs however among these latter a few exceptional occurrences of metazoan reefs are known one such reef complex occurs in the late middle early late cambrian furongian portion of the mila formation of northern iran in the otherwise low energy interval of this formation the anthaspidellid demosponge rankenella hamdii sp nov is associated with encrusting girvanella eocrinoid plates rhynchonelliformean brachiopod valves and subordinate hyoliths and trilobites in tempestite shell beds these beds underwent synsedimentary cementation on the seafloor to form hardgrounds in the succeeding higher energy interval a complex of metre scale bioherms and or taphoherms incorporates toppled or transported rankenella hamdii in association with brachiopods echinoderm plates trilobites and some red arg

geochemistry nd pb and sr isotope systematics and u pb zircon ages of the neoarchean bad vermilion lake greenstone belt and spatially associated granitic rocks western superior province canada abstract the ca 2720 ma neoarchean bad vermilion lake bvl greenstone belt in the western superior province canada is composed of a suite of tholeiitic to calc alkaline basalts to rhyolites volcaniclastic rocks gabbros and timiskaming type siliciclastic sedimentary rocks the greenstone belt was intruded by neoarchean granitic rocks and underwent greenschist facies metamorphism and intense deformation resulting in mobilization of many elements e g rb ba sr k u pb the high field strength element and rare earth element systematics of the volcanic and volcaniclastic rocks and gabbros are consistent with subduction zone geochemical signatures suggesting that the bvl greenstone belt formed in a magmatic arc setting on the basis of lithological associations and trace element systematics the bvl greenstone belt is defined as a fragment of a neoarchean subduction related ophiolite three rhyolite samples from the belt have yielded 2722 18 ma 2706 13 ma and 2710 28 ma u pb zircon ages representing the approximate age of the arc volcanism in the study area and development of a subduction zone between the western wabigoon terrane to the north and the wawa abitibi terrane to the south the intrusion of the ca 2664 15 ma late to post tectonic potassic ottertail lake granite marks the end of tectonic accretion in the study area both the volcanic rocks and gabbros display large ranges of nd 143 nd 144 nd 0 511600 0 512849 e nd 2720 ma 0 8 to 4 0 pb 206 pb 204 pb 13 80 60 67 and sr 87 sr 86 sr 0 701481 1 01154 isotopic compositions suggesting that these isotope systems were variably affected by post magmatic element mobility neither the sm nd 2921 200 ma nor rb sr 2130 610 ma system has yielded reliable regression isochron ages reflecting the open system behavior of these systems during metamorphism despite large uncertainties pb pb regression ages yielded by all rock types 2661 60 ma and basalts and gabbros 2725 83 ma agree with the zircon u pb ages of the rhyolites suggesting that the u pb system was the most robust among all three systems

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

pressure solution deformation of the purgatory conglomerate rhode island u s a quantification of volume change real strains and sedimentary shape factor abstract pressure solution has caused substantial volume redistribution within the purgatory conglomerate from rhode island material has been removed from quartzite cobble surfaces parallel to the fold axes and mostly redeposited as fibrous pressure shadows at the long axis terminations of the cobbles in the hinges of folds 23 of the mean cobble volume has been removed and in highly deformed and overturned fold limbs up to 55 volume reduction has occurred the initial cobble shape and orientation can be measured at an undeformed locality and the deformation path can be easily deduced thus these are real cobble volume reductions apparent volume losses initial shapes not removed range from 70 to 89 real strains for cobbles axial ratios ranging from 1 0 65 0 38 to 1 0 47 0 15 have values of e x e y e z which range from 0 20 11 to 0 37 42 respectively depending on structural position the conglomerate itself has been extended parallel to the fold axes e r but the extension is not recorded by the cobble shapes

tectonic implications of the 1419 ma nova guarita mafic intrusives paleomagnetic pole amazonian craton on the longevity of nuna abstract the nuna supercontinent was probably assembled in the paleoproterozoic but its paleogeography and the timing for its demise are still a matter of debate a paleomagnetic and geochronological study carried out on the mesoproterozoic nova guarita dyke swarm northern mato grosso state sw amazonian craton provides additional constraints on the duration of this supercontinent paleomagnetic af and thermal treatment revealed south southwest northeast magnetic directions with downward upward inclinations for 19 analyzed sites these directions are carried by psd magnetite with high unblocking temperatures as indicated by additional magnetic tests including thermomagnetic curves hysteresis loops and the progressive acquisition of isothermal remanence in selected samples a positive contact test with the host granite in one of the studied dykes further attests to the primary origin of the characteristic magnetic component a mean site direction was calculated at d m 220 5 i m 45 9 95 6 5 k 27 7 which yielded a paleomagnetic pole located at 245 9 e 47 9 s a 95 7 0 40 ar 39 ar dating carried out on biotites from four analyzed dykes yielded well defined plateau ages with a mean of 1418 5 3 5 ma the nova guarita pole precludes a long lived nuna configuration in which laurentia baltica north china and amazonia formed a long and continuous block as previously proposed for the paleoproterozoic it is nevertheless fully compatible with a samba amazonia baltica link at mesoproterozoic times

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

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

low temperature demagnetization of saturation remanence in magnetite bearing dolerites of high coercivity we studied 16 magnetite bearing dolerite dyke samples of high coercive force hc ranging from 11 to 50 mt that had been used successfully in precambrian palaeomagnetic studies each dolerite was given a saturation remanent magnetization whose change was measured as the sample was cooled to 77 k in zero field and warmed back to room temperature only the three dolerites of highest hc 40 mt show little change on cooling suggesting that their magnetite is mostly in elongated single domain grains the rest of the dolerites are likely to be dominated by pseudosingle domain magnetite cooling to 135 k causes their remanence to decrease by 37 per cent on average in rough proportion to the decrease in saturation magnetostriction as expected if internal stresses oppose domain wall motion cooling from 135 k to 77 k causes remanence to decrease further by 26 per cent on average probably mostly because of domain reorganization forced by magnetite s verwey crystallographic transition warming back to room temperature causes some of the remanence loss to be recovered perhaps because internal stresses act as a bridge between different easy axes below the verwey temperature 122 k and above the isotropic point 135 k this recoverable low temperature demagnetization averages 23 6 per cent of the initial saturation remanence while the permanent demagnetization averages 40 9 per cent recoverable low temperature demagnetization is even larger for natural remanence averaging 46 9 per cent for the six dolerites measured while the corresponding permanent demagnetization averages 13 6 per cent large recoverable low temperature demagnetization seems to be characteristic of pseudosingle domain magnetite in which high internal stresses block domain wall motion and may be common in mafic igneous rocks like our dolerite samples whose magnetite is intergrown with ilmenite lamellae measuring natural remanence of such rocks before after and while at 77 k should help separate remanence carried by multidomain magnetite mostly permanently demagnetized by single domain magnetite mostly unchanged and by pseudo single domain magnetite mostly responsible for recoverable demagnetization

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

phase boundary mobility in naturally deformed high grade quartzofeldspathic rocks evidence for diffusional creep abstract grain shape fabrics and optical microstructures of some quartzofeldspathic rocks deformed under upper amphibolite facies conditions in the southwestern grenville province ontario canada suggest that quartz and feldspar have accommodated intracrystalline plastic strains by both diffusional and dislocation creep in these rocks quartz and feldspar form polycrystalline domains separated by gently curved and locally cuspate phase boundaries whose morphology is similar in certain respects to the phase boundary morphology of rocks annealed experimentally under hydrostatic stress conditions in the naturally deformed rocks however phase boundary cusps consistently point along the foliation and parallel to the mineral fibre lineation i e in directions of inferred finite extension which implies that phase boundary motion and cusp formation occurred during deformation optical microstructures in feldspar and crystallographic preferred orientations in quartz are consistent with the accommodation of some intracrystalline plastic strains by dislocation creep however the morphology of quartz feldspar phase boundaries cannot be explained by either dislocation creep or static annealing alone we propose that phase boundary motion resulted from a diffusion assisted process involving dissolution at foliation parallel quartz feldspar phase boundaries mass transfer over length scales of the order of feldspar domain size 200 m or greater and precipitation at quartz feldspar phase boundary cusps this study extends the range of natural deformation conditions under which diffusional creep has been identified in quartzofeldspathic rocks it also has important implications for the natural rheological behavior of the mid and lower continental crust

germanium silicon of the ediacaran cambrian laobao cherts implications for the bedded chert formation and paleoenvironment interpretations sedimentary strata of the terminal ediacaran 635 542 ma to early cambrian 542 488 ma laobao liuchapo bedded cherts in the south china block include the ediacaran oxidation event and the cambrian explosion understanding the origin and depositional environment of the bedded cherts may provide insight into how the earth s surface environment changed between the proterozoic and phanerozoic we measured major and trace element compositions and ge si ratios of the laobao cherts from northern guangxi province the laobao cherts were deposited in the deep basinal environment of the south china block we show that the composition of the laobao cherts is determined by a mixture of four components quartz clay carbonate and pyrite iron oxide the quartz component is the dominant component of the laobao cherts the maximum estimated ge si of the quartz component is between 0 4 and 0 5 mol mol which is close to the ge si of modern seawater and biogenic silica but 1 order of magnitude lower than that of hydrothermal fluids these ge si systematics suggest that normal seawater rather than mid ocean ridge hydrothermal fluids is the primary si source for the laobao cherts the ge si of the clay component varies between 1 and 10 mol mol which is comparable to the ge si of typical marine clays but 10 100 times lower than that of chert nodules from early ediacaran beds the doushantuo formation predating the terminal ediacaran laobao cherts studied here our observations indicate that the clay component ge si ratio decreased from the early ediacaran to the late ediacaran we speculate that high ge si ratios in clays reflect the preferential chelation of ge by dissolved organic compounds adsorbed onto clays if so this suggests that the decrease in ge si ratio of the clay component in the ediacaran signifies a decrease in the total dissolved organic carbon content of seawater toward the ediacaran cambrian transition consistent with oxidation of the oceans during the late ediacaran finally the seawater origin of the laobao cherts also suggests that replacement of carbonate may not be the primary cause for bedded chert formation instead direct precipitation from seawater or early diagenetic silicification of calcareous sediments perhaps due to the emergence of si accumulation bacteria may have been responsible for the bedded laobao liuchapo chert formation in south china block

new and extraordinary early cambrian sponge spicule assemblage from china the fossil record of siliceous sponges compared with that of other skeleton secreting metazoa is poorly known based as it is on disarticulated spicules and sporadically preserved body fossils abundant spicules recovered from lower cambrian strata in shaanxi china essentially double the known morphological diversity of siliceous sponges for that interval of geologic time these fossils along with a comparable coeval fauna from south australia have a remarkably modern aspect thereby demonstrating that the principal siliceous sponge groups and styles of body architecture were established quickly in the earliest phanerozoic as part of the cambrian explosion and that they inhabited a variety of low energy relatively deep water settings the similarity of spicule shape and variation to that of younger assemblages reflects a conservative architecture for the siliceous sponges

the relationship of strain and preferred orientation of phyllosilicate grains in rocks a review abstract the calculation of strain from the preferred orientation of platy phyllosilicate grains taking strain to be homogeneous is surprisingly often successful from a pragmatic point of view although numerous theoretical objections to this procedure can be and have been raised for the calculation to succeed the original distribution of mica or clay mineral grains must be known or assumed the usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation or if it was laid down in still anoxic waters from the moment of first deposition it therefore includes compaction as part of the strain history which is usually neglected by structural geologists originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways exceptions probably exist but their frequency has not been estimated biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition with the exception of whole beds or concretions in which pores are early filled with cement mudrocks are almost always compacted by the weight of later deposits the strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes compaction acts on soft unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains tectonic deformation may start early before the rock is fully compacted or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial up to a certain level of mild metamorphism however phyllosilicate grains seem to be rather permanent if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra given this permanence the mechanisms of rock deformation whether intergranular slip cataclasis or dissolution and reprecipitation and various intragranular processes in the non phyllosilicate matrix hardly matter as far as the recording of strain is concerned even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains the same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another if distinct domains are not equant and have themselves a strong preferred orientation of shape nevertheless rocks containing an anastomosing network of zones rich in phyllosilicate grains as slates and other deformed mudrocks commonly do have in many cases been found to have average preferred orientations of mica or chlorite which if used to estimate the strain give results similar to those obtained from independent strain markers in the same rocks thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain the limit to such verification is the rarity of strain markers so that the coexistence of two independent ones is exceptionally rare for this reason however it is important to make full use of the orientation of phyllosilicate grains where they are available the alternative to have no strain measurements at all causes structural geologists to miss potential information

separating geometry from stress induced remanent magnetization in magnetite with ilmenite lamellae from the stardalur basalts iceland

low temperature magnetic properties of magnetite using first order reversal curve analysis implications for the pseudo single domain state 1 low temperature magnetic hysteresis properties of nearly stoichiometric magnetite were studied using the first order reversal curve forc technique forc distributions were measured at 20 k after cooling in zero magnetic field or cooling in the presence of a strong 1 5 t magnetic field five polycrystalline samples ranged in mean grain size and shape representing mostly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the forc distributions measured from psd magnetite after zero field cooling zfc manifest an unusual behavior characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram in contrast the low temperature forc distributions for sd and md samples have a single maximum located at the hu 0 axis the appearance of two maxima on low temperature forc distributions is interpreted as a manifestation of a perminvar like behavior due to stabilization of the magnetic domain structure by irregularities of crystalline lattice induced by twinning in monoclinic magnetite the experimental data suggest that the effect of monoclinic twinning on remagnetization is only pronounced within a certain grain size range the double maxima zfc forc distribution appears to be a distinctive property of psd hysteresis behavior which cannot be reproduced by mixing sd and md magnetites

grain size dependence of initial susceptibility and saturation magnetization related parameters of four natural magnetites in the psd md range summary initial susceptibility coercive force and several types of remanent coercive forces were measured on a set of artificial rock specimens containing grain size fractions of a natural magnetite and titanomagnetite both of which are optically homogeneous and a natural titanomagnetite and titanomaghemite both of which show exsolution phenomena it was found subsequently that the optically homogeneous titanomagnetite contains some submicroscopic fe rich inclusions these inclusions are magnetically screened in coarse grains only for the lower grain sizes studied they have an appreciable influence on the data the size of the fractions varied from 5 to 250 m the parameters and several parameter ratios are plotted as a function of grain size several of these parameters can be used as mineralogical and grain size indicators the varying degree of exsolution of the minerals is reflected in the gradients of the plots from plots of initial susceptibility versus grain size it is concluded that the shape of the grains does not play a dominant role in determining the change of this parameter with grain size variation of the various parameters with grain size is explained in terms of a gradual transition from large psd grains to md grains without there being any sharp transition for titanomagnetite x 0 55 the transition takes place at grain sizes that are about 15 m larger than for magnetite

diagenetic magnetite carries ancient yet secondary remanence in some paleozoic sedimentary carbonates many sedimentary carbonate rocks carry stable magnetizations that can be shown to reside in magnetite when such magnetizations are observed it is often argued or demonstrated that the magnetite was incorporated into the sediment during deposition however paleomagnetic and rock magnetic studies in conjunction with analyses of magnetic extracts from the helderberg and bonneterre carbonates united states indicate that the magnetite present in these rocks is most likely of diagenetic i e postdepositional origin

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

the effects of dissolution precipitation creep on quartz fabrics within the purgatory conglomerate rhode island abstract quartz crystallographic preferred orientation cpo patterns are most commonly a result of deformation by dislocation creep we investigated whether dissolution precipitation creep dpc a process that occurs at lower differential stresses and temperatures may result in cpo in quartz the purgatory conglomerate is part of the se narragansett basin where strain intensity increases from west to east and is associated with top to the west transport and folding during the alleghanian orogeny within the purgatory conglomerate dpc led to quartz dissolution along cobble surfaces perpendicular to the shortening direction and quartz precipitation in overgrowths at the ends of the cobbles strain shadows parallel to the maximum extension direction quartz c axis orientations as revealed by electron backscatter diffraction ebsd methods were random in all analyzed domains within the cobbles and strain shadows irrespective of the intensity of strain or metamorphic grade of the sample quartz dissolution probably occurred exclusively along the cobbles margins leaving the remaining grains unaffected by dpc the fact that quartz precipitated in random orientations may indicate that the strain shadows were regions of little or no differential stress

determination of pressure solution shortening in sandstones abstract a new method for the determination of pressure solution shortening in sandstones uses the geometry of grain to grain interpenetrations and grains truncated against solution surfaces these features are used to construct plots from which the magnitude and direction of the pressure solution shortening can be determined using simulated pressure solution deformation of artificial and natural grain populations the new method is shown to correctly assess a variety of coaxial and non coaxial shortenings although primarily intended to determine shortening the method can also quantify extension related to growth of beards or overgrowths during pressure solution application of the method to naturally deformed quartz arenite samples shows that pressure solution shortening of up to 26 occurred during compaction and 22 during layer parallel shortening

microborings in early cambrian phosphatic and phosphatized fossils abstract a small proportion of phosphatic and phosphatized fossils mostly microfossils extracted from lower cambrian qiongzhusian lime mudstones of deeper water aspect belonging to the yu anshan and shuijingtuo formations of south china is perforated by microborings these microborings are present in bradoriid and phosphatocopine arthropod carapaces lingulid brachiopod valves microdictyon plates and tommotiid sclerites their absence in co occurring phosphatized eodiscid trilobite exoskeletons and palaeoscolecid worm trunk fragments indicates a substrate specificity that suggests the boring microorganism was heterotrophic extracting particular residual organic compounds present in only certain bioclast types microborings occur as meandering tunnels and grooves on bioclast surfaces and most are 10 20 m wide and circular in cross section the euendoliths were thus able to live partly exposed as well as within the substrate tunnel and groove walls bear a distinct longitudinal sculpture of straight striae separated by cuspate ridges 1 2 m apart that parallel the microboring never twisting the microborings accord with the ichnotaxon microptychoites fuquanensis dong in dong et al dong y ji q wang s 1984 the discovery of microscopic trace fossils in the sinian doushantuo formation in southwestern china scientia geologica sinica 346 347 in chinese the fluted sculpture appears to have no modern analogue but may have arisen by localized secretions at the tip of a bundle of filaments this kind of microboring had been previously reported from phosphatic substrates of ediacaran earliest cambrian middle cambrian and probably early devonian age but appears to have no recent counterpart

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

proterozoic basalt dikes in the nain anorthosite complex labrador two sets of proterozoic between 1290 and 1042 ma alkali to transitional basaltic dikes occur sparsely in an area of at least 5000 km2 within the nain anorthosite complex both are compositionally similar in terms of major elements but the set that trends roughly north south hp dikes contains much higher concentrations of incompatible elements than the more abundant dikes that trend roughly east northeast west southwest lp dikes both types have low normative clinopyroxene relative to olivine a characteristic shared with widespread troctolitic rocks in the nain complex low mg numbers and low ni contents suggest that the magmas producing these dikes have undergone extensive fractional crystallization the low normative di content of these dikes suggests that this fractional crystallization occurred at depths equivalent to at least 10 kbar 1000 mpa high equilibration temperatures for coexisting fe ti oxides in the matrices of these dikes indicate that the dikes cooled rapidly and were emplaced near

low paleointensities recorded in 1 to 2 4 ga proterozoic dykes superior province canada abstract paleointensity studies have been conducted on six mafic dyke swarms from the superior province in canada with u pb ages between 1141 ma and 2473 ma buchan et al tectonophysics 319 2000 167 198 the mean direction of the characteristic magnetization for each dyke swarm coincides with results of earlier studies in which the primary origin of the magnetization was established on the basis of contact and secular variation tests this primary component was isolated over a very narrow range of high unblocking temperatures usually 550 580 c indicating that it is carried by magnetite or low titanium titanomagnetite magnetic mineralogy and grain size experiments suggest that the magnetization is dominated by pseudo single domain or single domain grains taken together the characteristics of this component suggest that it is thermal in origin paleointensity experiments were conducted with a specially designed oven using a revised approach of the thellier coe method eighty five successful determinations of paleointensity were obtained for 20 sites from six dyke swarms doubling the number of existing well constrained precambrian data the paleofield estimates for the dyke swarms vary between 5 and 11 2 t yielding virtual dipole moments between 0 85 0 1 and 2 8 0 87 1022 am2 the overall convergence between these new results and the rest of the database strongly reinforces the existence of a weaker geomagnetic field during the 1000 2400 ma period than has been recorded between 0 3 and 400 ma however additional determinations are required particularly for the 400 2000 ma interval in order to establish whether a long term evolution in the time averaged field intensity can be linked to the onset of the growth of the earth s inner core

oxide mineral formation during the serpentinization of a cr spinel seam insights from rock magnetic experiments 1 serpentinization of ultramafic rocks results in the growth of serpentine and magnetite following hydration of magmatic olivine cr spinel seams in ultramafic intrusions that also contain olivine might produce magnetite during the serpentinization process however hysteresis and temperature dependence of susceptibility experiments on serpentinized chromitite 80 cr spinel from the dawros peridotite western ireland reveal the presence of small quantities of a fe ti oxide phase likely titanomaghemite reflected light petrography reveals that the oxides occur as 0 1 m to 20 m grains that are typically intergrown with serpentine it is suggested that the formation of the minute quantities of low susceptibility high curie point titanomaghemite occurred by low temperature alteration of the magnetite produced during serpentinization the presence of such low quantities of the oxide phase may be either a function of the very high mg content of the magmatic olivine or the decoupling of oxide growth from the process of serpentinization where magnetite formed later than the reaction of olivine with the infiltrating fluids our data highlight the usefulness of rock magnetic techniques to the investigation of oxide mineral phases in serpentinized ultramafic rocks

palaeomagnetism of early triassic limestones from the huanan block south china no evidence for separation between the huanan and yangtze blocks during the early mesozoic summary there has been extensive debate on the timing of the collision between the huanan and yangtze blocks ever since hsu and co workers hsu et al 1987 1988 proposed a mesozoic collision model for south china tectonics we report new rock and palaeomagnetic data from an early triassic limestone formation from the huanan block that help to constrain its tectonic history the samples are from two localities jiupi jt and huangben lt in lianzhou county northern guangdong province only thermal demagnetization is eective in decomposing a multicomponent remanent magnetization the majority of samples show three components a low temperature component a an intermediate temperature component b and a high temperature characteristic remanent magnetization chrm the a component has a steep inclination and is probably drilling induced the b component clusters around the present geomagnetic field direction and is a brunhes age viscous remanence fold test results suggest that the jt chrm is pre tectonic and that the lt chrm is a syn or prefolding remagnetization samples from the two localities show quite similar acquisition and thermal demagnetization of the isothermal remanent magnetization irm and suggest that magnetite carries the chrm component however partial anhysteretic remanent magnetization parm spectra anisotropy of anhysteretic remanence aar rock fabric and morphology of magnetic extracts are quite dierent for the two localities the aar of jt samples shows low anisotropy average 2 per cent and reflects a composite compaction and strain fabric the aar of lt samples shows a high degree of anisotropy average 10 per cent and is clearly consistent with rock deformation fabrics the magnetic extracts of lt samples are dominated by spherical botryoidal ti poor magnetite which is probably precipitated from tectonic fluids during deformation while the jt magnetic extracts are dominated by rod shaped ti poor magnetite probably derived from the erosion of igneous rocks although minor amounts of botryoidal magnetite and rod like magnetite particles can be seen in jt and lt extracts respectively the rock fabric and magnetic fabric data and sem observations of magnetic extracts corroborate the results of the fold test the best clustered lt chrm direction is d 294 i 12 a 95 12 its pole position is 187 e 19 s dp 6 dm 12 the assumption of a reversed polarity for the jt chrm direction d 198 i 20 a 95 9 implies that the huanan block was in the northern hemisphere during the early triassic and its pole position 239 e 68 n dp 5 dm 10 is close to early triassic pole positions from the yangtze block a normal polarity for the jt site chrm would require tectonic movements inconsistent with the local geology therefore the huanan and yangtze blocks were not separated in the early triassic the rotation of the lt locality may have been caused by the collision of the southeast china coastal terranes and or subduction of the pacific plate in the late mesozoic

evidence for magnetostrictive control of intrinsic susceptibility and coercive force of multidomain magnetite in rocks abstract it is shown that intrinsic susceptibility xi susceptibility in the absence of self demagnetizing fields as well as coercive force hc of multidomain magnetite in rocks are likely to be commonly magnetostrictively controlled presumably through internal stresses opposing domain wall motion we use observations of the thermal variation of observed susceptibility x per cm3 of magnetite in the sample and coercive force hc on cooling to 130 k for seven basaltic rocks and for 0 25 0 21 mm magnetite grains dispersed in cement as expected theoretically with magnetostrictively controlled xi a plot of 1 x versus j s 2 for each sample where is the polycrystalline magnetostriction coefficient and js is saturation magnetization gives a reasonably straight line whose 1 x intercept agrees with n average self demagnetizing factor of magnetite grains in the sample given by h c j r n h c j r h c j ri where jr is saturation remanence and ri is jr when n 0 as expected theoretically with magnetostrictively controlled hc a plot of hc versus j s for each sample reveals an approximate proportionality this also implies magnetostrictive control of xi because a plot of 1 x versus h c j s for each sample yields a straight line of 1 x intercept agreeing with n from h c j r n h c j r h c j ri as expected if hc and xi are both magnetorestrictively controlled

possible mechanisms causing failure of thellier palaeointensity experiments in some basalts summary the normally magnetized zone of the jurassic lesotho basalts although providing apparently quite reliable palaeofield directions kosterov perrin 1996 shows anomalous behaviour when studied in vacuum using the thellier palaeointensity method typically the slope of the natural remanent magnetization thermoremanent magnetization nrm trm curves is very steep at intermediate temperatures 200 to 400 460 c in order to elucidate the reasons for such an anomalous behaviour six representative samples from a total of 74 studied using this method were subjected to a variety of analyses these experiments indicate that the magnetic properties are dominated by pseudo single domain psd magnetite grains some 1 mm in size resulting from high temperature oxidation of titanomagnetite laboratory heatings in vacuum up to the curie point do not change significantly the roomtemperature hysteresis characteristics or the initial susceptibility k similarly the k t curves in vacuum are with a single exception rather reproducible since the laboratory trms yield almost ideal nrm trm plots the anomalous nrm trm plot is presumably due to some peculiarity of the natural trm the partial trm ptrm acquisition capacity in the moderate temperature range cooling from 200 to 20 c is generally very strongly reduced after heating to 270 c which indicates that some magnetic alteration has already occurred at these temperatures hysteresis measurements between room temperature and the curie temperature t c show that some small less than 10 per cent but significant irreversible changes in hysteresis characteristics also occur during heating in particular the coercive force h c0 at room temperature is typically reduced after heating at a moderate temperature 175 c but increases after treatments at 475 c and more pronouncedly at 580 c the saturation magnetization j s0 remains unchanged except for a very small decrease less than 5 per cent occurring in some samples after the two latter treatments these changes are most clearly seen on h c t j s t bilogarithmic plots which show that the moderate temperature change in coercivity can extend up to 200 250 c thus hysteresis measurements as a function of temperature over a promising tool for sample pre selection for thellier experiments alternating field demagnetization and cycling of ptrms at liquid nitrogen temperature suggest that the blocking mechanism is largely multidomain like near room temperature but becomes less so as the curie point is approached the main reason for the failure of the thellier experiments is the loss of a fraction of the nrm natural trm at temperatures apparently lower than the blocking temperatures in nature it is suggested that this anomalous behaviour results from the reorganization of the domain structure of the psd grains during heating this transformation which seems to be triggered by the coercivity decrease observed at very moderate temperatures can reduce the nrm intensity without requiring any correlated ptrm acquisition

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

determination of domain structure in igneous rocks by alternating field and other methods abstract af alternating field demagnetization arm anhysteretic remanent magnetization and strong field hysteresis properties of a large collection of mostly continental igneous rocks are reported here the collection included rocks whose magnetic carriers were believed from previous work to be of one of three types md multidomain sd psd single domain pseudo single domain or a bimodal mixture of md grains e g discrete opaques and sd psd material e g silicate inclusions two series of subaerial basalts with a full range of deuteric oxidation classes included examples of all three classes of behaviour sd psd rocks have relatively hard inflected af decay curves decay rate initially increasing then decreasing md rocks have soft exponential like decay curves and bimodal rocks have a combination of these characteristics relative hardnesses of normalized decay curves of remanences acquired in weak intermediate and strong fields the lowrie fuller test are also distinctively different for the three classes and the results support the theory developed in an accompanying paper 1 that lowrie fuller characteristics are an expression of the shapes of decay curves the lowrie fuller test although its result can be expressed as a numerical parameter is not capable of fine scale classification of domain structure or grain size the shape of the arm induction curve does have a quasi continuous variation with grain size however the parameter ar j rs initial anhysteretic susceptibility normalized to saturation remanence which is easily measured with standard paleomagnetic instrumentation is potentially useful for magnetic granulometry although ar itself was not diagnostic of grain size

determination of self demagnetizing factor n for multidomain magnetite grains in rock abstract the magnetic poles on a multidomain grain with a magnetization j produce a field nj tending to demagnetize the grain strongly affecting many magnetic properties of rocks bearing multidomain magnetite the average self demagnetizing factor n of multidomain grains in a rock is thus an important parameter which is shown to be simply measured from a high field hysteresis loop using h c j r n h c j r h c j r i where h c is the coercive force j r the saturation remanence and j r i an estimate of j r when n 0 measuring magnetic susceptibility x and coercive force h c as a function of low temperature and then plotting 1 x versus h c j s where j s is saturation magnetization gives a straight line whose intercept on the 1 x axis also estimates n these two independent estimates of n are shown to agree for the seven magnetite bearing basaltic rocks studied yielding an average n 1 7 e m u and for the 0 2 mm natural magnetite grains yielding n 2 6 0 2 e m u in contrast estimating n from h c j r 1 x i where x i is x when n 0 is shown to be invalid and even use of n 1 x h c 45 is not recommended measured n is much below the 4 e m u expected of large equidimensional magnetite grains which is understandable for the rocks of higher coercive force but makes further study of rocks of low coercive force desirable

effect of the magnetic field applied during cooling on magnetic hysteresis in the low temperature phase of magnetite first order reversal curve forc analysis the strength of a magnetic field applied during cooling through the verwey transition can control the low temperature lt magnetic hysteresis properties of magnetite this effect is investigated using the first order reversal curve forc technique lt forc distributions were measured at 20 k from four polycrystalline samples after cooling from 300 k in zero magnetic field or in the presence of a magnetic field hfc from a 0 01 to 1 5 t range the samples represented nearly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the lt forc distributions measured from psd magnetite after zero field cooling are characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram application of an intermediate magnetic field 10 mt to 50 mt during cooling results in a shift of the peaks to near symmetry with respect to the hu hfc line the manifestation of the field controlled offset of forc distribution strongly depends on the direction of the field hfc the effect is best expressed when a forc diagram is measured parallel or antiparallel to the cooling field direction for hfc 100 mt the upper peak disappears and the lt forc distribution becomes single peaked and symmetrical with respect to the hu 0 axis in contrast no correlation between hfc and features of forc distributions is observed for sd and md magnetite these experimental results provide further support to a phenomenological model which suggests that magnetic hysteresis behavior of monoclinic magnetite is controlled by an intrinsic interplay between the magnetic and crystallographic twin domain structures

the hysteresis properties of multidomain magnetite and titanomagnetite titanomaghemite in mid ocean ridge basalts abstract hysteresis measurements have been carried out on a suite of ocean floor basalts with ages ranging from quaternary to cretaceous approximately linear yet separate relationships between coercivity bc and the ratio of saturation remanence saturation magnetization mrs ms are observed for massive doleritic basalts with low ti magnetite and for pillow basalts with multi domain titanomagnetites with x 0 6 even when the morb has undergone low temperature oxidation resulting in titanomaghemite the parameters are still distinguishable although offset from the trend for unoxidized multidomain titanomagnetite the parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types this plot provides a previously underutilized and non destructive method to detect the presence of low titanium magnetite in igneous rocks notably morb

organic walled microfossils from the chert phosphorite member tal formation precambrian cambrian boundary india the chert member also designated chert phosphorite member of the tal formation in the lesser himalaya contains well preserved organic walled microfossil assemblage the assemblage includes small 25 m acanthomorphic acritarchs sphaeromorphic acritarchs cyanobacterial filaments sponge spicules and thallophytic remains identified as micrhystridium lanceolatum m dissimilare m tornatum m lanatum cf lophosphaeridium sp paracymatiosphaera irregularis cymatiosphaera sp estiastra minima leiosphaeridia sp siphonophycus robustum thalloidal fragments and hexactinellid sponge spicules the present assemblage rich in small acanthomorphs appears to be best developed in upper nemakit daldynian beds based on acritarch biostratigraphy the present work suggests a nemakit daldynian to early tommotian age for the chert member which is in agreement with the formerly proposed age based mainly on small shelly fossils the assemblage is correlatable to the precambrian cambrian boundary microbiotic assemblage from china central asia and east european platform the assemblage shows low diversity of species the characteristic of inshore biota

pressure solution deformation of the purgatory conglomerate from rhode island pressure solution is the dominant mechanism for the alleghenian age deformation of pennsylvanian quartzite conglomerates from the narragansett basin rhode island this deformation is accompanied by inter cobble rotation and occurred at low metamorphic grades no microscopic evidence for internal penetrative deformation of cobbles in the conglomerate is observed three major pressure solution features are characteristic of this conglomerate adjacent cobbles show indentation relationships with no change in quartz fabrics within the individual cobbles insoluble material is concentrated between cobbles in mutual contact both in the matrix and within the outer margins of the cobbles quartz overgrowths are found at the long axis terminations of the cobbles many other pressure solution features are discussed these features remain recognizable after subsequent metamorphism review of other studies suggests that pressure solution may well be a common conglomerate deformation mechanism during early deformati

holocene paleointensities thellier experiments on submarine basaltic glass from the east pacific rise a complete description of the geomagnetic field requires both paleodirectional and paleointensity data although the paleointensity data base has grown steadily over the last three decades it remains limited in time and space the majority of data are of holocene age and come from europe furthermore it has been difficult to assess the reliability of the paleointensity determinations here we present paleointensity determinations on precisely dated holocene 0 to 3500 years old submarine basaltic glass from the east pacific rise 15 s to 22 s although hysteresis measurements and low temperature isothermal remanent magnetization irm acquisition experiments document a significant contribution of superparamagnetic grains high blocking temperatures above 400 c and curie temperatures between 490 c and 550 c indicate a single domain low ti magnetite as the carrier of the remanent magnetization this notion is further supported by the fact that saturation of remanence is achieved in moderate fields of about 200 300 mt submarine basaltic glass proves to be nearly ideal for paleointensity determinations in that it produces a high success rate for thellier experiments twenty six out of 30 samples resulted in acceptable paleointenisty determinations multiple experiments on splits from the same sample show good reproducibility the paleointensities for zero age glasses correspond precisely with the present field intensity at the site of recovery the results of the remaining samples range from 16 7 to 53 9 t with corresponding virtual axial dipole moments vadm of 3 61 1022 to 11 9 1022 a m2 the intensities vary rapidly with time excluding a westward drifting nondipole component as the source for these fluctuations basaltic glass is frequently recovered in both dredged and drilled material from the ocean floor the availability of submarine basaltic glass throughout the world oceans therefore holds great potential for a better distribution of paleointensity data through time and space