comment on magnetic studies of magma supply and sea floor metamorphism troodos ophiolite dikes by g j borradaile and d gauthier anisotropy of magnetic susceptibility is used as a proxy for the determination of magmatic flow direction in mafic dykes here we take advantage of the dataset from g j borradaile and d gauthier to comment three points 1 the sampling strategy 2 the geometric relationship between magnetic axis dyke and 3 an alternative interpretation to obtain a flow direction the magnetic lineations published by borradaile and gauthier correspond to the zone axis of the dyke and magnetic foliation poles questioning the reliability of the magnetic lineation as a flow estimate an alternative interpretation is based on the use of the tiling of the magnetic foliation plane against the dyke wall

flow directions in dikes from anisotropy of magnetic susceptibility data the bootstrap way one of the first applications of anisotropy of magnetic susceptibility ams was an attempt to determine flow directions from mafic dikes khan 1962 since the seminal work of knight and walker 1988 defining the expected behavior of ams in response to magma flow there has been increasing interest in using ams for this purpose here we present a quantitative method for interpretation of ams data from dikes using a parametric bootstrap first dikes must be sampled with at least five and preferrably more samples from within 10 cm of the dike margin the distributions of the eigenvalues and eigenvectors of the ams tensor are delineated by calculating eigenparameters of many bootstrapped paradata sets we generate paradata sets by first selecting a sample at random then calculating a replacement set of data by drawing tensor elements from normal distributions with the mean and standard deviation of the entire site the bounds containing 95 of the eigenparameters of the bootstrapped data serve as confidence limits for the parameter of interest classification of dikes proceeds as follows sites whose maximum and intermediate eigenvalues could not be distinguished are deemed uninterpretable in addition sites with principal eigenvectors with angles 45 away from the dike margin inverse or with markedly different directions on either side of the dike scissored are excluded the remaining dikes are classified as having unique flow direction information if the principal eigenvectors from at least one side are distinct from the dike plane based on the distribution of the bootstrapped principal eigenvectors if neither side has principal eigenvectors distinct from the dike plane the dikes are classified as having lineation information only a study comprising 251 dikes from the troodos ophiolite has 151 sites with directional data 38 sites with lineations only 7 inverse sites 5 scissored sites and 55 sites not fitting into any other category the flow directions interpreted from the data were generally southerly toward a fossil transform zone

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

comment on unresolved issues with the assessment of multidecadal global land surface temperature trends by roger a pielke sr et al 2 according to pielke et al 2007 the similarity of the different global surface temperature trends is unsurprising in view of an estimated overlap of 90 to 95 in the raw data regional temperature trends are considered to be less robust in data sparse regions such as parts of africa and the arctic and antarctic 3 effectively independent analyses of land surface global temperature trends have already been carried out peterson et al s 1999 subset of rural stations showed very similar trends to those derived from the full ghcn data set using a worldwide network of about 270 stations parker 2006 obtained very similar trends to those produced by jones and moberg 2003 from their full network the sensitivity of estimated land surface global temperature trends to analysis technique was estimated by vose et al 2005 this paper showed that the smith and reynolds 2005 and the jones and moberg 2003 analyses yield comparable trends but the hansen et al 2001 and k m lugina et al monthly surface air temperature time series area averaged over the 30 degree latitudinal belts of the globe 1881 2005 in trends a compendium of data on global change 2006 an online report available at http cdiac ornl gov trends temp lugina lugina html analyses yield reduced trends in recent decades because of the greater oceanic influence on the former and the damping influence of optimal interpolation on the latter trenberth et al 2007 the generally good agreement between the global trends of land surface global temperature and the independent sea surface temperature and marine air temperature is also illustrated clearly by trenberth et al 2007 here we conduct an extra experiment the crutem3 land surface air temperatures jones and moberg 2003 brohan et al 2006 have been subsampled by taking in each month alternate 5 5 areas in alternate 5 latitude belts figure 1 shows annual global figure 1a and hemispheric figures 1b and 1c average land surface air temperature anomalies for 1850 2007 based on the full database with uncertainty estimates as in work by brohan et al 2006 along with series based on the two sets of locations in figure 1d the subsampled series despite being based on essentially independent data lie well within the uncertainty estimates since 1900 and are usually within the uncertainty estimates even in the sparsely sampled 19th century confirming the robustness of all the series not only do these analyses use completely different stations but any adjustments applied are also independent owing to the distances between the stations that contributed to the two sets of 5 5 areas in essence the independence of the station data in the subsampled 5 5 areas cannot be compromised by the homogeneity adjustments reported in earlier papers see sources given by jones et al 1986a 1986b or through the inclusion of many additional adjusted data sets by jones and moberg 2003 this is because adjustments were made to only a limited number of stations 20 and almost all of these adjustments will have been based on stations within the same or surrounding 5 5 areas not on remoter stations furthermore the sum total of the adjustments has a near zero effect on large scale temperature averages see brohan et al 2006 figure 4 4 the spatial coherence of mapped temperature trends transcending international borders as illustrated for example in figures 3 9 and 3 10 of trenberth et al 2007 suggests strongly that recent global and regional trends are not severely affected by national biases or incomplete coverage the latter inference can also be drawn from the relatively small numbers of spatial degrees of freedom estimated by jones et al 1997 brohan et al 2006 estimated global and hemispheric incomplete coverage errors by subsampling complete reanalysis fields since the mid 20th century the 95 confidence range for decadally smoothed global land surface air temperature anomalies has been about 0 25 c and this is mainly due to incomplete coverage error brohan et al 2006 figure 12 however the robustness of estimated trends for earlier periods and smaller regions is journal of geophysical research vol 114 d05104 doi 10 1029 2008jd010450 2009 click here for full article

magma flow directions in dikes of the koolau complex oahu determined from magnetic fabric studies we compared the magnetic fabric with the macroscopic surface lineation direction previously assumed to be parallel with magma flow direction for 35 dikes of the koolau complex oahu anisotropy of magnetic susceptibility ams was determined for 530 samples from 59 dikes including a sill and a pluglike intrusive body and two filled lava tubes and statistically significant ams clusters were found in all but five dikes for the 35 lineated dikes the mean maximum ams direction 1 coincides with the macroscopic lineations to within 25 in 18 and to within 15 in eight the ams ellipsoid shape in about half of the samples is prolate cigar shaped and in about half is oblate pancake shaped we interpret the ams to be determined by shape anisotropy of ferromagnetic grains the mean total anisotropy is 2 2 and total anisotropy h ranges from 0 24 to as high as 14 12 the principal ams axes for samples from a dike are tightly grouped on equal area plots and for most of the dikes the minimum ams axes plot near to the pole of the dike our ams measurements enable us to infer absolute magma flow directions for half the dikes studied the maximum ams axes of samples from one side of the dike form a tight cluster on an equal area plot and those from the other side of the dike form another tight cluster the two clusters are close together separated by 10 30 for narrow dikes 100 cm wide and are farther apart 30 60 for wide dikes and are symmetrically disposed on either side of the plane of the dike we infer that these paired clusters are caused by imbrication of nonequant ferromagnetic crystals against each margin of the dike during deposition in a velocity gradient and hence indicate the flow azimuth i e the absolute magma flow direction nearly half of the dikes measured 25 gave paired ams plots in 12 the magma flowed up at low oblique angles 5 30 from the horizontal in five it flowed down at 5 20 in four it flowed horizontally 5 in three it flowed up at a steep angle 50 75 and in one it flowed down at a steep angle the magma flow in general was rarely vertical or horizontal

fabric analysis using the intercept method abstract the intercept method is a convenient yet powerful method for the numerical analysis of fabrics for geological applications it can be used when a set of objects e g pores grains or aggregates of a specific mineral lineaments etc with unique characteristics can be identified in an image the intercept method analyses boundaries of objects as a population of lines how intercepts are definined is therefore important in order to establish the significance of the intercept counts when the method is applied to digital images the subdivision of the information into square pixels must also be considered we apply a linear filter which minimizes the effects of grid anisotropy on the counting of intercepts analysis of the resulting rose of intercept counts by a fourier series considerably facilitates the interpretation of the data a rose of directions can readily be derived for the phase boundaries if a population of objects can be assumed to have started with an originally isotropic orientation and to have been deformed passively the fourier components of the rose of intercepts also give the sectional strain the fourier components also permit an objective test of the validity of describing a fabric as resulting from a homogeneous strain on an initially isotropic section two computer programs for the counting and analysis of intercepts on digital images are presented and applied in four examples which illustrate the usefulness of the intercept method for magmatic fabric measurement strain analysis and lineament analysis we also illustrate the importance of fourier series decomposition of rose of intercept counts for fabric interpretations intercept results are also shown to be compatible with those derived by other methods eigenvector surfor inertia tensor autocorrelation methods

crustal structure and magmatism of north atlantic continental margins enormous variations exist along rifted margins of the north atlantic in the magmatic activity that accompanied continental breakup the north atlantic margins exhibit a full range of magmatic behaviour one end of the spectrum commonly termed volcanic margins is found in the northern north atlantic off greenland norway and northwest britain where huge volumes of igneous rock were added to the continental crust as it rifted and the adjacent oceanic crust is considerably thicker than normal the other end of the spectrum so called non volcanic margins occurs further south off france and spain where only minor volcanism accompanied rifting and the oceanic crust immediately adjacent to the continental margin is thinner than normal oceanic crust results from seismic profiles are used to determine the volume and distribution of igneous crust accumulated as the continents broke up and seafloor spreading commenced the volume of melt produced during rifting is controlled mainly by the temperature of the underlying asthenospheric mantle and by the amount and rate of decompression as it rises beneath stretching and thinning lithosphere large quantities of igneous rock were produced as the continents broke up to form the north atlantic where the newly initiated iceland plume brought abnormally hot mantle beneath the rift much smaller quantities of melt were produced further south where mantle temperatures were normal during rifting the distribution of igneous rocks across the continental margin and the adjacent oceanic crust is controlled by the rate at which melt is produced and by the ease with which it can intrude laterally in the crust where the rate of stretching is very slow and the rifting is spread over many millions or even tens of millions of years as is common during the initial stages of rifting then the rising asthenospheric mantle has time to cool by conduction heat is lost from the asthenospheric mantle both vertically into the overlying water layer and laterally into the colder wedges of continental lithosphere on either side of the rifted region as a consequence of the reduced mantle temperatures considerably less partial melting occurs in the slowly rising asthenospheric mantle than in mantle which rises rapidly beneath rifts this explains the abnormally thin oceanic crust found adjacent to the non volcanic rifted continental margins in the north atlantic as the new ocean widens the spreading centre becomes isolated from the continental margins on either side by newly formed warm oceanic lithosphere and the rate of rifting generally increases the oceanic crust then attains its equilibrium thickness as conductive heat loss from the rising asthenospheric mantle reduces and all the melt formed by decompression of the mantle solidifies close to the spreading axis basaltic melts can intrude continental crust far more easily than oceanic crust basaltic dykes may extend many hundreds of kilometres away from the magma source beneath continental rifts this redistributes melt laterally towards the continent in both volcanic and to a lesser extent in non volcanic margins and may be a contributory factor in leaving unusually thin crust in the first formed ocean immediately adjacent to some rifted margins it also allows melt formed under volcanic rifts to migrate large distances laterally as is seen in the rockall area northwest of britain

seaward dipping reflectors and the continent ocean boundary at passive continental margins abstract the definition of the continent ocean boundary at passive continental margins has proved to be an elusive task even the relatively direct method of seismic refraction experiments has yielded results that cannot always be interpreted unequivocally multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors dipping seaward to form a wedge shaped structure their general characteristics and velocity structure suggest that they may be largely volcanic in nature but there is no agreed upon model for their origin nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean and as such they are extremely important structural markers that may yield important evidence concerning the structure and evolution of passive margins

dike surface lineations as magma flow indicators within the sheeted dike complex of the troodos ophiolite cyprus mesoscopic flow lineations and anisotropy of magnetic susceptibility ams have been measured for dikes within the cretaceous age troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge and groove structures termed hot slickenlines found throughout the complex a unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back flow of magma during the waning stages of intrusion surface lineations generally lie subparallel to the direction of flow inferred from ams determinations on cores within 5 cm of dike margins surface lineations also lie subparallel to the long axis e1 of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from ams cores correlation of surface lineations with interior indicators of flow ams plagioclase trachytic texture indicate that the surface features are good proxies for grain scale magma flow directions during dike propagation in troodos dikes orientations of surface flow features in the dikes of the troodos ophiolite indicate an approximately equal mix of subhorizontal to near vertical magma flow contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest

magma flow directions of shallow dykes from the east greenland volcanic margin inferred from magnetic fabric studies abstract the role played by plume generated crustal magmatic complexes in the segmentation of volcanic margins is highlighted by a preliminary study of magma flow directions in shallow intrusives from the east greenland volcanic margin we investigate the magmatic texture using anisotropy of magnetic susceptibility for eight dykes of tholeitic affinity belonging to a dyke swarm associated with the tertiary opening of the north atlantic ocean the thickness of the sampled dykes ranges from 3 to 37 m the dykes are of doleritic texture and contain up to 12 of opaque minerals and 35 of plagioclase laths dykes showing a magnetic foliation plane within the dyke plane i e a magnetic fabric usually attributed to magmatic processes represent 40 of the samples from the studied swarm these dykes have a low degree of anisotropy and their ellipsoid of magnetic susceptibility is strongly oblate inverse magnetic fabrics where the maximum principal susceptibility axis lies near the pole to the dyke account for 60 of the data we interpret these inverse fabrics either as alteration minerals with highly prolate ellipsoids of magnetic susceptibility or primary titanomagnetites with oblate ellipsoid of low anisotropy the flow direction is inferred for normal fabric dykes using the mirror imbrication of magnetic lineation analysis of thin sections shows a good agreement between magnetic fabric directions and phenocryst preferred orientations the inferred flow directions are predominantly horizontal throwing a new light on volcanic margin development

shallow intrusive directions of sheeted dikes in the troodos ophiolite anisotropy of magnetic susceptibility and structural data sheeted dikes play a central role in the formation of oceanic crust it is commonly assumed that sheeted dikes intrude vertically upward from elongated mid ocean ridge mor magma chambers but there are no direct observational data bearing on this hypothesis this assumption contrasts with the intrusive behavior of subaerial volcanoes where magmas rise into shallow central magma chambers that laterally feed vertically oriented dikes the authors have studied intrusive directions of sheeted dikes in a structural analogue to oceanic crust the troodos ophiolite structural and magnetic fabric data of 65 dikes provide consistent results and suggest a broad distribution of shallow 20 degree to nearly vertical upward magma transport directions these data suggest that horizontal emplacement has to be considered for sheeted dikes at mors implying more centralized mor plumbing systems than previously thought such plumbing systems provide ample opportunity for complex mixing fractionation and contamination of mor lavas in magma chambers and tabular magma storage volumes whether the mor magma supply is linear or centralized also has a fundamental effect on crustal accretion processes and the geometry of hydrothermal convection systems

southeast baffin volcanic margin and the north american greenland plate separation plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at moho level and extruded as thick piles of seaward dipping lava formations the disko svartenhuk area in west greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward we present a quantitative tectonic study of this margin which we tentatively restored in its geodynamic position during the different stages of plate separation between greenland and north america our structural data are constrained with recently published 39ar 40ar and new and coherent 40k 40ar geochronology in dikes of different orientations the first order structure is that of a tectonically driven seaward crustal flexure linked to definitive plate breakup between greenland and baffin island during the eocene coeval with the formation of the upper part of the exposed seaward dipping volcanic prism this flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults this across strike structure is correlated to a fundamental along strike segmentation with the three segments adopting a zigzag strike there is a clear increase of extensional strain at the extremities of the segments eocene extension trended n060 on average in the northern and southern segments but was nw trending in the central ne trending nugssuaq segment we discuss the interpretation of such an extension perpendicular to the different margin segments from a regional point of view the n060 extension is distinct in orientation from the approximately n s trend of plate separation between north america and greenland in the baffin bay during the eocene however the extension recorded in the margin is nearly perpendicular to the obliquely spreading eocene accretion axis in the baffin bay this latter point suggests a mantle or ridge control of the development of the margin over regional far field lithospheric stress models

magmatic tectonic evolution of a volcanic rifted margin abstract many north atlantic continental margins are underlain by huge volcanic edifices near the continent ocean boundary a deep crustal hole drilled on the voring plateau odp leg 104 has provided important constraints on the initial breakup history and the subsequent margin evolution by penetrating more than 900 m of igneous rocks and interbedded sediments below a post early eocene cover the rocks constitute two distinct different volcanic series the upper series comprising a seaward dipping reflector wedge consists of transitional mid oceanic tholeiitic lava flows and thin volcaniclastic sediments dacitic flows some dikes and thicker interbedded sediments constitute the lower series the margin evolved by paleocene crustal extension accompanied by uplift and pervasive intrusion of the rift zone just prior to breakup magma from shallow crustal melts produced the lower series the upper series was constructed during an intense rapidly waning subaerial volcanic surge following breakup near the paleocene eocene transition it covers both the new oceanic crust and large areas of the continental crust the dipping wedge was formed by subsidence due to loading and thermal contraction possibly amplified by tectonism when the surge abated the injection center rapidly subsided and normal oceanic crust was formed there is a direct temporal and compositional relationship between the onshore north atlantic volcanic province and the margin volcanism compared to the transverse part of the province including the iceland hotspot which has been active over 60 m y the volcanic margins reflect a 2500 km long transient segment related to continental breakup lasting only about 3 m y the volcanic surge and uplift at breakup are related to a modest regional increased temperature at the base of the lithosphere this mode of margin formation has had important paleoenvironmental implications

north atlantic volcanic margins dimensions and production rates early tertiary lithospheric breakup between eurasia and greenland was accompanied by a transient 3 m y igneous event emplacing both the onshore flood basalts of the north atlantic volcanic province navp and huge extrusive complexes along the continent ocean transition on the rifted continental margins seismic data show that volcanic margins extend 2600 km along the early eocene plate boundary in places underlain by high velocity 7 2 7 7 km s lower crustal bodies quantitative calculations of navp dimensions considered minimum estimates reveal an areal extent of 1 3 106 km2 and a volume of flood basalts of 1 8 106 km3 yielding a mean eruption rate of 0 6 km3 yr or 2 4 km3 yr if two thirds of the basalts were emplaced within 0 5 m y the total crustal volume is 6 6 106 km3 resulting in a mean crustal accretion rate of 2 2 km3 yr thus navp ranks among the world s larger igneous provinces if the volcanic margins are considered the velocity structure of the expanded crust seaward of the continent ocean boundary differs from standard oceanic and continental crustal models based on seismic velocities this volcanic margin crust can be divided into three units of which the upper unit corresponds to basaltic extrusives the regionally consistent velocity structure and geometry of the crustal units suggest that the expanded crust including the high velocity lower crust which extends some distance landward of the continent ocean boundary was emplaced during and subsequent to breakup the volcanic margin crust was formed by excess melting within a wide zone of asthenospheric upwelling probably reflecting the interaction of a mantle plume and a lithosphere already extending

determination of flow directions by combining ams and thin section analyses implications for oligocene volcanism in the kerguelen archipelago southern indian ocean we report an anisotropy of magnetic susceptibility ams study carried out on seven oligocene volcanic sections widely distributed on the surface of the kerguelen archipelago the statistical results at flow scale and at section scale were checked with ams density diagrams and thin section analysis because of the axis permutations frequently observed for lava flows two possible flow directions were nicely defined for the majority of the sections we then used two alternative approaches imbrication of the minimum axis and symmetry of the ams distribution to infer a flow direction for each section these two methods and thin section analysis not being decisive for choosing between the two solutions geological and geomorphological observations were finally used to obtain confident interpretations the results of this study enable the supposed eruptive centres of the kerguelen archipelago to be located more precisely

is this magnetic fabric normal a review and case studies in volcanic formations abstract when studying weakly anisotropic rocks non standard relationships between magnetic and expected petrofabric axes are sometimes observed in particular the so called inverse or intermediate fabrics a review of possible explanations for these effects is presented and leads to the conclusion that a majority of abnormal magnetic fabrics often encountered in magnetite rich volcanic rocks are due to abnormal preferred orientations or distributions of magnetite grains through case studies in dikes from skye island yemen and the iberian peninsula and magma and ash flows from the ethiopian traps and southern corsica the various possible origins of such fabrics are examined it is possible to retrieve reliable magma flow directions from these sites provided proper data selection is performed leading to non unique interpretations

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

rock magnetic and magnetic fabric variations across three thin alkaline dykes from sunnhordland western norway influence of initial mineralogy and secondary chemical alterations summary profiles of natural remanent magnetization nrm rock magnetic properties and anisotropy of magnetic susceptibility ams across three thin 26 to 109 cm alkaline dykes from sunnhordland western norway are presented the magnetic remanence is carried by pseudo single domain psd lowtemperature oxidized titanomagnetite with circa 6 per cent of cations substituted by aluminium systematic variations in magnetic properties across the dykes are caused by differences in titanomagnetite concentration as well as grain size lowtemperature alteration and laboratory oxidation by heating in air to a maximum temperature of 600 c do not homogenize the magnetic properties across the dykes fine grained regions near dyke margins carry the most stable remanent magnetization but are also most amenable to secondary chemical remanent magnetization crm resetting between dyke variations in magnetic properties are larger than within dyke variations a stable primary remanence is found in two of the dykes while the third is totally reset by a soft viscous remanent magnetization vrm despite similar optical grain sizes magnetic mineralogy and oxidation state the difference is attributed to a lower titanium content in the latter dyke preventing effective partitioning of magnetic grains during low temperature alteration laboratory heating affects anisotropy variations across the dykes resulting in more consistent axis orientations within each dyke and symmetric edge to edge variations in the degree of anisotropy ams directions are different from those in the country rock and vary systematically across the thinnest dyke before heating

magnetic and shape fabrics of magnetite in simple shear flows abstract the magnetite fabrics measured by anisotropy of magnetic susceptibility ams and by shape preferred orientation spo optical methods are classically used as flow kinematics indicators in lava flows the development of magnetite fabrics during simple shear strains 20 was performed using a suspension of 1 volume fraction of multidomain magnetite randomly contained in a mixture of silicone and wax we measured ams fabric and spo ellipsoids by calculating a quadratic shape tensor from oriented thin sections for jeffery 1922 fabrics are usable to determine the flow kinematics including the amount of applied finite strain for 8 fabric elements foliation and lineation are stabilised closely parallel to the flow plane and the shear direction respectively two and three dimensional numerical simulations using measured aspect ratios of magnetite point out that the large scattering of aspect ratios and the initial orientation distribution of particles are together responsible for a wide ranging loss of periodicity the stable ams and spo fabrics observed at large strains in experiments are the result of these primary fabric properties combined to collisions between particles and possibly their complex three dimensional shapes in addition the constant angular relationship observed at large strains between fabrics and flow components is related to the transient collisions consequently the determination of the lava flow kinematics by using fabric properties measured either by ams or by spo analyses should be indubitably associated to a detailed study of the three dimensional shape of the solid carriers regularly shaped populations of low elongated particles will be capable to produce cyclic to oscillating fabrics while the fabric of elongated particles will be more sensitive to the shape parameters and collisions ultimately favouring stable fabrics at large strains

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

multi scale stratigraphic forward modelling of the surat basin for geological storage of co2 underground geological storage of co2 gsc requires a high level of subsurface understanding that is often hindered by a lack of data this study demonstrates the use of stratigraphic forward modelling sfm in generating and characterising a static reservoir model using limited well data with multiple potential applications within the gsc workflow sedsim sfm software was used to create a static model of the surat basin including a high resolution nested model of the epq 7 gsc tenement within the basin deposition and burial of the jurassic precipice sandstone evergreen formation and hutton sandstone were simulated modelling results show a close match with gamma ray well logs in the tenement area and the model can be considered a credible model of the subsurface the sedsim predicted formation thicknesses and porosity and permeability distributions meet criteria set for gsc suggesting that the epq 7 tenement may be a prospective gsc location

40ar39ar geochronology of tertiary mafic intrusions along the east greenland rifted margin relation to flood basalts and the iceland hotspot track abstract the east greeland tertiary igneous province includes the largest exposed continental flood basalt sequence within the north atlantic borderlands more than ten layered gabbro complexes including the 55 ma skaergaard intrusion and a large dolerite sill complex are the plutonic equivalents of flood basalts both lavas and intrusions have been regarded as synchronous with continental breakup at 57 54 ma we report ten new ages of the mafic intrusions determined by40ar 39ar incremental heating experiments demonstrating that the mafic intrusions formed in two distinct time windows only intrusion ii of the imilik gabbro complex the skaergaard intrusion and the sorgenfri glestcher sill complex formed at 57 55 ma coeval with the eruption of regional flood basalts and continental breakup other layered gabbro intrusions at imilik intrusion iii kruuse fjord igtutarajik nordre aputite q kap edvard holm and lilloise are distinctly younger and formed between 50 and 47 ma plate kinematic models indicate the axis of the ancestral iceland mantle plume was located under central greenland at 60 ma and subsequently crossed the east greenland rifted continental margin we propose that tholeiitic magmatism along the east greenland rifted margin largely occurred in three distinct pulses at 62 59 ma lavas and dykes 57 54 ma lavas dykes sills and some gabbros and 50 47 ma gabbros dykes and rare lavas related to discrete mantle melting episodes triggered by plume impact continental breakup and passage of the plume axis respectively this model implies northwestward continental drift of greenland relative to the plume axis by 3 9 5 0 cm yr between 60 and 49 ma consistent with estimates from seismic studies of submerged flood basalts

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

evidence from magnetic fabric for the flow pattern of magma in the mackenzie giant radiating dyke swarm continental flood basalts and their associated swarms of mafic dykes are thought to result from melting in mantle plumes1 3 but the processes by which the resulting magma makes its way from mantle to crust remain obscure here we use magnetic anisotropy measurements to reconstruct the pattern of magma flow in the proterozoic mackenzie dyke swarm our results indicate that magma was injected vertically within 500 km of the focal point of the dyke swarm and then travelled horizontally at all further distances out to at least 2 100 km the sharp transition from vertical to horizontal flow at between 500 and 600 km may correspond to the outer boundary of melt generation in the mantle plume believed to be responsible for the mackenzie igneous events

the e greenland continental margin a transition between oceanic and continental magmatism the e greenland continental margin differs from typical margins in having a superimposed regional uplift which has exposed the early rift products for direct examination most of the igneous rocks in this region find counterparts in the oceanic setting with the proviso that the area is part of an aseismic ridge thus picrites basalts sheeted dyke swarms and layered gabbros are present and these can be used to shed light on oceanic magmatic events nephelinitic rocks are products of off axis volcanism and find no counterparts in the present oceanic rift system although they occur in marginal regions of the ocean basins and were generated in distal regions at the initiation of rifting in icelandic anomaly voluminous salic rocks in the area owe their generation largely to continental crust which has undergone extensive melting they find no direct counterparts in oceanic areas the outstanding degree of exposure in this area combined with its transitional nature and wide spectrum of rock types make it a unique laboratory for the elucidation of a variety of problems regarding both oceanic and continental volcanism together with the nature of rifted continental margins in general

magmatism at rifted continental margins the intense volcanism and uplift observed on many rifted continental margins forming basaltic seaward dipping reflector sequences is accompanied by the emplacement of a thick igneous section at depth partial melting by decompression of passively upwelling asthenosphere that is hotter than normal because it is near a hotspot explains both the thickness of the new igneous crust and the initial elevation of the rifted margins

magmatic flow direction indicators in a giant diabase feeder dike connecticut recent studies have suggested that many large diabase dikes have been emplaced laterally rather than vertically these studies report only unidirectional flow which is usually based on measurements of anisotropy of magnetic susceptibility however in the 250 km long higganum dike of southern new england united states seven independent flow direction indicators record a complex history of forward and backward moving magma we would expect other similar size dikes to have had equally complex histories measurements within one en echelon segment of the higganum dike show that as the magma rose it spread laterally toward the ends of the segment and that backflow was in the reverse direction of the forward flow the higganum dike is interpreted to have formed from a linear array of intrusive fingers each finger spreading laterally to form an en echelon dike segment as it rose through the lithosphere

magnetic fabric and its significance in the 1400 ma mealy diabase dykes of labrador canada an anistropy of magnetic susceptibility ams study of the relatively fresh mealy diabase dyke swarm indicates a correlation between the ams and the deformational effects in the rocks several ams components were revealed during experimental work on 211 samples that had been heated to 640 c during a paleomagnetic study and 22 fresh samples progressively treated in alternating fields seventeen sites with slight or no deformation have a magnetic fabric composed of two major components the stable component attributed to a primary shape anisotropy has minimum and maximum principal susceptibility directions preferentially oriented vertically and along dyke strike n 60 e respectively this configuration is consistent with an origin due to a post emplacement stress field whereby horizontal extension acted on a vertical sheet of partially crystallized magma the orientation of this component with respect to the present horizontal suggests that the dykes have not been tilted since their emplacement the unstable component removed by fields up to 250 mt is attributed to induced domain anisotropy produced by mild tectonic stress during the grenville orogeny the approximate orientation of its principal components as found by the method of substitutional ellipsoids suggests an east west directed maximum stress four other sites with noticeable deformation have magnetic fabric that could not be reliably analyzed but which may be largely composed of domain and stress induced anisotropies developed during the grenville orogeny

magma flow in the east greenland dyke swarm inferred from study of anisotropy of magnetic susceptibility magmatic growth of a volcanic margin summary volcanic passive margins vpms are characterized by large volumes of melt emplaced within the lithosphere during break up processes several data and a recently proposed conceptual model of volcanic margin development suggest that vpms are fed from localized crustal zones of magma storage underlying large polygenetic volcanoes localized above diapir like instabilities of the asthenosphere we investigated the magma flow pattern within the coastparallel dyke swarm of the east greenland vpm which is the only outcropping vpm over a distance of 125 km the 44 sampled dykes are representative of the successive families of intrusions igneous petrofabrics are constrained by the measurements of the anisotropy of magnetic susceptibility the magnetic fabrics are of medium to low anisotropy p 0 flow related fabrics are recorded in 75 per cent of the sampled dykes we infer the flow directions from the imbrication geometry of the magnetic foliation planes at the dyke margins and check the results by measuring the preferred orientation of plagioclase in thin sections cut in the magnetic principal planes due to probable fabric superposition the magnetic lineation represents the zone axis for the distribution of magnetic foliation plane we obtained 23 reliable flow directions that are predominantly horizontal and directed away from identified crustal reservoirs this flow pattern supports the proposed model of vpm growth and emphasizes the localized nature of the magma sources in the mantle the entire flood basalt sequence appears to have been fed by a restricted number of crustal reservoirs and associated dyke swarms