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

distinguishing diapirs from inflated plutons an integrated rock magnetic fabric and structural study on the roundstone pluton western ireland granitoid plutons account for much of the continental crust and are critical in the generation of several economic resources despite over a century of research the fundamental process by which large felsic bodies intrude remains controversial two contrasting models persist 1 mass ascent and subsequent cooling of magma diapirism 2 sheet ascent and subsequent emplacement laccoliths the latter hypothesis is different from the former as distinct ascent and emplacement processes must occur and be identifiable traditional field methods rarely allow this important distinction to be made we present field petrographic and anisotropy of magnetic susceptibility data from the roundstone pluton connemara western ireland which was previously considered a diapiric intrusion the new data reveal anomalous subvertical magmatic lineations in the core of this circular intrusion as well as a suite of magmatic and submagmatic fabrics that parallel nnw sse country rock faults and coeval g2 sheets within the pluton we deduce that these structures reflect a distinct centralized ascent process along a nnw sse conduit lateral emplacement and pluton inflation followed ultimately forming a punched laccolith we conclude that the combined application of the above techniques provides a unique insight into pluton architecture that removes ambiguity between contrasting diapiric and inflated pluton models supplementary material complete rock magnetic data tables and sample site coordinates irish grid are available at http www geolsoc org uk sup18837

characterization of the magnetic fabric of rocks abstract an inextensive system of magnetic susceptibility anisotropy factors is suggested which is sufficient in the majority of practical applications for characterizing the magnetic fabric of rocks the system involves three newly devised factors the corrected anisotropy degree the shape factor and the difference shape factor the first of these characterizes the quantity of anisotropy somewhat better than the commonly used anisotropy degree the shape factor characterizes the quality of anisotropy i e the shape of the susceptibility ellipsoid in a convenient way its definition is based on a certain analogy between the anisotropy ellipsoid and the strain ellipsoid the difference shape factor can replace the shape factor when only the differences between the principal susceptibilities are given while the mean susceptibility is unknown

magnetic versus crystallographic fabrics in a basaltic lava flow combined anisotropy of magnetic susceptibility ams and crystallographic studies were performed within a quaternary lava flow for which we have a thorough knowledge of the flow direction microscope examinations microprobe analyses and studies of rock magnetic properties show that ams is carried by homogeneous multidomain ti rich titanomagnetite xc0 6 this mineral forming either subhedral isolate individuals or grouped in various elongated aggregates represents a late crystallized interstitial phase ams determined from samples collected in the lower the middle and the upper part of the flow is characterized by a well defined magnetic foliation whereas magnetic lineation is generally poorly grouped and therefore meaningless the foliation plane of the lower and upper part of the flow show an obliquity which is interpreted as an opposite imbrication indicating a westward flow in agreements with the known flow direction in the lower part of the lava flow the tight correlation between ams and plagioclase lattice preferred orientation suggests that a silicate framework control the titanomagnetite crystallization and thus the orientation of the ams ellipsoid d 2005 elsevier b v all rights reserved

theoretical considerations on the anhysteretic remanent magnetization of interacting particles with uniaxial anisotropy 1 the anhysteretic remanent magnetization arm is widely used in rock magnetism and paleomagnetism because of its sensitivity to the domain state of magnetic particles and the close analogy to natural remanent magnetizations on the other hand the arm shares with other weak field magnetizations the property of being extremely sensitive to magnetostatic interactions therefore it is desirable to model the effects of interactions on natural assemblages of magnetic particles direct micromagnetic calculations of the arm acquisition process are not practicable therefore an analytical approach is used calculate the arm susceptibility of a system of interacting single domain sd particles the model is based on a statistical description of the interaction field the equations obtained have been used to evaluate the dependence of the arm on the packing fraction of the magnetic particles the effect of interactions on the anisotropy of arm aarm was evaluated as well the aarm of densely packed particles is complex and depends critically on the microcoercivity a physical interpretation of the aarm of highly interacting particles is therefore difficult

magnetic fingerprint of southern portuguese speleothems and implications for paleomagnetism and environmental magnetism environmental magnetism of speleothems is still in its early stage of development here we report on our investigation of the environmental and paleomagnetic information that has been recorded in speleothems and what are the factors that control its preservation and reliability to address these issues we used a multidisciplinary approach including rock magnetism petrography scanning electron microscopy stable carbon and oxygen isotope compositions and major and trace element concentrations we applied this to a set of samples from different stages of speleothem evolution present day dripwater glass plates a weathered stalactite a fresh stalagmite cave sediments and terra rossa soils these samples come from the penico and excentricas caves located in two distinct aquifers of the algarve region south portugal our results show that the main magnetic carriers of the speleothems under study are primary detrital and consist of maghemite and magnetite similarities in coercivity and temperature dependence of the studied set of samples suggest that iron oxides are inherited from the terra rossa soils that cap the cave and were transported to the speleothems by dripwater hence they represent a regional environmental signature interestingly a stable and probably detrital remanent magnetization could be isolated in the fresh stalagmite whereas the weathered stalactite yielded chaotic magnetic directions and very low remanent intensities we propose that these low intensities can be the result from i different remanence acquisition mechanisms between stalagmite and stalactite and or ii iron dissolution by fungal activity we also suggest that magnetic properties and color and the content in detrital elements in the fresh speleothem inform about environmental processes acting on the interface of rock soil atmosphere while oxygen isotope composition and alkaline earth element concentrations inform about calcite water interaction processes these results provide a better understanding of how environmental information is recorded in speleothems and what the factors are that control the reliability of the paleomagnetic and paleo environmental signal

shape fabrics of particles in low concentration suspensions 2d analogue experiments and application to tiling in magma abstract jeffery s equations ascribe a theoretical cyclic nature to the shape fabric of non interacting rigid particles immersed in a viscous fluid undergoing simple shear flow this theoretical behaviour is confirmed at low shear strains 5 first experiment 134 identical particles or 1 second experiment 178 particles with two size classes then become stable at 17 first experiment and at more than 50 second experiment of the population of particles due to the contribution of the tiled particles the shape fabric becomes asymmetrical in its orientation distribution with a maximum lying above the shear plane the latter result provides a new shear sense indicator in addition to the statistical determination of the tiled features the study also suggests that crystalline fabrics in magmas could be acquired at high melt fractions i e early in the crystallization history of the magma

some aspects of magnetic viscosity in subaerial and submarine volcanic rocks summary the magnetic viscosity of 334 upper tertiary and pre bruhnes quaternary volcanic rocks from the massif central france and the steens mountain oregon and of 40 basaltic cores from dsdp leg 37 has been investigated thellier s viscosity index follows a log normal distribution with mean values equal to 6 and 3 5 per cent for subaerial and submarine rocks respectively for subaerial rocks the average intensity of the viscous remanent magnetization vrm acquired in the earth s field since the beginning of the bruhnes polarity epoch t 0 7 myr is estimated to be equal to one quarter of the average intensity of the primary remanence alternating field demagnetization of vrms acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t neel s diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard vrm is carried by single domain grains and soft vrm carried by multidomain particles single domain particles carrying hard vrm in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold resulting from high temperature oxidation of titanomagnetite soft vrm is carried by low curie point homogeneous titanomagnetite unlike his single domain theory neel s multidomain theory of magnetic viscosity does not account quantitatively for the resistance of vrm to alternating fields

magnetic properties of olivine basalt application to mars abstract the natural remanent magnetisation nrm of basalt lava containing oxidised magnetic phases is usually assumed to be proportional to the weight percent of magnetic phases it is shown here that olivine basalt has a different behaviour the nrm intensity increases at an increasing rate with the amount of magnetic phases this is attributed to the oxidation of olivine during cooling of the basalt that leads to the exsolution of magnetite in a single domain state in this way olivine basalt is found to become an order of magnitude more magnetic than basalt that does not contain olivine a simple explanation for the magnetic anomalies on mars is offered based upon these findings and mossbauer spectroscopy data from gusev crater on mars

critical analysis of the holocene palaeointensity database in central america impact on geomagnetic modelling abstract thanks to its rich archaeological heritage central america is a key region to recover the past secular variation of the geomagnetic field this article presents 13 new palaeointensity data on epiclassic 650 900 ce pottery sherds from central mexico archaeointensities were determined using the thellier thellier protocol with anisotropy and cooling rate corrections average results between 25 and 42 t reveals a fast secular variation in the second half of the first millennium ce but are not in agreement with global geomagnetic models that predict a higher geomagnetic field strength to check the reasons of this discrepancy we compiled all intensity data over the last millennia published in central america the bayesian curve calculated from 194 data covering the last 4 millennia highlights a rapid succession of oscillations of the geomagnetic field strength between 20 and 80 t but a critical analysis of the dataset shows a large influence of data quality 74 of them having a poor cooling unit consistency and experimental quality the small number of specimens per cooling unit and the anisotropy correction absent or incorrectly made increase the scatter between data whereas the absence of cooling rate correction biases the dataset towards higher palaeointensity discarding these data results in a lower secular variation by removing most extreme values and several intensity oscillations the weaknesses of the dataset are likely the main reason of the limitations of global models in central america pending the acquisition of new high quality data archaeomagnetic dating seems premature in central america

palaeomagnetic study of oligocene 24 30 ma lava flows from the kerguelen archipelago southern indian ocean directional analysis and magnetostratigraphy we report palaeomagnetic determinations in order to estimate the palaeosecular variation of the geomagnetic field for the upper oligocene period from a southern site we measured the anisotropy of magnetic susceptibility ams of basalts from five sections to check the reliability of applying tectonic correction and concluded that no stratigraphic corrections were needed a positive reversal test implies that secondary components were sufficiently removed from the characteristic remanent magnetization also the sampling was sufficiently random to accurately reflect upper oligocene palaeosecular variation calculations of the between flow angular standard deviation sb from the geographic pole show values between 21 2 and 21 8 this palaeosecular variation estimate agrees with model g for palaeosecular variation j geophys res 96 b3 1991 3923 which is axisymmetric and assumes an equatorial symmetry of the geomagnetic field the virtual geomagnetic poles vgps from the mont des ruches section seem to exhibit two polarity changes reverse normal r n first and in more detail a second normal reverse n r one finally assuming that no polarity chrons are missed by any of the sections we compare the magnetostratigraphy we defined to the global geomagnetic polarity scale of geophys j int 129 1 1997 176 using previously obtained radiometric dates to correlate the sections j petrol 39 4 1998 711 earth planet sci lett 174 2000 313 j petrol 43 7 2002 1341 the precise age constraints furnished by these correlations allow an estimation of eruption rates from 0 30 to at least 1 31 km my with wide variations from section to section and an increase of eruption rate for younger sections however due to the non uniqueness of the correlations the age limits of the sections and consequently the extrusion rate estimations can vary moreover assuming the mont des ruches section that has probably recorded all the magnetic polarity intervals the mont de la rabouillere and mont de la tourmente intermediate vgp directions may indicate unrecorded polarity chrons the eruption rates calculated can thus be overestimated 2002 elsevier science b v all rights reserved

internal strain within lava flows from analogue modelling internal strain within lava flows can be measured in the field from deformed bubbles or vesicle foliations preferred crystal orientation or ams studies these measurements have important implications for the determination of vertical velocity profiles and flow directions recent field studies show very contrasting strain patterns especially in vertical profiles an experimental study was conducted to relate variations in internal strain to different boundary conditions during flow results show that viscous flow in a straight channel along a basal slope produces three successive stretch trajectories in vertical flow planes these trajectories are related to the distribution with time of a simple shear component and a pure shear component viscous flow in a straight channel along a horizontal base reveals convex stretch trajectories in the vertical flow plane radial spreading at the mouth of a straight channel reveals concentric stretching perpendicular to the radial direction of displacement in the upper part of the flow strain components deduced from experiments may also explain fold formation during motion these experimental results provide a few end member strain patterns which may facilitate the interpretation of strain measurements in natural lava flows in terms of emplacement mechanisms

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

the internal structure of lava flows insights from ams measurements ii hawaiian pahoehoe toothpaste lava and a ā abstract we studied the anisotropy of magnetic susceptibility ams of 22 basaltic flow units including s type pahoehoe p type pahoehoe toothpaste lava and a emplaced over different slopes in two hawaiian islands systematic differences occur in several aspects of ams mean susceptibility degree of anisotropy magnetic fabric and orientation of the principal susceptibilities among the morphological types that can be related to different modes of lava emplacement ams also detects systematic changes in the rate of shear with position in a unit allowing us to infer local flow direction and some other aspects of the velocity field of each unit a flows are subject to stronger deformation than pahoehoe and also their internal parts behave more like a unit according to ams the central part of pahoehoe commonly reveals a different deformation history than the upper and lower extremes probably resulting from endogenous growth

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

paleomagnetic full vector record of four consecutive mid miocene geomagnetic reversals abstract seventy mid miocene lava flows from flood basalt piles near neskaupstadur east iceland were sampled which provide a quasi continuous record of geomagnetic field variations samples were collected along the profile b of watkins and walker watkins n walker g p l 1977 magnetostratigraphy of eastern iceland am j sci 277 513 584 which was extended about 250 m farther down in a neighboring stream bed published radiometric age determinations harrison c mcdougall i watkins n 1979 a geomagnetic field reversal time scale back to 13 0 million years before present earth planet sci lett 42 143 152 range from 12 2 to 12 8 ma for the sampled sequence four reversals were recorded in this profile with 18 transitional lavas found within or between 17 normal and 30 reversed polarity flows the large amount of transitional lavas and the large virtual geomagnetic pole dispersion for stable field directions are noteworthy as such features are commonly observed in icelandic lavas and manifest in a far sidedness of the average vgp the reason for this characteristic which could be related to an anomaly beneath iceland a global field phenomenon local tectonics and or non horizontal flow emplacement is scrutinized non horizontal flow emplacement is likely in volcanic environments particularly if the sampled lavas are located on the paleoslopes of a central volcano from the difference of the observed paleomagnetic mean directions to the expected directions assuming a geocentric axial dipole gad a paleoslope which would explain the observed difference was calculated numerically the obtained dip and dip direction point consistently to a possible volcanic extrusion center of the lavas the determined paleodip however proved to be significantly too high compared to the usual slope of a central volcano suggesting further reasons for deviations from the gad other datasets of this age from europe also show enhanced vgp dispersion suggesting further contributions of geomagnetic origin for this observation basically all reversal paths move across the pacific transitions were identified as belonging to c5an 1r c5ar 3r based on the astronomically tuned neogene timescale lourens l hilgen f j laskar j shackleton n j wilson d 2004 a geological time scale cambridge university press we selected 122 samples for paleointensity measurements using a modified thellier method including tests for alteration and multidomain bias 85 of the measured samples yielded data of sufficient quality to calculate paleointensities for 26 lava flows the average paleointensity for stable field directions was 23 3 t whereas the intensity drops to a minimum of 5 8 t during field transitions the stable field intensities represent only about half of the present day field the saw tooth pattern of intensities which is characterized by a sharp increase of intensity directly after a reversal and then followed by a gradual decrease towards the next reversal was not found in this study

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

factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments abstract the effect of magnetic interactions has been commonly neglected in the interpretation of the anisotropy of magnetic susceptibility ams of igneous rocks despite the likely occurrence of magnetic interactions due to their relatively high content of ferromagnetic minerals current models of magnetic interactions apply to highly simplified systems and are not directly applicable to natural rocks on this work a model that includes variables previously neglected is used to assess the relative importance of magnetically interactive and non interactive fractions in a rock results of the model show that a variable proportion of magnetically interactive grains the relative values of the bulk susceptibility degree of anisotropy and orientation of the anisotropies characteristic for each fraction all play an important role in controlling the rock ams experimental evidence illustrating the importance of the distribution of the ferromagnetic grains within a specimen is also reported and supports the model predictions both theory and experiments indicate that if a rock has unequant magnetic grains and clusters of magnetic grains the contribution to the ams of the rock will depend on the relative values of all the model variables and not only on the occurrence of magnetic interactions as previously presumed

effects of magnetic interactions in anisotropy of magnetic susceptibility models experiments and implications for igneous rock fabrics quantification besides granites of the ilmenite series in which the anisotropy of magnetic susceptibility ams is mainly controlled by paramagnetic minerals the ams of igneous rocks is commonly interpreted as the result of the shape preferred orientation of unequant ferromagnetic grains in a few instances the anisotropy due to the distribution of ferromagnetic grains irrespective of their shape has also been proposed as an important ams source former analytical models that consider infinite geometry of identical and uniformly magnetized and coaxial particles confirm that shape fabric may be overcome by dipolar contributions if neighboring grains are close enough to each other to magnetically interact on these bases we present and experimentally validate a two grain macroscopic numerical model in which each grain carries its own magnetic anisotropy volume orientation and location in space compared with analytical predictions and available experiments our results allow to list and quantify the factors that affect the effects of magnetic interactions in particular we discuss the effects of i the infinite geometry used in the analytical models ii the intrinsic shape anisotropy of the grains iii the relative orientation in space of the grains and iv the spatial distribution of grains with a particular focus on the inter grain distance distribution using documented case studies these findings are summarized and discussed in the framework of the generalized total ams tensor recently introduced by canon tapia canon tapia e 2001 factors affecting the relative importance of shape and distribution anisotropy in rocks theory and experiments tectonophysics 340 117 131 the most important result of our work is that analytical models far overestimate the role of magnetic interaction in rock fabric quantification considering natural rocks as an assemblage of interacting and non interacting grains and that the effects of interaction are reduced by i the finite geometry of the interacting clusters ii the relative orientation between interacting grains iii their heterogeneity in orientation shape and bulk susceptibility and iv their inter distance distribution we reconcile analytical models and experiments with real case studies that minimize the role of magnetic interaction onto the measured ams limitations of our results are discussed and guidelines are provided for the use of ams in geological interpretation of igneous rock fabrics where magnetic interactions are likely to occur

single domain and superparamagnetic titanomagnetite with variable ti content in young ocean floor basalts no evidence for rapid alteration high resolution electron microscope studies have been carried out on zero age new flow basalts from the juan de fuca ridge and on young 20 ka basalts from the axis of the east pacific rise at 12 n such data lead to characterization of the magnetic minerals especially those of smaller grain size which have been hypothesized by kent and gee to have undergone grain size dependent alteration in addition to larger titanomagnetite grains abundant submicrometer titanomagnetite has been observed in globules within a glassy matrix these grains likely to be single domain sd or superparamagnetic are associated with apatite uncommon pyrrhotite and residual glass the submicrometer titanomagnetite grains have a wide compositional range 0 x 0 8 where x is the fraction of ulvospinel component whereas the larger multi domain md sized titanomagnetite grains have a narrow composition range of approximately x 0 6 this variability in ti content provides a ready explanation for the thermal rock magnetic properties observed by kent and gee and eliminates the need to invoke extremely rapid 20 ka alteration of these young basalts

ams nrm interferences in the deccan basalts toward an improved understanding of magnetic fabrics in flood basalts the evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies a statistically significant sample set of titanomagnetite bearing lava flows from the malwa plateau the northern part of the deccan traps in india has been investigated for a possible interference of induced and natural remanent magnetization nrm the nrm alters the scalar anisotropy of magnetic susceptibility ams parameter and the orientations of the ams principal magnetic axes which are crucial for the evaluation of the flow direction for cleaning of the nrm component the lava samples have been demagnetized by use of an alternating field af tumbling demagnetizer peak fields of 100 mt as previous studies have shown that static af demagnetization can bias the results samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization the evaluated flow directions show a more differentiated flow pattern of the malwa area which seems to fit better into the regional geological setting in samples with inverse magnetic fabrics carrying a higher portion of single domain particles ams principal axes remain unchanged after the demagnetization indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations according to these results we propose that the ams measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers at least for samples with normal magnetic fabrics and therefore a more precise and better reflection of the actual mineral fabric

the effects of heating on low temperature susceptibility and hysteresis properties of basalts abstract the variation of the low field susceptibility of basalts down to liquid nitrogen temperature always falls into one of three types that depend on the composition and grain size of the titanomagnetite grains present group 1 basalts contain predominantly unoxidised multidomain homogeneous titanomagnetites having x 0 3 group 2 basalts contain predominantly titanomagnetite grains with many exsolved ilmenite lamallae that subdivide the grains so that they act similarly to single domains group 3 basalts contain predominantly multidomain magnetite or magnetite rich titanomagnetite having x 0 15 after repeated heating to 615 c the group 1 basalts gradually oxidise above 300 c to produce the characteristics of group 2 basalts owing to the exsolution of ilmenite on the other hand both group 2 and 3 basalts are stable to oxidation until at least 500 c they are therefore the most useful material for palaeointensity studies

petrogenesis of the flood basalts forming the northern kerguelen archipelago implications for the kerguelen plume the thick 20 km crust of the kerguelen archipelago formed as the tectonic setting of the kerguelen plume changed from an oceanic ridge centered location at 43 ma to its present location beneath the antarctic plate the uppermost crust is dominantly flood basalt with a thickness of up to 10 km inverse isochron 40ar 39ar ages for upper and lower lavas in a 630 m section of basalt flows from mont bureau are 30 4 and 29 0 ma re os isotopic systematics are consistent with this age most of the lavas in two stratigraphic sections mont bureau and mont rabouillere from the northern part of the archipelago have sr nd and pb isotopic characteristics similar to the youngest upper miocene to pleistocene lavas erupted in the southeast part of the archipelago i e initial 87sr 86sr 0 7050 143nd 144nd 10 mgo alkalic lavas in the southeast province of the archipelago in these northern sections it is confined to transitional lavas with

statistical properties of paleomagnetic directions in kerguelen lava flows implications for the late oligocene paleomagnetic field 40 ar 39 ar ages are reported to aid in the calibration of the paleomagnetic results the primary contribution of this report however is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the kerguelen flood basalts in total 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 ma window from 25 to 30 ma the composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale our approach is to investigate the average normal and reversed polarity field directions over this 5 ma window we calculated a paleomagnetic pole found to be located at l 85 5n f 189 3 e a95 2 3 k 16 5 n 233 this pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the indo atlantic hot spot reference frame in the statistical analysis we tackle the specific question is the secular variation isotropic this question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles which is still a matter of debate among paleomagnetists by means of statistical tests we show that the paleomagnetic data from kerguelen agree with an isotropic model for paleosecular

ams detection of inverse fabrics without aarm in ophiolite dikes inverse fabric contributions from single domain sd magnetite complicate interpretation of magmatic flow from anisotropy of magnetic susceptibility ams in the s troodos ophiolite dike swarm flow directions and magma chamber dimensions may only be inferred correctly if magnetic fabrics correspond to the orientation distribution od of minerals magnetite shape fabrics were isolated by anisotropy of anhysteretic remanence aarm but ams ambiguously merges magnetite and silicate fabrics however with the retrospective benefit of aarm in this area we recommend avoiding sd complications by excluding magnetite rich samples for which the bulk low field susceptibility 27 500 si magma flow was steeply upwards to the sw proximal to an axial magma chamber

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

magnetic fabrics and rock magnetism of the xiong er volcanic rocks and their implications for tectonic correlation of the north china craton with other crustal blocks in the nuna columbia supercontinent abstract the tectonic background of the paleoproterozoic xiong er volcanic rocks xvr is important for understanding the tectonic evolution of the north china craton ncc as well as its paleogeographic position during the assembly of the nuna columbia supercontinent here we report the results of the first anisotropy of magnetic susceptibility ams study of the xvr and use the interpreted flow directions to constrain the emplacement mechanism and to assess its geological significance for the reconstruction of the nuna columbia supercontinent thirty lavas were sampled from three sections in western henan province detailed rock magnetic analyses including measurements of hysteresis loops magnetization versus temperature curves and first order reverse curves were performed to identify the main magnetic phases and grain sizes the inferred directions from the ams results reveal a radial flow pattern with an eruption center probably located near xiong er mountain our data suggest that the xvr may have been emplaced in a triple conjugated continental rift on the south margin of the ncc probably initiated from a paleoplume based on this interpretation a comparison of geological and paleomagnetic results among the proposed crustal blocks in the nuna columbia supercontinent suggests a close linkage of the ncc with sao francisco congo rio de la plate and siberia

magnetic interactions at the origin of abnormal magnetic fabrics in lava flows a case study from kerguelen flood basalts summary anisotropy of low field magnetic susceptibility ams of basaltic lava flows can give some clues about post emplacement tilting occurring in volcanic sequences such a study has been carried out on a sequence of 19 successive lava flows from kerguelen archipelago surprisingly two different patterns were observed the first one herein called normal fabric is consistent with the lava flow direction inferred from geological observations whereas the second one observed for about 70 per cent of the analysed samples herein called abnormal fabric appears to be unrelated to the shear history of the lava flows during their emplacements these abnormal fabrics are not strictly inverse fabric but seem to have a more complex origin the unusual nature of these abnormal fabrics has been confirmed by lattice preferred orientation of plagioclases obtained from high resolution electron backscattered diffraction measurements the aim of this study is to propose a physical interpretation of these fabrics a first step in our understanding was to clearly identify the nature and the magnetic properties of thefe tioxidesassemblage thus wepresentacomprehensive rockmagneticanalysisrelying on low and high temperature thermomagnetic curves k t crossover diagrams first order

the duration of a yellowstone super eruption cycle and implications for the age of the olduvai subchron abstract small volume rhyolitic eruptions preceding and following a caldera forming eruption can provide insights into the tempo of eruption cycles and timing of magmatic recharge in this contribution high precision 40 ar 39 ar eruption ages were obtained on the three effusive eruptions bracketing the huckleberry ridge tuff which comprise yellowstone s first volcanic cycle these dates are supplemented with detailed paleomagnetic and rock magnetic analyses to resolve discrepancies with previous reported stratigraphy the huckleberry ridge tuff 2 08 ma was preceded by an eruption at 2 14 ma and followed by eruptions at 1 98 and 1 95 ma all of which occurred during four distinct periods of geomagnetic instability within the matuyama chron the first volcanic cycle of yellowstone has now been constrained to within a 200 kyr timespan or half of the previously proposed duration and similar to the duration of volcanic activity for caldera forming systems in the jemez volcanic field the maximum duration for magmatic recharge for the first yellowstone volcanic cycle is no greater than 100 kyr and likely closer to 40 kyr furthermore the combined 40 ar 39 ar eruption ages and paleomagnetic results provide polarity anchors for the pre olduvai excursion and olduvai subchron which are often used as tie points in studies of early pleistocene hominin evolution

the anisotropy of magnetic susceptibility of lava flows an experimental approach measurements of the anisotropy of magnetic susceptibility ams of natural lavas have shown that ams varies with depth within a lava flow we have investigated the reasons for such variation by studying the effects of temperature and strain rate on the ams of recent lava in the laboratory samples of lava from kilauea were melted and subjected to a range of strain rate and cooling histories the results show that the degree of anisotropy is a function of both the thermal and shearing history of a sample high degrees of anisotropy were found only in samples that were deformed at temperatures close to those encountered during eruption and then rapidly quenched lavas subjected to similar shear stresses at high temperatures had low degrees of anisotropy if allowed to cool down slowly without further deformation additionally lava subjected to complex shearing yield a lower degree of anisotropy even when high strain rates were imposed on it these results lead to the conclusion that only the last phase of deformation is detectable using ams and that high strain rates will not result in high degrees of anisotropy if either deformation ends while lava is still fluid or if the orientation of the maximum shear stress varies with time the relation between the orientation of the principal susceptibilities and that of shear is less sensitive to variation on shear with time consequently flow directions can be inferred confidently with this type of measurements

post emplacement tilting of lava flows inferred from magnetic fabric study the example of oligocene lavas in the jeanne d arc peninsula kerguelen islands abstract statistical analysis of the magnetic fabric of samples from several successive lava flows emplaced under similar conditions can allow determination of the mean flow direction when magnetic fabric data from individual flows do not lead to reliable results a difference between the obtained flow direction and the present dip direction indicates that the flows were tilted after emplacement for 2 successive series of flows on the jeanne d arc peninsula presently nnw dipping this method shows lava emplacement along a ssw nne direction this indicates a gentle tilting acquired during a period of weak deformation in the whole archipelago additionally the magnetic fabric data allow the reconstruction of the different conditions of emplacement of these two series of lava flows and of formation of the local thick conglomerate interbedded between these series