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

weak palaeointensity results over a pliocene volcanic sequence from lesser caucasus georgia transitional record or time averaged field author s sanchez moreno elisa m calvo rathert manuel goguitchaichvili avto tauxe lisa vashakidze george t lebedev vladimir a abstract summary a palaeointensity study has been carried out on a pliocene sequence of 20 consecutive lava flows where previous directional results seem to reflect anomalous behaviour of the earth s magnetic field emf which can be explained by a polarity transition record or non averaged palaeosecular variation or both here we perform a total of 55 palaeointensity determinations using the original thellier thellier tt method and 100 with the izzi method we assess the performance of our selection criteria using a set of strict threshold values applied to a set of test data whose trms were acquired in known fields absolute palaeointensity determinations that passed our selection criteria were obtained on four specimens with the tt method and on 41 specimens with the izzi method application of reliability criteria at a site level yielded palaeointensity results in 8 of 20 studied lava flows we obtained median values of vadm between 28 9 and 45 6 zam2 for the reverse polarity lower apnia section while the normal polarity upper section displayed a single value of 54 6 zam2 the low palaeointensity values before a transitional direction lava flow and the higher value after it suggest the common behaviour at the start of a polarity reversal and the recovery after it however an isolated record of a stable emf where the intensity is lower than the current for the same location 83 7 zam2 cannot be discarded consequently this interpretation would support a weak time averaged field

the variation of intensity of earth s magnetic field with time abstract a new and larger database is established to assess the variation of paleointensity with time it is shown that submarine basaltic glass probably contains a grain growth chemical remanent magnetization in magnetite intensity estimates from these samples are used to establish a lower bound estimate on intensities statistical tests on various subsets of the database are used to establish a more reliable database for finding long term trends of the paleointensity we can obtain a reasonable estimate for intensity versus time for the cenozoic but not for the mesozoic or late paleozoic the distributions of data as a function of time also provide valuable information in particular the two well documented superchrons exhibit different intensity properties which suggests that there is not a simple correlation between reversal rate and intensity

long term variations in palaeointensity we compile a dataset of reliable palaeointensity estimates based both on published work and on new data from basaltic glass the basaltic glass data more than double the number of reliable thellie

paleomagnetic field intensity recorded in submarine basaltic glass from the east pacific rise the last 69 ka paleointensity determinations were obtained from basaltic glass recovered from the northern east pacific rise using the thelliers 1959 method as applied by pick and tauxe 1993a the samples are up to a69 ka old based on their distance from the ridge axis and u series disequilibria measurements the results show a general increase in paleointensity of the field from 69 ka to the present with values ranging from 50 higher than the present field to as low as 26 of the present dipole field the average dipole moment is 8 32 1022 am which is similar to the present axial dipole moment the results are in general agreement with other absolute and relative paleointensity records

geology geochemistry geochronology and economic potential of neogene volcanic rocks in the laguna pedernal and salar de aguas calientes segments of the archibarca lineament northwest argentina abstract this study presents new geochemical geochronological isotopic and mineralogical data combined with new geological mapping for a 2400 km 2 area of neogene volcanic rocks in northwestern argentina near the border with chile between 25 10 s and 25 45 s the area covers the zone of intersection between the main axis of the cordillera occidental and a set of nw se trending structures that form part of the transverse archibarca lineament this lineament has localized major ore deposits in chile e g the late eocene la escondida porphyry cu deposit and large volcanic centers such as the active llullaillaco and lastarria volcanoes on the border between chile and argentina and the neogene archibarca antofalla and cerro galan volcanoes in argentina neogene volcanic rocks in the laguna pedernal and salar de aguas calientes areas are mostly high k calc alkaline in composition and range from basaltic andesites through andesites and dacites to rhyolites magmatic temperatures and oxidation states estimated from mineral compositions range from 1000 c and fmq 1 0 1 5 in andesites to 850 c and fmq 1 5 2 0 in dacites and rhyolites the oldest rocks consist of early middle miocene andesite dacite plagioclase pyroxene phyric lava flows and ignimbrites with 40 ar 39 ar ages ranging from 17 14 0 10 ma to 11 76 0 27 ma their major and trace element compositions are typical of the andean central volcanic zone and show strong crustal contamination trends for highly incompatible elements such as cs rb th and u these rocks are geochemically grouped as sub suite 1 this widespread intermediate composition volcanism was followed in the middle late miocene by a period of more focused rhyodacitic flow dome complex formation these felsic rocks are characterized by less extreme enrichments in highly incompatible elements and increasing depletion of heavy rare earth elements these rocks are geochemically grouped as sub suite 2 the youngest rocks in this sub suite show the highest la yb ratios and are characterized by abundant hornblende phenocrysts not commonly seen in other rocks from the area in the pliocene pleistocene there was a return to more typical andesite dacite volcanism with geochemical characteristics similar to the early middle miocene lavas and are also grouped in sub suite 1 finally extensional tectonics in the quaternary led to localized outpouring of mafic basaltic andesitic to andesitic monogenetic lava flows and cones one particularly large flow the vega aguas calientes lava flow covers approximately 90 km 2 and samples form two groupings with affinities similar to the least evolved samples from sub suites 1 and 2 sub groups ba1 and ba2 respectively nd and sr isotopic compositions indicate moderate to strong crustal contamination especially in more felsic rocks and extend from 87 sr 86 sr 0 706 and end 2 4 values typical of central volcanic zone rocks to more evolved compositions 0 709 and 6 8 respectively typical of large volume ignimbrites of the altiplano puna volcanic complex and cerro galan the latter compositions are thought to be derived by extensive interaction between mantle derived arc magmas and paleozoic granitoid rocks that form much of the crustal column in this region the distinctive mineralogy and geochemistry of the sub suite 2 middle late miocene rhyodacitic flow dome complexes indicate that these magmas had higher water content than both the earlier and later sub suite 1 andesites dacites they were erupted during a period of tectonic quiescence following the quechua orogenic phase and geophysical evidence suggests that they were proximally derived from a large upper crustal magma chamber which partially collapsed to form a trap door caldera strong fumarolic alteration associated with the youngest of these felsic volcanoes cerro abra grande suggests the potential for the existence of epithermal type mineralization within the volcanic edifice or porphyry type mineralization at depth

the implications of long lived asymmetry of remanent magnetization across the north pacific fracture zones article i nfo large marine magnetic anomalies accompany the pacific fracture zones fzs for thousands of kilometers although the origin of these anomalies is poorly understood their underlying magnetization contrasts should reflect the temporal record of crustal accretion as well as geomagnetic field variations here we present an analysis of archival and newly collected magnetic anomaly profiles measured across three fzs from the north pacific cretaceous quiet zone 120 6 to 83 ma that are characterized by a remarkably uniform shape forward and inverse modeling indicate that these anomalies arise from remanent magnetization with enhanced remanence located on one side of each fz along the entire studied area a comparison of geochemical and magnetic data from active ridge discontinuities and transform faults suggests that elevated iron content near segment ends is likely responsible for the observed anomalies in the cretaceous quiet zone as well a more complex magnetization setting is observed where the fzs contain multiple faults there the simple model of one sided enhancements is only partly valid comparison between 3d forward modeling of the quiet zone magnetization and the calculated magnetization contrasts found across the pioneer and pau fzs suggests that the intensity of the geomagnetic field during the cretaceous superchron had less than 50 percent variability about its average value no major trends in the strength of the geomagnetic field during the superchron are observed the presence of long duration 30 m y zones of enhanced magnetization along the young old sides of the pioneer pau fzs both left stepping requires some long lived asymmetry in crustal construction processes near ridge transform intersections although the underlying mechanism that controls this long lived asymmetry remains unclear absolute plate motions might explain this asymmetry shorter period few m y variations in the amplitudes of the enhancements probably result from oscillations in crustal construction

long term trends in paleointensity the contribution of dsdp odp submarine basaltic glass collections abstract the deep sea drilling project and the ocean drilling program have been collecting fresh appearing submarine basaltic glass from the world s oceans for over three decades this glass has proved nearly ideal for estimating paleointensity variations of the earth s magnetic field we compile here data for 726 paleointensity experiments from six publications on paleointensity using dsdp odp glass we also include new data for an additional 225 specimens these were obtained through the so called izzi paleointensity experiment of tauxe l staudigel h 2004 strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite geochem geophys geosyst 5 2 q02h06 doi 10 1029 2003gc000635 whereby infield zerofield steps are alternated with the zerofield infield steps to enhance quality assessment of the resulting data the entire collection of data from 951 experiments was prepared for uploading to the magic data base http earthref org including original measurements interpretations and useful metadata excellent results were obtained throughout the depth 1400 mbsf and age 0 160 ma range sampled dsdp odp glass data are compared with published paleointensity data meeting minimal acceptance criteria from the time interval 1 160 ma paleolatitudes were estimated for all cooling units in a self consistent manner for use in calculating virtual axial dipole moments we conclude 1 there is about a 20 difference in mean values between the sbg and the lava flow data 48 36 and 57 29 zam2 respectively the difference is caused by the fact that there are more higher values in the lava flow data than in the sbg data set rather than a difference in the minimum values lava flows cooling over a periods of days to months can account for the discrepancy 2 the positive relationship between polarity interval length and average paleofield intensity first hypothesized by cox a v 1968 lengths of geomagnetic polarity intervals j geophys res 73 3247 3260 is supported by data compiled here the brunhes data for which we have only a minimum estimate for polarity interval length are consistent with a long polarity interval suggesting that instead of racing toward reversal hulot g eymin c langlais b mandea m olsen n 2002 small scale structure of the geodynamo inferred from oersted and magsat satellite data nature 416 620 623 we could instead be in the midst of a long stable polarity interval 3 because the average value appears to be a function of polarity interval length it is probably not useful to calculate a mean value nonetheless it appears that most of the time apart from the brunhes and the longest polarity intervals the average dipole moment is substantially less than the present day value as suggested by juarez t tauxe l gee j s pick t 1998 the intensity of the earth s magnetic field over the past 160 million years nature 394 878 881

paleointensity in hawaiian scientific drilling project hole hsdp2 results from submarine basaltic glass 1 paleointensity estimates based on the high quality thellier thellier data from the early brunhes 420 780 ka are rare only 30 in the published literature the second hawaiian scientific drilling project hsdp2 drill hole recovered submarine volcanics spanning the approximate time period of 420 550 ka these are of particular interest for absolute paleointensity studies owing to the abundance of fresh submarine basaltic glass which can preserve an excellent record of ancient geomagnetic field intensity we present here new results of thellier thellier paleointensity experiments that nearly double the number of reliable paleointensity data available for the early brunhes we also show that the magnetizations of the associated submarine basalts are dominated by viscous magnetizations and therefore do not reflect the true ancient geomagnetic field intensity at the time of extrusion the viscous contamination is particularly severe because of a combination of low blocking temperatures in the basalts and relatively high temperatures in the deeper parts of the drill core our new data when placed on the approximate timescale available for hsdp and hsdp2 are at odds with other contemporaneous paleointensity data the discrepancy can be reconciled by adjusting the hsdp timescales to be younger by about 35 kyr

high geomagnetic intensity during the mid cretaceous from thellier analyses of single plagioclase crystals recent numerical simulations have yielded the most efficient geodynamo having the largest dipole intensity when reversal frequency is low reliable paleointensity data are limited but heretofore have suggested that reversal frequency and paleointensity are decoupled we report data from 56 thellier thellier experiments on plagioclase crystals separated from basalts of the rajmahal traps 113 to 116 million years old of india that formed during the cretaceous normal polarity superchron these data suggest a time averaged paleomagnetic dipole moment of 12 5 1 4 1022 amperes per square meter three times greater than mean cenozoic and early cretaceous late jurassic dipole moments when geomagnetic reversals were frequent this result supports a correlation between intervals of low reversal frequency and high geomagnetic field strength

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

intensity of the earth s magnetic field evidence for a mesozoic dipole low the variation of the strength of the earth s magnetic field throughout geological time is known only poorly large uncertainties spoil indirect paleointensity determinations based on magnetic anomaly amplitude or remanence of sedimentary rocks even when paleointensity data are obtained from direct thermomagnetic studies of magmatic units fundamental problems related to the actual nature of the primary remanence and to the effects of different cooling rates may result in erroneous determinations these uncertainties combined with well known experimental difficulties can explain the large scatter in paleointensity data reported in most previous review papers because experimental difficulties are more rigourously dealt with using the thellier method which also offers some opportunities to minimize some fundamental uncertainties we carried out a compilation of all triassic and younger paleointensity determinations made using this method this selected data set indicates the occurrence of large long term changes of the geomagnetic dipole strength with time the average dipole strength seems to have been approximately constant since late cretaceous time but a dipole strength only one third of the cenozoic value prevailed during most of mesozoic time following a period of larger strength ending possibly during early jurassic times the mesozoic dipole low may be correlated with a cessation of relative motion between the geomagnetic axis and the mantle the terrestrial dynamo seems therefore to exhibit different long term stable states which are probably controlled by the thermal structure in the lowermost mantle

the cretaceous superchron geodynamo observations near the tangent cylinder abstract if relationships exist between the frequency of geomagnetic reversals and the morphology secular variation and intensity of earth s magnetic field they should be best expressed during superchrons intervals tens of millions of years long lacking reversals here we report paleomagnetic and paleointensity data from lavas of the cretaceous normal polarity superchron that formed at high latitudes near the tangent cylinder that surrounds the solid inner core the time averaged field recorded by these lavas is remarkably strong and stable when combined with global results available from lower latitudes these data define a time averaged field that is overwhelmingly dominated by the axial dipole octupole components are insignificant these observations suggest that the basic features of the geomagnetic field are intrinsically related superchrons may reflect times when the nature of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency

toward an optimal geomagnetic field intensity determination technique 1 paleointensity determinations based on double heating techniques in field zero field cooling zero field in field cooling and two in field steps with opposite laboratory fields are generally considered to be functionally interchangeable producing equally reliable paleointensity estimates to investigate this premise we have developed a simple mathematical model we find that both the zero field first and in field first methods have a strong angular dependence on the laboratory field parallel orthogonal and anti parallel while the two in field steps method is independent of the direction of the laboratory produced field contrary to common practice each method yields quite different outcomes if the condition of reciprocity of blocking and unblocking temperatures is not met even with marginal 10 tails of partial thermoremanence our calculations suggest that the zero field first method with the laboratory produced field anti parallel to the natural remanence nrm is the most robust paleointensity determination technique when the intensity of the lab induced field is smaller than ancient field however the zero field first method with the laboratory field parallel to the nrm is the optimum approach when the intensity of the lab induced field is larger than the ancient field by far the best approach however is to alternatethe infield zerofield iz steps with zerofield infield zi steps

comparison of the coe thellier thellier and microwave palaeointensity techniques using high titanium titanomagnetites results from a tertiary basaltic intrusion from the sydney basin new south wales abstract conventional thellier and microwave mw palaeointensity experiments accompanied by rock magnetic tests were conducted on samples from an early tertiary ca 49 ma basaltic intrusion the peats ridge pr basalt from the sydney basin thermomagnetic and low temperature susceptibility measurements suggest that the dominant magnetic mineral is ti enriched titanomagnetite tm65 tm50 and hysteresis results indicate that the majority of samples exhibit psd like behaviour some samples show evidence of alteration during laboratory heating but only at temperatures in excess of 400 c no alteration is observed in the dominant temperature range 70 300 c used for calculation of palaeointensities stepwise thermal demagnetisation studies reveal single component after removal of a small viscous reverse polarity magnetisations in all samples with a mean characteristic remanence chrm direction of d i 197 60 95 5 k 62 coe thellier thellier ctt experiments yielded a high success rate with 68 of samples conforming to standard acceptance criteria microwave mw palaeointensity experiments predominantly using the perpendicular applied field technique produced a greater success rate of 82 mean palaeointensities of 27 1 4 4 t n 21 and 27 4 4 0 t n 28 were obtained from the ctt and mw techniques respectively these means could not be distinguished statistically and the respective quality indicators were similar these results suggest that the new microwave technique for calculating palaeointensities is in this case equivalent to the well established ctt technique this is an intuitive outcome given that rock magnetic analyses suggest that alteration does not occur below the curie temperature of the samples our results therefore provide a positive test for the equivalence of the mw and ctt techniques the virtual dipole moment vdm value for the intrusion is 4 7 0 7 10 22 am 2 suggesting that the dipole field was approximately 59 of the present day value at ca 49 ma this provides a valuable data point from the southern hemisphere for a sparsely covered time period in the global palaeointensity database

paleointensity during periods of rapid reversal a case study from the middle jurassic shamrock batholith western nevada paleointensity is one of the least deter mined parameters in geomagnetism yet this information has the potential to address many fundamental geophysical problems that bear on the evolution of the earth s core and lower mantle we test two important hy potheses that affect our understanding of how paleointensity has changed in earth s past 1 the mesozoic dipole low mdl hypoth esis and 2 the inverse relationship between geomagnetic polarity reversal rate and paleo intensity we report paleointensity results determined using both the coe modified thellier method and the izzi protocol on the medium grained hornblende biotite granite of the middle jurassic shamrock batholith in the southern singatse range western nevada usa previous zircon u pb dating gives an age of batholith emplacement at ca 165 8 0 4 ma this age estimate is coincident with a period of unusually high reversal frequency 10 reversals ma in the middle jurassic remanence in the shamrock batholith is held primarily by exsolved submicron magnetite inclusions in plagioclase the labora tory un blocking temperature range for the well de fined thermal remanent magnetization is nar row between 540 and 575 c and median destructive fields are between 40 and 50 mt thus we infer that the age of rema nence ac quisition is approximated by the u pb age estimate for pluton emplacement two sets of quality criteria were used to assess the paleo intensity results the first set yielded an 87 success rate while the second set using more stringent criteria resulted in only a 20 suc cess rate mean paleointensity values for the shamrock batholith using the loose and strict quality criteria are 34 0 6 4 t n 13 and 33 6 9 6 t n 3 respectively with errors reported at one standard deviation after correction for both remanence anisot ropy and cooling rate the mean values that pass loose and strict criteria are respec tively 17 9 2 7 t and 17 5 2 6 t the uncorrected and corrected paleointensity values for the strict estimates yield respec tive mean virtual dipole moments vdms of 62 7 18 zam 2 z 10 21 and 32 7 4 9 zam 2 the uncorrected value is within error of the vdms reported for plagioclase grains isolated from ca 160 ma basalts from ocean drilling program site 801 however these values have an absolute difference ranging from 6 to 20 zam 2 while the corrected vdm value is more consistent with the esti mated average jurassic field strength of 29 zam 2 using a recent estimate for the long term stable field strength of 42 zam 2 our corrected values add further credence to both the mdl and the inverse relationship between geomagnetic polarity reversal rate and dipole moment hypotheses

analysis of long term variations in the geomagnetic poloidal field intensity and evaluation of their relationship with global geodynamics summary the 1167 published cooling unit cu palaeointensity estimates contained in the 400 10 ma portion of the pint global database were rigorously filtered according to accurate age determinations palaeodirectional reliability recognition of polarity and the method of palaeointensity acquisition the remaining 865 estimates group 1 were further filtered to ensure self consistency reducing the data set to 425 estimates group 2 group 1 and 2 data were clustered into temporally and or spatially distinct rock suites rs enabling each part of the record to be assessed for potential biasing by overrepresentation of palaeosecular variation psv the record was segmented according to the distribution of the data rather than using arbitrary time windows to ensure quasi consistent behaviour within each segment differences between these segments clearly indicate that a significant long timescale 10 7 and 10 8 yr variation of the mean geomagnetic poloidal field intensity gpfi occurred during the 400 10 ma period and hence that changing lowermost mantle conditions affect the capacity of the geodynamo to generate a poloidal field both the mean dipole moment and its standard deviation appear to be a function of the range of values each cu may adopt at one particular time this range is itself controlled by the variation of the maximum limit of dipole moment while the value of the minimum limit remains relatively constant tentative support is provided for the recent suggestion that psv may have been reduced during the cretaceous normal superchron cns though more data are needed in the range 120 60 ma to confirm this no conclusive evidence was found to support the suggestion that the gpfi record may be biased towards low or high values by palaeointensity determinations obtained using methods that do not adopt ptrm checks indeed offsets caused by unreliable data in well represented parts of the record are likely to be random and cancel one another out when gpfi variation is analysed at a sufficiently high resolution to allow comparisons with the geomagnetic polarity reversal frequency rf it is not possible to confirm whether the two parameters are anticorrelated decoupled or related in some more complex way however it is clear that gpfi and rf are definitely not positively correlated as has been previously suggested the present database documents sharp increases in gpfi around the onset times of the two recognized superchrons itself implying an anticorrelation the implications for geodynamo and mantle modelling of both an anticorrelation and a decoupling of the geomagnetic parameters are discussed briefly a generic geodynamic model is proposed to explain the relationship between observed longterm changes in gpfi and global geodynamic processes the model predicts that changes in gpfi result from a chain of geodynamic processes extending from crust to core beginning with plate reorganizations at the surface and culminating in increases in the vigour of outer core convection supercontinents are transient surface expressions of such geodynamic processes and provide the potential to test the generic model four time stages are proposed to describe the major long term changes in gpfi since the early devonian 400 350 350 250 250 175

the effect of remanence anisotropy on paleointensity estimates a case study from the archean stillwater complex paleomagnetism of archean rocks potentially provides information about the early development of the earth and of the geodynamo precambrian layered intrusive rocks are good candidates for paleomagnetic studies such complexes are commonly relatively unaltered and may contain some single domain magnetite armored by silicate mineral grains however layered intrusives often have a strong petrofabric that may result in a strong remanence anisotropy magnetic anisotropy can have particularly disastrous consequences for paleointensity experiments if the anisotropy is unrecognized and if its effects remain uncorrected here we examine the magnetic anisotropy of an anorthosite sample with a well developed magmatic foliation the effect of the sample s remanence fabric on paleointensity determinations is significant paleointensities estimated by the method of thellier and thellier range from 17 to 55 t for specimens magnetized in a field of 25 t we describe a technique based on the remanence anisotropy tensor to correct paleointensity estimates for the effects of magnetic fabric and use it to estimate a paleointensity for the stillwater complex mt usa of 32 t adjusted for the effects of slow cooling

some global features of palaeointensity in geological time summary a global palaeointensity data base was constructed from all published data from volcanic rocks in geological time older than 0 03 ma the data base contains a total of 1123 flow mean data retrieved from 83 original papers various features of the earth s dipole moment were examined from the data which are based on thellier and shaw methods long term variation of the earth s dipole moment seems to have existed in the past 300 ma with a broad minimum at 120 180 ma as suggested by prevot et al 1990 however due to limited site distribution we cannot regard this mesozoic dipole low as being completely established precambrian palaeointensity data are still insufficient to conclude any long term variation in this time range although the geodynamo processes of moderate magnitude definitely existed in the early time of the earth s history the 2 test was applied to the distribution of the virtual dipole moment for the past 5 ma in which the transitional data were excluded the results indicate that the distribution of virtual dipole moment is better represented by a log normal distribution rather than a normal distribution and this tendency seems to be true for the past 20 ma the relation of mean palaeointensity versus palaeomagnetic colatitude was examined for the past 10 ma for the data excluding transitions the relation is concordant with a theoretical curve from a geocentric axial dipole this reconfirms that the dipole field was dominant in the past geomagnetic field which is the dipole hypothesis in palaeomagnetism on the other hand the virtual dipole moment is much smaller for transitional palaeomagnetic fields and a virtual geomagnetic pole lower than 45 seems to be a reasonable criterion to be categorized as a transition the mean dipole moment for the pole latitude of 30 n 50 n band is larger than that for 30 s 50 s indicating that there might have been a persistent asymmetry of palaeointensity between normal and reversed states or some kind of geometrical asymmetry between the northern and southern hemispheres

cenozoic plate reconstruction of southeast asia abstract the india eurasia collision alone set up a series of chain reactions and caused the formation and destruction of sedimentary basins within the domain of the collision belt changes in the rate and angle of convergence between the india and eurasia plates reflect different stages of tectonic development in southeast asia for example extrusion of the indochina block induced the consumption of the pre existing proto south china sea along northeastern kalimantan and led to the eventual opening of the south china sea along the south china margin subsequent motions of the sino burma thailand malay peninsula sumatra and kalimantan blocks have produced a succession of basins stretching from north sumatra to central thailand and on to the natuna area we present reconstructions of the southeast asia region at 60 ma 50 ma 40 ma 30 ma 20 ma 15 ma 10 ma and 5 ma it is clear from the reconstructions that the impact between greater india and southeast asia took place in the northwestern part of southeast asia the duration for the impact was probably from the middle eocene to early miocene this timing is in good agreement with the dating of the main red river fault motion wu et al 1989 scharer et al 1990 since the impact between greater india and southeast asia was basically west of the burma block there is no reason to assume that the sumatra malay peninsula and kalimantan should extrude to the southeast first along the left laterally displaced mae ping and three pagodas fault zones as suggested by peltzer and tapponnier 1988 if the opening of the central thailand basins the gulf of thailand and the malay basin are taken into consideration a dextral megashear zone is required to compensate the relative motion between indochina and the malay peninsula this dextral megashear zone might even extend into western kalimantan and serve as a boundary between the indochina block and kalimantan

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

an integrated paleomagnetic study of rio grande de santiago volcanic succession trans mexican volcanic belt revisited abstract we carried out an integrated paleomagnetic rock magnetic and paleointensity study of miocene volcanic succession from the trans mexican volcanic belt tmvb north of guadalajara a total of 37 consecutive basaltic lava flows 326 oriented standard paleomagnetic cores were collected at lazo locality continuous susceptibility measurements with temperature and hysteresis experiments yield in most cases reasonably reversible curves with curie points close to that of pseudo single domain magnetite two geomagnetic reversals were observed in the 300 m thick composite section paleosecular variation was lower than the one observed in general during miocene it appears that the volcanic units have been emplaced during a relatively short time span of about 1 ma the mean paleomagnetic directions obtained from this study do not differ significantly from that expected for the middle miocene the mean paleomagnetic direction calculated from all data is i 31 1 d 354 6 k 124 and 95 2 1 n 37 seventy two samples with apparently preserved primary magnetic mineralogy and without secondary magnetization mostly belonging to reverse polarity chron were pre selected for thellier paleointensity determination the flow mean paleointensity values are ranging from 22 4 3 4 to 53 8 6 0 t and the corresponding virtual dipole moments vdms are ranging from 5 4 0 8 to 12 0 1 4 1022 a m2 this corresponds to mean value of 7 7 2 2 1022 a m2 which is close to present day geomagnetic field strength altogether our data suggest the existence of relatively high geomagnetic field strength undergoing low fluctuations

a geochemical traverse across the eastern carpathians romania constraints on the origin and evolution of the mineral water and gas discharges abstract the inner sector of the eastern carpathians displays a large number of na hco 3 co 2 rich meteoric originated cold springs soda springs and bore wells as well as dry mofettes they border the southern part of the pliocene quaternary calimani gurghiu harghita cgh calc alkaline volcanic chain both volcanic rocks and co 2 rich emissions are situated between the eastern part of the transylvanian basin and the main east carpathian range where active compression tectonics caused diapiric intrusions of miocene halite deposits and associated saline co 2 rich waters along active faults the regional patterns of the distribution of co 2 in spring waters as calculated p co 2 and the distribution pattern of the 3 he 4 he ratio in the free gas phases up to 4 5 r m r a show their maximum values in coincidence with both the maximum heat flow measurements and the more recent volcanic edifices moving towards the eastern external foredeep areas where oil fields and associated brines are present natural gas emissions become ch 4 dominated such a change in the composition of gas emissions at surface is also recorded by the 3 he 4 he ratios that in this area assume typical crustal values r m r a 0 02 in spite of the fact that thermal springs are rare in the harghita volcanic area and that equilibrium temperature estimates based on geothermometric techniques on gas and liquid phases at surface do not suggest the presence of shallow active hydrothermal systems a large circulation of fluids gases is likely triggered by the presence of mantle magmas stored inside the crust if total 3 he comes from the mantle or from the degassing of magmas stored in the crust co 2 might be associated to both volcanic degassing and thermometamorphism of recently subducted limestones

distinctive field behavior following geomagnetic reversals the paleomagnetism of lava flows from kauai hawaii erupted about 4 my ago immediately following a geomagnetic reversal reveals that the post transitional field had several distinctive characteristics while the directional dispersion was identical to that displayed by stable field over the past 5 my the post transitional field intensity was unusually high in both these respects the field at kauai resembles the one recorded at 15 ma by lava flows at steens mountain in se oregon the two records also differ in an important way the large intensity oscillations that immediately followed the steens mountain reversal are not apparent in the data from kauai these results suggest that while the stability of the newly established dipole may vary significantly from one reversal to the next strong dipolar field and normal directional dispersion may be systematic features of the post transitional geodynamo furthermore the results lend support to the recent suggestion that the core remains in an unusual state for many tens of kyr following a reversal

holocene geomagnetic paleointensities a blind test of absolute paleointensity techniques and materials through several decades of research absolute paleointensity estimates have been obtained from a wide range of rocks with varying ages covering the last 3 45 ga years these paleointensity data are crucial to study the past geodynamo and the geological evolution of the earth s deep interior however paleointensity data are often difficult to interpret and on going discussions concerning the past geomagnetic field invariably focus on data reliability and which kind of material and method is best suited for absolute paleointensity experiments in this paper we blind test paleointensity methods and materials using the new geomagia50 database that contains all available experimental information about published paleointensity estimates covering the last 50 000 years our analysis is built on simple comparisons of results obtained from different materials and methodologies to investigate their possible influence on the paleointensity estimate we also study the effect of various numbers of samples and dating accuracies we focus on paleointensity estimates from the last 12 000 years which includes the holocene epoch the advantage of this period is that data have been obtained from a wide array of methodological techniques as well as a wide selection of materials including both natural ones e g lava flows and various archaeological artifacts our main observations are i that well fired archaeological materials bricks potteries clays and ceramics show the best correlation with the rest of the dataset which we interpret to suggest that these data are the most reliable and ii although paleointensity data obtained from lava flows are slightly more scattered there is no evidence for paleointensity data from lavas to be significantly lower or higher than other data irrespective of the methodology c 2007 elsevier b v all rights reserved less

new absolute paleointensity results from the parana magmatic province uruguay and the early cretaceous geomagnetic paleofield 1 thellier paleointensity experiments are carried out on the early cretaceous arapey formation which belongs to the parana magmatic province pmp in northern uruguay twenty seven samples out of 225 analyzed yield reliable absolute intensity determinations for these samples the nrm fraction f used for determination ranges between 0 33 to 0 83 and the quality factor q ranges from 4 5 to 14 1 being generally greater than 5 the flow mean paleointensity values obtained in this study range from 16 3 2 3 to 41 5 3 8 t with corresponding virtual dipole moments vdms from 3 4 0 6 to 8 2 0 8 1022 am2 this corresponds to mean vdm value of 6 4 2 3 1022 am2 40ar 39ar dating on the pmp sequence by feraud et al 1999 gave a mean age of 132 2 0 5 ma our results in conjunction with previous studies kosterov et al 1998 goguitchaichvili et al 2002 suggest that the paleostrength during the early cretaceous might be as variable and strong as brunhes data and not anomalously low as previously suggested the scatter observed in our study is similar to recent findings by granot et al 2007 for the troodos ophiolite

cooling rate effects on paleointensity estimates in submarine basaltic glass and implications for dating young flows 1 cooling rate effects on the intensity of thermoremanent magnetization trm have been well documented in ceramics in that case laboratory cooling is generally more rapid than the initial cooling leading to an overestimate of the paleofield by 5 10 in thellier type paleointensity experiments the reverse scenario however has never been tested we examine the effects of cooling rate on paleointensity estimates from rapidly quenched submarine basaltic glass sbg samples from 13 sites at 17 30 18 30 s on the east pacific rise absolute cooling rates determined by relaxation geospeedometry at five of these sites range from 10 to 330 c min 1 at the glass transition 650 c over the dominant range of remanence blocking temperatures 200 400 c the natural cooling rates are approximately equal to or slightly slower than the laboratory cooling rates during the thellier experiment these results suggest that while the cooling rate effect might introduce some within site scatter it should not result in a systematic bias in paleointensity from sbg paleointensity estimates from the 15 sites range from 29 to 59 t with an average standard error of 1 t comparison with models of geomagnetic field intensity variations at the site indicate the youngest group of samples is very recent indistinguishable from present day and the oldest is at least 500 and probably several thousand years old these age estimates are consistent with available radiometric ages and geologic observations

palaeointensity of west greenland palaeocene basalts asymmetric intensity around the c27n c26r transition abstract palaeomagnetic sampling of the lower part of the palaeocene flood basalts of west greenland covering a composite succession of 1 6 km and close to 350 individual lava flows revealed two polarity zones and a 200 m zone of intermediate directions 40 ar 39 ar ages indicate that the transition is c27n c26r following palaeodirectional and rock magnetic analysis 102 samples from 64 flows from the pre transitional normal polarity zone the zone of intermediate directions and the post transitional reverse polarity zone were chosen for thellier palaeointensity experiments altogether 83 samples yielded reliable palaeointensity estimates the mean intensity of the pre transitional c27n field is 16 1 6 9 t 17 flows which is only twice the mean transitional field strength 9 9 3 7 t 10 flows the post transitional c26r field intensity is generally high with a mean 43 3 15 0 t 21 flows the asymmetric palaeointensity around the c27n c26r transition with a high post transitional field resembles the only other detailed absolute palaeointensity studies of geomagnetic transitions recorded in volcanic sequences in hawaii 4 ma and steens mountain 15 5 ma these new data therefore suggest that a strong post transitional field might be a systematic feature of geomagnetic reversals probably indicating a prolonged period of an unusual state of the geodynamo following polarity transitions

a paleomagnetic power spectrum we construct a power spectrum of geomagnetic dipole moment variations or their proxies that spans the period range from some tens of million down to about 100 years empirical estimates of the spectrum are derived from the magnetostratigraphic time scale from marine sediment relative paleointensity records and from a time varying paleomagnetic field model for the past 7 kyr the spectrum has the most power at long periods reflecting the influence of geomagnetic reversals and in general decreases with increasing frequency decreasing period the empirical spectrum is compared with predictions from simple models discrepancies between the observed and predicted spectra are discussed in the context of i changes in reversal rate ii overall average reversal rate iii cryptochrons iv the time taken for a reversal to occur and v long term paleosecular variations and average estimates of the field strength and variance from other sources 2005 elsevier b v all rights reserved

small scale structure of the geodynamo inferred from oersted and magsat satellite data the geodynamo in the earth s liquid outer core produces a magnetic field that dominates the large and medium length scales of the magnetic field observed at the earth s surface1 2 here we use data from the currently operating danish oersted3 satellite and from the us magsat2 satellite that operated in 1979 80 to identify and interpret variations in the magnetic field over the past 20 years down to length scales previously inaccessible projected down to the surface of the earth s core we found these variations to be small below the pacific ocean and large at polar latitudes and in a region centred below southern africa the flow pattern at the surface of the core that we calculate to account for these changes is characterized by a westward flow concentrated in retrograde polar vortices and an asymmetric ring where prograde vortices are correlated with highs and retrograde vortices with lows in the historical 400 year average magnetic field4 5 this pattern is analogous to those seen in a large class of numerical dynamo simulations6 except for its longitudinal asymmetry if this asymmetric state was reached often in the past it might account for several persistent patterns observed in the palaeomagnetic field7 8 9 10 we postulate that it might also be a state in which the geodynamo operates before reversing

magnetic hysteresis monitoring of cretaceous submarine basaltic glass during thellier paleointensity experiments evidence for alteration and attendant low field bias on the basis of studies of holocene samples submarine basaltic glass sbg is thought to be an ideal paleointensity recorder because it contains unaltered single domain magnetic inclusions that yield thellier paleointensity data of exceptional quality to be useful as a recorder of the long term geomagnetic field older sbg must retain these optimal properties here we examine this issue through rock magnetic and transmission electron microscope tem analyses of cretaceous sbg recovered at ocean drilling program site 1203 northwestern pacific ocean these sbg samples have very low natural remanent magnetization intensities nrm 2 g and tem analyses indicate a correspondingly low concentration of crystalline inclusions thellier experiments on samples with the strongest nrm intensity 5 10 11 am 2 show a rapid acquisition of thermoremanent magnetization trm with respect to nrm demagnetization taken at face value this behavior implies magnetization in a very weak 17 t ambient field but monitoring of magnetic hysteresis properties during the thellier experiments on subsamples of the sbg samples used for paleointensity determinations indicates systematic variations in values over the same temperature range where the rapid trm acquisition is observed a similar change in properties during heating is observed on monitor sbg specimens using low temperature data with progressive heatings the verwey transition becomes more distinct we suggest that these experimental data record the partial melting and neocrystallization of magnetic grains in sbg during the thermal treatments required by the thellier method resulting in paleointensity values biased to low values we further propose that this process is pronounced in cretaceous and jurassic sbg relative to holocene sbg because devitrification on geologic time scales i e tens of millions of years lowers the transition temperature at which the neocrystallization can commence magnetic hysteresis monitoring may provide a straightforward means of detecting the formation of new magnetic inclusions in sbg during thellier experiments

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

the intensity of the time averaged geomagnetic field the last 5 myr the existing database for paleointensity estimates of the ancient geomagnetic field contains more than 1500 data points collected through decades of effort despite the huge amount of work put into obtaining these data there remains a strong bias in the age and global distribution of the data toward very young results from a few locations also few of the data meet strict criteria for reliability and most are of unknown quality in order to improve the age and spatial distribution of the paleointensity database we have carried out paleointensity experiments on submarine basaltic glasses from a number of dsdp sites of particular interest are the sites that provide paleointensity data spanning the time period 0 3 5 ma a time of relatively few high quality published data points our new data are concordant with contemporaneous data from the published literature that meet minimum acceptance criteria and the combined data set yields an average dipole moment of 5 49 2 36u10 22 am 2 this average value is comparable to the average paleofield for the period 5 160 ma 4 2 2 3u10 22 am 2 t juarez l tauxe j s gee and t pick 1998 nature 394 878 881 and is substantially less than the value of approximately 8u10 22 am 2 often quoted for the last 5 myr e g mcfadden

paleointensity record in zero age submarine basalt glasses testing a new dating technique for recent morbs abstract thellier thellier paleointensity experiments were conducted on a collection of glasses from three very recent submarine axial flows two were erupted along the juan de fuca ridge at around 46 n and one along the east pacific rise south at around 18 s the within sample dispersion of paleointensity results from the animal farm flow epr south is very low and leads to a well defined mean value of 35 6 1 t 95 error on the mean based on 11 glass chips from four independent samples today s geomagnetic field intensity in the area is 31 2 t comparing animal farm results with published field model reference curves developed for the past 400 yr suggests an eruptive date estimated between 1880 a d and 1950 a d taking into account different sources of errors this is consistent with qualitative evidence for the age of this flow and constitutes the first precise demonstration of using paleointensity as a dating tool for very recent mid ocean ridge basalts morbs however in the juan de fuca area results show a more erratic pattern with samples varying by up to 30 higher and lower from the expected value of about 55 t the dispersion is attributed to the large local crustal magnetic anomalies in this area that can lead to inconsistent intensity values over the same unit local magnetic anomalies should thus always be checked when doing paleointensity on morb samples which should also be distributed as widely as possible in a flow unit when no significant magnetic anomalies are detected the paleointensity dating tool is anticipated to be especially efficient to investigate the volcanic cyclicity along the epr axis during the last several hundred years

detecting multidomain magnetic grains in thellier palaeointensity experiments the thellier technique is the most commonly used method for absolute palaeointensity studies it is based on thellier s laws stating that partial thermoremanent magnetisations ptrms are independent and additive which however only are true for non interacting single domain sd grains for multidomain md grains individual ptrms are not independent and thellier palaeointensity experiments on rocks with a significant proportion of md grains may therefore lead to erroneous palaeofield estimates by introducing a ptrm tail check in the coe version of the thellier technique j geophys res 72 1967 3247 we present a simple method to detect and discard such rocks the usefulness of the ptrm tail check is exemplified by thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy miocene baked sediments from french massif central are shown to be ideal for thellier studies with the remanent magnetisation residing in thermally stable sd grains for palaeocene eocene basaltic lavas from faeroe islands 64 of otherwise acceptable palaeointensity estimates are discarded due to the failure of ptrm tail checks indicating the presence of significant concentrations of md grains 2001 elsevier science b v all rights reserved

absolute geomagnetic paleointensity after the cretaceous normal superchron and just prior to the cretaceous tertiary transition 1 we report a detailed rock magnetic and thellier paleointensity study from autlan volcanic succession in western mexico the whole rock k ar age obtained for these lavas 67 4 1 2 ma together with the magnetostratigraphy results indicates that the lava flows were erupted during chron 30n just prior to cretaceous tertiary transition thirty nine samples from eight cooling units were preselected for paleointensity experiments on the basis of their low viscosity index stable remanent magnetization and close to reversible continuous thermomagnetic curves high quality determinations were obtained for 18 individual samples from four independent lava flows the mean virtual dipole moment vdm obtained in this study is 4 9 0 6 x 10 22 a m 2 which is significantly lower than the present geomagnetic field strength but in reasonably good accord with other comparable quality determinations between 35 and 80 ma our data are not significantly different from almost all published paleointensities between 5 and 90 ma moreover rigorously selected worldwide paleointensity data in the same interval better fit trend 2 of heller et al 2002 a careful examination of high quality paleointensity data from cretaceous indicates that there is no simple relation between magnitude of earth s magnetic field and reversal rate

a view into the cretaceous geomagnetic field from analysis of gabbros and submarine glasses the nature of the geomagnetic field during the cretaceous normal polarity superchron cns has been a matter of debate for several decades numerical geodynamo simulations predict higher intensities but comparable variability during times of few reversals than times with frequent reversals published geomagnetic paleointensity data from the cns are highly scattered suggesting that additional studies are required here we present new paleointensity results from 18 sites collected from the lower oceanic crust of the troodos ophiolite cyprus 92 1 ma old together with recently published data from the troodos upper crust we obtain three independent paleointensity time series these sequences reveal quasi cyclic variations of intensities about a mean value of 54 20 z am 2 providing insight into the fluctuating nature of the cretaceous magnetic field our data suggest the cns field was both weaker and more variable than predicted by geodynamo simulations the large amplitudes of these variations may explain the wide range of dipole moments previously determined from the cns 2007 elsevier b v all rights reserved

magnetic alteration of zero age oceanic basalt the youngest sampled submarine lava flow which erupted june 1993 on the juan de fuca ridge provides the basis for a tight constraint on the initial or zero age magnetization state of morb detailed profiles of magnetic hysteresis parameters curie temperatures and unblocking temperatures of nrm with respect to the chilled margin of a pillow fragment show evidence of significant oxidation which preferentially affected the finest grain size fraction and principal remanence carrier of the titanomagnetite magnetic mineralogy the oxidation must have occurred during or immediately after initial cooling implying that morb is already appreciably magnetically altered before aging nevertheless successful results of thellier paleointensity experiments on the basalt sample lend support to the idea that crustal magnetization represented by morb preserves a record of geomagnetic intensity variations that may be reflected in small scale magnetic anomalies

characteristics of magnetite in submarine basaltic glass summary we have compared the rock magnetic characteristics of cretaceous submarine basaltic glass sbg from dsdp odp holes 417d 418a 807c and 543a to data from holocene glasses from the east pacific rise both groups are very similar in that they contain single domain low ti magnetite as the carrier of a strong and stable remanent magnetization the behaviour of magnetic hysteresis in sbg is however dominated by superparamagnetic grain size populations transmission electron microscopy revealed spherically shaped grains of 10 20 nm diameter with crystal lattice properties consistent with magnetite these grains could be responsible for the magnetic behaviour of sbg

the signature of inner core nucleation on the geodynamo abstract energy considerations indicate that the power delivered to the present day geodynamo comes mainly from the growth of the solid inner core through light element and latent heat releases the nucleation of the inner core was therefore a major transition for the geodynamo here we use numerical dynamo simulations linked by thermochemical evolution of the core to investigate the effects of inner core nucleation icn on the geodynamo and identify possible icn footprints in the palaeomagnetic field our results suggest that a strong field dipole dominated dynamo prior and after icn the uniformitarian scenario is most likely we predict little footprint of icn on the surface field intensity consistent with the observed lack of a long term trend in paleointensity data instead we find that the best proxy for inner core growth is an increase in axial octupole strength with age from present day to icn which might be resolvable in the palaeomagnetic record we confirm the existence of weak field multipolar dynamos prior to icn suggesting the dynamo might have transiently reached a weak field state the catastrophic scenario the narrow stability domain of these weak field dynamos implies that the catastrophic scenario is less probable than the uniformitarian scenario

cretaceous geomagnetic paleointensities thellier experiments on pillow lavas and submarine basaltic glass from the ontong java plateau 1 we present new thellier paleointensity results on pillow lavas and pillow rim submarine basaltic glass sbg from 120 ma basalts erupted on the ontong java plateau all pillow lavas have suffered low temperature alteration and are unsuitable for thellier paleointensity experiments in contrast pillow rim sbg is virtually impermeable to seawater and therefore has suffered little or no low temperature alteration out of 21 selected sbg samples with magnetic moment larger than 10 10 am2 18 samples from 12 chilled sbg margins yielded paleointensity estimates of good technical quality rock magnetic measurements show the primary carrier of remanence is low ti titanomagnetite occurring in a continuous range of compositions starting from nearly pure magnetite as suggested by a recent study magnetic hysteresis parameters were monitored during the entire paleointensity experiment and a drastic change of parameters was observed above 400 440 c which is in accordance with the partial thermoremanent magnetization checks failing significantly above the same temperature we suggest that paleointensity estimates obtained below this critical temperature represent true determination of the paleomagnetic field strength for several pillow margins two to three samples were measured and in such cases the data agree well altogether we obtain reliable estimates from 12 sbg pillow rims yielding a mean paleointensity of 27 1 13 4 t and a corresponding virtual axial dipole moment of 5 8 2 8 1022 am2 this value is slightly higher than whole rock data from similar aged subaerial lava flows the paleosecular variation of the intensity estimates is slightly larger than previously reported for paleointensity determinations for the cretaceous normal superchron

strength of the geomagnetic field in the cretaceous normal superchron new data from submarine basaltic glass of the troodos ophiolite 1 we present here new paleointensity data from 39 sampling sites collected from the quenched margins of pillow lavas and dikes exposed within the troodos ophiolite similar to 92 ma formed during the cretaceous normal superchron cns a period of approximately 40 million years when the geomagnetic field reversed extremely infrequently if at all monte carlo simulations suggest that a minimum of 25 estimates are necessary for a reasonably robust estimate for the average field strength our data suggest a dipole strength equivalent to the present field or nearly twice the post cns average the mean and standard deviation of the dipole moment 81 43 zam 2 z 10 21 from the 57 data points compiled here agree remarkably well with those predicted from the long paleointensity record derived from dsdp site 522 the new data set for the cns suggests a picture of a strong and stable field during the period of time when it stopped reversing moreover the similarity of the cns data with the present geomagnetic field suggests that it is presently in a state of unusual polarity stability

the intensity of the earth s magnetic field over the past 160 million years in contrast to our detailed knowledge of the directional behaviour of the earth s magnetic field during geological and historical times1 2 data constraining the past intensity of the field remain relatively scarce this is mainly due to the difficulty in obtaining reliable palaeointensity measurements a problem that is intrinsic to the geological materials which record the earth s magnetic field although the palaeointensity database has grown modestly over recent years3 4 5 these data are restricted to a few geographical locations and more than one third of the data record the field over only the past 5 myr the most recent database5 covering the time interval from 5 to 160 myr contains only about 100 palaeointensity measurements here we present 21 new data points from the interval 5 160 myr obtained from submarine basalt glasses collected from locations throughout the world s oceans whereas previous estimates for the average dipole moment were comparable to that of the earth s present field6 the new data suggest an average dipole moment of 4 2 2 3 1022 a m2 or approximately half the present magnetic field intensity this lower average value should provide an important constraint for future efforts to model the convective processes in the earth s core which have been responsible for generating the magnetic field

geomagnetic palaeointensities during the cretaceous normal superchron measured using submarine basaltic glass high quality palaeointensity data have been obtained from thellier thellier experiments on recent and cretaceous submarine basaltic glasses whereas the recent samples faithfully yield today s geomagnetic intensity at the site the palaeointensities for the beginning and end of the cretaceous normal superchron are only 45 and 25 respectively of today s value the data thus extend the mesozoic dipole low into the cretaceous superchron and confirm that submarine basaltic glass is an excellent material for palaeointensity studies

a new approach for geomagnetic archaeointensity research insights on ancient metallurgy in the southern levant we present results from an archaeointensity investigation based on a relatively unexploited recording medium copper slag deposits together with a recently improved experimental design for the archaeointensity experiment we demonstrate the applicability of this medium as well as other archaeometallurgical artifacts for the study of the ancient geomagnetic field intensity in addition to archaeointensity data from well dated archaeological contexts we obtained reliable archaeointensity results from poorly dated or contentious archaeometallurgical sites in the southern levant these results shed new light on the dating of these sites among them the copper smelting installation of timna 39b a site that has important implications for the beginning of metallurgy during the fifth millennium bce the paper also aims to introduce archaeointensity research to the archaeologist scholar and to encourage further collaboration between the disciplines in future research

hyperquenched volcanic glass from loihi seamount hawaii abstract explosive submarine basaltic eruptions occurring in water depths of several kilometres commonly result in the formation of layered volcaniclastic deposits in order to quantify the cooling of such volcaniclastics we have identified and separated two types of glassy fragments from these deposits on loihi seamount 1 fine 30 80 m limu o pele bubble wall fragments and 2 coarser 0 8 1 2 mm dense angular fragments these pristine basaltic glasses have been subjected to differential scanning calorimetry dsc from room temperature up to temperatures above their glass transition heat capacity c p in j g 1 k 1 data reveal glassy and liquid regimes separated by clear hysteresis behaviour within the glass transition the transient c p in the glass transition interval exhibits a deep trough before the peak indicating very high cooling rates it is a classic feature of so called hyper quenching having been observed in dsc experiments on glasses produced using the splat quench d b dingwell p courtial d giordano and a r l nichols viscosity of peridotite liquid earth and planetary science letters 226 1 2 127 138 2004 and cascade fibre spinning y z yue j d christiansen and s l jensen determination of the fictive temperature for a hyperquenched glass chemical physics letters 357 1 2 20 24 2002 techniques the trough is deepest for the limu o pele fragments energy matching methods developed for the estimation of cooling rates for such glasses yield a rate of 10 5 31 k s 1 for the cooling of the fine sieve fraction of limu glass on the sea floor at loihi thus limu o pele volcaniclastics believed to be formed during mild submarine pyroclastic eruptions have experienced the fastest cooling of any natural volcanic glass measured to date such extreme cooling rates are likely to impact on many chemical and physical aspects of the stability of these glasses on the sea floor as well as their use as proxies for field variables of the earth s physical state and as monitors of the efficiency of chemical lithosphere hydrosphere exchange

paleointensity estimates from ignimbrites an evaluation of the bishop tuff 1 ash flow tuffs or ignimbrites typically contain fine grained magnetite spanning the superparamagnetic to single domain size range that should be suitable for estimating geomagnetic field intensity however ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history we examined three stratigraphic sections through the 0 76 ma bishop tuff where independent information on postemplacement cooling and alteration is available as a test of the suitability of ignimbrites for paleointensity studies thermomagnetic curves suggest that low ti titanomagnetite tc 560 c 580 c is the dominant phase with a minor contribution from a higher tc phase s significant remanence unblocking above 580 c suggests that maghemite and or titano maghemite is an important contributor to the remanence in most samples we obtained successful paleofield estimates from remanence unblocked between 440 c and 580 c for 46 of 89 specimens 15 sites at two of three total localities these specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43 0 t 3 2 equivalent to a vadm of 7 8 1022 am2 the most densely welded sections of the tuff have emplacement temperatures inferred to be as high as 660 c suggesting that the remanence may be primarily thermal in origin though a contribution from thermochemical remanence cannot be excluded these results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations

11 million years of oligocene geomagnetic field behaviour summary an 11 million year long record of the oligocene geomagnetic field has been obtained from pelagic sediments of dsdp hole 522 an average sample spacing of 4 cm yielded approximately one specimen per 4 to 8 kyr the rock magnetics are remarkably consistent across the entire interval previous work demonstrated a magnetic mineralogy dominated by magnetically stable magnetite the natural remanent magnetism nrm carries an oligocene polarity timescale that is in excellent agreement with the oligocene reversal record as determined from marine magnetic anomalies mmas including many of the so called crypto chrons normalized nrm intensities from the undisturbed portions of the record yield a time series of variations with features consistent with a number of other palaeointensity time series derived from both sedimentary and lava sequences these features include consistent major decreases in palaeointensity dips at reversal boundaries and occasional dips between reversal boundaries that could correspond to lineated tiny wiggles in the mma records the data set suggests that the overall field strength was 40 per cent higher in the first half of the oligocene when the average reversal frequency was 1 6 myr l than in the second half when the reversal frequency was 4myr there is also a weak dependence of average field strength on length of polarity interval finally in the three cores suited to spectral analysis of coherent polarity and relative intensity independent of lithological contamination there is a persistent ca 30 50 ka periodicity in the variations of the relative intensity suggesting that the geomagnetic field pulses at about this frequency not only during the brunhes as demonstrated by tauxe shackleton 1994 but in the oligocene as well

geomagnetic field intensity how high can it get how fast can it change constraints from iron age copper slag article the intensity of the geomagnetic field varies over different time scales yet constraints on the maximum intensity of the field as well as for its maximum rate of change are inadequate due to poor temporal resolution and large uncertainties in the geomagnetic record the purpose of this study is to place firm limits on these fundamental properties by constructing a high resolution archaeointensity record of the levant from the 11th century to the early 9th century bce a period over which the geomagnetic field reached its maximum intensity in eurasia over the past 50 000 years we investigate a 14 c dated sequence of ten layers of slag material which accumulated within an ancient industrial waste mound of an iron age copper smelting site in southern israel depositional stratigraphy constrains relative ages of samples analyzed for paleointensity and 14 c dates from different horizons of the mound constrain the age of the whole sequence the analysis yielded 35 paleointenisty data points with accuracy better than 94 and precision better than 6 covering a period of less than 350 years most probably 200 years we construct a new high resolution quasi continuous archaeointensity curve of the levant that displays two dramatic spikes in geomagnetic intensity each corresponding to virtual axial dipole moment vadm in excess of 200 zam 2

magnetic anomaly lineations from late jurassic to early cretaceous in the west central pacific ocean summary late jurassic to early cretaceous mesozoic magnetic anomaly lineations the phoenix magellan mid pacific mountains hawaiian and japanese lineation sets with fracture zones in the west central pacific ocean were identified more comprehensively than in any previous studies we fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks two remarkable fracture zones the phoenix and central pacific fracture zones were mapped and newly named our newly identified lineations from mlon to mo around the mid pacific mountains which belong to the hawaiian lineation set illustrated that the sea floor south of the mid pacific mountains has the same age as that of the north 132 118 ma our analysis of skewness parameters revealed that the older part of the phoenix set m17 m29 has skewness different from that of the younger part ml m14 implying an effect of magnetic overprints by the cretaceous volcanism it was confirmed that the spreading rate of the mesozoic pacific spreading system was the fastest in the world in the mesozoic a drastic change in spreading rates occurred simultaneously at the period between chrons m21 and m20 149 5 148 5 ma in all the mesozoic pacific spreading systems the event appears to be synchronous with events in other oceans such as the mesozoic atlantic and indian oceans