a 2 14 myr astronomically tuned record of relative geomagnetic paleointensity from the western philippine sea 1 we present a 2 14 myr astronomically tuned relative geomagnetic paleointensity record from the western philippine sea pseudosingle domain titanomagnetite is the only magnetic mineral identified and variations in titanomagnetite concentration fall well within the accepted limits for relative paleointensity variations no significant temporally persistent periodicities are observed in wavelet analyses of the paleointensity time series or in the rock magnetic parameters used for relative paleointensity normalization this suggests that our paleointensity record is largely free of rock magnetic or lithological artefacts and that it represents a reliable record of geomagnetic behavior with no evidence for modulation of the field at earth orbital periods the paleointensity record is highly coherent with the sint 800 global paleointensity stack for the last 800 kyr and with a coeval record from the west caroline basin our record confirms that it is normal for the geomagnetic field to undergo dynamic changes within polarity intervals with relatively frequent collapses of the field to low paleointensities and concomitant deviations away from the stable field direction we do not observe an asymmetrical sawtooth form in our paleointensity record which might suggest that previously observed asymmetrical sawtooth paleointensities result from rock magnetic artefacts also we do not observe a persistent 100 kyr inclination periodicity in contrast to the suggestion that geomagnetic field directions are modulated by orbital eccentricity good agreement between our paleointensity record and the coeval west caroline basin record provides the beginning of a detailed view of geomagnetic field behavior between 0 8 and 2 14 ma

magnetic properties of pleistocene sediments from the mediterranean sea a correlation with the palaeoclimatic record the magnetic properties of the late quaternary hemipelagic sediment from piston core bs77 15 taken in the tyrrhenian sea were investigated and the results compared with palaeoclimatic records the magnetic mineralogy of the sediment was studied and the magnetic carriers were identified by their spectra of coercivity and unblocking temperature investigations of rock magnetic parameters such as the natural remanent magnetization nrm the low field magnetic susceptibility the frequency dependent susceptibility fd the acquisition of anhysteretic magnetization arm and isothermal magnetization irm and the median destructive field mdf have established the downcore variations in terms of magnetic mineralogy grain size and coercivity in the sediments the rock magnetic parameters were compared with an alternative climatic record from the same core based on the faunal association of planktonic foraminifera a significant correlation was found between faunal climatic record and fd together with the magnetic properties of the sediment this suggests that a palaeoclimatic signal is recorded by a fine grained low coercivity fraction of the magnetic minerals

comment on a relative geomagnetic paleointensity stack from ontong java plateau sediments for the matuyama by yvo s kok and lisa tauxe

evolution of the geomagnetic field prior to the matuyama brunhes transition radiometric dating of a 820 ka excursion at la palma we present cassignol technique k ar dating of lava flows from la palma canary islands spain that bracket the matuyama brunhes transition an age of 821 13 ka obtained for a transitionally magnetized flow ls118 provides the first volcanic evidence for a geomag netic excursion occurring about 40 kyr prior to the transition this interval has been successfully correlated with intensity minima present in high resolution deep sea records before the matuyama brunhes transition this study along with the growing number of well dated excursions reported for the brunhes and matuyama chrons shows that the occurrence of excursions is a common feature of the evolution of the geomagnetic field during stable polarity intervals however the presence of two successive excursional states within 40 kyr prior to the actual reversal conjugated with moderate averaged palaeointensity in this interval as deduced from deep sea records could suggest that favorable conditions for the reversal to occur existed since about 820 ka the present study highlights the importance of detailed chronological constraints for our understanding of the transition of the geomagnetic field from a stable polarity to a reversal state

fingerprinting the water site in mantle olivine nominally anhydrous minerals such as olivine contain trace amounts of water and may accommodate the entire water budget of the upper mantle here we report for the first time synthetic olivines crystallized experimentally under upper mantle conditions that reproduce the most common and intense infrared hydroxyl stretching bands at 3572 and 3525 cm 1 observed in spinel peridotite mantle olivines these bands arise from water accommodated at point defects associated with the trace element ti and we suggest that this is the most important defect site in the shallow upper mantle additional hydrated defects may occur at higher pressures we also identify bands related to water associated with fe3 these are unlikely to reflect equilibrium with the mantle and indicate water incorporation during exhumation or retrogression water must be present at the defect site appropriate for the mantle at the conditions of interest for partitioning seismic wave speed and deformation experiments on hydrous olivine to be relevant

relative geomagnetic intensity of the field during the last 140 ka relative magnetic field palaeointensities have been obtained from three marine cores in the somali basin western indian ocean the correlation between the records based on changes in the low field susceptibility and the inclination is extremely detailed a time versus depth correlation has been established from the 18o record the remanence intensity was normalized with respect to the anhysteretic remanent magnetization with special attention to changes in grain sizes coercivities and mineralogy the synthetic record of relative palaeointensity has been extended to cover the last 140 ka the quasi cyclic pattern observed during the last 80 ka confirms the results previously obtained in the mediterranean sea and thereby establishes its dominantly dipolar character between 140 and 80 ka the field intensity exhibits a much longer cycle spectral analysis of the intensity records shows the presence of two dominant periodicities but the length of the signal is too short to document the presence of a stationary process

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

a geomagnetic record over the last 3 5 million years from deep tow magnetic anomaly profiles across the central indian ridge high resolution records of the geomagnetic field intensity over the last 4 myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short lived low field intensity features associated with excursions or short polarity intervals in order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events we have collected six deep tow 500 m above the seafloor and several sea surface magnetic anomaly profiles from the central indian ridge across the brunhes matuyama and gauss chrons i e from the ridge axis to anomaly 2a after removal of topography latitude and azimuth effects we converted distances into time sequences using well dated polarity reversal anomalies as tie points we calculated the average signal to test the robustness of the short wavelength anomalies the resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the central indian ridge the east pacific rise and the pacific antarctic ridge our results suggest that in addition to polarity reversals to previously suggested geomagnetic events subchrons or excursions within the brunhes and matuyama chrons a new small scale magnetic anomaly likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas we observe a dense succession of microanomalies that are correlated excursions ontong java 1 and 2 and gilsa is found after the olduvai chron the near bottom results support the existence of three geo magnetic features between the gauss matuyama boundary and olduvai they also suggest three geomagnetic events during the c2a i n subchron within the gauss chron this study emphasizes the potential of deep tow magnetic surveys in detecting fluctuations in geo magnetic field intensity and in particular short lived excursions a poorly constrained part of the geomagnetic field temporal variation spectrum

birth death and resurrection the life cycle of suprasubduction zone ophiolites 1 suprasubduction zone ssz ophiolites display a consistent sequence of events during their formation and evolution that suggests that they form in response to processes that are common to all such ophiolites this sequence includes the following 1 birth which entails the formation of the ophiolite above a nascent or reconfigured subduction zone this stage is typically characterized by the eruption of arc tholeiite lavas and the formation of layered gabbros and sheeted dike complex 2 youth during which is continued melting of refractory asthenosphere depleted during birth occurs in response to fluid flux from the subducting slab with extensional deformation of the older plutonic suite eruption of refractory lavas and the intrusion of wehrlite pyroxenite 3 maturity with the onset of semistable arc volcanism typically calc alkaline as the subduction zone matures and stabilizes and the intrusion of quartz diorite and eruption of silicic lavas and 4 death which is the sudden demise of active spreading and ophiolite related volcanism which in many cases is linked to collision with an active spreading center and the onset of shallow underthrusting of the buoyant spreading axis expressed as dikes and lavas with oceanic basalt compositions that crosscut or overlie rocks of the older suites 5 resurrection with emplacement by obduction onto a passive margin or accretionary uplift with continued subduction the early stages 1 3 may be diachronous and each stage may overlap in both time and space the existence of this consistent progression implies that ophiolite formation is not a stochastic event but is a natural consequence of the ssz tectonic setting

thermoremanence anhysteretic remanence and susceptibility of submicron magnetites nonlinear field dependence and variation with grain size we have measured initial susceptibility 0 and the dependence of anhysteretic remanent magnetization arm and thermoremanent magnetization trm on applied field for seven samples of magnetite with mean grain sizes from 40 to 540 nm trm acquisition is nonlinear in geomagnetically relevant weak fields contrary to the usual assumption made in paleointensity determination over the 40 540 nm range 0 varies with particle shape but only weakly with particle size d and can be used to correct for varying magnetite concentrations in sediment cores arm is strongly size dependent over the same range and is the best parameter for monitoring grain size variations in natural samples arm and trm have similar variations as d 1 for d 1 m suggesting a common source for this pseudo single domain psd dependence on grain size however trm is 10 20 times more intense than arm in grains around 0 2 m in size the trm microstate seems to be a two domain structure whereas the arm microstate may be a vortex structure which has never before been convincingly demonstrated by magnetic measurements independent evidence comes from theoretical fits to trm and arm field dependence data in moderate and strong fields trm in 215 540 nm magnetites is explained by two domain theory but arm is not in smaller grains d 0 1 m a psd theory predicts that trm is carried by single domain sd moments of entire grains but arm resides in moments 1 per grain 20 40 times weaker these remanence levels match those of metastable sd and vortex states respectively we therefore propose that magnetite grains in the 0 1 0 5 m size range can remain in metastable sd or two domain states following acquisition of trm but revert to a vortex ground state when field cycled for example in arm acquisition or alternating field demagnetization

magnetostratigraphic data from late quaternary sediments from the yermak plateau arctic ocean evidence for four geomagnetic polarity events within the last 170 ka of the brunhes chron summary palaeomagnetic investigations of two sediment cores recovered from rv polarstern near the eastern slope of the yermak plateau sites ps 1533 and ps 2212 reveal convincing evidence for four polarity events of the earth s magnetic field during the last 170 ka a comprehensive rock magnetic study of the sediments proved that fine grained magnetite is the principal carrier of the remanent magnetization no changes in magneto mineralogy across the polarity transitions in the sediments investigated were found calcareous nannofossil biostratigraphy ams 14c accelerated mass spectrometry and oxygen isotope data and 10be and 230th stratigraphies yielded age ranges of 24 29 ka for the mono lake event 34 43 ka for the laschamp event 72 86 ka for the norwegian greenland sea event and 118 128 ka for the blake event two reverse polarity samples at the base of core ps 2212 3 kal are interpreted as the termination of the biwa i event 171 181 ka the events exhibit full inversion of inclination in both cores the data suggest that the transition process of the earth s magnetic field during such polarity events requires some 1 ka

geomagnetic dipole strength and reversal rate over the past two million years independent records of relative magnetic palaeointensity from sediment cores in different areas of the world can be stacked together to extract the evolution of the geomagnetic dipole moment1 2 and thus provide information regarding the processes governing the geodynamo so far this procedure has been limited to the past 800 000 years 800 kyr ref 3 which does not include any geomagnetic reversals here we present a composite curve that shows the evolution of the dipole moment during the past two million years this reconstruction is in good agreement with the absolute dipole moments derived from volcanic lavas which were used for calibration we show that at least during this period the time averaged field was higher during periods without reversals but the amplitude of the short term oscillations remained the same as a consequence few intervals of very low intensity and thus fewer instabilities are expected during periods with a strong average dipole moment whereas more excursions and reversals are expected during periods of weak field intensity we also observe that the axial dipole begins to decay 60 80 kyr before reversals but rebuilds itself in the opposite direction in only a few thousand years

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

sedimentary records of relative paleointensity of the geomagnetic field theory and practice sediments have proved irresistible targets for attempts at determining the relative variations in the earth s magnetic field because of the possibility of long and continuous sequences that are well dated and have a reasonable global distribution the assumption underlying paleointensity studies using sedimentary sequences is that sediments retain a record reflecting the strength of the magnetic field when they were deposited early theoretical work suggested that because the time required for an assemblage of magnetic particles in water to come into equilibrium with the ambient magnetic field was quite short no dependence on magnetic field was expected nonetheless a number of experiments showed that sedimentary magnetizations varied in accordance with the field albeit not always in a simple linear fashion experiments in which the sediments were stirred in the presence of a field to simulate bioturbation showed a reasonably linear relationship with the applied field and these results spurred the hope that variations in the earth s magnetic field might indeed be recoverable from appropriate sedimentary sequences examination of existing paleointensity data sets allows a few general conclusions to be drawn it appears that sedimentary sequences can and do provide a great deal of information about the variations in relative paleointensity of the earth s magnetic field the dynamic range of sedimentary data sets is comparable to those acquired from thermal remanences moreover when compared directly with such independent measures of magnetic field variations as beryllium isotopic ratios and thermally blocked remanences there is considerable agreement among the various records when viewed over timescales of hundreds to thousands of years relative paleointensity data sets from more than a few thousand kilometers apart bear little resemblance to one another suggesting that they are dominated by nondipole field behavior when viewed over timescales of a few tens of thousands to hundreds of thousands of years however the records show coherence over large distances at least thousands of kilometers and may reflect changes in the dipole field on the basis of a sequence spanning the brunhes and matuyama chrons the magnetic field has oscillated with a period of about 40 ka for the last few hundred thousand years but these oscillations are not clear in the record prior to about 300 ka thus they are probably not an inherent feature in the geomagnetic field and the correspondence of the period of oscillation to that of obliquity is probably coincidence

a short reverse polarity interval within the jaramillo subchron evidence from the jingbian section northern chinese loess plateau 1 a high resolution paleomagnetic and rock magnetic investigation supported by scanning electron microscopy observations x ray diffraction determinations and anisotropy of magnetic susceptibility analyses has been carried out on a loess paleosol sequence from jingbian 37 4 n 108 8 e at the northern extremity of the chinese loess plateau the results indicate that the magnetic assemblage is dominated by pseudo single domain magnetite with associated maghemite and traces of a higher coercivity phase probably hematite the brunhes matuyama and upper jaramillo transitions have been carefully documented and a record of a short reverse polarity interval sri entirely within the jaramillo normal polarity subchron has been obtained the duration of the sri recorded over a stratigraphic interval of 48 cm is 10 1 kyr 29 kyr if the polarity transitions are included these results seem to exclude that rock magnetic variations are adversely affecting the recording process and compel us to reconsider the jaramillo subchron as characterized by a pair of normal polarity intervals interrupted by a short reverse polarity interval

new advances for paleomagnetic studies of sediment cores using u channels accurate and detailed paleomagnetic data can be obtained from continuous measurements of u channels extracted from sedimentary cores using a small diameter pass through magnetometer and high resolution signal response pick up coils the spatial resolution is not significantly lower than that obtained by measuring standard paleomagnetic samples and features on the order of 3 5 cm can be confidently resolved we propose a method to analyze the demagnetization data based on the angular dispersion of the directions between successive demagnetization steps and the downcore changes in the rate of intensity decrease this technique allows us to distinguish very rapidly between intervals with well identified characteristic magnetic components and zones characterized by a more complex behavior during demagnetization it is also possible to directly measure the magnetic susceptibility and to induce and measure most rock magnetic properties along the u channels thus all parameters necessary for a complete paleomagnetic study of sediment cores can be confidently obtained directly from u channels without extracting individual samples finally this equipment makes possible the measurement and complete demagnetization of up to 10 single samples at a time in less than one hour by using techniques similar to those developed for the u channels

evaluation of kimberlite diamond potential using ftir spectroscopy of xenocrystic olivine abstract xenocrysts or phenocrysts of olivine in kimberlite show a diversity of hydroxyl related infrared absorption bands that reflect compositional variation of their mantle source and conditions of last equilibration interpretation of infrared spectra of olivine in terms of equilibration crystallization conditions may help to isolate spectral features associated with good diamond carrying and diamond preservation potential as an exploratory study we examine by ftir the spectroscopic features of olivine inclusions in diamond olivine coexisting with diamond in peridotite xenoliths and olivine from diamond free peridotite xenoliths the three environments are respectively characterized by spectroscopically anhydrous olivine olivine with low water concentrations h 2 o 2 o up to 260 ppm previous studies have shown that the majority of olivine macrocrysts in kimberlite contain 100 ppm h 2 o with a significant proportion containing 250 ppm h 2 o our new results suggest that hydrogenation of olivine occurs in the mantle during equilibration with kimberlite related hydrous fluids and or during equilibration with or crystallization from kimberlite magmas during ascent the association of gem diamonds with water poor mantle olivine suggests that such samples experienced limited interaction with hydrous fluids melts over a short time span thus preventing diffusional equilibration future ftir studies of olivine from kimberlites with widely variable diamond grades and reconciliation with diamond resorption features may permit development of new ftir based procedures to assist in the evaluation of new kimberlite discoveries

a comparison of different techniques for relative paleointensity parallel relative paleointensity records that are supposed to display the same geomagnetic variations often show discrepancies we compared various techniques of normalization with different parameters involving alternating fields a f and thermal demagnetization of sediment samples spanning the same period from the indian and the pacific oceans the results are basically unchanged using any technique there is also no causal relationship between changes in rock magnetic properties and mismatch between the two records thus discrepancies between parallel records do not depend on methods for relative paleointensity and any technique can be used with confidence provided that the characteristic component has been properly isolated and the sediments are magnetically homogeneous complications are more likely due to subtle variations in the sediment remanent magnetization acquisition these results point to the importance of sedimentological lithology and physical properties factors in controling the process of magnetization

a 200 kyr record of cosmogenic radionuclide production rate and geomagnetic field intensity from 10be in globally stacked deep sea sediments the reconstruction of geomagnetic field intensity variations during the last 200 kyr from paleomagnetic data is at present the subject of numerous studies and major debate there is currently no generally accepted record here we present a global stacked record of 230thex normalized 10be deposition in marine sediments representing relative variations in 10be production rate which are translated into field intensity variations the record shows major periods during which the field intensity was between 10 and 40 of the present day value namely 30 42 60 75 85 110 and 180 192 kyr b p our results are compared to independently derived paleomagnetic studies and th u calibrations of 14c dates on corals during most of the observed period the geomagnetic field intensity was weaker than today resulting in an overall 30 reduced value for the last 200 kyr

rock magnetic properties and relative paleointensity stack for the last 300 ka based on a stratigraphic network from the subtropical and subantarctic south atlantic abstract we present a detailed study of natural remanence and rock magnetic properties of eight sediment cores from a south north profile across the subtropical front stf in the south atlantic which previously have been combined into a stratigraphic network based on these measurements we construct a first relative paleointensity rpi stack for the central south atlantic sas 300 covering the last 300 ka the degree of down core homogeneity of magnetic mineral concentration as well as magnetic mineral content and grain sizes vary between all cores and are quantified by high resolution rock magnetic measurements in the cores north of and close to the stf the magnetic remanence is mainly carried by magnetite the cores in the south also contain a high coercive mineral fraction based on a linear model of the sediment matrix we propose to apply principal component analysis pca of natural remanent magnetization nrm anhysteretic and isothermal remanent magnetization arm and irm to determine a homogeneity interval where remanences are only affected by a single environmental signal this reduces lithologic and climatic influence upon the relative paleointensity record for our data set pca indicates that all remanences in the 30 mt to 80 mt demagnetization interval are dominated by a single environmental influence single core rpi estimates are then obtained by normalizing the nrm fraction in this homogeneity interval with respect to either irm arm in the same interval or by by direct averaging we obtain the stack sas 300 which is compared to other paleointensity series such as sint 800 or the rpi records from odp sites 1089 subantarctic south atlantic and 983 gardar drift the detailed documentation of environmental influences combined with the presentation of seven different recordings of the same geomagnetic signal allows to connect southern and northern hemisphere rpi data also with respect to possible common environmental distortions

petrology and origin of primitive lavas from the troodos ophiolite cyprus parental magmas to the troodos ophiolite are characterised by low tio2 and al2o3 and high sio2 extremely fresh and chemically primitive high mgo rocks are found within the upper pillow lavas and along the arakapas fault belt of cyprus and contain forsteritic olivine enstatite and groundmass clinopyroxene set in glass or plagioclase with accessory magnesiochromite and sometimes hornblende they are quartz normative and may have originally contained up to 3 wt h2o geochemically there are three distinct groups of primitive lavas based on tio2 and zr contents but also reflected by cao na2o and ree abundances these groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region the parental magma to the least depleted group group i was that of the major portion of the troodos plutonic complex and is similar to those postulated for other low ti ophiolites chemically it has close affinities with komatiitic basalts the most depleted lavas group iii all have u shaped ree profiles and variable 143nd 144nd ratios interpreted in terms of metasomatism of the source region by an incompatible element enriched component which was probably derived from a subducted slab these lavas represent an intermediate step in the development of boninite series rocks

long term secular variation of the geomagnetic field during the last 200 kyr recorded in sediment cores from the western equatorial pacific we have conducted a paleomagnetic study of five hemipelagic clay cores of 4 7 m in length obtained from the west caroline basin western equatorial pacific the purpose of this study is to clarify the long term secular variation of the geomagnetic field during the last 200 kyr using both intensity and directional changes one core was dated using oxygen isotope ratios 18o and characteristic magnetic susceptibility variations of about a factor of four allowed precise correlation between all cores the magnetic mineralogy is estimated to be dominated by magnetite of single domain sd and or pseudo single domain psd state the magnetic mineralogy and grain size are uniform throughout the cores which enabled us to obtain relative paleointensity variations as well as directional changes remanent intensity normalized by anhysteretic remanent magnetization arm indicates that sharp intensity drops to 20 of the present field occurred at about 40 and 190 kyr b p with a broader decrease around 110 kyr b p the former two are accompanied by anomalous remanent directions suggesting short polarity reversals or excursions their ages are close to those of the previously reported laschamp excursion and biwa i event a prominent intensity peak occurred immediately preceding the possible excursion at 40 kyr b p our results are in general similar to the recently reported datasets spanning the last 80 140 kyr from the somali basin the mediterranean and the sulu sea 1 3 the inclination record shows long term cyclic changes of several degrees in amplitude the intervals of the recurring variation are 40 50 kyr which is longer than the core s memory and is close to the milankovitch frequency associated with the change in obliquity of earth s rotational axis we infer that an external force such as orbital forcing could be a cause of the long term secular variation

late brunhes polarity excursions mono lake laschamp iceland basin and pringle falls recorded at odp site 919 irminger basin abstract component natural remanent magnetizations derived from u channel and 1 cm3 discrete samples from odp site 919 irminger basin indicate the existence of four intervals of negative inclinations in the upper brunhes chronozone according to the age model based on planktic oxygen isotope data these excursional intervals occur in sediments deposited during the following time intervals 32 34 ka 39 41 ka 180 188 ka and 205 225 ka these time intervals correspond to polarity excursions detected elsewhere known as mono lake laschamp iceland basin and pringle falls the isotope based age model is supported by the normalized remanence paleointensity record that can be correlated to other calibrated paleointensity records for the 0 500 ka interval such as that from odp site 983 for the intervals associated with the mono lake and laschamp excursions virtual geomagnetic poles vgps reach equatorial latitudes and mid southerly latitudes respectively for intervals associated with the iceland basin and pringle falls excursions repeated excursions of vgps to high southerly latitudes indicate rapid directional swings rather than a single short lived polarity reversal the directional instability associated with polarity excursions is not often recorded probably due to smoothing of the sedimentary record by the process of detrital remanence drm acquisition

the santa rosa event 40ar 39ar and paleomagnetic results from the valles rhyolite near jaramillo creek jemez mountains new mexico the jaramillo event was originally defined by doell and dalrymple in 1966 on the basis of k ar ages from sanidine in the normally transitionally and reversely magnetized rhyolite domes named cerro del abrigo cerro santa rosa i and cerro santa rosa ii respectively that erupted following collapse of the archetypal valles caldera new mexico we have collected new paleomagnetic data from the three domes and new 40 ar 39 ar laser fusion and furnace incremental heating experiments on sanidine crystals from the cerro santa rosa i rhyolite step wise alternating field and thermal demagnetization techniques applied to 52 samples from seven sites indicate that the original paleomagnetic results of doell and dalrymple science 152 1966 1060 1061 are valid cerro del abrigo is normally magnetized whereas the santa rosa i dome is transitional with an inclination of 363 toward the east and the santa rosa ii dome is of reversed polarity twenty five laser fusion experiments on sanidine crystals from the the cerro santa rosa i dome together with the saddle shaped spectrum obtained by incrementally heating the sanidine in a furnace reveal that this rhyolite contained a small but significant component of excess argon prior to eruption our preferred age of 936 8 ka 2c for the santa rosa i rhyolite is based on the concordant laser fusion isochron and incremental heating plateau ages this age is significantly older than was inferred on the basis of earlier laser fusion 40 ar 39 ar results that suggested a trapped component characterized by a 40 ar 36 ar ratio lower than the atmospheric value of 295 5 our new age distinguishes the santa rosa i dome by 65 kyr from the termination of the jaramillo normal subchron 1001 10 ka and by 37 kyr from basaltic lavas at haleakala and tahiti which record the kamikatsura event 899 6 ka moreover this determination is a factor of three more precise than the best previous estimate for a proposed geomagnetic event in this time period the cerro santa rosa i rhyolite dome once intimately linked with the end of the jaramillo event and the acceptance of plate tectonic theory now defines a highly resolved feature the santa rosa event in a terrestrial geomagnetic reversal time scale that is consistent with the global record of magnetic field intensity from marine sediments fl 2002 elsevier science b v all rights reserved

timing of quaternary geomagnetic reversals and excursions in volcanic and sedimentary archives abstract we review the ages of quaternary magnetic polarity reversals and magnetic excursions improvements in radioisotopic 40ar 39ar dating of reversals and excursions reflect the use of multi collector mass spectrometers along with ultra low backgrounds laser based incremental heating methods and adoption of 28 201 ma for the fish canyon sanidine fcs standard age kuiper et al 2008 these improvements have brought 40ar 39ar ages of reversals and excursions into close correspondence with astrochronological ages based on the correlation of 18o data to lr04 lisiecki and raymo 2005 and in a few cases correlation to ice core chronologies ages of two polarity reversals the matuyama brunhes boundary and base olduvai are at odds with ages given in popular geomagnetic polarity timescales sedimentary records of the laschamp and iceland basin excursion are numerous and widespread and attest to global manifestation and global synchronicity of these excursions records of other quaternary excursions are less numerous the blake excursion 115 ka has been recorded in north atlantic sediments chinese loess indian ocean volcanics and spanish speleothems the santa rosa excursion 932 ka has been recorded in north atlantic and phillipine basin sediments as well as in volcanic rocks from new mexico and galapagos the punaruu excursion 1115 ka has been observed in sediments from the north atlantic and california margin as well as in volcanic rocks on tahiti uniformity of individual excursion ages using astrochronological 40ar 39ar and u th methods argues for synchronous global manifestation of excursions strengthening the usefulness of magnetic excursions and reversals in quaternary stratigraphy

saw toothed pattern of relative paleointensity records and cumulative viscous remanence several studies of relative paleointensity data derived from marine sediments spanning the last 4 myr display an asymmetrical saw toothed pattern polarity reversals of the geomagnetic field are associated with low points in paleofield intensity preceded by a long term progressive decay of the field and followed by a rapid post transitional recovery 1 3 since similar behavior is observed in far flung sites whose rock magnetic records are different it is argued that the world wide saw toothing represents geomagnetic field behavior 3 we present an alternative explanation calling on the effect of hard viscous remanence

40ar 39ar dating of the brunhes matuyama geomagnetic field reversal magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale gpts to date events in the range 0 to 5 million years ago a critical tie point on the gpts is the potassium argon age of the most recent brunhes matuyama geomagnetic field reversal astronomical values for the forcing frequencies observed in the oxygen isotope record in ocean drilling project site 677 suggest that the age of this last reversal is 780 ka thousand years ago whereas the potassium argon based estimate is 730 ka results from 4039 ar incremental heating studies on a series of lavas from maui that straddle the brunhes matuyama reversal give an age of 783 11 ka in agreement with the astronomically derived value the astronomically based technique appears to be a viable tool for dating young sedimentary sequences

a high resolution marine sedimentary record of geomagnetic intensity during the brunhes chron abstract this investigation of paleointensity employs data from four cores taken from two sites in the sulu sea during leg 124 of the ocean drilling program these sediments accumulated at a rate of about 10 cm ky yet they provide strong stable magnetizations a combination rarely found in the deep sea environment this study examines the interval of the last glacial cycle since 130 ka in most detail using paleomagnetic measurements from discrete samples to explore a variety of normalization parameters paleomagnetic remanence and susceptibility measurements taken aboard ship at odp site 769 extend the paleointensity record to the brunhes matuyama boundary after normalization by susceptibility the shipboard intensity data compare well with the more detailed measurements from discrete samples in the intervals for which such data are available and so complete a brunhes age 0 0 78 ma history of paleointensity for the sulu sea that record shows a number of short duration decreases in paleointensity that appear to be useful stratigraphic markers these results also indicate that the average geomagnetic dipole moment during the brunhes chron may have been considerably lower than the present value

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

evidence for the kamikatsura and santa rosa excursions recorded in eolian deposits from the southern chinese loess plateau 1 a detailed paleomagnetic investigation of a loess paleosol sequence in baoji southern chinese loess plateau reveals two distinct geomagnetic excursions accompanied by inferred relative paleointensity minima in the late matuyama chron rock magnetic experiments demonstrate that the sediments of the excursion intervals in loess unit l9 have the same magnetic properties as those from surrounding normal and reversed polarity intervals assuming constant sedimentation rate between the jaramillo matuyama and matuyama brunhes magnetic polarity boundaries the midpoint ages and durations for these two excursions are estimated at 0 89 ma and 16 kyr and 0 92 ma and 7 kyr respectively in addition these two events are separated by an interval with fully reversed polarity that had a duration of 16 kyr these estimates are supported by paleoclimatologically inferred age constraints the two excursions are probably correlated with the kamikatsura and santa rosa excursions respectively

relative paleointensity of the geomagnetic field during the brunhes chron abstract we conducted a paleomagnetic study of four sediment cores taken from different regions in the low latitudes of the pacific ocean in order to recover relative paleointensity variations during the brunhes chron three cores have good age control from oxygen isotope stratigraphy we extensively examined rock magnetic uniformity by the iso thermal remanent magnetization irm acquisition and the alternating field demagnetization of the anhysteretic remanent magnetization arm and the irm only rock magnetically homogeneous sediments were used biogenic magnetites were found in all but one core and their dissolution in early diagenesis was not significant the four cores showed coherent variations of the remanent intensity normalized by the arm implying that these variations represent the behavior of the geomagnetic field long term paleointensity variations during the brunhes chron are characterized by an asymmetric pattern rapid increases in the intensity occurred just after the brunhes matuyama transition in the mid brunhes from about 500 to 400 ka and during the last 100 ka and gradual decreases between them the first one supports the idea of rapid regeneration of the geomagnetic field after polarity reversals 1 the other two may have been triggered by very short polarity reversals

melting experiments on the udachnaya kimberlite at 6 3 7 5 gpa implications for the role of h2o in magma generation and formation of hydrous olivine abstract melting experiments on kimberlite from the udachnaya pipe have been performed at 6 3 7 5 gpa and 1300 1600 c using a split sphere multianvil apparatus the water content in kimberlite varied from 2 5 to 11 6 wt and the co 2 co 2 h 2 o molar ratio was from 0 61 to 0 23 the samples were placed in a graphite container inside a pt capsule the oxygen fugacity f o 2 during the experiment was controlled by the equilibrium between graphite and water bearing carbonate silicate melt at relatively low temperatures f o 2 was close to the emog d equilibrium at higher temperatures it shifted for approximately 1 log unit to more reduced conditions an olivine garnet clinopyroxene assemblage was present at 100 c below the liquidus of the udachnaya kimberlite with 6 8 wt h 2 o at the pressure 6 3 gpa and 6 10 wt h 2 o at 7 5 gpa at 2 5 wt h 2 o the same assemblage appeared at 150 c below liquidus both at 6 3 and 7 5 gpa orthopyroxene did not form at any temperature and pressure of the experiments the presence of clinopyroxene near the liquidus was due to the calcic nature and a high degree of silica undersaturation in the udachnaya kimberlite at the supra solidus conditions garnet and clinopyroxene compositionally distinct from minerals of the megacryst macrocryst suite crystallized in equilibrium with low h 2 o carbonated melt near the liquidus of high h 2 o kimberlite the stable olivine composition was from fo 91 to fo 98 the garnet composition cao 8 wt tio 2 2 o 3 up to 2 2 wt approached that of ti bearing garnet typical of cr poor garnet megacrysts while the clinopyroxene was an analog of clinopyroxene megacrysts in kimberlite infrared absorption measurements showed that crystallized olivines contained water in the form of ti clinohumite like and oh clinohumite like defects the h 2 o content of olivine was found to depend mainly on water content in kimberlite melt and pressure olivine with 120 170 ppm h 2 o crystallized at 6 gpa in equilibrium with the melt compositionally close to the archetypical kimberlite rather than a carbonate one and water as high as 6 wt olivine with 270 300 ppm h 2 o crystallized near kimberlite liquidus only at 7 5 gpa and 9 5 10 1 wt h 2 o the experimental results imply that hydrous olivine which is typical to kimberlites including the udachnaya pipe very unlikely can crystallize from anhydrous carbonated magma judging by the conditions for multiphase saturation of the udachnaya kimberlite melt its generation was possible by partial melting of carbonated garnet wehrlite at pressures from 6 to 7 5 gpa and temperatures within 1450 1600 c in the presence of aqueous fluids thus water must have been a trigger for generation of the primary kimberlite magma

the origin of high hydrogen content in kimberlitic olivine evidence from hydroxyl zonation in olivine from kimberlites and mantle xenoliths abstract olivine macrocrysts in kimberlites are the most h rich natural olivines known their xenocrystic provenance makes unclear whether h enrichment occurred in the mantle prior to entrainment in kimberlite magma or during ascent we present a fourier transform infrared ftir spectroscopy based study of h zonation in kimberlite derived olivine macrocrysts and or olivine in xenoliths from the kimberlites jericho nunavut canada beartooth northwest territories canada and pipe 200 and matsoku lesotho the objective of the study was to determine whether changes in the concentration or speciation of h defects in proximity to the host kimberlite could yield insights into the origin of h enrichment within the group 1 region of the ftir spectrum 3420 3700 cm 1 we find that olivines within xenoliths and macrocrysts show weak zonation or significant h depletions in rims peaks in the group 2 region 3260 3420 cm 1 in beartooth olivines show marked decrease in the crystal margins in comparison to group 1 peaks we find no preserved evidence for kimberlite related h enrichment in our dataset we ascribe h depletion to diffusive h loss to infiltrating kimberlitic media diffusion models using coefficients previously applied to dehydrogenation of olivine and assuming a dry environment produce extremely short ascent durations incompatible with the results of other geospeedometers this mismatch suggests that the poorly developed zonation in jericho and beartooth olivines indicates water enrichment and or faster ascent of these melts in comparison to pipe 200 and matsoku resulting in greater retention of mantle derived h a further significant factor in the mismatch is the differing speciation of h in most natural mantle derived olivines in comparison to those used in diffusion experiments we argue that the presence of presumably mantle derived group 2 h in olivine macrocrysts indicates rapid ascent of magma exsolving water rich fluid permitting preservation of these faster diffusing defects only in particularly rapidly ascending kimberlite magmas this interpretation explains empirical correlations between the presence of group 2 bands water derived diamond resorption features and volcaniclastic kimberlite facies

late pleistocene relative geomagnetic paleointensity from the deep labrador sea regional and global correlations abstract three long piston cores from the deep labrador sea appear to record late pleistocene variations of relative paleointensity of the geomagnetic field two cores extend at least into isotope stage 6 the other into stage 5 in addition to 18o 13c and ams 14c stratigraphies correlation between cores is facilitated by the presence of rapidly deposited high detrital carbonate dc and low detrital carbonate ldc layers some of which correlate to the north atlantic heinrich events hysteresis ratios and rock magnetic parameters indicate that the magnetic assemblage in the cores is pseudo single domain magnetite except for deglacial intervals and dc and ldc layers normalized remanence records show strong correlation for a range of normalizers k karm and sirm and no correlation with bulk magnetic parameters paleointensity records from the three cores are consistent when only dc and ldc layers are used for correlation suggesting that relative paleointensity can be used as a regional correlation tool consistent paleointensity features from the labrador sea indian ocean and mediterranean sea suggest that the sediments are recording dipole field variations although the composite record from two labrador sea cores is strongly correlated to a composite of the mediterranean indian ocean records the chronologies are inconsistent indicating apparent discrepancies in the isotopic and ams 14c chronologies used in the mediterranean indian ocean and labrador sea respectively

hydroxyl in olivines from mantle xenoliths in kimberlites of the siberian platform from a total of 335 olivine crystal grains crystallographically orientated platelets and where possible parallelepipeds were prepared chemically analysed by electron microprobe examined under the polarisation microscope and studied by polarised ftir microscope absorption spectrometry in the oh vibrational range 3 000 3 800 cm 1 the 335 crystal grains were extracted from 174 different specimens of yakutian upper mantle material including 97 xenoliths that represent all the rock types occurring in all the kimberlites of the siberian shield the other specimens were mega and macrocrysts or inclusions in diamonds and garnets analysis of the polarised oh spectra allowed distinction between hydroxyl in non intrinsic separate inclusions nsi and in isolated local or condensed extended defects intrinsic to the olivines ild or ced respectively as the two latter types cannot be distinguished by vibrational spectroscopy and as they are presumably interconnected by intracrystalline condensation reactions of the type n ild ced n we propose to symbolise them as ild ced of the total of 70 polarised oh bands that were found in the whole set 17 are caused by nsi 53 by ild ced total mean integrated oh band intensities int total with int a b c int 3 were determined from the spectra they yielded the contents of structurally unallocated water using the recent calibration of the ir method bell et al 2003 the range is 0 a b at 3 327 3 328 cm 1 an energy typical of oh in hydrous wadsleyite we assume this phase to be present as nis in the respective olivines possibly as relic phase 4 weak bands between 3 175 and 3 260 cm 1 polarised along c are tentatively assigned to molecular water nsi with relatively strong hydrogen bonds to the matrix we did not find larger clusters of molecular water i e liquid like water with its characteristic broad band centred at ca 3 400 cm 1 we did also not find any humite minerals as an nsi of the 53 oh bands intrinsic to olivine the 29 most abundant and strong ones were subject to further analysis in terms of oh bearing structural defects ild ced nearly all these bands are strongly polarised along a two bands at 3 672 and 3 535 cm 1 are assigned to boron related defects ild ced b five bands at 3 573 3 563 3 541 3 524 and 3 512 cm 1 are intensity correlated and are assigned to si depleted titan clinohumite like defects ild ced thl the other so far unidentified oh of ild ced are suggested to originate from oh in different types of mg fe depleted defects recently predicted and discovered by tem these are called ild ced kwk eight mostly strong bands of them occur at energies higher than 3 573 cm 1 ild ced kwk h 13 strong to medium strong bands occur below 3 500 cm 1 ild ced kwk l such intrinsic defects may occur alone ild ced thl and ild ced kwk h or in different combinations with each other ild ced kwk h ild ced thl ild ced kwk h ild ced kwk l and ild ced kwk h ild ced thl ild ced kwk l though there are indications that the occurrences of such types and combinations of the intrinsic oh bearing defects in the olivines are related to the types and genetic peculiarities of their host rocks straightforward and simple correlations do not exist the reasons for this and also for the great number of varieties of intrinsic ild ced are discussed

relative geomagnetic paleointensity from the jaramillo subchron to the matuyama brunhes boundary as recorded in a mediterranean piston core piston core lc07 located west of the sicily strait in the mediterranean sea unambiguously records the matuyama brunhes m b and the upper jaramillo polarity reversals with similar average sediment accumulation rates sars for the brunhes chron 2 29 cm kyr and late matuyama chron c1r 1r 2 19 cm kyr we report a relative paleointensity record for the interval spanning the m b boundary down into the jaramillo subchron which is unique in the mediterranean because existing records from this basin cover only the last 80 kyr the average sar in core lc07 is used to translate the depth related paleointensity record to the time domain the ratio of anhysteretic remanent magnetization to low field magnetic susceptibility is climatically sensitive and is used to tune the age model this correlation produces a good fit to the global ice volume model derived for summer insolation at 65 n with this age model a paleointensity minimum in association with the m b boundary has a duration of about 4 5 kyr while the directional change has a duration of 6 3 kyr a second paleointensity minimum of similar duration is found about 16 kyr below the m b boundary this feature precursor or dip in the literature has previously been recognized at the same time interval in many marine records which reinforces the validity of our age model other relative paleointensity minima are found within chron c1r 1r and within the uncertainties of the respective age models these minima coincide with those observed from the few published coeval paleointensity records in particular there is good correspondence between the ages of minima at about 0 92 and 0 89 ma which probably correlate with two geomagnetic excursions santa rosa and kamikatsura respectively that have been recorded in lava flows and dated using the 40 ar 39 ar technique in contrast a recently dated excursion at 0 83 ma from la palma seems to correspond to a paleointensity maximum this observation is opposite to that expected and this excursion needs to be confirmed in contrast to some recently published paleointensity records spectral analysis of the lc07 record does not reveal identification of significant power at the orbital obliquity frequency fl 2002 elsevier science b v all rights reserved

evidence for geomagnetic excursions recorded in brunhes and matuyama chron lavas from the trans mexican volcanic belt 1 this study presents paleomagnetic data from 59 independent lava flows from the trans mexican volcanic belt tmvb with ages from 6 4 ma to recent 52 being younger than 1 ma and 11 new 40ar 39ar age determinations most remanence carriers are ti poor titanomagnetite of pseudosingle domain magnetic structure nine lavas contain small amounts of titanomaghemite and four lavas additional titano hematite paleosecular variation of lava flows younger than 1 7 ma is consistent with latitude dependent model g and also in agreement with other pleistocene paleomagnetic data from the tmvb the directional record of brunhes and matuyama chrons lavas was correlated to the geomagnetic polarity timescale and there is evidence for at least four geomagnetic excursions one lava flow dated at 592 20 ka has a fully reversed paleodirection and most likely erupted during the big lost excursion another fully reversed flow dated at 671 12 ka gives new volcanic evidence for the delta stage 17 excursion this excursion is supported by a reversed intermediate direction of another flow from a different volcanic field but of very close age of 673 10 ka from the matuyama age lavas one flow with normal polarity magnetization dated at 949 37 ka could either be related to the kamikatsura or the santa rosa excursion and a normal polarity flow dated at 1628 56 ka could have been emplaced during the gilsa excursion the results presented here confirm in one case but disagree in four cases with results presented in two previous studies of the same lava flows and interpreted as geomagnetic excursions

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

dating transitionally magnetized lavas of the late matuyama chron toward a new 40ar 39ar timescale of reversals and events the k ar based geomagnetic polarity timescale was constructed using data from lavas and tuffs that bracketed but rarely dated the transitions between polarity intervals subsequent 40ar 39ar dating indicated that the ages of some polarity transitions had been underestimated by about 6 although the accepted ages of the polarity chron boundaries have increased their precise temporal definition remained uncertain we have taken a different approach and used incremental heating techniques to obtain 18 new 40ar 39ar ages from basaltic lavas within flow sequences at punaruu valley tahiti and haleakala volcano hawaii these lavas record transitional paleomagnetic directions corresponding to four mid pleistocene polarity reversals or events three lavas from punaruu valley previously thought to record the cobb mountain normal polarity subchron cmns gave a mean age of 1 105 0 005 ma indicating that they were erupted about 76 kyr after the cmns this period of transitional field behavior is designated the punaruu event in addition seven new 40ar 39ar ages from the punaruu valley indicate that the jaramillo normal polarity subchron jns lasted about 67 kyr starting at 1 053 0 006 ma and ending 0 986 0 005 ma this agrees with astronomical estimates but conflicts with jns ages proposed by spell and mcdougall 1992 and izett and obradovich 1994 on the basis of 40ar 39ar dating of rhyolite domes in the valles caldera indistinguishable 40ar 39ar ages of seven lavas including one from punaruu valley and six from haleakala that record broadly similar intermediate paleodirections suggest that the kamikatsura event occurred at 0 886 0 003 ma moreover these data indicate that the kamikatsura event occurred 20 40 kyr after another geomagnetic event most probably taking place at 0 92 ma we designate this earlier field behavior the santa rosa event adopting its name from that of a transitionally magnetized rhyolite dome which happened to figure prominently in the original definition of the end of the jns in the 1968 study of doell et al 1968 the discovery of these new short lived polarity events during the matuyama reversed chron suggests that the 400 kyr period between 1 18 and 0 78 ma experienced no less than 7 and perhaps more than 11 attempts by the geodynamo to reverse this newly determined higher frequency of geomagnetic activity illustrates vividly the importance of obtaining precise age control directly from transitionally magnetized rocks

cosmogenic signature of geomagnetic reversals and excursions from the reunion event to the matuyama brunhes transition 0 7 2 14 ma interval long term variations of the geomagnetic dipole moment gdm during periods of stable polarity and in transitional states reversals and excursions provide key information for understanding the geodynamo regime following several studies dealing with the brunhes chron and the matuyama brunhes transition this study presents a new authigenic 10 be 9 be ratio be ratio record obtained from the md97 2143 core western pacific ocean this new be ratio series yields a record of gdm variations covering the early brunhes and mid to late matuyama time period i e 700 2140 ka independently from the relative paleointensity rpi record obtained from the same core that can be compared with available rpi records and stacks stratigraphic offsets measured between the be ratio peaks and the corresponding rpi minima reach 2 to 14 cm they can be assigned to post detrital remanent magnetization pdrm effects leading to magnetization locking in delays varying from 2 to 12 ka in the studied core 10 be overproduction episodes triggered by geomagnetic dipole moment lows gdl linked to polarity reversals and excursions confirm the global control exerted by the gdm on cosmogenic radionuclides production a dipole moment reconstruction derived from the beryllium 10 bedimo was compiled and calibrated using absolute paleointensity data this independent record complements the available paleomagnetic rpi records permitting 1 to overcome the pdrm lock in offsets induced below the mixing layer 2 to confront and increase the robustness and precision of gdm reconstructions and 3 to better constrain the chronology of geomagnetic field instabilities during the mid to late matuyama chron our new 10 be derived inventory is fully compatible with the gdl series linked to geomagnetic polarity reversals and events matuyama brunhes transition jaramillo and olduvai subchron boundaries cobb mountain reunion and it strengthens the occurrence of several excursions kamikatsura santa rosa punaruu bjorn gilsa gardar that were until now reported from only sparse locations

a sedimentary paleomagnetic record of the matuyama chron from the western antarctic margin odp site 1101 a high resolution paleomagnetic record for part of the matuyama chron 0 7 2 1 ma is reported for ocean drilling program site 1101 leg 178 off the antarctic peninsula pacific margin a rock magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization nrm is carried by pseudo single domain magnetite progressive alternating field demagnetization of 83 m of u channels provided a polarity stratigraphy down to the olduvai subchron two geomagnetic events preceding the jaramillo subchron were identified including the cobb mountain polarity interval the bulk magnetic parameters vary by more than a factor of 20 over the entire time interval but by less than a factor of 6 over the 0 7 1 1 ma interval this latter interval was selected for paleointensity determinations which were carried out by normalizing the nrm by the anhysteretic remanent magnetization arm direct comparison of the site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record a more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0 95 1 1 ma interval this test yielded no outlier for the period considered confirming the geomagnetic character of the records we have constructed a lowresolution stack revealing some of the characteristic paleointensity features of the jaramillo subchron fl 2001 elsevier science b v all rights reserved

paleointensity of the geomagnetic field during the last 80 000 years high resolution records of the relative paleointensity of the geomagnetic field have been obtained from five marine cores three duplicate records were used to estimate the regional coherency of the data within a single area tyrrhrenian sea while the two others document the field variations in the eastern mediterranean and the southern indian ocean careful investigations of distinct rock magnetic parameters have established the downcore uniformity of the sediments in terms of magnetic mineralogy and grain sizes the time depth control was provided by oxygen isotopes and small scale variations in the deposition rates were constrained by means of tephrachronology the synthetic curve calculated from the mediterranean records provides a continuous record of the intensity variations during the last 80 000 years 80 kyr which correlates well with the sparse volcanic data available for the period 0 40 kyr the fact that identical behavior is seen in both data sets and that they also compare quite well with results from a core collected in the pacific ocean establishes the truly dipolar character of these variations the dipole field moment is characterized by large scale changes as shown by the existence of pronounced drops at 39 and 60 kyr alternating with periods of higher intensity the record suggests a periodic nature for these intensity variations however the period studied is not sufficiently long to state this conclusively these results demonstrate the potential of sediments for such studies and constitute a first step towards obtaining a global paleointensity record over a long period of time

paleointensity record from pleistocene sediments 1 4 0 ma off the california margin high resolution magnetic measurements of 45 m of sediment from the delgada fan area at ocean drilling program site 1021 39 5 n 127 46 w were performed in order to extract a reliable signal of relative paleointensity a detailed chronology was established by correlation of the magnetic susceptibility variations to the insolation curve for the past 1 4 myr covered by the record the remanence carrier is pseudo single domain sized magnetite that varies in concentration by a factor 3 stepwise alternating field demagnetization isolated a very stable natural remanent magnetization nrm which showed the succession of the cobb and jaramillo subchrons and directional changes associated with the punaruu event 1 105 ma two techniques have been used to extract the signal of relative paleointensity a significant climatic component remained present in the curve obtained from the slopes of the nrm versus the anhysteretic remanent magnetization arm in contrast the nrm and the susceptibility k lf both being driven by orbital forcing normalization by susceptibility is not contaminated by climatically driven changes and is thus taken as our best estimate of relative paleointensity comparison with other high resolution records and compilations reinforces the actual geomagnetic origin of the variations displayed by the nrm k signal overall this record confirms the succession of distinct highs and lows which are concomitant with full field reversals and short term geomagnetic events present during the matuyama chron the presence of large oscillations of the field with no periodicity confirms that the geomagnetic field at least his dipolar part is highly unstable and may have remained in this mode for a long period of time

testing the accuracy of the geomagnetic polarity time scale gpts at 2 5 ma utilizing40ar 39ar incremental heating data on whole rock basalts abstract recent40ar 39ar dating and astrochronologic calculations indicate that the k ar derived gpts underestimates the ages of geomagnetic field reversals in the interval 2 0 ma we report tests to check the accuracy of k ar data utilized to date the reunion event 2 1 ma and part of the early pliocene portion 4 6 ma of the gpts 40ar 39ar incremental heating studies on whole rock basalts from the island of reunion yield an age of2 14 0 03 ma for the reunion event somewhat older than the earlier k ar value of2 07 0 02 ma on the same set of lava flows study of a single specimen from the newer volcanics australia indicates that the earlier k ar date was accurate but the new40ar 39ar results yield a plateau age 4 56 ma with a standard error of 0 6 as compared to the k ar value 3 the improved precision and accuracy resulting from such40ar 39ar incremental heating studies will play a key role in calibrating more precisely the gpts for the interval 10 0 ma

olivine hydration in the deep upper mantle effects of temperature and silica activity 1 although water controls the biology and geology of the surface hydrogen is perhaps the most poorly constrained compositional variable in the bulk earth its concentration in the upper mantle appears to be controlled by its solubility as hydroxyl in the nominally anhydrous silicate phases olivine pyroxene garnet wadsleyite and ringwoodite here we describe a series of experiments showing that the solubility of h2o in olivine at 12 gpa increases with temperature to 8900 ppm by weight at 1250 c and decreases at higher temperature with the onset of melting sample characterization by infrared spectroscopy indicates that the primary hydration mechanism is the substitution of 2h for mg2 similar results obtained from samples coexisting with clinohumite low silica and with clinoenstatite high silica indicate that silica activity has minimal effect on hydration under these conditions single crystal x ray diffraction measurements constrain the volume of hydration and indicate significant m site vacancies hydrogen thus appears to become a geochemically compatible element as depths approach 400 km

relative geomagnetic paleointensity and δ18o at odp site 983 gardar drift north atlantic since 350 ka abstract at odp site 983 relative geomagnetic paleointensity and planktic and benthic 18o records have been acquired for the last 350 kyr the mean sedimentation rate in this interval is 11 3 cm kyr magnetic properties and hysteresis ratios indicate that pseudo single domain magnetite is the remanence carrier volume susceptibility k anhysteretic arm and isothermal irm remanence values vary by a factor of 3 4 well within the criteria usually cited for paleointensity studies natural remanent magnetization nrm is normalized by arm and irm to acquire the paleointensity proxy arithmetic means of nrm arm and nrm irm calculated for five demagnetization steps in the 25 45 mt range constitute the relative paleointensity estimates some paleointensity lows particularly those at 40 120 and 188 ka are associated with directional excursions of the field especially the event at 188 ka referred to here as the iceland basin event that constitutes a short lived polarity reversal for the last 200 kyr the records can be correlated with other high resolution paleointensity records such as those from the labrador sea mediterranean somali basin and sulu sea implying that the millennial scale features are globally synchronous a labeling system for paleointensity features is proposed that ties prominent highs and lows to oxygen isotope stages

the partitioning of water between olivine orthopyroxene and melt synthesised in the system albite forsterite h2o coexisting forsterite enstatite and silicate glass were synthesised from a water bearing basalt analogue in programmed cooling experiments at 1 0 1 5 2 0 and 2 5 gpa hydrous species in each phase were characterised and quantified using infrared absorbance spectroscopy al 3 related hydroxyl defects with vibrational energies between 3400 3200 cm 1 dominate the hydroxyl budget of synthetic forsterite the vibrational characteristics of these defects indicate that similar absorptions in febearing systems result from fe 3 related oh species this interpretation resolves a previously recognised but unexplained relationship between oxygen fugacity and hydroxyl speciation in experimentally annealed olivines and suggests that the intensities of bands at 3400 3300 cm 1 could form the basis of an oxybarometer oh defect speciation in enstatite is similar to that in aluminous crystals synthesised at high water fugacity and many natural orthopyroxenes hydroxyl speciation in orthopyroxene is not therefore controlled by water activity total water contents of synthetic enstatites from any single experiment vary enormously this heterogeneity is attributed to variable contributions from a non intrinsic broad band hydrous component intrinsic hydroxyl concentrations dissolved in enstatite are identical in all crystals from an experiment the partitioning of water between forsterite enstatite and coexisting silicate melt is independent of pressure temperature and total water content over the range of conditions studied as long as the influence of these parameters on phase compositions is taken into account dh2o fo melt 0 0001 0 0003 n 4 and is insensitive to pressure or olivine aluminium contents dh2o en melt is strongly controlled by al2o3 concentration and ranges from 0 003 0 016 n 6 two samples contain both forsterite and enstatite and yield dh2o en fo 25 1

geomagnetic modulation of the 36cl flux in the grip ice core greenland geomagnetic field strength is expected to affect the production rate of cosmogenic isotopes such as beryllium 10 carbon 14 or chlorine 36 chlorine 36 data from the greenland ice core project grip ice core agree well with a production rate calculation based on a paleomagnetic reconstruction for the past 100 000 years over both long and short term variations a chlorine 36 peak at 38 000 years ago previously found in the beryllium 10 record from the vostok ice core can be explained by a period of low geomagnetic field intensity

the effect of silica activity on the incorporation mechanisms of water in synthetic forsterite a polarised infrared spectroscopic study pure forsterite crystals were grown from hydrous melts using controlled cooling experiments at 2 0 gpa and varying the bulk mg si ratio from 2 0 to 1 5 oriented single crystals were then studied by polarised infrared spectroscopy the spectra of the samples with the lowest silica activity asio2 contain the main oh bands in the range 3 620 3 450 cm 1 only in contrast the spectra of the samples synthesised with the highest asio2 contain additional pleochroic bands at 3 160 3 220 and 3 600 cm 1 the variations are interpreted in terms of protonated silicon vacancies being dominant at low asio2 and mg vacancies dominant at high asio2 xenolithic mantle olivines generally do not have the spectrum expected for orthopyroxene buffered conditions suggesting that they re equilibrated with their host melts during ascent but mantle olivine from the zabargad peridotite massif probably is in equilibrium with the coexisting orthopyroxene

link between excursions and paleointensity inferred from abnormal field directions recorded at la palma around 600 ka abstract an excursion of the geomagnetic field characterized by strongly abnormal directions associated with a field intensity as low as 7 t has been identified in a lava flow sequence from la palma canary islands the age of 602 12 ka obtained by k ar dating using the cassignol technique for a transitional flow represents our best estimate for the age of the occurrence of this episode of excursional directions this age is concordant with a minimum observed in the global sint800 composite record derived from worldwide deep sea records of relative paleointensity we show that a geomagnetic field model composed of a present day like non dipole field superimposed on an axial dipole reduced by a factor of 2 compared to its present value can account for excursional directions associated with low paleointensity in some areas while most other parts of the globe display apparently normal polarity directions we next generate through linear inversion a synthetic field model constrained by the actual la palma data and a set of directions at 30 sites distributed worldwide based on an axial dipole field with half present day intensity the energy spectrum of this synthetic field model is compatible with most of the non dipole part of the present day field and in that sense is earth like i e represents an acceptable configuration of the past geomagnetic field at 600 ka the model shows that very anomalous excursional directions covering 1 5 of the earth s surface may co exist with apparently normal directions elsewhere the new data and modeling strongly support the hypothesis that excursions are an integral aspect of secular variation there may be a continuum between normal sv and field reversals primarily controlled by the ratio of axial dipole field intensity relative to the remaining non axial dipole part

40ar 39ar ages from the rhyolite of alder creek california age of the cobb mountain normal polarity subchron revisited new 40 ar 39 ar age determinations on sanidine from the rhyolite of alder creek california indicate a 1 186 0 006 ma age for the cobb mountain normal polarity subchron the new age is statistically older 0 05 than the previously reported k ar age 1 12 0 02 ma and agrees with the age suggested by the astronomical polarity time scale incomplete extraction of radiogenic 40 ar 40 ar from the sanidine is the most likely reason for the disparity between the 40 ar 39 ar and k ar ages because the cobb mountain subchron is a worldwide short duration event and because no widely used interlaboratory 40 ar 39 ar standard younger than 27 ma exists we propose that sanidine from the rhyolite of alder creek be considered for use as a new quaternary 40 ar 39 ar mineral standard

effect of oxygen fugacity on the h2o storage capacity of forsterite in the carbon saturated systems abstract high pressure experiments have been performed in the systems mg 2 sio 4 c o h and mg 2 sio 4 k 2 co 3 c at 6 3 gpa and 1200 to 1600 c using a split sphere multi anvil apparatus in the mg 2 sio 4 c o h system the composition of fluid was modeled by adding different amounts of water and stearic acid the f o 2 was controlled by the mo moo 2 or fe feo oxygen buffers several experiments in the mg 2 sio 4 c o h system and all experiments in the mg 2 sio 4 k 2 co 3 c system have been conducted without buffering the f o 2 forsterite in the system mg 2 sio 4 k 2 co 3 c does not reveal oh absorption bands in the ir spectra while forsterite coexisting with carbon bearing fluid and silicate melt at log f o 2 from fmq 2 to fmq 5 from 2 to 5 log units below fayalite magnetite quartz oxygen buffer contains 800 1850 wt ppm h 2 o the maximum concentrations were detected at 1400 c and fmq 3 5 we observed an increase in the solidus temperature in the system mg 2 sio 4 c o h from 1200 to above 1600 c with log f o 2 decreasing from fmq 2 to fmq 5 the increase of the solidus temperature and the broadening of the stability field of the h 2 o h 2 ch 4 subsolidus fluid phase at 1400 1600 c explain the high h 2 o storage capacity of forsterite relative to that crystallized from carbon free oxidized hydrous silicic melt at temperatures above 1400 c liquidus forsterite precipitated along with diamond from oxidized fmq 1 carbonate silicate melt and from silicate melt dissolving the moderately reduced c o h fluid from fmq 2 to fmq 3 5 formation of diamond was not detected under ultra reduced conditions fmq 5 at 1200 1600 c olivine co precipitating with diamond from dry carbonate silicate or hydrous silicic fluid melt can provide information on the h 2 o contents and speciation of the diamond forming media in the mantle the conditions for minimum post crystallization alteration of olivine and its hydrogen content are discussed

in situ infrared spectra of oh in olivine to 1100 c the infrared spectra of hydrated san carlos olivine were measured from room temperature to 1100 c at 1 bar using a heating stage the spectra show that even at the highest temperatures studied there are still two well separated groups of oh bands centered between 3200 and 3300 cm 1 and between 3500 and 3600 cm 1 respectively the distinction between group i 3450 cm 1 and group ii bands 3450 cm 1 is therefore still meaningful at upper mantle temperatures however a prominent band at 3612 cm 1 already loses intensity by 100 c and nearly disappears by 300 c suggesting that it corresponds to a particular h environment that only formed during cooling of the sample and that it is not stable at high temperature a similar band is prominent in many olivines from mantle xenoliths and previously it has sometimes been assigned to oh groups surrounding si vacancies we show that structural models of water dissolution in olivine derived from infrared spectra at ambient conditions may not be fully applicable for the upper mantle

relative paleointensity of the geomagnetic field over the last 200 000 years from odp sites 883 and 884 north pacific ocean ocean drilling program sites 883 and 884 were cored as part of a three site depth transect down the slopes of detroit seamount in the north pacific ocean continuous mineral magnetic and paleomagnetic measurements were made using u channel samples for sediments that span the last 200 kyr thermomagnetic and high field hysteresis data indicate that the magnetic mineralogy of the clay rich sediments at the two sites is dominated by pseudo single domain magnetite in a narrow range of grain sizes and concentrations thereby meeting established criteria for relative paleointensity studies the natural remanent magnetization nrm has been normalized using the anhysteretic remanent magnetization arm and the low field magnetic susceptibility the nrm arm and nrm records are nearly identical for both cores furthermore coeval horizons in the two cores can be correlated by matching 100 magnetic susceptibility features over the 15 m length of both cores coherence function analysis indicates that the records are not significantly affected by local environmental conditions these factors suggest that the large scale variations in normalized remanence are most likely due to geomagnetic paleointensity fluctuations dating constraints are provided by a 18o stratigraphy from site 883 our north pacific paleointensity versus age curve is similar to a published record from the western caroline basin and a recently proposed global paleointensity curve offsets up to 10 kyr in the timing of paleointensity features between our composite north pacific record and the global curve may result from imprecisions in the dating of our record nevertheless the correspondence between the paleointensity records suggests that in suitable sediments paleointensity of the geomagnetic field can give a globally coherent dominantly dipolar signal

ftir spectrum of phenocryst olivine as an indicator of silica saturation in magmas fourier transform infrared ftir absorption spectra of hydroxyl were measured on olivine phenocrysts from hydrous basaltic melts that originated in island arc tectonic settings the basaltic melts encompass a wide range of silica activities from orthopyroxene saturated hypersthene normative to nepheline normative compositions the intensities and wavenumber placement of hydroxyl absorption bands correlate with the degree of silica saturation of the parent melt from which the olivine crystallized olivines from silica undersaturated nepheline normative melts absorb ir radiation in the wavenumber range 3430 3590 cm 1 group 1 in contrast olivines from orthopyroxene saturated boninitic melts exhibit hydroxyl absorption bands in the wavenumber range 3285 3380 cm 1 group 2 olivines crystallized at intermediate silica activities exhibit a combination of the two groups of hydroxyl ir bands where the proportion of group 2 bands increases with increasing silica saturation of the parent melt the positions of hydroxyl absorption peaks observed here for natural samples are consistent with previous measurements on experimentally annealed olivines thus protonation experiments can be employed to make spectroscopically dry olivine structures visible by ir yielding information on the silica saturation of the parental magmas hydroxyl concentrations in the studied olivines were estimated to be 1 2 ppm corresponding to an olivine melt partition coefficient of similar to 1 0 0 3 x 10 4

isotopic and trace element composition of volcanic glasses from the akaki canyon cyprus implications for the origin of the troodos ophiolite abstract we report major elements k rb cs sr ba sc cr ni hf ta th and ree concentrations and isotopic compositions of sr nd pb and o of carefully handpicked volcanic glasses from the akaki river section of the troodos ophiolite complex on the basis of sr and o isotopic composition and fe 2 o 3 feo ratios nine of the ten glasses analyzed are considered to be fresh even with respect to those elements that are easily affected by alteration the glasses range in composition from basaltic andesite to dacite incompatible element concentrations in the mafic compositions are low 10 chondritic and the light ree are strongly depleted however the ratios of low field strength elements k rb cs sr th to high field strength elements ta hf ti and rare earth elements are higher than those in n morb nd isotopic compositions show a very small range with e nd t 80ma varying from 6 5 to 7 5 initial 87 sr 86 sr ratios range from 0 7033 to 0 7040 pb isotope ratios have a restricted range with 206 pb 204 pb 18 60 18 70 207 pb 204 pb 15 55 15 59 and 208 pb 204 pb 38 08 38 51 the oxygen isotopic composition for fresh glasses falls within the range reported for fresh mafic mantle derived volcanics with 18 o varying from 5 4 to 6 5 all geochemical characteristics of the troodos volcanic glasses are similar to those observed in supra subduction zone volcanics but we cannot distinguish between an early stage island arc or back arc origin for the troodos ophiolite on a geochemical basis only however considering our data and previously published geologic and geochemical data we suggest that the troodos ophiolite formed in the earliest stages of island arc development

accelerating mineral carbonation in ultramafic mine tailings via direct co2 reaction and heap leaching with potential for base metal enrichment and recovery accelerated carbonation of ultramafic mine tailings has the potential to offset co2 emissions produced by mining ores from cu ni platinum group element podiform chromite diamondiferous kimberlite and historical chrysotile deposits treatments such as acid leaching reaction of tailings with elevated concentrations of gaseous co2 and optimization of tailings pore water saturation have been shown to enhance co2 sequestration rates in laboratory settings the next challenge is to deploy treatment technologies on the pilot and field scale while minimizing cost energy input and adverse environmental impacts implementation of accelerated tailings carbonation at field scale will ideally make use of in situ treatments or modified ore processing routes that employ conventional technology and expertise and operate at close to ambient temperatures and pressures here we describe column experiments designed to trial two geochemical treatments that address these criteria 1 direct reaction of partially saturated ultramafic tailings with synthetic flue gas from power generation 10 co2 in n 2 and 2 repeated heap leaching of ultramafic tailings with dilute sulfuric acid in the first experiment we report rapid carbonation of brucite mg oh 2 in the presence of 10 co2 gas within tailings sampled from the woodsreef chrysotile mine new south wales australia within four weeks we observe a doubling of the amount of co2 stored within minerals relative to what is achieved after three decades of passive mineral carbonation via air capture in the field our simulated heap leaching experiments treated daily with 0 08 m h2so4 produce high mg leachates that have the potential to sequester 21 2 kg co2 m 2 which is approximately one to two orders of magnitude higher than the rate of passive carbonation of the wood sreef mine tailings although some nesquehonite mgco3 center dot 3h 2 o forms from these leachates most of the mg is precipitated as mg sulfate minerals instead therefore an acid other than h2so4 could be used otherwise sulfate removal would be required to maximize co2 sequestration potential from acid heap leaching treatments reactive transport modeling min3p is employed to simulate acid leaching experiments and predict the effects of heap leaching for up to five years finally our synchrotron x ray fluorescence microscopy results for leached tailings material reveal that valuable trace metals fe ni mn co cr become highly concentrated within secondary fe hydr oxide minerals at the ph neutralization horizon within our column experiments this discrete horizon migrates downward and our reactive transport models indicate it will become increasingly enriched in first row transition metals in response to continued acid leaching acid leaching treatments for accelerated mineral carbonation could therefore be useful for ore processing and recovery of base metals from tailings waste rock or low grade ores

water in natural olivine determined by proton proton scattering analysis here we present water concentration data for olivine from different host rocks measured with a nuclear technique using proton proton scattering this method which is used here for the first time on olivine is very powerful for determining trace amounts of water the studied olivine specimens differ in their h 2 o contents ranging from 4 to 51 wt ppm 10 117 atom ppm h the lowest concentrations are found in olivine from spinel peridotite xenoliths the highest concentrations in olivine from alpine type peridotite the contents of an ophiolitic and a hydrothermal olivine are intermediate infrared spectroscopy was applied to ensure that the measured water contents stem solely from hydroxyl defects in the mineral structure the infrared spectra differ from sample to sample five of six olivine specimens show absorption bands typical of hydroxyl groups associated with ti defects these olivines differ in their ti contents by two orders of magnitude however a correlation of water and ti content was not observed

the infrared signature of water associated with trivalent cations in olivine abstract forsterite crystals were synthesised under water saturated conditions at 1400 c and 1 5 gpa doped with trace amounts of either b al sc ti v cr mn fe co ga y zr in sm gd dy tm or lu the common and intense hydroxyl stretching bands in the infrared spectra of spinel peridotite olivine at 3572 and 3525 cm 1 were only reproduced in the presence of ti those samples where the trace element substitutes as the trivalent cation on the mg 2 site were identified from a systematic variation in concentration with the trivalent ionic radius the hydroxyl region of all samples is essentially identical except for between 3300 and 3400 cm 1 this region is characterised by one or more bands with the energy of the most intense feature being correlated with the ionic radius of the trivalent cation the integrated intensity of these hydroxyl bands also correlates with the concentration of the trivalent cation these correlations provide unambiguous evidence that bands or peaks in this region correspond to water at defect sites associated with trivalent cations trivalent peaks are sometimes observed in samples of mantle olivine and most likely indicate water associated with fe 3 the water at this site is not incorporated under normal mantle conditions and should not be included in estimates of the water capacity of mantle olivine these results emphasise the importance of identifying the infrared signature of different water substitution mechanisms

saw toothed variations of relative paleointensity and cumulative viscous remanence testing the records and the model a recent model kok and tauxe 1996a that attempts to explain asymmetrical sawtooth patterns of relative paleointensity from sediments valet and meynadier 1993 as the consequence of cumulative long term viscosity effects has been tested theoretically and experimentally a mathematical description of the model demonstrates that only a sharp distribution of relaxation times would be able to match the paleomagnetic record which implies that all the magnetic grains would be identical this has direct consequences for the distribution of the blocking temperatures tb which would not exceed 260 c and for the coercivities of the natural remanent magnetization nrm this tb spectrum derived from the model is inconsistent with the unblocking temperatures of the nrm and with the tb spectrum of the experimental thermoremanent magnetization trm of the sediment that recorded the sawtooth new results including thermal demagnetization and thellier thellier experiments across brunhes matuyama confirm the overall stability of the original records the stability of the older part of the record 2 to 4 ma has also been strengthed the sawtooth is unchanged between 20 mt and 65 mt and thermal demagnetization in a neutral atmosphere at 300 c and 450 c duplicates the results obtained by alternation field a f demagnetization thus viscous processes at least within the framework of a cumulative viscous model and for the existing data sets are not responsible for saw toothed paleointensity records

the effect of oxygen fugacity on hydroxyl concentrations and speciation in olivine implications for water solubility in the upper mantle the effect of oxygen fugacity on hydroxyl speciation and solubility in san carlos olivine was investigated in a series of piston cylinder experiments at 2 0 gpa and 1100 and 1300 c the presence of enstatite was intended to fix silica activity and experimental fo2 in most runs was regulated by lining the inner walls of the sample containing pt capsules with either fe ni or re foil polarised infrared absorbance spectra show that olivine annealed in capsules lined with fe foil only contain hydroxyl defects with vibrational energies between 3650 and 3400 cm 1 relatively high frequencies spectra of crystals annealed in capsules lined with ni or re foil show that these samples contain hydroxyl defects with vibrational energies between 3650 and 3400 cm 1 high frequency and at 3355 and 3325 cm 1 low frequency the concentrations of hydroxyl defects responsible for absorptions at 3355 and 3325 cm 1 change as a function of oxygen fugacity and are therefore interpreted as fe 3 related oh centres hydroxyl solubility in our experiments is substantially less than current solubility laws might suggest these differences may be attributed to variations in experimental procedure but they also demonstrate that the incorporation of hydroxyl in olivine must be betterunderstoodbeforeanaccuratemodeltodescribewatersolubilityinthedeepearthcanbeconstructed ourdatashowthat when experiments conductedat identicalp tconditionsarecompared olivine annealedat lowf o2 below fe feo contains lessthanhalf as much water as crystals from relatively more oxidised experiments because natural mantle conditions approximate to those of our low fo2 experiments current solubility laws which are based on data from relatively high f o2 experiments overestimate the amount of water that can be hosted in olivine in the upper mantle from a consideration of defect equilibria we propose a mechanism for the incorporation of oh in olivine in a way that is appropriate for the actual range of conditions expected in the mantle 2007 elsevier b v all rights reserved

relative variations in geomagnetic intensity from sedimentary records the past 200 000 years abstract eighteen records of relative paleointensity from marine sediments have been compiled in order to retrace the global changes in geomagnetic intensity during the past 200 kyr before dealing with any other source of noise we focused on problems inherent to the depth versus time correlations the accuracy and resolution of dating mostly deduced from isotopic stratigraphy were identified as significant factors of distortion we found also that different time scales have been used by authors after readjustment with respect to the same time scale the records were stacked together and the statistical significance of the results tested by a bootstrap technique seventeen records shown to be coherent were combined into a synthetic curve defined as sint 200 other stacks combining various subsets of data were found to be identical to sint 200 except for changes in dispersion in fact the dispersion around sint 200 could be reduced further by improving the correlations without violating the stratigraphy a procedure which at least for some records emphasizes the need for more detailed depth time correlations another characteristic emerging from the results is that paleointensity features as short as 10 kyr are observed between 120 200 ka and can thus in principle be resolved however subsequent refinement is still necessary for this period which is documented by fewer records than the following 120 kyr finally we compared sint 200 with the records from the labrador sea and the lac du bouchet ldb both of which rely mostly on 14c chronology the two records show satisfactory agreement with sint 200 when transferred with respect to the age model established by correlating their 18o labrador and susceptibility features ldb with the 18o reference record the offset of 7 kyr at 40 ka measured in both cases between the 14c and 18o ages is higher than the value deduced from a recent model of a geomagnetic field control of 14c production 1 but is not inconsistent with recent u th dates

thermodynamics of water solubility and partitioning when it was realized that nominally anhydrous minerals could be a major storage site of water in the mantle smyth 1987 bell and rossman 1992 it also became rather obvious that many mantle samples probably have lost most of their water during ascent e g ingrin and skogby 2000 while analyses of natural samples therefore in many circumstances only provide a lower limit of the actual water content in the mantle measurements of water solubility give an upper limit of the amount of water that might be stored in a mineral experimental measurements of water solubility have therefore naturally evolved in the last decades as a major tool for understanding the water storage capacity of the mantle closely related to studies of water solubility are studies of water partitioning among minerals if the water solubility in two minerals is known under given conditions the partition coefficient is just the ratio of the water solubilities under the same conditions provided that the composition of the fluid coexisting with the two minerals is the same since the pioneering work of bai and kohlstedt 1992 and of kohlstedt et al 1996 the water solubility in all major upper mantle minerals has been studied as well as the partitioning of water between the minerals of the lower mantle and the transition zone solubility studies by themselves do not directly give the water content in the mantle they constrain the storage capacity of the mantle i e the maximum amount of water that may be stored if the mantle were water saturated however as most of the mantle is very likely water undersaturated the actual water content is probably far below the storage capacity estimates of the actual abundance and distribution of water in the mantle may be obtained by combining solubility and partitioning data with direct

relative palaeointensity records from the ontong java plateau summary rock magnetic palaeomagnetic and oxygen isotopic results are presented from core rndb 75p which was recovered from the ontong java plateau ojp a high degree of uniformity in magnetic properties characterized by relatively small changes in concentration and grain size in the upper 4 m of the core combined with a lack of coherence between the normalized remanence and rock magnetic data suggests that the natural remanence normalized by saturation remanence reflects variations in relative palaeointensity of the geomagnetic field the record from rndb 75p replicates other ontong java records spanning the last 400ka and extends the record back to some 700ka spectral analysis of the ontong java record suggests periodic behaviour in the relative palaeointensity record with a dominant period of between 30 and 40ka which appears not be be an artefact of lithologic variability this dominant period lies between functions describing climatic precession and obliquity changes in the earth s orbit comparison of the normalized remanence record with astronomical precession 26 ka period however is much more favorable none the less tuning the palaeointensity record to that of astronomical precession appears inconsistent with existing isotopic age constraints derived from the specmap time scale based on these data we must choose between assuming that the earth s orbit controlled ice volume inherent in the specmap time scale and assuming that the earth s magnetic field is driven by astronomical precession the former assumption has a substantial theoretical and observational base and we prefer to interpret the data presented here as suggesting that the earth s orbit has not played a detectable role in the modulation of the magnetic field plots of saturation remanence and magnetic susceptibility are very sensitive to quite subtle changes in magnetic grain size a slight shift within the pseudo singledomain grain size range toward the multidomain or superparamagnetic field was detected at about 4m in rndb 75p this change in grain size may reflect a diagenetic alteration of the magnetite such as dissolution and may be related to the phenomenon responsible for the loss of magnetic remanence at depth detected in other cores from the region

three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism 1 infrared spectroscopy on synthetic olivines has established that there are at least four different mechanisms by which hydrogen is incorporated into the crystal structure two mechanisms occur in the system mgo sio2 h2o associated with silicon and magnesium vacancies respectively a third mechanism is associated with trivalent cation substitution commonly fe3 in natural olivine while the fourth mechanism which is the one most prevalent in natural olivines from the spinel peridotite facies of the earth s upper mantle is associated with ti4 berry et al 2005 here first principles calculations based on density functional theory are used to derive the structure and relative energies of the two defects in the pure mgo sio2 h2o system and possible hydrogen bearing and hydrogen free point defects in ti4 doped forsterite calculated structures are used to compare the predicted orientation of the o h bonds with the experimentally determined polarization the energies are used to discuss how different regimes of chemical environment temperature t pressure p and both water content and water fugacity f h2o impact on which of the different hydroxyl substitution mechanisms are thermodynamically stable we find that given the presence of ti impurities the most stable mechanism involves the formation of silicon vacancies containing two protons charge balanced by a ti4 cation occupying an adjacent octahedral site this mechanism leads to the water mediated formation of silicon vacancies as silicon is known to be the most slowly diffusing species in olivine this provides a credible explanation of the observed water weakening effect in olivine