GP11C-0835 0800h
Proterozoic Magnetic Field Intensities Obtained Using the Microwave Palaeointensity Technique.
Geomagnetic field intensities have been obtained using the microwave palaeointensity technique from a number of Precambrian dyke swarms. This method uses high-frequency microwaves resonant with the magnetic carrier to demagnetise and remagnetise the sample. As the microwaves only interact the magnetic grains (which make up only a small component of the rock) bulk heating of the specimen is reduced, minimising sample alteration and increasing the chance of experimental success. The conventional Thellier palaeointensity method has also been applied as an independent standard. Chilled margin samples from a selection of Proterozoic dolerite dykes ranging in age 2.5 - 1.2Ga have been studied to ascertain the magnetic field strength at the time of formation. Rock magnetic analysis has been carried out to evaluate magnetic mineralogy and grain size. Curie point analysis shows single distinct Curie temperatures at $580\deg$C for the majority of samples, indicating pure magnetite. Hysteresis and backfield IRM measurements indicate grains within the pseudo-single domain (PSD) grain size range. Lowrie-Fuller tests conducted to determine the grain size of remanence suggest that for samples that pass acceptance criteria, remanence is carried by single domain (SD) grains. High quality field estimates were obtained, passing alteration and multi-domain checks, with palaeointensities generally being lower than present. The corresponding virtual dipole moment (VDM) for the geomagnetic field across the Proterozoic ranges from 2 - 5 x10$^{22}$ Am$^{2}$, showing the geodynamo to be stronger in intensity now (8 x10$^{22}$ Am$^{2}$) than it was earlier in geologic history.
GP11C-0836 0800h
The Intensity of the Earth's Magnetic Field in the Earliest Triassic: Results from the East Siberian Trap Basalts in the Maimecha-Kotuy Region, Russia.
A reliable and detailed record of the variation of the Earth's virtual dipole moment (VDM) provides important constraints for the understanding of geodynamo processes. However, the dataset available is very scarce. This holds true especially for the Mesozoic. An excellent opportunity to study the behaviour of the Earth's magnetic field with high temporal resolution are the Siberian Trap basalts, erupted at the Permo-Triassic boundary. So far, most studies of this large igneous province (LIP) and the palaeomagnetic information recorded by this huge pile of extrusives are limited to the sequence exposed in the Noril'sk area. The study presented here focuses on the East Siberian Trap basalts of the Maimecha-Kotuy region. The section sampled (21 flows) covers the Kogotoksky suite (upper Onkuchaksky to Tyvankitsky suite). Recent radiometric dates suggest that the section was emplaced in the earliest Triassic, postdating the main volcanic activity of this LIP. A transition from normal to reversed polarity is observed within the section. Therefore, it probably corresponds to the very upper part of the volcanics of the Noril'sk area implying that volcanism remained active significantly longer in the Maimecha-Kotuy region. From the paleomagnetic point of view, flows yield reliable directional information (overall VGP: 152.5\ensuremath{^{\circ}}/ 48.7\ensuremath{^{\circ}} with d$_\mathrm{p}$/d$_\mathrm{m}$: 8.5 / 9.6) carried by Ti-rich titanomagnetites of variable composition. Secondary overprints are rare and restricted to blocking temperatures below 150\ensuremath{^{\circ}} C. Extracting paleointensities using the Thellier-Thellier method showed to be more difficult. Reliable values were obtained from 9 flows. The mean VDM calculated on the basis of the data obtained from this section is $1.6 \pm 0.3 \times 10^{22} \;\mathrm{Am}^2$. This value suggests that the Earth's magnetic field was very weak around the Latest Paleozoic/Earliest Mesozoic for a considerable time as it is comparable to data from the western (and older) part of the Traps ($2.2 \pm 0.9 \times 10^{22}\;\mathrm{Am}^2$).
GP11C-0837 0800h
Spot Reading of the Absolute Paleointensity of the Geomagnetic Field During the Kiaman Superchron: The Exeter Lavas Case
It is well known that Superchrons provide an opportunity to study the geomagnetic field in an extreme state, i.e. when reversals are at their lowest frequency or totally absent. Such Superchrons represent features that may provide constraints for theoretical calculations and numerical models of the geodynamo. Thus, one way to contribute to the understanding of the generation of the paleofield particularly during Superchrons is to determine the absolute paleointensity of rocks formed during those periods of time. We have sampled a member of the Exeter lavas (ca 290.8+/-0.8 Ma, Ar/Ar date) from the large, abandoned quarry in the northeastern slope of the southernmost hill of Killerton Park, 10 km northeast of Exeter where we collected three samples from three different stratigraphic levels. We stepwise demagnetized the samples collected from the three different levels by both alternating field (5mT to 100mT) and thermal (from 28$^{o}$C to 575-650$^{o}$ C) methods, and the mean directions obtained by principal component analysis (D=198$^{o}$ and I=-25$^{o}$). All samples yielded a strong and stable ChRM trending towards the origin based on no less than seven to nine steps, with thermal and AF results agreeing to a very high degree. Low field susceptibility versus temperature (k-T) analyses were conducted for individual samples and the majority of them show reversible curves. Curie point determinations revealed a temperature close to or equal to 580$^{o}$ C, indicative of almost pure magnetite for most of the samples. Magnetic grain sizes analysis indicated SD-PSD sizes. We used the modified Thellier-Coe double heating method to determine paleointensities. pTRM checks were performed systematically one temperature step down the last pTRM acquisition in order to document magnetomineralogical changes during heating. The temperature was incremented by steps of 50$^{o}$ C between room temperature and 500$^{o}$C and every 25-30$^{o}$ C. The paleointensity determinations were obtained from the slope of the Arai diagrams. Special care was taken to interpret the Arai diagrams within the same range of temperatures lower than 300$^{o}$C unless a clear and unique slope would be present. We were able to obtain paleointensity determinations for the three set of samples. The three independent estimates of paleofield range between 23.5 to 30 uT ( i.e. VADM 5.5 to 9.03 X10$^{22}$ Am$^{2}$). The range of these values includes the present dipole moment (i.e. 8X10$^{22}$ Am$^{2}$), indicating that the paleofield during the studied period of time was strong, stable and is consistent with available time-averaged intensity and directional data.
GP11C-0838 0800h
Palaeointensity study of the R3/N3 reversal recorded in Icelandic lavas using the microwave technique.
The microwave palaeointensity technique has been used to determine the palaeointensity during the R3/N3 geomagnetic field reversal using lavas from the Esja region of S.W. Iceland. The resulting intensity determinations have been compared to two previous studies (Shaw, 1974; Goguitchaichvili et al., 1999), which report similar low field intensities during the reversal except for 4 flows that produced high values (20.4-36.8 microTesla) in the study of Shaw, 1974, compared to a single maximum intermediate values of 22.8 microTesla in the study of Goguitchaichvili et al., 1999. In this study, using samples from both previous studies collections, we present results obtained using the microwave palaeointensity technique.
http://www.liv.ac.uk/geomagnetism
GP11C-0839 0800h
Thellier-Thellier Paleointensity and Directional Results From Hawaiian Lava Flows
During the last 25,000 years there has been low secular variation in the Pacific region. A large directional dataset for this specific time window already exists but will be enlarged by the inclusion of results from our recent analysis of rocks from Hawaii. In addition to the directional data, we have also obtained paleointensity values for a significant number of the samples studied. This study uses samples that have been collected by the US Geological Survey from 35 lava flows erupted from Kilauea, Mauna Loa and Hualalai volcanoes on the Big Island of Hawaii. All of the flows have been radiocarbon dated and the ages range from 290 to 25,000 years old. Paleomagnetic analysis over a 3 month period was carried out on a total of 216 half-size samples at the Laboratoire des Sciences du Climat et de l'Environnement in Gif-sur-Yvette, France. Thellier-Thellier paleointensity experiments were performed in two large capacity furnaces that have been designed to specifically reduce any oxidation of samples. A new, strict selection criteria based on the analysis of Arai, Zijderveld and stereo plots has also been applied to improve the reliability and robustness of the results. Failure to meet the selection criteria is generally found to be due to to the alteration of the sample and the sample's capacity to record magnetic fields during the heating process. Despite this rigorous selection criteria, a high success rate of 62% has been achieved, well above the average of 30% usually reported for this method. The resulting new paleointensities and directions will be presented within the context of augmenting the secular variation record and improving the dating of lava flows. Interpretion with reference to theories on the secular variation of the Pacific will also be discussed.
GP11C-0840 0800h
Preliminary Microwave Palaeointensity Results From the Tatara-San Pedro Volcanic Complex, Chile - Intensity Variation Through the Matuyama-Brunhes Transition?
A sequence of lavas from the Tatara-San Pedro volcanic complex (Andes, Chile) recording transitional directions has been dated and palaeomagnetic directions obtained (Brown et al., in press. J. Geophys. Res.). Ar/Ar dates from this study suggest this sequence records an event preceding the main Matuyama-Brunhes transition (MBT), though some normal polarity directions are recorded from the upper part of the lava sequence. Microwave palaeointensity determinations at the Liverpool University Geomagnetism Laboratory have been made to ascertain whether these normal polarities are related to the pre-cursor event or represent the end of the main MBT. Preliminary results yielding high intensity values from the normal polarity flows (up to ten times that of the transitional field intensity) suggest that the post-reversal field is recorded in these flows.
GP11C-0841 0800h
Paleodirectional and Paleointensity Variations During the Brunhes-Matuyama Polarity Reversal From the Lava Sequence in Punaruu Valley, Tahiti Island
The polarity reversal is considered to show a large drop in the geocentric axial dipole (GAD) moment. Since GAD is the greatest component of the main field, its large change possibly gives key information about the geodynamo. To clarify behaviors of the geomagnetic field during the reversal, we have studied the Brunhes-Matuyama (B-M) polarity reversal recorded in 21 lavas of the lava succession in the northern side of Punaruu valley, Tahiti Island. Although Chauvin et al. (1990) reported the B-M reversal and older events from the lava sequence in the southern side of the same valley, only a few paleointensities have been obtained for the B-M reversal record. We firstly carried out rock magnetic experiments. Curie temperatures were observed to be 500-600 and/or 100-200 $\deg$C, suggesting that a single phase of titanium-poor or titanium-rich titanomagnetite, or a mixture of them was contained in the samples. Hysteresis parameters of the samples were mostly plotted in PSD or SD regions of the Day plot. The samples were subjected to thermal demagnetization, or alternating field (AF) demagnetization following the low temperature demagnetization (LTD). Secondary components were generally erased at low temperatures ($\leq$ 300-400 $\deg$C) or low AFs ($\leq$ 10 mT). Several lavas showed significant amount of secondary components, so that a high coercivity or a high blocking-temperature component was carefully extracted as a primary one. For paleointensity determination, the double heating technique of the Shaw method combined with LTD (LTD-DHT Shaw method) was applied to 107 samples from those lavas. This is because AF demagnetization was more effective for removal of secondary components than thermal one and also because the reliability of the method was supported by recent studies of historical lavas. Mean paleodirections (N$\geq$3) were obtained for 18 lavas and mean paleointensities (N=2-6) for 13 lavas. The results show two stages of paleodirection: the directionally stable period of the reversed polarity and the directionally unstable period of reversed-intermediate-normal-reversed (R-I-N-R) polarity change. The second reversed polarity was recorded in the uppermost lava, suggesting another reversal to the Brunhes normal polarity after the studied sequence. While the field strength seems to be weak (4.7 $\mu$T) for the directionally unstable period, it varies in oscillation-like manner between 5.9 and 42.9 $\mu$T for the directionally stable period. During the directionally stable period, virtual geomagnetic pole (VGP) latitudes repeatedly changed between 60 and 90$\deg$S. When compared with these VGPs, the oscillation-like change in virtual dipole moments shows a strong correlation. This behavior is suggested to be one of the characteristics of the reversal process.
GP11C-0842 0800h
Variations in magnetic properties over a thin lava flow profile: implications for palaeo-direction and palaeo-intensity recordings
Reversals and excursions of the Earth's magnetic field have previously been reported to be recorded in an 14.1 Ma old lava sequence of the mid-Miocene shield basalts from Gran Canaria (Canary Islands). Earlier palaeomagnetic and rock magnetic investigations on standard cylindrical inch samples showed that the rocks are excellent recorders of the past Earth's magnetic field. We have resampled parts of the sequence by taking complete sections of thin lava flows ($ <1.5$ meters) in order to determine the influence of variations in rock magnetic properties within a flow on palaeo-direction and in particular on palaeo-intensity determinations. Preliminary results obtained on mini-cores drilled every 0.5 cm in a 33 cm coherent lava section indicate significant variations in saturation magnetisation and coercivity over the profile. From bottom to top, M${}_{\rm s}$ and M${}_{\rm rs}$ increase moderately from $\sim 0.25$~Am${}^2$/kg and $\sim 6.0 \cdot 10^{-2}$ Am${}^2$/kg to $\sim 0.46$~Am${}^2$/kg and $\sim 8.8\cdot 10^{-2}$ Am${}^2$/kg, respectively from 0 to 14 cm over the profile. Between 14 and 16 cm, values are nearly tripled. M${}_{\rm s}$ and M${}_{\rm rs}$ then reach $\sim 1.3$~Am${}^2$/kg and $ \sim 0.25$~Am${}^2$/kg, before gently decreasing to $ \sim 1.0$~Am${}^2$/kg and 0.1~Am${}^2$/kg respectively from 16 to 33 cm. Variations in H${}_{\rm c}$ and H${}_{\rm cr}$ also exist but appear to be less pronounced, and the general trend is a decrease from bottom to top. Furthermore, the H${}_{\rm c}$/H${}_{\rm cr}$ ratio increases from $\sim$2.5 to $\sim$3.5 whereas M${}_{\rm rs}$/M${}_{\rm s}$ decreases from $\sim$2.0 to $\sim$1.0 up in the profile. The bulk mineralogical composition changes also notably, resulting in a variation of Curie temperatures. This is confirmed by similar variations in unblocking temperature spectra measured both by standard palaeomagnetic thermal demagnetisation and by continuous thermal demagnetisation. In particular, alteration processes appear to be more prominent near the base of the flow. The influence of such changes on palaeo-direction and palaeo-intensity determinations are finally addressed in this poster.
GP11C-0843 0800h
A Geomagnetic Estimate of Mean Paleointensity
To test a statistical hypothesis about Earth's magnetic field against paleomagnetism, the present field is used to estimate time averaged paleointensity. The estimate uses the modern magnetic multipole spectrum R(n), which gives the mean square induction represented by spherical harmonics of degree n averaged over the sphere of radius a = 6371.2 km. The hypothesis asserts that low degree multi-pole powers of the core-source field are distributed as chi-squared with 2n+1 degrees of freedom and expectation values \{R(n)\} = K[(n+1/2)/n(n+1)](c/a)$^{(2n+4)}$, where c is the 3480 km radius of Earth's core. (This is compatible with a usually mainly geocentric axial dipolar field). Amplitude K is estimated by fitting theoretical to observational spectra through degree 12. The resulting calibrated expectation spectrum is summed through degree 12 to estimate expected square intensity \{F$^{2}$\}. The sum also estimates \{F$^{2}$\} averaged over geologic time, in so far as the present magnetic spectrum is a fair sample of that generated in the past by core geodynamic processes. Voorhies & Conrad (1996) excluded the dominant dipole and quadrupole from the fit, but not the sum, to predict mean paleointensity from the 1980 non-dipole field. Those predictions have a 30% standard deviation and a 9% standard error, yet are within 5% of the new estimates: \{F$^{2}$\} = (37,300 nT)$^{2}$, an expected paleointensity \{F\} of about 34,400 nT, and an expected Virtual Axial Dipole Moment \{VADM\} of about 6.13x10$^{22}$ Am$^{2}$. The values reflect the refined theoretical spectrum [Voorhies, 2004], self-consistent weights for all degrees 1-12, and the 41% increase in R(12) from 1980 Magsat to 2000 Oersted epochs. Both past predictions and new estimates are well within the range of published paleomagnetic determinations of mean paleointensity; therefore, the statistical hypothesis passes this test.
GP11C-0844 0800h
Geomagnetic Dipole Moment of the Past 7000 Years - New Results From a Global Model
Evolution of the geomagnetic field's dipole strength is studied by geomagnetists from global spherical harmonic models and by paleomagnetists using virtual (axial) dipole moments (VDM, VADM). Based on a recently published global model of the past 7000 years we study whether these three dipole moment descriptions can be considered equivalent, and compare the results to previous global VADM studies and recent global model dipole moments. We conclude that VADM and VDM results averaged over centennial and millennial time scales are systematically higher than the true dipole moment by about 20 percent. The current dipole decrease is part of a process that has been going on for about 1700 years. The average rate of decrease is lower than the current one, but has varied significantly so that the current rate cannot be regarded as exceptional.
GP11C-0845 0800h
Cooling Rate Effects on Paleointensity Estimates in Submarine Basaltic Glass and Implications for Dating of Young Flows
Cooling rate effects on the intensity of thermoremanent magnetization (TRM) have been well documented in ceramics. In that case, lab cooling is generally more rapid than the initial cooling, leading to an overestimate of the paleofield by 5-10% in Thellier-type paleointensity experiments. For slowly-cooled plutons, this effect may be as great as 50%. The reverse scenario, however, has never been tested. We investigate the theory that rapidly-quenched submarine basaltic glass (SBG) may produce an underestimate of the paleofield when subjected to Thellier experiments cooled in air. If attempts to use SBG paleointensity as a dating tool are to be successful, this question must be addressed. We combine Thellier-Thellier paleointensity experiments with relaxation geospeedometry determinations of natural cooling rate. Experiments were performed on a set of young SBG from 6 sites at $17\deg$S on the East Pacific Rise, where the present-day field is 30.8 $\mu$T, and the field has been monotonically decreasing for the past $\sim$200 yr. Cooling rate determinations were made on chips taken from the Thellier specimens, as well as on previously unheated specimens. Paleointensity values range from 26.9 $\pm$ 1.4 $\mu$T (1$\sigma$) to 44.1 $\pm$ 4.9 $\mu$T. Age estimates based on these paleointensity values corroborate radiometric ages and geologic observations that place the samples in 3 age groups. The youngest group is very recent (indistinguishable from present day) and the oldest at least 100, and probably several hundred, years old. Absolute cooling rate determinations range from $\sim$10 to $\sim$$250\deg$C min$^{-1}$, as captured at the glass transition at $\sim$$650\deg$C. Such absolute determinations were not possible for all specimens. However, using an estimate of relative cooling rate variation, several sites suggest a weak correlation between cooling rate and paleointensity, the magnitude of which roughly agrees with theory. We estimate lab cooling rates to be on the order of 10-100$\deg$C min$^{-1}$ over the blocking temperature range ($\sim$200-400$\deg$C). Taking the differing temperature ranges into account (650$\deg$C vs. 200-400$\deg$C), the cooling rate data suggest that natural cooling rates may be only slightly faster than the lab rates. The implications are that while the cooling rate effect might produce some within-site scatter, it should not often result in a significant bias in paleointensity from SBG.
GP11C-0846 0800h
Detecting ultra-low magnetic fields with common magnetic minerals
Growing volume of extraterrestrial material is being used to analyze magnetic paleo-intensities. They are important for estimation of paleo-fields that once existed in extraterrestrial environment. The extraterrestrial field can be several orders of magnitudes weaker than a terrestrial field. The data demonstrating that the TRM linear acquisition is valid for such low fields are virtually not existent. We tested the linear relation ship of the thermal remanent magnetization (TRM) acquisition in extremely week fields ranging from 1 nT to 100,000 nT. We used multidomain samples of hematite, magnetite, and iron. We also used mineral assemblages containing mixtures of these minerals. Our results indicate that multidomain material did not record magnetic fields below 1,000 nT. Assemblages carrying single domain (SD) material did record fields as low as 3 nT. When assemblages contain a magnetic phase with blocking temperature higher than the blocking temperatures of the rest of the magnetic minerals within the assemblage it can be detected by deviation from the linear magnetic acquisition. Our results provide rare experimental data for TRM acquisition in fields below 1000 nT.