Understanding non-ideal paleointensity recording in igneous rocks: Insights from aging experiments on lava samples and the causes and consequences of ‘fragile’ curvature in Arai plots

The theory for recording of thermally blocked remanences predicts a quasilinear relationship between low fields like the Earth's in which rocks cool and acquire a magnetization. This serves as the foundation for estimating ancient magnetic field strengths. Addressing long-standing questions concerning Earth's magnetic field requires a global paleointensity data set, but recovering the ancient field strength is complicated because the theory only pertains to uniformly magnetized particles. A key requirement of a paleointensity experiment is that a magnetization blocked at a given temperature should be unblocked by zero-field reheating to the same temperature. However, failure of this requirement occurs frequently and the causes and consequences of failure are understood incompletely. Recent experiments demonstrate that the remanence in many samples typical of those used in paleointensity experiments is unstable, exhibiting an “aging” effect in which the (un)blocking temperature spectra can change over only a few years resulting in nonideal experimental behavior. While a fresh remanence may conform to the requirement of equality of blocking and unblocking temperatures, aged remanences may not. Blocking temperature spectra can be unstable (fragile), which precludes reproduction of the conditions under which the original magnetization was acquired. This limits our ability to acquire accurate and precise ancient magnetic field strength estimates because differences between known and estimated fields can be significant for individual specimens, with a low field bias. Fragility of unblocking temperature spectra may be related to grain sizes with lower energy barriers and may be detected by features observed in first-order reversal curves

Geomagnetic paleointensity in historical pyroclastic density currents: Testing the effects of emplacement temperature and postemplacement alteration

Thellier-type paleointensity experiments were conducted on welded ash matrix or pumice from the 1912 Novarupta (NV) and 1980 Mt. St. Helens (MSH) pyroclastic density currents (PDCs) with the intention of evaluating their suitability for geomagnetic paleointensity studies. PDCs are common worldwide, but can have complicated thermal and alteration histories. We attempt to address the role that emplacement temperature and postemplacement hydrothermal alteration may play in nonideal paleointensity behavior of PDCs. Results demonstrate two types of nonideal behavior: unstable remanence in multidomain (MD) titanomagnetite, and nonideal behavior linked to fumarolic and vapor phase alteration. Emplacement temperature indirectly influences MSH results by controlling the fraction of homogenous MD versus oxyexsolved pseudo-single domain titanomagnetite. NV samples are more directly influenced by vapor phase alteration. The majority of NV samples show distinct two-slope behavior in the natural remanent magnetization—partial thermal remanent magnetization plots. We interpret this to arise from a (thermo)chemical remanent magnetization associated with vapor phase alteration, and samples with high water content (\u3e0.75% loss on ignition) generate paleointensities that deviate most strongly from the true value. We find that PDCs can be productively used for paleointensity, but that—as with all paleointensity studies—care should be taken in identifying potential postemplacement alteration below the Curie temperature, and that large, welded flows may be more alteration-prone. One advantage in using PDCs is that they typically have greater areal (spatial) exposure than a basalt flow, allowing for more extensive sampling and better assessment of errors and uncertainty

Paleomagnetic recording fidelity of nonideal magnetic systems

A suite of near-identical magnetite nanodot samples produced by electron-beam lithography have been used to test the thermomagnetic recording fidelity of particles in the 74–333 nm size range; the grain size range most commonly found in rocks. In addition to controlled grain size, the samples had identical particle spacings, meaning that intergrain magnetostatic interactions could be controlled. Their magnetic hysteresis parameters were indicative of particles thought not to be ideal magnetic recorders; however, the samples were found to be excellent thermomagnetic recorders of the magnetic field direction. They were also found to be relatively good recorders of the field intensity in a standard paleointensity experiment. The samples' intensities were all within ∼15% of the expected answer and the mean of the samples within 3% of the actual field. These nonideal magnetic systems have been shown to be reliable records of the geomagnetic field in terms of both direction and intensity even though their magnetic hysteresis characteristics indicate less than ideal magnetic grains

Absolute Paleointensity Study of Miocene Tiva Canyon Tuff, Yucca Mountain, Nevada: Role of Fine‐Particle Grain‐Size Variations

Fine‐grained, Ti‐poor titanomagnetite in the ~12.7 Ma Tiva Canyon (TC) Tuff systematically increases in grain size from superparamagnetic (SP) at the flow base to single domain (SD) at a few meters height. This allows us to examine the role of grain‐size variation on paleointensity, within the transition from SP to stable SD. We present magnetic properties from two previously unreported sections of the TC Tuff, as well as Thellier‐type paleointensity estimates from the lowermost ~7.0 m of the flow. Magnetic hysteresis, frequency‐dependent susceptibility, and thermomagnetic data show that sample grain‐size distribution is dominated by SP in the lower ~3.6 m, transitioning upwards to mostly stable SD. Paleointensity results are closely tied to stratigraphic height and to magnetic properties linked to domain state. SD samples have consistent absolute paleointensity values of 28.5 ± 1.94 μT (VADM of 51.3 ZAm2) and behaved ideally during paleointensity experiments. The samples including a significant SP fraction have consistently higher paleointensities and less ideal behavior but would likely pass many traditional quality‐control tests. We interpret the SD remanence to be a primary thermal remanent magnetization but discuss the possibility of a partial thermal‐chemical remanent magnetization if microcrystal growth continued at T \u3c Tc and/or the section is affected by post‐emplacement vapor‐phase alteration. The link between paleointensity and domain state is stronger than correlations with water content or other evidence of alteration and suggests that the presence of a significant SP population may adversely impact paleointensity results, even in the presence of a stable SD fraction

Covariant Giant Gaussian Process Models With Improved Reproduction of Palaeosecular Variation

A commonly used family of statistical magnetic field models is based on a giant Gaussian process (GGP), which assumes each Gauss coefficient can be realized from an independent normal distribution. GGP models are capable of generating suites of plausible Gauss coefficients, allowing for palaeomagnetic data to be tested against the expected distribution arising from a time‐averaged geomagnetic field. However, existing GGP models do not simultaneously reproduce the distribution of field strength and palaeosecular variation estimates reported for the past 10 million years and tend to underpredict virtual geomagnetic pole (VGP) dispersion at high latitudes unless trade‐offs are made to the fit at lower latitudes. Here we introduce a new family of GGP models, BB18 and BB18.Z3 (the latter includes non‐zero‐mean zonal terms for spherical harmonic degrees 2 and 3). Our models are distinct from prior GGP models by simultaneously treating the axial dipole variance separately from higher degree terms, applying an odd‐even variance structure, and incorporating a covariance between certain Gauss coefficients. Covariance between Gauss coefficients, a property both expected from dynamo theory and observed in numerical dynamo simulations, has not previously been included in GGP models. Introducing covariance between certain Gauss coefficients inferred from an ensemble of “Earth‐like” dynamo simulations and predicted by theory yields a reduced misfit to VGP dispersion, allowing for GGP models which generate improved reproductions of the distribution of field strengths and palaeosecular variation observed for the last 10 million years

Paleoproterozoic Geomagnetic Field Strength From the Avanavero Mafic Sills, Amazonian Craton, Brazil

A recent hypothesis has suggested that Earth's inner core nucleated during the Mesoproterozoic, as evidenced by a rapid increase in the paleointensity (ancient geomagnetic field intensity) record; however, paleointensity data during the Paleoproterozoic and Mesoproterozoic period are limited. To address this problem, we have determined paleointensity from samples from three Paleoproterozoic Avanavero mafic sills (Amazonian Craton, Brazil): Cotingo, 1782 Ma, Puiuà 1788, and Pedra Preta, 1795 Ma. We adopted a multi-protocol approach for paleointensity estimates combining Thellier-type IZZI and LTD-IZZI methods, and the non-heating Preisach protocol. We obtained an average VDM value of 1.3 ± 0.7 × 1022Am2 (Cotingo) of 2.0 ± 0.4 × 1022Am2 (Puiuà) and 6 ± 4 × 1022Am2 (Pedra Preta); it is argued that the Cotingo estimate is the most robust. Our results are the first data from the upper Paleoproterozoic for South America and are comparable to data available from other regions and similar periods. The new data do not invalidate the hypothesis of that Earth's inner core nucleated during the Mesoproterozoic

Palaeomagnetism of the Ordovician dolerites of the Crozon Peninsula (France)

In order to obtain a Lower Palaeozoic pole for the Armorican Massif and to test the origin of the Ibero-Armorican arc, the Ordovician dolerites of the Crozon peninsula have been palaeomagnetically studied. The samples show a multicomponent magnetization which has been revealed by AF and thermal demagnetization and thoroughly investigated with rock magnetic experiments, polished section examinations and K/Ar dating. Four groups of directions have been recognized, often superimposed on each other in an individual sample. One component (D) has always the lowest blocking temperatures and coercivities and is considered to be of viscous origin, acquired recently in situ or in the laboratory during storage. Two components (A and B) are interpreted to be of secondary origin and to correspond to the observed K/Ar age distribution between 300 and 190 Myr. These ages represent the time interval between two regional thermo-tectonic events, associated with the Hercynian orogeny and the intrusion of dykes related to the early opening of the Central Atlantic Ocean and the Bay of Biscay. A fourth component (C) could be of Ordovician or younger Palaeozoic age; it is not clear whether the age of the magnetization is pre- or post-folding, but a pre-folding age would yield a direction of magnetization similar to Ordovician results from the Iberian peninsula. The latter interpretation suggests a fairly high palaeolatitude, which is in agreement with a glacio-marine postulated for sediments overlying the dolerite sills.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/73214/1/j.1365-246X.1983.tb03785.x.pd

New archaeointensity data from Italy and geomagnetic field intensity variation in the Italian Peninsula

We present new archaeointensity results from three Italian kilns situated at Ascoli Satriano, Vagnari and Fontanetto Po obtained with the Thellier modified by Coe double heating method. These data complement the directional results previously published. All sites are dated on the basis of archaeological information and/or thermoluminescence dating. The results are corrected for the anisotropy of the thermoremanent magnetization and the cooling rate effects. The new data are compared with previously published archaeointensity data from Italy and nearby countries within 900 km radius from Viterbo. An initial data set including archaeointesity data mainly coming from Italy, France, Switzerland, Czech Republic, Slovakia, Hungary, Greece and Bulgaria has been compiled. After the application of strict selection criteria, the most reliable data have been used for the calculation of a preliminary Italian intensity secular variation (SV) curve for the last 3000 yr. The new curve covers the 300 BC–400 AD and 1200–1900 AD periods. It is established by means of sliding windows of 200 yr shifted by 100 yr. The lack of reliable data for the 1000–200 BC and 400–1200 AD time intervals does not permit the calculation of a continuous curve. Clearly, more high-quality archaeointensity data from Italy and Europe are still needed to draw a robust intensity SV curve for the Italian Peninsula that could be used for archaeomagnetic dating in combination with the directional data