Review of archaeointensity methods

Most of the traditional methods of determining the intensity of the ancient geomagnetic field from archaeological materials utilized thermal demagnetization of the natural remanent magnetization (NRM) and of the laboratory induced thermoremanent magnetization (TRM). When applied rigorously these methods are foolproof. They are, however, very time consuming and the number of samples with which they can be used is limited. Attempts to speed up these traditional methods have generally led to the use of subjective criteria in assessing the reliability of the results and archaeomagnetic research has recently been concentrated on extending the range of samples to which the method can be applied. Through the use of alternating field, rather than thermal, demagnetization of NRM and TRM it has become possible to apply corrections for alteration occurring during laboratory firing of the archaeological samples and develop objective criteria of reliability. Recent research has shown that it may be possible to determine archaeointensities the laboratory redeposition of lake sediments.     

Magnetic properties of some tertiary basalts from West Greenland

Summary Measurements of bulk magnetic properties, including the natural remanent magnetization (NRM), susceptibility and the Königsberger ratio, on over 250 samples of Tertiary basalts from Disko and Nûgssuaq, West Greenland are reported.The NRM intensities in basalts (geometric mean value 3.3 A/m in SI units) were on average three to four times as large as the induced magnetization intensities. The susceptibilities (geometric mean value 2.1×10^–2 SI units) were much more uniform than the NRM intensities. In the majority of samples, the NRM was predominantly of reverse (R) polarity, but samples from a few sites showed a remanence of normal (N) polarity.The NRM of both polarity classes (N, R) was very stable against alternating field (AF) demagnetization with median destructive fields of the order of 20,000–30,000 A/m (250–350 Oe), comparable to those for many stable continental and oceanic basalts. The viscous remanence intensity, as studied by storage tests on some specimens, was found to be an insignificant fraction of the original NRM, except in few cases.The low field hysteresis loops (Rayleigh loops) were studied for some specimens. A qualitative association was noted between wide hysteresis loop and relatively low AF stability, but no correlation was apparent between the loop type and the Königsberger ratio (Q _n) of a specimen.Contribution no. 6 Institute of Geophysics, University of Copenhagen.     

Self-reversal explanation for the Laschamp/Olby geomagnetic field excursion

The reversed natural remanent magnetisation (NRM) of the volcanic features at Laschamp and Olby (Chaîne des Puys, Auvergne, France) is commonly believed to document the youngest excursion of the geomagnetic field during the present Brunhes epoch. Recent radiometric dating determines the age of these lavas to be 35000–45000 y. Continuous thermal demagnetisation indicates that under laboratory conditions the NRM of many Olby samples undergoes complete or partial self-reversal; to a lesser extent this is also true for the Laschamp material. Thus self-reversal may be another possible explanation of the reversed NRM directions. Magnetic, optical and microprobe analyses suggest that the self-reversal mechanism is based upon magnetostatic interaction between differently oxidised titanomagnetite phases.     

Thermal remagnetization and the paleointensity record of metamorphic rocks

We present a quantitative relationship between blocking temperature and time that, in principle, provides a calibration of thermal remagnetization in nature. For a given metamorphic temperature-time regime, one can decide whether a given laboratory blocking temperature (or for paleointensity work, a range of blocking temperatures) is consistent with primary natural remanence (NRM) or with a metamorphic overprint. Independent of the domain structure or the chemical composition of the magnetic minerals, two general types of behaviour are predicted. If the primary NRM possesses laboratory (or primary cooling) blocking temperatures within 100°C or so of the Curie temperature, thermal remagnetization at lower temperatures, even over times as long as 10⁶ years, is improbable. If the blocking temperatures are lower, viscous remagnetization is pronounced at temperatures well below those indicated by laboratory thermal demagnetization. An approximate scale of the “survival potential” of primary NRM in rocks of different metamorphic grades indicates that primary paleointensities are unlikely to be recovered from rocks metamorphosed above high-greenschist facies if the predominant magnetic mineral is nearly pure magnetite, or above middle-amphibolite facies if nearly pure hematite is predominant. Evidence from laboratory experiments and paleomagnetic field studies in metamorphic regions suggests, however, that these survival estimates are unduly optimistic. Chemical remagnetization through the destruction of primary magnetic minerals, and not thermal remagnetization, probably sets an effective upper temperature for the survival of primary NRM.     

Ancient magnetic field determinations on selected chondritic meteorites

Paleofield intensity determinations involving a comparison of the stable natural remanence (NRM) component with a laboratory thermoremanence (TRM) were carried out on nine chondrites selected in Brecher and Fuhrman (1979a, this issue, hereafter called Paper I), as well as on two manifestly unsuitable controls. To judge their reliability: (1) heat-alteration was monitored by comparing saturation coercivity spectra before and after heating; and (2) the NRM and TRM intensity and stability were compared to those of residual magnetization following zero-field cooling (TRM₀) from above the Curie point of kamacite (Ni---Fe). The latter criterion separates the role of an external magnetic field (of 0.43 Oe) at cooling from intrinsic contributions to magnetic grain alignments, due to accretionary, metamorphic or shock-oriented petrofabrics.

In some chondrites (e.g., Brownfield, H3B; Holyoke, H4C; Farley, H5A), a surprisingly large (10% NRM) and stable TRM₀ proved so similar to NRM and TRM, that sizeable spurious “paleofields” — comparable to paleointensities obtained — were derived by the standard method for zero-field cooling. In other chondrites, with negligible TRM₀ (1% of NRM) and irregular AF demagnetization curves, more reliable paleofield strengths in the range 0.01–0.09 Oe were obtained (e.g., Cavour, H6C). These seem representative of magnetic fields at the end of metamorphism intervals (10⁷ years after accretion) and/or at post-shock cooling. Thus, field strengths obtained from ordinary chondrites are typically weaker (by factors of 10–100) than those reliably determined from carbonaceous chondrites and ureilites, suggesting temporal decay of nebular magnetic fields, from the end of accretion until the end of metamorphism and early catastrophic-collisional stages.     

Magnetic fields of the solar nebula as recorded in chondrules from the Allende meteorite

The natural remanent magnetization (NRM) in individual chondrules from the Allende meteorite was measured. These had previously been oriented relative to each other. The NRM directions of the chondrules are not initially random, but they become scattered after either alternating field (AF) or thermal demagnetization. The NRM is less stable than anhysteretic remanent magnetization (ARM) against AF-demagnetization.

The bulk of the NRM in the matrix is erased by 300°C. For the larger chondrules it is erased by 550°C, but for the smaller chondrules and the white inclusion a substantial decrease in NRM occurs by 350°C leaving about 20% up to 600°C. The behavior of the laboratory-induced ARM and the NRM under alternating field demagnetization suggest that the NRM of the chondrules consists of at least two components of TRM. One is a high-temperature component which was acquired when the individual chondrules were cooled through the Curie temperature and before they were assembled into the Allende meteorite. The other is a low-temperature component which was probably acquired in a field of about 1 Oe when the meteorite experienced thermal metamorphism or during the assembly of the meteorite.     

Paleointensity determinations of lunar samples

Evidence for the existence of an ancient lunar field comes from the NRM of lunar rocks and from measurements of local lunar surface magnetic fields. Even when the Fe particles present in the rocks possess enough magnetic stability to preserve a record of the original magnetizing field, there are considerable difficulties in estimating the strength of the field, primarily because of the chemical changes which take place on heating in the laboratory. These difficulties are discussed together with a paleointensity method involving heating in a more indirect way. A tentative interpretation of the results of paleointensity-age measurements is given.     

Single-domain magnetite in the Jimberlana Norite,Western Australia

The feeder series gabbros of the Jimberlana Norite possess a large, stable NRM. Koenigsberger ratios of 20 or greater suggest that single-domain grains carry most of the NRM. Coercivity spectra extend up to 2000 Oe. Volumes of magnetite satisfying single-domain requirements occur principally as intergrowths of host magnetite between exsolution phases. Small discrete magnetite grains do occur in the Ca-rich pyroxenes but are thought to contribute only a minor fraction of the NRM.     

Magnetic properties of the Olivenza meteorite—possible implications for its evolution and an early solar system magnetic field

The magnetic properties of samples of the Olivenza chondrite (LL5) obtained from four collections have been investigated. The natural remanent magnetization (NRM) consists of a very stable primary component, which is randomly scattered in direction on a scale of 1 mm³ or less within the samples, and a secondary magnetization widely varying in intensity, and probably also in direction. The origin of the secondary NRM is not clear, and may be of terrestrial origin. It is concluded that the NRM is carried by the ordered nickel-iron mineral, tetrataenite. The origin of the primary NRM could be a magnetic field associated with the solar nebula, out of which the metal grains condensed and acquired a thermo-remanent magnetization (TRM), or Olivenza could be a fine-grained breccia, the constituent fragments possessing randomly directed magnetization. The implications for the origin and evolution of Olivenza and its parent body if the former magnetizing process has occurred are discussed.     

菲律宾海西北部岩芯记录的125 ka以来的地磁场强度及其影响因素

沉积物记录的地磁场强度首先提供了模拟地磁场演化的数据约束,其次提供了沉积物的年龄信息. 本文报道了菲律宾海西北部岩芯记录的地磁场相对强度,并结合岩石磁学和沉积学性质探讨了影响强度的各个因素. 除底部红粘土层的局部磁偏角偏转可能揭示了沉积后改造以外,磁化率各向异性和地磁场方向特征表明沉积物为原状沉积. 岩石磁学性质表明沉积物符合磁性均一性,可以记录可靠的地磁场强度. 由于红粘土层及其下部的磁偏角异常,本文讨论其上部约125 ka的结果. 常规归一方法获得的两个地磁场强度参数NRM/ARM(特征剩磁和非磁滞剩磁比值)和NRM/κ(特征剩磁和磁化率比值)与其它记录对比得到时间-深度对比点,对比点之间的年龄为线性内推或者外推. 地磁场强度时间模型上的岩芯氧同位素与全球氧同位素综合曲线一致证明强度结果的有效性和对比的正确性. 磁化率为归一参数的强度大多低于以非磁滞剩磁为归一参数的强度,频谱和相关分析证明NRM/ARM不与ARM和磁性矿物粒度(ARM/κ)相关,也没有轨道周期性,而NRM/κ却与κ和ARM/κ相关,而且有13~12 ka的周期. 由此我们认为NRM/ARM记录的地磁场强度比NRM/κ更好地消除了气候印记. 进一步探讨了超顺磁含量、碳酸钙含量、磁性矿物组成以及磁性矿物粒度变化与地磁场强度差值的关系,发现末次间冰期较高的超顺磁含量和磁性矿物粒度的较大范围变化造成了地磁场强度差值,后者至少造成了90%差异. 中等含量的碳酸钙和较小的磁性矿物组成变化不是磁场强度差值产生的原因. 如何校正磁性矿物粒度变化的影响将是下一步工作的重点.     

Complete and partial self-reversal of natural remanent magnetization of basaltic rocks from Lower Silesia,Poland

Summary Experiments of heating-cooling cycles in zero magnetic field were performed in order to study self-reversal of NRM in basaltic rocks from Lower Silesia. Complete self-reversal occurred in one sample containing titanomagnetite withT _c of 170°C and a small amount of a phase with higher Curie point. During consecutive heat treatments the phenomenon became less conspicuous. In three samples of higher oxidation level, containing several magnetic phases, only partial self-reversal of NRM occurred. For the most oxidized sample no changes of direction of NRM were observed. We suggest that the investigated phenomenon of self-reversal of NRM is due to a negative magnetic interaction between primary titanomagnetite and products of its oxidation. It seems that complete self-reversal can take place in a restricted state of oxidation.     

Reliability of geomagnetic paleointensity data: the effects of the NRM fraction and concave-up behavior on paleointensity determinations by the Thellier method

To test the reliability of the Thellier method for paleointensity determinations, we studied six historic lavas from Hawaii and two Gauss-age lava flows from Raiatea Island (French Polynesia). Our aim is to investigate the effects of the NRM fraction and concave-up behavior of NRM–thermal remanent magnetization (TRM) diagrams on paleointensity determinations. For the Hawaiian samples, the paleointensity results were investigated at both sample and site levels. For consistency and confidence in the paleointensity results, it is important to measure multiple samples from each cooling unit. The results from the Raiatea Island samples confirm that reliable paleointensities can be obtained from NRM–TRM diagrams with concave-up curvature, provided the data are accompanied by successful partial TRM (pTRM) checks and no significant chemical remanent magnetization (CRM) production. We conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40–50% of the total NRM. This new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models. Using this condition together with other commonly employed selection criteria, the observed mean site paleointensities are typically within 10% of the Definitive Geomagnetic Reference Field (DGRF). Our new results for the Hawaii 1960 lava flow are in excellent agreement with the expected value, in contrast to significant discrepancies observed in some earlier studies.

Overestimates of paleointensity determinations can arise from cooling-rate dependence of TRM acquisition, viscous remanent magnetization (VRM) at elevated temperatures, and TRM properties of multidomain (MD) particles. These outcomes are exaggerated at lower temperature ranges. Therefore, we suggest that, provided the pTRM checks are successful and there is no significant CRM production, it is better to increase the NRM fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the NRM–TRM diagrams.

We introduce the factor, Q = Nq, to assess the quality of the weighted mean paleointensity, H_w, for each cooling unit.     

On the possibility of using lunar fines to determine the intensity of the ancient lunar magnetic field

The remanent magnetization of iron grains in lunar fines can be studied by allowing a sample of fines to acquire a depositional remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a natural remanent magnetization (NRM) in the Thellier determination of an ancient lunar field intensity. Although a palaeointensity of ca. 1.0 Oe was determined by this method, several factors complicate the interpretation of the result. These include the origin and nature of the iron metal in lunar fines, and the time and acquisition process of its magnetization.     

Inversion of titanomaghemite in oceanic basalt during heating

We studied the change of magnetic behaviour upon laboratory heating of altered mid-ocean ridge basalt (MORB) samples in the age range of 16-35 Ma to determine the influence of titanomaghemite inversion on the thermal demagnetisation of natural remanent magnetisation (NRM) of these basalts. MORB samples were heated to successively higher temperatures and at the same time the temperature dependence of either saturation magnetisation or NRM was monitored continuously. After each heating step, hysteresis loops and remanent magnetisation curves between 10 K and room temperature were measured. With this procedure, it is shown that the dominant magnetic remanence carrier in our MORB samples is cation deficient titanomaghemite. Moreover, it is demonstrated that the titanomaghemite is gradually changing to a Ti-poor titanomagnetite as the final inversion product. During inversion, both the Curie temperature as well as the maximum unblocking temperature of the NRM are gradually increasing. We show that the paradox of unblocking temperatures above the Curie temperatures often observed for altered MORBs is an artefact of this gradual, heating induced inversion process.     

Magnetostratigraphy of the late Tertiary Verde Formation,central Arizona

Paleomagnetic polarity data were obtained from nine sections of the Verde Formation, a late Tertiary carbonate-bearing lacustrine unit in central Arizona. This study tested the applicability of magnetostratigraphy as a geochronologic technique in a restricted terrestrial sedimentary basin, and its objective was to better define the age of the Verde Formation.Intensities of natural remanent magnetism (NRM) ranged from <10^?7 to >10^?4 gauss. Although secondary components of viscous magnetization commonly were observed, alternating field demagnetization was successful in revealing the polarity of the primary NRM at almost all sites. Thermomagnetic analysis, partial thermal demagnetization of NRM, and polished section analysis together indicate that the primary NRM is a depositional remanence carried by detrital magnetite. Intrabasin stratigraphic correlation of the sections, together with K-Ar ages on interbedded and underlying volcanic rocks has allowed construction of a composite magnetostratigraphic column for the Verde Formation that is correlated with the late Cenozoic polarity time scale. The correlation indicates nearly continuous sedimentation in the Verde basin from ～7.5 to ～2.5 m.y. ago.     

Anomalous directions and paleointensity of the geomagnetic field based on paleomagnetic investigations of rocks from the Karadzha Range (Azerbaijan) of the age 45–20 ka

Paleomagnetic investigations of marine and subaqueous deposits in the 12-meter marine terrace of a section of the Karadzha Range (Azerbaijan) are performed. These deposits correspond to OI stage 3 and encompass the time interval ～45–20 ka. Four anomalous deviations of the magnetization from the dipole field at the sampling site are recorded in the upper and lower transgressive members of deposits. Investigations of the influence of the anisotropy of the magnetic susceptibility (AMS) on directions of the natural remanent magnetization (NRM) showed that only three of the four identified intervals can actually reflect geomagnetic field changes. The fourth interval of the anomalous NRM behavior is recorded in samples demonstrating the presence of the identified AMS direction pointing to a possible deformation of layers, which could turn the NRM vector toward the direction of the acting factor. Based on the age of the terrace under investigation, three other anomalous horizons could correspond to heavily reduced records of the Mono and Lashamp excursions of the geomagnetic field.     

Palaeomagnetic study of some charnockites of the Palni Hills

The intensity of natural remanence magnetisation (NRM) is measured along the three mutually-perpendicular directions using an astatic magnetometer. The intensity of induced magnetisation is measured using an apparatus fabricated in the laboratory. The Koenigsberger ratio has been calculated for all the samples and the ratio has been used to test the stability of NRM in the samples. The mean direction of magnetisation is determined from the three components of the NRM intensity. The palaeomagnetic pole position of the samples is determined using the direction of magnetisation and the site location. An attempt has been made to fix the geological age of the charnockites using palaeomagnetic methods.     

Holocene Earth’s magnetic field variations recorded in marine sediments of the NW African continental margin

Holocene records documenting variations in direction and intensity of the geomagnetic field during the last about seven and a half millennia are presented for Northwest Africa. High resolution paleomagnetic analyses of two marine sediment sequences recovered from around 900 meter water depth on the upper continental slope off Cape Ghir (30°51′N, 10°16′W) were supplemented by magnetic measurements characterizing composition, concentration, grain size and coercivity of the magnetic mineral assemblage. Age control for the high sedimentation rate deposits (∼60 cm/kyr) was established by AMS radiocarbon dates. The natural remanent magnetization (NRM) is very predominantly carried by a fine grained, mostly single domain (titano-)magnetite fraction allowing the reliable definition of stable NRM inclinations and declinations from alternating field demagnetization and principal component analysis. Predictions of the Korte and Constable (2005) geomagnetic field model CALS7K.2 for the study area are in fair agreement with the Holocene directional records for the most parts, yet noticeable differences exist in some intervals. The magnetic mineral inventory of the sediments reveals various climate controlled variations, specifically in concentration and grain size. A very strong impact had the mid-Holocene environmental change from humid to arid conditions on the African continent which also clearly affects relative paleointensity (RPI) estimates based on different remanence normalizers. To overcome this problem the pseudo-Thellier RPI technique has been applied. The results represent the first Holocene record of Earth’s magnetic field intensity variations in the NW Africa region. It displays long term trends similar to those of model predictions, but also conspicuous millennium scale differences.     

Magnetic parameters and variations in the composition of igneous rocks of the Franz Josef Land archipelago

A combined study of magnetic parameters of basalt and andesite samples is performed in the framework of geological investigations of the Franz Josef Land at the paleomagnetic laboratory of Munich University. The study included the determination of the coercivity, saturation magnetization, Curie points, natural remanent magnetization (NRM), and magnetic susceptibility and the examination of ferromagnetic minerals with a microscope. Data on the chemical composition of rocks are obtained for all samples, and radiological ages are determined for the majority of rocks.Thermomagnetic curves of samples are subdivided into four types depending on the composition of ferromagnetic NRM carriers.The data obtained point to multiple changes in the predominant composition of igneous rocks. Each stage of magmatism is characterized by a specific type of the ferromagnetic component in the rocks and, therefore, magnetomineralogical investigations can be used for differentiation and correlation of the igneous rocks.     

Author index 1984 (volumes 67–71)

Magnetic properties of samples from Bell Island sedimentary rocks have been studied. X-ray analysis indicates that the main magnetic mineral is hematite in all samples. The other iron-bearing minerals identified are siderite and chamosite. Microscope observations of thin sections suggest that the rocks consist of oolitic hematite in a matrix of siderite or calcite. The intensity of natural remanent magnetization (NRM) varies in the range of (0.03–0.4 A m^?1), depending on the percentage of hematite. The thermal demagnetization curves of NRM show in some cases a sharp increase in magnetization at temperatures in the range 500–600°C. The peaks that occur in these demagnetization curves are due to a chemical change of siderite during repeated laboratory heating. X-ray analysis confirmed that the newly formed material is magnetite. Since the original NRM has been masked by the new intergrown material, this would result in a serious error in the determination of paleomagnetic pole positions. The samples showing this behaviour were not considered for paleomagnetic study. The samples containing oolitic hematite in a calcite matrix exhibit very high stability of NRM, including directional stability until almost 670°C. For these samples, a virtual pole position based on N = 6 samples (32 specimens) demagnetized to 665°C is 34°N, 114°E, not far from published Ordovician poles for the North American craton.