Determination of paleointensity from sedimentary and igneous rocks by the method of alternating-field remagnetization

The remanent magnetization of igneous and sedimentary rocks, if not changed by heating or by alteration of magnetic minerals, keep the information about the intensity of that magnetic field in which initial magnetization took place.It has been determined that the dependence of anhysteretic remanent magnetization of such a rock on d.c. magnetic field permits us to find the paleointensity. A method of investigation of rock specimens by means of such remagnetization is suggested and applied to determining the paleointensity for a series of Permo-Triassic rocks.     

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.     

河西走廊早新生代红层次生剩磁及其对原生剩磁影响评价

系统研究了河西走廊火烧沟组陆相红层200个采点岩石的热退磁行为和17个代表性样品的岩石磁学特征,结果表明有102个采点可以分离出A、B、C三个剩磁分量,有82个采点只有一个分量(C分量),16个采点只能分离出A、B分量.A、B分量分别由针铁矿和磁赤铁矿携带,C分量在砂岩中由磁铁矿携带,在泥质砂岩和泥岩中由磁铁矿和赤铁矿共同携带.A、B分量剩磁方向随机分布,不能获得置信水平的古地磁平均方向,为次生剩磁.它们的存在并不影响岩石原生剩磁的分离,也不影响原生剩磁信号的稳定性和获得的古地磁数据的可靠性.16个砂岩采点中不能分离出C分量,是由于后生的赤铁矿彻底改变了由磁铁矿携带的原生剩磁组分,在高密度采样的情况下剔除这些采点并不影响古地磁极性柱的构建和解释.     

Tectonic rotations during the Chile Ridge collision and obduction of the Taitao ophiolite (southern Chile)

Abstract The < 6 Ma young Taitao ophiolite, exposed at the westernmost promontory of the Taitao Peninsula, is located approximately 40 km southeast of the Chile triple junction and consists of a complete sequence of oceanic lithosphere. Systematic sampling for paleomagnetic study was performed to understand the complex obduction processes of the ophiolite onto the forearc of the South American Plate. Two representative demagnetization paths of remanent magnetization vectors were observed. One is characterized by stable univectorial demagnetization paths and was observed in volcaniclastic rocks and dyke complexes. Orientations of their remanent magnetization vectors indicate various degrees of counterclockwise rotations. The other is characterized by multivectorial demagnetization paths and was observed in the plutonic units (gabbros and ultramafic rocks). From these, two distinct stable remanent magnetization vectors were isolated; one has high coercivity and the other has low coercivity along the demagnetization paths with little influence of viscous magnetizations. This suggests that the complex deformation history involved at least two rotational events. The clockwise rotation, inferred from high coercivity remanent magnetization vectors, was attributed to a ridge collision event and the counterclockwise rotation, inferred from the low coercivity remanent magnetization vectors, was attributed to an accommodation phase into the South American forearc during obduction and final emplacement of the ophiolite. Folds developed during this period. Paleomagnetic restorations of the internal structures of the plutonic units and dyke complexes suggest that they probably originated in a mid‐oceanic ridge environment near a transform fault. The counterclockwise rotation of the plutonic and dyke complex units during the obduction generated tectonic gaps between these and the basement. The volcaniclastic rocks must have been deposited at nearly their present location, filling the tectonic gaps, as less effect of tectonic rotation was identified on these rocks.     

Magnetism of rocks and volumetric strain in uniaxial failure tests

Summary The relation between remanent magnetization and volumetric strain for gabbro samples stressed in uniaxial compression inside a near zero-field -metal shield has been examined. For samples with an induced IRM parallel to the axis of compression, remanent magnetization decreased linearly up to the onset of dilatancy. As increased stress produced additional dilatancy, the variation of remanent magnetization became nonlinear, and the stress dependence continually decreased until the rock failed. Stress cycling with the peak stress augmented for each cycle produced a continuous decrease in the zero stress value of the IRM although an appreciable amount of recovery was observed during unloading. When the sample was loaded in constant stress increments after the onset of dilatancy and held for several minutes at each level, time-dependent variations in remanent magnetization coincided with time-dependent increases in inelastic volumetric strain. In general as the inelastic creep rate increases, the rate of change in remanent magnetization increases. These results suggest that dilatancy related effects of the intensity of rock magnetization should be observed in magnetic rocks in epicentral regions prior to earthquakes and may serve as both long- and short-term precursors.     

Experimental modeling of the chemical remanent magnetization and Thellier procedure on titanomagnetite-bearing basalts

The results of the experimental studies on creating chemical and partial thermal remanent magnetizations (or their combination), which are imparted at the initial stage of the laboratory process of the oxidation of primary magmatic titanomagnetites (Tmts) contained in the rock, are presented. For creating chemical remanent magnetization, the samples of recently erupted Kamchatka basalts were subjected to 200-h annealing in air in the temperature interval from 400 to 500°С under the action of the magnetic field on the order of the Earth’s magnetic field. After creation of this magnetization, the laboratory modeling of the Thellier–Coe and Wilson–Burakov paleointensity determination procedures was conducted on these samples. It is shown that when the primary magnetization is chemical, created at the initial stage of oxidation, and the paleointensity determined by these techniques is underestimated by 15–20% relative to its true values.     

Magsat anomaly data over the Kursk region,U.S.S.R.

A new method of reducing Magsat orbits was developed to produce a map of the Kursk magnetic anomaly at satellite altitude. Two different techniques to estimate the direction of magnetization were applied to the components of the anomaly to determine the presence of remanent magnetization. We propose that there is a significant component of remanent magnetization, however, we are unable to determine the importance of remanence relative to induced magnetization.     

Palaeomagnetism of Precambrian rocks from Southeast Norway and South Sweden

Precambrian amphibolite and hyperite rocks from the Bamble and Kongsberg areas in SE Norway, and amphibolite rocks from SW Sweden were investigated for evidence of remagnetization by the Sveconorwegian metamorphic episode. The similarity of the characteristic natural remanent magnetization directions, shown by the various rocks from the Bamble and Kongsberg areas, indeed supports the idea of remagnetization on a regional scale. Therefore the average pole position at 3°S, 153°W, determined from six sites in these areas, is considered to reflect the average virtual pole position for the post-Sveconorwegian period of uplift and cooling (1,120–975) · 10⁶ year ago. The pole positions determined from the characteristic natural remanent magnetization directions of amphibolite rocks in SW Sweden are indicative of being somewhat younger.In addition, two hyperite dikes were studied near Karlshamn in SE Sweden. Their characteristic natural remanent magnetization is consistent with that of the hyperite dikes in central south Sweden (Mulder, 1971).The Precambrian apparent polar wandering path for Europe is reconstructed on the basis of twenty-three pole positions from the Baltic Shield and three pole positions from Great Britain. This pole path requires an average angular rate of apparent polar wandering of 0.2–0.3° per 10⁶ year.     

Assessment of soils pollution extent in surroundings of ironworks based on magnetic analysis

This paper reports on magnetic and magnetomineralogical studies of soils influenced by ironworks activity. Researches were conducted in five areas, of which Ostrowiec Świętokrzyski ironworks area is described in detail. A map of magnetic susceptibility was created based on field measurements and samples taken from soil profile were analyzed in laboratory. Measurements of magnetic susceptibility in two frequency ranges, anhysteretic remanent magnetization, isothermal remanent magnetization thermomagnetic and geochemical analysis were carried out. SEM was used to identify ferromagnetic fractions. As a result the horizontal and vertical extent of heavy metals pollution was established.     

The effect of uniaxial elastic deformation on the remanent magnetization of basalts

Summary The effect of uniaxial elastic deformation on the remanent magnetization of some basalts, containing generalized titanomagnetities of different chemical composition, was studied. The irreversible changes of the saturated and natural remanent magnetizations were measured. With the deformed samples no permanent change of the spectrum of micro-coercive forces, or change in the magnitude of the specific saturated remanent magnetization were observed. The experimental results agree with the model of mutual interaction of structural defects with magnetic domatins [10] under the assumption of predominantly reversible changes of the dislocation structure of the sample due to elastic deformation.     

Magnetism of natural pyrrhotite,haematite and ilmenite

Summary The paper deals with the magnetic properties of the natural minerals haematite, ilmenite and pyrrhotite. The natural remanent magnetization J_n, the volume susceptibility , the specific magnetization of saturation _so, the Curie temperature , the A.C. demagnetization of natural remanent magnetization, the thermal demagnetization J_n, the temperature dependence of the specific magnetization of saturation, and several other parameters were determined for the individual sets of minerals. Since natural minerals can and do contain various heterogeneous and isomorphic admixtures, their magnetic properties may change depending on the content and type of these admixtures. Therefore, all the investigations of the magnetic properties described in this paper were carried out with extensive sets of minerals, representing various Czechoslovak and world localities. A number of interesting results were obtained from the executed experiments, e.g., the different stability of the natural remanent magnetization of haematite with respect to A.C. demagnetization, the presence and type of heterogeneous inclusions in ilmenite, the phase changes connected with the -transition in natural pyrrhotites, etc. The types of distributions of the investigated values of the magnetic properties were also determined.     

Impact-pressure controlled orientation of shatter cone magnetizations in Sierra Madera,Texas, USA

A suite of Sierra Madera Impact deformed rocks was studied and magnetic analyses were performed. We characterized the magnetic signatures of two locations, sites A and B that have different physical characteristics of shock fractured structures as well as the magnetic signatures. Shatter cone at site A has a fine-scale (few to ∼10 mm) distributed array of complete shatter cones with sharp apex. Natural remanent magnetization (NRM) of site A shatter cone is distributed within the plane that is perpendicular to the apexes of the cones. Shatter cone at site B shows no apparent cone shape or apex, instead, a relatively larger scale and multiple striated joint set (MSJS) and sinusoidal continuous peak. NRM of site B shatter cone is clustered along the apexes. The difference in magnetization direction is a likely indicator of the shock pressure where parallel to apex indicates pressures larger than 10 GPa and perpendicular to apex indicate pressures less than 10 GPa. Intensities of NRM and saturation isothermal remanent magnetization (SIRM) contrast and fluctuate within a shatter cone as well as in between two sites. We observed a random orientation of magnetic vector directions and amplitudes changing over small scales leading to the absence of coherent macro-scale signature.     

The use of a portable fluxgate magnetometer for the estimation in the field of the direction and stability of remanent magnetization of rock specimens

Summary A portable magnetometer of the fluxgate type is described, which is capable of measuring in the field the direction and intensity of remanent magnetization of rock samples of 2 cm diameter with an accuracy of the order of 6° for intensities of 10^–3 emu/cc or more. The measurement and calculation can be carried out in less than five minutes, and a precision of the order of 2° can be obtained if more readings are taken so that ten minutes are required for a complete measurement. The chief limitation to the precision of measurement was found to be inhomogeneity of magnetization of the specimens. The instrument has been used to estimate stability of natural remanent magnetization where A.C. cleaning apparatus is not available, and the method of doing so is briefly described.Now with English Electric Computers.     

La de´termination de l'aimantation re´manente visqueuse et le test de E. et O. Thellier applique´ aux roches

A method is described for measurement of the magnetic viscosity of rocks, which is considered in its three forms: induced magnetization viscosity, remanent magnetization viscosity and viscous remanent magnetization. An application is then presented in the form of an experimental verification of the E. and O. Thellier test of approximate elimination of the viscous magnetization effect in rocks. It is shown that, used in the conventional manner, the employment of this test is fully justified, although not allowing measurement of the VRM, the values obtained being seriously underestimated.     

Origin of a magnetic lineation on Kyushu Island, Japan

Abstract Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of ∼1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.     

Magnetic properties of a freshly fallen LL ordinary chondrite: the Bensour meteorite

A comprehensive rock magnetic, magnetic anisotropy and paleomagnetic study has been undertaken in the brecciated LL6 Bensour ordinary chondrite, a few months only after its fall on Earth. Microscopic observations and electronic microprobe analyses indicate the presence of Ni-rich taenite, tetrataenite and rare Co-rich kamacite. Tetrataenite is the main carrier of remanence. Magnetization and anisotropy measurements were performed on mutually oriented 125 mm³ sub-samples. A very strong coherent susceptibility and remanence anisotropy is evidenced and interpreted as due to the large impact responsible for the post-metamorphic compaction of this brecciated material and disruption of the parent body. We show that the acquisition of remanent magnetization postdates metamorphism on the parent body and predates the entering of the meteorite in Earth’s atmosphere. Three components of magnetization could be isolated. A soft coherent component is closely related to the anisotropy of the meteorite and is interpreted as a shock remanent magnetization acquired during the same large impact on the parent body. Two harder components show random directions at a few mm scale. This randomness is attributed either to the formation mechanism of tetrataenite or to post-metamorphic brecciation. All components are likely acquired in very low (≈μT) to null ambient magnetic field, as demonstrated by comparison with demagnetization behavior of isothermal remanent magnetization. Two other LL6 meteorites, Kilabo and St-Mesmin, have also been studied for comparison with Bensour.     

Distribution of the values of natural remanent magnetization and magnetic susceptibility of some minerals

Summary A statistical treatment is presented of the observed values of natural remanent magnetization and of magnetic susceptibility of natural minerals: magnetite, chromite, ilmenite pyrrhotite, haematite, cassiterite and garnets. It was found that for most minerals the distribution of the natural remanent magnetization as well as the magnetic susceptibility is logarithmically normal at a significance level of p=0.05. The typical values of J_n and x, the limits of the intervals of reliability of these typical values for p=0.05, and the standard deviations of the distribution were determined for the individual minerals. The end values of the sets were tested by two independent tests of extreme deviations at a level of significance of p=0.05. Following statistical deliberations it was proved that the lognormal distribution of the J_n and x values depended on the number of factors affecting these values, independently of the type of distribution of these so-called disturbing factors. By generalizing for rocks it was shown that the lognormal and normal types of distribution of J_n and x values are extreme cases as regards the observable types with rocks.     

Separation of magnetic anomalies into induced and remanent magnetization contributions

Inversion of magnetic data is complicated by the presence of remanent magnetization, and it provides limited information about the magnetic source because of the insufficiency of data and constraint information. We propose a Fourier domain transformation allowing the separation of magnetic anomalies into the components caused by induced and remanent magnetizations. The approach is based on the hypothesis that each isolated source is homogeneous with a uniform and specific Koenigsberger ratio. The distributions of susceptibility and remanent magnetization are subsequently recovered from the separated anomalies. Anomaly components, susceptibility distribution and distribution of the remanent and total magnetization vectors (direction and intensity) can be achieved through the processing of the anomaly components. The proposed method therefore provides a procedure to test the hypotheses about target source and magnetic field, by verifying these models based on available information or a priori information from geology. We test our methods using synthetic and real data acquired over the Zhangfushan iron-ore deposit and the Yeshan polymetallic deposit in eastern China. All the tests yield favourable results and the obtained models are helpful for the geological interpretation.     

Paleosecular variations 12–20 Kyr as recorded by sediments from Lake Moreno (Southern Argentina)

We present results of paleomagnetic and sedimentological studies carried out on three cores Lmor1, Lmo98-1, Lmor98-2 from bottom sediments of Lake Moreno (south-western Argentina), and integrate them with data from our previous studies. Measurements of directions (declination D and inclination I) and mass specific intensity of natural remanent magnetization (NRM intensity), magnetic susceptibility (specific, χ and volumetric, κ), isothermal remanent magnetization (IRM), saturation of isothermal remanent magnetization (SIRM), and back field remanent coercivity (B_0CR) were performed. The stability of the NRM was investigated using alternating-field demagnetization. The results show that these sediments meet the criteria required to construct a reliable paleomagnetic record. The cores were correlated very well based on magnetic parameters, such as χ and NRM intensity, as well as with lithological features. Tephra layers were identified from the lithological profiles and magnetic susceptibility logs. We obtained the D and I logs of the characteristic remanent magnetization for the cores as a function of shortened depth. The data from the three cores were combined to form a composite record using the Fisher method. A comparison between stacked inclination and declination records of Lake Moreno and those obtained in previous works on Lake Escondido and Lake El Trébol shows good agreement. This agreement made it possible to transform the stacked curves into time series spanning the interval 12–20 kyr. The results obtained improved our knowledge of SV and the behaviour of the geomagnetic field and also allowed us to determine the range of past inclination variations from −70° to −45° for the southern hemisphere, where data are scarce.     

Variation of magnetic directions within pillow basalts

Relative directions of magnetization have been measured within individual pillow basalts collected from the Atlantic Ocean and Caribbean Sea. The angle between the magnetic directions was determined and is referred to as the directional difference. Although one pillow contained a directional difference of 44°, the remaining ten pillows had differences less than 14°. The maximum orientation and measurement error was 7°. Dispersion on the scale found in these fine-grained pillow basalts would not appreciably affect the magnetic anomaly pattern on the sea floor. We detected no reversals of magnetization despite the sometimes large and variable low-temperature oxidation. Comparison of directions within homogeneous segments of the pillow, viscous remanent magnetization (VRM) acquisition experiments, and alternating field (AF) demagnetization indicate a large portion of the dispersion was due to the acquisition of a viscous component in the larger grained, less oxidized portion of the pillows. Evidence from one variably weathered pillow suggests that extreme low-temperature oxidation may lead to the acquisition of a secondary component with high coercivities (20–80 mT). We could not determine whether this was a chemical remanent magnetization (CRM) or a VRM acquired by single domain grains near the superparamagnetic threshold. Hysteresis properties confirmed by microscopic examination indicated that the magnetic grain size in all the pillows was at least as small as pseudo-single domain.