The magnetic properties and mineralogy of kimberlites from four pipes (BK53, BK56, AK08, and AK06) in Botswana are studied. It is shown that magnetic characteristics of the kimberlites as well as chemical and phase compositions of the oxides from kimberlite groundmass reflect the difference in evolution of kimberlite melts and formation conditions of these bodies and are associated with the grade of these diamoniferous bodies. The relatively low-grade (0.22 carats per ton) kimberlites from the southern lobe of the AK06 pipe contain picrochromites with high chrome and low titanium contents (up to 49.4 wt % Cr2O3 with 3.9 wt % TiO2), which occur in the paramagnetic state at a temperature of T > −180°C. Their magnetic properties above room temperature are determined by titanomagnetites with the Curie point at +370 to + 500°C, which were formed at the later stages of crystallization as a result of the gradual decline in the PT-parameters. Most spinelides in the Botswana kimberlites from the low-grade pipes (below 0.07 carats per ton) were crystallized at higher PT-parameters than in the southern lobe of pipe AK06, in an environment with highly active titanium. The chromspinelides that were segregated previously turned out to be unstable, which led to the formation of magnesian-chrome and magnesian ulvospinels (15.9–29.7 wt % TiO2; 12.8–22.8 wt % MgO; 0–11.3 wt % Cr2O3; and 1.2–12.4 wt % Al2O3) with Curie points T C = −50 to −80°C. These conditions were unfavorable for preservation of chrome-spinelides and diamonds. At lower PT-parameters, a small amount of finely dispersed magnetite with a single-domain and pseudo-single domain structure was formed.

Исследованы магнитные свойства и минералогия кимберлитов из четырех трубок Ботсваны ВК53, ВК56, АК08 и АК06. Показано, что особенности их магнитных характеристик, а также химического и фазового составов оксидов из связующей массы отражают различия в эволюции кимберлитовых расплавов и условиях становления изученных тел и связаны с продуктивностью алмазов в этих телах. Для кимберлитов из южного тела трубки АК06 с относительно высокой продуктивностью алмазов (0.22 кар/т) характерно наличие пикрохромитов с высоким содержанием хрома и низким – титана (до 49.4 мас. % Cr2O3 при 3.9 мас. % TiO2) находящихся в парамагнитном состоянии при температурах Т > –180°C, а магнитные свойства при температурах выше комнатной обусловлены магнезиальным титаномагнетитом с точкой Кюри +370... +500°С, сформировавшимся в поздние этапы кристаллизации в результате медленного снижения РТ-параметров. Основная часть шпинелидов в кимберлитах Ботсваны из трубок с продуктивностью алмазов менее 0.07 карат/т кристаллизовалась при более высоких РТ-параметрах, чем в южном теле трубки АК06, в среде с высокой активностью титана. Ранее выделившиеся хромшпинелиды оказались неустойчивыми, что привело к образованию магнезиально-хромовой и магнезиальной ульвошпинелей (15.9–29.7 мас. % TiO2; 12.8–22.8 мас. % MgO; 0–11.3 мас. % Cr2O3; 1.2–12.4 мас. % Al2O3) с точками Кюри Тс = –50... –80°C. Эти условия оказались неблагоприятны не только для сохранности хромшпинелидов, но и алмаза. При этом, при снижении РТ-параметров образовалось небольшое количество мелкодисперсного магнетита с однодоменной и псевдооднодоменной структурой.

A simple finite-dimensional geodynamo model, obtained from the equations of the mean field electrodynamics and reproducing the phenomenon of geomagnetic reversals, is proposed. It has been indicated that the reversal scale obtained in the scope of this model is rather close to the observed scale in its properties. The reversal mechanism is related to the α-effect fluctuations. It is not necessary to substantially change the hydrodynamic parameters of the problem so that a reversal originates in the scope of such a model, but it is only sufficient to take the α-effect fluctuations into account. If the rms deviation of fluctuations accounts for 10% of the average α value, a fluctuation of two-three standard deviations is sufficient for the origination of a reversal, which quite agrees with the concept that reversals are rather rare phenomena. Another factor resulting in the regime with reversals is that the model can generate magnetic fields with different behaviors in different regions of the parametric space in linear mode: monotonically increasing fields and fields increasing with oscillations.

We suggest a simple dynamical system which mimics a nonlinear dynamo which is able to provide (in specific domains of its parametric space) the temporal evolution of solar magnetic activity cycles as well as evolution of geomagnetic field including its polarity reversals. A qualitative explanation for the physical nature of both phenomena is presented and discussed.

Предлагается простая конечномерная модель геодинамо, полученная из уравнений электродинамики средних полей и воспроизводящая феномен инверсий геомагнитного поля. Показано, что шкала инверсий, получаемая в рамках этой модели, достаточно близка по своим свойствам к наблюдаемой. Механизм инверсий связан с влиянием флуктуаций -эффекта. Для возникновения инверсии в рамках такой модели не нужно существенно менять гидродинамические параметры задачи, а достаточно учесть флуктуации -эффекта. Если среднеквадратичное отклонение флуктуаций составляет 10% среднего значения , для возникновения инверсии достаточно флуктуации в 2–3 стандартных отклонения, что вполне согласуется с представлением о сравнительной редкости инверсий. Еще одним фактором, приводящим к режиму с инверсиями, является то, что в линейном режиме модель в разных областях параметрического пространства может генерировать магнитные поля с различным поведением – монотонно-растущие и растущие с осцилляциями.

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Research was carried out to estimate the possibility of determining the direction and strength of the ancient geomagnetic field (H an ) by natural remanent magnetization (I n ) of nine oriented samples from traps of the Minor Botuoba Region (Yakutia) aged 260 Ma. Five samples (Pi-10, K-4, K-6, 315–13, and Ki-2) are characterized by negative polarity of In, while four samples (nos. 334-5, 331–2, 315–11, 299–2) have positive polarity as does the recent geomagnetic field in this region. The ferrimagnetic constituent of the samples with reverse I n polarity appears to be quite variable: samples K-4 and K-5 are characterized by low Curie points (T c ≈ 200°C) of the ferrimagnetic phase, sample Ki-2 contains single-phase oxidized titanomagnetite with T c ≈ 310°C, and the T c of the Pi-10 ferrimagnetic phase is 540°C. Hence, it may be concluded that the primary remanent magnetization of the first two samples was formed in a reverse polarity field. These samples also may be used to determine the paleostrength of the geomagnetic field. The properties of traps containing single-phase oxidized (sample Ki-2) and disintegrated (sample Pi-10) titanomagnetite require additional investigation. Samples with positive In polarities are characterized by the self-reversal phenomenon upon thermal demagnetization of the natural remanent magnetization, which was most likely caused by the occurrence of titanomagnetite exsolution textures in ferrimagnetic grains. The paleoinformation value of the I n of these samples is doubtful.

We carried out shock experiments on macroscopic spherical samples of the L4 ordinary chondrite Saratov (natural shock stages S2-S3), using explosively generated spherical shock waves with maximum peak pressures of 400 GPa and shock-induced temperatures > 800 degrees C (up to several thousands degrees C). The evolution of shock metamorphism within a radius of the spherical samples was investigated using optical and scanning electron microscopy, microprobe and magnetic analyses as well as Mossbauer spectroscopy and X-ray diffraction techniques. Petrographic analyses revealed a shock-induced formation of three different concentric petrographic zones within the shocked samples: zone of total melting (I), zone of partial melting (II), and zone of solid-state shock features (III). We found a progressive pressure-induced oxidation of Fe-Ni metal, whose degree increased with increasing shock peak pressure. The amount of FeO within zone I increased the factor of 1.4 with respect to its amount in the unshocked Saratov sample. This suggests that within zone I about 70 wt% of the initial metallic iron was oxidized, whereas magnetic analyses showed that about 10 wt% of it remained intact. This strongly supports the hypothesis that, in addition to oxidation, a migration of metallic iron from the central heavily shocked zone I toward less shocked peripheral zone took place as well (likely through shock veins where metallic droplets were observed). Magnetic analyses also showed a shock-induced transformation of tetrataenite to taenite within all shocked subsamples, resulting in magnetic softening of these subsamples (decrease in remanent coercivity). These results have important implications for extraterrestrial paleomagnetism suggesting that due to natural impact processes, the buried crustal rocks of heavily cratered solid solar system bodies can have stronger remanent magnetism than the corresponding surface rocks.

Paleomagnetic studies have revealed that natural ferrimagnetics, which are constituents of igneous rocks, can be magnetized antiparallel to the magnetizing field. This phenomenon is called self-reversal of magnetization. The question of the actual existence of inversions of the geomagnetic field is still under investigation. This paper considers the physics of the anomalous behavior of natural ferrimagnetic thermomagnetization (TM). The existence of thermomagnetization arising in natural ferrites against the direction of a magnetizing field is experimentally confirmed. It is shown that even after the demagnetization of a ferrimagnetic by alternating magnetic field a significant increase in residual magnetization can occur when it is heated in the absence of a magnetic field.

The paleointensity of a geomagnetic field is determined on the basis of the standard Thellier’s method using the remanent magnetization (H a ) of basalts of the rift zone of the Red Sea (with an age of 0.5 million years) and south of the Mid-Atlantic Ridge (with an age of 0.1–0.3 million years). Samples were selected whose natural remanent magnetization (NRM) was mainly one-component and the measured Curie temperatures of the titanium-magnetite fraction (T C = 205–250°C) were close to the calculated values. Samples with T C = 580°C were also used. It was shown that NRM of basalts with both high and low T C values have a thermoremanent nature. The studies have revealed that the law of independence and additivity of partial thermoremanent magnetizations (PTRMs) is satisfied for the collected samples with an error of no more than 6% and the error of determination of the field of TRM creation using the Arai-Nagata diagram is no more than 10%. The value H a = 32 A/m determined from NRM of basalts south of the Mid-Atlantic Ridge appears to be approximately equal to the contemporary intensity of the geomagnetic field (H c ) in this region. In the Red Sea region, H a = 77.5 A/m, which is 2.5 times higher than the current value. It is suggested that during formation of the basalts of the Red Sea rift zone the geomagnetic pole was located in that region and the virtual dipole moment was 35% higher than its contemporary value.

We carried out hydrostatic pressure demagnetization experiments up to 1.24GPa on samples of terrestrial and extraterrestrial rocks and minerals of different lithologies as well as on synthetic samples. The magnetic remanence of samples was measured directly under pressure using a non-magnetic high-pressure cell of piston-cylinder type that was inserted into a high sensitivity SQUID magnetometer. In order to bring light on the pressure demagnetization effect, we investigated 50 samples with different magnetic mineralogies, remanent coercivities (Bcr) and hysteresis parameters. The samples consisted of pyrrhotite-, magnetite- and titanomagnetite-bearing Martian meteorites, taenite-, tetrataenite- and kamacite-bearing ordinary chondrites and pyrrhotite-bearing Rumuruti chondrite; magnetite- and titanomagnetite-bearing basalts, andesites, ignimbrites, obsidians and granites; a variety of pyrrhotite- and hematite-bearing rocks and minerals (jasper, schist, rhyolite, radiolarite); samples of goethite and greigite as well as synthetic samples of dispersed powders of magnetite, hematite, pyrrhotite and native iron set into epoxy resin. Under hydrostatic pressure of 1.24GPa, applied in a low magnetic field (<5μT), the samples lost up to 84% of their initial saturation isothermal remanent magnetization (SIRM) without any changes in their intrinsic magnetic properties. We found that the efficiency of the pressure demagnetization is not exclusively controlled by the magnetic hardness of the samples (Bcr), but that it is strongly dependent on their magnetic mineralogy. For a given magnetic mineralogy the resistance to hydrostatic pressure is roughly proportional to ln(Bcr). It was shown that there is no simple equivalence between pressure demagnetization and alternating field demagnetization effects. The pressure demagnetization was shown to be time-independent but repeated application of the same pressure level resulted in further demagnetization.

The behavior of two-sublattice natural ferromagnetics under the effects of static and variable fields and high and low temperatures is studied. Novel unknown peculiarities of thermoremanent magnetization (TRM) and partial thermoremanent magnetization (pTRM) dependences on fields and temperatures are obtained. Self-reversal of TM and pTRM in the sample, where self-reversal has not been observed prior to the application of fields and temperatures, is revealed.

We studied the change of remanent saturation magnetization of ocean basalts exposed to different types of compression: uniaxial, quasihydrostatic, and hydrostatic. It is found that the character of remanent magnetization changes depends on the compression type and magnetization direction in the space. The stability of the remanent magnetization grows both with the increase of coercive force of ferrimagnetic grains and in passing from titanomagnetite to magnetite. It is shown that from the change of the remanent magnetization during laboratory specimen compression one can judge the in situ tectonic impacts suffered by rocks.

We report here the results of laboratory analog experiments to consider the potential effects of solar energetic particles (SEP or solar-flare-associated particles) and galactic cosmic rays (GCR) on the magnetic properties of extraterrestrial materials. We carried out proton bombardment experiments (with irradiation energies E 1=400, E 2 =850 keV and three irradiation fluences in 1014–1016 p/cm2 range) and lead-ion bombardment experiments (E =1 GeV) on (previously demagnetized by 120 mT alternating magnetic field) rock and synthetic samples with the following magnetic carriers: metallic iron and nickel iron, Ti-rich and Ti-free magnetite, pyrrhotite. Irradiation experiments resulted in either further demagnetization or magnetization of irradiated samples depending on the type of magnetic mineralogy and type of ionizing radiation involved. Apart for the formation of radiation-induced remanent magnetization (RIRM), we observed major changes in bulk magnetic properties, i.e., a moderate to dramatic decrease (up to 93%) in the coercivity of remanence B cr for all iron-bearing phases (iron-in-epoxy and Bensour meteorite samples). Contrary to iron-bearing samples, several magnetite-bearing samples experienced a radiation-induced magnetic hardening (increase in B cr ). Magnetic hardening was also observed for Ar2+ ion-irradiated nickel iron-bearing HED meteorites, measured for comparison with the previously stated results. Therefore, the combined effect of SEP with GCR may magnetically soften iron-bearing materials and harden magnetite-bearing materials. In order to answer the question wether RIRM may account for natural remanent magnetization of meteorites and lunar samples, physical mechanism of RIRM formation and potential dependence of RIRM intensity on the background magnetic field present during irradiation event should be investigated.