V. A. Tsel’movich

Extraterrestrial magnetic minerals

Thermomagnetic and microprobe analyses are carried out and a set of magnetic characteristics are measured for 25 meteorites and 3 tektites from the collections of the Vernadsky Geological Museum of the Russian Academy of Sciences and Museum of Natural History of the North-East Interdisciplinary Science Research Institute, Far Eastern Branch of the Russian Academy of Sciences. It is found that, notwithstanding their type, all the meteorites contain the same magnetic minerals and only differ by concentrations of these minerals. Kamacite with less than 10% nickel is the main magnetic mineral in the studied samples. Pure iron, taenite, and schreibersite are less frequent; nickel, various iron spinels, Fe-Al alloys, etc., are very rare. These minerals are normally absent in the crusts of the Earth and other planets. The studied meteorites are more likely parts of the cores and lower mantles of the meteoritic parent bodies (the planets). Uniformity in the magnetic properties of the meteorites and the types of their thermomagnetic (MT) curves is violated by secondary alterations of the meteorites in the terrestrial environment. The sediments demonstrate the same monotony as the meteorites: kamacite is likely the only extraterrestrial magnetic mineral, which is abundant in sediments and associated with cosmic dust. The compositional similarity of kamacite in iron meteorites and in cosmic dust is due to their common source; the degree of fragmentation of the material of the parent body is the only difference.

Native iron and other magnetic minerals in the sediments of the northwestern Atlantic: Thermomagnetic and microprobe evidence

The thermomagnetic and microprobe analyses of sedimentary samples from DSDP 386, 387, 391A, and 391C boreholes in the northwestern Atlantic reveal the ubiquitous occurrence of particles of native iron. The concentrations of native iron are bimodal everywhere with the zero mode necessarily present. The nickel admixture in native iron forms two groups, one represented by pure iron and the comprising native iron with 5–6% Ni. The redeposition of iron particles manifests itself in the correlation between the concentrations of these particles and terrestrial minerals (magnetite), as well as in the equalization and reduction of the concentration of the iron particles. Pyrite and pyrrhotite are widespread in the studied sediments, and the distribution of native iron does not depend on the presence of pyrite (i.e., on redox conditions) in them. At the same time, the distributions of pyrite and particles of magnetite + titanomagnetite are inversely correlated, which can probably be accounted for by the partial dissolution of magnetite and titanomagnetite in the reducing conditions. The increased concentration of particles of volcanogenic homogeneous titanomagnetite is revealed in the volcanoclastic turbidites of the Oligocene and early and middle Miocene age at the base of the Bermuda Rise (borehole 386). The titanomagnetite composition is characteristic of the basalts of plume magmatism; it corresponds to the depth of the magmatic source in the interval of 50–25 km.

Cosmic magnetite microspherules and metallic particles near the Permian-Triassic boundary in a global stratotype section and point (Stratum 27, Meishan, China)

631 The study of standard stratigraphic sections, where the most complete sequences of geological, cosmic, cli matic, and biotic events are observed, is one of the pri ority directions in fundamental geology. On the one hand, the study of such objects allows us to develop new methodologies for reconstruction of events in the past and to develop or close hypotheses of biospheric crises. On the other hand, this study is very important for prac tical stratigraphy, because in many cases new marking stratigraphic levels are revealed and many of them are applied for division and correlation of old series.

The First U-Pb (LA-ICP-MS) isotope data of detrital zircons from the basal levels of the Riphean stratotype

Attempts at dating detrital zircons from Late Pre� cambrian terrigenous rocks of the Western Urals have already been made. In (1-3), the age data of detrital zircons from Upper Middle-Upper Riphean and Lower and Upper Vendian (Upper Vendian-Middle Cambrian) deposits were summarized. This work pre� sents the first results of U-Pb LAICPMS isotope dating of detrital zircons, extracted from sandstones of the basal horizons of the Riphean stratotype sequence of the Southern Urals (Ai Formation, Burzyan Group of the Bashkir Meganticlinorium). The Bashkir meganticlinorium, a major structural unit of the structure of the Uralian foldthrust belt is located in the western segment of the Southern Urals (Fig. 1a). The Bashkir Meganticlinorium is composed of unevenly dislocated Paleozoic rocks. Within the Meganticlinorium, PreOrdovician complexes, divided distinctly into two units, are exposed on the surface. The lower horizons of these complexes are represented by Early Precambrian granitoides, amphibolites, and gneisses with relics of granulite� facies rocks (4, 5). This rock assemblage is referred to the Taratash metamorphic complex. In the northeast� ern part of the Bashkir Meganticlinorium, the Taratash complex composes the core of the Taratash Anticline. The rocks of the Taratash complex are over� lapped with stratigraphic and structural unconformity by a complex of terrigenous, terrigenouscarbonate, carbonate, and rarely volcanogenic and volcano� genic-sedimentary rocks with a total thickness of up to 12-15 km (4, 6, 7). The basal part of the complex is represented by conglomerates. The large lower part of this complex is the typical Riphean Erathem sequence (4, 6-8). The upper part is represented by the stratum of alternating aleurolites, sandstones, and rare arkose, and polymictic conglomerates (up to 1500 m thick) are referred as Upper Vendian (4, 7) or Upper Ven� dian-Middle Cambrian Asha (9).

Nanostructures in the deep xenolite before and after straining

A search for nanocrystals in the sample of deep rock, mantle xenolite from the kimberlitic tube, was carried out. With the use of the Raman spectroscopy method pyrope nanocrystals measuring ∼18 nm and omphacite nanocrystals measuring ∼13 nm were identified. The dimensions of the crystallographic cell in nanocrystals were increased in comparison with macrocrystals. The internal tensile stresses, which could cause these changes, were evaluated by a value of ∼1.1 GPa. The action of quasihydrostatic compression pressure with a value up to 2.5 GPa on a change in the structure and properties of nanocrystals was investigated. As a result of the compression, the sizes of pyrope nanocrystals did not change, but the dimensions of the crystallographic cell increased. The method applied did not make it possible to reliably determine the changes as a result of the pressure of the internal stresses and the sizes of the omphacite nanocrystals.

The common nature of native iron in terrestrial rocks and meteorites: Microprobe and thermomagnetic data

The microprobe and thermomagnetic data for native iron in terrestrial rocks (xenoliths from mantle hyperbasites, Siberian traps, and oceanic basalts) are generalized and compared to the data for native iron from sediments and meteorites. It is found that the native iron particles contained in the continental and oceanic hyperbasites and basites are similar in composition, shape, and grain size. These particles are predominantly nickel-free and similar to the extraterrestrial iron from sediments and meteorites. This similarity suggests the similar sources of origin of these particles. This means that the terrestrial conditions were close to the conditions that existed at the source planets of the meteorites, e.g., the bodies from the asteroid belt which have been subsequently disintegrated, and crushed into cosmic interplanetary dust, which has entered the terrestrial sediments. This similarity originates from the homogeneity of the gasdust cloud at the early stage of the Solar System. The predominance of extraterrestrial native iron in sediments is accounted for by the fact that the interplanetary dust is mainly contributed by the material from the upper mantle of the source planets of cosmic dust.

Testing the Models of Late Vendian Evolution of the Northeastern Periphery of the East European Craton Based on the First U/Pb Dating of Detrital Zircons from Upper Vendian Sandstones of Southeastern White Sea Region

1073 The basement of the Northeastern Periphery of the East European Craton (EEC) is composed of relics of the fold belt called the “Timan orogen” [1] or “Pre Uralides Timanides orogen” [2] in publications of the past 20 years. The age interval of crystalline complexes of the orogen is estimated at 510–750 Ma [3]. The tec tonic nature of the orogen is still disputable: it is inter preted as either an accretionary structure, which evolved in multiple stages at the active Timan margin of Baltica (ancient carcass of the EEC) in the second half of the Neoproterozoic [1], or a collisional unit formed at the Precambrian–Cambrian boundary as a result of collision between the Bol’shezemel’skaya active margin of Arctida and the Timan passive margin of Baltica [2]. Based on the results of dating for detrital zircons (dZr) from sandstones of the Upper Vendian Tamitsa Subformation, Southeastern White Sea Region (SWSR), we will test these versions of the tec tonic origin.

Pure iron and other magnetic minerals in meteorites

The results of thermomagnetic and microprobe analyses of 37 samples from 25 different types of meteorite are analyzed with the focus on the presence of pure (nickel-free) iron in them. It is established that the metallic particles in the studied meteorites cluster in three isolated groups: (1) pure iron, (2) kamacite with mode 3–6% Ni, and (3) taenite with mode ∼50% Ni. The hiatuses in the Fe-Ni alloy compositions between these groups of magnetic grains contradict the Fe-Ni phase diagram, which predicts a continuous series of solid solutions in this system. This isolated distribution of the compositions of the metallic particles in the meteorites is reasonably accounted for by the specific properties of the melt (melts) and the processes of their crystallization and decomposition in space. It is suggested that pure iron in the meteorites could have been formed by either of two scenarios. According to the “primary,” pure iron crystallizes from the melt, and according to the “secondary” scenario, it is produced by the decomposition of the solid solution.

Titanomagnetites and ilmenites from the Early Cenozoic basalts and limburgites of the Northern Tien Shan

Based on X-ray spectrometry and thermomagnetic analysis, the chemical composition is studied and the Curie points are determined for the Early Cenozoic basalts and limburgites of the Northern Tien Shan. The microprobe analysis used in combination with the thermomagnetic analysis unambiguously identified a series of homogeneous primarily magmatic titanomagnetites in the studied samples against the broad variety of grains of titanomagnetite that underwent single-phase heterophase oxidation, which are often undetectable by a microprobe alone. The titanium content or TiO2/FeO ratio in titanomagnetites and, correspondingly, the Curie points reflect the depth of the most recent equilibrium state of the magma, i.e., the depth of the magma sources. According to the dependence of the content of primarily magmatic titanomagnetite on the source depth, the latest equilibrium state of the magmatic melt of the Northern Tien Shan occurred at a depth of 40 ± 5 km. The obtained results agree with the reduction in the seismic velocities by a few percent below the Moho revealed by seismic tomography.

Cosmic particles (micrometeorites) and nanospheres from the Cretaceous-Paleogene (K/T) boundary clay layer at the Stevns Klint Section, Denmark

This paper presents new data on numerous small metallic particles of iron, copper, Fe-Ni, Fe-Ni-Co, and Fe-Cr alloys, magnetite, and aluminosilicate balls of cosmic origin found in the black clay boundary layer between the Cretaceous and Paleogene in the Stevns Klint Section (Denmark). The findings imply that a fall of an asteroid to Earth 65 Ma ago was accompanied with falling of finely dispersed metallic particles of extraterrestrial nature related to the asteroid fragments or to micrometeorites following the asteroid or to the intense supply of cosmic dust. The huge amount of finely dispersed matter that fell to Earth at that time should be considered in further reconstructions of events at the boundary of the Cretaceous and Paleogene.