Pitch-angle scattering of energetic protons in the magnetotail current sheet as the dominant source of their isotropic precipitation into the nightside ionosphere

Characteristics of the nightside isotropic precipitation of energetic protons during a period of 4 quiet days has been studied using data from the ESRO 1A satellite. The observed features of the equatorward precipitation boundary (its thickness, energy dependence, dynamics, dependence of its latitudinal position on the magnetic field at the geosynchronous orbit, etc.) were found to be in good agreement with calculations based on recent magnetospheric magnetic field models. We argue that the mechanism of non-adiabatic pitchangle scattering in the equatorial current sheet is a dominant source of isotropic precipitation of energetic protons observed in the nightside auroral zone. Observations of the isotropic precipitation boundary can be used for monitoring the changes in the magnetotail current intensity.     

New Riphean microbiotas of the Billyakh Group,the north Anabar region (Fomich River basin): To Riphean biostratigraphy of the Siberian platform

Excellently preserved organic-walled and silicified microfossils are first found in the Lower Riphean Ust-Il’ya and Kotuikan formations of the Billyakh Group in the northern slope of the Anabar Uplift (the Fomich River basin). Similar assemblages were previously known only from sections located southward in the Kotuikan River basin, and taxonomic composition of organic-walled microbiotas from the Ust-Il’ya and Kotuikan formations became a corner stone in competitive microphytological models that are based on different approaches. In their composition and general appearance, microbiotas from the Kotuikan and Ust-Il’ya formations in the Fomich River basin are similar to microbiotas reported from the Kotuikan River basin, although northern sections of the above formations characterize deeper sedimentation settings than in localities known before. The Ust-Il’ya and Kotuikan assemblages of organic-walled microfossils include sphaeromorphic Chuaria circularis and Leiosphaeridia, two-layer vesicles the genus Simia, filamentous Plicatidium and Taenitrichoides, and some others. The silicified microbiota from the lower Kotuikan Subformation is largely composed of akinetes of Anabaena-like cyanobacteria Archaeoellipsoides, spherical Myxococcoides grandis, and short trichomes Filiconstrictosus and Orculiphycus representing initial germination stages of Anabaena-like cyanobacterial spores. Acanthomorphic acritarchs known from lithology-similar Lower and Middle Riphean (Mesoproterozoic) formations of Australia and China have not been observed in the Ust-Il’ya and Kotuikan microbiotas, which are probably of older age. The found microbiotas outline substantially wider distribution area of organic-walled and silicified microfossils, supplement microphytological characteristics of Riphean sediments in the Anabar Uplift, provide information on taxonomic composition of microbiotas from a wider spectrum of facies, and specify relationships between Early and Middle Riphean assemblages of microorganisms from different continents.     

Microstructural Patterns of Ophiolitic Chromitite of the Kraka Massif,South Urals. I. Banded Disseminated Ores

Geology of Ore Deposits - The microstructure and composition of accessory chromian spinels from peridotite and ore-forming chromian spinels from disseminated ores in dunite near the boundary...     

Biogenic Microstructures in Stromatolites of the Baikal–Patom Highland: Results of Complex Study

Precambrian stromatolites were studied with a complex approach including two complementary methods. The biogenic origin of ultramicrostructures examined with SEM was supported by the traditional optical microscopy. The paper addresses columnar-stratiform stromatolites of the Vendian Chencha Formation in the Ura Uplift (Baikal–Patom Highland, Central Siberia) and fossilized remains of coccoidal and filamentous microorganisms therein: cyanobacteria Eoentophysalis, hormogonian cyanobacteria Siphonophycus and, probably, Eomicrocoleus. The unraveled community of stromatolite builders includes the major organisms commonly observed in the Precambrian microbiota. Stromatolites of the Chencha Formation contained not only cyanobacteria, but also remains of eukaryotic microorganisms, including likely testate amoebae and acanthomorphic acritarchs. It is shown that the complex approach rules out incorrect determination of biota and significantly enhances concepts of the origin and formation of stromatolites, as well as the participation of microorganisms in their formation.     

Unusual Photometric Activity of the Weak-line T Tauti Star V715 Per

The results of many-year photometric observations of the weak-line T Tauri star V715 Per in the optical (V RI) and infrared (JHK) photometric bands are presented. The period of the star’s lowamplitude brightness variations in the V RI bands detected earlier, 5.23d, is confirmed. This period persists during the entire 14-year time interval of the observations, but its amplitude varies with time. The same period was found in the variations of JHK infrared fluxes, with the brightness variations in the two ranges being almost synchronous. The most probable origin of these variations is periodic eclipses of the star by its own disk, perturbed by its interaction with the magnetosphere (AA Tau-type variations). In addition to the periodic brightness variations, the star also displayed sporadic Algol-like minima characteristic of UX Ori stars during several years (2005–2011). The amplitude of these variations increasedmonotonically, reaching approximately 1^m in the V band by the end of 2010, after which the deep fadings ended. During this entire time interval, the amplitude of the periodic variations was observed to decrease. After 2011, the periodic component began to become stronger again. These changes of the photometric activity of V715 Per can be explained by an increasing rate of gas accretion onto the star, decreasing the radius of warped regions of the disk and leading to accompanying Algol-like fadings. This model assumes a low inclination of the circumstellar disk in V715 Per to the line of sight.     

U-Pb SHRIMP-II ages of titanite and timing constraints on apatite-nepheline mineralization in the Khibiny and Lovozero alkaline massifs (Kola Peninsula)

Results of this study of titanite samples collected from silicate rocks and apatite-nepheline-(sphene) ores from Paleozoic polyphase alkaline nepheline syenite complexes of the Khibiny and Lovozero massifs revealed the possibility of their in-situ U-Pb dating using sensitive high-resolution ion microprobe SHRIMP-II with an accuracy of 1.0-1.5%, which is comparable with that of U-Pb zircon analysis. Employing different approaches to age determination of the formation of the U-Pb system of titanites, the combined isochrons and mixing lines were plotted from the data obtained from the differentiated complex samples (121 analyses of five Khibiny samples and 52 analyses of one Lovozero sample) and apatite-nepheline ores (120 analyses of five Khibiny samples and 88 analyses of three Lovozero samples). They indicate synchronous crystallization of titanite in silicate rocks throughout the complexes: 374.1 ± 3.7 Ma for the Khibiny massif and 380.9 ± 4.5 Ma for the Lovozero massif, and attest to the later formation of phosphate-rare-metal ores: 371.0 ± 4.2 and 361.4 ± 3.2 Ma, respectively. The relatively delayed ore mineralization specific to the Lovozero massif can be accounted for the significantly lower volumes of magmatic melt and ore fluid involved, different thermal conditions, and the pattern of the investigated mineralization. As such, the obtained U-Pb data from titanite make it possible to limit significantly the time interval (most likely, not exceeding 15-20 Ma) comprising the evolution and activity of the ore-magmatic system of major agpaitic complexes, which is probably associated with plume magmatism.     

Paleoproterozoic Keulik–Kenirim Ore-Bearing Gabbro–Peridotite Complex,Kola Region: A New Occurrence of Ferropicritic Magmatism

Comprehensive research of ore-bearing differentiated intrusions of the Keulik–Kenirim structural unit, which represents a fragment of the Paleoproterozoic Pechenga–Varzuga Belt, has been carried out for the first time. The intrusions are subvolcanic by type and lenticular in shape, nearly conformable and steeply dipping. They are made up of peridotite, olivine and plagioclase pyroxenites, and gabbro metamorphosed under amphibolite facies conditions along with host basic volcanics. All intrusive rocks are enriched in TiO2 and FeO. Sulfide Cu–Ni mineralization is represented by disseminated, pocket, and stringer-disseminated types, which are clustered in the peridotitic zone as hanging units and bottom lodes. The Ni content in disseminated ore is estimated at 0.45–0.55 wt % and 1.15–3.32 wt % in ore pockets; the Cu grades are 0.17–0.20 and 0.46–5.65 wt %, respectively. To determine the age of intrusions and metamorphism of intrusive and volcanic rocks, various isotopic systems have been used: Sm–Nd (TIMS) in rock and U–Pb (SIMS SHRIMP) and Lu–Hf (LA-ICP-MS) in zircon. Conclusions on the origin of zircons are based on concentrations of trace elements including REE therein and Hf–Nd correlation in zircons and rocks. The U–Pb system of zircons reflects episodes of igneous rock formation (1982 ± 12 Ma) and their postmagmatic transformation (1938 ± 20 Ma). The last disturbance of the U–Pb isotopic system occurred 700 and 425 Ma. Xenogenic zircons dated from 3.17 to 2.65 Ga have been revealed in the studied samples. These zircons were captured by magma from the Archean basement during its ascent. The intrusions were emplaced synchronously with economic ore formation in the Pechenga ore field (1985 ± 10 Ma). The peak metamorphism of intrusive rocks under amphibolite facies conditions is recorded at 40 Ma later. The differentiated intrusions of the Keulik–Kenirim structural unit are close in their internal structure, mineralogy, and geochemistry, as well as in age and features of related Cu–Ni mineralization to ore-bearing intrusions of the Pechenga ore field, which are derivatives of ferropicritic (ferriferous) magmatism.