Geometric Aspects and Testing of the Galactic Center Distance Determination from Spiral Arm Segments

We consider the problem of determining the geometric parameters of a Galactic spiral arm from its segment by including the distance to the spiral pole, i.e., the distance to the Galactic center (R₀). The question about the number of points belonging to one turn of a logarithmic spiral and defining this spiral as a geometric figure has been investigated numerically and analytically by assuming the direction to the spiral pole (to the Galactic center) to be known. Based on the results obtained, in an effort to test the new approach, we have constructed a simplified method of solving the problem that consists in finding the median of the values for each parameter from all possible triplets of objects in the spiral arm segment satisfying the condition for the angular distance between objects. Applying the method to the data on the spatial distribution of masers in the Perseus and Scutum arms (the catalogue by Reid et al. (2014)) has led to an estimate of R₀ = 8.8 ± 0.5 kpc. The parameters of five spiral arm segments have been determined from masers of the same catalogue. We have confirmed the difference between the spiral arms in pitch angle. The pitch angles of the arms revealed by masers are shown to generally correlate with R₀ in the sense that an increase in R₀ leads to a growth in the absolute values of the pitch angles.     

The Pechora Sea: Past,recent, and future

The main stages in the development of the Pechora Sea are discussed. It is established that, during the high sea level stand corresponding to the warmest epoch of the Mikulino Interglacial, the Pechora Sea represented a more spacious, as compared with its present-day size, basin owing to the flooded valleys of river lower reaches. No sea in its present-day configuration existed during the last (Valdai) glaciation. At that time, the sea could have occupied only a narrow area along the southern coast of Novaya Zemlya, where marine sedimentation was in progress during the Late Pleistocene and Holocene. During the glaciation and postglacial time, the dried bottom of the former Pechora Sea accumulated large volumes of sand that are now concentrated largely in the accretion structures along its southern coast. In the current century, changes will occur mainly in the coastal zone of the Russkii Zavorot Peninsula, Pesyakov Island, the Varandei Settlement area, and the Medynskii Zavorot Peninsula, where a shoreline retreat for a distance of 0.5 km is expected.     

Constructing a Stäckel Model of the Galaxy: Solving the Problem of Whether the Vertical Density Distribution Is Realistic

Astronomy Letters - Techniques for solving the problem of constructing a Stäckel model by generalizing the potential from the equatorial plane to the entire space are considered. The initial...     

Electroconducting Elements of the Ultradeep Fluid–Fault Systems as Indicators of Seismically Active Zones of the Eastern Margin of the Eurasian Continent: Evidence from Magnetotelluric Data

Doklady Earth Sciences - A fundamentally new conclusion that zones of high seismic activity are controlled by electroconducting (fluid-saturated) fragments rather than by the entire deep fault is...     

A New Dinosaur Fossil from the Southern Urals

Doklady Earth Sciences - A caudal vertebra of a dinosaur (Iguanodontia indet.) is described from Upper Cretaceous (lower Campanian) coastal marine deposits of Izhberda Quarry near Orsk (Orenburg...     

Isotopic and geochemical zoning of Devonian magmatism in the Altai–Sayan rift system: Composition and geodynamic nature of mantle sources

Based on the systematic investigation of the geochemical and isotopic (Sr and Nd) characteristics of basic rocks from various volcanic areas of the Devonian Altai-Sayan rift system, the compositions of mantle magma sources were characterized, and the geodynamic scenarios of their entrainment into rifting processes were reconstructed. It was found that the titanium-rich basic rocks (2.5 < TiO₂ < 4.2 wt %) of this region are enriched in lithophile trace elements, including the rare earth elements, compared with N-MORB and are similar in composition to intraplate subalkali basalts of the OIB type. In contrast, moderate-titanium basic rocks (1.1 < TiO₂ < 2.5 wt %) are mainly depleted in the highly charged incompatible elements Th, U, Nb, and Ta (La/Yb = 1.2−2.2) and, to a lesser extent, in Zr, Hf, and LREE ((La/Yb)_N < 7), but are enriched in Ba. With respect to these characteristics, the moderate-titanium basites are similar to rocks formed in subduction environments. The geochemical parameters of the basites are strongly variable, which probably reflects the heterogeneity of the mantle sources that contributed to the formation of the rift system. In particular, the most notable variations in rock composition related to an increase in the contribution of an OIB-type source to magma composition were observed in the eastern direction, i.e., inland from the paleocontinent margin. The isotopic composition of the basites is relatively stable within individual rift zones and significantly variable at a comparison of rocks from different zones of the region. Based on the isotopic characteristics of the rocks, three melt sources were identified. One of them is chemically similar to the PREMA and is a common component in all observed trends of isotopic variations, irrespective of the position of the particular assemblage in the structure of the region. This component dominates the composition of the titaniumrich basalts with geochemical signatures of the derivatives of enriched mantle reservoirs of the E-MORB and OIB types and is considered as a plume source. Two other isotopic melt sources are related to subduction processes, which is indicated by their dominance during the formation of the moderate-titanium basalts showing the geochemical signatures (primarily, Ta-Nb depletion) of typical volcanic-arc rocks. These differences are consistent with the formation of the Altai-Sayan rift system in a complex geodynamic setting, which developed under the influence of intraplate magma sources (mantle plume) on the region of melt generation in an active continental margin (subduction zone).     

Intermediate sulfates in barite-celestite isomorphic series: Composition and mode of occurrence

Intermediate (mixed) sulfates of the BaSO₄-SrSO₄ isomorphic series are relatively rare in nature. Nevertheless, large resources of such sulfates have been revealed in Russia at carbonatite-type deposits (Karasug in Tuva and Khalyuta in the western Transbaikal region) and at the Oktyabr’skoe skarn magnetite deposit in the Angara region. The compiled database of chemical compositions of mixed Ba-Sr sulfates (876 items) comprises data related to the above deposits and other worldwide occurrences. Ba-Sr sulfates are formed in a wide range of conditions from sedimentary basins to high-temperature magmatic systems, and their generalized compositional intervals completely overlap. At the same time, sulfates of various natural assemblages and generations occupy discrete intervals separated from one another in isomorphic series. The statistical boundaries of these intervals correspond to 15, 36, and 66 (±1) mol % SrSO₄. Many Ba-Sr sulfates contain up to 7–11 mol % CaSO₄.