Study of the Asteroid (13553) Masaakikoyama

Solar System Research - The asteroid (13553) Masaakikoyama was observed with the ZA-320M and MTM-500M telescopes of the Pulkovo observatory in August and September 2018. Its axial rotation period...     

An Early Jurassic age for the Puchezh‐Katunki impact structure (Russia) based on 40Ar/39Ar data and palynology

The Puchezh‐Katunki impact structure, 40–80 km in diameter, located ~400 km northeast of Moscow (Russia), has a poorly constrained age between ~164 and 203 Ma (most commonly quoted as 167 ± 3 Ma). Due to its relatively large size, the Puchezh‐Katunki structure has been a prime candidate for discussions on the link between hypervelocity impacts and extinction events. Here, we present new ⁴⁰Ar/³⁹Ar data from step‐heating analysis of five impact melt rock samples that allow us to significantly improve the age range for the formation of the Puchezh‐Katunki impact structure to 192–196 Ma. Our results also show that there is not necessarily a simple relationship between the observed petrographic features of an impact melt rock sample and the obtained ⁴⁰Ar/³⁹Ar age spectra and inverse isochrons. Furthermore, a new palynological investigation of the postimpact crater lake sediments supports an age significantly older than quoted in the literature, i.e., in the interval late Sinemurian to early Pliensbachian, in accordance with the new radioisotopic age estimate presented here. The new age range of the structure is currently the most reliable age estimate of the Puchezh‐Katunki impact event.     

Volatiles in basaltic magmas of ocean islands and their mantle sources: II. Estimation of contents in mantle reservoirs

In this paper, we address the average compositions (including the contents of H₂O, Cl, F, and S) and the compositional structure of oceanic mantle plumes on the basis of element contents and ratios in ocean island magmas. The average contents of incompatible volatile and nonvolatile elements were calculated for the material of mantle plumes using a thermal and a more plausible moderately enriched model. The following average contents were estimated for the plume mantle: 510 ppm K₂O, 520 ppm H₂O, 21 ppm Cl, 55 ppm F, and 83 ppm S. These values are significantly higher than those of the depleted mantle (except for S). The primitive mantle normalized average content of water in mantle plumes is similar to those of La and Ce but lower than those of K, Cl, and Sr. This is at odds with the hypothesis of “wet” mantle plumes. Three types of basaltic magmas distinguished in our previous study (Part I) characterize three types of plume sources (MI, MII, and MIII). Using the favored moderately enriched model, the average contents of H₂O, Cl, F, and S were estimated for the three sources (ppm): 130, 33, 11, and 110 for MI; 110, 12, 65, and 45 for MII; and 530, 29, 49, and 110 for MIII, respectively. The plume mantle is heterogeneous and its heterogeneity can be described by the presence of three main types of compositions, one of which (MI) is similar to the composition of the mid-ocean ridge mantle and the other two types (MII and MIII) are moderately enriched in K, Ti, P, F, and incompatible trace elements but depleted in Cl, H₂O, and sometimes S. The compositions of MII and MIII have different H₂O, Cl, and S contents: MII is significantly depleted in these components compared with MIII. The MII component is probably similar to the enriched mantle (EM). In addition to the aforementioned three main components, the plume mantle probably contains high-Cl and low-F materials, which are related to the recycling of the oceanic and continental crust. All the observed characteristics of the mantle plumes are in adequate agreement with the model of a zonal mantle plume including a central part hot and depleted in H₂O, Cl, and S; a periphery enriched in volatile components; and the enclosing mantle interacting with the plume material.     

Impact lithologies – a key for reconstruction of rock-forming processes and a geological model of the Popigai crater,northern Siberia

The 100-km diameter Popigai impact crater (astrobleme), which formed 35.7?Ma ago as a result of the collision of an ordinary chondrite asteroid, was discovered in the 1970s. The impact site was studied in detail for nearly two decades, and various geological investigations were performed there. They included drilling of numerous wells (about 850), geophysical surveys, and investigations of impact breccias and impactites. This research was generally performed in connection with the identification of the unique resource of industrial impact diamonds, which were found in impact rocks for the first time in the world. The extensive research data acquired over 20–30 years include geological maps, collections of rock samples and thin sections, core samples, etc. All these materials are stored in the Russian Research Geological Institute in St Petersburg. Although a lot of data on the Popigai crater have already been published, the available materials and new analytical methods offer the opportunity to obtain some new data on mechanisms of rock-forming processes during an impact event, to improve existing geological models, and to compare in detail all these features to those established in other large craters on the Earth. Modelling of the physical processes of impact cratering has been extended and new data contribute significantly to the study of impact cratering and other problems of comparative planetology. In particular, many different hypotheses of rock-forming mechanisms are tested, especially those of impact melting of various target lithologies, homogenisation of huge volumes of melt products, and their mode of ejection, deposition, cooling, etc.     

The Schlema–Alberoda five-element uranium deposit,Germany: An example of self-organizing hydrothermal system

As a result of integrating geological, mineralogical, and geochemical data on the unique Schlema–Alberoda five-element uranium deposit situated in Federal Republic of Germany and explored in detail down to a depth of 2 km, it has been shown that its formation for more than 100 Ma has been caused by combination of internal and external factors. The latter comprise favorable metallogenic specialization of the region, injection of intrusive bodies bearing the necessary stock of energy, and periodic pulses of tectonic reactivation. The internal factors of self-development involve evolutionary processes, which occur in host rocks at the consecutive stages of prograde and retrograde metamorphism giving rise to alteration of rocks in consistence with physical and chemical laws at variable temperature and degree of system opening.     

Deposits of the 1983 and 1993 tsunamis on the coast of Primorye

Deposits of the two strongest tsunamis of the 20th century have been found on the eastern coast of Primorye. The tsunamis had epicenters in the Sea of Japan west of the coast of Hokkaido. The distribution and preservation of deposits in bays of different geomorphological structure have been analyzed. The best defined sedimentary covers occur in the upper part of sections in low-lying areas of bay shores, where the wave runup was more than 3 m. The best preserved deposits have been observed in bays attributed to loworder streams. Variations of the structural composition of tsunami deposits formed by traction processes associated with the tsunamis have been analyzed depending on distance from the shoreline; the sources of material have been identified. Tsunami waves transported sand not only from beaches, ancient storm ridges, and terraces, but also from the underwater coastal slope; waves also grabbed material from estuarine lagoons and lakes located in the shore inundation zone. Deposits include marine diatoms with dominant sublittoral planktonic and benthic species, which suggests that the material was transported from a depth no more than 15 m. Deep-sea species of diatoms and their fragments have been encountered. Among freshwater diatoms are species with different ecological identities, indicating erosion and redeposition of material transported from various sources.     

Physicochemical parameters and geochemical features of fluids of precamrbian gold deposits

This paper analyzes literature data on physicochemical parameters and chemical composition of fluids of Precambrian endogenous gold deposits. The average values and ranges of temperature, pressure, and salinity of fluids from the Archean and Proterozoic gold deposits are estimated. It is revealed that fluids of Archean deposits are dominated by methane, while those of Proterozoic deposits, by nitrogen. It is proposed that the accumulation of nitrogen in the atmosphere is related to the intense nitrogen degassing from the Earth’s interior. The highest pressures of endogenous fluids in this period could reflect specifics of deep geodynamics of the planet in the Proterozoic. The large gold deposits (>100 tons) are characterized by narrower range of physicochemical parameters as compared to small deposits. The contribution of heated chloride brines in the formation of majority of large Proterozoic deposits is established.