Geohimiâ

This study focuses on the igneous rocks composing the Odikhincha massif. The massif is typical ring alkaline–ultrabasic massif with carbonatites, second largest in the Maimecha-Kotui province. The Sr-Nd isotopic values of the traps of the Arydzhang Formation and the host dolomites were also determined for comparison. The Rb–Sr isotope system of phlogopite and calcite from the Od-16-19 carbonatite of the Odikhincha massif is disturbed; the obtained age on the mineral isochrone (245 ± 3 Ma) is close to the time of formation of the Siberian traps and rocks of the ultrabasic–alkaline Maimecha-Kotui complex, but the large scatter of analytical points (MSWD = 22) does not allow this date to be considered as reliable. The disturbance of the isotope system is probably related to the fact that the strontium isotope ratio in the fluid was not constant during autometasomatic phlogopitization of carbonatite. The U–Pb isotopic system of titanite and perovskite from the same carbonatite sample Od-16-19 also appeared to be disturbed, since data points formed discordia. The U–Pb age obtained for titanite and perovskite are 244 ± 5 Ma (MSWD = 1.8) and 247 ± 18 Ma (MSWD = 4), respectively. Apparently, the age values provided by the two isotopic systems (245 ± 3 Ma by Rb–Sr and 247 ± 18 and 244 ± 5 Ma by U–Pb) are consistent with each other and reflect the time of metasomatic processes, i.e., phlogopitization and iolitization. Rb–Sr and Sm–Nd isotope data for ultrabasic–alkaline intrusive rocks with carbonatites of the Odikhincha massif and volcanics of the Arydzhang Formation indicate an enriched, relative to the composition of the convecting mantle, isotopically heterogeneous source of their parent melts. This source could be a combination of ultrabasic mantle rocks and rocks of basic composition (basites). The latter played the role of an enriched component. No signs of contamination of the melts with the host sedimentary rocks in situ were found, however, variations of Sr and Nd isotopic ratios in the rocks of the Odikhincha massif may indicate that during the introduction of deep magmas their interaction and substance exchange with the surrounding rocks of the lithosphere continued up to complete solidification of the melts, as indicated by the nature of local isotopic heterogeneity within the Odikhincha intrusion.

Data from our original database, which includes more than 2 600 000 analyses for 75 elements of mineral-hosted melt inclusions and quench glasses in volcanic rocks, are generalized to calculate the mean concentrations of major, volatile, ore, and trace elements in magmatic melts from the following dominant geodynamic environments: (I) spreading zones of oceanic plates (mid-oceanic ridges), (II) environments affected by mantle plumes in oceanic plates (oceanic islands and lava plateaus), (III, IV) environments related to subduction processes (III is zones of arc magmatism on the oceanic crust, and IV is zones of magmatism in active continental margins in which magma-generating processes involve the continental crust), (V) environments of continental rifts and areas with continental hotspots, and (VI) environments of backarc spreading. A histogram of SiO2 distribution in natural magmatic melts shows a bimodal distribution: one of the maxima falls onto SiO2 concentrations of 50–52 wt % and the other onto 72–76 wt %. The most widely spread melts contain 62–66 wt % SiO2. Mean temperatures and pressures are calculated for each of the environments. The normalized multielemental patterns presented for environments I through VI show the ratios of the mean concentrations of elements in magmatic melts of mafic, intermediate, and felsic composition to the concentrations in the primitive mantle. Mean ratios of incompatible, trace, and volatile components (H2O/Ce, K2O/Cl, Nb/U, Ba/Rb, Ce/Pb, etc.) are evaluated for the melts of each of the environments. The variations in these ratios are calculated, and it is demonstrated that the ratios of incompatible elements are mostly statistically significantly different in the different environments. The differences are particularly significant between the ratios of the most differently incompatible elements (e.g., Nb/Yb) and some ratios involving volatile components (e.g., K2O/H2O).

The results of study and comparison of geochemical characteristics of quartzite sandstones of the Upper Riphean Khobeyu and Lower Paleozoic Obeiz formations of the Circumpolar Urals are presented. It has been established that the composition of quartzite sandstone of both formations was formed mainly from recycled material of ancient metaterrigenous rocks, with the participation of destruction products of igneous rocks of felsic (Obeiz Formation) and basic (Khobeyu Formation) composition and weathering crust material. The accumulation of the Lower Paleozoic psammites was accompanied by a gradual change of clastic sources, with increasing contribution of granitoid clastics.

The chemical composition of the snow cover in the area of industrial development of the apatite–nepheline deposit is analyzed to estimate the ecological and geochemical environmental impact of the mining enterprise. It has been established that the snow of the studied area of the Khibiny is enriched in Cl– and Na+ ions (on average 38 and 41 µeq/L), and relations between basic ions (Cl– > 
 > 
 and Na+> Ca2+> K+ = Mg2+) and mineralization value (from 1.7 to 6.4 mg/L) are typical for precipitates in the coastal regions of the northern European Russia. The average content of total nitrogen and phosphorus in the snow of the impact zone is 495 and 26 μg/L, respectively, which is 3 and 5 times higher than in the background zone. This is explained by their influx into the atmosphere with dust emissions from the mining enterprise. The content of organic matter (CODMn and TOC 5.5 and 5.8 mg/L) in the snow of the impact zone is about two times higher than in the snow of the background zone and in the water of the Khibiny water bodies. Probably, the elevated content of organic matter in the snow is associated with the supply of organic substances-reagents from the tailing dump, which are used to obtain apatite concentrate, as well as the intensive growth of unicellular green algae Chlamydomonas nivalis (Bauer) Wille under conditions of an increased content of nutrients and long daylight hours. The concentrations of a number of heavy metals (Zn, Mn, Cu, Cr, Pb, Cd) in the snow of the impact zone exceed their contents in the water of water body of the impact zone (13.4, 5.4, 3.8, 0.8, 0.65, 0.035 μg/L, respectively). These metals enter the snow as a part of dust emissions from the mine, and as polluted air masses from the industrial regions of Eurasia.