N. N. Mel’nikov

Mössbauer characteristics, mineralogy and isotopic age (Rb-Sr, K-Ar) of Upper Riphean glauconites from the UK Formation, the southern Urals

Comprehensive mineralogical analysis, Mössbauer spectroscopy and isotopic-geochronological study have been carried out for globular phyllosilicates (GPS) of glauconite group from the Uk Formation, the second one below the top of the Upper Riphean stratotype in the southern Urals. Glauconites have been sampled in the Kurtaza and Kulmas sections remote from each other in the Alatau anticlinorium that corresponds to western facies zone of the Bashkirian meganticlinorium. As is shown, size and density monomineral fractions of globules are represented by Al-glauconite according to established structural formula. The isotopic (Rb-Sr and K-Ar) dating of glauconites from the Uk Formation is performed for the first time along with computer simulation of cation arrangements in their crystal lattice and comparison of the results obtained with data of Mössbauer spectroscopy. It has been assumed by simulation that origin and transformation of the Rb-Sr and K-Ar systems in glauconite are concurrent to stages in structural evolution of this mineral, which have been controlled by geological and geochemical events in the history of sedimentary successions. The approach has been aimed at recognition of stratigraphically meaningful isotopic dates corresponding to the glauconite formation at the stage of the early diagenesis close to sedimentation time and the “rejuvenated” dates characterizing ages of subsequent geological events. The comparison of simulated cation arrangements with data of Mössbauer spectroscopy shows that the Rb-Sr (663 ± 9 Ma) and K-Ar (669 ± 16 Ma) dates established for glauconites correspond to the time of early diagenesis in their host sediments, being suitable for age assessment of the Uk Formation. The dates obtained are of interregional and wider significance, as they must be taken into consideration when constructing the general curve of Sr isotope variations in the Late Riphean oceans.

Rb-Sr, K-Ar, H-and O-Isotope systematics of the Middle Riphean shales from the Debengda Formation, the Olenek Uplift, North Sibera

Clay subfractions (SFs) of <0.1, 0.1–0.2, 0.2–0.3, 0.3–0.6, 0.6–2 and 2–5 μm separated from Middle Riphean shales of the Debengda Formation are studied using the TEM, XRD, K-Ar and Rb-Sr isotopic methods. The oxygen and hydrogen isotope compositions in the SFs are studied as well. The low-temperature illite-smectite is dominant mineral in all the SFs except for the coarsest ones. The XRD, chemical and isotopic data imply that two generations of authigenic illite-smectite different in age are mixed in the SFs. The illite crystallinity index decreases in parallel with size diminishing of clay particles. As compared to coarser SFs, illite of fine-grained subfractions is enriched in Al relative to Fe and Mg, contains more K, and reveals higher K/Rb and Rb/Sr ratios. The Rb-Sr age calculated by means of the leachochron (“inner isochron”) method declines gradually from 1254-1272 Ma in the coarsest SFs to 1038-1044 Ma in finest ones, while the K-Ar age decreases simultaneously from 1225–1240 to 1080 Ma. The established positive correlation of δ18O and δD values with dimensions of clay particles in the SFs seems to be also consistent with the mixing systematics. The isotopic systematics along with data on mineral composition and morphology lead to the conclusion that mixedlayer illite-smectite was formed in the Debengda shales during two periods 1211–1272 and 1038–1080 Ma ago. The first period is likely close to the deposition time of sediments and corresponds to events of burial catagenesis, whereas the second one is correlative with the regional uplift and changes in hydrological regime during the pre-Khaipakh break in sedimentation.

Geodynamic aspects of the development of offshore oil and gas deposits: Case study of Barents region

Geodynamic aspects of the development of offshore deposits in the Russian sector of Barents region are considered. The main technogenic geohazards at oil hydrocarbon extraction are shown to be land subsidence and earthquakes. A concept of studying geodynamic manifestations is substantiated. This concept is based on the evolution of oil-and-gas-producing enterprise with appropriate portion of geological environment, regarded as a complex open natural-engineering system. The geodynamic situation at the Shtokman gas-condensate field was simulated using a mathematical model. The model showed some regularities in the deformation of the fluid-bearing massif because of gas extraction, including the up to 30% volumetric compaction of the production strata, resulting in a regression deflection (subsidence) of seabed and considerable subhorizontal deformations and displacements of bottom rock beds. The specific features of gas pipeline laying from the Shtokman field through Murmansk province are discussed, and the structure of geodynamic monitoring is proposed. Substantiation is given to the need to carry out special geodynamic studies in the Barents region with the aim to ensure operation geosafety and stability of structures of oil-and-gas facilities and pipeline transportation of hydrocarbons materials.

Rb-Sr and U-Pb Systematics of Metasedimentary Carbonate Rocks: The Paleoproterozoic Kuetsjarvi Formation of the Pechenga Greenstone Belt, Kola Peninsula

The Rb-Sr and U-Pb systematics have been studied in the metasedimentary carbonate rocks from the Paleoproterozoic Kuetsjarvi Formation. Samples were taken from the borehole drilled in the northern zone of the Pechenga Greenstone Belt in the northwestern Kola Peninsula. The carbonate section of the formation is made up of three units (from the bottom to top): (I) dolomite (68 m), (II) calcareous-dolomite (9 m), and (III) clayey calcareous (1 m) ones. Dolomites (Mg/Ca = 0.55–0.61) from the lowermost unit I contain 70.3–111 ppm Sr. Initial 87Sr/86Sr ratio in them varies within 0.70560–0.70623 and characterizes the primary continental-lacustrine carbonate sediments. Calcareous dolomites (Mg/Ca = 0.39–0.59) and dolomitic limestones of units II and III (Mg/Ca = 0.02–0.36) are enriched in Sr (285–745 and 550–1750 ppm, respectively). Initial 87Sr/86Sr ratios in these rocks (0.70406–0.70486 and 0.70407–0.70431, respectively) fall within the range typical of the Jatulian seawater, which indicates that the carbonate sediments of two upper units were formed in an open marine basin. Study of dolomites from unit I showed that the Svecofennian metamorphism more significantly affected the U-Pb systems of carbonate rocks as compared to their Rb-Sr systems. In the 207Pb/204Pb-206Pb/204Pb diagram, most data points corresponding to the carbonate constituent of dolomites define isochron with an age of 1900 ± 25 Ma (MSWD = 0.5). The same samples define a positive correlation in the 208Pb/204Pb-206Pb/204Pb plot. Since sedimentary carbonates usually do not contain Th, this correlation points to secondary enrichment of the studied dolomites in Th or thorogenic 208Pb. Hence, the obtained Pb-Pb dating can be regarded as the age of the Svecofennian metamorphic event. Three samples from dolomites of unit I lack any disturbance of the initial U-Th-Pb systematics, but their trend in the 207Pb/204Pb-206Pb/204Pb diagram deviates from the 1900 Ma isochron. Based on these samples, the model U-Pb premetamorphic age of the Kuetsjarvi carbonate sediments is 2075–2100 Ma. This interval is consistent with the age range of the Lomagundi-Jatulian event, which was responsible for the formation of carbonate sediments with high positive δ13C values.

Strontium isotope composition of the lower proterozoic carbonate concretions: The Zaonega Formation, Southeast Karelia

The middle part of the volcanosedimentary Zaonega Formation of the Ludikovian Suprahorizon (approximately 2.0 Ga) includes large carbonates concretions and lenses in shungite layers. Carbonate lenses and concretions are primarily elongated and flattened, and their thickness varies from tens of centimeters to a few meters. Some lenses retain relicts of lamination. Concretions are composed of calcite or dolomite. They contain abundant organic matter, as well as mica, talc, chlorite, quartz, and pyrite crystals. The calcite concretions contain some dolomite admixture (Mg/Ca = 0.011−0.045) and differ from sedimentary limestones by a low Fe/Mn value (0.3–2.1). The Sr content is as much as 385–505 μg/g in most samples and is low (86 μg/g) only in one sample. The Rb-Sr systematics of carbonate concretions was studied with the stepwise dissolution procedure, which included processing with the ammonium acetate solution (AMA fraction) to partially remove the secondary carbonate material, with dissolution of the residue in acetic acid (ACA fraction). In individual calcite samples, discrepancy between the measured 87Sr/86Sr values in the AMA and ACA calcite fractions shows a variation range of 0.0008–0.0033. The initial 87Sr/86Sr ratio in the ACA fractions of the studied samples varies from 0.7053 to 0.7162. The ratio shows a positive correlation with Mg/Ca and the proportion of siliciclastic admixture and negative correlation with the Mn content. The concretions were formed when the sediments subsided, probably, during the transition from a zone with “mild” reductive conditions to zones with active sulfate reduction and methanogenesis. In the sulfate reduction zone, where most pyrite-bearing concretions were formed, the sediment was not geochemically exchaged with the bottom water and was evolved into a closed or semiclosed system. Processes of diagenesis in this zone promoted the release of the radiogenic 87Sr from the associated siliciclastic minerals, resulting in growth of the initial 87Sr/86Sr in concretions up to 0.7108–0.7162. Some calcite concretions, which lacked pyrite (or contained its minimal amount) were likely formed in a thin surficial sediment layer located above the sulfate reduction zone. Therefore, they precipitated Sr in isotope equilibrium with Sr of the bottom water. However, large concretions and carbonate lenses with an insignificant siliciclastic admixture could retain the signature of early diagenesis or even sedimentation. The initial 87Sr/86Sr ratio in one of such samples with the siliciclastic admixture of 6.2% makes it possible to estimate the maximal value of this ratio (0.7053) in the Ludikovian paleobasin.

Rb-Sr Geochronology of Vendian Shales, the Staraya Rechka Formation, Anabar Massif, Northern Siberia

Fine-grained clayey subfractions (SF) with particle sizes of 1–2, 0.6–1.0, 0.3–0.6, 0.2–0.3, 0.1–0.2, and <0.1 μm were extracted from shales of the Vendian Staraya Rechka Formation in the Anabar Massif and studied by XRD and Rb-Sr methods. All the clayey subfractions are represented by illite with high crystallinity indices, which are characteristic of the low-temperature diagenesis/catagenesis zone and grow with the decrease of the particle size. The Rb-Sr systematics in clayey subfractions combined with mineralogical data provide grounds for the conclusion that illite from clayey rocks of the Staraya Rechka Formation was forming during two periods: approximately 560 and 391–413 Ma ago. The first illite generation was likely formed in the course of lithostatic subsidence of the Staraya Rechka sediments and the second one, during the Devonian lithogenesis stage. It is assumed that age of the first generation (∼560 Ma) is close to that of the Staraya Rechka Formation. This inference is consistent with biostratigraphic, chemostratigraphic, and geochronological data obtained for both rocks of the Anabar Massif and Vendian sediments from other regions of Siberia.

Sm-Nd systematics of fine-grained fractions of the Lower Cambrian blue clay in northern Estonia

Fine-grained clay subfractions, SFs (particle size <0.1, 0.1–0.4, 0.4–0.6, and 0.6–2.0 μm) separated from a sample of the Lower Cambrian blue clay of the Lontova Formation were studied with XRD and Sm-Nd methods. The relatively coarse-grained SFs include illite with a small admixture of chlorite, while the finegrained SF (<0.1 μm) consists of mixed-layer illite-smectite. The illite crystallinity index (Ic) increases with decreasing particle size. The leaching of SFs with 1N HCl, analysis of Sm-Nd systematics of leachates and residues, as well as XRD data and results of chemical analysis show that the studied rocks contain at least two generations of minerals. The first (detrital) generation is related to the transformation of provenance material, whereas the second (authigenic) generation was formed at the postsedimentary stage of the evolution of the Lontova sediments. The Sm-Nd date of the first generation (790±90 Ma) is considered a minimal age of rocks in the northeastern and southwestern regions of the East European Platform that served as a source of sedimentary material of the Lower Cambrian blue clay. The date of the second generation of minerals reflects the timing of authigenic mineral formation in the course of burial and diagenetic and catagenetic reworking of clay sediments.

Methodological approach to restoration of ecosystem functions in the industrial lands

The methodological approach to rehabilitation of disturbed lands in mining areas in accordance with the evolutionary program of soil formation on mineral substrates by generating biologically active medium has been substantiated and approved based on the data of many-years monitoring at various objects.