Zircon reaction and stability of the U-Pb isotope system during interaction with carbonate fluid: experimental hydrothermal study

 The interpretation of metamorphically induced U-Pb isotopic discordance requires a thorough understanding of zircon-fluid interactions. With this aim we have studied the behaviour of metamict and crystalline zircon phases and their U-Pb systems by cathodoluminescence after treatment by 2M Na₂CO₃ solution at T = 200–800 °C and P = 1–5 kbar for 3–14 days, X-ray diffraction, microprobe and isotope dilution analysis. The data indicate that zircon transformation under hydrothermal conditions depends on the experimental conditions and the degree of structural damage. Reconstitution of defective and impurity-enriched zones of metamict zircon (homogenization of impure element concentrations and increase of crystallinity) was observed at 400 °C and P = 1 kbar. Considerable lead and uranium loss occurred under these conditions. As a result of zircon dissolution, newly formed baddeleyite accommodating U from 2M Na₂CO₃ solution and Zr-Na-silicate were recognized. This process intensified with increasing pressure. Study of crystalline zircon indicates that migration of U and Pb took place only during dissolution of zircon at T above 650 °C. In the presence of carbonate-ions essential U and Pb amounts are lost from metamict zircon at a lower P-T than is typical for greenschist facies metamorphism. Received: 4 October 1997 / Accepted: 6 December 1999     

Archeological Monuments: Evidence of Strong Earthquakes in the Past (Using the Example of the Southwestern Part of the Issyk-Kul Lake Region,the Tien Shan)

Izvestiya, Atmospheric and Oceanic Physics - This paper presents new materials and summarizes existing ones on archeoseismology studies of stony walls conducted in the Alabash-Konurolyong...     

Role of microstructure in clay emanation

The influence of clay microstructure on radon emanation has been studied based on the analysis of literary data and direct measurements of the emanation coefficient of different types of clay rocks. Experiments have shown that modern and Holocene clay and silt with a cellular microstructure have the highest emanation coefficient (more than 60%). The majority of clay rocks transformed by diagenesis (showing either matrix or turbulent microstructure), have the emanation coefficient of about 50%. The emanation coefficients of lithified clay and argillite (laminar microstructure) do not exceed 10%. Thus, the emanation capacity of clay depends on clay microstructure and regularly decreases in the following row: cellular > matrix or turbulent > laminar microstructure.     

Early Paleozoic batholiths in the northern part of the Kuznetsk Alatau: Composition,age, and sources

The paper reports geological, chemical, and geochronological data on the Early Paleozoic granitoid and gabbro-granite associations, which compose the Kozhukhovskii and Dudetskii batholiths in the northern part of the Kuznetsk Alatau. The Kozhukhovskii batholith located in the Alatau volcanoplutonic belt is made up of tholeiitic, calc-alkaline, and subalkaline rocks that were formed in two stages. The first stage corresponded to the formation of granitoids of the Tylinskii quartz diorite-tonalite-plagiogranite complex (～530 Ma, Tylinskii Massif, tholeiitic type) in an island arc setting. The second stage (～500 Ma) produced the Martaiga quartz diorite-tonalite-plagiogranite complex (Kozhukhovskii Massif, calc-alkaline type) and the Krasnokamenskii monzodiorite-syenite-granosyenite complex (Krasnokamenskii Massif, subalkaline type) in an accretionary-collisional setting. The Dudetskii batholith is situated in the Altai-Kuznetsk volcanoplutonic belt and contains widespread subalkaline intrusive rocks (Malodudetskii monzogabbro-monzodiorite-syenite and Karnayul’skii granosyenite-leucogranite complexes) and less abundant alkaline rocks (Verkhnepetropavlovskii carbonatite-bearing alkaline-gabbroid complex), which were formed within the age range of 500–485 Ma. Our Nd isotopic studies suggest mainly a subduction source of the rocks of the Kozhukhovskii batholith (ε_Nd from + 4.8 to + 4.2). Subalkaline rocks of the Dudetskii batholith exhibit wide isotopic variations. The Nd isotopic composition of monzodiorites and monzogabbro of the Malodudetskii Complex (ε_Nd = + 6.6), in association with the elevated alkalinity and high Nb and Ta contents of these rocks, testifies to the predominant contribution of an enriched mantle source at the participation of a depleted mantle source. The lower ε_Nd (from + 3.2 to + 1.9) in its syenites possibly indicates their generation through melting of metabasic rocks derived from enriched mantle protolith. The rocks of the Karnayul’skii Complex have lower Nb and Ta contents at similar ε_Nd (+3.6), which suggests some crustal contribution to their formation.     

Interaction of metamict zircon with fluids of various composition

Interaction of metamict zircon with solutions of various composition (H₂O, 1 m NaCl + 0.5 m HCl, 1 m NaCl, 1 m NaHCO₃, 0.1 m Na₂CO₃, 1 m Na₂CO₃, 2 m Na₂CO₃, and 1 m NaOH) was examined at temperatures of 25–800°C, pressures of 1 and 5 kbar, and exposure times of 3, 7, and 14 days. The choice of fluid compositions was determined by the fact that aqueous, chloride, and carbonate solutions are the principal components of postmagmatic, metamorphic, and metasomatic fluids that can affect zircon in nature. Occurring in contact with these solutions, metamict zircon congruently and incongruently dissolves competing with the sorption of elements, crystallization of newly formed phases, and the restoration of the crystal structure. The intensities of these processes are controlled by the temperature, pressure, exposure time, and the pH and composition of the solution.     

Forecasting Water Levels in Krasnodar Krai Rivers with the Use of Machine Learning

Water Resources - The potential of construction of machine learning models was considered as applied to water level forecasting in mountain river reaches in Krasnodar Krai based on observation data...     

Geological structure and isotopic—geochronologic study of rocks from the Strel’na segment of the Terskii Greenstone belt,Kola Peninsula

The geological structure, age, and genesis of sedimentary—volcanogenic, metamorphic, and metasomatic rocks from the Terskii greenstone belt fringing the southern Imandra—Varzuga structure in the southeastern Kola Peninsula are discussed with defining main stages in endogenic activity of the region in the Late Archean and Early Proterozoic. The U-Pb method (SHRIMP-II, ID-TIMS, and Pb-LS techniques) was used to determine the age of volcano-sedimentary rocks of the Imandra Group as well as that of magmatic and superimposed metamorphic and metasomatic processes. The basic—intermediate metavolcanics of the Imandra Group are dated at 2.67 Ga, which corresponds to the Lopingian Gimol’skii Superhorizon (Late Archean). The Archean metavolcanics were subjected to Early Proterozoic regional metamorphism 2.1 Ga ago and metasomatic processes in the period of 1.85 to 1.77 Ga ago. The obtained data indicate multistage evolution of rock formation in the Terskii greenstone belt located in the southern flank of the Imandra—Varzuga structure in the Kola Peninsula.     

Age of titanium deposits of the northeastern part of the eastern European Platform: Rb—Sr data

Doklady Earth Sciences -