A new approach to obtaining (U–Th)/He thermochronological information is proposed. The content and mobility of radiogenic helium in concentrates of pyrite, magnetite, hematite, and amphibole from albitites and their host rocks from gold-bearing uranium ore occurrences in the Salla-Kuolayarvi belt (northern Karelia) were estimated based on the results of incremental heating of samples with simultaneous registration of the helium flux. The values of the apparent U–Th–He ages were calculated taking into account the contribution of the trapped component. The suitability of the mineral species for placing the age boundaries on the evolution of ore-bearing metasomatic rocks was considered on the basis of the calculated parameters of helium mobility in the lattice, as well as the petrographic features of each individual sample.

The results of geological, geochemical, and geochronological studies of granophyre rocks from the Jarva-Varaka Massif (Kola region) are presented. The 2-km section of the massif is composed of mafic and felsic norites, hypersthene diorite, pigeonite-augitic diorite, quartz diorite, and granodiorite. All these rocks contain a variable amount of granophyre (micropegmatite), from 10

Recently experiments on He extraction from an amphibole by the incremental heating unexpectedly revealed that the He release pattern depends on the heating rate. During slow heating (~4 K·min−1) of the amphibole grains, one smooth peak of the He flux from the mineral was observed; in contrast, during fast heating (~40 K·min−1) an additional sharp peak appeared at a temperature about 750 °C. In order to explain these observations, we developed a model of He diffusion from the amphibole, which allowed the calculated He fluxes from the mineral to be reconciled with those observed. From the modelling we derived: (i) the helium diffusion domain size distribution, and evolution of the distribution in the course of incremental heating; (ii) occurrence of the tensile stresses, operating under enhanced temperatures above 700 °C. The stresses are different in sites with the different local thermal expansion of the crystalline lattice and they increase the He diffusion flux. The model can be applied to other minerals (materials).

The Vulvara anorthosites occur among the rocks of intermediate composition, in contrast to most Lapland granulite belt (LGB) anorthosites, associated with mafic granulites. The U-Pb, Sm-Nd and REE geochemical data were used to compare Vulvara with other LGB anorthosites. The intrusion of the Vulvara massif occurred in the interval of 2100-1965 Ma. The first metamorphic processing took place 1947 +/- 11 Ma ago under granulite facies. Postgranulite transformations occurred 1900-1850 Ma ago under amphibolite facies; earlier (1896 +/- 15 Ma) and later (1852 +/- 17 Ma, 1846 +/- 16 Ma) thermal events were established. The Vulvara anorthosites, in comparison with other LGB anorthosites, are distinguished by a more acidic composition of plagioclase, a higher F number of rocks and mafic minerals, a higher content of SiO2, Na2O, K2O, Ba, Sr, Zr, Nb, Rb, and a lower content of CaO. The Sm-Nd and REE geochemistry suggest that there are three groups of LGB anorthosites, distinguished both by age and magma source.

Усовершенствована методика реконструкции концентраций радиогенного гелия в глубинных подземных водах путем изучения гелия в газово-жидких включениях (внутреннем объеме) кварца, извлеченного из керна скважин

5 Лаборатория свойств стекла, Санкт-

Zircon grains from various metagranitoids (plagio- and monzo-granites, gneisses, metasomatic rocks, and pegmatoid veins) from the Skal’noe and Dikoe sites of the Litsa uranium ore area (Kola Region, Russia) were studied in order to reconstruct the sequence and timing of events in the area and to observe effects of hydrothermal process related to uranium mineralization on structure and composition of zircon. Individual zircon grains were studied by means of laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS), ion microprobe and Raman spectroscopy. Isotopic LA–ICPMS data for the Skal’noye and Dikoe ore occurrences suggest the following age sequence of events in the area: intrusion of plagiogranites—2829 ± 12 Ma, formation of magmatic protolith of gneisses—2781 ± 17 Ma, metamorphism of plagiogranites—2636 ± 34 Ma; intrusion of monzogranites and pegmatoid veins—2549–2526 Ma, hydrothermal event with uranium input—2276 ± 21 Ma, last metamorphism of plagio- and monzo-granites—1892–1696 Ma. Ore-bearing rocks in the area are pegmatoid veins and quartz–feldspar metasomatites which contain uraninite. During a 2.3 Ga hydrothermal process, newly formed zircon rims grew simultaneously with the precipitation of uraninite in the veins and metasomatites. These rims are characterized by high U and rare earth elements (REE) contents (up to 6560 and 8760 ppm, respectively), dark cathodoluminescence, low Th-U ratios (0.1–0.007) and a flat LREE-enriched pattern, in some cases inherited from minerals, dissolved during a hydrothermal event (magmatic plagioclase and probably monazite). Hydrothermal zircon rims grew with partial dissolution of the magmatic zircon, as evidenced by the rounded and curved shapes of zircon cores. The degree of alteration caused by hydrothermal events depends on the uranium content in the pre-existing zircon. The effects of zircon alteration and newly formed zircon composition reflect the redistribution of uranium in rocks.

A relation of uranium mineralization to structural, textural and physical properties of rocks was investigated using two uranium ore occurrences (Beregovoe and Dikoe) in the Litsa ore area (Kola region, Russia) as an example. Study of the rock samples collected on the surface was carried out using X-ray computer tomography (CT), petrography and petrophysics. Petrophysical properties (density and elastic anisotropy index) as well as petrographic characteristics of 25 rock samples were studied; six samples from this collection were studied by CT method. The samples from the Beregovoe site display general positive correlation between magnitude of the elastic anisotropy index and uranium concentration. The samples from the Dikoe ore occurrence, however, do not follow this trend. Comparison of CT data with that obtained from petrophysical measurements shows that the elastic anisotropy index can be low in highly deformed rock, if microfractures and micropores were sealed with secondary (including uranium) minerals; while the uneven distribution of the heavy mineral phases in weakly deformed rock can significantly increase its elastic anisotropy. The CT method combined with petrographic and petrophysical methods has proved to be useful for studying ore deposits. In particular, the CT method allows the influence of spatial variations of minerals of different specific weight on the elastic properties of rocks (elastic anisotropy) to be ascertained. The data obtained for the Litsa area suggest the course of further research involving the construction of geological structural models of the crust blocks with subsequent selection of areas with the most favorable conditions for the formation of uranium ore.

The article presents the preliminary results of a study of the U-Th-He system in a sample of apatite from the Kovdor massif.The potential usefulness of such studies for learning early thermal history of rocks of the massif is discussed.

Helium isotopes are useful for tracing terrestrial water movement and calculating residence times, which have important implications for radioactive waste disposal and carbon capture and storage projects. Helium concentrations are generally measured directly in water samples; however, this is not always possible. In this scenario, the He concentration equilibrium between pore waters and He accessible volumes (e.g., fluid inclusions, hereafter HAV) in minerals (e.g., quartz) can be utilized to determine He concentrations. In this study, the He partial pressure was measured in HAVs of quartz grains collected from cores of the Kola Super Deep Borehole (KSDB) using isothermal He extraction/saturation experiments. A large fraction of He was released from these samples during the first interval of isothermal heating, indicating a destruction of the unstable HAVs, likely due to decrepitation of fluid inclusions. We present a new approach to calculate He partial pressures using only the He fraction released from the stable HAVs. This approach gives a He partial pressure p(He) of congruent to 1.5 atm at depths of similar to 10 km. When combined with solubility data, this value suggests a high He concentration in the deep pore waters nearby the KSDB, congruent to 1.3 x 10(-6) mol (cc H2O)(-1), indicating a long residence time of He atoms in a deep water - mineral system, close to the age of the regional metamorphism (congruent to 1700 Ma). The He-3/He-4 in the deep KSDB quartz samples (and the pore waters) are similar to 3 x 10(-8), typical of radiogenic He generated in 10 km deep Archean rocks of the KSDB. Importantly, external fluxes are not needed to explain He isotope abundances in rocks, minerals and pore waters at these depths.

Abundances of radiogenic He and Ar isotopes, observed in ground waters of the Permian-Carboniferous terrigenous sediments (PCT) in Northern Switzerland, exceed those in the overlaying (Muschelkalk) and underlying (Crystalline basement) aquifers, eliminating external sources of these species. In this case a comparison of the observed abundances with those in situ generated sheds light on mobility of noble gases generated in the PCT.Detailed studies of the parent element and He isotope concentrations in the PCT rocks, mineral separates, pore and ground waters along with the data on γ-logging and porosity allow He isotope production and redistribution in the PCT rock-water system to be understood and quantified. The PCT shales generate most of 3He and 4He and both isotopes are almost completely released from the shales into pore space and migrate either into the aquifers with movable waters and further out of the PCT, or into gas-fluid inclusions of the rock forming minerals, quartz and plagioclase.Here we present a He concentration profile, based on: (i) helium measured concentrations in ground water samples from 5 aquifers available within the cross section studied; (ii) helium concentrations (in 15 PCT pore water “samples”), derived from the He partial pressure in gas-fluid inclusions, recovered by special extraction/saturation experiments.In order to estimates the rates of He migration through the PCT, we compare the produced and observed abundances within the frame of 1D diffusion model. Only two parameters, the diffusion coefficient for the whole sequence, DALL = 4.1 × 10−3 m2 year−1, and the rate of He removal from the crystalline aquifers, θCR ≈ 1.4 × 10−6 year−1, are required to obtained a good agreement between the observed and calculated He concentrations in PCT waters: the minimal square deviations are ≈(2 to 3) × 10−8 mol cm−3 H2O, comparable with the accuracy of measurements. The following parameters were derived from the modeling: the reduction factor DALL/D0 = 0.013; the He diffusion fluxes into the overlaying Muschelkalk and underlying Crystalline aquifers, FMU ≈ 1.4 × 10−7 mol m−2 year−1 and FCR = 0.24 × 10−7 mol m−2 year−1, respectively. From these values we calculated the mean helium residence times in PCT, <τPCT> ≈ 120 Ma, and in Muschelkalk aquifer, <τMU> ≈ 6 ka. The relationships between He diffusivities of the PCT and the underlying Crystalline basement are discussed. We further compare our results with other estimates of He mobility in low-permeable sediments.

The paper presents newly acquired data on the budget and mobility of isotopes of noble gases in samples of amphibole, a mineral commonly characterized by relatively high He concentrations compared to other rock-forming minerals of Earth’s crust. In the amphibole samples from alkaline granites of the Ponoy Massif, Kola Peninsula, 3He, 4Не, and 40Ar* isotopes were mostly radiogenic, generated by radioactive decay and nuclear reactions. Retention ability of two helium isotopes was found to be different: since the time of metamorphic event approximately 38% 3Не and only approximately 16% 4Не have been preserved. A small He fraction (≈3% of its total concentration) and a larger part of radiogenic 40Ar* (≈35%) are hosted in fluid inclusions. Relatively high 3Не/4He ratios (up to ≈1 × 10–6) in the amphibole were caused by the high Li concentrations and better preservation of 3Не. The 3He/4He ratios in helium extracted from the samples by melting and crushing are indistinguishable, while the 4He/40Ar* ratios in the fluid inclusions (opened by sample crushing) (~0.6) are notably lower than those in the bulk samples (melting) (5) and those calculated from the U, Th, and K concentrations (15). Two He release peaks were observed on the curves of rapid (12–40°C min–1) liner heating of the samples. At a lower heating rate (~5°C min–1), the high-temperature peak disappeared. Further investigation of this peculiarity of He migration has shown that (1) it is also typical of some other amphibole samples (not only those from Ponoy granites); (2) He amount in the “disappearing” peak varies from sample to sample and also depends on the heating rate and grain size (powdered samples with an average grain size of ≤50 µm did not yield the second peak, regardless of the heating rate); (3) the temperature of He release from powdered samples is approximately 50°C lower than that for mineral grains; and (4) preliminary long-term heating of the sample to 400°C (a temperature at which He starts to diffuse from the amphibole structure) results in a significant decrease in the low-temperature peak and a shift of the “disappearing” peak toward lower temperatures. The probable reasons for this phenomenon are discussed.

The Kola region (NE of Fennoscandian Shield) has high uranium potential. The most promising structures within the Kola region in respect to uranium enrichment are the Litsa area and the Salla-Kuolajarvi zone. The principal objective of the present study was to define sequence and timing of uranium deposition within these areas. Isotopic (U-Pb and Rb-Sr) exploration of the rocks from Skal’noe and Dikoe U occurrences of the Litsa area and Ozernoe occurrences of the Salla-Kuolajarvi zone was carried out. As it follows from isotopic dating, the principal stages of uranium mineralization had taken place 2.3–2.2, 1.75–1.65, and 0.40–0.38 Ga ago, simultaneously with the stages of alkaline magmatism in the Kola region, which provided the uranium input. Uranium mineralization was related to hydrothermal and metasomatic events under medium to low temperature of ~550 °С at 2.3 Ga to ~280 °С at 0.4 Ga.

The first evidence for a specific role of amphiboles in He isotope balance of crustal rocks was presented in early contributions by Gerling et al. (1971, 1976). Since then it was shown that 4He and 3He concentrations in amphiboles generally exceed those in the host rock samples. Recently amphibole was considered as an important carrier of noble gases and other volatiles components in the course of their subduction into the mantle. This paper presents new data on the balance and mobility of noble gas isotopes and major gas constituents in amphibole separates in order to understand sources and evolution of volatile components of 2666Ma old alkaline granites from Ponoy massif (Kola Peninsula), which underwent metamorphism 1802Ma ago.In the amphiboles 3He, 4He and 40Ar* were dominantly produced in situ due to radioactive decay of the parent isotopes and associated nuclear reactions. A small fraction of He (≈3% of the total) is liberated by crushing and shows 3He/4He ratio indistinguishable from that found by total extraction. The fraction of trapped 40Ar* amounts to ≈40%; both these fractions presumably occupy fluid inclusions and show rather low 4He/40Ar*≈0.1, a factor of ≈ 150 below the production ratio (calculated assuming no loss/gain of the species has happened since the time of metamorphism).3He has been better preserved in amphiboles compared with 4He: the retention parameter (measured amount of He/totally produced amount) for 3He (≈0.4) exceeds that for 4He (≈0.15).He extraction by fast and slow linear heating of amphiboles resulted in different release patterns. The fast heating (within 12 to 40°Cmin−1) revealed a superposition of two peaks. When heating with slower heating rate (below 8°Cmin−1) was applied, the high-temperature peak disappeared (the “disappearing site”). Extractions of He atoms from grain and powder samples at different heating rates have shown that: (1) the “disappearing site” is revealed by the fast heating analyses of different amphibole samples but not only those from the Ponoy massif; (2) amount of He liberated from the “disappearing site” is variable and generally much less than the total amount of He in the sample; (3) analysis of the powder produced in the crushing experiments never reveals the “disappearing site”; the temperature of He release from the powder is lower than that from the mm grain size sample by ≈50°C. Possible explanations of the nature of the “disappearing site” are discussed. However, independently on nature of this effect, repeated gas extractions by heating at different rates would give additional information about structure and its transformation during heating of amphiboles.The simplest explanation of the observed abundances of noble gas isotopes in the amphibole separates from Ponoy granites suggests local production, redistribution and partial loss of noble gases during evolution of the massif.

In a rock–water system an apparent residence time for He can be derived from the bulk He concentration in the rock and the porewater and the He production rate provided that (1) the system is at steady state and (2) the He flux from external sources is negligible. This second condition is crucial and needs to be assessed by identifying the various He-sources in a given hydrologic environment.He and Ar isotope abundances were investigated in whole rock samples and mineral separates of the alternating shale–sandstone rock sequence of the Permo-Carboniferous Trough (PCT) of the Molasse Basin in northern Switzerland. 4He concentrations and 40Ar/36Ar ratios in groundwater in the bounding aquifers are lower than those in PCT groundwaters, thus eliminating a possible external source of radiogenic noble gases.The elements producing radiogenic helium isotopes, U, Th and Li, are mainly concentrated in the shales, where they reside in biotite, muscovite, clay minerals and organic matter. Contents of these elements as well as chronological and chemical data allow us to calculate maximum “closed system” He isotope concentrations and the retention coefficients, e.g. 3Hemeasured/3Hecalculated ratios. As deduced from the low retention coefficients, almost all helium-3 that has been produced in these minerals since their deposition has been lost into the complementary reservoir, i.e. the porewater; the same conclusion is valid for 4He. This is also indicated by the similarity between the calculated 4He/3He production ratio of the shales and that observed in PCT groundwater (1.3×107). Moreover, similar 4He/3He ratios are measured in quartz and plagioclase mineral grains from the sandstone layers, by far exceeding the production ratios derived for these minerals. Also, the measured concentrations of He isotopes (especially 3He) are higher than the calculated production capacities of these minerals. These observations suggest that some of the He produced in the shale and released from these rocks migrates via the porewater into the sandstone layers and penetrates further into some rock-forming minerals, such as quartz and plagioclase.Measurements of the migration rate of He atoms through quartz crystals showed that equilibrium between internal (gas–fluid vesicles) and external (porewater) He-concentrations is reached on a short time scale of ~104years at an in-situ temperature of ≈70°C. The equilibrium allows the He concentrations in the porewater across the PCT sequence to be quantified using quartz crystals as detectors. For the PCT segment opened by the Weiach borehole these “quartz-derived” concentrations as well as those directly measured in PCT groundwaters are rather high implying a long residence time for the He-atoms in the PCT rock-porewater system, well exceeding 107years.

The estimation of the mobility of ancient groundwater provides clues to a number of fundamental and applied problems. The U(Th)-He system is a promising tool for dating ancient waters. This study focused on the recently proposed method of the reconstruction of helium concentration in groundwater. The method is based on equilibrium in the mineral-water system which allows us to avoid direct sampling of water from boreholes. Some minerals contain excess helium migrating from the ambient groundwater into the internal helium-accessible volume (HAV) of grains. For this study, quartz was sampled from the sandstones of the 1000-m-thick Permian-Carboniferous sedimentary sequence of the Molasse Basin in northern Switzerland. The HAV of the quartz concentrate was determined via helium saturation of samples under controlled conditions and subsequent helium analysis as 0.066 ± 0.037% of the bulk sample volume (hereafter, mean values for the sequence are given). The measurement of helium concentration in initial samples allowed us to determine the partial pressure of helium in the gas phase of HAV as 0.47 ± 0.16 atm; the concentration of helium in the pore water was calculated as the product of pressure and the Henry solubility constant corresponding to the conditions of natural rock occurrence: 0.0036 ± 0.0016 cm 3 STP (He)/cm 3 (H 2 O). This estimate based on mineral-water equilibrium is in good agreement with the results of direct measurements if helium content in water from the two aquifers of the Permian-Carboniferous sequence of the Molasse Basin, where helium concentrations were found to be 0.0045 and 0.0016 cm 3 STP (He)/cm 3 (H 2 O). The obtained results validated the method. A necessary prerequisite for its application is the equilibrium of helium concentrations in the water-HAV system. The investigation of helium mobility in quartz grains (through the measurement of helium release rates from the grains under isothermal heating at different temperatures) allowed us to estimate the time of equilibration as approximately 103 yr, which is much shorter than the helium residence time in the Permian-Carboniferous sequence of the basin (more than 10 7 yr). Using quartz grains as detectors of helium concentration in pore waters, it was shown that the waters of the whole sequence are stagnant, and the sequence is appropriate for the disposal of high-level radioactive wastes.

The importance of shales layers being a principal source of helium isotopes in adjacent sandstone layers has been shown for the aquifer-aquitard system of the Permo-Carboniferous Trough (PCT) in the Molasse Basin, Northern Switzerland (e.g., Tolstikhin et al., 1996; Lehmann et al., 2003). In this contribution we define processes responsible for He atoms behaviour in PCT and model these processes in order to quantify helium migration parameters. Using U, Th, K and Li concentrations as well as He and Ar isotope abundances obtained for the shale-sandstonewater system of the PCT, we show that: (i) the inventory of the parent elements and He isotopes indicates an intra-basin production and loss of radiogenic helium, (ii) sandstones contribute only little to the inventory of both radiogenic helium isotopes, but serve also as a sink (!) for shale-produced helium and (iii) the simplest estimate of the residence time of He-atoms (as a ratio of He abundance over its production) is long, approaching 100 Ma. Processes controlling the He isotope abundance in PCT include: (i) the production of radiogenic He isotopes in the rocks, mainly in shales, which are enriched in the parent elements, (ii) the loss of radiogenic He atoms from the rock minerals into porewaters, (iii) the migration of He along concentration gradients from the porewater of the shale layers into the porewater of the sandstone layers and there into lattice defects of sandstone-forming minerals such as quartz and feldspars; (iv) the migration of He atoms in the porewater through the connected interstitial pore space towards the aquifer groundwaters with low He concentrations from where it is removed from the system. We model these processes using simple diffusion model envisaging the following conditions: (i) last hydrothermal overprint of the Crystalline basement (infinite thick layer, 315 Ma ago) and subsequent formation of the Permian (285 Ma) to Muschelkalk (240 Ma) sediment layers, each of them including observed aquifers; (ii) helium production in each layer as a function of the average U, Th and Li concentrations; (iii) helium diffusion in each layer along the concentration gradients; (iv) helium removal from the layers via migration into sandstone minerals and/or by flowing groundwater in the aquifers. Considering diffusion as the only mechanism of helium migration across the different rock layers yields reasonable agreement between the calculated concentrations and those observed in the PCT groundwaters for diffusion coefficients with a best fit value of 0.001 square m / yr. Such diffusion coefficient is similar to that derived by Rubel et al. (2002) for Opalinus Clay aquitard at Mont Terri, Switzerland. However, diffusion alone is not able to remove enough He from rock-groundwater system of the Crystalline basement. There the required advective removal of He by the groundwater suggest a He residence time within the Crystalline aquifers of approximately 1 Ma. These estimates indicate that the PCT rock-groundwater system is an appreciably stagnant one: the residence time of the highly mobile helium atoms in this system is longer than 10 Ma.