B. G. Polyak

Isotopic Composition of Thermal Waters in Chukotka

Twenty three groups of thermomineral springs in eastern Chukotka with the discharge temperature of 2 to 97°C and mineralization of 1.47 to 37.14 g/l are studied and compared with surface freshwater from their localities. The δD and δ18O values in surface waters vary from −121.4 to −89.5‰ and from −16.4 to −11.1‰, respectively, while respective values in thermomineral waters range from −134.2 to −92.5‰ and from −17.6 to −10.5‰. The δD value in surface waters decreases from the east to west, i.e., toward interior areas of the peninsula. Hydrothermal springs most depleted in deuterium (δD < −120‰) are localized in the geodynamically active Kolyuchinskaya-Mechigmen Depression. According to the proposed formation model of Chukotka thermomineral waters, their observed chemical and isotopic characteristics could result from the mixing (in different proportions) of surface waters with the deep-sourced isotopically light mineralized component (δD ≈ −138‰, δ18O ≈ −19‰, M = 9.5−14.7 g/l). The latter originates most likely from subpermafrost waters subjected to slight cryogenic metamorphism.

Recent tectonomagmatic reactivation of the Kolyuchino-Mechigmen zone of the Chukchi Peninsula from data on the composition of gases in hydrothermal springs

Thirty-three groups of thermal mineral springs in the Chukchi Peninsula are studied. The thermal fluids of the Kolyuchino-Mechigmen Zone (KMZ) show specific characteristics. The zone corresponds to recent depression with occurrences of basic volcanism and high seismicity. Characteristic features of the gases in the springs of the KMZ include the predominance of carbon dioxide, high percentage of mantle-derived helium, enrichment of CO2 in 13C, admixture of nonatmospheric nitrogen, excess in 40Ar and 15N relative to the atmosphere, maximal depletion of thermal water in deuterium and heavy oxygen isotope, and the highest estimates of deep temperature in reservoirs of hydrothermal fluid. All these features indicate the contemporary ascent of a mantle diapir that supplied helium enriched in 3He and thermal energy to the crust, thus provoking thermal metamorphism of rocks and degradation of permafrost.

Waters from mud volcanoes of Azerbaijan: Isotopic-geochemical properties and generation environments

New data on chemical and isotopic properties of waters from 35 mud volcanoes of Azerbaijan show that they are represented by two contrasting types characterized by different mineralization: Cl-Na (M = ∼30–80 g/L) and HCO3-Cl-Na (M = ∼8–15 g/L). Waters of the last type are usually enriched with alkaline metals, B, and Br. Waters of the Cl-Na type are similar in their Cl/Br coefficient with seawater. According to Mg-Li and Na-Li geothermometers, waters were formed at temperatures varying from ∼20 to ∼140°C. Taking into consideration these estimates, “roots” of the volcanoes under consideration should be located at depths of 6.0–7.5 km. The fluid generation temperatures exhibit a tendency for their growth toward the Greater Caucasus orogen. The δD, δ18O, and δ13C (TDIC) values in dissolved inorganic carbon vary from −32 to −12 and −0.6 to +10.4‰ (V-SMOW) or −12.9 to +37.3‰ (V-PDB), respectively. The highest δD, δ18O, and δ13C values are typical of HCO3-Na waters. It is shown that the growth of formation (Mg-Li) temperatures is accompanied by a significant increase of the δ18O(H2O) and δ13C(HCO3−) values along with HCO3− concentrations. The degree of water enrichment with Br, B, and alkaline metals also increases. This allows the formation of soda waters from mud volcanoes with elevated 18O and 13C contents to be attribute to relatively high-temperature transformations of mineral and organic matter during the catagenic alteration sedimentary rocks.

Role of radiogenic heat generation in surface heat flow formation

Heat generation due to decay of long-lived radioactive isotopes is considered in the Earth’s crust of the Archean–Proterozoic and Paleozoic provinces of Eurasia and North America. The heat flow that forms in the mantle is calculated as the difference between the heat flow observed at the boundary of the solid Earth and radiogenic heat flow produced in the crust. The heat regime in regions with anomalously high radiogenic heat generation is discussed. The relationship between various heat flow components in the Precambrian and Phanerozoic provinces has been comparatively analyzed, and the role of erosion of the surfaceheat- generating layer has been estimated.

Geothermal models of various geodymanic settings

The distribution of heat flow, deep temperature, and helium isotope ratios in axial spreading zones of mid-ocean ridges and in scattered spreading zones in backarc basins are considered, as well as in active segments of transform fracture zones, intra- and pericontinental rift zones, linear and mosaic Paleozoic fold-belts, and loading and extension sedimentary basins. The heat flow in these structural elements varies widely from 15 to 1500 mW/m2, and the thickness of the thermal lithosphere is correspondingly variable. The quantitatively estimated radiogenic heat generation in Paleozoic foldbelts provides 40–50% of the background heat flow. A time-dependent heat flow is characteristic of not only recent but also Late Paleozoic tectonic belts. The origin of positive and negative geothermal anomalies has been explained. Localization of hydrocarbon fields in sedimentary basins is linked to these anomalies.

Isotope and chemical composition of gases from mud volcanoes in the Taman Peninsula and problem of their genesis

Variations in the carbon isotope composition in gases and waters of mud volcanoes in the Taman Peninsula are studied. The δ13C values in CH4 and CO2 vary from −59.5 to −44.0‰ (δ13Cav = −52.4 ± 5.4‰) and from −17.8 to +22.8‰ (δ13Cav = +6.9 ± 9.3‰), respectively. In waters from most mud volcanoes of the peninsula, this parameter ranges from +3.3 to +33.1‰, although locally lower values are also recorded (up to −12‰. Fractionation of carbon isotopes in the CO2-HCO3 system corresponds to the isotope equilibrium under Earth’s surface temperatures. The growth of carbon dioxide concentration in the gaseous phase and increase in the HCO3 ion concentration in their water phase is accompanied by the enrichment of the latter with the heavy 13C isotope. The δ13CTDIC value in the water-soluble carbon depends on the occurrence time of water on the Earth’s surface (exchange with atmospheric CO2, methane oxidation, precipitation of carbonates, and other processes), in addition to its primary composition. In this connection, fluctuations in δ13CTDIC values in mud volcanoes with stagnant waters may amount to 10–20‰. In the clayey pulp, concentrations of carbonate matter recalculated to CaCO3 varies from 1–4 to 36–50 wt %. The δ13C value in the latter ranges from −3.6 to +8.4‰. Carbonate matter of the clayey pulp represents a mixture of sedimentogenic and authigenic carbonates. Therefore, it is usually unbalanced in terms of the carbon isotope composition with the water-soluble CO2 forms.

Strontium and oxygen isotopic systems in waters of mud volcanoes of the Taman Peninsula

Ten of eleven analyzed water samples from mud volcanoes of the Taman Peninsula are characterized by 87Sr/86Sr ratio within 0.70734–0.70957, which overlaps the values typical of the Mesozoic and Cenozoic sedimentary carbonates, but sharply differs from the value in the clayey sediments of the Maikop Group (0.7157 ± 0.0022). These data indicate that the strontium isotopic composition is mainly defined by carbonate reservoirs, with relatively little effect of elision solutions, input of which is noticeable only in the water of Gladkovsky Volcano (87Sr/86Sr = 0.71076). The high δ18O in mud volcanic waters (up to 14.2‰) can also be attributed to ionic exchange with sedimentary carbonates at temperatures around 150°C.

Isotopic-geochemical peculiarities of gases in mud volcanoes of eastern Georgia

Isotopic-geochemical study revealed the presence of mantle He (3He/4He up to 223 × 10−8) in gases from mud volcanoes of eastern Georgia. This fact confirms that the Middle Kura basin fill encloses an intrusive body previously distinguished from geophysical data. Wide variations in the carbon isotopic composition δ13C of CH4 and CO2 and the chemical composition of gas and water at a temporally constant 3He/4He ratio indicate their relation with crustal processes. Unusual direct correlations of the 3He/4He ratio with the contents of He and CH4 and the 40Ar/36Ar ratio can be explained by the generation of gas in the Cenozoic sequence of the Middle Kura basin.

Special features of heat flow in transform faults of the North Atlantic and Southeast Pacific

The paper reports on the morphostructure and heat flow in zones of transform faults of the North Atlantic and the Southeast Pacific, focusing on the fundamental difference between heat flow in active and inactive parts of the faults. In the active parts, which are located between segments of the mid-ocean ridge (MOR), the measured heat flow is close to that observed in the rift zones of MORs. The heat flow is considered a joint effect of the thermal conductivity of the oceanic crust and convective heat and mass transfer by thermal waters inside the oceanic crust. In the inactive parts of the faults, with distance from the MOR, the heat flow decreases to the background rates typical of thalassocratons. The sedimentation rate in a fault zone and conductive heat flow refraction resulting from the heterogeneous thermal characteristics of the geological section are the factors that deflect heat flow.

Conditions of the formation of thermomineral waters in the Talysh fold zone of the Lesser Caucasus (Azerbaijan) based on isotope-geochemical data (3Не/4Не,

It is shown that the gas and water phases of the thermal nitrogen–methane waters in the Talysh fold zone of the Lesser Caucasus mountain system contain helium and strontium with mantle isotope signatures (3Не/4Не from 200 × 10–8 to 401 × 10–8 and 87Sr/86Sr from 0.70490 to 0.70562). At the same time, clear signs of the mantle component in other gases (nitrogen, methane, and carbon dioxide) are absent. The δ15N value in nitrogen varies from +0.3 to +1.7‰, methane is mainly characterized by δ13C from–57.4 to–38.0‰, while δ13C(CО2) varies from–24.4 to–11.3‰. An increase of the CО2 content is accompanied by the decrease of δ13C in CО2, against the background of increasing SO4 content in the salt composition of waters. This indicates a microbial nature of CO2 in the studied gases. Thus, the presence of mantle helium and strontium in the thermal waters is likely related to their leaching from the Pleogene–Neogene host volcanic rocks. The studies of the oxygen and hydrogen isotope composition in water revealed quite different mechanisms for the formation of cold and thermal waters of the region. The cold waters are mainly fed by local infiltration, whereas the feeding of thermal nitrogen–methane waters is strongly provided by transit atmogenic waters (>50%), which are formed in the mountain ranges at altitudes no less than 1600 m and spaced at 20–40 km or more from the thermal discharge sites.It is shown that the gas and water phases of the thermal nitrogen–methane waters in the Talysh fold zone of the Lesser Caucasus mountain system contain helium and strontium with mantle isotope signatures (3Не/4Не from 200 × 10–8 to 401 × 10–8 and 87Sr/86Sr from 0.70490 to 0.70562). At the same time, clear signs of the mantle component in other gases (nitrogen, methane, and carbon dioxide) are absent. The δ15N value in nitrogen varies from +0.3 to +1.7‰, methane is mainly characterized by δ13C from–57.4 to–38.0‰, while δ13C(CО2) varies from–24.4 to–11.3‰. An increase of the CО2 content is accompanied by the decrease of δ13C in CО2, against the background of increasing SO4 content in the salt composition of waters. This indicates a microbial nature of CO2 in the studied gases. Thus, the presence of mantle helium and strontium in the thermal waters is likely related to their leaching from the Pleogene–Neogene host volcanic rocks. The studies of the oxygen and hydrogen isotope composition in water revealed quite different mechanisms for the formation of cold and thermal waters of the region. The cold waters are mainly fed by local infiltration, whereas the feeding of thermal nitrogen–methane waters is strongly provided by transit atmogenic waters (>50%), which are formed in the mountain ranges at altitudes no less than 1600 m and spaced at 20–40 km or more from the thermal discharge sites.