Geochemistry of isotopes (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and depositional environment of the upper Kazanian carbonate sediments in the Volga-Vyatka interfluve

This work reports the results of lithological and isotopic study of carbonate rocks from the Pechishchi stratotype section (Kazan) and three adjacent sections of Kazanian rocks of the Volga-Vyatka region at the eastern Russian Platform. These sections were recovered by the Kremeshki, Popovtsevo, and Chimbulat quarries (near the town of Sovetsk, southeastern Kirov district). Lithological features and wide variations of δ¹³C (from −6.0 to 6.8‰) and δ¹⁸ O (from 22.9 to 33.4 ‰) indicate that the rocks were formed in a shallow-marine basin with rapidly varying conditions of sedimentation which characterize different facies (and/or paleoecological) zones: lagoonal, supralittoral, littoral, shoal. They also suggest processes of postsedimentary alteration (mainly, under supergene conditions). Numerous short-term hiatuses are also recorded.     

Isotope (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genetic features of manganese carbonates of the Mazul deposit (Krasnoyarsk region)

New isotope and mineral data on manganese carbonates of the Mazul deposit (Krasnoyarsk region) in combination with morphology of ore bodies suggest that the ores were formed in several stages with the involvement of meteoric solutions through infiltration and, possibly, exfiltration mechanisms. Based on the geological–geochemical data, manganese carbonates of the Mazul deposit may be ascribed to a new genetic subtype of the catagenesis (epigenesis) zone.     

Isotopic composition (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis of carbonates from the famennian manganiferous formation of Pai-Khoi

The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ¹³C values from–6.6 to 1.3‰ and δ¹⁸O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ¹³C from–5.2 to–2.9 and δ¹⁸O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ¹³C from–16.4 to–13.1 and δ¹⁸O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ¹³C from–13.0 to–10.4‰ and δ¹⁸O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ¹⁸O from 18.7 to 17.6‰ and δ¹³C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO₂. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.     

A superlarge deposit—Kalahari manganese ore field (Northern Cape,South Africa): Geochemistry of isotopes (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis

Study of carbonate inclusions in the Mamatwan-type oxide ores (braunite lutites) made it possible to reveal residual microbial structures in them. Their presence in manganese ores, the type of manganese oxide mineralization, isotope signatures of manganese carbonates, and available literature data on the geology of the Kalahari manganese field suggest that manganese ore formation was widespread at the postsedimentary stage of transformation of the primary microbial sediment. The deposit can be referred to the hydrothermal-metasomatic type.     

Sr chemostratigrphy, δ<Superscript>13</Superscript>C,and δ<Superscript>18</Superscript>O of rocks in the Crimean carbonate platform (Late Jurassic,northern Peri-Tethys)

The paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The ⁸⁷Sr/⁸⁶Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ¹⁸O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in ¹⁸O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ¹³C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ¹³C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys.     

Isotopic Composition (δ<Superscript>13</Superscript>C, δ<Superscript>18</Superscript>O) and the Origin of Carbonates from Manganese Deposits of the Southern Urals

Isotopic compositions of carbon (δ¹³C from −51.4 to −10.8 PDB) and oxygen (δ¹⁸O from 14.4 to 21.4 SMOW) were studied in rhodochrosite and calcite from manganese ores in the South Faizuly and Kyzyltash deposits of the southern Urals. The geological, petrographic, and isotopic data indicate that the studied carbonates are diagenetic formations. It is suggested that the main ore element (Mn) was delivered to the marine basin with hydrothermal solutions percolating in the oceanic crust. Manganese precipitated on the oceanic bottom as oxides near solution discharge zones. Manganese carbonates formed in sediments as a result of the oxidation of organic matter by manganese oxides. High biological productivity of the environment was caused by proximity to the hydrothermal vent that provided favorable biogeochemical conditions for the development of biocoenosis. Anomalously low ¹³C values in the South Faizuly deposit testify to the large-scale oxidation of methane in the course of manganese carbonate formation.__________Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2005, pp. 416–429.Original Russian Text Copyright © 2005 by Kuleshov, Brusnitsyn.     

Using stable isotopes (δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr) to identify sources of water in abandoned mines in the Fengfeng coal mining district,northern China

With depleted coal resources or deteriorating mining geological conditions, some coal mines have been abandoned in the Fengfeng mining district, China. Water that accumulates in an abandoned underground mine (goaf water) may be a hazard to neighboring mines and impact the groundwater environment. Groundwater samples at three abandoned mines (Yi, Er and Quantou mines) in the Fengfeng mining district and the underlying Ordovician limestone aquifer were collected to characterize their chemical and isotopic compositions and identify the sources of the mine water. The water was HCO₃·SO₄-Ca·Mg type in Er mine and the auxiliary shaft of Yi mine, and HCO₃·SO₄-Na type in the main shaft of Quantou mine. The isotopic compositions (δD and δ¹⁸O) of water in the three abandoned mines were close to that of Ordovician limestone groundwater. Faults in the abandoned mines were developmental, possibly facilitating inflows of groundwater from the underlying Ordovician limestone aquifers into the coal mines. Although the Sr²⁺ concentrations differed considerably, the ratios of Sr²⁺/Ca²⁺ and ⁸⁷Sr/⁸⁶Sr and the ³⁴S content of SO₄^2? were similar for all three mine waters and Ordovician limestone groundwater, indicating that a close hydraulic connection may exist. Geochemical and isotopic indicators suggest that (1) the mine waters may originate mainly from the Ordovician limestone groundwater inflows, and (2) the upward hydraulic gradient in the limestone aquifer may prevent its contamination by the overlying abandoned mine water. The results of this study could be useful for water resources management in this area and other similar mining areas.     

Reconstruction of palaeohydrological conditions in a lagoon during the 2nd Zechstein cycle through simultaneous use of δD values of individual n-alkanes and δ<Superscript>18</Superscript>O and δ<Superscript>13</Superscript>C values of carbonates

For the first time ¹⁸O and ¹³C values from carbonates and D values of individual n-alkanes were used to reconstruct palaeohydrological conditions in a lagoon at the southern margin of the Central European Zechstein Basin (CEZB). A 12-m core covering the complete Ca2 interval and adjacent anhydrites (A1 and A2) was analyzed for ¹⁸O and ¹³C values of dolomitized carbonates and D values of individual n-alkanes. ¹⁸O_carb values (+2 to +5 vs. VPDB) were strongly influenced by evaporation and temporal freshwater input into the lagoon. The ¹³C_carb values (–1 to +4 vs. VPDB) were controlled mainly by changes in primary production. Both isotopic ratios show an inverse relationship throughout most of the core, contradicting diagenetic alteration, since ¹³C_carb values are not altered significantly during dolomitization. Assuming a temperature range of 35–40 °C in the lagoon, ¹⁸O_carb values of +2.5 to +8 (vs. VSMOW) for the lagoonal water can be reconstructed. The lagoon may have desiccated twice during the Ca2 interval, as indicated by very high ¹⁸O_carb and low ¹³C_carb values, coinciding with increasing amount of anhydrite in the analyzed sample. These events seem to reflect not just local but a regional intra-Ca2 cyclicity. Measured D values of the short-chain n-alkanes, namely n-C₁₆ and n-C₁₈ which are widely used as indicators for photosynthetic bacterial and algal input, reflect the isotopic composition of the lagoonal water. Assuming constant fractionation during incorporation of hydrogen into lipids of –160, an average D value of +70 (vs. VSMOW) can be reconstructed for the lagoonal water, accounting for very arid conditions. The long-chain n-alkanes n-C₂₇, n-C₂₈, n-C₂₉ and n-C₃₀, thought to be derived from the leaf waxes of terrestrial higher plants, were shown to be depleted in D relative to the short-chain alkanes, therefore indicating a different hydrogen source. Terrestrial plants in arid areas mainly use water supplied by precipitation. By using a smaller fractionation of –120 due to evaporation processes in the leaves, reconstructed values vary between –74 and –9 (vs. VSMOW). These values are not indicating extremely arid conditions, implying that the long-chain n-alkanes were transported trough wind and/or rivers into the lagoon at the Zechstein Sea coast. D_water values, reconstructed using the n-C₁₆ alkane and ¹⁸O _water values, independently reconstructed on the same sample using the temperature dependant fractionation for dolomites are good agreement and suggest high amounts of evaporation affecting the coastal lagoon. Altogether, our results indicate that hydrogen isotopic ratios of n-alkanes give information on their origin and are a useful proxy for palaeoclimatic reconstruction.     

Isotope (δD, δ<Superscript>18</Superscript>О) systematics in waters of the Russian Arctic seas

Oxygen and hydrogen isotope analysis was performed to study the processes of distribution of water masses and modification of their salinity in the Russian Arctic seas. A wealth of new isotopic data was obtained for freshwater (river runoff, Novaya Zemlya glaciers) and seawater samples collected along a set of extended 2D profiles in the Barents, Kara, and Laptev Seas. The study presents the first δD values measured for the Northeast Atlantic Deep Water NEADW dominated the water column of the Barents Sea (S = 34.90 ± 0.05, δD = +1.55 ± 0.4‰, δ¹⁸O = +0.26 ± 0.1‰, n = 44). This water mass is present in the Kara Sea and western Laptev Sea. The relationship between δD, δ18О, and salinity data was used to calculate the fractions of waters of different origin, including the fractions of continental runoff in waters of the Barents, Kara, and Laptev Seas. It was shown that the relationships between the isotopic parameters (δD, δ¹⁸О) and salinity in waters of the Kara and Laptev Seas is controlled by the intensity of continental runoff and sea ice processes. Sea ice formation is the main factor controlling the formation of the water column on the Laptev Sea shelf, whereas the surface waters of the middle Kara Sea are dominated by the contribution of river runoff. A very strong stratification in the Kara Sea is caused by the presence of a relatively fresh surface layer mostly contributed by estuarine water inputs from the Ob and Yenisei Rivers. The contribution of river waters reaches 40–60% in the surface layer in the central part of the sea and decreases to a few percent down 100 m water depth. Stratification in the western part of the Laptev Sea is controlled by the contribution of freshwater input from the Lena River and modification of salinity by sea ice formation.     

Carbonate Rocks from the Riphean–Vendian Boundary Deposits of the Anabar Uplift: Isotope (<Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>13</Superscript>C, δ<Superscript>18</Superscript>O) Systematics and Chemostratigraphic Consequences

New ⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ¹⁸О chemostratigraphic data were obtained for carbonate rocks of the Lower Riphean Yusmastakh and the Vendian Starorechenskaya formations. The δ¹³С values in dolomites of the Yusmastakh Formation varies from–0.6 to–0.1‰ and in dolomites and dolomitic limestones of the Starorechenskaya Formation, from–1.2 to–0.4‰ PDB, and δ¹⁸О values, from 24.4 to 26.4‰ and from 25.3 to 27.6‰ SMOW, respectively. The Rb–Sr systematics of carbonate rocks was studied using the refined method of stepwise dissolution of samples in acetic acid, including chemical removal of up to one-third of the ground sample by preliminary acid leaching and subsequent partial dissolution of the rest of the sample. Owing to this procedure, secondary carbonate material is removed, which enables one to improve the quality of the Sr-chemostratigraphic data obtained. The initial ⁸⁷Sr/⁸⁶Sr ratios in carbonate rocks of the Yusmastakh (0.70468–0.70519) and Starorechenskaya (0.70832–0.70883) formations evidence the Riphean–Vendian boundary in the Precambrian sequence of the Anabar Uplift.     

Behavior of isotope (<Superscript>18</Superscript>O/<Superscript>16</Superscript>O, <Superscript>234</Superscript>U/<Superscript>238</Superscript>U) systems during the formation of uranium deposits of the “sandstone” type

The uneven character of the distribution of ¹⁸O/¹⁶O and ²³⁴U/²³⁸U values was established in the vertical cross section of the productive sequence of the Dybryn uranium deposit (Vitim uranium-ore region, Buryatia). Both a deficiency and an excess of ²³⁴U in relation to the equilibrium ²³⁴U/²³⁸U ratio in the vertical sequence may provide evidence for the extremely low rate of the infiltration water flow. The behavior of oxygen isotope characteristics for different size fractions of terrigenous rocks provides evidence for active uranium redistribution and openness of the isotope system of this element during interaction of terrigenous–sedimentary rocks with infiltration waters.     

Sources and formation conditions of sulfide-silicate magmas in the Noril’sk district

Geology, tectonomagmatic reactivation of the Noril??sk district, as well as stratigraphy and geochemistry of the volcanic sequence are considered. Sources and formation mechanism of ore-bearing magma and the scope of ore formation are discussed. The Permian-Triassic flood-basalt magmatism of the Noril??sk district developed in part of the Siberian Platform with Archean-Paleoproterozoic basement broken into blocks and overlapped by a sedimentary cover up to 13 km thick and a volcanic sequence reaching 3.7 km in thickness. The geophysical data show that remnants of the subducted ancient oceanic crust exist in the mantle and fragments of transitional magma chambers and conduits are retained at different levels of the Earth??s crust. The cyclic tectonomagmatic evolution of the territory was characterized by alternation of extension with intense volcanic activity and compression accompanied by waning of volcanic eruptions. The early rifting, transitional stage, and late dispersed spreading are distinguished. The associations of volcanic (lavas and tuffs) and intrusive rocks were formed during each stage. The volcanic sequence is subdivided into 11 formations. The intrusions of the Talnakh and Noril??sk ore fields are distinguished by two-level structure with the Upper Noril??sk ore-bearing intrusions above and the Lower Noril??sk barren intrusions below. Two types of primary magmas differ in geochemistry of lavas and intrusions: (1) OIB-type high-Ti magma (iv, sv, gd formations of the first stage from bottom to top) and (2) low-Ti magma (hk, tk, nd formations of the second stage and mr-mk formations of the third stage). The nd formation depleted in ore elements and the ore-bearing cumulus composed of silicate and sulfide melts in combination with early silicate minerals and chromite are products of the fractionation of the primary low-Ti magma. As follows from geochemical parameters, intrusions of the Lower Noril??sk type are comagmatic to the evolved lavas of the nd₃ subformation, whereas intrusions of the Upper Noril??sk type are comagmatic to the lavas of the mr-mk formations. Geochemical similarity with volcanic rocks provides evidence for the composition of the initial magma and the time of intrusion emplacement. The ore-bearing intrusions of the Upper Noril??sk type were formed at the onset of the third stage, when the primitive low-Ti magma similar to the lavas of mr-mk formations in composition was emplaced. When intruding, this melt captured and transported ore-bearing cumulus (drops of sulfide melt, early olivine and chromite grains) into the magma chamber. Separate portions of sulfide liquid were involved into movement as a self-dependent intrusive subphase during formation of the Talnakh and Kharaelakh intrusions. An extremal effect of pressure on sulfur concentration in fluid-bearing and sulfide-saturated mafic magmas has been established in experiments to be P = 1?2 GPa. In this interval of pressure, the S concentration in sulfide-saturated magmas increases in the following sequence: dry magma ??(H₂O + CO₂)-bearing magma <H₂O-bearing magma. In the regions of low (<0.3 GPa) and high (>2.5 GPa) pressures, the S contents (0.1?C0.2 wt %) are commensurable. The extremal baric relationship of S concentration in fluid-bearing and sulfide-saturated mafic magmas may be important for the formation of ore-bearing magmas. The calculation results show that the amount of sulfides in the known deposits does not exceed 2% of geological resources of the sulfides separated from the flood basalts. Therefore, the chance of discovery of new deposits remains rather high. Proceeding from the conditions of ore-bearing magma formation and geological setting of the known deposits, criteria for recognition of potentially ore-bearing areas are proposed and such areas are outlined.     

Geochronological (U–Pb,U–Th–total Pb,Sm–Nd) and geochemical (REE, <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>18</Superscript>O, δ<Superscript>13</Superscript>C) tracing of intraplate tectonism and associated fluid flow in the Warburton Basin,Australia

The Warburton Basin of central Australia has experienced a complex tectonic and fluid-flow history, resulting in the formation of various authigenic minerals. Geochemical and geochronological analyses were undertaken on vein carbonates from core samples of clastic sediments. Results were then integrated with zircon U–Pb dating and uraninite U–Th–total Pb dating from the underlying granite. Stable and radiogenic isotopes (δ¹⁸O, Sr and εNd), as well as trace element data of carbonate veins indicate that >200 °C basinal fluids of evolved meteoric origin circulated through the Warburton Basin. Almost coincidental ages of these carbonates (Sm–Nd; 432 ± 12 Ma) with primary zircon (421 ± 3.8 Ma) and uraninite (407 ± 16 Ma) ages from the granitic intrusion point towards a substantial period of active tectonism and an elevated thermal regime during the mid Silurian. We hypothesise that such a thermal regime may have resulted from extensional tectonism and concomitant magmatic activity following regional orogenesis. This study shows that the combined application of geochemical and geochronological analyses of both primary and secondary species may constrain the timing of tectonomagmatic events and associated fluid flow in intraplate sedimentary basins. Furthermore, this work suggests that the Sm–Nd-isotopic system is surprisingly robust and can record geologically meaningful age data from hydrothermal mineral species.     

Chemical and isotopic investigations (δ<Superscript>18</Superscript>O, δ<Superscript>2</Superscript>H, <Superscript>3</Superscript>H, <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr) to define groundwater processes occurring in a deep-seated landslide in flysch

Deep-seated landslides are complex systems. In many cases, multidisciplinary studies are necessary to unravel the key hydrological features that can influence their evolution in space and time. The deep-seated Berceto landslide, in the northern Apennines of Italy, has been investigated in order to define the origin and geochemical evolution of groundwater (GW), to identify the slope system hydrological boundary, and to highlight the GW flow paths, transit time and transfer modalities inside the landslide body. This research is based on a multidisciplinary approach that involves monitoring GW levels, obtaining analyses of water chemistry and stable and unstable isotopes (δ¹⁸O-δ²H, ³H, ⁸⁷Sr/⁸⁶Sr), performing soil leaching tests, geochemical modelling (PHREEQC), and principal component analysis (PCA). The results of δ¹⁸O-δ²H and ⁸⁷Sr/⁸⁶Sr analyses show that the source of GW recharge in the Berceto landslide is local rainwater, and external contributions from a local stream can be excluded. In the landslide body, two GW hydrotypes (Ca-HCO₃ and Na-HCO₃) are identified, and the results of PHREEQC and PCA confirm that the chemical features of the GW depend on water–rock interaction processes occurring inside the landslide. The ³H content suggests a recent origin for GW and appears to highlight mixing between shallow and deep GW aliquots. The ³H content and GW levels data confirm that shallow GW is mainly controlled by a mass transfer mechanism. The ³H analyses with GW levels also indicate that only deep GW is controlled by a pressure transfer mechanism, and this mechanism is likely the main influence on the landslide kinematics.     

Central Paratethys paleoenvironment during the Badenian (Middle Miocene): evidence from foraminifera and stable isotope (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) study in the Vienna Basin (Slovakia)

Stable isotope data of the foraminiferal carbonate shells and bulk sediment samples from the Central Paratethys were investigated to contribute to better knowledge of the paleoenvironmental changes in Badenian (Middle Miocene). Five benthic (Uvigerina semiornata, U. aculeata, Ammonia beccarii, Elphidium sp. and Heterolepa dutemplei) and three planktonic taxa (Globigerina bulloides, G. diplostoma and Globigerinoides trilobus), characterising the bottom, intermediate and superficial layers of the water column, were selected from the Vienna Basin (W Slovakia). The foraminiferal fauna and its isotope signal point out to temperature-stratified, nutrient-rich and consequently less-oxygenated marine water during the Middle/Late Badenian. Negative carbon isotope ratios indicate increased input of ¹²C-enriched organic matter to the bottom of the Vienna Basin. Positive benthic δ¹⁸O implies that the global cooling tendency recorded in the Middle Miocene has also affected the intramountain Vienna Basin. In this time, the Central Paratethys has been in the process of isolation. Our stable isotope trend suggests that the communication with Mediterranean Sea has been still more or less active on the south of Vienna Basin (Slovak part) in the Late Badenian, whereas the seawater exchange towards north was apparently reduced already during the Middle Badenian.     

Origin and evolution of oilfield brines from Tertiary strata in western Qaidam Basin: Constraints from <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and water chemistry

Chemistry of major and minor elements, ⁸⁷Sr/⁸⁶Sr, δD, δ¹⁸O and δ³⁴S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry data show that all oilfield brines are CaCl₂ type. They were enriched in Ca²⁺, B³⁺, Li⁺, Sr²⁺, Br⁻, and were depleted in Mg²⁺, SO₄ ²⁻, which indicated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ¹⁸O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line (QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from meteoric water, and then might be affected by water-rock metamorphose, because most oilfield brines distribute in the range of metamorphosing water. The ⁸⁷Sr/⁸⁶Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition, all oilfield brines have obviously much positive δ³⁴S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting in positive shift of δ³⁴S value and depleteed SO₄ ²⁻ in oilfield brines. Combined with water chemical data and δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, δ³⁴S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected by salt dissolution, water-rock metamorphose, sulfate reduction and dolomitization during the process of migration. These processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so on) for sustainable utilization of salt lake resources in the Qaidam Basin.     

Uniformly mantle-like δ<Superscript>18</Superscript>O in zircons from oceanic plagiogranites and gabbros

Lower ocean crust is primarily gabbroic, although 1–2% felsic igneous rocks that are referred to collectively as plagiogranites occur locally. Recent experimental evidence suggests that plagiogranite magmas can form by hydrous partial melting of gabbro triggered by seawater-derived fluids, and thus they may indicate early, high-temperature hydrothermal fluid circulation. To explore seawater–rock interaction prior to and during the genesis of plagiogranite and other late-stage magmas, oxygen-isotope ratios preserved in igneous zircon have been measured by ion microprobe. A total of 197 zircons from 43 plagiogranite, evolved gabbro, and hydrothermally altered fault rock samples have been analyzed. Samples originate primarily from drill core acquired during Ocean Drilling Program and Integrated Ocean Drilling Program operations near the Mid-Atlantic and Southwest Indian Ridges. With the exception of rare, distinctively luminescent rims, all zircons from ocean crust record remarkably uniform δ¹⁸O with an average value of 5.2 ± 0.5‰ (2SD). The average δ¹⁸O(Zrc) would be in magmatic equilibrium with unaltered MORB [δ¹⁸O(WR) ~ 5.6–5.7‰], and is consistent with the previously determined value for equilibrium with the mantle. The narrow range of measured δ¹⁸O values is predicted for zircon crystallization from variable parent melt compositions and temperatures in a closed system, and provides no indication of any interactions between altered rocks or seawater and the evolved parent melts. If plagiogranite forms by hydrous partial melting, the uniform mantle-like δ¹⁸O(Zrc) requires melting and zircon crystallization prior to significant amounts of water–rock interactions that alter the protolith δ¹⁸O. Zircons from ocean crust have been proposed as a tectonic analog for >3.9 Ga detrital zircons from the earliest (Hadean) Earth by multiple workers. However, zircons from ocean crust are readily distinguished geochemically from zircons formed in continental crustal environments. Many of the >3.9 Ga zircons have mildly elevated δ¹⁸O (6.0–7.5‰), but such values have not been identified in any zircons from the large sample suite examined here. The difference in δ¹⁸O, in combination with newly acquired lithium concentrations and published trace element data, clearly shows that the >3.9 Ga detrital zircons did not originate by processes analogous to those in modern mid-ocean ridge settings.     

Spatio-temporal variations of δ<Superscript>2</Superscript>H and δ<Superscript>18</Superscript>O in precipitation and shallow groundwater in the Hilly Loess Region of the Loess Plateau,China

Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of δ²H and δ¹⁸O in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004–2006 at 13 sites in the Chabagou Catchment (187 km²). In the Caopingxigou Experimental Watershed (0.1 km²), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The δ²H–δ¹⁸O relation of groundwater was found to be δ²H = 3.22 × δ¹⁸O − 38.1, deviating from the local meteoric water line δ²H = 7.57 × δ¹⁸O + 3.9. The range of δ values in groundwater is shrunken to be 15–21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of δ values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being −69 and −9.7‰, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.     

δ<Superscript>18</Superscript>O and δD variations in Holocene massive ice in the Sabettayakha river mouth,northern Yamal Peninsula

The conditions of formation of massive ice near the South Tambey gas-condensate field in northern Yamal Peninsula are studied. It is shown that massive ice bodies up to 4.5 m thick occur in the Holocene deposits of the high laida and the first terrace. Therefore, they cannot be the remains of glaciers; they are ground ice formations. All three types of massive ice have quite various isotopic compositions: the values of δD range from–107 to–199.7, and δ¹⁸O from–15.7 to–26.48‰. Such a significant differentiation in isotopic composition is a result of cryogenic fractionation in a freezing water-saturated sediment. The most negative isotope values are even lower in this Holocene massive ice than in the Late Pleistocene ice-wedge ice of Yamal Peninsula.