Authigenic carbonates in methane seeps from the Norwegian sea: Mineralogy, geochemistry, and genesis

Authigenic carbonates in the caldera of an Arctic (72°N) submarine mud volcano with active CH₄bearing fluid discharge are formed at the bottom surface during anaerobic microbial methane oxidation. The microbial community consists of specific methane-producing bacteria, which act as methanetrophic ones in conditions of excess methane, and sulfate reducers developing on hydrogen, which is an intermediate product of microbial CH₄ oxidation. Isotopically light carbon (δ¹³C_av =−28.9%0) of carbon dioxide produced during CH₄ oxidation is the main carbonate carbon source. Heavy oxygen isotope ratio (δ¹⁸O_av = 5%0) in carbonates is inherited from seawater sulfate. A rapid sulfate reduction (up to 12 mg S dm⁻³ day⁻¹) results in total exhausting of sulfate ion in the upper sediment layer (10 cm). Because of this, carbonates can only be formed in surface sediments near the water-bottom interface. Authigenic carbonates occurring within sediments occur do notin situ. Salinity, as well as CO ₃ ²⁻ /Ca and Mg/Ca ratios, correspond to the field of nonmagnesian calcium carbonate precipitation. Calcite is the dominant carbonate mineral in the methane seep caldera, where it occurs in the paragenetic association with barite. The radiocarbon age of carbonates is about 10000 yr.     

The Darasun gold deposit,Eastern Transbaikal region: Chemical composition,REE patterns,and stable carbon and oxygen isotopes of carbonates from ore veins

The chemistry, REE patterns, and carbon and oxygen isotopic compositions of carbonates from ore veins of the Darasun deposit are discussed. In addition to the earlier described siderite, calcite, and carbonates of the dolomite-ankerite series, kutnahorite is identified. The total REE content in Fe-Mg carbonates of the dolomite-ankerite series (2.8–73 ppm) is much lower than in later calcite (18–390 ppm). δ¹³C of Fe-Mg carbonates and calcite varies from +1.1 to −6.7‰ and from −0.9 to −4.9‰, respectively. δ¹⁸O of Fe-Mg carbonates and calcite varies from +17.6 to 3.6‰ and from +15.7 to −0.5‰, respectively. The REE sum and carbon and oxygen isotopic compositions reveal zonal distribution relative to the central granodiorite porphyry stock. The correlation between the carbon and oxygen isotopic compositions and REE sum reflects variations in the physicochemical formation conditions and composition of ore-forming fluid. The isotopic composition of fluid is calculated, and possible sources of its components are considered. Earlier established evidence for a magmatic source of ore-forming fluid and participation of meteoric water in ore formation is confirmed. Geochemical evidence for interaction of ore-forming fluid with host rocks is furnished. The relationships between the REE sum, on the one hand, and carbon and oxygen isotopic compositions of hydrothermal ore-forming fluid, on the other, are established.     

Origin of ultramafic-hosted magnesite on Margarita Island,Venezuela

Ultramafic-hosted deposits of magnesite (MgCO₃) have been studied on Margarita Island, Venezuela, to elucidate the source of carbon and conditions of formation for this type of ore. Petrographic, mineralogic, and δ¹⁸O data indicate that magnesite precipitated on Margarita in near-surface environments at low P and T. δ¹³C ranges from −9 to −16‰ PDB within the magnesite and −8 to −10‰ PDB within some calcite and dolomite elsewhere on the island. The isotopically light dolomite fills karst and the calcite occurs as stock-work veins which resemble the magnesite deposits. These carbon isotopic ratios are consistent with a deep-seated source rather than an overlying source from a zone of surficial weathering. However, there is not much enrichment of precious metals and no enrichment of heavy rare-earth elements, as would be expected if the carbon had migrated upward as aqueous carbonate ions. The carbon probably has risen as a gaseous mixture of CO₂ and CH₄ which partially dissolved in near-surface water before leaching cations and precipitating as magnesite and other carbonates. The process probably is ongoing, given regional exhalation of carbonaceous gases.     

The genesis of calcite in carbonate rocks of the sedimentary basins: Evidence from the stable carbon and oxygen isotopes composition

The distributions of stable carbon and oxygen isotopes in modern and ancient limestones of various types were studied. Carbonate samples from modern sediments were collected in the Black and Barents Seas. Ancient carbonates were represented by Upper Jurassic (Kimmeridgian-Tithonian) limestones from the central part of the West Siberian basin. Carbonate samples include remains of modern and Upper Jurassic fauna, carbonate crust from sediments of the Black Sea, carbonate tube from sediments of the Barents Sea, and Upper Jurassic limestone from the carbonate layer found at top of Abalak, bottom of Bazhenov deposits in the central part of the West Siberian basin. According to the results of the isotope analysis and comparison with modern carbonates, Upper Jurassic limestones of the West Siberian basin belong to the group of methane-derived carbonates and precipitated as a result of anaerobic oxidation of methane (AOM). Fractures in limestones are filled with secondary calcite.     

Stable isotope characterisation of co-existing carbonates from the Holocene Gallocanta lake (NE Spain): palaeolimnological implications

The sedimentary succession of Gallocanta lake, a closed saline lake located in the Iberian Range (NE Spain), documents two successive lacustrine stages: (1) brackish lake stage and (2) shallow saline lake stage. The saline stage corresponds to the present-day situation in which the lake water properties are mainly controlled by a strongly negative annual water balance. The carbonates of the brackish lake stage have relatively constant δ¹⁸O values, however, they are rather high (δ¹⁸O_Do = 2.4‰ and δ¹⁸O_Cc = 4.5‰ mean values) suggesting a hydrologically closed lake with a long residence time of the waters. δ¹⁸O values of carbonates from the saline stage vary greatly, and are lighter than in the previous stage (δ¹⁸O_Do = 0.5‰, δ¹⁸O_Cc = −0.7‰, δ¹⁸O_Mgs = −2.3‰ mean values). These carbonates also precipitated in a hydrologically closed lake, but in equilibrium with a lake water of more variable isotopic composition. The δ¹³C values for carbonates of both stages reflect a mixing of different pools of carbon, but during saline stage δ¹³C values have been more controlled by the equilibrium of the lake waters with atmospheric CO₂. During the current stage, calcite and dolomite precipitate in Gallocanta lake mainly during spring and summer, although dolomite precipitation is more favoured towards the summer. Magnesite precipitates at the beginning of autumn, when the first rainfall re-dissolves the saline surface crust, producing saline waters with a high Mg²⁺ content. The isotopic composition of lake waters sampled in 2005 are far higher than those calculated from the carbonates. It is considered that this could be due to two factors: either because there have not been many extremely dry years (like the year 2005) during the development of the lake, or because the physical and chemical characteristics of the lake waters in such conditions are not appropriate for the development of these minerals.     

Diagenesis of an Upper Triassic reef complex, Wilde Kirche, Northern Calcareous Alps, Austria

The Wilde Kirche reef complex (Early-Late Rhaetian) grew as an isolated carbonate structure within the shallow Kössen Basin. At the Triassic/Jurassic boundary a single brief (c. 10–50 ka) period of subaerial exposure occurred. The preserved karst profile (70 m thick) displays a vadose zone, enhanced dissolution at a possible palaeo-watertable (5–15 m below the exposure surface), and a freshwater phreatic zone. Karst porosity was predominantly biomouldic. Primary cavities and biomoulds were enlarged and interconnected in the freshwater phreatic zone; cavity networks developed preferentially in patch reef facies. Resubmergence of the reef complex allowed minor modification of the palaeokarst surface by sea floor dissolution and Fe-Mn crust deposition on a sediment-starved passive margin. Fibrous calcite (FC). radiaxial fibrous calcite (RFC) and fascicular optic calcite (FOC) cements preserved as low Mg calcite (LMC) are abundant in primary and karst dissolution cavities. FC cement is restricted to primary porosity, particularly as a synsedimentary cement at the windward reef margin. FC, RFC and FOC contain microdolomite inclusions and show patchy non-/bright cathodoluminescence. δ¹⁸O values of non-luminescent portions (interpreted as near original) are − 1.16 to − 1.82%0 (close to the inferred δ¹⁸O of calcite precipitated from Late Triassic sea water). δ¹³C values are constant (+3 to + 2.2%0). These observations suggest FC, RFC and FOC were originally marine high Mg calcite (HMC) precipitates, and that the bulk of porosity occlusion occurred not in the karst environment but in the marine environment during and after marine transgression. The HMC to LMC transition may have occurred in contact with meteoric water only in the case of FC cement. The most altered (brightly luminescent) portions of RFC/FOC cements yield δ¹⁸O=−2.44 to − 5.8%0, suggesting HMC to LMC alteration at up to 34°C. in the shallow burial environment at depths of 180–250 m. Abundant equant cements with δ¹⁸O =−4·1 to −7.1%0 show crisp, uniform or zoned dull luminescence. They are interpreted as unaltered cements precipitated at 33–36°C at 200–290 m burial depth, from marine-derived fluids under a slightly enhanced geothermal gradient. Fluids carrying the equant cements may have induced the HMC to LMC transition in the fibrous cements.     

Isotopic constraints on fluid evolution and precipitation mechanisms for the Boléo Cu–Co–Zn district, Mexico

Stable and radiogenic isotope composition of stratiform Cu–Co–Zn mineralization and associated sedimentary rocks within the Boléo district of the Miocene Santa Rosalía basin, Baja California Sur, constrains the evolution of seawater and hydrothermal fluids and the mechanisms responsible for sulfide and oxide deposition. Stable isotope geochemistry of limestone and evaporite units indicates a strong paleogeographic influence on the chemistry of the water column. Near-shore limestone at the base of the Boléo Formation is characterized by modified marine carbon (δ ¹³C_PDB=−6.0 to +4.4‰) and oxygen (δ ¹⁸O_SMOW=+19.5 to +26.2‰) isotope composition due to the influx of ¹³C- and ¹⁸O-depleted fluvial water. Sulfate sulfur isotope composition (δ ³⁴S_CDT=+17.21 to +22.3‰ and δ ¹⁸O_SMOW=+10.7 to +13.1‰) for basal evaporite and claystone facies are similar to Miocene seawater. Strontium isotopes are less radiogenic than expected for Miocene seawater due to interaction with volcanic rocks. Low S/C ratios, high Mn contents and sedimentological evidence indicate the basin water column was oxidizing. The oxygenated basin restricted sulfide precipitation to within the sedimentary pile by replacement of early diagenetic framboidal pyrite and pore-space filling by Cu–Co–Zn sulfides to produce disseminated sulfides. Quartz–Mn oxide oxygen isotope geothermometry constrains mineralization temperature between 18 and 118°C. Sulfur isotopes indicate the following sources of sulfide: (1) bacterial sulfate reduction within the sedimentary pile produced negative δ ³⁴S values (<−20‰) in framboidal pyrite; and (2) bacterial sulfate reduction at high temperature (80–118°C) within the sedimentary pile during the infiltration of the metal-bearing brines produced Cu–Co–Zn sulfides with negative, but close to 0‰, δ ³⁴S values. Isotope modeling of fluid-rock reaction and fluid mixing indicates: (1) sedimentary and marine carbonates (δ ¹³C=−11.6 to −3.2‰ and δ ¹⁸O=+19.0 to +21.8‰) precipitated from basin seawater/pore water that variably mixed with isotopically depleted meteoric waters; and (2) hydrothermal calcite (δ ¹³C=−7.9 to +4.3‰ and δ ¹⁸O=+22.1 to +25.8‰) formed by dissolution and replacement of authigenic marine calcite by downward-infiltrating metalliferous brine and brine-sediment exchange, that prior to reaction with calcite, had mixed with isotopically depleted pore water. The downward infiltration of metalliferous brine is inferred from lateral and stratigraphic metal distributions and from the concentration of Cu sulfides along the upper contact of pyrite-bearing laminae. The co-existence and textural relationships among framboidal pyrite, base metal sulfides, carbonate and Mn–Fe oxides (including magnetite) within mineralized units are consistent with carbonate replacement and high-temperature bacterial reduction within the sedimentary pile occurring simultaneously below a seawater column under predominantly oxygenated conditions.     

Contrasting stable isotope behavior between calcite and dolomite marbles,Lone Mountain,Nevada

 Late Proterozoic to Cambrian carbonate rocks from Lone Mountain, west central Nevada, record multiple post-depositional events including: (1) diagenesis, (2) Mesozoic regional metamorphism, (3) Late Cretaceous contact metamorphism, related to the emplacement of the Lone Mountain granitic pluton and (4) Tertiary hydrothermal alteration associated with extension, uplift and intrusion of silicic porphyry and lamprophyre dikes. Essentially pure calcite and dolomite marbles have stable isotopic compositions that can be divided into two groups, one with positive δ¹³C values from+3.1 to +1.4 ‰ (PDB) and high δ¹⁸O values from +21.5 to +15.8 ‰ (SMOW), and the other with negative δ¹³C values from –3.3 to –3.6‰ and low δ¹⁸O values from +16.9 to +11.1‰. Marbles also contain minor amounts of quartz, muscovite and phlogopite. Brown and blue luminescent, clear, smooth textured quartz grains from orange luminescent calcite marbles have high δ¹⁸O values from +23.9 to +18.1‰, while brown luminescent, opaque, rough textured quartz grains from red luminescent dolomite marbles typically have low δ¹⁸O values from +2.0 to +9.3‰. The δ¹⁸O values of muscovite and phlogopite from marbles are typical of micas in metamorphic rocks, with values between +10.4 and +14.4‰, whereas mica δD values are very depleted, varying from −102 to −156‰. No significant lowering of the δ¹⁸O values of Lone Mountain carbonates is inferred to have occurred during metamorphism as a result of devolatilization reactions because of the essentially pure nature of the marbles. Bright luminescence along the edges of fractures, quartz cements and quartz overgrowths in dolomite marbles, low δD values of micas, negative δ¹³C values and low δ¹⁸O values of calcite and dolomite, and depleted δ¹⁸O values of quartz from dolomite marbles all indicate that meteoric fluids interacted with Lone Mountain marbles during the Tertiary. Partial oxygen isotopic exchange between calcite and low ¹⁸O meteoric fluids lowered the δ¹⁸O values of calcite, resulting in uniform quartz-calcite fractionations that define an apparent pseudoisotherm. These quartz-calcite fractionations significantly underestimate both the temperature of metamorphism and the temperature of post-metamorphic alteration. Partial oxygen isotopic exchange between quartz and meteoric fluids also resulted in ¹⁸O depletion of quartz from dolomite marbles. This partial exchange was facilitated by an increase in the surface area of the quartz as a result of its dissolution by meteoric fluids. The negative δ¹³C values in carbonates result from the oxidation of organic material by meteoric fluids following metamorphism. Stable isotopic data from Lone Mountain marbles are consistent with the extensive circulation of meteoric hydrothermal fluids throughout western Nevada in Tertiary time. Received: 1 February 1994/Accepted: 12 September 1995     

Anaerobic oxidation of methane in coastal sediment from Guishan Island (Pearl River Estuary), South China Sea

The concentrations of CH₄, SO₄²⁻, σCO₂ and the carbon isotope compositions of ΣCO₂ and CH₄ in the pore-water of the GS sedimentary core collected from Guishan Island (Pearl River Estuary), South China Sea, were determined. The methane concentration in the pore-water shows dramatic changes and sulfate concentration gradients are linear at the base of the sulfate reduction zone for the station. The carbon isotope of methane becomes heavier at the sulfate-methane transition (SMT) likely because of the Raleigh distillation effect; ¹²CH₄ was oxidized faster than ¹³CH₄, and this caused the enrichment of residual methane δ ¹³C and δ ¹³C-ΣCO₂ minimum. The geochemical profiles of the pore-water support the existence of anaerobic oxidation of methane (AOM), which is mainly controlled by the quality and quantity of the sedimentary organic matter. As inferred from the index of δ ¹³C-TOC value and TOC/TN ratio, the organic matter is a mix of mainly refractory terrestrial component plus some labile alga marine-derived in the study area. A large amount of labile organic matter (mainly labile alga marine-derived) is consumed via the process of sedimentary organic matter diagenesis, and this reduces the amount of labile organic matter incorporated into the base of the sulfate reduction zone. Due to the scarcity of labile organic matter, the sulfate will in turn be consumed by its reaction with methane and therefore AOM takes place. Based on a diffussion model, the portion of pore-water sulfate reduction via AOM is 58.6%, and the percentage of ΣCO₂ in the pore-water derived from AOM is 41.4%. Thus, AOM plays an important role in the carbon and sulfur cycling in the marine sediments of Pearl River Estuary.     

西沙群岛晚第四纪碳酸盐岩淡水成岩作用

基于西沙永兴岛上最新钻孔（SSZK1）取得的55.92 m岩芯的 U?Th定年、矿物、薄片、主微量元素及碳氧稳定同位素等资料,开展了西沙群岛晚第四纪碳酸盐岩沉积相和淡水成岩作用的研究。根据不同的矿物组成特征,可将SSZK1钻孔岩芯分为上、中、下三段: 下 段（33.89~55.92 m,主要为低镁方解石）、中 段（18.39~33.89 m,主要为文石和低镁方解石）、上段（0~18.39 m,主要为文石、高镁方解石和低镁方解石）。由于下段碳酸盐岩几乎全为稳定的低镁方解石组成,碳氧同位素值的严重负偏和小幅度变化,可推断其经历了程度较大的淡水成岩作用。中段和上段还存在不稳定的文石和高镁方解石,碳氧同位素值相对下段正偏和高幅高频变化,推测其淡水成岩作用的程度比下段要小。中段碳氧同位素值高幅高频变化同时也说明该段的矿物纵向变化较复杂。这种矿物组成的复杂变化可能是由于晚第四纪海平面频繁变化,该段被大气水渗流带和潜流带交替占据引起的。主微量元素的变化同时受到淡水成岩作用和沉积环境的影响。在中段、下段中可识别出sq1、sq2、sq3、sq4四个完整的相旋回。Na₂O,S,Sr 和碳氧同位素受到的淡水成岩作用而被消耗和负偏,且由于老一期的旋回经历了更长时间的淡水成岩作用,新、老旋回间的 Na₂O,S,Sr含量值和碳氧同位素值有明显差异。利用新、老时期形成的旋回间淡水成岩作用剩余Na₂O,S,Sr含量和碳氧同位素值的差别可以将新、老两个旋回区分开来。     

Methane and sulfide fluxes in permanent anoxia: In situ studies at the Dvurechenskii mud volcano (Sorokin Trough, Black Sea)

The Dvurechenskii mud volcano (DMV) is located in permanently anoxic waters at 2060 m depth (Sorokin Trough, Black Sea). The DMV was studied during the RV Meteor expedition M72/2 as an example of an active mud volcano system, to investigate the significance of submarine mud volcanism for the methane and sulfide budget of the anoxic Black Sea hydrosphere. Our studies included benthic fluxes of methane and sulfide, as well as the factors controlling transport, consumption and production of both compounds within the sediment. The pie-shaped mud volcano showed temperature anomalies as well as solute and gas fluxes indicating high fluid flow at its summit north of the geographical center. The anaerobic oxidation of methane (AOM) coupled to sulfate reduction (SR) was repressed in this zone due to the upward flow of sulfate-depleted fluids through recently deposited subsurface muds, apparently limiting microbial methanotrophic activity. Consequently, the emission of dissolved methane into the water column was high, with an estimated rate of 0.46 mol m⁻² d⁻¹. On the wide plateau and edge of the mud volcano surrounding the summit, fluid flow and total methane flux were lower, allowing higher SR and AOM rates correlated with an increase in sulfate penetration into the sediment. Here, between 50% and 70% of the methane flux (0.07-0.1 mol m⁻² d⁻¹) was consumed within the upper 10 cm of the sediment. The overall amount of dissolved methane released from the entire mud volcano structure into the water column was significant with a discharge of 1.3 × 10⁷ mol yr⁻¹. The DMV maintains also high areal rates of methane-fueled sulfide production and emission of on average 0.05 mol m⁻² d⁻¹. This is a difference to mud volcanoes in oxic waters, which emit similar amounts of methane, but not sulfide. However, based on a comparison of this and other mud volcanoes of the Black Sea, we conclude that sulfide and methane emission into the hydrosphere from deep-water mud volcanoes does not significantly contribute to the sulfide and methane inventory of the Black Sea.     

Geochemical characteristics of sediment pore water from Site XS-01 in the Xisha trough of South China Sea and their significance for gas hydrate occurrence

Gas hydrate is a recently-found new source of energy that mostly exists in marine sediments. In recent years, we have conducted gas hydrate exploration in the South China Sea. The Xisha trough, one of the promising target areas for gas hydrate, is located in the northern margin of the South China Sea, adjacent to several large oil and gas fields. The Xisha trough extends 420 km long with the water depth of 1 500 m in the west part and 3 400 m in the east part and deposits thick sediments with organic matter content of 0.41%–1.02%. Previous studies on topographical features, geological P-T conditions, structural geology, sedimentary geology and geophysical bottom simulating reflectors (BSR) in the Xisha trough suggest that this area is favorable for the formation and accumulation of gas hydrate. In this paper, we present geochemical analyses for the sediment and pore water from a piston core at Site XS-01 in the Xisha trough. Seven pore water samples were analyzed for their anion (Cl⁻, SO₄ ²⁻, Br⁻, I⁻) contents, cation (Na, K, Ca, Mg) contents and trace element (Li, B, Sr, Ba, Rb, Mn) contents. Eight sediment samples were analyzed for stable carbon and oxygen isotopic compositions. A number of geochemical anomalies such as anions (e.g. Cl⁻, SO₄ ²⁻), cations (e.g. Ca, Mg) and trace elements (e.g. Sr, Ba, B) were found in this study. For example, the concentrations of Cl⁻ and SO₄ ²⁻ in pore water show a decreasing trend with depth. The estimated sulfate/methane interface (SMI) is only 18 m, which is quite similar to the SMI value of 23 m in the ODP164 Leg 997 at Blake Ridge. The Ca, Mg and Sr concentrations of pore water also decrease with depth, but concentrations of Ba, and Mg/Ca and Sr/Ca ratios increase with depth. These geochemical anomalies are quite similar to those found in gas hydrate locations in the world such as the Blake Ridge and may be related to the formation and dissociation of gas hydrates. The salt exclusion effect during the gas hydrate formation will cause an increase in major ion concentrations in the pore waters that diffused upward such as Cl. The anaerobic methane oxidation (AMO) may lead to the change of SO₄ ²⁻ and other cations such as Ca, Mg, Sr and Ba in pore water. Low δ ¹³C value of authigenic carbonates is a good indicator for gas hydrate occurrence. However, the bulk sediment samples we analyzed all show normal δ ¹³C values similar to biogenic marine carbonates, and this may also suggest that no gas hydrate-related authigenic carbonates exist or their amount is so small that they are not detectable by using this bulk analytical method. In conclusion, we suggest that the Site XS-01 in the Xisha trough of the northern margin of the South China Sea is a potential target for further gas hydrate exploration. Translated from Quaternary Sciences, 2006, 26(3): 442–448 [译自: 第四纪研究]     

Phase equilibrium and stable isotope constraints on the formation of metasomatic garnet-vesuvianite veins (SW Adamello,N Italy)

 Metasomatic garnet-vesuvianite veins occur within the contact metamorphic marble sequence of the Lower Triassic Prezzo formation in a narrow, 1–5 m wide zone along an intrusive marble-granodiorite contact at the southwestern border of the Tertiary Adamello batholith. The metasomatic mineral assemblage is comprised of garnet, vesuvianite, clinopyroxene, wollastonite, and pyrrhotite, which were precipitated from the vein-forming fluid in a preexisting calcite matrix at conditions of about 2800 bars and 630° C. The veins are enriched in silicon, aluminum, iron, magnesium, titanium and depleted in calcium with respect to the unaltered contact metamorphic marble. Graphite, which is present in the unaltered Prezzo Marble is absent in the veins. Irregularly shaped mineralogically distinct zones with different degrees of silicification can be distinguished within the veins. The isotopic compositions of calcite (cc) in the unaltered marble are about δ¹⁸O (SMOW; Standard mean Ocean Water)=21.0‰ and δ¹³C(PDB; Peedee belemnite)=0.0‰. They are reset to significantly lower values within the veins, where δ¹⁸O_cc is 15.0 to 16.0‰ and δ¹³C_cc is −4.5 to −3.5‰. The isotopic front coincides with an abrupt change in the microscopic texture of matrix carbonate which occurs at the sharp boundary between graphite-bearing and graphite-free material. Within the veins the oxygen isotope fractionation between calcite and garnet (gar) varies systematically with distance from highly silicified zones. The variations in Δ¹⁸O_cc-gar are as large as 2‰, on a millimeter scale, indicating garnet-calcite isotopic disequilibrium. Vein formation was due to the infiltration of a water rich fluid of magmatic provenance into the carbonate country rock along fractures. Removal of graphite from the wall rock by dissolution through the metasomatic fluid induced recrystallization of matrix calcite. Permeability was enhanced during calcite recrystallization facilitating material transport into the wall rock and metasomatic alteration. Vein garnet was precipitated in isotopic equilibrium with the metasomatic fluid. The isotopic composition of preexisting calcite was initially out of equilibrium with the vein-forming fluid and it was shifted towards equilibrium by surface-reaction controlled calcite-fluid isotopic exchange during calcite recrystallization. Due to the short lifetime of the metasomatic system, calcite-fluid isotopic equilibrium was generally not attained. Within the veins, oxygen and carbon transport was fast relative to mineral-fluid exchange of their isotopes and the geometry of the isotopic pattern is largely controlled by the kinetics of mineral-fluid exchange. Received: 16 June 1994/Accepted: 20 May 1995     

Biogeochemistry of the Russian Arctic. Kara Sea: Research results under the SIRRO project, 1995–2003

The Kara Sea is an area uniquely suitable for studying processes in the river-sea system. This is a shallow sea into which two great Siberian rivers, Yenisei and Ob, flow. From 1995 to 2003, the sea was studied by six international expeditions aboard the R/V Akademik Boris Petrov. This publication summarizes the results obtained, within the framework of this project, at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Various hydrogeochemical parameters, concentrations and isotopic composition of organic and carbonate carbon of the sediments, plankton, particulate organic matter, hydrocarbons, and dissolved CO₂ were examined throughout the whole sea area at more than 200 sites. The δ¹³C varies from −22 and −24‰ where Atlantic waters enter the Kara Sea and in the North-eastern part of the water area to −27‰ in the Yenisei and Ob estuaries. The value of δ¹³C of the plankton is only weakly correlated with the δ¹³C of the organic matter from the sediments and is lower by as much as 3–4‰. The paper presents the results obtained from a number of meridional river-sea profiles. It was determined from the relations between the isotopic compositions of plankton and particulate matter that the riverwaters carry material consisting of 70% detrital-humus matter and 30% planktonogenic material in the river part, and the material contained in the off-shore waters consists of 30% terrigenous components, with the contribution of bioproducers amounting to 70%. The carbon isotopic composition of the plankton ranges from −29 to −35‰ in the riverine part, from −28 to −27‰ in the estuaries, and from −27.0 to −25% in the marine part. The relative lightness of the carbon isotopic composition of plankton in Arctic waters is explained by the temperature effect, elevated CO₂ concentrations, and long-distance CO₂ supply to the sea with riverwaters. The data obtained on the isotopic composition of CO₂ in the surface waters of the Kara Sea were used to map the distribution of δ¹³C_{CO 2}. The complex of hydrocarbon gases extracted from the waters included methane, C₂–C₅, and unsaturated C₂₌–C₄₌ hydrocarbons, for which variations in the concentrations in the waters were studied along river-estuary-sea profiles. The geochemistry of hydrocarbon gases in surface fresh waters is characterized by comparable concentrations of methane (0.3–5 μl/l) and heavier hydrocarbons, including unsaturated ones. Microbiological methane with δ¹³C from −105 to −90‰ first occurs in the sediments at depths of 40–200 cm. The sediments practically everywhere display traces of methane oxidation in the form of a shift of the δ¹³C of methane toward higher values and the occurrence of autogenic carbonate material, including ikaite, enriched in the light isotope. Ikaite (δ¹³C from −25 to −60‰) was found and examined in several profiles. The redox conditions in the sediments varied from normal in the southern part of the sea to highly oxidized along the Novaya Zemlya Trough. Vertical sections through the sediments of the latter exemplify the complete suppression of the biochemical activity of microorganisms. Our data provide insight into the biogeochemistry of the Kara Sea and make it possible to specify the background values needed for ecological control during the future exploration operations and extraction of hydrocarbons in the Kara Sea. Original Russian Text ? E.M. Galimov, L.A. Kodina, O.V. Stepanets, G.S. Korobeinik, 2006, published in Geokhimiya, 2006, No. 11, pp. 1139–1191.     

Stratotype of the Lower Riphean, the Burzyan Group of the Southern Urals: Lithostratigraphy, paleontology, geochronology, Sr- and C-isotopic characteristics of its carbonate rocks

The main objective of this work is the generalization of lithostratigraphic, biostratigraphic and isotopic-geochronological data characterizing carbonate rocks from type succession of the broadly acknowledged chronostratigraphic subdivision of the Lower Riphean, such as the Burzyan Group of the Southern Urals and its analogs. Using an original approach to investigation of the Rb-Sr and Pb-Pb isotopic systems in carbonates and strict criteria of their retentivity, we studied the least altered (“best”) samples of the Burzyan carbonates, which retain the ⁸⁷Sr/⁸⁶Sr ratio of the sedimentation environment. As long ago as 1550 ± 30 and 1430 ± 30 Ma, that ratio corresponded to 0.70460–0.70480 and 0.70456–0.70481. The results confirm the influx of the mantle material predominantly into the World Ocean of the Early Riphean. The influence of meteoric diagenesis was likely responsible for local declines of δ¹⁸O in the Burzyan carbonates down to the values of −2.5 to −1.5‰ V-PDB. In the “best” samples, this parameter ranges from −0.7 to 0‰, which is consistent with the assumption that δ¹⁸O values (0 ± 1‰) characterized the stasis of the carbonate carbon isotopic composition in oceanic water 2.06–1.25 Ga ago. C-isotopic data on carbonate from the Paleoproterozoic-Lower Riphean boundary formations of the Urals, India, North America and Siberia suggest that the mentioned stasis ended by the commencement of the Early Riphean ca. 1.6–1.5 Ga ago. In the least altered carbonates of the Early Riphean, the δ¹⁸O variation range corresponds to 4.0–4.5‰.     

The Gumeshevo skarn-porphyry copper deposits in the Central Urals,Russia: Evolution of the ore-magmatic system as deduced from isotope geochemistry (Sr,Nd, C,O, H)

The Early Devonian Gumeshevo deposit is one of the largest ore objects pertaining to the dioritic model of the porphyry copper system paragenetically related to the low-K quartz diorite island-arc complex. The (⁸⁷Sr/⁸⁶Sr)_t and (ɛ_Nd)_t of quartz diorite calculated for t = 390 Ma are 0.7038–0.7045 and 5.0–5.1, respectively, testifying to a large contribution of the mantle component to the composition of this rock. The contents of typomorphic trace elements (ppm) are as follows: 30–48 REE sum, 5–10 Rb, 9–15 Y, and 1–2 Nb. The REE pattern is devoid of Eu anomaly. Endoskarn of low-temperature and highly oxidized amphibole-epidote-garnet facies is surrounded by the outer epidosite zone. Widespread retrograde metasomatism is expressed in replacement of exoskarn and marble with silicate (chlorite, talc, tremolite)-magnetite-quartz-carbonate mineral assemblage. The ⁸⁷Sr/⁸⁶Sr ratios of epidote in endoskarn and carbonate in retrograde metasomatic rocks (0.7054–0.7058 and 0.7053–0.7065, respectively) are intermediate between the Sr isotope ratios of quartz dioritic rocks and marble (⁸⁷Sr/⁸⁶Sr = 0.70784 ± 2). Isotopic parameters of the fluid equilibrated with silicates of skarn and retrograde metasomatic rocks replacing exoskarn at 400°C are δ¹⁸O = +7.4 to +8.5‰ and δD = −49 to −61‰ (relative to SMOW). The δ¹³C and δ¹⁸O of carbonates in retrograde metasomatic rocks after marble are −5.3 to +0.6 (relative to PDB) and +13.0 to +20.2% (relative to SMOW), respectively. Sulfidation completes metasomatism, nonuniformly superimposed on all metasomatic rocks and marbles with formation of orebodies, including massive sulfide ore. The δ³⁴S of sulfides is 0 to 2‰ (relative to CDT);⁸⁷Sr/⁸⁶Sr of calcite from the late calcite-pyrite assemblage replacing marble is 0.704134 ± 6. The δ¹³C and ⁸⁷Sr/⁸⁶Sr of postore veined carbonates correlate positively (r = 0.98; n = 6). The regression line extends to the marble field. Its opposite end corresponds to magmatic (in terms of Bowman, 1998b) calcite with minimal δ¹³C, δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr values (−6.9 ‰, +6.7‰, and 0.70378 ± 4, respectively). The aforementioned isotopic data show that magmatic fluid was supplied during all stages of mineral formation and interacted with marble and other rocks, changing its Sr, C, and O isotopic compositions. This confirms the earlier established redistribution of major elements and REE in the process of metasomatism. A contribution of meteoric and metamorphic water is often established in quartz from postore veins.     

Chemostratigraphy and lithological characters of Neoproterozoic cap carbonates from the Kuruktag Mountain, Xinjiang, western China

The Neoproterozoic Era includes some of the most largest ice ages in the geological history. The exact number of glaciations is unknown, though there were at least two events of global glaciation. Neoproterozoic glacial deposits in the Kuruktag Mountain, Xinjiang, western China have proven that there had occurred three discrete Neoproterozoic glaciations. Diamictite units occurred in the Bayisi, Tereeken, and Hankalchough formations, carbonate units were recognized among the diamictites and immediately overlied the Bayisi, Tereeken and Hankalchough diamictites. Carbonates at the top of the Bayisi Formation are characterized by the dolo-sility stones with negative δ13C values ranging from -4.10‰ to -8.17‰ (PDB), comparable to the Sturtian cap carbonates that overlie the Sturtian glacial deposits from other Neoproterozoic sequences. Carbonates overlying the Tereeken Formation are characterized by the pinkish cap dolostones (ca. 10 m thick) with negative δ13C values ranging from -2.58‰ to -4.77‰ (PDB), comparable to the Marinoan cap carbonates. The cap is also characterized by tepee-like structures, barite precipitates and pseudomorphous aragonite crystal fan limestones. Carbonates at the top of the Hankalchough Formation are characterized by subaerial exposure crust (vadose pisolite structure, calcareous crust structure) dolostones with negative δ13C values ranging from -4.56‰ to -11.45‰ (PDB) and the calcareous crust dolostones, implying that the Hankalchough cap carbonates differ from either the Sturtian or Marinoan cap carbonates in sedimentary environment and carbon isotopic composition. In addition, it is suggested the Hankalchough glaciation belongs to a terrestrial glaciation and it is the third largest glaciation during the Neoproterozoic period on the Tarim platform.     

Ore fluid geochemistry of the Jinlongshan Carlintype gold ore belt in Shaanxi Province, China

THEJINLONGSHANGOLDOREBELTINZHEN’ANCOUN TY,SOUTHERNSHAANXIPROVINCE,ISLOCATEDINTHEWEST ERNQINLINGGOLDPROVINCE(NO.16INFIG.1;CHEN YANJINGETAL.,2004).ITWASDISCOVEREDINTHEDEVO NIANSTRATAINTHELATE1980S).ITSGEOLOGICALSETTING ANDMETALLOGENICEVOLUTIONARESIMILARTOT…     

Hydrogeochemistry,environmental isotopes and the origin of the Hamamayagi-Ladik thermal spring (Samsun,Turkey)

Hamamayagi thermal spring (HTS) is located along the North Anatolian Fault Zone. The thermal spring has a temperature of 36°C, with total dissolved solids ranging from 485.6 to 508.5 mg/L. Hard, brittle, and gray limestones Permian aged are the reservoir rocks of the HTS. δ¹⁸O–δ²H isotope ratios clearly indicate a meteoric origin for the waters. The δ³⁴S value of sulfate in the thermal water is nearly 4.1‰ and implies a diagenetic environment characterized by reduced sulfur compounds. The δ¹³C ratio for dissolved inorganic carbonate in the HTS lies between −1.78 and −1.62‰, showing that it originates from the dissolution of fresh-water carbonates. Quartz geothermometry suggests a reservoir temperature of 52–85°C for the Hamamayagi geothermal field, but chalcedony geothermometers suggest reservoir temperatures between 30 and 53°C.     

Carbonate alteration associated with talc-chlorite mineralization in the eastern Pyrenees, with emphasis on the St. Barthelemy Massif

Summary The eastern Pyrenees host a large number of talc-chlorite mineralizations of Albian age (112–97 Ma), the largest of which occur in the St. Barthelemy massif. There talc develops by hydrothermal replacement of dolostones, which were formed by alteration of calcite marbles. This alteration is progressive. Unaltered calcite marbles have oxygen isotope composition of about 25‰ (V-SMOW). The δ¹⁸O values decrease down to values of 12‰ towards the contact with dolostones. This ¹⁸O depletion is accompanied by Mg enrichment, LREE fractionation and systematic shifts in the Sr isotope compositions, which vary from ⁸⁷Sr/⁸⁶Sr = 0.7087–0.7092 in unaltered calcite marbles to slightly more radiogenic compositions with ⁸⁷Sr/⁸⁶Sr = 0.7094 near dolomitization fronts. Dolostones have δ¹⁸O values (about 9‰) lower than calcitic marbles, higher REE content and more radiogenic Sr isotope composition (⁸⁷Sr/⁸⁶Sr = 0.7109 to 0.7130). Hydrothermal calcites have δ¹⁸O values close to dolostones but substantially lower δ¹³C values, down to −6.5‰, which is indicative of the contribution of organic matter. The REE content of hydrothermal calcite is one order of magnitude higher than that of calcitic marbles. Its highly radiogenic Sr composition with ⁸⁷Sr/⁸⁶Sr = 0.7091 to 0.7132 suggests that these elements were derived from silicate rocks, which experienced intense chlorite alteration during mineralization. The chemical and isotopic compositions of the calcite marbles, the dolostones and the hydrothermal calcites are interpreted as products of successive stages of fluid-rock interaction with increasing fluid-rock ratios. The hydrothermal quartz, calcite, talc and chlorite are in global mutual isotopic equilibrium. This allows the calculation of the O isotope composition of the infiltrating water at 300 °C, which is in the δ¹⁸O = 2–4.5‰ range. Hydrogen isotope compositions of talc and chlorite indicate a δD = 0 to −20‰. This water probably derived from seawater, with minor contribution of evolved continental water.