Intermediate sulfidation epithermal gold-base metal deposits in Tertiary subaerial volcanic rocks,Sahinli/Tespih Dere (Lapseki/Western Turkey)

Gold in the Sahinli and Tespih Dere intermediate sulfidation gold-base metal deposits in Western Turkey occurs in relatively deep epithermal quartz veins along with base metal minerals which have epithermal textures, including plumose quartz, vug infills, comb and cockade textures and matrix-supported milled breccias. The total sulfide content of the veins in the area is variable ranging from < 1% to 60% and is dominated by pyrite, galena, sphalerite and chalcopyrite. Sphalerite is Fe-poor (0.6 to 1.4 mol% FeS). Minor amounts of Ag-rich tetrahedrite are present. Primary hydrothermal alteration minerals include illite/muscovite, mixed-layer illite/smectite (11.6 Å) and clinochlore towards the east and, alunite, dickite/nacrite and pyrophyllite towards the west at Sahinli; major illite/muscovite and dickite occur at Tespih Dere and Sarioluk, respectively.Fluid inclusions in main-stage quartz at Sahinli are only liquid-rich, with homogenization temperatures ranging from 220 to 322 °C and the majority of T_h values between 250 and 300 °C. Salinity ranges from 4.3 to 6.9 wt.% NaCl equiv. First ice-melting temperatures (T_mf) between ?24.5 and ?19.0 °C indicate that the fluids were dominated by NaCl – H₂O during mineralization. The relatively higher average T_h at the Tespih Dere deposit (295 °C) is attributed to a relatively deeper level of exposure.Calculated δ¹⁸O values indicate that ore-forming hydrothermal fluids in the study area had δ¹⁸O_H2O ranging from + 1.1 to + 9.7‰ (average = 3.8‰), strongly ¹⁸O-enriched compared with present-day hydrothermal meteoric water in the area (δ¹⁸O = ?8.5‰). δD values of fluid inclusions in quartz range from ?58 to ?93‰ and δD values of clay minerals and alunite from ?40 to ?119‰. δD values from intermediate argillic alteration (average = ?68‰) in the study area are very similar to δD values of the present-day local geothermal system (average δD = ?54‰) whereas δD values from advanced-argillic alteration (average δD = ?33‰) are very different from the present-day local geothermal system.The δ³⁴S values in samples from the Sahinli and Tespih Dere deposits average ?2.9‰ for pyrite; ?3.3‰ for chalcopyrite; ?5.4‰ for sphalerite and ?7.6‰ for galena. These data are consistent with derivation of the sulfur from either igneous rocks or possibly from local wallrock.     

Geochemical and isotopic approach to maturity/source/mixing estimations for natural gas and associated condensates in the Thrace Basin,NW Turkey

The Tertiary Thrace Basin located in NW Turkey comprises 9 km of clastic-sedimentary column ranging in age from Early Eocene to Recent in age. Fifteen natural gas and 10 associated condensate samples collected from the 11 different gas fields along the NW–SE extending zone of the northern portion of the basin were evaluated on the basis of their chemical and individual C isotopic compositions. For the purpose of the study, the genesis of CH₄, thermogenic C₂₊ gases, and associated condensates were evaluated separately.Methane appears to have 3 origins: Group-1 CH₄ is bacteriogenic (Calculated δ¹³C_C1–C = −61.48‰; Silivri Field) and found in Oligocene reservoirs and mixed with the thermogenic Group-2 CH₄. They probably formed in the Upper Oligocene coal and shales deposited in a marshy-swamp environment of fluvio-deltaic settings. Group-2 (δ¹³C_C1–C = −35.80‰; Hamitabat Field) and Group-3 (δ¹³C_1–C = −49.10‰; Değirmenköy Field) methanes are thermogenic and share the same origin with the Group-2 and Group-3 C₂₊ gases. The Group-2 C₂₊ gases include 63% of the gas fields. They are produced from both Eocene (overwhelmingly) and Oligocene reservoirs. These gases were almost certainly generated from isotopically heavy terrestrial kerogen (δ¹³C = −21‰) present in the Eocene deltaic Hamitabat shales. The Group-3 C₂₊ gases, produced from one field, were generated from isotopically light marine kerogen (δ¹³C = −29‰). Lower Oligoce ne Mezardere shales deposited in pro-deltaic settings are believed to be the source of these gases.The bulk and individual n-alkane isotopic relationships between the rock extracts, gases, condensates and oils from the basin differentiated two Groups of condensates, which can be genetically linked to the Group-2 and -3 thermogenic C₂₊ gases. However, it is crucial to note that condensates do not necessarily correlate to their associated gases.Maturity assessments on the Group-1 and -2 thermogenic gases based on their estimated initial kerogen isotope values (δ¹³C = −21‰; −29‰) and on the biomarkers present in the associated condensates reveal that all the hydrocarbons including gases, condensates and oils are the products of primary cracking at the early mature st age (R_eq = 0.55–0.81%). It is demonstrated that the open-system source conditions required for such an early-mature hydrocarbon expulsion exist and are supported by fault systems of the basin.     

Post-Chicxulub depositional and diagenetic history of the northwestern Yucatan Peninsula,Mexico

The Chicxulub Sedimentary Basin of the northwestern Yucatan Peninsula, Mexico, which was formed because of the largest identified Phanerozoic bolide impact on Earth, became a site of deposition of dominantly marine carbonate sediments during most of the Cenozoic Era. This is a study of the filling and diagenetic history of this basin and surrounding areas. The study makes use of lithologic, biostratigraphic, petrographic, and geochemical data obtained on core samples from boreholes drilled throughout the northwestern Yucatan Peninsula.The core sample data indicate that: 1) The Chicxulub Sedimentary Basin concentrated the deposition of pelagic and outer-platform sediments during the Paleocene and Eocene, and, in places, during the Early Oligocene, as well, and filled during the Middle Miocene, 2) deeper-water limestone also is present within the Paleocene and Lower Eocene of the proposed Santa Elena Depression, which is located immediately south of the Basin, 3) shallow-water deposits are relatively more abundant outside the Basin and Depression than inside, 4) the autigenic and allogenic silicates from the Paleogene formations are the most abundant inside the Depression, 5) sediment deposition and diagenesis within the Basin also were controlled by impact crater topography, 6) the abundance of the possible features of subaerial exposure increases upward and outward from the center of the Basin, and 7) the formation of replacive low-magnesium calcite and dolomite, dedolomitization, dissolution, and precipitation of vug-filling calcite and dolomite cement have been more common outside the Basin than inside.δ¹⁸O in whole-rock (excluding vug-filling) calcite from core samples ranges from − 7.14‰ to + 0.85‰ PDB. δ¹³C varies from − 6.92‰ to + 3.30‰ PDB. Both stable isotopes correlate inversely with the abundance of subaerial exposure features indicating that freshwater diagenesis has been extensive especially outside and at the edge of the Chicxulub Sedimentary Basin.δ¹⁸O and δ¹³C in whole-rock (excluding vug-filling) dolomite ranges from − 5.54‰ to + 0.87‰ PDB and − 4.63‰ to + 3.38‰ PDB, respectively. Most dolomite samples have negative δ¹⁸O and positive δ¹³C suggesting that replacive dolomitization involved the presence of a fluid dominated by freshwater and/or an anomalously high geothermal gradient.Most dolomite XRD-determined mole percent CaCO₃ varies between 51 and 56. Replacive dolomite is larger, more euhedral, and less stoichiometric inside the Chicxulub Sedimentary Basin than outside.     

Inorganic,isotopic, and organic composition of high-chloride water from wells in a coastal southern California aquifer

Chloride concentrations were as high as 230 mg/L in water from the surface discharge of long-screened production wells in Pleasant Valley, Calif., about 100 km NW of Los Angeles. Wells with the higher Cl⁻ concentrations were near faults that bound the valley. Depending on well construction, high-Cl⁻water from different sources may enter a well at different depths. For example, Cl⁻ concentration in the upper part of some wells completed in overlying aquifers influenced by irrigation return were as high as 220 mg/L, and Cl⁻ concentrations in water sampled within wells at depths greater than 450 m were as high as 500 mg/L. These high-Cl⁻ waters mix within the well during pumping and produce the water sampled at the surface discharge. Changes in the major ion, minor ion, trace element, and δ³⁴S and δ¹³C isotopic composition of water in wells with depth were consistent with changes resulting from SO₄ reduction, precipitation of calcite, and cation exchange. The chemical and isotopic composition of high-Cl⁻ water from deep wells trends towards the composition of oil-field production water from the study area. Chloride concentrations in oil-field production water present at depths 150 m beneath freshwater aquifers were 2200 mg/L, and Cl⁻ concentrations in underlying marine rock were as high as 4400 mg/L. High-Cl⁻ concentrations in water from deeper parts of wells were associated with dissolved organic C composed primarily of hydrophobic neutral compounds believed to be similar to those associated with petroleum in underlying deposits. These compounds would not be apparent using traditional sampling techniques and would not be detected using analytical methods intended to measure contamination.     

Stable oxygen and carbon isotopes of carbonate concretions of the Miocene Yeonil Group in the Pohang Basin,Korea: Types of concretions and formation condition

Carbonate concretions in the Miocene sedimentary rocks of the Yeonil Group in the Pohang Basin (Korea) were investigated in terms of stable oxygen and carbon isotope composition to delineate the origin and associated diagenetic environment for their formation. Carbonate concretions are widely distributed in all the sedimentary rocks in the Pohang Basin, showing that the calcitic concretions are preserved within the mass-flow deposits and the dolomitic ones mostly in the hemipelagic siliceous rocks (diatomites). Concretions can be classified into four different types, on the basis of the stable isotopic signatures, each of which represents its own geochemical range.Type I concretions are calcitic and are composed of micrite to microspar. They occur in the conglomerates and sandstones which were deposited by mass flows (debris flow to turbidity current). It shows relatively lower δ¹⁸O (− 14.0 to − 9.3‰) and δ¹³C (− 19.6 to − 8.4‰) values. These concretions grew in a sulfate reducing zone under the influence of residual ambient seawater which had been significantly modified by volcanogenic sediments. Type II concretions are also calcitic, composed mostly of micrite with minor microspar and found in the sandstones. These concretions are characterized by relatively high δ¹⁸O (+ 1.8 to + 2.4‰) and variable δ¹³C (− 17.3 to − 0.4‰) values. These isotopic signatures reflect that Type II concretions formed from just beneath the sediment/water interface down to the sulfate reducing zone through the early stage of methanogenesis. Type III concretions are also calcitic, and composed largely of micrite with a minor contribution of microspar. They are observed in hemipelagic mudrocks which were deposited under the influence of mass flows. They are characterized by intermediate to high δ¹⁸O (− 4.6 to + 1.6‰) and high δ¹³C (− 1.3 to + 8.8‰) values. These concretions grew in a methanogenic zone by residual ambient seawater and/or seawater slightly modified by reaction with volcanogenic sediments. Type IV concretions are dolomite with calcite inclusion, and occur in hemipelagic siliceous rocks. These concretions are mostly composed of micrite and characterized by variable δ¹⁸O (− 9.1 to + 0.7‰) and high δ¹³C (+ 3.1 to + 17.9‰) values, suggesting formation in the methanogenic zone, although the residual ambient seawater is slightly modified by volcanogenic sediments.The same type of the concretions is widely distributed throughout the basin and always shows its own distinctive stable isotopic signature. This means that the formation of the given type depends upon the lithology and composition of host sediments that are closely related to the depositional process of the fan-delta systems regardless of their localities. Further, the different types of concretions are also found at the different, but closely spaced stratigraphic levels in the same locality, displaying the distinctive diagenetic conditions for each type. Such preservation of the unique diagenetic signatures in individual type of concretion suggests that the concretions formed in a completely closed diagenetic system. Therefore, caution should be made to simplify and generalize the diagenetic condition for the formation of any concretions in a large sedimentary basin.     

An experimental comparison of gas generation from three oil fractions: Implications for the chemical and stable carbon isotopic signatures of oil cracking gas

Although oil cracking has been documented as one of the important sources of gas in many overmature marine sedimentary basins, the chemical and carbon isotopic signatures of gases of this origin are still open to question. In this study a Cambrian crude oil from the central Tarim basin, along with its main separated fractions (saturates, aromatics and asphaltenes), were pyrolyzed in sealed gold tubes to investigate how generated gases vary in chemical and carbon isotopic composition and how this variation would influence the genetic interpretation of oil cracking gas. The results indicate that the gases from cracking of aromatics and asphaltenes are much drier and more enriched in ¹³C than the gases from the cracking of saturates and crude oil at the same level of thermal maturity. In the experimental run of 20 °C/h, the dryness index of the gases (defined as the volume percentage of C₁ in C_1–5) from the cracking of saturates ranges from 26.2–90.6% with the methane carbon isotope change ranging from −54.8‰ to −35.5‰, whereas the dryness index is never lower than 60.6% for the gases from the cracking of aromatics with methane carbon isotope ranging from −39.9‰ to −32.2‰. Correspondingly, experimental data for the four samples plot in different areas in diagrams designed to distinguish oil cracking gas from kerogen cracking gas, such as ln(C₂/C₃) vs. δ¹³C₂–δ¹³C₃ and δ¹³C₁ vs. δ¹³C₂–δ¹³C₃, indicating compositional variability of crude oil could assert an important influence in these diagrams. Therefore it is prudent to bring other geological constraints into consideration to avoid misinterpretation.The kinetic parameters for the bulk generation of C_1–5 gas and the methane carbon isotope fractionation extrapolated to geological conditions of 2 °C/Ma and an initial temperature of 50 °C show that the temperatures of C_1–5 gas generation from the aromatics and asphaltenes are lower than those from the saturates and crude oil due to their lower activation energies and frequency factors. Generation of C_1–5 gases from the aromatics is modeled to be initiated about 122 °C whereas the initiation temperature for the saturates sample is 176 °C. Below 189 °C (EasyRo = 1.8%), the yields of C_1–5 gases follow the order: aromatics > asphaltenes > crude oil > saturates. At similar thermal maturity levels, the methane carbon isotopic compositions are significantly different for the four samples, with an order of ¹³C enrichment: aromatics > asphaltenes > crude oil > saturates, however the difference in methane carbon isotopes becomes smaller with increasing temperature. This indicates that methane carbon isotopic values can be significantly different for gases cracked from oils that are compositionally diverse, especially in the early stage of methane generation.     

Isotope geochemistry of Swan Lake Basin in the Nebraska Sandhills,USA: Large 13C enrichment in sediment-calcite records

This study presents isotope geochemical analyses conducted on water column samples and core sediments collected from the Swan Lake Basin. Water analyses include the dissolved methane (CH₄) content and the ratio of carbon-13 to carbon-12 (δ¹³C) in dissolved inorganic carbon (DIC). The core sediments – sandy muds containing inorganic calcite, organic matter, and opal phases ± ostracods – were examined by X-ray diffraction, dated by radiocarbon (¹⁴C), analyzed for wt% organic carbon, wt% organic nitrogen, wt% organic matter, wt% calcite, δ¹³C of bulk-sediment insoluble organic matter (kerogen), ¹⁸O:¹⁶O ratio (δ¹⁸O) and δ¹³C of bulk and ostracod calcite. Of particular significance is the large enrichment in carbon-13 (δ¹³C = +4.5 to +20.4‰ V-PDB) in the calcite of these sediments. The ¹³C-enriched calcite is primarily formed from DIC in the water column of the lake as a result of the following combined processes: (i) the incorporation of ¹³C enriched residual carbon dioxide (CO₂) after partial reduction to CH₄ in the sediments and its migration into the water column-DIC pool; (ii) the preferential assimilation of ¹²C by phytoplankton during photosynthesis; (iii) the removal of ¹³C-depleted CH₄ by ebullition and of organic matter by sedimentation and burial. The ¹³C enrichment was low between 3624 and 2470 yr BP; high between 2470 and 1299 yr BP; and moderate since 1299 yr BP. Low ¹³C enrichment was formed under low water-column carbon levels while higher ones were formed under elevated rates of biomass and calcite deposition. These associations seem to imply that biological productivity is the main reason for carbon-13 enrichments.     

A ∼25 ka Indian Ocean monsoon variability record from the Andaman Sea

Recent paleoclimatic work on terrestrial and marine deposits from Asia and the Indian Ocean has indicated abrupt changes in the strength of the Asian monsoon during the last deglaciation. Comparison of marine paleoclimate records that track salinity changes from Asian rivers can help evaluate the coherence of the Indian Ocean monsoon (IOM) with the larger Asian monsoon. Here we present paired Mg/Ca and δ¹⁸O data on the planktic foraminifer Globigerinoides ruber (white) from Andaman Sea core RC12-344 that provide records of sea-surface temperature (SST) and δ¹⁸O of seawater (δ¹⁸O_sw) over the past 25,000 years (ka) before present (BP). Age control is based on nine accelerator mass spectrometry (AMS) dates on mixed planktic foraminifera. Mg/Ca-SST data indicate that SST was ∼3 °C cooler during the last glacial maximum (LGM) than the late Holocene. Andaman Sea δ¹⁸O_sw exhibited higher than present values during the Lateglacial interval ca 19–15 ka BP and briefly during the Younger Dryas ca 12 ka BP. Lower than present δ¹⁸O_sw values during the BØlling/AllerØd ca 14.5–12.6 ka BP and during the early Holocene ca 10.8–5.5 ka BP are interpreted to indicate lower salinity, reflect some combination of decreased evaporation–precipitation (E–P) over the Andaman Sea and increased Irrawaddy River outflow. Our results are consistent with the suggestion that IOM intensity was stronger than present during the BØlling/AllerØd and early Holocene, and weaker during the late glaciation, Younger Dryas, and the late Holocene. These findings support the hypothesis that rapid climate change during the last deglaciation and Holocene included substantial hydrologic changes in the IOM system that were coherent with the larger Asian monsoon.     

In situ LA–MC–ICP–MS boron isotope and zircon U–Pb age determinations of Paleoproterozoic borate deposits in Liaoning Province,northeastern China

A large number of Paleoproterozoic borate deposits are hosted by the lower units of a volcanic-sedimentary sequence in Liaoning Province, northeastern China, and are a major source of boron in China. The ore-bearing wall rocks in the deposits are serpentinized ultrabasic rocks and carbonates, with layered leptynites, leptites, amphibolites, and migmatites adjacent to the ore. Both the borate ores and country rocks contain tourmaline, although the country rocks have much lower abundances of the mineral. Based on in situ boron isotope measurements using laser ablation–multi-collector–inductively coupled plasma–mass spectrometry (LA–MC–ICP–MS), boron isotope data show that: (1) δ¹¹B values of borate ores range from + 6.8‰ to + 13.9‰ (mean + 10.8‰); (2) tourmalines from the borate ores have δ¹¹B values from + 9.5‰ to + 12.7‰; and (3) the wall rocks within the borate ores yield slightly lower δ¹¹B values ranging from + 5.7‰ to + 7.6‰, and those outside the deposits from − 9.9‰ to − 5.9‰. Positive δ¹¹B values in borates as well as in tourmalines inside the mining area indicate that boron in these Paleoproterozoic borate deposits was derived from marine evaporites. δ³⁴S_V-CDT (where V-CDT is Vienna Canyon Diablo Troilite) values of borate ores, serpentinized marbles, and anhydrites range from + 16.1‰ to + 24.7‰, whereas δ¹³C_V-PDB (where V-PDB is Vienna Pee Dee Belemnite) values of marbles range from + 3.2‰ to + 5.9‰. These isotopic characteristics are interpreted to reflect formation in a marine evaporative environment. LA–MC–ICP–MS zircon weighted²⁰⁷Pb/²⁰⁶Pb ages of leptite and serpentinized olivine basalt from the hanging wall of the borate deposits are 2139 ± 13 Ma and 2130 ± 19 Ma, respectively. Therefore, the (~ 2.2 Ga) borate deposits may have originated from marine evaporative boron-bearing sediments, which were interbedded within bimodal volcanic rocks during the early stages of development of the Liaoji rift.     

Geochemical and oxygen isotope signatures of mantle corundum megacrysts from the Mbuji-Mayi kimberlite,Democratic Republic of Congo,and the Changle alkali basalt,China

Oxygen isotope signatures of ruby and sapphire megacrysts, combined with trace-element analysis, from the Mbuji-Mayi kimberlite, Democratic Republic of Congo, and the Changle alkali basalt, China, provide clues to specify their origin in the deep Earth. At Mbuji-Mayi, pink sapphires have δ¹⁸O values in the range 4.3 to 5.4‰ (N = 10) with a mean of 4.9 ± 0.4‰, and rubies from 5.5 to 5.6‰ (N = 3). The Ga/Mg ratio of pink sapphires is between 1.9 and 3.9, and in rubies, between 0.6 and 2.6. The blue or yellow sapphires from Changle have δ¹⁸O values from 4.6 to 5.2 ‰, with a mean of 4.9 ± 0.2‰ (N = 9). The Ga/Mg ratio is between 5.7 and 11.3. The homogenous isotopic composition of ruby suggests a derivation from upper mantle xenoliths (garnet lherzolite, pyroxenite) or metagabbros and/or lower crustal garnet clinopyroxenite eclogite-type xenoliths included in kimberlites. Data from the pink sapphires from Mbuji-Mayi suggest a mantle origin, but different probable protoliths: either subducted oceanic protolith transformed into eclogite with δ¹⁸O values buffered to the mantle value, or clinopyroxenite protoliths in peridotite. The Changle sapphires have a mantle O-isotope signature. They probably formed in syenitic magmas produced by low degree partial melting of a spinel lherzolite source. The kimberlite and the alkali basalt acted as gem conveyors from the upper mantle up to the surface.     

Late- and postglacial environment of the Buzuluk area,middle Volga region,Russia

A 13 m long core from the lake-swamp at Pobochnoye (53°01′30″ N, 51°50′30″ E) in the Buzuluk pine forest in the middle part of the Volga River basin, Russia was studied for pollen, peat stratigraphy, mollusc, δ¹⁸O/δ¹⁶O and δ¹³C/δ¹²C analyses and radiocarbon dating. For the first time the environment history of the east European Russia has been reconstructed for the last 14,000 years; ca 14,000–13,000 BP cold dry steppes spread across the basin of the Samara River. Isotope data indicate that the main climate shift occurred ca 10,000 BP at the Lateglacial–Holocene transition when climate became warmer and forests expanded. Pinus sylvestris L. expanded 10,000 BP. Ca 9,000 BP Ulmus, Quercus and Corylus appeared in the Buzuluk forest followed at ca. 7,000 BP by Alnus, then Tilia and Acer at 6000 BP. Between 6000 and 4500 BP the climatic conditions were optimal for the forest growth in the Samara River basin. 5500–5000 BP the lake became shallower and was transformed into the eutrophic peat swamp. Between 4500–3500 BP climate became drier and hotter and forest less abundant. Between 3500–2400 BP the forest cover again increased. Between 2400–2000 BP the pine forest area has reduced, apparently due to increased dryness, and around 2000 BP the modern environment in the Buzuluk area has been in existence.     

The impact of African aridity on the isotopic signature of Atlantic deep waters across the Middle Pleistocene Transition

A high resolution analysis of benthic foraminifera as well as of aeolian terrigenous proxies extracted from a 37 m-long marine core located off the Mauritanian margin spanning the last ~ 1.2 Ma, documents the possible link between major continental environmental changes with a shift in the isotopic signature of deep waters around 1.0–0.9 Ma, within the so-called Mid-Pleistocene Transition (MPT) time period. The increase in the oxygen isotopic composition of deep waters, as seen through the benthic foraminifera δ¹⁸O values, is consistent with the growth of larger ice sheets known to have occurred during this transition. Deep-water mass δ¹³C changes, also estimated from benthic foraminifera, show a strong depletion for the same time interval. This drastic change in δ¹³C values is concomitant with a worldwide 0.3‰ decrease observed in the major deep oceanic waters for the MPT time period. The phase relationship between aeolian terrigeneous signal increase and this δ¹³C decrease in our record, as well as in other paleorecords, supports the hypothesis of a global aridification amongst others processes to explain the deep-water masses isotopic signature changes during the MPT. In any case, the isotopic shifts imply major changes in the end-member δ¹⁸O and δ¹³C values of deep waters.     

Hydro-geochemical and isotopic fluid evolution of the Los Azufres geothermal field,Central Mexico

Hydrothermal alteration at Los Azufres geothermal field is mostly propylitic with a progressive dehydration with depth and temperature increase. Argillic and advanced argillic zones overlie the propylitic zone owing to the activity of gases in the system. The deepest fluid inclusions (proto-fluid) are liquid-rich with low salinity, with NaCl dominant fluid type and ice melting temperatures (T_mi) near zero (0 °C), and salinities of 0.8 wt% NaCl equivalent. The homogenization temperature (T_h)  = 325 ± 5 °C. The boiling zone shows T_h = ±300 °C and apparent salinities between 1 and 4.9 wt% NaCl equivalent, implying a vaporization process and a very important participation of non-condensable gases (NCGs), mostly CO₂. Positive clathrate melting temperatures (fusion) with T_h = 150 °C are observed in the upper part of the geothermal reservoir (from 0 to 700 m depth). These could well be the evidence of a high gas concentration. The current water produced at the geothermal wells is NaCl rich (geothermal brine) and is fully equilibrated with the host rock at temperatures between T = 300 and 340 °C. The hot spring waters are acid-sulfate, indicating that they are derived from meteoric water heated by geothermal steam. The NCGs related to the steam dominant zone are composed mostly of CO₂ (80–98% of all the gases). The gases represent between 2 and 9 wt% of the total mass of the fluid of the reservoir.The authors interpret the evolution of this system as deep liquid water boiling when ascending through fractures connected to the surface. Boiling is caused by a drop of pressure, which favors an increase in the steam phase within the brine ascending towards the surface. During this ascent, the fluid becomes steam-dominant in the shallowest zone, and mixes with meteoric water in perched aquifers. Stable isotope compositions (δ¹⁸O–δD) of the geothermal brine indicate mixing between meteoric water and a minor magmatic component. The enrichment in δ¹⁸O is due to the rock–water interaction at relatively high temperatures. δ¹³C stable isotope data show a magmatic source with a minor meteoric contribution for CO₂. The initial isotopic value δ³⁴S_RES = −2.3‰, which implies a magmatic source. More negative values are observed for shallow pyrite and range from δ³⁴S (FeS₂) = −4‰ to −4.9‰, indicating boiling. The same fractionation tendencies are observed for fluids in the reservoir from results for δ¹⁸O.     

Organic carbon accumulation over the last 16 kyr off Vancouver Island,Canada: evidence for increased marine productivity during the deglacial

The organic carbon content of marine sediments is commonly used as a proxy for export production. However, in continental margin sediments a large fraction of the organic matter may be of terrestrial origin, and it is necessary to correct the total organic carbon data accordingly. Radiocarbon dating of bulk organic carbon, organic geochemistry and isotope data (δ¹³C and δ¹⁵N) are used here to characterize the type of organic matter present in Core JT96-09 collected at a water depth of 920 m on the slope off Western Canada. The quantities of marine and terrestrial organic carbon are then estimated using the δ¹³C data. The 16 kyr record obtained from Core JT96-09 suggests that accumulation of total organic carbon was highest during the late glacial and deglacial, but geochemical data indicate that as much as 70% of this carbon is terrestrial in origin. When the palaeo-record is corrected for this terrigenous input it is observed that accumulation of marine organic matter, and presumably marine export production, increased at the end of the last glacial contemporaneous with the Bølling, and that it peaked during the Allerød. Data from other palaeoproductivity proxies (i.e., bio-barium, opal and alkenones) also indicate relatively high productivity during the deglacial. These results indicate a return to modern upwelling conditions and high marine export production at ∼14.3 calendar kyr BP and a period of enhanced upwelling, relative to the present, during the Allerød.     

Sulfur cycling at the Mid-Atlantic Ridge: A multiple sulfur isotope approach

The role of sulfur in two hydrothermal vent systems, the Logatchev hydrothermal field at 14°45′N/44°58′W and several different vent sites along the southern Mid-Atlantic Ridge (SMAR) between 4°48′S and 9°33′S and between 12°22′W and 13°12′W, is examined by utilizing multiple sulfur isotope and sulfur concentration data. Isotope compositions for sulfide minerals and vent H₂S from different SMAR sites range from + 1.5 to + 8.9‰ in δ³⁴S and from + 0.001 to + 0.051‰ in Δ³³S. These data indicate mixing of mantle sulfur with sulfur from seawater sulfate. Combined δ³⁴S and Δ³³S systematics reveal that vent sulfide from SMAR is characterized by a sulfur contribution from seawater sulfate between 25 and 33%. This higher contribution, compared with EPR sulfide, indicates increased seawater sulfate reduction at MAR, because of a deeper seated magma chamber and longer fluid upflow path length, and points to fundamental differences with respect to subsurface structures and fluid evolution at slow and fast spreading mid-ocean ridges.Additionally, isotope data uncover non-equilibrium isotopic exchange between dissolved sulfide and sulfate in an anhydrite bearing zone below the vent systems at fluid temperatures between 335 and 400 °C. δ³⁴S values between + 0.2 to + 8.8‰ for dissolved and precipitated sulfide from Logatchev point to the same mixing process between mantle sulfur and sulfur from seawater sulfate as at SMAR. δ³⁴S values between ? 24.5 and + 6.5‰ and Δ³³S values between + 0.001 and + 0.125‰ for sulfide-bearing sediments and mafic/ultramafic host rocks from drill cores taken in the region of Logatchev indicate a clear contribution of biogenic sulfides formed via bacterial sulfate reduction. Basalts and basaltic glass from SMAR sites with Δ³³S = ? 0.008‰ reveal lower Δ³³S lower values than suggested on the basis of previously published isotopic measurements of terrestrial materials.We conclude that the combined use of both δ³⁴S and Δ³³S provides a more detailed picture of the sulfur cycling in hydrothermal systems at the Mid-Atlantic Ridge and uncovers systematic differences to hydrothermal sites at different mid-ocean ridge sites. Multiple sulfur isotope measurements allow identification of incomplete isotope exchange in addition to isotope mixing as a second important factor influencing the isotopic composition of dissolved sulfide during fluid upflow. Furthermore, based on Δ³³S we are able to clearly distinguish biogenic from hydrothermal sulfides in sediments even when δ³⁴S were identical.     

Persistent sea surface temperature and declined sea surface salinity in the northwestern tropical Pacific over the past 7500 years

To understand Holocene climate evolutions in low-latitude region of the western Pacific, paired δ¹⁸O and Mg/Ca records of planktonic foraminifer Globigerinoides ruber (250–300 μm, sensu stricto, s.s.) from a marine core ORI715-21 (121.5°E, 22.7°N, water depth 760 m) underneath the Kuroshio Current (KC) off eastern Taiwan were analyzed. Over the past 7500 years, the geochemical proxy-inferred sea surface temperature (SST) hovered around 27–28 °C and seawater δ¹⁸O (δ¹⁸O_W) slowly decreased 0.2–0.4‰ for two KC sites at 22.7° and 25.3°N. Comparison with a published high-SST and high-salinity equatorial tropical Pacific record, MD98-2181 located at the Mindanao Current (MC) at 6.3°N, reveals an anomalous time interval at 3.5–1.5 kyr ago (before 1950 AD). SST gradient between the MC site and two KC site decrease from 1.5–2.0 °C to only 0–1 °C, and δ¹⁸O_W from 0.1–0.3‰ to 0‰ for this 2-kyr time window. The high SST and low gradient could result from a northward shift of the North Equatorial Current, which implies a weakened KC. The long-term descending δ¹⁸O_W and increasing precipitation in the entire low-latitude western Pacific and the gradually decreasing East Asian summer monsoonal rainfall during middle-to-late Holocene is likely caused by different land and ocean responses to solar insolation and/or enhanced moisture transportation from the Atlantic to Pacific associated with the southward movement of ITCZ.     

Oxygen isotopic composition of lacustrine carbonates since 130 ka bp from a Tianshuihai Lake core,Tibet: an overall increasing δ18O trend and its implications

The oxygen-isotopic composition of lacustrine authigenic carbonates from a core taken from Lake Tianshuihai in western Qinghai-Tibet Plateau shows an overall increasing δ¹⁸O trend (OIT) from about 130 to 15 ka bp. Based on the argument by Wei and Gasse (1999) that the δ¹⁸O of lacustrine carbonates from lakes in west China is controlled mainly by the δ¹⁸O of the local precipitation, we propose that this OIT may have resulted from a similar OIT of δ¹⁸O of precipitation over the drainage area of the Tianshuihai Lake. This OIT suggests that from about 130 to 15 ka bp, variations of δ¹⁸O of precipitation on the Qinghai-Tibet Plateau were controlled mainly by changes, not in temperature, but in moisture sources, especially in the ratio of precipitation of moisture from marine sources (MMS) to that from continental evaporation (MCE). The changes in the sources of the moisture and in the MMS/MCE ratio may be mainly a function of the uplift of the Qinghai-Tibet Plateau.     

The Cipoeiro gold deposit,Gurupi Belt,Brazil: Geology,chlorite geochemistry,and stable isotope study

The Cipoeiro gold deposit, located in the Gurupi Belt, northern Brazil, is hosted by tonalites of 2148 Ma. The deposit is controlled by splays related to the major strike-slip Tentugal shear zone, and at the deposit scale, the mineralization is confined to ductile–brittle shear zones. Mineralization style comprises thick quartz veins and narrow and discontinuous quartz-carbonate veinlets associated with disseminations in altered host rocks. The postmetamorphic hydrothermal paragenesis is composed of quartz, calcite, chlorite, white mica (phengite), pyrite, and minor albite. Electron microprobe analysis of chlorites reveals a relatively uniform chemical composition at depths of more than 100 m. The chlorites are characterized by (Fe + Mg) ratios between 0.37 and 0.47 and Al^IV ranging between 2.22 and 2.59 a.p.f.u. and are classified as Fe-chlinochlore. Temperatures calculated by applying the Al^IV contents of chlorites yield a relatively narrow interval of 305 ± 15°C. Stable isotope (O, H, C, S) compositions have been determined in silicate, carbonate, and sulfide minerals. The δ¹⁸O and δD values of the mineralizing fluid range from +2.4 to +5.7 and from −43‰ to −20‰, respectively, and are interpreted as having a metamorphic origin. The δ¹³C values of fluid CO₂ are in the range −10.7‰ to −3.9‰, whereas the fluid δ³⁴S is around 0‰. Carbon and sulfur compositions are not diagnostic of their sources, compatible as they are with mantle, magmatic, or average crustal reservoirs. The hydrothermal paragenesis, chlorite–pyrite coexistence, temperature of ore formation, and sulfur isotope evidence indicate relatively reduced fO₂ conditions for the mineralizing fluid. Geologic, chemical, and isotopic characteristics of the Cipoeiro deposit are compatible with the class of orogenic gold deposits.     

Geochemical assessment of hydrocarbon migration phenomena: Case studies from the south-western margin of the Dead Sea Basin

Calcite veins with fluid and solid bitumen inclusions have been discovered in the south-western shoulder of the Dead Sea rift within the Masada-Zohar block, where hydrocarbons exist in small commercial gas fields and non-commercial fields of heavy and light oils. The gas–liquid inclusions in calcite are dominated either by methane or CO₂, and aqueous inclusions sometimes bear minor dissolved hydrocarbons. The enclosed flake-like solid bitumen matter is a residue of degraded oil, which may be interpreted as “dead carbon”. About 2/3 of this matter is soot-like amorphous carbon and 1/3 consists of n-C₈C₁₈ carboxylic acids and traces of n-alkanes, light dicarboxylic acids, and higher molecular weight (>C₂₀) branched and/or cyclic carboxylic acids. Both bitumen and the host calcites show genetic relationship with mature Maastrichtian chalky source rocks (MCSRs) evident in isotopic compositions (δ¹³C, δ³⁴S, and δ¹⁸O) and in REE + Y patterns. The bitumen precursor may have been heavy sulfur-rich oil which was generated during the burial compaction of the MCSR strata within the subsided blocks of the Dead Sea graben. The δ¹⁸O and δ¹³C values and REE + Y signatures in calcites indicate mixing of deep buried fluids equilibrated with post-mature sediments and meteoric waters. The temperatures of fluid generation according to Mg–Li-geothermometer data range from 55 °С to 90 °С corresponding to the 2.5–4.0 km depths, and largely overlap with the oil window range (60–90 °С) in the Dead Sea rift (Hunt, 1996; Gvirtzman and Stanislavsky, 2000; Buryakovsky et al., 2005). The bitumen-rich vein calcites originated in the course of Late Cenozoic rifting and related deformation, when tectonic stress triggers damaged small hydrocarbon reservoirs in the area, produced pathways, and caused hydrocarbon-bearing fluids to rise to the subsurface; the fluids filled open fractures and crystallized to calcite with entrapped bitumen. The reported results are in good agreement with the existing views of maturation, migration, and accumulation of hydrocarbons, as well as basin fluid transport processes in the Dead Sea area.     

Geology,geochemistry and age of the Hukeng tungsten deposit,Southern China

The Hukeng tungsten deposit, located in the Wugongshan area in central part of Jiangxi province, South China, is a large-scale quartz-vein wolframite deposit. It is hosted in the Hukeng granitic intrusion. Based on the mineral assemblage and crosscutting relationship of the veins, three mineralization stages are identified, including: (1) quartz–wolframite stage, (2) quartz–fluorite–wolframite stage, and (3) quartz–pyrite–sphalerite–wolframite stage.The homogenization temperatures of fluid inclusions in vein quartz vary from 220 to 320 °C, and the salinities are from 0 to 10 wt.% NaCl equiv.; corresponding densities range from 0.7 to 1 g/cm³. These features indicated that the ore-forming fluids in the Hukeng tungsten deposit have medium temperature, low density and low salinity.The δ¹⁸OSMOW values of quartz range from 10.8‰ to 14.4‰, with corresponding δ¹⁸O_fluid values of 3.7‰ to 7.7‰, and δD values of fluid inclusions of between ? 70‰ and ? 55‰. The combined isotopic data indicate that the ore-forming fluids of the Hukeng tungsten deposit were mainly derived from magmatic water, with some minor input from meteoric water.We have carried out molybdenite Re–Os and muscovite ⁴⁰Ar/³⁹Ar dating to constrain the timing of mineralization. Re–Os dating of six molybdenite samples yielded model ages ranging from 149.1 ± 2.0 to 150.7 ± 3.7 Ma, with an average of 150.0 Ma. The Re–Os analyses give a well-defined ¹⁸⁷Re/¹⁸⁷Os isochron with an age of 150.2 ± 2.2 Ma (MSWD = 0.60). Hydrothermal muscovite yields a plateau ⁴⁰Ar/³⁹Ar age of 147.2 ± 1.4 Ma. ⁴⁰Ar/³⁹Ar age is in good agreement with the Re–Os age. These ages show that the timing of tungsten mineralization occurred at about 150 Ma. Our new data, when combined with published geochronological results from the other major deposits in this region, suggest that widespread W mineralization occurred in the Late Jurassic throughout South China.