The isotopic composition of nitrate produced from nitrification in a hardwood forest floor

Dual isotopic analysis of nitrate (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) is increasingly used to investigate the environmental impacts of human-induced elevated atmospheric nitrate deposition. In forested ecosystems, the nitrate found in surface water and groundwater can originate from two sources: (1) atmospheric deposition, and (2) nitrate produced from nitrification in forest soils (microbial nitrate). Application of the dual nitrate isotope technique for determining the relative importance of nitrate sources in forested catchments requires knowledge of the isotopic composition of microbial nitrate. We excluded precipitation inputs to three zero-tension lysimeters installed below the F-horizon (Oe) at the Turkey Lakes Watershed (TLW) in order to measure the isotopic composition of microbial nitrate produced in situ. To our knowledge, this is the first in situ study of the isotopic composition of microbial nitrate in forest soils. Over a 2-week period, nitrate produced by nitrification was periodically flushed to the lysimeters by watering the area with a nitrogen-free solution. Nitrate produced in the forest floor had δ¹⁸O values ranging from +3.1‰ to +10.1‰ with a mean of +5.2‰. These values were only slightly higher than from the expected value of +1.0‰ calculated for chemolithoautotrophic nitrification, which depends on the δ¹⁸O of available O₂ and H₂O. In addition to nitrate, we also collected soil gas to determine if soil respiration and O₂ diffusion affected soil gas δ¹⁸O-O₂, which is typically assumed to be identical to atmospheric O₂ (+23.5‰) when calculating microbial nitrate δ¹⁸O values. No significant difference in δ¹⁸O-O₂ from atmospheric O₂ was found in forest soils to a depth of 55 cm, and therefore ¹⁸O-enrichment of soil gas O₂ could not explain the modest enrichment of nitrate ¹⁸O. Evaporative ¹⁸O-enrichment of soil water available to nitrifiers in the forest floor is a plausible mechanism for slightly elevated nitrate δ¹⁸O values. However, the observed nitrate δ¹⁸O values could also be explained by a minor contribution of nitrate from heterotrophic nitrifiers. The δ¹⁵N of nitrate produced ranged from −10.4 to −7.3‰ and, as expected, was depleted in ¹⁵N relative to soil organic nitrogen. Microbial nitrate produced in the forest floor was also significantly depleted in ¹⁵N relative to microbial nitrate exported in groundwater and headwater streams at the TLW. We hypothesize that ¹⁵N-depleted forest floor nitrate is not detected in groundwaters largely because of: (1) the immobilization of forest floor nitrate in the mineral soil and (2) the mixing of the remaining forest floor nitrate with nitrate generated in the mineral soil, which is expected to have higher δ¹⁵N values. This study demonstrates that current methods of calculating a priori the δ¹⁸O of microbial nitrate provide a reasonable value for nitrate produced by nitrification at the TLW.     

Discovery of Mass Independent Oxygen Isotopic Compositions in Superscale Nitrate Mineral Deposits from Turpan‐Hami Basin,Xinjiang, China and Its Significance

<正>The Turpan-Hami Basin in eastern Xinjiang is one of the driest regions on Earth and a premier environment to form and preserve nitrate.A large nitrate ore field in this basin was found recently.It is estimated there are about 2.5 billion tons of resources of nitrate,and the amount is as much as the Atacama Desert super-scale nitrate deposit in Chile.Nitrate is one of a few minerals with mass-independent fractionation(MIF),and the oxygen isotope MIF is an effective method to determine the source of nitrate.Theδ~(17)O,δ~(18)O of nitrate were measured by fluorination and thermal decomposition method.The date indicated that this is the first time that oxygen isotope MIF has been located in inland nitrate minerals.The results obtained by two methods are similar,⊿~(17)=δ~(17)O-0.52×δ~(18)O=12‰-17‰.The experiment and observation data proved that oxygen isotope MIF of nitrate are the result of photochemical reactions in the troposphere and stratosphere.Thus, evidence from MIF oxygen isotopic compositions indicate that long term atmospheric deposition of nitrate aerosol particles produced by photochemical reactions is the source of the deposits.     

Natürliche Isotopengehalte von Nitrat als Indikatoren für dessen Herkunft

The δ¹⁵N-value has often tentatively been used for the assignment of nitrate to its origin. However, the very complex correlations between the different nitrogen pools, mostly accompanied by isotope discriminations, oppose a very limited application of this method. On the other hand, the oxygen isotope abundance should be more indicative, because industrially produced NO⁻₃ must nearly exclusively contain oxygen from O₂ (δ¹⁸O = +23.5%.), while NO⁻₃ originating from a nitrification process must have water (δ¹⁸O ≅ -10%.) as the main oxygen source.For the proof of this reflection a method for the precise oxygen isotope analysis of NO⁻₃ was developed. Its application to the δ-value determination of commercial fertilizers and NO⁻₃ formed by nitrification absolutely confirmed the above predictions. Similarly, the isotope abundance of NO⁻₃-samples from ground and drinking water of known origin corresponded to the expected values. On the basis of these results and taking into account the known isotope abundance shifts due to isotope effects of nitrification and denitrification, a diagram between δ¹⁵N- and δ¹⁸O-values for NO⁻₃ was developed, which permitted the assignment of NO⁻₃ in unknown water samples to its probable source and origin.     

Modelling nitrogen and oxygen isotope fractionation during denitrification in a lacustrine redox-transition zone

The stable isotope composition (δ¹⁵N and δ¹⁸O) of nitrate was measured during Summer 1999 in the anaerobic hypolimnion of eutrophic Lake Lugano (Switzerland). Denitrification was demonstrated by a progressive nitrate depletion coupled to increasing δ¹⁵N and δ¹⁸O values for residual nitrate. Maximum δ¹⁵N and δ¹⁸O values amounted to 27.2 and 15.7‰, respectively.¹⁵N and ¹⁸O enrichment factors for denitrification (ε) were estimated using a closed-system model and a dynamic diffusion-reaction model. Using the Rayleigh equation (closed-system approach), we obtained ε values of −11.2 and −6.6‰ for nitrogen and oxygen, respectively. The average ε values derived using the diffusion-reaction model were determined to be −20.7 ± 3.8 for nitrogen and −11.0 ± 1.7 for oxygen. Both N and O isotope fractionation appeared to be lower when denitrification rates where high, possibly in association with high organic carbon availability. In addition, variations in the isotope effects may be attributed to the variable importance of sedimentary denitrification having only a small isotope effect on the water column. The combined measurement of N and O isotope ratios in nitrate revealed that coupled nitrification-denitrification in the open-water was of minor importance. This is the first study of nitrogen and oxygen isotope effects associated with microbial denitrification in a natural lake. Moreover, this study confirms the high potential of δ¹⁸O of nitrate as a valuable biogeochemical tracer in aquatic systems, complementing nitrate δ¹⁵N.     

Oxygen isotope composition of syngenetic inclusions in diamond from the Finsch Mine,RSA

Oxygen isotope data have been obtained for silicate inclusions in diamonds, and similar associated minerals in peridotitic and eclogitic xenoliths from the Finsch kimberlite by laser-fluorination. Oxygen isotope analyses of syngenetic inclusions weighing 20–400 μg have been obtained by laser heating in the presence of ClF₃. ¹⁸O/¹⁶O ratios are determined on oxygen converted to CO₂ over hot graphite and, for samples weighing less than 750 μg (producing <12 μmoles O₂) enhanced CO production in the graphite reactor causes a systematic shift in both δ¹³C and δ¹⁸O that varies as a function of sample weight. A “pressure effect” correction procedure, based on the magnitude of δ¹³C (CO₂) depletion relative to δ¹³C (graphite), is used to obtain corrected δ¹⁸O values for inclusions with an accuracy estimated to be ±0.3‰ for samples weighing 40 μg.Syngenetic inclusions in host diamonds with similar δ¹³C values (−8.4‰ to −2.7‰) have oxygen isotope compositions that vary significantly, with a clear distinction between inclusions of peridotitic (+4.6‰ to +5.6‰) and eclogitic paragenesis (+5.7‰ to +8.0‰). The mean δ¹⁸O composition of olivine inclusions is indistinguishable from that of typical peridotitic mantle (5.25 ± 0.22‰) whereas syngenetic purple garnet inclusions possess relatively low δ¹⁸O values (5.00 ± 0.33‰). Reversed oxygen isotope fractionation between olivine and garnet in both diamond inclusions and diamondiferous peridotite xenoliths suggests that garnet preserves subtle isotopic disequilibrium related to genesis of Cr-rich garnet and/or exchange with the diamond-forming fluid. Garnet in eclogite xenoliths in kimberlite show a range of δ¹⁸O values from +2.3‰ to +7.3‰ but garnets in diamondiferous eclogites and as inclusions in diamond all have values >4.7‰.     

Long term atmospheric deposition as the source of nitrate and other salts in the Atacama Desert, Chile: New evidence from mass-independent oxygen isotopic compositions

Isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the Atacama Desert in northern Chile has shown anomalous ¹⁷O enrichments in both minerals. Δ¹⁷O values of 14-21 ‰ in nitrate and 0.4 to 4 ‰ in sulfate are the most positive found in terrestrial minerals to date. Modeling of atmospheric processes indicates that the Δ¹⁷O signatures are the result of photochemical reactions in the troposphere and stratosphere. We conclude that the bulk of the nitrate, sulfate and other soluble salts in some parts of the Atacama Desert must be the result of atmospheric deposition of particles produced by gas to particle conversion, with minor but varying amounts from sea spray and local terrestrial sources. Flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200,000 to 2.0 M years during hyper-arid conditions similar to those currently found in the Atacama Desert. Correlations between Δ¹⁷O and δ¹⁸O in nitrate salts from the Atacama Desert and Mojave Desert, California, indicate varying fractions of microbial and photochemical end-member sources. The photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless, whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture, biologic activity, and nitrogen cycling. These isotopic signatures have important implications for paleoclimate, astrobiology, and N cycling studies.     

The nature and origin of sideritic ironstone bands in the Tertiary Lowmead and Duaringa Basins,Queensland

Sideritic ironstones in Tertiary lacustrine oil shale from the Lowmead and Duaringa Basins in Queensland, contain two distinctive types of siderite in the ironstone bands: sphaerosiderite in the mudstone and coal, and finely crystalline siderite in the lamosite. The petrological evidence indicates that the siderite in the ironstone bands formed eogenetically by growing displacively within the soft sediment. Chemically the siderite is very pure though the sphaerosiderite sometimes shows compositional zoning. Stable oxygen and carbon isotope analyses of the siderite show a wide range of values from ‐12.8‰ to ‐2.4 %₀ δ¹⁸O (PDB) and ‐5.5‰ to +12.9‰ δ¹³C (PDB) for the Lowmead Basin; and ‐9.6‰ to ‐1.2‰ δ¹⁸O (PDB) and ‐18.6‰ to +16.4‰ δ¹³C (PDB) for the Duaringa Basin. The oxygen isotope data indicate that the siderite formed in freshwater environments but not in isotopic equilibrium with the formation waters. Kinetic factors offer the most plausible explanation for the anomalously light δ¹⁸O values of many of the siderites. The carbon isotope data show that the carbonate for the formation of the siderite originated predominantly from methanogenic fermentation processes but there was also the varying influence of bacterial oxidation processes. The different petrological and isotopic characteristics of the ironstones broadly reflect variations in their depositional environments and the variable eogenetic conditions in which the siderite formed. There is no suitable single model to explain the genesis of all the different types of ironstones other than that a synsedimentary iron‐enrichment process is involved.     

Stable isotopic compositions of Recent freshwater cyanobacterial carbonates from the British Isles: local and regional environmental controls

Recent (<50 years old) freshwater cyanobacterial carbonates from diverse environments (streams, lakes, waterfalls) throughout Britain and Ireland were analysed for their stable carbon and oxygen isotope compositions. The mean δ¹⁸O value of ?5–9‰ PDB for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central Britain (?7–5‰SMOW) assuming a mean water temperature of 9°C. The mean δ¹⁸O of lake data, ?4–5‰ PDB, is statistically different, reflecting the effects of residence time and/or variations in the oxygen isotopic composition of rainfall. Carbon isotopes have wide variations in both fluviatile and lake data sets (+ 3 to ?12‰ PDB). These variations are principally controlled in the fluviatile samples by contribution of isotopically light ‘soil zone’ carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution. Lake samples have the heaviest carbon isotope values, reflecting a trend toward isotopic equilibrium between atmospheric CO₂ and aqueous HCO^?₃. We infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information, although the effects of stabilization and diagenesis on primary δ¹⁸O values will need careful consideration. Primary carbon isotope compositions should be well preserved, although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate.     

Nitrogen and oxygen isotopic compositions of water-soluble nitrate in Taihu Lake water system, China: implication for nitrate sources and biogeochemical process

The stable isotope nitrogen-15 (¹⁵N) is a robust indicator of nitrogen (N) source, and the joint use of δ¹⁵N and δ¹⁸O–NO₃ ^? values can provide more useful information about nitrate source discrimination and N cycle process. The δ¹⁵N and δ¹⁸O–NO₃ ^? values, as well as major ion tracers, from Taihu Lake in east China were investigated to identify the primary nitrate sources and assess nitrate biogeochemical process in the present study. The results show that the nitrate concentration in West Taihu Lake (WTL) was generally higher than those in East Taihu Lake (ETL) and its upstream inflow rivers. The NO₃ ^?/Cl^? value combined with mapping of δ¹⁵N–NO₃ ^? and NO₃ ^? concentration suggest that the mixing process should play a major effect in WTL, and denitrification was the dominant N transformation process in WTL. A linear relationship of close to ~1: 2 was observed between δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? values in WTL, confirming the occurrence of denitrification in WTL. The δ¹⁵N–NO₃ ^? data imply that sewage and manure were the principal nitrate sources in WTL and its feeder rivers, while the nitrate in ETL might derive from soil organic nitrogen and atmospheric deposition. The δ¹⁸O–NO₃ ^? data indicate most of nitrate from microbial nitrification of organic nitrogen matter possibly make a significant contribution to the lake.     

Modelling of the oxygen isotope evolution of seawater: implications for the climate interpretation of the δ18O of marine sediments

It is often argued that the δ¹⁸O value of oceanic water was maintained close to 0‰ for hundreds of millions of years, as a consequence of oxygen isotope exchange between oceanic crust and seawater. However, for several decades, the interpretation of the biosedimentary oxygen isotope record has conflicted with the igneous record because, with increasing age, a general trend of decreasing δ¹⁸O values (about 6‰) is observed in most carbonates, cherts and phosphates, especially for the Paleozoic and early Mesozoic. We developed a dynamical model of seawater-crust interaction that computes the δ¹⁸O value in these two reservoirs as function of time. This model takes into account the continuous production of crust at oceanic ridges, its expansion rate, the permeability profile with space and time, the mineralogical mode of the crust, and the kinetics of oxygen isotope exchange between rock-forming minerals and seawater. The model indicates that the δ¹⁸O value of seawater may vary by ±2‰ with a time response ranging from 5 to 50 Ma for expansion rates of 1 to 10 cm.a⁻¹. The variation of ±2‰ is fixed by both integrated water-rock ratio and closure time of the seawater-crust system by sediments. Variations in the oxygen isotope ratio of seawater through time have important implications for the interpretation of the systematically low δ¹⁸O values of pre-Jurassic marine sediments. According to our model, marine paleotemperatures could be up to 10°C lower than those expected when applying the classical hypothesis of an ice-free ocean with a δ¹⁸O value of −1‰.     

Local distribution of oxygen isotopes and fluid exchange during genesis of the corundum-bearing rocks of Khitostrov Island

New data are presented on the distribution of oxygen isotopes and conditions of the local isotope equilibrium in high-Al rocks rocks of Khitostrov Island showing abnormally low δ¹⁸O values (below–25‰). The temperatures of isotope equilibrium are within 400–475°C. The minimum δ¹⁸O values have been registered in the in plagioclase, whereas the same phases in kyanite-bearing rocks lacking corundum demonstrate δ¹⁸O values usually 3–5‰ higher. The fluid δ¹⁸O value varies from–22 to–16‰ at 475 ± 15°C, from–18 to–23‰ at 425 ± 25°C, and from–17 to–22‰ at 380 ± 15°C. The results obtained do not require abnormal depletion of δ¹⁸O values owing to the infiltration of an external fluid under the Svecofennian transformations. The association of corundum-bearing rocks with the basic intrusions, the presence of zircon cores of older ages compared to these rocks, and the peculiarities of rock chemistry may be ascribed to the fact that lower crustal layers of ancient rocks depleted in δ¹⁸O before metamorphism were captured by basite melts.     

Seasonal variability of oxygen and hydrogen stable isotopes in precipitation and cave drip water at Guilin,southwest China

The interpretation of climatic information from stalagmites has traditionally been a complex research problem, with oxygen isotopes playing a particularly important role in global climate change studies. This study investigates the relationship between oxygen isotope composition of the atmospheric in precipitation and cave drip water at Panlong cave in southwest China on seasonal timescales of variability. Time series seasonal variability was derived from Panlong cave in Guilin by collecting daily precipitation samples for stable isotope analysis during 2012. Results indicate that δ¹⁸O of precipitation contains a clear seasonal variation whereby higher values are mainly distributed during winter and lower values during summer. Seasonal variations in water sources affect the precipitation δ¹⁸O values. Drip water δ¹⁸O also displayed a seasonal cycle which is attenuated relative to δ¹⁸O of precipitation. Drip water time series display seasonal cycle ranges from 1.5 to 3.5 ‰ relative to Vienna Standard Mean Ocean Water, which mainly follow the precipitation δ¹⁸O seasonal cycle. Seasonal variation in drip water δ¹⁸O supports interpretations of the stalagmite δ¹⁸O record as a paleoclimate proxy sensitive to the local environment. This monitoring experiment revealed that drip water must be transported through the epikarst in approximately 1.5 months during cold periods, and <0.5 months during warm periods. Different residence time percolation is mainly affected by the atmospheric precipitation amount, depending on whether soil moisture reaches saturation.     

Evaluation of nitrate source in surface water of southwestern China based on stable isotopes

Stable isotope tracing and analysis play an important role in interpretation of hydrological and ecological processes at the watershed scale and can provide information regarding the flow path, water source, nutrient loss and biogeochemical cycles of a system. In this study, environmental isotopes (δ¹⁸O-H₂O, δD, δ¹⁵N-NO₃ ^?, δ¹⁸O-NO₃ ^?) and chemical compositions of surface water in Guizhou Province, China, were measured to evaluate the primary sources of nitrate and characterize the processes affecting nitrate as well as its correlation with vegetation cover in karstic areas. The δ¹⁵N and δ¹⁸O-NO₃ ^? levels ranged from +1.3 to +9.8 ‰ and +4.7 to +16.9 ‰, respectively, which indicated that nitrate in water from the investigated area primarily originated from nitrification of soil organic matter during the sampling period. There was also a wide range of isotopes in the water and high contents of nitrate in karstic areas with poor vegetation cover, indicating that water and nutrient loss were serious problems hindering plant growth in the study areas. For example, there was a positive relationship between isotopic composition and nitrate content in the natural forest and negative relationship in Libo County nearby, which suggested that the nitrate fate was affected by land use and human disturbance.     

Changes to depositional palaeoenvironments within the Qikou depression (Bohaiwan Basin,China): carbon and oxygen isotopes in lacustrine carbonates of the Palaeogene Shahejie Formation

This paper reports the carbon and oxygen isotope compositions of lacustrine carbonate sediments from the Palaeogene Shahejie Formation, Qikou depression, Bohaiwan Basin, with the aim of determining the palaeoenvironmental conditions in the region. Results from Es₂, the second member of the Shahejie Formation, showed values of δ¹³C and δ¹⁸O from –1.2‰ to +2.4‰ (average +0.6‰) and from –6.8‰ to –4.7‰ (average –5.7‰), respectively, suggesting a relatively hot climate attending deposition. The slightly closed nature of the lake, which contains brackish water, resulted in higher carbonate δ¹³C and δ¹⁸O values than in a meteoric environment. The values of δ¹³C and δ¹⁸O preserved within the carbonates of the overlying lower Shahejie I (Es₁) varied between +1.3‰ and +4.9‰ (average +3.2‰) and from ?4.4‰ to ?1.8‰ (average ?3.1‰), respectively, indicating that the climate became colder at that time. Subsequently, a marine transgression caused the salinity of the lake water to increase. The values of δ¹³C and δ¹⁸O were controlled by salinity. The high δ¹³C values were also influenced by the rapid burial of the lake organisms and by algal photosynthesis. Values of δ¹³C and δ¹⁸O from carbonates in upper Es₁ ranged from ?8.0‰ to +11.0‰ (average +10.1‰) and from ?5.0‰ to ?1.5‰ (average ?3.4‰), respectively, indicating a slight increase in the temperature over time. In the closed and reducing environment, extremes in δ¹³C values resulted from biochemical fermentation. The positive δ¹³C excursion recorded in the carbonates of the Shahejie Formation in the Qikou depression indicates that the palaeoclimate underwent a significant transformation during the Eocene and the Oligocene.     

Application of multiple-isotope and groundwater-age data to identify factors affecting the extent of denitrification in a shallow aquifer near a river in South Korea

The extent of denitrification in a small agricultural area near a river in Yangpyeong, South Korea, was determined using multiple isotopes, groundwater age, and physicochemical data for groundwater. The shallow groundwater at one monitoring site had high concentrations of NO₃-N (74–83 mg L^?1). The δ¹⁵N-NO₃ values for groundwater in the study area ranged between +9.1 and +24.6‰ in June 2014 and +12.2 to +21.6‰ in October 2014. High δ¹⁵N-NO₃ values (+10.7 to +12.5‰) in both sampling periods indicated that the high concentrations of nitrate in the groundwater originated from application of organic fertilizers and manure. In the northern part of the study area, some groundwater samples showed elevated δ¹⁵N-NO₃ and δ¹⁸O-NO₃ values, which suggest that nitrate was removed from the groundwater via denitrification, with N isotope enrichment factors ranging between ?4.8 and ?7.9‰ and O isotope enrichment factors varying between ?3.8 and ?4.9‰. Similar δD and δ¹⁸O values of the surface water and groundwater in the south appear to indicate that groundwater in that area was affected by surface-water infiltration. The mean residence times (MRTs) of groundwater showed younger ages in the south (10–20 years) than in the north (20–30 years). Hence, it was concluded that denitrification processes under anaerobic conditions with longer groundwater MRT in the northern part of the study area removed considerable amounts of nitrate. This study demonstrates that multi-isotope data combined with physicochemical data and age-dating information can be effectively applied to characterize nitrate contaminant sources and attenuation processes.     

The origin of variable-δ18O zircons in Jurassic and Cretaceous Mo-bearing granitoids in the eastern Xing–Meng Orogenic Belt,Northeast China

This study reports new zircon U–Pb ages, Lu–Hf isotope data, and oxygen isotope data for Mesozoic Mo-bearing granitoids in the eastern Xing–Meng Orogenic Belt (XMOB) of Northeast China, within the eastern Central Asian Orogenic Belt. Combining these new laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb ages with the results of previous research indicates that two stages of Mo-bearing granitoid magmatism occurred in the eastern XMOB, during the Early–Middle Jurassic (200–165 Ma) and the Early Cretaceous (ca. 111 Ma). The eastern XMOB also contains Mo-bearing granitoids with variable δ¹⁸O compositions that record variations in source oxygen isotopic compositions. Combining δ¹⁸O data with zircon U–Pb and Hf isotopic data provides evidence of the origin of these granitoids. Three types of zircon have been identified within these granitoids. Type 1 zircons formed during the Mesozoic and having high δ¹⁸O values (5.71–7.05‰) that are consistent with the compositions of magmatic zircons from the Luming, Jiapigou, and Kanchuangou areas. These zircons suggest that the Mo-bearing granitoids were derived from a source containing supracrustal materials. The type 2 zircons have extremely low and heterogeneous δ¹⁸O values (4.64–4.89‰) that are consistent with the compositions of magmatic zircons from the Jidetun and Fuanpu areas. These magmas were generated by the remelting of juvenile crustal material that was previously significantly modified by interaction with fluids. Type 3 zircons generally have mantle-like δ¹⁸O values (5.42–5.57‰), with several zircons yielding higher δ¹⁸O values, suggesting that these intrusions formed from mantle-derived magmas that assimilated and were metasomatized by crustal material. Combining these geochemical data with the geology of this region indicates that the Mo-bearing granitoids were generated as a result of subduction of the Palaeo-Pacific Plate beneath the Eurasian continent.     

A study of oxygen isotopic fractionation during bio-induced calcite precipitation in eutrophic Baldeggersee,Switzerland

In order to better understand environmental factors controlling oxygen isotope shifts in autochthonous lacustrine carbonate sequences, we undertook an extensive one-year study (March, 1995 to February, 1996) of water-column chemistry and daily sediment trap material from a small lake in Central Switzerland. Comparisons between calculated equilibrium isotope values, using the fractionation equation of Friedman and O’Neil, (1977) and measured oxygen isotope ratios of calcite in the sediment-traps reveal that oxygen isotopic values of autochthonous calcite (δ¹⁸O) are in isotopic equilibrium with ambient water during most of the spring and summer, when the majority of the calcite precipitates. In contrast, small amounts of calcite precipitated in early-spring and again in late-autumn are isotopically depleted in ¹⁸O relative to the calculated equilibrium values, by as much as 0.8‰. This seasonally occurring apparent isotopic nonequilibrium is associated with times of high phosphorous concentrations, elevated pH (∼8.6) and increased [CO₃²⁻] (∼50 μmol/l) in the surface waters. The resulting weighted average δ¹⁸O value for the studied period is −9.6‰, compared with a calculated equilibrium δ¹⁸O value of −9.4‰. These data convincingly demonstrate that δ¹⁸O of calcite are, for the most part, a very reliable proxy for temperature and δ¹⁸O of the water.     

江西德兴铜厂铜矿水－岩体系氢氧同位素演化

 江西铜厂铜矿床露天采场岩石76个全岩氧同位素组成表明,该超大型铜矿的形成与具有5个水-岩交换成矿体系汇集在一起有关。水-岩体系计算表明,成矿流体储库形成是大气降水与千枚岩和花岗闪长斑岩在300℃以及W/R比值为0.5左右条件下形成,而后上升进入矿化沉积体系时温度降低,W/R比值超过10.0,计算的最少水量达1.9×10¹⁰t以上。     

Carbon and Oxygen Isotopic Composition of Surface‐Sediment Carbonate in Bosten Lake (Xinjiang,China) and its Controlling Factors

Bosten Lake is a mid-latitude lake with water mainly supplied by melting ice and snow in the Tianshan Mountains. The depositional environment of the lake is spatially not uniform due to the proximity of the major inlet and the single outlet in the western part of the lake. The analytical results show that the carbon and oxygen isotopic composition of recent lake sediments is related to this specific lacustrine depositional environment and to the resulting carbonate mineralogy. In the southwestern lake region between the Kaidu River inlet and the Kongqi River outlet, carbon isotope composition (δ13C) values of the carbonate sediment (?1‰ to ?2‰) have no relation to the oxygen isotope composition of the carbonate (δ18O) values (?7‰ to ?8‰), with both isotopes showing a low variability. The carbonate content is low (<20%). Carbonate minerals analyzed by X-ray diffraction are mainly composed of calcite, while aragonite was not recorded. The salinity of the lake water is low in the estuary region as a result of the Kaidu River inflow. In comparison, the carbon and oxygen isotope values are higher in the middle and eastern parts of the lake, with δ13C values between approximately +0.5‰ and +3‰, and δ18O values between ?1‰ and ?5‰. There is a moderate correlation between the stable oxygen and carbon isotopes, with a coefficient of correlation r of approximately 0.63. This implies that the lake water has a relatively short residence time. Carbonate minerals constitute calcite and aragonite in the middle and eastern region of the lake. Aragonite and Mg–calcite are formed at higher lake water salinity and temperatures, and larger evaporation effects. More saline lake water in the middle and eastern region of the lake and the enhanced isotopic equilibrium between water and atmospheric CO2 cause the correlating carbon and oxygen isotope values determined for aragonite and Mg–calcite. Evaporation and biological processes are the main reasons for the salinity and carbonate mineralogy influence of the surface-sediment carbonate in Bosten Lake. The lake water residence time and the CO2 exchange between the atmosphere and the water body control the carbon and oxygen isotope composition of the carbonate sediment. In addition, organic matter pollution and decomposition result in the abnormally low carbon isotope values of the lake surface-sediment carbonate.     

Oxygen Isotopic Study on the Date‐Nagai Skarn‐Type Tungsten Deposit,Northeastern Japan

The skarn‐type tungsten deposit of the Date‐Nagai mine is genetically related to the granodiorite batholith of the Iidateyama body. Skarn is developed along the contact between pelitic hornfels and marble that remains as a small roof pendant body directly above the granodiorite batholith. Zonal arrangement of minerals is observed in skarn. The zonation consists of wollastonite, garnet, garnet‐epidote, and vesuvianite‐garnet zones, from marble to hornfels. Sheelite is included in garnet, garnet‐epidote, and vesuvianite‐garnet zones. The oxygen isotope values of skarn minerals were obtained as δ¹⁸O = 4.2–7.7‰ for garnet, 5.9–6.9‰ for vesuvianite, ?0.3–3.4‰ for scheelite, 6.0–10.9‰ for quartz, and 8.2‰ for muscovite. The temperature of skarn‐formation was calculated from oxygen isotopic values of scheelite‐quartz pairs to be 288°C. Calculated oxygen isotope values of fluid responsible for skarn minerals were 6.1–9.5‰ for garnet, 1.2–4.8‰ for scheelite, ?1.3‐3.6‰ for quartz, and 4.5‰ for muscovite. Garnet precipitated from the fluids of different δ¹⁸O values from scheelite, quartz, and muscovite. These δ¹⁸O values suggest that the origin of fluid responsible for garnet was magmatic water, while evidence for the presence of a meteoric component in the fluids responsible for middle to later stages minerals was confirmed.