Oxygen Isotope Determinations Research Articles

Comparison of different preparation methods for oxygen isotope determination of phosphate in mammal tooth enamel

AbstractWe tested several sample pre-treatment protocols for the study of oxygen isotope ratios in the phosphate phase of mammalian enamel of ten different fossil samples. We investigated the effect of different pre-treatment methods and the duration of the hydrogen fluoride treatment on enamel samples from skeletal phosphate with known δ18O values. The samples had been measured previously, so we could compare the ratios measured in our laboratory with the previous values to choose the best chemical preparation procedure. Four pre-soaking methods and two different time intervals of 2 mol dm−3 hydrogen fluoride treatment were compared during our experiments. In our experimental conditions, the distilled water wash and the 6 h of soaking in hydrogen fluoride gave the closest results to the expected δ-values. The steps of the tested preparation processes were repeated at least three times on each sample, so the reproducibility of the process could be also investigated.

Study on the complex magmatic and hydrothermal metallogenic processes in the giant Huayangchuan uranium (U)-polymetallic ore district: Constraints from geochemical and geochronological evidences

The supergiant Huayangchuan uranium (U)-polymetallic deposit is situated in the west part of Lesser Qinling Orogen, Central China, which is renowned for its unique metallogenic type and complex mineralization processes. This newly-discovered deposit contains economic endowments of U, Nb, Pb, Se, Sr, Ba and REEs. The genesis of this deposit remains hugely controversial and the related magmatic and metallogenic framework have not been well established. In our study, we present newly obtained zircon U-Pb age data, whole-rock trace elements, detailed SEM observation, carbon and oxygen isotope determination and titanite U-Pb age result to provide geochemical and geochronological constraints on the magmatic and hydrothermal metallogenic processes in Huayangchuan ore district and give insight into the REE, Nb and U mineralization and remobilization mechanism. Carbonatite in Huayangchuan deposit is rich in Ba, Sr, Nb, U and REE and the δ18OSMOW and δ13CPDB (‰) values of carbonatite vary between + 7.30 ‰ to + 9.80 ‰ and − 6.00 ‰ to − 7.20 ‰, respectively, which is the unique characteristic of igneous carbonatite. The total REE contents of carbonatite range from 1319.5 to 2535.8 ppm, significantly higher than that of the granite pegmatite dykes and Taihua Group granitic gneiss. Two zircon grains from the Huayangchuan carbonatite yield the 206 Pb / 238 U age of 223 ± 5.6 Ma and 217 ± 8.6 Ma. Uranium and niobium are mainly hosted in the primary mineral of betafite, minorly in uraninite and fergusonite. REE are remained in various REE carbonates, phosphates and silicates, e.g. allanite, parite, monazite, apatite, yttrialite and xenotime. Betafite contains numerous mineral inclusions of calcite, allanite and apatite, which displays as vermicular relicts with metasomatic corrosion texture. The intimate relationship between betafite, calcite, apatite, allanite and REE carbonates implies that U and Nb mineralization occurred during carbonatite-related magmatic stage. Late Triassic carbonatite magmatism (∼220 Ma) was responsible for the world-class U-Nb-Mo-REE mineralization in the Qinling Orogen. Whereas, LA-ICP-MS U-Pb data of titanite collected from Huayangchuan carbonatite reveal two sets of ages in the Huayangchuan deposit: during the Late Triassic and the Early Cretaceous. LA-ICP-MS data of the Triassic titanite and Cretaceous titanite in Huayangchuan deposit yield the lower intercept age of 223.1 ± 9.4 Ma and 141.9 ± 4.7 Ma respectively in the Tera-Wasserburg (238U/206Pb - 207Pb/206Pb) diagram. Cretaceous hydrothermal titanites show enrichment in Y, HREEs and depletion in TiO2 and LREEs relative to the Triassic magmatic titanites. Primary betafite and titanite were subjected to later hydrothermal alteration. The close genetic relation between altered titanite and Ti-Fe-Nb-Pb-U mineral inclusions demonstrates that Early Cretaceous hydrothermal fluids contributed to the Nb, U and Ti remobilization. Early Cretaceous regional granitic magmatic activity brought hydrothermal fluids to alter the early carbonatite magmatic pyrochlore and titanite. U, Nb, Ti, Y, Pb and HREEs were transferred and subsequently re-precipitated into uraninite, and hydrothermal titanite.

Epidote Reference Material Development Calibrated for Oxygen Isotope Determination by Secondary Ion Mass Spectrometry (SIMS).

Epidote Reference Material Development Calibrated for Oxygen Isotope Determination by Secondary Ion Mass Spectrometry (SIMS).

Collection of Nitrogen Dioxide for Nitrogen and Oxygen Isotope Determination─Laboratory and Environmental Chamber Evaluation

The family of atmospheric oxides of nitrogen, NOy (e.g., nitrogen oxides (NOx) + nitric acid (HNO3) + nitrous acid (HONO) + peroxyacetyl nitrate (PAN) + particulate nitrate (pNO3-) + other), have an influential role in atmospheric chemistry, climate, and the environment. The nitrogen (δ15N) and oxygen (δ18O and Δ17O) stable isotopes of NOy are novel tools for potentially tracking emission sources and quantifying oxidation chemistry. However, there is a lack of well-established methods, particularly for speciated gas-phase components of NOy, to accurately quantify δ15N, δ18O, and Δ17O. This work presents controlled laboratory experiments and complex chamber α-pinene/NOx oxidation experiments of a sampling apparatus constructed for the simultaneous capture of multiple NOy species for isotope analysis using a series of coated denuders, with a focus on nitrogen dioxide (NO2•). The laboratory tests indicate complete NO2• capture for the targeted concentration of 15 ppbv for at least 24 h collections at 10 liters per minute, with δ15N and δ18O precisions of ±1.3‰ and 1.0‰, respectively, and minimal (2.2% ± 0.1%) NO2• collection on upstream denuders utilized for the capture of HNO3 and other acidic gases. The multispecies NOy collection system showed excellent concentration correlations with online instrumentation for both HNO3 and NO2• and isotope reproducibility of ±1.7‰, ±1.8‰, and ±0.7‰ for δ15N, δ18O, and Δ17O, respectively, for replicate experiments and highly time-resolved collections. This work demonstrates a new method that can enable the simultaneous collection of HNO3 and NO2• for accurate quantification of concentration and isotopic composition.

LM-19 lawsonite: a potential reference material for in situ oxygen isotope determination in lawsonite by ion microprobe

A natural lawsonite sample LM-19 is presented as a potential reference material for in situ O isotope analysis.

Investment of needle nitrogen to photosynthesis controls the nonlinear productivity response of young Chinese fir trees to nitrogen deposition

Plant carbon (C) assimilation is expected to nonlinearly increase with continuously increasing nitrogen (N) deposition, causing a N saturation threshold for productivity. However, the response of plant productivity to N deposition rates and further the N saturation threshold still await comprehensive quantization for forest ecosystem. Here, we tested the effect of N addition on aboveground net primary productivity (ANPP) of three-year old Chinese fir (Cunninghamia lanceolata) trees by adding N at 0, 5.6, 11.2, 22.4, and 44.8 g N m−2 yr−1 for 2.5 years. The N saturation threshold was estimated based on a quadratic-plus-plateau model. Results showed that ANPP transitioned from an increasing stage with increasing N addition rate to a plateaued stage at an N rate of 16.3 g N m−2 yr−1. The response of ANPP to N addition rates was well explained by the net photosynthetic rates of needles. Results from the dual isotope measurement [simultaneous determination of needle stable carbon (δ13C) and oxygen (δ18O) isotopes] indicated that the photosynthetic capacity, rather than the stomatal conductance, mediated the response of photosynthesis and ANPP of the young Chinese fir trees to N addition. Accordingly, the amount of needle N partitioning to water-soluble fraction, which is associated with the photosynthetic capacity, also responded to N enrichment with a nonlinear increase. Our study will contribute to a more accurate prediction on the influence of N deposition on C cycles in Chinese fir plantations.

Isotope partitioning between cow milk and farm water: A tool for verification of milk provenance

RationaleThe oxygen and hydrogen isotope compositions of the water component of the milk from nine Italian dairy farms were studied together with the farm water for one year. The aim was to verify the importance of farm water and seasonal temperature variation on milk isotope values and propose mathematical relations as new tools to identify the milk origin.MethodsMilk was centrifuged to separate the solids and then distilled under vacuum to separate water. δ(18O/16O) and δ(2H/1H) analyses of the water molecules were carried out using a water equilibrator online with a mass spectrometer. For oxygen and hydrogen isotope determination, water was equilibrated with pure CO2 for 7.5 h and with pure H2 for 5 h, respectively. The isotope ratio value is indicated with δ (expressed on the VSMOW/SLAP scale) as defined by IUPAC.ResultsThe average annual isotope value of milk at the different cattle sheds is mostly related to the farm water suggesting that the drinking water is the most important factor influencing the isotopic values of the milk water. The milk/water fractionation factor correlates with the milking time and, thus, the seasonal temperature is best described by a 4th order polynomial regression line. A two‐level check model was used to verify the milking provenance.ConclusionsThis study shows that it is essential to analyze both milk and farm water to indicate provenance. A two‐step verification tool, based on the difference between the measured and calculated δ(18O/16O)M values, and the difference between the calculated and estimated milk‐water fractionation factors, allowed the source determination of milk. Both conditions must be met if the milk is considered to be from the Parmigiano‐Reggiano production region. Although this approach was developed for this region, it can easily be tested and adapted to other dairy production areas.

Blank experimental study on the determination of nitrogen and oxygen isotopes by chemical conversion method.

The reduction of nitrate to nitrous oxide by chemical conversion has the characteristics of simple operation and high sensitivity. Therefore, it is widely used in the pretreatment of nitrogen and oxygen isotopes. In this paper, a series of blank experiments were performed to determine various factors influencing the determination process for the chemical conversion method. To determine blank background values for reagents such as deionized water, NaCl and cadmium powder used in the experiment, the optimal experimental conditions of NaN3, NaCl and cadmium powder were determined to maximize the nitrate reduction rate. The results showed that NaCl was burned at 450 °C for 48 hours, and the placement time was not more than one day, which could minimize the pollution introduced by NaCl. When 0.3 g of cadmium powder and 600 nmol of NaN3 were added, the overall reduction efficiency reached 90%. After measuring actual standard samples, the method demonstrated a good accuracy and applicability.

Impact of nitrate contamination on the analysis of carbon and oxygen isotopes in carbonate and a low-temperature phosphoric acid digestion approach for online sample preparation.

High-temperature phosphoric acid digestion combined with continuous-flow isotope ratio mass spectrometry (CF-IRMS) is one of the standard methods for the determination of carbon and oxygen isotopes in carbonate. However, the routine purification protocol has not been tested by samples with nitrate contamination. Here we show that the standard protocol is not capable of measuring such samples with satisfactory accuracy and precision. We tested the low-temperature (25°C) phosphoric acid digestion method combined with CF-IRMS for the measurement of natural carbonate samples with ~1% nitrate concentration and nitrate-doped carbonate standards. We also recorded the Raman spectra of the gases extracted from the nitrate-doped carbonate standards to identify the gas species that affect the results. The accuracy and precision of δ18 O values for nitrate-doped carbonate standards digested at 72°C are much poorer than routine measurements. For the measurements of doped standards reacted at 25°C, the accuracy and precision of the δ13 C and δ18 O values are indistinguishable from normal results. Raman spectra demonstrate that NO2 generated from high-temperature digestion is the possible interference species for oxygen isotope measurement. Low-temperature digestion is a reliable method for the measurement of carbon and oxygen isotopes in nitrate-contaminated carbonate samples because the formation of NO2 can be largely reduced. This approach can be applied to some nitrate-rich speleothems, evaporites and other samples with nitrate contamination.

The Diamantina Monazite: A New Low‐Th Reference Material for Microanalysis

Most monazite reference materials (RMs) for in situ U‐Pb geochronology are rich in Th; however, many hydrothermal ore deposits contain monazite that is low in trace element contents, including Th, U and Pb. Because of potential problems with matrix effects and the lack of appropriate matrix‐matched RMs, such variations can bias dating of hydrothermal deposits. In this paper, we describe a polycrystalline low‐U and low‐Th Diamantina monazite from the Espinhaço Range, SE Brazil. It has a U‐Pb ID‐TIMS weighted mean 207Pb*/235U ratio of 0.62913 ± 0.00079, 206Pb*/238U of 0.079861 ± 0.000088 and 207Pb*/206Pb* of 0.057130 ± 0.000031, yielding a weighted mean 206Pb*/238U date of 495.26 ± 0.54 Ma (95% c.l.). In situ dates acquired with different methods (LA‐(Q, SF, MC)‐ICP‐MS and SIMS) are within uncertainty of the ID‐TIMS data. U‐Pb LA‐(Q, MC)‐ICP‐MS runs, using Diamantina as a primary RM, reproduced the ages of other established RMs within < 1% deviation. The LA‐MC‐ICP‐MS analyses yielded homogeneous Sm‐Nd isotopic compositions (143Nd/144Nd = 0.511427 ± 23, 2s; 147Sm/144Nd = 0.1177 ± 13, 2s) and εNd(495 Ma) of −18.7 ± 0.5 (2s). SIMS oxygen isotope determinations showed measurement reproducibility better than ± 0.3‰ (2s), confirming Diamantina's relative homogeneity at test portion masses below 1 ng.

Secondary Ion Mass Spectrometry Analyses of Diamond and Moissanite in Ophiolite

The Cameca 1280‐HR large geometry SIMS instrument is a highly versatile analytical tool which can support a broad range of geochemical applications. Research using the Potsdam 1280 instrument focuses primarily on isotope ratio determinations in geomaterials. Optimized measurement protocols have already been established for δ18O determinations in zircon, and we are also working towards routine oxygen isotope determinations for quartz, calcite, mica, apatite and titanite. The primary challenge in developing such measurement systems are the identification and characterization of suitable reference materials (RMs), and this is made particularly challenging due to the matrix dependent ion yields of the SIMS ion source.Here we wish to report our progress towards establishing new analytical protocols for the determination of δ13C in both diamond and moissanite. In the case of diamond, our facility possesses three natural RMs with which we are able to produce data with a typical analytical repeatability of ∼0.15 ‰ (1sd). An inter‐comparison of our three diamond RMs demonstrates an overall data quality of better than 0.5‰ in terms of systematic offset between the various materials characterized using gas source mass spectrometry (Palot et al., 2012). A single such δ13C determination in diamond requires 80 s of data acquisition and involves a test portion mass of ∼400 pg of material. In‐house diamond reference materials for δ15N calibration allow us to measure this isotopic system to a total analytical uncertainty of ± 1.6 ‰ (1sd) at nitrogen concentrations reaching down to 250 μg/g. Due to the relatively low abundance of nitrogen in diamonds, such isotope ratio determinations require around 9 minutes of data collection.With respect to δ13C determinations in moissanite, we use a kimberlitic SiC as calibrant (Mathez et al., 1995), on which we achieve a repeatability of ∼0.2 ‰ (1sd) on a ∼350 pg test portion mass. Total data acquisition time for such measurements is 80 s. We are currently in the process of developing a second moissanite RM based on a synthetic, coarse‐grained powder. We will also investigate this new material for its δ30Si characteristics.

Enhanced Oxygen Isotope Determination in Uranium Oxides Using BrF5 Fluorination

A new method for accurate determination of oxygen isotopes in uranium oxides encountered in the nuclear fuel cycle was developed using the conventional BrF5 fluorination technique. Laser‐assisted fluorination was tested for comparison. We focused on fine powders of triuranium octoxide (U3O8), uranium dioxide (UO2±x with 0 ≤ x ≤ 0.25), uranium trioxide (UO3.nH2O, with 0.8 ≤ n ≤ 2) and diuranates (M2U2O7.nH2O, with M = NH4, Na or Mg0.5 and 0 ≤ n ≤ 6). Fluorination at room temperature and heating under vacuum at 150 °C are shown to eliminate both adsorbed and structural water from the powder samples. Precision fit for purpose of δ18O values (± 0.3‰, 1s) and oxygen yields (close to 100%) were obtained for U3O8 and UO2 where oxygen is only bound to uranium. A lower precision was observed for UO3.nH2O and M2U2O7.nH2O where oxygen is both present in the structural H2O and bonded to uranium and where the extracted O2(g) can be contaminated by NF3 and NOx compounds. Laser‐assisted fluorination gave shifted δ18O values between +0.8 and +1.4‰ for U3O8, around −0.8‰ for UO3.nH2O and between −3.9 and −4.5‰ for M2U2O7.nH2O (± 0.3‰, 1s) compared with the conventional method.

Biotite Reference Materials for Secondary Ion Mass Spectrometry 18O/16O Measurements

Five new biotite reference materials were calibrated at the SwissSIMS laboratory (University of Lausanne) for oxygen isotope determination by secondary ion mass spectrometry (SIMS) and are available to the scientific community. The oxygen isotope composition of the biotites, UNIL_B1 to B5, was determined by laser‐heating fluorination to be 11.4 ± 0.11‰, 8.6 ± 0.15‰, 6.1 ± 0.04‰, 7.1 ± 0.05‰ and 7.6 ± 0.04‰, respectively. SIMS analyses on spots smaller than 20 μm gave a measurement repeatability of 0.3‰ (2 standard deviation, 2s). The matrix effect due to solid solution in natural biotite could be expressed as a linear function of XMg and XF for biotite. No effect was found for different crystallographic orientations. SIMS analysis allows the oxygen isotope composition of biotite to be measured with a measurement uncertainty of 0.3–0.4‰ (2s) for biotites with similar major element compositions. A measurement uncertainty of 0.5‰ (2s) is realistic when F poor biotites (lower than 0.2% m/m oxides) within the compositional range of XMg of 0.3–0.9 were compared from different sessions. The linear correlation with F content offers a reasonable working curve for F‐rich biotites, but additional reference materials are needed to confirm the model.

Root‐related rhodochrosite and concretionary siderite formation in oxygen‐deficient conditions induced by a ground‐water table rise

AbstractSedimentological, mineralogical, stable carbon and oxygen isotope determinations and biomarker analyses were performed on siderite concretions occurring in terrestrial silts to understand their formation and to characterize the sedimentary and diagenetic conditions favouring their growth. High δ13C values (6·4‰ on average) indicate that siderite precipitated in an anoxic environment where bacterial methanogenesis operated. The development of anoxic conditions during shallow burial was induced by a change in sedimentary environment from flood plain to swamp, related to a rise of the ground‐water table. Large amounts of decaying plant debris led to efficient oxygen consumption within the pore‐water in the peat. Oxygen depletion, in combination with a decrease in sedimentation rate, promoted anoxic diagenetic conditions under the swamp and favoured abundant siderite precipitation. This shows how a change in sedimentary conditions can have a profound impact on the early‐diagenetic environment and carbonate authigenesis. The concretions contain numerous rhizoliths; they are cemented with calcium‐rhodochrosite, a feature which has not been reported before. The rhodochrosite cement has negative δ13C values (−16·5‰ on average) and precipitated in suboxic conditions due to microbial degradation of roots coupled to manganese reduction. The exceptional preservation of the epidermis/exodermis and xylem vessels of former root tissues indicates that the rhodochrosite formed shortly after the death of a root in water‐logged sediments. Rhodochrosite precipitated during the initial stages of concretionary growth in suboxic microenvironments within roots, while siderite cementation occurred simultaneously around them in anoxic conditions. These suboxic microenvironments developed because oxygen was transported from the overlying oxygenated soil into sediments saturated with anoxic water via roots acting as permeable conduits. This model explains how separate generations of carbonate cements having different mineralogy and isotopic compositions, which would conventionally be regarded as cements precipitated sequentially in different diagenetic zones during gradual burial, can form simultaneously in shallow burial settings where strong redox gradients exist around vertically oriented permeable root structures.

Precise oxygen and hydrogen isotope determination in nanoliter quantities of speleothem inclusion water by cavity ring-down spectroscopic techniques

Speleothem inclusion-water isotope compositions are a promising new climatic proxy, but their applicability is limited by their low content in water and by analytical challenges. We have developed a precise and accurate isotopic technique that is based on cavity ring-down spectroscopy (CRDS). This method features a newly developed crushing apparatus, a refined sample extraction line, careful evaluation of the water/carbonate adsorption effect. After crushing chipped speleothem in a newly-developed crushing device, released inclusion water is purified and mixed with a limited amount of nitrogen gas in the extraction line for CRDS measurement. We have measured 50–260nL of inclusion water from 77 to 286mg of stalagmite deposits sampled from Gyokusen Cave, Okinawa Island, Japan. The small sample size requirement demonstrates that our analytical technique can offer high-resolution inclusion water-based paleoclimate reconstructions. The 1σ reproducibility for different stalagmites ranges from ±0.05 to 0.61‰ for δ18O and ±0.0 to 2.9‰ for δD. The δD vs. δ18O plot for inclusion water from modern stalagmites is consistent with the local meteoric water line. The 1000 ln α values based on calcite and fluid inclusion measurements from decades-old stalagmites are in agreement with the data from present-day farmed calcite experiment. Combination of coeval carbonate and fluid inclusion data suggests that past temperatures at 9–10 thousand years ago (ka) and 26ka were 3.4±0.7°C and 8.2±2.4°C colder than at present, respectively.

Simultaneous determination of stable carbon, oxygen, and hydrogen isotopes in cellulose.

A technological development is described through which the stable carbon-, oxygen-, and nonexchangeable hydrogen-isotopic ratios (δ(13)C, δ(18)O, δ(2)H) are determined on a single carbohydrate (cellulose) sample with precision equivalent to conventional techniques (δ(13)C 0.15‰, δ(18)O 0.30‰, δ(2)H 3.0‰). This triple-isotope approach offers significant new research opportunities, most notably in physiology and medicine, isotope biogeochemistry, forensic science, and palaeoclimatology, when isotopic analysis of a common sample is desirable or when sample material is limited.

40Ar/39Ar ages and stable isotopes of supergene jarosite from the Baiyin VHMS ore field, NE Tibetan Plateau with paleoclimatic implications

Supergene jarosite is widely distributed in weathering profiles derived from hypogene sulfide ores. Precise 40Ar/39Ar dating and determination of hydrogen and oxygen isotopes of supergene jarosite can not only place tight constraints on the timing and history of continental weathering and supergene enrichment of sulfide ore deposits, but also shed significant insights into paleoclimatic conditions and tectonic uplift responsible for the weathering. In this paper, we present a preliminary study of 40Ar/39Ar geochronology and stable isotope geochemistry of supergene jarosite from the early Paleozoic Zheyaoshan volcanic-hosted massive sulfide deposit of the Baiyin ore field located in the eastern portion of the Qilian Mountain along the northeastern (NE) margin of the Tibetan Plateau in an attempt to better understand the climatic conditions prevailed during the pervasive weathering in the region. Jarosite from the Zheyaoshan mining area occurs either as gravels within alluvial deposits over low-relief flat-top hills or cm-wide veins crosscutting weathered sulfide ores or their wall rocks. The gravels consist of massive jarosite aggregates with tabular crystal morphology and contain 8.21 wt%–8.31 wt% K2O on average. Two jarosite grains extracted from the gravels (08-02-2 and 08-02-5) yield well-defined 40Ar/39Ar plateau ages of 37.1 ± 0.3 and 41.2 ± 0.4 Ma (2σ), respectively. They have δD values of −133 ‰ and −156 ‰, and \({\delta ^{18}}{{\rm{O}}_{{\rm{S}}{{\rm{O}}_4}}}\) values of 2.6 ‰ and 2.5 ‰. The vein-type jarosite consists of hexagonal pyramid crystals and contains much lower K2O, averaging at 2.44 wt%–2.72 wt%. Two jarosite veins (06-16-3 and 06-16-4) yield similar plateau ages of 3.2 ± 0.1 and 3.3 ± 0.1 Ma (2σ), respectively. These jarosite veins have δD values of −158 ‰ and −160 ‰, and \({\delta ^{18}}{{\rm{O}}_{{\rm{S}}{{\rm{O}}_4}}}\) values of 2.8 ‰ and 2.5 ‰. The 40Ar/39Ar ages indicate that sulfide ores of the Zheyaoshan deposit must have been brought to the surface since the late Eocene, and subsequently subjected to prolonged oxidation and supergene enrichment. The precipitation and preservation of jarosite in weathering profiles indicate that arid–semiarid climatic conditions must have been prevailed since the late Eocene. The hydrogen and oxygen isotopes of the jarosite further point to variation of climatic conditions in the area, with the Pliocene being slightly cooler relative to the late Eocene. Given that weathering profiles overlying sulfide ore deposits are widely distributed along the Qilian Mountain, systematic studies of 40Ar/39Ar geochronology and stable isotope geochemistry of jarosite-group minerals from these profiles would provide valuable information on the timing and evolution of regional tectonic uplift, paleoclimatic changes, and supergene mineralization in this mountainous belt along the NE edge of the Tibetan Plateau.

Determination of carbon and oxygen isotopes of geological samples with a complicated matrix: comparison of different analytical methods

Several automated on-line carbonate reaction devices coupled with isotope ratio mass spectrometers (IRMS) have been frequently used to determine the isotopic compositions of the carbon and oxygen in carbonates because of their high efficiency and small sample size.

Origin of fluids in the Hetaoping Pb–Zn deposit, Baoshan–Narong–Dongzhi block metallogenic belt, Yunnan Province, SW China

The Hetaoping skarn type Pb–Zn deposit is located in the Baoshan–Narong–Dongzhi block metallogenic belt (BND belt), a belt between the Tengchong terrane and the Lanping basin. The deposit is hosted by marble of the upper Cambrian Hetaoping Formation and there are no outcrops of plutonic rocks present. This deposit is one of two large Pb–Zn deposits recently discovered in the BND belt. The Hetaoping deposit is a high Mn skarn. Four types of fluid inclusions were recognized in quartz from the deposit: vapor-rich inclusions (Type I), liquid-rich inclusions (Type II), pure vapor inclusions (Type III), and pure fluid inclusions (Type IV). The coexistence of Type I and Type III inclusions in Stage I (pre-ore stage) and Stage II (main ore stage) shows evidence of fluid boiling. Quartz-hosted fluid inclusions (Stage I and Stage II) display high homogenization temperatures and salinities (134–315°C; 3.7–18.6wt% NaCl equivalent) but calcite-hosted fluid inclusions in Stage III (post-ore stage) record lower homogenization temperatures and salinities (85–214°C; 0.5–5.4wt% NaCl equivalent). These data suggest a possible mixing between primary magmatic water and meteoric water. Based on chromatography data, the fluid inclusions in quartz contain abundant CO2 and O2 and subordinate CO, CH4 and C2H2+C2H4, suggesting an oxidizing environment. Based on their Na/K and Cl/SO4 ratios, fluids contained in fluid inclusions are similar to volcanic spring waters. The low Na/K ratios (0.40–1.34) of the ore-forming fluids may have resulted from interaction with a deep alkaline intermediate-acid intrusion. Hydrogen and oxygen isotope determinations on quartz from different ore stages show low δ18O and δD values relative to VSMOW (−4.3‰ to 2.3‰; −109‰ to −91‰), indicating that the ore-forming fluids were diluted by external fluid sources as the skarn system cooled. Overall, geological and geochemical interpretations suggest that the Hetaoping deposit is a distal manganese skarn Pb–Zn deposit related to concealed intrusions.

Growth model of a hydrothermal low-temperature Si-rich chimney: Example from the CDE hydrothermal field, Lau Basin

The CDE hydrothermal field was first discovered during a Chinese cruise to the East Lau Basin Spreading Centre in 2007. Apart from significant amounts of loose Fe-Si-Mn (oxyhydr) oxide (referred to as oxide below) precipitates, a small Si-rich oxide chimney was also recovered on this cruise. In this study, we report on the mineralogical and geochemical analyses of this chimney and a model for its growth that has been developed. Based on the mineralogy and O isotope results, the chimney walls can be divided into four growth generations (layers) from the inner to the outer layers: amorphous opal and barite layer (precipitation temperature 68.5°C based on oxygen isotope determinations), a rod-like amorphous layer (precipitation temperature 39.6°C), a filamentous Fe-Si oxide layer, and an outer Fe-Mn oxide layer. Investigations based on SEM and EDS showed that neutrophilic Fe-oxidizing bacteria play an important role in the formation of this chimney, particularly in the outer two generations. In the first stage, the metabolic activity of the microbes results in the pervasive precipitation of the filamentous Fe-rich oxides inside a ring formed by some amorphous opal and barite; therefore, a loose porous layer forms. In the second stage, amorphous opal then precipitates inside this wall as a result of conductive cooling and gradually controls the mixing between the hydrothermal fluids and ambient seawaters. In the third stage, barite and some amorphous opal form from the higher temperature fluids at the summit of the chimney growth history. In the last stage, the chimney wall becomes thicker and denser and the exchange of hydrothermal fluids and seawater ceases. As a result, a Fe-Mn oxide layer precipitates onto the outer surface of the chimney wall as neutrophilic Fe-oxidizing bacteria reoccupy the surface of the chimney. This mineral sequence and the resultant growth generations are confirmed by the chemical characteristics of the chimney wall. Sr isotopes extracted from the Fe oxides of the four-generation wall generally show a decreasing trend of the 87Sr/86Sr ratios from the second layer to the inner layer (from 0.707008 to 0.705877) except for the outer layer (0.706502). The Sr isotope and chondrite normalized REE patterns of the corresponding bulk samples from the chimney wall also display a similar trend. Our study shows that the biogenic filament network plays a key role in the formation of the chimney in contrast to previous growth models of higher temperature chimneys, which often ignore the influence of biogenic factors.