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1.
The linkage between the iron and the carbon cycles is of paramount importance to understand and quantify the effect of increased CO2 concentrations in natural waters on the mobility of iron and associated trace elements. In this context, we have quantified the thermodynamic stability of mixed Fe(III) hydroxo-carbonate complexes and their effect on the solubility of Fe(III) oxihydroxides. We present the results of carefully performed solubility measurements of 2-line ferrihydrite in the slightly acidic to neutral–alkaline pH ranges (3.8–8.7) under constant pCO2 varying between (0.982–98.154 kPa) at 25 °C.The outcome of the work indicates the predominance of two Fe(III) hydroxo carbonate complexes FeOHCO3 and Fe(CO3)33−, with formation constants log*β°1,1,1 = 10.76 ± 0.38 and log β°1,0,3 = 24.24 ± 0.42, respectively.The solubility constant for the ferrihydrite used in this study was determined in acid conditions (pH: 1.8–3.2) in the absence of CO2 and at T = (25 ± 1) °C, as log*Ks,0 = 1.19 ± 0.41.The relative stability of the Fe(III)-carbonate complexes in alkaline pH conditions has implications for the solubility of Fe(III) in CO2-rich environments and the subsequent mobilisation of associated trace metals that will be explored in subsequent papers.  相似文献   

2.
Titratable actual acidity (TAA) is a technique commonly used to estimate the existing pool of exchangeable H+ in acid sulfate soils (ASS). A widely adopted version of the TAA method involves titrating a 1M KCl suspension of oven-dry soil (1:40) with NaOH to a known pH endpoint. However, when ASS are subject to long term re-flooding during wetland remediation, former sulfuric horizons can develop substantial quantities of porewater Fe2+, non-sulfidic solid-phase Fe(II) and a variety of reduced inorganic sulfur (RIS) species (e.g. pyrite, mackinawite, greigite and elemental sulfur). For these sediments, an oven-drying approach may induce oxidation of the abundant Fe(II) and/or reactive RIS species, thereby generating H+ and leading to overestimation of existing in situ exchangeable H+. In this study, we compare TAA via the standard approach (1M KCl; 1:40; oven-dry soil, 4 hr extract; TAAD) with an identical O2-free extraction approach using wet-sediment (TAAW). We apply both methods to former sulfuric horizon sediments from freshwater re-flooded ASS wetlands. There are significant (α = 0.01) differences (up to 12×) between TAA measured by the two methods, with the oven-dried standard approach overestimating TAA relative to the wet, O2-free approach in 85% of cases. Despite the fact that all AVS-S and some S(0) was oxidised during the oven-drying process, the increases in TAA (TAAD–TAAW) show very weak correlation(s) with corresponding losses in RIS species or increases in water soluble sulfate and KCl extractable sulfate. However, oven-drying caused substantial loss of 1M KCl exchangeable Fe(II) and 1 M HCl-extractable Fe(II) and led to large increases in 1 M HCl-extractable Fe(III). These changes in Fe fractions displayed strong positive linear correlation (α = 0.01) with increases in TAA. Although this is not evidence of causality, it suggests that oxidation of Fe(II) species are playing an important role in the development of additional exchangeable H+ and may be largely responsible for the contrasting TAA derived by the two methods. The differences in TAA between the two methods are greatest in organic-rich surface sediments and are significantly positively correlated with total organic carbon content. These findings have major implications for accurately assessing TAA in re-flooded ASS wetlands.  相似文献   

3.
Small angle neutron scattering techniques have been applied to investigate the phase behavior of CO2 injected into coal and possible changes in the coal pore structure that may result from this injection. Three coals were selected for this study: the Seelyville coal from the Illinois Basin (Ro = 0.53%), Baralaba coal from the Bowen Basin (Ro = 0.67%), and Bulli 4 coal from the Sydney Basin (Ro = 1.42%). The coals were selected from different depths to represent the range of the underground CO2 conditions (from subcritical to supercritical) which may be realized in the deep subsurface environment. The experiments were conducted in a high pressure cell and CO2 was injected under a range of pressure conditions, including those corresponding to in-situ hydrostatic subsurface conditions for each coal. Our experiments indicate that the porous matrix of all coals remains essentially unchanged after exposure to CO2 at pressures up to 200 bar (1 bar = 105 Pa). Each coal responds differently to the CO2 exposure and this response appears to be different in pores of various sizes within the same coal. For the Seelyville coal at reservoir conditions (16 °C, 50 bar), CO2 condenses from a gas into liquid, which leads to increased average fluid density in the pores (ρpore) with sizes (r) 1 × 105 ≥ r ≥ 1 × 104 Å (ρpore ≈ 0.489 g/cm3) as well as in small pores with size between 30 and 300 Å (ρpore ≈ 0.671 g/cm3). These values are by a factor of three to four higher than the density of bulk CO2 (ρCO2) under similar thermodynamic conditions (ρCO2 ≈ 0.15 g/cm3). At the same time, in the intermediate size pores with r ≈ 1000 Å the average fluid density is similar to the density of bulk fluid, which indicates that adsorption does not occur in these pores. At in situ conditions for the Baralaba coal (35 OC, 100 bar), the average fluid density of CO2 in all pores is lower than that of the bulk fluid (ρpore / ρCO2 ≈ 0.6). Neutron scattering from the Bulli 4 coal did not show any significant variation with pressure, a phenomenon which we assign to the extremely small amount of porosity of this coal in the pore size range between 35 and 100,000 Å.  相似文献   

4.
Streams and rivers are major exporters of C and other dissolved materials from watersheds to coastal waters. In streams and rivers, substantial amounts of terrigenous organic C is metabolized and degassed as CO2 to the atmosphere. A long-term evaluation of CO2 dynamics in streams is essential for understanding factors controlling CO2 dynamics in streams in response to changes in climate and land-use. Long-term changes in the partial pressure of CO2 (pCO2) were computed in the Anacostia River and the lower Potomac River in the Chesapeake Bay watershed. Long-term estimates were made using routine monitoring data of pH, total alkalinity, and dissolved nutrients from 1985 to 2006 at 14 stations. Longitudinal variability in pCO2 dynamics was also investigated along these rivers downstream of the urban Washington D.C. metropolitan area. Both rivers were supersaturated with CO2 with respect to atmospheric CO2 levels (392 μatm) and the highly urbanized Anacostia waters (202–9694 μatm) were more supersaturated than the Potomac waters (557–3800 μatm). Long-term variability in pCO2 values may be due to changes in river metabolism and organic matter and nutrient loadings. Both rivers exchange significant amounts of CO2 with the atmosphere (i.e., Anacostia at 0.2–72 mmol m−2 d−1 and Potomac at 0.12–24 mmol m−2 d−1), implying that waterways receiving organic matter and nutrient subsidies from urbanized landscapes have the potential to increase river metabolism and atmospheric CO2 fluxes along the freshwater–estuarine continuum.  相似文献   

5.
It is challenging to predict the degree to which shallow groundwater might be affected by leaks from a CO2 sequestration reservoir, particularly over long time scales and large spatial scales. In this study observations at a CO2 enriched shallow aquifer natural analog were used to develop a predictive model which is then used to simulate leakage scenarios. This natural analog provides the opportunity to make direct field observations of groundwater chemistry in the presence of elevated CO2, to collect aquifer samples and expose them to CO2 under controlled conditions in the laboratory, and to test the ability of multi-phase reactive transport models to reproduce measured geochemical trends at the field-scale. The field observations suggest that brackish water entrained with the upwelling CO2 are a more significant source of trace metals than in situ mobilization of metals due to exposure to CO2. The study focuses on a single trace metal of concern at this site: U. Experimental results indicate that cation exchange/adsorption and dissolution/precipitation of calcite containing trace amounts of U are important reactions controlling U in groundwater at this site, and that the amount of U associated with calcite is fairly well constrained. Simulations incorporating these results into a 3-D multi-phase reactive transport model are able to reproduce the measured ranges and trends between pH, pCO2, Ca, total C, U and Cl− at the field site. Although the true fluxes at the natural analog site are unknown, the cumulative CO2 flux inferred from these simulations are approximately equivalent to 37.8E−3 MT, approximately corresponding to a .001% leak rate for injection at a large (750 MW) power plant. The leakage scenario simulations suggest that if the leak only persists for a short time the volume of aquifer contaminated by CO2-induced mobilization of U will be relatively small, yet persistent over 100 a.  相似文献   

6.
Effective radium-226 concentration (ECRa) has been measured in soil samples from seven horizontal and vertical profiles of terrace scarps in the northern part of Kathmandu Valley, Nepal. The samples belong to the Thimi, Gokarna, and Tokha Formations, dated from 50 to 14 ky BP, and represent a diverse fluvio-deltaic sedimentary facies mainly consisting of gravelly to coarse sands, black, orange and brown clays. ECRa was measured in the laboratory by radon-222 emanation. The samples (n = 177) are placed in air-tight glass containers, from which, after an accumulation time varying from 3 to 18 days, the concentration of radon-222, radioactive decay product of radium-226 and radioactive gas with a half-life of 3.8 days, is measured using scintillation flasks. The ECRa values from the seven different profiles of the terrace deposits vary from 0.4 to 43 Bq kg?1, with profile averages ranging from 12 ± 1 to 27 ± 2 Bq kg?1. The values have a remarkable consistency along a particular horizon of sediment layers, clearly demonstrating that these values can be used for long distance correlations of the sediment horizons. Widely separated sediment profiles, representing similar stratigraphic positions, exhibit consistent ECRa values in corresponding stratigraphic sediment layers. ECRa measurements therefore appear particularly useful for lithologic and stratigraphic discriminations. For comparison, ECRa values of soils from different localities having various sources of origin were also obtained: 9.2 ± 0.4 Bq kg?1 in soils of Syabru–Bensi (Central Nepal), 23 ± 1 Bq kg?1 in red residual soils of the Bhattar-Trisuli Bazar terrace (North of Kathmandu), 17.1 ± 0.3 Bq kg?1 in red residual soils of terrace of Kalikasthan (North of Trisuli Bazar) and 10 ± 1 Bq kg?1 in red residual soils of a site near Nagarkot (East of Kathmandu). The knowledge of ECRa values for these various soils is important for modelling radon exhalation at the ground surface, in particular in the vicinity of active faults. Importantly, the study also reveals that, above numerous sediments of Kathmandu Valley, radon concentration in dwellings can potentially exceed the level of 300 Bq m?3 for residential areas; a fact that should be seriously taken into account by the governmental and non-governmental agencies as well as building authorities.  相似文献   

7.
Pure-iron end-member hibbingite, Fe2(OH)3Cl(s), may be important to geological repositories in salt formations, as it may be a dominant corrosion product of steel waste canisters in an anoxic environment in Na–Cl- and Na–Mg–Cl-dominated brines. In this study, the solubility of Fe2(OH)3Cl(s), the pure-iron end-member of hibbingite (FeII, Mg)2(OH)3Cl(s), and Fe(OH)2(s) in 0.04 m to 6 m NaCl brines has been determined. For the reactionFe2(OH)3Cl(s) + 3H+ ? 3 H2O + 2 Fe2+ + Cl?,the solubility constant of Fe2(OH)3Cl(s) at infinite dilution and 25 °C has been found to be log10 K = 17.12 ± 0.15 (95% confidence interval using F statistics for 36 data points and 3 parameters). For the reactionFe(OH)2(s) + 2H+ ? 2 H2O + Fe2+,the solubility constant of Fe(OH)2 at infinite dilution and 25 °C has been found to be log10 K = 12.95 ± 0.13 (95 % confidence interval using F statistics for 36 data points and 3 parameters). For the combined set of solubility data for Fe2(OH)3Cl(s) and Fe(OH)2(s), the Na+–Fe2+ pair Pitzer interaction parameter θNa+/Fe2+ has been found to be 0.08 ± 0.03 (95% confidence interval using F statistics for 36 data points and 3 parameters). In nearly saturated NaCl brine we observed evidence for the conversion of Fe(OH)2(s) to Fe2(OH)3Cl(s). Additionally, when Fe2(OH)3Cl(s) was added to sodium sulfate brines, the formation of green rust(II) sulfate was observed, along with the generation of hydrogen gas. The results presented here provide insight into understanding and modeling the geochemistry and performance assessment of nuclear waste repositories in salt formations.  相似文献   

8.
《Applied Geochemistry》2004,19(8):1217-1232
Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO2 (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m−2 h−1 at pH 2.78 in the batch experiments to 0.009 μmol Si m−2 h−1 at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H+ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO2 demonstrated that dissolution rates were independent of CO2 within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO2 have been elevated over the last decade. This release of magmatic CO2 has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO2 may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO2 can be excluded.  相似文献   

9.
CO2, CH4, and N2 adsorption and gas-induced swelling were quantified for block Blind Canyon, Pittsburgh #8 and Pocahontas Argonne Premium coals that were dried and structurally relaxed at 75 °C in vacuum. Strain measurements were made perpendicular and parallel to the bedding plane on ~ 7 × 7 × 7 mm3 coal blocks and gravimetric sorption measurements were obtained simultaneously on companion coal blocks exposed to the same gaseous environment. The adsorption amount and strain were determined after equilibration at P  ≤ 1.8 MPa. There is a strong non-linear correlation between strain and the quantity of gas adsorbed and the results for all gases and coals studied follow a common pattern. The dependence of the coal matrix shrinkage/swelling coefficient (Cgc) on the type and quantity of gas adsorbed is seen by plotting the ratio between the strain and the adsorbate concentration against the adsorbate concentration. In general, Cgc increases with increasing adsorbate concentration over the range of ~ 0.1 to 1.4 mmol/g. Results from the dried block coals are compared to CO2 experiments using native coals with an inherent level of moisture as received. The amount of CO2 adsorbed using native coals (assuming no displacement of H2O by CO2) is significantly less than the dried coals. The gas-induced strain (S) and adsorption amount (M) were measured as a function of time following step changes in CO2, CH4, and N2 pressure from vacuum to 1.8 MPa. An empirical diffusion equation was applied to the kinetic data to obtain the exponent (n) for time dependence for each experiment. The data for all coals were pooled and the exponent (n) evaluated using an ANOVA statistical analysis method. Values for (n) near 0.5 were found to be independent on the coal, the gas or type of measurement (e.g., parallel strain, perpendicular strain, and gas uptake). These data support the use of a Fickian diffusion model framework for kinetic analysis. The kinetic constant k was determined using a unipore diffusion model for each experiment and the data were pooled for ANOVA analysis. For dry coal, statistically significant differences for k were found for the gases (CO2 > N2 > CH4) and coals (Pocahontas >Blind Canyon > Pittsburgh #8) but not for the method of the kinetic measurement (e.g., strain or gas uptake). For Blind Canyon and Pittsburgh #8 coal, the rate of CO2 adsorption and gas-induced strain for dry coal was significantly greater than that of the corresponding native coal. For Pocahontas coal the rates of CO2 adsorption and gas-induced strain for dry and native coal were indistinguishable and may be related to its low native moisture and minimal amount of created porosity upon drying.  相似文献   

10.
The Great Falls-Lewistown Coal Field (GFLCF) in central Montana contains over 400 abandoned underground coal mines, many of which are discharging acidic water with serious environmental consequences. Areas of the mines that are completely submerged by groundwater have circum-neutral pH and relatively low concentrations of metals, whereas areas that are only partially flooded or freely draining have acidic pH (< 3) and high concentrations of metals. The pH of the mine drains either decreases or increases after discharging to the surface, depending on the initial ratio of acidity (mainly Al and Fe2+) to alkalinity (mainly HCO3?). In acidic, Fe-rich waters, oxidation of Fe2+ after exposure to air is microbially catalyzed and follows zero-order kinetics, with computed rate constants falling in the range of 0.97 to 1.25 mmol L? 1 h? 1. In contrast, Fe2+ oxidation in near-neutral pH waters appears to be first-order with respect to Fe2+ concentration, although insufficient data were collected to constrain the rate law expression. Rates of Fe2+ oxidation in the field are dependent on temperature such that lower Fe2+ concentrations were measured in down-gradient waters during the day, and higher concentrations at night. Diel cycles in dissolved concentrations of Zn and other trace metals (Mn, Ni) were also noted for down-gradient waters that were net alkaline, but not in the acidic drains.The coal seams of the GFLCF and overlying Cretaceous sandstones form a perched aquifer that lies ~ 50 m above the regional water table situated in the underlying Madison Limestone. The δD and δ18O values of flooded mine waters suggest local derivation from meteoric water that has been partially evaporated in agricultural soils overlying the coal mines. The S and O isotopic composition of dissolved sulfate in the low pH mine drains is consistent with oxidation of biogenic pyrite in coal under aerated conditions. A clear distinction exists between the isotopic composition of sulfate in the acid mine waters and sulfate in the adjacent sedimentary aquifers, making it theoretically possible to determine if acid drainage from the coal mines has leaked into the underlying Madison aquifer.  相似文献   

11.
14C measurements of CH4 in environmental samples (e.g. soil gas, lake water, gas hydrates) can advance understanding of C cycling in terrestrial and marine systems. The measurements are particularly useful for detecting the release of old C from climate sensitive environments such as peatlands and hydrate fields. However, because 14C CH4 measurements tend to be complex and time consuming, they are uncommon. Here, we describe a novel vacuum line system for the preparation of CH4 and CO2 from environmental samples for 14C analysis using accelerator mass spectrometry (AMS). The vacuum line is a flow-through system that allows rapid preparation of samples (1 h for CH4 and CO2, 30 min for CH4 alone), complete separation of CH4 and CO2 and is an easy addition to multipurpose CO2 vacuum lines already in use. We evaluated the line using CH4 and CO2 standards with different 14C content. For CH4 and CO2, respectively, the total line blank was 0.4 ± 0.2 and 1.4 ± 0.6 μg C, the 14C background 51.1 ± 1.2 and 48.4 ± 1.5 kyr and the precision (based on pooled standard deviation) 0.9‰ and 1.3‰. The line was designed for sample volumes of ca. 180 ml containing 0.5–1% CH4 and CO2, but can be adjusted to handle lower concentration and larger volume samples. This rapid and convenient method for the preparation of CH4 and CO2 in environmental samples for 14C AMS analysis should provide more opportunities for the use of 14C CH4 measurements in C cycle studies.  相似文献   

12.
The dissolution kinetics of carbonate rocks sampled from the Keg River Formation in Northeast British Columbia were measured at 50 bar pCO2 and 105 °C, in both natural and synthetic brines of 0.4 M ionic strength. Natural brines yielded reaction rates of −12.16 ± 0.11 mol cm−2 s−1 for Log RCa, and −12.64 ± 0.05 for Log RMg. Synthetic brine yielded faster rates of reaction than natural brines. Experiments performed on synthetic brines, spiked with 10 mmol of either Sr or Zn, suggest that enhanced reaction rates observed in synthetic brines are due to a lack of trace ion interaction with mineral surfaces. Results were interpreted within the surface complexation model framework, allowing for the discrimination of reactive surface sites, most importantly the hydration of the >MgOH surface site. Dissolution rates extrapolated from experiments predict that CO2 injected into the Keg River Formation will dissolve a very minor portion of rock in contact with affected formation waters.  相似文献   

13.
Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone.Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C1–C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] > 1000) and stable isotopic compositions of methane (δ13C = ? 53.5‰ V-PDB; D/H around ? 175‰ SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C1–C4, CO2]) relative to the other gas types and the virtual lack of 14C–CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely.As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C1–C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C–CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.  相似文献   

14.
We investigate the helium, carbon and oxygen–hydrogen isotopic systematics and CO2/3He ratios of 8 water and 6 gas samples collected from 12 geothermal fields in western Anatolia (Turkey). 3He/4He ratios of the samples (R) normalized to the atmospheric 3He/4He ratio (RA = 1.39 × 10? 6) range from 0.27 to 1.67 and are significantly higher than the crustal production value of 0.05. Fluids with relatively high R / RA values are generally found in areas of significant heat potential (K?z?ldere and Tuzla fields). CO2/3He ratios of the samples, ranging from 1.6 × 109 to 2.3 × 1014, display significant variation and are mostly higher than values typical of an upper mantle source (2 × 109). The δ13C (CO2) and δ13C (CH4) values of all fluids vary from ? 8.04 to + 0.35‰ and ? 25.80 to ? 23.92‰ (vs. PDB), respectively. Stable isotope values (δ18O–δD) of the geothermal waters are conformable with the Mediterranean Meteoric Water Line and indicate a meteoric origin. The temperatures calculated by gas geothermometry are significantly higher than estimates from chemical geothermometers, implying that either equilibrium has not been attained for the isotope exchange reaction or that isotopic equilibration was disturbed due to gas additions en route to the surface.Evaluation of He–CO2 abundances indicates that hydrothermal degassing and calcite precipitation (controlled probably by adiabatic cooling due to degassing) significantly fractionate the elemental ratio (CO2/3He) in geothermal waters. Such processes do not affect gas phase samples to anywhere near the same extent. For the gas samples, mixing between mantle and various crustal sources appears to be the main control on the observed He–C systematics: however, crustal inputs dominate the CO2 inventory. Considering that limestone is the main source of carbon (～ 70 to 97% of the total carbon inventory), the carbon flux from the crust is found to be at least 20 times that from the mantle. As to the He-inventory, the mantle-derived component is found to vary up to 21% of the total He content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with the elevated geotherm and adiabatic melting accompanying current extension. The range of 3He/enthalpy ratios (0.000032 to 0.19 × 10? 12 cm3 STP/J) of fluids in western Anatolia is consistent with the release of both helium and heat from contemporary additions of mantle-derived magmas to the crust. The deep faults appear to have facilitated the deep circulation of the fluids and the transport of mantle volatiles and heat to the surface.  相似文献   

15.
The Qiangma gold deposit is hosted in the > 1.9 Ga Taihua Supergroup metamorphic rocks in the Xiaoqinling terrane, Qinling Orogen, on the southern margin of the North China Craton. The mineralization can be divided as follows: quartz-pyrite veins early, quartz-polymetallic sulfide veinlets middle, and carbonate-quartz veinlets late stages, with gold being mainly introduced in the middle stage. Three types of fluid inclusions were identified based on petrography and laser Raman spectroscopy, i.e., pure carbonic, carbonic-aqueous (CO2–H2O) and aqueous inclusions.The early-stage quartz contains pure carbonic and CO2–H2O inclusions with salinities up to 12.7 wt.% NaCl equiv., bulk densities of 0.67 to 0.86 g/cm3, and homogenization temperatures of 280−365 °C. The early-stage is related to H2O–CO2 ± N2 ± CH4 fluids with isotopic signatures consistent with a metamorphic origin (δ18Owater = 3.1 to 5.2‰, δD = − 37 to − 73‰). The middle-stage quartz contains all three types of fluid inclusions, of which the CO2–H2O and aqueous inclusions yield homogenization temperatures of 249−346 °C and 230−345 °C, respectively. The CO2–H2O inclusions have salinities up to 10.9 wt.% NaCl equiv. and bulk densities of 0.70 to 0.98 g/cm3, with vapor bubbles composed of CO2 and N2. The isotopic ratios (δ18Owater = 2.2 to 3.6‰, δD = − 47 to − 79‰) suggest that the middle-stage fluids were mixed by metamorphic and meteoric fluids. In the late-stage quartz only the aqueous inclusions are observed, which have low salinities (0.9−9.9 wt.% NaCl equiv.) and low homogenization temperatures (145−223 °C). The isotopic composition (δ18Owater = − 1.9 to 0.5‰, δD = − 55 to − 66‰) indicates the late-stage fluids were mainly meteoric water.Trapping pressures estimated from CO2–H2O inclusions are 100−285 MPa for the middle stage, suggesting that gold mineralization mainly occurred at depths of 10 km. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO2-rich to CO2-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ18Owater values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Qiangma gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic−Early Cretaceous continental collision between the North China and Yangtze cratons.  相似文献   

16.
Pristine diorite drill cores, obtained from the Äspö Hard Rock Laboratory (HRL, Sweden), were used to study the retention properties of fresh, anoxic crystalline rock material towards the redox-sensitive uranium. Batch sorption experiments and spectroscopic methods were applied for this study. The impact of various parameters, such as solid-to-liquid ratio (2–200 g/L), grain size (0.063–0.2 mm, 0.5–1 mm, 1–2 mm), temperature (room temperature and 10 °C), contact time (5–108 days), initial U(VI) concentration (3 × 10−9 to 6 × 10−5 M), and background electrolyte (synthetic Äspö groundwater and 0.1 M NaClO4) on the U(VI) sorption onto anoxic diorite was studied under anoxic conditions (N2). Comparatively, U(VI) sorption onto oxidized diorite material was studied under ambient atmosphere (pCO2 = 10−3.5 atm). Conventional distribution coefficients, Kd, and surface area normalized distribution coefficients, Ka, were determined. The Kd value for the U(VI) sorption onto anoxic diorite in synthetic Äspö groundwater under anoxic conditions by investigating the sorption isotherm amounts to 3.8 ± 0.6 L/kg which corresponds to Ka = 0.0030 ± 0.0005 cm (grain size 1–2 mm). This indicates a weak U sorption onto diorite which can be attributed to the occurrence of the neutral complex Ca2UO2(CO3)3(aq) in solution. This complex was verified as predominating U species in synthetic Äspö groundwater by time-resolved laser-induced fluorescence spectroscopy (TRLFS). Compared to U sorption at room temperature under anoxic conditions, U sorption is further reduced at decreased temperature (10 °C) and under ambient atmosphere. The U species in aqueous solution as well as sorbed on diorite were studied by in situ time-resolved attenuated total reflection Fourier-transform infrared (ATR FT-IR) spectroscopy. A predominant sorbing species containing a UO2(CO3)34− moiety was identified. The extent of U sorption onto diorite was found to depend more on the low sorption affinity of the Ca2UO2(CO3)3(aq) complex than on reduction processes of uranium.  相似文献   

17.
Lumbricus terrestris earthworms exposed to 11 soils of contrasting properties produced, on average, 0.8 ± 0.1 mgCaCO3 earthworm?1 day?1 in the form of granules up to 2 mm in diameter. Production rate increased with soil pH (r2 = 0.68, p < 0.01). Earthworms could be a significant source of calcite in soils.  相似文献   

18.
Natural gas reservoirs in organic-rich shales in the Appalachian and Michigan basins in the United States are currently being produced via hydraulic fracturing. Stratigraphically-equivalent shales occur in the Canadian portion of the basins in southwestern Ontario with anecdotal evidence of gas shows, yet there has been no commercial shale gas production to date. To provide baseline data in the case of future environmental issues related to hydraulic fracturing and shale gas production, such as leakage of natural gas, saline water, and/or hydraulic fracturing fluids, and to evaluate hydrogeochemical controls on natural gas accumulations in shallow groundwater in general, this study investigates the origin and distribution of natural gas and brine in shallow aquifers across southwestern Ontario. An extensive geochemical database of major ion and trace metal chemistry and methane concentrations of 1010 groundwater samples from shallow, domestic wells in bedrock and overburden aquifers throughout southwestern Ontario was utilized. In addition, select wells (n = 36) were resampled for detailed dissolved gas composition, δ13C of CH4, C2, C3, and CO2, and δD of CH4. Dissolved gases in groundwater from bedrock and overburden wells were composed primarily of CH4 (29.7–98.6 mol% of total gas volume), N2 (0.8–66.2 mol%), Ar + O2 (0.2–3.4 mol%), and CO2 (0–1.2 mol%). Ethane was detected, but only in low concentrations (<0.041 mol%), and no other higher chain hydrocarbons were present, except for one well in overburden overlying the Dundee Formation, which contained 0.81 mol% ethane and 0.21 mol% propane. The highest methane concentrations (30 to >100 in situ % saturation) were found in bedrock wells completed in the Upper Devonian Kettle Point Formation, Middle Devonian Hamilton Group and Dundee Formation, and in surficial aquifers overlying these organic-rich shale-bearing formations, indicating that bedrock geology is the primary control on methane occurrences. A few (n = 40) samples showed Na–Cl–Br evidence of brine mixing with dilute groundwater, however only one of these samples contained high (>60 in situ % saturation) CH4. The relatively low δ13C values of CH4 (−89.9‰ to −57.3‰), covariance of δD values of CH4 and H2O, positive correlation between δ13C values of CH4 and CO2, and lack of higher chain hydrocarbons (C3+) in all but one dissolved gas sample indicates that the methane in groundwater throughout the study area is primarily microbial in origin. The presence or absence of alternative electron acceptors (e.g. dissolved oxygen, Fe, NO3, SO4), in addition to organic substrates, controls the occurrence of microbial CH4 in shallow aquifers. Microbial methane has likely been accumulating in the study area, since at least the Late Pleistocene to the present, as indicated by the co-variance and range of δD values of CH4 (−314‰ to −263‰) and associated groundwater (−19‰ to −6‰ δD-H2O).  相似文献   

19.
The Aerhada Pb-Zn-Ag deposit is located in the western segment of the Great Hinggan Range Ag-Pb-Zn-Cu-Mo-Au-Fe metallogenic belt in NE China. Orebodies occur mainly as vein type and are hosted by sandstone and siliceous slate. Three stages of primary mineralization, including an early arsenopyrite-pyrite-quartz, a middle polymetallic and silver sulfides-quartz and a late sphalerite-pyrite-calcite-fluorite are recognized. Four types of fluid inclusions have been identified in the ore-bearing quartz and fluorite veins, i.e., liquid-rich, gas-rich, three-phase CO2 aqueous inclusions, and pure gas or liquid aqueous inclusions. Microthermometric studies on fluid inclusions reveal that homogenization temperatures from early to late stages range from 253° to 430 °C, 195° to 394 °C and 133° to 207 °C, respectively. Fluid salinities range from 2.9 to 14.0 wt.% NaCl equiv. The vapor composition of the ore fluid is dominated by H2O, CO2 and CH4, with minor proportions of N2. The fluid δ18OH2O and δDH2O values vary from +1.6 to +9.3‰ and −122 to −56‰, respectively, and reflect a magmatic fluid and a meteoric fluid dominant hydrothermal system for the early and late stages of mineralization, respectively. The calculated δ34SH2S values of hydrothermal fluids in equilibrium with sulfides range from +5.2 to +7.1‰, suggesting a mixed source for sulfur, i.e., the local magmatic and sedimentary rocks. The Pb isotope compositions of sulfides are similar to those of the local magmatic and sedimentary rocks, implying that lead and possibly silver relate to these sources. The noble gas isotope compositions of fluid inclusions hosted in ore minerals suggest that the ore-forming fluids were dominantly derived from a deep mantle source. Fluid mixing and dilution are inferred as the dominant mechanisms for ore deposition. The Aerhada Pb-Zn-Ag deposit can be classified as a medium to low temperature hydrothermal vein type deposit.  相似文献   

20.
The Wangfeng gold deposit is located in Western Tian Shan and the central section of the Central Asian Orogenic Belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Caledonian granites and is structurally controlled by the Shenglidaban ductile shear zone. The gold orebodies consist of gold-bearing quartz veins and altered mylonite. The mineralization can be divided into three stages: quartz–pyrite veins in the early stage, sulfide–quartz veins in the middle stage, and quartz–carbonate veins or veinlets in the late stage. Ore minerals and native gold mainly formed in the middle stage. Four types of fluid inclusions were identified based on petrography and laser Raman spectroscopy: CO2–H2O inclusions (C-type), pure CO2 inclusions (PC-type), NaCl–H2O inclusions (W-type), and daughter mineral-bearing inclusions (S-type). The early-stage quartz contains only primary CO2–H2O fluid inclusions with salinities of 1.62 to 8.03 wt.% NaCl equivalent, bulk densities of 0.73 to 0.89 g/cm3, and homogenization temperatures of 256 °C–390 °C. Vapor bubbles are composed of CO2. The middle-stage quartz contains all four types of fluid inclusions, of which the CO2–H2O and NaCl–H2O types yield homogenization temperatures of 210 °C–340 °C and 230 °C–300 °C, respectively. The CO2–H2O fluid inclusions have salinities of 0.83 to 9.59 wt.% NaCl equivalent and bulk densities of 0.77 to 0.95 g/cm3, with vapor bubbles composed of CO2, CH4, and N2. Fluid inclusions in the late-stage quartz are NaCl–H2O solution with low salinities (0.35–3.87 wt.% NaCl equivalent) and low homogenization temperatures (122 °C–214 °C). The coexistence of inclusions of these four types in middle-stage quartz suggests that fluid boiling occurred in the middle-stage mineralization. Trapping pressures estimated from CO2–H2O inclusions are 110–300 MPa and 90–250 MPa for the early and middle stages, respectively, suggesting that gold mineralization mainly occurred at depths of about 10 km. In general, the Wangfeng gold deposit originated from a metamorphic fluid system characterized by low salinity, low density, and enrichment of CO2. Depressurized fluid boiling caused gold precipitation. Given the regional geology, ore geology, fluid-inclusion features, and ore-forming age, the Wangfeng gold deposit can be classified as a hypozonal orogenic gold deposit.  相似文献   

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