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1.
Iron isotopes fractionate during hydrothermal processes. Therefore, the Fe isotope composition of ore-forming minerals characterizes either iron sources or fluid histories. The former potentially serves to distinguish between sedimentary, magmatic or metamorphic iron sources, and the latter allows the reconstruction of precipitation and redox processes. These processes take place during ore formation or alteration. The aim of this contribution is to investigate the suitability of this new isotope method as a probe of ore-related processes. For this purpose 51 samples of iron ores and iron mineral separates from the Schwarzwald region, southwest Germany, were analyzed for their iron isotope composition using multicollector ICP-MS. Further, the ore-forming and ore-altering processes were quantitatively modeled using reaction path calculations. The Schwarzwald mining district hosts mineralizations that formed discontinuously over almost 300 Ma of hydrothermal activity. Primary hematite, siderite and sulfides formed from mixing of meteoric fluids with deeper crustal brines. Later, these minerals were partly dissolved and oxidized, and secondary hematite, goethite and iron arsenates were precipitated. Two types of alteration products formed: (1) primary and high-temperature secondary Fe minerals formed between 120 and 300 °C, and (2) low-temperature secondary Fe minerals formed under supergene conditions (<100 °C). Measured iron isotope compositions are variable and cover a range in δ56Fe between −2.3‰ and +1.3‰. Primary hematite (δ56Fe: −0.5‰ to +0.5‰) precipitated by mixing oxidizing surface waters with a hydrothermal fluid that contained moderately light Fe (δ56Fe: −0.5‰) leached from the crystalline basement. Occasional input of CO2-rich waters resulted in precipitation of isotopically light siderite (δ56Fe: −1.4 to −0.7‰). The difference between hematite and siderite is compatible with published Fe isotope fractionation factors. The observed range in isotopic compositions can be accounted for by variable fractions of Fe precipitating from the fluid. Therefore, both fluid processes and mass balance can be inferred from Fe isotopes. Supergene weathering of siderite by oxidizing surface waters led to replacement of isotopically light primary siderite by similarly light secondary hematite and goethite, respectively. Because this replacement entails quantitative transfer of iron from precursor mineral to product, no significant isotope fractionation is produced. Hence, Fe isotopes potentially serve to identify precursors in ore alteration products. Goethites from oolitic sedimentary iron ores were also analyzed. Their compositional range appears to indicate oxidative precipitation from relatively uniform Fe dissolved in coastal water. This comprehensive iron isotope study illustrates the potential of the new technique in deciphering ore formation and alteration processes. Isotope ratios are strongly dependent on and highly characteristic of fluid and precipitation histories. Therefore, they are less suitable to provide information on Fe sources. However, it will be possible to unravel the physico-chemical processes leading to the formation, dissolution and redeposition of ores in great detail.  相似文献   

2.
The isotopic composition of dissolved Cu and solid Cu-rich minerals [δ65Cu (‰) = (65Cu/63Cusample/65Cu/63Custd) - 1)*1000] were monitored in batch oxidative dissolution experiments with and without Thiobacillus ferrooxidans. Aqueous copper in leach fluids released during abiotic oxidation of both chalcocite and chalcopyrite was isotopically heavier (δ65Cu = 5.34‰ and δ65Cu = 1.90‰, respectively, [±0.16 at 2σ]) than the initial starting material (δ65Cu = 2.60 ± 0.16‰ and δ65Cu = 0.58 ± 0.16‰, respectively). Isotopic mass balance between the starting material, aqueous copper, and secondary minerals precipitated in these experiments explains the heavier isotopic values of aqueous copper. In contrast, aqueous copper from leached chalcocite and chalcopyrite inoculated with Thiobacillus ferrooxidans was isotopically similar to the starting material. The lack of fractionation of the aqueous copper in the biotic experiments can best be explained by assuming a sink for isotopically heavy copper present in the bacteria cells with δ65Cu = 5.59 ± 0.16‰. Consistent with this inference, amorphous Cu-Fe oxide minerals are observed surrounding cell membranes of Thiobacillus grown in the presence of dissolved Cu and Fe.Extrapolating these experiments to natural supergene environments implies that release of isotopically heavy aqueous Cu from oxidative leach caps, especially under abiotic conditions, should result in precipitates in underlying enrichment blankets that are isotopically heavy. Where iron-oxidizing cells are involved, isotopically heavy oxidized Cu entrained in cellular material may become associated with leach caps, causing the released aqueous Cu to be less isotopically enriched in the heavy isotope than predicted for the abiotic system. Rayleigh fractionation trends with fractionation factors calculated from our experiments for both biotic and abiotic conditions are consistent with large numbers of individual abiotic or biotic leaching events, explaining the supergene chalcocites in the Morenci and Silver Bell porphyry copper deposits.  相似文献   

3.
Copper isotope fractionation in acid mine drainage   总被引：4，自引：0，他引：4  
We measured the Cu isotopic composition of primary minerals and stream water affected by acid mine drainage in a mineralized watershed (Colorado, USA). The δ65Cu values (based on 65Cu/63Cu) of enargite (δ65Cu = −0.01 ± 0.10‰; 2σ) and chalcopyrite (δ65Cu = 0.16 ± 0.10‰) are within the range of reported values for terrestrial primary Cu sulfides (−1‰ < δ65Cu < 1‰). These mineral samples show lower δ65Cu values than stream waters (1.38‰ ? δ65Cu ? 1.69‰). The average isotopic fractionation (Δaq-min = δ65Cuaq − δ65Cumin, where the latter is measured on mineral samples from the field system), equals 1.43 ± 0.14‰ and 1.60 ± 0.14‰ for chalcopyrite and enargite, respectively. To interpret this field survey, we leached chalcopyrite and enargite in batch experiments and found that, as in the field, the leachate is enriched in 65Cu relative to chalcopyrite (1.37 ± 0.14‰) and enargite (0.98 ± 0.14‰) when microorganisms are absent. Leaching of minerals in the presence of Acidithiobacillus ferrooxidans results in smaller average fractionation in the opposite direction for chalcopyrite (Δaq-mino=-0.57±0.14‰, where mino refers to the starting mineral) and no apparent fractionation for enargite (Δaq-mino=0.14±0.14‰). Abiotic fractionation is attributed to preferential oxidation of 65Cu+ at the interface of the isotopically homogeneous mineral and the surface oxidized layer, followed by solubilization. When microorganisms are present, the abiotic fractionation is most likely not seen due to preferential association of 65Cuaq with A. ferrooxidans cells and related precipitates. In the biotic experiments, Cu was observed under TEM to occur in precipitates around bacteria and in intracellular polyphosphate granules. Thus, the values of δ65Cu in the field and laboratory systems are presumably determined by the balance of Cu released abiotically and Cu that interacts with cells and related precipitates. Such isotopic signatures resulting from Cu sulfide dissolution should be useful for acid mine drainage remediation and ore prospecting purposes.  相似文献   

4.
Significant, systematic Cu isotopic variations have been found in the Northparkes porphyry Cu-Au deposit, NSW, Australia, which is an orthomagmatic porphyry Cu deposit. Copper isotope ratios have been measured in sulfide minerals (chalcopyrite and bornite) by both solution and laser ablation multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The results from both methods show a variation in δ65Cu of hypogene sulfide minerals of greater than 1‰ (relative to NIST976). Significantly, the results from four drill holes through two separate ore bodies show strikingly similar patterns of Cu isotope variation. The patterns are characterized by a sharp down-hole decrease from up to 0.8‰ (0.29 ± 0.56‰, 1σ, n = 20) in the low-grade peripheral alteration zones (phyllic-propylitic alteration zone) to a low of ∼−0.4‰ (−0.25 ± 0.36‰, 1σ, n = 30) at the margins of the most mineralized zones (Cu grade >1 wt%). In the high-grade cores of the systems, the compositions are more consistent at around 0.2‰ (0.19 ± 0.14‰, 1σ, n = 40). The Cu isotopic zonation may be explained by isotope fractionation of Cu between vapor, solution and sulfides at high temperature, during boiling and sulfide precipitation processes. Sulfur isotopes also show an isotopically light shell at the margins of the high-grade ore zones, but these are displaced from the low δ65Cu shells, such that there is no correlation between the Cu and S isotope signatures. Fe isotope data do not show any discernable variation along the drill core. This work demonstrates that Cu isotopes show a large response to high-temperature porphyry mineralizing processes, and that they may act as a vector to buried mineralization.  相似文献   

5.
Copper isotopes may prove to be a useful tool for investigating bacteria-metal interactions recorded in natural waters, soils, and rocks. However, experimental data which attempt to constrain Cu isotope fractionation in biologic systems are limited and unclear. In this study, we utilized Cu isotopes (δ65Cu) to investigate Cu-bacteria interactions, including surface adsorption and intracellular incorporation. Experiments were conducted with individual representative species of Gram-positive (Bacillus subtilis) and Gram-negative (Escherichia coli) bacteria, as well as with wild-type consortia of microorganisms from several natural environments. Ph-dependent adsorption experiments were conducted with live and dead cells over the pH range 2.5-6. Surface adsorption experiments of Cu onto live bacterial cells resulted in apparent separation factors (Δ65Cusolution-solid = δ65Cusolution − δ65Cusolid) ranging from +0.3‰ to +1.4‰ for B. subtilis and +0.2‰ to +2.6‰ for E. coli. However, because heat-killed bacterial cells did not exhibit this behavior, the preference of the lighter Cu isotope by the cells is probably not related to reversible surface adsorption, but instead is a metabolically-driven phenomenon. Adsorption experiments with heat-killed cells yielded apparent separation factors ranging from +0.3‰ to −0.69‰ which likely reflects fractionation from complexation with organic acid surface functional group sites. For intracellular incorporation experiments the lab strains and natural consortia preferentially incorporated the lighter Cu isotope with an apparent Δ65Cusolution-solid ranging from ∼+1.0‰ to +4.4‰. Our results indicate that live bacterial cells preferentially sequester the lighter Cu isotope regardless of the experimental conditions. The fractionation mechanisms involved are likely related to active cellular transport and regulation, including the reduction of Cu(II) to Cu(I). Because similar intracellular Cu machinery is shared by fungi, plants, and higher organisms, the influence of biological processes on the δ65Cu of natural waters and soils is probably considerable.  相似文献   

6.
Copper stable isotope ratios are fractionated during various biogeochemical processes and may trace the fate of Cu during long-term pedogenetic processes. We assessed the effects of oxic weathering (formation of Cambisols) and podzolization on Cu isotope ratios (δ65Cu). Two Cambisols (oxic weathered soils without strong vertical translocations of soil constituents) and two Podzols (soils showing vertical translocation of organic matter, Fe and Al) were analyzed for Cu concentrations, partitioning of Cu in seven fractions of a sequential extraction and δ65Cu values in bulk soil. Cu concentrations in the studied soils were low (1.4-27.6 μg g−1) and Cu was mainly associated with strongly bound Fe oxide- and silicate-associated forms. Bulk δ65Cu values varied between −0.57‰ and 0.44‰ in all studied horizons. The O horizons had on average significantly lighter Cu isotope compositions (−0.21‰) than the A horizons (0.13‰) which can either be explained by Cu isotope fractionation during cycling through the plants or deposition of isotopically light Cu from the atmosphere. Oxic weathering without pronounced podzolization in both Cambisols and a weakly developed Podzol (Haplic Podzol 2) caused no significant isotope fractionation in the single profiles, while a slight tendency to lower δ65Cu values with depth was visible in all four profiles. This is the opposite depth distribution of δ65Cu values to that we observed in hydromorphic soils (soils which show indication of redox changes because of the influence of water saturation) in a previous study. In a more pronounced Podzol (Haplic Podzol 1), δ65Cu values and Cu concentrations decreased from Ah to E horizons and increased again deeper in the soil. Humus-rich sections of the Bhs horizon had higher Cu concentrations (2.8 μg g−1) and a higher δ65Cu value (−0.18‰) than oxide-rich sections (1.9 μg g−1, −0.35‰) suggesting Cu translocation between E and B horizons as organo-Cu complexes. The different depth distributions in oxic weathered and hydromorphic soils and the pronounced vertical differences in δ65Cu values in Haplic Podzol 1 indicate a promising potential of δ65Cu values to improve our knowledge of the fate of Cu during long-term pedogenetic processes.  相似文献   

7.
We examined the copper isotope ratio of primary high temperature Cu-sulfides, secondary low temperature Cu-sulfides (and Cu-oxides) as well as Fe-oxides in the leach cap, which represent the weathered remains of a spectrum of Cu mineralization, from nine porphyry copper deposits. Copper isotope ratios are reported as δ65Cu‰ = ((65Cu/63Cusample/65Cu/63CuNIST 976 standard) − 1) ? 103. Errors for all the analyses are ± 0.14‰ (determined by multiple analyses of the samples) and mass bias was corrected through standard-sample-standard bracketing. The overall isotopic variability measured in these samples range from − 16.96‰ to 9.98‰.  相似文献   

8.
We measured Ca stable isotope ratios (δ44/40Ca) in an ancient (2 My), hyperarid soil where the primary source of mobile Ca is atmospheric deposition. Most of the Ca in the upper meter of this soil (3.5 kmol m−2) is present as sulfates (2.5 kmol m−2), and to a lesser extent carbonates (0.4 kmol m−2). In aqueous extracts of variably hydrated calcium sulfate minerals, δ44/40CaE values (vs. bulk Earth) increase with depth (1.4 m) from a minimum of −1.91‰ to a maximum of +0.59‰. The trend in carbonate-δ44/40Ca in the top six horizons resembles that of sulfate-δ44/40Ca, but with values 0.1-0.6‰ higher. The range of observed Ca isotope values in this soil is about half that of δ44/40Ca values observed on Earth. Linear correlation among δ44/40Ca, δ34S and δ18O values indicates either (a) a simultaneous change in atmospheric input values for all three elements over time, or (b) isotopic fractionation of all three elements during downward transport. We present evidence that the latter is the primary cause of the isotopic variation that we observe. Sulfate-δ34S values are positively correlated with sulfate-δ18O values (R2 = 0.78) and negatively correlated with sulfate δ44/40CaE values (R2 = 0.70). If constant fractionation and conservation of mass with downward transport are assumed, these relationships indicate a δ44/40Ca fractionation factor of −0.4‰ in CaSO4. The overall depth trend in Ca isotopes is reproduced by a model of isotopic fractionation during downward Ca transport that considers small and infrequent but regularly recurring rainfall events. Near surface low Ca isotope values are reproduced by a Rayleigh model derived from measured Ca concentrations and the Ca fractionation factor predicted by the relationship with S isotopes. This indicates that the primary mechanism of stable isotope fractionation in CaSO4 is incremental and effectively irreversible removal of an isotopically enriched dissolved phase by downward transport during small rainfall events.  相似文献   

9.
The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U-233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41 ± 0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U = −0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U-Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3 Ma, and up to 2 Ma for minerals that are 3 billion years old.  相似文献   

10.
Groundwater is the most important source of water supply in southern Tunisia. Previous hydrogeologic and isotopic studies carried out in this region revealed the existence of two major aquifer systems: the “Complex Terminal” (CT) and the “Continental Intercalaire” (CI). Turonian carbonates constitute one of the major aquifer levels of the CT multilayered aquifer. It extends over most of southern Tunisia, and its hydrodynamic regime is largely influenced by tectonics, lithology and recharge conditions. Forty-eight groundwater samples from the CI and Turonian aquifers were collected between January and April 2004 for chemical and isotopic analyses. Hydrochemistry and isotopic tools were combined to get an insight into the processes controlling chemical composition of groundwater and wide-scale interaction of these two aquifer systems. Analysis of the dissolved constituents revealed that several processes control the observed chemical composition: (i) incongruent dissolution of carbonate minerals, (ii) dissolution of evaporitic minerals, and (iii) cation exchange. Dissolution alone cannot account for the observed high supersaturation states of groundwater with respect to calcite and dolomite. The observed supersaturation is most probably linked to geogenic CO2 entering water-bearing horizons of the CT and CI aquifers via deep tectonic faults and discontinuities and subsequent degassing in the exploitation wells. Presence of geogenic CO2 in the investigated region was confirmed by C isotope data of the DIC reservoir. The radiocarbon content of the Turonian samples varied between 9.5 and 43 pmc. For CI samples generally lower values were recorded, between 3.8 and 22.5 pmc. Stable isotope composition of Turonian groundwater samples varied from −8.3 to −5.3‰ for δ18O and from −60 to −25‰ for δ2H. The corresponding ranges of δ values for the Continental Intercalaire samples were from −8.9‰ to −6.9‰ for δ18O and from −68.2‰ to −45.7‰ for δ2H. Stable isotope composition of groundwater representing CT and CI aquifers provide strong evidence for regional interaction between both systems.  相似文献   

11.
The Reykjanes geothermal system is located on the landward extension of the Mid-Atlantic Ridge in southwest Iceland, and provides an on-land proxy to high-temperature hydrothermal systems of oceanic spreading centers. Previous studies of elemental composition and salinity have shown that Reykjanes geothermal fluids are likely hydrothermally modified seawater. However, δD values of these fluids are as low as −23‰, which is indicative of a meteoric water component. Here we constrain the origin of Reykjanes hydrothermal solutions by analysis of hydrogen and oxygen isotope compositions of hydrothermal epidote from geothermal drillholes at depths between 1 and 3 km. δDEPIDOTE values from wells RN-8, -9, -10 and -17 collectively range from −60 to −78‰, and δ18OEPIDOTE in these wells are between −3.0 and 2.3‰. The δD values of epidote generally increase along a NE trend through the geothermal field, whereas δ18O values generally decrease, suggesting a southwest to northeast migration of the geothermal upflow zone with time that is consistent with present-day temperatures and observed hydrothermal mineral zones. For comparative analysis, the meteoric-water dominated Nesjavellir and Krafla geothermal systems, which have a δDFLUID of ∼ −79‰ and −89‰, respectively, show δDEPIDOTE values of −115‰ and −125‰. In contrast, δDEPIDOTE from the mixed meteoric-seawater Svartsengi geothermal system is −68‰; comparable to δDEPIDOTE from well RN-10 at Reykjanes.Stable isotope compositions of geothermal fluids in isotopic equilibrium with the epidotes at Reykjanes are computed using published temperature dependent hydrogen and oxygen isotope fractionation curves for epidote-water, measured isotope composition of the epidotes and temperatures approximated from the boiling point curve with depth. Calculated δD and δ18O of geothermal fluids are less than 0‰, suggesting that fluids of meteoric or glacial origin are a significant component of the geothermal solutions. Additionally, δDFLUID values in equilibrium with geothermal epidote are lower than those of modern-day fluids, whereas calculated δ18OFLUID values are within range of the observed fluid isotope composition. We propose that modern δDEPIDOTE and δDFLUID values are the result of diffusional exchange between hydrous alteration minerals that precipitated from glacially-derived fluids early in the evolution of the Reykjanes system and modern seawater-derived geothermal fluids. A simplified model of isotope exchange in the Reykjanes geothermal system, in which the average starting δDROCK value is −125‰ and the water to rock mass ratio is 0.25, predicts a δDFLUID composition within 1‰ of average measured values. This model resolves the discrepancy between fluid salinity and isotope composition of Reykjanes geothermal fluids, explains the observed disequilibrium between modern fluids and hydrothermal epidote, and suggests that rock-fluid interaction is the dominant control over the evolution of fluid isotope composition in the hydrothermal system.  相似文献   

12.
We present some of the first analyses of the stable isotopic composition of dissolved silicon (Si) in groundwater. The groundwater samples were from the Navajo Sandstone aquifer at Black Mesa, Arizona, USA, and the Si isotope composition of detrital feldspars and secondary clay coatings in the aquifer were also analyzed. Silicon isotope compositions were measured using high-resolution multi-collector inductively coupled mass spectrometry (HR-MC-ICP-MS) (Nu1700 & NuPlasma HR). The quartz dominated bulk rock and feldspar separates have similar δ30Si of −0.09 ± 0.04‰ and −0.15 ± 0.04‰ (±95% SEM), respectively, and clay separates are isotopically lighter by up to 0.4‰ compared to the feldspars. From isotopic mass-balance considerations, co-existing aqueous fluids should have δ30Si values heavier than the primary silicates. Positive δ30Si values were only found in the shallow aquifer, where Si isotopes are most likely fractionated during the dissolution of feldspars and subsequent formation of clay minerals. However, δ30Si decreases along the flow path from 0.56‰ to −1.42‰, representing the most negative dissolved Si isotope composition so far found for natural waters. We speculate that the enrichment in 28Si is due to dissolution of partly secondary clay minerals and low-temperature silcretes in the Navajo Sandstone. The discovery of the large range and systematic shifts of δ30Si values along a groundwater flow path illustrates the potential utility of stable Si isotopes for deciphering the Si cycling in sedimentary basins, tracing fluid flow, and evaluating global Si cycle.  相似文献   

13.
Mg isotope ratios (26Mg/24Mg) are reported in soil pore-fluids, rain and seawater, grass and smectite from a 90 kyr old soil, developed on an uplifted marine terrace from Santa Cruz, California. Rain water has an invariant 26Mg/24Mg ratio (expressed as δ26Mg) at −0.79 ± 0.05‰, identical to seawater δ26Mg. Detrital smectite (from the base of the soil profile, and therefore unweathered) has a δ26Mg value of 0.11‰, potentially enriched in 26Mg by up to 0.3‰ compared to the bulk silicate Earth Mg isotope composition (although within the range of all terrestrial silicates). The soil pore-waters show a continuous profile with depth for δ26Mg, ranging from −0.99‰ near the surface to −0.43‰ at the base of the profile. Shallow pore-waters (<1 m) have δ26Mg values that are similar to, or slightly lower than the rain waters. This implies that the degree of biological cycling of Mg in the pore-waters is relatively small and is quantified as <32%, calculated using the average Mg isotope enrichment factor between grass and rain (δ26Mggrass-δ26Mgrain) of 0.21‰. The deep pore-waters (1-15 m deep) have δ26Mg values that are intermediate between the smectite and rain, ranging from −0.76‰ to −0.43‰, and show a similar trend with depth compared to Sr isotope ratios. The similarity between Sr and Mg isotope ratios confirms that the Mg in the pore-waters can be explained by a mixture between rain and smectite derived Mg, despite the fact that Mg and Sr concentrations may be buffered by the exchangeable reservoir. However, whilst Sr isotope ratios in the pore-waters span almost the complete range between mineral and rain inputs, Mg isotopes compositions are much closer to the rain inputs. If Mg and Sr isotope ratios are controlled uniquely by a mixture, the data can be used to estimate the mineral weathering inputs to the pore-waters, by correcting for the rain inputs. This isotopic correction is compared to the commonly used chloride correction for precipitation inputs. A consistent interpretation is only possible if Mg isotope ratios are fractionated either by the precipitation of a secondary Mg bearing phase, not detected by conventional methods, or selective leaching of 24Mg from smectite. There is therefore dual control on the Mg isotopic composition of the pore-waters, mixing of two inputs with distinct isotopic compositions, modified by fractionation. The data provide (1) further evidence for Mg isotope fractionation at the surface of the Earth and (2) the first field evidence of Mg isotope fractionation during uptake by natural plants. The coherent behaviour of Mg isotope ratios in soil environments is encouraging for the development of Mg isotope ratios as a quantitative tracer of both weathering inputs of Mg to waters, and the physicochemical processes that cycle Mg, a major cation linked to the carbon cycle, during continental weathering.  相似文献   

14.
Copper and Zn metals are produced in large quantities for different applications. During Cu production, large amounts of Cu and Zn can be released to the environment. Therefore, the surroundings of Cu smelters are frequently metal-polluted. We determined Cu and Zn concentrations and Cu and Zn stable isotope ratios (δ65Cu, δ66Zn) in three soils at distances of 1.1, 3.8, and 5.3 km from a Slovak Cu smelter and in smelter wastes (slag, sludge, ash) to trace sources and transport of Cu and Zn in soils. Stable isotope ratios were measured by multicollector inductively coupled plasma mass spectrometry (MC-ICP-MS) in total digests. Soils were heavily contaminated with concentrations up to 8087 μg g−1 Cu and 2084 μg g−1 Zn in the organic horizons. The δ65Cu values varied little (−0.12‰ to 0.36‰) in soils and most wastes and therefore no source identification was possible. In soils, Cu became isotopically lighter with increasing depth down to 0.4 m, likely because of equilibrium reactions between dissolved and adsorbed Cu species during transport of smelter-derived Cu through the soil. The δ66ZnIRMM values were isotopically lighter in ash (−0.41‰) and organic horizons (−0.85‰ to −0.47‰) than in bedrock (−0.28‰) and slag (0.18‰) likely mainly because of kinetic fractionation during evaporation and thus allowed for separation of smelter-Zn from native Zn in soil. In particular in the organic horizons large variations in δ66Zn values occur, probably caused by biogeochemical fractionation in the soil-plant system. In the mineral horizons, Zn isotopes showed only minor shifts to heavier δ66Zn values with depth mainly because of the mixing of smelter-derived Zn and native Zn in the soils. In contrast to Cu, Zn isotope fractionation between dissolved and adsorbed species was probably only a minor driver in producing the observed variations in δ66Zn values. Our results demonstrate that metal stable isotope ratios may serve as tracer of sources, vertical dislocation, and biogeochemical behavior in contaminated soil.  相似文献   

15.
We have developed a method for iron isotope analysis by multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) using a 58Fe-54Fe double spike. A 20 min analysis produces mass-bias-corrected iron isotope data with an external reproducibility of ±0.05 (2 SD) on δ56Fe, which represents a decrease in analysis time compared to sample-standard bracketing techniques. The estimation of external reproducibility is based on replicate analysis of the ETH hematite in-house standard. The double spike method has two advantages. First, matrix effects during MC-ICP-MS analysis are decreased with tests showing that accurate iron isotope data can, in some cases, be obtained even when matrix levels exceed iron concentration (Na/Fe, Mg/Fe, and Ca/Fe up to 5, 2, and 0.1, respectively). Because chemical separation reduces matrix/Fe to levels more than three orders of magnitude lower than this, measured Fe isotope compositions are unlikely to be compromised by matrix effects. Second, it is possible to spike samples before chemical purification, which enables any isotopic fractionation effect because of incomplete recovery of iron from a sample to be accounted for. This may be important where obtaining quantitative iron yields from samples is difficult, such as the extraction of dissolved iron from water samples. Fe isotope data on a set of standard reference materials (igneous rocks, ferromanganese nodules, sedimentary rocks, and ores) are presented, which are in agreement with previously published data considering analytical uncertainties. Mantle-derived standard rock samples that are the source of iron for surficial, (bio)geochemical cycling yield a mean δ56Fe of 0.041 ± 0.11‰ (n = 8; 2 SD) with reference to IRMM-14. Hydrothermal and metamorphic calcium carbonate rocks with a relatively low iron content (100-4000 ppm) have δ56Fe = −1.25 to −0.07‰. Structural Fe(II) in hydrothermal calcites has δ56Fe = −1.25 to −0.27‰. The light iron in this range of carbonate minerals may reflect the iron isotope composition of the hydrothermal fluids from which the carbonate precipitated, or the presence of Fe(III) and/or organic material in the hydrothermal fluids during calcite precipitation.  相似文献   

16.
Li concentrations and isotopic compositions of coexisting minerals (ol, opx, and cpx) from peridotite xenoliths entrained in the Hannuoba Tertiary basalts, North China Craton, provide insight into Li isotopic fractionation between mantle minerals during melt-rock interaction in the considerably thinned lithospheric mantle. Bulk analyses of mineral separates show significant enrichment of Li in cpx (2.4-3.6 ppm) relative to olivine (1.2-1.8 ppm), indicating that these peridotites have been affected by mantle metasomatism with mafic silicate melts. Bulk olivine separates (δ7Li ∼ +3.3‰ to +6.4‰) are isotopically heavier than coexisting pyroxenes (δ7Li ∼ −3.3‰ to −8.2‰ in cpx, and −4.0‰ to −6.7‰ in opx). Such large variation suggests Li elemental and isotopic disequilibrium. This conclusion is supported by results from in situ SIMS analyses of mineral grains where significant Li elemental and isotopic zonations exist. The olivine and opx have lower Li concentrations and heavier Li isotopes in the rims than in the cores. This reverse correlation of δ7Li with Li concentrations indicates diffusive fractionation of Li isotopes. However, the zoning patterns in coexisting cpx show isotopically heavier rims with higher Li abundances. This positive correlation between δ7Li and Li concentrations suggests a melt mixing trend. We attribute Li concentration and isotope zonation in minerals to the effects of two-stage diffusive fractionation coupled with melt-rock interaction. The earliest melts may have been derived from the subducted oceanic slab with low δ7Li values produced by isotopic fractionation during the dehydration of the seawater-altered slab. Melts at later stages were derived from the asthenosphere and interacted with the peridotites, producing the Li elemental and isotopic zoning in mineral grains. These data thus provide evidence for multiple-stage peridotite-melt interaction in the lithospheric mantle beneath the northern North China Craton.  相似文献   

17.
We determined the stable carbon and hydrogen isotope fractionation factors for methane oxidation under oxic conditions using strains with known degradation pathways. The aerobic oxidation of methane can be initiated by two different forms of enzymes known as methane monooxygenases (MMO). The expression of these enzymes is type-specific and dependent upon the adjusted copper concentration in the medium (or environment). In this study, the expression of either the soluble MMO or the particulate MMO was supported by adjusting the copper concentrations in the growth medium. Taxonomically different aerobic methanotrophic strains, mainly belonging to the alpha- and gamma- classes of Proteobacteria, produced methane isotope enrichment factors (εbulk) ranging from −14.8 to −27.9‰ for carbon, and from −110.0 to −231.5‰ for hydrogen. The ratio of hydrogen versus carbon discrimination (Λ = (αH−1 − 1)/(αC−1 − 1) ≈ Δ(δ2H)/Δ(δ13C)) were similar for all tested cultures, and are also identical to values calculated from previously published enrichment factors for aerobic and anaerobic methane degradation. In contrast, Λ-values for the abiotic oxidation of methane with OH radicals (this process is considered as the main removal process for methane from the atmosphere) were significantly higher than the values derived from biotic oxidation. Due to the low variability of microbial methane isotope fractionation patterns, we propose that combined carbon and hydrogen isotope fractionation analyses can be used to monitor and assess the occurrence of microbial methane oxidation in marine or terrestrial environments. However, it is not possible to distinguish distinct aerobic or anaerobic methane-oxidation pathways by this approach.  相似文献   

18.
We report high precision Cu isotope data coupled with Cu concentration measurements for metal, troilite and silicate fractions separated from magmatic and non-magmatic iron meteorites, analysed for Fe isotopes (δ57Fe; permil deviation in 57Fe/54Fe relative to the pure iron standard IRMM-014) in an earlier study (Williams et al., 2006). The Cu isotope compositions (δ65Cu; permil deviation in 65Cu/63Cu relative to the pure copper standard NIST 976) of both metals (δ65CuM) and sulphides (δ65CuFeS) span much wider ranges (−9.30 to 0.99‰ and −8.90 to 0.63‰, respectively) than reported previously. Metal-troilite fractionation factors (Δ65CuM-FeS = δ65CuM − δ65CuFeS) are variable, ranging from −0.07 to 5.28‰, and cannot be explained by equilibrium stable isotope fractionation coupled with either mixing or reservoir effects, i.e. differences in the relative proportions of metal and sulphide in the meteorites. Strong negative correlations exist between troilite Cu and Fe (δ57FeFeS) isotope compositions and between metal-troilite Cu and Fe (Δ57FeM-FeS) isotope fractionation factors, for both magmatic and non-magmatic irons, which suggests that similar processes control isotopic variations in both systems. Clear linear arrays between δ65CuFeS and δ57FeFeS and calculated Cu metal-sulphide partition coefficients (DCu = [Cu]metal/[Cu]FeS) are also present. A strong negative correlation exists between Δ57FeM-FeS and DCu; a more diffuse positive array is defined by Δ65CuM-FeS and DCu. The value of DCu can be used to approximate the degree of Cu concentration equilibrium as experimental studies constrain the range of DCu between Fe metal and FeS at equilibrium to be in the range of 0.05-0.2; DCu values for the magmatic and non-magmatic irons studied here range from 0.34 to 1.11 and from 0.04 to 0.87, respectively. The irons with low DCu values (closer to Cu concentration equilibrium) display the largest Δ57FeM-FeS and the lowest Δ65CuM-FeS values, whereas the converse is observed in the irons with large values DCu that deviate most from Cu concentration equilibrium. The magnitudes of Cu and Fe isotope fractionation between metal and FeS in the most equilibrated samples are similar: 0.25 and 0.32‰/amu, respectively. As proposed in an earlier study (Williams et al., 2006) the range in Δ57FeM-FeS values can be explained by incomplete Fe isotope equilibrium between metal and sulphide during cooling, where the most rapidly-cooled samples are furthest from isotopic equilibrium and display the smallest Δ57FeM-FeS and largest DCu values. The range in Δ65CuM-FeS, however, reflects the combined effects of partial isotopic equilibrium overprinting an initial kinetic signature produced by the diffusion of Cu from metal into exsolving sulphides and the faster diffusion of the lighter isotope. In this scenario, newly-exsolved sulphides initially have low Cu contents (i.e. high DCu) and extremely light δ65CuFeS values; with progressive equilibrium and fractional crystallisation the Cu contents of the sulphides increase as their isotopic composition becomes less extreme and closer to the metal value. The correlation between Δ65CuM-FeS and Δ57FeM-FeS is therefore a product of the superimposed effects of kinetic fractionation of Cu and incomplete equilibrium between metal and sulphide for both isotope systems during cooling. The correlations between Δ65CuM-FeS and Δ57FeM-FeS are defined by both magmatic and non-magmatic irons record fractional crystallisation and cooling of metallic melts on their respective parent bodies as sulphur and chalcophile elements become excluded from crystallised solid iron and concentrated in the residual melt. Fractional crystallisation processes at shallow levels have been implicated in the two main classes of models for the origin of the non-magmatic iron meteorites; at (i) shallow levels in impact melt models and (ii) at much deeper levels in models where the non-magmatic irons represent metallic melts that crystallised within the interior of a disrupted and re-aggregated parent body. The presence of non-magmatic irons with a range of Fe and Cu isotope compositions, some of which record near-complete isotopic equilibrium implies crystallisation at a range of cooling rates and depths, which is most consistent with cooling within the interior of a meteorite parent body. Our data therefore lend support to models where the non-magmatic irons are metallic melts that crystallised in the interior of re-aggregated, partially differentiated parent bodies.  相似文献   

19.
Ion-exchange fractionation of copper and zinc isotopes   总被引：5，自引：0，他引：5  
Whether transition element isotopes can be fractionated at equilibrium in nature is still uncertain. Standard solutions of Cu and Zn were eluted on an anion-exchange resin, and the isotopic compositions of Cu (with respect to Zn) of the eluted fractions were measured by multiple-collector inductively coupled plasma mass spectrometry. It was found that for pure Cu solutions, the elution curves are consistent with a 63Cu/65Cu mass fractionation coefficient of 0.46‰ in 7 mol/L HCl and 0.67‰ in 3 mol/L HCl between the resin and the solution. Batch fractionation experiments confirm that equilibrium fractionation of Cu between resin and 7 mol/L HCl is ∼0.4‰ and therefore indicates that there is no need to invoke kinetic fractionation during the elution. Zn isotope fractionation is an order of magnitude smaller, with a 66Zn/68Zn fractionation factor of 0.02‰ in 12 mol/L HCl. Cu isotope fractionation results determined from a chalcopyrite solution in 7 mol/L HCl give a fractionation factor of 0.58‰, which indicates that Fe may interfere with Cu fractionation.Comparison of Cu and Zn results suggests that the extent of Cu isotopic fractionation may signal the presence of so far unidentified polynuclear complexes in solution. In contrast, we see no compelling reason to ascribe isotope fractionation to the coexistence of different oxidation states. We further suggest that published evidence for iron isotopic fractionation in nature and in laboratory experiments may indicate the distortion of low-spin Fe tetrahedral complexes.The isotope geochemistry of transition elements may shed new light on their coordination chemistry. Their isotopic fractionation in the natural environment may be interpreted using models of thermodynamic fractionation.  相似文献   

20.
Sulfide mineralization in the Voisey’s Bay Intrusion, Labrador, Canada, is closely associated with country rock xenoliths that have extensively reacted with basaltic magma. In order to better understand the processes that control the assimilation of country rocks by mafic magma, a detailed study of oxygen isotope systematics related to magma-country rock interaction in the Voisey’s Bay area was undertaken. Protracted interaction of the xenoliths with magma produced refractory mineral assemblages in the xenoliths (2-10 cm in diameter) composed of Ca-rich plagioclase, corundum, hercynite, and minor magnetite. Overgrowth rims of plagioclase and biotite that surround most xenoliths separate the restites from the enclosing igneous matrix. The δ18O values of minerals from regionally metamorphosed pelitic and quartzofeldspathic protoliths are: plagioclase (8.7-12.3‰), orthoclase (9.5-9.8‰), biotite (5.2-8.7‰), garnet (8.3-10.8‰), pyroxene (8.0-10.1‰), and quartz (9.6-14.0). The δ18O values of minerals from the hornfels in the contact aureole of the intrusion are consistent with modeling which indicates that as a result of essentially closed system contact metamorphism oxygen isotope values should differ only slightly from those of the protoliths. Hercynite, plagioclase, and corundum separates from the xenoliths have δ18O values that vary from 2.9‰ to 10.5‰, 5.6‰ to 10.9‰, and 2.0‰ to 6.8‰, respectively. Although a siliceous 18O-enriched melt has been lost from the xenoliths, corundum, and feldspar δ18O values are significantly lower than expected through melt loss alone. The relatively low δ18O values of minerals from the xenoliths may be a function of incomplete isotopic exchange with surrounding mafic magma which had a δ18O value of ∼5.5‰ to 6.0‰. The high-18O melt that was released from the xenoliths is partially recorded in the plagioclase overgrowth on the margin of the xenoliths (δ18O values from 6.2‰ to 10.7‰), and in hercynite that replaced corundum. However, mass balance calculations indicate that a portion of the partial melt must have been transferred to magma that was moving through the conduit system. δ18O and δD values of biotite surrounding the plagioclase overgrowth range from 5.0‰ to 6.2‰ and −58‰ to −80‰, respectively. These data suggest that the outermost rim associated with many xenoliths has closely approached isotopic equilibrium with uncontaminated mafic magma. The current gabbroic to troctolitic matrix of the xenoliths shows no evidence for contamination by the high-18O partial melt from the xenoliths. The feldspar and biotite overgrowths on the xenoliths that formed after the motion of the xenoliths relative to the magma had stopped prevented further isotopic exchange between the xenoliths and final magma. The minerals within the xenoliths are not in oxygen isotopic equilibrium with each other, due in part to rapid thermal equilibration, partial melting, and partial exchange with flow through magma.  相似文献   

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