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1.
Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ18O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO2, Al2O3). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ56Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ18O and δ56Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ56Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ18O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ18O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ18O and δ56Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).  相似文献   

2.
Bulk-rock chlorine content and isotopic composition (δ37Cl) were determined in oceanic serpentinites, high-pressure metaperidotites and metasediments in order to gain constraints on the global chlorine cycle associated with hydrothermal alteration and subduction of oceanic lithosphere. The distribution of insoluble chlorine in oceanic serpentinites was also investigated by electron microprobe. The hydrothermally-altered ultramafic samples were dredged along the South West Indian Ridge and the Mid-Atlantic Ridge. The high-pressure metamorphic samples were collected in the Western Alps: metaperidotites in the Erro-Tobbio unit and metasediments in the Schistes Lustrés nappe.Oceanic serpentinites show relatively large variations of bulk-rock Cl contents and δ37Cl values with mean values of 1105 ± 596 ppm and −0.7 ± 0.4‰, respectively (n = 8; 1σ). Serpentines formed after olivine (meshes) show lower Cl content than those formed after orthopyroxene (bastites). In bastites of two different samples, Cl is positively correlated with Al2O3 and negatively correlated with SiO2. These relationships are interpreted as reflecting preferential Cl-incorporation into the bastite structure distorted by Al (substituted for Si) rather than different alteration conditions between olivine and orthopyroxene minerals. High-pressure metaperidotites display relatively homogeneous Cl contents and δ37Cl values with mean values of 467 ± 88 ppm and −1.4 ± 0.1‰, respectively (n = 7; 1σ). A macroscopic high-pressure olivine-bearing vein, formed from partial devolatilization of serpentinites at ∼2.5 GPa and 500-600 °C, shows a Cl content and a δ37Cl value of 603 ppm and −1.6‰, respectively. Metasediments (n = 2) show low whole-rock Cl contents (<15 ppm Cl) that did not allow Cl isotope analyses to be obtained.The range of negative δ37Cl values observed in oceanic serpentinites is likely to result from water-rock interaction with fluids that have negative δ37Cl values. The homogeneity of δ37Cl values from the high-pressure olivine-bearing vein and the metaperidotite samples implies that progressive loss of Cl inherited from oceanic alteration throughout subduction did not significantly fractionate Cl isotopes. Chlorine recycled in subduction zones via metaperidotites should thus show a range of δ37Cl values similar to the range found in oceanic serpentinized peridotites.  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.
We present high-precision measurements of Mg and Fe isotopic compositions of olivine, orthopyroxene (opx), and clinopyroxene (cpx) for 18 lherzolite xenoliths from east central China and provide the first combined Fe and Mg isotopic study of the upper mantle. δ56Fe in olivines varies from 0.18‰ to −0.22‰ with an average of −0.01 ± 0.18‰ (2SD, n = 18), opx from 0.24‰ to −0.22‰ with an average of 0.04 ± 0.20‰, and cpx from 0.24‰ to −0.16‰ with an average of 0.10 ± 0.19‰. δ26Mg of olivines varies from −0.25‰ to −0.42‰ with an average of −0.34 ± 0.10‰ (2SD, n = 18), opx from −0.19‰ to −0.34‰ with an average of −0.25 ± 0.10‰, and cpx from −0.09‰ to −0.43‰ with an average of −0.24 ± 0.18‰. Although current precision (∼±0.06‰ for δ56Fe; ±0.10‰ for δ26Mg, 2SD) limits the ability to analytically distinguish inter-mineral isotopic fractionations, systematic behavior of inter-mineral fractionation for both Fe and Mg is statistically observed: Δ56Feol-cpx = −0.10 ± 0.12‰ (2SD, n = 18); Δ56Feol-opx = −0.05 ± 0.11‰; Δ26Mgol-opx = −0.09 ± 0.12‰; Δ26Mgol-cpx = −0.10 ± 0.15‰. Fe and Mg isotopic composition of bulk rocks were calculated based on the modes of olivine, opx, and cpx. The average δ56Fe of peridotites in this study is 0.01 ± 0.17‰ (2SD, n = 18), similar to the values of chondrites but slightly lower than mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB). The average δ26Mg is −0.30 ± 0.09‰, indistinguishable from chondrites, MORB, and OIB. Our data support the conclusion that the bulk silicate Earth (BSE) has chondritic δ56Fe and δ26Mg.The origin of inter-mineral fractionations of Fe and Mg isotopic ratios remains debated. δ56Fe between the main peridotite minerals shows positive linear correlations with slopes within error of unity, strongly suggesting intra-sample mineral-mineral Fe and Mg isotopic equilibrium. Because inter-mineral isotopic equilibrium should be reached earlier than major element equilibrium via chemical diffusion at mantle temperatures, Fe and Mg isotope ratios of coexisting minerals could be useful tools for justifying mineral thermometry and barometry on the basis of chemical equilibrium between minerals. Although most peridotites in this study exhibit a narrow range in δ56Fe, the larger deviations from average δ56Fe for three samples likely indicate changes due to metasomatic processes. Two samples show heavy δ56Fe relative to the average and they also have high La/Yb and total Fe content, consistent with metasomatic reaction between peridotite and Fe-rich and isotopically heavy melt. The other sample has light δ56Fe and slightly heavy δ26Mg, which may reflect Fe-Mg inter-diffusion between peridotite and percolating melt.  相似文献   

5.
This study investigates the application of sulphur isotope ratios (δ34S) in combination with carbon (δ13C) and nitrogen (δ15N) ratios to understand the influence of environmental sulphur on the isotopic composition of archaeological human and faunal remains from Roman era sites in Oxfordshire, UK. Humans (n = 83), terrestrial animals (n = 11), and freshwater fish (n = 5) were analysed for their isotope values from four locations in the Thames River Valley, and a broad range of δ34S values were found. The δ34S values from the terrestrial animals were highly variable (−13.6‰ to +0.5‰), but the δ34S values of the fish were clustered and 34S-depleted (−20.9‰ to −17.3‰). The results of the faunal remains suggest that riverine sulphur influenced the terrestrial sulphur isotopic signatures. Terrestrial animals were possibly raised on the floodplains of the River Thames, where highly 34S-depleted sulphur influenced the soil. The humans show the largest range of δ34S values (−18.8‰ to +9.6‰) from any archaeological context to date. No differences in δ34S values were found between the males (−7.8 ± 6.0‰) and females (−5.3 ± 6.8‰), but the females had a linear correlation (R2 = 0.71; p < 0.0001) between their δ15N and δ34S compositions. These δ34S results suggest a greater dietary variability for the inhabitants of Roman Oxfordshire than previously thought, with some individuals eating solely terrestrial protein resources and others showing a diet almost exclusively based on freshwater protein such as fish. Such large dietary variability was not visible by analysing only the carbon and nitrogen isotope ratios, and this research represents the largest and most detailed application of δ34S analysis to examine dietary practices (including breastfeeding and weaning patterns) during the Romano-British Period.  相似文献   

6.
A laser fluorination microprobe system has been constructed for high-accuracy, high-precision multisulfur isotope analysis with improved spatial resolution. The system uses two lasers: (a) a KrF excimer laser for in situ spot analysis by ultraviolet (UV) photoablation with λ = 248 nm and (b) a CO2 laser for whole-grain analysis of powdered samples by infrared heating at λ = 10.6 μm. A CO2 laser is necessary for the analysis of interlaboratory isotope reference materials because they are supplied as powders. The δ34S and δ33S compositions of reference materials measured with a CO2 laser fluorination system agree (±0.2‰, 1σ) with the recommended values by the Sulfur Isotope Working Group of the International Atomic Energy Agency [Ding et al 2001] and [Taylor]. The precision of replicate analyses of powdered sulfide minerals with the CO2 laser is typically ±0.2‰ (1σ) for δ34S.The in situ fluorination of sulfides with a KrF excimer laser (λ = 248 nm) was validated by comparison of measurements of side-by-side laser craters and powders excavated from drill holes. Powders from drill holes were analyzed with the CO2 laser. In situ laser craters and drill hole powders give the same δ34SV-CDT and δ33SV-CDT values within 0.2‰. The δ34SV-CDT and δ33SV-CDT values of both powders and in situ analyses are independent of F2 gas pressure over a range of 15 to 65 torr. No dependence of δ34SV-CDT and δ33SV-CDT values on UV laser energy fluence has been observed. Mineral-specific fractionation of sulfur isotopes in analyzing pyrite, sphalerite, galena, troilite, and chalcopyrite has not been observed with a KrF excimer laser (λ = 248 nm). Test analyses with an ArF excimer laser (λ = 193 nm), however, gave fractionated sulfur isotope ratios.A range of Δ33S anomalies of from - 1.5 to +3.0‰ in Archean samples from the North Pole district, Pilbara Craton, Australia, and from black shale of the Lokamonna Formation, South Africa, were verified by in situ analysis of individual pyrite grains with a KrF excimer laser. These results show that a combination of high-accuracy, high-precision analyses with improved spatial resolution permits locating and analyzing host minerals of non-mass-dependent sulfur isotope anomalies.  相似文献   

7.
The concentrations of mercury (Hg) and other trace metals (Ni, Cu, Zn, Mo, Ba, Re, U) and the Hg isotopic composition were examined across a dramatic redox and productivity transition in a mid-Pleistocene Mediterranean Sea sapropel sequence. Characteristic trace metal enrichment in organic-rich layers was observed, with organic-rich sapropel layers ranging in Hg concentration from 314 to 488 ng/g (avg = 385), with an average enrichment in Hg by a factor of 5.9 compared to organic-poor background sediments, which range from 39 to 94 ng/g Hg (avg = 66). Comparison of seawater concentrations and sapropel accumulations of trace metals suggests that organic matter quantitatively delivers Hg to the seafloor. Near complete scavenging of Hg from the water column renders the sapropel Hg isotopic composition representative of mid-Pleistocene Mediterranean seawater. Sapropels have an average δ202Hg value of −0.91‰ ± 0.15‰ (n = 5, 1 SD) and Δ199Hg value of 0.11‰ ± 0.03‰ (n = 5, 1 SD). Background sediments have an average δ202Hg of −0.76‰ ± 0.16‰ (n = 5, 1 SD) and Δ199Hg of 0.05‰ ± 0.01‰ (n = 5, 1 SD), which is indistinguishable from the sapropel values. We suggest that the sapropel isotopic composition is most representative of the mid-Pleistocene Tyrrhenian Sea.  相似文献   

8.
Calcium isotopes in tissues are thought to be influenced by an individual’s diet, reflecting parameters such as trophic level and dairy consumption, but this has not been carefully assessed. We report the calcium isotope ratios (δ44/42Ca) of modern and archaeological animal and human bone (n = 216). Modern sheep raised at the same location show 0.14 ± 0.08‰ higher δ44/42Ca in females than in males, which we attribute to lactation by the ewes. In the archaeological bone samples the calcium isotope ratios of the herbivorous fauna vary by location. At a single site, the archaeological fauna do not show a trophic level effect. Humans have lower δ44/42Ca than the mean site fauna by 0.22 ± 0.22‰, and the humans have a greater δ44/42Ca range than the animals. No effect of sex or age on the calcium isotope ratios was found, and intra-individual skeletal δ44/42Ca variability is negligible. We rule out dairy consumption as the main cause of the lower human δ44/42Ca, based on results from sites pre-dating animal domestication and dairy availability, and suggest instead that individual physiology and calcium intake may be important in determining bone calcium isotope ratios.  相似文献   

9.
We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) meteorites. The accuracy of our new Mg isotope ratio measurement protocol is substantiated by a combination of standard addition experiments, the absence of mass independent effects in terrestrial samples and our obtaining identical values for rock standards using two different separation chemistries and three different mass-spectrometric introduction systems. Carbonaceous, ordinary and enstatite chondrites have irresolvable mean stable Mg isotopic compositions (δ25Mg = −0.14 ± 0.06; δ26Mg = −0.27 ± 0.12‰, 2SD), but our enstatite chondrite samples have lighter δ7Li (by up to ∼3‰) than our mean carbonaceous and ordinary chondrites (3.0 ± 1.5‰, 2SD), possibly as a result of spallation in the early solar system. Measurements of equilibrated, fertile peridotites give mean values of δ7Li = 3.5 ± 0.5‰, δ25Mg = −0.10 ± 0.03‰ and δ26Mg = −0.21 ± 0.07‰. We believe these values provide a useful estimate of the primitive mantle and they are within error of our average of bulk carbonaceous and ordinary chondrites. A fuller range of fresh, terrestrial, ultramafic samples, covering a variety of geological histories, show a broad positive correlation between bulk δ7Li and δ26Mg, which vary from −3.7‰ to +14.5‰, and −0.36‰ to + 0.06‰, respectively. Values of δ7Li and δ26Mg lower than our estimate of primitive mantle are strongly linked to kinetic isotope fractionation, occurring during transport of the mantle xenoliths. We suggest Mg and Li diffusion into the xenoliths is coupled to H loss from nominally anhydrous minerals following degassing. Diffusion models suggest that the co-variation of Mg and Li isotopes requires comparable diffusivities of Li and Mg in olivine. The isotopically lightest samples require ∼5-10 years of diffusive ingress, which we interpret as a time since volatile loss in the host magma. Xenoliths erupted in pyroclastic flows appear to have retained their mantle isotope ratios, likely as a result of little prior degassing in these explosive events. High δ7Li, coupled with high [Li], in rapidly cooled arc peridotites may indicate that these samples represent fragments of mantle wedge that has been metasomatised by heavy, slab-derived fluids. If such material is typically stirred back into the convecting mantle, it may account for the heavy δ7Li seen in some oceanic basalts.  相似文献   

10.
A detailed oxygen isotope study of detrital quartz and authigenic quartz overgrowths from shallowly buried (<1 km) quartz arenites of the St. Peter Sandstone (in SW Wisconsin) constrains temperature and fluid sources during diagenesis. Quartz overgrowths are syntaxial (optically continuous) and show complex luminescent zonation by cathodoluminescence. Detrital quartz grains were separated from 53 rocks and analyzed for oxygen isotope ratio by laser fluorination, resulting in an average δ18O of 10.0 ± 0.2‰ (1SD, n = 109). Twelve thin sections were analyzed by CAMECA-1280 ion microprobe (6-10 μm spot size, analytical precision better than ±0.2‰, 1SD). Detrital quartz grains have an average δ18O of 10.0 ± 1.4‰ (1SD, n = 91) identical to the data obtained by laser fluorination. The ion microprobe data reveal true variability that is otherwise lost by homogenization of powdered samples necessary for laser fluorination. Laser fluorination uses samples that are one million times larger than the ion microprobe. Whole rock (WR) samples from the 53 rocks were analyzed by laser fluorination, giving δ18O between 9.8‰ and 16.7‰ (n = 110). Quartz overgrowths in thin sections from 10 rocks were analyzed by ion microprobe and average δ18O = 29.3 ± 1.0‰ (1SD, n = 161).Given the similarity, on average, of δ18O for all detrital quartz grains and for all quartz overgrowths, samples with higher δ18O(WR) values can be shown to have more cement. The quartz cement in the 53 rocks, calculated by mass balance, varies from <1 to 21 vol.% cement, with one outlier at 33 vol.% cement. Eolian samples have an average of 11% cement compared to marine samples, which average 4% cement.Two models for quartz cementation have been investigated: high temperature (50-110 °C) formation from ore-forming brines related to Mississippi Valley Type (MVT) mineralization and formation as silcretes at low temperature (10-30 °C). The homogeneity of δ18O for quartz overgrowths determined by ion microprobe rules out a systematic regional variation of temperature as predicted for MVT brines and there are no other known heating events in these sediments that were never buried to depths >1 km. The data in this study suggest that quartz overgrowths formed as silcretes in the St. Peter Sandstone from meteoric water with δ18O values of −10‰ to −5‰ at 10-30 °C. This interpretation runs counter to conventional wisdom based on fibrous or opaline silica cements suggesting that the formation of syntaxial quartz overgrowths requires higher temperatures. While metastable silica cements commonly form at high degrees of silica oversaturation following rapid break-down reactions of materials such as of feldspars or glass, the weathering of a clean quartz arenite is slower facilitating chemical equilibrium and precipitation of crystallographically oriented overgrowths of α-quartz.  相似文献   

11.
Silicon isotopes in meteorites and planetary core formation   总被引：1，自引：0，他引：1  
The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ30Si differences are resolved between chondrites (δ30Si = −0.49 ± 0.15‰, 2σSD) and achondrites (δ30Si = −0.47 ± 0.11‰, 2σSD), although enstatite chondrites are consistently lighter (δ30Si = −0.63 ± 0.07‰, 2σSD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ30SiBSE−meteorite∗ = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal-silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor (ε ∼0.89), the Δ30SiBSE−meteorite∗ value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.  相似文献   

12.
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   

13.
An 18 million year record of the Ca isotopic composition (δ44/42Ca) of planktonic foraminiferans from ODP site 925, in the Atlantic, on the Ceara Rise, provides the opportunity for critical analysis of Ca isotope-based reconstructions of the Ca cycle. δ44/42Ca in this record averages +0.37 ± 0.05 (1σ SD) and ranges from +0.21‰ to +0.52‰. The record is a good match to previously published Neogene Ca isotope records based on foraminiferans, but is not similar to the record based on bulk carbonates, which has values that are as much as 0.25‰ lower. Bulk carbonate and planktonic foraminiferans from core tops differ slightly in their δ44/42Ca (i.e., by 0.06 ± 0.06‰ (n = 5)), while the difference between bulk carbonate and foraminiferan values further back in time is markedly larger, leaving open the question of the cause of the difference. Modeling the global Ca cycle from downcore variations in δ44/42Ca by assuming fixed values for the isotopic composition of weathering inputs (δ44/42Caw) and for isotope fractionation associated with the production of carbonate sediments (Δsed) results in unrealistically large variations in the total mass of Ca2+ in the oceans over the Neogene. Alternatively, variations of ±0.05‰ in the Ca isotope composition of weathering inputs or in the extent of fractionation of Ca isotopes during calcareous sediment formation could entirely account for variations in the Ca isotopic composition of marine carbonates. Ca isotope fractionation during continental weathering, such as has been recently observed, could easily result in variations in δ44/42Caw of a few tenths of permil. Likewise a difference in the fractionation factors associated with aragonite versus calcite formation could drive shifts in Δsed of tenths of permil with shifts in the relative output of calcite and aragonite from the ocean. Until better constraints on variations in δ44/42Caw and Δsed have been established, modeling the Ca2+ content of seawater from Ca isotope curves should be approached cautiously.  相似文献   

14.
Molybdenum (Mo) isotope studies in black shales can provide information about the redox evolution of the Earth’s oceans, provided the isotopic consequences of Mo burial into its major sinks are well understood. Previous applications of the Mo isotope paleo-ocean redox proxy assumed quantitative scavenging of Mo when buried into sulfidic sediments. This paper contains the first complete suite of Mo isotope fractionation observations in a sulfidic water column and sediment system, the meromictic Lake Cadagno, Switzerland, a small alpine lake with a pronounced oxygen-sulfide transition reaching up to H2S ∼ 200 μM in the bottom waters (or about 300 μM total sulfide: ΣS2− = H2S + HS− + S2−). We find that Mo behaves conservatively in the oxic zone and non-conservatively in the sulfidic zone, where dissolved Mo concentrations decrease from 14 nM to 2-8 nM across this transition. Dissolved Mo in the upper oxic waters has a δ98Mooxic = 0.9 ± 0.1‰, which matches that of the riverine input, δ98Moriver = 0.9 ± 0.1‰. In the deeper sulfidic waters, a subaquatic source delivers Mo at 1.55 ± 0.1‰, but the dissolved Mo is even heavier at δ98Mosulfidic = 1.8‰. Sediment traps in the sulfidic zone of the lake collect particles increasingly enriched in Mo with depth, with δ98Mo values significantly fractionated at −0.8‰ to −1.2‰ both near the chemocline and in the deepest trap. Suspended particulates in the sulfidic waters carry lighter Mo than the ambient dissolved Mo pool by ∼0.3-1.5‰. Sedimentary Mo concentrations correlate with total organic carbon and yield Mo levels which are two orders of magnitude higher than typical crustal values found in rocks from the catchment area. Solid-phase Mo in the sediment shows a slightly positive δ98Mo trend with depth, from δ98Mo = 1.2‰ to 1.4‰ while the pore waters show dramatic enrichments of Mo (>2000 nM) with a relatively light isotope signature of δ98Mo = 0.9-1.0‰.These data are explained if Mo is converted to particle-reactive oxythiomolybdates in the sulfidic waters and is fractionated during removal from solution onto particles. Isotope fractionation is expressed in the water column, despite the high sulfide concentrations, because the rate of Mo removal is fast compared to the slow reaction kinetics of thiomolybdate formation. However, elemental and isotopic mass balances show that Mo is indeed quantitatively removed to the lake sediments and thus the isotopic composition of the sediments reflects sources to the sulfidic water. This efficient Mo drawdown is expected to occur in settings where H2S is very much in excess over Mo or in a restricted setting where the water renewal rate is slow compared to the Mo burial rate. We present a model for the Mo isotope fractionation in sulfidic systems associated with the slow reaction kinetics and conclude that quantitative removal will occur in highly sulfidic and restricted marine systems.  相似文献   

15.
Zinc isotopes have been studied along two smelter-impacted soil profiles sampled near one of the largest Pb and Zn processing plants in Europe located in northern France, about 50 km south of Lille. Mean δ66Zn values along these two soil profiles range from +0.22 ± 0.17‰ (2σ) to +0.34 ± 0.17‰ (2σ) at the lowest horizons and from +0.38 ± 0.45‰ (2σ) to +0.76 ± 0.14‰ (2σ) near the surface. The δ66Zn values in the lowest horizons of the soils are interpreted as being representative of the local geochemical background (mean value +0.31 ± 0.38‰), whereas heavier δ66Zn values near the surface of the two soils are related to anthropogenic Zn. This anthropogenic Zn occurs in the form of franklinite (ZnFe2O4)-bearing slag grains originating from processing wastes at the smelter site and exhibiting δ66Zn values of +0.81 ± 0.20‰ (2σ). The presence of franklinite is indicated by EXAFS analysis of the topsoil samples from both soil profiles as well as by micro-XANES analysis of the surface horizon of a third smelter-impacted soil from a distant site. These results indicate that naturally occurring Zn and smelter-derived Zn exhibit significantly different δ66Zn values, which suggests that zinc isotopes can be used to distinguish between geogenic and anthropogenic sources of Zn in smelter-impacted soils. In addition to a possible influence of additional past sources of light Zn (likely Zn-sulfides and Zn-sulfates directly emitted by the smelter), the light δ66Zn values in the surface horizons compared to smelter-derived slag materials are interpreted as resulting mainly from fractionation processes associated with biotic and/or abiotic pedological processes (Zn-bearing mineral precipitation, Zn complexation by organic matter, and plant uptake of Zn). This conclusion emphasizes the need for additional Zn isotopic studies before being able to use Zn isotopes to trace sources and pathways of this element in surface environments.  相似文献   

16.
Silicon isotopes in dissolved silicic acid were measured in the upper four kilometers between 4°N and 3°S latitude at 110°W longitude in the eastern Equatorial Pacific. Silicon isotopes became progressively heavier with silicic acid depletion of surface water as expected from biological fractionation. The value of ε estimated by applying a steady-state isotope fractionation model to data from all stations between 4°N and 3°S was −0.77 ± 0.12‰ (std. err.). When the analysis was restricted to those stations whose temperature and salinity profiles indicated that they were directly influenced by upwelling of the Equatorial Undercurrent (EUC), the resulting value of ε was −1.08 ± 0.27‰ (std. err.) similar to the value established in culture studies (−1.1‰). When the non steady state Rayleigh model was applied to the same restricted data set the resulting value of ε was significantly more positive, −0.61 ± 0.16‰ (std. err.). To the extent that the equatorial system approximates a steady state these results support a value of −1.1‰ for the fractionation factor for isotopes of Si in the sea. Without the assumption of steady state the value of ε can only be constrained to be between −0.6 and −1.1‰. Silicic acid in Equatorial Pacific Deep Water below 2000 m had a near constant δ30Si of +1.32 ± 0.05‰. That value is significantly more positive than obtained for North Pacific Deep Water at similar depths at stations to the northwest of our study area (0.9-1.0‰) and it is slightly less positive than new measures of the δ30Si of silicic acid from the silicic acid plume centered over the Cascadia basin in the Northeast Pacific (Si(OH)4 > 180  μM, δ30Si = +1.46 ± 0.12‰ (SD, n = 4). We show that the data from the equator and Cascadia basin fit a general trend of increasing δ30Si(OH)4 with increasing silicic acid concentration in the deep sea, but that the isotope values from the Northeast Pacific are anomalously light. The observed level of variation in the silicon isotope composition of deep waters from this single ocean basin is considerably larger than that predicted by current models based on fractionation during opal formation with no isotope effect during dissolution. Confirmation of such high variability in deep water δ30Si(OH)4 within individual ocean basins will require reassessment of the mechanisms controlling the distribution of isotopes of silicon in the sea.  相似文献   

17.
We examined oxygen three-isotope ratios of 48 extraterrestrial chromite (EC) grains extracted from mid-Ordovician sediments from two different locations in Sweden, and one location in south-central China. The ages of the sediments (∼470 Ma) coincide with the breakup event of the L chondrite parent asteroid. Elemental compositions of the chromite grains are generally consistent with their origin from L or LL chondrite parent bodies. The average Δ17O (‰-deviation from the terrestrial mass-fractionation line, measured in situ from 15 μm spots by secondary ion mass spectrometry; SIMS) of EC grains extracted from fossil meteorites from Thorsberg and Brunflo are 1.17 ± 0.09‰ (2σ) and 1.25 ± 0.16‰, respectively, and those of fossil micrometeorites from Thorsberg and Puxi River are 1.10 ± 0.09‰, and 1.11 ± 0.12‰, respectively. Within uncertainty these values are all the same and consistent with the L chondrite group average Δ17O = 1.07 ± 0.18‰, but also with the LL chondrite group average Δ17O = 1.26 ± 0.24‰ (Clayton et al., 1991). We conclude that the studied EC grains from correlated sediments from Sweden and China are related, and most likely originated in the same event, the L chondrite parent body breakup. We also analyzed chromites of modern H, L and LL chondrites and show that their Δ17O values coincide with averages of Δ17O of bulk analyses of H, L and LL chondrites. This study demonstrates that in situ oxygen isotope data measured by SIMS are accurate and precise if carefully standardized, and can be used to classify individual extraterrestrial chromite grains found in sediments.  相似文献   

18.
We investigate the Logatchev Hydrothermal Field at the Mid-Atlantic Ridge, 14°45′N to constrain the calcium isotope hydrothermal flux into the ocean. During the transformation of seawater to a hydrothermal solution, the Ca concentration of pristine seawater ([Ca]SW) increases from about 10 mM to about 32 mM in the hydrothermal fluid endmember ([Ca]HydEnd) and thereby adopts a δ44/40CaHydEnd of −0.95 ± 0.07‰ relative to seawater (SW) and a 87Sr/86Sr isotope ratio of 0.7034(4). We demonstrate that δ44/40CaHydEnd is higher than that of the bedrock at the Logatchev field. From mass balance calculations, we deduce a δ44/40Ca of −1.17 ± 0.04‰ (SW) for the host-rocks in the reaction zone and −1.45 ± 0.05‰ (SW) for the isotopic composition of the entire hydrothermal cell of the Logatchev field. The values are isotopically lighter than the currently assumed δ44/40Ca for Bulk Earth of −0.92 ± 0.18‰ (SW) [Skulan J., DePaolo D. J. and Owens T. L. (1997) Biological control of calcium isotopic abundances in the global calcium cycle. Geochim. Cosmochim. Acta61,(12) 2505-2510] and challenge previous assumptions of no Ca isotope fractionation between hydrothermal fluid and the oceanic crust [Zhu P. and Macdougall J. D. (1998) Calcium isotopes in the marine environment and the oceanic calcium cycle. Geochim. Cosmochim. Acta62,(10) 1691-1698; Schmitt A. -D., Chabeaux F. and Stille P. (2003) The calcium riverine and hydrothermal isotopic fluxes and the oceanic calcium mass balance. Earth Planet. Sci. Lett. 6731, 1-16]. Here we propose that Ca isotope fractionation along the fluid flow pathway of the Logatchev field occurs during the precipitation of anhydrite. Two anhydrite samples from the Logatchev Hydrothermal Field show an average fractionation of about Δ44/40Ca = −0.5‰ relative to their assumed parental solutions. Ca isotope ratios in aragonites from carbonate veins from ODP drill cores indicate aragonite precipitation directly from seawater at low temperatures with an average δ44/40Ca of −1.54 ± 0.08‰ (SW). The relatively large fractionation between the aragonite precipitates and seawater in combination with their frequent abundance in weathered mafic and ultramafic rocks suggest a reconsideration of the marine Ca isotope budget, in particular with regard to ocean crust alteration.  相似文献   

19.
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   

20.
This work is aimed at quantifying the main environmental factors controlling isotope fractionation of Cu during its adsorption from aqueous solutions onto common organic (bacteria, algae) and inorganic (oxy(hydr)oxide) surfaces. Adsorption of Cu on aerobic rhizospheric (Pseudomonas aureofaciens CNMN PsB-03) and phototrophic aquatic (Rhodobacter sp. f-7bl, Gloeocapsa sp. f-6gl) bacteria, uptake of Cu by marine (Skeletonema costatum) and freshwater (Navicula minima, Achnanthidium minutissimum and Melosira varians) diatoms, and Cu adsorption onto goethite (FeOOH) and gibbsite (AlOOH) were studied using a batch reaction as a function of pH, copper concentration in solution and time of exposure. Stable isotopes of copper in selected filtrates were measured using Neptune multicollector ICP-MS. Irreversible incorporation of Cu in cultured diatom cells at pH 7.5-8.0 did not produce any isotopic shift between the cell and solution (Δ65/63Cu(solid-solution)) within ±0.2‰. Accordingly, no systematic variation was observed during Cu adsorption on anoxygenic phototrophic bacteria (Rhodobacter sp.), cyanobacteria (Gloeocapsa sp.) or soil aerobic exopolysaccharide (EPS)-producing bacteria (P. aureofaciens) in circumneutral pH (4-6.5) and various exposure times (3 min to 48 h): Δ65Cu(solid-solution) = 0.0 ± 0.4‰. In contrast, when Cu was adsorbed at pH 1.8-3.5 on the cell surface of soil the bacterium P. aureofacienshaving abundant or poor EPS depending on medium composition, yielded a significant enrichment of the cell surface in the light isotope (Δ65Cu (solid-solution) = −1.2 ± 0.5‰). Inorganic reactions of Cu adsorption at pH 4-6 produced the opposite isotopic offset: enrichment of the oxy(hydr)oxide surface in the heavy isotope with Δ65Cu(solid-solution) equals 1.0 ± 0.25‰ and 0.78 ± 0.2‰ for gibbsite and goethite, respectively. The last result corroborates the recent works of Mathur et al. [Mathur R., Ruiz J., Titley S., Liermann L., Buss H. and Brantley S. (2005) Cu isotopic fractionation in the supergene environment with and without bacteria. Geochim. Cosmochim. Acta69, 5233-5246] and Balistrieri et al. [Balistrieri L. S., Borrok D. M., Wanty R. B. and Ridley W. I. (2008) Fractionation of Cu and Zn isotopes during adsorption onto amorhous Fe(III) oxyhydroxide: experimental mixing of acid rock drainage and ambient river water. Geochim. Cosmochim. Acta72, 311-328] who reported heavy Cu isotope enrichment onto amorphous ferric oxyhydroxide and on metal hydroxide precipitates on the external membranes of Fe-oxidizing bacteria, respectively.Although measured isotopic fractionation does not correlate with the relative thermodynamic stability of surface complexes, it can be related to their structures as found with available EXAFS data. Indeed, strong, bidentate, inner-sphere complexes presented by tetrahedrally coordinated Cu on metal oxide surfaces are likely to result in enrichment of the heavy isotope on the surface compared to aqueous solution. The outer-sphere, monodentate complex, which is likely to form between Cu2+ and surface phosphoryl groups of bacteria in acidic solutions, has a higher number of neighbors and longer bond distances compared to inner-sphere bidentate complexes with carboxyl groups formed on bacterial and diatom surfaces in circumneutral solutions. As a result, in acidic solution, light isotopes become more enriched on bacterial surfaces (as opposed to the surrounding aqueous medium) than they do in neutral solution.Overall, the results of the present study demonstrate important isotopic fractionation of copper in both organic and inorganic systems and provide a firm basis for using Cu isotopes for tracing metal transport in earth-surface aquatic systems. It follows that both adsorption on oxides in a wide range of pH values and adsorption on bacteria in acidic solutions are capable of producing a significant (up to 2.5-3‰ (±0.1-0.15‰)) isotopic offset. At the same time, Cu interaction with common soil and aquatic bacteria, as well as marine and freshwater diatoms, at 4 < pH < 8 yields an isotopic shift of only ±0.2-0.3‰, which is not related to Cu concentration in solution, surface loading, the duration of the experiment, or the type of aquatic microorganisms.  相似文献   

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