首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 46 毫秒
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.
At the Earth’s surface, Fe(II) often oxidises and forms insoluble Fe(III)-(oxyhydr)oxides, whose particle size and structure depend on solution composition and temperature during formation and afterwards. Bacterial processes and exposure to reducing environments reduces them again, releasing dissolved iron to the groundwater. During such cycling, the Fe isotopes fractionate to an extent that is expected to depend on temperature. In this study, we report on the use of Fe-oxides as paleo-redox indicators, using their structure, morphology and Fe-composition as a clue for formation conditions. In samples taken from ∼120 m drill cores in granite from SE Sweden, X-ray amorphous, superparamagnetic, nanometre-sized Fe-oxides are confined to fractures of the upper ∼50 m, whereas well-crystalline Fe-oxides, with particle sizes typical for soils, occur down to ∼110 m. We also identified hematite with a particle size of 100 nm, similar to hematite of hydrothermal origin. The Fe isotope composition of the fine-grained Fe-oxides (−1‰ < δ56Fe < 1‰, IRMM-14 referenced) scatter significantly compared to the distribution previously observed for hydrothermal material (−0.26‰ < δ56Fe < 0.12‰) and they are dominantly heavier than Fe-bearing silicates from fractures (−0.56‰ < δ56Fe < −0.35‰). This is consistent with formation by low-temperature weathering, where Fe-silicates dissolve, Fe(II) oxidises and Fe(III)-oxides precipitate. The X-ray amorphous, nanometre-sized nature of near-surface Fe-oxides suggests recent formation. The deeper situated, well-crystalline Fe-oxides are more mature and we interpret that they record earlier oxidising events. They exist in fractures that are not significantly altered, indicating formation during periods of oxidation. Our results show that oxygenated water may reach depths of ∼110 m in fractured granite. The absence of natural, low-temperature Fe-oxides from deeper drill cores suggests that oxygenated waters do not readily penetrate beyond about 100 m and suggests that radioactive waste repositories located at a depth of ∼500 m should be well-protected from oxygenated waters.  相似文献   

3.
The feasibility of in situ stable Fe isotope ratio measurements using UV-femtosecond laser ablation connected to a multiple-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) has been investigated. Different types of matrices, independently determined by solution MC-ICP-MS after chromatographic separation of Fe, have been analysed by laser ablation using the isotopically certified iron reference material IRMM-014 as the bracketing standard. The samples have been pure iron metal (JM Puratronic), Fe-meteorites (North Chile, Glenormiston and Toluca), the meteorite phases kamacite and taenite in Toluca and Fe-sulphides. Furthermore, Fe isotope ratios from hydrothermal hematite, siderite and goethite from an old mining area in the Schwarzwald, Germany, and of magnetite from the metamorphic Biwabik iron formation have been determined. The results show that a precision of better than 0.1‰ (2 sigma) can be achieved with laser ablation and that all the results obtained agree with those determined by solution ICP to better than 0.1‰. This precision and accuracy is achievable in both raster and spot ablation mode. A matrix-matched bracketing standard is not required , and all these materials can be measured accurately against a metal standard. The hydrothermal minerals show significant Fe isotope zonations. In some samples the range of δ56Fe in a single aggregate encompasses the entire spectrum of ratios found by bulk solution analyses in multiple samples distributed over the whole mining district. For example, isotopic zonations found in secondary fibrous hematites show a continuous change in the δ56Fe values from −0.5‰ in the core to −1.8‰ in the rim. Primary hydrothermal siderite shows the reverse pattern with lighter values in the core than in the rim. While the siderite is thought to record primary fluid histories, the hematite pattern is interpreted as a reworked isotopic signature generated by oxic dissolution of primary zoned siderite and immediate close range re-precipitation of the oxidized Fe. Abrupt changes are documented for secondary goethite showing a distinct overgrowth that is 0.4‰ lighter than the core of the grain. If indeed Fe isotopes in secondary minerals from hydrothermal ore deposits record the initial isotopic signatures of their precursor minerals, and these in turn record hydrothermal fluid histories, then the tools are in place for a detailed reconstruction of the deposit‘s genesis. We expect similar observations from other Fe-rich deposits formed at intermediate and low-temperatures (e.g. banded iron formations). Laser ablation now provides us with the spatial resolution that adds a further dimension to our interpretation of stable Fe-isotope fractionation.  相似文献   

4.
Iron isotope compositions in marine pore fluids and sedimentary solid phases were measured at two sites along the California continental margin, where isotope compositions range from δ56Fe = −3.0‰ to +0.4‰. At one site near Monterey Canyon off central California, organic matter oxidation likely proceeds through a number of diagenetic pathways that include significant dissimilatory iron reduction (DIR) and bacterial sulfate reduction, whereas at our other site in the Santa Barbara basin DIR appears to be comparatively small, and production of sulfides (FeS and pyrite) was extensive. The largest range in Fe isotope compositions is observed for Fe(II)aq in porewaters, which generally have the lowest δ56Fe values (minimum: −3.0‰) near the sediment surface, and increase with burial depth. δ56Fe values for FeS inferred from HCl extractions vary between ∼−0.4‰ and +0.4‰, but pyrite is similar at both stations, where an average δ56Fe value of −0.8 ± 0.2‰ was measured. We interpret variations in dissolved Fe isotope compositions to be best explained by open-system behavior that involves extensive recycling of Feflux. This study is the first to examine Fe isotope variations in modern marine sediments, and the results show that Fe isotopes in the various reactive Fe pools undergo isotopic fractionation during early diagenesis. Importantly, processes dominated by sulfide formation produce high-δ56Fe values for porewaters, whereas the opposite occurs when Fe(III)-oxides are present and DIR is a major pathway of organic carbon respiration. Because shelf pore fluids may carry a negative δ56Fe signature it is possible that the Fe isotope composition of ocean water reflects a significant contribution of shelf-derived iron to the open ocean. Such a signature would be an important means for tracing iron sources to the ocean and water mass circulation.  相似文献   

5.
Recent studies have suggested that rivers may present an isotopically light Fe source to the oceans. Since the input of dissolved iron from river water is generally controlled by flocculation processes that occur during estuarine mixing, it is important to investigate potential fractionation of Fe-isotopes during this process. In this study, we investigate the influence of the flocculation of Fe-rich colloids on the iron isotope composition of pristine estuarine waters and suspended particles. The samples were collected along a salinity gradient from the fresh water to the ocean in the North River estuary (MA, USA). Estuarine samples were filtered at 0.22 μm and the iron isotope composition of the two fractions (dissolved and particles) were analyzed using high-resolution MC-ICP-MS after chemical purification. Dissolved iron results show positive δ56Fe values (with an average of 0.43 ± 0.04‰) relative to the IRMM-14 standard and do not display any relationships with salinity or with percentage of colloid flocculation. The iron isotopic composition of the particles suspended in fresh water is characterized by more negative δ56Fe values than for dissolved Fe and correlate with the percentage of Fe flocculation. Particulate δ56Fe values vary from −0.09‰ at no flocculation to ∼0.1‰ at the flocculation maximum, which reflect mixing effects between river-borne particles, lithogenic particles derived from coastal seawaters and newly precipitated colloids. Since the process of flocculation produces minimal Fe-isotope fractionation in the dissolved Fe pool, we suggest that the pristine iron isotope composition of fresh water is preserved during estuarine mixing and that the value of the global riverine source into the ocean can be identified from the fresh water values. However, this study also suggests that δ56Fe composition of rivers can also be characterized by more positive δ56Fe values (up to 0.3‰) relative to the crust than previously reported. In order to improve our current understanding of the oceanic iron isotope cycling, further work is now required to determine the processes controlling the fractionation of Fe-isotopes during continental run-off.  相似文献   

6.
Holocene sediments from the Gotland Deep basin in the Baltic Sea were investigated for their Fe isotopic composition in order to assess the impact of changes in redox conditions and a transition from freshwater to brackish water on the isotope signature of iron. The sediments display variations in δ56Fe (differences in the 56Fe/54Fe ratio relative to the IRMM-14 standard) from −0.27 ± 0.09‰ to +0.21 ± 0.08‰. Samples deposited in a mainly limnic environment with oxygenated bottom water have a mean δ56Fe of +0.08 ± 0.13‰, which is identical to the mean Fe isotopic composition of igneous rocks and oxic marine sediments. In contrast, sediments that formed in brackish water under periodically euxinic conditions display significantly lighter Fe isotope signatures with a mean δ56Fe of −0.14 ± 0.19‰. Negative correlations of the δ56Fe values with the Fe/Al ratio and S content of the samples suggest that the isotopically light Fe in the periodically euxinic samples is associated with reactive Fe enrichments and sulfides. This is supported by analyses of pyrite separates from this unit that have a mean Fe isotopic composition of −1.06 ± 0.20‰ for δ56Fe. The supply of additional Fe with a light Fe isotopic signature can be explained with the shelf to basin Fe shuttle model. According to the Fe shuttle model, oxides and benthic ferrous Fe that is derived from dissimilatory iron reduction from shelves is transported and accumulated in euxinic basins. The data furthermore suggest that the euxinic water has a negative dissolved δ56Fe value of about −1.4‰ to −0.9‰. If negative Fe isotopic signatures are characteristic for euxinic sediment formation, widespread euxinia in the past might have shifted the Fe isotopic composition of dissolved Fe in the ocean towards more positive δ56Fe values.  相似文献   

7.
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   

8.
The carbon, oxygen, and strontium isotope compositions of carbonate rocks from the upper Miocene Kudankulam Formation, southern India, were measured to understand palaeoenvironment and carbonate diagenesis of this formation. Both carbon and oxygen isotope ratios of various carbonate phases including whole rocks, ooids, molluscan mold-fill and sparry pore-fill calcite cements are depleted in 18O and 13C compared to those of contemporaneous seawater, indicating that the Kudankulam carbonates underwent extensive meteoric diagenesis. Based on δ13C and δ18O values for sparry calcite cements (pore-fill and molluscan mold-fill) formed in the meteoric diagenetic realm (δ13C from −7.8‰ to −6.0‰ and −9.0‰ to −7.0‰; δ18O from −9.2‰ to −6.5‰ and −9.4‰ to −2.6‰, respectively), it is interpreted that the diagenetic system was open and was proximal to the vadose water recharge zone. The negative δ18O values of various carbonate components (about −9.4‰ to −4.1‰ for whole rocks; about −8.4‰ to −2.6‰ for ooids) suggest that during the late Miocene the paleoclimate of the study area was humid, unlike today, probably due to the intense Indian monsoon system. The carbon isotope compositions (−7.9‰ to −3.6‰ for whole rocks; −4.9‰ to −1.5‰ for ooids) are consistent with the interpretation that the paleo-ecosystem comprised a significant proportion of C4 type plants, supporting a scenario of expansion of C4 plants during the late Miocene in the Indian subcontinent as far south as the southern tip of India. The 87Sr/86Sr ratios of the Kudankulam carbonates (0.70920 to 0.72130) are much greater than those of the contemporaneous or modern seawater (between 0.7089 and 0.7091) and show a general decrease up-sequence. Such high Sr isotope ratios indicate significant radiogenic 87Sr influx to the system from the Archean rocks exposed in the drainage area, implying that the deep-seated Archean rocks were already exposed in southern India by the late Miocene.  相似文献   

9.
Dissolved Fe concentrations in subterranean estuaries, like their river-seawater counterparts, are strongly controlled by non-conservative behavior during mixing of groundwater and seawater in coastal aquifers. Previous studies at a subterranean estuary of Waquoit Bay on Cape Cod, USA demonstrate extensive precipitation of groundwater-borne dissolved ferrous iron and subsequent accumulation of iron oxides onto subsurface sands. Waquoit Bay is thus an excellent natural laboratory to assess the mechanisms of Fe-isotope fractionation in redox-stratified environments and determine potential Fe-isotope signatures of groundwater sources to coastal seawater. Here, we report Fe isotope compositions of iron-coated sands and porewaters beneath the intertidal zone of Waquoit Bay. The distribution of pore water Fe shows two distinct sources of Fe: one residing in the upward rising plume of Fe-rich groundwater and the second in the salt-wedge zone of pore water. The groundwater source has high Fe(II) concentration consistent with anoxic conditions and yield δ56Fe values between 0.3 and −1.3‰. In contrast, sediment porewaters occurring in the mixing zone of the subterranean estuary have very low δ56Fe values down to −5‰. These low δ56Fe values reflect Fe-redox cycling and result from the preferential retention of heavy Fe-isotopes onto newly formed Fe-oxyhydroxides. Analysis of Fe-oxides precipitated onto subsurface sands in two cores from the subterranean estuary revealed strong δ56Fe and Fe concentration gradients over less than 2m, yielding an overall range of δ56Fe values between −2 and 1.5‰. The relationship between Fe concentration and δ56Fe of Fe-rich sands can be modeled by the progressive precipitation of Fe-oxides along fluid flow through the subterranean estuary. These results demonstrate that large-scale Fe isotope fractionation (up to 5‰) can occur in subterranean estuaries, which could lead to coastal seawater characterized by very low δ56Fe values relative to river values.  相似文献   

10.
Lithological, chemical, and stable isotope data are used to characterize lacustrine tufas dating back to pre-late Miocene and later unknown times, capping different surfaces of a Tertiary carbonate (Sinn el-Kedab) plateau in Dungul region in the currently hyperarid south-western Egypt. These deposits are composed mostly of calcium carbonate, some magnesium carbonate and clastic particles plus minor amounts of organic matter. They have a wide range of (Mg/Ca)molar ratios, from 0.03 to 0.3. The bulk-tufa carbonate has characteristic isotope compositions: (δ13Cmean = −2.49 ± 0.99‰; δ18Omean = −9.43 ± 1.40‰). The δ13C values are consistent with a small input from C4 vegetation or thinner soils in the recharge area of the tufa-depositing systems. The δ18O values are typical of fresh water carbonates. Covariation between δ13C and δ18O values probably is a reflection of climatic conditions such as aridity. The tufas studied are isotopically similar to the underlying diagenetic marine chalks, marls and limestones (δ13Cmean = −2.06 ± 0.84‰; δ18Omean = −10.06 ± 1.39‰). The similarity has been attributed to common meteoric water signatures. This raises large uncertainties in using tufas (Mg/Ca)molar, δ13C and δ18O records as proxies of paleoclimatic change and suggests that intrinsic compositional differences in material sources within the plateau may mask climatic changes in the records.  相似文献   

11.
Although iron isotopes provide a new powerful tool for tracing a variety of geochemical processes, the unambiguous interpretation of iron isotope ratios in natural systems and the development of predictive theoretical models require accurate data on equilibrium isotope fractionation between fluids and minerals. We investigated Fe isotope fractionation between hematite (Fe2O3) and aqueous acidic NaCl fluids via hematite dissolution and precipitation experiments at temperatures from 200 to 450 °C and pressures from saturated vapor pressure (Psat) to 600 bar. Precipitation experiments at 200 °C and Psat from aqueous solution, in which Fe aqueous speciation is dominated by ferric iron (FeIII) chloride complexes, show no detectable Fe isotope fractionation between hematite and fluid, Δ57Fefluid-hematite = δ57Fefluid − δ57Fehematite = 0.01 ± 0.08‰ (2 × standard error, 2SE). In contrast, experiments at 300 °C and Psat, where ferrous iron chloride species (FeCl2 and FeCl+) dominate in the fluid, yield significant fluid enrichment in the light isotope, with identical values of Δ57Fefluid-hematite = −0.54 ± 0.15‰ (2SE) both for dissolution and precipitation runs. Hematite dissolution experiments at 450 °C and 600 bar, in which Fe speciation is also dominated by ferrous chloride species, yield Δ57Fefluid-hematite values close to zero within errors, 0.15 ± 0.17‰ (2SE). In most experiments, chemical, redox, and isotopic equilibrium was attained, as shown by constancy over time of total dissolved Fe concentrations, aqueous FeII and FeIII fractions, and Fe isotope ratios in solution, and identical Δ57Fe values from dissolution and precipitation runs. Our measured equilibrium Δ57Fefluid-hematite values at different temperatures, fluid compositions and iron redox state are within the range of fractionations in the system fluid-hematite estimated using reported theoretical β-factors for hematite and aqueous Fe species and the distribution of Fe aqueous complexes in solution. These theoretical predictions are however affected by large discrepancies among different studies, typically ±1‰ for the Δ57Fe Fe(aq)-hematite value at 200 °C. Our data may thus help to refine theoretical models for β-factors of aqueous iron species. This study provides the first experimental calibration of Fe isotope fractionation in the system hematite-saline aqueous fluid at elevated temperatures; it demonstrates the importance of redox control on Fe isotope fractionation at hydrothermal conditions.  相似文献   

12.
The continental shelf benthic iron flux and its isotope composition   总被引:1,自引:0,他引:1  
Benthic iron fluxes from sites along the Oregon-California continental shelf determined using in situ benthic chambers, range from less than 10 μmol m−2 d−1 to values in excess of ∼300 μmol m−2 d−1. These fluxes are generally greater than previously published iron fluxes for continental shelves contiguous with the open ocean (as opposed to marginal seas, bays, or estuaries) with the highest fluxes measured in the regions around the high-sediment discharge Eel River and the Umpqua River. These benthic iron fluxes do not covary with organic carbon oxidation rates in any systematic fashion, but rather seem to respond to variations in bottom water oxygen and benthic oxygen demand. We hypothesize that the highest rates of benthic iron efflux are driven, in part, by the greater availability of reactive iron deposited along these river systems as compared to other more typical continental margin settings. Bioirrigation likely plays an important role in the benthic Fe flux in these systems as well. However, the influence of bottom water oxygen concentrations on the iron flux is significant, and there appears to be a threshold in dissolved oxygen (∼60-80 μM), below which sediment-ocean iron exchange is enhanced. The isotope composition of this shelf-derived benthic iron is enriched in the lighter isotopes, and appears to change by ∼3‰ (δ56Fe) during the course of a benthic chamber experiment with a mean isotope composition of −2.7 ± 1.1‰ (2 SD, n = 9) by the end of the experiment. This average value is slightly heavier than those from two high benthic Fe flux restricted basins from the California Borderland region where δ56Fe is −3.4 ± 0.4‰ (2 SD, n = 3). These light iron isotope compositions support previous ideas, based on sediment porewater analyses, suggesting that sedimentary iron reduction fractionates iron isotopes and produces an isotopically light iron pool that is transferred to the ocean water column. In sum, our data suggest that continental shelves may export a higher efflux of iron than previously hypothesized, with the likelihood that along river-dominated margins, the benthic iron flux could well be orders of magnitude larger than non-river dominated shelves. The close proximity of the continental shelf benthos to the productive surface ocean means that this flux is likely to be essential for maintaining ecosystem micronutrient supply.  相似文献   

13.
The stable copper isotope composition of 79 samples of primary and secondary copper minerals from hydrothermal veins in the Schwarzwald mining district, South Germany, shows a wide variation in δ65Cu ranging from −2.92 to 2.41‰. We investigated primary chalcopyrite, various kinds of fahlores and emplectite, as well as supergene native copper, malachite, azurite, cuprite, tenorite, olivenite, pseudomalachite and chrysocolla. Fresh primary Cu(I) ores have at most localities copper isotope ratios (δ65Cu values) of 0 ± 0.5‰ despite the fact that the samples come from mineralogically different types of deposits covering an area of about 100 by 50 km and that they formed during three different mineralization events spanning the last 300 Ma. Relics of the primary ores in oxidized samples (i.e., chalcopyrite relics in an iron oxide matrix with an outer malachite coating) display low isotope ratios down to −2.92‰. Secondary Cu(I) minerals such as cuprite have high δ65Cu values between 0.4 and 1.65‰, whereas secondary Cu(II) minerals such as malachite show a range of values between −1.55 and 2.41‰, but typically have values above +0.5‰. Within single samples, supergene oxidation of fresh chalcopyrite with a δ value of 0‰ causes significant fractionation on the scale of a centimetre between malachite (up to 1.49‰) and relict chalcopyrite (down to −2.92‰). The results show that—with only two notable exceptions—high-temperature hydrothermal processes did not lead to significant and correlatable variations in copper isotope ratios within a large mining district mineralized over a long period of time. Conversely, low-temperature redox processes seriously affect the copper isotope compositions of hydrothermal copper ores. While details of the redox processes are not yet understood, we interpret the range in compositions found in both primary Cu(I) and secondary Cu(II) minerals as a result of two competing controls on the isotope fractionation process: within-fluid control, i.e., the fractionation during the redox process among dissolved species, and fluid-solid control, i.e., fractionation during precipitation involving reactions between dissolved Cu species and minerals. Additionally, Rayleigh fractionation in a closed system may be responsible for some of the spread in isotope compositions. Our study indicates that copper isotope variations may be used to decipher details of natural redox processes and therefore may have some bearing on exploration, evaluation and exploitation of copper deposits. On the other hand, copper isotope analyses of single archeological artefacts or geological or biological objects cannot be easily used as reliable fingerprint for the source of copper, because the variation caused by redox processes within a single deposit is usually much larger than the inter-deposit variation.  相似文献   

14.
Although commonly utilized in continental geothermal work, the water-hydrogen and methane-hydrogen isotope geothermometers have been neglected in hydrothermal studies. Here we report δD-CH4 and δD-H2 values from high-temperature, black smoker-type hydrothermal vents and low-temperature carbonate-hosted samples from the recently discovered Lost City Hydrothermal Field. Methane deuterium content is uniform across the dataset at − 120 ± 12‰. Hydrogen δD values vary from − 420‰ to − 330‰ at high-temperature vents to − 700‰ to − 600‰ at Lost City. The application of several geothermometer equations to a suite of hydrothermal vent volatile samples reveals that predicted temperatures are similar to measured vent temperatures at high-temperature vents, and 20-60 °C higher than those measured at the Lost City vents. We conclude that the overestimation of temperature at Lost City reflects 1) that methane and hydrogen are produced by serpentinization at > 110 °C, and 2) that isotopic equilibrium at temperatures < 70 °C is mediated by microbial sulfate reduction. The successful application of hydrogen isotope geothermometers to low-temperature Lost City hydrothermal samples encourages its employment with low-temperature diffuse hydrothermal fluids.  相似文献   

15.
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   

16.
We present a double-spike isotope dilution MC-ICP-MS technique for the determination of germanium (Ge) isotope fractionation. Using this technique we determined Ge isotope compositions of geothermal spring fluids, a Columbia River Basalt sample, and an in-house diatom standard. Our technique uses a 73Ge/70Ge double spike in combination with hydride generation for Ge extraction from the sample matrix. Fractionation is determined on the 74Ge/72Ge mass ratio. The double spike allows us to effectively correct analytical isotope fractionation. Our external standard reproducibility is 0.4‰ (2 SD) over the course of several months. The minimum quantity of Ge needed for isotope analysis is approximately 2 ng. Consistent with previous work on geothermal fluids, Ge in the geothermal spring samples presented here is enriched over Si as compared to low temperature weathering signatures. This observation is typically interpreted as Ge exclusion during silicate mineral precipitation (e.g., quartz). Our isotope results indicate that the analyzed high temperature fluids fractionate Ge isotopes with a range in δ74Ge between −0.4‰ and −1.4‰ relative to a Columbia River basalt. We cautiously interpret the observed fractionation as preferential removal of heavy Ge isotopes out of solution during cooling of the hydrothermal fluid and subsequent precipitation of quartz.  相似文献   

17.
The application of stable Fe isotopes as a tracer of the biogeochemical Fe cycle necessitates a mechanistic knowledge of natural fractionation processes. We studied the equilibrium Fe isotope fractionation upon sorption of Fe(II) to aluminum oxide (γ-Al2O3), goethite (α-FeOOH), quartz (α-SiO2), and goethite-loaded quartz in batch experiments, and performed continuous-flow column experiments to study the extent of equilibrium and kinetic Fe isotope fractionation during reactive transport of Fe(II) through pure and goethite-loaded quartz sand. In addition, batch and column experiments were used to quantify the coupled electron transfer-atom exchange between dissolved Fe(II) (Fe(II)aq) and structural Fe(III) of goethite. All experiments were conducted under strictly anoxic conditions at pH 7.2 in 20 mM MOPS (3-(N-morpholino)-propanesulfonic acid) buffer and 23 °C. Iron isotope ratios were measured by high-resolution MC-ICP-MS. Isotope data were analyzed with isotope fractionation models. In batch systems, we observed significant Fe isotope fractionation upon equilibrium sorption of Fe(II) to all sorbents tested, except for aluminum oxide. The equilibrium enrichment factor, , of the Fe(II)sorb-Fe(II)aq couple was 0.85 ± 0.10‰ (±2σ) for quartz and 0.85 ± 0.08‰ (±2σ) for goethite-loaded quartz. In the goethite system, the sorption-induced isotope fractionation was superimposed by atom exchange, leading to a δ56/54Fe shift in solution towards the isotopic composition of the goethite. Without consideration of atom exchange, the equilibrium enrichment factor was 2.01 ± 0.08‰ (±2σ), but decreased to 0.73 ± 0.24‰ (±2σ) when atom exchange was taken into account. The amount of structural Fe in goethite that equilibrated isotopically with Fe(II)aq via atom exchange was equivalent to one atomic Fe layer of the mineral surface (∼3% of goethite-Fe). Column experiments showed significant Fe isotope fractionation with δ56/54Fe(II)aq spanning a range of 1.00‰ and 1.65‰ for pure and goethite-loaded quartz, respectively. Reactive transport of Fe(II) under non-steady state conditions led to complex, non-monotonous Fe isotope trends that could be explained by a combination of kinetic and equilibrium isotope enrichment factors. Our results demonstrate that in abiotic anoxic systems with near-neutral pH, sorption of Fe(II) to mineral surfaces, even to supposedly non-reactive minerals such as quartz, induces significant Fe isotope fractionation. Therefore we expect Fe isotope signatures in natural systems with changing concentration gradients of Fe(II)aq to be affected by sorption.  相似文献   

18.
Iron isotopic compositions measured in chondrules from various chondrites vary between δ57Fe/54Fe = +0.9‰ and −2.0‰, a larger range than for igneous rocks. Whether these compositions were inherited from chondrule precursors, resulted from the chondrule-forming process itself or were produced by later parent body alteration is as yet unclear. Since iron metal is a common phase in some chondrules, it is important to explore a possible link between the metal formation process and the observed iron isotope mass fractionation. In this experimental study we have heated a fayalite-rich composition under reducing conditions for heating times ranging from 2 min to 6 h. We performed chemical and iron isotope analyses of the product phases, iron metal and silicate glass. We demonstrated a lack of evaporation of Fe from the silicate melt in similar isothermal experiments performed under non-reducing conditions. Therefore, the measured isotopic mass fractionation in the glass, ranging between −0.32‰ and +3.0‰, is attributed to the reduction process. It is explained by the faster transport of lighter iron isotopes to the surface where reduction occurs, and is analogous to kinetic isotope fractionation observed in diffusion couples [Richter, F.M., Davis, A.M., Depaolo, D.J., Watson, E.B., 2003. Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochim. Cosmochim. Acta67, 3905-3923]. The metal phase contains 90-99.8% of the Fe in the system and lacks significant isotopic mass fractionation, with values remaining similar to that of the starting material throughout. The maximum iron isotope mass fractionation in the glass was achieved within 1 h and was followed by an isotopic exchange and re-equilibration with the metal phase (incomplete at ∼6 h). This study demonstrates that reduction of silicates at high temperatures can trigger iron isotopic fractionation comparable in its bulk range to that observed in chondrules. Furthermore, if metal in Type I chondrules was formed by reduction of Fe silicate, our observed isotopic fractionations constrain chondrule formation times to approximately 60 min, consistent with previous work.  相似文献   

19.
Transport of iron (Fe) within hydrothermal and soil environments involves the transferral into aqueous solutions by leaching of complex, polyminerallic rocks. Understanding the isotope fractionation mechanisms during this process is key for any application of the Fe-isotope system to biogeochemical studies. Here, we reacted biotite granite and tholeiite-basalt with 0.5 M hydrochloric acid and 5 mM oxalic acid solutions at ambient temperature. Solution aliquots were recovered over a seven-day period and analysed for major and trace element concentrations and Fe isotopic compositions. In all experiments, Fe initially released into solution was isotopically lighter, with Δ56Fesolution-rock as low as −1.80‰ in the granite-hydrochloric acid system. The oxalic acid experiments showed similar patterns but smaller fractionation. In all experiments, the Δ56Fesolution-rock reduced over time, which would be in line with the formation of a leached layer as proposed before [Brantley S. L., Liermann L. J., Guynn R. L., Anbar A., Icopini G. A., and Barling J. (2004) Fe isotopic fractionation during mineral dissolution with and without bacteria. Geochim. Cosmochim. Acta68(15), 3189-3204]. Granite and basalts reacting with hydrochloric acid reached apparent steady-state values of −0.60 ± 0.15‰ and −0.40 ± 0.20‰, respectively, whilst experimental values with oxalic acid were −1.0 ± 0.15‰ and −0.50 ± 0.15‰. During the granite experiments, alteration of biotite to chlorite, followed by dissolution of chlorite, were likely the dominant processes, whilst in the basalt experiments, dissolution of pigeonite was likely the principal source of Fe. Variations in pH during the hydrochloric acid experiments were minimal, remaining below 0.5 at all times. In oxalic acid solutions, the pH increased to over 4, leading likely to precipitation of secondary minerals and adsorption/co-precipitation of Fe onto mineral surfaces. These processes could contribute to the greater fractionation observed in the final stages of the oxalic acid experiments. Our results highlight the importance of mineralogy and fluid composition on the Fe-isotope systematics during weathering. The fractionation processes identified for granites and basalts are in line with those inferred from field observations in soils, sediments, groundwater and hydrothermal deposits and from laboratory studies of single-mineral leaching.  相似文献   

20.
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   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号