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1.
An Fe isotope study of ordinary chondrites   总被引：3，自引：0，他引：3  
The Fe isotope composition of ordinary chondrites and their constituent chondrules, metal and sulphide grains have been systematically investigated. Bulk chondrites fall within a restricted isotopic range of <0.2‰ δ56Fe, and chondrules define a larger range of >1‰ (−0.84‰ to 0.21‰ relative to the IRMM-14 Fe standard). Fe isotope compositions do not vary systematically with the very large differences in total Fe concentration, or oxidation state, of the H, L, and LL chondrite classes. Similarly, the Fe isotope compositions of chondrules do not appear to be determined by the H, L or LL classification of their host chondrite. This may support an origin of the three ordinary chondrite groups from variable accretion of identical Fe-bearing precursors.A close relationship between isotopic composition and redistribution of Fe during metamorphism on ordinary chondrite parent bodies was identified; the largest variations in chondrule compositions were found in chondrites of the lowest petrologic types. The clear link between element redistribution and isotopic composition has implications for many other non-traditional isotope systems (e.g. Mg, Si, Ca, Cr). Isotopic compositions of chondrules may also be determined by their melting history; porphyritic chondrules exhibit a wide range in isotope compositions whereas barred olivine and radial pyroxene chondrules are generally isotopically heavier than the ordinary chondrite mean. Very large chondrules preserve the greatest heterogeneity of Fe isotopes.The mean Fe isotope composition of bulk ordinary chondrites was found to be −0.06‰ (±0.12‰ 2 SD); this is isotopically lighter than the terrestrial mean composition and all other published non-chondritic meteorite suites e.g. lunar and Martian samples, eucrites, pallasites, and irons. Ordinary chondrites, though the most common meteorites found on Earth today, were not the sole building blocks of the terrestrial planets.  相似文献   

2.
Chondrules and chondrites provide unique insights into early solar system origin and history, and iron plays a critical role in defining the properties of these objects. In order to understand the processes that formed chondrules and chondrites, and introduced isotopic fractionation of iron isotopes, we measured stable iron isotope ratios 56Fe/54Fe and 57Fe/54Fe in metal grains separated from 18 ordinary chondrites, of classes H, L and LL, ranging from petrographic types 3-6 using multi-collector inductively coupled plasma mass spectrometry. The δ56Fe values range from −0.06 ± 0.01 to +0.30 ± 0.04‰ and δ57Fe values are −0.09 ± 0.02 to +0.55 ± 0.05‰ (relative to IRMM-014 iron isotope standard). Where comparisons are possible, these data are in good agreement with published data. We found no systematic difference between falls and finds, suggesting that terrestrial weathering effects are not important in controlling the isotopic fractionations in our samples. We did find a trend in the 56Fe/54Fe and 57Fe/54Fe isotopic ratios along the series H, L and LL, with LL being isotopically heavier than H chondrites by ∼0.3‰ suggesting that redox processes are fractionating the isotopes. The 56Fe/54Fe and 57Fe/54Fe ratios also increase with increasing petrologic type, which again could reflect redox changes during metamorphism and also a temperature dependant fractionation as meteorites cooled. Metal separated from chondrites is isotopically heavier by ∼0.31‰ in δ56Fe than chondrules from the same class, while bulk and matrix samples plot between chondrules and metal. Thus, as with so many chondrite properties, the bulk values appear to reflect the proportion of chondrules (more precisely the proportion of certain types of chondrule) to metal, whereas chondrule properties are largely determined by the redox conditions during chondrule formation. The chondrite assemblages we now observe were, therefore, formed as a closed system.  相似文献   

3.
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   

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.
Very precise silver (Ag) isotopic compositions have been determined for a number of terrestrial rocks, and high and low Pd/Ag meteorites by utilizing multicollector inductively coupled plasma mass spectrometry (MC-ICPMS). The meteorites include primitive chondrites, the Group IAB iron meteorites Canyon Diablo and Toluca, and the Group IIIAB iron meteorite Grant. Silver isotopic measurements are primarily of interest because 107Ag was produced by decay of the short-lived radionuclide 107Pd during the formation of the solar system and hence the Pd-Ag chronometer has set constraints on the timing of early planetesimal formation. A 2σ precision of ±0.05‰ can be obtained for analyses of standard solutions when Ag isotopic ratios are normalized to Pd, to correct for instrumental mass discrimination, and to bracketing standards. Caution must be exercised when making Ag isotopic measurements because isotopic artifacts can be generated in the laboratory and during mass spectrometry. The external reproducibility for geological samples based on replicate analyses of rocks is ±0.2‰ (2σ).All chondrites analyzed have similar Ag isotopic compositions that do not differ significantly (>0.3‰) from the ‘terrestrial’ value of the NIST SRM 978a Ag isotope standard. Hence, they show no evidence of excess 107Ag derived from 107Pd decay or, of stable Ag isotope fractionation associated with volatile element depletion within the accretion disk or from parent body metamorphism. The Group IAB iron meteorite samples analyzed show evidence of complex behavior and disturbance of Ag isotope systematics. Therefore, care must be taken when using this group of iron meteorites to obtain chronological information based on the Pd-Ag decay scheme.  相似文献   

6.
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   

7.
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   

8.
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   

9.
New precise Te isotope data acquired by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) are presented for selected extraterrestrial and terrestrial materials. Bulk samples of carbonaceous, ordinary and enstatite chondrites as well as the metal and sulfide phases of iron meteorites were analyzed to search for nucleosynthetic isotope anomalies and to find evidence of formerly live 126Sn, which decays to 126Te with a half-life of 234,500 yr. None of the meteorites show evidence of mass dependent Te isotope fractionations larger than 2‰ for δ126/128Te. Following internal normalization of the data to 125Te/128Te, the Te isotope ratios of all analyzed meteorites were found to be identical to a terrestrial standard, within uncertainties. This provides evidence that the regions of the solar disk that were sampled during accretion of the meteorite parent bodies were well mixed and homogeneous on a large scale, with respect to Te isotopes. The data acquired for bulk carbonaceous chondrites indicate that the initial 126Sn/118Sn ratio of the solar system was <4 × 10−5, but this is dependent on the assumption that no redistribution of Sn and Te occurred since the start of the solar system. Five Archean sedimentary sulfides that display both mass dependent and mass-independent isotope effects for S yield internally normalized Te isotope data, which indicate that mass-independent Te isotope effects are absent. The mass dependent fractionations in these samples are constrained to be less than ∼1‰ for δ126/128Te.  相似文献   

10.
We report a study of the oxygen isotope ratios of chondrules and their constituent mineral grains from the Mokoia, oxidized CV3 chondrite. Bulk oxygen isotope ratios of 23 individual chondrules were determined by laser ablation fluorination, and oxygen isotope ratios of individual grains, mostly olivine, were obtained in situ on polished mounts using secondary ion mass spectrometry (SIMS). Our results can be compared with data obtained previously for the oxidized CV3 chondrite, Allende. Bulk oxygen isotope ratios of Mokoia chondrules form an array on an oxygen three-isotope plot that is subparallel to, and slightly displaced from, the CCAM (carbonaceous chondrite anhydrous minerals) line. The best-fit line for all CV3 chondrite chondrules has a slope of 0.99, and is displaced significantly (by δ17O ∼ −2.5‰) from the Young and Russell slope-one line for unaltered calcium-aluminum-rich inclusion (CAI) minerals. Oxygen isotope ratios of many bulk CAIs also lie on the CV-chondrule line, which is the most relevant oxygen isotope array for most CV chondrite components. Bulk oxygen isotope ratios of most chondrules in Mokoia have δ18O values around 0‰, and olivine grains in these chondrules have similar oxygen isotope ratios to their bulk values. In general, it appears that chondrule mesostases have higher δ18O values than olivines in the same chondrules. Our bulk chondrule data spread to lower δ18O values than any ferromagnesian chondrules that have been measured previously. Two chondrules with the lowest bulk δ18O values (−7.5‰ and −11.7‰) contain olivine grains that display an extremely wide range of oxygen isotope ratios, down to δ17O, δ18O around -50‰ in one chondrule. In these chondrules, there are no apparent relict grains, and essentially no relationships between olivine compositions, which are homogeneous, and oxygen isotopic compositions of individual grains. Heterogeneity of oxygen isotope ratios within these chondrules may be the result of incorporation of relict grains from objects such as amoeboid olivine aggregates, followed by solid-state chemical diffusion without concomitant oxygen equilibration. Alternatively, oxygen isotope exchange between an 16O-rich precursor and an 16O-poor gas may have taken place during chondrule formation, and these chondrules may represent partially equilibrated systems in which isotopic heterogeneities became frozen into the crystallizing olivine grains. If this is the case, we can infer that the earliest nebular solids from which chondrules formed had δ17O and δ18O values around -50‰, similar to those observed in refractory inclusions.  相似文献   

11.
We report Ni isotopic data, for 58,60-62Ni, on (1) FeNi metal and sulfides in different groups of iron meteorites, (2) sulfides and a whole rock sample of the St. Séverin chondrite, and (3) chondrules from the Chainpur chondrite. We have developed improved, Multiple-Collector, Positive ion Thermal Ionization Mass Spectrometric (MC-PTIMS) techniques, with Ni+ ionization efficiency at 1‰, and chemical separation techniques for Ni which reduce mass interferences to the 1 ppm level, so that no mass interference corrections need be applied, except for 64Ni (from 64Zn, at the 0.1‰ level), for which we do not report results. We normalize the data to 62Ni/58Ni to correct for mass dependent isotope fractionation. No evidence was found for resolved radiogenic or general Ni isotope anomalies at the resolution levels of 0.2 and 0.5 εu (εu = 0.01%) for 60Ni/58Ni and 61Ni/58Ni, respectively. From the 56Fe/58Ni ratios and ε(60Ni/58Ni) values, we calculate upper limits for the initial value of (60Fe/56Fe)0 of (a) <2.7 × 10−7 for Chainpur chondrules, (b) <10−8 for the St. Séverin sulfide, and (c) <4 × 10−9 for sulfides from iron meteorites. We measured some of the same meteorites measured by other workers, who reported isotopic anomalies in Ni, using Multiple-Collector, Inductively-Coupled Mass Spectrometry. Our results do not support the previous reports of Ni isotopic anomalies in sulfide samples from Mundrabilla by Cook et al. [Cook D. L., Clayton R. N., Wadhwa M., Janney P. E., and Davis A. M. (2008). Nickel isotopic anomalies in troilite from iron meteorites. Geophy. Res. Lett. 35, L01203] and in sulfides from Toluca and Odessa by Quitté et al. [Quitté G., Meier M., Latkoczy C., Halliday A. N., and Gunther D., (2006). Nickel isotopes in iron meteorites-nucleosynthetic anomalies in sulfides with no effects in metals and no trace of 60Fe. Earth Planet. Sci. Lett. 242, 16-25]. Hence, we find no need for specialized physical-chemical planetary processes for the preservation of different Ni isotope compositions, between FeNi metal and sulfides in the same iron meteorites, as proposed by the above reports nor for complex astrophysical scenarios to provide the very peculiar Ni isotope anomalies reported by these workers for sulfides.  相似文献   

12.
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   

13.
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   

14.
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   

15.
Isotope fractionation of electroplated Fe was measured as a function of applied electrochemical potential. As plating voltage was varied from −0.9 V to 2.0 V, the isotopic signature of the electroplated iron became depleted in heavy Fe, with δ56Fe values (relative to IRMM-14) ranging from −0.18(±0.02) to −2.290(±0.006) ‰, and corresponding δ57Fe values of −0.247(±0.014) and −3.354(±0.019) ‰. This study demonstrates that there is a voltage-dependent isotope fractionation associated with the reduction of iron. We show that Marcus’s theory for the kinetics of electron transfer can be extended to include the isotope effects of electron transfer, and that the extended theory accounts for the voltage dependence of Fe isotope fractionation. The magnitude of the electrochemically-induced fractionation is similar to that of Fe reduction by certain bacteria, suggesting that similar electrochemical processes may be responsible for biogeochemical Fe isotope effects. Charge transfer is a fundamental physicochemical process involving Fe as well as other transition metals with multiple isotopes. Partitioning of isotopes among elements with varying redox states holds promise as a tool in a wide range of the Earth and environmental sciences, biology, and industry.  相似文献   

16.
In this paper, we applied a reliable technique for measuring Fe isotope variations in coastal seawater at nanomolar levels. Iron was directly pre-concentrated from acidified seawater samples onto a nitrilotriacetic acid chelating resin and further purified using anion-exchange resin. Sample recovery, determined using a standard addition method, was essentially quantitative. Iron was then determined using a high-resolution multicollector ICP-MS (Neptune) coupled to an ApexQ desolvation introduction system. The external precision for δ56Fe values was 0.11‰ (2s) when using total a Fe quantity between 25 and 100 ng. We initially applied this technique to measure the Fe isotope composition of dissolved Fe from several coastal environments in the north-eastern United States and we observed a range of δ56Fe values between -0.9‰ and 0.1‰ relative to the IRMM-14 reference material. Iron isotope compositions of several reference water materials for inter-laboratory comparisons were also reported. Our results suggest that iron in coastal seawater, derived from benthic diagenesis and/or groundwater has negative Fe isotopic signatures that are distinct from other iron sources such as atmospheric deposition and rivers.  相似文献   

17.
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   

18.
The oxygen three-isotope systematics of 36 chondrules from the Allende CV3 chondrite are reported using high precision secondary ion mass spectrometer (CAMECA IMS-1280). Twenty-six chondrules have shown internally homogenous Δ17O values among olivine, pyroxene, and spinel within a single chondrule. The average Δ17O values of 19 FeO-poor chondrules (13 porphyritic chondrules, 2 barred olivine chondrules, and 4 chondrule fragments) show a peak at −5.3 ± 0.6‰ (2SD). Another 5 porphyritic chondrules including both FeO-poor and FeO-rich ones show average Δ17O values between −3‰ and −2‰, and 2 other FeO-poor barred olivine chondrules show average Δ17O values of −3.6‰ and 0‰. These results are similar to those for Acfer 094 chondrules, showing bimodal Δ17O values at −5‰ and −2‰. Nine porphyritic chondrules contain olivine grains with heterogeneous Δ17O values as low as −18‰, indicating that they are relict olivine grains and some of them were derived from precursors related to refractory inclusions. However, most relict olivine grains show oxygen isotope ratios that overlap with those in homogeneous chondrules. The Δ17O values of four barred olivine chondrules range from −5‰ to 0‰, indicating that not all BO chondrules plot near the terrestrial fractionation line as suggested by previous bulk chondrule analyses. Based on these data, we suggest the presence of multiple oxygen isotope reservoirs in local dust-rich protoplanetary disk, from which the CV3 parent asteroid formed.A compilation of 225 olivine and low-Ca pyroxene isotopic data from 36 chondrules analyzed in the present study lie between carbonaceous chondrite anhydrous mineral (CCAM) and Young and Russell lines. These data define a correlation line of δ17O = (0.982 ± 0.019) × δ18O − (2.91 ± 0.10), which is similar to those defined by chondrules in CV3 chondrites and Acfer 094 in previous studies. Plagioclase analyses in two chondrules plot slightly below the CCAM line with Δ17O values of −2.6‰, which might be the result of oxygen isotope exchange between chondrule mesostasis and aqueous fluid in the CV parent body.  相似文献   

19.
Atmospheric heating alters the compositions and textures of micrometeorites. To understand the changes and to test a proposed relationship between a micrometeorite’s petrographic texture and its degree of heating, we made elemental and multiple isotope analyses of stony cosmic spherules (sCS) collected from the South Pole Water well. Specifically, we analyzed the elemental compositions of 94 sCS and the isotopic ratios of Fe, K and O, on 43, 12 and 8 of these sCS, respectively.Our results show that sCS classified as strongly heated generally have lower concentrations of volatile and moderately volatile elements. Of the 43 spherules analyzed for Fe isotopes, only 5 have δ57Fe >5‰. In contrast, enrichment of 41K is pervasive (δ41K >0 in all 12 spherules analyzed) and large (up to 183‰). The determination of K isotope abundances in sCS may therefore be useful in deciphering thermal histories. Three of the eight sCS analyzed for O isotopes are mass fractionated with δ18O >30‰. We attribute two of these three δ18O enrichments to evaporative losses of oxygen in the atmosphere and the third to the presence in the parent material of an exotic phase, perhaps a sulfate or a carbonate. The K isotope and O isotope data are broadly consistent with the proposed textural classification.Because most spherules were not heated enough to fractionate Al, Mg, or Si, we compared the measured Mg/Al and Si/Al ratios directly to those of conventional meteorites and their matrices. ∼30% of the sCS have compositions outside the range defined by the bulk and the matrix compositions of known meteorite groups but consistent with those of pyroxene- and olivine-rich materials and may be samples of chondrules. The other 70% have Mg/Al and Si/Al ratios similar to those of CI, CM, CO, and CV chondrites. Natural variability of the Mg/Al and Si/Al ratios precludes the assignment of an individual sCS to a particular class of C-chondrite.  相似文献   

20.
《Geochimica et cosmochimica acta》1999,63(11-12):1653-1660
We present the analytical methods that have been developed for the first high-precision Fe isotope analyses that clearly identify naturally-occurring, mass-dependent isotope fractionation. A double-spike approach is used, which allows rigorous correction of instrumental mass fractionation. Based on 21 analyses of an ultra pure Fe standard, the external precision (1-SD) for measuring the isotopic composition of Fe is ±0.14 ‰/mass; for demonstrated reproducibility on samples, this precision exceeds by at least an order of magnitude that of previous attempts to empirically control instrumentally-produced mass fractionation (Dixon et al., 1993). Using the double-spike method, 15 terrestrial igneous rocks that range in composition from peridotite to rhyolite, 5 high-Ti lunar basalts, 5 Fe-Mn nodules, and a banded iron formation have been analyzed for their iron isotopic composition. The terrestrial and lunar igneous rocks have the same isotopic compositions as the ultra pure Fe standard, providing a reference Fe isotope composition for the Earth and Moon. In contrast, Fe-Mn nodules and a sample of a banded iron formation have iron isotope compositions that vary over a relatively wide range, from δ56Fe = +0.9 to −1.2 ‰; this range is 15 times the analytical errors of our technique. These natural isotopic fractionations are interpreted to reflect biological (“vital”) effects, and illustrate the great potential Fe isotope studies have for studying modern and ancient biological processes.  相似文献   

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