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1.
High-precision Zn isotopic variations are reported for carbonaceous chondrites (CC), equilibrated (EOC) and unequilibrated (UOC) ordinary chondrites, iron meteorites from the IAB-IIICD (nonmagmatic) and IIIA (magmatic) groups, and metal from the Brenham pallasite. For irons, δ65Cu values are also reported. Data have also been obtained on a coarse-grained type-B calcium-, aluminum-rich refractory inclusion (CAI) from Allende and on acid leaches of Allende (CV3), Krymka (LL3), and Charsonville (H6). Variations expressed as δ66Zn (deviation in parts per thousand of 66Zn/64Zn in samples relative to a standard) spread over a range of 0.3‰ for carbonaceous chondrites, 2‰ for ordinary chondrites, and 4‰ for irons.The measured 66Zn/64Zn, 67Zn/64Zn, and 68Zn/64Zn ratios vary linearly with mass difference and define a common isotope fractionation line with terrestrial samples, which demonstrates that Zn was derived from an initially single homogeneous reservoir. The δ66Zn values are correlated with meteorite compositions and slightly decrease in the order CI, CM, CV-CO, and to UOC. The isotopically light Zn of Allende CAI and the acid-resistant residues of Allende and Krymka show that the light component is associated with refractory material, presumably minerals from the spinel-group. This, together with the reverse correlation between relative abundances of light Zn isotopes and volatile element abundances, suggests that Zn depletion in planetary bodies with respect to CI cannot be ascribed to devolatilization of CI-like material. These observations rather suggest that refractory material reacted with a gas phase enriched in the lighter Zn isotopes. Alternatively, chondrules with their associated rims should carry a light Zn isotopic signature. The δ66Zn values of unequilibrated chondrites are rather uniform, whereas equilibrated chondrites show distinctly more isotopic variability.The values of δ65Cu-δ66Zn in irons define two trends. The moderate and positively correlated Cu and Zn isotope variations in IIIA and pallasite samples probably reflect crystallization of silicate, sulfide, and solid metal from the liquid metal. The range of δ66Zn values of the IAB-IIICD group is large (>3‰) and contrasts with the moderate fractionation of Cu isotopes. We interpret this feature and the negative δ66Zn-δ65Cu correlation as reflecting mixing, possibly achieved by percolation, between metals from a regolith devolatilized at low temperature (enriched in heavy zinc) and metallic liquids formed within the parent body.  相似文献   

2.
The first cold plasma ICP-MS (inductively coupled plasma mass spectrometer) Fe isotope study is described. Application of this technique to the analyses of Fe isotopes in a number of meteorites is also reported. The measurement technique relies on reduced temperature operation of the ICP source to eliminate pervasive molecular interferences from Ar complexes associated with conventional ICP-MS. Instrumental mass bias corrections are performed by sample-standard bracketing and using Cu as an external mass bias drift monitor. Repeated measurements of a terrestrial basalt reference sample indicate an external reproducibility of ± 0.06 ‰ for δ56Fe and ± 0.25 ‰ for δ58Fe (1 σ). The measured iron isotopic compositions of various bulk meteorites, including irons, chondrites and pallasites are identical, within error, to the composition of our terrestrial basalt reference sample suggesting that iron mass fractionation during planet formation and differentiation was non-existent. Iron isotope compositions measured for eight chondrules from the unequilibrated ordinary chondrite Tieschitz range from −0.5 ‰ < δ56Fechondrules < 0.0 ‰ relative to the terrestrial/meteorite average. Mechanisms for fractionating iron in these chondrules are discussed.  相似文献   

3.
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   

4.
We report on the abundances of Ru isotopes in (1) iron meteorites, (2) stony-iron meteorites (pallasites), (3) ordinary and carbonaceous chondrites, and (4) in refractory inclusions from the carbonaceous meteorite Allende. We have developed improved Multiple-Collector, Negative-ion Thermal Ionization Mass Spectrometric (MC-NTIMS) techniques for Ru, with high ionization efficiency of 4% and with chemical separation techniques for Ru, which reduce mass interferences to the ppm level, so that no mass interference corrections needed to be applied. Our data were normalized to 99Ru/101Ru to correct for mass-dependent fractionation. We find no Ru isotopic effects in the ordinary chondrites and group IAB iron meteorites we have measured. There are significant effects (deficits) in the pure s-process nuclide 100Ru, in the Allende whole-rock and in refractory inclusions of up to 1.7 parts in 10,000 (εu). There are also endemic deficits in 100Ru in iron meteorites and in pallasites of up to 1.1 εu. The Ru data suggest a wide spread and large scale heterogeneity in p-, s-, and r-process components resulting in a deficit in s-process nuclides or enhancements in both p- and r-process nuclides, in refractory siderophiles condensing in the early solar nebula. In contrast, the data on bulk Murchison suggest an excess in 100Ru and in 104Ru, which are distinct from the rest of the measured patterns. Our results establish the presence of significant isotopic heterogeneity for Ru in the early solar nebula. The observation of endemic Ru effects in planetary differentiates, such as iron meteorites and pallasites, must reflect the siderophile nature of Ru and the preservation in condensing FeNi metal of refractory metal condensate grains formed in the early solar nebula. Once incorporated in the metal phase, the refractory siderophiles remained in the metal phase through the melting and differentiation of planetesimals to form FeNi cores and silicate mantles and crusts.  相似文献   

5.
Recent developments in multiple-collector magnetic-sector ICP-MS (inductively coupled plasma-mass spectrometry) have permitted the relative abundances of the two isotopes 63 and 65 of copper to be measured with unprecedented precision (40 ppm). Here, we report Cu isotopic variations among eight carbonaceous chondrites (CCs) from the CI, CM, CO, and CV groups and the presently ungrouped Tagish Lake, and 10 ordinary chondrites (OCs) from the H, L, and LL groups. The widest isotopic range of ∼0.8‰ per a.m.u. is observed for the carbonaceous chondrites. Copper in carbonaceous chondrites becomes isotopically lighter with petrologic type in the order 1 to 3 but seems extremely homogeneous for each type. The Cu isotopic composition of Tagish Lake confirms its other characteristics that are intermediate between CI and CM. In three of the groups (CI-CM-CO), as well as for Tagish Lake, 63Cu excess over terrestrial mantle abundances correlates well with 16O excess. For all four groups, 63Cu excess also correlates remarkably well with elemental refractory/volatile ratios (e.g., Ca/Mn). For ordinary chondrites, small differences exist between the H, L, and LL groups, with Cu becoming isotopically heavier in that order. Equilibrated and unequilibrated samples, however, exhibit the same Cu isotopic signature within each group. Although the range of Cu isotopic compositions in ordinary chondrites is smaller than in carbonaceous chondrites, 63Cu excesses still correlate with 16O excesses. The observed trends of isotopic variation seem incompatible with a single-stage fractionation process by either volatilization or low-temperature metamorphism. The correlations between 63Cu excesses and 16O excesses suggest the presence of at least two and perhaps three isotopically distinct Cu reservoirs in the early Solar System: (1) an Earth-like reservoir common to the CI and LL probably representing the main Cu stock of the inner Solar System, (2) a reservoir present in all carbonaceous chondrites, but most abundant in CV, with large 63Cu and 16O excesses (this reservoir is probably hosted in refractory material), and (3) possibly a third reservoir present in ordinary chondrites. The OC trend may also be explained as a mixture of the first two Cu reservoirs if its oxygen was first equilibrated with nebular gas. The coexistence of 63Cu and 16O excesses in the same component raises the issue of how volatile Cu was preserved in refractory material. A strong correlation between 63Cu/65Cu and Ni/Cu ratios suggests that 63Cu excess may have originated as more refractory 63Ni (T1/2 = 100 yr) upon irradiation of refractory grains by electromagnetic flares and particle bursts during the T-Tauri phase of the Sun.  相似文献   

6.
Enstatite meteorites include the undifferentiated enstatite chondrites and the differentiated enstatite achondrites (aubrites). They are the most reduced group of all meteorites. The oxygen isotope compositions of both enstatite chondrites and aubrites plot along the terrestrial mass fractionation line, which suggests some genetic links between these meteorites and the Earth as well.For this study, we measured the Zn isotopic composition of 25 samples from the following groups: aubrites (main group and Shallowater), EL chondrites, EH chondrites and Happy Canyon (impact-melt breccia). We also analyzed the Zn isotopic composition and elemental abundance in separated phases (metal, silicates, and sulfides) of the EH4, EL3, and EL6 chondrites. The different groups of meteorites are isotopically distinct and give the following values (‰): aubrite main group (−7.08 < δ66Zn < −0.37); EH3 chondrites (0.15 < δ66Zn < 0.31); EH4 chondrites (0.15 < δ66Zn < 0.27); EH5 chondrites (δ66Zn = 0.27 ± 0.09; n = 1); EL3 chondrites (0.01 < δ66Zn < 0.63); the Shallowater aubrite (1.48 < δ66Zn < 2.36); EL6 chondrites (2.26 < δ66Zn < 7.35); and the impact-melt enstatite chondrite Happy Canyon (δ66Zn = 0.37).The aubrite Peña Blanca Spring (δ66Zn = −7.04‰) and the EL6 North West Forrest (δ66Zn = 7.35‰) are the isotopically lightest and heaviest samples, respectively, known so far in the Solar System. In comparison, the range of Zn isotopic composition of chondrites and terrestrial samples (−1.5 < δ66Zn < 1‰) is much smaller ( [Luck et al., 2005] and [Herzog et al., 2009]).EH and EL3 chondrites have the same Zn isotopic composition as the Earth, which is another example of the isotopic similarity between Earth and enstatite chondrites. The Zn isotopic composition and abundance strongly support that the origin of the volatile element depletion between EL3 and EL6 chondrites is due to volatilization, probably during thermal metamorphism. Aubrites show strong elemental depletion in Zn compared to both EH and EL chondrites and they are enriched in light isotopes (δ66Zn down to −7.04‰). This is the opposite of what would be expected if Zn elemental depletion was due to evaporation, assuming the aubrites started with an enstatite chondrite-like Zn isotopic composition. Evaporation is therefore not responsible for volatile loss from aubrites. On Earth, Zn isotopes fractionate very little during igneous processes, while differentiated meteorites show only minimal Zn isotopic variability. It is therefore very unlikely that igneous processes can account for the large isotopic fractionation of Zn in aubrites. Condensation of an isotopically light vapor best explains Zn depletion and isotopically light Zn in these puzzling rocks. Mass balance suggests that this isotopically light vapor carries Zn lost by the EL6 parent body during thermal metamorphism and that aubrites evolved from an EL6-like parent body. Finally, Zn isotopes suggest that Shallowater and aubrites originate from distinct parent bodies.  相似文献   

7.
The extinct radionuclide 107Pd decays to 107Ag (half-life of 6.5 Ma) and is an early solar system chronometer with outstanding potential to study volatile depletion in the early solar system. Here, a comprehensive Ag isotope study of carbonaceous and ordinary chondrites is presented. Carbonaceous chondrites show limited variations (ε107Ag = −2.1 to +0.8) in Ag isotopic composition that correlate with the Pd/Ag ratios. Assuming a strictly radiogenic origin of these variations, a new initial 107Pd/108Pd of 5.9 (±2.2) × 10−5 for the solar system can be deduced. Comparing the Pd-Ag and Mn-Cr data for carbonaceous chondrites suggests that Mn-Cr and Pd-Ag fractionation took place close to the time of calcium-aluminium-rich inclusion (CAI) and chondrule formation ∼4568 Ma ago. Using the new value for the initial 107Pd abundance, the revised ages for the iron-rich meteorites Gibeon (IVA, 8.5 +3.2/−4.6 Ma), Grant (IIIAB, 13.0 +3.5/−4.9 Ma) and Canyon Diablo (IA, 19.5 +24.1/−10.4 Ma) are consistent with cooling rates and the closure temperature of the Pd-Ag system. In contrast to carbonaceous chondrites, ordinary chondrites show large stable isotope fractionation of order of 1 permil for 107Ag/109Ag. This indicates that different mechanisms of volatile depletion were active in carbonaceous and ordinary chondrites. Nebular processes and accretion, as experienced by carbonaceous chondrites, did not led to significant Ag isotope fractionation, while the significant Ag isotope variations in ordinary chondrites are most likely inflicted by open system parent body metamorphism.  相似文献   

8.
Stable isotopic data of meteorites are critical for understanding the evolution of terrestrial planets. In this study, we report high-precision vanadium (V) isotopic compositions of 11 unequilibrated and equilibrated L chondrites. Our samples show an average δ51V of ??1.25‰?±?0.38‰ (2SD, n?=?11), which is ~?0.5‰ lighter than that of the bulk silicate Earth constrained by mantle peridotites. Isotopic fractionation in type 3 ordinary chondrites vary from ??1.76‰ to ??1.29‰, whereas the δ51V of equilibrated chondrites vary from ??1.37‰ to ??1.08‰. δ51V of L chondrites do not correlate with thermal metamorphism, shock stage, or weathering degree. Future studies are required to explore the reason for V isotope variation in the solar system.  相似文献   

9.
Ion-exchange fractionation of copper and zinc isotopes   总被引：5，自引：0，他引：5  
Whether transition element isotopes can be fractionated at equilibrium in nature is still uncertain. Standard solutions of Cu and Zn were eluted on an anion-exchange resin, and the isotopic compositions of Cu (with respect to Zn) of the eluted fractions were measured by multiple-collector inductively coupled plasma mass spectrometry. It was found that for pure Cu solutions, the elution curves are consistent with a 63Cu/65Cu mass fractionation coefficient of 0.46‰ in 7 mol/L HCl and 0.67‰ in 3 mol/L HCl between the resin and the solution. Batch fractionation experiments confirm that equilibrium fractionation of Cu between resin and 7 mol/L HCl is ∼0.4‰ and therefore indicates that there is no need to invoke kinetic fractionation during the elution. Zn isotope fractionation is an order of magnitude smaller, with a 66Zn/68Zn fractionation factor of 0.02‰ in 12 mol/L HCl. Cu isotope fractionation results determined from a chalcopyrite solution in 7 mol/L HCl give a fractionation factor of 0.58‰, which indicates that Fe may interfere with Cu fractionation.Comparison of Cu and Zn results suggests that the extent of Cu isotopic fractionation may signal the presence of so far unidentified polynuclear complexes in solution. In contrast, we see no compelling reason to ascribe isotope fractionation to the coexistence of different oxidation states. We further suggest that published evidence for iron isotopic fractionation in nature and in laboratory experiments may indicate the distortion of low-spin Fe tetrahedral complexes.The isotope geochemistry of transition elements may shed new light on their coordination chemistry. Their isotopic fractionation in the natural environment may be interpreted using models of thermodynamic fractionation.  相似文献   

10.
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   

11.
A new technique for the precise and accurate determination of Cd stable isotope compositions has been developed and applied to geological materials and meteorites. The Cd isotope analyses are performed by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) using external normalization to Ag for mass bias correction. The accuracy of the new procedure was ascertained by the comparison of data for meteorites with published results acquired by thermal ionization mass spectrometry and double spiking. Some results were also confirmed by measurements using external normalization to Sb on a different MC-ICPMS instrument. A long-term reproducibility of ± 1.1 εCd/amu (2 sd) was obtained for separate dissolutions and multiple analyses of several rock and meteorite samples (εCd/amu represents the deviation of a Cd isotope ratio of a sample relative to the JMC Cd standard in parts per 104, normalized to a mass difference of 1 amu). As little as 5-20 ng of Cd are sufficient for the acquisition of precise and accurate data.Terrestrial rock and mineral samples display little variations in Cd isotope compositions (εCd/amu between −1 and +1.2), except for a tektite sample that was found to be enriched in the heavy Cd isotopes by +7.6 εCd/amu. The carbonaceous chondrites Orgueil, Murchison and Allende have Cd isotope ratios that are unfractionated relative to the JMC Cd standard and terrestrial rocks. The ordinary chondrites analyzed in this study and a Rumuruti chondrite display Cd isotope fractionations, ranging from −19 to +36 εCd/amu.These results suggest that substantial (inorganic) natural Cd isotope fractionations are generated only by evaporation and/or condensation processes. The lack of resolvable Cd isotope variations between the different carbonaceous chondrites, despite large differences in Cd concentrations, implies that the primary depletion of Cd in the early solar system did not involve Rayleigh evaporation. The Cd isotope fractionation in ordinary and Rumuruti chondrites is probably due to the redistribution of Cd by evaporation and condensation processes during thermal metamorphism on the parent bodies. Models that explain the enrichments of highly volatile elements in unequilibrated ordinary chondrites by primary equilibrium condensation appear to be inconsistent with the Cd isotope data.  相似文献   

12.
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   

13.
The absolute isotope abundance of Ti has been determined in Ca-Al-rich inclusions from the Allende and Leoville meteorites and in samples of whole meteorites. The absolute Ti isotope abundances differ by a significant mass dependent isotope fractionation transformation from the previously reported abundances, which were normalized for fractionation using 46Ti48Ti. Therefore, the absolute compositions define distinct nucleosynthetic components from those previously identified or reflect the existence of significant mass dependent isotope fractionation in nature. We provide a general formalism for determining the possible isotope compositions of the exotic Ti from the measured composition, for different values of isotope fractionation in nature and for different mixing ratios of the exotic and normal components. The absolute Ti and Ca isotopic compositions still support the correlation of 50Ti and 48Ca effects in the FUN inclusions and imply contributions from neutron-rich equilibrium or quasi-equilibrium nucleosynthesis. The present identification of endemic effects at 46Ti, for the absolute composition, implies a shortfall of an explosive-oxygen component or reflects significant isotope fractionation. Additional nucleosynthetic components are required by 47Ti and 49Ti effects. Components are also defined in which 48Ti is enhanced.Bulk samples of carbonaceous meteorites (C2 and C3 types) show distinct excesses at 50Ti but no nonlinear effects at the other Ti isotopes. Other chondrites, including Orgueil (Cl), show no nonlinear effects. Relative to terrestrial Ti, a small isotope fractionation is found for only an enstatite chondrite. The Ti absolute compositions in Ca-Al-rich inclusions show significant isotope fractionation effects corresponding to an enhancement in the heavier isotopes relative to the lighter isotopes as compared to Ti in a TiO2 standard and in chondrites. The absence of a correlation of Ti isotope fractionation effects with those for Ca and Mg is indicative of multiple processes of condensation, volatilization and recondensation; however, the mechanisms causing the isotope fractionation are not well understood.  相似文献   

14.
We report high precision Cu isotope data coupled with Cu concentration measurements for metal, troilite and silicate fractions separated from magmatic and non-magmatic iron meteorites, analysed for Fe isotopes (δ57Fe; permil deviation in 57Fe/54Fe relative to the pure iron standard IRMM-014) in an earlier study (Williams et al., 2006). The Cu isotope compositions (δ65Cu; permil deviation in 65Cu/63Cu relative to the pure copper standard NIST 976) of both metals (δ65CuM) and sulphides (δ65CuFeS) span much wider ranges (−9.30 to 0.99‰ and −8.90 to 0.63‰, respectively) than reported previously. Metal-troilite fractionation factors (Δ65CuM-FeS = δ65CuM − δ65CuFeS) are variable, ranging from −0.07 to 5.28‰, and cannot be explained by equilibrium stable isotope fractionation coupled with either mixing or reservoir effects, i.e. differences in the relative proportions of metal and sulphide in the meteorites. Strong negative correlations exist between troilite Cu and Fe (δ57FeFeS) isotope compositions and between metal-troilite Cu and Fe (Δ57FeM-FeS) isotope fractionation factors, for both magmatic and non-magmatic irons, which suggests that similar processes control isotopic variations in both systems. Clear linear arrays between δ65CuFeS and δ57FeFeS and calculated Cu metal-sulphide partition coefficients (DCu = [Cu]metal/[Cu]FeS) are also present. A strong negative correlation exists between Δ57FeM-FeS and DCu; a more diffuse positive array is defined by Δ65CuM-FeS and DCu. The value of DCu can be used to approximate the degree of Cu concentration equilibrium as experimental studies constrain the range of DCu between Fe metal and FeS at equilibrium to be in the range of 0.05-0.2; DCu values for the magmatic and non-magmatic irons studied here range from 0.34 to 1.11 and from 0.04 to 0.87, respectively. The irons with low DCu values (closer to Cu concentration equilibrium) display the largest Δ57FeM-FeS and the lowest Δ65CuM-FeS values, whereas the converse is observed in the irons with large values DCu that deviate most from Cu concentration equilibrium. The magnitudes of Cu and Fe isotope fractionation between metal and FeS in the most equilibrated samples are similar: 0.25 and 0.32‰/amu, respectively. As proposed in an earlier study (Williams et al., 2006) the range in Δ57FeM-FeS values can be explained by incomplete Fe isotope equilibrium between metal and sulphide during cooling, where the most rapidly-cooled samples are furthest from isotopic equilibrium and display the smallest Δ57FeM-FeS and largest DCu values. The range in Δ65CuM-FeS, however, reflects the combined effects of partial isotopic equilibrium overprinting an initial kinetic signature produced by the diffusion of Cu from metal into exsolving sulphides and the faster diffusion of the lighter isotope. In this scenario, newly-exsolved sulphides initially have low Cu contents (i.e. high DCu) and extremely light δ65CuFeS values; with progressive equilibrium and fractional crystallisation the Cu contents of the sulphides increase as their isotopic composition becomes less extreme and closer to the metal value. The correlation between Δ65CuM-FeS and Δ57FeM-FeS is therefore a product of the superimposed effects of kinetic fractionation of Cu and incomplete equilibrium between metal and sulphide for both isotope systems during cooling. The correlations between Δ65CuM-FeS and Δ57FeM-FeS are defined by both magmatic and non-magmatic irons record fractional crystallisation and cooling of metallic melts on their respective parent bodies as sulphur and chalcophile elements become excluded from crystallised solid iron and concentrated in the residual melt. Fractional crystallisation processes at shallow levels have been implicated in the two main classes of models for the origin of the non-magmatic iron meteorites; at (i) shallow levels in impact melt models and (ii) at much deeper levels in models where the non-magmatic irons represent metallic melts that crystallised within the interior of a disrupted and re-aggregated parent body. The presence of non-magmatic irons with a range of Fe and Cu isotope compositions, some of which record near-complete isotopic equilibrium implies crystallisation at a range of cooling rates and depths, which is most consistent with cooling within the interior of a meteorite parent body. Our data therefore lend support to models where the non-magmatic irons are metallic melts that crystallised in the interior of re-aggregated, partially differentiated parent bodies.  相似文献   

15.
The carbon isotopic composition of the total carbon in the enstatite chondrites Indarch, Abee, St. Marks, Pillistfer, Hvittis and Daniel's Kuil and the enstatite achondrite Cumberland Falls has been measured. The empirical relationhip between carbon isotopic composition and total carbon content is distinct from that of carbonaceous and ordinary chondrites. Within the enstatite chondrite group the average 13C content increases with petrographic type: E4 < E5 < E6. Daniel's Kuil shows the largest 13C enrichment in the bulk carbon of any meteorite. The carbon isotopic composition is most clearly correlated with the abundance of the elements Zn, Cd and In. Insofar as these elements may hold the key to the understanding of enstatite chondrites, more detailed combined carbon isotope and trace element studies of these meteorites will play an important role in the deciphering of their history.  相似文献   

16.
Separated magnetic and nonmagnetic components from the ordinary chondrites Dhajala (H3.8) and Ochansk (H4) were analyzed for their Re-Os isotopic compositions, as well as for the abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt and Pd. The Re-Os isotopic systematics of these components are used to constrain the timing of HSE fractionations, and assess the level of open-system behavior of these elements in each of the different components. The high precision, isotope dilution mass spectrometric analyses of the HSE are used to constrain the origins of, and possible relations between some of the diverse components present in these chondrites. The relative and absolute abundances of the HSE differ considerably among the components. Metal fractions have Re/Os that are factors of ∼2 (Dhajala) to ∼3 (Ochansk) higher than those of their nonmagnetic fractions. The isotopic data for both meteorites are consistent with the largest Re-Os fractionations occurring between metal and nonmagnetic components early in solar system history, although minor to moderate late stage, open-system behavior, and limited variations in Re/Os preclude a precise determination of the age for that fractionation. Open-system behavior is generally absent to minor in the metal fractions, and highly variable in nonmagnetic fractions. Re/Os ratios of nonmagnetic fractions deviate as much as 40% from a primordial isochron. Although some deviations are large for isochron applications, nearly all are negligible with respect to consideration of fractionation processes controlling the HSE.Metal from both meteorites contains about 90% of the total budget of HSE. Metal in Ochansk has ∼2 to 10 times the abundances of the bulk meteorite, while metal from the matrix of Dhajala has ∼2 to 4 times the abundances of the bulk. Fine metal in both meteorites has higher abundances than coarse metal, as has been previously observed. Nonmagnetic components, consisting of chondrules and matrix from which metal was removed in the laboratory, have highly fractionated HSE, characterized by much lower Re/Os than the bulk meteorites, as well as large relative depletions in Pd. The abundances of Re, Os, Ir, Ru and Pt in the nonmagnetic fractions are 14-120 ng/g, much higher than would be expected if they had equilibrated with the metal phases present (150-16,000 ng/g). Collectively, the data are consistent with the HSE budget in ordinary chondrites being dominated by two HSE-bearing carrier phases with distinct compositions. These phases formed separately, and never subsequently equilibrated. Metal components incorporated a HSE carrier that formed at high through moderate temperatures and relatively high pressures, such that the relatively volatile Pd behaved coherently with the more refractory HSE. Nonmagnetic fractions from both chondrules and matrix have HSE compositions that likely require at least two processes that fractionated the HSE. Depletions in Pd are consistent with the presence of HSE carriers that formed as either highly refractory condensates, or residues of high degrees of metal melting. Depletions in Re may implicate a period of relatively high fO2 during which a volatile form of Re was separated from the other HSE.  相似文献   

17.
The concentration of Pd in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites, 29 iron meteorites and other samples has been determined by stable isotope dilution using solid source mass spectrometry. The Cl chondrite Orgueil gives a ‘cosmic’ abundance for Pd of 1.5 (Si = 106 atoms), in good agreement with the currently accepted value.The concentration of Pd shows little variation among the carbonaceous chondrites, but in ordinary chondrites decreases from the H to L to LL groups. Pd in achondrites is approx 100 times lower than in chondrites. Data for iron meteorites plot around the ‘cosmic’ PdNi ratio; however the Pd data falls into distinct groups, corresponding to the chemical group classification. These results support the hypothesis that at least two fractionation processes have occurred during the formation of iron meteorites.  相似文献   

18.
Lead-205 decays to 205Tl with a half-life of 15 Myr and should have been present in the early solar system according to astrophysical models. However, despite numerous attempts, Tl isotopic measurements of meteorites have been unable to demonstrate convincingly its former presence. Here, we report large (∼5‰) variations in Tl isotope composition in metal and troilite fragments from a range of iron meteorites that were determined at high precision using multiple collector inductively coupled plasma mass spectrometry. The Tl isotopic compositions of seven metal samples of the IAB iron meteorites Toluca and Canyon Diablo define a correlation with 204Pb/203Tl. When interpreted as an isochron, this corresponds to an initial 205Pb/204Pb ratio of (7.4 ± 1.0) × 10−5. Alternative explanations for the correlation, such as mixing of variably mass-fractionated meteorite components or terrestrial contamination are harder to reconcile with independent constraints. However, troilite nodules from Toluca and Canyon Diablo contain Tl that is significantly less radiogenic than co-existing metal with isotope compositions that are variable and decoupled from 204Pb/203Tl. These effects are similar to those recently reported by others for Fe and Ni isotopes in iron meteorite sulfides and appear to be the result of kinetic stable isotope fractionation during diffusion. Though it cannot conclusively be shown that the metal fragments are unaffected by the secondary processes that disturbed the troilites, mass balance modeling indicates that the alteration of the troilites is unlikely to have significantly affected the Tl isotope compositions of the co-existing metals. It is therefore reasonable to conclude that the IAB metal isochron is a product of the in situ decay of 205Pb. If the I-Xe ages of IAB silicate inclusions record the same event as the 205Pb-205Tl chronometer then crystallization of the IAB metal was probably completed between 10 and 20 Myr after the condensation of the first solids. This implies an initial solar system 205Pb/204Pb of (1.0-2.1) × 10−4, which is in excellent agreement with recently published astrophysical predictions. Similar calculations yield an initial solar system Tl isotope composition of ε205Tl = −2.8 ± 1.7. The Tl isotopic composition and concentration of the silicate Earth depends critically on the timing and mechanism of core formation and Earth’s volatile element depletion history. Modeling of the Earth’s accretion and core formation using the calculated initial solar system Tl isotope composition and 205Pb/204Pb, however, does not yield reasonable results for the silicate Earth unless either the Earth lost Tl and Pb late in its accretion history or the core contains much higher concentrations of Pb and Tl than are found in iron meteorites.  相似文献   

19.
Oxygen isotopic compositions of silicate inclusions in IVA iron meteorites have been measured with an in situ UV laser microprobe technique. The homogeneity of oxygen isotopic compositions within and among individual mineral grains has also been examined. Oxygen isotope fractionations between coexisting mineral pairs were utilized in oxygen isotope thermometry. Our measured Δ17O values, ranging from 0.97 to 1.25‰, are characteristic of a single reservoir and fully confirm the oxygen isotopic similarity between IVA irons and L/LL chondrites. Steinbach and São João Nepomuceno, containing inclusions of two silicate minerals in mutual contact, exhibit a mass-dependent fractionation of 18O/16O between tridymite and bronzite with apparent oxygen isotopic heterogeneity. The SiO2-bearing member, Gibeon, gives homogeneous oxygen isotopic compositions without detectable fractionation of 18O/16O between tridymite and quartz. Oxygen isotope equilibrium temperatures are estimated for coexisting tridymite and bronzite in the same sample slabs or clusters in Steinbach and São João Nepomuceno. The fractionations of 18O/16O between bronzite and tridymite range from 1.6 to 2.3‰ in different sample slabs or clusters. On the basis of the closure temperature concept, cooling rates are estimated at approximately 20 to 1000°C/Myr between 800 and 1000°C, a range of temperatures not accessible to other cooling rate methods. Using the Fast Grain Boundary diffusion model, we have demonstrated that significant oxygen heterogeneity both in tridymite and bronzite is probably due to isotope exchange during cooling between minerals with various grain sizes and mineral abundances in different regions of the samples. The new estimates of cooling rate by oxygen isotope thermometry refine previous cooling curves of IVA irons and support the breakup-reassembly model for the IVA parent body.  相似文献   

20.
Here we report the elemental and isotopic compositions of the insoluble organic material (IOM) isolated from several previously unanalyzed meteorites, as well as the reanalyses of H isotopic compositions of some previously measured samples (Alexander et al., 2007). The IOM in ordinary chondrites (OCs) has very large D enrichments that increase with increasing metamorphism and decreasing H/C, the most extreme δD value measured being almost 12,000‰. We propose that such large isotopic fractionations could be produced in the OC parent bodies through the loss of isotopically very light H2 generated when Fe was oxidized by water at low temperatures (<200 °C). We suggest that similar isotopic fractionations were not generated in the IOM of CV and CO chondrites with similar metamorphic grades and IOM H/C ratios because proportionately less water was consumed during metamorphism, and the remaining water buffered the H isotopic composition of the IOM even a H was being lost from it.Hydrogen would also have been generated during the alteration of CI, CM and CR carbonaceous chondrites. The IOM in these meteorites exhibit a considerable range in isotopic compositions, but all are enriched in D, as well as 15N, relative to terrestrial values. We explore whether these enrichments could also have been produced by the loss of H2, but conclude that the most isotopically anomalous IOM compositions in meteorites from these groups are probably closest to their primordial values. The less isotopically anomalous IOM has probably been modified by parent body processes. The response of IOM to these processes was complex and varied, presumably reflecting differences in conditions within and between parent bodies.The D enrichments associated with H2 generation, along with exchange between D-rich IOM and water in the parent bodies, means that it is unlikely that any chondrites retain the primordial H isotopic composition of the water ice that they accreted. The H isotopic compositions of the most water-rich chondrites, the CMs and CIs, are probably the least modified and their compositions (δD ? −25‰) suggest that their water did not form at large radial distances from the Sun where ice is predicted to be very D-rich. Yet models to explain the O isotopic composition of inner Solar System bodies require that large amounts of ice were transported from the outer to the inner Solar System.  相似文献   

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