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1.
Asuka 881394 is a unique basaltic meteorite that originated in the crust of a differentiated planetesimal in the early Solar System. We present high precision Pb, Mg, and Cr isotopic compositions of bulk samples and mineral separates from this achondrite. A 207Pb-206Pb internal isochron obtained from the radiogenic pyroxene and whole-rock fractions of Asuka 881394 yields an absolute age of 4566.5 ± 0.2 Ma, which we consider to be the best estimate for the crystallization age of this basaltic achondrite. The 26Al-26Mg systematics show some evidence of disturbance, but 5 of the 6 analyzed whole-rock and mineral fractions define an isochron corresponding to a 27Al/26Al ratio of (1.28 ± 0.07) × 10−6. Comparison with the 26Al-26Mg and Pb-Pb systematics in the D’Orbigny achondrite translates to a 26Al-26Mg age of 4565.4 ± 0.2 Ma for Asuka 881394. The 53Mn-53Cr systematics in whole-rock, silicate and chromite fractions correspond to a 53Mn/55Mn ratio of (3.85 ± 0.23) × 10−6. Compared to the most precise 53Mn-53Cr and Pb-Pb systematics available for the D’Orbigny angrite, this translates to a 53Mn-53Cr age of 4565.3 ± 0.4 Ma; similarly, a comparison with the NWA 4801 angrite yields a 53Mn-53Cr age of 4565.5 ± 0.4 Ma, in agreement with the age obtained relative to D’Orbigny. While the 26Al-26Mg and 53Mn-53Cr ages appear to be concordant in Asuka 881394, these ages are ∼1 Ma younger than its 207Pb-206Pb age. This discordance might have been caused by one or more of several reasons, including differences in the closure temperatures for Pb versus Cr and Mg diffusion in their host minerals combined with slow cooling of the parent body as well as differential resetting of isotopic systems by a process other than volume diffusion, e.g., shock metamorphism. The ancient age of Asuka 881394 suggests that basaltic volcanism on its parent planetesimal occurred within ∼3 Ma of the formation of earliest solids in the Solar System, essentially contemporaneously with chondrule formation. This requires that the Asuka 881394 parent body was fully accreted within ∼500,000 yrs of Solar System formation.  相似文献   

2.
We have reinvestigated the Mn-Cr systematics in a number of primitive meteorites, differentiated planetesimals and terrestrial planets in order to address the chronology of the early stages of protoplanetary disk evolution and planetary formation. Our analytical procedure is based on the assumption of terrestrial abundances for 50Cr and 52Cr only; recognizing that a data reduction scheme based on Earth-like 54Cr/52Cr abundances in all meteorites is not tenable. Here we show that initial ε53Cr compositions of 54Cr-rich and 54Cr-poor acid leach fractions in the primitive carbonaceous chondrite Orgueil differ by 0.9ε, reflecting primordial mineral-scale heterogeneity. However, asteroidal processing effectively homogenized any ε53Cr variations on the planetesimal scale, providing a uniform present-day solar ε53Cr=0.20±0.10. Thus, our 53Mn-53Cr data argue against the previously suggested 53Mn heliocentric gradient. Instead, we suggest that inner Solar System objects possessed an initially homogeneous 53Mn/55Mn composition, which determined by two independent means is estimated at (6.28 ± 0.66) × 10−6. Our revised Mn-Cr age for Ste. Marguerite (SM) metamorphism of 4562.9 ± 1.0 Ma is identical to the Pb-Pb age of SM phosphates. Using this age, we confirm that mantle differentiation of the eucrite parent body occurred 4564.9 ± 1.1 Ma ago, and revise the time interval between this event and CAI formation to 2.2 ± 1.1 Ma. We also constrain metamorphism in carbonaceous chondrites of type 2 and 3 to have occurred between 1 and 6 Ma after CAI formation. The 53Mn-53Cr correlation among chondrites, planetesimals and terrestrial planets (the eucrite parent body, Mars and Earth) provides evidence for Mn/Cr fractionation within the protoplanetary disk recorded by all precursor materials of the terrestrial planets and primitive asteroids. This fractionation appears to have occurred within 2 Ma of CAI formation.  相似文献   

3.
We report here the results of a study of trace element microdistributions and 53Mn-53Cr systematics in several basaltic and orthopyroxenitic clasts from the Vaca Muerta mesosiderite. Ion microprobe analyses of selected trace and minor element abundances in minerals of the silicate clasts indicate that, following igneous crystallization, these clasts underwent extensive metamorphic equilibration that resulted in intra- and inter-grain redistribution of elements. There is also evidence in the elemental microdistributions that these clasts were subsequently affected to varying degrees by alteration resulting from redox reactions involving the indigenous silicates and externally derived reducing agents (such as phosphorus, derived from the mesosiderite metal) at the time of metal-silicate mixing. Furthermore, our results suggest that the varying degrees of alteration by redox reactions recorded in the different clasts were most likely facilitated by different degrees of remelting induced by heating during the metal-silicate mixing event. After taking into account the effects of these postmagmatic secondary processes, comparison of the trace and minor element concentrations and distributions in minerals of basaltic and orthopyroxenitic clasts with those of noncumulate eucrites and diogenites, respectively, suggests that the primary igneous petrogenesis, including parent magma and source compositions, of Vaca Muerta silicates were similar to those of achondritic meteorites of the Howardite-Eucrite-Diogenite (HED) association. Internal 53Mn-53Cr isochrons obtained for two basaltic (pebble 16 and 4679) and two orthopyroxenitic (4659 and 4670) clasts show that chromium isotopes are equilibrated within each clast. Nevertheless, just as for noncumulate eucrites and diogenites, 53Cr excesses in whole-rock samples of the basaltic clasts (∼1.01 ε in pebble 16; ∼1.07 ε in 4679) are significantly higher than in the orthopyroxene-rich clasts (∼0.62 ε in 4659; ∼0.53 ε in 4670). As in the case of the HED parent body, this suggests that Mn/Cr fractionation in the parent body of the Vaca Muerta silicate clasts occurred very early in the history of the solar system, when 53Mn was still extant. However, the slope of the 53Mn-53Cr isochron defined by the whole-rock samples of Vaca Muerta clasts (corresponding to a 53Mn/55Mn ratio of 3.3 ± 0.6 × 10−6) is distinctly lower than that defined by the HED whole-rock samples (corresponding to a 53Mn/55Mn ratio of 4.7 ± 0.5 × 10−6), indicating that the global Mn/Cr fractionation event that established mantle source reservoirs on the parent body of the Vaca Muerta silicate clasts occurred ∼2 Ma after a similar event on the HED parent body.  相似文献   

4.
Origin and chronology of chondritic components: A review   总被引：1，自引：0，他引：1  
Mineralogical observations, chemical and oxygen-isotope compositions, absolute 207Pb-206Pb ages and short-lived isotope systematics (7Be-7Li, 10Be-10B, 26Al-26Mg, 36Cl-36S, 41Ca-41K, 53Mn-53Cr, 60Fe-60Ni, 182Hf-182W) of refractory inclusions [Ca,Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)], chondrules and matrices from primitive (unmetamorphosed) chondrites are reviewed in an attempt to test (i) the x-wind model vs. the shock-wave model of the origin of chondritic components and (ii) irradiation vs. stellar origin of short-lived radionuclides. The data reviewed are consistent with an external, stellar origin for most short-lived radionuclides (7Be, 10Be, and 36Cl are important exceptions) and a shock-wave model for chondrule formation, and provide a sound basis for early Solar System chronology. They are inconsistent with the x-wind model for the origin of chondritic components and a local, irradiation origin of 26Al, 41Ca, and 53Mn. 10Be is heterogeneously distributed among CAIs, indicating its formation by local irradiation and precluding its use for the early solar system chronology. 41Ca-41K, and 60Fe-60Ni systematics are important for understanding the astrophysical setting of Solar System formation and origin of short-lived radionuclides, but so far have limited implications for the chronology of chondritic components. The chronological significance of oxygen-isotope compositions of chondritic components is limited. The following general picture of formation of chondritic components is inferred. CAIs and AOAs were the first solids formed in the solar nebula ∼4567-4568 Myr ago, possibly within a period of <0.1 Myr, when the Sun was an infalling (class 0) and evolved (class I) protostar. They formed during multiple transient heating events in nebular region(s) with high ambient temperature (at or above condensation temperature of forsterite), either throughout the inner protoplanetary disk (1-4 AU) or in a localized region near the proto-Sun (<0.1 AU), and were subsequently dispersed throughout the disk. Most CAIs and AOAs formed in the presence of an 16O-rich (Δ17O ∼ −24 ± 2‰) nebular gas. The 26Al-poor [(26Al/27Al)0 < 1 × 10−5], 16O-rich (Δ17O ∼ −24 ± 2‰) CAIs - FUN (fractionation and unidentified nuclear effects) CAIs in CV chondrites, platy hibonite crystals (PLACs) in CM chondrites, pyroxene-hibonite spherules in CM and CO chondrites, and the majority of grossite- and hibonite-rich CAIs in CH chondrites—may have formed prior to injection and/or homogenization of 26Al in the early Solar System. A small number of igneous CAIs in ordinary, enstatite and carbonaceous chondrites, and virtually all CAIs in CB chondrites are 16O-depleted (Δ17O > −10‰) and have (26Al/27Al)0 similar to those in chondrules (<1 × 10−5). These CAIs probably experienced melting during chondrule formation. Chondrules and most of the fine-grained matrix materials in primitive chondrites formed 1-4 Myr after CAIs, when the Sun was a classical (class II) and weak-lined T Tauri star (class III). These chondritic components formed during multiple transient heating events in regions with low ambient temperature (<1000 K) throughout the inner protoplanetary disk in the presence of 16O-poor (Δ17O > −5‰) nebular gas. The majority of chondrules within a chondrite group may have formed over a much shorter period of time (<0.5-1 Myr). Mineralogical and isotopic observations indicate that CAIs were present in the regions where chondrules formed and accreted (1-4 AU), indicating that CAIs were present in the disk as free-floating objects for at least 4 Myr. Many CAIs, however, were largely unaffected by chondrule melting, suggesting that chondrule-forming events experienced by a nebular region could have been small in scale and limited in number. Chondrules and metal grains in CB chondrites formed during a single-stage, highly-energetic event ∼4563 Myr ago, possibly from a gas-melt plume produced by collision between planetary embryos.  相似文献   

5.
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   

6.
CM chondrites contain carbonates and other secondary minerals such as phyllosilicates, sulfides, sulfates, oxides and hydroxides that are believed to have formed by aqueous alteration reactions on their parent asteroid. We report in situ Mn-Cr isotope measurements in the highly aqueously altered CM2.1 chondrites QUE 93005 and ALH 83100 using secondary ion mass spectrometry (Cameca ims-1270 ion microprobe). The 53Cr excesses are correlated with the 53Mn/55Mn ratio and result from the in situ decay of 53Mn, a short-lived radioisotope with a half-life of 3.7 Ma. If we assume that carbonate grains in samples QUE 93005 and ALH 83100 are cogenetic, then the excesses define initial 53Mn/55Mn ratios ((53Mn/55Mn)0) of (4.1 ± 1.2) × 10−6 and (5.1 ± 1.7) × 10−6, respectively. These values are comparable to those in carbonates from other CM chondrites as reported in the literature. Initial 53Mn/55Mn ratios for calculated model isochrones for individual carbonate grains range from (3.8 ± 1.4) × 10−6 to (4.8 ± 2.1) × 10−6 for QUE 93005 and from (3.1 ± 1.6) × 10−6 to (1.3 ± 0.5) × 10−5 for ALH 83100. A possible interpretation for the ranges in (53Mn/55Mn)0 could be that alteration in individual CM chondrites was episodic and occurred over an extended period of time. However, isochrones based on the entire set of carbonate grains in each of the CM chondrites imply that the degree of aqueous alteration is roughly correlated with the age of carbonate formation in CM chondrites of different subtypes and that alteration on the CM parent asteroid started contemporaneously with or shortly after CAI formation and lasted at least 4 Ma.  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.
Tidal inundation was restored to a severely degraded tropical acid sulfate soil landscape and subsequent changes in the abundance and fractionation of Al, Fe and selected trace metals were investigated. After 5 a of regular tidal inundation there were large decreases in water-soluble and exchangeable Al fractions within former sulfuric horizons. This was strongly associated with decreased soil acidity and increases in pH, suggesting pH-dependent immobilisation of Al via precipitation as poorly soluble phases. The water-soluble fractions of Fe, Zn, Ni and Mn also decreased. However, there was substantial enrichment (2–5×) of the reactive Fe fraction (FeR; 1 M HCl extractable) near the soil surface, plus a closely corresponding enrichment of 1 M HCl extractable Cr, Zn, Ni and Mn. Surficial accumulations of Fe(III) minerals in the inter-tidal zone were poorly crystalline (up to 38% FeR) and comprised mainly of schwertmannite (Fe8O8(OH)6SO4) with minor quantities of goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). These Fe (III) mineral accumulations provide an effective substrate for the adsorption/co-precipitation and accumulation of trace metals. Arsenic displayed contrary behaviour to trace metals with peak concentrations (∼60 μg g−1) near the redox minima. Changes in the abundance and fractionation of the various metals can be primarily explained by the shift in the geochemical regime from oxic–acidic to reducing-circumneutral conditions, combined with the enrichment of reactive Fe near the soil surface. Whilst increasing sequestration of trace metals via sulfidisation is likely to occur over the long-term, the current abundance of reactive Fe near the sediment–water interface favours a dynamic environment with respect to metals in the tidally inundated areas.  相似文献   

9.
The results from batch sorption experiments on montmorillonite systems have demonstrated that bivalent transition metals compete with one another for sorption sites. For safety analysis studies of high level radioactive waste repositories with compacted bentonite near fields, the importance of competitive sorption on the migration of radionuclides needs to be evaluated. Under reducing conditions, the bentonite porewater chosen has a Fe(II) concentration of ∼5.3 × 10−5 M through saturation with siderite. The purpose of this paper is to assess the influence of such high Fe(II) concentrations on the transport of Ni(II) through compacted bentonite, Ni(II) was chosen as an example of a bivalent transition metal. The one-dimensional calculations were carried out at different Ni(II) equilibrium concentrations at the boundary (Ni(II)EQBM) with the reactive transport code MCOTAC incorporating the two site protolysis non electrostatic surface complexation/cation exchange sorption model, MCOTAC-sorb. At a Ni(II)EQBM level of 10−7 M without Fe(II) competition, the reactive transport calculations using a constant Kd approach and the MCOTAC-sorb calculation yielded the same breakthrough curves. At higher Ni(II)EQBM (10−5 M), the model calculations with MCOTAC-sorb indicated a breakthrough which was shifted to later times by a factor of ∼5 compared with the use of the constant Kd approach.When sorption competition was included in the calculations, the magnitude of the influence depended on the sorption characteristics of the two competing sorbates and their respective concentrations. At background Fe(II) concentrations of 5.3 × 10−5 M, and a Ni(II)EQBM level of 10−7 M, the Ni(II) breakthrough time was ∼15 times earlier than in the absence of competition. At such Fe(II) concentrations the Ni(II) breakthrough curves at all source concentrations less than 3.5 × 10−5 M (fixed by the NiCO3,S solubility limit) are the same i.e. Ni(II) exhibits linear (low) sorption.Competitive sorption effects can have significant influences on the transport of radionuclides through compacted bentonite i.e. reduce the migration rates. Since, for the case considered here, the Fe(II) concentration in the near field of a high-level radioactive waste repository may change in time and space, the transport of bivalent transition metal radionuclides can only be properly modelled using a multi-species reactive transport code which includes a sorption model.  相似文献   

10.
Interpretation of the origins of iron-bearing minerals preserved in modern and ancient rocks based on measured iron isotope ratios depends on our ability to distinguish between biological and non-biological iron isotope fractionation processes. In this study, we compared 56Fe/54Fe ratios of coexisting aqueous iron (Fe(II)aq, Fe(III)aq) and iron oxyhydroxide precipitates (Fe(III)ppt) resulting from the oxidation of ferrous iron under experimental conditions at low pH (<3). Experiments were carried out using both pure cultures of Acidothiobacillus ferrooxidans and sterile controls to assess possible biological overprinting of non-biological fractionation, and both SO42− and Cl− salts as Fe(II) sources to determine possible ionic/speciation effects that may be associated with oxidation/precipitation reactions. In addition, a series of ferric iron precipitation experiments were performed at pH ranging from 1.9 to 3.5 to determine if different precipitation rates cause differences in the isotopic composition of the iron oxyhydroxides. During microbially stimulated Fe(II) oxidation in both the sulfate and chloride systems, 56Fe/54Fe ratios of residual Fe(II)aq sampled in a time series evolved along an apparent Rayleigh trend characterized by a fractionation factor αFe(III)aq-Fe(II)aq ∼ 1.0022. This fractionation factor was significantly less than that measured in our sterile control experiments (∼1.0034) and that predicted for isotopic equilibrium between Fe(II)aq and Fe(III)aq (∼1.0029), and thus might be interpreted to reflect a biological isotope effect. However, in our biological experiments the measured difference in 56Fe/54Fe ratios between Fe(III)aq, isolated as a solid by the addition of NaOH to the final solution at each time point under N2-atmosphere, and Fe(II)aq was in most cases and on average close to 2.9‰ (αFe(III)aq-Fe(II)aq ∼ 1.0029), consistent with isotopic equilibrium between Fe(II)aq and Fe(III)aq. The ferric iron precipitation experiments revealed that 56Fe/54Fe ratios of Fe(III)aq were generally equal to or greater than those of Fe(III)ppt, and isotopic fractionation between these phases decreased with increasing precipitation rate and decreasing grain size. Considered together, the data confirm that the iron isotope variations observed in our microbial experiments are primarily controlled by non-biological equilibrium and kinetic factors, a result that aids our ability to interpret present-day iron cycling processes but further complicates our ability to use iron isotopes alone to identify biological processing in the rock record.  相似文献   

11.
Contributors to chromium isotope variation of meteorites   总被引：3，自引：0，他引：3  
We report the results of a comprehensive, high precision survey of the Cr isotopic compositions of primitive chondrites, along with some differentiated meteorites. To ensure complete dissolution of our samples, they were first fused with lithium borate-tetraborate at 1050-1000 °C. Relative to the NIST Cr standard SRM 3112a, carbonaceous chondrites exhibit excesses in 54Cr/52Cr from 0.4 to 1.6ε (1ε = 1 part in 10,000), and ordinary chondrites display a common 54Cr/52Cr deficit of ∼0.4ε. Analyses of acid-digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54Cr-enriched carrier, which is characterized by a large excess in 54Cr/52Cr (up to 200ε) associated with a very small deficit in 53Cr/52Cr (<2ε). We did not find 54Cr anomalies in either bulk enstatite chondrites or in leachates of their acid-digestion residues. This either requires that the enstatite chondrite parent bodies did not incorporate the 54Cr anomaly carrier phase during their accretion, or the phase was destroyed by parent body metamorphism. Chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the NIST SRM 3112a Cr standard. The eucrite and Martian meteorites studied exhibit small deficits in 54Cr/52Cr. The 54Cr/52Cr variations among different meteorite classes suggest that there was a spatial and/or temporal heterogeneity in the distribution of a 54Cr-rich component in the inner Solar System.We confirm the correlated excesses in 54Cr/52Cr and 53Cr/52Cr for bulk carbonaceous chondrites, but the new data yield a steeper slope (∼6.6) than that reported in Shukolyukov and Lugmair (2006). The correlated excesses may affect the use of the Mn-Cr chronometer in carbonaceous chondrites. We could not confirm the bulk carbonaceous chondrite Mn-Cr isochron reported by Shukolyukov and Lugmair (2006) and Moynier et al. (2007), mostly because we find much smaller total variations in ε53Cr (∼0.2). All bulk chondrites have small ε53Cr excesses (up to 0.3) relative to the Earth, most likely reflecting the sub-chondritic Mn/Cr ratio of the Earth. The ε53Cr variations in chondrites do seem to grossly correlate with Mn/Cr and yield an initial Solar System 53Mn/55Mn value of 5.4(±2.4) × 10−6, corresponding to an absolute age of 4566.4 (±2.2) Ma.Nuclear interactions with cosmic rays result in coupled excesses in ε54Cr and ε53Cr with a ∼4:1 ratio in phases with high Fe/Cr. These are most dramatically demonstrated in the iron meteorite Carbo, showing excesses in ε54Cr of up to 140ε. These new results show that the Mn-Cr chronometer should be used with caution in samples/minerals with high Fe/Cr and long cosmic ray exposure ages.  相似文献   

12.
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   

13.
The effect of sulfur dissolved as sulfide (S2−) in silicate melts on the activity coefficients of NiO and some other oxides of divalent cations (Ca, Cr, Mn, Fe and Co) has been determined from olivine/melt partitioning experiments at 1400 °C in six melt compositions in the system CaO-MgO-Al2O3-SiO2 (CMAS), and in derivatives of these compositions at 1370 °C, obtained from the six CMAS compositions by substituting Fe for Mg (FeCMAS). Amounts of S2− were varied from zero to sulfide saturation, reaching 4100 μg g−1 S in the most sulfur-rich silicate melt. The sulfide solubilities compare reasonably well with those predicted from the parameterization of the sulfide capacity of silicate melts at 1400 °C of O’Neill and Mavrogenes (2002), although in detail systematic deviations indicate that a more sophisticated model may improve the prediction of sulfide capacities.The results show a barely discernible effect of S2− in the silicate melt on Fe, Co and Ni partition coefficients, and also surprisingly, a tiny but resolvable effect on Ca partitioning, but no detectable effect on Cr, Mn or some other lithophile incompatible elements (Sc, Ti, V, Y, Zr and Hf). Decreasing Mg# of olivine (reflecting increasing FeO in the system) has a significant influence on the partitioning of several of the divalent cations, particularly Ca and Ni. We find a remarkably systematic correlation between and the ionic radius of M2+, where M = Ca, Cr, Mn, Fe, Co or Ni, which is attributable to a simple relationship between size mismatch and excess free energies of mixing in Mg-rich olivine solid solutions.Neither the effect of S2− nor of Mg#ol is large enough by an order of magnitude to account for the reported variations of obtained from electron microprobe analyses of olivine/glass pairs from mid-ocean ridge basalts (MORBs). Comparing these MORB glass analyses with the Ni-MgO systematics of MORB from other studies in the literature, which were obtained using a variety of analytical techniques, shows that these electron microprobe analyses are anomalous. We suggest that the reported variation of with S content in MORB is an analytical artifact.Mass balance of melt and olivine compositions with the starting compositions shows that dissolved S2− depresses the olivine liquidus of haplobasaltic silicate melts by 5.8 × 10−3 (±1.3 × 10−3) K per μg g−1 of S2−, which is negligible in most contexts. We also present data for the partitioning of some incompatible trace elements (Sc, Ti, Y, Zr and Hf) between olivine and melt. The data for Sc and Y confirm previous results showing that and decrease with increasing SiO2 content of the melt. Values of average 0.01 with most falling in the range 0.005-0.015. Zr and Hf are considerably more incompatible than Ti in olivine, with and about 10−3. The ratio / is well constrained at 0.611 ± 0.016.  相似文献   

14.
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   

15.
An understanding of the biogeochemical behaviour of metals in mine spoil materials is a prerequisite to rehabilitate Ni mining sites. The objective of this study was to characterize the fate of metals in different Ni ore spoil materials as influenced by hydrological conditions and fertilisation practices. In tropical ultramafic complexes, the different stages of lateritic weathering lead to two types of ores, and therefore, to two spoil types. They are mainly either a clay-rich saprolite, so-called “garnierite”, enriched in phyllosilicates, or a limonitic material, enriched in Fe oxides. Lysimeter columns were designed to monitor leaching waters through both spoil materials. The garnieritic spoil released higher concentrations of Mg (mean = 2.25 mg L−1), Ni (0.39 mg L−1) and Cr (1.19 mg L−1) than the limonitic spoil (Mg = 0.5 mg L−1; Ni = 0.03 mg L−1 and Cr = 0.25 mg L−1). Chromium was mainly in an anionic form in leaching solutions. As exchangeable pools of Cr(VI) in limonite (980 mg kg−1 of KH2PO4-extractable Cr) are considerable its release in water may still occur in the case of a pH increase. In mixed spoil, metal concentrations were almost as low as in the limonitic one. The effect of mineral-N fertilisation was a strong release of cations (Ni, Mg) into the leachate. Phosphate amendment did not affect the soil solution composition under experimental conditions.  相似文献   

16.
We evaluate initial (26Al/27Al)I, (53Mn/55Mn)I, and (182Hf/180Hf)I ratios, together with 207Pb/206Pb ages for igneous differentiated meteorites and chondrules from ordinary chondrites for consistency with radioactive decay of the parent nuclides within a common, closed isotopic system, i.e., the early solar nebula. The relative initial isotopic abundances of 26Al, 53Mn, and 182Hf in differentiated meteorites and chondrules are consistent with decay from common solar system initial values, here denoted by I(Al)SS, I(Mn)SS, and I(Hf)SS, respectively. I(Mn)SS and I(Hf)SS = 9.1 ± 1.7 × 10−6 and 1.07 ± 0.08 × 10−4, respectively, correspond to “canonical” I(Al)SS = 5.1 × 10−5. I(Hf)SS so determined is consistent with I(Hf)SS = 9.72 ± 0.44 × 10−5 directly determined from an internal Hf-W isochron for CAI minerals. I(Mn)SS is within error of the lowest value directly measured for CAIs. We suggest that erratically higher values measured for CAIs in carbonaceous chondrites may reflect proton irradiation of unaccreted CAIs by the early Sun after other asteroids destined for melting by 26Al decay had already accreted. The 53Mn incorporated within such asteroids would have been shielded from further “local” spallogenic contributions from within the solar system. The relative initial isotopic abundances of the short-lived nuclides are less consistent with the 207Pb/206Pb ages of the corresponding materials than with one another. The best consistency of short- and long-lived chronometers is obtained for (182Hf/180Hf)I and the 207Pb/206Pb ages of angrites. (182Hf/180Hf)I decreases with decreasing 207Pb/206Pb ages at the rate expected from the 8.90 ± 0.09 Ma half-life of 182Hf. The model solar system age thus determined is TSS,Hf-W = 4568.3 ± 0.7 Ma. (26Al/27Al)I and (53Mn/55Mn)I are less consistent with 207Pb/206Pb ages of the corresponding meteorites, but yield TSS,Mn-Cr = 4568.2 ± 0.5 Ma relative to I(Al)SS = 5.1 × 10−5 and a 207Pb/206Pb age of 4558.55 ± 0.15 Ma for the LEW86010 angrite. The Mn-Cr method with I(Mn)SS = 9.1 ± 1.7 × 10−6 is useful for dating accretion (if identified with chondrule formation), primary igneous events, and secondary mineralization on asteroid parent bodies. All of these events appear to have occurred approximately contemporaneously on different asteroid parent bodies. For I(Mn)SS = 9.1 ± 1.7 × 10−6, parent body differentiation is found to extend at least to ∼5 Ma post-TSS, i.e., until differentiation of the angrite parent body ∼4563.5 Ma ago, or ∼4564.5 Ma ago using the directly measured 207Pb/206Pb ages of the D’Orbigny-clan angrites. The ∼1 Ma difference is characteristic of a remaining inconsistency for the D’Orbigny-clan between the Al-Mg and Mn-Cr chronometers on one hand, and the 207Pb/206Pb chronometer on the other. Differentiation of the IIIAB iron meteorite and ureilite parent bodies probably occurred slightly later than for the angrite parent body, and at nearly the same time as one another as shown by the Mn-Cr ages of IIIAB irons and ureilites, respectively. The latest recorded episodes of secondary mineralization are for carbonates on the CI carbonaceous chondrite parent body and fayalites on the CV carbonaceous chondrite parent body, both extending to ∼10 Ma post-TSS.  相似文献   

17.
We present results of a study of the 53Mn-53Cr isotope systematics in the enstatite chondrites and achondrites (aubrites). The goal of this study was to explore the capabilities of this isotope system to obtain chronological information on these important classes of meteorites and to investigate the original distribution in the inner solar system of the short-lived radionuclide 53Mn. Our earlier work (Lugmair and Shukolyukov, 1998; Shukolyukov and Lugmair, 2000a) has shown that the asteroid belt bodies are characterized by essentially the same initial 53Mn abundance. However, we have found the presence of a gradient in the abundance of the radiogenic 53Cr between the earth-moon system, Mars, and the asteroid Vesta. If this gradient is considered as a function of the heliocentric distance a linear radial dependence is indicated. This can be explained either by an early, volatility controlled Mn/Cr fractionation in the nebula or by an original radially heterogeneous distribution of 53Mn. The enstatite chondrites are suggested to form in the inner zones of the solar nebula, much closer to the Sun than the ordinary chondrites. Therefore, their investigation may be an important test on the hypothesis on a radial heterogeneity in the initial 53Mn.We have studied the bulk samples of the EH4-chondrites Indarch and Abee and the EL6-chondrite Khairpur. Although these meteorites have essentially the same Mn/Cr ratio as the ordinary chondrites, the relative abundance of the radiogenic 53Cr is three times smaller than in the ordinary chondrites. Because these meteorites are primitive (undifferentiated) and no Mn/Cr fractionation had occurred within their parent bodies, this difference is a strong argument in favor of an initially heterogeneous distribution of 53Mn in the early inner solar system. This finding is also consistent with formation of the enstatite chondrites in the inner zones of the solar nebula. Using the characteristic 53Cr excess of the enstatite chondrites and the observed gradient, their place of origin falls at about 1.4 AU or somewhat closer to the Sun (i.e. >1.0-1.4 AU).We also present chronological results for the enstatite chondrites and achondrites. The ‘absolute’ 53Mn-53Cr ages of the EH4-chondrites are old: ∼4565 Ma. The EL6-chondrite Khairpur is ∼4.5 Ma younger, which is in good agreement with the 129I-129Xe data from the literature. The age of the aubrite Peña Blanca Spring appears to be similar to those of the enstatite chondrites while that of the aubrite Bishopville is at least ∼10 Ma younger, which is also in agreement with the 129I-129Xe data. The results from bulk samples of aubrites indicate that the last Mn/Cr fractionation in their parent body occurred ∼ 4563 Ma ago and imply an evolution of the Mn-Cr isotope system in an environment with an higher than chondritic Mn/Cr ratio for several millions of years.  相似文献   

18.
Waters from abandoned Sb-Au mining areas have higher Sb (up to 2138 μg L−1), As (up to 1252 μg L−1) and lower Al, Zn, Li, Ni and Co concentrations than those of waters from the As-Au mining area of Banjas, which only contain up to 64 μg L−1 As. In general, Sb occurs mainly as SbO3− and As H2AsO4−. In general, waters from old Sb-Au mining areas are contaminated in Sb, As, Al, Fe, Cd, Mn, Ni and NO2−, whereas those from the abandoned As-Au mining area are contaminated in Al, Fe, Mn, Ni, Cd and rarely in NO2−. Waters from the latter area, immediately downstream of mine dumps are also contaminated in As. In stream sediments from Sb-Au and As-Au mining areas, Sb (up to 5488 mg kg−1) and As (up to 235 mg kg−1) show a similar behaviour and are mainly associated with the residual fraction. In most stream sediments, the As and Sb are not associated with the oxidizable fraction, while Fe is associated with organic matter, indicating that sulphides (mainly arsenopyrite and pyrite) and sulphosalts containing those metalloids and metal are weathered. Arsenic and Sb are mainly associated with clay minerals (chlorite and mica; vermiculite in stream sediments from old Sb-Au mining areas) and probably also with insoluble Sb phases of stream sediments. In the most contaminated stream sediments, metalloids are also associated with Fe phases (hematite and goethite, and also lepidocrocite in stream sediments from Banjas). Moreover, the most contaminated stream sediments correspond to the most contaminated waters, reflecting the limited capacity of stream sediments to retain metals and metalloids.  相似文献   

19.
An excellent 53Mn-53Cr isochron for bulk CI, CM, CO, CV, CB, and ungrouped C3 chondrites seems to suggest that each carbonaceous chondrite group acquired its Mn/Cr ratio 4568 ± 1 Myr ago. This age is indistinguishable from the age of Ca-Al-rich inclusions (CAIs), which is considered to be the start of the solar system (t0). However, carbonaceous chondrites were not assembled until at least 1.5-5 Myr after t0, to judge by the 207Pb-206Pb and 26Al-26Mg ages of the chondrules within them, and by the fact that they were not melted by heat from the decay of 26Al. Presumably, therefore, these meteorites inherited their bulk Mn-Cr isochron from precursor materials which experienced Mn-Cr fractionation at t0. As a possible physical mechanism for how the isochron was established initially, and later inherited by the carbonaceous chondrites, we propose the rapid formation at t0 of planetesimals that were variably depleted in moderately volatile elements, and hence had variably low Mn/Cr. The planetesimals and the undepleted (high Mn/Cr) primitive dust from which they were made shared the same initial ε53Cr, and therefore evolved on an isochron. We suggest that later impact-disruption of the planetesimals produced dusty debris, which became mixed, in various proportions, with unprocessed (high Mn/Cr) dust before accreting to the carbonaceous chondrite parent bodies. With mixing in a closed system, the isochron was unchanged. We infer that some debris-rich material was converted to chondrules prior to accretion. The chondrules could have been formed by flash melting of the mixed dust, or could instead have been made directly by the impact splashing of molten planetesimals, or by condensation from impact-generated vapor plumes.  相似文献   

20.
Precise U-Pb ages, determined with double spike (202Pb-205Pb) thermal ionization m1ass spectrometry, are reported for angrites Angra dos Reis (AdoR), Lewis Cliff 86010 (LEW), and D’Orbigny. Nineteen of 23 acid-washed pyroxene fractions from these meteorites and whole rock fractions from D’Orbigny contain between 0.5 and 1.3 pg of total common Pb, indistinguishable from analytical blank. Measured 206Pb/204Pb ratios in these fractions are between 6300 and 14,100 for AdoR, 1160-4500 for LEW, and 608-8500 for D’Orbigny. Blank-corrected 206Pb/204Pb ratios for all three meteorites vary from 2160 to over 100,000. These fractions yielded precise and reproducible 207Pb∗/206Pb∗ dates with the average values of 4557.65 ± 0.13 Ma for AdoR, 4558.55 ± 0.15 Ma for LEW, and 4564.42 ± 0.12 Ma for D’Orbigny. Pb-Pb isochrons including data with slightly elevated common Pb, and U-Pb upper concordia intercepts, yield similar dates. The implications of these new Pb-isotopic ages of angrites are threefold. First, they demonstrate that AdoR and LEW are not coeval, and the group of “slowly cooled” angrites is therefore genetically diverse. Second, the new age of LEW suggests an upward revision of 53Mn-53Cr “absolute” ages by 0.7 Ma. Third, a precise age of D’Orbigny allows consistent linking of the 53Mn-53Cr and 26Al-26Mg extinct nuclide chronometers to the absolute lime scale.  相似文献   

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