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1.
The behavior of Na and K during evaporation from chondrule composition melts was studied using a vacuum furnace. Though Na is the less volatile of the two as an element, it is lost more rapidly than K from silicate melts. Mass fractionation of K isotopes was measured by ion microprobe and Rayleigh fractionation is observed for vacuum evaporation (10−5 atm). With higher pressures of air, the K loss rate decreases but with increasing hydrogen pressure, K is lost more rapidly. δ41K decreases with higher pressures, because of back reaction between melt and K in the gas. With long heating duration, the release of light K condensed within the furnace leads to interaction with the K-depleted melt and a further reduction of δ41K. Natural chondrules differ in some ways from our experimental residues. Some (especially type IIA) have superchondritic Na and K, despite their assumed formation in nebular hydrogen, which enhances volatile loss, and chondrules do not show K isotopic fractionation. Type I chondrules in Semarkona (LL3.0) either plot on our evaporation trend, or are depleted in K but slightly enriched in Na, relative to K. In Bishunpur (LL3.1), type I chondrules are mostly K-depleted but moderately to strongly enriched in Na. In petrologic type 3.2 to 3.4 chondrites they are enriched in both K and Na, like type II chondrules. The alkali contents suggest type I chondrules experienced evaporation and subsequent metasomatism. Their normal δ41K values suggest closed-system evaporation of a chondritic precursor in a gas with relatively high K pressures due to vaporization of dust accompanying chondrule precursor aggregates. Type II chondrules are volatile-rich, as well as unfractionated in K isotopes. They probably formed in a gas with higher pK than in the case of type I chondrules, due to heating of a more dust-rich parcel of gas.  相似文献   

2.
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   

3.
The sulfur isotopic compositions of putative primary troilite grains within 15 ferromagnesian chondrules (10 FeO-poor and 5 FeO-rich chondrules) in the least metamorphosed ordinary chondrites, Bishunpur and Semarkona, have been measured by ion microprobe. Some troilite grains are located inside metal spherules within chondrules. Since such an occurrence is unlikely to be formed by secondary sulfidization processes in the solar nebula or on parent bodies, those troilites are most likely primary, having survived chondrule-forming high-temperature events. If they are primary, they may be the residues of evaporation at high temperatures during chondrule formation and may have recorded mass-dependent isotopic fractionations. However, the supposed primary troilites measured in this study do not show any significant sulfur isotopic fractionations (<1 ‰/amu) relative to large troilite grains in matrix. Among other chondrule troilites that we measured, only one (BI-CH22) apparently has a small excess of heavy isotopes (2.7 ± 1.4 ‰/amu) consistent with isotopic fractionation during evaporation. All other grains have isotopic fractionations of <1 ‰/amu. Because sulfur is so volatile that evaporation during chondrule formation is probably inevitable, non-Rayleigh evaporation most likely explains the lack of isotopic fractionation in putative primary troilite inside chondrules. Evaporation through the surrounding silicate melt would have suppressed the isotopic fractionation after silicate dust grains melted. At lower temperatures below extensive melting of silicates, a heating rate of >104-106 K/h would be required to avoid a large degree of sulfur isotopic fractionation in the chondrule precursors. This heating rate may provide a new constraint on the chondrule formation processes.  相似文献   

4.
We review the oxygen isotopic compositions of minerals in chondrules and compound objects composed of a chondrule and a refractory inclusion, and bulk oxygen isotopic compositions of chondrules in unequilibrated ordinary, carbonaceous, enstatite, and Kakangari-like chondrites, focusing on data acquired using secondary ion mass-spectrometry and laser fluorination coupled with mass-spectrometry over the last decade. Most ferromagnesian chondrules from primitive (unmetamorphosed) chondrites are isotopically uniform (within 3–4‰ in Δ17O) and depleted in 16O (Δ17O>−7‰) relative to amoeboid olivine aggregates (AOAs) and most calcium–aluminum-rich inclusions (CAIs) (Δ17O<−20‰), suggesting that these classes of objects formed in isotopically distinct gaseous reservoirs, 16O-poor and 16O-rich, respectively. Chondrules uniformly enriched in 16O (Δ17O<−15‰) are exceptionally rare and have been reported only in CH chondrites. Oxygen isotopic heterogeneity in chondrules is mainly due to the presence of relict grains. These appear to consist of chondrules of earlier generations and rare refractory inclusions; with rare exceptions, the relict grains are 16O-enriched relative to chondrule phenocrysts and mesostasis. Within a chondrite group, the magnesium-rich (Type I) chondrules tend to be 16O-enriched relative to the ferrous (Type II) chondrules. Aluminum-rich chondrules in ordinary, enstatite, CR, and CV chondrites are generally 16O-enriched relative to ferromagnesian chondrules. No systematic differences in oxygen isotopic compositions have been found among these chondrule types in CB chondrites. Aluminum-rich chondrules in carbonaceous chondrites often contain relict refractory inclusions. Aluminum-rich chondrules with relict CAIs have heterogeneous oxygen isotopic compositions (Δ17O ranges from −20‰ to 0‰). Aluminum-rich chondrules without relict CAIs are isotopically uniform and have oxygen isotopic compositions similar to, or approaching, those of ferromagnesian chondrules. Phenocrysts and mesostases of the CAI-bearing chondrules show no clear evidence for 16O-enrichment compared to the CAI-free chondrules. Spinel, hibonite, and forsterite of the relict refractory inclusions largely retained their original oxygen isotopic compositions. In contrast, plagioclase and melilite of the relict CAIs experienced melting and 16O-depletion to various degrees, probably due to isotopic exchange with an 16O-poor nebular gas. Several igneous CAIs experienced isotopic exchange with an 16O-poor nebular gas during late-stage remelting in the chondrule-forming region. On a three-isotope diagram, bulk oxygen isotopic compositions of most chondrules in ordinary, enstatite, and carbonaceous chondrites plot above, along, and below the terrestrial fractionation line, respectively. Bulk oxygen isotopic compositions of chondrules in altered and/or metamorphosed chondrites show evidence for mass-dependent fractionation, reflecting either interaction with a gaseous/fluid reservoir on parent asteroids or open-system thermal metamorphism. Bulk oxygen isotopic compositions of chondrules and oxygen isotopic compositions of individual minerals in chondrules and refractory inclusions from primitive chondrites plot along a common line of slope of 1, suggesting that only two major reservoirs (gas and solids) are needed to explain the observed variations. However, there is no requirement that each had a permanently fixed isotopic composition. The absolute (207Pb–206Pb) and relative (27Al–26Mg) chronologies of CAIs and chondrules and the differences in oxygen isotopic compositions of most chondrules (16O-poor) and most refractory inclusions (16O-rich) can be interpreted in terms of isotopic self-shielding during UV photolysis of CO in the initially 16O-rich (Δ17O−25‰) parent molecular cloud or protoplanetary disk. According to these models, the UV photolysis preferentially dissociates C17O and C18O in the parent molecular cloud and in the peripheral zones of the protoplanetary disk. If this process occurs in the stability field of water ice, the released atomic 17O and 18O are incorporated into water ice, while the residual CO gas becomes enriched in 16O. During the earliest stages of evolution of the protoplanetary disk, the inner solar nebula had a solar H2O/CO ratio and was 16O-rich. During this time, AOAs and the 16O-rich CAIs and chondrules formed. Subsequently, the inner solar nebula became H2O- and 16O-depleted, because ice-rich dust particles, which were depleted in 16O, agglomerated outside the snowline (5 AU), drifted rapidly towards the Sun and evaporated. During this time, which may have lasted for 3 Myr, most chondrules and the 16O-depleted igneous CAIs formed. We infer that most chondrules formed from isotopically heterogeneous, but 16O-depleted precursors, and experienced isotopic exchange with an 16O-poor nebular gas during melting. Although the relative roles of the chondrule precursor materials and gas–melt isotopic exchange in establishing oxygen isotopic compositions of chondrules have not been quantified yet, mineralogical, chemical, and isotopic evidence indicate that Type I chondrules may have formed in chemical and isotopic equilibrium with nebular gas of variable isotopic composition. Whether these variations were spatial or temporal are not known yet.  相似文献   

5.
The petrological properties, and O and Al-Mg isotopic compositions of two spinel-bearing chondrules from the Allende CV chondrite were investigated using scanning electron microscopy and secondary ion mass spectrometry. A coarse spinel grain in a barred-olivine (BO) chondrule is less enriched in 16O (Δ17O ∼ −5‰; Δ17O = δ17O - 0.52 δ18O), whereas smaller spinel grains in a plagioclase-rich chondrule member of a compound chondrule are extremely 16O-rich (Δ17O ∼ −17‰) and the spinels have a strongly serrated character. The petrological features and 16O-enrichments of the spinels in the plagioclase-rich chondrule indicate that spinels originating in coarse-grained Ca-Al-rich Inclusions (CAIs) were incorporated into chondrule precursors and survived the chondrule-forming event. The degree of 16O-excesses among minerals within each chondrule is correlated to the crystallization sequences. This evidence suggests that the O isotopic variation among minerals may have resulted from incomplete exchange of O isotopes between 16O-rich chondrule melt and 16O-poor nebular gas. Aqueous alteration also has changed the O-isotope compositions in the mesostasis. The feldspathic mesostasis in the BO chondrule shows a disturbed Mg-Al isochron indicating that the BO chondrule experienced secondary alteration. While plagioclase in the plagioclase-rich chondrule member of the compound chondrule shows slight 26Mg-excesses corresponding to (26Al/27Al)0 = [4.6±4.0(2σ)] × 10−6, nepheline formed by secondary alteration shows no detectable excess. The Al-Mg isotopic system of these chondrules was disturbed by aqueous alteration and thermal metamorphism on the Allende parent body.  相似文献   

6.
U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The 206Pb/204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the 207Pb/204Pb vs 204Pb/204Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed 87Sr/86Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970).The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.  相似文献   

7.
Experiments were conducted under canonical nebular conditions to see whether the chemical compositions of the various chondrule types can be derived from a single CI-like starting material by open-system melting and evaporation. Experimental charges, produced at 1580 °C and PH2 of 1.31×10−5 atm over 1 to 18 hours, consisted of only two phases, porphyritic olivine crystals in glass. Sulfur, metallic-iron and alkalis were completely evaporated in the first minutes of the experiments and subsequently the main evaporating liquid oxides were FeO and SiO2. Olivines from short runs (2-4 hours) have compositions of Fo83-Fo89, as in Type IIA chondrules, while longer experimental runs (12-18 hours) produce ∼Fo99 olivine, similar to Type IA chondrules. The concentration of CaO in both olivine (up to 0.6 wt.%) and glass, and their Mg#, increased with increasing heating duration. Natural chondrules also show increasing CaO with decreasing S, alkalis, FeO and SiO2. The similarities in bulk chemistry, mineralogy and textures between Type IIA and IA chondrules and the experimental charges demonstrate that these chondrules could have formed by the evaporation of CI precursors. The formation of silica-rich chondrules (IIB and IB) by evaporation requires a more pyroxene-rich precursor.Based on the FeO evaporation rates measured here, Type IIA and IA chondrules, were heated for at least ∼0.5 and ∼3.5 h, respectively, if formed at 1580 °C and PH2 of 1.31×10−5 atm. Type II chondrules may have experienced higher cooling-rates and less evaporation than Type I.The experimental charges experienced free evaporation and exhibited heavy isotopic enrichments in silicon, as well as zero concentrations of S, Na and K, which are not observed in natural chondrules. However, experiments on potassium-rich melts at the same pressure but in closed capsules showed less evaporation of K, and less K isotopic mass fractionation, than expected as a function of decreasing cooling rate. Thus the environment in which chondrules formed is as important as the kinetic processes they experienced. If chondrule formation occurred under conditions in which evaporated gases remained in the vicinity of the residual melts, the extent of evaporation would be reduced and back reaction between the gas and the melt could contribute to the suppression of isotopic mass fractionation. Hence chondrule formation could have involved evaporative loss without Rayleigh fractionation. Volatile-rich Type II and volatile-poor Type I chondrules may have formed in domains with high and low chondrule concentrations, and high partial pressures of lithophile elements, respectively.  相似文献   

8.
A correlation of petrography, mineral chemistry and in situ oxygen isotopic compositions of fine-grained olivine from the matrix and of fine- and coarse-grained olivine from accretionary rims around Ca-Al-rich inclusions (CAIs) and chondrules in CV chondrites is used here to constrain the processes that occurred in the solar nebula and on the CV parent asteroid. The accretionary rims around Leoville, Vigarano, and Allende CAIs exhibit a layered structure: the inner layer consists of coarse-grained, forsteritic and 16O-rich olivine (Fa1-40 and Δ17O = −24‰ to −5‰; the higher values are always found in the outer part of the layer and only in the most porous meteorites), whereas the middle and the outer layers contain finer-grained olivines that are more fayalitic and 16O-depleted (Fa15-50 and Δ17O = −18‰ to +1‰). The CV matrices and accretionary rims around chondrules have olivine grains of textures, chemical and isotopic compositions similar to those in the outer layers of accretionary rims around CAIs. There is a correlation between local sample porosity and olivine chemical and isotopic compositions: the more compact regions (the inner accretionary rim layer) have the most MgO- and 16O-rich compositions, whereas the more porous regions (outer rim layers around CAIs, accretionary rims around chondrules, and matrices) have the most MgO- and 16O-poor compositions. In addition, there is a negative correlation of olivine grain size with fayalite contents and Δ17O values. However, not all fine-grained olivines are FeO-rich and 16O-poor; some small (<1 μm in Leoville and 5-10 μm in Vigarano and Allende) ferrous (Fa>20) olivine grains in the outer layers of the CAI accretionary rims and in the matrix show significant enrichments in 16O (Δ17O = −20‰ to −10‰). We infer that the inner layer of the accretionary rims around CAIs and, at least, some olivine grains in the finer portions of accretionary rims and CV matrices formed in an 16O-rich gaseous reservoir, probably in the CAI-forming region. Grains in the outer layers of the CAI accretionary rims and in the rims around chondrules as well as matrix may have also originated as 16O-rich olivine. However, these olivines must have exchanged O isotopes to variable extents in the presence of an 16O-poor reservoir, possibly the nebular gas in the chondrule-forming region(s) and/or fluids in the parent body. The observed trend in isotopic compositions may arise from mixtures of 16O-rich forsterites with grain overgrowths or newly formed grains of 16O-poor fayalitic olivines formed during parent body metamorphism. However, the observed correlations of chemical and isotopic compositions of olivine with grain size and local porosity of the host meteorite suggest that olivine accreted as a single population of 16O-rich forsterite and subsequently exchanged Fe-Mg and O isotopes in situ in the presence of aqueous solutions (i.e., fluid-assisted thermal metamorphism).  相似文献   

9.
The properties of ordinary chondrites (OC) reflect both nebular and asteroidal processes. OC are modeled here as having acquired nebular water, probably contained within phyllosilicates, during agglomeration. This component had high Δ17O and acted like an oxidizing agent during thermal metamorphism. The nebular origin of this component is consistent with negative correlations in H, L, and LL chondrites between oxidation state (represented by olivine Fa) and bulk concentration ratios of elements involved in the metal-silicate fractionation (e.g., Ni/Si, Ir/Si, Ir/Mn, Ir/Cr, Ir/Mg, Ni/Mg, As/Mg, Ga/Mg). LL chondrites acquired the greatest abundance of phyllosilicates with high Δ17O among OC (and thus became the most oxidized group and the one with the heaviest O isotopes); H chondrites acquired the lowest abundance, becoming the most reduced OC group with the lightest O isotopes.Chondrule precursors may have grown larger and more ferroan with time in each OC agglomeration zone. Nebular turbulence may have controlled the sizes of chondrule precursors. H-chondrite chondrules (which are the smallest among OC) formed from the smallest precursors. In each OC region, low-FeO chondrules formed before high-FeO chondrules during repeated episodes of chondrule formation.During thermal metamorphism, phyllosilicates were dehydrated; the liberated water oxidized metallic Fe-Ni. This caused correlated changes with petrologic type including decreases in the modal abundance of metal, increases in olivine Fa and low-Ca pyroxene Fs, increases in the olivine/pyroxene ratio, and increases in the kamacite Co and Ni contents. As water (with its heavy O isotopes) was lost during metamorphism, inverse correlations between bulk δ18O and bulk δ17O with petrologic type were produced.The H5 chondrites that were ejected from their parent body ∼7.5 Ma ago during a major impact event probably had been within a few kilometers of each other since they accreted ∼4.5 Ga ago. There are significant differences in the olivine compositional distributions among these rocks; these reflect stochastic nebular sampling of the oxidant (i.e., phyllosilicates with high Δ17O) on a 0.1-1 km scale during agglomeration.  相似文献   

10.
A first order characteristic of the relative abundance of the elements in solar system materials ranging in size from inclusions in primitive meteorites to planetary sized objects such as the Earth and the Moon is that they are very much like that of the Sun for the more refractory elements but systematically depleted to varying degrees in the more volatile elements. This is taken as evidence that evaporation and and/or condensation were important processes in determining the distinctive chemical properties of solar system materials. In some instances there is also isotopic evidence suggesting evaporation in that certain materials are found enriched in the heavy isotopes of their more volatile elements. Here model calculations are used to explore how the relative rates of various key processes determine the relationship between elemental and isotopic fractionation during partial evaporation and partial condensation. The natural measure of time for the systems considered here is the evaporation or condensation timescale defined as the time it would take under the prevailing conditions for evaporation or condensation to completely transfer the element of interest between the two phases of the system. The other timescales considered involve the rate of change of temperature, the rate at which gas is removed from further interaction with the condensed phase, and the rates of diffusion in the condensed and gas phases. The results show that a key determinant of whether or not elemental fractionations have associated isotopic effects is the ratio of the partial pressure of a volatile element (Pi) to its saturation vapor pressure (Pi,sat) over the condensed phase. Systems in which the rate of temperature change or of gas removal are slow compared to the evaporation or condensation timescale will be in the limit Pi ∼ Pi,sat and thus will have little or no isotopic fractionation because at the high temperatures considered here there is negligible equilibrium fractionation of isotopes. If on the other hand the temperature changes are relatively fast, then Pi ≠ Pi,sat and there will be both elemental and isotopic fractionation during partial evaporation or partial condensation. Rapid removal of evolved gas results in Pi ? Pi,sat which will produce isotopically heavy evaporation residues. Diffusion-limited regimes, where transports within a phase are not sufficiently fast to maintain chemical and or isotopic homogeneity, will typically produce less isotopic fractionation than had the phases remained well mixed. The model results are used to suggest a likely explanation for the heavy silicon and magnesium isotopic composition of Type B CAIs (as due to rapid partial melting and subsequent cooling at rates of a few °C per hour), for the uniformity of the potassium isotopic composition of chondrules despite large differences in potassium depletions (as due to volatilization of potassium by reheating in regions of large but variable chondrules per unit volume), and that the remarkable uniformity of the potassium isotopic composition of solar system materials is not a measure of the relative importance of evaporation and condensation but rather due to the solar nebula having evolved sufficiently slowly that materials did not significantly depart from chemical equilibrium.  相似文献   

11.
High-precision Ni isotopic variations are reported for the metal phase of equilibrated and unequilibrated ordinary chondrites, carbonaceous chondrites, iron meteorites, mesosiderites, and pallasites. We also report new Zn and Cu isotopic data for some of these samples and combine them with literature Fe, Cu, and Zn isotope data to constrain the fractionation history of metals during nebular (vapor/solid) and planetary (metal/sulfide/silicate) phase changes.The observed variations of the 62Ni/58Ni, 61Ni/58Ni, and 60Ni/58Ni ratios vary linearly with mass difference and define isotope fractionation lines in common with terrestrial samples. This implies that Ni was derived from a single homogeneous reservoir. While no 60Ni anomaly is detected within the analytical uncertainties, Ni isotopic fractionation up to 0.45‰ per mass-difference unit is observed. The isotope compositions of Ni and Zn in chondrites are positively correlated. We suggest that, in ordinary chondrites, exchange between solid phases, in particular metal and silicates, and vapor followed by mineral sorting during accretion are the main processes controlling these isotopic variations. The positive correlation between Ni and Zn isotope compositions contrasts with a negative correlation between Ni (and Zn) and Cu isotope compositions, which, when taken together, do not favor a simple kinetic interpretation. The observed transition element similarities between different groups of chondrites and iron meteorites are consistent with the genetic relationships inferred from oxygen isotopes (IIIA/pallasites and IVA/L chondrites). Copper is an exception, which we suggest may be related to separate processing of sulfides either in the vapor or during core formation.  相似文献   

12.
We report in situ ion microprobe analyses of oxygen isotopic compositions of olivine, low-Ca pyroxene, high-Ca pyroxene, anorthitic plagioclase, glassy mesostasis, and spinel in five aluminum-rich chondrules and nine ferromagnesian chondrules from the CR carbonaceous chondrites EET92042, GRA95229, and MAC87320. Ferromagnesian chondrules are isotopically homogeneous within ±2‰ in Δ17O; the interchondrule variations in Δ17O range from 0 to −5‰. Small oxygen isotopic heterogeneities found in two ferromagnesian chondrules are due to the presence of relict olivine grains. In contrast, two out of five aluminum-rich chondrules are isotopically heterogeneous with Δ17O values ranging from −6 to −15‰ and from −2 to −11‰, respectively. This isotopic heterogeneity is due to the presence of 16O-enriched spinel and anorthite (Δ17O = −10 to −15‰), which are relict phases of Ca,Al-rich inclusions (CAIs) incorporated into chondrule precursors and incompletely melted during chondrule formation. These observations and the high abundance of relict CAIs in the aluminum-rich chondrules suggest a close genetic relationship between these objects: aluminum-rich chondrules formed by melting of spinel-anorthite-pyroxene CAIs mixed with ferromagnesian precursors compositionally similar to magnesium-rich (Type I) chondrules. The aluminum-rich chondrules without relict CAIs have oxygen isotopic compositions (Δ17O = −2 to −8‰) similar to those of ferromagnesian chondrules. In contrast to the aluminum-rich chondrules from ordinary chondrites, those from CRs plot on a three-oxygen isotope diagram along the carbonaceous chondrite anhydrous mineral line and form a continuum with amoeboid olivine aggregates and CAIs from CRs. We conclude that oxygen isotope compositions of chondrules resulted from two processes: homogenization of isotopically heterogeneous materials during chondrule melting and oxygen isotopic exchange between chondrule melt and 16O-poor nebular gas.  相似文献   

13.
It has been recently suggested that (1) CH chondrites and the CBb/CH-like chondrite Isheyevo contain two populations of chondrules formed by different processes: (i) magnesian non-porphyritic (cryptocrystalline and barred) chondrules, which are similar to those in the CB chondrites and formed in an impact-generated plume of melt and gas resulted from large-scale asteroidal collision, and (ii) porphyritic chondrules formed by melting of solid precursors in the solar nebula. (2) Porphyritic chondrules in Isheyevo and CH chondrites are different from porphyritic chondrules in other carbonaceous chondrites ( [Krot et al., 2005], [Krot et al., 2008a] and [Krot et al., 2008b]). In order to test these hypotheses, we measured in situ oxygen isotopic compositions of porphyritic (magnesian, Type I and ferroan, Type II) and non-porphyritic (magnesian and ferroan cryptocrystalline) chondrules from Isheyevo and CBb chondrites MAC 02675 and QUE 94627, paired with QUE 94611, using a Cameca ims-1280 ion microprobe.On a three-isotope oxygen diagram (δ17O vs. δ18O), compositions of chondrules measured follow approximately slope-1 line. Data for 19 magnesian cryptocrystalline chondrules from Isheyevo, 24 magnesian cryptocrystalline chondrules and 6 magnesian cryptocrystalline silicate inclusions inside chemically-zoned Fe,Ni-metal condensates from CBb chondrites have nearly identical compositions: Δ17O = −2.2 ± 0.9‰, −2.3 ± 0.6‰ and −2.2 ± 1.0‰ (2σ), respectively. These observations and isotopically light magnesium compositions of cryptocrystalline magnesian chondrules in CBb chondrites (Gounelle et al., 2007) are consistent with their single-stage origin, possibly as gas-melt condensates in an impact-generated plume. In contrast, Δ17O values for 11 Type I and 9 Type II chondrules from Isheyevo range from −5‰ to +4‰ and from −17‰ to +3‰, respectively. In contrast to typical chondrules from carbonaceous chondrites, seven out of 11 Type I chondrules from Isheyevo plot above the terrestrial fractionation line. We conclude that (i) porphyritic chondrules in Isheyevo belong to a unique population of objects, suggesting formation either in a different nebular region or at a different time than chondrules from other carbonaceous chondrites; (ii) Isheyevo, CB and CH chondrites are genetically related meteorites: they contain non-porphyritic chondrules produced during the same highly-energetic event, probably large-scale asteroidal collision; (iii) the differences in mineralogy, petrography, chemical and whole-rock oxygen isotopic compositions between CH and CB chondrites are due to various proportions of the nebular and the impact-produced materials.  相似文献   

14.
We found thirty compound chondrules in two CV3 carbonaceous chondrites. The abundance in each meteorite relative to single chondrules is 29/1846 (1.6%) in Allende and 1/230 (0.4%) in Axtell. We examined petrologic features, major element concentrations and oxygen isotopic compositions. Textural, compositional and isotopic evidence suggests that multiple, different mechanisms are responsible for the formation of compound chondrules.Seven compound chondrules are composed of two conjoined porphyritic chondrules with a blurred boundary. At the boundary region of this type of compounds, a poikilitic texture is commonly observed. This suggests that the two chondrules were melted when they came to be in contact. On the other hand, seventeen compound chondrules consist of two conjoined chondrules with a discrete boundary. The preservation of spherical boundary planes of an earlier-formed chondrule of this type implies that it already solidified before fusing with a later-formed chondrule that was still melted. Six samples out of 17 compound chondrules of this type are composed of two BO chondrules. The BO-BO compound chondrules have a unique textural feature in common: the directions of the barred olivines are mostly parallel between two chondrules. This cannot be explained by a simple collision process and forces another mechanism to be taken into consideration.The remaining six compound chondrules differ from the others; they consist of an earlier-formed chondrule enclosed by a later-formed chondrule. A large FeO enrichment was observed in the later-formed chondrules and the enrichment was much greater than that in the later-formed chondrules of other types of compounds. This is consistent with the relict chondrule model, which envisages that the later-formed chondrule was made by a flash melting of a porous FeO-rich dust clump on an earlier-formed chondrule. The textural evidence of this type of compound shows that the earlier-formed chondrule has melted again to varying degrees at the second heating event. This implies that FeO concentrations in bulk chondrules increases during the second heating event if an earlier-formed chondrule was totally melted together with the FeO-rich dust aggregates.Silicate minerals such as olivine and low-Ca pyroxene in compound chondrules have oxygen isotope compositions similar to those in single chondrules from CV3 chondrites. The oxygen isotope composition of each part of the compound chondrule is basically similar to their chondrule pair, but silicates in some chondrules show varying degrees of 16O-enrichment down to −15‰ in δ18O, while those in their partners have 16O-poor invariable compositions near 0 ‰ in δ18O. This implies that the two chondrules in individual compounds formed in the same environments before they became conjoined and the heterogeneous oxygen isotope compositions in some chondrules resulted from incomplete exchange of oxygen atoms between 16O-rich chondrule melts and 16O-poor nebular gas.  相似文献   

15.
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   

16.
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   

17.
Samples from ten refractory inclusions in Murchison, some of which are splits of inclusions whose mineralogical and petrographic characteristics are known, have been analysed for thirty-six elements by neutron activation. Six inclusions have group II or group III patterns or variants of such patterns. Two inclusions, BB-5 and MUCH-1, have large negative Yb anomalies unaccompanied by correspondingly large negative Eu anomalies. It is possible that the latter condensed originally with group III patterns and preferentially took up Eu in later exchange processes under reducing conditions. One inclusion, SH-2, has heavy REE enrichment factors that increase with the refractoriness of the REE, indicating the presence of an extremely high-temperature, or ultrarefractory, REE condensate, but it also has a heavy REE/light REE ratio that indicates mixing of that component with a lower-temperature REE condensate. The frequency of highly fractionated REE patterns and absence of group I patterns suggest that refractory inclusions in Murchison stopped equilibrating with the nebular gas at higher temperatures than most Allende coarse-grained inclusions. The lower Ir/Os and Ru/Re ratios of some Murchison inclusions compared to those of Allende coarse-grained inclusions indicate that condensate alloys that contributed noble metals to the former also stopped equilibrating with the nebular gas at higher temperatures than those that contributed noble metals to the latter. Murchison inclusions tend to be lower in non-refractory elements than Allende coarse-grained inclusions, suggesting that, on average, the former underwent less severe secondary alteration than the latter.  相似文献   

18.
We report the oxygen-isotope compositions of relict and host olivine grains in six high-FeO porphyritic olivine chondrules in one of the most primitive carbonaceous chondrites, CO3.0 Yamato 81020. Because the relict grains predate the host phenocrysts, microscale in situ analyses of O-isotope compositions can help assess the degree of heterogeneity among chondrule precursors and constrain the nebular processes that caused these isotopic differences. In five of six chondrules studied, the Δ17O (=δ17O −0.52 · δ18O) compositions of host phenocrysts are higher than those in low-FeO relict grains; the one exception is for a chondrule with a moderately high-FeO relict. Both the fayalite compositions as well as the O-isotope data support the view that the low-FeO relict grains formed in a previous generation of low-FeO porphyritic chondrules that were subsequently fragmented. It appears that most low-FeO porphyritic chondrules formed earlier than most high-FeO porphyritic chondrules, although there were probably some low-FeO chondrules that formed during the period when most high-FeO chondrules were forming.  相似文献   

19.
Isotopic fractionation of Cu in tektites   总被引：1，自引：0，他引：1  
Tektites are terrestrial natural glasses of up to a few centimeters in size that were produced during hypervelocity impacts on the Earth’s surface. It is well established that the chemical and isotopic composition of tektites is generally identical to that of the upper terrestrial continental crust. Tektites typically have very low water content, which has generally been explained by volatilization at high temperature; however, the exact mechanism is still debated. Because volatilization can fractionate isotopes, comparing the isotopic composition of volatile elements in tektites with those of their source rocks may help to understand the physical conditions during tektite formation.Interestingly, volatile chalcophile elements (e.g., Cd and Zn) seem to be the only elements for which isotopic fractionation is known so far in tektites. Here, we extend this study to Cu, another volatile chalcophile element. We have measured the Cu isotopic composition for 20 tektite samples from the four known different strewn fields. All of the tektites (except the Muong Nong-types) are enriched in the heavy isotopes of Cu (1.98 < δ65Cu < 6.99) in comparison to the terrestrial crust (δ65Cu ≈ 0) with no clear distinction between the different groups. The Muong Nong-type tektites and a Libyan Desert Glass sample are not fractionated (δ65Cu ≈ 0) in comparison to the terrestrial crust. To refine the Cu isotopic composition of the terrestrial crust, we also present data for three geological reference materials (δ65Cu ≈ 0).An increase of δ65Cu with decreasing Cu abundance probably reflects that the isotopic fractionation occurred by evaporation during heating. A simple Rayleigh distillation cannot explain the Cu isotopic data and we suggest that the isotopic fractionation is governed by a diffusion-limited regime. Copper is isotopically more fractionated than the more volatile element Zn (δ66/64Zn up to 2.49‰). This difference of behavior between Cu and Zn is predicted in a diffusion-limited regime, where the magnitude of the isotopic fractionation is regulated by the competition between the evaporative flux and the diffusive flux at the diffusion boundary layer. Due to the difference of ionic charge in silicates (Zn2+ vs. Cu+), Cu has a diffusion coefficient that is larger than that of Zn by at least two orders of magnitude. Therefore, the larger isotopic fractionation in Cu than in Zn in tektites is due to the significant difference in their respective chemical diffusivity.  相似文献   

20.
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   

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