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1.
Abstract— Nitrogen and noble gas isotopic compositions and C abundance of ureilites were analyzed using a stepwise combustion technique. Four Antarctic ureilites, ALHA77257, Asuka 881931, Yamato 791538 and Yamato 790981 were analyzed. Multiple N isotopic components were observed in these ureilites. The δ15N values of these N components ranged from +160 to ?120%. The minimum δ15N values of typically ?120% were observed at combustion temperatures at 700–900 °C where large amounts of C were released. A heavy N component was observed in only two ureilites, ALHA77257 and Asuka 881931. Silicate-enriched fractions and C-concentrated fractions were prepared for these two ureilites. We conclude that both the light N and the heavy N are trapped in the carbonaceous vein minerals. The lack of correction between the N/C ratio and the 36Ar/C ratio suggests that the primary carrier phase of the light N does not correspond to that of the planetary noble gases. We consider that the isotopically heavy N, which was observed in this study, is related to the heavy N observed among polymict ureilites. Small amounts (<0.5 ppm) of light N with the minimum δ15N value of ?120% were observed among the silicate fractions at the highest combustion temperature of 1200 °C, although the exact carrier phase of this light N is not known. We consider that the currently observed ureilites were produced by injection of several volatile-rich objects into volatile-poor ureilitic silicates.  相似文献   

2.
This work is the first detailed study of carbon phases in the ureilite Almahata Sitta (sample #7). We present microRaman data for diamond and graphite in Almahata Sitta, seven unbrecciated ureilites, and two brecciated ureilites. Diamond in Almahata Sitta was found to be distinct from that in unbrecciated and brecciated ureilites, although diamond in unbrecciated and brecciated ureilites is indistinguishable. Almahata Sitta diamond shows a peak center range of 1318.5–1330.2 cm?1 and a full width at half maximum (FWHM) range of 6.6–17.4 cm?1, representing a shock pressure of at least 60 kbar. The actual peak shock pressure may be higher than this due to postshock annealing, if shock synthesis is the source of ureilite diamonds. Diamond in unbrecciated and brecciated ureilites have peak center wave numbers closer to terrestrial kimberlite diamond, but show a wider range of FWHM than Almahata Sitta. The larger peak shift observed in Almahata Sitta may indicate the presence of lonsdaleite. Alternatively, the lower values in brecciated ureilites may be evidence of an annealing step either following the initial diamond‐generating shock or as a consequence of heating during reconsolidation of the breccia. Graphite in Almahata Sitta shows a G‐band peak center range of 1569.1–1577.1 cm?1 and a G‐band FWHM range of 24.3–41.6 cm?1 representing a formation temperature of 990 ± 120 °C. Amorphous carbon was also found. We examine the different theories for diamond formation in ureilites, such as chemical vapor deposition and shock origin from graphite, and explore explanations for the differences between Almahata Sitta and other ureilites.  相似文献   

3.
Angrites are a small group of ancient basaltic achondrites, notable for their unusual chemistry and extreme volatile depletion. No comprehensive study of indigenous light elements currently exists for the group. Measurement of the abundances and isotopic composition of carbon and nitrogen could provide information pertaining to the evolution of the angrite parent body. Bulk‐sample stepped combustion analyses of five angrites and a glass separate from D'Orbigny were combined with earlier data and acid dissolution experiments of carbonates found in D'Orbigny to compile an inventory of indigenous carbon and nitrogen. Indigenous carbon combusted between 700 °C and 1200 °C, with abundances of 10–140 ppm and a mass‐weighted δ13C of ?25 to ?20‰ with the exception of D'Orbigny (δ13C approximately ?5‰). Nitrogen was released at 850–1200 ºC, 1–20 ppm with a δ15N ?3‰ to +4‰; again, D'Orbigny (δ15N approximately +20 to +25‰) was an exception. We interpret these components as largely indigenous and decoupled; the carbon in graphitic or amorphous form, while the nitrogen is present as a dissolved component in the silicates. No relationship with the textural sub‐classification of angrites is apparent. We suggest that the angrite parent body contains a reservoir of reduced carbon and thus may have undergone a change in redox conditions, although the timing and mechanism for this remain unclear.  相似文献   

4.
Abstract— Nitrogen and noble gases were measured in a bulk sample and in acid‐resistant carbon‐rich residues of the ureilite Allan Hills (ALH) 78019 which has experienced low shock and is free of diamond. A small amount of amorphous carbon combusting at ≤500 °C carries most of the noble gases, while the major carbon phase consisting of large crystals of graphite combusts at ≥800 °C, and is almost noble‐gas free. Nitrogen on the other hand is present in both amorphous carbon and graphite, with different δ15N signatures of ?21%o and +19%o, respectively, distinctly different from the very light nitrogen (about ?100%o) of ureilite diamond. Amorphous carbon in ALH 78019 behaves similar to phase Q of chondrites with respect to noble gas release pattern, behavior towards oxidizing acids as well as nitrogen isotopic composition. In situ conversion of amorphous carbon or graphite to diamond through shock would require an isotopic fractionation of 8 to 12% for nitrogen favoring the light isotope, an unlikely proposition, posing a severe problem for the widely accepted shock origin of ureilite diamond.  相似文献   

5.
Abstract— We report nitrogen isotopic data obtained from a stepwise gas release of two grain-size fractions of the gas-rich meteorite Pesyanoe. Cosmic-ray-produced 15Nc may be present in all temperature steps ≥600 °C, and we correct this component using spallation 21Ne data. The resulting ratios reveal the presence of more than one trapped N component. Indigenous N is released above 1000 °C with an isotopic signature of δ15N = ?33‰. This is consistent with the rather uniform signatures of indigenous nitrogen in enstatite meteorites. There is no evidence for the presence of “very light” N of δ15N ? ?200‰. On the other hand, a “heavy” nitrogen component appears in the temperature range 700–800 °C, and coincides with a major release of solar-type noble gases. For a two-component mixture, the isotopic shifts in this temperature range define a lower limit δ15Ncorr = ?6‰ for the second component (e.g., solar-type nitrogen). However, for the case of a solar-type component, the calculated δ15N signature depends on the adopted elemental abundances. For example, adoption of the relative abundances of 14N and noble gases in lunar ilmenite 71501 yields δ15N ? +170, which is in the range of the heavier nitrogen signatures observed on the lunar surface.  相似文献   

6.
Abstract— Polymict ureilites contain various mineral and lithic clasts not observed in monomict ureilites, including plagioclase, enstatite, feldspathic melt clasts and dark inclusions. This paper investigates the microdistributions and petrogenetic implications of rare earth elements (REEs) in three polymict ureilites (Elephant Moraine (EET) 83309, EET 87720 and North Haig), focusing particularly on the mineral and lithic clasts not found in monomict ureilites. As in monomict ureilites, olivine and pyroxene are the major heavy (H)REE carriers in polymict ureilites. They have light (L)REE‐depleted patterns with little variation in REE abundances, despite large differences in major element compositions. The textural and REE characteristics of feldspathic melt clasts in the three polymict ureilites indicate that they are most likely shocked melt that sampled the basaltic components associated with ureilites on their parent body. Simple REE modeling shows that the most common melt clasts in polymict ureilites can be produced by 20–30% partial melting of chondritic material, leaving behind a ureilitic residue. The plagioclase clasts, as well as some of the high‐Ca pyroxene grains, probably represent plagioclase‐pyroxene rock types on the ureilite parent body. However, the variety of REE patterns in both plagioclase and melt clasts cannot be the result of a single igneous differentiation event. Multiple processes, probably including shock melting and different sources, are required to account for all the REE characteristics observed in lithic and mineral clasts. The C‐rich matrix in polymict ureilites is LREE‐enriched, like that in monomict ureilites. The occurrence of Ce anomalies in C‐rich matrix, dark inclusions and the presence of the hydration product, iddingsite, imply significant terrestrial weathering. A search for 26Mg excesses, from the radioactive decay of 26Al, in the polymict ureilite EET 83309 was negative.  相似文献   

7.
Abstract— For most elements, polymict ureilite EET83309 shows no significant compositional difference from other ureilites, including ordinary (“monomict”) ureilites. Polymict ureilites appear to be mixtures of a wide variety of ordinary ureilites, with little dilution by “foreign” extra-ureilitic materials. Thus, they apparently were mixed (i.e., the ureilites in general formed) on a very small number of parent bodies. In one respect, polymict ureilites do stand out. Along with the only other polymict ureilite that has been analyzed for REE (Nilpena), EET83309 has much higher concentrations of light-middle REE than most ordinary ureilites. Despite these relative enrichments in LREE, polymict ureilites are nearly devoid of basaltic (Al-rich) material. A basaltic component should have formed along with (and presumably above) the ultramafic ureilites, in any closed-system differentiation of an originally chondritic asteroid. This scarcity of complementary basaltic materials may be an important clue to ureilite origins. We suggest that ureilites originated as paracumulates (mushy, cumulate-like, partial melt residues) deep within a primordially-heated asteroid or asteroids. While still largely molten, the asteroid was severely disrupted, and most of its external basaltic portion was permanently blown away, by impact of a large, C-rich projectile. This partially-disruptive impact tended to permeate the paracumulates with C-rich, noble-gas-rich, and 16O-rich magma derived mainly from shock-melting of the projectile. After reaccumulation and cooling, the resultant mixtures of cumulus mafic silicates with essentially “foreign” C-matrix became “monomict” ureilites. Further small impacts produced polymict ureilites as components of a newly-developed, basalt-poor megaregolith. The consistently moderate pyroxene/olivine ratios of the ureilites are as expected for partial melt residues, but not for cumulate (sensu stricto) rocks. The final projectile/target mixing ratio tended to be greatest among the more magnesian and pyroxene-rich portions of the paracumulate, because these portions were lowest in density, and thus concentrated toward the upper surface of the paracumulate layer. As a result, ureilites show correlations among C, Δ17O, and silicate-core mg. This model appears to reconcile many paradoxical aspects of ureilite composition (primitive, near-chondritic, except depleted in basalt, diverse Δ17O) and petrography (igneous, cumulate-like).  相似文献   

8.
Abstract— Isotopic analysis of nesquehonite recovered from the surface of the LEW 85320 H5 ordinary chondrite shows that the δ13C and δ18O values of the two generations of bicarbonate (Antarctic and Texas) are different: δ13C = +7.9‰ and +4.2‰; δ18O = +17.9‰ and + 12.1‰ respectively. Carbon isotopic compositions are consistent with equilibrium formation from atmospheric carbon dioxide at ?2 ± 4 °C (Antarctic) and +16 ± 4 °C (Texas). Oxygen isotopic data imply that the water required for nesquehonite precipitation was derived from atmospheric water vapour or glacial meltwater which had locally exchanged with silicates, either in the meteorite or in underlying bedrock. Although carbonates with similar δ13C values have been identified in the SNC meteorites EETA 79001 and Nakhla, petrographic and temperature constraints argue against their simply being terrestrial weathering products.  相似文献   

9.
We report the bulk C abundances, and C and O isotopic compositions of carbonates in 64 CM chondrites, 14 CR chondrites, 2 CI chondrites, LEW 85332 (C2), Kaba (CV3), and Semarkona (LL3.0). For the unheated CMs, the total ranges of carbonate isotopic compositions are δ13C ≈ 25–75‰ and δ18O ≈ 15–35‰, and bulk carbonate C contents range from 0.03 to 0.60 wt%. There is no simple correlation between carbonate abundance and isotopic composition, or between either of these parameters and the extent of alteration. Unless accretion was very heterogeneous, the uncorrelated variations in extent of alteration and carbonate abundance suggests that there was a period of open system behavior in the CM parent body, probably prior to or at the start of aqueous alteration. Most of the ranges in CM carbonate isotopic compositions can be explained by their formation at different temperatures (0–130 °C) from a single fluid in which the carbonate O isotopes were controlled by equilibrium with water (δ18O ≈ 5‰) and the C isotopes were controlled by equilibrium with CO and/or CH4 (δ13C ≈ ?33‰ or ?20‰ for CO‐ or CH4‐dominated systems, respectively). However, carbonate formation would have to have been inefficient, otherwise carbonate compositions would have resembled those of the starting fluid. A quite similar fluid composition (δ18O ≈ ?5.5‰, and δ13C ≈ ?31‰ or ?17‰ for CO‐ or CH4‐dominated systems, respectively) can explain the carbonate compositions of the CIs, although the formation temperatures would have been lower (~10–40 °C) and the relative abundances of calcite and dolomite may play a more important role in determining bulk carbonate compositions than in the CMs. The CR carbonates exhibit a similar range of O isotopes, but an almost bimodal distribution of C isotopes between more (δ13C ≈ 65–80‰) and less altered samples (δ13C ≈ 30–40‰). This bimodality can still be explained by precipitation from fluids with the same isotopic composition (δ18O ≈ ?9.25‰, and δ13C ≈ ?21‰ or ?8‰ for CO‐ or CH4‐dominated systems, respectively) if the less altered CRs had higher mole fractions of CO2 in their fluids. Semarkona and Kaba carbonates have some of the lightest C isotopic compositions of the meteorites studied here, probably because they formed at higher temperatures and/or from more CO2‐rich fluids. The fluids responsible for the alteration of chondrites and from which the carbonates formed were almost certainly accreted as ices. By analogy with cometary ices, CO2 and/or CO would have dominated the trapped volatile species in the ices. The chondrites studied are too oxidized for CO‐dominated fluids to have formed in their parent bodies. If CH4 was the dominant C species in the fluids during carbonate formation, it would have to have been generated in the parent bodies from CO and/or CO2 when oxidation of metal by water created high partial pressures of H2. The fact that the chondrite carbonate C/H2O mole ratios are of the order predicted for CO/CO2‐H2O ices that experienced temperatures of >50–100 K suggests that the chondrites formed at radial distances of <4–15 AU.  相似文献   

10.
Abstract— The LEW 88774 ureilite is extraordinarily rich in Ca, Al, and Cr, and mineralogically quite different from other ureilites in that it consists mainly of exsolved pyroxene, olivine, Cr-rich spinel, and C. The presence of coarse exsolved pyroxene in LEW 88774 is unique because pyroxene in most other ureilites is not exsolved. The pyroxene has bulk Wo contents of 15–20 mol% and has coarse exsolution lamellae of augite and low-Ca pyroxene, 50 μm in width. The compositions of the exsolved augite (Ca33.7Mg52.8Fe13.5) and host low-Ca pyroxene (Ca4.4Mg75Fe20.6) show that these exsolution lamellae were equilibrated at 1280 °C. A computer simulation of the cooling rate, obtained by solving the diffusion equation for reproducing the diffusion profile of CaO across the lamellae, suggests that the pyroxene was cooled at 0.01 °C/year until the temperature reached 1160 °C. This cooling rate corresponds to a depth of at least 1 km in the parent body, assuming it was covered by a rock-like material. Therefore, LEW 88774 was held at this high temperature for 1.2 × 104years. The proposed cooling history is consistent with that of other ureilites with coarsegrained unexsolved pigeonites. Lewis Cliff 88774 includes abundant Cr-rich spinel in comparison with other ureilites. The range of FeO content of spinels in LEW 88774 is from 1.3 wt% to 21 wt% [Fe/(Fe + Mg) = 0.04–0.6]. The Cr-rich and Fe-poor spinel in LEW 88774 has less Fe (FeO, 1.3 wt%) than spinels in other achondrites. We classify this spinel as an Fe, Al-bearing picrochromite. Most ureilites are depleted in Ca and Al, but this meteorite has high-Ca and Al concentrations. In this respect, as well as mineral assemblage and the presence of coarse exsolution lamellae in pyroxene, LEW 88774 is a unique ureilite. Most differentiated meteorites are poor in volatile elements such as Zn, but the LEW 88774 spinels contain abundant Zn (up to 0.6 wt%). We note that such a high Zn concentration in spinel has been observed in the carbonaceous chondrites and recrystallized chondrites. This unusual ureilite has more primitive characteristics than most other ureilites.  相似文献   

11.
Abstract— Isotopic variations have been reported for many elements in iron meteorites, with distinct N signatures found in the metal and graphite of IAB irons. In this study, a dozen IAB/IIICD iron meteorites (see Table 1 for new classifications) were analyzed by stepwise pyrolysis to resolve nitrogen components. Although isotopic heterogeneity has been presumed to be lost in thermally processed parent objects, the high‐resolution nitrogen isotopic data indicate otherwise. At least one reservoir has a light nitrogen signature, δ15N = ?(74 ± 2)‰, at 900 °C to 1000 °C, with a possible second, even lighter, reservoir in Copiapo (δ15N ≤ ?82‰). These releases are consistent with metal nitride decomposition or low‐temperature metal phase changes. Heavier nitrogen reservoirs are observed in steps ≤700 °C and at 1200 °C to 1400 °C. The latter release has a δ15N signature with a limit of ≥?16‰. Xenon isotopic signatures are sensitive indicators for the presence of inclusions because of the very low abundances of Xe in metal. The combined high‐temperature release shows 131Xe and 129Xe excesses to be consistent with shifts expected for Te(n,γ) reaction in troilite by epithermal neutrons, but there are also possible alterations in the isotopic ratios likely due to extinct 129I and cosmic‐ray spallation. The IAB/IIICD iron data imply that at least one light N component survived the formation processes of iron parent objects which only partially exchanged nitrogen between phases. Preservation of separate N reservoirs conflicts with neither the model of impact‐heating effects for these meteorites nor reported age differences between metal and silicates.  相似文献   

12.
The bulk chlorine concentrations and isotopic compositions of a suite of non‐carbonaceous (NC) and carbonaceous (CC) iron meteorites were measured using gas source mass spectrometry. The δ37Cl values of magmatic irons range from ?7.2 to 18.0‰ versus standard mean ocean chloride and are unrelated to their chlorine concentrations, which range from 0.3 to 161 ppm. Nonmagmatic IAB irons are comparatively Cl‐rich containing >161 ppm with δ37Cl values ranging from ?6.1 to ?3.2‰. The anomalously high and low δ37Cl values are inconsistent with a terrestrial source, and as Cl contents in magmatic irons are largely consistent with derivation from a chondrite‐like silicate complement, we suggest that Cl is indigenous to iron meteorites. Two NC irons, Cape York and Gibeon, have high cooling rates with anomalously high δ37Cl values of 13.4 and 18.0‰. We interpret these high isotopic compositions to result from Cl degassing during the disruption of their parent bodies, consistent with their low volatile contents (Ga, Ge, Ag). As no relevant mechanisms in iron meteorite parent bodies are expected to decrease δ37Cl values, whereas volatilization is known to increase δ37Cl values by the preferential loss of light isotopes, we interpret the low isotope values of <?5‰ and down to ?7.2‰ to most closely represent the primordial isotopic composition of Cl in the solar nebula. Similar conclusions have been derived from low δ37Cl values down to ?6, and ?3.8‰ measured in Martian and Vestan meteorites, respectively. These low δ37Cl values are in contrast to those of chondrites which average around 0‰ previously explained by the incorporation of isotopically heavy HCl clathrate into chondrite parent bodies. The poor retention of low δ37Cl values in many differentiated planetary materials suggest that extensive devolatilization occurred during planet formation, which can explain Earth's high δ37Cl value by the loss of approximately 60% of the initial Cl content.  相似文献   

13.
Ureilite meteorites are abundant, carbon‐rich, primitive achondrites made of coarse‐grained, equilibrated olivine and pyroxene (usually pigeonite). They probably sample the baked, heterogeneous, melt‐depleted mantle of a large, once‐chondritic parent body that was broken up catastrophically while still young and hot. Heterogeneity in the parent body is inferred from a considerable “slope‐1” variation from one meteorite to another in oxygen isotopes (?2.5‰ < Δ17O < ?0.2‰), which correlates with both molar FeO/MgO (range 0.03–0.35) and molar FeO/MnO (range 3–57), i.e., Δ17O correlates with the redox state. No consensus has yet emerged on the cause of these correlated trends. One view favors their inheritance via silicates from hot nebular (preaccretion) processes. Another invokes smelting (reduction of FeO by C in the hot parent body). Here, guided mainly by similar trends among equilibrated ordinary and R chondrites, studies of their unequilibrated counterparts, and work on other primitive achondrites, we propose a new model for ureilites in which the parent body accreted nebular ice with high‐?17O, Mg‐rich silicates with low ?17O, and varying amounts of metallic iron. Water from the thawing ice then oxidized the metal yielding secondary FeO‐bearing minerals with high ?17O that, with metamorphism, became incorporated into the ureilite silicates. FeO/MgO, FeO/MnO, and ?17O correlate because they rose in unison by amounts that varied spatially, depending on the local amount of metal that was oxidized. We suggest that the parent body was so large (radius ? 100 km) that smelting was inhibited and that carbon played a passive role in ureilite evolution. Although ureilites are regarded as complicated meteorites, we believe our analysis explains their mass‐independent oxygen isotope trend and related FeO variation through well‐understood processes and enlightens our understanding of the evolution of early planetesimals from cold, wet bodies to hot, dry ones.  相似文献   

14.
Abstract— We have studied the carbon and nitrogen stable isotope geochemistry of a small pristine sample of the Tagish Lake carbonaceous chondrite by high‐resolution stepped‐combustion mass spectrometry, and compared the results with data from the Orgueil (CI1), Elephant Moraine (EET) 83334 (CM1) and Murchison (CM2) chondrites. The small chip of Tagish Lake analysed herein had a higher carbon abundance (5.81 wt%) than any other chondrite, and a nitrogen content (?1220 ppm) between that of CI1 and CM2 chondrites. Owing to the heterogeneous nature of the meteorite, the measured carbon abundance might be artificially high: the carbon inventory and whole‐rock carbon isotopic composition (δ13C ? +24.4%o) of the chip was dominated by 13C‐enriched carbon from the decomposition of carbonates (between 1.29 and 2.69 wt%; δ13C ? +67%o and δ18O ? +35%o, in the proportions ?4:1 dolomite to calcite). In addition to carbonates, Tagish Lake contains organic carbon (?2.6 wt%, δ13C ? ?9%o; 1033 ppm N, δ15N ? +77%o), a level intermediate between CI and CM chondrites. Around 2% of the organic material is thermally labile and solvent soluble. A further ?18% of the organic species are liberated by acid hydrolysis. Tagish Lake also contains a complement of presolar grains. It has a higher nanodiamond abundance (approximately 3650–4330 ppm) than other carbonaceous chondrites, along with ?8 ppm silicon carbide. Whilst carbon and nitrogen isotope geochemistry is not diagnostic, the data are consistent with classification of Tagish Lake as a CI2 chondrite.  相似文献   

15.
Lithium isotope and abundance data are reported for Apollo 15 and 17 mare basalts and the LaPaz low‐Ti mare basalt meteorites, along with lithium isotope data for carbonaceous, ordinary, and enstatite chondrites, and chondrules from the Allende CV3 meteorite. Apollo 15 low‐Ti mare basalts have lower Li contents and lower δ7Li (3.8 ± 1.2‰; all uncertainties are 2 standard deviations) than Apollo 17 high‐Ti mare basalts (δ7Li = 5.2 ± 1.2‰), with evolved LaPaz mare basalts having high Li contents, but similar low δ7Li (3.7 ± 0.5‰) to Apollo 15 mare basalts. In low‐Ti mare basalt 15555, the highest concentrations of Li occur in late‐stage tridymite (>20 ppm) and plagioclase (11 ± 3 ppm), with olivine (6.1 ± 3.8 ppm), pyroxene (4.2 ± 1.6 ppm), and ilmenite (0.8 ± 0.7 ppm) having lower Li concentrations. Values of δ7Li in low‐ and high‐Ti mare basalt sources broadly correlate negatively with 18O/16O and positively with 56Fe/54Fe (low‐Ti: δ7Li ≤4‰; δ56Fe ≤0.04‰; δ18O ≥5.7‰; high‐Ti: δ7Li >6‰; δ56Fe >0.18‰; δ18O <5.4‰). Lithium does not appear to have acted as a volatile element during planetary formation, with subequal Li contents in mare basalts compared with terrestrial, martian, or vestan basaltic rocks. Observed Li isotopic fractionations in mare basalts can potentially be explained through large‐degree, high‐temperature igneous differentiation of their source regions. Progressive magma ocean crystallization led to enrichment in Li and δ7Li in late‐stage liquids, probably as a consequence of preferential retention of 7Li and Li in the melt relative to crystallizing solids. Lithium isotopic fractionation has not been observed during extensive differentiation in terrestrial magmatic systems and may only be recognizable during extensive planetary magmatic differentiation under volatile‐poor conditions, as expected for the lunar magma ocean. Our new analyses of chondrites show that they have δ7Li ranging between ?2.5‰ and 4‰. The higher δ7Li in planetary basalts than in the compilation of chondrites (2.1 ± 1.3‰) demonstrates that differentiated planetary basalts are, on average, isotopically heavier than most chondrites.  相似文献   

16.
Abstract— Ureilites are coarse-grained ultramafic rocks whose petrography, mineral chemistry, lithophile element bulk chemistry, and Sm-Nd isotopic systematics suggest that they are highly fractionated igneous rocks and thus are products of common planetary differentiation processes. However, they also have primitive characteristics that are difficult to reconcile with extensive igneous processing. These include high abundances of siderophile elements, planetary-type noble gases, and the oxygen isotopic signature of unequilibrated solar system materials. The incongruity between igneous and primitive features constitutes the most important problem in understanding ureilite petrogenesis. In this review the petrographic, chemical, and isotopic characteristics of ureilites are summarized, and the petrogenetic implications of these characteristics are discussed. The most important constraints on ureilite petrogenesis are: 1) Ureilites have lost a basaltic complement; 2) Ureilites had a two-stage cooling history; 3) Ureilites are probably residues but partly crystallized from melts; 4) Ureilites are derived from a minimum of six reservoirs which were distinct in oxygen isotopic composition and did not equilibrate with one another; 5) A correlation between oxygen isotopic composition and mg ratio was established in ureilite parent material in the solar nebula; 6) If carbon-metal-silicate-CO/CO2 equilibrium was maintained then the mg ratios of ureilites were pressure/depth-dependent; however, if the pressure was sufficiently high (> 100–200 bars) that a CO/CO2 gas phase was not present then carbon and metal could have been at equilibrium with all ureilite mg ratios at the same pressure; 7) Ureilites either lost a low-melting temperature metal fraction or gained a refractory-rich metal component; 8) Primordial noble gases were retained in carbon in ureilites; 9) The ultramafic ureilite assemblage formed at ～4.55 Ga, but Sm-Nd and Rb-Sr isotopic systematics have been subsequently disturbed. Recently proposed models for ureilite petrogenesis are evaluated in terms of how well they satisfy these constraints; no models unequivocally satisfy all of them. Reconciling constraints 5 and 6 requires a large ureilite parent body.  相似文献   

17.
The present study has shown that the dependence of the isotopic composition of nitrogen on the N/C ratio, revealed from the data for bulk samples of meteoritic nanodiamond, can be obtained within the framework of the following model of the composition of populations of nanodiamond grains: (a) initial nanodiamond, i.e., the nanodiamond in the protoplanetary cloud before the accretion of the meteorite parent bodies, was composed mainly of grains of two populations (denoted as CN and CF), the ratio of which changed in meteorites depending on the degree of hydrothermal metamorphism; (b) only the grains of one of these populations (CN) contain volume-bound nitrogen with δ15N = ?350‰; (c) the grains of both populations contain surface-bound nitrogen (δ15N ≡ 0). The calculations revealed the following properties of population grains in this model. (1) The grains of the CN and CF populations are most likely the same in isotopic composition of carbon and heterogeneous in distribution of its isotopes: the central part of grains is enriched with the δ12C isotope relative to the remainder of the grain. While the value of δ13C is ?37.3 ± 1.1‰ for carbon in the central part, it is ?32.8 ± 1.5‰ for the whole volume of the grains. (2) The noble gases of the HL component, specifically Xe-HL, are anomalous in isotopic composition and are most likely contained in the third population of nanodiamond grains (denoted as CHL), the mass fraction of which is negligible relative to that for other grain populations. Only the grains of the CHL population have an undoubtedly presolar origin, while the grains of the other nanodiamond populations could have formed at the early stages of the evolution of the protoplanetary cloud material before the accretion of the meteoritic parent bodies.  相似文献   

18.
We have investigated the H and Cl systematics in apatite from four brecciated lunar meteorites. In Northwest Africa (NWA) 4472, most of the apatites contain ～2000–6000 ppm H2O with δD between ?200 and 0‰, except for one grain isolated in the matrix, which contains ～6000 ppm H2O with δD of ～500–900‰. This low‐δD apatite contains ～2500–7500 ppm Cl associated with δ37Cl of ～15–20‰, while the high‐δD grain contains ～2500 ppm Cl with δ37Cl of ～7–15‰. In NWA 773, apatites in a first group contain ～700–2500 ppm H2O with δD values averaging around ～0 ± 100‰, while apatites in a second group contain ～5500–16500 ppm H2O with δD ～250 ± 50‰. In Sayh al Uhaymir (SaU) 169 and Kalahari (Kal) 009, apatites are similar in terms of their H2O contents (～600–3000 ppm) and δD values (?100 to 200‰). In SaU 169, apatites contain ～6000–10,000 ppm Cl, characterized by δ37Cl of ～5–12‰. Overall, most of the analyzed apatite grains have δD within the range reported for carbonaceous chondrites, similar to apatite analyzed in ancient (>3.9 Ga) lunar magmatic. One grain in NWA 4472 has H and Cl isotope compositions similar to apatite from mare basalts. With an age of 4.35 Ga, this grain could be a representative of the oldest known lunar volcanic activity. Finally, since numerous evolved clasts in NWA 773 formed through silicate liquid immiscibility, the apatite grains with extremely high H2O contents, reaching pure hydroxylapatite composition, could provide insights into the effects of such process on the evolution of volatiles in lunar magmas.  相似文献   

19.
High‐precision Zn isotopic compositions measured by MC‐ICP‐MS are documented for 32 iron meteorites from various fractionally crystallized and silicate‐bearing groups. The δ66Zn values range from ?0.59‰ up to +5.61‰ with most samples being slightly enriched in the heavier isotopes compared with carbonaceous chondrites (0 < δ66Zn < 0.5). The δ66Zn versus δ68Zn plot of all samples defines a common linear fractionation line, which supports the hypothesis that Zn was derived from a single reservoir or from multiple reservoirs linked by mass‐dependent fractionation processes. Our data for Redfields fall on a mass fractionation line and therefore refute a previous claim of it having an anomalous isotopic composition due to nonmixing of nucleosynthetic products. The negative correlation between δ66Zn and the Zn concentration of IAB and IIE is consistent with mass‐dependent isotopic fractionation due to evaporation with preferential loss of lighter isotopes in the vapor phase. Data for the Zn concentrations and isotopic compositions of two IVA samples demonstrate that volatile depletion in the IVA parent body is not likely the result of evaporation. This is important evidence that favors the incomplete condensation origin for the volatile depletion of the IVA parent body.  相似文献   

20.
We present high‐precision measurements of the Mg isotopic compositions of a suite of types I and II chondrules separated from the Murchison and Murray CM2 carbonaceous chondrites. These chondrules are olivine‐ and pyroxene‐rich and have low 27Al/24Mg ratios (0.012–0.316). The Mg isotopic compositions of Murray chondrules are on average lighter (δ26Mg ranging from ?0.95‰ to ?0.15‰ relative to the DSM‐3 standard) than those of Murchison (δ26Mg ranging from ?1.27‰ to +0.77‰). Taken together, the CM2 chondrules exhibit a narrower range of Mg isotopic compositions than those from CV and CB chondrites studied previously. The least‐altered CM2 chondrules are on average lighter (average δ26Mg = ?0.39 ± 0.30‰, 2SE) than the moderately to heavily altered CM2 chondrules (average δ26Mg = ?0.11 ± 0.21‰, 2SE). The compositions of CM2 chondrules are consistent with isotopic fractionation toward heavy Mg being associated with the formation of secondary silicate phases on the CM2 parent body, but were also probably affected by volatilization and recondensation processes involved in their original formation. The low‐Al CM2 chondrules analyzed here do not exhibit any mass‐independent variations in 26Mg from the decay of 26Al, with the exception of two chondrules that show only small variations just outside of the analytical error. In the case of the chondrule with the highest Al/Mg ratio (a type IAB chondrule from Murchison), the lack of resolvable 26Mg excess suggests that it either formed >1 Ma after calcium‐aluminum‐rich inclusions, or that its Al‐Mg isotope systematics were reset by secondary alteration processes on the CM2 chondrite parent body after the decay of 26Al.  相似文献   

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