Stony-iron meteorites: History of the metal phase according to tungsten isotopes

The global composition of the early solar system is thought to be roughly chondritic in terms of refractory components, and this means that metal and silicate should be present together in early planetesimals. To fully understand the metal-silicate differentiation process within the eucrite parent body (EPB), it is important to try and identify the metal reservoir that is complementary to the silicate part. The isotope 182 of tungsten (W), a siderophile element, is partly formed from the decay of ¹⁸²Hf, and W isotopes are useful for examining metal-silicate segregation. The W isotopic composition expected for the metal that is complementary to eucrites falls in the range of iron meteorites. However, mesosiderites seem to be genetically linked to eucrites based on petrologic and oxygen isotopic similarities. Therefore, we undertook the analysis of the metal phase of these stony-irons. Here we present tungsten isotopic data for mesosiderite and pallasite metal to characterize their parent body (bodies) and to assess possible relationships with eucrites.All stony-iron metals are depleted in radiogenic tungsten by −1.3 to −4.2 ε units, relative to the terrestrial standard, while chondrites, for comparison, are depleted by −1.9 ε units. In addition to W isotopic heterogeneity from one stony-iron to another, there is also W isotopic heterogeneity within individual meteorites. A formation model is tentatively proposed, where we show that mesosiderites, pallasites, and eucrites could possibly come from the same parent body. Several hypotheses are discussed to explain the isotopic heterogeneity: the production of cosmogenic tungsten, the in situ decay of hafnium present in inclusions, and tungsten diffusion processes after metal-silicate mixing during the cooling of the meteorites. The two latter hypotheses provide the best explanation of our data.     

Planet formation processes revealed by meteorites

The history of the solar system is locked within the planets, asteroids and other objects that orbit the Sun. While remote observations of these celestial bodies are essential for understanding planetary processes, much of the geological and geochemical information regarding solar system heritage comes directly from the study of rocks and other materials originating from them. The diversity of materials available for study from planetary bodies largely comes from meteorites; fragments of rock that fall through Earth's atmosphere after impact‐extraction from their parent planet or asteroid. These extra‐terrestrial objects are fundamental scientific materials, providing information on past conditions within planets, and on their surfaces, and revealing the timing of key events that affected a planet's evolution. Meteorites can be sub‐divided into four main groups: (1) chondrites, which are unmelted and variably metamorphosed ‘cosmic sediments’ composed of particles that made up the early solar nebula; (2) achondrites, which represent predominantly silicate materials from asteroids and planets that have partially to fully melted, from a broadly chondritic initial composition; (3) iron meteorites, which represent Fe‐Ni samples from the cores of asteroids and planetesimals; and (4) stony‐iron meteorites such as pallasites and mesosiderites, which are mixtures of metal and dominantly basaltic materials. Meteorite studies are rapidly expanding our understanding of how the solar system formed and when and how key events such as planetary accretion and differentiation occurred. Together with a burgeoning collection of classified meteorites, these scientific advances herald an unprecedented period of further scientific challenges and discoveries, an exciting prospect for understanding our origins.     

Hf-W chronometry of primitive achondrites

Metal segregation and silicate melting on asteroids are the most incisive differentiation events in the early evolution of planetary bodies. The timing of these events can be constrained using the short-lived ¹⁸²Hf-¹⁸²W radionuclide system. Here we present new ¹⁸²Hf-¹⁸²W data for major types of primitive achondrites including acapulcoites, winonaites and one lodranite. These meteorites are of particular interest because they show only limited evidence for partial melting of silicates and are therefore intermediate between chondrites and achondrites.For acapulcoites we derived a ¹⁸²Hf-¹⁸²W age of Δt_CAI = 4.1 ^+1.2/_−1.1 Ma. A model age for winonaite separates calculated from the intercept of the isochron defines an age of Δt_CAI = 4.8 ^+3.1/_−2.6 Ma (assuming a bulk Hf/W ratio of ∼1.2). Both ages most likely define primary magmatic events on the respective parent bodies, such as melting of metal, although metal stayed in place and did not segregate to form a core. A later thermal event is responsible for resetting of the winonaite isochron, yielding an age of Δt_CAI = 14.3 ^+2.7/_−2.2 Ma, significantly younger than the model age. Assuming a co-genetic relationship between winonaites and silicates present in IAB iron meteorites (based on oxygen isotope composition) and including data by Schulz et al. (2009), a common parent body chronology can be established. Magmatic activity occurred between ∼1.5 and 5 Ma after CAIs. More than 5 Ma later, intensive thermal metamorphism has redistributed Hf-W. Average cooling rates calculated for the winonaite/IAB parent asteroid range between ∼35 and ∼4 K/Ma, most likely reflecting different burial depths. Cooling rates obtained for acapulcoites were ∼40 K/Ma to ∼720 K and then ∼3 K/Ma to ∼550 K.Accretion and subsequent magmatism on the acapulcoite parent body occurred slightly later if compared to most achondrite parent bodies (e.g., angrites, ureilites and eucrites), in this case supporting the concept of an inverse correlation between accretion-age of asteroids and intensity of heating in their interiors as expected from heating by ²⁶Al and ⁶⁰Fe decay. However, the early accretion of the parent asteroid of primitive IAB silicates (∼1.0 Ma after CAIs; Schulz et al., 2009) and the possibly impact-induced melting-history of winonaites show that this concept is too simplistic. Parent body size, impact-driven melting as well as heat-insulating regolith cover also need to be considered in the early history of asteroid differentiation.     

Spectroscopic evidence for two achondrite parent bodies: asteroids 349 Dembowska and 4 Vesta

A Fourier spectrometer was used to obtain IR spectra of asteroids 349 Dembowska and 4 Vesta (0.8–2.5μm; 25cm ^?1 resolution). The spectrum of Dembowska shows olivine and pyroxene (Fs_{24 ± 8}) with an olivine/pyroxene abundance ratio greater than 2, and possibly as high as 10. This is probably an unsampled achondritic composition, similar to the unique achondrite ALHA 77005. Similar olivine-rich compositions have also been proposed for the mantles of basaltic achondrite parent bodies. Dembowska's mineralogy therefore appears related in some way to the achondrites. but a direct link cannot as yet be made. Our IR observations of Vesta have more complete spectral coverage than those first obtained by Larson and Fink (1975b). Pyroxene (Fs_{50 ± 5}) and plagioclase feldspar are both seen, with a pyroxene/feldspar abundance ratio between 1.5 and 2.0. Vesta's mineralogy is consistent with a mixture of eucrites and howardites on its surface. Time-resolved observations over one-half of the rotation period show no spectral variations at the 2% level, indicating compositional homogeneity. Both 349 Dembowska and 4 Vesta can be considered as candidates for the parent bodies of igneous meteorites.     

Early core formation in asteroids and late accretion of chondrite parent bodies: Evidence from Hf-W in CAIs, metal-rich chondrites, and iron meteorites

The ¹⁸²Hf-¹⁸²W isotopic systematics of Ca-Al-rich inclusions (CAIs), metal-rich chondrites, and iron meteorites were investigated to constrain the relative timing of accretion of their parent asteroids. A regression of the Hf-W data for two bulk CAIs, various fragments of a single CAI, and carbonaceous chondrites constrains the ¹⁸²Hf/¹⁸⁰Hf and ε_W at the time of CAI formation to (1.07 ± 0.10) × 10⁻⁴ and −3.47 ± 0.20, respectively. All magmatic iron meteorites examined here have initial ε_W values that are similar to or slightly lower than the initial value of CAIs. These low ε_W values may in part reflect ¹⁸²W-burnout caused by the prolonged cosmic ray exposure of iron meteorites, but this effect is estimated to be less than ∼0.3 ε units for an exposure age of 600 Ma. The W isotope data, after correction for cosmic ray induced effects, indicate that core formation in the parent asteroids of the magmatic iron meteorites occurred less than ∼1.5 Myr after formation of CAIs. The nonmagmatic IAB-IIICD irons and the metal-rich CB chondrites have more radiogenic W isotope compositions, indicating formation several Myr after the oldest metal cores had segregated in some asteroids.Chondrule formation ∼2-5 Myr after CAIs, as constrained by published Pb-Pb and Al-Mg ages, postdates core formation in planetesimals, and indicates that chondrites do not represent the precursor material from which asteroids accreted and then differentiated. Chondrites instead derive from asteroids that accreted late, either farther from the Sun than the parent bodies of magmatic iron meteorites or by reaccretion of debris produced during collisional disruption of older asteroids. Alternatively, chondrites may represent material from the outermost layers of differentiated asteroids. The early thermal and chemical evolution of asteroids appears to be controlled by the decay of ²⁶Al, which was sufficiently abundant (initial ²⁶Al/²⁷Al >1.4 × 10⁻⁵) to rapidly melt early-formed planetesimals but could not raise the temperatures in the late-formed chondrite parent asteroids high enough to cause differentiation. The preservation of the primitive appearance of chondrites thus at least partially reflects their late formation rather than their early and primitive origin.     

陨石分类研究进展及其地学意义

近40年来陨石分类学经历了3个发展阶段，60－70年代，由根据陨石的矿物结构的分类方法发展为球粒陨石的化学一岩石学分类法和铁陨石的化学群分类法；70－80年代，提出了分异型陨石和未分异型陨石的概念，球粒陨石被认为是未分异型陨石，而其它陨石（铁陨石，石铁陨石和无球粒陨石）大多被划入分异型陨石，80－90年代以来，陨石氧同位素组成成为了陨石成因分类的一个主要依据，使陨石分类学进入了一个新的成因分类阶段，作者对80－90年代以来新确立的R群，K小群，CR群和CK群球粒陨石，以及根据氧同位素划分出的原始型无球粒陨石系列：A－L无球粒陨石，Winonaites无球粒陨石和Brachinites无球粒陨石进行了介绍，笔者对陨石研究和陨石分类学的发展在估算地球整体成分，探讨地球成因和早期演化历史方面的重要意义进行了说明，并建议地球科学家应对陨石学和陨石分类的发展现状给以关注。     

Evidence from polymict ureilite meteorites for a disrupted and re-accreted single ureilite parent asteroid gardened by several distinct impactors

Ureilites are ultramafic achondrites that exhibit heterogeneity in mg# and oxygen isotope ratios between different meteorites. Polymict ureilites represent near-surface material of the ureilite parent asteroid(s). Electron microprobe analyses of >500 olivine and pyroxene clasts in several polymict ureilites reveal a statistically identical range of compositions to that shown by unbrecciated ureilites, suggesting derivation from a single parent asteroid. Many ureilitic clasts have identical compositions to the anomalously high Mn/Mg olivines and pyroxenes from the Hughes 009 unbrecciated ureilite (here termed the “Hughes cluster”). Some polymict samples also contain lithic clasts derived from oxidized impactors. The presence of several common distinctive lithologies within polymict ureilites is additional evidence that ureilites were derived from a single parent asteroid.In situ oxygen three isotope analyses were made on individual ureilite minerals and lithic clasts, using a secondary ion mass spectrometer (SIMS) with precision typically better than 0.2-0.4‰ (2SD) for δ¹⁸O and δ¹⁷O. Oxygen isotope ratios of ureilitic clasts fall on a narrow trend along the CCAM line, covering the range for unbrecciated ureilites, and show a good anti-correlation with mineral mg#. SIMS analysis identifies one ferroan lithic clast as an R-chondrite, while a second ferroan clast is unlike any known meteorite. An exotic enstatite grain is derived from an enstatite chondrite or aubrite, and another pyroxene grain with Δ¹⁷O of −0.4 ± 0.2‰ is unrelated to any known meteorite type.Ureilitic olivine clasts with mg#s < 85 are much more common than those with mg# > 85 which include the melt-inclusion-bearing “Hughes cluster” ureilites. Thus melt was present in regions of the parent ureilite asteroid with a bulk mg# > 85 when the asteroid was disrupted by impact, giving rise to two types of ureilites: common ferroan ones that were residual after melting and less common magnesian ones that were still partially molten when disruption occurred. One or more daughter asteroids re-accreted from the remnants of the mantle of the proto-ureilite asteroid. Polymict ureilite meteorites represent regolith that subsequently formed on the surface of a daughter asteroid, including impact-derived material from at least six different meteoritic sources.     

Comparative stable isotope geochemistry of Ni, Cu, Zn, and Fe in chondrites and iron meteorites

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.     

Mesosiderites—I. Compositions of their metallic portions and possible relationship to other metal-rich meteorite groups

The metal from 17 mesosiderites has been analyzed for Ni, Ga, Ge and Ir by the techniques of atomic-absorption spectrometry and neutron activation. Most mesosiderite metal samples fall in a narrow compositional range: Ni, 7·0–9·0 per cent; Ga, 13–16 ppm; Ge, 47–58 ppm; and Ir, 2·4–4·4 ppm. Most of those falling outside these ranges belong to Powell's (1971) least-metamorphosed type. Mesosiderite metal falls in the same general composition range as IIIAB irons, IIIE irons, pallasites and H-group chondrite metal. There are distinct differences in detail, however, and firm evidence for a close genetic relationship between any of these groups and the mesosiderites is lacking. Metallic portions of Weekeroo-type irons tend to have slightly higher Ni, Ga, Ge and Ir contents than found in mesosiderite metal, and the two groups tend to form a single trend on all plots. The Weekeroo-type silicates closely resemble mesosiderites in terms of orthopyroxene composition and oxygen-isotope ratio. We interpret these similarities to indicate that the silicate and metallic portions of these two groups are closely related; if the mesosiderite silicates and metal were initially formed in separate parent bodies, these were of similar composition and formed at about the same distance from the Sun.     

Oxygen isotope measurements of individual unmelted Antarctic micrometeorites

We present oxygen isotope measurements of 28 unmelted Antarctic micrometeorites measuring 150-250 μm (long axis) collected in the South Pole water well. The micrometeorites were all unmelted and classified as either fine-grained, scoriaceous, coarse-grained or composite (a mix of two other classes). Spot analyses were made of each micrometeorite type using an ion microprobe. The oxygen isotope values were measured relative to standard mean ocean water (SMOW) and range from δ¹⁸O = 3‰ to 60‰ and δ¹⁷O = −1‰ to 32‰, falling along the terrestrial fractionation line (TFL) within 2σ errors. Several analytical spots (comprising multiple phases) were made on each particle. Variability in the oxygen isotope ratios was observed among micrometeorite types, between micrometeorites of the same type and between analytical spots on a single micrometeorite indicating that micrometeorites are isotopically heterogeneous. In general, the lowest isotope values are associated with the coarse-grained micrometeorites whereas most of the fine-grained and scoriaceous micrometeorites have an average δ¹⁸O ? 22‰, suggesting that the matrix in micrometeorites is isotopically heavier than the anhydrous silicate phases. The oxygen isotope values for the coarse-grained micrometeorites, composed mainly of anhydrous phases, do not lie along the carbonaceous chondrite anhydrous mineral (CCAM) line, as observed for olivines, pyroxenes and some kinds of chondrules in carbonaceous chondrites, suggesting that coarse-grained MMs are not related to chondrules, as previously thought. Our measurements span the same range as values found for melted micrometeorites in other studies. Although four of the micrometeorites have oxygen isotope values lying along the TFL, close to the region where the bulk CI carbonaceous chondrites are found, 21 particles have very enriched ¹⁷O and ¹⁸O values that have not been reported in previous analyses of chondrite matrix material, suggesting that they could be a new type of Solar System object. The parent bodies of the micrometeorites with higher ¹⁸O values may be thermal metamorphosed carbonaceous asteroids that have not been found as meteorites either because they are friable asteroids that produce small particles rather than rocks upon collision with other bodies, or because the rocks they produce are too friable to survive atmospheric entry.     

New constraints on early Solar System chronology from Al-Mg and U-Pb isotope systematics in the unique basaltic achondrite Northwest Africa 2976

We report elemental abundances and the isotopic systematics of the short-lived ²⁶Al-²⁶Mg (half-life of ∼0.73 Ma) and long-lived U-Pb radiochronometers in the ungrouped basaltic meteorite Northwest Africa (NWA) 2976. The bulk geochemical composition of NWA 2976 is clearly distinct from that of the eucrites and angrites, but shows broad similarities to that of the paired NWA 001 and 2400 ungrouped achondrites indicating that it is likely to also be paired with these two samples. The major and trace element abundances in NWA 2976 further indicate that it formed by extensive melting and magmatic fractionation processes on its parent body. The Al-Mg and Pb-Pb isotope systematics indicate that this meteorite represents the earliest stages of crust formation on a differentiated parent body in the early Solar System. The absolute Pb-Pb internal isochron age of NWA 2976, obtained from acid leaching residues of three whole-rock samples and two pyroxene separates, is 4562.89 ± 0.59 Ma (MSWD = 0.02). This Pb-Pb age is calculated using the measured ²³⁸U/²³⁵U ratio of a NWA 2976 whole-rock of 137.751 ± 0.018 (2σ) which was determined relative to the recently revised value of 137.840 ± 0.008 for the SRM 950a U isotope standard. The Al-Mg systematics reveal the presence of ²⁶Mg isotopic anomalies produced by the decay of ²⁶Al with an (²⁶Al/²⁷Al)₀ of (3.94 ± 0.16) × 10⁻⁷, and indicate a time of formation of 0.26 ± 0.18 Ma after the D’Orbigny angrite. Using the revised Pb-Pb age of 4563.36 ± 0.34 Ma for the D’Orbigny anchor (corrected for its U isotopic composition), we deduce an Al-Mg model age of 4563.10 ± 0.38 Ma for NWA 2976, which is consistent with its Pb-Pb internal isochron age.The concordance of the Pb-Pb and Al-Mg chronometers, when taking into account the differences in the U isotopic compositions of the D’Orbigny and NWA 2976 achondrites (whose parent bodies likely formed in distinct regions of early Solar System as indicated by their different oxygen isotopic compositions), implies that ²⁶Al was homogeneously distributed in the early Solar System. It also suggests that igneous processes on planetesimals, as represented by the formation of various basaltic meteorite groups that likely originated on distinct parent bodies (e.g., eucrites and angrites, as well as ungrouped achondrites), were widespread throughout the protoplanetary disk within the first ∼5 Ma of the history of the Solar System.     

Silicate-bearing iron meteorites and their implications for the evolution of asteroidal parent bodies

Silicate-bearing iron meteorites differ from other iron meteorites in containing variable amounts of silicates, ranging from minor to stony-iron proportions (∼50%). These irons provide important constraints on the evolution of planetesimals and asteroids, especially with regard to the nature of metal–silicate separation and mixing. I present a review and synthesis of available data, including a compilation and interpretation of host metal trace-element compositions, oxygen-isotope compositions, textures, mineralogy, phase chemistries, and bulk compositions of silicate portions, ages of silicate and metal portions, and thermal histories. Case studies for the petrogeneses of igneous silicate lithologies from different groups are provided. Silicate-bearing irons were formed on multiple parent bodies under different conditions. The IAB/IIICD irons have silicates that are mainly chondritic in composition, but include some igneous lithologies, and were derived from a volatile-rich asteroid that underwent small amounts of silicate partial melting but larger amounts of metallic melting. A large proportion of IIE irons contain fractionated alkali-silica-rich inclusions formed as partial melts of chondrite, although other IIE irons have silicates of chondritic composition. The IIEs were derived from an H-chondrite-like asteroid that experienced more significant melting than the IAB asteroid. The two stony-iron IVAs were derived from an extensively melted and apparently chemically processed L or LL-like asteroid that also produced a metallic core. Ungrouped silicate-bearing irons were derived from seven additional asteroids. Hf–W age data imply that metal–silicate separation occurred within 0–10 Ma of CAI formation for these irons, suggesting internal heating by ²⁶Al. Chronometers were partly re-set at later times, mainly earlier for the IABs and later for the IIEs, including one late (3.60 ± 0.15 Ga) strong impact that affected the “young silicate” IIEs Watson (unfractionated silicate, and probable impact melt), Netschaëvo (unfractionated, and metamorphosed), and Kodaikanal (fractionated). Kodaikanal probably did not undergo differentiation in this late impact, but the similar ages of the “young silicate” IIEs imply that relatively undifferentiated and differentiated materials co-existed on the same asteroid. The thermal histories and petrogeneses of fractionated IIE irons and IVA stony irons are best accommodated by a model of disruption and reassembly of partly molten asteroids.     

Mass-dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles

Sulfur isotope studies of post-Archean terrestrial materials have focused on the ratio ³⁴S/³²S because additional isotopes, ³³S and ³⁶S, were thought to carry little information beyond the well-known mass-dependent relationship among multiple-isotope ratios. We report high-precision analyses of Δ³³S and Δ³⁶S values, defined as deviations of ³³S and ³⁶S from ideal mass-dependent relationships, for international reference materials and sedimentary sulfides of Phanerozoic age by using a fluorination technique with a dual-inlet isotope ratio mass spectrometer. Measured variations in Δ³³S and Δ³⁶S are explained as resulting from processes involve branching reactions (two or more reservoirs formed) or mixing. Irreversible processes in closed systems (Rayleigh distillation) amplify the isotope effect. We outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle, including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation. The isotope systematics discussed here cannot explain the much larger variation of Δ³³S and Δ³⁶S observed in Archean rock records. Furthermore, Phanerozoic samples we have studied show a characteristic Δ³³S and Δ³⁶S relationship that differs from those measured in Archean rocks and laboratory photolysis experiments. Thus, high precision analysis of Δ³³S and Δ³⁶S can be used to distinguish small non-zero Δ³³S and Δ³⁶S produced by mass-dependent processes from those produced by mass-independent processes in Archean rocks and extraterrestrial materials.     

Discovery of Mass Independent Oxygen Isotopic Compositions in Superscale Nitrate Mineral Deposits from Turpan‐Hami Basin,Xinjiang, China and Its Significance

<正>The Turpan-Hami Basin in eastern Xinjiang is one of the driest regions on Earth and a premier environment to form and preserve nitrate.A large nitrate ore field in this basin was found recently.It is estimated there are about 2.5 billion tons of resources of nitrate,and the amount is as much as the Atacama Desert super-scale nitrate deposit in Chile.Nitrate is one of a few minerals with mass-independent fractionation(MIF),and the oxygen isotope MIF is an effective method to determine the source of nitrate.Theδ~(17)O,δ~(18)O of nitrate were measured by fluorination and thermal decomposition method.The date indicated that this is the first time that oxygen isotope MIF has been located in inland nitrate minerals.The results obtained by two methods are similar,⊿~(17)=δ~(17)O-0.52×δ~(18)O=12‰-17‰.The experiment and observation data proved that oxygen isotope MIF of nitrate are the result of photochemical reactions in the troposphere and stratosphere.Thus, evidence from MIF oxygen isotopic compositions indicate that long term atmospheric deposition of nitrate aerosol particles produced by photochemical reactions is the source of the deposits.     

Cadmium stable isotope cosmochemistry

Cadmium stable isotope compositions are reported for a comprehensive suite of carbonaceous, ordinary, enstatite, and Rumuruti chondrites as well as achondrites and lunar samples (soils, breccias, pristine anorthosite). The Cd isotope analyses were performed by multiple collector ICP-MS with an external reproducibility of ±0.43‰ (2 sd) for δ^114/110Cd. None of the samples shows evidence of nucleosynthetic anomalies and cosmogenic isotope effects from neutron-capture by ¹¹³Cd were only observed for two lunar samples.The Cd stable isotope compositions of type 1, 2, and some type 3 carbonaceous chondrites, EH4 enstatite chondrites, eucrites, and the Earth are essentially identical at δ^114/110Cd ≈ 0.0 ± 0.4. This suggests that the portion of the solar nebula from which the inner solar system bodies accreted was homogeneous with respect to its Cd isotope composition. It also indicates that the primary volatile element depletion of the inner solar system did not involve partial kinetic Rayleigh evaporation or condensation. Furthermore no resolvable Cd isotope effects were generated during the accretion and initial differentiation of the planetary bodies.In contrast, the analyses reveal large Cd isotope effects for ordinary and some enstatite chondrites, which display δ^114/110Cd values between about −8 and +16. Smaller fractionations are observed for the Rumuruti and some type 3 to 5 carbonaceous chondrites. These Cd isotope variations are thought to reflect secondary depletion or redistribution of Cd, due to open system thermal metamorphism on the meteorite parent bodies.One CAI and chondrule separates from the Allende meteorite have unexpectedly high Cd concentrations and fractionated light Cd isotope compositions with δ^114/110Cd ≈ −1 to −4. These characteristics may have been established by the interaction of originally Cd-poor materials with a volatile-rich gas prior to the final accretion of the Allende parent body. The general Cd enrichment of the lunar soil and regolith mainly reflects early volcanic activity. However, decreasing Cd abundances in lunar soils correlate well with an enrichment of the heavy Cd isotopes. This relationship is best explained by suppressed Rayleigh fractionation in response to space weathering.     

陨石氧同位素组成及其地学意义

介绍了各类陨石氧同位素组成的特点,对陨石氧同位素组成的主要成因观点进行了评述,结合地球的原始物质组成,讨论了陨石氧同位素组成的地球科学意义。     

On the thermal and redox history of the Yamato diogenite Y-74013

The Yamato diogenite, Y-74013, shows a high degree of textural equilibrium with the apparent crystallization sequence: troilite and metal → orthopyroxene → plagioclase. The position of the large chromite crystals in this sequence is unclear. Except chromite, all other minerals have composition similar to common orthopyroxene achondrites. The chromite is more magnesian than in common diogenites, strongly zoned and, on the whole, intermediate in composition between chromites of diogenites and pallasites. Texture, mineral composition data and an equilibrium thermodynamic analysis of the mineral association strongly indicate that the chromite crystallized earlier than the silicates at a much higher temperature (possibly above 1100°C) and rapidly grew in a medium which was progressively enriched in Mg, Al and Ti. But the chromite failed to reach chemical equilibrium, even at its outermost rim, with the orthopyroxene. The calculated equilibrium log fO₂ of the Yamato diogenite, ?20·21 to ?11·08 for temperatures between 880°C and 1500°C is well within the normal oxygen fugacity range of pyroxene achondrites.     

Excimer laser isotope-ratio-monitoring mass spectrometry for in situ oxygen isotope analysis

A technique using isotope-ratio-monitoring gas chromatography-mass spectrometry (irmGCMS) and excimer laser fluorination for in situ oxygen isotope analysis of silicates is described. The irmGCMS and oxygen extraction line is connected by a newly developed interface, reducing the time for a single analysis to less than 10 min. The precision obtained for δ¹⁸O is similar to what has been reported for excimer laser fluorination using dual inlet systems. δ¹⁸O values of two olivine standards had 1σ precision of ±0.14‰ (n=19 and n=10) and that of Dörentrup quartz had ±0.17‰. Eleven analyses of a large zircon crystal had a precision of ±0.12‰. However, between 300 and 600 nmol oxygen was liberated for a single analysis, equivalent to cylindrical laser holes 250 to 350 μm in diameter and depth. In the future it will be feasible to measure the isotope ratio of cylindrical volumes 150 μm in diameter simply by reducing the volume of the extraction line. While this is still significantly larger than what is possible with ion probes, the ratios obtained by excimer laser-irmGCMS are highly accurate and precise without correction. The value of this technique for in situ oxygen isotope measurements is demonstrated with two rock slabs from metamorphic rocks of the Dabie–Sulu ultra-high-pressure belt, China.     

Multiple parent bodies of polymict brecciated meteorites

In three brecciated meteorites, Bencubbin, Cumberland Falls and Plainview, the oxygen isotopic compositions of different rock types within each meteorite were determined to seek genetic relationships between them. In all cases the isotopic compositions are not consistent with derivation from a single parent body. There is no evidence that chondrites and achondrites could be derived from a common parent body. The chondritic inclusions in Bencubbin and Cumberland Falls cannot be identified with any of the ordinary chondritic meteorites. The carbonaceous chondritic fragments in Bencubbin are smilar to, but not identical with, C2 meteorites. The achondritic portion of Bencubbin has a very unusual isotopic composition, which, along with its close relative Weatherford, sets it in a class distinctly apart from other achondrites. Lithic fragments in brecciated meteorites provide a wider range of rock types than is represented by known macroscopic meteorites. Collisions between some meteorite parent bodies were of sufficiently low velocity that fragments of both are preserved in breccias.