The compositional classification of chondrites: III. Ungrouped carbonaceous chondrites

Seven carbonaceous chondrites (Allan Hills A77307, Adelaide, Al Rais, Coolidge, Grosnaja, Karoonda and Renazzo) with uncertain classifications were analyzed by instrumental and radiochemical neutron activation analysis for 29 elements: Na, Mg, Al, K, Ca, Sc, V, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Se, Br, Ru, Cd, Sb, La, Sm, Eu, Yb, Lu, Os, Ir and Au. Five of these chondrites (A77307, Adelaide, Al Rais, Karoonda and Renazzo) are unique ‘grouplets’, not closely related to other groups or to each other. Only Coolidge (CV4) and Grosnaja (CV3-an) are members of previously established groups. A77307 and Adelaide have refractory lithophile abundances similar to those in the CM-CO clan; A77307 probably is a member of that clan, but Adelaide, which shows CV-like petrographic characteristics, cannot as yet be assigned to a clan. Al Rais and Renazzo have similar refractory lithophile abundances (essentially at CI levels) and probably belong to the same clan, i.e., formed in the same region of the nebula. There are insufficient data to determine whether they formed at the same general region as the CI chondrites, but separates having O-isotope compositions near the terrestrial fractionation line indicate that this is plausible. Karoonda has refractory lithophile abundances ~ 1.21 × CI and appears to belong to a new clan distinct from CM-CO (1.11 × CI) and CV (1.34×).     

The compositional classification of some Chinese iron meteorites

Based on structural observations and the concentrations of Cr, Co, Ni, Cu, Ga, Ge, As, Sb, Re, Ir, and Au by neutron-activation analysis we have classified 14 Chinese iron meteorites. Thirteen are members of the large groups IAB, IIICD, IIIAB and IVA. Leshan is an ungrouped iron meteorite that falls within the IIE field on some element-Ni diagrams, but is distinctly outside this field on plots of Cu, W, and Ir vs. Ni; it is very similar in composition to Techado, another ungrouped iron. The high Cu content of Leshan in consistent with other evidence indicating that Cu is a valuable parameter for classifying iron meteorites. IIICD Dongling appears not to be a new meteorite, but to be paired with Nantan; Dongling was recovered about 50 km from the location of the Nantan shower. In view of the fact that Yongning is highly oxidized, we assign it to group IAB but cannot rule out IIICD. IVA-An Longchang has many characteristics of IVA irons, but has been remelted, probably in a terrestrial setting. Five irons belong to group IVA, a remarkably large number. Three are identical in composition, and we suspect that the two from Hubei, Guanghua and Huangling, are paired. Thus this set of 14 irons includes 12 independent falls.     

The compositional classification of chondrites: II The enstatite chondrite groups

We present new data from a neutron activation analysis of four enstatite chondrites including the taxonomically important St. Sauveur, and discuss the classification of enstatite chondrites. The enstatite chondrites can be divided into two compositionally distinct sets; in one set abundances of nonrefractory siderophiles and moderately volatile chalcophiles and alkalis are 1.5–2.0× higher than in the other. A well-resolved compositional hiatus separates these two sets. The differences in composition are as great as those between the groups of ordinary chondrites, and therefore it appears best to treat these sets as separate groups. By analogy with the symbols used for ordinary chondrites we propose to designate the high-Fe, high siderophile group EH and the low-Fe, low-siderophile group EL. Known members of the EH group belong to petrologic types 4 and 5, whereas all EL members are petrologic type 6. Within the EH group no correlation is observed between petrologic type and abundance of nonrefractory siderophiles or moderately volatiles or alkalis.Two physical properties show only modest overlap between the EH and EL groups. Cosmic-ray ages for EH chondrites are 0.5–7 Ma, while those for EL chondrites are 4–18 Ma. Relative to Bjurböle, I-Xe formation intervals are $\text{?1.3 ± 0.6}$ Ma for EH chondrites and $\text{2.9 ± 0.5}$ Ma for EL chondrites. The weight of the chemical and physical evidence indicates that the EH and EL groups formed separate bodies at similar distances from the Sun.The available evidence for Shallowater and Happy Canyon, two strongly recrystallized silicate-rich meteorites containing > 40 mg/g Fe-Ni, indicates that the former is an enstatite-clan chondrite altered by loss of sulfide- and plagioclase-rich melts, whereas the latter is intermediate in composition between EL chondrites and the chondritic silicates in the Pine River IAB-anomalous meteorite.     

The compositional classification of chondrites—I. The carbonaceous chondrite groups

Twenty carbonaceous chondrites were analyzed by instrumental and radiochemical neutron activation analysis for Na, Mg, Al, K, Ca, Sc, V. Cr, Mn. Fe, Co, Ni, Zn, Ga, Ge, As, Se. Br. Ru, Cd, In, Sb, La, Sm, Eu, Yb, Lu, Os, Ir, and Au. Analysis of 2 or more samples of all but 2 chondrites has helped yield a high precision that allowed the resolution of numerous previously unrecognized trends. Refractory lithophile abundances decrease through the sequence CV (1.33 × CI), CM-CO (1.11 × CI) and CI. The abundances of the common siderophiles Fe, Ni and Co follow the order CI >CM >CO >CV, with CV chondrites depleted about 15% relative to CI. Volatile lithophile (Mn to K) and volatile siderophile (As to Ge) abundances decrease in the order CI >CM >CO >CV. The volatile trends in CO and CV chondrites reverse for the more volatile elements (Br to Cd) producing the sequence CI >CM >CV >CO. These three different sequences in the ordering of group elemental abundances can be used to resolve compositionally the four carbonaceous chondrite groups.We define clans to consist of one or more groups formed at a narrow range of heliocentric distances. Quantization of refractory lithophile abundances indicates the existence of three carbonaceous chondrite clans: CI, CM-CO, and CV. Despite similarities in parameters such as volatile abundances and O-isotope compositions differences in chondrule size and refractory abundances suggest that CO and CV chondrites are indeed best placed in separate clans. The relative heliocentric distance at which CI chondrites formed cannot be inferred, thus it seems safer to assign them to a separate clan.     

Cumberland Falls chondritic inclusions—II. Trace element contents of forsterite chondrites and meteorites of similar redox state

Mineralogic study of black inclusions in the Cumberland Falls enstatite achondrite revealed that they constitute a highly unequilibrated chondritic suite distinct from other chondrite groups. This highly shocked suite, the forsterite (F) chondrites, exhibits mineralogic trends apparently produced during primary nebular condensation and accretion over a broad redox range. We analyzed these samples and possibly related meteorites for Ag, As, Au, Bi, Cd, Co, Cs, Ga, In, Rb, Sb, Se, Te, Tl, U and Zn, trace elements known to yield important genetic information. The results demonstrate the compositional coherence and distinctiveness of the F chondrite suite relative to other chondrites. The Antarctic aubrite, ALH A78113, may include more F chondrite material. Trace element contents do not vary with mineral compositions hence do not reflect redox variations during formation of F chondrite parental matter. Trace element mobilization—during secondary heating episodes in the F chondrite parent or during its disruptive collision with the enstatite meteorite parent body—is not detectable. Chemical trends in F chondrites apparently reflect primary nebular processes. Cosmochemical fractionation of lithophiles from siderophiles and chalcophiles occurred at moderately high temperatures, certainly higher than those existing during formation of primitive carbonaceous, enstatite and ordinary chondrites of petrologic type ≤3.     

The isotopic compositions and concentrations of lead in some chondritic stone meteorites

Microgram quantities of lead were volatilized from the chondrites Richardton, Holbrook, Beardsley and Plainview and purified for mass spectrometric analysis. Aliquots of approximately 1 μg were surface-ionized by use of the boric acid technique. The isotopic compositions found range from highly radiogenic lead in the case of Richardton to leads in the other three chondrites which are substantially less radiogenic than average lead in the crust of the earth. p]From our recent concentration determinations by the isotopic dilution method, it seems that the amount of lead in Beardsley is about 0·13 p.p.m. and that in Richardton about 0·06 p.p.m. The leads in these meteorites are consistent within a factor of approximately two with the lead-lead ages and the independently measured uranium and thorium concentrations. The lead—lead age for Richardton is the same as that found by (1955) for Nuevo Laredo, being 4·6 Æ. The lead concentration measured for Forest City is 0·09 p.p.m., while that for Holbrook is 0·28 p.p.m. Holbrook is quite anomalous in containing 2·5 times the amount of radiogenic lead that can be explained by the accumulation of decay products in situ with a primeval lead which had the isotopic composition of the lead from the troilite in the Canyon Diablo and Henbury iron meteorites. p]The total amount of lead on the surface of a meteorite appears to be comparable to the total amount inside. The isotopic composition of this surface lead indicates that it is principally terrestrial lead.     

Isotopic anomalies of noble gases in meteorites and their origins—IV. C3 (Ornans) carbonaceous chondrites

The C3O chondrites Kainsaz, Lancé and Ornans were studied by an acid dissolution technique, to characterize the noble-gas components in 3 mineral fractions: HF, HCl-solubles (99% of the meteorite), chromite and carbon (0.3–0.9%), and ‘phase Q’, a poorly characterized trace mineral (0.05–0.4%) containing most of the Ar, Kr, Xe. For all fractions, gas contents decline in the order Kainsaz > Lancé > Ornans; this trend parallels volatile contents but not heterogeneity of olivine composition or degree of metamorphism and seems to reflect progressively higher condensation temperatures from the solar nebula.Solubles contain nearly unfractionated Xe, and show ${}^{136}\text{Ar}{}^{132}\text{Xe}$ ratios up to 850. Hence the high $\text{Ar}\text{Xe}$ ratios (200–400) of bulk C3O chondrites must be due to an HF-soluble mineral (possibly magnetite). Phase Q contains ordinary planetary gases and a Ne component of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 10.3 ± 0.4.}$Chromite and carbon contain Ne of ${}^{20}\text{Ne}{}^{22}\text{Ne}\text{= 8.6 ± 0.1}$ and ‘CCF’ xenon (a peculiar component of possibly fissiogenic origin, enriched in the heavy isotopes but accompanied by a component enriched in the light isotopes).In all primitive chondrites, both the amount and the chemical separability of CCFXe parallel the abundance of promordial noble gases and other volatiles, such as C, N, Tl, Bi and In. The close correlation of CCFXe with various properties of undoubtedly local origin (volatile content, petrologic type, presence of ferrichromite and carbon, etc.) is more consistent with a local than with an extrasolar origin of this component. A volatile superheavy element seems to be the most plausible source, but the evidence is not conclusive.     

Chemical fractionations in meteorites—IX. C3 chondrites

Four C3V chondrites (Grosnaja, Kaba, Mokoia, Vigarano) and three C3O chondrites (Felix, Kainsaz, and Lancé) were analyzed by radiochemical neutron activation for 17 trace elements. Both classes show a typical chondritic step pattern, reflecting loss of volatiles during chondrule formation. Elements condensing above 1300 K (U, Re, Ir, Ni) are present in essentially Cl chondrite proportions, while moderately volatile elements condensing between 1300 K and 800 K (Ge, Rb, Ag) are depleted by a factor of 0.44. However, elements condensing below 700 K (S, Cs, Bi, Tl, Br, Se, Te, In, Cd) are depleted to a still greater degree, and more so in the Ornans subclass (factor of 0.24, except Cd 0.007) than in the Vigarano subclass (factor of 0.29). This additional depletion may be due to a slight (less than 3-fold) dust-gas fractionation, by settling of dust to the median plane of the solar nebula. Among other chondrite classes, ordinary chondrites show a similar depletion, but C2 chondrites do not. Possibly the undepleted meteorites formed in one of the convection zones of the nebula predicted by Cameron and Pine, whereas the depleted meteorites formed in a quiescent region.The condensation of chalocophile elements as a function of H₂S partial pressure is discussed, in an attempt to explain the drastic difference in Cd abundance between the two subclasses. It appears that the $\text{H}{}_2\text{S}\text{H}{}_2$ ratio is the key variable. C3O's seem to have condensed in a region where enough metallic Fe was present to buffer the H₂S pressure, while C3V's condensed in a more oxidized region, where H₂S was in excess. Accretion temperatures, for an assumed nebular pressure of 10^?5 atm, were between 415 and 430 K for C3O's and less than 440 K for C3V's.Two slightly volatile elements, Sb and Au, show variable depletion, presumably reflecting variable loss during chondrule formation. Indeed, their depletion correlates with the abundance of iron-poor olivine, a measure of the peak temperature and time during chondrule formation.     

Chemical fractionations in meteorites—XI. C2 chondrites

Six C2M chondrites (Boriskino, Cold Bokkeveld, Erakot, Essebi, Haripura and Santa Cruz) and the C2R chondrite Al Rais were analyzed by radiochemical neutron activation analysis for Ag, Au, Bi, Cd, Cs, Ge, In, Ir, Ni, Os, Pd, Rb, Re, Sb, Se, Sn, Te, Tl, U, and Zn. Abundances (relative to Cl chondrites) show a systematic dependence on volatility, apparently reflecting volatile loss during formation of chondrules and other high-T components. Elements of nebular condensation temperature (T_c) > 1200 K are undepleted, those of T_c < 700 K are depleted by a constant factor (0.482 ± 0.049 for C2M's) and elements of intermediate volatility are depleted by intermediate factors. The abundances do not “tend to fall monotonically as a function of [T_c],” as previously claimed by Wai and Wasson (1977) for a more restricted temperature range. For meteorites that have suffered little aqueous alteration (Mighei, Murchison, Murray), the mean abundance of volatiles agrees with the matrix content, but for the more altered meteorites, matrix contents are 20–30% higher. Only a few meteorites deviate appreciably from the mean abundance pattern. Al Rais, a C2R chondrite with a significant metal content, is systematically lower in 12 volatiles, but is enriched in Ni and Pd. Haripura and Erakot are enriched in Bi and Tl, possibly from the late condensate, mysterite.     

Rhodium, gold and other highly siderophile element abundances in chondritic meteorites

The abundances of the highly siderophile elements (HSE) Re, Os, Ir, Ru, Pt, Rh, Pd and Au, and ¹⁸⁷Os/¹⁸⁸Os isotope ratios have been determined for a set of carbonaceous, ordinary, enstatite and Rumuruti chondrites, using an analytical technique that permits the precise and accurate measurement of all HSE from the same digestion aliquot. Concentrations of Re, Os, Ir, Ru, Pt and Pd were determined by isotope dilution ICP-MS and N-TIMS analysis. The monoisotopic elements Rh and Au were quantified relative to the abundance of Ir.Differences in HSE abundances and ratios such as Re/Os, ¹⁸⁷Os/¹⁸⁸Os, Pd/Ir and Au/Ir between different chondrite classes are further substantiated with new data, and additional Rh and Au data, including new data for CI chondrites. Systematically different relative abundances of Rh between different chondrite classes are reminiscent of the behaviour of Re. Carbonaceous chondrites are characterized by low average Rh/Ir of 0.27 ± 0.03 (1s) which is about 20% lower than the ratio for ordinary (0.34 ± 0.02) and enstatite chondrites (EH: 0.33 ± 0.01; EL: 0.32 ± 0.01). R chondrites show higher and somewhat variable Rh/Ir of 0.37 ± 0.07.Well-defined linear correlations of HSE, in particular for bulk samples of ordinary and EL chondrites, are explained by binary mixing and/or dilution by silicates. The HSE carriers responsible for these correlations have a uniform chemical composition, indicating efficient homogenization of local nebular heterogeneities during or prior to the formation of the host minerals in chondrite components. Excepting Rumuruti chondrites and Au in carbonaceous chondrites, these correlations also suggest that metamorphism, alteration and igneous processes had negligible influence on the HSE distribution on the bulk sample scale.Depletion patterns for Rh, Pd and Au in carbonaceous chondrites other than CI are smoothly related to condensation temperatures and therefore consistent with the general depletion of moderately volatile elements in carbonaceous chondrites. Fractionated HSE abundance patterns of ordinary, enstatite and Rumuruti chondrites, however, are more difficult to explain. Fractional condensation combined with the removal of metal phases at various times, and later mixing of early and late formed metal phases may provide a viable explanation. Planetary fractionation processes that may have affected precursor material of chondrite components cannot explain the HSE abundance patterns of chondrite groups. HSE abundances of some, but not all Rumuruti chondrites may be consistent with solid sulphide-liquid sulphide fractionation processes during impact induced melting.     

Ruthenium endemic isotope effects in chondrites and differentiated meteorites

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

Foreign inclusions in stony meteorites—I. Carbonaceous chondritic xenoliths in the Kapoeta howardite

Small, 1–3 millimeter-sized, black xenoliths containing chondrules and chondrule fragments embedded in a fine-grained matrix are present in the Kapoeta howardite. Forsteritic olivine, pentlandite and a nickel-rich (0.3–2.8 wt. %) matrix indicate that these xenoliths are carbonaceous ohondrite material.     

Origin of rapidly solidified metal-troilite grains in chondrites and iron meteorites

Inclusions of troilite and metallic Fe,Ni 0.2–4 mm in size with a dendritic or cellular texture were observed in 12 ordinary chondrites. Cooling rates in the interval 1400?950°C calculated from the spacing of secondary dendrite arms or cell widths and published experimental data range from 10^?7 to 10⁴°C/sec. In 8 of these chondrites, which are breccias containing some normal slow-cooled metal grains, the inclusions solidified before they were incorporated into the breccias. Their cooling rates of 1–300 °C/sec indicate cooling by radiation, or by conduction in contact with cold silicate or hot silicate volumes only 6–40 mm in size. This is quantitative evidence that these inclusions and their associated clasts were melted on the surface of a parent body (by impact), and were not formed at depth from an internally derived melt. In Ramsdorf, Rose City and Shaw, which show extensive reheating to ? 1000°C, Fe-FeS textures in melted areas are coarser and indicate cooling rates of 10^?1 to 10^?4°C/sec during solidification. This metal may have solidified inside hot silicate volumes that were 10–300 cm in size. As Shaw and Rose City are breccias of unmelted and melted material, their melted metal did not necessarily cool through 1000°C within a few m of the surface. Shock-melted, fine-grained, irregular intergrowths of metal and troilite formed in situ in many irons and some chondrites by rapid solidification at cooling rates of ? 10⁵°C/sec. Their kamacite and taenite compositions may result from annealing at ~250°C of metallic glass or exceedingly fine-grained quench products.     

A search for nickel isotopic anomalies in iron meteorites and chondrites

We report Ni isotopic data, for ^58,60-62Ni, on (1) FeNi metal and sulfides in different groups of iron meteorites, (2) sulfides and a whole rock sample of the St. Séverin chondrite, and (3) chondrules from the Chainpur chondrite. We have developed improved, Multiple-Collector, Positive ion Thermal Ionization Mass Spectrometric (MC-PTIMS) techniques, with Ni⁺ ionization efficiency at 1‰, and chemical separation techniques for Ni which reduce mass interferences to the 1 ppm level, so that no mass interference corrections need be applied, except for ⁶⁴Ni (from ⁶⁴Zn, at the 0.1‰ level), for which we do not report results. We normalize the data to ⁶²Ni/⁵⁸Ni to correct for mass dependent isotope fractionation. No evidence was found for resolved radiogenic or general Ni isotope anomalies at the resolution levels of 0.2 and 0.5 εu (εu = 0.01%) for ⁶⁰Ni/⁵⁸Ni and ⁶¹Ni/⁵⁸Ni, respectively. From the ⁵⁶Fe/⁵⁸Ni ratios and ε(⁶⁰Ni/⁵⁸Ni) values, we calculate upper limits for the initial value of (⁶⁰Fe/⁵⁶Fe)₀ of (a) <2.7 × 10⁻⁷ for Chainpur chondrules, (b) <10⁻⁸ for the St. Séverin sulfide, and (c) <4 × 10⁻⁹ for sulfides from iron meteorites. We measured some of the same meteorites measured by other workers, who reported isotopic anomalies in Ni, using Multiple-Collector, Inductively-Coupled Mass Spectrometry. Our results do not support the previous reports of Ni isotopic anomalies in sulfide samples from Mundrabilla by Cook et al. [Cook D. L., Clayton R. N., Wadhwa M., Janney P. E., and Davis A. M. (2008). Nickel isotopic anomalies in troilite from iron meteorites. Geophy. Res. Lett. 35, L01203] and in sulfides from Toluca and Odessa by Quitté et al. [Quitté G., Meier M., Latkoczy C., Halliday A. N., and Gunther D., (2006). Nickel isotopes in iron meteorites-nucleosynthetic anomalies in sulfides with no effects in metals and no trace of ⁶⁰Fe. Earth Planet. Sci. Lett. 242, 16-25]. Hence, we find no need for specialized physical-chemical planetary processes for the preservation of different Ni isotope compositions, between FeNi metal and sulfides in the same iron meteorites, as proposed by the above reports nor for complex astrophysical scenarios to provide the very peculiar Ni isotope anomalies reported by these workers for sulfides.     

Chemical and physical studies of type 3 chondrites—IV: Annealing studies of a type 3.4 ordinary chondrite and the metamorphic history of meteorites

Samples of a type 3.4 chondrite have been annealed at 400–1000°C for 1–200 hours, their thermoluminescence properties determined and analyzed for K, Na, Mn, Sc and Ca by instrumental neutron activation analysis. After annealing at ?900°C, the samples showed a 50% decrease in TL sensitivity, while after annealing at 1000°C it fell to 0.1-0.01 times its unannealed value and loss of Na and K occurred. The TL and compositional changes resemble those observed for the equilibrated Kernouve chondrite after similar annealing treatments, except that the sharp TL decrease, and element loss, occurred at ~ 1100°C; this difference is presumably due to petrographic differences in the feldspar of the two meteorites. The temperature and the width of the TL peak showed a discontinuous increase after annealing at 800°C; peak temperature jumped from 130 to 200°C and peak width increased from 90 to 150°C. The activation energies for these TL changes are 7–10 kcal/mole. Similar increases in the TL peak temperature have been reported in TL studies of Amelia, VA, albite, where they were associated with the low to high-temperature transformation. However, the activation energy for the transformation is ~80 kcal/mole. These changes in TL emission characteristics resemble trends observed in type 3 ordinary chondrites and it is suggested that type 3.3–3.5 chondrites have a low-feldspar as TL phosphor and > 3.5 have high-feldspar as the phosphor. Thermoluminescence therefore provides a means of palaeothermometry for type 3 ordinary chondrites.     

Where do the meteorites come from? A re-evaluation of the earth-crossing apollo objects as sources of chondritic meteorites

The total number of Earth-crossing Apollo objects larger than 500 m in radius is estimated to be 600, based on failure of chance rediscovery, lunar crater frequency and completeness-of-search results of Shoemaker, Helin and Gillett. The number of Amor objects (perihelion between 1.0 and 1.3 A.U.) is estimated to be about 500. These estimates are about an order of magnitude higher than those given by previous workers, and these objects appear sufficiently numerous to dominate post-mare lunar and terrestrial cratering (d ≥ 10 km).The terrestrial meteorite and meteorite yield of 100-10⁶ g bodies derived from fragmentation of Apollo objects is re-evaluated using this estimate, together with more recent data on asteroid albedos and on hypervelocity impact. Terrestrial rate of impacts of these fragments at sufficiently low velocities to penetrate the atmosphere is estimated to be ~2 × 10⁸g/yr. This is in the middle of the range of the actual extraterrestrial impact rate based on photographic fireball surveys (Prairie Network), lunar seismometry, and recovery of meteorites. It is likely that most ordinary chondrites are fragments of Apollo objects, provided that these fragments are sufficiently strong to survive atmospheric entry.Possible asteroidal and cometary sources of Apollo objects are reviewed. Several mechanisms for the removal of asteroids into Earth-crossing orbit are qualitatively acceptable, but appear inadequate by at least an order of magnitude to supply the required number. Most Apollo objects are probably the cores of comets which have lost their volatile material by repeated solar evaporation, as proposed by Öpik.The distribution of the component of the Apollo objects' angular momentum perpendicular to the plane of the solar system is tabulated. It is found that considerable non-random clustering of these values exists, for which no adequate explanation is known.     

Oxygen isotopic composition of chondritic interplanetary dust particles: A genetic link between carbonaceous chondrites and comets

Oxygen isotopes were measured in four chondritic hydrated interplanetary dust particles (IDPs) and five chondritic anhydrous IDPs including two GEMS-rich particles (Glass embedded with metal and sulfides) by a combination of high precision and high lateral resolution ion microprobe techniques.All IDPs have isotopic compositions tightly clustered around that of solar system planetary materials. Hydrated IDPs have mass-fractionated oxygen isotopic compositions similar to those of CI and CM carbonaceous chondrites, consistent with hydration of initially anhydrous protosolar dust. Anhydrous IDPs have small ¹⁶O excesses and depletions similar to those of carbonaceous chondrites, the largest ¹⁶O variations being hosted by the two GEMS-rich IDPs. Coarse-grained forsteritic olivine and enstatite in anhydrous IDPs are isotopically similar to their counterparts in comet Wild 2 and in chondrules suggesting a high temperature inner solar system origin. The small variations in the ¹⁶O content of GEMS-rich IDPs suggest that most GEMS either do not preserve a record of interstellar processes or the initial interstellar dust is not ¹⁶O-rich as expected by self-shielding models, although a larger dataset is required to verify these conclusions.Together with other chemical and mineralogical indicators, O isotopes show that the parent-bodies of carbonaceous chondrites, of chondritic IDPs, of most Antarctic micrometeorites, and comet Wild 2 belong to a single family of objects of carbonaceous chondrite chemical affinity as distinct from ordinary, enstatite, K- and R-chondrites. Comparison with astronomical observations thus suggests a chemical continuum of objects including main belt and outer solar system asteroids such as C-type, P-type and D-type asteroids, Trojans and Centaurs as well as short-period comets and other Kuiper Belt Objects.     

Metamorphism of the H-group chondrites: implications from compositional and textural trends in chondrules

Major and minor element bulk compositions of 373 individual chondrules from 18 H3 to H6 chondrites were determined in polished thin sections by broad-beam electron probe analysis. Bulk chondrule FeO and Al₂O₃ increase and TiO₂ and Cr₂O₃ decrease with increasing petrologic type; normative fayalite, albite and plagioclase increase through the petrologic sequence. Chondrule diameters correlate with phenocryst sizes in porphyritic chondrules of type 3 chondrites, but this correlation is diminished in the higher petrologic types. Furthermore, for a given chondrule diameter, phenocryst sizes are larger in the higher petrologic types. We attribute most compositional trends in chondrules through the petrologic sequence to diffusion and equilibration among chondrules and between chondrules and matrix in response to increasing degrees of thermal metamorphism. Increased phenocryst sizes in the higher petrologic types are probably the result of grain growth during metamorphism.We suggest that H-group chondrites formed by accretion of high-temperature (chondrules) and low-temperature (matrix) materials. Parent materials of each of the petrologic types resembled type 3 chondrites, but had slight compositional differences (e.g. volatiles, rare gases, total iron) inherited during accretion. These differences were predominantly functions of decreasing temperature in the nebula as accretion progressed. Internal reheating of the parent materials to different temperatures and (probably) for different times, as a function of depth in the parent body, caused compositional equilibration, grain coarsening, and reduction of FeO to Fe° by carbon.     

南极三块（L3）陨石球粒结构类型及其矿物学物征

我国第16次南极考察队回收到6块稀少种类陨石-13型,GRV99001,GRV99019,GRV99020,GRV99021,GRV99022,GRV99026。本文对其中3块陨石进行研究,研究它们的球粒结构和矿物化学成分．它们虽然部属于非平衡普通球粒陨石（L3）,但它们的亚类不同,GRV99001为13．4,GRV99026为L3．5,GRV99019为L3．6．它们的球粒结构和球粒内的矿物晶体完整性和矿物组合变化比较大．橄榄石和辉石以高镁为特征．这三块陨石的球粒结构种类比较多．有班状的、炉条状的、扇形的和隐晶质的等．在GRV99001陨石中班状结构的球粒内能见到一个或两个以上完整的单晶橄榄石构成的球粒,也能见到多个细小的或是破碎橄榄石,被包襄在辉石晶体内．而在GRV99019和GRV99026陨石中只能见到多个细小单晶体或是破碎的橄榄石晶体．GRV99001陨石的炉条状结构,好象是由一条带状长石矿物,穿插在单个橄榄石晶体中构成．扇形和伞形结构的球粒,以一个点为中心,向外放射呈扇形．如GRV99019陨石中扇形结构球粒,它们是以辉石为主,陨硫铁充填在低钙辉石缝隙中,形成扇形．另一种是以多个点为中心,如GRV99001陨石,它们是由橄榄石、低钙辉石和长石质的玻璃．构成多个小伞形,形状类似三维立体的球,裂缝中也充填有金属矿物．隐晶质的球粒在GRV99026陨石中有两种,一种是在一厘米等于100un时呈现隐晶质矿物,而放大到一厘米等于5un时,就可以清楚看到两种低钙辉石矿物,在低钙辉石中还有金属矿物．另一种隐晶质结构球粒由极细小破碎的橄榄石和辉石矿物构成．这三块陨石中的橄榄石和辉石都以高镁为特征．班状结构球粒,在GRV99001陨石中橄榄石的MgO～33．37～51．21,辉石为35．9～36．61;GRV99019陨石中橄榄石23．33～56．58．辉石21.38-33.07,GRV99026陨石中榄橄榄石45.91-52.63,辉石34.48-37.35,扇形结构球粒,在GRV99001陨石中橄榄石29.94-46.22,辉石28.17-30.36;GRV99019陨石中橄榄石29.17-34.38,辉石23.11-27.79,炉条状结构的球粒,在GRV99001陨石中橄榄石51.84-56.03,隐晶质结构球粒,在GRV99026陨石中辉石20.17-21.54,橄榄石30.84-32.66,由此看出矿物晶体完整性越好镁的含量越高。