The abundance of barium in stony meteorites

The concentration of Ba in 7 carbonaceous chondrites, 18 ordinary chondrites, 3 achondrites and 1 stony-iron meteorite has been determined by the stable isotope dilution technique using solid source mass spectrometry. Analysis of the C1 chondrite Orgueil indicates a small adjustment of the “cosmic” abundance of Ba to 4.2 on the Si=10⁶ abundance scale. The present work provides a more complete coverage of a number of meteorite classes than has so far been available for the abundance of Ba in stony meteorites.     

Mass spectrometric isotope dilution analyses of tin in stony meteorites and standard rocks

The abundance of tin in 21 stony meteorites and 12 standard rocks has been determined by the stable isotope dilution technique. The measurements of the C1 chondrite Orgueil gives a cosmic abundance for tin of 3.7 (with respect to Si= 10⁶ atoms) and thus substantiates the peak in the cosmic abundance curve due to the closed proton shell forZ = 50. The low abundance of tin in ordinary chondrites allows tin to be reassigned to the strongly depleted group of elements in the context of the Larimer-Anders model. Our values for the standard rocks are in reasonable agreement with the presently accepted abundances of tin, except for the diabase W-1 and tonalite T-1 for which significantly lower values were obtained.     

A classification of meteorites based on oxygen isotopes

On the basis of¹⁸O/¹⁶O and¹⁷O/¹⁶O ratios, meteorites and planets can be grouped into at least six categories, as follows: (1) the terrestrial group, consisting of the earth, moon, differentiated meteorites and enstatite chondrites; (2) types L and LL ordinary chondrites; (3) type H ordinary chondrites; (4) anhydrous minerals of C2, C3, C4 carbonaceous chondrites; (5) hydrous matrix minerals of C2 carbonaceous chondrites; (6) the ureilites. Objects of one category cannot be derived by fractionation or differentiation from the source materials of any other category.     

Nitrogen abundances and isotopic compositions in stony meteorites

Nitrogen contents range from a few parts per million in ordinary chondrites and achondrites to several hundred parts per million in enstatite chondrites and carbonaceous chondrites. Four major isotopic groups are recognized: (1) C1 and C2 carbonaceous chondrites have δ¹⁵N of+30to+50%.; (2) enstatite chondrites have δ¹⁵N of?30to?40‰; (3) C3 chondrites have low δ¹⁵N with large internal variations; (4) ordinary chondrites have δ¹⁵N of?10to+20‰. The major variations are primary, representing isotopic abundances established at the time of condensation and accretion. Secondary processes, such as spallation reactions, solar wind implantation and metamorphic loss may cause small but observable isotopic variations in particular cases. The large isotopic difference between enstatite chondrites and carbonaceous chondrites cannot be accounted for by equilibrium condensation from a homogeneous nebular gas, and requires either unusually large kinetic effects, or a temporal or spatial variation of isotopic composition of the nebula. Nitrogen isotopic heterogeneity in the nebula due to nuclear processes has not been firmly established, but may be required to account for the large variations found within the Allende and Leoville meteorites. The unique carbonaceous chondrite, Renazzo, has δ¹⁵N of+170%., which is well beyond the range of all other data, and also requires a special source. It is not yet possible, from the meteoritic data, to establish the mode of accretion of nitrogen onto the primitive Earth.     

Noble gases,KrKr ages, andBe of chrondrites from China

A comprehensive study of the cosmic-ray exposure history of five ordinary chondrites from China was carried out using measurements of the noble gas isotopic abundances and¹⁰Be concentrations. The following average cosmic-ray exposure ages, based on cosmogenic²¹Ne and on⁸¹KrKr dating were obtained: Zhaodong (L4) — 15.7 ± 3.0 m.y., Nan Yang Pao (L6) — 48 ± 10.0 m.y., Guangrao (L6) — 16.8 ± 3.5 m.y., and Lunan (H6) — 26.7 ± 5.0 m.y. The H5 chondrite Zaoyang was exposed for only 0.90 ± 0.12 m.y. to galactic cosmic rays as calculated from the¹⁰Be activity and from the low amounts of cosmic-ray-produced noble gases. The Zhaodong chondrite contains large amounts of⁸⁰Kr and⁸²Kr produced by neutron capture of bromine. From the high slowing down density for neutrons we derive a preatmospheric mass of more than 1800 kg for this meteorite.     

Cosmic-ray-produced40K and50V in the metal phase of chondrites

Cosmic-ray-produced⁴⁰K in the metal phase of six chondrites and⁵⁰V in that of one chondrite were determined using a surface ionization mass spectrometer. The²²Ne_total/⁴⁰K_metal ratios of the chondrites are explained in part by shielding effects during cosmic-ray irradiation. The wide variation of this ratio in some groups of meteorites is explained in terms of partial loss of rare-gas nuclides. Radiation ages for the chondrites were determined using⁴⁰K measurements and production-rate estimates from thick target calculations.     

Iron-silicate fractionation within ordinary chondrite groups

A comparison of recent bulk chemical analyses of fresh, well-classified ordinary chondrites reveals that the unequilibrated H-3 and LL-3 chondrites tend to be iron-poor relative to equilibrated H- and LL-group chondrites (types 4–6). A more complex relationship in the L-group suggests that it consists of two chemical subgroups, in each of which iron is deficient in the lower petrologic types. The available data suggest that the chondrite parent bodies accreted inhomogeneously.     

Moderately volatile siderophiles in ordinary chondrites

The contents of the moderately volatile elements Ga, Ge, Cu and Sb in ordinary chondrites give us some clues with regard to the metal-silicate fractionation process. Their concentration in coexisting magnetic and non-magnetic portions of members of each ordinary chondrite group will be discussed. Germanium and Sb are mostly siderophilic, but Ga is strongly lithophilic in unequilibrated chondrites; its partition coefficient between magnetic and non-magnetic portions is positively correlated with petrologic type in L and LL chondrites, but not in H4–6 chondrites. From 25 to 50% of the total Cu is found in the non-magnetic fraction of chondrites, but there is no correlation between Cu content and petrologic type. The abundances of Ga, Cu and Sb (relative to Si) are constant in ordinary chondrites, independent of the amount of metal present, indicating that these elements were not in solid solution in the metal phase of chondrites when the metal-silicate fractionation process occurred. Germanium, which is the most volatile among the four elements analyzed, is more abundant in H than in L and LL chondrites, indicating that it was fractionated by this process. Nebular oxidation processes can be responsible for the behavior of Ga if this element was in oxidized form when loss of metal occurred, but cannot explain the results for Cu and Sb which are predicted to condense as metals and accrete mostly in metallic form. It is possible that Cu and Sb, upon condensation, did not form solid solutions with metallic Ni-Fe until after the separation of metal from silicates took place.     

Lu-Hf and Sm-Nd isotopic systematics in chondrites and their constraints on the Lu-Hf properties of the Earth

Sm-Nd and Lu-Hf isotopic data are presented for 19 chondritic meteorites: six carbonaceous chondrites, five L-chondrites, seven H-chondrites, and a single enstatite chondrite. The primary goal of the study is to better define the Bulk Silicate Earth (BSE) reference values for Hf isotopes. Except for one sample with lower Sm/Nd, the Sm-Nd data define a cluster around the accepted reference values for chondrites and terrestrial planets, giving a mean ¹⁴⁷Sm/¹⁴⁴Nd of 0.1960±0.0005, and a mean ¹⁴³Nd/¹⁴⁴Nd of 0.512631±0.000010 (uncertainties are two standard errors). It seems appropriate to retain the presently accepted Sm-Nd reference parameters, ¹⁴⁷Sm/¹⁴⁴Nd=0.1966 and ¹⁴³Nd/¹⁴⁴Nd=0.512638 (when fractionation-corrected to ¹⁴⁶Nd/¹⁴⁴Nd=0.7219).Lu-Hf isotopic data are not clustered, but spread along an approximate 4.5-Ga isochron trend, with a range of ¹⁷⁶Lu/¹⁷⁷Hf from 0.0301 to 0.0354. The data are similar to many of the samples of chondrites presented by Bizzarro et al. [Nature 421 (2003) 931], but lack the range to lower Lu/Hf shown by those authors. Our chondrite data define a regression line of 4.44±0.34 Ga when 1.867×10⁻¹¹ year⁻¹ is used for the decay constant of ¹⁷⁶Lu [Science 293 (2001) 683; Earth Planet. Sci. Lett. 219 (2004) 311-324]. Combining our data with the main population of analyses from Bizzarro et al. [Nature 421 (2003) 931] yields 4.51±0.24 Ga. Unless samples of eucrite meteorites and deviating replicates of chondrites with ¹⁷⁶Lu/¹⁷⁷Hf less than 0.030 are employed, no combination of the main population of chondrite Lu-Hf data yields a regression with sufficiently low error to constrain the decay constant of ¹⁷⁶Lu. Sample heterogeneity seems to hinder the acquisition of reproducible Lu-Hf analyses from small, manually ground pieces of chondrites, and we suggest that analysis of powders prepared from large volumes of meteorite will be needed to adequately characterize the Lu-Hf isotope systematics of chondritic reservoirs and of BSE. Our results for carbonaceous chondrites show higher average ¹⁷⁶Lu/¹⁷⁷Hf and ¹⁷⁶Hf/¹⁷⁷Hf than ordinary chondrites, and the mean of carbonaceous chondrites also coincides with replicate analyses of a powder representing a large volume of meteorite, the Allende powder from the Smithsonian Institution. Use of the carbonaceous chondrite mean for BSE Lu-Hf characteristics results in a BSE Hf-Nd point that lies well within the array of terrestrial compositions, and leads to plausible initial ε_Hf values for Precambrian rocks. An improved objective resolution of meteorite data and of meteoritic models for the Earth needs to occur before BSE can be established for Lu-Hf.     

Tungsten in ordinary chondrites

In ordinary chondrites tungsten displays both lithophile and siderophile characteristics. Its concentration in the metal phase is positively correlated with petrologic type, and with the distribution coefficientK_D =W in metal/W in silicates plus troilite. The oxidation-reduction reactions involved are temperature-dependent and the recrystallization temperature recorded on the basis of the partition of W between coexisting metal and silicate plus troilite fractions are950° ± 100°C for equilibrated chondrites (types 5 and 6), and800° ± 50°C for type 4, while Shaw (L7) records the highest recrystallization temperature (>1200°C).The different metallic content of the three groups of ordinary chondrites has been attributed to a metal-silicate fractionation process. Such a process appears to have fractionated W and Ir, but not W and Fe as these elements were partly oxidized when the fractionation process took place.     

New kind of type 3 chondrite with a graphite-magnetite matrix

We have discovered four clasts in three ordinary-chondrite regolith breccias which are a new kind of type 3 chondrite. Like ordinary and carbonaceous type 3 chondrites, they have distinct chondrules, some of which contain glass, highly heterogeneous olivines and pyroxenes, and predominantly monoclinic low-Ca pyroxenes. But instead of the usual fine-grained, Fe-rich silicate matrix, the clasts have a matrix composed largely of aggregates of micron- and submicron-sized graphite and magnetite. The bulk compositions of the clasts as well as the types of chondrules (largely porphyritic) are typical of type 3 ordinary chondrites, although chondrules in the clasts are somewhat smaller (0.1–0.5 mm). A close relationship with ordinary chondrites is also indicated by the presence of similar graphite-magnetite aggregates in seven type 3 ordinary chondrites. This new kind of chondrite is probably the source of the abundant graphite-magnetite inclusions in ordinary-chondrite regolith breccias, and may be more common than indicated by the absence of whole meteorites made of chondrules and graphite-magnetite.     

Trace element content of metals from L-group chondrites

Eight L- and one LL-group chondrites were selected for a major and trace element content study of their metals by instrumental neutron activation techniques. The elements Ni, Co, Fe, Cu, As, Ga, W, Au and Ir were determined. For each meteorite three metallic fractions were analyzed: (1) coarse, >100 mesh; (2) intermediate, 100–200 mesh; (3) fine, <200 mesh. The composition of the metals varies considerably with grain size, as a result of a preferential concentration of kamacite in the coarse and of taenite in the intermediate and fine fractions.A third metallic component, consisting of very fine plessitic inclusions, was observed in chondrules of equilibrated chondrite types 5 and 6. This component is probably responsible for the decrease of Ni, Cu, Ga and Au observed in the fine metallic fractions of the equilibrated chondrite types.W, as well as Ga, increases in the bulk metals with the petrologic type, suggesting that a substantial amount of this element, as already observed for Ga by previous authors, is not in the metal, but in some silicate phases in the lower metamorphic petrologic types 3 and 4.Ir is always concentrated in the fine metallic fractions of all meteorites, independent of petrologic type, suggesting the presence of a fine-grained metallic component enriched in this element.     

The unique cosmic-ray history of the Malakal chondrite

Cosmogenic radionuclides, including²²Na,²⁶Al and⁵⁴Mn, were measured in a sample of the recently-fallen Malakal hypersthene chondrite. The high²⁶Al activity, 79 ± 2 dpm/kg, greatly exceeds the levels expected from elemental production rates, shielding considerations, or comparisons with other ordinary chondrites, and can only be explained by exposure to a uniquely high cosmic-ray flux. Calculations including noble gas,³H, and⁵³Mn data from other laboratories require a two-stage irradiation. Malakal's most probable history is: exposure in excess of 4 m.y. to an effective cosmic-ray flux approximately three times that experienced by other chondrites, an orbit change (very possibly caused by a collision), and a final period of about 2 m.y. during which it was exposed to a “normal” cosmic-ray flux.     

Zirconium isotope evidence for incomplete admixing of r-process components in the solar nebula

Isotopic anomalies in Mo and Zr have recently been reported for bulk chondrites and iron meteorites and have been interpreted in terms of a primordial nucleosynthetic heterogeneity in the solar nebula. We report precise Zr isotopic measurements of carbonaceous, ordinary and enstatite chondrites, eucrites, mesosiderites and lunar rocks. All bulk rock samples yield isotopic compositions that are identical to the terrestrial standard within the analytical uncertainty. No anomalies in ⁹²Zr are found in any samples including high Nb/Zr eucrites and high and low Nb/Zr calcium-aluminum-rich inclusions (CAIs). These data are consistent with the most recent estimates of <10⁻⁴ for the initial ⁹²Nb/⁹³Nb of the solar system. There exists a trace of isotopic heterogeneity in the form of a small excess of r-process ⁹⁶Zr in some refractory CAIs and some metal-rich phases of Renazzo. A more striking enrichment in ⁹⁶Zr is found in acetic acid leachates of the Allende CV carbonaceous chondrite. These data indicate that the r- and s-process Zr components found in presolar grains were well mixed on a large scale prior to planetary accretion. However, some CAIs formed before mixing was complete, such that they were able to sample a population of r-process-enriched material. The maximum amount of additional r-process component that was added to the otherwise well-mixed Zr in the molecular cloud or disk corresponds to ∼0.01%.     

Zirconium and hafnium in meteorites

The abundances of zirconium and hafnium have been determined in nine stony meteorites by a new, precise neutron-activation technique. The Zr/Hf abundance ratios for the chondrites vary in a rather narrow range, consistent with previously published observations from our group. Replicate analyses of new, carefully selected clean interior samples of the C1 chondrite Orgueil yield mean zirconium and hafnium abundances of 5.2 and 0.10 ppm, respectively. These abundances are lower than we reported earlier for two C1 chondrite samples which we now suspect may have suffered contamination. The new C1 zirconium and hafnium abundances are in closer agreement with predictions based on theories of nucleosynthesis than the earlier data.     

The oxygen isotope record in Murchison and other carbonaceous chondrites

The ranges of δ¹⁸O and δ¹⁷O in components of the Murchison (C2) chondrite exceed those in all other meteorites analyzed. Previous authors have proposed that C2 chondrites are the products of aqueous alteration of anhydrous silicates. A model is presented to determine whether the isotopic variations can be understood in terms of such alteration processes. The minimum number (two) of initial isotopic reservoirs is assumed. Two major stages of reservoir interaction are required: one at high temperature to produce the¹⁶O-mixing line observed for the anhydrous minerals, and another at low temperature to produce the matrix minerals. The isotopic compositions severely constrain the conditions of the low-temperature process, requiring temperatures < 20°C and volume fractions of water > 44%. Extension of the model to the data on C1 chondrites requires aqueous alteration in a warmer, wetter environment.     

87Rb87Sr dating of LL chondrites

⁸⁷Rb⁸⁷Sr analyses of LL chondrites have been made in 10 whole rock meteorites, chondrules from Chainpur (LL3) and Soko Banja (LL4), density separates and chondrules from Guidder (LL5) and density separates from Jelica (LL6) and Ensisheim (LL6). Whole rocks define an isochron of age 4.486±0.020 Ga (λ⁸⁷Rb=1.42×10^?11a^?1) and initial ratio (⁸⁷Sr/⁸⁶Sr)_I=0.69887±0.00012. This is in agreement with the results for H- and E-type chondrites. Analyses for chondrules from Soko Banja yield a very good isochron of age 4.452±0.020 Ga and strontium initial ratio 0.69954±0.00024, and give an interval for metamorphism of (37±10)×10⁶ a. A more poorly defined isochron is obtained for Jelica; the age is 4.423±0.041 Ga and the strontium initial ratio 0.69959±0.00029, indicating an interval for metamorphism of (70±60)×10⁶ a. No isochron could be obtained for Chainpur. This could be due to terrestrial alteration or to a late isotopic disturbance of the meteorite. The⁸⁷Rb-⁸⁷Sr system is also disturbed in Guidder and Ensisheim, probably as a consequence of shock. These results are discussed in comparison with our former studies, and in relation with thermal metamorphism in the LL chondrite parent body(ies).     

Genetic relations between iron and stony meteorites

The main group pallasites and the mesosiderites fall within the oxygen isotope group previously determined for the calcium-rich achondrites (eucrites, howardites and diogenites), consistent with derivation from a common source material, and perhaps a common parent body. The group IIE iron meteorites were derived from the same source material as H-group ordinary chondrites. The chondrite-like silicate inclusions in group IAB iron meteorites are not related to the ordinary chondrites, but may be related to the enstatite chondrites. Several meteorites previously considered “anomalous” fall into these groups: Pontlyfni and Winona with the IAB irons, and Netschaëvo possibly with the H chondrites and IIE irons. The unusual pallasites Eagle Station and Itzawisis have remarkable oxygen isotopic compositions, and have more of the ¹⁶O-rich component than any other meteorite. Bencubbin and Weatherford are also unusual in their isotopic compositions, and may bear some relationship to the C2 carbonaceous chondrites. Lodran and Enon are isotopically similar to one another and are close to the achondrite-mesosiderite-pallasite group.     

Composition and classification of clasts in the St. Mesmin LL chondrite breccia

Seven samples of the unique St. Mesmin meteorite have been analyzed by instrumental and radiochemical neutron activation analysis for Na, Ca, Sc, Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, Se, In, Sm, Yb, Ir and Au. St. Mesmin is unique in being the only ordinary chondrite known to contain an unmelted xenolith of another ordinary chondrite. Data for two host matrix samples and three light clasts are consistent with their classification as LL chondrite material. The composition of the large dark xenolith confirms earlier evidence that it is an H chondrite; volatile abundances are consistent with it being highly shocked, petrologic type-4 material. In an olivine microporphyry, siderophile abundances are mostly about 0.13 times LL abundances, an apparent indication of metal loss during the shock melting which produced the clast. As in other regolithic chondrites, the dark host has higher contents of highly volatile elements than do the light clasts. We suggest that this results from a combination of differences in intensity of preexisting metamorphism as well as a redistribution of volatiles during regolith gardening.The H-group xenolith in St. Mesmin is a relatively recent addition to the parent body (< 1.4 Ga ago), but it is argued that this does not require regolith activity at that time. Rather the view is supported that the regolith period occurred very early in the meteorite's history (&gsim;4.0 Ga ago) and may have been related to the growth of the parent body. The H-group fragment may be part of the projectile whose impact excavated the St. Mesmin meteoroid from the LL parent body.     

87Rb87Sr chronology of H chondrites: Constraint and speculations on the early evolution of their parent body

A precise⁸⁷Rb-⁸⁷Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and⁸⁷Sr/⁸⁶Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(⁸⁷Rb) = 1.42 × 10^?11yr^?1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the⁸⁷Rb/⁸⁶Sr vs.⁸⁷Sr/⁸⁶Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and⁸⁷Sr/⁸⁶Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and⁸⁷Sr/⁸⁶Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a⁸⁷Sr/⁸⁶Sr homogeneous nebula. Data from Guaren?a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by²⁶Al. An²⁶Al/²⁷Al ratio of 4–6 × 10^?6 is enough for heating such a body. Further tests for this model are proposed.