The Shaw chondrite,I. The case of the missing metal

The mineralogy, elemental and isotopic composition of the Shaw meteorite indicate that it is a highly metamorphosed L-group chondrite which has lost a portion of its metal and sulfide. The metal which remains has an unusual composition relative to that in other L-group chondrites. It is enriched in Ga, Ge, Ir, Mo, Os, Pt, Re and Ru and depleted in As, Au, Cu and Sb. A comparison of the relative enrichments and depletions in Shaw to those observed in San Cristobal, the extreme end-member of group IAB iron meteorites, shows that the metal phases in these two meteorites have complementary compositions. This implies that the metal in Shaw represents the residual solid of a partial melting process while the missing metal, which drained away, may have gone to form an iron meteorite, like San Cristobal.     

Tungsten in iron meteorites

Tungsten concentrations have been determined by instrumental neutron activation in 104 iron meteorites, and range from 0.07 to 5 μg/g. In individual groups, concentrations vary by factors of between 1.5 and 8, but there are negative W-Ni correlations in 8 groups: IAB, IC, IIAB, IID, IIE, IIIAB, IIICD, and IIIF. The lowest W concentrations are found in groups IAB and IIICD, which also have the smallest slopes on a W-Ni plot. Eighteen anomalous irons have W concentrations between 5 μg/g (Butler) and 0.11 μg/g (Rafrüti). The distribution of W in irons shows similarities to that of other refractory siderophilic elements (except Mo), but is closest to the distribution of Ru and Pt.Assuming that chemical trends in group IIIAB were produced by fractional crystallization, a value of 1.6 can be deduced for the distribution coefficient of W between solid and liquid metal, cf. 0.89 for Mo. Experimental evidence in support of these values is tenuous.     

Nickel isotope heterogeneity in the early Solar System

We report small but significant variations in the ⁵⁸Ni/⁶¹Ni-normalised ⁶⁰Ni/⁶¹Ni and ⁶²Ni/⁶¹Ni ratios (expressed as ε⁶⁰Ni and ε⁶²Ni) of bulk iron and chondritic meteorites. Carbonaceous chondrites have variable, positive ε⁶²Ni (0.05 to 0.25), whereas ordinary chondrites have negative ε⁶²Ni (− 0.04 to − 0.09). The Ni isotope compositions of iron meteorites overlap with those of chondrites, and define an array with negative slope in the ε⁶⁰Ni versus ε⁶²Ni diagram. The Ni isotope compositions of the volatile-depleted Group IVB irons are similar to those of the refractory CO, CV carbonaceous chondrites, whereas the other common magmatic iron groups have Ni isotope compositions similar to ordinary chondrites. Only enstatite chondrites have identical Ni isotope compositions to Earth and so appear to represent the most appropriate terrestrial building material. Differences in ε⁶²Ni reflect distinct nucleosynthetic components in precursor solids that have been variably mixed, but some of the ε⁶⁰Ni variability could reflect a radiogenic component from the decay of ⁶⁰Fe. Comparison of the ε⁶⁰Ni of iron and chondritic meteorites with the same ε⁶²Ni allows us to place upper limits on the ⁶⁰Fe/⁵⁶Fe of planetesimals during core segregation. We estimate that carbonaceous chondrites had initial ⁶⁰Fe/⁵⁶Fe < 1 × 10^− 7. Our data place less good constraints on initial ⁶⁰Fe/⁵⁶Fe ratios of ordinary chondrites but our results are not incompatible with values as high as 3 × 10^− 7 as determined by in-situ measurements. We suggest that the Ni isotope variations and apparently heterogeneous initial ⁶⁰Fe/⁵⁶Fe results from physical sorting within the protosolar nebula of different phases (silicate, metal and sulphide) that carry different isotopic signatures.     

Lithium isotope composition of ordinary and carbonaceous chondrites, and differentiated planetary bodies: Bulk solar system and solar reservoirs

In order to better constrain the Li isotope composition of the bulk solar system and Li isotope fractionation during accretion and parent body processes, Li isotope compositions and concentrations were determined on a number of meteorite falls and finds. This is the first comprehensive study that systematically investigates a representative set of samples from carbonaceous chondrites (CI, CM2, CO3, CV3, CK4 and one ungrouped member), enstatite chondrites (EH, EL), ordinary chondrites (H, L, LL), and achondrites (one eucrite, diogenites, one pallasite, and a silicate inclusion from a IAB iron).

Carbonaceous chondrites have an average isotope composition of δ⁷Li = + 3.2‰ ± 1.9 (2σ) which agrees with the average composition of relatively pristine olivines (representative for the bulk composition) from the Earth primitive upper mantle (PUM). This is lighter than the average δ⁷Li of the basaltic differentiates of the Earth, Moon and Mars and the achondrites. It is an important observation, however, that the lighter end of the isotopic range of the differentiates always coincides with the averages of the mantle olivines and the carbonaceous chondrites. From this we conclude that the bulk of the inner solar system consists mostly of material from carbonaceous chondrites and that the variation seen in the differentiates is due to planetary body processes. Ordinary chondrites are significantly lighter than carbonaceous chondrites. No significant differences in δ⁷Li exist between enstatite chondrites (n = 3) and carbonaceous or ordinary chondrites. The difference between carbonaceous and ordinary chondrites and the variability within the chondrites could indicate the existence of distinct Li isotope reservoirs in the early solar nebula.     

Stable calcium isotopic composition of meteorites and rocky planets

New measurements of mass-dependent calcium isotope effects in meteorites, lunar and terrestrial samples show that Earth, Moon, Mars, and differentiated asteroids (e.g., 4-Vesta and the angrite and aubrite parent bodies) are indistinguishable from primitive ordinary chondritic meteorites at our current analytical resolution (± 0.07‰ SD for the ⁴⁴Ca/⁴⁰Ca ratio). In contrast, enstatite chondritic meteorites are slightly enriched in heavier calcium isotopes (ca. + 0.5‰) and primitive carbonaceous chondritic meteorites are depleted in heavier calcium isotopes (ca. ? 0.5‰). The calcium isotope effects cannot be easily ascribed to evaporation or intraplanetary differentiation processes. The isotopic variations probably survive from the earliest stages of nebular condensation, and indicate that condensation occurred under non-equilibrium (undercooled nebular gas) conditions. Some of this early high-temperature calcium isotope heterogeneity is recorded by refractory inclusions (Niederer and Papanastassiou, 1984) and survived in planetesimals, but virtually none of it survived through terrestrial planet accretion. The new calcium isotope data suggest that ordinary chondrites are representative of the bulk of the refractory materials that formed the terrestrial planets; enstatite and carbonaceous chondrites are not. The enrichment of light calcium isotopes in bulk carbonaceous chondrites implies that their compositions are not fully representative of the solar nebula condensable fraction.     

Fe isotope fractionation in iron meteorites: New insights into metal-sulphide segregation and planetary accretion

Magmatic iron meteorites are considered to be remnants of the metallic cores of differentiated asteroids, and may be used as analogues of planetary core formation. The Fe isotope compositions (δ^57/54Fe) of metal fractions separated from magmatic and non-magmatic iron meteorites span a total range of 0.39‰, with the δ^57/54Fe values of metal fractions separated from the IIAB irons (δ^57/54Fe 0.12 to 0.32‰) being significantly heavier than those from the IIIAB (δ^57/54Fe 0.01 to 0.15‰), IVA (δ^57/54Fe − 0.07 to 0.17‰) and IVB groups (δ^57/54Fe 0.06 to 0.14‰). The δ^57/54Fe values of troilites (FeS) separated from magmatic and non-magmatic irons range from − 0.60 to − 0.12‰, and are isotopically lighter than coexisting metal phases. No systematic relationships exist between metal-sulphide fractionation factor (Δ^57/54Fe_M-FeS = δ^57/54Fe_metal − δ^57/54Fe_FeS) metal composition or meteorite group, however the greatest Δ^57/54Fe_M-FeS values recorded for each group are strikingly similar: 0.79, 0.63, 0.76 and 0.74‰ for the IIAB, IIIAB, IAB and IIICD irons, respectively. Δ^57/54Fe_M-FeS values display a positive correlation with kamacite bandwidth, i.e. the most slowly-cooled meteorites, which should be closest to diffusive equilibrium, have the greatest Δ^57/54Fe_M-FeS values. These observations provide suggestive evidence that Fe isotopic fractionation between metal and troilite is dominated by equilibrium processes and that the maximum Δ^57/54Fe_M-FeS value recorded (0.79 ± 0.09‰) is the best estimate of the equilibrium metal-sulphide Fe isotope fractionation factor. Mass balance models using this fractionation factor in conjunction with metal δ^57/54Fe values and published Fe isotope data for pallasites can explain the relatively heavy δ^57/54Fe values of IIAB metals as a function of large amounts of S in the core of the IIAB parent body, in agreement with published experimental work. However, sequestering of isotopically light Fe into the S-bearing parts of planetary cores cannot explain published differences in the average δ^57/54Fe values of mafic rocks and meteorites derived from the Earth, Moon and Mars and 4-Vesta. The heavy δ^57/54Fe value of the Earth's mantle relative to that of Mars and 4-Vesta may reflect isotopic fractionation due to disproportionation of ferrous iron present in the proto-Earth mantle into isotopically heavy ferric iron hosted in perovskite, which is released into the magma ocean, and isotopically light native iron, which partitions into the core. This process cannot take place at significant levels on smaller planets, such as Mars, as perovskite is only stable at pressures > 23 GPa. Interestingly, the average δ^57/54Fe values of mafic terrestrial and lunar samples are very similar if the High-Ti mare basalts are excluded from the latter. If the Moon's mantle is largely derived from the impactor planet then the isotopically heavy signature of the Moon's mantle requires that the impacting planet also had a mantle with a δ^57/54Fe value heavier than that of Mars or 4-Vesta, which then implies that the impactor planet must have been greater in size than Mars.     

The type three ordinary chondrities: A review

The ordinary chondrites are the largest group of meteorites, and the type 3 ordinary chondrites are those which experienced only very mild parent metamorphism; their study provides a unique means of studying the first solid material to from in the early solar system which is either free from the effects of mild metamorphism, or in which the effects of mild metamorphism can be distinguished from primary, nebular effects. In this paper we list all known type 3 ordinary chondrites and references to their study, their compositional data and data relating to the metamorphic history. We review current theories on their formation and the effects of metamorphism, with emphasis on quantitative considerations. Studies on the thermoluminescence properties of these meteorites, which have provided many new insights into their metamorphic history, are reviewed. Some of the least metamorphosed meteorites show evidence for aqueous alteration, which provides a link between the type 3 ordinary chondrites and objects containing water in various forms the carbonaceous chondrites, comets and planets with wet mantles.     

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.     

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%.     

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.     

Meteorite magnetism and the early solar system magnetic field

The ferromagnetism of irons, stony-irons, E-, H-, L- and LL-chondrites and achondrites is due to a metallic phase comprising mostly Fe and Ni and small amounts of Co and P. The ferromagnetic constituent in non-metamorphosed C-chondrites is magnetite, but some metamorphosed C-chondrites contain FeNi metallic grains too.

Among the stony meteorites, the content of metals as determined by their saturation magnetization (I_S) sharply decreases in the order E → H → L → LL → achondrites, whereas the I_S value for magnetite and additional metals in C-chondrites ranges from the I_S value of achondrites to that of L-chondrites.

With an increase of Ni-content in the metallic phase in chondrites of the order E → H → L → LL → C, the relative amount of Ni-poor kamacite magnetization, I_S(), in the total I_S decreases in the same order, from I_S()/I_S 1 for E-chondrites to I_S()/I_S 0 for C-chondrites. Thus, E-, H-, L-, LL- and C-chondrites and achondrites are well separated in a diagram of I_S()/I_S versus I, which could be called a magnetic classification diagram for stony meteorites.

As the surface skin layer of all meteorites is anomalously magnetized, it must be removed and the natural remanent magnetization (NRM) of the unaltered interior only must be examined for the paleomagnetic study. The NMR of C-chondrites is highly stable and that of achondrites is reasonably stable against AF-demagnetization, whereas the NMR of E-chondrites and ordinary chondrites as well as stony-iron meteorites is not very stable in most cases. Although the NRM of iron meteorites is reasonably stable, it is not attributable to the extraterrestrial magnetic field.

The paleointensity for Allende C₃-chondrite is estimated to be about 1.0 Oe assuming that its NRM is of TRM origin. The paleointensity for other reasonably reliable C-chondrites (Orgueil, Mighei, Leoville and Karoonda) is also around 1 Oe.

The paleointensity for two achondrites has been determined to be about 0.1 Oe. The NRM of other achondrites also suggests that their paleointensity is roughly 0.1 Oe.

The NRM of ordinary chondrites is less stable than that of C-chondrites and achondrites so that the estimated paleointensity for ordinary chondrites is less reliable. The paleointensity for comparatively reliable ordinary chondrites ranges from 0.1 to 0.4 Oe.

The paleointensity values of 1 Oe for C-chondrites and 0.1 Oe for achondrites may represent the early solar nebula magnetic field about 4.5 × 10⁹ years ago. A possibility that the paleomagnetic field for achondrites was a magnetic field attributable to a dynamo within a metallic core of their parent planet may also not be rejected.     

Investigations on cosmic-ray-produced nuclides in iron meteorites, 2. New results on41K/40K-4He/21Ne exposure ages and the interpretation of age distributions

The cosmic ray exposure ages of 16 iron meteorites were determined by the⁴¹K/⁴⁰K-⁴He/²¹Ne method. The ages measured in the present and in previous experiments are summarized and presented in form of various histograms characterizing the age distributions of the different chemical groups separately. Age clustering at 650 Ma (mega years) is typical for the group IIIAB. Age clustering at 400 Ma is observed for the IVA irons. Quasi-continuous age distributions are found for the groups IA, IIA, IIB, IVB and for the anomalous irons. The following interpretation is offered. The IIIA and IIIB irons have initially been core material of the same parent asteroid and were ejected in consequence of a single impact event about 650 Ma ago. The IVA irons represent core material of another asteroid which was hit and partially disrupted in consequence of an impact event about 400 Ma ago. The group IA exhibits meteorites with ages between 200 and 1200 Ma. The quasi-continuous character of this age distribution and cosmochemical evidence indicate for these irons a raisin-bread-like character of their initial distribution within the silicate mantle of their parent asteroid. In consequence of several or, perhaps, of many crater-forming impact events the mantle material was gradually destructed and ejected. In the age distribution of the IIA hexahedrites, ages <300 Ma predominate and ages >600 Ma seem to be missing. In attempting to understand this, the possibility must be taken into consideration that the mean life-time of hexahedrites in the interplanetary space might be shorter than that of other irons. The cause might be that the hexahedrite single crystals are perhaps easier cleavable in the space environment. A similar kind of selective mass wastage appears also to be the cause for the absence of stone meteorites with high exposure ages.     

The compositions of incipient shock melts in L6 chondrites

Microprobe analyses of 33 melt pocket glasses in five L6d and L6e chondrites show them to be chemically varied but typically enriched in the constituents of plagioclase relative to the host meteorites. This enrichment appears to increase with the degree of melting (0–6.5 vol.%), but other chemical variations among the glasses (sodium depletion, reduction of ferrous iron) appear to be unrelated to shock intensity and melt abundance.Chemical trends for melt pocket glasses differ sharply from those reported for chondrules in ordinary chondrites. Thus partial shock melting of chondritic material is an inadequate explanation for the chemical properties of chondrules.     

Accretion of the ordinary chondrites

The narrow size distributions of silicate and metal particles in 19 unequilibrated ordinary chondrites and other textural properties of these meteorites strongly suggest that chondritic material was sorted before or during its accumulation in parent bodies. Gravitational sorting during accretion is possible, but the conditions which it requires are implausible. Aerodynamic sorting - exclusion of small and/or low-density particles from a planetesimal moving through a mixture of gas and dust - can account for the textures of ordinary chondrites. It may also explain observed variations of siderophile element contents among and within the three groups of ordinary chondrites.     

Nickel isotopic studies in meteorites

We have analyzed the nickel isotopic composition of meteoritic materials by high-precision mass spectrometry. The samples analyzed include almost all meteorite types for which large isotopic anomalies have been reported for oxygen, silver, magnesium and titanium. These samples are C₁, C₃, L, LL, H and E chondrites, IVB irons, Eagle Station pallasite and inclusion, matrix and “whole rock” samples of the Allende meteorite. The result is that we have not found any anomaly for nickel isotopic compositions within our accuracy of 0.7‰ for⁶¹Ni/⁶⁰Ni, 0.4-0.08‰ for⁶²Ni/⁶⁰Ni and 1–1.5‰ for⁶⁴Ni/⁶⁰Ni.     

Pontlyfni: a differentiated meteorite related to the group IAB irons

The abundances of 23 major and trace elements in the Pontlyfni meteorite have been measured by instrumental neutron activation analysis. The compositions of the metal and silicate fractions suggest a genetic relationship between Pontlyfni and the group IAB irons.     

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.     

Oxygen isotopic constraints on the origin of Mg-rich olivines from chondritic meteorites

Chondrules are the major high temperature components of chondritic meteorites which accreted a few millions years after the oldest solids of the solar system, the calcium–aluminum-rich inclusions, were condensed from the nebula gas. Chondrules formed during brief heating events by incomplete melting of solid dust precursors in the protoplanetary disk. Petrographic, compositional and isotopic arguments allowed the identification of metal-bearing Mg-rich olivine aggregates among the precursors of magnesian type I chondrules. Two very different settings can be considered for the formation of these Mg-rich olivines: either a nebular setting corresponding mostly to condensation–evaporation processes in the nebular gas or a planetary setting corresponding mostly to differentiation processes in a planetesimal. An ion microprobe survey of Mg-rich olivines of a set of type I chondrules and isolated olivines from unequilibrated ordinary chondrites and carbonaceous chondrites revealed the existence of several modes in the distribution of the ?¹⁷O values and the presence of a large range of mass fractionation (several ‰) within each mode. The chemistry and the oxygen isotopic compositions indicate that Mg-rich olivines are unlikely to be of nebular origin (i.e., solar nebula condensates) but are more likely debris of broken differentiated planetesimals (each of them being characterized by a given ?¹⁷O). Mg-rich olivines could have crystallized from magma ocean-like environments on partially molten planetesimals undergoing metal–silicate differentiation processes. Considering the very old age of chondrules, Mg-rich olivine grains or aggregates might be considered as millimeter-sized fragments from disrupted first-generation differentiated planetesimals. Finally, the finding of only a small number of discrete ?¹⁷O modes for Mg-rich olivines grains or aggregates in a given chondrite suggests that these shattered fragments have not been efficiently mixed in the disk and/or that chondrite formation occurred in the first vicinity of the breakup of these planetary bodies.     

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.