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Abstract—We present laboratory mid‐infrared absorption spectra (2.5 urn to 16.0 μm) of powdered calcium‐aluminum‐rich inclusions (CAIs) and matrix separated from the carbonaceous chondrites Allende (CV3.2), Vigarano (CV3.3), and Ornans (C03.3). Two groups of spectra with different features were found for the CAI: in the first group spectra are dominated by spinel, pyroxene, and sodalite ± nepheline, where main features occur at 9.3 μm, 10.3 μm, and 11.3 μm. In the second group, characteristic minerals are spinel and melilite with typical band maxima at 11.0 μm and 12.3 μm, and a broad feature between 14.0 μn and 15.0 μn. The position of the broad spinel feature probably depends on its iron content.

Abstract—Infrared spectra of mineral grains from primitive meteorites could be useful for comparison with astronomical infrared spectra since some of their grains might be similar to those formed in the planet‐forming disks around young stars or in the envelopes surrounding late‐type stars. To assess the usefulness of meteorite spectra, olivine grains separated from primitive meteorites have been analyzed using FTIR microscope techniques in the 2–16 μm wavelength range. The sub‐micron sizes of the grains made a complex preparation process necessary.

Abstract—Previous studies of unmelted micrometeorites (>50 μm) recovered from Antarctic ice have concluded that chondrules, which are a major component of chondritic meteorites, are extremely rare among micrometeorites. We report the discovery of eight micrometeorites containing chondritic igneous objects, which strongly suggests that at least a portion of coarse‐grained crystalline micrometeorites represent chondrule fragments. Six of the particles are identified as composite micrometeorites that contain chondritic igneous objects and fine‐grained matrix. These particles suggest that at least some coarse‐grained micrometeorites (cgMMs) may be derived from the same parent bodies as fine‐grained micrometeorites. The new evidence indicates that, contrary to previous suggestions, the parent bodies of micrometeorites broadly resemble the parent asteroids of chondrulebearing carbonaceous chondrites.

Abstract—Zag is an H3‐6 chondrite regolith breccia within which we have studied 14 halite grains ≤3 mm. The purity of the associated NaCl‐H2O brine is implied by freezing characteristics of fluid inclusions in the halite and EPMA analyses together with a lack of other evaporite‐like phases in the Zag H3–6 component. This is inconsistent with multi‐stage evolution of the fluid involving scavenging of cations in the Zag region of the parent body. We suggest that the halite grains are clastic and did not crystallize in situ. Halite and water‐soluble extracts from Zag have light Cl isotopic compositions, δ37Cl = −1.4 to −2.8%. Previously reported bulk carbonaceous chondrite values are approximately δ37Cl = +3 to +4%. This difference is too great to be the result of fractionation during evaporation, and instead, we suggest that Cl isotopes in chondrites are fractionated between a light reservoir associated with fluids and a heavier reservoir associated with higher temperature phases such as phosphates and silicates. Extraterrestrial carbon released at 600 °C from the H3–4 matrix has δ13C = −20%, consistent with poorly graphitized material being introduced into the matrix rather than indigenous carbonate derived from a brine. We have also examined 28 other H chondrite falls to ascertain how widespread halite or evaporite‐like mineral assemblages are in ordinary chondrites. We did not find any more to add to Zag (H3‐6) and Monahans (H5), which suggests that such highly soluble phases were not usually preserved on the parent bodies.

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

Abstract—We measured the concentrations of the cosmogenic radionuclides 14C (half‐life = 5.73 × 103years) in the bulk and of 10Be (1.5 × 106years), 26Al (7.05 × 105years), 36C1 (3.01 × 105years) and the light noble gases in metal and stone fractions of the Chinguetti meteorite to investigate the controversial claim that the 4.5 kg mesosiderite is part of a much larger mass in the Mauritanian desert. Based on the 36C1‐36Ar, 10Be‐21Ne and 26Al‐21Ne pairs in the metal fraction, we derive an average cosmic‐ray exposure age of 66 ± 7 Ma. Chinguetti is now the third out of 20 mesosiderites with an exposure age between 60 and 70 Ma. This may be the first hint of a major impact on the parent body of the mesosiderites, which show ages ranging from 10 to 300 Ma (Terribilini et al., 2000). From the 14C‐10Be pair we derive a terrestrial age of 18 ± 1 ka, which seems too recent to be consistent with the original description of the main mass having a heavily wind‐eroded base, overhung by the upper part of the meteorite. Finally, from the radionuclide concentrations in combination with Monte‐Carlo based calculations, we conclude that our sample of Chinguetti was irradiated at a depth of ∼15 cm in an object not larger than 80 cm in radius. This is the most compelling evidence against the reports that the Chinguetti mesosiderite is a small fragment of a mass 100 m long and 40 m high.

Abstract—We report the results of a study of a halite‐siderite‐anhydrite‐chlorapatite assemblage in the Shergotty‐Nakhla‐Chassigny (SNC) Martian meteorite Nakhla. These minerals are found associated with each other in interstitial areas, with halite often being adjacent to or enclosing siderite. We suggest the halite and other minerals are Martian in origin because (1) the conditions of fall preclude significant amounts of terrestrial contamination or weathering having taken place; (2) textures indicate that the minerals within this assemblage crystallized at the same stage as some silicate and oxide minerals within the Nakhla parent; and (3) the association with siderite, which previous studies have shown has C and O‐isotopic compositions incompatible with an origin on the Earth. Siderite has the range of compositions: CaCO3, 0.1–5.7; MgCO3, 2.0–40.9; FeCO3, 23.2–87.0; MnCO3, 1.0–39.9 mol%. There are two compositional groupings: high‐MnCO3(≥30 mol%) and low‐MnCO3/high‐FeCO3, with a gap identified between the two. This may be a miscibility gap or, alternatively, the two compositional groupings may mark separate generations of carbonate. We have not found any textural evidence for the latter explanation, but acceptance of the presence of a miscibility gap would require independent work on Fe‐Mn carbonates to verify its existence. Trace element abundances have been determined by ion microprobe analysis on three siderite and one anhydrite grains. Siderite has light rare earth elements (2.2–7.3 × C1) greater than heavy rare earth elements (0.32–0.79 × C1) without Ce or Eu anomalies, and the anhydrite has a similar pattern. These abundances reflect the source composition rather than partitioning or complexing controls. They are not typical of hydrothermal signatures which generally do not have such smooth REE abundance patterns. The nature of the mineral assemblage suggests that its source rocks on Mars were evaporites. These may be common in the craters and flood plains of the Martian southern highlands. Two models are suggested in this paper to explain the incorporation of evaporitic material into the Nakhla igneous parent. It may have happened as a low‐temperature process (<200 °C) by crystallisation from an aqueous fluid. An origin at low temperature is compatible with the available experimental data on siderite stability. Alternatively, we suggest evaporitic material may have been incorporated into the Nakhla parent while melt was still present and crystallized ∼800 °C. The latter model can more readily explain the trace element abundances and also the siderite textures that imply intergrowth with residual interstitial melt. Both high‐ and low‐temperature models are consistent with the presence of evaporite sediments on Mars.

AbstractThe fusion crusts of meteorites form due to heating during atmospheric entry and have mineralogies which are strongly influenced by changes in oxidation state. We have studied the fusion crusts of the most reduced primitive meteorite group, the enstatite chondrites, since they should demonstrate pronounced changes on oxidation. The fusion crusts are dominated by highly unequilibrated assemblages of sulphide, metal and oxide with compositions indicative of progressive oxidation by the reaction with atmospheric oxygen. Troilite becomes depleted in Ti, Cr and Mn, and enriched in Ni with increasing oxidation. Enrichments in the Ni-contents of kamacite and depletions in Si-content also occur with oxidation, although contemporaneous enrichments in Si within metal droplets occurs by partial melting. Assemblages dominated by Fe-oxide are found within fusion crusts as reaction rims on metal, as veins and within troilite-metal assemblages as droplets and formed by oxidation of metal during heating. Despite the evidence for large increases in redox state during heating, fusion crusts also contain lithophile sulphides indicating a high degree of disequilibrium during the atmospheric reprocessing of enstatite chondrite materials. Based on comparisons with experimental phase relations, ablation rates of 0.08–0.25 cm s-1are predicted from the thermal gradients recorded in fusion crusts; these rates are broadly similar to those suggested for other meteorite groups.

Abstract—The N and C abundances and isotopic compositions of acid‐insoluble carbonaceous material in thirteen primitive chondrites (five unequilibrated ordinary chondrites, three CM chondrites, three enstatite chondrites, a CI chondrite and a CR chondrite) have been measured by stepped combustion. While the range of C isotopic compositions observed is only ∼δ13C = 30%, the N isotopes range from δ15N ' ‐40 to 260%. After correction for metamorphism, presolar nanodiamonds appear to have made up a fairly constant 3–4 wt% of the insoluble C in all the chondrites studied. The apparently similar initial presolar nanodiamond to organic C ratios, and the correlations of elemental and isotopic compositions with metamorphic indicators in the ordinary and enstatite chondrites, suggest that the chondrites all accreted similar organic material. This original material probably most closely resembles that now found in Renazzo and Semarkona. These two meteorites have almost M‐shaped N isotope release profiles that can be explained most simply by the superposition of two components, one with a composition between δ15N = ‐20 and ‐40% and a narrow combustion interval, the other having a broader release profile and a composition of δ15N ∼ 260%. Although isotopically more subdued, the CI and the three CM chondrites all appear to show vestiges of this M‐shaped profile. How and where the components in the acid‐insoluble organics formed remains poorly constrained. The small variation in nanodiamond to organic C ratio between the chondrite groups limits the local synthesis of organic matter in the various chondrite formation regions to at most 30%. The most 15N‐rich material probably formed in the interstellar medium, and the fraction of organic N in Renazzo in this material ranges from 40 to 70%. The isotopically light component may have formed in the solar system, but the limited range in nanodiamond to total organic C ratios in the chondrite groups is consistent with most of the organic material being presolar.

Nitrogen isotopes have played an important part in the acceptance of the hypothesis that SNC meteorites derive from Mars. As a result, these meteorites can be investigated for their carbon, sulphur, and hydrogen systematics with a view to learning something about the environmental conditions on the planet. Important aspects of the role of carbon, present in the form of carbon dioxide as an atmospheric gas and leading to the formation of carbonates by weathering or hydrothermal activity, can be established. The presence of indigenous organics is an intriguing possibility. A variety of new or emerging techniques which could improve our understanding of SNC meteorites and might be applied to a returned Martian sample are discussed.

Abstract—The C contents and isotopic compositions of four eucrites, four diogenites and two howardites have been determined. Stepped heating in an O atmosphere was employed to convert selectively different carbonaceous materials to CO2gas at various temperatures. This technique successfully distinguishes between terrestrial contaminants and indigenous C. With the exception of the Kapoeta howardite, the howardite, eucrite, and diogenite (HED) meteorites contain ∼10–30 ppm indigenous C with δ13C between −29% and −19%. Kapoeta (a regolith breccia) has an elevated C content and δ13C, due to the presence of 13C‐enriched carbonate minerals (δ13C ∼ +20%) in CM2‐ or CR2‐like clasts. The range in δ13C displayed by HED samples is similar to that of other solar system basalts, such as lunar rocks and Martian meteorites but distinctly different from that of the terrestrial mantle.

SNC meteorites—four shergottites, three nakhlites and Chassigny—are postulated to have originated on Mars1. Their late crystallization ages (<1,300 Myr compared with 4,600 Myr for other igneous meteorites) and the presence, in shock-produced glass in EETA79001, of noble gas2and nitrogen3components resembling the martian atmosphere provide evidence for such a provenance. If this interpretation is correct then carbon dioxide, by far the most abundant constituent of the martian atmosphere4, should also be present. The stepped combustions described here show that most of the carbon present in the samples can be ascribed to terrestrial contamination. EETA79001, however, contains 4.6 p.p.m. of an isotopically distinct component enriched in 13C (δ13C = +36‰), whereas high-temperature carbon of inferred igneous origin in this meteorite and other SNCs has a δ13C value of about −30‰. The 12C/13C ratio of the isotopically heavy component is within the error limits of Viking measurements4in the martian atmosphere and, thus, strengthens the case for a planetary origin. Another carbon-containing species, believed to be carbonate, has been found in Nakhla (δ13C = between +12 and +24‰) and may be a product of atmospheric weathering on Mars.

The stepwise oxidation of the Allende meteorite reported here has shown that there are at least three isotopically distinct carbon phases, each combustible over different temperature ranges. The temperature of combustion is dependent both on grain size of the fragments studied and/or on the nature of the carbonaceous material and its availability to the oxygen. Thus we attempt here to correlate the isotopic signatures recognized with phases whose existence has been postulated to explain noble gas systematics1,2and to provide some insight into the nature and location of these phases. Most of the carbon (δ13C= −20‰), possibly a highly cross-linked polymer, exists predominantly in the matrix and has been reported by others3to be the host of most of the normal planetary noble gases. Carbon protected in mineral grains may be isotopically heavier at −13 to −7‰, but is still more easily oxidized than graphite when exposed by HF/HCI demoralization. A very easily oxidized phase, as yet unidentified, has a δ13C value typically ∼ −23 to −27‰.

Chondritic meteorites contain grains which condensed in circumstellar environments and carry unusual isotope signatures1providing clues to the location in which grain formation took place. Isotope measurements on grains that are composed entirely of carbon or carbon compounds have, up to now, given compositions either close to typical Solar System values or enriched in 13C (refs 2–5). Despite the existence of astrophysical environments where 12C production predominates, highly 12C-enriched carbon com-ponents in meteorites are uncommon. Here we report the results of an analysis of an acid-resistant residue from the Allende (CV3) meteorite1,6, extensively combusted in oxygen at low temperatures (<500 °C) to remove isotopically normal carbon, which revealed the presence of a small amount (<0.1 p.p.m.) of carbon with a 12C/13C ratio close to 120, well above the value of 89 ± 3 found in terrestrial samples or bulk meteorites. This component is denoted Cλ. Stellar evolutionary processes favour lowering of the 12C/13C ratio; hence Cλ, which has a combustion temperature consistent with graphite, might be a primitive type of interstellar material.