L. Schultz

Acfer 182 and paired samples, an iron-rich carbonaceous chondrite: Similarities with ALH85085 and relationship to CR chondrites

Three samples of a new, Fe-rich chondrite were found in the Sahara in 1990 and 1991 (Acfer 182, Acfer 207, Acfer 214). The samples are paired and the meteorite will be designated as Acfer 182. The chondrite is chemically, texturally, and mineralogically similar to the Allan Hills meteorite ALH85085. One important difference between the two meteorites is the smaller average chondrule size in ALH85085. The major components of Acfer 182 (in decreasing abundance) are 1. (1) highly altered (by terrestrial weathering) matrix 2. (2) mineral and polymineralic silicate fragments and aggregates 3. (3) chondrule fragments 4. (4) chondrules 5. (5) metal 6. (6) fine-grained, dark inclusions. The abundance of chondrules is lower and the average chondrule size (

Paired Renazzo-type (CR) carbonaceous chondrites from the Sahara

90 μm) smaller than in most other chondrites. Chondrule fragments are often so large that they cannot be derived from the present chondrule population. Apparently, size sorting has prevented accumulation of the larger parent chondrules. Several spectacular Ca,Al-rich inclusions were found, rich in Ca-dialuminate, hibonite, or Zr-, Y-, Sc-bearing phases. The chemical composition of Acfer 182 and of ALH85085 are almost indistinguishable. Major chemical signatures are 1. (1) uniform enrichment of Fe and other nonvolatile metals relative to CI-chondrites by about 70% 2. (2) absence of enrichment in refractory lithophiles, characteristic of most type 2 and 3 carbonaceous chondrites 3. (3) strong depletion of volatile and moderately volatile elements. Based on the oxygen isotopic composition, the chemical composition, and the abundances of chondrules and matrix, Acfer 182 should be classified as a carbonaceous chondrite. Considering their affinity to carbonaceous chondrites and their high bulk iron content the two meteorites, Acfer 182 and ALH85085, are designated as CH-chondrites. There are mineralogical and chemical similarities among Acfer 182, ALH85085, and CR chondrites which distinguish these meteorites from other types of carbonaceous chondrites: 1. (1) low FeO contents of olivine and pyroxene and correspondingly high metal contents 2. (2) high Cr-content in olivine 3. (3) abundant fine-grained dark inclusions 4. (4) abundant Ca-dialuminate (CaAl4O7) in CAIs 5. (5) similarities in oxygen isotopic composition 6. (6) low contents of moderately volatile elements 7. (7) low refractory element contents 8. (8) presence of a unique component of subsolar rare gases. These observations suggest similar conditions of formation for the components of these meteorites. A single common parent body is unlikely in view of the differences in chemical composition and in the size distribution of individual components.

The Acapulco meteorite: Chemistry, mineralogy and irradiation effects

Ten chondrites with chemical and mineralogical similarities to the carbonaceous chondrite Renazzo were recovered at two locations of the Sahara: Acfer 059, 087, 097, 114, 139, 186, 187, 209, 270 and El Djouf 001. Although the El Djouf location is more than 500 km away from the Acfer location, all samples appear to result from a single fall based on chemical and petrographic similarities and supported by light element stable isotope geochemistry, noble gas record, and similar 26Al contents. The Acfer-El Djouf meteorite is classified as a CR (Renazzo-type) carbonaceous chondrite. This group presently comprises three non-Antarctic members (Al Rais, Renazzo, Acfer-El Djouf) and five Antarctic meteorites. The major lithological components of the Acfer-El Djouf meteorite are large chondrules (up to 1 cm in size; mean diameter: 1.0 ± 0.6 mm), chondrule and mineral fragments, Ca,Al-rich inclusions, FeNi-metal (about 8–10 vol%) and dark inclusions embedded in a fine-grained fragment-bearing groundmass. Mineral compositions of the ten Acfer-El Djouf samples are similar to those of other CR chondrites. Most of the Ca,Al-rich inclusions are below 300 μm in size and rich in melilite and spinel. In some CAIs the rare phase CaAl4O7 is dominant. Fo-rich, Cr-bearing olivine (Fa0–4) and enstatite (Fs0–4) are the major phases of the chondrite. The meteorite is mildly shocked with a shock stage of S2 indicating a peak shock pressure of 5–10 GPa for the bulk meteorite. The oxygen isotopic compositions and carbon and nitrogen stable isotope geochemistry of the Acfer-El Djouf samples are very similar to those of the other CR-type chondrites. The major element composition of the Acfer-El Djouf meteorite is indistinguishable from CR chondrites. When compared to Renazzo the Acfer-El Djouf samples, however, have systematically lower contents of the moderately volatile elements Zn, Ga, As, Au, Sb, and Se and the highly volatile elements Br, C, and N. This is thought to reflect primary differences between Renazzo and the Acfer-El Djouf meteorite.

Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy

Abstract The Acapulco meteorite fell in August, 1976, at El Quemado, near Acapulco, Mexico. It is a unique object with chondritic composition but achondritic texture. High degree of recrystallisation and mineral chemical data indicate formation of the meteorite under redox conditions intermediate between those of H- and E-chondrites at ~ 1100°C, from which it cooled at a rate > 10°C/Myr. The major element composition is within the range of H-chondrites. Troilite and metal, and associated trace elements, are inhomogeneously distributed. Chromium is a factor of two higher than in H-chondrites. Enrichments of P and U indicate high phosphate content. Limited extent of partial melting may explain the light REE enrichment. However other incompatible elements have normal H-chondritic abundances or are even depleted like K and Rb. Moderately volatile or volatile elements (e.g. Mn, Ga, Ge, Zn) are enriched nearly to the level of C1-chondrites. Planetary noble gases are also significantly higher than in equilibrated ordinary chondrites. High temperature recrystallisation has not affected volatile element abundances. Compared to H-chondrites Acapulco is enriched in refractory siderophile elements. The distribution of W and other siderophile elements between metal and silicate phases are indicative of the higher temperature and lower oxygen fugacity of the assemblage. However, contrary to previous claims, the distribution of W cannot be used to calculate the equilibration temperature. Low K and high U contents are also reflected in the anomalous amounts of 40 Ar and 4 He. The old K-Ar age (4.7 ± 0.3 Gyr) and high 244 Pu track densities indicate mobilisation of U and Pu-rich phases shortly after formation of the parent material. This and other evidence suggests that Acapulco may represent a rock formed in the early stages of incipient melting of a chondritic parent body. However, since compositional differences between Acapulco and H-chondrites cannot be explained by fractionation processes on the Acapulco parent body. Acapulco must have originated from a different parent body. Lack of depletion of volatile elements, absence of chondrules and reduced mineral composition indicate some relationship of Acapulco to silicate inclusions in iron meteorites and to other unusual meteorites. Oxygen isotopes and chemical data suggest that there are at least three different groups of reduced chondritic meteorites: 1. (a) Acapulco, Lodran, and probably Allan Hills A 77081; 2. (b) Pontlyfni, Mount Morris, Winona and silicate inclusions in IAB iron meteorites, and 3. (c) Kakangari. An exposure age of 5 × 10 6 yr is deduced from spallogenic rare gas data.

Terrestrial 81Kr-Kr ages of Antarctic meteorites

21Ne cosmic-ray exposure ages have been calculated from literature data for 201 H, 203 L and 38 LL chondrites, corrected for shielding differences when possible. The distributions of exposure ages again show the familiar peaks at 4.5 and 20 Myr for the Hs, but no outstanding events for the Ls and LLs. If the L-chondrite distribution is interpreted as a series of discrete events, then at least 6 peaks between 1 and 35 Myr are needed to model it. The observations, that every petrologic type occurs in every large peak and that even the higher petrologic types contain solar wind gases, suggest that the parent bodies have been fragmented and reassembled into a megabreccia. For the H chondrites, both large and small peaks contain about 15% solar-gas bearing meteorites, which could mean that surface material has been mixed to depths represented by the largest event, on the order of a kilometer. In contrast, only 3% of the Ls contain solar wind, which may be related to breakup of their parent planet. Those Ls with especially low radiogenic He (U,Th-He ages < 1 AE) tend to have low exposure ages: their distribution may be biased by a subgroup that had orbits coupling short capture lifetimes with significant solar heating.

Noble gases in the St. Mesmin chondrite: Implications to the irradiation history of a brecciated meteorite

The production rate of 38Ar in meteorites—P(38)—has been determined, as a function of the samples chemical composition, from 81Kr-Kr exposure ages of four eucrite falls. The cosmogenic 78Kr/83Kr ratio is used to estimate the shielding dependence of P(38). From the “true” 38Ar exposure ages and the apparent 81Kr-Kr exposure ages of nine Antarctic eucrite finds, terrestrial ages are calculated. They range from about 3 × 105a (Pecora Escarpment 82502) to very recent falls (Thiel Mountains 82502). Polymict eucrites from the Allan Hills (A78132, A79017 and A81009) have within the limits of error the same exposure age (15.2 × 106a) and the same terrestrial age (1.1 × 105a). This is taken as strong evidence that these meteorites are fragments of the same fall. A similar case are the Elephant Moraine polymict eucrites A79005, A79006 and 82600 with an exposure age of 26 × 106a and a terrestrial age of 1.8 × 105a. EETA79004 may be different from this group because its exposure age and terrestrial age are 21 × 106a and 2.5 × 105a, respectively. The distribution of terrestrial ages of Allan Hills meteorites is discussed. Meteorites from this blue ice field have two sources: Directly deposited falls and meteorites transported to the Allan Hills inside the moving Antarctic ice sheet. During the surface residence time meteorites decompose due to weathering processes. The weathering “half-life” is about 1.6 × 105a. From the different age distributions of Allan Hills and Yamato meteorites, it is concluded that meteorite concentrations of different Antarctic ice fields need different explanations.

Allan Hills 77081—an unusual stony meteorite

Abstract Helium, neon and argon were analysed in matrix samples and in different clasts of the polymict-brecciated LL-chondrite St. Mesmin. All clasts have high K-Ar ages with a mean value of 4.40 ± 0.26Ga . One exotic H-group xenolith, however, has a K-Ar age of only 1.36 ± 0.05Ga . The low age indicates that the St. Mesmin breccia was compacted to its present structure relatively late in its history and that the St. Mesmin meteorite developed from regolith material on the meteorites parent body. This is further demonstrated by the high concentrations of solar noble gases in the matrix and the cosmic ray pre-exposure of one individual clast.

Radiogenic, fissiogenic and nucleogenic noble gases in zircons

Abstract Allan Hills (ALHA) 77081 is achondritic in texture while the mineral composition and the chemistry are chondritic with the exception of a few elements. An assignment to one specific group of ordinary chondrites is therefore difficult. In many respects this meteorite is similar to the unusual stone meteorite Acapulco. The REE pattern of ALHA 77081 is essentially flat and the distribution ratios of siderophile elements between metal and silicates are high compared to ordinary chondrites. Gas retention ages are 3.5±0.5 AE for U, Th-He and 4.50±0.15 AE for K-Ar. In spite of the high degree of recrystallisation the meteorite contains trapped noble gases in amounts comparable to type 4 chondrites. Cosmic ray tracks and spallogenic noble gases indicate a small preatmospheric radius of about 2–3 cm. Spallogenic nuclides produced by solar cosmic rays or stopped solar flare ions may be present.

Helium- und Neonisotope in Eisenmeteoriten und der Tritiumverlust in Hexaedriten

The concentrations and isotopic compositions of argon, krypton and xenon have been determined in a grain size suite of zircons separated from pyroxene syenite of the Botnavatn Igneous Complex, southwestern Norway. The UPb systematics of these zircons has been studied previously. Kr and Xe are mixtures of fissiogenic gas from the spontaneous fission of238U and a component with atmospheric isotopic composition. From correlation diagrams the fissiogenic component is determined to be:83Kr :84Kr :86Kr = (4.6 ± 1.3) : (11.0 ± 2.0) : 100 and129Xe :131Xe :132Xe :134Xe :136Xe = (0.6 ± 0.3) : (8.8 ± 0.2) : (56.8 ± 0.3) : (82.8 ± 0.4) : 100. The fissiogenic136Xe/86Kr is 6.0 ± 0.4. The Ar isotopic composition shows radiogenic40Ar and a small excess of38Ar. The excess38Ar of about 1 × 10−11 cm3 STP/g can be explained by reactions of α-particles with chlorine. Asymmetric fission of238U which has been postulated to cause argon isotope anomalies in U-rich minerals is unnecessary to explain the observed38Ar concentrations. UXe ages are (1.19 ± 0.07) Ga, in agreement with UPb ages. However, if the recoil loss of fissiogenic Xe is considered the UXe ages of these zircons are about 1.53 Ga, which is comparable with the KAr ages and some RbSr ages observed in basement rocks in this region. The uncertainty of the product of fission yield times spontaneous fission decay constant of238U prevents to decide which age is the true crystallization age.

Noble gases and H chondrite meteoroid streams: No confirmation

The He- and Ne-contents as well as their isotopic composition have been measured of 40 iron meteorites, including 12 hexahedrites. These hexahedrites were of particular interest because many of them showed a definite 3He-deficiency which we explain by a loss of tritium. This effect was already found by us previously for the hexahedrite Braunau. The tritium loss depends on the temperature the meteorites were exposed to, hence, on their orbital elements. Some of the hexahedrites have lost practically all their tritium produced during their exposure to the cosmic rays, indicating that they always had been in the vicinity of the Earth or even closer to the Sun. Two octahedrites, Anoka and Staunton, suffered an appreciable loss of tritium too. From the rare gas data of meteorites with a nearly complete loss of tritium the cross-section ratios of the direct production of tritium, 3He, and 4He by the cosmic rays in iron meteorites could be calculated. In the ataxite Washington County a portion of the rare gas content was identified to be of primordial origin.