Neutron-induced prompt gamma ray analysis (PGA) was applied to several lump samples of the Kobe meteorites, along with powdered sample and separated chondrules, and a total of 15 elements (H, B, Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Mn, Fe, Co and Ni) were determined non-destructively. Except for chondrule samples, uniformity in chemical composition is remarkable for most elements. Exceptions are B and Cl, which appear to be heterogeneously distributed in the Kobe meteorite. Comparing Mg/Si, Al/Si, Ca/Si and Ti/Si ratios among carbonaceous chondrites (CI, CM, CV, CO and CK), CK chondrites are characterized by the highest Mg/Si ratio. Abundances of S and Zn were observed to coincidently change among carbonaceous chondrites, as suggested by similar condensation temperatures. Boron abundances in CK are even lower than those for S and Zn, being contrary to the prediction from thermodynamic calculation. For the classification of carbonaceous chondrites, we propose a S/Mn vs. Al/Mn diagram, on which individual groups of CI, CM, CV, CO and CK cluster with discrete locality. These three key elements are all determined for voluminous and irregularly shaped chondrite samples by PGA, showing that PGA has a great merit in analyzing new fall meteorites like Kobe.

View full abstract

The Kobe (CK4) meteorite and other CK chondrites are characterized with respect to rare earth element (REE) abundances using 12 bulk samples from 10 CK chondrites including Ningqiang (CK3), Karoonda (CK4), Kobe (CK4), Y-693 (CK4), A-882113 (CK4), Maralinga (CK4), Y-82102 (CK5), Y-82105 (CK5), Y-82191 (CK6) and A-881551 (CK6). REE, Ba, Sr, Ca, Mg, Fe, Rb and K were analyzed by high-precision isotope dilution. The CK chondrites examined exhibit systematically higher REE abundances compared to ordinary chondrites, comparable to CV and CO, flat REE patterns with minor negative anomalies up to −15% on Cc and −25% on Eu. These chondrites also exhibit an appreciable light/heavy REE gap and Yb anomaly compared to CI chondrites. CK3-4 chondrites in general are found to exhibit a larger negative Cc anomaly (mean anomaly −9.6 ± 3.8%) compared to CK5-6 chondrites (3.3 ± 1.0%), and some CK5-6 show a slightly light-REE depleted pattern with less-pronounced negative Ce anomaly. It is suggested that these CK chondrites had a larger negative Cc anomaly prior to early thermal metamorphism, which yielded the minor redistribution of light REEs. No clear correlation is found between Sm and Eu/Sm or between Eu and Sr. From these observations, it is suggested that Eu existed in the trivalent state due to higher oxidizing conditions, suppressing redistribution of REEs among minerals compared to metamorphosed ordinary chondrites during the early thermal metamorphic event on the parent body. The systematic REE anomalies observed in CK chondrites are considered to be inherited properties from the refractory precursors produced in the early solar nebula.

View full abstract

Petrography and mineralogy of one of the polished sections of the Kobe meteorite (CK4) are described as a part of the Kobe meteorite consortium. Porphyritic olivine chondrules are dominant and one barred olivine chondrule was observed. Olivine grains both in chondrules and matrix are homogeneous and have compositions of Fo₆₉. Pyroxenes are also abundant in chondrules and matrix. Orthopyroxene and augite grains except for augite grains in chondrules are also homogeneous and have the average compositions of about En₇₃Fs₂₆Wo₁ and En₄₄Fs₁₀Wo₄₆, respectively. The homogeneity of these two pyroxene grains is due to parent body metamorphism. The equilibration temperature of the chondrite was estimated to be around 800°C based on pyroxene geothermometers and an olivine-spinel geothermometer. Plagioclase shows compositional variations. Albite-rich (about An₄₀) and anorthite-rich (∼An₆₀) grains were observed in regions of the studied section. The inhomogeneity may be attributed to the miscibility gap close to the inferred metamorphic temperature.

View full abstract

Cosmogenic radionuclides in the Kobe chondrite (CK4) fell on September 26th, 1999, have been measured immediately after the fall (21 hours) using a ultra low-background Ge-detector. Nineteen cosmogenic nuclides including the very short-lived ²⁴Na were detected and their activities place constrains on the exposure history of the meteorite and reflect effect of solar modulation of galactic cosmic rays during solar maximum. Two nuclides, ²⁸Mg and ⁴³K were possibly detected for the first time and signals probably due to ⁵⁶Ni and ⁵⁷Ni were present although statistical uncertainties were very large. On the other hand, low activities of ⁶⁰Co (∼1 dpm/kg) and ²⁶Al (∼38 dpm/kg) mainly suggest a small preatmospheric size (≤10 cm) of the Kobe meteorite.

View full abstract

We have investigated elemental and isotopic compositions of noble gases in the Kobe (CK4) chondrite by a stepwise heating method. The previously observed inconsistency in cosmic-ray exposure ages derived from cosmogenic ²¹Ne and ³⁸Ar was eliminated in this study, and the averaged value of 38.2 Ma was obtained. Based on neutron-produced ⁸⁰Kr, a pre-atmospheric size of Kobe is estimated to be larger than ∼23 cm radius, whereas the results from cosmogenic radionuclides indicate a pre-atmospheric radius of ∼10 cm. These results indicate a two-stage exposure history for the Kobe meteorite. The release pattern of the primordial noble gas from Kobe indicates that Kobe has lost its primordial gas by thermal metamorphism. However, the observed Q-Xe concentration is lower than that expected from the HL-Xe as a carbonaceous chondrite, indicating preferential loss of Q-Xe. This cannot be explained solely by thermal metamorphism, suggesting that shock events at high temperature have caused the preferential loss of Q-Xe over HL-Xe in the Kobe meteorite.

View full abstract

The Kobe CK4 carbonaceous chondrite is depleted in spallogenic and radiogenic He as well as trapped noble gases. The loss of spallogenic and radiogenic He is likely due to heating by solar radiation. Concordant cosmic-ray exposure ages are obtained from spallogenic ²¹Ne, ³⁸Ar and ⁸³Kr. The ²¹Ne-basis exposure age is 42.3 Myr, using the production rate for CV chondrites which are compositionally similar to CK chondrites. We have 3.3 Ga for K-Ar age. Kobe contains neutron-induced ⁸⁰Kr, ⁸²Kr and ¹²⁸Xe. The neutron-induced ⁸⁰Kr/⁸²Kr ratio agrees with that for Kr produced by epithermal neutron captures on ⁷⁹Br and ⁸¹Br. From the slowing-down density and the Fermi age of epithermal neutrons, we have the minimum radius (or mass) of 24 cm (or 200 kg with the density of 3.5 g/cm³) for the precursor meteoroid during neutron-capture reactions. Kobe was also analyzed for He, Ne and Ar by laser-microprobe techniques. The preliminary result shows heterogeneous distribution of noble gas components. A barred olivine chondrule is highly enriched in spallogenic gases and radiogenic He and Ar. It is suggested that a potential source for these components in the chondrule is mesostasis, because it is usually enriched in target elements O, Na and Ca for spallogenic gases and parent nuclides K and U for radiogenic gases, though the chondrule has never been analyzed for concentrations of these elements. A search for carriers of the trapped noble gas component, including matrix minerals, magnetite and also sulfides, olivine and mesostasis of chondrules did not yield positive results.

View full abstract

The Kobe meteorite was imaged by a micro X-ray CT scanner to examine the possibility of three-dimensional curation of meteorite samples without destruction. Materials including major minerals in meteorites were also imaged to obtain a quantitative relation between CT value, which is numerical expression of CT contrast, and X-ray linear attenuation coefficient (LAC) of the materials under polychromatic X-ray beams. CT values of the samples are normalized by that of an olivine crystal from San Carlos, U.S.A. (SCO = San Carlos olivine: Fo_91.8), which was imaged under the same condition as a standard material (normalized CT value = CT value(sample)/CT value(SCO) × 100 [OU]: OU = olivine unit). The normalized CT values almost equal to similarly normalized LAC values calculated for the samples at <150–250 OU. We can estimate materials by the normalized CT value-LAC relation even if we use polychromatic X-ray beams. Successive 380 CT slice images of apiece of the Kobe meteorite (Kobe C-5-1; 1.144 g and 7–9 mm) were taken to obtain the three-dimensional structure. The pixel size of 31.6 μm × 31.6 μm and the slice width of 25.3 μm determine the spatial resolution for the images of about 60–90 μm. Thin sections were made to compare the CT images with images by an optical microscope, SEM and X-ray fluorescent microscope. In the CT images, we can distinguish plagioclase, ferromagnesian silicates (mainly olivine), magnetite-pentlandite and molybdenite. The CT value for the ferromagnesian silicates is approximately the same as that expected from olivine of FO_∼69 in the meteorite. Orthopyroxene and clinopyroxene were not distinguished from olivine due to their small grain sizes and LAC values similar to olivine. Magnetite and pentlandite cannot be distinguished from each other due to their similar LAC values. The CT values of magnetite-pentlandite aggregates are smaller than those expected from their LAC values because the aggregates have inclusions of silicate minerals and Ca phosphates. The CT value of plagioclase is slightly larger than that expected due to small inclusions too. Chondrules with rims with different contrasts can be recognized while chondrules without rims cannot. The two-dimensional shapes of some magnetite-pentlandite aggregates are circular like opaque chondrules under an optical microscope. However, they are not in spherical shape three-dimensionally. Their sub-rounded shapes might be formed by partial melting of these aggregates probably at the same time as a local melting of plagioclase and some olivine. The sample was cut into seven plates for different analysis at different laboratories. Three-dimensional CT images were made for the plates to understand their internal structures. The present results suggest that curation of meteorites is possible by micro X-ray CT.

View full abstract