Nobuo Takaoka

The global accretion rate of extraterrestrial materials in the last glacial period estimated from the abundance of micrometeorites in Antarctic glacier ice

The accretion rate of micrometeorites in the last glacial period was estimated from the concentrations of micrometeorites in the blue ice around the Yamato Mts. in Antarctica. The samples from this study were collected from the five sampling points (M03, K02, K11, J09 and J10) in the blue ice. The blue ice was melted and filtered, and the micrometeorites were handpicked from the collected “glacial sands”. The weight of the micrometeorites in the blue ice was estimated from the abundance of recovered micrometeorites and the solar noble gas concentrations in the “residue” after handpicking. The age of the blue ice from the K area was estimated to be 27–33 kyr before present based on oxygen isotope data. The estimated accretion rate to the whole Earth ranges from 5300 × 103kg/a to 16000 × 103kg/a. However, the lower end of this range probably represents lower limits due to possible loss of solar noble gases during long residence in the glacier ice. Hence, we estimate that the accretion rate of micrometeorites 27–33 kyr before present to be in the range between (11000 ± 6600) × 103kg/a and (16000 ± 9100) × 103kg/a. These results, as well as the other estimates, suggest that the accretion rate of micrometeorites in the last glacial period was comparable to that in the present. Micrometeorite k]accretion rate k]Antarctica k]last glacial periods k]noble gas k]interplanetary dust particle

Bulk mineralogy of individual micrometeorites determined by X-ray diffraction analysis and transmission electron microscopy

Abstract Bulk mineralogy of individual fine-grained micrometeorites from 50 to 200 μm in diameter was determined on the basis of the powder X-ray diffraction patterns and the observation of internal textures by a transmission electron microscope (TEM). X-ray diffraction analysis of 56 micrometeorites indicated that 42, 11, and 3 samples are olivine-rich, pyroxene-rich, and phyllosilicate-rich micrometeorites, respectively. Among the phyllosilicate-rich micrometeorites, one contains saponite and other two contain serpentine. No samples contain both saponite and serpentine. We found that saponite-rich micrometeorite was weakly heated, which results in shrinkage of 001 basal spacing of saponite down to 9.7 A, and that cronstedtite, which is commonly contained in CM chondrites, occurs in serpentine-rich micrometeorites. Micrometeorites that consist entirely of anhydrous minerals and amorphous phases are predominant in the samples studied. The major phases of such micrometeorites are olivine, low-Ca pyroxene, magnetite, and Fe-sulfide and the average abundances are 65, 17, 11, and 7 wt%, respectively, when the total abundance of the four minerals are normalized to 100 wt%. The relative mineral abundance varies greatly between samples: low-Ca pyroxene/olivine ratios range from 0 to 3.5, with a mean of 0.3. TEM observations of inner portions of some micrometeorites revealed that they are aggregates of very small equigranular grains (∼100 nm) of olivine + magnetite, or low-Ca pyroxene + olivine + magnesiowustite. The textures are very similar to those of hydrous carbonaceous chondrite that was experimentally heated to temperature below melting point, thus suggesting that the micrometeorites had been hydrous particles but were decomposed by the brief heating upon atmospheric entry. It is newly found that magnesiowustite was formed in micrometeorites instead of magnetite as a product of phyllosilicate decomposition under low oxygen fugacity. The decomposed hydrous micrometeorites gave two types of characteristic X-ray diffraction patterns: (1) broad olivine and magnetite reflections or (2) variable intensities of magnesiowustite reflections together with magnetite, low-Ca pyroxene, and olivine reflections. Twenty-nine olivine- or pyroxene-rich micrometeorites showed such diffraction patterns, thus suggesting that more than half of micrometeorites investigated must be decomposed hydrous particles. The results confirmed that hydrous dust particles are much more abundant in the interplanetary space than in the micrometeorites recovered on the Earth.

Rare gas isotopic compositions in diamonds

RARE gas isotopic compositions such as 40Ar/36Ar, 3He/4He and 129Xe/132Xe in the Earth have provided a powerful tool for understanding the origin and evolution of the terrestrial atmosphere1–4. The isotopic information may be obtained from rare gases trapped in some mantle-derived materials such as volcanic rocks, volcanic xenoliths or volcanic gases. Among these mantle-derived materials, diamond seems to be unique due to its almost complete inertness to any known chemical and to its enormous stability against high temperature. Although the presence of O2, H2, CH4, H2O, CO, N2, Ar and CO2 in diamonds has been reported5,6. No previous measurement has been made either on elemental compositions or on isotopic ratios of rare gases in diamonds. Here we report on rare gas elemental composition and isotopic ratios in diamond. We found that 3He/4He ratio is more than an order of magnitude larger than the atmospheric value and also 40Ar/36Ar ratio is significantly higher.

Evaluating the thermal metamorphism of CM chondrites by using the pyrolytic behavior of carbonaceous macromolecular matter

The degrees of thermal metamorphism of 10 CM chondrites and of the Allende CV3 chondrite were evaluated from the viewpoint of “graphitization” of the carbonaceous macromolecular matter by means of flash pyrolysis-gas chromatography (GC). The unheated chondrites, Yamato- (Y-) 791198, Murray and Cold Bokkeveld, yielded larger amounts and wider varieties of pyrolyzates than the chondrites strongly heated in the parent asteroids, Y-82054, Y-86695, and Belgica- (B-) 7904, and Asuka- (A-) 881334 (more strongly heated than Y-793321, which has been weakly heated, but lesser than the other strongly heated meteorites). The weakly heated chondrites, Y-793321 and A-881458, showed intermediate features. The data indicate that graphitization of the carbonaceous matter is most extreme in the strongly heated chondrites and that during graphitization, the matter has lost its labile portion, which can generate pyrolyzates such as naphthalene. In order to establish a new method for the evaluation of the degree of graphitization of chondritic carbonaceous matter, a diagram was developed to show the relationship between the total amounts of pyrolyzates with retention times later than 5 min (=SRT>5) and the ratio of the amount of naphthalene, a pyrolysis product, to SRT>5 (=SN/SRT>5). The diagram indicates a possible evolutionary pathway of graphitization of the carbonaceous matter in carbonaceous chondrites.

Microdistribution of primordial noble gases in CM chondrites determined by in situ laser microprobe analysis: decipherment of nebular processes

Abstract In situ noble gas analyses using a laser microprobe with a beam diameter of 50–100 μm were performed on thin slices of Murchison and Yamato (Y-) 791198 CM carbonaceous chondrites in order to see microdistribution of primordial noble gases. Petrographic observations prior to noble-gas analyses showed that the two meteorite slices are entirely composed of chondrules, PCPs (poorly characterized phases) and other chondritic components that are rimmed by layers of fine-grained dust. Based on an existing classification scheme for texture of CM chondrites, the two samples are classified as primary accretionary rocks (PARs). The noble gas analyses showed that primordial noble gases are rich in the fine-grained rims around chondrules in the two meteorite slices. This suggests that large amounts of carrier phases of the primordial noble gases in CM chondrites are located in these rims which seem to have accreted on the surfaces of chondrules prior to the formation of the meteorite parent bodies. Rims on different chondrules within a given meteorite showed similar concentrations of heavy primordial noble gases, suggesting that phase Q is distributed homogeneously in the rims and possibly in the nebular region where the chondrules had acquired their dust rims. ( 20 Ne) diamond /( 20 Ne) Q ratios are relatively constant among rims in a given meteorite, indicative of homogeneous mixing of interstellar diamonds and phase Q on a 10 μg scale. One location in a rim around a chondrule in the Murchison sample showed an enrichment of 22 Ne, which suggests presence of carrier phases of Ne-E in the rim. A stepped heating analysis of a chip of Y-791198 was carried out to characterize the noble gases of bulk PAR. Abundances of primordial noble gases are only ∼50% of those of rims around chondrules, consistent with the relative abundances of gas-rich fine-grained material and gas-poor chondrules and inclusions in PAR. Solar gases were not detected, which implies that PARs, the primitive CM bodies, had grown to at least centimeter size in a dense solar nebula which shielded these objects from direct exposure to solar wind.

Heterogeneous distribution of solar and cosmogenic noble gases in CM chondrites and implications for the formation of CM parent bodies

Abstract Distribution of solar, cosmogenic, and primordial noble gases in thin slices of Murchison, Murray, and Nogoya CM carbonaceous chondrites was determined by the laser microprobe analysis so as to put some constraints on the parent-body processes in the CM chondrite formation. The main lithological units of the three meteorite slices were located by electron microscope observations and classified into clastic matrix and clasts of primary accretionary rocks (PARs) based on the classification scheme of texture of CM chondrites. All sample slices contain both clastic matrix and PARs. Clastic matrix shows a comminuted texture formed by fragmentation and mechanical mixing of rocks due to impacts, whereas PARs preserve the original textures prior to the mechanical disruption. Solar-type noble gases are detected in all sample slices. They are located preferentially in clastic matrix. The distribution of solar gases is similar to that in ordinary chondrites where these gases reside in clastic dark portions of these meteorites. The heterogeneous distribution of solar gases in CM chondrites suggests that these gases were acquired not in a nebular accretion process but in parent body processes. Solar energetic particles (SEP) are predominant in CM chondrites. The low abundance of low energy solar wind (SW) component relative to SEP suggests preferential loss of SW from minerals comprising the clastic matrix, due to aqueous alteration in the parent bodies. Cosmogenic noble gases are also enriched in some portions in clastic matrix, indicating that some parts of clastic matrix were exposed to solar and galactic cosmic rays prior to the final consolidation of the CM parent bodies. Primordial noble gases are rich in fine-grained rims around chondrules in all three meteorites. However, average concentrations of heavy primordial gases in the rims differ among meteorites and correlate inversely to the degree of aqueous alteration that the meteorites have experienced. This appears to have been caused by aqueous alteration reactions between fluids and carbonaceous carrier phases of noble gases.

Noble-gas-rich chondrules in an enstatite meteorite

Chondrules are silicate spherules that are found in abundance in the most primitive class of meteorites, the chondrites. Chondrules are believed to have formed by rapid cooling of silicate melt early in the history of the Solar System, and their properties should reflect the composition of (and physical conditions in) the solar nebula at the time when the Sun and planets were forming. It is usually believed that chondrules lost all their noble gases at the time of melting. Here we report the discovery of significant amounts of trapped noble gases in chondrules in the enstatite chondrite Yamato-791790, which consists of highly reduced minerals. The elemental ratios 36Ar/132Xe and 84Kr/132Xe are similar to those of ‘subsolar’ gas, which has the highest 36Ar/132Xe ratio after that of solar-type noble gases. The most plausible explanation for the high noble-gas concentration and the characteristic elemental ratios is that solar gases were implanted into the chondrule precursor material, followed by incomplete loss of the implanted gases through diffusion over time.

Collisional destruction experiment of chondrules and formation of fragments in the solar nebula

Collisional destruction experiments with chondrules from the Allende CV3 chondrite were performed over a range of velocities (10 m/s to 76 m/s). Electron microscopy shows that two types of chondrules were affected by low-velocity impacts: (1) reactivated pre-existing cracks filled with iron-oxides and (2) poorly crystallized finegrained silicates in glass. The relatively-well crystallized chondrules were destroyed at higher impact velocities. Based on the range of velocities causing chondrule destruction, we theoretically examined the condition of the solar nebula in the chondrule destruction periods and suggest that collisional destruction of chondrules can occur during abrupt and/or localized strong turbulence, in a nebular shock, by a collision between a chondrule and an object larger than 1 m in the laminar solar nebula.

Full-length sequence of mouse acupuncture-induced 1-L (aig1l) gene including its transcriptional start site.

We have been investigating the molecular efficacy of electroacupuncture (EA), which is one type of acupuncture therapy. In our previous molecular biological study of acupuncture, we found an EA-induced gene, named acupuncture-induced 1-L (Aig1l), in mouse skeletal muscle. The aims of this study consisted of identification of the full-length cDNA sequence of Aig1l including the transcriptional start site, determination of the tissue distribution of Aig1l and analysis of the effect of EA on Aig1l gene expression. We determined the complete cDNA sequence including the transcriptional start site via cDNA cloning with the cap site hunting method. We then analyzed the tissue distribution of Aig1l by means of northern blot analysis and real-time quantitative polymerase chain reaction. We used the semiquantitative reverse transcriptase-polymerase chain reaction to examine the effect of EA on Aig1l gene expression. Our results showed that the complete cDNA sequence of Aig1l was 6073 bp long, and the putative protein consisted of 962 amino acids. All seven tissues that we analyzed expressed the Aig1l gene. In skeletal muscle, EA induced expression of the Aig1l gene, with high expression observed after 3 hours of EA. Our findings thus suggest that the Aig1l gene may play a key role in the molecular mechanisms of EA efficacy.

Mass Spectrometrical Study of Rare Gas Compositions and Neutron Capture Effects in Yamato-74191 (L 3) Chondrite

Abstract The unequilibrated hypersthene chondrite Yamato-74191 was studied mass spectrometrically for rare gases released at various temperatures. Cosmogenic gases dominate in He and Ne. The meteorite contains large amounts of trapped Ar, Kr and Xe, and radiogenic 40Ar and 129Xe. Cosmic-ray irradiation and K-Ar ages were determined. In addition to spallogenic components of Kr and Xe, isotopic excesses of 80Kr, 82Kr, 128Xe and 126Xe relative to AVCC-Kr and -Xe were found. The ratio of 80Kr-excess to 82Kr-excess is 2.66 after correction for spallogenic Kr. A correlation between 128Xe/132Xe and 129Xe/132Xe was found The 129Xe/132Xe ratio for trapped Xe in Yamato-74191 was determined as 1.12 ± 0.29 with the correlation plot. The excesses found in Yamato-74191 are best explained by epithermal neutron capture on Br and I, and by the 127I (n, 2nβ) 126 Xe reaction. Using neutron-produced 80Kr, the neutron slowing -down density was estimated to be 0.14 ± 0.03 cm-3sec-1. A minimum mass and a preatmospheric radius was estimated to be 470 kg and 32 cm, respectively.