Hideyasu Kojima

The Carlisle Lakes-type chondrites - A new grouplet with high Delta(17)O and evidence for nebular oxidation

Abstract Carlisle Lakes, ALH85151, and Y75302 are similar ungrouped chondrites which have petrologic and bulk compositional similarities to the ordinary chondrites, but are more oxidized; and their oxygen isotopic compositions differ. They represent a new grouplet which we call the Carlisle Lakes-type chondrites. They have the highest Δ17O values (up to 2.91) measured to date. The whole chondrites and most of their chondrules plot on the same mass fractionation line on an oxygen 3-isotope diagram. They are olivine rich (>70 vol%), essentially metal free, and most olivine is FeO rich, equilibrated at Fa38. Rare olivine and pyroxene grains in chondrules and fragments are zoned, and these are important in discerning the history of these chondrites. The zoning does not appear to have formed during crystallization from a melt droplet chondrule, but post-dated chondrule formation. Two hypotheses are postulated to explain the zoning: 1. (1) parent-body thermal metamorphism 2. (2) nebular gas-solid exchange reactions accompanied by condensation of new FeO-rich olivine, utilizing existing olivine surfaces as nucleation sites. The occurrence of steep Fe-Mg compositional gradients of core-to-rim profiles, oscillatory zoning in olivine, fayalitic rims of Fa45 that exceed instead of approach the equilibrium composition of the matrix (Fa38), and olivine-filled veins in zoned pyroxenes are more compatible with the nebular hypothesis. The Carlisle Lakes-type chondrites may have originally been derived from an ordinary chondrite-like precursor which was later oxidized, prior to its final lithification. However, the oxygen isotopic compositions of the whole chondrites and most of their chondrules suggest that the precursor probably formed in an oxygen isotopically distinct environment.

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

Petrography and oxygen isotopic compositions in refractory inclusions from CO chondrites

Fine-grained Ca-Al-rich inclusions (FGIs) in Yamato-81020 (CO3.0) and Kainsaz (CO3.1–CO3.2) chondrites have been studied by secondary ion mass spectrometry. The FGIs from Yamato-81020 consist of aggregates of hibonite, spinel, melilite, anorthite, diopside and olivine grains with no petrographic evidence of alteration. In contrast, the FGIs from Kainsaz commonly contain alteration products such as nepheline. From replacement textures and chemical compositions of altered and unaltered FGIs, we conclude that the alteration products formed by decomposition of melilite and anorthite. All phases in the Yamato-81020 FGIs are enriched in 16O, with δ17, 18O = ∼−40‰ except for one FGI that experienced melting. Oxygen isotopic compositions of melilite, anorthite, some spinel and diopside in Kainsaz FGIs changed from δ17, 18O = ∼−40‰ toward 0‰ by aqueous alteration. Alteration products in FGIs are depleted in 16O relative to primary phases, with δ17, 18O = ∼0‰. These results show that FGIs in CO chondrites commonly had 16O-rich compositions in the solar nebula. The original 16O-rich FGIs were modified to 16O-poor compositions during aqueous alteration in the parent body.

Neutron activation analysis of iron meteorites

A simple and effective instrumental neutron activation analysis for iron meteorites is presented in this study. In order to design the analytical procedure, self-absorption of gamma-rays on samples, interfering nuclides and their respective reactions were examined. Seventeen elements (Fe, Co, Ni, Cu, Ga, Ge, As, Mo, Ru, Rh, Sb, W, Re, Os, Ir, Pt and Au) could be nondestructively determined in Canyon Diablo and Cape York iron meteorites. Our data of these two iron meteorites are consistent with the corresponding literature values. This study confirms that our analytical procedure with INAA is simple and effective in classification of iron meteorites.

An evaluation method of reflectance spectra to be obtained by Hayabusa2 Near-Infrared Spectrometer (NIRS3) based on laboratory measurements of carbonaceous chondrites

We conducted ground-based performance evaluation tests of the Near-Infrared Spectrometer (NIRS3) onboard Hayabusa2 spacecraft in November 2013 and from April to May 2014 and established a method for evaluating its measured reflectance spectra. Reflectance spectra of nine powdered carbonaceous chondrite samples were measured by both NIRS3 and a Fourier transform infrared (FT-IR) spectrometer. We have established two methods for correcting the NIRS3 data by comparing them with the corresponding FT-IR data because raw data obtained by NIRS3 underwent spectral distortion caused by systematic offsets in sensitivity of individual pixels. The corrected NIRS3 spectra of carbonaceous chondrite samples are comparable with their FT-IR spectra. The depth of each band component Dλ is defined for each wavelength λ (μm) to characterize the absorption bands in NIRS3 spectra. It is suggested that the relationship between the D2.72/D2.79 ratio and the D2.76/D2.90 ratio would be useful for estimating the degree of heating of the asteroid surface, if contributions of terrestrial adsorbed water on D2.79 and D2.90 are properly corrected. The degrees of heating and space weathering are also comprehensively evaluated by the relationship between D2.90 and the D2.76/D2.90 ratio. Reflectance spectra of asteroid Ryugu, the target asteroid of Hayabusa2, to be recorded by the NIRS3 instrument are expected to reveal the characteristics of the surface materials by using the evaluation technique proposed in this paper. Such information will be used for choosing the touchdown points for sampling and also for investigating the distribution of the materials similar to the returned samples on Ryugu.Graphical abstract.