Masako Shima

Distributions of alkali, alkaline earth and rare earth elements in component minerals of chondrites

Abstract The distribution of alkali, alkaline earth and rare earth elements was studied in ordinary chondrites and an enstatite chondrite by means of wet chemical fractionation methods of powdered meteorite samples. An ordinary chondrite, Bruderheim, and a Type I enstatite chondrite, Abee, were studied most extensively. A stepwise fractional dissolution scheme was used to decompose the various main component minerals successively, using water or acetate buffer (pH 5.5), EDTA in an ammoniacal solution, bromine water, HCl, aqua regia and finally HF + HClO4. The potential of fractional dissolution was critically examined from several points of view. In both types of meteorites, Li, Ba, and rare earths except for Eu were found in readily soluble phases. The behaviour of rare earth and alkaline earth elements in the Type I enstatite chondrite seems to be unique. These data have a bearing on the thermal histories of the chondrites.

Determination of rubidium-strontium age of chondrites using their separated components

Abstract The method for the determination of the solidification age of some chondrites was developed through the measurements of various rubidium-strontium pairs in component minerals. A fractional dissolution technique was applied to separate the minerals according to their solubilities in water, EDTA, bromine water, HCl, and HF. Peace River and Bruderheim as the typical ordinary chondrites, and Abee, an enstatite chondrite, were used for the samples. By this method, the Rb-Sr age of these chondrites and the initial isotope ratio of strontium were obtained from the individual sample independently.

DISTRIBUTION OF SPALLATION-PRODUCED CHROMIUM BETWEEN ALLOYS IN IRON METEORITES

Abstract Chromium nuclides produced by cosmic rays were measured mass spectrometrically in iron meteorites. A surface ionization method was applied with chromium, finally isolated from the samples by a solvent extraction method. A marked enrichment due to cosmic ray products of 54 Cr and 53 Cr could be observed in the samples obtained by fractional dissolution with dilute sulfuric acid. The acid dissolved a bulk of meteorite selectively, leaving a small residue of a nickel rich metal phase which contained relatively high concentrations of natural chromium. An isotopic dilution analysis using enriched 50 Cr or 52 Cr was made to obtain absolute contents of those nuclides. In the cases of Grant and other meteorites, the nickel rich (taenite rich) fraction was found to contain a higher concentration of these products than in the nickel poor (kamacite rich) fraction which is readily soluble in acid. The over-all contents of 54 Cr were found to be in the range of 13–25 ppb in several meteorites, and the ratios of the products, 50 Cr: 53 Cr: 54 Cr were always found to be approximately 0.2:1:1 respectively. The enrichment of these chromium nuclides in taenite was confirmed by several methods. The interpretation of this phenomenon is discussed from a cosmochemical viewpoint.

Distribution of rare earths among component minerals of bruderheim chondrite

Abstract The distribution of rare earth elements was studied among component minerals in an ordinary chondrite, Bruderheim. For this purpose a wet fractional phase separation method was applied to a powdered sample of Bruderheim. Eleven members of this group of elements in the fractionated samples were determined mass-spectrometrically and by neutron activation, without individual chemical separation. Most of the elements were found to be concentrated in the phosphate fraction which was isolated mainly in EDTA solution. On the other hand, practically all of Eu and an important part of Yb were found in the HCl-insoluble silicate fraction. This result may indicate a lower oxidation level of the system in which the minerals were formed.

Radial Distribution of Spallogenic K, Ca, Ti, V and Mn Isotopes in Iron Meteorites

Abstract Cosmic-ray-produced stable nuclides of Ca (mass number: 42, 43, 44 and 46), Ti (46, 47, 49 and 50), V (50), Cr (50, 53 and 54) and the long-lived nuclides, 40K and 53Mn were determined along the radial axes of the iron meteorites Grant and Treysa. Grant was extensively examined and the results compared with rare gas data. Although Treysa does not include enough samples to allow detailed analysis, the depth profile shows typical features for a small meteorite. The results were compared with calculated profiles of 40K, 49Ti and 53Mn using thick bombardment data. The approximate pre-atmospheric radii of Grant and Treysa were determined to be 30 cm and 14 cm, respectively. The effect of space erosion was also estimated by comparing the data of 49Ti and radioactive 53Mn in Grant and Treysa with the calculated patterns. It is suggested that space erosion of both meteorites is small (≲ 0 . 8 Å /y ) during the cosmic-ray exposure of several hundred million years.

Some Stable and Long-Lived Nuclides Produced by Spallation in Meteoritic Iron

The concentrations of cosmic-ray-produced stable nuclides of chromium, vanadium, titanium, and calcium, and two long-lived radioactive nuclides, K-40 and Mn-53, were measured in iron meteorites. In the Grant iron meteorite they were determined as a function of the depth of the specimen.

Cosmic-ray-produced40K and50V in the metal phase of chondrites

Abstract Cosmic-ray-produced 40 K in the metal phase of six chondrites and 50 V in that of one chondrite were determined using a surface ionization mass spectrometer. The 22 Ne total / 40 K metal ratios of the chondrites are explained in part by shielding effects during cosmic-ray irradiation. The wide variation of this ratio in some groups of meteorites is explained in terms of partial loss of rare-gas nuclides. Radiation ages for the chondrites were determined using 40 K measurements and production-rate estimates from thick target calculations.

Noble Gas Record of Japanese Chondrites

Abstract Concentrations and isotopic ratios of noble gases are reported for nineteen Japanese chondrites. Among those, Nio (H3-4) is a solar-gas-rich meteorite. U/Th - He ages are younger than K - Ar ages for all meteorites studied. Six of the nine L-chondrites give significantly young K-Ar ages, suggesting gas loss by impact shock heating. The remaining three L-chondrites and seven of the ten H-chondrites have K-Ar ages older than 4 Ga. The L-chondrite Nogata and the H-chondrites Numakai, Ogi and Higashi-Koen have concordant ages. Cosmic-ray exposure ages for six of the H-chondrites show clustering around the 6-Myr peak in the distribution of exposure ages, while those for the L-chondrites, ranging from 8.2 to 64 Myr, do not show clustering. Fukutomi (L4) contains trapped 36Ar in excess, 3.5 times enriched compared to the highest value so far reported for type-4 ordinary chondrites except solar-gas-rich chondrites. The 36Ar/132Xe and 84Kr/132Xe ratios fit along a mixing line between a planetary and a sub-solar (or argon-rich) component found in separates of E-chondrites [43], The Xe isotopic composition is identical with that in Abee and Kenna. The isotopic signatures suggest that this meteorite may contain mineral fragments bearing the noble gas component found in E-chondrites or ureilites. Fukutomi also contains 80Kr, 82Kr and 128Xe produced by epithermal neutron captures on 79Kr, 81Kr and 127I, respectively. From the neutron-produced Kr, the preatmospheric minimum radius is estimated to be 20 cm with an assumption of a spherical meteoroid.