Noble gas isotopic compositions and concentrations were measured for two monomict eucrites Yamato-791186 and -792510 and for three polymict eucrites Yamato-791960, -791962 and -792769. Terrestrial ages calculated on the concentrations of cosmogenic radionuclide ⁸¹Kr are in the range of 0.23-0.35 Ma, which are older than those reported for Yamato meteorites including eucrites. The noble gas result suggests that the monomict eucrites, Y-791186 and Y-792510, and the polymict eucrites, Y-791960 and Y-791962, are paired. If shielding effect on the cosmogenic ⁷⁸Kr/⁸³Kr ratio was considered, the preatmospheric radius of Y-791186 and Y-792510 was estimated to be small (about 10 cm in diameter). It was also calculated that Y-791960 was near the surface and Y-791962 was at the center of a preatmospheric body of about 1m in diameter. At least five pairs are identified for eucrites in the Yamato ice field referred to the published data.

抄録全体を表示

Air standard neon (3.7×10^-9 cm³ STP or 1.7×10^-13 mol of Ne) was repeatedly analyzed with a sector type mass spectrometer VG5400 under various partial pressures of helium with the ⁴He/²⁰Ne ratio from 0.24 to 27500 (or total pressure in the mass spectrometer, P(MS) from 1×10^-7 Pa to 4.3×10^-4 Pa by ion gauge reading). The measured ²⁰Ne/²²Ne ratios were kept almost constant when ⁴He/²⁰Ne 400 or P(MS) 7×10^-6 Pa (by ion gauge reading), but increased by up to a factor of ~3% at higher partial pressure of helium. The present result demonstrates the importance of He-Ne separation for a precise measurement of neon isotopic ratios in samples with high He/Ne ratios, such as MORB (Mid Oceanic Ridge Basalt) glasses.

抄録全体を表示

We measured noble gases in artificially shock-produced diamonds in order to examine the size effect of diamond grains on noble gas concentrations and to get the diffusion coefficients of noble gases in diamonds. Noble gases in diamonds were degassed by diffusion process at low temperature below 1700°C, while major degassing occurred at 1700-1900°C by the graphitization of diamonds. As the shock-produced diamonds contain large amounts of noble gases, the diffusion coefficients can be determined at low temperatures even for heavy noble gases. The diffusion coefficients of Ne, Ar and Kr have been estimated to be lower than 10^-21 to 10^-18 (cm²/sec) at 1000-1400°C.

抄録全体を表示

The assumption that the ⁴⁰Ar/³⁶Ar ratio of Ar trapped in volcanic rocks at eruption is atmospheric often gives a large systematic error in the K-Ar dating of Quaternary volcanic rocks younger than 1 Ma. There are two possible sources of error, the existence of excess Ar and mass-fractionated, initial Ar.
The major source of excess Ar is supposed to be magma. The ⁴⁰Ar/³⁶Ar ratio for magmatic Ar in the North-East Japan arc is tentatively estimated to be 340±10 from measurements of Ar in large phenocrysts separated from Quaternary volcanic rocks. Separation of phenocrysts is the most effective to decrease the systematic error caused by excess Ar.
The mass-fractionation of initial Ar should give the ³⁸Ar/³⁶Ar ratio which was changed from the atmospheric ratio. This can be checked by determining the ³⁸Ar/³⁶Ar ratio in samples. A peak-height comparison method is preferential to an isotope dilution method in the K-Ar dating of very young volcanic rocks because with the former, both the amount of Ar and the ³⁸Ar/³⁶Ar ratio in a sample can be determined at one measurement, while with the latter, an additional measurement is needed to know the ³⁸Ar/³⁶Ar ratio in the sample. Further, the statistical error in the isotope dilution method overwhelms that of the peakheight comparison method for large atmospheric Ar correction (e.g., 83% correction for 1% relative errors of isotopic ratio measurements). Since the systematic error caused by the mass-fractionated, initial Ar increases greatly with the increasing atmospheric Ar correction, it is important to correct the result for the mass-fractionation. Correction formulae are given.
The error sources other than those mentioned above are uncertainty in the blank correction and instabilities in the sensitivity of spectrometer and the Ar isotopic ratio measurement. Especially, Ar in Al- or other metal foil should be taken into consideration for the blank correction. In order to increase accuracy of the K-Ar age obtained, to check the systematic errors by measuring samples together with standard samples is of great importance in the K-Ar dating of very young volcanic rocks.
A measurable limit of K-Ar age is affected greatly by the atmospheric Ar correction. For andesites containing 5×10^-11 ccSTP ³⁶Ar/g and 2 wt% K, the limit is estimated to be 0.004 Ma for 98% atmospheric Ar correction. For most of Japanese Quaternary lavas which contain 2×10^-10 ccSTP ³⁶Ar/g and 2 wt% K, the limit is 0.015 Ma for the same atmospheric Ar correction.

抄録全体を表示

Precise determination of argon isotopic ratios was made on twenty-seven historic lavas from Japan. Their isotopic ratios were mostly mass fractionated from that of the atmospheric argon toward the enrichment in lighter isotopes. This suggests that the correction for the mass fractionated initial ⁴⁰Ar/³⁶Ar ratio is very important for the K-Ar dating of volcanic rocks younger than 0.1 Ma. The limit of measurable K-Ar age is estimated to be 0.03 Ma for a normal andesitic lava having a fractionated initial argon.

抄録全体を表示

Samples of eleven andesites and one dacite were collected from six lava flows from Takahara Volcano, central Japan, and were dated by the K-Ar method. Some of these lavas are intercalated with intra-caldera Shiobara lake sediments, thus providing an estimation of the age of the sediments which contain a cool-temperate Pleistocene flora. The validity of the analytical technique for K-Ar age dating of young volcanic rocks is discussed, and some important problems concerning lava samples with anomalous argon contents and isotopic ratios are addressed. Processes of argon release during cooling of lava flows are also examined in relation to their petrographic charcteristics.

抄録全体を表示

K-Ar ages were determined for 6 GSJ rock reference materials and 3 mineral standards for K-Ar dating. The results give reasonably good reproducibility, and generally agree with the reported values within experimental errors.

抄録全体を表示

Significance of He-Pb isotope systematics in Earth Sciences is discussed by showing some examples, which has not well been developed so far in spite of its usefulness. Both ⁴He and lead isotopes (²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) are decay products from U and Th, and Th/U ratios in the Earth's interior can be regarded to be varied in a relatively narrow range (2-4). Hence, the production of ⁴He and ^206-208Pb can be approximated to be a function of U-content in the system. However, this approximation does not change the shape of the growth curve in the ⁴He-^206-208Pb diagrams significantly. In such diagrams, the relative difference in the mobility of He and Pb during the formation of the system or later loss from the system is the main factor to control the variation of the growth curve. Hence, such a diagram can be used as a sensitive tool to trace the processes which might occur in the Earth's interior.

抄録全体を表示

We have constructed a mass spectrometer system for noble gas isotopes. This system consists of a noble gas extraction and purification system for an extremely small amount of extraterrestrial sample and a conventional (30 cm radius, 90° magnetic sector) single focus mass spectrometer, which are made of stainless steel.
The sample is heated in a small tungsten basket. Extracted noble gases are purified by hot Ti-Zr getters and a getter pump.
The mass spectrometer is controlled by a personal computer to obtain high precision. The computer is connected with a programmable voltage current source and digital multimeters through GP-IB interface. The computer controls these instruments and the isotope data are recorded on a floppy disk. The software was developed with N88-basic (86).
In order to improve the system sensitivity, ion counting method was introduced. The number of ions is counted with a multichannel scaler by means of a constant fraction circuit.

抄録全体を表示