Crude oils and condensates from the Japanese Neogene Tertiary sedimentary basins were analyzed for steroid and triterpenoid hydrocarbons, using GC/MS. Most crude oils show incomplete side chain epimerization of C₂₉ steranes. This is possibly due to an insufficient reaction time and temperature for epimerization since the Miocene age. All oils show a good correlation between nuclear isomerization and side chain epimerization of C₂₉ steranes. The correlation curve appears to be different from that previously reported, which may have resulted from a difference in thermal histories. On the basis of the extents of side chain cleavages of saturated and triaromatic steroid hydrocarbons as well as upon the extents of configurational isomerizations of steranes and hopanes, the condensates are considered to be distinctly maturer than the crude oils. This fact suggests that the condensates have undergone higher temperatures than the crude oils. Among the crude oils, those from the Niigata basin are significantly maturer than those from the Akita-Yamagata basin. This is probably due to the difference between the oil source rocks in the two basins, which may cause a difference in the stage of primary migration of oils. The conversion of monoaromatic to triaromatic steroid hydrocarbons does not correlate well with the side chain epimerization of steranes. This may be due to preferred migration of monoaromatics by geochromatographic separation. There is a significant difference between the crude oils from the Niigata basin and those from the Akita-Yamagata basin in respect of the C₂₇/C₂₉ ratios of steroid hydrocarbons. This difference is consistent with the previous report on visual kerogen compositions of oil source rocks from these basins. The relative abundance of C₂₇ to C₂₉ of monoaromatic steroid hydrocarbons is thought to be less affected by thermal stress than that of steranes. This could thus be a more useful organic source indicator in the case off matured oils such as condensates in Japanese Tertiary sedimentary basins.

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A distinct hydrothermal plume was found in the Iheya Graben in the middle Okinawa Trough in May, 1987, by using methane, manganese and the helium isotope ratio (³He/⁴He) as the geochemical tracers. The plume rises to an altitude of about 300-400 m above the sea floor with maximum methane and manganese anomalies of up to 650 nl kg^-1, and 0.54μg kg^-1, respectively. The ³He/⁴He ratio in the center of the plume is 1.65 times that of atmospheric helium. These anomalies are significantly higher than those previously observed for the shimmering water from the hydrothermal mounds discovered in the same area by the submersible “SHINKAI 2000” in July, 1986. This strongly suggests that a more active hydrothermal system should exist nearby the hydrothermal mounds. Together with previous geophysical and geological studies, the present results further support the view that the middle Okinawa Trough is in an active stage of back-arc spreading behind the Ryukyu trench.

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In this pilot study an extension of whole-rock K-Ar dating methods of volcanic rocks is established. Whole-rock sieve fractions 80-250 μm of basaltic rocks were etched for durations up to 20 min in 6% hydrofluoric acid at room temperature and cleaned ultrasonically. Aliquots thereof were further separated into magnetic and nonmagnetic subfractions. Conventional K-Ar analyses reveal disturbed K-Ar systems in the rocks and nonmagnetic fractions, as both Ar loss and extraneous Ar are observed. Ar loss occurred in high-potassium matrix phases, while olivine- and clinopyroxene-phenocrysts, which are mostly xenocrysts, are carriers of extraneous Ar. Magnetic fractions etched for 5 min exhibit no significant Ar losses nor extraneous Ar and show internally consistent ages or age concordance with minerals of proven suitable quality for K-Ar dating. The method is suited for unravelling extrusion ages of volcanic rocks in which the whole-rock K-Ar system is disturbed and should be applied whenever adequate minerals cannot be separated for dating purposes. The eight basaltic rocks investigated stem from four localities in the Hocheifel Area, FRG; all of them show significant disturbances in their K-Ar system.

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Even though the sphalerite geobarometer enjoys wide support as a very reliable parameter which gives pressure values consistent with the pressures obtained by different silicates equilibrium, thermodynamically the internal consistency of its experimental data may be questioned. The pressure read from the geobarometer should be regarded tentatively.

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Commercial aluminum foil may contain a high concentration of uranium, up to 1.4 μg/g. Samples or sample tubes in contact with aluminum foil during neutron irradiation can be contaminated with recoil-implanted fission products of ²³⁵U from uranium in the foil. This can lead to errors in the determination of Zr, Mo, La, Ce, Nd in neutron activation analysis.

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Zashu et al. (1986) found an isochron-like high correlation between ⁴⁰Ar and potassium in Zaire cubic diamonds, which appears to correspond to 6.0 Gyr. Theoretically, in view of the covalent C-C bonding of diamond and the chemical natures of electrically neutral Ar atoms and positively charged K⁺ ions, the partition coefficient of Ar must be much higher than that of K⁺ in the diamond. As a result, this effect leads to a false K-Ar ages for the diamonds. Along this line of reasoning, somewhat modified interpretations are also presented.

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