Five gabbros from the Mid-Atlantic Ridge near 30°N were analyzed for the rare-earth elements. The pattern analysis indicates that these gabbros represent the solid-type materials which were separated from the melt with mutually almost unfractionated RE abundances relative to chondrite. The bulk partition coefficient functions pertaining to those rocks have the common feature in an inflection at Tb (Lm = 8.0). It is noted that the RE pattern of the North American shale (HASKIN et al., 1966) reflects the similar partition coefficient function. Thus this allows us to speculate that the Mid-Atlantic Ridge gabbros ever underlay the continental crust. Meanwhile, it is difficult to consider that the abyssal tholeiites had the direct genetic relation with the gabbros studied. The systematic notation to express the RE bulk partition coefficient function is presented at the same time.

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Iron-wollastonites from skarns have an approximate composition of Ca₅FeSi₆-O₁₈. Iron-wollastonites and ordinary wollastonites from the Kagata mine are associated with clinopyroxenes of hedenbergite and ferrosalite compositions, respectively. This fact could be interpreted as that a miscibility gap exists between iron-wollastonite of Ca₅FeSi₆-O₁₈ composition and ordinary wollastonite. Thus iron-wollastonite of Ca₅FeSi₆-O₁₈ composition appears to be a stable phase in the CaSiO₃-CaFeSi₂O₆ join at temperatures as low as those of the skarn formation, probably 300°∼600°C.

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A precise determination of the extremely low concentration of radium in marine carbonate samples via ²²²Rn counting using a ZnS(Ag) scintillation chamber was described. Carbonate sample was dissolved with hydrochloric acid and the solution was sealed in a Curie bottle for the period of accumulation of ²²²Rn from ²²⁶Ra. The radon in the bottle was separated from the solution and collected in a trap cooled with liquid nitrogen. Finally the radon was transferred to the scintillation chamber which is coated with a thin layer of ZnS(Ag) powder. The luminescence signals by alpha rays were well discriminated from background. Advantages of this method are as follows; its simple and reliable operation, low background count (4.5±0.5cph) and low detection limit of I × 10^-14 g Ra. Some results of radium content determinations of modern and fossil shell samples are reported.

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Sulphide globules were found in glassy rim as well as in more crystalline interior of pillowed abyssal tholeiite from the Mid-Atlantic Ridge near 30°N latitude. The magma reached saturation with respect to sulphide at the time of eruption and immiscible silicate and sulphide liquids were quenched when the magma was extruded on the sea floor. Saturated sulphur content of the basaltic liquid was estimated at about 0.15 ± 0.03 percent by weight. Sulphur isotopic abundance of the rocks expressed in δ³⁴S ranges from +0.3 to +1.6‰. This value may represent the isotopic ratio of mantle sulphur.

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The relation in the chemical composition between coexisting sphalerite and carbonate, which occur together with pyrite, from some hydrothermal vein deposits, was investigated on the basis of the data of both theoretical considerations and electron microprobe analyses. The chemical environment of hydrothermal ore deposition was then discussed. Concentrations of total dissolved sulfur and total dissolved carbon, NaCl-equivalent concentration and pH play an important role as the factors limiting the hydrothermal condition, reflecting the chemical nature of ore-forming solution. Therefore, the relation between the activity of FeS in sphalerite and that of FeCO₃ in carbonate should be considered as a function not only of fugacities of gaseous phases concerned, but also of chemical composition of the ore-forming solution. For refined theoretical considerations, activity-concentration relations for components in the solid phases must be known. The relation between activity and concentration of FeS in sphalerite can be obtained from the recent experimental works in the Zn-Fe-S system. That of carbonate can be approximately obtained on the basis of the experimental work for the CaCO₃-FeCO₃ system by ROSENBERG (1963) and the theoretical study by THOMPSON (1967). Analytical data are plotted on logm'^sp_FeS (m' and sp mean gram formula weight and sphalerite, respectively)-logm'^cb_FeCO3 (cb means carbonate) diagram. In general, the results of the measurements are consistent with the theoretical consideration. Namely, a positive correlation of the logm'^sp_FeS and the logm'^cb_FeCO3 was obtained. Comparing the theoretical consideration with the analytical data obtained, it can be concluded that the fugacity of oxygen gas during ore deposition varies in a very wide range. About eight orders in the range for the fugacity of oxygen gas in logarithmic scale is deduced as a result. This result tells us that the fugacity of oxygen gas could be the most important parameter limiting the physicochemical environment among many parameters. The fugacity of oxygen gas at the time of deposition is increased from the Toyoha (Pb-Zn) thorough the Oe (Mn-Pb-Zn), Inakuraishi (Mn) and Jyokoku (Mn) to the Omori (Au-Ag) vein deposits. The relation may indicate decreasing order of the depth of formation.

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Isotope ratios measured for selected Japanese ore leads are corrected to the absolute scale and compared with some available data for other Japanese samples. The isotopic feature pointed out in the previous work is confirmed: The data for the Miocene vein-type deposits are very similar to the lead isotope ratios available for the Miocene strata-bound (Kuroko) deposits, northern and central Honshu volcanic rocks and pelagic sediments near the Japan Trench.

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