岩石鉱物鉱床学会誌

We have been measuring the K-Ar ages of the granitic rocks taken from the all granitic areas in Japan, since 1964. The number of granitic samples measured, at present, amounts to 400 and they are representative of each granitic area of Japan.
The ages of the granitic rocks of Japan can be grouped into the following nine periods: 1) Older than 370 m.y. (Pre-Devonian), 2) 170-180 m.y. (Early Jurassic), 3) 110-120 m.y. (Late stage of Lower-Cretaceous), 4) 90-100 m.y. (Early stage of Upper-Cretaceous, ) 5) 75-95 m.y. (Middle stage of Upper-Cretaceous), 6) 50-65 m.y. (Late Cretaceous-Early Tertiary), 7) 30-40 m.y. (Late Eocene-Early Oligocene), 8) 25-25 m.y. (Miocene), 9) 3-13 m.y. (Pliocene).
Some groups of the granitic rocks belonging to the above periods show zonal distribution.
In northeastern Japan, four zones are arranged en echelon. They successively become younger toward the Japan Sea; the oldest one occupies the outermost side along the Pacific coast.
In southwestern Japan, the zonal distribution of the granitic rocks is more complicated than that in northeastern Japan.

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Some hydrothermal experiments, especially rate studies, were made on hydration of olivines in pure water and also in dilute sodium hydroxide solution. Oxide mixtures were also used some purposes. The results are as follows.
1. Several natural olivines held at 250°C to 350°C and 1000kg/cm² for 30 days to 80 days in pure water were not hydrated except in the case of iron-poor specimen. In dilute sodium hydroxide solution, however, all specimens used in experiments were changed to chrysotile+magnetite+brucite+amorphous substance at about 330°C and 300kg/cm².
2. At about 450°C, forsterite+quartz( serpentine composition) in pure water changed to talc+forsterite in one day and the latter mineral assemblage remained stable even after 30 days. Change to chrysotile as expected from the phase diagram of MgO-SiO₂-H₂O by Bowen and Tuttle has not been observed in the present experiment.
3 Ratlstudies for hydration of synthetic forsterite were carried out in dilute sodium hydroxide solution at 250-330°C and 20-300kg/cm². It is indicated that the decomposition velocity of forsterite is proportional to concentration of sodium hydroxide solution. It is also indicated that crystallization velocity of chrysotile is smaller than the decomposition velocity of forsterite. Hence it is estimated that there is some kind of amorphous substance which coexists with chrysotile and brucite.
4. Rate studies on crystallization of chrysotile from the oxide mixture (2MgO+SiO₂) in pure water at 250°C and 300°C and 50kg/cm² were made. Solid phase which was amorphous or poorly crystallized was washed with 10% NH₄C1 solution, and free Mg ions in the solution was analyzed by means of chelatometric titration. The composition of amorphous substance after 7 hours was almost the same as that of serpentine.
It is concluded that the amorphous substrance is formed at first by the hydration process of forsterite, followed by the crystallization of chrysotile from it.

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