1.アルカリ岩の輝石に関する2,3の問題

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Several aspects of the genetic problems on pyroxenes from alkalic rocks have been studied from the view point of both the experimental petrology and the natural occurrences. The crystallization courses of pyroxenes from alkalic rocks can be divided into the following types: 1. diopsidic augite→augite→aegirine augite→aegirine, 2. augite→hedenbergite, 3. augite→ferropigeonite, and 4. augite→salite. The first course is typically developed in the pyroxenes from alkalic rocks of Morotu, Shiant Isles, Shonkin Sag, Black Jack and east Uganda, whereas the second course is observed in the pyroxenes from Gough Island, Pantelleria, and Shiant Isles, all of which are volcanic rocks formed at relatively high temperatures. The range of these courses have been extended considerably by means of EPMA determination, by which it becomes possible to analyze the thin outermost margins of zoned crystals. The third course is known only from alkalic rocks of northern Kyushu. Recently the fourth course of augite→salite, entirely different from the other courses was observed in the mafic and ultramafic alkalic rocks from Blue Mountain, New Zealand and Hocheifel, Germany. A large immiscibility region in the central part of the system Di-Hd-Ac proposed by Aoki has not been supported by these observations and synthetic studies in the systems Ac-Di, Ac-Hd and Ac-Di-Hd at moderate temperatures. This, however, does not necessarily rule out the possibility of the presence of such immiscibility in the subsolidus regions at much lower temperatures. The difference of the courses augite→aegirine and augite-hedenbergite can be ascribed to the difference of the f_<O2> of the host rocks. When f_<O2> decreases at constant rate, keeping the Fe^<2+>/Fe^<3+> ratio nearly constant, hedenbergite crystallizes out from the rocks, whereas if decrease of f_<O2> occurs at decreasing rate, then concentration of Fe^<3+> is increased, resulting in the formation of aegirine in the later stages. Many examples of natural pyroxenes show that the SiO_2 concentration in the rocks has a considerable effect on the solubility of Al and Ti in the pyroxenes which crystallize out from them. This effect has been studied experimentally in the system CaMgSi_2O_6-CaTiAl_2O_6(Tp)-SiO_2 at atmospheric pressure. The system can be divided into region I, undersaturated with SiO_2 region III, oversaturated with Si02 and region II, transitional between I and III. Pvroxenes from region I have maximum content of Tp molecule, associated with anorthite and perovskite, with or without sphene, and those from region II have much less content of Tp molecule, associated with anorthite and sphene. To the contrary pyroxenes from region III are almost pure diopside, associated with anorthite, sphene, tridymite, with or without rutile. Thus the experimental result clearly indicates that with increasing total SiO_2 content, the pyroxenes change compositions from a diopsidic solid solution containing maximum amount of Tp to almost pure diopside. Pyroxenes from SiO_2-undersaturated alkalic rocks, such as melilite nephelinites have generally high concentration of CaAl_2-SiO_6 (CT), CaFe^<3+>SiO_6 (ferri-CT) and CaFe^<3+>AlSiO_6 (ferri-Al-CT). Pyroxenes in the system Di-CT- Ferri CT, which are more or less similar to natural pyroxenes, change to pyroxene+garnet+oxide at about 10 kb, and Ti-bearing diopside also exsolves perovskite at nearly the same pressure. This may suggest that these rocks should have been formed at pressures less than about 10 kb. Thermal experiment on the titanaugite with exsolved ilmenite lamellae at atmospheric pressure indicates that the exsolution occurs at temperature range of 1150-1200℃, though the effect of pressure is unknown.
日本地質学会の論文
1974-09-30

1.アルカリ岩の輝石に関する2,3の問題

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