F contents in about 500 samples of various types of basalts and related rocks from continental (southwestern U. S. A., Zaire, Deccan and South Africa), island arc (Japan) and oceanic regions (Hawaii and Mid-Atlantic Ridge) were determined by a selective ionelectrode method.
Of all of the major components in these basaltic rocks, F shows good correlation only with K2O. It increases regularly from low potassic tholeiite (island arc tholeiite) to perpotassic basalt (60-7600 ppm) on continents and island arcs, and from tholeiite to nephelinite (180-960 ppm) on Hawaii. In the F-K
2O diagram all the basaltic rocks from continents, island arc and Hawaii plot between the origin of the coordinate axes and the field of phlogopite in upper mantle peridotite xenoliths in South African kimberlite. Accordingly, the major proportion of F, K
2O and also H
2O in these basaltic magmas are derived from phlogopite at the soruce regions in the mantle. On the other hand, F in abyssal tholeiites is relatively higher than that of the other tholeiites at equal K
2O contents, and it is suggested that most of F, K
2O and H
2O are derived from pargasites.
Japanese and Deccan tholeiitic series show remarkable F enri c h ment with increasing K
2O due to separation of anhydrous and K-free minerals during fractionation. F in tholeiitic and alkali basalt magmas in Hawaii also increases regularly with K
2O during progressive fractionation until the later stages, where rhyodacite and trachyte exhibit a relative decrease owing to the effective subtraction of F-bearing amphibole and apatite in addition to anhydrous minerals.
However, when Fcontents of the Japanese Quaternary calc-alkaline series are plotted on the F-K
2O and SiO
2-F diagrams, they scatter in wide areas, and they indicate no clear relations among F, K
2O and SiO, as shown in basaltic magmas and their crystallization products. It seems likely that any simple interpretation cannot apply to the calc-alkaline series based on these three components.
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