Fluorine geochemistry of basaltic rocks from continental and oceanic regions and petrogenetic application |
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| Authors: | K Aoki K Ishiwaka S Kanisawa |
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| Institution: | (1) Institute of Mineralogy, Petrology and Economic Geology, Tohoku University, 980 Sendai, Japan;(2) Department of Earth Sciences, College of General Education, Tohoku University, 980 Sendai, Japan |
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| Abstract: | Fluorine contents in about 300 samples of various types of basalts and related rocks from continental (southwestern U.S.A.; Zaire; Deccan and South Africa) and oceanic regions (Hawaii and Mid-Atlantic Ridge between 23° N and 40° N) were determined by a selective ion-electrode method.Of all of the major components in these basaltic rocks, F shows good correlation only with K2O. It increases regularly from tholeiite to perpotassic basalt on continents, and from tholeiite to nephelinite on Hawaii. In the F-K2O diagram all the basaltic rocks from continents and Hawaii plot between the origin of the coordinate axes and the field of phlogopite in peridotite xenoliths in South African kimberlites. Accordingly, the major proportions of F, K2O and also H2O in these basaltic magmas are derived from phlogopite at the source regions in the upper mantle. On the other hand, F in abyssal tholeiites is relatively higher than that of the other tholeiites at equal K2O content, and it is suggested that most of F, K2O and H2O are derived from pargasites.When it is assumed that the upper mantle phlogopite contains about 10% K2O, 0.4% (0.3–0.5%) F and 4% H2O, H2O content in basaltic magmas from continental including island arc and oceanic island regions can be qualitatively estimated based on their proportions of K2O FH2O. Similarly, H2O content in abyssal basaltic rocks is also estimated on the basis of FH2O in pargasites (Table 2).A suite of Deccan tholeiites shows remarkable F enrichment with increasing K2O due to separation of anhydrous and K-free minerals during fractionation. F in tholeiitic and alkali basalt magmas in Hawaii also increases regularly with K2O 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. |
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