Absolute and Relative Abundances of K, Rb, Sr and Ba in Circum-Pacific Island-Arc Magmas, with Special Reference to the Marianas

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Absolute and relative abundances of K, Rb, Sr and Ba in 22 specimens representing lavas from six of the ten active and dormant subaerial volcanoes from the northern Mariana island arc are presented. Average concentration of K, Rb, Sr and Ba is about 7000, 15, 300 and 219 ppm, respectively. K, Rb and Ba covary strongly with silica while Sr is relatively constant. Ratios of K/Rb and K/Ba average about 500 and 31, respectively. The Mariana lavas are enriched in K, Rb, Sr and Ba, and their elemental ratios are markedly different from those of the “island-arc tholeiites” and are similar to values typical of the calc-alkaline rock series. Ranges of elemental ratios among different island arcs, and among islands of the Mariana arc, are often distinctive. In many arc lavas, K/Ba is more distinctive than K/Rb in distinguishing these rocks from those of mid-ocean ridge basalts. Consideration of incompatible element ratios shows that magmas of the Mariana arc occupy a range intermediate between the two contrasting oceanic magma-types of mid-ocean ridge basalt and alkali basalt. These data, together with isotopic constraints, support the conclusion that the lavas of the Mariana arc are derived by fractional fusion of a mantle source intermediate in incompatible element and isotopic characteristics between that responsible for the generation of mid-ocean ridge basalts and oceanic alkalic rocks.

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Chemical Geology, v. 28, p. 111-121