Strontium isotope ratios in volcanic rocks of Three Island Arc Areas

Abstract

The initial Sr87/Sr86 ratios in several non-ignimbrite calc-alkaline volcanic rocks from Saipan, Central America, and the Lesser Antilles, with two exceptions, lie in the range 0.703 to 0.705 (average very close to 0.704). Such isotope ratios are characteristic of most young volcanic rocks. Similar ratios are found in several tholeiitic lavas in the northern Mariana, the Bonin, and the Izu islands as well. This suggests that all these lavas are formed in a source area in the upper mantle or possibly in basaltic rocks of the oceanic crust and lower continental crust. Formation of the calc-alkaline series by contamination of such magmas with continental material is not required by the isotope evidence, but it may take place to the extent that the contaminating radiogenic strontium in the continental material can be diluted beyond detection by magmatic strontium. The calc-alkaline series of Nuclear Central America cannot form by direct melting of the underlying continental rocks. Two lavas in the Lesser Antilles were erupted with initial Sr87/Sr86 ratios markedly greater than 0.704. It is suggested that the Puerto Rico trough became filled with sedimentary material composed of authigenic carbonate-rich material and continental detritus deposited by the Orinoco River. This material was then buckled to form the Barbados ridge, possibly by convective currents in the upper mantle, and some of it became incorporated in magma chambers.