Recent, fresh, volcanic rocks of the intra-oceanic Mariana and Volcano Arcs were analyzed for O and Sr isotopic compositions in order to determine the source of these magmas. Fresh, non-arc, volcanic rocks from the regions surrounding the Mariana-Volcano Arcs and some DSDP sediments were also analyzed for comparison. The oxygen isotopic ratios of the arc lavas (5.5-6.8‰) exhibited a small inter-island variation that cannot be entirely explained by fractional crystallization. The Sr isotopic composition of the arc lavas is remarkably uniform (0.70332-0.70394 for the Marianas). Three models are considered in order to explain the observed isotopic characteristics: (1) bulk mixing and melting of MORB-type mantle with (a) subducted sediments, and (b) subducted oceanic crust (excluding sediments); (2) melting of a mixture of sediment-derived fluids and MORB-type mantle; and (3) melting of a mixture of sediment-derived fluids and oceanic island or "hot-spot" type mantle. The last model fits the data best. The conclusion that very small, and variable, amounts of sediment-derived fluid (≤ 1%) are required to explain the observed inter-island O isotopic variation, is consistent with that of other workers who used different isotopic and trace element methods. The generation of magmas in the Mariana-Volcano Arcs involves very little sediment and the source region of Mariana lavas is isotopically indistinguishable from that of hot-spot basalts.
Bibliographical noteFunding Information:
Much of the work for this study was accomplished while we were post-doctoral fellows at the Department of Terrestrial Magnetism, Carnegie Institution of Washington. E.I. acknowledges a fellowship grant from the Graduate School of the University of Minnesota, and an NSF grant OCE82-8201077. We thank Dr. T.C. Hoering of the Geophysical Laboratory for assistance with the stable isotope mass spectrometer, Drs. R.G. Coleman, G. Corwin, D. Fornari, P. Fryer, and J. Ossaka for the donation of samples, and Dr. J.D. Morris for her constructive review.