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Samples dredged from the forearc west of the Mariana Trench include boninite and island-arc tholeiite series volcanic rocks. These are part of a late Eocene–early Oligocene arc complex that forms most of the forearc basement; the complex has been exposed by tectonic erosion. The boninites are depleted in TiO2 (0.20%), Y (5–9 ppm), and heavy rare-earth elements (YbN = 2.4), and have low Ti/Zr and Y/Zr ratios. They are variously enriched in alkali metals, alkaline earths, and light rare-earth elements. These boninitic samples, in common with other such suites, appear to be hydrous melts of a once-melted peridotitic mantle that was enriched prior to the boninite-melting event by a light rare-earth element Ba-, Sr-, and Zr-rich fluid or melt. The tholeiitic series samples, which are spatially associated with the boninites, include basalts, andesites, and dacites. The least fractionated basalts have higher concentrations of TiO2 (0.4–0.5%), Y (10–15 ppm), and heavy rare-earth elements (YbN = 6) than the boninites, but similar concentrations of Ba, Sr, Zr, and Ce. The least fractionated samples from both series have similar ratios of moderately incompatible elements (Ti/Y, Ti/Yb) and highly incompatible elements (Ce/Zr, Ba/Zr); values of Ti/Zr and Ce/Sm vary antithetically. The melts parental to the boninite and arc-tholeiite series cannot be derived by different degrees of melting of a single, homogeneous source; their distinct chemical compositions reflect initially different sources. The similarities of some element ratios and the continuous variation of others, between the two suites, suggest that there is some similarity or gradation between the boninite source and that of the arc tholeiites.

Calculations indicate that many of the characteristics of the two suites can be produced by melting mixtures of variously depleted mantle residual from the production of ocean-ridge basalts, and an incompatible element–enriched fluid. The postulated fluid has high Ce/Sm; is enriched in Ba, Sr, and Zr; and is nearly devoid of Ti, Y, V, and the heavy rare-earth elements. The more depleted portions of the mantle yield boninitic melts; the less depleted portions, tholeiitic melts. Concentrations of high field-strength cations are controlled principally by the percentage of melting of the mixed source and by the initial degree of depletion of the ORB-type mantle. Light rare-earth enrichments, Ti/Zr, and Ba and Sr abundances are controlled primarily by the amount of fluid added. The postulated incompatible element–enriched fluid may be derived from a less depleted mantle, from the subducted slab, or from both; this fluid is not by itself adequate to produce the Ba enrichments in typical arc tholeiites. An additional enrichment process may be required.

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