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 (Yb_N = 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...