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A series of water-de®cient partial melting experiments on a low-K tholeiite were carried out under lower crustal P±T±H 2 O conditions (900±1200°C, 0.7±1.5 GPa, 2 and 5 wt% H 2 O added) using a piston-cylinder apparatus. With increasing temperature at 1.0 GPa, supersolidus mineral assemblages vary from amphibolitic to pyroxenitic. Garnet crystallizes in the higher pressure runs (> 1.2 GPa). Melt compositions show low-K calc-alkalic trends, and are classi®ed as metaluminous or peraluminous tonalite. These features are similar to the felsic rocks in the Izu±Bonin ± Mariana (IBM) arc, for example Tanzawa plutonic rocks. The anatectic origin of Tanzawa tonalites is consistent with geochemical modeling, which demonstrates that the rare earth element (REE) characteristics of Tanzawa plutonic rocks (which represent the middle crust of the IBM arc) can be generated by partial melting of amphibolite in the lower crust ($ 50% melting at 1050°C and below 1.2 GPa). Estimated densities of pyroxenitic restites ($ 3.9 g/cm 3 ) after extraction of andesitic melts are higher than that of mantle peridotite beneath the island arc (3.3 g/cm 3 ). The high density of the restite could cause delamination of the IBM arc lower crust. Rhyolitic magmas in the IBM arc (e.g. Niijima) could be formed by low degrees of partial melting of the amphibolitic crust at a temperature just above the solidus (10% melting at or below 900°C).

Hawaiian volcanoes are exceptional examples of intraplate hotspot volcanism. Hotspot volcanoes, which frequently host large eruptions and related earthquakes, flank‐failure landslides, and associated tsunamis, can present severe hazards to populated regions. Many studies have focused on subaerial parts of Hawaiian volcanoes, but the deep‐water flanks of the edifices, which can reach 5700 m below sea level, remain poorly understood because they are so inaccessible. In 1998 a collaborative program between Japan and the United States was initiated to explore the evolution of Hawaiian volcanoes, including their growth and degradation.

We have developed a sputter ion pump (SIP) to obtain a lower ultimate pressure.