Magma origin and evolution of White Island (Whakaari) volcano, Bay of Plenty, New Zealand

White Island is an active composite stratovolcano in the Bay of Plenty, New Zealand, that comprises many small volume (<0.1 km(3)) andesite-dacite lav a flows and pyroclastic deposits with phenocryst contents of similar to 15- 44%. Minor high-Mg basaltic andesite explosive eruptions, such as those of 1976-1992, may have occurred at intervals throughout the history of White I sland, but are rarely preserved. These alternate with major episodes of and esite-dacite lava extrusion. The high-Mg magmas form by hydrous melting of mantle, metasomatized by fluids from the dehydrating slab at the slab-mantl e wedge interface, that rise rapidly to shallow magma chambers (2 7 km?) wh ere limited mixing and contamination occurs before eruption. Some of this m agma remains in the magma chamber where it interacts with the crystal mush, from which it inherits phenocrysts, to form so-called 'dirty' lavas. Total phenocryst content of these lavas is correspondingly higher. As more magma is intruded into the chamber, the heat flux will increase and melt fractio n will eventually rise to the surface to form high-silica andesite-dacite m agma ('clean' lavas) with fewer inherited phenocrysts. Similar multi-magma chamber plumbing systems, with complex evolution involving fractionation an d contamination, probably occur in most andesite-dacite arc volcanoes.