WATER SOURCES FOR SUBDUCTION ZONE VOLCANISM - NEW EXPERIMENTAL CONSTRAINTS

Despite its acknowledged importance, the role of water in the genesis of subduction zone volcanism is poorly understood. Amphibole dehydrati on in subducting oceanic crust at a single pressure is assumed to gene rate the water required for melting, but experimental constraints on t he reaction are limited, and little attention has been paid to reactio ns involving other hydrous minerals. Experiments on an oceanic basalt at pressure-temperature conditions relevant to subducting slabs demons trate that amphibole dehydration is spread over a depth interval of at least 20 kilometers. Reactions involving other hydrous minerals, incl uding mica, epidote, chloritoid, and lawsonite, also release water ove r a wide depth interval, and in some subduction zones these phases may transport water to deep levels in the mantle.