Geophysical and geochemical constraints at converging plate boundaries: Part I. Dehydration in the downgoing slab

Geochemical constraints based upon compositions and mechanisms of formation and eruption of island arc volcanic rocks strongly suggest hydrous conditions in the region of primary magma generation. The source of most of the water is probably dehydration of the oceanic crust during underthrusting. Chemical reactions involving dehydration of representa-tive oceanic crustal assemblages absorb enough heat to possibly negate any frictional heat which may be generated along the Benioff-Wadati zone in the upper 75 to 100 km of the underthrust zone. Calculations of thermo-dynamic parameters of relevant reactions at elevated pressures and temperatures show that N 50cal of heat are absorbed per gramme of oceanic crust dehydrated. The result of this heat absorption is suppression of isotherms not only in the oceanic crustal section of the downgoing slab but also in the overlying mantle wedge. Therefore, low heat fiow measured from the trenches to the volcanic arcs may be caused in part by the heat-sink effect of dehydration in the downgoing slab. The water released by dehydration either remains in the slab promoting its melting and the generation of andesitic primary magmas and/or escapes into the overlying wedge of mantle promoting its melting and the generation of the basaltic or andesitic primary magmas. The major thermal effect of the released water is to cool the wedge overlying the slab. The progressive geochemical changes in arc volcanism both in space and time may be controlled by the thermal conditions at the downgoing slab-peridotite wedge interface