Experiments on dehydration of subducting oceanic crust: Implications for arc volcanism

In subduction zones, oceanic lithosphere progressively dehydrates as it sinks deep into the underlying mantle. Fluids released from the subducting slab are thought to trigger partial melting in the overlying mantle wedge, leading to the formation of volcanic arcs.; Experiments were conducted in the ranges of 2.2–3.4 GPa (70 to 100 km) and 625–750°C to determine the dehydration reactions that control fluid release from the basaltic layer of the subducting slab.; The transformation of a hydrated eclogite into a nominally dry eclogite occurs through the decomposition of three hydrous phases: amphibole, lawsonite, and zoisite. Chloritoid, a mineral described as an H₂O carrier in previous experimental studies, is found to be metastable in the examined pressure-temperature (P-T) range and therefore should not be involved in the global fluid release from the basaltic crust. A detailed chemical analysis reveals that amphiboles are sodic-calcic (barroisite) at low pressures (2.2 to 2.4 GPa), but become sodic (glaucophane) with increasing pressure. This observation is the first experimental confirmation of the high-pressure stability of glaucophane in metabasalt compositions.; Dehydration of a slab is described by combining the experimental data obtained in the present study with published thermal models of subduction zones. In a cold subduction zone, the lawsonite dehydration boundary may never be crossed and H₂O could therefore be transported to depths exceeding 250 km. A slab following an intermediate temperature path that crosses the lawsonite dehydration reaction could release up to 0.9 wt% H₂O into the overlying mantle wedge in the sub-arc region. A warm P-T path would instead cross the amphibole and zoisite dehydration reactions, liberating a maximum of 0.3 wt% H₂O before the slab reaches the sub-arc region and would leave the basaltic crust completely dehydrated around 90 kilometers depth. In this scenario, an additional fluid source is therefore necessary to explain the presence of a volcanic arc and is most probably the result of dehydration of the slab mantle or of the down-dragged hydrated mantle wedge. Finally, in “hot” subduction zones, partial melting of the sediments and crust could occur, leading to the formation of adakites.