Water and magma generation at subduction zones

Abstract

The basaltic ocean crust, metasomatized and metamorphosed during and after generation at the ocean ridge, contains H_2O stored in minerals and pore fluid. Phase equilibrium data establish the conditions for dehydration, and the conditions for melting of amphibole-gabbro or amphibole-quartz-eclogite, or for quartz-eclogite or mantle peridotite if aqueous fluids are available. But there is no concensus about the temperature distribution through the subducted crust, or within the overlying mantle wedge. Therefore, a variety of magmatic models can be derived from the experimental data. According to some calculations, endothermic dehydration reactions in the depth interval 75–125 km cool the oceanic crust to such an extent that it cannot be a major source of magmas; instead, concentrated aqueous fluids released from the crust generate magmas in the overlying peridotite. However, according to most existing thermal models, if temperatures in ocean crust are cool enough to prohibit melting of amphibolite, then temperatures in the mantle above the main sources of expelled fluids are too low for hydrous melting. The ocean crust appears to be effectively dehydrated by 100–125 km depth. Dense hydrous magnesian silicates are not likely candidates for deeper H_2O transport. The extent to which H_2O can be fixed in metasomatic phlogopite in crust or mantle is a significant but undetermined factor. Experimental data on minerals and liquid compositions do not support the concept of primary magmas for andesites and associated lavas from mantle or subducted crust. Complex, multi-stage processes appear to be more likely, which is consistent with recent interpretations of geochemical data.