HR: 10:50h
AN: V12A-03    [Abstracts]
TI: The Effect of Solid Mantle Flow Above a Subducting Plate on Melting and Fluid Migration
AU: * Cagnioncle, A
EM: amandine@brown.edu
AF: Brown University, Department of Geological Sciences Box 1846 , Providence, RI 02912 United States
AU: Parmentier, E
EM: EM_Parmentier@brown.edu
AF: Brown University, Department of Geological Sciences Box 1846 , Providence, RI 02912 United States
AU: Elkins-Tanton, L T
EM: Linda_Elkins_Tanton@brown.edu
AF: Brown University, Department of Geological Sciences Box 1846 , Providence, RI 02912 United States
AB: Arc magmas are generally thought to be derived from hydrous fluids rising buoyantly from the subducting plate causing hot overlying mantle to melt. The purpose of our study is to model the migration of water released by the dehydration of slab minerals, the subsequent melting of the mantle wedge at subduction zones and the overall distribution of water in the mantle wedge. Hydrous fluids with wetting angles smaller than 60\deg (e.g. Mibe et al, 1999) form a connected network along mineral grain edges, hence allowing for Darcy flow with grain size-melt fraction dependent permeability (Wark et al, 2003). The effect of solid flow pressure gradients on melt migration can be neglected for mantle viscosities smaller than 10$^{20}$ Pa-s. The volume of water released by the dehydration of minerals of the slab between depths of 80 and 150 km is derived from the estimates of Schmidt and Poli (1998). The volume of melt generated by the interaction of the hydrous fluids and the mantle is parameterized using the results of the program pMELTS (Ghiorso et al, 1998). Using a temperature-dependent viscosity, we explore for varying subduction rates and overlying crust thicknesses the effect of solid flow near the wedge corner on the temperature distribution of both the mantle wedge and the subducting plate. The temperature distributions thus obtained restrict in turn the region in the mantle wedge where wet melting can occur. The amount and distribution of melt in the mantle wedge is also strongly influenced by depleted mantle material. Indeed, mantle depleted by melting flows through the wedge corner and is entrained along the top of the down-going plate, hence constraining the region where wet melting can occur. Melt production is therefore dependent on mantle flow rate. The strong dependency of fluid migration on permeability requires that we explore a range of parameters, particularly grain size. Grain size in deforming solid mantle is expected to depend on stress, and mantle flow models indicate that a large range of grain sizes should be present. Small grain sizes in high stress regions near the top of the slab can enhance the amount of water carried downward into the mantle where it can be incorporated in transition zone mineral phases. Furthermore, the competition between the increasing solubility of water with increasing pressure and the buoyant upward fluid migration could control the amount of water that rises and causes wet melting region or that is entrained downward to the transition zone.
DE: 8434 Magma migration
DE: 8155 Plate motions--general
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting