E. M. Parmentier

Effect of solid flow above a subducting slab on water distribution and melting at convergent plate boundaries

[1] Hydrous fluids derived by dehydration of the downgoing slab at convergent plate boundaries are thought to provoke wet melting in the wedge above the downgoing plate. We have investigated the distribution of hydrous fluid and subsequent melt in the wedge using two-dimensional models that include solid mantle flow and associated temperature distributions along with buoyant fluid migration and melting. Solid mantle flow deflects hydrous fluid from their buoyant vertical migration through the wedge. Melting therefore does not occur directly above the region where hydrous fluids are released from the slab. A melting front develops where hydrous fluids first encounter mantle material hot enough to melt. Wet melting is influenced by solid flow through the advection of fertile mantle material into the wet melting region and the removal of depleted material. The region of maximum melting occurs where the maximum flux of water from slab mineral dehydration reaches the wet melting region. The extent of melting (F) and melt production rates increase with increasing convergence rate and grain size due to increased temperatures along the melting front and to increased fractions of water reaching the melting front, respectively. The position of isotherms above the wet solidus varies with increasing slab dip and thereby also influences F and melt production rates. Applying the understanding of wet melting from this study to geochemical studies of the Aleutians may help elucidate the processes influencing fluid migration and melt production in that region. Estimates of the timescale of fluid migration, seismic velocity variation, and attenuation are also investigated.

A high-order numerical study of reactive dissolution in an upwelling heterogeneous mantle: 2. Effect of shear deformation

High-porosity dunite channels produced by orthopyroxene dissolution may provide pathways for orthopyroxene-undersaturated melt generated in the deep mantle to reach shallower depth without extensive chemical reequilibration with surrounding mantle. Previous studies have considered these high-porosity channels and melt localization in the presence of a uniform upwelling mantle flow through the process of melt-rock reaction as well as shear deformation, but not both simultaneously. In this Part 2 of a numerical study of high-porosity melt and dunite channel formation during reactive dissolution, we considered the effect of shear deformation on channel distribution and channel geometry in an upwelling and viscously compacting mantle column. We formulated a high-order numerical experiment using conditions similar to those in Part 1, but with an additional prescribed horizontal shearing component in the solid matrix, as could be present in flowing mantle beneath spreading centers. Our focus was to examine orthopyroxene dissolution to determine the behavior of dunite formation and its interaction with melt flow field, by varying the upwelling and shear rate, orthopyroxene solubility gradient, and domain height. Introduction of shearing tilts the developing dunite, causing asymmetry in the orthopyroxene gradient between the dunite channels and the surrounding harzburgite. The downwind gradient is sharp, nearly discontinuous, whereas the upwind gradient is more gradual. For higher shear rates, a wave-like pattern of alternating high and low-porosity bands form on the downwind side of the channel. The band spacing increases with increasing shear rate, relative melt flow rate, and orthopyroxene solubility gradient, whereas the band angle is independent of solubility gradient and increases with increasing shear rate and decreasing relative melt flow rate. Such features could be observable in the field and provide evidence for mantle shearing. Standing wave-like patterns of melt fraction also develop on the downwind side with possible implications for the interpretation of seismic velocities in upwelling mantle.