No abstract

[1] Fractionated isotopic ratios in some oceanic basalts indicates the presence of recycled oceanic crust in the mantle. This crust must have escaped complete remixing for a significant period of time. Gravitational settling into a dense layer at the base of the mantle may facilitate this preservation. Christensen and Hofmann (1994) first demonstrated the dynamics of this process by developing scaling laws for extrapolating low convective vigor models to conditions estimated for the mantle. Here this sequestration is studied in models with more Earth-like convective vigor. Scaling laws for geodynamic parameters are updated and the isotopic evolution of the U-Pb and Sm-Nd systems examined. Significant accumulation is still found at high Rayleigh number, but only when the excess density of oceanic crust in the lower mantle is larger than currently suggested from laboratory experiments. These accumulations are found to maintain the fractionated isotopic signature of ancient crust for models with moderate to moderately high convective vigor relative to mantle estimates. At the highest convective vigor tested, the accumulations are not isotopically distinct.Citation: Brandenburg, J. P., and P. E. van Keken (2007), Deep storage of oceanic crust in a vigorously convecting mantle,

Deformation-band networks at Buckskin Gulch, Utah, and the Big Hole fault, Utah, both formed in the Navajo Sandstone with similar initial porosity and permeability, at similar burial depths, and result in similar reductions in effective permeability. However, the band networks at Buckskin Gulch, which formed in a contractional tectonic setting, appear to be much more areally extensive and are not associated with any discrete faults having displacements greater than at most a few meters and more likely only a few tens of centimeters. In contrast, the bands at Big Hole fault are generally limited to the damage zone of a about 25-m (82-ft) displacement normal fault formed in a locally extensional environment. These results suggest that deformation bands in well core from extensional settings may be indicative of discrete damage zones associated with normal faults, whereas deformation bands in well core from contractional settings may be indicative of much more areally extensive deformation-band networks. The band networks in both cases will affect similar reductions in reservoir effective permeability, but only in the latter case will the affected area be sufficiently large to affect well performance.

No abstract

[1] Models of convection in the mantle can be used to study the effects of differentiation and remixing on the geochemical evolution of the Earth. Implementation of melting and degassing at mid-ocean ridges and subduction zones requires an adequate approximation of plate tectonics as well as temperature-dependent rheology. We have developed a new two-dimensional cylindrical model that combines a force-balance method for energetically consistent stiff plates with tracer-discretized chemical buoyancy. Basaltic crust is extracted at distinct spreading centers and is subducted into the lower mantle. We find that the unmodified implementation of the force-balance equations in a full cylinder causes occasional spurious rotations by amplification of numerical discretization errors. The method is stable if a single internal symmetry boundary condition is used, but this causes artificial pooling of dense crust near the boundary where it is easily disrupted. This results in artificially enhanced remixing of dense crust. We modify the force-balance equations to damp net lateral plate movement. The energetic consistency of this modification is then demonstrated by comparison to a one-plate, single convection cell calculation. With the removal of the symmetry boundary condition a more continuous rate of crustal pooling is observed. This suggests that models with symmetry boundary conditions may overpredict the rate of pooling and remixing of ancient crust.