Deep storage of oceanic crust in a vigorously convecting mantle

摘要

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.