Oceanic migration history of uranium and thorium
Mantle convection is well documented and described in the literature on plate tectonics. Strong evidence is the creation of oceanic crust by effusion along oceanic spreading ridges (Fig. 5), magnetic imprinting of the crust in marker strips of alternately reversed polarity (Vine, 1968), and the transfer of these strips, in a conveyor-belt manner, across the ocean basin to continental margins (Fig. 6).
The rise of mafic lava along ridges is evidence of the rising arm of a convection cell. More properly, the rising environment along midocean ridges’has been interpreted as the common, joined rising arms of two convection cells, as illustrated by the spread of seafloor away from the ridge on each side. However, the lavas are mantle differentiates (basalt and various olivine-pyroxene dunites which constitute ophiolites or oceanic crust) rather than original mantle material (pyrolite), so ridge effusives appear to be material transferred from the cell rather than part of the cell itself. The ocean floor can be considered to be riding passively on top of a mantle cell. Because ocean-floor material somewhere returns down subduction planes, presumably to the mantle, the ocean floor can be considered as a crustal subcell, subsidiary to the mantle cell and in many places receiving or interchanging material with it, particularly the volatiles. However, no reverse movement within the crustal subcell is envisioned.
At orogenic continental margins (e.g., the South American Andean margin; Fig. 21), the ocean floor descends along well-defined Benioff subduction zones, presumably to join with the descending arm of the mantle convection cell. The vertical and horizontal dimensions of subduction zones extending beneath orogenic margins are unknown, as is the depth of the returning current at the bottom of the cell.