Radionuclide gradients in two Mn oxide deposits from the Mid-Atlantic Ridge; possible influence of a hydrothermal plume

Two Mn-Fe oxide samples, one having characteristics generally attributed to hydrothermal origin and the other comparable to material of hydrogenous formation, were recovered in the same dredge haul in 1973, 135 km W of the Mid-Atlantic Ridge axis at 23 degrees N. For both samples, the observed decrease of (super 230) Th excess with depth in the sample suggests a hydrogenous origin, with growth rates around 20 mm/m.y. However a hydrothermal origin for one sample is indicated by it consisting mainly of todorokite, with a very low U content (1 ppm) and no detectable Th. Decreasing gradients with depth of (super 232) Th and (super 238) U, as well as near constant (super 230) Th ratios, in both samples cast doubt on the excess (super 230) Th method for calculating growth rates. The observed gradients with depth are explained by a qualitative model based on the scavenging properties of oxyhydroxides for trace elements. In the model, it is suggested that the gradients rather than being due to radioactive decay, arise from primary chemical gradients that reflect differentiation and contribution from waning hydrothermal plumes.