Rapidly accumulating manganese deposit from the Median Valley of the Mid‐Atlantic Ridge

A manganese oxide crust from an extensive deposit in the median valley of the Mid‐Atlantic Ridge was found to be unusually high in manganese (up to 39.4% Mn), low in Fe (as low as .01% Fe), low in trace metals and deficient in Th 230 and Pa 231 with respect to the parent uranium isotopes in the sample. The accumulation rate is 100 to 200 mm/10 6 y, or 2 orders of magnitude faster than the typical rate for deep‐sea ferromanganese deposits. The rapid growth rate and unusual chemistry are consistent with a hydrothermal origin or with a diagenetic origin by manganese remobilized from reduced sediments. Because of the association with an active ridge, geophysical evidence indicative of hydrothermal activity, and a scarcity of sediment in the sampling area, we suggest that a submarine hot spring has created the deposit.