RAPIDLY ACCUMULATING MANGANESE DEPOSIT FROM THE MEDIAN VALLEY OF THE MID-ATLANTIC RIDGE

A manganeseo xide crust from an exdeposit in the median valley of the MidAtlantic 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 uraniumi sotopesin the sample. T hea ccumulation rate is 100 to 200 mm/10(6) y or 2 orderso f magnirude 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.