. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography Vol. 1, No. 8 GEOPHYSICAL RESEARCH LETTERS December 1974 RAPIDLY ACCUMULATING MANGANESE DEPOSIT FROM THE MEDIAN VALLEY OF THE MID-ATLANTIC RIDGE Martha R. Scott, Robert B. Scott Department of Oceanography and Department of Geology, Texas A & M University College Station, Texas 77843 Peter A. Rona, Louis W. Butler NOAA, 15 Rickenbacker Causeway, Miami, Florida 33149 Andrew J. Nalwalk Marine Sciences Institute, University of Connecticut, Groton, C
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography Vol. 1, No. 8 GEOPHYSICAL RESEARCH LETTERS December 1974 RAPIDLY ACCUMULATING MANGANESE DEPOSIT FROM THE MEDIAN VALLEY OF THE MID-ATLANTIC RIDGE Martha R. Scott, Robert B. Scott Department of Oceanography and Department of Geology, Texas A & M University College Station, Texas 77843 Peter A. Rona, Louis W. Butler NOAA, 15 Rickenbacker Causeway, Miami, Florida 33149 Andrew J. Nalwalk Marine Sciences Institute, University of Connecticut, Groton, Connecticut 06340 Abstract A manganese oxide crust from an ex- tensive deposit in the median valley of the Mid- Atlantic Ridge was found to be unusually high in manganese (up to Mn), low in Fe (as low as ..017. Fe), low in trace metals and deficient in Th230 and Pa231 with respect to the parent uran- ium isotopes in the sample. The accumulation rate is 100 to 200 mm/105y, or 2 orders of magni- tude 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. Introduction Typical deep-sea ferromanganese nodules and crusts grow at a rate of about 1-10 mm/icf years and have an Fe to Mn ratio of .5 to 2. A man- ganese crust recently recovered from the median valley of the Mid-Atlantic Ridge has a growth rate two orders of magnitude faster than the typical rate and an extremely low iron concentra- tion; it appears to have formed under conditions which are not typical of deep-sea manganese de- position. The location of the sampling site and the nature of the material immediately bring to mind a po
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