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ABSTRACT: Development of Cobalt-Rich Ferromanganese Crusts on Hawaiian Seamounts

Alexander Malahoff, Kevin M. Kelly

Cretaceous age off-ridge seamounts are located adjacent to the axis of the Hawaiian Ridge. These seamounts were formed 84 million years ago as a result of short-lived hot spot volcanism in the vicinity of an R-R-R triple junction, then located at 195°W, 28°S. These seamounts, as represented by the flat summited, 500 m deep Cross Seamount, were formed at or near the ocean surface and subsequently subsided with the cooling of the oceanic crust to their present location at 158°15^primeW, 18°40^primeN, west of the Island of Hawaii. During their 84 million year traverse, the seamounts have undergone continuous mass wasting, exposing the radial or elongate dike structures within the edifices and forming long, wide talus chutes extending from the summits to b yond the bases of these seamounts. SEABEAM bathymetric, photogeological, shipboard dredging studies, followed by detailed submersible traverses and sampling of the cobalt-rich ferromanganese crusts, were conducted over Cross Seamount. Shipboard photogeological studies of the dike-intruded ridges as well as the talus-filled chutes show patches of ferromanganese crust with the largest continuous patches located along the base of the chutes. Extensive shipboard dredging produced crust specimens ranging up to 6 cm in thickness. The substratum below the crusts consists of calcite- and phosphate-encrusted talus fragments. Stratigraphic analyses of the layers of these specimens show discontinuous crust stratigraphy with numerous unconformities. Detailed stratigraphic correlation sample-to-sampl is difficult in the specimens. Crust specimens collected by the submersible PISCES V from dike rock substratum showed conformable crust layers from 1 to 2 cm thick. The average composition of these crust specimens is 24% Mn, 15-20% Fe, 0.4% Ni, 0.8% Co, .05% Cu, and 100-547 ppb Pt. The comprehensive geological studies conducted on the crusts of Cross Seamount suggest that for the geological setting of Cross Seamount, cobalt-rich ferromanganese crust formation has been severely affected by a continuous process of downslope mass wasting, giving rise to highly variable crust thicknesses from site to site.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990