PALEOCEANOGRAPHIC IMPLICATIONS OF RARE-EARTH ELEMENT VARIABILITY WITHIN A FE MN CRUST FROM THE CENTRAL PACIFIC-OCEAN

Results of detailed geochemical analyses of thick Fe-Mn oxide crust accumulations from the central Pacific Ocean lead to the suggestion that variations in their chemical composition and microstructure reflect the influence of prevailing environmental conditions at the time of formation of the deposit. These findings imply that well-preserved Fe-Mn crusts represent a geologic record that may extend back to the onset of Fe-Mn oxide precipitation on the substrate material. Current crust growth rate estimates of 1-3 mm/m.y. imply that 6-10 cm thick Fe-Mn oxide accumulations represent a growth record of 20 to a maximum of 100 m.y. Detailed chemical and mineralogical analyses were performed on successive hand-picked layers of a 95-mm-thick Fe-Mn crust recovered from Schumann Seamount in the Hawaiian Archipelago. The analyses reveal significant variations in the elemental distribution and abundance of major constituents and REEs as a function of depth within the crust. The trivalent REEs generally exhibit sympathetic concentration changes in pure Fe-Mn oxides but the fractionation observed between the light and heavy REEs differs in layers enriched in phosphatic matter. The compositional variability and the extent of REE fractionation can be explained in large part by mineralogical control but can also be inferred to reflect differing depositional conditions in the seawater from which the deposits formed. Variations in the Ce anomaly and REE fractionation patterns within the individual layers of the crust are related to their major element composition and mineralogy and to the anomalies observed by other workers. These are used to attempt to reconstruct the growth history of the crust.