Influence of substrate rocks on Fe-Mn crust composition

Principal Component and other statistical analyses of chemical and mineralo gical data of Fe-Mn oxyhydroxide crusts and their underlying rock substrate s in the central Pacific indicate that substrate rocks do not influence cru st composition. Two ridges near Johnston Atoll were dredged repetitively an d up to seven substrate rock types were recovered from small areas of simil ar water depths. Crusts were analyzed mineralogically and chemically for 24 elements, and substrates were analyzed mineralogically and chemically for the 10 major oxides. Compositions of crusts on phosphatized substrates are distinctly different from crusts on substrates containing no phosphorite. H owever, that relationship only indicates that the episodes of phosphatizati on that mineralized the substrate rocks also mineralized the crusts that gr ew on them. A two-fold increase in copper contents in crusts that grew on p hosphatized elastic substrate rocks, relative to crusts on other substrate rock types, is also associated with phosphatization and must have resulted from chemical reorganization during diagenesis. Phosphatized crusts show in creases in Sr, Zn, Ca, Ba, Cu, Ce, V, and Mo contents and decreases in Fe, Si, and As contents relative to non-phosphatized crusts. Our statistical re sults support previous studies which shovel that crust compositions reflect predominantly direct precipitation from seawater (hydrogenetic), and to le sser extents reflect detrital input and diagenetic replacement of parts of the older crust generation by carbonate fluorapatite. (C) 1999 Elsevier Sci ence Ltd. All rights reserved.