Abstract
Most hydrogenetic ferromanganese crusts in the central Pacific consist of two growth generations: a phosphatized older growth generation that is impregnated with carbonate fluorapatite (CFA), and a younger non-phosphatized growth generation. Geochemical and mineralogical investigations of non-phosphatized and phosphatized crust layers revealed significant differences that are not simply dilution effects by CFA. Certain elements are depleted in old crusts compared to young crusts in the order Si > Fe > Al > Ti ≥ Co > Mn ≥ Pb; others like Ni, Zn, Cu, Y, and REEs (rare earth elements) are mostly enriched. Suboxic conditions in the phosphate-rich water of the oxygen-minimum-zone that infiltrated the crusts caused a partial redissolution of crust phases, resulting in mobilization of associated elements, a partial recrystallization of more stable phases, and a secondary input of nutrient-type elements. The frequent occurrence of todorokite in phosphatized crust layers indicates that Mn mobilized from the vernadite crust phase was partially recrystallized as todorokite, which is more stable under suboxic conditions, incorporating Ni, Cu, and Zn into the crystal lattice, but rejecting Co. Other metals like Pb, Y, and REEs probably form stable phosphate phases in phosphatized crusts. Our investigations emphasize that under the influence of phosphatization, diagenetic remobilization and reorganization in FeMn crusts took place. In contrast to non-phosphatized crusts, the older phosphatized crust generation no longer represents a complete primary crust precipitate. Therefore, young non-phosphatized and old phosphatized crust generations should be considered as two different sample types with respect to geochemical composition, mineralogy, and evolution. We also suggest that most methods for age determination like the 87 Sr 86 Sr isotopic dating method or Co chronometry cannot be used for phosphatized crusts without caution.
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