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Metalliferous sediments from the East Pacific Rise and the Bauer Deep on the northwestern Nazca plate, as well as size-separated sediment components, have been analyzed for uranium (U) and thorium (Th) isotopes by alpha spectrometry in an attempt to determine the origin and mode of emplacement of uranium in the sediments. In addition, the partition of uranium between different sediment components has been investigated, using fission-track techniques.

The results indicate that the uranium in metalliferous sediments has been derived from sea water and detrital sources in various proportions, depending on time and location. Along the crest of the East Pacific Rise, the 234U/238U activity ratio is indistinguishable from that in modern sea water, indicating that the uranium on the rise crest is dominated by the uranium of hydrogenous origin, whereas lower 234U/238U and U/Th ratios in one area on the east flank of the East Pacific Rise and everywhere in the Bauer Deep may reflect relatively more input of uranium from detrital sources, coupled with a reduced hydrogenous input.

Isotopic analysis of individual sediment components, coupled with fission-track studies, reveals that the hydrogenous uranium in the Bauer Deep resided predominantly in fish-bone apatite, with smaller amounts in manganese micronodules, whereas the uranium of detrital origin appears to be restricted largely to the clay fraction. By contrast, all size fractions on the East Pacific Rise are dominated by hydrogenous uranium. Moreover, the distribution of uranium in metalliferous sediments is quite heterogeneous. This suggests that locally reducing conditions, perhaps associated with fish debris, fecal pellets, or decaying organic matter inside foraminiferal shell cavities, may enhance the fixation of uranium in the sediments.

On the basis of these data, coprecipitation of uranium from sea water with volcanogenic iron oxides, followed by fixation in the sediments by local reducing conditions, is considered to be the predominant mechanism for the enrichment of uranium in metalliferous sediments on the East Pacific Rise.

The mechanism of hydrogenous uranium input in the Bauer Deep, on the other hand, probably involves redeposition of laterally transported rise-crest sediments, followed by diagenetic release of uranium and its fixation in the sediment by fish-bone apatite and manganese micronodules. It is estimated that a maximum of 70% of the total uranium content in the Bauer Deep may have originated in this way, the rest having been supplied from detrital sources.

The data presented in this study do not provide any support for significant contributions of uranium from hydrothermal solutions on the Nazca plate.

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