Abstract: Marine Metalliferous Sediments from Nazca Plate
E. J. Dasch, J. B. Corliss, J. Dymond
The Nazca plate has been studied to understand the metallogenic processes which accompany plate generation on the East Pacific Rise (EPR). Nazca plate sediments are a mixture of hydrothermal, detrital, hydrogenous, and biogenous components. Because no one of these sources dominates the entire plate, analyses of sediments from many locations permit the development of a normative model which can partition sources of any given element in any sample. Hydrothermal sources greatly dominate near the EPR, biogenic sources are important in the northern parts of the plate, and hydrogenous sources are most important in the metal-rich sediments of the Bauer Deep east of the EPR. The unique composition of the hydrothermal sediments can be explained by precipitation of metal hydroxides and oxides from seawater hydrothermal solutions that are hypothesized to cool, leach, and alter the newly formed lithosphere at the EPR.
This model is supported by accumulation-rate studies which demonstrate that the highest accumulations of many metals occur near the rise crest. Accumulation of manganese and other transition metals within the Bauer Deep, however, is sufficiently low that it could result largely from hydrogenous precipitation. Factor analysis demonstrates that more than 98% of the variability in chemical composition can be described by mixtures of three statistically determined factors. One factor, enriched in Fe, Mn, Cu, and Zn, predominates in sediments from the EPR, and is interpreted as a product of hydrothermal source. A second factor, enriched in Ni and Al, is associated with the Bauer Deep and other areas of slow sediment accumulation where detrital and hydrogenous sources can predominate. A thi d factor, enriched in Si, is limited to the northern Nazca plate in regions of high equatorial productivity.
The isotopic composition of Pb in the sediments varies as a function of U/Pb and Th/Pb ratios and the age of the source materials, and this isotopic patterning has been established as an indicator of geologic provenance. "Mantle" Pb from fresh, mid-ocean ridge basalts varies within relatively narrow isotopic limits and is significantly less radiogenic than the "marine" or "seawater" Pb of many manganese nodules. Metalliferous sediments of the Nazca plate range in Pb isotopic composition from "mantlelike," along the EPR, to "marinelike," in sediments which have had extensive contact with seawater. The geographic and temporal texture of the Pb isotope data can be explained in terms of a magmatic component, to which have been added marine, and, of lesser importance, detrital components.< P>
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC