Lead isotopic composition of metalliferous sediments from the Nazca plate

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Abstract
Metal-rich sediments recently deposited over the northern part of the Nazca plate have lead isotopic compositional ranges as follows: 206Pb/204Pb = 18.123 to 18.734; 207Pb/204Pb = 15.472 to 15.640; 208Pb/204Pb = 37.796 to 38.729. On 207Pb/204Pb versus 206Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb graphs, data for these sediments plot both within and between the otherwise separate fields on the diagrams that represent Pb isotopic ranges from mid-ocean ridge basalts (MORB; mantle-derived Pb) and ferromanganese nodules (MNOD; marine Pb). Least radiogenic (MORB) Pb is derived from the most volcanogenic sediments—along the East Pacific Rise and within the Bauer Deep. These leads exhibit extensive isotopic ranges that may in part reflect isotopic variations in mantle source rocks. Scatter plots of Pb isotopic composition of the samples versus a variety of elemental abundances, ratios, and combinations of elemental and other data show highest (positive) correlation coefficients with parameters that can best be described as “detrital.” Pb isotopic data for the sediments may be viewed graphically as mixing bands between mantle and marine Pb. Least radiogenic (but isotopically variable) mantle Pb is incorporated within the more volcanogenic sediments by coprecipitation with and absorption on Fe and Mn oxides and hydroxyoxides. Pb abundance in the sediments is increased by the adsorption and precipitation of Pb from sea water and pore waters. The marine Pb is more radiogenic and apparently has been of rather uniform isotopic composition in this region of the Pacific during the time of deposition and diagenesis of the sediment. These most radiogenic leads are isotopically indistinquishable from lead derived from deep ocean manganese nodules.

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