Lead Isotopic Composition of Paleozoic and Late Proterozoic Marine Carbonate Rocks in the Vicinity of Yucca Mountain, Nevada

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by Robert E. Zartman, U.S. Geological Survey, Denver, United States,
Loretta M. Kwak, U.S. Geological Survey, Denver, United States,

Document Type: Proceeding Paper

Part of: High Level Radioactive Waste Management 1993:

Abstract: Paleozoic and Late Proterozoic marine carbonate rocks (limestones, dolomites, and their metamorphic equivalents) cropping out in the vicinity of Yucca Mountain contain lead with an isotopic composition strongly suggesting them to be a major source of the lead observed at Trench 14 in the carbonate phase of carbonate-silica veins and nearby surficial calcrete deposits. Six whole-rock samples of marine carbonate rocks yield 206Pb/204Pb = 19.21-29.06, 207Pb/204Pb = 15.74-16.01, and 208Pb/204Pb = 37.90-39.25, and leachate and residue fractions of the rocks reveal additional isotopic heterogeneity within individual samples. Two samples of eolian dust also have isotopic compositions lying along a 'carbonate' to 'silicate' mixing trend that appears to arise entirely from pedogenic processes. The tendency for the marine carbonate rocks to evolve highly uranogenic, but no thorogenic, lead results in a distinctive isotopic composition that serves as a tracer in eolian dust and secondary carbonate minerals derived from the marine carbonate rocks.

Subject Headings: Carbonate rocks | Lead (chemical) | Radioactive wastes | Carbonation | Limestone | Dust | Nevada | North America | United States

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