Sediments, Whole-Rock and HCl-Soluble Lead

Cenozoic Sediments, Whole-Rock and HCl-Soluble Lead 

Lead isotopic composition of the main sediment types (for a compilation see Appendix C) is more a function of age of source areas for the sediment than of rock type because the source rocks contain significant quantities of U and Th relative to lead (Fig. 17). Young sediments in restricted basins draining Precambrian terranes, such as Hudson Bay, Great Slave Lake, Great Bear Lake, Lake Superior, and the Baltic Sea (Chow, 1965 Hart and Tilton, 1966), tend to be highly radiogenic and variable in lead isotope ratios. 

On the other hand, sediments, well-mixed because they are far from their source, tend to be isotopically rather uniform even if the detritus is Precambrian in age, but the lead is still somewhat radiogenic (Doe et al, 1966). The isotopic composition in the insoluble residues of these sediments, all roughly of the same sedimentation age, appears to be more a function of grain size but which may in reality be more a function of K-feldspar content. The coarse fractions are less radiogenic than the silt- or clay-sized fractions, and the lead isotope ratios still reflect the Precambrian ancestry of the detritus (Muffler and Doe, 1968). The finer fractions of detritus approach more closely the isotopic composition of the HCl-soluble fraction. 

Pelagic sediments from the ocean basins (Chow and Patterson, 1962a Chow and Tatsumoto, 1964 Wampler and Kulp, 1964 Chow, 1968b) fall in restricted ranges of lead isotopic composition (Fig. 17), although the means from the Atlantic, Indian, and Antarctic Oceans and the Red Sea are somewhat more radiogenic than those of 

Cenozoic age from various localities. (References to data are given in Section III, part 6 — Cenozoic sediments whole-rock and HCl soluble lead)  

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