SYSTEMATIC CHANGES IN LEAD ISOTOPIC COMPOSITION WITH SOIL AGE IN GLACIAL GRANITIC TERRAINS

Variations in the isotopic composition of Pb in soils developed on gla cial moraines in granitoid terrains were investigated. Samples were co llected from the Mesozoic Sierra Nevada batholith, California and the Archean Wind River Mountains, Wyoming, both in USA. Lead isotopes were measured in whole soil samples and in weak-acid leaches of both soils and fresh bedrock (granitoid) samples. In the Sierra Nevada, Pb-206/P b-204 and Pb-206/Pb-207 ratios of the acid-leached fraction of three s oils of varying ages (approximately 20, 120, and greater than or equal to 330 kyr) decreases systematically with moraine age from 20.73 +/- .03 to 20.14 +/- .02, and from 1.314 +/- .001 to 1.279 +/- .003, respe ctively. In contrast, Pb-208/Pb-204 ratios do not change systematicall y with soil age. In the Wind River Mountains, lead isotopes were measu red in soils developed on six moraines of varying ages (similar to 0.4 , similar to 2, similar to 12, similar to 22, similar to 140, and grea ter than or equal to 300 kyr). Pb-208/Pb-204 ratios of the acid-leache d fraction of soils decrease systematically with moraine age from 61.0 +/- 7.8 to 38.7 +/- .4 in soils ranging in age from similar to 12kyr to greater than or equal to 300 kyr. On the other hand, Pb-206/Pb-204 and Pb-207/Pb-204 ratios of the acid-leached fraction do not change sy stematically with soil age, nor do the Pb-208/Pb-204 ratios of the aci d-leached fractions of the three younger soils (similar to 0.4, simila r to 2, and similar to 12 kyr). The isotopic composition of the acid-l eached Pb in soils developed on the oldest moraines (greater than or e qual to 300 kyr) in both the Sierra Nevada and the Wind River Mountain s is very close to the isotopic composition of the whole soil. The fra ction of radiogenic Pb in the weatherable pool is approximately 2% in the Sierra Nevada and 40% in the Wind River Mountains. This explains w hy changes in the isotopic composition of total Pb can be detected in the Wind River Mountains soils (ages 2 to greater than or equal to 300 kyr) but not in the Sierra Nevada soils. The Pb-207- Pb-206 isochron age obtained from whole soil samples in the Wind River Mountains is in agreement with reported ages of various granitoids in the area and is consistent with the proposition that the soil is largely derived from in situ sources, namely from the granitoids of the Wind River Mountai ns. We suggest that Pb-Pb isochrons of the acid-leached fraction in th e soil can be used to predict whether the isotopic ratios of Pb in the acid-leached fraction of a soil will change systematically with soil age in a given area and whether U-or Th-rich accessory phases are the major source of radiogenic Pb in the soil. Copyright (C) 1998 Elsevier Science Ltd.