Systematic Changes in Lead Isotopic Composition with Soil Age in Glacial Granitic Terrains

Abstract

Variations in the isotopic composition of Pb in soils developed on glacial moraines in granitoid terrains were investigated. Samples were collected 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, 206Pb/ 204Pb and 206Pb/ 207Pb ratios of the acid-leached fraction of three soils of varying ages (approximately 20, 120, and ≥330 kyr) decreases systematically with moraine age from 20.73 ± .03 to 20.14 ± .02, and from 1.314 ± .001 to 1.279 ± .003, respectively. In contrast, 208Pb/ 204Pb ratios do not change systematically with soil age. In the Wind River Mountains, lead isotopes were measured in soils developed on six moraines of varying ages (∼0.4, ∼2, ∼12, ∼22, ∼140, and ≥300 kyr). 208Pb/ 204Pb ratios of the acid-leached fraction of soils decrease systematically with moraine age from 61.0 ± 7.8 to 38.7 ± .4 in soils ranging in age from ∼12kyr to ≥300 kyr. On the other hand, 206Pb/ 204Pb and 207Pb/ 204Pb ratios of the acid-leached fraction do not change systematically with soil age, nor do the 208Pb/ 204Pb ratios of the acid-leached fractions of the three younger soils (∼0.4, ∼2, and ∼12 kyr). The isotopic composition of the acid-leached Pb in soils developed on the oldest moraines (≥300 kyr) in both the Sierra Nevada and the Wind River Mountains is very close to the isotopic composition of the whole soil. The fraction of radiogenic Pb in the weatherable pool is approximately 2% in the Sierra Nevada and 40% in the Wind River Mountains. This explains why changes in the isotopic composition of total Pb can be detected in the Wind River Mountains soils (ages 2 to ≥300 kyr ) but not in the Sierra Nevada soils. The 207Pb- 206Pb 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 Mountains. We suggest that Pb-Pb isochrons of the acid-leached fraction in the 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.