The coupled release of REE and Pb to the soil labile pool with time by weathering of accessory phases, Wind River Mountains, WY

Abstract

Rare earth element (REE) distributions and Pb isotope compositions were explored in soils varying in age from ca. 0.4 to ⩾300 ka, developed on moraines in the Wind River Mountains, Wyoming. Soil extracts (0.6 M HCl) were used to examine the soil labile pool while the major element distribution in soil profiles was used to determine the extent of weathering at different soil depths. The results show that the chondrite-normalized REE patterns of the deepest bulk soil within each profile reflects the composition of the moraine till, except for the oldest soil. Up to ca. 12 ka, the soil extract fraction is enriched in light REE, indicating early release of light REE to the soil labile pool while that of the two oldest soils are relatively enriched in heavy REE. In the soil extracts the La/Sm ratio normalized to the deepest soil (La D/Sm D) decreases systematically with soil age. Similarly, the Eu-anomaly in the deepest soil from each profile (Eu D/Eu D*) decreases slightly with soil age in the three young soils; however, Eu D/Eu D* increases with soil age in the older soils. The systematic trends of these two ratios indicate the depletion of light REE in young soils and the enrichment of Eu and heavy REE in the older soils. Based on the Pb isotope ratios, the relative contribution of Pb to the soil labile pool via mineral weathering of U- or Th-rich phases was assessed for the different stages of weathering. The whole-soil profile 208Pb/ 204Pb ratio was found to decrease with soil age and with La D/Sm D, whereas it increased with the Eu D/Eu D* ratio. In each horizon, Pb isotope ratios ( 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb) ratio generally decrease with soil age. In order to overcome possible effects from parent material heterogeneity, the amount of radiogenic Pb as compared to the whole-soil composition was calculated and this was found to decrease systematically with soil age.