Loess geochemistry and its implications for particle origin and composition of the upper continental crust

Chemical and Nd-Sr isotopic compositions of loess samples from Argentina, Europe and Spitsbergen were analyzed to examine the nature of source terrains, the origin of silt-size particles and the suitability of using loess as starting material for estimating the average chemical composition of the upper continental crust. From the relations between Na(2)O/Al(2)O(3) and K(2)O/Al(2)O(3) ratios and CIA values (chemical index of alteration), the loess protoliths must have undergone previous sedimentary differentiation and subjected to moderate chemical weathering. REE patterns are remarkably uniform with (La/Yb)(N) approximate to 10, which is characteristic of the upper continental crust (UCC). Negative Eu anomalies, expressed in Eu/Eu* ratios, vary from 0.65 for European loess to 0.8 for Pampean loess from Argentina. All loess deposits have nearly constant La/Th or Th/U ratios, which are very similar to those of the average UCC or post-Archean shales. These ratios are not fractionated in size-fractions relative to the whole-rock values. Nd and Sr isotopic compositions clearly distinguish Argentinean loess ((87)Sr/(86)Sr = 0.706-0.709, epsilon(Nd)(0) = -6 to -1.5) from all other loess deposits ((87)Sr/(86)Sr = 0.712-0.730, epsilon(Nd)(0) = -13 to -8). The REE and isotopic results clearly indicate a significant contribution of young Andean volcanics to the Pampean loess deposits, whereas multi-recycled and well-mixed ancient sediments are principal sources for the other deposits. The present results reinforce the earlier conclusion reached by S.R. Taylor, S.M. McLennan and M.T. McCulloch [Geochemistry of loess, continental crustal composition and crustal model ages, Geochim. Cosmochim. Acta 47 (1983) 1597-1905] that the average chemical composition of UCC can be obtained from eolian deposits as well as from fine-grained elastic sediments. (C) 1998 Elsevier Science B.V. All rights reserved.