New perspectives on the Li isotopic composition of the upper continental crust and its weathering signature

Abstract Lithium isotopes are increasingly used to trace both present-day and past weathering processes at the surface of the Earth, and could potentially be used to evaluate the average degree of past weathering recorded by the upper continental crust (UCC). Yet the previous estimate of average δ 7 Li of the UCC has a rather large uncertainty, hindering the use of Li isotopes for this purpose. New δ 7 Li for desert and periglacial loess deposits (windblown dust) from several parts of the world (Europe, Argentina, China and Tajikistan) demonstrate that the former are more homogeneous than the latter, and may therefore serve as excellent proxies of the average composition of large tracts of the UCC. The Li isotopic compositions and concentrations of desert loess samples are controlled by eolian sorting that can be quantified by a binary mixing between a weathered, fine-grained end-member, dominated by phyllosilicates and having low δ 7 Li, and an unweathered, coarse-grained end-member, that is a mixture of quartz and plagioclase having higher δ 7 Li. We use correlations between insoluble elements (REE, Nd/Hf and Fe 2 O 3 /SiO 2 ), Li concentrations (henceforth referred as [Li]), and δ 7 Li to estimate a new, more precise, average Li isotopic composition and concentration for the UCC: [ Li ] = 30.5 ± 3.6 ( 2 σ ) ppm , and δ Li 7 = + 0.6 ± 0.6 ( 2 σ ) . The δ 7 Li for desert loess deposits is anti-correlated with the chemical index of alteration (CIA). Using this relationship, along with our average δ 7 Li, we infer that (1) the present-day CIA of the average UCC is 61 − 2 + 4 ( 2 σ ) , higher than the common reference value of 53, and (2) the average proportion of chemically weathered components is as high as 37 − 10 + 17 ( 2 σ ) % at the surface of the Earth.