The lithium isotope response to the variable weathering of soils in Iceland

This study has analysed Li isotopes ratios from well-studied soil and pore water profiles from Iceland that have the same parent material but have experienced different degrees of chemical weathering. Thus, from least to most weathered, we have analysed vitrosols (V), gleyic andosols (GA), brown andosols (BA), Histosols (H) and Histic Andosols (HA). Although the most weathered H and HA soils have the highest content in clay-sized material, they have the least fractionated δ⁷Li_{pore water} values. In contrast, the least weathered GA and BA pore waters are most fractionated for Li isotopes. Given that Li isotope ratios are fractionated by clay mineral formation, this appears counter-intuitive. A single trend for all samples of δ⁷Li as a function of Li/Na ratios suggests that they are all controlled by a process with a single fractionation factor, in this case likely the formation of poorly-crystalline allophane, which dominates in the “least weathered” soils. This rapidly forming secondary mineral dominates Li isotope fractionation over more slowly-forming crystalline clays. The fractionation along a single path shows that the key process here in controlling the Li isotope ratio of surface waters is the degree of Li uptake by secondary minerals. This does not necessarily correspond to the amount of clay minerals present in the soil, but to the amount of clay minerals that are being newly formed in a single passage of the pore water through the soil, or are in equilibrium with soil solutions at the time of sampling.