The relationship between riverine lithium isotope composition and silicate weathering rates in Iceland

This study presents lithium isotope and elemental data for the dissolved phase and suspended and bedload sediments of the major Icelandic rivers. For the dissolved phase, delta Li-7 values range between 10.1 parts per thousand and 23.8 parts per thousand, while river sediments display lower and much more homogeneous values (delta Li-7 = 3.1 parts per thousand-4.8 parts per thousand), close to the composition of unweathered Mid-Ocean Ridge Basalt (MORB). High delta Li-7 values are associated with high K/Li, Na/Li and Mg/Li ratios, in waters draining mainly old and weathered basalt catchments, whereas low delta Li-7 rivers are located in younger parts of the island. Simple mixing between precipitation, Li-rich hydrothermal springs and basalt weathering is unable to explain the entire range of delta Li-7 values. Instead, a simple model of Li uptake by secondary minerals, associated with clay-water Li isotope fractionation (Delta Li-7 ranging from -1 parts per thousand to - 7.5 parts per thousand) can explain both water and sediment delta Li-7 values. A negative correlation is observed between basalt chemical erosion rates and delta Li-7 measured in Icelandic rivers, and an empirical law is inferred. Comparison with literature data suggests that this relationship may be applicable at a more global and scale, if confirmed, could be of particular use for estimating the evolution of continental weathering preserved in marine sedimentary records. However, more data are now needed for rivers draining silicates typical of the continental crust, in order to refine large scale modelling. (C) 2009 Elsevier B.V. All rights reserved.