Behaviour of Li isotopes during continental weathering:: the Bidar laterite profile, India

Weathering of large basaltic provinces is a major sink of atmospheric carbon dioxide, and is also an important part of the geochemical cycle of many other elements. This study aims to improve our understanding of basaltic weathering by analysis of trace elements and Li isotopes in a lateritic profile developed upon the Deccan Traps flood basalt. At its base, the profile consists of unaltered basalt that has a Li concentration and delta(7)Li value of -5 ppm and +2.5parts per thousand, respectively, which is upwards progressively altered to a highly weathered laterite (iron-rich) residuum at the surface. High concentrations of Fe2O3 (85 wt.%) at 11 m depth indicate the presence of a palaeowatertable; the transition metals, the rare earth elements (REEs) and U are also strongly enriched in this horizon, whereas fluid mobile elements, such as Li and Rb, are depleted relative to the precursor basalt, and the bulk delta(7)Li value is very low (-5.5parts per thousand). A high proportion of leachable Li is found in this horizon, and this is enriched in Li-6 relative to the bulk. We suggest that leaching extracts the labile weathering products, and these have high Li-6 as Li-7 has been preferentially mobilised during basalt weathering. Throughout the rest of the profile, Li concentrations and Li isotope values are best explained by two-component mixing between the precursor basalt and aeolian material with high Li and low delta(7)Li. Mass balance calculations show that at least 50% of the Li present in the profile could have been supplied by a mineral aerosol that has a delta(7)Li value between 0parts per thousand and +1.3parts per thousand. The depth-integrated 67 Li value of the profile is -0.6parts per thousand, which suggests that Li-7 has been preferentially leached from the aerosol after its deposition. The source of this aeolian material is most likely to be ancient continental crust. It is possibly derived from Archaen/Proterozoic terrain to the south and southeast of the Deccan, and/or material from central Asia, similar to that which forms Chinese loess and which is supplied to the North Pacific basin. (C) 2004 Elsevier B.v. All rights reserved.