Quantifying Li isotope fractionation during smectite formation and implications for the Li cycle

Tri-octahedral Li-Mg smectites (hectorites) were synthesized at temperatures ranging from 25 to 250 degrees C, in the presence of solutions highly enriched in lithium. After removing all the exchangeable lithium from the synthesized clays, Li isotope fractionation (Delta Li-7(clay-solution)) was determined. This fractionation was linked to Li incorporation into the structural octahedral site, substituting for Mg2+. As predicted, experimental Delta Li-7(clay-solution) inversely correlates with temperature, and ranges from -1.6 parts per thousand +/- 1.3 parts per thousand at 250 degrees C to -10.0 parts per thousand +/- 1.3 parts per thousand at 90 degrees C, and then stays relatively constant down to 25 degrees C. The relatively constant isotope fractionation factor below 90 degrees C may be due to high concentrations of edge octahedra in low crystallinity smectites. The isotopic fractionation factor (a), for a given temperature, does not depend on the solution matrix, nor on the amount of structural Li incorporated into the clay. Empirical linear laws for a as a function of 1/T (K) were inferred. Smectite Li contents and smectite-solution distribution coefficients (D-Li/Mg) increase with temperature, as expected for a substitution process. The fractions of dissolved Li incorporated into the smectite octahedral sites are small and do not depend on the duration of the experiment. In a seawater-like matrix solution, less Li is incorporated into the smectites, probably as a result of competition with dissolved Mg2+ ions for incorporation into the octahedral sites. The high Li contents observed in marine smectites are therefore best explained either by a significant contribution from basalts, by adsorption processes, or by the influence of seawater chemical composition on distribution coefficients. We also calculate, using present-day estimates of hydrothermal water and river fluxes, that a steady-state ocean would require a relatively large global clay-water Li isotope fractionation (-12 parts per thousand to -21 parts per thousand). This study demonstrates the ability of laboratory experiments to quantify the impact of secondary phases on the Li geochemical cycle and associated isotope fractionations. (c) 2007 Elsevier Ltd. All rights reserved.