Experimental determination of Li isotope behaviour during basalt weathering

Abstract Silicate weathering is the primary control of atmospheric CO 2 concentrations on multiple timescales. However, tracing this process has proven difficult. Lithium isotopes are a promising tracer of silicate weathering. This study has reacted basalt sand with natural river water for ~9 months in closed experiments, in order to examine the behaviour of Li isotopes during weathering. Aqueous Li concentrations decrease by a factor of ~10 with time, and δ 7 Li increases by ~19‰, implying that Li is being taken up into secondary phases that prefer 6 Li. Mass balance using various selective leaches of the exchangeable and secondary mineral fractions suggest that ~12–16% of Li is adsorbed, and the remainder is removed into neoformed secondary minerals. The exchangeable fractionation factors have a Δ 7 Li exch-soln  = −11.6 to −11.9‰, while the secondary minerals impose Δ 7 Li secmin-soln  = −22.5 to −23.9‰. Overall the experiment can be modelled with a Rayleigh fractionation factor of α = 0.991, similar to that found for natural basaltic rivers. The mobility of Li relative to the carbon-cycle-critical cations of Ca and Mg changes with time, but rapidly evolves within one month to remarkably similar mobilities amongst these three elements. Th evolution shows a linear relationship with δ 7 Li (largely due to a co-variation between aqueous [Li] and δ 7 Li), suggesting that Li isotopes have the potential to be used as a tracer of Ca and Mg mobility during basaltic weathering, and ultimately CO 2 drawdown.