Experimental determination of Li isotope behaviour during basalt weathering

Silicate weathering is the primary control of atmospheric CO2 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 similar to 9 months in closed experiments, in order to examine the behaviour of Li isotopes during weathering. Aqueous Li concentrations decrease by a factor of similar to 10 with time, and delta Li-7 increases by similar to 19 parts per thousand, implying that Li is being taken up into secondary phases that prefer Li-6. Mass balance using various selective leaches of the exchangeable and secondary mineral fractions suggest that similar to 12-16% of Li is adsorbed, and the remainder is removed into neoformed secondary minerals. The exchangeable fractionation factors have a Delta Li-7(exch-soln) = -11.6 to -11.9 parts per thousand, while the secondary minerals impose Delta Li-7(secmin-soln) = -22.5 to - 23.9 parts per thousand. Overall the experiment can be modelled with a Rayleigh fractionation factor of alpha = 0.991, similar to that found for natural basaltic rivers. The mobility of Li relative to the carboncycle-critical cations of Ca and Mg changes with time, but rapidly evolves within one month to remarkably similar mobilities amongst these three elements. This evolution shows a linear relationship with delta Li-7 (largely due to a co-variation between aqueous [Li] and delta Li-7), suggesting that Li isotopes have the potential to be used as a tracer of Ca and Mg mobility during basaltic weathering, and ultimately CO2 drawdown.