Experimental determination of Li isotope behaviour during basalt weathering

Abstract

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 ~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 d 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 D 7 Liexch-soln =-11.6 to-11.9‰, while the secondary minerals impose D 7 Lisecmin-soln =-22.5 to-23.9‰. Overall the experiment can be modelled with a Rayleigh fractionation factor of a = 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 d 7 Li (largely due to a co-variation between aqueous [Li] and d 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 CO2 drawdown.

The lithium and magnesium isotope signature of olivine dissolution in soil experiments

“…Results are given as ‰ deviation from the standard L-SVEC. To assess accuracy and precision, seawater and USGS standards BCR-2 (basalt) and SGR-1 (shale) were analysed, giving d 7 Li = 31.3 ± 0.6‰ (n = 50, chemistry = 50), 2.7 ± 0.4‰ (n = 4, chemistry = 4) and 3.6 ± 0.4‰ (n = 3, chemistry = 3), respectively, which are in agreement with other studies (Dellinger et al, 2015;Phan et al, 2016;Pogge von Strandmann et al, 2019a;Pogge von Strandmann et al, 2012).…”

“…Overall, the exchangeable pool will have been exchanging Li with pore waters, with an isotope fractionation on the order of 10-20‰ towards lighter values (Pogge von Strandmann et al, 2019a;Hindshaw et al, 2019), as also calculated by mass balance below. This process should theoretically drive both pore waters and the exchangeable pool isotopically heavy with time (and depth).…”

Lithium Occurrences in Brines from Two German Salt Deposits (Upper Permian) and First Results of Leaching Experiments

“…These reaction times are quite different to other rock types, such as basalt. For example, the reaction between basalt sand and river water seams to reach equilibrium after 126 days at 20 • C in closed experiments [52].…”

The hydrochemical signature of incongruent weathering in Iceland

“…As a result, the elemental mobility inferred from Equation 8 might be artificially decreased. Identifying this potential bias is important as it underlies the calibration of some isotopic tools for reconstructing weathering processes in the past (e.g., δ 7 Li; P. A. E. Pogge von Strandmann, Fraser, et al, 2019).…”