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Experimental determination of Li isotope behaviour during basalt weathering

Pogge von Strandmann, Philip A.E.; Fraser, Wesley T.; Hammond, Sam; Tarbuck, Gary; Wood, Ian G.; Oelkers, Eric H. and Murphy, Melissa J. (2019). Experimental determination of Li isotope behaviour during basalt weathering. Chemical Geology, 517 pp. 34–43.

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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 δ7Li increases by ~19‰, implying that Li is being taken up into secondary phases that prefer 6Li. 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 Δ7Liexch-soln = −11.6 to −11.9‰, while the secondary minerals impose Δ7Lisecmin-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. This evolution shows a linear relationship with δ7Li (largely due to a co-variation between aqueous [Li] and δ7Li), 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.

Item Type: Journal Item
Copyright Holders: 2019 Elsevier B.V
ISSN: 0009-2541
Keywords: Chemical weathering; Weathering experiments; Isotope fractionation; Basalt weathering.
Academic Unit/School: Faculty of Science, Technology, Engineering and Mathematics (STEM)
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Item ID: 60899
Depositing User: ORO Import
Date Deposited: 03 May 2019 09:15
Last Modified: 05 Jun 2019 08:59
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