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The effect of hydrothermal spring weathering processes and primary productivity on lithium isotopes: Lake Myvatn, Iceland

Pogge von Strandmann, PAE; Burton, KW; Opfergelt, S; Eiríksdóttir, ES; Murphy, MJ; Einarsson, A; Gislason, SR; (2016) The effect of hydrothermal spring weathering processes and primary productivity on lithium isotopes: Lake Myvatn, Iceland. Chemical Geology , 445 pp. 4-13. 10.1016/j.chemgeo.2016.02.026. Green open access

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Abstract

Lithium isotopes are rapidly becoming one of the most useful tracers of silicate weathering processes, but little is known on their behaviour in groundwaters and hydrothermal springs, and how these sources might influence the weathering signal in surface waters. This study presents lithium isotope compositions (δ7Li) for cold groundwaters (3–7 °C) and hydrothermal springs that were at geothermal temperatures (200–300 °C) but have cooled during transport (17–44 °C). Both represent an important source of water and nutrients for Lake Myvatn, Iceland. We also present a time-series from the Laxa River, which is the single outflow from the lake. The δ7Li values in the input springs to Lake Myvatn are highly variable (5–27‰), and correlate inversely with temperature and total dissolved solids. These co-variations imply that even in such waters, the processes controlling δ7Li variations during weathering still operate: that is, the ratio of primary rock dissolution to secondary mineral formation, where the latter preferentially incorporates 6Li with a temperature-dependent fractionation factor. In high-temperature geothermal waters (> 300 °C) secondary mineral formation is inhibited, and has a low fractionation factor, leading to little δ7Li fractionation. Even in waters that have cooled considerably over several months from their geothermal temperatures, fractionation is still low, and δ7Li values are similar to those reported from waters measured at > 350 °C. In contrast, cooler groundwaters promote relatively high proportions of clay formation, which scavenge dissolved solids (including 6Li). The time series on the Laxa River, the single outflow from Lake Myvatn, shows little δ7Li variation with time over the 12 month sampling period (17–21‰), demonstrating that in contrast to tracers such as Si isotopes, Li isotopes are unaffected by the significant seasonal phytoplankton blooms that occur in the lake. Thus, these results clearly illustrate that Li isotopes are ideally suited to constrain silicate weathering processes, because fractionation by secondary mineral formation operates even when groundwater and hydrothermal inputs are significant, and because Li isotopes are demonstrably unaffected by phytoplankton or plant growth.

Type: Article
Title: The effect of hydrothermal spring weathering processes and primary productivity on lithium isotopes: Lake Myvatn, Iceland
Open access status: An open access version is available from UCL Discovery
DOI: 10.1016/j.chemgeo.2016.02.026
Publisher version: http://dx.doi.org/10.1016/j.chemgeo.2016.02.026
Language: English
Additional information: © 2016 Elsevier B.V. This manuscript is made available under a Creative Commons Attribution Non-commercial Non-derivative 4.0 International license (CC BY-NC-ND 4.0). This license allows you to share, copy, distribute and transmit the work for personal and non-commercial use providing author and publisher attribution is clearly stated. Further details about CC BY licenses are available at http://creativecommons.org/ licenses/by/4.0. Access may be initially restricted by the publisher.
Keywords: Hydrothermal; Weathering; Lithium; Silicon; Primary productivity
UCL classification: UCL > Provost and Vice Provost Offices
UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Earth Sciences
URI: https://discovery.ucl.ac.uk/id/eprint/1477340
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