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

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• 作者：Philip A.E. Pogge von Strandmann^a ; ^{p.strandmann@ucl.ac.uk" class="auth_mail" title="E-mail the corresponding author} ; Kevin W. Burton^b ; Sophie Opfergelt^c ; Eydí ; s S. Eirí ; ksdó ; ttir^d ; Melissa J. Murphy^e ; Arni Einarsson^f ; ^g ; Sigurdur R. Gislason^d
• 关键词：Hydrothermal ; Weathering ; Lithium ; Silicon ; Primary productivity
• 刊名：Chemical Geology
• 出版年：2016
• 出版时间：16 December 2016
• 年：2016
• 卷：445
• 期：Complete
• 页码：4-13
• 全文大小：847 K

文摘

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.