Behaviour of lithium and its isotopes during weathering in the Mackenzie Basin, Canada

Abstract

International audienceWe report Li isotopic compositions, for river waters and suspended sediments, of about 40 rivers sampled within the Mackenzie River Basin in northwestern Canada. The aim of this study is to characterize the behaviour of Li and its isotopes during weathering at the scale of a large mixed lithology basin. The Mackenzie River waters display systematically heavier Li isotopic compositions relative to source rocks and suspended sediments. The range in delta Li-7 is larger in dissolved load (from +9.3 parts per thousand to +29.0 parts per thousand) compared to suspended sediments (from 1.7 parts per thousand to +3.2 parts per thousand), which are not significantly different from delta Li-7 values in bedrocks. Our study shows that dissolved Li is essentially derived from the weathering of silicates and that its isotopic composition in the dissolved load is inversely correlated with its relative mobility when compared to Na. The highest enrichment of Li-7 in the dissolved load is reported when Li is not or poorly incorporated in secondary phases after its release into solution by mineral dissolution. This counterintuitive observation is interpreted by the mixing of water types derived from two different weathering regimes producing different Li isotopic compositions within the Mackenzie River Basin. The incipient weathering regime characterizing the Rocky Mountains and the Shield areas produces Li-7 enrichment in the fluid phase that is most simply explained by the precipitation of oxyhydroxide phases fractionating Li isotopes. The second weathering regime is found in the lowland area and produces the lower delta Li-7 waters (but still enriched in Li-7 compared to bedrocks) and the most Li-depleted waters (compared to Na). Fractionation factors suggest that the incorporation of Li in clay minerals is the mechanism that explains the isotopic composition of the lowland rivers. The correlation of boron and lithium concentrations found in the dissolved load of the Mackenzie Rivers suggests that precipitation of clay minerals is favoured by the relatively high residence time of water in groundwater. In the Shield and Rocky Mountains, Li isotopes suggest that clay minerals are not forming and that secondary minerals with stronger affinity for Li-7 appear. Although the weathering mechanisms operating in the Mackenzie Basin need to be characterized more precisely, the Li isotope data reported here clearly show the control of Li isotopes by the weathering intensity. The spatial diversity of weathering regimes, resulting from a complex combination of factors such as topography, geology, climate and hydrology explains, in fine, the spatial distribution of Li isotopic ratios in the large drainage basin of the Mackenzie River. There is no simple relationship between Li isotopic composition and chemical denudation fluxes in the Mackenzie River Basi

Lithium-isotope evidence for enhanced silicate weathering during OAE 1a (Early Aptian Selli event)

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“…In nature, lithium is composed of two stable isotopes -6 Li and 7 Li -and is predominantly found in silicate minerals (Kisakürek et al, 2005;Millot et al, 2010). In modern oceans, Li has a residence time of about 1 Myr and the principal input fluxes are continental weathering and hydrothermal activity, which are roughly equivalent (Hathorne & James, 2006;Misra & Froelich, 2012).…”

“…Dellinger Huh et al, 1998;Millot et al, 2010), and suggested for some case histories from the geological record (Li & West, 2014;Wanner et al, 2014), makes it possible to model the observed features, whereas changing the residence times of Os, Sr and Ca do not change the main signal (Fig. 6).…”

Constraints on continental crustal mass loss via chemical weathering using lithium and its isotopes

“…Because the mass difference between these two isotopes is relatively large (approximately 16%), they show significant mass dependent fractionation in nature (>50‰) (12), expressed in δ 7 Li notation: δ 7 Lið‰Þ ¼ ð½ 7 Li∕ 6 Li sample ∕½ 7 Li∕ 6 Li standard − 1Þ × 1000, where the standard used is a synthetic Li carbonate, L-SVEC (13). During chemical weathering, secondary minerals, such as clays, take 6 Li preferentially into their structure, resulting in heavier Li isotopic composition in rivers and lighter isotopic composition in the regolith (14)(15)(16)(17)(18)(19)(20)(21)(22). The lithium concentration and isotopic composition of the continental crust, as well as river waters, are well documented by various authors (17,(19)(20)(21)(22)(23)(24)(25)(26).…”

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“…As Li is 59 3 a recently developed isotopic tracer, not all the processes that could induce and control 60 isotopic fractionation are as yet well constrained. However, in the context of water/rock 61 interactions, numerous studies (Huh et al, 1998(Huh et al, , 2001Pistiner et al, 2003; Kisakurek et al, 62 2004 Kisakurek et al, 62 , 2005 Pogge von Strandmann et al, 2006;Millot et al, 2007Millot et al, , 2010aMillot et al, , 2010b have 63 clearly shown that isotopic fractionation supports the enrichment of the heavy isotope ( 7 Li) in 64 solution, the light isotope ( 6 Li) being preferentially retained in secondary weathering 65 minerals. Silicate rocks display Li isotope compositions ranging from -2 to +10‰ (Coplen et 66 al., 2002;Teng et al 2004), that of seawater is ~ +31‰ (Millot et al, 2004), river water has 67 intermediate isotopic compositions (+6 to +23‰, Huh et al, 1998), and saline (thermo-68 mineral) water generally has isotopic compositions in the range of 0 to +15‰ 69 Négrel, 2007, Millot et al, 2010b) although Falkner et al (1997) found Li isotopic 70 compositions in the range of +17 to +35‰ in hot spring water from Lake Baikal.…”