『Abstract
We 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 of δ7Li
is larger in dissolved load (from +9.3‰ to +29.0‰) compared to
suspended sediments (from -1.7‰ to +3.2‰), which are not significantly
different from δ7Li 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 7Li 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 7Li 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 δ7Li
waters (but still enriched in 7Li 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 7Li
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 Basin.』
1. Introduction
2. Geological setting and sample locations
3. Analytical methods
3.1. Sampling methodology, major and trace element concentration
measurements
3.2. Lithium isotope measurements
4. Results
4.1. Dissolved phase
4.2. River sediments and bedrocks
5. Discussion
5.1. Sources of dissolved lithium
5.2. Li isotopes and silicate weathering regimes
5.2.1. Evidence for isotope fractionation during chemical weathering
5.2.2. Influence of the weathering regime
6. Conclusions
Acknowledgments
References