『Abstract
Magnesium isotopic compositions of a profile through saprolites
developed on a diabase dike from South Carolina have been measured
in order to study the behavior of Mg isotopes during continental
weathering. As weathering progresses, Mg isotopes are greatly
fractionated and are correlated with Mg concentration, clay mineral
proportions and density of the saprolites, δ26Mg values
increase from -0.22 in the unweathered diabase to +0.65 in the
most weathered saprolite. These observations are consist with
the release of light Mg to the hydrosphere and formation of isotopically
heavy Mg in the weathered products. The loss of Mg during weathering
can be modeled by Rayleigh distillation with an apparent fractionation
factor between the saprolite and fluid (α) of 1.00005 to 1.0004,
i.e., up to 0.4‰ fractionation in the 26Mg/24Mg
ratio between the saprolite and fluid. The large variation in
α value reflects a mineralogical control on Mg isotope fractionation
during primary dissolution of Mg-rich minerals and formation of
secondary minerals during continental weathering. Like Mg isotopes,
Li isotopes in the saprolite profile are also greatly fractionated,
with δ7Li values ranging from -6.7 down to -20. The
large Li isotope fractionation and variation in Li concentration,
as well as irregularities in the δ7Li profile with
depth, however, cannot be explained by Li loss during weathering
alone. Instead, Li can be modeled by a two-step process: (1) equilibrium
isotope fractionation during continental weathering, which lowered
δ7Li and Li concentrations and produced a Li concentration
gradient in the saprolites like that seen in Mg, and (2) subsequent
kinetic isotope fractionation produced by diffusion of Li in the
saprolites, possibly across a paleo-water table. The results presented
here suggest that continental weathering will shift the Mg isotopic
composition of the continental crust to values higher than the
mantle value, whereas crustal recycling over the history of the
Earth will have no discernible effect on the Mg isotopic composition
of the mantle.
Keywords: lithium; magnesium; isotope fractionation; continental
weathering』
1. Introduction
2. Samples
3. Analytical methods
4. Results
5. Discussion
5.1. Magnesium isotope fractionation during weathering
5.2. Diffusion-driven Li isotope fractionation during weathering
5.3. Implications for the Mg isotopic composition of the crust
and mantle
6. Conclusions
Acknowledgements
References