『Abstract
The lithium concentration and isotopic composition of two saprolites
developed on a granite and diabase dike from South Carolina have
been measured in order to document the behavior lithium isotopes
during continental weathering. Both saprolites show a general
trend of decreasing δ7Li with increasing weathering
intensity, as measured by both bulk density and the chemical index
of alteration (CIA). The saprolite developed on the granite is
isotopically lighter than the fresh igneous rock (δ7Li
= -6.8‰ to +1.4‰ vs. +2.3‰, respectively), and is generally depleted
in lithium. These observations are consistent with leaching of
lithium via Rayleigh distillation during progressive weathering;
most saprolites fall on a Rayleigh distillation curve corresponding
to an apparent fractionation factor (α) of 0.997. However, two
samples have higher lithium contents than the fresh granite and
thus point to additional processes affecting lithium in the saprolite
(e.g., sorption of lithium on to clay minerals). The saprolite
profile developed on the diabase dike shows highly variable δ7Li
values, ranging down to extremely light compositions (-20‰). Previous
work has identified a chemical and mineralogical discontinuity
at a depth of 2m, but our lithium data show a marked discontinuity
at 6m depth. saprolite sample at or above 6m depth are highly
weathered (CIA = 88-95), depleted in lithium (having <50% of the
original diabase lithium}) and isotopically light (-10‰ to -20‰
vs. -4.3‰ for the unweathered diabase}). Most of the data are
consistent with leaching of lithium via Rayleigh distillation
during intense weathering, with apparent α values of 0.995 to
0.980. Samples experiencing lower apparent α values tend to have
higher kaolinite/smectite ratios, suggesting a mineralogical control
on isotopic fraction. However, the lightest sample (at -20‰) is
only slightly depleted in lithium and would require extremely
low α values to explain via Rayleigh distillation. This extreme
composition remains enigmatic. Saprolite samples below 6m have
highly variable δ7Li (-5‰ to -14‰) and, importantly,
lithium concentrations that are higher than that of the unweathered
diabase (up to 2.4 times the concentration of the fresh diabase).
These deeper saprolites thus cannot be explained by Rayleigh distillation.
A positive correlation between δ7Li and Li concentration
suggests these samples formed by mixing between an isotopically
light saprolite and heavy, groundwater lithium, and we thus suggest
that the 6-m discontinuity may mark the position of a paleo water
table.
Keywords: Lithium isotopes; Continental weathering; Saprolite;
South Carolina』
1. Introduction
2. Samples
2.1. Cayce diabase
2.2. Liberty Hill granite
3. Analytical methods
4. Results
4.1. Presentation of data
4.2. Cayce diabase saprolite
4.3. Liberty Hill granite saprolite
5. Discussion
5.1. Granite saprolite
5.2. Diabase saprolite
6. Conclusions
Acknowledgements
References