『Abstract
Weathering of large basaltic provinces is a major sink of atmospheric
carbon dioxide, and is also an important part of the geochemical
cycle of many other elements. This study aims to improve our understanding
of basaltic weathering by analysis of trace elements and Li isotopes
in a lateritic profile developed upon the Deccan Trap flood basalt.
At its base, the profile consists of unaltered basalt that has
a Li concentration and δ7Li value of 〜5 ppm and +2.5‰,
respectively, which is upwards progressively altered to a highly
weathered laterite (iron-rich) residuum at the surface. High concentrations
of Fe2O3 (85 wt.%) at 11m depth indicate the presence of a palaeowatertable;
the transition metals, the rare earth element (REEs) and U are
also strongly enriched in this horizon, whereas fluid mobile elements,
such as Li and Rb, are depleted relative to the precursor basalt,
and the bulk δ7Li value is very low (-5.5‰). A high
proportion of leachable Li is found in this horizon, and this
is enriched in 6Li relative to the bulk. We suggest
that leaching extracts the labile weathering products, and these
have high 6Li as 7Li has been preferentially
mobilised during basalt weathering. Throughout the rest of the
profile, Li concentrations and Li isotope values are best explained
by two-component mixing between the precursor basalt and aeolian
material with high Li and low δ7Li. Mass balance calculations
show that at least 50‰ of the Li present in the profile could
have been supplied by a mineral aerosol that has a δ7Li
value between 0‰ and +1.3‰. The depth-integrated δ7Li
value of the profile is -0.6‰, which suggest that 7Li
has been preferentially leached from the aerosol after its deposition.
The source of this aeolian material is most likely to be ancient
continental crust. It is possibly derived from Archaen/Proterozoic
terrain to the south and southeast of the Deccan, and/or material
from central Asia, similar to that which forms Chinese loess and
which is supplied to the North Pacific basin.
Keywords: Lithium isotopes; Weathering; Laterites; Deccan』
1. Introduction
2. Geological setting
2.1. The Bidar weathering profile and laterization process
3. Methods
3.1. Sampling and sample preparation
3.2. Major and trace element analysis
3.3. Lithium isotope analysis
4 Results
4.1. Bulk analyses
4.1.1. Zone I (47-35m)
4.1.2. Zones II and IV (35-10m)
4.1.3. Zone III (10-2m)
4.1.4. Granite samples
4.2. Leachable fraction
5. Discussion
5.1. Palaeowatertable
5.2. Behaviour of Li and Li isotopes during weathering
5.3. Input of Li to the weathering profile
6. Conclusions
Acknowledgements
References