『Abstract
Fe-nodules occur within saprolites formed from weathering of
granodioritic gneisses in the rain-shadow region of the Mysore
Plateau adjacent to the Sahyadri Mountains in Southern India.
These nodules and their host saprolites were studied for their
geochemistry, including chemical speciation, to understand nodule
formation and chemical redistribution processes during rock weathering.
From their mode of occurrence, and mineralogical and geochemical
data, we infer that the nodules originated by a two-stage process
in which the initial extensive weathering of gneisses likely facilitated
subsequent ferrolysis weathering and nodule formation. Nodules
originated by precipitation of goethite, hematite and gibbsite
along with several amorphous phases within the matrix of weathered
gneisses. This is possible only under hydromorphic conditions,
suggesting that parts of the plateau must have gone through a
humid phase prior to the present aridity. In the saprolites, Al,
Fe, and Ti become enriched because of the removal of Si, Ca, Na,
and K. However within the nodule, Fe, Ti, Cr, and Ni are deposited
after their chemical transport from the saprolite. Titanium, known
for its immobile nature, was also mobilized and concentrated under
the conditions of nodule formation. The most important elements
in the nodule constitution are Fe, Al, Ti, and Mn, each having
both crystalline and amorphous phases. Fe-Ti and Mn oxyhydroxides
grain coatings in the saprolites and discrete amorphous Mn and
Ti phases in the nodules seem to have scavenged trace elements
from the weathering profile. REE were mobilized during weathering
and nodule genesis in which Ce and Ti show a strong geochemical
coherence. The enrichment of only HREE in saprolite, and both
HREE and LREE with significant Ce in the nodule, indicate the
control of evolving secondary minerals in the REE redistribution
during rock weathering. Strong enrichment of Ce in the weathering
profile and in nodules has important implications to the REE chemistry
of river waters.』
1. Introduction
2. Geology and climate of the region
3. Materials and methods
4. Results and discussion
4.1. Mineralogy of weathering profile and nodules
4.2. Major element chemistry
4.3. Speciation analysis and redistribution of elements
4.4. Statistics of the elemental distribution
4.5. Chemical phases of the nodules
4.6. Rare earth elements (REE) geochemistry
5. Conclusions
Acknowledgments
Appendix A. Supplementary data
References