『Abstract
Titanium and Zr were analyzes in 10 weathering and pedogenetic
soil profiles developed on granitic rocks from NW Spain. Concentrations
were obtained by XRF and the Ti and Zr-bearing minerals were studied
by petrographical, SEM and EDS analysis. Both elements were measured
in rock, bulk soil and granulometric fractions (sand: 2000-50μm,
silt: 50-2μm, clay: ＜2μm).
Titanium concentrations in the rock varied between 0.5 and 4.2
g kg-1, and Zr concentrations from 52 to 173 mg kg-1,.
Concentrations in bulk soil for the different horizons remain
almost unchanged and are strongly related to the element content
of the parent rock. In some epipedons Ti content is relatively
lower than that of the underlying horizons due to a dilution effect
caused by the high organic matter content and the addition of
fresh minerals - from erosion upslope. The mineralogical study
revealed that Zr is present exclusively in zircons, while Ti occurs
in rutile, ilmenite and titanite but mainly as a structural component
of the biotite.
Regarding the particle-size fractions both elements behave differently.
Zirconium is enriched in the silt fraction of all soil horizons,
with concentrations up to 5 times that of the parent rock; while
Ti is enriched in the silt (up to 5 times) and clay fractions
(up to 12 times) of the solum horizons, and depleted in
the sand fraction of the A horizons. These results indicate that
Zr concentrations are mainly controlled by the abundance of zircons
in the rock and their physical detachment from the host minerals
upon weathering. Ti mainly depends on the release from the structure
of biotites, through chemical weathering, and its reprecipitation
as neoformed mineral phases. This physicochemical fractionation
of Ti and Zr in soils bears important implications for studies
on soil genesis, continuity of soil profiles and the calculation
on enrichment factors which are based on the conservative behaviour
of both elements.
Keywords: Titanium; Zirconium; Granites; Weathering; Pedogenesis;
Particle-size fractionation』
1. Introduction
2. Materials and methods
2.1. Geographic setting and sampling
2.2. Analytical methods
3. Results
3.1. Fresh rocks
3.2. Soil horizons
3.2.1. Geochemical analysis
3.2.2. Mineralogical study
3.3. Ti and Zr as mass-balance strain indicators in granitic
soils
3.4. Titanium and Zr in different particle-size fractions
4. Discussion
5. Conclusions
Acknowledgments
References