Particle-size fractionation of titanium and zirconium during weathering and pedogenesis of granitic rocks in NW Spain

Abstract

Titanium and Zr were analyzed 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: 2000–50 μm, silt: 50–2 μm, clay: < 2 μm). 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.