Redistribution and mobilization of titanium, zirconium and thorium in an intensely weathered lateritic profile in Western Australia

Abstract

The mobility of titanium, zirconium and thorium, elements commonly considered insoluble during supergene weathering, is still not well understood, especially in intensely weathered regolith. Thus, an intensely weathered lateritic profile (JG) developed on meta-granitoids in Jarrahdale, Western Australia, was investigated. The mobility of Ti, Zr and Th has been assessed at both mineral assemblage and profile scales and the mode of occurrence has been investigated through the combined use of geochemical data from bulk regolith, particle size fractions and sequential extractions, with in situ data determined by electron probe micro-analyzer and synchrotron X-ray powder diffraction. Neoformed poorly crystalline phases containing trace to minor amounts of Zr, Ce and Th unassociated with silicates or phosphates were identified on the walls of Al/Fe-rich pores in the ferruginous duricrust. This implies that some mobilization and redistribution of Zr and Th occurs within a sample scale. Breakdown of primary thorite and rare earth element rich fluorocarbonates is thought to be the source for Zr and Th in the neoformed phases rather than zircon. Thus, the mineral hosts of Zr, Ti and Th in the parent rock and their relative susceptibility to weathering are the fundamental controls on subsequent mobility during initial weathering. Trace amounts of Th in secondary phases, such as rhabdophane and florencite, show translocation of Th at the mineral scale, while strong partitioning of Th into gravel rather than matrix reflects redistribution of Th at the profile scale. The absence of primary sphene from the regolith and the presence of partially dissolved ilmenite and rutile grains in the ferruginous mottled zone suggest mobilization and translocation of Ti at a mineral assemblage scale. Furthermore, the fluctuation of Ti/Zr in the ferruginous zone is in contrast to the consistency of Zr/Hf throughout the profile in general (within the range of parent meta-granitoid). This suggests that Ti and Zr fractionate from each other and partition between gravel and matrix during extreme weathering and advanced lateritization. This study demonstrates that Ti, Zr and Th are mobile at a variety of scales, an important consideration that is often overlooked when calculating element mass flux in intensely weathered regolith.