The occurrence and behaviour of rare earth and associated elemets in lateritic regolith profiles in Western Australia

[Truncated abstract] The nature of the redistribution and fractionation of rare earth elements (REE) during supergene weathering is not fully understood, especially in lateritic weathering which is characterised by intense weathering and formation of ferruginous materials. Therefore, the geochemical characteristics and mode of occurrence of REE were investigated in four lateritic regolith profiles (GE, MQ I, MQ II and JG) developed on granitoids with dolerite dykes in Western Australia. The outcomes of this study are important factors to consider when using REE as tracers for the regolith weathering, pedogenesis and sedimentation. A high deficiency of REE relative to parent granitoids is typical of the regolith studied, especially in the GE and JG profiles, suggesting high mobility of REE. Breakdown of weathering-susceptible light REE (LREE)-rich minerals, such as allanite and/or REE-rich fluorocarbonate facilitates depletion of LREE at early stages of weathering. The released REE either are partially leached away by solutions, or precipitate as secondary phosphates (e.g. rhabdophane and florencite). These secondary phosphates play an important role in sequestering REE and hence limiting their further mobility, especially for LREE. Trace to minor amounts of REE are associated with clay minerals, Fe oxides/oxyhydroxides and organic ligands, and thus are retained in regolith, as revealed by the sequential extraction experiment. Heavy REE (HREE)-rich minerals, such as zircon, are relatively weathering-resistant and thus HREE hosted by these minerals are not susceptible to inter-horizon transport. Residual accumulation of weathering-resistant minerals is the main control on the retention of HREE in intensely weathered regolith.