Disentangling intense weathering profile evolution in Western Australia: Insights from REE geochemistry and neodymium stable isotopes

Rare earth element (REE) mobility, fractionation and enrichment during weathering is poorly constrained, particularly within deeply and intensely weathered landscapes (e.g., Australia). Mobility of REE has traditionally been traced through REE geochemistry and radiogenic Sm-Nd isotopes, but intense long-term weathering compromises the effectiveness of these tools. Therefore, this study applies novel stable Nd isotopes (δ 146 Nd) as a weathering and REE mobility tracer, alongside traditional approaches, to reconstruct weathering profile evolution at Lady Ada Au open pit, Yilgarn Craton, Western Australia. Transition metal and REE geochemistry indicates two relict groundwater tables within the profile. In the upper profile a shallow, ‘fresh’ and circumneutral to alkaline groundwater table prior to climatic changes in the middle Neogene to Quaternary caused appreciable REE fractionation. Subsequent climatic changes facilitated lowering of the groundwater table, acidification and salinisation, with REE remobilised to the lower profile with negligible REE fractionation. The saline groundwater, alongside cation leaching and alkaline pore water from pedogenic carbonate dissolution, promoted inner-sphere REE-clay complexation as demonstrated by light δ 146 Nd fractionation in the upper profile. The δ 146 Nd composition of the weathering profile is well correlated with traditional weathering tracers (e.g., CIA and clay content), while unaffected by pH and redox changes. However, radiogenic Nd isotopes were ineffective in constraining pedogenesis and REE mobility, given its time- and lithology-dependence. Consequently, utilising stable Nd isotopes alongside traditional weathering and REE mobility tracers broadens our understanding of weathering processes across diverse climatic regimes.