An in situ, micro-scale investigation of inorganically and organically driven rare-earth remobilisation during weathering

Abstract

AbstractAt present, a significant portion of rare-earth elements (REEs) are sourced from weathering profiles. The mineralogy of the protolith plays an important role in controlling the fate of REEs during weathering, as accessory minerals contain the bulk the REE budget in most rocks, and different minerals vary in their susceptibilities to weathering processes. REE supergene deposits (‘adsorption clay deposits’) are associated with deep weathering in tropical environments, which often precludes characterisation of the incipient steps in REE liberation from their host minerals in the protolith. Here we have targeted a weathered REE-enriched lithology from a sub-arid environment undergoing relatively rapid uplift, namely the Yerila Gneiss from the Northern Flinders Ranges, Australia, where regolith was shallow or absent and parent rock material had yet to completely break down. Results from X-ray fluorescence mapping, scanning electron microscopy (SEM), SEM-focussed ion beam milling (FIB-SEM), inductively-coupled plasma mass spectrometry (ICP-MS) and laser ablation ICP-MS highlight the migration pathways of REEs and associated U and Th from allanite-(Ce) grains that are the main REE host within Yerila Gneiss material. Migration of light REEs and Th away from the allanite-(Ce) grains via radial cracks resulting from allanite-(Ce) metamictisation was interpreted to result from weathering, as Ce is partially present in its tetravalent oxidation state and Th mobility is most easily explained by the involvement of organic ligands. FIB-SEM provides further evidence for the importance of biogenic processes in REE+U/Th mobility and fractionation in uranothorite-associated spheroidal structures associated with the weathering of allanite-(Ce). Organic carbon was also found in association with a xenotime-(Y) grain; in this case, REE liberation is most likely a by-product of biogenic phosphate utilisation. These results highlight that local controls (at mineral interfaces) mediated by biota and/or biogenic organic matter can control the initiation of REE (+Th,U) mobilisation during weathering.