At 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.

目前，稀土元素（REE）的很大一部分来自风化剖面。原生岩的矿物学在控制风化过程中稀土元素的命运方面起着重要作用，因为辅助矿物占大多数岩石中稀土元素预算的大部分，而且不同的矿物对风化过程的敏感性也各不相同。REE超基因沉积物（“吸附黏土沉积物”）与热带环境中的深层风化有关，这常常使人们无法表征从原生岩中的宿主矿物释放REE的初期步骤。在这里，我们针对的是来自处于相对较快隆起的亚干旱环境中的风化REE富集岩性，即来自澳大利亚北弗林德斯山脉的耶里拉片麻岩，灰岩很浅或没有，母岩材料尚未完全分解。X射线荧光作图，扫描电子显微镜（SEM），聚焦于SEM的离子束铣削（FIB-SEM），电感耦合等离子体质谱（ICP-MS）和激光烧蚀ICP-MS的结果突出了REE的迁移途径以及来自尿素-（Ce）晶粒的相关的U和Th，这些晶粒是Yerila片麻岩材料中主要的REE主体。轻质稀土元素和Th通过尿囊石-（Ce）代谢过程产生的径向裂纹从尿囊石-（Ce）晶粒中迁移出的原因被认为是风化的结果，因为铈部分以四价氧化态存在，Th的迁移性最容易解释。通过有机配体的参与。FIB-SEM为REE + U / Th流动性和与尿石（Ce）的风化作用相关的与尿钙石相关的球状结构的分级分离中的生物发生过程的重要性提供了进一步的证据。还发现有机碳与Xenotime-（Y）谷物相关。在这种情况下，稀土元素的释放很可能是生物磷酸盐利用的副产品。这些结果表明，由生物区系和/或生物有机质介导的局部控制（在矿物界面处）可以控制风化过程中REE（+ Th，U）动员的启动。