Geochemical and mineralogical evolution of cerium during chemical weathering

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This data contains in situ electron probe micro analyses (EPMA) and selective extraction.

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For EPMA, major element compositions of accessory and secondary minerals in different weathering horizons were determined by a JEOL JXA-8230 EPMA in GIGCAS, using an accelerating voltage of 15 kV, probe current of 20 nA and electron beam diameter of 1 μm. Peak and background elemental counting differ for the analyzed elements: 20 s and 10s for Si, Al, Ca, Fe and P, 10 s and 5 s for F and Cl, 60 s and 30 s for Y, and 40 s and 20 s for Ti, Th, U, Mn, Hf and REEs. The Kα X-ray line was used for Ti, Fe, F, Si, Cl, Ca, P, Mn and Al; the Lα X-ray line for La, Ce, Dy and Hf; the Lβ X-ray line for Tb, Ho, Yb, Pr, Nd, Eu, Lu, Sm, Gd, Er and Tm; the Mα line for Th and U. The standards for calibration are rutile (Si), magnetite (Fe), kaersutite (Al, Si), rhodonite (Mn), BaF2 (F), and tugtupite (Cl), Th-metal (Th), U-metal (U), Cr-diopside (Ca), apatite (P) and synthetic REE-glass for REEs. For selective extration, we primarily focused on the specific species of Ce, including exchangeable Ce, Fe-Mn oxides bound Ce, and residual Ce. MgCl2 solution (1.0 mol L-1) was used to extract ion-exchangeable species. NH2OH-HCl solution (0.25 mol L-1, pH = 2 ± 0.2) was used to extract Ce species bound to Fe-Mn oxide, and the remaining solid in the last step was completely dissolved using a 1:1:1 mixture of concentrated HCl, HNO3, HClO4 and concentrated HF (all of which are high-purity) to extract the residual Ce. The exchangeable species refers to hydrated Ce cations primarily adsorbed by secondary clay minerals through electrostatic force owing to their permanent or variable negative charges (Li and Zhou, 2020; Yang et al., 2019). The species of Ce bound to Fe-Mn oxides represents those scavenged by Fe-Mn oxides due to high specific areas and adsorption capabilities (Ohta and Kawabe, 2001; Yu et al., 2017). As for the residual Ce, it represents secondary or unweathered Ce-bearing minerals. After the extraction, the supernatants were pipetted out, filtered by a 0.22 μm filter, and diluted using 2% HNO3 (trace metal grade) solutions, followed by the analysis of REE concentrations using inductively coupled plasma-mass spectrometry (Thermo Icap Qc) at the Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (GIGCAS).

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