This study concerns the simultaneous detection of five rare-earth elements (REEs) – Ce, La, Nd, Pr, Sm – in ores and nodules using two X-ray fluorescence techniques (XRF): total reflection (TXRF) and wavelength-dispersive (WDXRF). The issues of high pairwise correlation between REE contents in certified reference materials of natural origin, resulting in a poor quality of calibration sample set, and the significant overlapping of REE lines, especially in TXRF spectra, which have low spectral resolution, are addressed in the study. For the first time, this is done by employing a specially designed calibration set, based on Latin hypercube sampling. Its applicability was assessed for univariate and multivariate (PCR, PLS) calibration models to overcome the above issues. A feature of our calibration set is the use of artificial mixtures of REE oxides and Fe₂O₃ + TiO₂ (as a matrix) providing extremely low correlation (<0.03) between the content of different REEs. Simultaneously, the validation set consists of natural certified reference materials of REE-rich ores and oceanic ferromanganese nodules. The content of each REE in the calibration set varies in the range of 0–40 000 ppm. Our PLS/PCR-based calibration models provide a quantitative determination of Ce, La, and Nd content using both XRF techniques. Pr and Sm were special cases, which cannot be quantitatively determined using univariate calibration, while our multivariate models were able to detect these REEs at a level above 100 ppm.

The determination of the chemical composition of meimechites – which are unique and rarely occurring ultra-high MgO igneous rocks – can be complicated due to their porphyric structure, the presence of acid-insoluble minerals, and wide variation of major and trace element contents. In the present study the optimal analytical strategy based on a combination of X-ray fluorescence (XRF) and inductively coupled plasma mass spectrometry (ICP-MS) methods was suggested for the determination of the elemental composition of meimechites. The preparation of glass beads using a lithium tetraborate and metaborate mixture proved to be suitable for the XRF determination of major oxides. A comparative study of the sample decomposition procedures for the determination of trace elements by ICP-MS clearly showed that fusion with lithium metaborate was the most appropriate sample preparation technique for complete digestion of meimechites. The open beaker HF-HNO ₃ -HClO ₄ acid digestion was insufficient because the results for Nb, Ta, V, Zr, Cr and Hf were underestimated by 20–80% compared to those determined using the fusion method due to the presence in the rock samples of acid-resistant accessory minerals. It is shown that using analytical data from acid digestion may lead to erroneous interpretation of geochemical data. Supplementary material: Comparing the results obtained by ‘wet chemistry’ and XRF methods; ICP-MS data for meimechite samples; ICP-MS analysis of reference materials are available at https://doi.org/10.6084/m9.figshare.c.4226786