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Determination of the Eu(II)/Eu(III) ratios in minerals by X-ray absorption near-edge structure (XANES) and its application to hydrothermal deposits

¹ Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
² Laboratory for Multiple Isotope Research for Astro- and Geochemical Evolution (MIRAGE), Hiroshima University, Hiroshima 739-8526, Japan
³ Institute of Mineralogy and Petrography, SB RAS, Prosp. Koptyuga 3, Novosibirsk, 630090, Russia
⁴ Fedorovskii All-Russia Research Institute of Mineral Resources, Staromonetnyi per. 31, Moscow, 119017, Russia
⁵ SPring-8, Japan Synchrotron Radiation Research Institute (JASRI), Mikazuki, Hyogo 679-5198, Japan

^* E-mail: takahasi{at}geol.sci.hiroshima-u.ac.jp

Europium L_III-edge X-ray absorption near-edge structure (XANES) was employed to determine the Eu(II)/Eu(III) ratios in minerals. This ratio can be determined based on the peak-area ratio of white lines, the resonance peak, in normalized XANES spectra for Eu(II) and Eu(III) species. For precise determination of the Eu(II)/Eu(III) ratios, however, it was revealed that the transition probabilities for each individual Eu(II) and Eu(III) species in the system must be quantified, because we found that the peak area in normalized XANES spectra is different in each Eu(II) and Eu(III) species. Despite this ambiguity, the method was applied to Eu in natural hydrothermal apatites (Eu = 39 and 64 ppm) and fluocerite (Eu = 282 ppm). The relationship between the Eu(II)/Eu(III) ratio in these hydrothermal minerals, and the distribution coefficients of Eu(II) and Eu(III) were discussed, taking into account Eu anomalies in their REE patterns. It is considered that by combining the Eu(II)/Eu(III) ratios determined by XANES and the degree of Eu anomaly in REE patterns, we can provide new information on the distribution of Eu(II) and Eu(III) in various geochemical studies.

KEYWORDS: XANES, hydrothermal deposits, europium, apatite, fluocerite, REE