XANES - IngentaConnect

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1,2. 1Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, Higashi-. Hiroshima, Hiroshima 739-8526, Japan.
Mineralogical Magazine, April 2005, Vol. 69(2), pp. 179±190

Determination of the Eu(II)/Eu(III) ratios in minerals by X-ray absorption near-edge structure (XANES) and its application to hydrothermal deposits 1,2,

Y. TAKAHASHI

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*, G. R. KOLONIN , G. P. SHIRONOSOVA , I. I. KUPRIYANOVA , T. URUGA

AND

H. SHIMIZU

1,2

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Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University, HigashiHiroshima, 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

ABS TR AC T

Europium LIII-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.

K EY WORDS :

XANES, hydrothermal deposits, europium, apatite, ¯uocerite, REE.

Introduction

THE abundances of rare earth elements (REE) have provided useful information for characterizing various geochemical samples such as rocks, minerals and water samples, with the purpose of estimating the source materials of the sample and environment where the sample was formed. Among various features involved in REE patterns (= relative abundances of REE normalized by proper reference materials), Eu anomalies can be used as an indicator of redox conditions, the source of the materials, and REE fractionation

* E-mail: [email protected] DOI: 10.1180/0026461056920245

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2005 The Mineralogical Society

during crystallization/differentiation (e.g. Sverjensky, 1984; McLennan, 1989; Terakado and Fujitani, 1995). The Eu anomaly is presumably caused by the formation of Eu(II) in a reducing environment, which induces the fractionation of Eu from other trivalent REEs. The extent of Eu anomaly is often expressed as EuN/ EuN* where EuN is the normalized abundance in the sample and EuN* is the theoretical abundance when assuming no fractionation involved in Eu(II) formation, which can be calculated as an interpolation in REE patterns, i.e. EuN/EuN* = 1/2 1/2 Eu N /{(Sm N ) (Gd N ) }, where N indicates normalized abundances (Taylor and McLennan, 1988). For example, the EuN/EuN* index can be used to characterize crustal materials qualitatively

Y. TAKAHASHI ET AL.

(e.g. Taylor and McLennan, 1985; Chang et al., 2000) or to characterize the evolution of melt during the formation of granitic rocks (e.g. Irber, 1999). However, the EuN/EuN* ratio cannot be used to determine the Eu(II)/Eu(III) ratio in the minerals if REE abundances in source material or coexisting equilibrated phases are not available. Philpotts (1970) proposed a method to determine the Eu(II)/Eu(III) ratio in mineral/melt systems based on the REE abundances in two equilibrated phases. However, it may be dif®cult to use this method to determine the Eu(II)/Eu(III) ratio in a disequilibrium system even if we can obtain the two phases (Terakado and Fujitani, 1995). The case is similar for the Eu(II)/Eu(III) ratio in a hydrothermal system, since it is usually not easy to ®nd coexisting equilibrated phases in its rapid mineralization process and the partition coef®cient depends on the composition of ligands which form complexes with Eu(II) and Eu(III) ÿ 2ÿ in hydrothermal solution such as CO3 , OH and ÿ F . Accordingly, the quantitative determination of the Eu(II)/Eu(III) ratio, by any direct methods, should be of great importance to our ability to utilize the information exhibited by the Eu anomaly. Numerous luminescence studies have been conducted to identify Eu(II) and Eu(III) in some minerals, using the characteristic luminescence properties of Eu (Mariano and Ring, 1975; Roeder et al., 1987; Burruss et al., 1992; Barbin et al., 1996; Mitchell et al., 1997; Gaft et al., 2001). Although the presence of Eu(II) in some minerals has been con®rmed by this technique, none of these studies was successful in determining the Eu(II)/Eu(III) ratio quantitatively. This is because the structural control on the luminescence properties of Eu(II) and Eu(III) and the large quantum ¯uorescence yields of Eu(II) (greater than that of Eu(III) by two or three orders of magnitude) prevent the precise determination of relative abundances of Eu(II) and Eu(III) in minerals. X-ray absorption near-edge structure is a promising tool for the speciation of trace elements in rocks, due to its high sensitivity and its selective detection of target elements. We have already used the XANES technique on Ce, another REE useful in the study of various geochemical issues (Takahashi et al., 2000a,b, 2002a). Similarly, we have employed this method to determine the Eu(II)/Eu(III) ratio in this study, which was applied to apatite and ¯uocerite in hydrothermal vein deposits from Malyshevskoe and Katuginskoe, Russia. It is expected that the

Eu(II)/Eu(III) ratio determined for the minerals can be used to estimate the Eu(II)/Eu(III) ratio in hydrothermal solutions where the minerals have formed, which in turn can be used to estimate fO2 in the hydrothermal solution, based on the relationship between the Eu(II)/Eu(III) ratio and fO2 (e.g. Bau, 1991). There has only been one previously published report, from a mineralogical point of view, of an attempt to determine the Eu(II)/Eu(III) ratio using XANES for natural minerals containing Eu, the concentration of which was 3000 ppm (Rakovan et al., 2001). However, the transition probabilities for the resonance peak in XANES for Eu(II) and Eu(III) species, which are essential for determining the Eu(II)/Eu(III) ratio, were not examined in that study. In addition, comparison between the Eu(II)/Eu(III) ratio and the extent of the Eu anomaly was not conducted in the study. Therefore, the aims of the present study are: (1) to establish the XANES method to determine the Eu(II)/Eu(III) ratio for the samples containing Eu at levels