144 A At to om mi ic c- -S Sc ca al le e E En nv vi ir ro on nm me en nt t o of f R Re e a an nd d R Ra ad di io og ge en ni ic c 1 18 87 7 O Os s i in n N Na at tu ur ra al l M Mo ol ly yb bd de en ni it te e M Mi in ne er ra al l R Re el la at te ed d t to o R Re el li ia ab bl le e R Re e- -O Os s D Da at ti in ng g The age of terrestrial and extraterrestrial materials is determined by the ratio of parent to daughter nuclides, which can vary as a function of time due to radioactive decay [1]. The radiometric dating method assumes the retention of both parent and daughter nuclides in the samples since their formation. The chemical properties of the daughter nuclides often differ from those of the parent, which may result in the separation of the parent and daughter nuclides. Therefore, their retention in the sample may depend on the chemical environment of both nuclides, which affects their stabilities against various processes such as metamorphism, alteration, and diffusion within a crystal. Therefore, the chemical states of parent and daughter nuclides in mineral or rock samples is important for accurate dating. Recently, Os geochemistry has attracted considerable interest due to the variation of Os isotopes related to the 187 Re- 187 Os decay system [2]. In particular, molybdenite (MoS 2 ) is known as a robust geochronometer resistant to regional deformation and metamorphism, as demonstrated by a trial of the 187 Re- 187 Os system for the dating of molybdenite [3]. In this method, however, it has been indicated that successful dating requires the careful selection of a representative sample from a specific geologic occurrence with a sufficient volume of the sample for the analysis to overcome the decoupling of 187 Os and Re in the molybdenite after their formation. This decoupling of Re and Os has been attributed to the high mobility of radiogenic 187 Os relative to Re in molybdenite. In this study, we examine the local structure of Re and radiogenic 187 Os at the atomic scale in molybdenite using their X-ray absorption fine structure (XAFS) to clarify the atomic-scale environment and chemical state of the parent and daughter nuclides (Fig. 1) [4]. The XAFS for Os in molybdenite is considered to originate from radiogenic 187 Os produced by the β -decay of 187 Re (half life: 4.4 × 10 10 years), since it is well known that the abundance of common Os initially contained in molybdenite is negligible. Thus, the local atomic structures of Re and radiogenic 187 Os in molybdenite from the Onganja Mine (Namibia) were examined by XAFS, which was conducted at beamlines BL37XU and BL01B1 . Rhenium L III -edge XANES (X-ray absorption near- edge structure) and EXAFS (extended X-ray absorption fine structure) showed that the oxidation state of Re, the interatomic distances between Re and the neighboring atoms, and the coordination number of Re to S are very similar to those of Mo in molybdenite. The results confirm that Re is substituted at the Mo site in molybdenite. We successfully measured the L III -edge XANES and EXAFS of the small quantity (8.55 ppm) of radiogenic Os in molybdenite in fluorescence mode by removing the interference of X-rays from Re and other elements using a crystal analyzer system at BL37XU (Fig. 2) [4,5]. The XANES data indicate that the oxidation state of radiogenic Os is either Os(IV) or Os(III), similar to Re(IV) and Mo(IV), but not Os(II), the state in Os II S 2 (= erlichmanite), naturally occurring stable Os sulfide (Fig. 3). XANES data also suggest that radiogenic Os does not form a secondary Os phase, such as OsS 2 or Os metal, in molybdenite. The EXAFS of radiogenic Os in molybdenite was successfully simulated assuming that Os is present at the Mo site in molybdenite using the parameters Absorption Energy (keV) Chemical environment of daughter nuclide in the crystal. Is it comfortable or not ? Os and Re L III - edge XAFS Os and Re L III - edge XAFS Re in natural molybdenite (MoS 2 ) Half life: 4.4 × 10 10 y β – decay 187 Re 187 Os Fig. 1. Schematic representation of this study illustrating the importance of determining the chemical environment of the daughter nuclide ( 187 Os in this study) in the mineral (molybdenite in this study) for the reliable dating of natural samples. Environmental science Incident X-ray Ionization chamber Log spiral analyzing crystal Si (111) Analyzing slit Ys Xs Sample D 19-element SSD Fig. 2. Schematic of experimental setup of the fluorescence XAFS using a crystal analyzer system at BL37XU. 145 generated by FEFF (Fig. 4). EXAFS data are not consistent with the presence of Os phases such as Os metal and OsS 2 . The EXAFS simulation showed that the interatomic distance between Os and S is 2.27 Å, which is 0.12 Å smaller than those of Re-S and Mo-S (2.39 Å) in the molybdenite. This shorter distance between radiogenic Os and S may be explained by the relatively small ionic radii of Os(IV) and Os(III). The interatomic distance of Os-S is 2.35 Å in Os II S 2 , and Os(III) or Os(IV) should have a shorter interatomic distance with S than that in Os II S 2 due to the larger charge. The similar valence and ionic radius between Re and Mo in molybdenite obtained from our XAFS data support the fact that a large amount of Re can be incorporated into the Mo site, as was indicated in previous studies (e.g., [3]), while the different geochemical properties of Os compared with Mo and Re suggested here are evidence for the much lower abundance of common Os in molybdenite [3]. This makes molybdenite an ideal mineral for the Re-Os geochronometer. However, the shorter distance between radiogenic Os and S compared with those of Re-S and Mo-S in molybdenite suggests that Os diffuses faster than Re and Mo ions in molybdenite, which may induce the decoupling of Re and radiogenic Os in molybdenite. This is consistent with the higher mobility of Os than Re in molybdenite suggested in previous studies. To our knowledge, this is the first study to clarify the chemical environment at atomic scale for the daughter nuclide produced by radioactive decay in natural samples. Atomic scale information is essential for every decay system to elucidate reliable dating data to understand the evolution of the solar system and the earth. Yoshio Takahashi Department of Earth and Planetary Systems Science, Hiroshima University E-mail: ytakaha@hiroshima-u.ac.jp References [1] G. Faure and M.T.G. Mensing: Isotopes: Principles and Applications (Wiley 2004). [2] Y. Yamashita et al. : Geochim. Cosmochim. Acta 71 (2007) 3458. [3] W. Herr and E. Merz: Z. Naturforschg 10a (1955) 613. [4] Y. Takahashi, T. Uruga, K. Suzuki, H. Tanida, Y. Terada, and K.H. Hattori: Geochim. Cosmochim. Acta 71 (2007) 5180. [5] Y. Takahashi et al. : Anal. Chim. Acta 558 (2006) 332. Normalized Absorption 10.84 10.86 10.88 10.90 OsCl 3 OsO 4 Os metal Energy (keV) Os(II)S 2 187 Os in OsO 2 molybdenite Fig. 3. Osmium L III -edge XANES for Os in molybdenite and reference materials such as Os metal, Os II S 2 , Os III Cl 3 , Os IV O 2 , and Os VIII O 4 . 0 5 3 4 5 3 4 5 0 1 2 6 7 Sample Fit Os-S Os-Mo (a) Sample Fit 0 1 2 3 Os-S Os-Mo (b) R + Δ R (Å) k 3 χ (k) k (Å -1 ) Intensity Fig. 4. EXAFS results for Os in molybdenite including (a) k 3 -weighted χ (k) with the simulated spectrum with the contribution of Os-S and Os-Mo shells considered therein and (b) radial structural function (RSF) for Os with the simulation curve.
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