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Determinations of Osmium Isotope Ratios in Iron Meteorites and Iridosmines by ICP-MS

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Determinations of Osmium Isotope Ratios in Iron Meteorites and Iridosmines by ICP-MS Geochemical Journal, Vol. 20, pp. 233 to 239, 1986 Determinations of osmium isotope ratios in iron meteorites and iridosmines by ICP-MS AKIMASA MASUDA, TAKAFUMI HIRATA and HIROSHI SHIMIZU Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Tokyo 113, Japan (Received August 15, 1986: Accepted September 10, 1986) Osmium isotopes in three Os reagents, four terrestiral Os-Ir alloys (iridosmine) and two iron mete orites have been measured on an inductively coupled plasma mass spectrometer (ICP-MS), with the preci sion of around 1% for 1870S/1920S. Comparisons are made of our isotopic ratios with the previous ones. Apart from the variation of 1870S abundance as naturally expected, there seems to be a slight systematic deviation for "'Os. Also some discussions are given to 184Os. The Os isotopic ratios for three iridosmine samples from Hokkaido, Japan, have been found quite similar to each other. The values for the Hokkaido iridosmines are 3-4% higher relative to the straight "'Os growth line obtained by Allegre and Luck (1980) . Finally, the isotopic "standard" abundances (%) for Os are presented by us: 1840s, 0.019; 1860s, 1.589; 187Os, 1.519; 1880s, 13.24; 1890s, 16.37; 190Os , 26.30; 192Os, 40.96. These give us the Os atomic weight = 190.24. INTRODUCTION determined the 117Re half-life ((4.35 ± 0.13) X 1010y) with these measurements. The half life Herr et al. (1961) started Re-Os cosmo of 117Re is similar to that of 8'Rb (4.88 X 101°y). chronology on the basis of the g decay of Further, Os isotope measurements by accelera 187Re, but the difficulty in Os isotope measure for mass spectrometer have been reported on ments prevented the popular use of this method. meteoritic and terrestrial samples (Teng et al., Recent developments in a variety of mass spec 1986). trometry have allowed us easier access to Os Re and Os are siderophile or chalcophile isotope measurements. Luck et al. (1980) and elements. Further, it has been pointed out that Allegre and Luck (1980) established Os isotope Re/Os ratio has a large fractionation factor measurements with secondary ion mass spectro during continental crust/mantle fractionation. meter (SIMS) and reported Re-Os age for These geochemical features indicate the im meteorites. Since these works, geochemical and portance of Re-Os systematics as an isotope cosmochemical studies on Re-Os systematics tracer and geo and cosmo-chronometer. Thus, have proceeded on the basis of Os isotope data it can be said that the recent developments in obtained by SIMS (Luck and Allegre, 1982, mass spectrometry are strongly promoting the 1983, 1984; Luck and Turekian, 1983; Palmer further progress of the Re-Os method. and Turekian, 1986). Luck and Allegre (1983) In this paper, we will report Os isotope reported the half life value of (4.56 ± 0.12) X ratios in iron meteorites and iridosmines as well 1010y based on the meteoritic isochron. On the as Os reagents measured by ICP-MS. ICP-MS other hand, Lindner et al. (1986) measured Os uses inductively coupled plasma (ICP) as an isotope ratios in HReO4 by two different types atmospheric ion source for a quadrupole mass of mass spectrometers, an inductively coupled spectrometer (Gray, 1975, 1985; Gray and Date, plasma mass spectrometer (ICP-MS) and a laser 1983; Houk et al., 1980). The most prelimina microprobe mass analyzer (LAMMA). They ry work of ours was presented at the 1985 233 234 A. Masuda et al. Annual Meeting of the Geochemical Society of affect Os' mass spectrometry. Japan. ICP-MS used in this study was VG Isotopes PlasmaQuad. Sample solution of 1 % HNO3 was introduced to ICP-MS through the nebuli SAMPLES AND EXPERIMENTAL METHOD zer. All of Os isotopes were measured; 184, Osmium tetroxide (Os04) reagents prepared 186, 187, 188, 189, 190 and 192. 192Oswas by Strem Chemicals Inc., E. Merck Co. Inc. and used as a reference isotope in this study. Os Nakarai Chemicals Ltd. were used for our exami isotopic abundance obtained by Nier (1937) nation. Os isotopes in these reagents were was used for correction of mass discrimination, measured for 1 ppm Os solution (1 % HNO3 solu which is currently inevitable as shown later. tion). Besides, Os isotopes were measured on The mass discrimination as observed in ICP-MS two iron meteorites, Canyon Diablo and Tlaco rests mostly on the temperature of RF/DC tepec. The iron meteorites were decomposed generator of quadrupole mass spectrometer and by aqua regia (1:3 mixture of HNO3 and HC1) on the condition of electrode bias supplied to at 80-100°C. In the decomposition process, the ion lens system. Generally, for the quadru Os was oxidized to volatile Os04. The evapo pole mass spectrometer like this, the mass dis rated Os04 was trapped with distilled water. crimination effect is inevitable and is not a The trapped OsO4 solution was free from Pt, W strictly linear function with the mass number. and Re. Four iridosmine samples were also However, for the rather short mass range dealt studied for Os isotope ratios; three samples are with here and within the precision under con from Hokkaido, Japan and another is from the sideration, one can assume the mass discrimina Urals, USSR. Iridosmine is a terrestrial Os-Ir tion effect as a linear function with the mass alloy. Iridosmine samples were decomposed by number. sodium peroxide (Na202) in a zirconium cru cible. After decomposition, Os concentration of RESULTS AND DISCUSSION each sample solution was adjusted to about 1 ppm. We have confirmed that (Zr2J ions do Os reagent not appear, but very weak Zr+ ions are recog Figure 1 shows two examples of Os isotopic nized in such a product. If (Zr2)+ appear, they measurement for Strem Os reagent. Ordinate in STREM CHEM. lppm Os c 1.05 0 0 a c E.c U) 1 0 U) U) 0 0.95 184 186 187 188 189 190 192 Mass number Fig. 1. Mass discrimination factor relative to Os isotope ratios of Nier (1937), employing 192 Os as a " reference l1 nuclide. Determination of Os isotope ratios 235 Fig. 1 refers to ratios of mOs/192Os (m = 184, and Table 2 shows Os isotopic abundances. The 186, 187, 188, 190 and 192) divided by the Os isotopic data by Nier (1937) and Luck and corresponding Os isotope ratios obtained by Allegre (1983) are also presented in Tables 1 Nier (1937). Three points of "'Os/'9'0s, 188Os/ and 2. Figure 3 shows Os isotope data for Os 192Os and 190Os/1920s fall close to a straight reagents , corrected by mass discrimination line and a point of "90s/"'Os appears to deviate factor as defined by the least-square line for from this line to a certain extent. This can be 186Os/1920s, 188Os/192Os and 190Os/1920s. As understood as implying that 1110s isotopic shown in Fig. 3 and Table 1, our data for abundance obtained here differs from that by 186Os/1920S, 188Os/1920s and 190Os/1920s agree Nier. well with those obtained by Nier (1937) . and Correction factors of mass discrimination Luck and Allegre (1983). As for 189Os/192Os for each isotope can be obtained from the ratio, our value is close to that of Luck and straight line drawn by a least-squares method Allegre, but it is 1.7% higher than that of with respect to points of 1860S/1920S, 188Os/ Nier. Precision of 1110S/192Os ratio is worse 192Os and 190Os/192Osas shown in Fig. 1. When than those for other Os isotope ratios, because dividing the actually observed ratios by the of very low isotopic abundance of 184Os. Any correction factors mathematically given by the how, 1140S/192Os ratio obtained here is close to least-square line, one can obtain the corrected the value by Nier (1937), but 184Os/192Os value ratios. Figure 2 shows "'Os/"'Os and 1860s/ obtained by Luck and Allegre (1983) is about "'Os ratios thus corrected , for Strem Os04 20-40% higher than that obtained by Nier and reagent during this work. In this diagram, the in this study. It is noted here that 184Wis the precision of 1870s/1920s is about 0.4% (2am). major isotope (30.7%) in tungsten and 184Os From Fig. 2, it can be seen that the precision (0.018%) is subject to the interference by W. for 187Os/1920s is much worse than that for The data cited in Table 1, from the work by 11110S/192Os. This is due to a memory effect, Luck and Allegre (1983) is for their Merck because, while measuring the Strem Os reagent, standard. we measured intermittently other two reagents Specifically, in our measurements, any W' having different 1170S/192Os ratios from that peaks have not been observed for Os reagents. of Strem. If more caution is taken against However, when using the solutions simply such an effect, better data will be obtained. prepared by dissolving the iridosmines from Table 1 lists Os isotopic ratios against 1920s Nuppu River and Nizhni Tagil, we could recog 1860s/1920a=0.03881±0.00002 0.03885 ~TTT 0.03880 0.03875 1870S/1920S.0 .04862±0.00022 0.04910 I 1 i I I T 0.04860 I I T 1 1 l~ I I 0.04810 L 1870s/192Os and 1860s/!92 Fig. 2. 1 Os ratios in repeated measurements of Strem Os reagent. Errors are 2am. 236 A. Masuda et al. Tabble 1. Os isotope ratios against 1920s for Os reagents, together with isotopic ratios by Nier (1937) and Luck and Allegre (1983) 1840s 1860s 1870s 1880s 1890s 19005 Strem 0.00042 0.03881 0.04862 0.3238 0.4002 0.6428 ±0.00002 ±0.00002 ±0.00022 ±0.0005 ±0.0005 ±0.0017 Nakarai 0.00050 0.0388 0.0463 0.3223 0.4002 0.6421 ±0.00010 ±0.0014 ±0.0010 ±0.0016 ±0.0028 ±0.0020 Merck 0.00045 0.0387 0.0371 0.3237 0.3992 0.6416 ±0.00008 ±0.0002 ±0.0010 ±0.0012 ±0.0038 ±0.0016 Average* 0.00046 0.0388 0.3233 0.3998 0.6422 (I) ±0.00004 ±0.0001 ±0.0009 ±0.0006 ±0.0007 Average" 0.00042 0.0388 0.3237 0.4000 0.6421 (II) ±0.00002 ±0.0001 ±0.0006 ±0.0002 ±0.0007 Nier 0.00049 0.0388 0.0393 0.3244 0.3927 0.6439 Luck & 0.00059 0.03904 0.03687 0.32439*** 0.39679 0.64382 Allegre ±0.00006 ±0.00002 ±0.00002 ±0.00011 ±0.00026 Errors are 2 0m.
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