Ammon, Katja

Halliday, A.N.

Leya, Ingo

Markowski, A.

Quitté, G.

Wieler, R.

Bern Open Repository and Information System (BORIS)

TUNGSTEN ISOTOPIC COMPOSITION CORRECTED FOR COSMIC RAY EFFECTS AND THE Hf-W AGE OF IRON METEORITES  A. Markowski1, I. Leya2, G. Quitté1, R. Wieler3, K. Ammon2, A.N. Halliday3, 1 Institute of Isotope Geochemistry and Mineral Resources, ETH, Sonneggstrasse 5, CH-8092 Zürich, Switzerland (markowski@erdw.ethz.ch), 2 University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland, 3 Dept. of Earth Sci-ences, Parks Road, Oxford, OX1 3PR UK.   Abstract: We report a method to correct W iso-topic compositions in iron meteorites for cosmic-ray-induced variations [1]. This allows us to deduce that at least some magmatic irons segregated within less than 1 Myr of CAI formation [2]. Introduction:  Magmatic iron meteorites are be-lieved to sample the metal cores of planetesimals that formed by metal–silicate segregation followed by frac-tional crystallization of the metallic melt [3]. Core for-mation in planetesimals can in principle be dated using the 182Hf-182W chronometer applied to iron meteorites [2, 4-9]. However, production and burnout of W iso-topes due to a long exposure of the sample to cosmic rays and interaction with thermal neutrons can lower the 182W/184W ratio in iron meteorites, resulting in ap-parently older ages [10-11]. Cosmogenic interactions affect all W isotopes, depending on the relevant cross-section and relative abundance of the isotope. Fur-thermore the  effect depends on meteorite size, sample depth and exposure age. Tungsten isotope variations have been reported in different iron meteorites [e.g. 2, 6-9], but their cause remains debated. In theory, differ-ences in 182W/184W ratio are expected if iron meteorites segregated at different times or if accretion and differ-entiation were protracted. However, isotopic heteroge-neities may also be due to nucleosynthetic anomalies or cosmogenic effects. It is therefore important to un-derstand and constrain the different processes that can modify the W isotopic composition of meteorites. We present here the first experimental evidence for cosmic ray isotopic effects on W, with a study of sam-ples taken from slabs from two large iron meteorites, Carbo and Grant, for more detail see [1]. The cos-mogenic noble gas concentrations across these slabs had previously been measured and hence the preat-mospheric center of the meteorites is roughly known [e.g. 12, 13]. Based on new nuclear physics parame-ters, we developed a physical model which allows us to correct for the cosmogenic effect on W isotopes provided that the 3He concentration of the sample and an independent estimate of the exposure age are known.  Nucleosynthetic effects:  Tungsten isotopes 182, 183, 184 and 186 are produced by s-and r-processes, whereas 180W is produced by the p-process [14-15]. Nucleosynthetic anomalies in iron meteorites, have been reported for several elements [16-18]. However, all 184W/183W ratios measured in this study are equal to the terrestrial standard within uncertainty. We can therefore assume that no nucleosynthetic effect on W due to s-and/or r-processes could be measured at our level of precision. Possible anomalies on 180W would not be indicative for any s-and/or r-nucleosynthetic contributions. Furthermore, the precision on 180W ob-tained up to now is not sufficient to detect a possible nucleosynthetic effect.  Results: In Figure 1, we present the first evidence of variable W isotopic composition in a single meteor-ite as a function of depth.     Fig. 1: Correlation between the ε182W value and the distance relative to the sample closest to the preatmospheric centre. For exact sample location see [1]. ε 182W is the deviation from the terrestrial standard value in parts per 10,000.  All W isotopic measurements were performed us-ing a Nu Plasma MC-ICPMS at ETH Zürich. Each sample has been measured up to 18 times in different sessions. The data were corrected for mass bias using 186W/183W = 1.985936 [19]. Along two profiles in Carbo (samples C1-C4 and C5-C6), ε182W increases by ~ 0.5ε units from the preatmospheric centre towards the preatmospheric surface (Fig. 1). The same trend is observed for Grant (~0.3ε units, samples G7-9). Figure 1 represents the first experimental evidence for the modification of W isotopic composition by galactic cosmic rays, because the effect varies with shielding [10-11]. Figure 2 shows ε182W versus the 3He concen-trations of the samples as found in the literature. Note that 3He and cosmogenic W are produced by two dif-ferent processes; while 3He is produced by high energy particles mainly from Fe and Ni, burnout and produc-Lunar and Planetary Science XXXVII (2006) 1984.pdftion of W isotopes mainly occurs by slow neutrons, the flux of which increases with shielding. Thus ε182W values inversely correlate with the 3He concentration.    Fig. 2: ε182W versus 3He concentration taken from published data. Solid symbols represent the measured ratios without cosmogenic correction, whereas the open symbols are corrected data as explained in the text. Error bars on all ratios are the 2σ error on the measure-ments combined with the error on the bracketing standards. The uncertainty of the model calculations is estimated to be about 50%.   Model: We modeled the modification of the W isotope composition due to cosmic ray interactions as a function of the 3He concentration and the exposure age for iron meteorites with radii between 5 and 85 cm (Fig. 3). We found that 3He concentrations inversely correlate with the effect on the ε182W [1].   Fig.3: Model of the cosmogenic effect on  ε182W versus [3He], for meteorites having different exposure ages and radii of  5 - 85cm.  Cosmic-ray effects were calculated by integrating depth- and size dependent spectra of primary and sec-ondary particles with the excitation functions of the relevant nuclear reactions. Particle spectra were de-rived by following the trajectories of primary GCR-particles and calculating production and transport of secondary particles using Monte-Carlo techniques. Excitation functions for (n,γ)- and (n,2n)-reactions and production cross sections of 3He from Fe and Ni are given in [20, 21]. In this model, samples close to the pre-atmospheric centre have a low 3He concentration and the largest effect on ε182W. The largest modelled meteorite here (r=85cm) also shows the largest effect, but for meteor-ites with even larger radii the cosmogenic effect must inverse, as had e. g. been shown by Masarik for Toluca [10]. As a consequence, curves increase very likely towards zero at very low 3He concentrations for very large meteorites. Similar calculations have also been done for the 186W/183W ratio, the ratio used for the mass bias cor-rection. The overall corrections on the ε182W also con-sider this latter correction as shown in Figure 2. We assumed an exposure age of 850 ±140 Ma for Carbo and 695±65 Ma for Grant [22]. The correction lies between 0.42ε and 0.22ε units for Carbo and between 0.09ε and 0.04ε units for Grant. We consider the sam-ple with the smallest correction as the best estimate for the true W isotopic composition of Carbo and Grant, which corresponds to ε182W =–3.64±0.11 and ε182W =–3.59±0.11 respectively. Finally, we applied this method to other iron meteorites for which we meas-ured [3He] and found that the real 182W deficits, are similar to those measured for Allende CAIs [2]. This indicates that at least some iron meteorites formed wi-thin 0.0±1.0 Myr of the start of the solar system. References: [1] Markowski et al. (submitted to EPSL) [2] Kleine et al. GCA 69, 5805-5818. [3] Scott and Wasson (1975) Rev. Geophys. Space Phys. 13, 526–546. [4] Lee and Halliday (1995) Nature 378, 771-774. [5] Harper and Jacobsen (1996) GCA 60, 1131-1153 [6] Horan et al (1998) GCA 62, 545-554. [7] Lee (2005) EPSL 237, 21-32. [8] Schérsten et al. EPSL (in press)  [9] Markowski et al. EPSL (in press). [10] Masarik (1997) EPSL 151, 181-185. [11] Leya et al. (2003) GCA 67, 529-541. [12] Signer and Nier (1960) J. Gephy. Res. 65, 2947-2964 [13] Ammon and Leya, (2006) LPSC XXXVII [14] Anders and Grevesse (1989) GCA 53, 197-214. [15] Norman and Schramm (1983) Nature 304, 515-517. [16] Quitté et al. (2006) EPSL (in press) [17] Dauphas et al. (2004) EPSL 226, 465-475 [18] Chen et al. (2003) LPI XXXIV #1789 [19] Völkening et al. (1991) Int. J. Mass Spectr. Ion Processes 107, 361-368 [20] Kinsey R. (1979) ENDF-102 Data formats and Procedured for Evalu-ated Nuclear Data File ENDF [21] Leya et al. (2004) MAPS 39, 367-386 [22] Voshage and Feldmann (1979) EPSL 45, 293-308    Lunar and Planetary Science XXXVII (2006) 1984.pdf

Tungsten Isotopic Composition Corrected for Cosmic Ray Effects and the Hf-W Age of Iron Meteorites

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Tungsten Isotopic Composition Corrected for Cosmic Ray Effects and the Hf-W Age of Iron Meteorites

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