Physics
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.546m&link_type=abstract
Meteoritics, vol. 30, no. 5, page 546
Physics
Cff-Xe, Mantle, Migration, Xe Precursors, Xe, Primordial
Scientific paper
We have already reported [7, 23] on the non-linear isotope mass-fractionation of fission Xe by migration of the precursors I, Te, Sn, and Sb and simultaneous fission of heavy nuclei. Xe with anomalous isotopic pattern was found in a number of meteorites and terrestrial materials and was named CFF-Xe (Chemically Fractionated Fission Xe). It is characterized by an up eightfold ^132Xe and ^131Xe excesses coupled with smaller ^134Xe and ^129Xe excesses. The present work is aimed to estimate the role of CFF-Xe in the terrestrial lithosphere and specifically deals with the problem of the isotopic composition of primordial terrestrial Xe. Due to variations of the migration conditions the isotopic structure of CFF-Xe is not well established and is even not reproducible in the same rock [2]. Nevertheless, we have tried to estimate the composition of CFF-Xe by investigating all available isotopic data of Xe of presumable mantle origin. This is Xe in MORB [29, 1, 12] and ocean island glasses [1, 28], in diamonds [17], in volcanic rocks [29, 8, 9, 21], in volcanic glasses from pillow basalts [16, 6], continental igneous rocks [1, 24, 10, 22], carbonatites and granitoids [1] as well as Xe in natural gases [3, 24, 11, 4, 15]. All data are plotted Fig. 1 where we also suggest end members of the observed scattering. Optimized slopes of CFF-lines are shown as well as the position of the initial points which we regard as primordial terrestrial Xe (Xe0). The isotopic composition of CFF-Xe and Xe0 are given in Tab. 1. The abundances of ^124Xe and ^126Xe in mantle derived samples are very uncertain, but since ^128Xe/^130Xe in Xea and Xe0 is very similar we propose the same ^124Xe/^130Xe and ^126Xe/^130Xe ratios for both Xea and Xe0. If so, AVCC-Xe is simply Xe0 with an admixture of L-Xe, and atmospheric xenon Xea consists of Xe0, CFF-Xe and a small amount of fission Xe (92.5%Xe0 + 5.3%CFF-Xe + 2.2%XeF). Thus, a number of old problems in xenology are removed. The hypothetic components U-Xe or atmosphere-like Xe are not required anymore. Instead, experimentally identified Xe0 can be regarded as primordial terrestrial Xe with an isotopic composition close to AVCC-Xe. Isotopic mass-fractionation is not needed to be involved. Concerning ^129Xe in the mantle, it seems to be part of CFF-Xe rather than the product of primordial 129I decay. This interpretation is supported by the observation of 129I excesses near uranium deposits that provides an additional argument in favor the CFF-Xe hypothesis [5, 14]. This work is supported by INTAS # 94-2397. References: [1] Allegre C. J. et al. (1983) Nature, 303, 762-766. [2] Azuma Sh. et al. (1993) EPSL, 114, 341-352. [3] Boulos M. S. et al. (1971) Science, 174, 1334-1336. [4] Caffee M. W. et al. (1988) AGU Meeting in San Francisco, reprint. [5] Fabrika-Martin J. et al. (1989) GCA, 53, 1817-1823.[6] Hiyagon H. et al. (1992) GCA, 56, 1301-1316. [7] Jessberger E. K. et al. (1992) LPS XXIII, 615-616. [8] Kaneoka I. et al. (1978) EPSL, 39, 382-386. [9] Kaneoka I. et al. (1983) EPSL, 66, 427-437. [10] Levsky L. K. (1993) personal communication. [11] Lin W. J. and Manuel O. K. (1987) Geochem. J., 2, 197-207. [12] Marty B. (1989) EPSL, 94, 45-56. [13] Meshik A. P. (1988) Ph.D. thesis , Vernadsky Institute, Moscow, 211 pp., in Russian. [14] Michelot J. L. et al. (1989) GCA, 53, 1803-1815. [15] Murty S. V. S. (1992) Chem. Geol., 94, 229-240. [16] Ozima M. and Podosek F. A. (1983) Noble Gas Geochemistry, Cambridge Univ., 367 pp. [17] Ozima M. and Zashu S. (1991) EPSL, 105, 13-27. [18] Ozima M. et al. (1983) EPSL, 62, 24-40. [19] Pepin R. O. (1993) preprint. [20] Phinney D. et al. (1978) JGR, 83, 2313-2319. [21] Poreda J. and Farley K. A. (1992) EPSL, 113, 129-144. [22] Schafer K. et al. (1993) Jahresbericht, 244-245, MPI fur Kernphysik, Heidelberg. [23] Shukolyukov Yu. A. et al. (1994) GCA, 58, 3075-3092. [24] Smith S. P. (1984) GCA, 48, 1033-1041. [25] Smith S. P.and Reinolds J. H. (1981) EPSL, 54, 236-238. [26] Staudacher Th. (1987) Nature, 325, 605-609. [27] Staudacher Th. and Allegre C. J. (1982) EPSL, 60, 389-406. [28] Staudacher Th. et al. (1986) Chem. Geol., 56, 193-205. [29] Thompson D. P. (1978) Phys. Earth Planet. Inter., 17, 98-107.
Jessberger Elmar K.
Meshik Alex P.
Shukolyukov Yu. A.
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