The composition of the trapped Ar, Kr, and Xe in the Martian meteorites Los Angeles, Say Al Uhaymir 005/008, and 094 is discussed and found to be consistent with a mixture of Martian mantle and atmosphere noble gases and terrestrial contamination

Busemann, H.

Eugster, O.

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Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe trapped heavy noble gases in recently foundMartian meteoritesConference or Workshop ItemHow to cite:Busemann, H. and Eugster, O. (2002). The trapped heavy noble gases in recently found Martian meteorites.In: 33rd Lunar and Planetary Science Conference, 11-15 Mar 2002, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1823.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.uk  THE TRAPPED HEAVY NOBLE GASES IN RECENTLY FOUND MARTIAN METEORITES.  H. Buse-mann and O. Eugster, University of Bern, Physics Institute, Space Research & Planetary Science, Sidlerstr. 5, 3012 Bern, Switzerland, (busemann@phim.unibe.ch).   Introduction:  Noble gases in Martian meteorites are an important source of our knowledge of composi-tion and evolution of the volatile inventory of Mars [1]. Ancient (ALH 84001), fractionated (nakhlites), and recent atmosphere (shock-produced glass in shergottite EETA 79001) as well as at least one component origi-nating from the interior (Chassigny) have been identi-fied based on analyses of the Ar-Xe elemental and iso-topic composition (e.g. [2-5]). While the interior con-tains Xe of solar isotopic composition, the atmosphere is fractionated relative to solar Xe favoring the heavier isotopes. Variations in the 129Xe/132Xe ratio (~2.0-2.6) represent the evolution of the atmosphere [6].  Tab. 1. Element concentrations and Xe isotopic composition (132Xe ≡ 100) in Los Angeles, SAU 005/008, SAU 094, and Nakhla. Assumed uncertainty for concentrations 15%.  L.A. 1 SAU 005/8 I SAU 005/8 II SAU 094 Nakhla 36Ar 1.06 0.26 0.22 0.28 1.3784Kr 77 73 138 150 24132Xe 8.2 10.3 16.9 11.5 9.0124Xe/132Xe 1.250.100.3620.0160.3800.0290.3730.0245.280.07126Xe/132Xe 1.700.140.410.030.3500.0140.460.038.700.10128Xe/132Xe 9.00.37.000.267.20.37.30.419.180.19129Xe/132Xe 1143127511241206152.41.1130Xe/132Xe 16.30.615.30.715.10.515.10.822.280.24131Xe/132Xe 83.82.979.13.278.72.480493.80.7134Xe/132Xe 38.51.639.01.437.91.339.22.037.10.4136Xe/132Xe 32.81.432.81.132.71.332.91.531.30.3In 10-12 cm3/g, Ar: 10-8 cm3/g. Uncertainty 1σ.  We present whole rock analyses of the basaltic shergottites [7, 8] Los Angeles/L.A. (stone 1), Sayh Al Uhaymir/SAU 005/008 (2 samples) and SAU 094 in order to decipher how their trapped noble gas records fit into the picture described above. Also given are additional data for Nakhla. 81Kr-Kr cosmic-ray expo-sure ages and the neutron-induced production of 80Kr in space or regolith for these samples have been pub-lished elsewhere [9-11]. NakhlaL.A. 1aSAU 005/008 ISAU 094Mars interiorMars atmosphereair0 500 1000 15001.01.52.02.5Influence ofterrestrial weathering?SAU 005/008 II36Ar/132Xe129 Xe/132 Xe Fig. 1 This plot suggets the presence of noble gases from the Martian interior in the measured samples. The Xe isotopic composition however indicates that the observations could be at least partially the result of terrestrial weathering and preferential Xe absorption (See [1] for ref.).  Results:  The samples (whole rocks of 84-810 mg) have been measured in three steps (pyrolysis at 600°C to remove absorbed air, 1700°C, and 1750°C). The gas poor steps at  1750°C indicate the complete extraction in the main steps at 1700°C. Table 1 shows the elemen-tal and Xe isotopic composition. Table 2 gives the cosmogenic (cos) and trapped (tr) concentrations and some Xe isotope ratios after correction for Xecos. We determined 36Arcos using (36Ar/38Ar)cos = 0.64±0.03 and (36Ar/38Ar)tr = 4.6±0.8. The range for Artr encompasses values deduced for Martian atmosphere (~5.3) and interior (~3.9, see [1] for ref.). For Nakhla and L.A., this correction amounts to ~80 and 50%, respectively, of the 36Ar. We calculated 84Krcos with (84Kr/83Kr)cos = 0.54±0.06, obtained by extrapolation of our data to (86Kr/83Kr)cos = 0.015±0.015. For trapped Kr we adopt both terrestrial atmosphere and solar wind Kr [12], because it is not clear to what extent Martian Kr is fractionated relative to solar [13]. The Xe isotope con-centrations are corrected for Xecos using the Xe spectra given in [14] and Ba and REE concentrations taken from [8] for L.A. and [15] for Nakhla. For SAU we used an average shergottite composition due to the lack of data for this shergottite [14]. We adopted terrestrial as well as Martian atmosphere [15] and solar wind Xe [12] as trapped component. The correction for 84Kr and 132Xe amount to 13 and 7% for Nakhla, 2 and 1% for L.A. and less than 1% for the SAU samples, respec-Lunar and Planetary Science XXXIII (2002) 1823.pdfTRAPPED HEAVY NOBLE GASES IN MARTIAN METEORITES:  H. Busemann and O. Eugster  tively. Uncertainties due to the choice of the trapped component are added.   Tab. 2. Cosmogenic 36Ar, 83Kr, 126Xe abundances, trapped elemental and Xe isotopic composition in L. A., SAU 005/008, SAU 094, and Nakhla, corrected for cosmogenic composition. See text for assumptions.  L. A. 1 SAU 005/8 I SAU 005/8 II SAU 094 Nakhla 36Arcos* 0.530.040.0500.0090.0490.0080.0310.0091.080.0783Krcos 3.030.160.290.0132.160.260.140.265.350.09126Xecos 0.1120.0040.0070.0060.0010.0080.0140.0050.7520.00536Artr* 0.53 0.21 0.17 0.25 0.3084Krtr 75 73 137 150 21132Xetr 8.1 10.3 16.9 11.4 8.440Ar/36Artr 995911421591969807391202861742(36Ar/132Xe)tr 6507520818101132183235389(84Kr/132Xe)tr 9.31.37.10.78.11.113.11.72.50.3128Xe/132Xe 7.00.56.890.287.20.37.10.46.90.9129Xe/132Xe 11331275112412061495130Xe/132Xe 15.00.715.20.715.10.515.00.815.40.8131Xe/132Xe 79.43.278.83.278.72.479467.53.0134Xe/132Xe 38.91.739.01.437.91.339.32.039.31.1136Xe/132Xe 33.21.532.91.132.71.332.91.533.50.9In 10-12 cm3/g, *10-8 cm3/g. Uncertainties of 15% for the are concentrations not included.  Discussion:  A first analysis reveals that all meas-ured elemental and isotopic abundances in the samples are consistent with a Martian origin. The signatures most indicative for a Martian origin can be identified: Large 40Ar/36Artr ratios (Table 2), an elemental compo-sition implying a mixture of Martian interior and at-mosphere (Fig. 1), and 129Xe/132Xe ratios higher than those found in terrestrial air or solar wind (Fig. 1). However, the data also indicate contamination with absorbed fractionated air, as expected at least for L. A. and SAU, both found in the deserts of California and Oman. Except for 129Xe/132Xe, all Xe isotopic ratios are consistent with air composition. The Kr isotopic com-position agrees with air or solar wind. Our samples of SAU contain significantly higher concentrations of trapped Kr and Xe than combusted samples from the interior of the meteorite [16], also indicating the pres-ence of terrestrial contamination. This agrees with the observation that SAU 005/008 I contains the smallest concentrations of trapped Kr and Xe but the most Mar-tian atmosphere-like component (40Ar/36Artr; Fig. 1&2) of all aliquots of SAU. Bulk samples of desert finds appear not to be the most suitable samples to determine trapped Martian noble gases. Further examinations on mineral separates and with improved analytical meth-ods will follow.  14.0 14.5 15.0 15.5 16.0 16.5 17.0100120140160180200220240260NakhlaL.A. 1aSAU 005/008 ISAU 094SWMars atmosphereair SAU 005/008 II129 Xe/132 Xe130Xe/132Xe  Fig. 2 After correction for Xecos, the Xe isotopic composition in all samples indicate a mixture of small amounts of Martian [15] and terrestrial atmosphere. Pure solar wind composition, which is diagnostic for Xe from the interior, can be excluded.  Acknowledgements:  We thank A. Schaller for technical support and J. A. Whitby for helpful discus-sions. Work supported by the Swiss NSF. References: [1] Bogard D. D. et al. (2001) Space Sci. Rev., 96, 425-458. [2] Swindle T. D. et al. (1995) GCA, 59, 793-803. [3] Drake M. J. et al. (1994) Mete-oritics, 29, 854-859. [4] Bogard D. D. and Johnson P. (1983) Science, 221, 651-654. [5] Ott U. and Bege-mann F. (1985) Nature, 317, 509-512. [6] Gilmour J. D. et al. (1998) GCA, 62, 2555-2571. [7] Zipfel J. (2000) Meteoritics & Planet. Sci., 35, A178. [8] Rubin A. E. et al. (2000) Geology, 28, 1011-1014. [9] Ter-ribilini D. et al. (2000) Meteoritics & Planet. Sci., 35, A155-156. [10] Nyquist L. et al. (2001) Space Sci. Rev., 96, 105-164. [11] Eugster O. et al. (2002) LPS, XXXIII, this volume. [12] Pepin R. O. (1995) GCA, 59, 4997-5022. [13] Garrison D. H. and Bogard D. D. (1998) Meteoritics & Planet. Sci., 33, 721-736. [12] Hohenberg C. M. et al. GCA, 45, 1909-1915. [14] Koblitz J. (2001) Metbase 5.0, Data Compilation, Fischerhude, Germany. [15] Swindle T. D. et al. (1986) GCA, 50, 1001-1015. [16] Mohapatra R. K. and Ott U. (2000) Meteoritics & Planet. Sci., 35, A113.  Lunar and Planetary Science XXXIII (2002) 1823.pdf

The trapped heavy noble gases in recently found Martian meteorites

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The trapped heavy noble gases in recently found Martian meteorites

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