Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27..271o&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 271
Other
11
Scientific paper
We report noble gas results obtained for the new shergottite LEW 88516 that has recently been identified in the Antarctic collection. First results obtained on mineralogy and chemical composition of this meteorite have shown it to be similar, if not identical to (i.e., paired with), ALHA 77005 (Lindstrom et al., 1992; Boynton et al., 1992). Table 1 summarizes our results. Cosmogenic gases. Cosmogenic noble gases are consistent with LEW 88516 being a shergottite of the 2.5-Ma exposure age group closely resembling ALHA 77005 (Bogard et al., 1984), with abundances almost identical (^3He(sub)c) or slightly lower (^21Ne(sub)c). Helium is almost purely cosmogenic. Complete loss at the time of ejection some 3 Ma ago of previously acquired radiogenic ^4He is compatible with the data. Trapped noble gases. Gases in LEW 88516 show the characteristics of shergottite trapped gases: high elemental ratios Ar/Xe, Kr/Xe and excess ^129Xe (^129Xe*). The ratio ^40Ar/^129Xe* of ~1.2x10^6 (1800 degrees C fraction) or 1.4x10^6 (1200 and 1800 degrees C fraction combined) agrees with that in EETA 79001 Lith.C (Becker and Pepin, 1984) thought to be representative of the martian atmosphere. Neon released at 800 degrees C is largely trapped (^20Ne/^22Ne=7.7; ^21Ne/^22Ne=0.27). A fit through this and the two largely spallogenic higher temperature extractions yields (^20Ne/^22Ne)trapped ~10.8 (for ^21Ne/^22Ne=0.03), similar to the 10.6+-0.6 inferred for trapped SPB-Ne by Swindle et al. (1986). Abundances of heavy noble gases as well as maximum observed ^40Ar/^36Ar(sub)trapped and ^129Xe/^132Xe ratios are higher than for ALHA 77005 (Bogard et al., 1984; Swindle et al., 1986). In a ^129Xe/^132Xe vs. ^84Kr/^132Xe plot (Fig. 1) the 1800 degrees C data point plots on the line joining Chassigny and gases measured in EETA 79001 Lith.C, a situation noted for Shergotty previously (Ott and Begemann, 1985). The 1200 degrees C data point plots off the line, having almost no ^129Xe*. (The situation for Zagami (Ott et al., 1988), which also plots below the mixing line, may be similar). Clearly noble gases in the shergottites are more than a simple two-component mixture of EETA79001-(Mars- atmosphere)-type and Chassigny-type gases. References: Becker R.H. and Pepin R.O. (1984) Earth Planet. Sci. Lett. 69, 225-242. Bogard D.D., Nyquist L.E. and Johnson P. (1984) Geochim. Cosmochim. Acta 48, 1723-1739. Boynton W.V., Hill D.H. and Kring D.A. (1992) Lunar Planet. Sci. (abstract) 23, 147-148. Lindstrom M.M., Mittlefehldt D.W., Treiman A.H., Wentworth S.J., Gooding J.L., Morris R.V., Keller L.P., and McKay G.A. (1992) Lunar Planet. Sci. (abstract) 23, 783-784. Ott U. and Begemann F. (1985) Nature 317, 509-512. Ott U., Lohr H.P., and Begemann F. (1988) Meteoritics (abstract), 295-296. Swindle T.D., Caffee M.W., and Hohenberg C.M. (1986) Geochim. Cosmochim. Acta 50, 1001-1015. Table 1. Noble gases (abundances in 10^-8 cm^3 STP/g) in 69.2 mg of chips. 3He..4He...21Ne.36Ar(sub)tr*.38/36*.40Ar*.84Kr*..132Xe*....129/13 2* 6.3..33.8..0.70.0.72........0.532..639....0.0134.0.00152...1.284 +-.3.+-1.6.+-.04.+-.04......+-.009.+-22...+-.0007.+-.00008.+-.015 Gases were extracted in three temperature steps (800 degrees C, 1200 degrees C, and 1800 degrees C). 36Ar(sub)tr = trapped 36Ar. *For Ar, Kr, and Xe gases released above 800 degrees C, another 0.58x10^-8 cm^3 SPT/g of 35Ar, 179x10^-8 cm^3 SPT/g of 40Ar, 0.015x10^-10 cm^3 STP/g of 84Kr, and 0.0013x10^-11 cm^3 STP/g of 132Xe were released, the isotopic compositions being indistinguishable from terrestrial air. Figure 1, which in the hard copy appears here, shows 129Xe/132Xe vs. 84Kr/132Xe (>800 degrees C fractions) in shergottites and Chassigny.
Löhr H. P.
Ott Ulrich
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