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Noble Gas Distribution in the Yamato 000593 Nakhlite Deciphered By 40th Lunar and Planetary Science Conference (2009) 1682.pdf NOBLE GAS DISTRIBUTIONS IN THE YAMATO 000593 NAKHLITE DECIPHERED BY LASER ABLATION ANALYSIS AND MINERAL SEPARATION. K. Nagao1, J. Park2, R. Okazaki3, N. Imae4 and H. Kojima4. 1Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan ([email protected]), 2ARES, NASA, Johnson Space Center, Houston, TX 77058, USA, 3Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Hakozaki, Fukuoka 812-8581, Japan, 4Antarctic Meteorite Research Center, National Institute of Polar Research, Kaga 1-chome, Itabashi-ku, Tokyo 173-8515, Japan. Introduction: Three meteorites Yamato (Y) steps from 400 to 1800ºC) was applied to the mineral 000593 (13.7 kg), Y000749 (1.3 kg) and Y000802 (22 separates; olivine (75.5 mg), plagioclase (18.1 mg), g) were discovered in Yamato ice field in Antarctica and pyroxene (187.9 mg). However, noble gases ex- by the Japanese Antarctic Research Expedition in 2000 tracted from olivine and plagioclase at 1800ºC gave (JARE-41). They were recognized as paired nakhlites meaningless results due to small amounts of extracted from view points of petrological and mineralogical noble gases, which were comparable with blank levels. observations as well as recovered locations [1]. For noble gas extraction by laser microprobe (Nd- Nakhlites are cumulates [2] and consist of mostly YAG pulse laser, 1064 nm), a meteorite plate with magnesian augite, microcrystalline mesostasis and ~300 µm thick was used. Optical microprobe and smaller amounts of Fe-rich olivine. Parent magma for Scanning electron microprobe observations on the nakhlites may be different from shergottites, because polished surface of the plate showed abundant nakhlites have high volatile contents, in contrast to mesostasis, probably weathering products, while id- shergottites, but are LREE-enriched [3]. Moreover, dingsite was minor. Three laser pits with diameter of nakhlites have essentially no feature for shock, whilst ~50 µm were produced through single crystal or phase shergottites, chassignites and ALH 84001 have heavily of the meteorite plate. Weight of fused material by the shocked. Nakhlites have the evidence for aqueous al- laser ablation was estimated as about 5 µg. teration on Mars. Iddingsite is an alteration product of Results and discussion: Laser ablation method olivine with aqueous fluid at the lower temperature showed higher concentrations of radiogenic 4He and than 150°C [4, 5]. 129Xe/132Xe value of iddingsite in 40Ar (40Ar/36Ar ratios were up to ~10,000) in mesosta- the Lafayette was 2.04 ± 0.28, while the value of sis than those in olivine and pyroxene, which indicates 84Kr/132Xe is as low as 6 ± 3 [6]. They concluded that high concentrations of U, Th and K in the mesostasis. high 129Xe/132Xe ratio in iddingsite was due to the 129Xe-excess and low 84Kr/132Xe ratios, which are char- trapped Martian atmosphere during aqueous alteration acteristic noble gas signature for nakhlites, were also on the surface of Mars. Therefore, nakhlites trapped observed in the mesostasis. The results imply that the Martian atmosphere by the aqueous mechanism, in- main trapping site of the elementally fractionated Mar- stead of shock mechanism. Hence the secondary al- tian atmosphere is mesostasis. Hence the formation of teration products should be important for understand- mesostasis would be closely related with the trapping ing aqueous process on Mars. mechanism of elementally fractionated atmosphere on Noble gas compositions of bulk sample and min- Mars. eral separates (Olivine, Pyroxene and Plagioclase) As noted above, the plagioclase separate contains from MIL 03346 have been reported [7]. Following weathering products, probably related with the the research work on nakhlite, we measured noble gas mesostasis observed in the meteorite plate used for compositions in different minerals and phases from Y laser ablation method. Release profiles of 3He, 4He and 000593 nakhlite by two different methods, laser abla- 3He/4He ratio are presented in Fig. 1. Though cos- tion and mineral separation. Noble gas data on bulk mogenic 3He is mostly released at 800°C from three samples from the three Yamato nakhlites have been mineral separates, the 3He concentration of plagioclase reported by [8], which show that the Yamato nakhlites (93×10-9 cc/g) is much lower than those in olivine were ejected from Mars by the common impact event (210×10-9 cc/g) and pyroxene (267×10-9 cc/g). This is for other nakhlites at about 12 Myr ago. also the case for cosmogenic 21Ne, i.e., the concentra- Experimental method: Noble gas elemental and tion is lower than half of those in olivine and pyroxene. isotopic compositions were analyzed using a noble gas This indicates low retentivity of noble gases in plagio- mass spectrometer (modified-VG5400/MS-II) at the clase compared with olivine and pyroxene. On the University of Tokyo. other hand, radiogenic 4He concentration is much Grains of 100-200 mesh weighing 573 mg were higher in plagioclase (2.7×10-5 cc/g) than those in oli- separated into three fractions, plagioclase + weathering vine (0.30×10-5 cc/g) and pyroxene (0.55×10-5 cc/g). products, pyroxene and olivine. Stepwise heating (8 40th Lunar and Planetary Science Conference (2009) 1682.pdf 200 2.6 Olv cc/g) 150 2.4 -9 Iddingsite Mars atmosphere Plg 2.2 100 3He Pyx 2.0 3He (10 50 1.8 0 14,000 1.6 129Xe/132Xe cc/g) 12,000 -9 1.4 10,000 8,000 1.2 Earth atmosphere 6,000 4He-rad 4,000 1.0 2,000 4He-rad (10 4He-rad 0.8 0 0 5 10 15 20 25 30 0.16 84Kr/132Xe 0.14 1.4 0.12 0.10 Olv 0.08 1.3 0.06 3He/4He Plg 3He/4He 0.04 0.02 1.2 Pyx 0.00 200 400 600 800 1,000 1,200 1,400 1,600 129Xe/132Xe Temperature (°C) 1.1 Chassigny Fig. 1. Release profiles of helium isotopes from min- 1.0 eral separates, olivine, plagioclase and pyroxene from Y000593 nakhlite. 0.9 0246810 84Kr/132Xe 129 132 84 132 Fig. 2. Plot of Xe/ Xe versus Kr/ Xe for mineral Neon is dominated by cosmogenic component. separates; olivine, plagioclase and pyroxene from Low 20Ne/22Ne and 21Ne/22Ne ratios, ~0.6 and ~0.7, Y000593 nakhlite. respectively, were observed for plagioclase at low temperatures, 600 and 800°C. This can be explained as spallogenic Ne from Na, and the results support the Such discrepancies between the data for bulk sam- interpretation for the Ne isotopic compositions meas- ple and mineral separates have been observed for ured on bulk samples [8]. MIL03346 nakhlite [7], for which high 129Xe/132Xe Radiogenic 40Ar concentrations also show big dif- (>2) observed for bulk sample at low temperature dis- ference among the minerals; 0.1, 9 and 0.2 (×10-5 cc/g) appeared in mineral separates (100-200 mesh) as well for olivine, plagioclase and pyroxene, respectively. as in finer grained materials separated before heavy The 40Ar concentrations reported for bulk samples are liquid procedures. It should be cautioned that fragile ~1×10-5 cc/g [8], which are intermediate among the weathering products might have been easily lost by the values for mineral separates. Accordingly, the He, Ne mineral separation procedures. and Ar data indicate that U, Th, Na and K are concen- trated in plagioclase. Fig. 2 is a plot of 129Xe/132Xe versus 84Kr/132Xe for the mineral separates from Y000593. All the samples References: [1] Imae N. et al. (2003) Antarct. Meteorite Res. show relatively low 129Xe/132Xe ratios compared with 16, 13-33. [2] McSween Jr. H. Y. (1994) Meteoritics & the reported values of ≥2 for nakhlites [e.g., 6, 7]. The Planet. Sci., 29, 757-779. [3] Nakamura N. et al. (1982) highest ratio of 1.3 observed for plagioclase at 1400°C Geochim. Cosmochim. Acta, 46, 1555-1573. [4] Gooding J. is still lower than the highest value of 1.4 observed for L. et al. (1991) Meteoritics, 26, 135-143. [5] Treiman A. H. bulk samples [8]. As noted above, laser microprobe and Gooding J. L. (1991) Meteoritics, 26, 402 (Abstract). analysis showed presence of high 129Xe/132Xe in some [6] Drake M. J. et al. (1994) Meteoritics & Planet. Sci., 29, mesostasis of this nakhlite. This might be explained as 854-859. [7] Nagao K. and Park J. (2008) LPSC XXXIX, a loss of fragile weathering products such as mesosta- #1614. [8] Okazaki R. et al. (2003) Antarct. Meteorite Res. sis-like materials during procedures of mineral separa- 16, 58-79. tion, because the bulk sample was crushed to small grains and then treated with water. .
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