Noble Gases in the Brachinites Eagles Nest and LEW 88763

Details Noble Gases in the Brachinites Eagles Nest and LEW 88763 Noble Gases in the Brachinites Eagles Nest and LEW 88763

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.445s&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 445

Mathematics

Logic

Ages, K-Ar, Brachinite, Eagles Nest, Exposure Ages, Iodine-Xenon, Noble Gases

Scientific paper

Although ultramafic regions should be plentiful on differentiated chondritic bodies, our collection of meteorites only contains a few meteorites that are dominated by olivine, the bulk of which are from the ureilite parent body. Another olivine-rich specimen, Chassigny, is believed to be a piece of Mars, while two others, Brachina and ALH84025, appear to represent a third parent body. We have analyzed two more finds, Eagles Nest and LEW 88763, which have been classified as brachinites [1,2]. We analyzed Ne, Ar, Kr, and Xe in unirradiated samples of each, primarily to study their exposure histories, and Ar and Xe in an irradiated sample of Eagles Nest, to use radiogenic gases to constrain its parent body history. It turns out that the noble gases in brachinites do not testify to such simple relationships as are seen among the SNC meteorites. Eagles Nest and LEW 88763 contain 17 and 22 x 10^-8 ccSTP/g, respectively, of cosmogenic 21Ne, considerably more than Brachina [3,4] or ALH 84025 [5]. However, if standard production rates without shielding corrections [6,7] are used, the exposure ages based on 21Ne (50-70 Ma) and 38Ar (15-20 Ma) are not concordant. LEW 88763 has a K-Ar age of about 4500 Ma, implying early crystallization. On the other hand, it has a puzzling lack of radiogenic 129Xe (129Xe/132Xe < 1.04 for all four temperature extractions). For Eagles Nest, the best chronological information comes from the irradiated sample. Most (94%) of the K-derived 39Ar came out in a single step, giving an apparent age of 955 +- 8 Ma. The next three steps (88% of remaining gas), give apparent ages of 1300-1500 Ma, suggesting that this sample's K-Ar system has been partially reset no more than 955 Ma ago. This may have been induced by the shock event that produced irregular and planar fractures in olivine and mechanical twinning of pyroxene in Eagles Nest. While these features only indicate very weak to weak shock (S2 or S3 levels [8]), this is sufficiently high to have degassed radiogenic 40Ar from many ordinary chondrites [8]. Eagles Nest has very high 129Xe/132Xe ratios (>10) in high-temperature extractions, like Brachina and ALH 84025 [3-5], but not LEW 88763. The "radiogenic" 129Xe does not correlate with iodine, again consistent with a shock disturbance. By comparison, 129Xe and iodine do correlate in Brachina, and the K-Ar system also appears to be less disturbed than that of Eagles Nest, although a 40Ar-39Ar study gave a mean age of 4100 Ma and evidence for more recent resetting [3]. The Rb-Sr system in Brachina is disturbed [3], and Brachina has a U-Th-He age of 400 Ma [4], suggesting a disturbance at least that recent. We can compare the noble gases in the four brachinites to those in the nine SNC meteorites. The brachinites have at least three different exposure ages, while all but one of the SNCs fall into one of two groups. All SNCs have crystallization ages of <1300 Ma, while most of the brachinites have some indication of much older crystallization ages. The Xe isotopes and Ar/Kr/Xe elemental systematics of SNCs appear to be relatively simple mixtures of a Chassigny-like component (129Xe/132Xe =1.0) and a component like the martian atmosphere (129Xe/132Xe = 2.4) [9,10], while there is no systematic relationship among elemental abundances or Xe isotopes of brachinites. Many of these differences probably reflect the difference between a planet-sized shergottite parent body (probably Mars) and the smaller body (or bodies) on which the brachinites evolved. References: [1] Kring D. A. and Boynton W. V. (1992) LPSC XXIII, 727-728. [2] Nehru C. E. et al. (1992) Meteoritics, 27, 267. [3] Bogard D. D. et al. (1983) Meteoritics, 18, 269-270. [4] Ott U. et al. (1985) Meteoritics, 20, 69-78. [5] Ott U. et al. (1987) Meteoritics, 22, 476-477. [6] Schultz L. and Freundel M. (1985) Isotopic Ratios in the Solar System, 27-33. [7] Freundel M. et al. (1986) GCA, 50, 2663-2673. [8] Stoffler D. et al. (1991) GCA, 55, 3845-3867. [9] Ott U. (1988) GCA, 52, 1937-1948. [10] Swindle T. D. et al. (1989) LPSC XX, 1097-1098.

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