Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.464e&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 464
Other
14
Chemical Composition, Galactic Cosmic Rays, Mars (Planet), Meteoritic Composition, Pyroxenes, Abundance, Radiation Effects, Rare Gases
Scientific paper
ALH 84001 was recognized to be a cumulate orthopyroxenite meteorite related to martian (SNC) meteorites. In the framework of a consortium study we obtained interior sample ALH 84001,99 (2.13 g) for noble gas isotope studies. A split of about 0.1 g was crushed to a grain size less than 750 mm and two samples were analyzed. The results for He, Ne, and Ar are in good agreement. Kr and Xe analyses will be performed later. He-3, Ne-21, and Ar-38 are essentially products of galactic cosmic irradiation. The main purpose of this work is to derive the duration of exposure to cosmic rays for ALH 84001. For the calculation of production rates we adopted the chemical abundances reported by Middlefehldt (1994) and Sack et al. (1991): 17.5% Mg, 0.355% Al, 23.4% Si, 1.22% Ca, 0.048% Ti, 0.6% Cr, 0.38% Mn, and 13.1% Fe. Production rates as functions of chemical abundances and shielding were derived according to the formulas given by Eugster and Michel (1994). Because the chemistry of ALH 84001 for the above elements is similar to that of diogenites, we adopted the shielding dependency of the production rates based on Ne-22/Ne-21 proposed for diogenites. The exposure ages based on He-3, Ne-21, and Ar-38 are in good agreement. The average exposure age for six analyses is 14.9 +/- 1.8 m.y. We suggest that ALH 84001 was ejected 15 Ma from its parent body, probably Mars. The ejection time of ALH 84001 differs from those of the other meteorites thought to be samples of the martian crust. The shergottites cluster around 3 m.y. and the nakhlites and Chassigny between 9 and 11 m.y. ALH 84001 does not originate from the same ejection event as the other martian meteorites.
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