ORTHOPYROXENITE ALH 84001:  EJECTION FROM MARS (?) 15 Ma.  O. Eugster, Physikalisches Institut, University of Bern, 3012 Bern, Switzerland.

Published in Meteoritics, 29, p. 464.

ALH 84001 was recognized to be a cumulate orthopyroxenite meteorite related to martian (SNC) meteorites [1]. 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 < 750 mm and two samples were analyzed (Table 1). The results for He, Ne, and Ar are in good agreement. Kr and Xe analyses will be performed later.

TABLE 1.  Noble gas results* and cosmic ray exposure age (m.y.) of ALH 84001 (preliminary data).

19.52 mg24.74714.570.9611.218131920921.2415.317.011.414.6
43.16 mg24.94204.050.8541.221143924191.0615.416.913.215.2

* Concentrations in 10-8 cm3STP/g. Experimental errors:  concentrations 5%, ratios 1-2%, ages 10%.

3He, 21Ne, and 38Ar 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 [1 and 2]: 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 [3]. 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 22Ne/21Ne proposed for diogenites. We obtain (10-8 cm3 STP/g per m.y.):  P3 = 1.617, P21 = 0.2295, P38 = 0.0389. The exposure ages based on 3He, 21Ne, and 38Ar (Table 1) are in good agreement. The average exposure age for six analyses is 14.9 ± 1.8 m.y. Miura et al. [4] reported 16 m.y. based on 21Ne alone. 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. (except for one younger member) and the nakhlites and Chassigny between 9 and 11 m.y., if we use the noble gas concentrations compiled by [5] and calculate the production rates by the same procedure as for ALH 84001. Consequently, ALH 84001 does not originate from the same ejection event as the other martian meteorites.

Using 80 ppm K [2] and 1379 × 10-8 cm3 STP/g 40Ar (Table 1) we calculate an apparent K-Ar age of 5850 m.y. Using 27 ppm K [1] this age becomes even larger. Obviously ALH 84001 contains excess 40Ar of the type observed in shergottite EET 79001 [6].

Acknowledgments:  We thank the MWG of NASA for the ALH 84001 sample. This work was supported by the Swiss NSF.

References:  [1] Middlefehldt D. W. (1994) Meteoritics, 29, in press. [2] Sack R. O. et al. (1991) GCA, 55, 1111-1120. [3] Eugster O. and Michel Th. (1994) GCA, in press. [4] Miura Y. N. et al. (1994) Abstracts Yamada Conf., 38, Kyoto, 13-14. [5] Schultz L. and Kruse H. (1992) A Data Compilation, Max-Planck-Institut, Mainz. [6] Bogard D. D. and Johnson P. (1983) Science, 221, 651-654.