ALH 84001:  ALIEN OR PROGENITOR OF THE SNC FAMILY?  E. Jagoutz, A. Sorowka, J. D. Vogel, and H. Wänke, Max-Planck-Institut für Chemie, Saarstraße 23, D-55122 Mainz, Germany.

Published in Meteoritics, 29, pp. 478-479.

We have analyzed two 50-mg samples of ALH 84001. One of the samples consisted of an aliquot of a finely pulverized 1.7-g WR sample (F), while the other one consisted of several coarse fragments (C). Leaches (2.5 N HCl) (L) and the residua (R) were analyzed for Sr, Nd, and Pb isotopic systematics. The results are shown in Figs. 1, 2, 3 and compared to angrite LEW 86010 and the other SNC meteorites. Terrestrial fields are indicated for orientation.

About 50% of the Nd and Pb is in the leach, while only 25% of the Sm and 10% of the Sr were found to be leachable. The Sm-Nd data plot along an isochron of 4.56 Ga with a chondritic initial. The whole rock is about 30 e radiogenic in Nd isotopes. Lead isotopes form a mixing line comparable to LEW 86010 [1], which indicates terrestrial contamination in the Antarctic.

Mittlefehldt [2] suggested that ALH 84001, initially classified as a diog-enite, might actually be a member of the SNC meteorites. If ALH 84001 were a diogenite, its volatile-element contents would be unreasonably high. Also, 87Sr/86Sr ratios are significantly higher in ALH 84001 than in diogenites. In addition, the radiogenic Nd in the WR aliquot does not fit HED meteorites.

If ALH 84001 is a member of the SNC family, there are profound implications for martian evolution. It must then be a sample from the early differentiation of Mars, since its crystallization age is 4.56 Ga. If the Nd in the leached sample is representative of the carbonates, then they are also products of the early differentiation.

As shown by [3], Mars differentiated in its early history and was not homogenized by convection like the Earth was. All SNCs are late magmatic products of early differentiated reservoirs, and ALH 84001 might be a sample of such a reservoir. Neodymium isotopes indicate that Nakhla could have formed 1.3 Ga from such a reservoir as ALH 84001. However, in contrast, the radiogenic Sr isotopes in ALH 84001 are close to those in shergottites. It also seems that ALH 84001 might contain some radiogenic Pb, as indicated by the Pb in the residues, a facet similar to HED meteorites.

References:  [1] Lugmair G. W. and Galer S. J. G. (1992) GCA, 56, 1673-1694. [2] Mittlefehldt D. W. (1994) Meteoritics, 29, 214-221. [3] Jagoutz E. (1991) Space Sci. Rev., 56, 13-22.