NUCLEAR TRACK STUDIES OF THE SNC METEORITE ALH 84001.  N. Sinha and J. N. Goswami, Physical Research Laboratory, Ahmedabad 380009, India.

Published in Meteoritics, 29, p. 534.

ALH 84001, classified initially as a diogenite, has recently been identified as a new unusual SNC meteorite based on petrographic and O isotopic composition [1]. In fact, the unusual Xe isotopic composition of ALH 84001, with extremely high 129Xe/132Xe, that did not match the expected signature in HED meteorites, was also noted by Miura et al.[2]. In this study we have analyzed nuclear track records in several samples of ALH 84001 to determine the extent of its atmospheric ablation. Dynamical modeling [3] suggests that martian ejecta that can give rise to SNC meteorites will enter the Earth only slightly above the Earth’s escape velocity and the expected ablation loss is small (~50%). This model is also consistent with the low exposure ages of the SNC meteorites. These suggestions were strengthened by the nuclear track and other cosmogenic data obtained for the shergottites [4]. In the present work, we have studied cosmic-ray-produced nuclear track records in five samples of ALH 84001, taken from close to the fusion crust (~0.5 cm away from the crust), and one sample from the interior region. The samples were gently crushed and individual pyroxene grains were hand-picked and processed to reveal nuclear tracks using standard procedures. The etching was done in steps in a boiling 30N NaOH solution. A large fraction of the pyroxenes were lost due to dissolution during the initial etching steps, reflecting the shocked nature of these grains. In three of the samples from the near-crust region, we could find well-developed tracks in several pyroxene grains. The track densities (number of tracks per cm2 ) in two of the samples are similar [(4.7 ± 0.6)×106 and (4.5 ± 0.6)×106 ], while the third sample had a slightly lower value [(3.6 ± 0.2)×106 ]. The track density in the interior sample is (2.5 ± 0.5)×106 . These track densities suggest atmospheric ablations of 4-5 cm for the sampled locations of ALH 84001, if we consider a cosmic ray exposure age of 16 Ma [2]. Assuming a symmetric ablation, the minimum mass ablation of ALH 84001 turns out to be ~85%. This is much higher than the predicted value based on dynamic modeling and the value of ~50% obtained for the shergottite ALHA 77005 and EETA 79001 and the value of 65-80% for the Shergotty meteorite [4]. However, in making the above estimate, we have assumed that ALH 84001 had a simple one-stage cosmic ray exposure and all the cosmogenic records seen in it were imprinted entirely during its space exposure. We note that the exposure age of 16 Ma for ALH 84001 is not typical of other SNC meteorites whose exposure ages group around 2.5 Ma (shergottite) and ~10 Ma (nakhlite and chassignite). If we assume an exposure age of 10 Ma for ALH 84001, the minimum mass ablation will still be ~80%. However, the high cosmogenic 22Ne/ 21Ne ratio of 1.192 measured in this meteorite [2] suggests the possibility that the shielding depths of the analyzed samples could be somewhat lower than those obtained by us. Although radionuclide data [e.g.,5] rule out the possibility of a two-stage exposure history for the shergottites, the same need not be true for the NC group. The possibility that ALH 84001 had a two-stage exposure history, with a significant production of cosmogenic nuclides during its exposure in martian regolith or in space before its final break-up, can be ascertained when additional data on cosmogenic stable and radionuclides become available.

References:  [1] Antarct. Meteorite Newsletter (1994) 16, 1-4. [2] Miura Y. N. et al. (1994) Intl. Workshop on Noble Gas Geochem. Cosmochem., 17-19, Kyoto. [3] Wetherill G. W. (1984) Meteoritics, 19, 1-13. [4] Bhandari N. et al. (1986) GCA, 50, 1023-1030. [5] Pal D. K. et al. (1986) GCA, 50, 2405-2409.