Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.202b&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 202
Mathematics
Logic
2
Scientific paper
Approximately 380 meteorite specimens have been retrieved from the Acfer region of the Sahara desert, Algeria. To date, 26 of these have been classified H5 (Bischoff et al., 1990, 1991, 1992). Being the most common meteorite type and having a tightly constrained mineralogy (Mason, 1965), H5 chondrites are ideal candidates for investigating terrestrial weathering products in meteorites. Arid climate, uniform topography, and lack of a concentration/movement mechanism makes it likely that meteorites throughout Reg el Acfer were weathered by a common mechanism. Jull et al. (1991) showed a correlation in meteorites from Roosevelt County between terrestrial ^14C ages and a qualitative weathering scale. An aim of the present study is to provide a quantitative measure of weathering for the Acfer region that might allow an estimate of terrestrial age, as well as information on pairing. Meteorite Specimens: Approximately 1 g of sample was used, prepared by grinding under acetone to prevent oxidation during crushing, until a homogenized powder was produced. Mossbauer spectra were recorded at 298 degrees K with a microprocessor controlled Mossbauer spectrometer using a ^57Co/Rh source. Drive velocity was calibrated with the same source and a metallic iron foil. Results: The H5 chondrite Acfer 146 (Bischoff, forthcoming Meteoritical Bulletin) was found on 19/11/90 at coordinates 27 degrees 38'N, 4 degrees 05'E. This meteorite gave a spectrum dominated by quadrupole split absorption characteristics of Fe^2+ in a forsteritic olivine structure. A sample of the outer crust showed the additional presence of Fe^3+. XRD was insensitive to the unequivocal identification of the phases present in the two samples and given that the Mossbauer parameters of the hydrolyzed Fe^3+ species and ferric oxyhydroxides are very similar it is not possible at this stage to identify the oxidized phase. Clearly, however, the results demonstrate the sensitivity of Mossbauer spectroscopy to the products of terrestrial weathering processes in meteorites. The investigation was extended to the H5 chondrite Acfer 098 (Bischoff, 1991) recovered on 19/4/90 from a location coordinates 27 degrees 28'N, 3 degrees 53'E. This meteorite was chosen because in hand specimen it "looked"' severely weathered. The sawn surface shows reddish brown staining, is friable, and cut by veins of alteration products. This sample showed the presence of a much greater proportion of Fe^3+ than with the Acfer 146 interior or crust, confirming the findings of Fisher and Burns (1992) in work on H5 chondrites from Antarctica. Given that samples recovered from the Reg el Acfer had resided in geologically and physiographically similar areas, the differences observed by Mossbauer spectroscopy may be estimating changes in the degree of weathering and hence possibly the relative time that the meteorite had spent on the Earth's surface. Considerably more work is needed, however, to ascertain whether a quantitatively useful weathering scale can be derived with sufficient resolution to be of value. Any correlation may be complicated by the climatic changes that occurred at the Pleistocene/Holocene boundary in North Africa (Butzer et al., 1972), as the results of Jull et al. (1990) in relation to the distribution of terrestrial ages of meteorites from western Libya have shown. References: Bischoff A. et al. (Analyst) (1990) Meteoritical Bulletin No. 69, Meteoritics 25, 237. (1991) Meteoritical Bulletin No. 71, Meteoritics 26. (1992) Meteoritical Bulletin No. 72, Meteoritics 27, 109-110. Butzer K. W., Isaac G. L., Richardson J. L., and Washbourn-Kamau C. (1972) Science 175, 1069-1076. Fisher S. D. and Burns R. G. (1992) Lunar Planet. Sci. XXIII, 367-368. Jull A. J. T., Wlotzka F., Palme H., and Donahue D. J. (1990) Geochim. Cosmochim. Acta. 54, 2895-2898. Jull A. J. T., Wlotzka F., and Donahue D. J. (1991) Lunar Planet. Sci. XXII, 667-668. Mason B. (1965) Am. Mus. Novitates 2223, 1-38.
Berry F.
Bland Phil
Oates Gordon
Pillinger Colin T.
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