Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.577s&link_type=abstract
Meteoritics, vol. 30, no. 5, page 577
Other
Chondrites, Carbonaceous, Compounds, Organic, Meteorites, Acfer 094, Acfer 182, Acfer 186, Acfer 187, Acfer 202, El Djouf, Pyrolysis-Gcms, Spectroscopy, Mossbauer, Weathering
Scientific paper
It is now well established for meteorites which fall in hot deserts that weathering brings about a readjustment of extra-terrestrial minerals. Assemblages which had achieved a level of equilibrium on the meteorite parent body now become unstable when exposed to new chemical and physical conditions[1] with FeO and Fe2+ minerals converting to Fe3+ species. Ash and Pillinger[2] have suggested that meteoritic organic matter may also become degraded in desert environments but this is less well substantiated and the processes involved far from clear. To investigate the effects of weathering on meteorite organics, five Saharan carbon-rich chondrites were studied by Mossbauer spectroscopy and pyrolysis-gas chromatography-mass spectrometry (Py-GCMS). Experimental conditions are given elsewhere. The samples chosen were El Djouf (CR), Acfer 186 and 187 (both CR, undoubtably related to one another and probably El Djouf, even though the latter was found some 500km away), Acfer 182 an anomalous chondrite (possibly in the CR clan) and Acfer 202 (a C03) . Table 1 shows the relative amounts (%) of iron containing assemblages in four of the meteorites analysed as determined by Mossbauer spectroscopy. El Djouf is substantially weathered with Acfer 187 and Acfer 182 perhaps less so but more weathered than Acfer 202. Fig 1 shows the pyrograms of four of the meteorites analyzed. El Djouf, Acfer 182 and Acfer 186 (=Acfer 187) yield very few discrete organic compounds. However Py-GCMS of unweathered CRs frequently detects a variety of organic fragments. Therefore it seems reasonable to suggest that, in the two CRs at least, macromolecular material has been present but has been degraded by weathering. Such a conclusion agrees well with the results from Mossbauer spectroscopy which indicate extensive oxidation in the CR meteorites. Acfer 202 clearly contains a number of organic components almost exclusively without oxygen, indicating that the macromolecule in Acfer 202 has escaped significant terrestrial oxidation. Again this is consistent with our Mossbauer results which show that Acfer 202 contains predominantly ferrous iron indicative of low levels of terrestrial oxidation. At face value we would argue that Acfer 202 is a relatively fresh carbonaceous chondrite worthy of detailed organic study. Clearly the above samples represent almost end-member cases where terrestrial weathering has either destroyed or has yet to affect the organic material present. Perhaps the most valuable information would come from a sample where oxidation of the macromolecule is at an intermediate stage. References: [1] Bland P. A. et al. (1995) LPS XXVI, 39. [2] Ash R. D. and Pillinger C. T. (1995) Meteoritics, 30, 85-92.
Bland Philip A.
Gilmour Iain
Pillinger Colin T.
Sephton Mark A.
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