Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.215d&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 215
Other
1
Scientific paper
Lewis Cliffs 88516 is a shergottite of harzburgite composition similar in many respects to ALH A77005. A previous study of the carbon in this latter sample (Wright et al., 1986) has shown it to contain one of the lowest carbon contents of any SNC meteorite (140 ppm carbon). At the very least this suggests that ALH A77005 is not heavily contaminated with terrestrial organic compounds or Antarctic weathering products. On the basis of stepped combustion results, no evidence was found in ALH A77005 for the presence of ^13C-rich carbonates, as have been found in Antarctic and non- Antarctic meteorites alike (Carr et al., 1985; Wright et al., 1988, 1992). However, ALH A77005 was observed to liberate a discrete release of carbon at 500-600 degrees C, a temperature interval that would be consistent with the presence of carbonate minerals. Interestingly, the delta^13C value at 500-600 degrees C was -29o/oo, which is hardly different to the bulk value of -28.7o/oo. Thus, it remains a possibility that ALH A77005 contains carbonate minerals of relatively light carbon isotopic composition, a proposition that will have to be clarified by making further analyses of this meteorite. As a preliminary investigation of LEW 88516 we elected to use stepped combustion to analyse the subsample LEW 88516,13, a powder prepared specially at NASA-JSC, using ca. 1 g of the meteorite, in order to provide a representative and homogeneous sample. The total carbon content of LEW 88516,13 is 1215 ppm, which is in fact the highest measured so far for any SNC meteorite. In light of the similarities between LEW 88516 and ALH A77005 the largest excesses of carbon found in the former meteorite pose a problem of interpretation. Most of this carbon is released at low temperatures and is thus most likely to be organic in nature. It would seem to be a possibility that some of this carbon was introduced during the crushing procedure, highlighting the problem of trying to ascertain the true bulk carbon content of SNC meteorites. However, the bulk delta^13C value of LEW 88516,13 is -21.2o/oo, which is relatively high for normal terrestrial contaminants. The reason for the elevated delta^13C is the presence of a ^13C-rich component that is liberated between 400 and 900 degrees C, broadly the region that results in the decrepitation of carbonate minerals. In order to obtain the delta^13C value of these minerals it will be necessary to undertake an acid-dissolution extraction of the meteorite. On the basis of the stepped combustion data it is apparent that the delta^13C of the carbonates is at least -14.6o/oo (the most extreme result obtained, at 500-600 degrees C). If the delta^13C of any carbonates was +lOo/oo, LEW 88516 could contain 90 ppm of carbon as carbonate, a concentration that is considerably higher than any observed in other SNC meteorites (Wright et al., 1992). If the delta^13C value of the putative carbonates in LEW 88516 was lower than +10o/oo, then the meteorite would contain even more of these minerals. Clearly, in spite of the potential problems of contamination, LEW 88516 is worthy of further study. The Antarctic shergottite, EET A79001, has been found to contain enrichments of organic compounds accompanying deposits of carbonate minerals (Wright et al., 1989). Since LEW 88516 also contains high levels of both organics and carbonates, it is apparent that analyses of this meteorite will assist the understanding of this association. Although we cannot rule out the possibility that the carbon components observed in LEW 88516 are indigenous, in light of the small size of the sample (13.2 g) it is anticipated that Antarctic weathering will have played an important role in affecting the carbon inventory of the sample. References: Carr, R.H., Grady, M.M., Wright, I.P. and Pillinger, C.T. (1985) Nature, 314, 248-250. Wright, I.P., Carr, R.H. and Pillinger, C.T. (1986) Geochim. Cosmochim. Acta, 50, 983-991. Wright, I.P., Grady, M.M. and Pillinger, C.T. (1989) Nature, 340, 220-222. Wright, I.P., Grady, M.M. and Pillinger, C.T. (1992) Geochim. Cosmochim. Acta, 56, 817-826.
Douglas Craig C.
Pillinger Colin T.
Wright Ian P.
Yates P. D.
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