CARBON, NITROGEN AND HYDROGEN IN SAHARAN CHONDRITES - THE IMPORTANCE OF WEATHERING

The Sahara Desert is a region of high diurnal temperature variation and sporadic rainfall that has recently yielded over 450 meteorites. Eighteen of these Saharan samples are carbonaceous chondrites, of which we have analysed 17 for C content and isotopic composition. Ten of the 18 are paired CR chondrites, of which four have also had N and H contents and compositions determined. A primitive ordinary chondrite (L/LL3.2) found in the region has also been analysed for C, N and H contents and isotopic composition. Saharan samples contain between 21% and 45% of the light elements of their non-Saharan counterparts. Paired Saharan samples show a greater heterogeneity in both C content and isotopic composition than multiple analyses of non-Saharan samples. The cause of the observed isotopic and abundance effects is due to the hot desert weathering processes experienced by these samples. Peak temperatures of meteorites on the desert floor may be in excess of 100-degrees-C, leading to low-temperature hydrous pyrolysis and oxidation reactions, liberating volatile organics and CO2. This may also cause the remaining material to become partially solubilised and ultimately lost during rainfall. The low deltaD of the CR and ordinary chondrites can be attributed to the destruction and loss of organic material through dehydrogenation and exchange reactions on the desert surface. The increased C-13 abundance suggests that the less tightly bound C from the macromolecular organic material is isotopically lighter than the remaining C. Carbon contents and isotopic compositions are also affected by the addition of terrestrial calcitic evaporite deposits, up to 10,000 ppm carbonate has been measured, with a deltaC-13 of between 0 and -10parts per thousand.