Stable carbon isotope ratio measurement of meteoritic carbonaceous material



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In order to produce stable carbon isotopic data with the high precision required to make meaningful conclusions concerning the source and mechanism of formation of meteoritic organic matter, a high precision isotope ratio measurement system was designed, constructed, and evaluated to determine its capabilities. The design objective was to construct a system capable of measuring carbon isotope ratio differences with a precision of t 0.005 percent. Some papers have been published in recent years describing various aspects of isotope ratio instrumentation, but no complete and comprehensive description of a high precision measurement system has been given in recent years. A detailed description sufficient to allow one to fabricate, assemble and operate a complete isotope ratio measurement system is given. The system described here incorporates the most modern electronics available; and though it has no more apparent precision than previously reported instruments it is simpler to operate, less time consuming, and more reliable. A new sample combustion technique developed during this study which greatly simplifies both the equipment required and the operating procedures is also described. In addition to instrumentation development sample preparation techniques designed to separate various carbonaceous fractions of carbonaceous chondrite meteorites are described. These separation techniques isolate the carbon containing material into inorganic, total organic, and insoluble organic matter and remove much of the confusion caused by inhomogeneous inorganic carbon distribution. Carbon content and isotopic composition data were then obtained on each of these fractions for representatives of each of the three types of carbonaceous chondrites. Isotopic data for graphitic material in other types of meteorites was also determined. Conclusions are drawn concerning the origin and history of organic matter in the solar system and meteorites, but can only be considered tentative since they are based on such a small sampling of meteorites. This work has shown that proper sample preparation can eliminate some of the confusion caused by the inorganic carbon phases and allow one to see trends in organic carbon phases heretofore unobserved in carbonaceous chondrites. The next logical step would be to extend this type of study to additional carbonaceous chondrites wherever sufficient sample can be obtained. Measurement of carbon isotope ratios of other chemical groups of meteorites is also indicated but will require instrumental development because of the decreasing carbon contents that will be encountered. It is hoped that other investigators will adopt separation techniques described here as standard so that data from different laboratories can be more readily compared in the future.