BULK AND COMPOUND SPECIFIC STABLE ISOTOPES AS PROBES FOR THE ORIGINS OF ORGANIC MATTER IN CARBONACEOUS METEORITES

Organic matter in carbonaceous meteorites consists primarily of insoluble, macromolecular material (kerogen) and lesser amounts of extractable compounds (e.g. hydrocarbons, carboxylic acids, amino acids). The structures of these compounds are commonly indistinguishable from those of biological origin on Earth. Given that all meteorites have been exposed to the Earth’s biosphere, distinguishing indigenous components from terrestrial contaminants remains a serious challenge. However, by fortuitous circumstances resulting from differences in precursor sources and distinct pathways of formation, organic matter in carbonaceous meteorites is enriched in the heavier isotopes of carbon, nitrogen and hydrogen. Thus, by establishing approximate end members for the stable isotope compositions of two or more of the light elements that comprise organic matter, it is possible to estimate the relative contributions of terrestrial and extraterrestrial sources for compounds in carbonaceous meteorites. For example, the average stable carbon isotope composition for organic matter of biological origin on Earth is approx. -26±7‰, and almost all kerogens in ancient rocks fall within this range. In contrast, the bulk macromolecular material in carbonaceous meteorites is enriched in 13C relative to terrestrial kerogens (e.g. Murchison meteorite, –11.3‰;Tagish Lake meteorite, -12.6‰). Similarly, the stable nitrogen isotope values for the bulk macromolecular material in carbonaceous meteorites are enriched in 15N (Murchison meteorite, +30.1‰; Tagish Lake meteorite, +54.8‰) relative to the range of stable nitrogen isotope values (approx. –10 to +20‰) for organic matter on Earth. Stable hydrogen isotope values for organic matter in meteorites are even more enriched in the heavier isotope (D) relative to the Earth’s biosphere. At the molecular level, meteoritic amino acids exhibit 13C and 15N enrichments that exceed that observed for their respective bulk, macromolecular materials. Thus, multi-element, stable isotope analyses of bulk and molecular components in carbonaceous meteorites affords a definitive method for establishing their authenticity, an approach that should be particularly useful for meteorites that have experienced extended residence times on Earth (e.g. Antarctic meteorites).