CARBONACEOUS MATTER IN MANTLE XENOLITHS: COMPOSITION AND RELEVANCE TO THE ISOTOPES

Abstract

Carbonaceous matter in xenoliths from alkalic basalts is present as particles in prominent cracks and as a semicontinuous film on microcracks. The particles are partially chlorinated, contain detectable amounts of Si and K, are thought to be the host phase for Rb, La and other minor and trace elements known from leaching experiments to be concentrated in microcracks and are believed to consist dominantly of graphite intercalation compounds. The carbonaceous films have been characterized by x-ray photoelectron spectroscopy. The films consist of complex mixtures of organic and possibly graphitic compounds and may include alcohols, organic acids, or other compounds unstable at elevated temperature. Nitrogen is frequently observed in photoelectron spectra of the films.Most cracks in xenoliths are likely to have developed during quenching, resulting in the exposure of chemically active surfaces. The carbonaceous films formed on them abiotically by condensation from hot volcanic gas and the low-temperature compounds formed by evolution from simpler high-temperature condensates during cooling. Such a process may have represented one step in the chemical evolution of life.The bulk of the C and other elements on crack surfaces should be regarded as contaminants from the host lava. Therefore, stepped heating procedures are necessary for the generation of meaningful isotopic data for C, N and H in xenoliths.