Study on the origin of mantle hydrocarbons

Analyses of 250 rocks from the world showed that mantle-derived rocks such as tectonized peridotite in ophiolite sequences (tectonite) and mantle xenolithes in alkali basalts contained high molecular weight hydrocarbons from C₁₄ to C₃₃. The occurrence of the hydrocarbons indicated that they were not derived either from laboratory contamination or from field contamination; those compounds found in the mantle-derived rocks were called here "mantle hydrocarbons." Possible origins of the mantle hydrocarbons were as follows. (1) They were delivered to the early earth by meteorites and comets. (2) They were recycled by subduction. (3) They were inorganically synthesized by Fischer-Tropsch type reaction in the mantle. The mantle hydrocarbons in the case of (1) and (3) are abiotic and (2) is mainly biotic. In order to examine the first possible origin for the mantle hydrocarbons, shock experiments for CO and H₂ mixture, benzene, and hexane as reactants were carried out. Shock waves generated by projectile impacts were transmitted into starting materials. The shock reaction for CO and H₂ mixtures produced hydrocarbon gases up to C₄H₁₀. The conversion of CO to hydrocarbons increased with increasing the projectile velocity. The composition of produced hydrocarbons depended on that of the starting material (CO/H₂). The shock reaction for benzene produced hydrogen, light alkanes from C₁ to C₃, light alkenes from C₁ to C₃, acetylene, aromatic hydrocarbons with high molecular weight from 102 (phenylacetylene) to 306 (quaterphenyl). Examination of the yield relationships among structural isomers in products suggested that concerted cycloaddition reactions controlled by Woodward-Hoffmann rules explained the formation of some products better than do radical addition reactions. The shock reaction for hexane produced hydrogen, light alkanes from C₁ to C₄, light alkenes from C₂ to C₃, heavy alkanes from C₈ to C₁₂ and soot-like materials. In this shock reaction, the dehydrogenation was one of the important reactions and the recombination of hexyl radicals might play a role in the formation of n-C₁₂. The shock reactions showed that organic materials in meteorites and comets would survive and would develop into organics with higher molecular weights than initial materials, when the meteorites and the comets impacted on the earth. However, it is still unclear whether organic materials can survive in high temperature-pressure environment such as the mantle or not. Author plans to analyze organic materials in high-grade metamorphic rocks. This study will provide us with important information on the stability of organic materials.