Ein Beitrag zur Geochemie des Kohlenstoffs in magmatischen und metamorphen Gesteinen

Abstract

A method for the determination of carbonate carbon and elementary carbon is described. If a finely ground sample is heated with phosphoric acid, carbon dioxide is liberated. This carbon dioxide is passed through dilute Ba(OH) 2 solution. The change of ion concentration during the precipitation of BaCO 3 is measured potentiometrically. After the determination of CO 2, the same sample is treated with a solution of CrO 2 in phosphoric acid for oxidation of elementary carbon to CO 2. Then elementary carbon is determined in the form of CO 2. This method is suitable to determine concentrations greater than 10 ppm of elementary carbon. 332 analysed rock and mineral samples vary in their CO 2-concentrations but have rather constant contents of elementary carbon. The CO 2-determinations are of minor geochemical importance due to the lack of indications for the primary or secondary origin of the carbon dioxide. The average elementary carbon values for igneous and metamorphic rocks scatter around 200 ppm C. A greater number of analyses from granites shows an inverse relationship between the CO 2-and C-contents. P o 2 is one factor controlling the ratio between CO 2-carbon and elementary carbon. In basalts a correlation between elementary carbon and chlorine could be detected. Mineral analyses show that Fe-bearing rock-forming minerals contain more elementary carbon than Fe-poor or Fe-free ones, e.g. in granites: biotites > feldspars > quartz, in basalts: magnetites > olivines > pyroxenes > feldspars. The kind of distribution of carbon in rock-forming minerals suggests a magmatic origin of the carbon although a secondary enrichment cannot be ruled out entirely. It is assumed that carbon occurs in the rocks mainly as graphite and/or as carbide, of these, graphite is the most probable one. Finally, some questions of balance are discussed.