Ein Beitrag zur Isotopengeochemie des Kohlenstoffs in magmatischen Gesteinen

Several arguments indicate that the mean carbon isotopic composition of the earth's crust and the upper mantle should be around-7‰. This agrees quite well with a balance calculation (Table 7) and with what we know about the carbon isotope composition of carbonatites and diamonds. Since fractionation factors decrease with increasing temperatures, the differences in isotopic compositions found in igneous rocks might be expected to be relatively slight and not to differ very much from the mean δ-value for the earth's crust. This also applies to the elements oxygen and sulfur, and to a lesser extent even for hydrogen, but not for carbon. Hoefs (1965) has shown that all igneous rocks contain carbon in at least two different forms:

1. an oxidized form mainly as carbonate and/or as CO₂ (in fluid and gaseous inclusions) in variable concentrations between <100 ppm and several thousand ppm CO₂, and
2. a reduced form with a relatively constant concentration around 200 ppm C. To 1). If the carbonate were of primary magmatic origin, we should expect, in analogy to carbonatites or to some hydrothermal carbonates, a δ ¹³C-value around-7 and a δ ¹⁸O-value between ?15 to ?25‰ relative to PDB, but on the contrary, the variable δ ¹³C- and the relatively heavy δ ¹⁸O-values make it seem probable that the carbonate is not of pirmary magmatic origin, but of secondary, maybe groundwater origin. This does not exclude the possibility that in some cases there may also be some carbonate which is of primary magmatic origin. To 2). If the reduced carbon found in igneous rocks is indigenous to these specimens, theoretically it may occur as elemental carbon (graphite), as carbides, and as organic compounds or as all three combined together.

This reduced carbon has a very light and fairly constant isotopic composition between ?24 and ?28‰ relative to PDB in all igneous rock types. There are two very different possible explanations for these values. The first and simplest one is that this carbon is also of secondary origin, or in other words of biogenic origin—some kind of assimilation of sedimentary organic material. But since this carbon is very evenly distributed, this means that all igneous rocks with a very small, but not negligible, porosity and permeability are impregnated by surface waters containing biogenic-derived organic substances in a concentration of around 200 ppm C. Since bore samples have also been analyzed, this also means that these waters penetrate into igneous rocks even at greater depths. Due to certain similarities in carbon isotopic composition found in extraterrestrial material, in meteorites and in lunar rocks (Table 9), I favor the second possibility of explaining the rather light δ ¹³C values: Several mechanisms have been postulated for the formation of organic matter in our solar system (Fischer-Tropsch type synthesis, Miller-Urey reactions etc.). Evidence supporting the hypothesis of inorganically formed organic matter on the earth has accumulated since Miller (1957) first demonstrated the synthesis of organic compounds from methane, ammonia and water. It is postulated that photosynthesis is not the only process leading to isotopically light carbon, but that some of these reactions (perhaps Fischer-Tropsch type synthesis) may also yield to isotopically light carbon. In addition to these data, some gaseous CO₂-samples of probably volcanic origin from Germany have been analyzed. The CO₂ discharged in areas of ancient volcanic activity shows δ ¹³C value between ?2 and ?5‰, typical for geothermal areas (e.g. Yellowstone, New Zealand). The CO₂ found in inclusions in evaporites, some of them near basaltic dikes, shows a strikingly different δ ¹³C composition (between ?15 and ?25%.) comparable to CO₂ sampled over liquid Hawaiian lavas. On the basis of the isotope-exchange reaction CH₄+2H₂O ? CO₂+4H₂, temperature seems to be the most important parameter, being responsible for the observed differences in isotopic composition.