Carbon in solid solution in forsterite—a key to the untractable nature of reduced carbon in terrestrial and cosmogenic rocks

Abstract

Carbon is an incompatible element in oxide and silicate lattices. Until now it has been believed to exist only in the form of CO 2− 3 ions, molecularily dissolved CO 2 or graphitic inclusions. Recently it has been shown that carbon can dissolve in refractory oxides, like MgO and CaO, in the form of carbon atoms. The experimental results obtained with carbonaceous MgO are reviewed and new results are presented which demonstrate that synthetic forsterite and natural olivines can also take up atomic carbon in solid solution. The incorporation of the carbon atoms is treated thermodynamically. Near the melting point they probably occupy cation vacancies, but with decreasing temperature they are progressively transferred on interstitial sites. On these sites they are very mobile and tend to segregate into the elastically relaxed subsurface zone, but exsolution to graphite is prevented by the strain fields surrounding each carbon atom. Upon heating, however, the atomic carbon may react with lattice oxygen to give CO 2 and with co-dissolved hydrogen to give a wide variety of hydrocarbons. The underlying reaction mechanisms, involving the formation and decay of O − ions, are discussed in view of the so-called ‘carbonatic carbon’ and ‘reduced carbon’ in magmatic minerals and meteorites, in view of the diamond genesis and also in view of the reversible CO 2 solubility in silicate magmas at high pressures and temperatures.