High-pressure fluid-absent melting experiments in the presence of graphite: oxygen fugacity, ferric/ferrous ratio and dissolved CO2

Abstract

Graphite capsules are commonly used to prevent loss of iron from iron-bearing samples in high-pressure melting experiments. The graphite is not inert in such experiments and reacts with Fe 2 O 3 in the melt to form dissolved CO 2 and FeO. This equilibrium can be quantified if the activity-composition relations of CO 2 in the melt are known. The presence of graphite is shown to limit the maximum fO 2 possible, but places no limit on the minimum fO 2 . The fO 2 in the system, the ferric/ferrous ratio in the melt, and the concentration of dissolved CO 2 are proportional to the concentration of ferric iron in the starting material. The dissolved CO 2 content cannot be controlled independently of fO 2 , but for values of fO 2 appropriate to mantle melting, the amount of dissolved CO 2 can be controlled over values ranging from about 0.1 wt% to saturation