Formation of carbon and hydrogen species in magmas at low oxygen fugacity

Abstract

Studies of iron-bearing silicate melt (ferrobasalt) + iron metallic phase + graphite + hydrogen equilibria show that carbon and hydrogen solubilities in melts are important for the evolution of the upper mantle. In a series of experiments conducted at 3·7 GPa and 1520-1600°C, we have characterized the nature (oxidized vs reduced) and quantified the abundances of C- and H-compounds dissolved in iron-bearing silicate melts. Experiments were carried out in an anvil-with-hole apparatus permitting the achievement of equal chemical potentials of H2 in the inner Pt capsule and outer furnace assembly. The fO2 for silicate melt-iron equilibrium was 2·32 ± 0·04 log units below iron-wüstite (IW). The ferrobasalt used as starting material experienced a reduction of its iron oxides and silicate network. The counterpart was a liberation of oxygen reacting with the hydrogen entering the capsule. The amount of H2O dissolved in the glasses was measured by ion microprobe and by step-heating and was found to be between 1 and 2 wt %. The dissolved carbon content was found to be 1600 ppm C by step-heating. The speciation of C and H components was determined by IR and Raman spectroscopy. It was established that the main part of the liberated oxygen was used to form OH- and to a much lesser extent H2O, and only traces of H2, CO2 and CO32-. Dissolved carbon is mainly present as atomic carbon or amorphous carbon. It was possible to measure an isotopic fractionation of 0·8‰ between graphite and dissolved or amorphous carbon at the temperatures of experiments. The Raman spectra also suggest that the network units might contain Si-C bonds. Comparison of our results with the literature demonstrates that the amount of dissolved species decreases as fO2 decreases. In the light of these experimental data, it appears that large-scale melting of the proto-Earth could be associated with melts containing an oxidized form of hydrogen. The early Earth, however, was likely to have been a very reducing environment, in which most of the carbon remained stable in the form of graphite. As the Earth became more and more oxidized, melts formed at depth would have dissolved larger amounts of water, and also carbon in the form of CO2, which would have made the degassing of the upper mantle more and more efficient. © Oxford University Press 2004; all rights reserved.