Oxygen fugacity of basaltic magmas and the role of gas‐forming elements

Abstract

It is probable that elemental carbon exists in the source region of a basaltic magma and is suspended in the magma during ascent. As carbon has a large redox capacity, it is probably in control of the magmatic f O 2 from the source region to the deep crustal environment. Isothermally carbon becomes more reducing with decreasing pressure, and thus reduces the host magma upon ascent. If a magma is anhydrous (e.g., lunar basalts), the reduction by carbon continues through the extrusive phase and the relative f O 2 decreases rapidly until buffered by the precipitation of a metallic phase. If a magma is hydrous (e.g., terrestrial basalts), reduction by carbon is eventually superceded by oxidation due to the loss of H 2 , which is generated by the reaction of C with H 2 O and also by the thermal dissociation of H 2 O, and ferric iron is produced. Cumulus crystallization of ferrous silicates also contributes to the oxidation of magma. The relative f O 2 of a hydrous magma initially decreases as the magma ascends from the source region, and then increases until magnetite crystallization curbs the rising trend of the relative f O 2 .