Formation of Iron-Carbon Alloys in Basaltic Magma at Uivfaq, Disko Island: The Role of Carbon in Mafic Magmas

Two types of metallic iron were identified in a collection of samples from the Tertiary iron-bearing basalts of Uivfaq, Disko Island, Greenland. High-carbon iron has 2.9-4.0 wt % carbon and eutectic textures of iron and cohenite (Fe₃C) similar to those of commercial white cast iron. It occurs in approximately spherical bodies, up to 15 mm in diameter, that appear to have been immiscible metallic liquids within basaltic magma. Low-carbon samples have ~1.8-2.0 wt % carbon and textures similar to those of commercial hypereutectoid steel. They occur principally as large (up to 22 tons) cumulate masses. Both types of metal could have crystallized at low pressure from iron-carbon liquids of their present bulk carbon contents. Fe, Ni, Co, Cu, and P oxides in basaltic magma were reduced by carbon derived principally from assimilated Cretaceous-Tertiary sediments. Niand Ni/Cu ratios of the metal are similar to those of terrestrial basalts. The magma contained excess carbon (graphite), so the metal formed as an immiscible C-saturated liquid (represented by the high-carbon samples). Carbon geobarometry indicates that reduction took place at pressures less than ~400 bars (1.2 km). Separation of the metal from excess carbon caused decarburization of most of the high-carbon metal to form low-carbon metal. The decarburization reaction, C(in metal) + O₂ = CO or CO₂, is exothermic and produced enough heat locally to raise the temperature of the low-carbon metal to at least its melting point (~1390 C). P and some Fe were also oxidized and removed from the metal as FeO and P₂O₅ and were trapped in silicate and oxide inclusions within the metal. Consequently the low-carbon metal has a lower P content than the high-C metal. Mm-sized grains of metal were carried upward and accumulated at the base of a subvolcanic intrusive body at ~0.6 km depth.