Formation of native iron in sediment-contaminated magma: I. A case study of the Hanekammen Complex on Disko Island, West Greenland

Abstract

Abstract For the first time a compositional range of native iron bodies is described in a cogenetic series of sediment-contaminated volcanic rocks from the Tertiary West Greenland Basalt Province. The iron-bearing rocks occur in a high-level composite intrusion, the Hanekammen Complex. Reaction between a tholeiitic parent magma with > 11% MgO and carbonaceous Al2O3-rich shale took place in a reservoir >3 km below the paleosurface and created magmatic layering with basaltic magma overlain by less dense andesitic magma. The contaminated rock series bears a strong imprint of assimilation but very little fractional crystallization, which implies that the two processes were not intimately coupled in the present case. Most of the iron was formed at depth in a range of fO2 from 10−13 atm 1200°C to 10−16 atm 1100°C as determined by the P distribution between solid iron and magma. This is far below the graphite-gas (CCO) buffer at the estimated pressure (>800 bars) and suggests a high hydrogen fugacity. Together, iron in basalt and andesite form a general trend, defined by Co versus Ni concentrations, that reflects the degree of assimilation, the amount of immiscible sulphide liquid, and the degree of reduction (in order of decreasing importance). The zoning of single iron grains reflects the dynamics of their growth and, to some extent, subsequent homogenization and reaction with the magma. Weakly zoned iron spherules in viscous andesite were formed and remained in situ, whereas iron grains in basalt settled through the layered magma and developed strong zoning. All iron types contain Co-rich domains (