Iron-titanium oxide minerals in layered gabbros of the Skaergaard intrusion, East Greenland : Part I. Chemistry and ore-microscopy
The microscopic characters and chemical compositions of coexisting magnetites and ilmenites in a suite of differentiated gabbros are discussed. Textures of the minerals suggest that some discrete crystals of ilmenite owe their origin to complete exsolution from an originally homogeneous high-temperature solid solution. The magnetites contain exsolution bodies of ilmenite and sometimes of ulvöspinel, Fe 2TiO 4, those of rocks rich in opaque oxides carrying much more ulvöspinel than those of average rocks at the same horizons in the intrusion; leading to the conclusion that the magnetite compositions are largely determined by oxidation-reduction equilibria between ferrous and ferric oxides and oxygen in the crystallizing magma. The variation in contents of Mg, Co, Mn, Zn, Cu, Al, Cr and V in both magnetites and ilmenites is discussed. In the magnetites these vary much less than FeO, Fe 2O 3 and TiO 2 between melanocratic and average bands at the same horizon and reflect both their ease of entry into the minerals and their availability in the magma at each stage.