An Experimental Study of the Influence of Oxygen Fugacity on Fe-Ti Oxide Stability, Phase Relations, and Mineral--Melt Equilibria in Ferro-Basaltic Systems

Equilibrium crystallization experiments at atmospheric pressure and over a range of oxygen fugacity (fO2) have been carried out on a ferro-basaltic composition similar to liquids proposed to have been parental to much of the exposed portion of the Skaergaard intrusion. Before Fe-Ti oxide saturation the liquid line of descent is little affected by fO2. However, the appearance temperatures of the magnetite-ulvöspinel solid solution (Mt) and the ilmenite-haematite solid solution (Ilm) depend strongly on fO2. Above the fayalite-magnetite-quartz (FMQ) buffer Mt is the first oxide phase to appear on the liquidus, but below the FMQ buffer Ilm is the first oxide to crystallize. The appearance temperature of Mt is ∼1100°C at FMQ and the Mt liquidus slope is ∼30°C/log fO2 unit between FMQ−;2 and FMQJ+1. The Ilm liquidus is at ∼1100°C between FMQ and FMQ−2, but moves to lower temperature at higher fO2 where Mt is the first oxide phase. The results indicate that the ferric iron content of Mt-saturated melts varies linearly with inverse temperature, and that Ilm saturation is closely related to melt TiO2 content. Mt saturation produces an immediate enrichment of SiO2 and depletion in FeO* in the melt phase, whereas Ilm saturation produces similar enrichment in SiO2, but inn enrichment may continue for ∼10°C below the ilmenite liquidus. The experimental liquids reach a maximum of ∼18 wt% FeO*, at ∼48 wt% SiO2 for ilmenite-saturated melts at low fO2, more differentiated melts having lower iron and higher silica. Cotectic proportions, derived from mass balance calculations, are in good agreement with data from natural samples and other experimental studies. Olivine resorption is inferred at all fO2, with the onset of resorption occurring ∼10°C higher than the appearance of magnetite. The effect of fO2 on silicate mineral compositions, and partitioning of elements between coexisting mineral-melt pairs, is small. Thermodynamic considerations suggest that variations of Fe-Mg partitioning between the iron-rich olivines, pyroxenes and melts produced in this study may be explained by known non-idealities of Fe-Mg mixing in the crystalline phases, rather than nonidealities in the coexisting melts. These experiments also provide insights into many features common to natural tholeiitic series of volcanic and plutonic rocks, and provide experimental data required for modelling of fractional crystallization and crystallization closed to oxygen, processes which are not easily investigated experimentally.