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 ov er a range of oxygen fugacity (f(O2)) have been carried out on a ferro -basaltic composition similar to liquids proposed to have been parenta l to much of the exposed portion of the Skaergaard intrusion. Before F e-Ti oxide saturation the liquid line of descent is little affected by f(O2). However, the appearance temperatures of the magnetite-ulvospin el solid solution (Mt) and the ilmenite-haematite solid solution (Ilm) depend strongly on f(O2). Above the fayalite-magnetite-quartz (FMQ) b uffer 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 similar to 1100 degrees C at FMQ and the Mt liqui dus slope is similar to 30 degrees C/log f(O2) unit between FMQ - 2 an d FMQ + 1. The Ilm liquidus is at similar to 1100 degrees C between FM Q and FMQ - 2, but moves to lower temperature at higher f(O2) where Mt is the first oxide phase. The results indicate that the ferric iron c ontent of Mt-saturated melts varies linearly with inverse temperature, and that nm saturation is closely related to melt TiO2 content. Mt sa turation Produces an immediate enrichment of SiO2 and depletion in FeO in the melt phase, whereas Ilm saturation Produces similar enrichmen t in SiO2, but iron enrichment may continue for similar to 10 degrees C below the ilmenite liquidus. The experimental liquids reach a maximu m of similar to 18 wt % FeO, at similar to 48 wt % SiO2 for ilmenite- saturated melts at lore f(O2), more differentiated melts having lower iron and higher silica. Cotectic proportions, derived from mass balanc e calculations, are in good agreement with data from natural samples a nd other experimental studies. Olivine resorption is inferred at all f (O2), with the onset of resorption occurring similar to 10 degrees C h igher than the appearance of magnetite. The effect of f(O2) on silicat e mineral compositions, and partitioning of elements between coexistin g mineral-melt pairs, is small. Thermodynamic considerations suggest t hat variations of Fe-Mg partitioning between the iron-rich olivines, p yroxenes and melts produced in this study may be explained by known no n-idealities of Fe-Mg mixing in the crystalline phases, rather than no n-idealities in the coexisting melts. These experiments also provide i nsights into many features common to natural tholeiitic series of volc anic and plutonic rocks, and provide experimental data required for mo delling of fractional crystallization and crystallization closed to ox ygen, processes which are not easily investigated experimentally.