DIFFERENTIATION OF FERRO-BASALTIC MAGMAS UNDER CONDITIONS OPEN AND CLOSED TO OXYGEN - IMPLICATIONS FOR THE SKAERGAARD INTRUSION AND OTHER NATURAL SYSTEMS

Because processes such as fractional crystallization and crystallizati on under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystalliz ation behaviour. Comparison of experimental data on an iron-rich basal tic composition with predictions of the MELTS free energy minimization algorithm shows that although liquidus temperatures and silicate mine ral equilibria are predicted relatively well, the saturation of Fe-Ti oxides is not. We have used the same experimental data to construct an alternative crystallization model based on known equilibrium phase re lations, mineral-melt partitioning of major elements, and mass balance constraints. The model is used to explore the consequences of equilib rium and fractional crystallization in systems open and closed to oxyg en. Liquid lines of descent for perfect equilibrium and perfect fracti onal crystallization are predicted to be very similar. In a system ope n to oxygen the model Predicts that magnetite saturation leads to stro ngly decreasing iron and increasing silica contents of residual liquid s, whereas systems closed to oxygen crystallize less abundant magnetit e, leading to a less pronounced iron depletion in the liquid Predicted bulk solid compositions and variations of f(O2) with falling temperat ure agree well with those observed or inferred from the cumulates of t he Skaergaard intrusion, but none of the predicted liquid lines of des cent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations. Compositional factors s uch as water and phosphorus are not thought to be the source of the di screpancy as the cumulates of the Basistoppen sill (which closely rese mble those of Skaergaard) may be used to calculate a liquid line of de scent in agreement with that predicted by the model for fractional cry stallization closed to oxygen. A comparison of the predicted T-f(O2) p aths and liquid lines of descent with those inferred from natural syst ems suggests that volcanic centres such as Iceland and Hawaii evolve u nder conditions open to oxygen, whereas evidence from plutonic environ ments (e.g. Skaergaard and Kiglapait layered intrusions) suggests that they evolved under conditions more closed to oxygen. The compositiona l evolution of the melt phase in volcanic and plutonic systems may the refore be different, although the results of this study suggest that m agnetite saturation will limit Fe enrichment in all environments to <2 0 wt % FeO, consistent with enrichments reported for volcanic glasses .