Differentiation of ferro-basaltic magmas under conditions open and closed to oxygen: Implications for the skaergaard intrusion and other natural systems

Abstract

Because processes such as fractional crystallization and crystallization under conditions closed to oxygen are difficult to simulate in the laboratory there is a need for quantitative models of magma crystallization behaviour. Comparison of experimental data on an iron-rich basaltic composition with predictions of the MELTS free energy minimization algorithm shows that although liquidus temperatures and silicate mineral 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 relations, mineral-melt partitioning of major elements, and mass balance constraints. The model is used to explore the consequences of equilibrium and fractional crystallization in systems open and closed to oxygen. Liquid lines of descent for perfect equilibrium and perfect fractional crystallization are predicted to be very similar. In a system open to oxygen the model predicts that magnetite saturation leads to strongly decreasing iron and increasing silica contents of residual liquids, whereas systems closed to oxygen crystallize less abundant magnetite, leading to a less pronounced iron depletion in the liquid. Predicted bulk solid compositions and variations of fO2 with falling temperature agree well with those observed or inferred from the cumulates of the Skaergaard intrusion, but none of the predicted liquid lines of descent are consistent with the extreme iron enrichment proposed for this intrusion based on mass balance calculations. Compositional factors such as water and phosphorus are not thought to be the source of the discrepancy as the cumulates of the Basistoppen sill (which closely resemble those of Skaergaard) may be used to calculate a liquid line of descent in agreement with that predicted by the model for fractional crystallization closed to oxygen. A comparison of the predicted T-fO2 paths and liquid lines of descent with those inferred from natural systems suggests that volcanic centres such as Iceland and Hawaii evolve under conditions open to oxygen, whereas evidence from plutonic environments (e.g. Skaergaard and Kiglapait layered intrusions) suggests that they evolved under conditions more closed to oxygen. The compositional evolution of the melt phase in volcanic and plutonic systems may therefore be different, although the results of this study suggest that magnetite saturation will limit Fe enrichment in all environments to <20 wt% FeO*, consistent with enrichments reported for volcanic glasses.