Differentiation of Ferro-Basaltic Magmas under Conditions Open and Closed to Oxygen: Implications for the Skaergaard Intrusion and Other Natural Systems |
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| Authors: | TOPLIS M J; CARROLL M R |
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| Institution: | DEPARTMENT OF GEOLOGY, UNIVERSITY OF BRISTOL BRISTOL BS8 1RJ, UK |
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| Abstract: | Because processes such as fractional crystallization and crystallizationunder conditions closed to oxygen are difficult to simulatein the laboratory there is a need for quantitative models ofmagma crystallization behaviour. Comparison of experimentaldata on an iron-rich basaltic composition with predictions ofthe MELTS free energy minimization algorithm shows that althoughliquidus temperatures and silicate mineral equilibria are predictedrelatively well, the saturation of Fe–Ti oxides is notWe have used the same experimental data to construct an alternativecrystallization model based on known equilibrium phase relations,mineral–melt partitioning of major elements, and massbalance constraints. The model is used to explore the consequencesof equilibrium and fractional crystallization in systems openand closed to oxygen. Liquid lines of descent for perfect equilibriumand perfect fractional crystallization are predicted to be verysimilar. In a system open to oxygen the model predicts thatmagnetite saturation leads to strongly decreasing iron and increasingsilica contents of residual liquids, whereas systems closedto oxygen crystallize less abundant magnetite, leading to aless pronounced iron depletion in the liquid. Predicted bulksolid compositions and variations of fo2, with falling temperatureagree well with those observed or inferred from the cumulatesof the Skaergaard intrusion, but none of the predicted liquidlines of descent are consistent with the extreme iron enrichmentproposed for this intrusion based on mass balance calculations.Compositional factors such as water and phosphorus are not thoughtto be the source of the discrepancy as the cumulates of theBasistoppen sill (which closely resemble those of Skaergaard)may be used to calculate a liquid line of descent in agreementwith that predicted by the model for fractional crystallizationclosed to oxygen. A comparison of the predicted T-fo2, pathsand liquid lines of descent with those inferred from naturalsystems suggests that volcanic centres such as Iceland and Hawaiievolve under conditions open to oxygen, whereas evidence fromplutonic environments (e.g. Skaergaard and Kiglapait layeredintrusions) suggests that they evolved under conditions moreclosed to oxygen. The compositional evolution of the melt phasein volcanic and plutonic systems may therefore be different,although the results of this study suggest that magnetite saturationwill limit Fe enrichment in all environments to <20wt% FeO*,consistent with enrichments reported for volcanic glasses. KEY WORDS: Skaergaard; ferro-basalt; iron enrichment; oxygen fugacity
*Bayerisches Geoinstitut, Universitt Bayreuth, D-95440 Bayreuth, Germany. Telephone: $ 49-921-553718. Fax $ 49-921-553769. e-mail: mike.toplis{at}uni-bayreuth.de |
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