EXPERIMENTAL-STUDY OF LIQUID EVOLUTION IN AN FE-RICH, LAYERED MAFIC INTRUSION - CONSTRAINTS OF FE-TI OXIDE PRECIPITATION ON THE T-FO2 AND T-RHO PATHS OF THOLEIITIC MAGMAS

The Newark Island layered intrusion, a composite intrusion displaying a similar fractionation sequence to the Skaergaard, has both dikes which preserved liquids fed into the intrusion and chilled pillows of liquids resident in the chamber. This study reports experimentally determined one atmosphere liquid lines of descent of these compositions as a function of oxygen fugacity which varies from QFM (quartz-fayalite magnetite) to 0.5 log10 units above IW (iron wustite). These experiments reveal a strong oxygen fugacity dependence on the order of appearance and relative abundances of the Fe-Ti oxide minerals. Titanomagnetite saturates prior to ilmenite at QFM, but the order is reversed at lower oxygen fugacities. In the layered series of the Newark Island intrusion, ilmenite arrives shortly before titanomagnetite and the titanomagnetite/ilmenite ratio decreases monotonically after the cumulus appearance of titanomagnetite. Comparison of the crystallization sequence in the intrusion with that of the experiments requires that the oxygen fugacity in the intrusion increased relative to QFM before titanomagnetite saturation and decreased afterward, but always remained between the QFM and IW buffers. Similar trends in the modes of the Fe-Ti oxides (ilmenite and titanomagnetite) in the Skaergaard, Kiglapait, and Somerset Dam intrusions along with Fe2O3/FeO ratios in MORBs suggest that such a temperature-oxygen fugacity path may be typical of tholeiitic magma differentiation. Calculations of the temperature-density paths of the experimental liquids indicate that, at all possible oxygen fugacities, the density must have decreased abruptly after Fe-Ti oxide saturation. Accordingly, liquids replenishing the intrusion after Fe-Ti oxide saturation should pond at the bottom of the chamber, quenching against older cumulates. Field observations at the Newark Island intrusion confirm this prediction. The similarities in the fractionation paths of several other layered intrusions to that of the Newark Island intrusion suggest that the density of the liquids in these intrusions also decreased after Fe-Ti oxide saturation. Experiments on a suggested initial Skaergaard liquid are consistent with this model.