RE-EVALUATION OF FE-TI OXIDE RELATIONSHIPS IN THE SKAERGAARD INTRUSION AND IMPLICATIONS FOR THE INTERPRETATION OF BULK-MINERAL AND IN-SITU FE ISOTOPE VARIATIONS

Recognizing the relationships among Fe-Ti oxide phases and the various scales of exsolutions in layered intrusions, as well as variations in mineral chemistry, is fundamental for interpreting Fe isotope compositions measured in bulk oxide mineral separates and by in-situanalysis. We present a summary of the different types of exsolutions and textural relationships among Fe-Ti oxides in the Skaergaard intrusion observed through detailed petrographic, SEM-EDS, and EMP analysis. They emphasize the need for specific sample preparation procedures defined for Fe isotope analysis of these minerals and for the particular goals of such study.

Ilmenite, magnetite, and ulvospinel occur in the following main types of associations: 1) isolated ilmenite crystals; 2) small aggregates of individual, euhedral titanomagnetite and ilmenite in mutual contact; 3) ilmenite inclusions (~<2 mm) within titanomagnetite; 4) magnetite inclusions (25 μm) with ilmenite exsolutions within ilmenite; 5) dendritic and symplectitic ilmenite (50 μm – 5 mm) next to magnetite; 6) isolated ilmenite with very thin exsolutions (1 μm) of magnetite and ulvospinel; 7) triangular-patterned exsolutions (~20-30 μm wide) of ilmenite with regular or irregular outlines within titanomagnetite; 8) irregular, aligned, semi-ellipsoidal exsolutions of ilmenite (~5-25 μm long x 3 μm wide) superimposed on 7) and 9); and 9) very fine-scale (0.1 μm), grid-like exsolutions of ilmenite and ulvospinel within titanomagnetite, visible under the SEM at 24,000x magnification.

Exsolutions of ilmenite and ulvospinel occur at various scales and superimposed on different generations of titaniferous magnetite and ilmenite. The diverse types of exsolutions are the result of sub-solidus reactions and, likely, late-stage hydrothermal alteration. They represent spatial variations in Fe redox state and fO₂ conditions within and among individual crystals of magnetite and ilmenite that, along with trace-element variations, can produce Fe isotope heterogeneities in magnetite and ilmenite throughout the Skaergaard intrusion and similar layered intrusions. In-situ and bulk powder Fe isotope studies of magnetite and ilmenite may require considering the scale of exsolutions and the relative amounts of ferric and ferrous Fe in the analyzed domains.