| Summary: | Fractionation between Fe isotopes in igneous systems and hydrothermally-altered rocks is small but significant, and detailed petrographic understanding of rocks is needed to discern the origin of such fractionations. One hypothesis proposed to explain the causes of Fe isotope fractionation in igneous rocks involves hydrothermal alteration. Movement of hydrothermal fluids through the system could result in removal of light Fe isotopes, leaving a heavier Fe isotope composition in the rocks. Previous oxygen isotope ([delta]18O) analyses of bulk-rocks, pyroxene, and plagioclase indicate that the Skaergaard intrusion hosted its own hydrothermal system. To provide petrographic context to interpret bulk-rock, bulk-mineral, and in-situ Fe isotope compositions (of Fe-Ti oxides), this study characterizes hydrothermal alteration via oxygen isotope geochemistry of rocks and minerals and the nature of fine-scale exsolution relationships among Fe-Ti oxides in the Skaergaard intrusion. Petrographic, SEM-EDS, TEM, and XRD analyses show exsolutions of ilmenite, hematite, and spinel of different sizes and shapes within Ti-rich magnetite and ilmenite. Characterization of the textural relationships show the following associations: 1) discrete magnetite and ilmenite crystals (0.2-1 mm) in mutual contact, in which ilmenite exsolved from original titanomagnetite or crystalized separately\; 2) very thin (1 [micro]m) exsolutions of hematite within ilmenite\; 3) coarse (~50 [micro]m) triangular exsolutions of ilmenite within magnetite\; 4) very-fine scale (0.1[micro]m), box-like exsolutions of Fe-Ti oxide within titanomagnetite visible only via SEM at 24,000x magnification\; 5) very fine-grained (0.5-5 [micro]m) exsolutions of a transparent mineral phase. TEM analysis indicates that the very fine, box-type exsolutions within magnetite are secondary ilmenite containing small amounts of Mg, Al, and Mn, whereas thicker, triangular lamellae are early ilmenite close to pure Ti-Fe oxide in composition. Very fine-grained transparent phases ...
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