| Summary: | Despite reports of significant fractionation of iron isotopes occurring under magmatic and hydrothermal conditions, the mechanisms that cause and control fractionation in magmatic systems remain poorly understood. Previous studies suggest that iron isotopes fractionate during fractional crystallization and hydrothermal alteration, but this has yet to be determined in a simple, closed igneous system. The Eocene-aged Skaergaard layered mafic intrusion, East Greenland underwent closed-system evolution, as it cooled and crystallized inward from the floor, roof, and walls, and later underwent hydrothermal alteration, making it a natural laboratory for studying the processes that control iron isotope fractionation under magmatic and hydrothermal conditions. This study focuses on the iron isotope composition of olivine, coexisting phases, and the host rocks of the Skaergaard intrusion, in addition to determining the extent and possible causes of fractionation. Presented here are new iron isotope compositions, relative to igneous rocks ([delta]56FeIgR), for bulk-mineral separates of olivine (n=9), coexisting Fe-Ti oxides (n=9), coexisting pyroxene (n=9), and bulk-rocks (n=6), obtained via multi-collector - inductively coupled plasma - mass spectrometry, for a suite of gabbros from the Layered Series of the Skaergaard intrusion. Samples were selected to represent the range of compositions and the varying extent of alteration within the Layered Series. The [delta]56Fe values range from -0.12 0.05[per mille] to +0.01 ± 0.01[per mille] (2-SE) for olivine, -0.09 ± 0.01[per mille] to +0.51 ± 0.01[per mille] for coexisting Fe-Ti oxides, -0.10 ± 0.03 to -0.01 ± 0.01[per mille] for coexisting pyroxene, and -0.02 ± 0.01[per mille] to +0.03 ± 0.03[per mille] for bulk-rocks. These values mostly fall within, but also vary from the average of mafic- to intermediate-composition crustal igneous rocks (0.00 ± 0.08[per mille]; 2-SD). Oxygen isotope compositions ([delta]18OVSMOW), obtained to investigate potential iron isotope ...
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