| Summary: | Iron isotopes show small fractionations in igneous rocks emplaced at high temperature, but the processes by which these fractionations originate are still debated. The Skaergaard layered mafic intrusion of East Greenland is a good system to study how iron stable isotopes may fractionate in igneous systems because it formed from a single pulse of tholeiitic magma. High-precision (0.023 [per mille], 2-SE) iron isotope ratios were obtained for pyroxene from fresh and altered gabbros and ferrodiorites representative of the Skaergaard intrusion. The [delta]56Fe values of pyroxene vary from -0.102 [per mille] 0.025 [per mille] to +0.114 [per mille] 0.018 [per mille] (2-SE, relative to igneous rocks), with an overall range of 0.216 [per mille] (n = 13). Poikilitic pyroxene from gabbros of the Lower Zone a and the Upper Border Series g have the lowest [delta]56Fe values, whereas mosaic pyroxene in an altered ferrodiorite of the Upper Border Series [gamma] has the highest [delta]56Fe value. The possible effect of hydrothermal alteration on the iron isotope compositions was investigated for this pyroxene, but it was determined that the pyroxene contains abundant inclusions of magnetite, which increase its [delta]56Fe value, and that hydrothermal alteration did not fractionate iron isotopes in pyroxenes from Skaergaard. The iron isotope composition of pyroxene was investigated in relation to the fractional crystallization of coexisting Fe-Ti oxides magnetite and ilmenite. In the Lower Zone, the [delta]56Fe values of pyroxene systematically increase with magma evolution from the Lower Zone a (-0.102 [plus or equal to] 0.025 [per mille]) to the Lower Zone c (-0.005 [plus or equal to] 0.012 [per mille]). Ferric iron is increasingly incorporated into pyroxene with magmatic evolution in the Lower Zone, allowing for incorporation of more heavy isotopes that result in the increase in [delta]56Fe values. When magnetite saturation occurs at the top of the Lower Zone c, Fe3+ becomes preferentially partitioned into magnetite, and ...
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