The iron isotope composition of pyroxene from the Skaergaard layered mafic intrusion , East Greenland

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 pyroxene [delta]56Fe values then start to generally decline slightly with evolution in the Middle Zone. Fractionation factors between Fe-Ti oxides and pyroxene were calculated and compared with theoretical Fe isotope fractionation factors to determine equilibration temperatures and their significance. Calculated fractionation factors between Fe-Ti oxides and pyroxene range from -0.08 [per mille] to 0.58 [per mille]. The ratio of magnetite:ilmenite was found to strongly control the calculations of the temperature of iron-isotope equilibration between Fe-Ti oxides and pyroxene. When magnetite:ilmenite ratios representative of each rock and various pyroxene varieties were used to calculate weighted theoretical b factors for the oxides , some Fe-Ti oxide - pyroxene pairs yield low temperatures (~200 °C - ~900 °C) , whereas others yield temperatures near the temperature of coexisting plagioclase crystallization (~1030 °C - ~1140 °C). Others yield calculated temperatures of equilibrium iron isotope fractionation >1300 °C. Anomalously low or high temperatures of fractionation calculated between Fe-Ti oxides and pyroxene pairs were found to be most likely the result of a lower magnetite:ilmenite ratio than estimated , and changing that ratio to include more ilmenite was shown to lower the calculated temperature of equilibrium iron isotope fractionation. Iron isotope fractionation factors between Fe-Ti oxides and coexisting pyroxene from Skaergaard are small , are affected by magnetite:ilmenite ratios and the composition of pyroxene , and reflect high-temperature magmatic fractionation