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

ECU Author/Contributor (non-ECU co-authors, if there are any, appear on document)

Brett Anthony Pertunen (Creator)

Institution

East Carolina University (ECU )

Web Site: http://www.ecu.edu/lib/

Abstract: 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.

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Publication

Thesis

Language: English

Date: 2023

Subjects

Iron Isotopes;Layered Mafic Intrusion

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