IRON ISOTOPE GEOCHEMISTRY OF BULK ROCKS AND Fe-Ti OXIDES FROM THE SKAERGAARD LAYERED MAFIC INTRUSION , SE GREENLAND

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

Alex Joseph Hammerstrom (Creator)

Institution

East Carolina University (ECU )

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

Abstract: Small , but significant fractionation of Fe isotopes occurs in high-temperature igneous rocks. Despite increasing study , the exact causes of this fractionation are still debated. Therefore , a systematic study of a complete igneous intrusive system is presented to evaluate the extent and causes of Fe isotope fractionation. The Eocene Skaergaard layered mafic intrusion , SE Greenland has a relatively simple magmatic history and localized superimposed alteration , which makes it a good setting to study Fe isotope fractionation. High-precision Fe isotope compositions ([delta]Feµ¶ , relative to igneous rocks , in €°) are presented for bulk rocks and bulk Fe-Ti oxides (magnetite-ilmenite) from a suite of 26 gabbros and ferrodiorites that encompasses the magmatic/alteration history of the Skaergaard intrusion. The [delta]Feµ¶ values of bulk rocks differ only by 0.074€° (-0.043±0.013€° to +0.031±0.027; avg. = -0.007±0.012€° , 2-SE; n = 11) and show a lack of or a slight variation with magmatic evolution as measured by plagioclase compositions and bulk-rock geochemistry. These bulk-rock [delta]Feµ¶ values remain within the 2-SE of the average mafic-intermediate composition of the Earth's crust (0.00±0.08€° ). The lack of clear measurable trends in [delta]Feµ¶ values of Skaergaard bulk rocks as a function of magmatic evolution outside of analytical error is consistent with data from other mafic and ultramafic intrusions in that [delta]Feµ¶ values do not vary as much as in high-silica igneous rocks. In contrast , [delta]Feµ¶ values of Fe-Ti oxides throughout the evolution vary significantly , up to 0.69€° (-0.179±0.010€° to +0.508±0.012€° , 2-SE; n = 26). A positive correlation between [delta]Feµ¶ values and Fe3+:Fe2+ ratios for bulk Fe-Ti oxides shows that their Fe isotope compositions are controlled by their Fe3+:Fe2+ ratio. Similar trends of [delta]Feµ¶ variations in Fe-Ti oxides and previously modeled fO2 values through the Layered Series seem to suggest that Fe isotope fractionation in Fe-Ti oxides may be driven by redox changes of the magma , but this is not seen in bulk rocks. The removal of iron via partial or complete magnetite dissolution by hydrothermal fluids may account for Fe isotope variations in bulk Fe-Ti oxides from highly altered rocks. Secondary minerals , not analyzed but replacing altered Fe-Ti oxides , likely have the light Fe isotopes removed from magnetite that results in high [delta]Feµ¶ values , and this difference can explain why altered and fresh bulk rocks do not differ in [delta]Feµ¶ values. Compared to the average Fe isotope composition of the Earth's mantle and chondritic meteorites , the average [delta]Feµ¶ value of Skaergaard bulk rocks is higher by 0.053€° and 0.081€° , respectively. The higher [delta]Feµ¶ average of Skaergaard rocks is similar to that of terrestrial basalts , which record an average [delta]Feµ¶ value (+0.006±0.007€° ) ~ 0.07€° higher than mantle values. Further detailed Fe isotope studies of large mafic igneous intrusions will help better understand the mechanisms that produce the observed differences in [delta]Feµ¶ values between terrestrial mafic rocks and those of other planetary bodies.

Additional Information

Publication

Thesis

Language: English

Date: 2019

Keywords

Fe isotopes, Magnetite and ilmenite, MC-ICP-MS, Gabbros, Layered mafic intrusion

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