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

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]Fe⁵⁶, 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]Fe⁵⁶ 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]Fe⁵⁶ 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]Fe⁵⁶ 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]Fe⁵⁶ values do not vary as much as in high-silica igneous rocks. In contrast, [delta]Fe⁵⁶ 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]Fe⁵⁶ 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]Fe⁵⁶ 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]Fe⁵⁶ values, and this difference can explain why altered and fresh bulk rocks do not differ in [delta]Fe⁵⁶ values. Compared to the average Fe isotope composition of the Earth's mantle and chondritic meteorites, the average [delta]Fe⁵⁶ value of Skaergaard bulk rocks is higher by 0.053‰ and 0.081‰, respectively. The higher [delta]Fe⁵⁶ average of Skaergaard rocks is similar to that of terrestrial basalts, which record an average [delta]Fe⁵⁶ 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]Fe⁵⁶ values between terrestrial mafic rocks and those of other planetary bodies.