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

• Main Author: Hammerstrom, Alex Joseph
• Other Authors: Heimann, Adriana, Geological Sciences
• Format: Master Thesis
• Language: English
• Published: East Carolina University 2019
• Subjects: Fe isotopes; Magnetite and ilmenite; MC-ICP-MS; Gabbros; Layered mafic intrusion; Igneous rocks > Greenland; SkærgaÌŠrden Channel (Greenland); Analytical geochemistry > Greenland; Greenland
• Online Access: http://hdl.handle.net/10342/7487

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��, 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�� 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�� 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�� 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�� values do not vary as much as in high-silica igneous rocks. In contrast, [delta]Fe�� 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�� 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�� variations in Fe-Ti oxides and previously modeled fO2 values through the Layered Series seem to suggest that Fe isotope ...