GSA 2020 Connects Online

Paper No. 28-3
Presentation Time: 6:00 PM

CONTROLS ON IRON ISOTOPE VARIATIONS OF COMPLEX FE-TI OXIDES AT HIGH-TEMPERATURE: EFFECT OF FE3+/FE2+ RATIOS AT THE SKAERGAARD LAYERED-MAFIC INTRUSION, GREENLAND


HEIMANN, Adriana1, HAMMERSTROM, Alexander2, CUMMINGS, Tiffany1, MURRIE, Allison3 and PERTUNEN, Brett1, (1)East Carolina University, Geological Sciences, 101 Graham Building, Greenville, NC 27858, (2)Geosciences, University of Massachusetts, Amherst, 627 North Pleasant Street, 233 Morrill Science Center, Amhers, MA 01060, (3)Geological Sciences, University of North Carolina, Chapel Hill, 104 South Road, Campus Box #3315, Chapel Hill, NC 27599

Large iron-isotope variations have been reported for magnetite (~1.1‰; -0.33 to +0.77‰; δ56FeIgRocks) and ilmenite (~0.97‰; -0.86 to +0.13‰) in high-temperature mafic/ultramafic intrusive igneous rocks. These Fe isotope variations recorded in magnetite and ilmenite grains, within individual crystals (in-situ), among crystals, and between rocks contrast with d56Fe values of their high-temperature host bulk rocks and Fe silicates. Explanations for Fe isotope variability in and among these minerals include isotope fractionation between magnetite, ilmenite, and Fe silicates during fractional crystallization, sub-solidus re-equilibration and cation exchange between magnetite and ilmenite, and hydrothermal alteration.

Titanomagnetite with complex, fine-scale (<0.1 mm - mm) ubiquitous exsolutions of ilmenite and Fe-bearing spinel, and isolated ilmenite, occur in gabbros and ferrodiorites of the Skaergaard layered-mafic intrusion, Greenland. Bulk titanomagnetite and ilmenite separates analyzed via high-precision MC-ICP-MS show δ56Fe variability from -0.18 ± 0.01‰ to +0.51 ± 0.01‰ (2-SE). Relationships among the Fe-Ti oxides, distribution, and abundance in each rock throughout Skaergaard aided in the interpretation of Fe isotope ratios of Fe-Ti oxide separates. Calculated titanomagnetite/ilmenite ratios using image analysis vary within and among rocks and within grains. Ilmenite-rich concentrates (Ilm > Mt) have the lowest δ56Fe values, whereas those with the highest titanomagnetite/ilmenite ratios yield the highest δ56Fe values. Calculated Fe3+/Fe2+ ratios using the abundance of titanomagnetite, ilmenite, and spinel in analyzed oxide concentrates correlate positively with measured δ56Fe values. High-δ56Fe values correspond with high Fe3+/Fe2+ ratios, whereas low-δ56Fe values were measured in oxides with low Fe3+/Fe2+ ratios. Titanomagnetite grains with heterogeneous exsolutions of ilmenite and spinel create low-δ56Fe and high-δ56Fe domains that, when separated, determine the δ56Fe values of bulk-mineral separates and can produce artificial δ56Fe variability. Variations in δ56Fe values of bulk titanomagnetite-ilmenite separates at Skaergaard are controlled by their Fe3+/Fe2+ ratios, consistent with results from theoretical predictions and experimental studies.