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

Large iron-isotope variations have been reported for magnetite (~1.1‰; -0.33 to +0.77‰; δ⁵⁶Fe_IgRocks) 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.