IRON ISOTOPE SYSTEMATICS OF THE SKAERGAARD LAYERED MAFIC INTRUSION, GREENLAND: MINERALOGIC CONSTRAINTS ON BULK-ROCK FE ISOTOPE COMPOSITIONS

To better understand the effects that magmatic differentiation has on Fe isotope fractionation in a tholeiitic intrusive magmatic system, a systematic high-precision Fe isotope study was conducted for bulk-rock gabbros and ferrodiorites and bulk-mineral separates that encompass the magmatic history of the Skaergaard layered mafic intrusion. Measured bulk-rock δ⁵⁶Fe_{Ig rocks} values exhibit little variation (-0.04±0.01‰ to +0.03±0.03‰; 2-SE), remain within the average Fe isotope composition of the mafic-intermediate Earth’s crust (0.00±0.08‰; 2-SE), and are similar to those of ferrous Fe-bearing silicates (Ol = -0.12±0.05‰ to +0.01±0.01‰; Px = -0.10±0.03‰ to -0.01±0.01‰). In contrast, the Fe isotope composition of bulk Fe-Ti oxide concentrates (Mt and Ilm) vary significantly (-0.18±0.02‰ to +0.51±0.02‰).

Iron isotope mass-balance modeling and Fe contents of minerals were used to assess the influence of the relative abundance and δ⁵⁶Fe values of Fe-bearing silicates and Fe-Ti oxides on bulk-rock δ⁵⁶Fe values throughout the intrusion. Model results show that Fe-Ti oxides exert the greatest influence on bulk-rock δ⁵⁶Fe values in Middle Zone gabbros where they are cumulus (12%-53%) and contribute the most Fe. In Lower Zone a and b and the Upper Border Series, magnetite and ilmenite occur primarily as interstitial phases (2-12%). Based on the mass balance, the higher abundance (< 95%) of Fe silicates relative to Fe-Ti oxides in these layers cause the bulk-rock Fe isotope composition to be close to the δ⁵⁶Fe values of olivine and pyroxene. In the Upper Zone, even though Fe-Ti oxides are more abundant (5-20%) and mostly present as cumulus phases, Fe-rich olivine and pyroxene contribute more Fe, which causes modeled bulk-rock δ⁵⁶Fe values to be less affected by Fe-Ti oxides than in the Middle Zone but more than in the Lower Zone. The higher abundance of low-δ⁵⁶Fe Fe²⁺-rich silicates and ilmenite over high-δ⁵⁶Fe Fe³⁺-bearing magnetite, and the balance between the two, explain the narrow variation in bulk-rock δ⁵⁶Fe values throughout the intrusion.