NON-TRADITIONAL STABLE ISOTOPE FRACTIONATION IN THE SKAERGAARD INTRUSION

The early Eocene Skaergaard intrusion, central east Greenland, is one of the most thoroughly studied layered mafic intrusions on Earth and an exceptional example of (near) closed-system magmatic differentiation. As such, it is ideally suited to test models of closed-system fractional crystallization on non-traditional stable isotope fractionation. Here we report iron and silicon isotope compositions of gabbros of the layered and upper border series, melanogranophyre and pegmatite associated with these gabbroic rocks, and rocks associated with the sandwich horizon thought to represent the product of extreme differentiation and/or liquid immiscibility. Whole rock samples from well-constrained stratigraphic levels in the intrusion were crushed, powdered and dissolved, followed by element purification by ion exchange chromatography and analysis by high-resolution MC-ICPMS using sample-standard bracketing. The δ⁵⁶Fe values for Skaergaard gabbros range from a low of -0.019 per mil to a high of 0.253 per mil with an external precision of ±0.05 per mil (2σ) or better. This range in δ⁵⁶Fe spans that reported for terrestrial igneous rocks. δ⁵⁶Fe varies systematically with stratigraphic position with the heaviest Fe isotope compositions found in upper zone gabbros and associated melanogranophyre. δ⁵⁶Fe for magnetite and ilmenite separates range 0.265 to 0.004 and 0.097 to -0.045, respectively, straddling bulk rock compositions, while melanogranophyres and pegmatites are consistently isotopically heavier then their host gabbro. Forward modeling of closed system fractional crystallization constrained by cumulate volumes, whole rock and mineral compositions, mineral modes and independent constraints on Fe isotope fractionation (factors) based on ferric/ferrous ratios of magma and cumulates can account for the stratigraphic relations, except during the final stages of differentiation. We postulate that the very large increase in δ⁵⁶Fe in the upper zone of Skaergaard intrusion is due to changes in the structural role of Fe⁺² in very iron-rich and reduced silicate magma during the terminal stages of crystallization. Complementary data for Si isotopes will also be reported.