Nd-, O-, and H-isotopic evidence for complex, closed-system fluid evolution of the peralkaline Ilı́maussaq intrusion, south Greenland

Abstract

Relatively homogeneous oxygen isotope compositions of amphibole, clinopyroxene, and olivine separates (+5.2 to +5.7‰ relative to VSMOW) and neodymium isotope compositions (ε Nd(T) = −0.9 to −1.8 for primary magmatic minerals and ε Nd(T) = −0.1 and −0.5 for mineral separates from late-stage pegmatites and hydrothermal veins) from the alkaline to agpaitic Ilı́maussaq intrusion, South Greenland, indicate a closed system evolution of this igneous complex and support a mantle derivation of the magma. In contrast to the homogeneous oxygen and neodymium isotopic data, δD values for hand-picked amphibole separates vary between −92 and −232‰ and are among the most deuterium-depleted values known from igneous amphiboles. The calculated fluid phase coexisting with these amphiboles has a homogeneous oxygen isotopic composition within the normal range of magmatic waters, but extremely heterogeneous and low D/H ratios, implying a decoupling of the oxygen- and hydrogen isotope systems. Of the several possibilities that can account for such unusually low δD values in amphiboles (e.g., late-stage hydrothermal exchange with meteoric water, extensive magmatic degassing, contamination with organic matter, and/or effects of Fe-content and pressure on amphibole-water fractionation) the most likely explanation for the range in δD values is that the amphiboles have been influenced by secondary interaction and reequilibration with D-depleted fluids obtained through late-magmatic oxidation of internally generated CH 4 and/or H 2. This interpretation is consistent with the known occurrence of abundant magmatic CH 4 in the Ilı́maussaq rocks and with previous studies on the isotopic compositions of the rocks and fluids.