Relatively homogeneous oxygen isotope compositions of amphibole,
clinopyroxene, and olivine separates (+5.2 to +5.7parts per thousand
relative to VSMOW) and neodymium isotope compositions (epsilon(Nd(T)) =
-0.9 to -1.8 for primary magmatic minerals and epsilon(Nd(T)) = -0.1 and
-0.5 for mineral separates from late-stage pegmatites and hydrothermal
veins) from the alkaline to agpaitic llimaussaq 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,
deltaD values for hand-picked amphibole separates vary between -92 and
-232parts per thousand 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
deltaD 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 deltaD
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 Ilimaussaq rocks and with previous studies on the
isotopic compositions of the rocks and fluids. Copyright (C) 2004
Elsevier Ltd.