O¹⁸/O¹⁶ Ratios in Rocks and Coexisting Minerals of the Skaergaard Intrusion, East Greenland

Abstract

The Skaergaard intrusion of East Greenland is a gravitationally stratified gabbroic mass that has undergone extreme fractional crystallization. Oxygen-isotopic analyses have been obtained for the various rock types of this intrusion and for several coexisting minerals of these rocks. The general relationships among the O18/O16 ratios of the minerals are the same as have been found for other igneous rocks, but the isotopic fracticnations are smaller, probably as a result of the higher temperature of formation of the Skaergaard rocks. The later differentiates are progressively depleted in O18 to a marked degree relative to the earlier-formed portions of the layered series; the late-stage granophyres are 4–5 per mil lower in O18/O16 than the layered Lower Zone gabbros, and are 7–9 per mil lower than normal granitic rocks from other localities. This progressive depletion in O18 is a result of crystallization and settling out of minerals that are, on the whole, about 1 per mil higher in O18/O16 than the magma liquid. Calculations based on a simple crystallization model are in agreement with the experimental results.