Major Element Variations in the Evolving Skaergaard Magma Chamber : A Computer Simulation to Test the Sequential Extraction Model

Abstract

Chemical evolution within basaltic magma chambers is still the subject of intense debate. The Skaergaard Intrusion in southeast Greenland has been the subject of considerable study for nearly a century, and yet today no consensus exists to explain several first-order observations of the crystallized magma body. Recently, a promising new model, the Sequential Extraction Model (SEM), was proposed to explain the many enigmatic features observed in the Skaergaard intrusion. The SEM invokes the process of liquid immiscibility inside segregated boundary layers that form at the bottom of the evolving magma chamber. Two resultant liquids with dramatically different densities physically separate from one another. Here, we test the SEM using a computer simulation that follows the crystallization sequence within the cooling magma body. To guide chemical evolution of the magma within the code, we use the observed modal abundances and chemical compositions of the main primocryst phases (olivine, pyroxene, and feldspar) measured within the intrusion. Our code allows us to set constraints on such parameters as the amount of buoyant liquid that accumulates above layered mafic intrusions and the relative amount of this material that is returned to the magma chamber upon ascent.