SEQUENTIAL EXTRACTION OF IMMISCIBLE LIQUIDS FROM THE SKAERGAARD CRYSTAL PILE: THE KEY TO UNDERSTANDING BASALT EVOLUTION AND THE ORIGIN OF GRANITE

Tholeiitic basalt is Earth’s most abundant magma type. Generated by partial melting of upwelling mantle material, tholeiite forms oceanic crust at spreading centers, ocean islands at hotspots, and massive flood basalts on continents. Surprisingly, a satisfying framework that accounts for the chemical evolution of crystallizing tholeiitic magma has not found consensus among petrologists. The tholeiitic Skaergaard intrusion of eastern Greenland, the most thoroughly studied igneous body on our planet, is the metric for testing theories of magmatic evolution. Here we review the important physical and chemical observations of the intrusion that have stimulated nearly a century of vigorous debate regarding its chemical evolution. We present a new model of tholeiite evolution that invokes the sequential extraction of late-stage immiscible liquids from successive isolated mushy boundary layers that develop beneath a well-mixed convecting magma chamber. The model builds on several recent studies of the Skaergaard that document the prevalence of late-stage conjugate immiscible liquids throughout the intrusion. We propose the volume of interstitial liquid that experienced immiscibility and subsequent vertical transport has been vastly underestimated to date. Our model explains the enigmatic abundances of both major and trace elements observed throughout the intrusion and reconciles several long-standing contentious issues that have kept the Skaergaard at the forefront of petrologic study, including the cause of extreme Fe-enrichment that distinguishes Fenner’s enigmatic tholeiite trend from Bowen’s iconic calc-alkaline trend. The model also offers a plausible mechanism for generating the large accumulations of homogeneous granite and rhyolite observed above nearly every large layered mafic intrusion and at active continental hotspots, and thus provides a new framework for understanding the origin of these felsic liquids.