Silicate Liquid Immiscibility within the Crystal Mush: Evidence from Ti in Plagioclase from the Skaergaard Intrusion

Abstract

A key target in the study of layered intrusions is to constrain the liquid line of descent of the magma. However, the evolution of the interstitial liquid is rarely considered, and its liquid line of descent is often assumed to be equivalent to that of the bulk magma. Because of extensive sub-solidus and diffusional changes that occur in slowly cooled rocks, clues to the composition of the interstitial liquid can only be obtained using very slowly diffusing trace elements and components. This study uses the Ti concentrations and anorthite contents of interstitial plagioclase to consider the compositional evolution of the interstitial liquid in the Skaergaard Intrusion. Ti-XAn zoning of interstitial plagioclase does not follow the same cryptic variations that develop in plagioclase primocrysts as a function of stratigraphic height, demonstrating that the bulk and interstitial liquid lines of descent are not equivalent. After Fe-Ti oxides start to crystallize, Ti concentrations decrease in both primocryst and interstitial plagioclase as a result of decreasing melt Ti. However, in the interstitial plagioclase within a single thin section, divergent trends develop adjacent to fine-grained interstitial pockets containing diverse mineral assemblages, which are interpreted to represent the crystallized products of late-stage immiscible liquids. These trends vary systematically as a function of stratigraphic height and spatial location within the intrusion. The distribution and compositions of these plagioclase zoning trends are used to comment on the spatial distribution and differential movement of interstitial immiscible liquids within the intrusion. © The Author 2010. Published by Oxford University Press. All rights reserved.