The Significance of the Mesostasis of Basic Layered Igneous Rocks

Abstract

Many layered igneous rocks consist of two main parts: the cumulus (plus adcumulus and heteradcumulus) material and the pore material, or mesostasis, produced by the crystallization of the trapped liquid. Knowledge of the chemistry of the mesostasis will be useful in deducing magma fractionation trends and in the determination of solid/liquid trace element partition coefficients, which in turn will be significant in helping to elucidate the petrogenetic relationship between certain basalts and layered gabbros. A new geochemical method for the determination of both the composition and amount of mesostasis in layered rocks is proposed. This method uses elements of contrasting behaviour and is best applied to rhythmically layered rocks rich in mafic or felsic minerals. Data on strontium, cobalt, and uranium are used to apply the method to two rocks from the Skaergaard intrusion. The results obtained are in agreement with published data.