A new method to quantify the real supply of mafic components to a hybrid andesite.

Abstract

We present a new method for quantifying the relative proportions of groundmass plagioclase derived from mafic and andesitic magma, based on analysis of back-scattered electron images of the groundmass.Preliminary results indicate that approximately 16 % of all groundmass plagioclase belongs genetically to the mafic inclusions. Together with the crystal clusters, disequilibrium phenocryst textures and mm-scale inclusions, there is a ‘cryptic’ mafic component in the andesite of approximately 6 % by volume. This is significant compared with the proportion of macroscopic mafic inclusions (typically* 1–5 %). The new method has the potential to allow tracking of the mafic fraction through time and thus to yield further insights into magma hybridisation.