Textures and chemistry of crystal cargo of The Pleiades Volcanic Field, Antarctica: potential influence of ice load in modulating the plumbing system

Abstract

The Pleiades Volcanic Field (PVF) of northern Victoria Land (Antarctica) is made up of a dozen scoria cones whose erupted products present an unusually complete evolutionary trend from alkali-basalt to trachyte. With the aim of reconstructing the evolution of the PVF plumbing system, we have investigated the petrography and chemistry of main mineral phases using SEM-EDS coupled with major (EPMA-WDS) element analyses. We further focussed on clinopyroxene phenocrysts obtaining a more detailed characterisation by means of trace (LA-ICP-MS) element analyses coupled with machine learning thermobarometry. The results indicate that fractional crystallisation and magma mixing are the major processes determining the development of the complete evolution trend. While fractional crystallisation is a persistently active process in all parts of the plumbing system, mixing among differently evolved magma batches pertaining to the same association is responsible for the formation of intermediate compositions in the differentiation lineage at a specific pressure range (0.4-0.5 GPa). These processes are compatible with significant residence time of magmas at depth, resulting in multiple episodes of magma mixing, as testified by resorption and overgrowth textures in phenocryst assemblage occurring under isobaric conditions. The prolonged residence time likely increased the efficiency of the mixing process, leading to the formation of magmas with intermediate composition. In turn, the build-up of volatiles during the magma differentiation at depth could have favoured the eruption of these (variably differentiated) magmas. Considering that the PVF is situated in a glacial region, a process forcing long magma residence time can be envisaged associated with increased ice loading during glacial stages. This study specifically considers the ice fluctuations in the last 100 ka, theorising the possibility of a climate-controlled volcano plumbing system.