Evolution of a plumbing system under the influence of variable ice load - The Pleiades volcanic complex, Antarctica

The climate-controlled variations of the glacio-lithostatic load in glaciated terrains can potentially affect the tempo of volcanic eruptions, by modifying the pressure conditions acting on the underlying plumbing system. During glacial periods, increasing ice load hinders magma eruption, thus leading to prolonged residence time in the crust. This allows the magma to crystallise, differentiate and accumulate volatiles over a longer time span with respect to non-glacial periods.

In Antarctica, volcanism in glaciated regions has been acting since the Miocene. In detail, in northern Victoria Land, volcanoes are located either in attenuated or thick cratonic lithosphere. Among the volcanic edifices built on thick crust, the quaternary Pleiades Volcanic Complex (PVC) is made up of some 20 monogenetic, partly overlapping scoria and spatter cones, that erupted over the last 900 ka. The erupted products range in composition from hawaiite to trachyte, defining a complete mildly Na-alkaline differentiation trend, which is quite unusual among alkaline monogenetic volcanic fields.

Six samples from the PVC, representative of the whole differentiation trend, have been investigated by means of electron microscopy, electron microprobe and laser ablation ICP-MS. The parageneses of the rocks includes dominant feldspar and clinopyroxene with minor olivine. The mafic phenocrysts are characterised by significant resorption textures: specifically, the olivine presents deep embayments with absence of compositional zoning, while clinopyroxene frequently shows spongy texture and compositionally zoned mantle and rims, often with patchy and convoluted patterns.

Petrography, textures and mineral chemistry suggest that the magma experienced first a rapid decompression, followed by a prolonged residence time, likely supported by increased ice load. During this time interval, resorption of the early formed mineral phases occurred, probably also enhanced by crustal assimilation processes, coupled with re-crystallization under isobaric conditions. Moreover, the prolonged residence time, coupled with the occurrence of (multiple) magma recharge(s) caused the mixing of a basaltic magma with other differentiated magmas stalling in the plumbing system, yielding to the formation of intermediate magma compositions. Finally, magma refilling of the plumbing system during an ice loss period, favoured the eruption. Machine-learning based thermo-barometric estimates consistently indicate crystallization of clinopyroxene at transcrustal pressures, ranging from the crust-mantle interface (early crystallization) to shallow crust (late crystallization in a shallow plumbing system under glacial load).