PERSISTENT MAGMA STORAGE IN THE MANTLE ACROSS 2.5 MYRS OF OCEAN-ISLAND VOLCANISM

Traditional models of ocean island evolution, largely based on observations from Hawai’i, link variations in magma chemistry, the rate of volcanic activity, and the depth of magma storage to the distance from the upwelling plume stem. However, Hawai’i represents an outlier in ocean island volcanism due to the high buoyancy flux and large distance from any plate boundary. Most mantle plumes display clear interaction with nearby plate boundaries, influencing the dynamics of solid plume material in the upper mantle and the distribution of melt across regions of active volcanism. Yet, the influence plume-ridge interaction, and the tectonic setting of mantle plumes, on the structure, characteristics, and evolution of magma storage beneath ocean island volcanoes remains poorly understood.

To examine the role of plume-ridge interaction, and the tectonic setting of near-ridge mantle plumes, we consider the evolution of magmatic systems in the Galápagos Archipelago, a region of mantle plume volcanism located 200-250 km south of the Galápagos Spreading Centre (GSC). Geochemical analysis of gabbro xenoliths from Isla Floreana in the south-eastern Galapagos reveal that they formed at ~2.5 Ma, when the island was located close to the centre of plume upwelling. The petrological record contained within the gabbro xenoliths thus provide insights into the magmatic evolution of the Isla Floreana as the Nazca plate carried it eastwards, away from the plume centre.

Mineral thermobarometry, thermodynamic modelling, and CO₂ fluid inclusion densities in the gabbroic xenoliths reveal that Isla Floreana’s plume-proximal stage of volcanic activity was characterized by the presence of high-pressure magma storage (> 25 km), well below the base of the crust. In fact, we find no petrological evidence that sustained, crustal-level magma storage ever occurred beneath Isla Floreana. Our results contrast with the characteristics of volcanic systems in the western Galápagos (above the current centre of plume upwelling) where mid-crust magma storage has been identified. We propose that the change in magmatic architecture is controlled by the increase in plume-ridge distance, from ~100 km at 2.5 Ma, with greater plume-ridge distances resulting in less diversion of plume-material to the GSC and thus a greater magma flux into the volcanic systems directly above the plume stem.