The influence of mantle melt flux on the physiochemical characteristics of magma storage

Abstract:

The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of lithospheric magma storage regions located above high buoyancy flux mantle plumes (such as Kīlauea, Hawai’i and Fernandina, Galápagos) are relatively well understood. However, far fewer constraints exist on the nature of magma storage beneath ocean island volcanoes that are distal to the main zone of mantle upwelling or above low buoyancy flux plumes, despite these systems representing a substantial proportion of global ocean island volcanism. To address this, we present a detailed petrological study of erupted material from Isla Floreana in the Galápagos Archipelago, which is characterised by an extremely low flux of magma into the lithosphere from the underlying mantle plume.

In-situ major and trace element analyses of crystal phases from exhumed cumulate xenoliths, lavas and scoria deposits, indicate that magma storage beneath Floreana is dominated by crystal-rich domains (i.e. mush). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveals that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within these crystal-rich storage regions. In addition, after filtering the dataset to avoid crystal compositions modified by reactive melts, multiple petrological barometers demonstrate that the Floreana mush zones are located in the upper mantle, at ~24 ±5 km depth. Comparing this result with recent barometric estimates from other ocean island volcanoes worldwide suggests that the flux of magma from the underlying mantle source represents a first-order control on the depth magma storage beneath ocean island volcanoes.