Abstract
Abstract The physicochemical characteristics of sub-volcanic magma storage regions have important implications for magma system dynamics and pre-eruptive behaviour. The architecture of magma storage regions located directly 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 ocean island volcanism globally. To address this, we present a detailed petrological study of Isla Floreana in the Galápagos Archipelago, which lies at the periphery of the upwelling mantle plume and is thus characterized by an extremely low flux of magma into the lithosphere. Detailed in situ major and trace element analyses of crystal phases within exhumed cumulate xenoliths, lavas and scoria deposits indicate that the erupted crystal cargo is dominated by disaggregated crystal-rich material (i.e. mush or wall rock). Trace element disequilibria between cumulus phases and erupted melts, as well as trace element zoning within the xenolithic clinopyroxenes, reveal that reactive porous flow (previously identified beneath mid-ocean ridges) is an important process of melt transport within crystal-rich magma storage regions. In addition, application of three petrological barometers reveals that the Floreana mush zones are located in the upper mantle, at a depth of 23·7 ± 5·1 km. Our barometric results are compared with recent studies of high melt flux volcanoes in the western Galápagos, and other ocean island volcanoes worldwide, and demonstrate that the flux of magma from the underlying mantle source represents a first-order control on the depth and physical characteristics of magma storage.
Highlights
While a substantial number of hotspot-related volcanic systems that have been active during the Holocene are located in regions characterised by a relatively low flux of magma into the lithosphere, only a small number of eruptions have been observed at these systems since the advent of modern volcano monitoring of August 2020)
Comparison of olivine and clinopyroxene major, minor and trace element contents between our different sample types reveal that a substantial portion of the erupted crystal cargo is derived from mush-dominated magma storage regions beneath Floreana
Highly enriched clinopyroxene trace element signatures and trace element zoning in the xenoliths reveals that reactive porous flow is an important process of chemical differentiation and melt transport within these mush-dominated regions
Summary
The physicochemical characteristics (such as size, pressure, volatile content and geochemical heterogeneity) of magma storage at volcanic centres located directly above high buoyancy flux mantle plumes (e.g. Kīlauea, Hawai’i and Isabela, Galápagos) have been subject to intense study over the past few decades (Bagnardi et al, 2013; Bernard et al, 2019; Clague and Denlinger, 1994; Geist et al, 1998; Naumann and Geist, 1999; Neal et al, 2019; Park et al, 2007; Pietruszka et al, 2015; Poland et al, 2015; Sides et al, 2014; Stock et al, 2018; Wieser et al, 2020, 2019). The abundance of olivine and absence of plagioclase in the Floreana lavas and scoria contrasts with basalts in the central, northern, and western parts of the Galápagos Archipelago, where plagioclase-phyric and ultraphyric basalts are common (Geist et al., 2002; Gibson et al, 2012; Harpp et al, 2014b). Despite their relatively simple mineralogy, Floreana lava and scoria samples contain texturally diverse olivine crystals which can be divided into five distinct groups (Fig. 2):. Group 5 olivines are only found in a minority of samples and are characterised by the presence of 4 compositional zones with alternating high and low forsterite contents (visible in back-scattered electron images; Fig. 2E and F)
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